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From Alan M. Haywood, John L. Smellie, Allan C. Ashworth, David J. Cantrill, Fabio Florindo, MichaelJ. Hambrey, Daniel Hill, Claus-Dieter Hillenbrand, Stephen J. Hunter, Robert D. Larter, Caroline H. Lear,Sandra Passchier and Roderick van de Wal, Middle Miocene to Pliocene History of Antarctica and the

Southern Ocean. In: Fabio Florindo and Martin Siegert, editors: Developments in Earth andEnvironmental Sciences, Vol 8, Antarctic Climate Evolution, Fabio Florindo and Martin Siegert.

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Developments in Earth & Environmental Sciences, 8F. Florindo and M. Siegert (Editors)r 2009 Elsevier B.V. All rights reservedDOI 10.1016/S1571-9197(08)00010-4

Chapter 10

Middle Miocene to Pliocene Historyof Antarctica and the Southern Ocean

Alan M. Haywood1,�, John L. Smellie2, Allan C. Ashworth3, DavidJ. Cantrill4, Fabio Florindo5, Michael J. Hambrey6, Daniel Hill2,Claus-Dieter Hillenbrand2, Stephen J. Hunter2,1, Robert D. Larter2,Caroline H. Lear7, Sandra Passchier8 and Roderick van de Wal9

1School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK2Geological Sciences Division, British Antarctic Survey, High Cross, Madingley Road,Cambridge CB3 0ET, UK3Department of Geosciences, North Dakota State University, Fargo,ND 58105-5517, USA4Royal Botanic Gardens Melbourne, Private Bag 2000, Birdwood Avenue,South Yarra, Victoria 3141, Australia5Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605,00143 Rome, Italy6Institute of Geography & Earth Sciences, University of Wales, Aberystwyth,Ceredigion SY23 3DB, UK7School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building,Park Place, Cardiff CF10 3YE, UK8Department of Earth and Environmental Studies, Mallory Hall 252, Montclair StateUniversity, Montclair, NJ 07043, USA9Institute for Marine and Atmospheric Research Utrecht, Utrecht University,Princetonplein 5, 3584 Utrecht, The Netherlands

ABSTRACT

This chapter explores the Middle Miocene to Pliocene terrestrial and marinerecords of Antarctica and the Southern Ocean. The structure of the chapter makesa clear distinction between terrestrial and marine records as well as proximal(on or around Antarctica) and more distal records (Southern Ocean). Particulargeographical regions are identified that reflect the areas for which the majorityof palaeoenvironmental and palaeoclimatic information exist. Specifically,

E-mail: a.haywood@see.leeds.ac.uk (A.M. Haywood).

�Corresponding author. Tel.: þ44(0)113 343 8657; Fax: þ44(0)113 343 6716;

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the chapter addresses the terrestrial sedimentary and fjordal environments of theTransantarctic Mountains and Lambert Glacier region, the terrestrial fossil recordof Antarctic climate, terrestrial environments of West Antarctica, and the marinerecords of the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet(WAIS) and the Antarctic Peninsula Ice Sheet (APIS), as well as the marinerecord of the Southern Ocean. Previous and current studies focusing on modellingMiddle Miocene to Pliocene climate, environments and ice sheets are discussed.

10.1. Introduction

The Middle-to-Late Miocene interval is believed to represent a time ofsignificant ice-sheet expansion on Antarctica (e.g. Miller et al., 1991, 2005;Lear et al., 2000; Turco et al., 2001; Billups and Schrag, 2002; Shevenell et al.,2004). The stable isotope record of the deep sea demonstrates that a mid-Miocene ‘‘climatic optimum’’, at B15Ma, was followed by strong enrich-ment in oceanic d18O and climatic cooling over the next 6Ma (e.g. Zachoset al., 2001). During this interval, the East Antarctic Ice Sheet (EAIS) isthought to have been a major and permanent ice sheet, although fluctuationsin the size of EAIS may still have occurred (e.g. Westerhold et al., 2005).Denton et al. (1984) proposed that during this time, the EAIS overrode the

Transantarctic Mountains (Fig. 10.1). Recent studies from the western DryValleys indicate that the atmosphere cooled by as much as B201C prior toan erosional event that is linked to the EAIS overtopping the mountains(Lewis et al., 2007). If such a sequence is repeated elsewhere (i.e. cooling anddevelopment of cold-based alpine glaciers preceded ice-sheet overriding),then it would suggest that the initial rise in deep-sea benthic d18O (Zachoset al., 2001) reflects deep-water cooling, followed later by ice-sheetexpansion. Furthermore, Lewis et al. (2006) propose that immensefreshwater floods to the Southern Ocean from large subglacial lakes beneaththe expanded EAIS occurred between 14.4 and 12.4Ma. The discharges arenot only considered to have been the erosive force that formed suchprominent Dry Valleys features as the Labyrinth, but are also consideredthrough their impact on oceanic circulation to be a cause of mid-Mioceneclimatic change.A different explanation for the enrichment in oceanic d18O is that it

represents the onset of significant glaciation on West Antarctica(Mercer, 1978; Ciesielski et al., 1982). This is supported by the firstoccurrence of ice rafted debris (IRD) at Deep Sea Drilling Project (DSDP)Site 325 in the Bellingshausen Sea during the Early to Middle Miocene

Figure 10.1: Map showing geographical locations discussed in the text.Antarctic bed topography also shown from the BEDMAP dataset (Lythe

et al., 2000).

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404 A. M. Haywood et al.

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(Hollister et al., 1976). However, the relative role that the EAIS and WestAntarctic Ice Sheet (WAIS) played in the Middle Miocene ClimateTransition (MMCT) has recently been questioned by new seismic-stratigraphic data from the Ross Sea revealing at least five major intervalsof grounded ice advance and retreat in the Middle Miocene (Bart, 2003;Chow and Bart, 2003). Much of this ice was sourced from West Antarctica,suggesting the presence of a large and dynamic WAIS prior to the MMCT(Bart, 2003; Chow and Bart, 2003).The stability of Antarctic Ice Sheets during the Late Miocene and Pliocene

has been the subject of almost continuous debate for more than 20 years. Thekey questions in this argument are when did the EAIS switch from beingpolythermal and dynamic to predominantly cold based and stable, and couldrelatively short-lived climatic warm intervals have been sufficient to influencethe Antarctic Ice Sheet? A long-standing view is that the EAIS became stableduring the Middle Miocene, evidence for which is primarily derived fromapparent landscape stability and well-dated surfaces and ash deposits in theDry Valleys region along the western border of the Ross Sea (e.g. Dentonet al., 1993; Marchant et al., 1993a; Sugden, 1996). An alternate view is thatterrestrial glacial deposits, known as the Sirius Group, scattered throughoutthe Transantarctic Mountains, indicate dynamic ice-sheet conditions evenduring the Pliocene. This conclusion is based on the occurrence of Pliocene(and older) diatoms reworked into glacial deposits (Harwood, 1983, 1986;Webb et al., 1984, 1996; Harwood and Webb, 1986; Wilson, 1995; Wilsonet al., 1998). The dynamic nature of the Pliocene Antarctic Ice Sheet issupported by the Pagodroma Group along the flanks of the Lambert Glacier(e.g. Hambrey and McKelvey, 2000a,b). Each view is internally consistentand scientists have been presented with a significant challenge in reconcilingthe different views. When considering the size and diverse landscapes ofAntarctica, we should not be surprised to see a degree of heterogeneity in theclimate and environmental response. Yet the current state of knowledge is socontradictory that the scientific community has become polarized into twocamps (‘‘stabilists and dynamicists’’) over the issue of Middle Miocene toPliocene conditions on Antarctica (e.g. Harwood and Webb, 1998; Stroevenet al., 1998a,b).

10.2. East Antarctic Terrestrial Environments

Two main pre-Quaternary sedimentary sequences recording glacial eventsare found on land in East Antarctica, the Sirius Group in the Transantarctic

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 405

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Mountains and the Pagodroma Group in the Prince Charles Mountainsflanking the Lambert Glacier (Fig. 10.2). The two sequences are quitedifferent in character. The Sirius Group is a mainly terrestrial glacigenicsuccession whose age has proved highly controversial, with Pliocene, mid-Miocene or even older ages suggested. The Pagodroma Group is an ice-proximal fjordal sequence up to several hundred metres thick, and is welldated on the basis of in situmarine fossils. All these well-exposed deposits areimportant in unravelling the history of the EAIS, as they are the nearest

Figure 10.2: (a) Map of Antarctica showing key sites of investigations ofon-shore pre-quaternary glacigenic sediments. (b) The distribution of knownSirius Group outcrops. (c) Location of the Pagodroma Group sites along theLambert Graben (redrawn from Hambrey and McKelvey, 2000a; Hambrey

et al., 2003).

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deposits to the heart of the ice sheet, and yield valuable informationconcerning palaeogeography and palaeoclimate.

10.2.1. Transantarctic Mountains

Outcrops of glacigenic sedimentary rocks of the Sirius Group occur aserosional remnants scattered throughout the Transantarctic Mountains asfar south as 861S (Fig. 10.3). The deposits are exposed in two typical settings:(1) as thin erosional remnants at high elevation in palaeovalleys or on flatmountain summits, or (2) as sequences more than 100m thick along the wallsof broad trunk valleys occupied by large outlet glaciers draining the modernEAIS. The main sedimentary facies are massive diamicts, massive bouldergravels, stratified pebbly sands and muds, and laminated sand/mud coupletswith dropstones (Fig. 10.3). Diamicts overlie spectacular grooved andstriated pavements in some areas, notably at Roberts Massif. In theDominion Range, bordering Beardmore Glacier, well-preserved Nothofagusleaves, stems and roots, plus mosses and insects are interbedded with thediamict (Francis and Hill, 1996; Ashworth et al., 1997). The diamicts aredominated by subangular to subrounded striated and facetted clasts. Theenvironment of deposition is interpreted as largely subglacial to proglacial,with extensive wet-based, probably polythermal glaciers being the agent ofsediment transport, although a marine influence is evident through marinefossil remains in part of the Beardmore Glacier succession (e.g. Mercer, 1972;Mayewski and Goldthwait, 1985; McKelvey et al., 1991; Webb et al., 1996;Stroeven and Prentice, 1997; Wilson et al., 1998, 2002; Hambrey et al., 2003).The lithostratigraphy and depositional setting of the Sirius Group at two ofthe southernmost locations is summarized in Table 10.1.A debate was ignited when Webb et al. (1984), based on recycled marine

diatoms, linked the Sirius Group to glacial expansion after early Pliocenedeglaciation and marine incursion of East Antarctica. In contrast, Dentonet al. (1984) attributed the Sirius Group to mid-Miocene overriding of theTransantarctic Mountains based on geomorphological evidence from theDry Valleys. A dynamic ice-sheet hypothesis and a stable ice-sheethypothesis developed, which represented contrasting views of NeogeneAntarctic climate and glacial dynamics. Pivotal to this debate is ourunderstanding of the uplift and erosional, hence unroofing, history of theTransantarctic Mountains (e.g. Webb et al., 1984; Behrendt and Cooper,1991; Sugden et al., 1995; Kerr and Huybrechts, 1999). The TransantarcticMountains are divided into several crustal blocks which vary in size on arange of scales and which probably experienced uplift at different rates and

Figure 10.3: Glacial erosional features and sedimentary characteristics of theSirius Group, Transantarctic Mountains. (A) Aerial view of sub-Sirius Groupglacially abraded surface on Roberts Massif, showing patch of diamict, post-depositional faults, and a superimposed recessional quaternary morainesystem. (B) Multiple beds of diamict comprising the W100m thick ShackletonGlacier formation at the type locality alongside upper Shackleton Glacier.(C) Large-Scale Glacial Grooves on Jurassic Dolerite, predating deposition ofthe Sirius Group. (D) Striated dolerite surface with overlying Sirius Groupdiamict, Roberts Massif. (E) Typical massive diamict facies of the SiriusGroup, showing alignment of clasts, interpreted as basal till, Roberts Massif.(F) Laminated silt and thin diamicts with large dropstone, interpreted as anice-contact lake deposit in the Shackleton Glacier formation, Bennett Platform

(redrawn from Hambrey et al., 2003).

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Table

10.1:RepresentativestratigraphyoftheSiriusandPagodromaGroupswithprincipallithofacies

and

interpretationofpalaeoenvironment.

Group

Form

ations

Principalfacies

Paleoenvironment

ShackletonGlacier

Beardmore

Glacier

BennettPlatform

Meyer

Desert(Pliocene)

Massiveandweakly

stratified

diamict,

massiveboulder

gravel,stratified

sandstone,

breccia;sand/mudlaminate

withdropstones

atShackletonGlacier.

Wellpreserved

Nothofagusflora

at

Beardmore

Glacier.Allfacies

well

indurated

Subglacial;some

supraglacial;proglacial

glaciofluvial;ice

contact

lake

ShackletonGlacier

Cloudmaker

(Pliocene–?M

iocene)

Sirius

Eroded

remnants

as

erratics

Massiveandstratified

diamictite;sandy

breccia/conglomerate;partiallysilicified

woodfragments;welllithified

Subglacialwithextensive

mass-m

ovem

entand

fluvialreworking;well-

developed

flora

Shackletonerosionsurface/dominionerosionsurface

Group

Form

ations

Principalfacies

Paleoenvironment

AmeryOasis

Fisher

Massif

Bardin

Bluffs

(Pliocene–?Pleistocene)

Massiveboulder

gravel;massivediamict;

gravel;laminate

withdropstones.Rich

diatom

microflora.Allweakly

towell

indurated

Ice-proxim

alfjordal,

grounding-linefan

complexes

predominant,to

distal

glaciomarine

Pago-droma

BattyeGlacier

(mid-M

iocene)

Fisher

Bench

(mid-M

iocene)

MountJohnston

(?Oligocene–early

Miocene)

Ages

ofform

ationsare

given

wheredetermined

usingdiatom

biostratigraphy.Nocorrelationisim

plied

intheSiriusGroup.Data

from

Webbet

al.(1996)for

theSiriusGroup

atBeardmore

Glacier;Hambreyet

al.(2003)fortheSiriusGroup

atShackleton

Glacier;Hambreyand

McK

elvey

(2000a)forthe

PagodromaGroup.

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Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 409

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during different periods, principally in the late Cretaceous and early Tertiary(prior to c. 40Ma; Stump and Fitzgerald, 1992; Fitzgerald, 1994; Bussettiet al., 1999; van der Wateren et al., 1999). The uplift histories of adjacentblocks might also be different. The younger uplift and erosional history isonly relatively well known for the Dry Valleys block, as a result of detaileddetrital studies by the Cape Roberts Project (CRP). Those studies showedthat at the deepest levels cored, equivalent to c. 34Ma, the kilometre-thickKirkpatrick Basalts (Jurassic) had already been almost completely removedand the sand modes were dominantly derived from sandstones of theVictoria Group, then the Taylor Group (Devonian–Triassic Beacon Super-group; Smellie, 2000a,b, 2001a,b, unpublished; Talarico et al., 2000;Sandroni and Talarico, 2001). By c. 33Ma, the 2 km-thick BeaconSupergroup had been cut through to expose outcrops of basement rock(early Palaeozoic and Precambrian granitoids and metamorphic rocks),which then began to contribute significant detritus. Between that time andc. 29Ma, tectonic stability and, presumably, little uplift-related erosion areinferred from the essentially unchanging detrital modes. Further changes inthe detrital modes between 24 and 16Ma suggest another phase of instabilityand possibly uplift/unroofing, after which the record is not preserved.Although the detrital record is interpreted here in terms of simple uplift andunroofing, an alternative explanation is that the changes observed mightreflect varied phases of climate-related erosion independent of uplift(e.g. enhanced downcutting by glaciers; cf. Kerr and Huybrechts, 1999).This is an ambiguity that has yet to be resolved.According to the dynamic ice-sheet hypothesis, the diatom assemblages

incorporated in the Sirius Group record periods when East Antarctic basinswere ice-free and became inundated by the sea (Harwood, 1986; Harwoodand Webb, 1998). The time intervals of which diatoms are lacking recordthe stages when either ice was covering the inland basins or the floors of thebasins were exposed. According to stabilists, however, the basic assumptionderived from ice-sheet modelling is that deglaciation requires considerableclimatic warming (Huybrechts, 1993). A substantial body of internallyconsistent evidence (including ash and cosmogenic dating) for a pre-middleMiocene landscape, subsequently unmodified by ice, has been published innumerous papers (e.g. Marchant et al., 1993a,b,c, 1996; Sugden et al., 1995;Sugden, 1996; Sugden and Denton, 2004). For example, isotopic ages ofW14Ma for the in situ, unweathered, ash deposits from the higher elevatedregions of the Dry Valleys (Marchant et al., 1993b) are in disagreement withthe concept of warming causing major deglaciation in the early–mid-Pliocene. The diatom evidence for a Pliocene age of the Sirius Group hasbeen disputed (Barrett, 1996; Gersonde et al., 1997; Stroeven et al., 1998a,b).

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Field studies and studies of aerial photographs provided evidence ofrejuvenation of faulting during and after deposition of the Sirius Group(Hambrey et al., 2003). Moreover, based on morphostratigraphic constraintsand provenance analyses, several authors pointed out that the Sirius Groupcomprises deposits of multiple glaciations within different wet-based ice-sheet drainage systems, which operated during consecutive stages of glacialdenudation in the Transantarctic Mountains (e.g. Mercer, 1968, 1972;Brady and McKelvey, 1979, 1983; McKelvey et al., 1991; Stroeven, 1997;Van der Wateren et al., 1999; Passchier, 2001, 2004; Hambrey et al., 2003).Unfortunately, the absolute ages of the individual formations within theSirius Group remain inconclusive and lithostratigraphic correlations arehampered by the complex tectonic framework of the TransantarcticMountains. Given the considerable linear geographical spread of the SiriusGroup over a distance of c. 1,500 km and spanning nearly 101 of latitude, thegrouping of these strata under a single group name has exacerbated thearguments concerning the age. Resolution of the age question remains a keychallenge for Antarctic geologists, but it is likely that, although the SiriusGroup may contain disputed Pliocene or Miocene elements, it could alsoextend back to Oligocene time in view of the presence of glaciomarinesediments of this age offshore (Barrett, 1996; Hambrey et al., 2002; Franciset al., 2008).

10.2.2. Lambert Glacier Region

The Pagodroma Group is a succession of massive diamicts and bouldergravels with minor stratified diamicts, laminites, sand and gravel, croppingout in the Prince Charles Mountains along the western margin of theLambert Graben (Figs. 10.2 and 10.4). The best known outcrops are atAmery Oasis and Fisher Massif (Bardin, 1982; Hambrey and McKelvey,2000a; McKelvey et al., 2001; Whitehead et al., 2003, 2006a), but equivalentexposures are known from Mount Menzies, the last exposed rock in thisregion heading polewards. The total outcrop distance of known glacigenicstrata along the flank of the graben is about 800 km. The graben is occupiedby the Lambert Glacier, a south–north-flowing outlet glacier, whose totalice-drainage area represents 13 per cent or 1million km, of the EAIS. It hasacted as a conduit for ice flowing into Prydz Bay since its initial formation atthe Eocene–Oligocene transition (Barron et al., 1991; Strand et al., 2003).Although the Pagodroma Group is regarded as the East Antarctic equivalentof the Sirius Group, there are major differences in lithofacies, facies thicknessand geometry, fossil content and palaeoenvironment. For example, these

Figure 10.4: Glacial erosional features and sedimentary characteristics of thePagodroma Group, Prince Charles Mountains. (A) Uplifted segment of apalaeofjord bottom and sidewall (arrowed), filled with nearly 300m of ice-proximal fjordal sediment of the Miocene Fisher Bench formation, FisherMassif. (B) Striated pavement on Amery Group Sandstone, Radok Lake,Amery Oasis; this surface is coeval with the lower part of the Pliocene BardinBluffs formation. (C) Typical massive to crudely stratified boulder gravel ofthe Miocene Fisher Bench formation, Fisher Massif, interpreted as ice-contact deposits associated with a grounding-line fan. (D) Weakly stratifieddiamict and boulder gravel of the Pliocene Bardin Bluffs formation at thetype locality, interpreted as ice-proximal deposits; the cliff face here is about60m high. (E) Laminated silt and thin diamictites representing depositionadjacent to a grounding-line fan as quiet water cyclopels and cyclopsams,interrupted by gravity-flow deposition, Miocene Battye Glacier formation,Dragons Teeth, Amery Oasis (redrawn from Hambrey and McKelvey, 2000a).

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deposits contain in situ diatom assemblages that can be used to date theformations directly (Whitehead et al., 2003, 2004).The four formally defined formations within the Pagodroma Group

(Table 10.1) have age ranges from early Miocene (or possibly Oligocene) toPliocene–Pleistocene (Hambrey and McKelvey, 2000b; Whitehead et al.,2003, 2006a). The strata at Mount Menzies (Whitehead and McKelvey,2002) have not yet yielded any datable fossils. The massive diamicts andboulder gravels indicate deposition in an ice-proximal environment near agrounding line, whereas the stratified facies represent more distal icebergdeposition. The diamicts contain striated and facetted clasts within a matrixthat is finer grained than modern Antarctic tills. Dropstone structures occurin thin laminated beds and resemble glacigenic marine rhythmites (cyclopelsand cyclopsams) that commonly occur to the sides and front of a grounding-line fan (Fig. 10.5). The depositional environments are considered to beanalogous to the modern fjords of East Greenland with fast-flowingpolythermal tidewater glaciers (Hambrey and McKelvey, 2000b). The BattyeGlacier Formation contains diatom-bearing beds (opal contents up to15wt.%) with in situ mollusc assemblages, which are assumed to have beendeposited in ice-distal settings (Whitehead et al., 2006a). The PagodromaGroup has a cumulative thickness of W800m and provides evidence formajor shifts in the position of the grounding line of the Lambert Glacierthroughout the Neogene (Hambrey and McKelvey, 2000a).The Pagodroma Group represents remnants of a much more extensive

fjord-fill sequence that has been uplifted above sea level (a.s.l.) (Fig. 10.4).Palaeofjord floors have been uplifted to different levels at Amery Oasis andFisher Massif, the highest being nearly 1,500m on the latter. In contrast, themain Lambert trough has been excavated to 2,000m below sea level, thesediment from which has been delivered into Prydz Bay since the Eocene/Oligocene transition, where it forms the prograded continental shelf,including the huge late Neogene Prydz Trough-Mouth Fan that was drilledduring Ocean Drilling Program (ODP) Leg 188 (Passchier et al., 2003;O’Brien et al., 2004). The Prydz Bay continental margin has been drilled byODP Legs 119 and 188 (see Barron et al., 1991; Cooper et al., 2004;Whitehead et al., 2006b). Numerical modelling of ice-sheet expansion andrecession in the Lambert Glacier–Prydz Bay region has demonstrated thatwhen major phases of erosion and sediment delivery took place, the icedynamics were strongly controlled by the changing bathymetry of the ice-eroded graben (Taylor et al., 2004).The clay mineralogy of the Pagodroma Group indicates a dominance of

illite and chlorite in the Middle Miocene or older Mount Johnston andFisher Bench Formations, indicative of physical weathering and erosion of

Figure 10.5: Distribution of Neogene (15–2Ma) terrestrial volcanic outcropsin West Antarctica.

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 413

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local metavolcanic and gneissic source rock terrains under glacial conditions(Ehrmann et al., 2003). In contrast, the Middle–Late Miocene Battye GlacierFormation contains significant contributions of smectite and kaolinite(Whitehead et al., 2006a), and the Pliocene–Pleistocene Bardin BluffsFormation has the highest kaolinite concentrations in the absence ofsmectite. Formation of kaolinite and smectite in source rock terrainsgenerally requires prolonged exposure of source rocks to chemical

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weathering. The chemically altered nature of the strata is confirmed bybulk chemical and rock magnetic studies (Bloemendal et al., 2003; Passchierand Whitehead, 2006). Recycling of weathered materials from localsedimentary source rocks cannot be ruled out (Ehrmann et al., 2003).The origin and significance of the chemically weathered materials in theMiddle Miocene and younger Pagodroma Group formations remainsinconclusive at this time and further field and laboratory studies arenecessary to resolve this issue.The two regions described above represent the best studied examples of

pre-Quaternary onshore glacigenic successions, but there are others else-where, e.g. the Antarctic Peninsula (Hambrey et al., 2008) and the GroveMountains (Fang et al., 2004). When a better-dated Sirius Group canbe linked to the offshore record in McMurdo Sound, and the data from thePrince Charles Mountains fully integrated with the emerging Prydz Bayrecord, we will have a good understanding of the behaviour of the EAISsince its inception in these segments of the Antarctic margin.

10.2.3. The Terrestrial Fossil Record of East Antarctic Climate

The onset of Antarctic glaciation during the Oligocene dramatically alteredterrestrial environments ultimately resulting in widespread extinction ofplant and animal taxa (Cantrill, 2001). Tundra environments, based onpollen from the Cape Roberts borehole, are known to have existed in theRoss Sea region through the Oligocene until at least the early Miocene(Askin and Raine, 2000; Raine and Askin, 2001). The final extinction ofplants and animals with more complex life histories is assumed to beassociated with the shift to the polar desert climate which prevails today.Whether or not this immediately followed the mid-Miocene warm interval orwas as late as the mid-Pliocene is the most outstanding problem in Neogeneclimatic history in need of resolution.Pre-Pleistocene glacigenic sequences (Sirius and Pagodroma Groups) are

patchily exposed throughout the Transantarctic and Prince Charlesmountains. They are dominated by glaciomarine fjordal sediments,lodgement tills and outwash deposits (see previous section for more details).At the Oliver Bluffs on the upper Beardmore Glacier, fossiliferous sequencesoccur within the Meyer Desert Formation. The discovery of wood(Carlquist, 1987) and later Nothofagus leaves (Hill and Truswell, 1993;Webb and Harwood, 1993; Francis and Hill, 1996; Hill et al., 1996) from alocation about 500 km from the South Pole makes this arguably the mostimportant fossil site in Antarctica. The fossils provide an unusual window

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 415

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into Neogene environments and biogeography but until questions surround-ing their age can be better resolved, their value as palaeoclimatic proxies islessened.The deposits have been assigned a Pliocene age using a biostratigraphic

argument based on reworked marine diatoms (Harwood, 1986; Webb et al.,1996). Recently, however, cosmogenic dates obtained on boulders frommoraines that post-date the Meyer Desert Formation indicate ages of greaterthan 5Ma (Ackert and Kurz, 2004). The authors propose various age modelsbased on different rates of weathering and discuss different amounts ofuplift, but based on their discussion, a Miocene age for the Meyer DesertFormation is preferred over a Pliocene age (Ackert and Kurz, 2004). If theolder age for the Meyer Desert Formation prevails, then it is tempting tospeculate that the degree of melting of the ice sheet that resulted in fjordsopening up into the Transantarctic Mountains was associated with the mid-Miocene climatic optimum. That interpretation would be more acceptable toresearchers in the Dry Valleys who present evidence that the shift from wet-to cold-based glaciation occurred during the Middle Miocene (Denton et al.,1993; Marchant et al., 1993a; Sugden, 1996).The fossil Nothofagus leaves from the Meyer Desert Formation were

described as a new species, N. beardmorensis, but with morphologicalsimilarities to extant species in South America and Tasmania (Hill et al.,1996). Originally, a krummholz growth form (scrubby, stunted growth form)was assigned to the Nothofagus (Webb and Harwood, 1993) but later studiesof the growth ring anatomy (Francis and Hill, 1996) suggested a lowprostrate and spreading habitus, more similar to ground-hugging shrubsgrowing today along exposed parts of treeline on Isla Navarino at thesouthern tip of South America (Ashworth and Kuschell, 2003, p. 193). Theprostrate growth forms implied colder growing conditions than estimatedusing the krummholz interpretation, and considerably lower than thecomparison to the cold-temperate rain forest of southern South Americamade on the basis of a pollen study (Askin and Markgraf, 1986; Mercer,1986). Nearest living relative comparisons, along with physiologicalexperiments into cold tolerance of Nothofagus, yielded theoretical estimatesof mean summer month temperatures of at least 51C for 3 months and lowwinter month temperatures of �15 to �221C. Mean annual temperature(MAT) was estimated to have been in the range of �8 to �121C (Francis andHill, 1996; Ashworth and Kuschel, 2003) compared to an interpolated sea-level MAT at latitude 851S today of �261C (Ashworth and Kuschel, 2003).Although much of the research to date has focused on Nothofagus, the

vegetation was more complex consisting of cushion plants, grasses,ranunculids (buttercups) and mosses (Ashworth and Cantrill, 2004).

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It should be emphasized that to date, only one site has been reported fromthe Beardmore Glacier and leaves and wood of Nothofagus have not beenreported from other Sirius Group sequences although they have beendiscovered recently in pre-Pleistocene glacial sequences of the western DryValleys (Ashworth et al., unpublished).The Meyer Desert Formation also contains fossils of beetles, including two

weevil species, a higher fly, single species each of a freshwater gastropod andbivalve, an ostracod and a fish (Ashworth and Kuschel, 2003; Ashworth andPreece, 2003; Ashworth and Thompson, 2003; Ashworth et al., 1997). Usingoverlapping autecological requirements for Nothofagus, listroderine weevilsand freshwater molluscs, mean summer temperatures are estimated to havebeen 4–51C for at least two summer months and the MAT was estimatedto be �81C, similar to the temperatures estimated from the physiologicalrequirements of the Nothofagus fossils (Ashworth and Kuschel, 2003;Ashworth and Preece, 2003). The palaeosols, which are stratigraphicallyhigher than the fossiliferous beds, indicate polar conditions (Retallack et al.,2001), and their development may signal the end of warmer and wetterclimates in Antarctica.Regardless of age, there is a question regarding the likelihood of a tundra

biome re-establishing itself in the interior of Antarctica during a warm phaseif it had been extirpated during an earlier cold phase. The question isimportant because it sets limits on how cold the climate of Antarctica couldhave been before the Meyer Desert Formation biota colonized the interior.Did the species disperse from refugia around the margins of Antarctica, inwhich case the climate had to have been continually warmer around themargins of the continent through at least the Early Miocene? Or, if theybecame extinct prior to the mid-Miocene climatic optimum or mid-Pliocenewarm interval (B3Ma), could they have reinvaded the continent?For Nothofagus, seed germination is considered to be critical to answering

this question. Studies suggest that Nothofagus seeds cannot surviveprolonged periods of immersion in salt water (Hill et al., 1996) andconsequently cannot be easily dispersed across oceanic barriers. Theimplications of this are that components of the vegetation survived onAntarctica even during earlier glacial phases. There is a scenario, however,based on phylogenetic and palynological studies, that allows for thepossibility that Nothofagus may have dispersed from Australia to NewZealand after the Tasman Sea formed (McGlone et al., 1996). Even if trans-Tasman dispersal occurred, it would have been from land areas with similarclimatic conditions that would favour successful colonization. For organismsto disperse from South America or New Zealand to Antarctica would requirenot only much greater distances of dispersal but for organisms to re-establish

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 417

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themselves in complex communities in a different climatic regime. Thismakes the long-distance dispersal scenario for recolonization of Antarcticaeven less probable. We note that since deglaciation at the end of thePleistocene, climatic conditions on many of the sub-Antarctic islands havebeen favourable for colonization by Nothofagus, listroderine weevils and thetypes of freshwater molluscs found in the Meyer Desert Formation, but ithas not occurred. This strongly implies that refugia were present on themargins of Antarctica, from which plants, insects and freshwater molluscscould migrate to the continental interior during a time or times of significantclimatic warming (cf. Convey et al., in press).

10.3. West Antarctic Terrestrial Environments

West Antarctica contains numerous outcrops that preserve, or potentiallypreserve, information on Miocene and Pliocene terrestrial palaeoenviron-ments. All are volcanic, although several contain associated, usually minor,sedimentary strata. Because of the long time gaps between eruptions inany volcanic centre, the environmental record they preserve is typicallyof a coarse resolution, and they have often been ignored as a potentiallyimportant source of palaeoenvironmental information. The outcrops areoften remote or inaccessible, and many have been rarely visited. Summaryinformation on all those outcrops of Middle Miocene and Pliocene age isgiven in Table 10.2 (see also Fig. 10.5). The eruptive environment is relativelywell understood for a few but remains uncertain for many.

10.3.1. Antarctic Peninsula

Most of the outcrops in the Antarctic Peninsula have been relatively wellinvestigated in recent years. They form two main types: outcrops dominatedby a major stratovolcano (James Ross Island Volcanic Group) and outcropsformed by multiple monogenetic volcanic fields and small isolated centres(Bellingshausen Sea Volcanic Group; Smellie, 1999). James Ross Island,situated at the northern end of the Antarctic Peninsula, comprises a singlelarge stratovolcano 1.5 km high and about 60 km in diameter, together withnumerous small satellite centres. About 50 individual eruptive episodes havebeen identified, with most retaining information on environmental condi-tions between 6.25Ma and today. All but a few show features diagnostic ofglacial conditions (Smellie, 2006). The associated ice sheets had a provenance

Table

10.2:Characteristics

ofvolcanic

outcropsofNeogeneAge(15–2Ma)in

WestAntarctica.

Outcrop

Age(M

a)

Dating

method

Environmen

tal

characteristics

aComments

Principalsources

AntarcticPeninsula

Alexander

Island

7.5–5.4;2.5–

2.7

K–Ar

Glacial(1

and2)b;

subaerial

Multiple

monogenetic

centres

SmellieandHole

(1997),Smellie

(1999)

Merrick

Mountains

6K–Ar

Subaerialc;glacial?

Tinydegraded

outcropremnants

Smellie(1999)

James

Ross

Island

region

o6.5

40/39Ar

Glacial(1)and

glaciomarine

(interglacial)

Dissected

stratovolcanoand

multiple

satellite

centres

Nelson(1975),

Skilling(1994)

Ellsw

orthLand

Snow

Nunataks

Undated

Glacial(1)

Multiple

monogenetic

centres

LeM

asurier

and

Thomson(1990)

Jones

Mountains

10–7?

(imprecise)

K–Ar

Glacial(2?)

HudsonMountains

8.5–3.7

K–Ar

Subaerial,subaqueous

and/orglacial

Multiple

smallcentres

LeM

asurier

and

Thomson(1990)

MarieByrd

Land

MountFlint

14.1

K–Ar

Unknown

Stratovolcano

LeM

asurier

and

Thomson(1990)

Ames

Range

14.7–10.0;

10.3–10.0;

7.6–5.7

K–Ar

Unknown

Threeoverlapping

stratovolcanoes

LeM

asurier

and

Thomson(1990)

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Whitney

Peak,

Executive

CommitteeRange

13.7

K–Ar

Subaerial?c

Undissected

stratovolcano

LeM

asurier

and

Thomson(1988)

MountHampton,

Executive

CommitteeRange

11.7–8.6

K–Ar

Subaerial?c

Undissected

stratovolcano

LeM

asurier

and

Thomson(1989)

MountHartigan,

Executive

CommitteeRange

8.5–6.4;7.86–

7.57

K–Ar

Subaerial?c

Twopoorlyexposed

volcanoes:Lavris

PeakandTusing

Peak

LeM

asurier

and

Thomson(1990)

MountCumming,

Executive

CommitteeRange

10.4–10.0;3.0

K–Ar

Subaerial?c

Undissected

stratovolcano

LeM

asurier

and

Thomson(1990)

MountSidley,

Executive

CommitteeRange

5.7–4.2

40/39Ar

Subaerialcandsnow

contact

Threecoalesced

stratovolcanoes

MountBursey

10.4–8.54;6.4

K–Ar

Unknown

Twooverlapping

undissected

stratovolcanoes

LeM

asurier

and

Thomson(1990)

Toney

Mountain

10.1–9.1

K–Ar

Unknown

Plateaulavabasal

succession

LeM

asurier

and

Thomson(1990)

HobbsCoast

9.56–3.19

K–Ar

Glacial

Numeroussm

all

dissected

centres

LeM

asurier

and

Thomson(1990)

MountMurphy

9.39–8.22

40/39Ar

Glacial(1

and

(mainly)2)and

subaerialc

Dissected

stratovolcano

Smellie(2000a,b),

LeM

asurier

(2002),

Wilch

andMcIntosh

(2002)

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 419

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Table

10.2:(C

ontinued

).

Outcrop

Age(M

a)

Dating

method

Environmen

tal

characteristics

aComments

Principalsources

Mt.Murphy

satellitecentres

6.8–5.95

40/39Ar

Glacial(1)

Smallmonogenetic

centres

Smellie(2001a,b),

Wilch

andMcIntosh

(2002)

Crary

Mountains

9.13–6.82;

8.46–5.74;

4.25–1.81

40/39Ar

Glacial(2);subaerialc

Threevolcanoes:Mt

Rees,MtFrakes

and

MtSteere

Wilch

andMcIntosh

(2002)

MountMoulton

4.9–4.7

K–Ar

Unknown

Twooverlapping

undissected

stratovolcanoes

LeM

asurier

and

Thomson(1990)

MountBerlin

2.58–2.32

K–Ar

Subaerialcandglacial?

Twocoalesced

undissected

stratovolcanoes

LeM

asurier

and

Thomson(1990)

FosdickMountains

4.5–3.4

K–Ar

Unknown

LeM

asurier

and

Thomson(1990)

aIgnoreslate-stagesubaerialparasiticcones

(MarieByrd

Land),often

verymuch

younger

(bym.y.)thanthemain

stratoconesuccessions,whichthey

overlie.

bGlacial1:‘‘thickice’’conditions(few

hundredmetres);Glacial2:‘‘thin

ice’’conditions(o

150m);after

Smellie(2000a,b).

cEnvironmentallyundiagnostic

since

subaerialvolcanicsmaybesupraglacial;evidence

for(lower

elevation)glacialconditionsnotexposed.

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Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 421

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rooted both in the Antarctic Peninsula and in a local ice cap centred on thevolcano itself (Hambrey and Smellie, 2006). The ice was wet based anderosive throughout, although there are few thermal regime data for unitsyounger than 2Ma. Conversely, a few of the volcanic units showed evidencefor eruptions in marine conditions, and marine fossils in interbeddedsedimentary deposits also suggest that warmer ice-poor conditions (inter-glacials) occurred during at least three broad periods: 6.5–5.9, 5.03–4.22 ando0.88Ma (Smellie et al., 2006a,b). The James Ross Island Volcanic Group isnow known to contain numerous sedimentary interbeds, mainly varieties ofdiamicts with a glacial association that are only beginning to be investigatedfully (Hambrey and Smellie, 2006; Hambrey et al., 2008).By contrast, the Bellingshausen Sea Volcanic Group contains multiple

small volcanic centres with ages ranging between 7.5 and 2.5Ma (Smellie,1999). The glacial environment was wet-based, temperate or subpolar,and varied between thin and thick ice conditions (o70 and W500m,respectively), although not all outcrops yielded the full range of information(Table 10.2; Smellie et al., 1993; Smellie and Hole, 1997). There also exists asmall outcrop of 2.5Ma tephra (Hornpipe Heights Formation, northernAlexander Island) erupted under essentially dry subaerial conditions,presumably when any associated glacial cover was below the presentoutcrop (i.e. below 750m a.s.l.). Scant evidence from elsewhere on AlexanderIsland suggests that glacial conditions may have been present at 2.5Ma,although few Pliocene samples are dated and the age resolution is poor(Smellie, 1999).

10.3.2. Ellsworth Land

Three outcrops occur in this region: Snow Nunataks, Jones Mountains andHudson Mountains (Table 10.2). The Hudson Mountains are particularlypoorly known. Conversely, Snow Nunataks comprises several small undatednunataks that have been divided into the Mount Benkert and MountMcCann formations, formed of subaqueous stratified hyalotuff andsubaerial lava, respectively (Smellie, 1999). The formations are cogeneticand resemble basaltic glaciovolcanic centres known as tuyas (Smellie,2000a,b) erupted probably under ice thicknesses that locally exceeded 350m.However, a marine environment, while probably less likely, cannot beexcluded since so little is known about the area. A 700m-thick Cenozoicvolcanic succession resting on a subhorizontal polished and striated glacialunconformity crops out at Jones Mountains (Rutford et al., 1972; Hole et al.,1994). Diamictite lenses interpreted as tillite overlie the unconformity and the

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overlying sequence has been divided into at least two units of pillow lava,hyaloclastite breccia and hyalotuff. Striated and facetted erratic boulders arealso scattered throughout the lower volcanic unit. The descriptions mostclosely resemble volcanic sequences formed under a thin glacial cover.However, since individual ‘‘thin ice sequences’’ are theoretically limited tothicknesses r150m (Smellie, 2000a,b, 2001a,b), the much greater totalthickness of volcanic rock present suggests that more than two volcanicsequences are present. This is also suggested by the presence of numerousreworked hyalotuff lenses dispersed within the sequence, and possibly by thewide range in the published isotopic ages (mainly 7–10Ma; Table 10.2),which might suggest an outcrop constructed from multiple eruptions thatoccurred over a long period.

10.3.3. Marie Byrd Land

Marie Byrd Land contains the largest Cenozoic volcanic field in Antarctica(LeMasurier and Thomson, 1990). It is also the most remote region and isamong the least visited and least well known of all Antarctic volcanoes.Numerous reports published in the 1970s and 1980s suggested a subglacialorigin for many centres (summarized by LeMasurier and Rocchi, 2005).While agreeing generally that the eruptive environment was probably glacial,the published criteria used most often (e.g. mainly presence of hyaloclastite;less often interbedded tillite and/or glacially scoured surfaces) are toosimplistic and permit only limited interpretation of the eruptive environment,except in rare outcrops where more modern detailed studies have beencarried out. In addition, most Marie Byrd Land volcanoes oc. 12Ma in agehave undergone only minimal dissection, probably as a result of a change toa polar thermal regime (LeMasurier and Rocchi, 2005; cf. Armienti andBaroni, 1999), so only the latest and topographically highest eruptedproducts are available for examination. As those units were usually eruptedsubaerially, they provide little of environmental use except a limitingelevation for any coeval ice sheet. However, caldera and sector collapseshave led to a few inland volcanoes being well exposed (e.g. Crary Mountains,Mount Sidley: Wilch and McIntosh, 2002), and many of the coastalvolcanoes are deeply dissected (e.g. Mount Murphy: LeMasurier et al., 1994;Smellie, 2000a,b, 2001a,b; LeMasurier, 2002; Wilch and McIntosh, 2002;LeMasurier and Rocchi, 2005). Only the better exposed and describedexamples are considered here.Mount Murphy is situated on the coast in eastern Marie Byrd Land. It is a

complex polygenetic edifice constructed from at least three centres, of which

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 423

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only the westernmost is well exposed. The basal section, up to c.1,400m a.s.l., is basaltic and it has a thinner capping sequence of mainlytrachyte lavas. The basaltic section is dominated by multiple unitsindividually several tens of metres thick and composed of stratified hyalotuff,hyaloclastite breccia and sheet or pillow lava. Each unit is separated bya glacial unconformity that is often draped by a thin diamict (tillite), and afew units are capped by strombolian scoria. Although LeMasurier (2002)interpreted the Mount Murphy succession in terms of lava-fed deltas (andtherefore, by implication, eruptions beneath ‘‘thick ice’’), the sequences aremuch more similar to those associated with a thin glacial cover, i.e.composed mainly of snow, firm and/or fractured ice that was never morethan tens of metres thick (Smellie, 2000a,b, unpublished). The sequencesextend down to below 400m a.s.l., and they reflect the local dynamics of asmall ice cap probably similar in appearance to that centred on MountMurphy today. Conversely, several satellite centres at Icefall Nunatak,Turtle Rock and Hedin Nunatak have younger ages of 6.8–5.95Ma(Table 10.2). They were associated with a low-elevation (o600m a.s.l.) icesheet composed of relatively thick ice (at least several hundred metres). Thatice was thicker than today and had a surface elevation that fluctuatedthrough 200m during the period (Smellie, 2000a,b, 2001a,b). A wet-basedthermal regime (temperate or subpolar ice) characterized the glacial coverassociated with the main (8–9Ma) Mount Murphy edifice, but its nature isunknown for the younger satellite centres. Finally, Mount Murphy isoverlain by a thin laminated mudstone interpreted as a glacial lake sequencerelated to a significant but undated overriding event (o3.5Ma) by anextraordinarily thick former ice sheet that was at least 1,500m thicker thanat present (LeMasurier et al., 1994).Crary Mountains (9.13–1.81Ma; Table 10.2) are situated 250 km inland of

Mount Murphy, where the surrounding WAIS surface elevation is1,600m a.s.l. Only the two older northern centres (Mount Rees and MountSteere) are well exposed and have ages in the range between 8 and 9Ma,i.e. coeval with Mount Murphy. The mafic to intermediate successionsare individually o100m thick and composed of alternating ‘‘dry’’ lava-dominated and ‘‘wet’’ hyaloclastite-rich lithofacies, interpreted as lavasinteracting with local slope ice and snow (Wilch and McIntosh, 2002).Thus, the coeval Crary Mountains and Mount Murphy successions arebroadly similar, being products of interaction with a thin glacial cover.They differ, however, in that the former lack the interbedded tillites andglacial surfaces that are so prominent at Mount Murphy, a difference thatwas tentatively interpreted to be due to the Crary Mountains ‘‘ice’’ beingcold based.

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Finally, sector collapse at Mount Sidley has exposed a magnificent1,300m-high caldera wall section that extends from c. 2,700 toW4,000m a.s.l. The outer slopes are essentially undissected and expose onlysubaerially erupted volcanic products, including a widespread ‘‘dry’’ trachyte–phonolite fall deposit. By contrast, the caldera wall section is mainly formed ofinterbedded phonolite and trachyte lavas, domes and ignimbrite ranging inage from 5.69 to 4.18Ma. The sequence is essentially subaerial. Evidencefor glacial conditions is absent, apart from minor signs of water interaction(stratified hyalotuffs and thin vitroclastic lava bases) in one of thesubsequences dated as 5.36–4.81Ma. The latter was interpreted as due tolimited glaciovolcanic interaction, presumably local slope ice/snow. However,the basal 1.5 km of the volcano is obscured by the present-day WAIS and theearlier (and lower elevation) environmental history is therefore unseen.

10.4. The Marine Record of the East Antarctic Ice Sheet

Due to the nearly complete ice coverage of the Antarctic continent, a recordof past Antarctic climate lies in the layers of sediments eroded over millionyears from the continent and deposited in sedimentary basins around itsmargin and in the Southern Ocean.During the past ca. 20 years, nine ODP legs have significantly advanced

our understanding of the Cenozoic tectonics and palaeoenvironments of theAntarctic region. These legs include Leg 113 in the Weddell Sea (Barkeret al., 1988, 1990), Leg 114 in the sub-Antarctic South Atlantic (Ciesielskiet al., 1988, 1991), Leg 119 in Prydz Bay and on Kerguelen Plateau(Barron et al., 1989, 1991), Leg 120 on Kerguelen Plateau (Schlich et al.,1989; Wise et al., 1992), Leg 177 in the southeast Atlantic sector of theSouthern Ocean (Gersonde et al., 1999, 2003), Leg 178 on the AntarcticPeninsula (Barker et al., 2002), Leg 181 (Carter et al., 2000), Leg 188 inPrydz Bay (O’Brien et al., 2001; Cooper et al., 2004) and Leg 189 in theTasmanian region (Exon et al., 2001, 2004).The pioneer of Antarctic drilling was the DSDP in 1973 (Hayes et al.,

1975). Following that, four distinct phases of drilling have taken place in theRoss Sea region, in sedimentary basins under the direct influence of EastAntarctica, by rigs mounted on the fast-ice that rings the southern part of theRoss Sea just north of the Ross Ice Shelf (see Chapter 3 for further details).These include the Dry Valley Drilling Project (DVDP) in 1970–1975(McGinnis, 1981), the McMurdo Sound Sediment and Tectonic Studies(MSSTS) in 1979 (Barrett, 1986), the Cenozoic Investigations in the Western

Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 425

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Ross Sea (CIROS) in 1984 and 1986 (Barrett, 1989; Barrett et al., 1992) andthe multinational CRP in 1997–1999 (Cape Roberts Science Team, 1998,1999, 2000; Hambrey et al., 1998; Barrett et al., 2000, 2001). These efforts arecontinuing through ANDRILL (Antarctic geological drilling) programme,which demonstrated ability to recover high quality marine and glacimarinesedimentary drill cores from high latitude ice-covered areas (Naish et al.,2007; Florindo et al., 2008; Harwood et al., 2008).To date, in spite of these efforts, important questions and problems remain

unresolved and are the subject of a considerable debate within the scientificcommunity. Among these are the causes of the mid-Miocene cooling event ataround 14Ma, and the warming–cooling climatic phases of the Pliocene.

10.4.1. Middle Miocene Cooling

Marine deep-sea oxygen isotope records indicate that a period of ice-sheetinstability during the Early and Middle Miocene is superimposed on a long-term warming trend that culminated in the mid-Miocene climatic optimumfrom B17 to 15Ma (Miller et al., 1987; Zachos et al., 2001). This phase wasfollowed by a gradual cooling with a major cooling event occurring at theMiddle–Late Miocene boundary B14Ma that continued for B1–2m.y.(MMCT; Flower and Kennett, 1995).The Prydz Bay area was visited during the Leg 119 (Barron et al., 1989,

1991) and Leg 188 (O’Brien et al., 2001; Cooper et al., 2004) drillingcampaigns of the ODP. Middle Miocene–Pliocene sediments were drilled atSites 739, 742, 1165 and 1166. An interesting stratigraphic record across thistransition was recovered during ODP Leg 188 when three sites were drilledproximal to the EAIS across the Prydz Bay continental shelf (Site 1166),slope (Site 1167) and rise (Site 1165). Of these, Site 1165 (64122uS; 67113uE)records a history of sedimentation on the continental rise extending back toearliest Miocene times (about 22Ma). Several long-term changes character-ize this record, including an overall trend of decreasing sedimentation ratesfrom the bottom to the top of the hole. There is a progressive decrease in thesedimentation rate above about 308m b.s.f. (metres below sea floor), which ismarked by a transition from predominantly dark-grey fissile claystones withabundant silt laminae to grey diatom-bearing clays with a higher abundanceof IRD (Fig. 10.6). At this transition, the total clay content also increases.The chronology of this sequence indicates a Middle Miocene age(ca. 14.3Ma) for the lithological transition. Correlation to ODP Hole747A on the Kerguelen Plateau shows that this transition coincides with thebase of the Mi-3/3a d18O event (Florindo et al., 2003). Middle Miocene

Figure

10.6:Lithostratigraphic

columnofSite1165andcorrelationbetweenMiddle

Mioceneintervals

toODP

Hole

747A

from

theKerguelen

Plateau(O

DPLeg

120).Thelithologicaltransitionatabout308m

b.s.f.,whichis

characterized

byanincrease

inice-rafted

debris,

sandgrainsandtotalclaycontent,coincides

withthebase

of

Mid-M

ioceneglacialevent(M

i-3/3a)(m

odified

from

Florindoet

al.,2003).

426 A. M. Haywood et al.

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Middle Miocene to Pliocene History of Antarctica and the Southern Ocean 427

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changes in sedimentation at Site 1165 may have been caused by thedevelopment of a cooler ice sheet characterized by decreased rates of glacialerosion and decreased production of melt water. In such an event, lesssuspended sediment is delivered to the continental rise, but ice at sea levelproduces more icebergs and, hence, more ice rafting of debris.

10.4.2. Late Miocene–Early Pliocene Ice-Sheet Fluctuations

Long-term variation in the marine oxygen isotopic data (e.g. Hodell andVenz, 1992; Kennett and Hodell, 1995; Zachos et al., 2001) are suggestive ofa warming in the earliest Pliocene, culminating in the mid-Pliocene ClimaticOptimum at B3Ma. However, the oxygen isotope ratios do not allow majorchanges in ice volume on Antarctica. Obliquity-driven oscillations (41 kyr) ind18O of up to 0.6m, which dominate the Pliocene period, while capableof producing B20–30m sea-level changes (depending on the temperaturecontribution to the d18O signal), are also considered insufficient to causelarge-scale deglaciation of Antarctica without a significant increase in thetemperature of the deep ocean.Marine sedimentary deposits exposed on land and on the continental shelf,

slope and rise show evidence of dynamic ice-sheet behaviour in the LateMiocene–Early Pliocene and Early Pliocene warming. At Marine Plain in theVestvold Hills, East Antarctica, a ca. 8m thick early Pliocene sequence ofdiatomaceous sands and silts is exposed (Pickard et al., 1988). Duringdeposition of the diatomaceous sediments, the ice margin must have been ca.50 km further inland and no floating ice was covering the site. The presenceof Cetacean skeletons, including dolphins and a right whale, and preliminaryisotope measurements led Quilty (1993) to infer warmer conditions duringdeposition than currently prevail. Water temperatures may have been as highas 51C. A marked increase in sea-ice indicator diatoms and a coarseningupward of the sediments in the upper 3m of the deposit indicate coolingconditions and glacial expansion.The sediments at ODP Sites 739, 742 and 1166 in Prydz Bay record

evidence of repeated advances and retreats of the Lambert ice stream acrossthe shelf in the Late Miocene through Early Pliocene (Hambrey et al., 1991;Passchier et al., 2003; O’Brien et al., 2004). During repeated advances of theLambert ice stream to the shelf break glacigenic debris flows build up atrough-mouth fan on the continental slope.It is worth mentioning that the IRD signal inferred from the W250-mm

size fraction at Site 1165, in a sediment drift on the continental riseoff Prydz Bay, suggests warmer conditions in the Early Pliocene

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(Grutzner et al., 2005). The low sea-ice concentrations derived from diatomassemblages (Whitehead et al., 2005) and the warm-water periods inferredfrom high abundances of silicoflagellates (Dictyocha) at Site 1165 (White-head and Bohaty, 2003) are consistent with isotopic estimates of warmerconditions than present during the Early Pliocene (Hodell and Venz, 1992).In addition, three periods of enhanced accumulations of sediment originatingfrom East Antarctica are recognized in the Late Miocene–Pliocene record ofthe East Kerguelen Ridge sediment drift, in the Indian Ocean to the northof Prydz Bay (Joseph et al., 2002). The sediment pulses are interpreted asperiodically warmer, less stable, ice-sheet conditions during this time.DVDP-10, 11 and 15, MSSTS-1 and CIROS-2 drill-holes in the Ross Sea

Region have recovered partial late Neogene records that suggest a dynamicPlio-Pleistocene history of glacial advance and retreat in the Taylor andFerrar valleys. Of these, a 166m thick Plio-Pleistocene sequence of diatom-bearing glaciomarine strata was cored in Ferrar Fiord by the CIROS-2drilling campaign (163131uE; 77141uS) (Barrett et al., 1992; Barrett andHambrey, 1992). The Pliocene sequence is B62m thick and is composed ofdiamictite dominated by basement lithologies interbedded with thinmudstone intervals, indicating several early Pliocene advances and retreatsof ice through the Transantarctic Mountains. Further evidence for PlioceneAntarctic ice volume fluctuation is recorded by glaciomarine strata from theDVDP-10 and 11 drill-holes (163132uE; 77135uS) (McGinnis, 1981; Ishmanand Reick, 1992; Wilson, 1993). Combined foraminiferal and magnetostrati-graphic analyses indicate highly fluctuating palaeoenvironmental conditionsin the Late Miocene–Early Pliocene with less ice-influenced conditions in theearly Pliocene (Ishman and Reick, 1992).Late Neogene trough-mouth fans have been identified in seismic data on

the Ross Sea and Weddell Sea continental slopes. The Crary Fan on theWeddell Sea margin was formed by glacial sediment supply from DronningMaud Land and ice drainage from East Antarctica through the Weddell Sea(Bart et al., 1999), whereas the Ross Sea fan was fed by ice draining throughthe Northern basin (Bart et al., 2000). In contrast, the geometry of LateNeogene sequences on the Wilkes Land margin is characterized as steepprograding foresets and trough-mouth fans that are less developed (Escutiaet al., 2005). However, the close proximity to the deep subglacial WilkesLand basin results in ice grounding below sea level, which may havecontributed to ice-sheet behaviour that is different from the other portions ofthe East Antarctic margin. Interpretations of the seismic data record adynamic Late Neogene EAIS with large expansions and contractions for theRoss Sea and Weddell Sea portions of the East Antarctic margin (Bart et al.,1999, 2000).

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10.5. The Marine Record of the West Antarctic and Antarctic

Peninsula Ice Sheets

The late Cenozoic glacial history of West Antarctica is archived in thesedimentary sequences of its continental margins. During the last 20 years,multichannel seismic data were acquired from the margins of the WeddellSea, the Antarctic Peninsula, the Bellingshausen and Amundsen seas, and theeastern Ross Sea. DSDP Legs 28 (Ross Sea margin) and 35 (BellingshausenSea) and ODP Legs 113 (Weddell Sea) and 178 (Pacific margin of theAntarctic Peninsula) recovered sedimentary sequences spanning the Neogeneto Quaternary. In the following, we summarize the main results of thesestudies and their implications for the history of WAIS and AntarcticPeninsula Ice Sheet (APIS) from the Middle Miocene to Pliocene.Seismostratigraphic studies suggest that an early stage of glaciation

characterized by presence of local ice caps and tidewater glaciers had affectedthe Ross Sea region already during the Late Oligocene and the EarlyMiocene (De Santis et al., 1995, 1999; Anderson and Shipp, 2001). Glacialand glaciomarine sediments interbedded with fluvial to deep-water mud-stones were deposited on the innermost part of the shelf (CIROS-1 drill site),while open-marine sedimentation dominated on the outer shelf (Abreu andAnderson, 1998; Anderson and Shipp, 2001). This depositional patternpoints to cool temperate interglacial conditions, but gives no direct evidencefor presence of the WAIS (Abreu and Anderson, 1998; Anderson and Shipp,2001). Based on seismic-stratigraphic investigations and sedimentary recordsdrilled at DSDP Sites 270 and 272, De Santis et al. (1999) inferred that aglacial transitional regime developed in the Ross Sea region during the Earlyand Middle Miocene. Most of the subglacial erosion and deposition still tookplace on the inner shelf, but occasionally local ice caps expanded onto theouter shelf. Around this time, the first significant amounts of IRD weredeposited in the Bellingshausen Sea (DSDP Leg 35, Sites 323 and 325) andthe Weddell Sea (ODP Leg 113, Sites 693 and 694) (Abreu and Anderson,1998). The onset of IRD sedimentation at Site 325 points to an establishmentof West Antarctic ice caps outside the Ross Sea region during the Early toMiddle Miocene (Hollister et al., 1976; Abreu and Anderson, 1998; Barkeret al., 2002).By analysing Mg/Ca ratios of calcareous benthic and planktonic

foraminifera tests, Lear et al. (2000) and Shevenell et al. (2004) demonstratedthat ice-sheet build-up in Antarctica between 14.2 and 13.8Ma contributedto the pronounced, global increase in d18O ratios in benthic foraminiferatests observed by Zachos et al. (2001), even though the related eustaticsea-level fall was rather indistinct (Abreu and Anderson, 1998; Miller et al.,

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2005). While most of the Middle Miocene ice-volume increase may haveoccurred in East Antarctica and possibly the Arctic (Moran et al., 2006),seismic studies also point to an expansion of ice caps in West Antarctica ortheir coalescence to a WAIS smaller than present (Abreu and Anderson,1998; Anderson and Shipp, 2001). In the Ross Sea region, seismic data andsedimentary records reveal that grounded ice advanced at least five timesacross the shelf (Chow and Bart, 2003). A comparison with foraminiferaloxygen isotope and sea-level data shows that at least two of these groundingevents fall into the early part of the Middle Miocene (Bart, 2003). Bart andAnderson (1995) proposed on the basis of seismic profiles from the westernAntarctic Peninsula outer shelf that glacial erosional surfaces formed duringthe Middle to Late Miocene, when huge sediments drifts started to grow onthe adjacent continental rise as indicated by seismic investigations (Rebescoet al., 1997, 2002). Bart et al. (2005) combined seismic data from the westernAntarctic Peninsula shelf with sedimentological data from ODP Leg 178 Site1097 drilled on the shelf, and concluded that the earliest grounding events onthe Antarctic Peninsula shelf are of Late Miocene age. Seismic studies of thesedimentary development of the Crary Fan in the southeastern Weddell Seaindicate a minimum of 5 long-term and 14 small-scale ice-sheet expansionsacross the adjacent shelf from the Middle Miocene to the Pleistocene(Anderson and Shipp, 2001, and references therein). However, present-dayice-drainage patterns raise the question of whether or not WAIS expansionmade a major contribution to the inferred ice advances (cf. Bart et al., 1999).Despite episodic ice expansion in parts of West Antarctica during the MiddleMiocene, benthic and neritic planktonic diatoms, which were reworked fromthe Antarctic shelf and deposited in the central Weddell Sea (Site 694), pointto the absence of a major WAIS or APIS from the Middle Miocene to theearly part of the Late Miocene (Kennett and Barker, 1990; cf. Chow andBart, 2003).Widespread glacial unconformities assumed to be Late Miocene in age are

observed in seismic profiles from the eastern Ross Sea margin (Cooper et al.,1991; De Santis et al., 1995; Anderson and Shipp, 2001). The grounded iceadvances were accompanied by the deposition of glacial and glaciomarinestrata (DSDP Leg 28 Site 270 and CIROS-1), which might exhibit an interiorWest Antarctic provenance (Anderson and Shipp, 2001) and which areinterbedded with meltwater deposits and diatom oozes documentingtemperate interglacial episodes (Abreu and Anderson, 1998). De Santiset al. (1999) correlated seismic data with sedimentary records drilled atDSDP Leg 28 sites and proposed an onset of fully glacial conditions in theRoss Sea region during the latest Miocene and Early Pliocene. A large andthick WAIS advanced up to the shelf edge (Accaino et al., 2005) and gave the

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Ross Sea shelf its typical modern seabed morphology, which is characterizedby landward dipping, overdeepening and troughs carved by grounded icestreams (De Santis et al., 1999). Shelf progradation observed in seismicprofiles west of the Antarctic Peninsula indicates a general expansion ofAPIS assumed to be Late Miocene to Early Pliocene in age (Larter andBarker, 1991; Larter and Cunningham, 1993; Bart and Anderson, 1995;Larter et al., 1997; Barker et al., 2002; Bart et al., 2005). Similar shelfprogradation is reported from seismic profiles elsewhere on the continentalmargin in the Bellingshausen and Amundsen seas although the age of theprograding sequences in these areas is unconstrained (Nitsche et al., 1997,2000; Scheuer et al., 2006). Episodic advance and retreat of grounded iceacross the eastern and western Antarctic Peninsula shelf throughout thelatest Miocene and Early Pliocene are evident from seismic unconformities(Sloan et al., 1995; Larter et al., 1997; Bart and Anderson, 2000; Bart, 2001;Bart et al., 2005) and the onset of shelf overdeepening west of the AntarcticPeninsula (Camerlenghi et al., 2002). Recovery of sediments deposited insubglacial and glaciomarine settings from the western Antarctic Peninsulashelf (ODP Leg 178 Sites 1097 and 1103) confirm episodic APIS groundingevents at least since the Latest Miocene to Early Pliocene (Barker et al.,1999a, 2002; Eyles et al., 2001). The general APIS expansion during the LateMiocene to Early Pliocene coincided with a northward shift of the AntarcticCircumpolar Current (ACC) (Hernandez-Molina et al., 2006), pronouncedgrowth of large sediment drifts (Rebesco et al., 1997), clay-mineralfluctuations (Hillenbrand and Ehrmann, 2005) and distinct maxima inaccumulation rates of terrigenous detritus (Grutzner et al., 2005) on thewestern Antarctic Peninsula rise. The sedimentary development of the driftssuggests episodic, enhanced supply of glacial-sourced sediments from theadjacent shelf from ca. 9.4Ma onwards. Considerable growth of APIS andWAIS during the Late Miocene is also indicated by the onset of IRDdeposition and sedimentation of a clay mineral assemblage pointing to thedominance of physical weathering in the South Orkney region (Site 696) andof rapid turbidite deposition with a West Antarctic provenance in the centralWeddell Sea (Site 694) (Kennett and Barker, 1990). Anderson and Shipp(2001) infer that during the Late Miocene, the WAIS reached its modern sizeand, at times, was even larger than today. The seismic and core data from thewestern Antarctic Peninsula margin mentioned above suggest a similarscenario for the APIS.At the Miocene/Pliocene boundary, the deposition of IRD at Site 325

increased (Hollister et al., 1976), while the first significant deposition of IRDstarted at Site 322 near Drake Passage (Abreu and Anderson, 1998). Kennettand Barker (1990) attributed a shift from pelagic to hemipelagic deposition

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at Site 696 and from turbiditic to hemipelagic sedimentation at Site 694during the earliest part of the Pliocene to the development of a stable APISand WAIS or climatic warming in West Antarctica. Seismic stratigraphycombined with sedimentological data from DSDP Leg 28 drill cores indicatesthat the eastern Ross Sea shelf reached a late fully glacial stage with anoverdeepened topography, which was mainly influenced by glacial processesof erosion and deposition resulting in relatively low sedimentation rates(De Santis et al., 1999). Throughout the Early Pliocene, high-frequencygrounding events are observed in seismic profiles from the eastern Ross Seashelf and the western Antarctic Peninsula shelf (Alonso et al., 1992; Larterand Cunningham, 1993; Larter et al., 1997; Bart and Anderson, 2000; Bart,2001). Cyclicity in drift sedimentation recorded in drill cores from theAntarctic Peninsula rise confirms that ice streams repeatedly advanced to theshelf break throughout the Pliocene (Barker et al., 2002; Hillenbrand andEhrmann, 2005; Hepp et al., 2006). During the Early Pliocene, sedimentationrates increased in the deep Bellingshausen Sea (DSDP Leg 35 Sites 322, 323and 325) and the Weddell Sea (ODP Leg 113 Sites, 693 and 697) (Barker,1995). On the western Antarctic Peninsula continental rise, down-slopetransport exceeded along-slope transport throughout the Pliocene(Hernandez-Molina et al., 2006), and the sediment supply from the shelfenhanced and maintained the sediment drifts (Rebesco et al., 1997, 2002).Seismic studies of the continental margin in the western Bellingshausen andAmundsen seas revealed truncations of foreset reflectors and extensiveprogradation, probably during the Pliocene (Nitsche et al., 1997, 2000;Scheuer et al., 2006), suggesting that grounded ice eroded the shelf andtransported the terrigenous detritus across the shelf edge. According toScheuer et al. (2006), the accumulation rate of terrigenous sediment on theadjacent continental rise reached its maximum during the Pliocene andQuaternary, which may indicate a relatively late establishment of WAIS inthe adjacent hinterland (cf. Nitsche et al., 1997). In contrast, sedimentationrates on other parts of the West Antarctic margin reached distinct maximaduring the Late Miocene (Site 1095; Barker et al., 2002) or during the EarlyPliocene (Barker, 1995).During the Late Pliocene, sedimentation rates decreased at the drifts on

the Antarctic Peninsula rise (ODP Leg 178, Sites 1095, 1096 and 1101; Barkeret al., 1999a, 2002), in the deep Bellingshausen Sea (DSDP Leg 35, Sites 322,323 and 325; Barker, 1995) and in the Weddell Sea (ODP Leg 113, Sites 694and 697; Kennett and Barker, 1990; Barker, 1995). A concomitant transitionfrom progradation to aggradation is observed in seismic profiles fromelsewhere on the West Antarctic margin, from the Antarctic Peninsula to theeastern Ross Sea (Larter and Barker, 1991; Larter and Cunningham, 1993;

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De Santis et al., 1995; Larter et al., 1997; Nitsche et al., 1997, 2000). Incontrast to these findings, Rebesco et al. (2006) compared changes in thegeometries of seismic reflection profiles and concluded that enhancedprogradation of the continental shelf and slope started almost all-aroundAntarctica at ca. 3Ma. These authors suggest that APIS, WAIS and EAISswitched from a predominantly polythermal to a predominantly cold-basedmode at this time of major ice-sheet build-up in the Northern Hemisphere(cf. Barker, 1995).

10.6. Marine Records of the Southern Ocean

Today there are four main water masses south of the polar front: AntarcticSurface Water; Warm Deep Water (WDW); Circumpolar Deep Water(CDW); and Antarctic Bottom Water (AABW) (see Chapter 4 for furtherdetails). WDW flows into the Southern Ocean from the North Atlantic,Pacific and Indian Oceans. This nutrient-rich water mass upwells at theAntarctic Divergence, replacing sinking Antarctic Surface Water. CDW, thedominant water mass in the Southern Ocean, is a mixture of WDW andAntarctic waters. AABW forms as cold, saline shelf waters sink and mix withCDW prior to flowing northwards out of the Southern Ocean. The Antarcticcoastal current flows from east to west around the continent. North of theAntarctic Divergence, prevailing westerlies drive surface waters towardsthe east, forming the ACC, which in places extends to the seafloor. In part,these oceanographic conditions are a response to climate on the Antarcticcontinent. For example, formation of sea ice increases the density of shelfwaters, aiding the formation of AABW. Conversely, changing ocean–atmosphere circulation patterns influence global climate by determining thetransport of heat and moisture onto the polar continent. A commonly citedexample is the thermal isolation of Antarctica by the ACC.

10.6.1. The MMCT

The MMCT witnessed a major reorganization of global deepwatercirculation patterns associated with the expansion of the Antarctic Ice Sheet(e.g. Woodruff and Savin, 1989; Wright et al., 1992; Flower and Kennett,1995). The widespread presence of hiatuses in Southern Ocean sedimentarycores is one indicator of this Middle Miocene reorganization of oceancirculation patterns. Although these hiatuses present difficulties in

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reconstructing past oceanographic conditions using marine cores, a broadconsensus regarding the history of some of the Southern Ocean water masseshas emerged in recent years.One proxy used to reconstruct ocean circulation patterns is the carbon

isotopic composition of benthic foraminifera, as water masses generallybecome enriched in the light isotope of carbon as they age. Interbasinaloffsets in benthic foraminiferal carbon isotopes indicate that the EarlyMiocene Southern Ocean received warm saline deep water sourced fromlow latitudes in the Tethyan region (Woodruff and Savin, 1989; Wrightet al., 1992). The flow of this warm saline water mass likely ceased in theMiddle Miocene, and the consequent reduction in meridional heattransport has been called upon as a trigger for the ice-sheet expansionin the Middle Miocene (Woodruff and Savin, 1989; Flower and Kennett,1995).The ACC connects the Pacific, Atlantic and Indian Oceans and thus

plays an important role with respect to global heat transport. Inception ofa deep circumpolar ACC possibly occurred near the Oligocene–Mioceneboundary, well before the MMCT (e.g. Pfuhl and McCave, 2005; Lyleet al., 2007). However, a variety of different proxies all point to temporalvariations in the intensity of the ACC since its inception. A reduction insurface-to-thermocline foraminiferal d18O gradients at DSDP Site 516indicates reduced thermal stratification in the surface waters of thesouthwestern South Atlantic around 16Ma (Pagani et al., 2000). At thesame time, the abundance of alkenones at this site decreased, implying areduced nutrient input to this area. These observations have beeninterpreted to reflect a reduction in Antarctic Intermediate Watercaused by a reduction in the strength of the ACC (Pagani et al., 2000).Re-establishment of a strong surface-to-thermocline temperature gradientin this region points to renewed flow of the ACC coincident with theexpansion of the Antarctic Ice Sheet in the Middle Miocene (Pagani et al.,2000).Because the oceanic residence time of neodymium is comparable to the

ocean mixing time, the neodymium isotope system can be used as aradiogenic isotope tracer of ocean water masses. A study of the neodymiumisotopic composition of sediments from DSDP Site 266 in the Australian–Antarctic basin indicates an increased contribution of neodymium from theKerguelen volcanic province during the Middle Miocene, suggestingenhancement of the ACC during the climate transition (Vlastelic et al.,2005).Pacific ferromanganese crusts that grew in equatorial Pacific bottom water

display a gradual change in their neodymium isotopic composition between

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38 and B20Ma, which has been interpreted as increasing flow of SouthernOcean waters into the Pacific Ocean via the Deep Western Boundary Current(DWBC) as a result of progressive build-up of the ACC (Van de Flierdtet al., 2004). The isotope trends in the ferromanaganese crusts reversed in theMiddle Miocene, likely reflecting increased mixing in the Pacific as a result ofincreased production and export of AABW associated with the build-up ofAntarctic ice (Van de Flierdt et al., 2004). Independent evidence forintensification of the DWBC associated with the Middle Miocene AntarcticIce Sheet growth is found in the record of sortable silt mean size from ODPSite 1123 situated on the Chatham Rise (Hall et al., 2003). The sortable siltmean size increases with stronger near bottom current flow and selectivedeposition and winnowing. The ODP Site 1123 record indicates increasedflow speeds during the MMCT, interpreted as reflecting increased export ofSouthern Ocean waters to the Pacific Ocean. Furthermore, the intensity ofthe DWBC was found to exhibit variability on the timescale of the 41 kyearorbital obliquity cycle (Hall et al., 2003).High-resolution foraminiferal proxy records from the South Tasman Rise

(southwest Pacific) give further independent evidence for a strengthening ofthe ACC associated with the ice-sheet expansion in the Middle Miocene.Shevenell et al. (2004) generated paired oxygen isotope (d18O) and Mg/Carecords using a planktonic foraminifera, G. bulloides, from ODP Site 1171.The d18O record reflects a combination of temperature and the oxygenisotopic composition of seawater, the latter a function of global ice volumein addition to regional salinity. The Mg/Ca record is used as a salinity-independent temperature proxy. The ODP Site 1171 Mg/Ca recorddisplays a B71C cooling (point-to-point) between 14.2 and 13.8Ma, whichsuggests that the paired d18O record reflects a concomitant surface waterfreshening above the South Tasman Rise. These records are interpreted asreflecting an intensification of the ACC between 14.2 and 13.8Ma in a seriesof steps paced by orbital cycles (Shevenell et al., 2004), although it has alsobeen suggested that a component of the surface water freshening mayrepresent massive melting pulses of the Antarctic Ice Sheet (Holbourn et al.,2005).

10.6.2. Late Miocene–Early Pliocene

The Late Miocene witnessed further high latitude cooling, summarized inHodell and Kennett (1986). A cooling of Southern Ocean upper circumpolarand intermediate water masses is documented in benthic foraminiferaloxygen isotope records from ODP Site 1088 in the sub-Antarctic South

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Atlantic, which display a two-step increase between B7.4 and 6.9Ma(Billups, 2002). Associated with this late Neogene cooling are furtherchanges in Southern Ocean oceanography.A 10–5Ma hiatus in the sediment package underlying the Bounty Fan at

ODP Site 1122 suggests an intensification of the ACC, associated with theLate Miocene climate cooling and expansion of the WAIS (Carter et al.,2004). The strengthening of North Atlantic Deep Water (NADW) in theLate Miocene led to global oceanic circulation patterns approaching themodern configuration (e.g. Wright et al., 1991). Late Miocene strengtheningof NADW is supported by benthic foraminiferal carbon isotope recordsfrom ODP Site 1088 in the sub-Antarctic South Atlantic, which indicate aninflux of a nutrient-depleted water mass into the Southern Ocean at B6.6Ma(Billups, 2002). The relative flux of NADW into the Southern Ocean reachedmodern proportions by B6Ma, and subsequently intensified yet further,supporting speculation that the climatic warmth of the Early Plioceneresulted from enhanced thermohaline overturn (Kwiek and Ravelo, 1999;Billups, 2002).The warm period in the Early Pliocene coincides with an apparent increase

in biological productivity as indicated by increased opal deposition rates atODP Sites 1165, 1095, 1096 and several gravity core sites in the SouthernOcean, which has been interpreted as reflecting a reduction in the extent ofsea ice (Hillenbrand and Futterer, 2002; Grutzner et al., 2005; Hillenbrandand Ehrmann, 2005; Hillenbrand and Cortese, 2006). Surprisingly, this warminterval appears to coincide with glacial advance (indicated by increasedterrigenous input from Antarctica), perhaps supporting the ‘‘snowgunhypothesis’’ of Prentice and Matthews (1991) (Grutzner et al., 2005). Largeoscillations in the Al/Ti ratio, the lead isotopic composition and theneodymium isotopic composition of sediments at DSDP Site 266 in theAustralian–Antarctic basin have been interpreted to reflect either variationsin the intensity of the ACC or variations in weathering fluxes resultingfrom periods of instability of the Antarctic Ice Sheet around 1.9 and 3.3Ma(Vlastelic et al., 2005).Diatom assemblages at ODP Sites 1165 and 1166 suggest that sea-ice cover

remained lower than today through much of the Pliocene (Whitehead et al.,2005), and silicoflagellate assemblages have been used to pinpoint threeintervals (3.7, 4.3–4.4 and 4.6–4.8Ma) within the Pliocene when sea surfacetemperatures in the Southern Ocean were roughly 51C warmer than today(Whitehead and Bohaty, 2003). Similar events are recorded in ODP Leg 178Sites, pointing to circum-Antarctic warming that may have been responsiblefor ice-sheet loss (Escutia et al., 2007).

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10.7. Modelling Antarctic Climates and Ice Sheets

Aspects of global Middle Miocene to Pliocene climates and environmentshave been examined in palaeoclimate modelling studies. For example, theimpact that oceanic gateways such as the Panama Gateway (Klocker et al.,2005), the Indonesian Throughflow (Cane and Molnar, 2001) and Tethyancircumglobal passage (Hotinski and Toggweiler, 2003) had on global oceaniccirculation patterns and heat/moisture transports has received considerableattention. The role of Tibetan uplift, as well as other regional orographic andpalaeogeographic variations, in changing atmospheric circulation patternsand the development of monsoon systems has also been a key area ofresearch (Prell and Kutzbach, 1992; Kutzbach et al., 1993; Fluteau et al.,1999; Bice et al., 2000; Zhisheng et al., 2001; Liu et al., 2003; Kutzbach andBehling, 2004; Ramstein et al., 2005). Reconstructing and predicting theclimatic implications of altered distributions of vegetation types (Dutton andBarron, 1996, 1997; Cosgrove et al., 2002; Haywood et al., 2002a; Haywoodand Valdes, 2006), sea surface temperature gradients (SST) and latitudinaltemperature gradients/heat transports have been studied in detail (Chandleret al., 1994; Sloan et al., 1996; Rind, 1998; Haywood et al., 2000; Barreiroet al., 2006). However, very few palaeoclimate modelling studies havespecifically addressed the question of how climates and environmentschanged on Antarctica during this interval. The majority of globalpalaeoclimate modelling experiments have used specified Greenland andAntarctic ice volumes based solely on estimates of global sea level. Therefore,modelling studies conducted thus far could be considered as representing asuite of sensitivity experiments that have tested the response of climatemodels to different initial conditions rather than accurate climate ‘‘retro-dictions’’.One example of a palaeoclimate modelling study that did focus on

Antarctica is Haywood et al. (2002b). In this study, they attempted to assessthe sensitivity of a climate model to different Antarctic Ice Sheet configura-tions and to address, indirectly at least, the controversy existing over thebehaviour of the EAIS during the Pliocene epoch and whether or not it waspossible for Nothofagus remains founds in Sirius Group deposits in theTransantarctic Mountains to be Pliocene in age. Three sensitivity experi-ments were performed for the Pliocene using the UK Meteorological Office’sGeneral Circulation Model (GCM). Each experiment used a differentconfiguration of Antarctic ice conforming to a range of sea-level estimatessupported by geological evidence (specifically 35, 25 and 12–15m higher thanmodern sea level). Climate outputs from these experiments were used to drive

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a biome model capable of predicting the equilibrium vegetation state for agiven climate forcing. For the experiments using an Antarctic Ice Sheetconfiguration equivalent to a +12 to 15 and +25m sea level, the biomemodel predicted the occurrence of an exclusively tundra type of vegetationon deglaciated regions of Antarctica (Fig. 10.7). In the +35m experiment,climate ameliorated sufficiently to allow the model to predict deciduous taigamontane forests in areas of Antarctica where fossil Nothofagus material hasbeen recovered from Sirius Group sediments (Fig. 10.7). This work suggestedthat it is feasible for Nothofagus to have existed in Antarctica during thePliocene with ice-sheet extents calibrated to global sea-level records.This research had certain limitations relating to the experimental design.

The configuration of Antarctic ice was specified within the GCM and theresulting climatologies were only used to drive an offline biome model. Amore sophisticated approach would use a coupled climate–ice and vegetationmodel capable of predicting, rather than being prescribed with, theequilibrium condition for Antarctic ice cover and vegetation with climate.As a step towards this, Francis et al. (2007) dynamically coupled the Top-down Representation of Interactive Flora and Foliage Including Dynamics(TRIFFID) dynamic global vegetation model to the HadAM3 GCM set upfor the mid-Pliocene. The results supported the earlier study by Haywoodet al. (2002b) since the TRIFFID model predicted tundra vegetation ondeglaciated regions of Antarctica.A number of studies have utilized ice-sheet models to predict the response

of the EAIS to increasing surface temperatures the results from which, in afew cases, have been related to the debate over how dynamic or stable theAntarctic Ice Sheet was likely to have been during the Middle Mioceneto Pliocene (e.g. Barker et al., 1999b). Huybrechts (1993) used a three-dimensional ice-sheet model to determine ice-sheet geometries under variouskinds of climatic conditions. A surface temperature warming of between17 and 201C was needed to generate the ice-free corridor over the Pensacolaand Wilkes subglacial basins hypothesized by Harwood (1983) and Harwoodand Webb (1986). For temperature rises of less than 51C, the modelpredicted that the EAIS increased in size due to an increase in snowfall. Evenwith the most favourable model set up for ice loss, a surface temperature riseof 151C was still required to make the Pensacola and Wilkes subglacialbasins ice-free which Huybrechts (1993) concluded was unlikely for theinterval in question.However, there are a number of limitations inherent in such an approach.

First, by assessing the response of the modern EAIS to a certain climaticforcing, an assumption is made that the EAIS during the Middle Mioceneand Pliocene was the same as it is today. Other important differences may

Pliocene control experiment

Pliocene dyanamic experiment

Pliocene stable experiment

Biome key:

Figure 10.7: Predictions of Antarctic vegetation distributions for each of thePliocene experiments using climatological means derived from the UKMeteorological Office General Circulation Model within a biome model

(BIOME 4; Kaplan, 2001).

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also have existed. For example, the Transantarctic Mountains may havebeen substantially lower in the past (McKelvey et al., 1991). Kerr andHuybrechts (1999) tested this possibility in an ice-sheet-modelling study andconcluded that the surface elevation of individual mountain blocks has onlya very local effect on EAIS dynamics. Second, by applying a uniform climateforcing within the model, the possibility for important regional-scalevariations in temperature and precipitation, resulting from the differencesin Middle Miocene and Pliocene boundary conditions, is ignored. Third, theice-sheet model used did not include a representation of ice shelves or icestreams. Ice-sheet theory states that the stability of an ice sheet groundedbelow sea level depends on its surrounding ice shelves that are believed tobuffer the ice sheet from oceanic changes (Lingle, 1984). Any weakening ofthese ice shelves may lead to a runaway process in which the entire marine icesheet potentially collapses. Support for this hypothesis comes from recentobservations of the terrestrial APIS. After the collapse of the major part ofthe Larsen A ice shelf in 1995, the flow of glaciers, which had fed the formerice shelf, accelerated significantly (De Angelis and Skvarca, 2003). The iceshelves provide a mechanism for linking ocean temperature change with theretreat of ice shelves and grounded ice-sheet instability that may have beenimportant during the Middle Miocene to Pliocene interval.Current research is using an alternative approach to explore the nature

and behaviour of the Antarctic Ice Sheet during the Middle Miocene toPliocene interval. Instead of attempting to reconstruct the actual state of theice sheet at a specific time, which is extremely difficult given the incompletegeological record and uncertainties inherent in the techniques used forpalaeoenvironmental reconstruction, modelling studies are now focusing ondeveloping ensemble predictions which encompass the range of plausiblebehaviour of the ice sheet for particular periods. For the Pliocene, a suite ofpalaeoclimate modelling experiments which cover the range of possibleboundary conditions, and which have been validated against global Pliocenepalaeoenvironmental proxies, are being used to provide three-dimensionalthermomechanical ice-sheet models with the required climatological forcingsto predict the extent of the both the Greenland Ice Sheet and EAIS. Rangesof different initial conditions are used within ice-sheet models so that ice-sheet hysteresis can be explored in detail. The dependence of these results onthe GCM boundary conditions and the initial state of the ice sheets leads to arange of ice-sheet reconstructions, but it seems likely that the mean state forthe Pliocene Greenland Ice Sheet and EAIS were, to some extent, smallerthan they are today (Fig. 10.8). For Antarctica, the model results clearlyshow that the Wilkes and Aurora Basins are the areas of the EAIS that aremost susceptible to increased temperatures. Nevertheless, even in the most

Figure 10.8: Examples of (A) smallest and (B) largest modelled EAIS inequilibrium with reconstructed Pliocene climate.

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extreme scenario, the predicted EAIS still covers all but the northernmostreaches of the Wilkes subglacial basin, which is suggested to be a source ofthe diatoms found within the Sirius Group sediments (Hill et al., 2007).Comparison of the model-predicted ice sheet and sea-level mean state toobserved Pliocene highstands (e.g. Dowsett and Cronin, 1990) provides apotential match, which may be improved in future modelling exercises thatincorporate variability in orbital forcing.

10.8. Summary

10.8.1. Terrestrial and Fjordal Sedimentary Environments of theTransantarctic Mountains and Lambert Glacier Region

Two main pre-Quaternary sedimentary sequences recording glacial eventsare found on land in East Antarctica, the Sirius Group in the TransantarcticMountains and the Pagodroma Group in the Prince Charles Mountainsflanking the Lambert Glacier. The Sirius Group is a mainly terrestrialglacigenic succession, commonly a few tens of metres thick or as a thindrape, but whose age has proved highly controversial, with Pliocene, MiddleMiocene or even older ages suggested. The Pagodroma Group is an

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ice-proximal fjordal sequence up to several hundred metres thick, and is welldated on the basis of in situmarine fossils. All these well-exposed deposits areimportant in unravelling the history of the EAIS, as they are the nearestdeposits to the heart of the ice sheet, and yield valuable informationconcerning palaeogeography and palaeoclimate.The two regions described above represent the best studied examples

of pre-Quaternary onshore glacigenic successions, but there are otherselsewhere, e.g. the Antarctic Peninsula and the Grove Mountains(Fang et al., 2004). When a better dated Sirius Group can be linked tothe offshore record in McMurdo Sound, and the data from the PrinceCharles Mountains fully integrated with the emerging Prydz Bay record, wewill have a good understanding of the behaviour of the EAIS since itsinception.

10.8.2. The Terrestrial Fossil Record of Antarctic Climate

The terrestrial Neogene biota of Antarctica is poorly known with fossilscoming only from exposures on the Beardmore Glacier and from boreholesin the Ross Sea region. This will soon change with the recent discoveries ofNeogene fossil assemblages in the Dry Valleys (Ashworth, A. C., Lewis, A. R.and Marchant, D. M., personal communication, 2006). What is known so faris that the Neogene biota had a low diversity and was very different than thediverse forested habitats of the early Palaeogene. Evidently, tundravegetation was present in the Transantarctic Mountains from the Oligoceneuntil at least the Early Miocene and during that time it appears that therewas a declining biodiversity (Askin and Raine, 2000; Raine and Askin, 2001).The final extinction of several vascular plants and animals with morecomplex life histories is assumed to be associated with the shift to the polardesert climate which prevails today in the interior of the continent. Whetheror not this immediately followed the mid-Miocene climatic optimum or wasas late as the mid-Pliocene is the most outstanding problem in Neogeneclimatic history in need of resolution.The extinction implies that a climatic threshold was passed in which

temperatures and available moisture were significantly reduced after theevent than those which supported the tundra biota. The most parsimoniousinterpretation to explain the occurrence of the Neogene biota is that theorganisms were descendents of the Gondwana biota that had existed on thecontinent for millions of years, long after the fragmentation of the continent.The conclusion that plants and animals continued to inhabit Antarctica formillions of years after the continent became isolated makes their extinction

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an especially significant event and supports the argument that it wasassociated with a major reorganization of the global climate system.

10.8.3. Terrestrial Environments of West Antarctica

Over the past 7Ma, the APIS has always been relatively thin (few hundredmetres). It was also wet based and erosive, at least until about 2Ma ago.There is also evidence for several ice-poor periods (interglacials) preserved innorthern volcanic sequences, particularly in latest Miocene and EarlyPliocene. A relatively thin ice cover (probably tens of metres) existed inEllsworth Land, and it may have locally reached a few hundred metres, but isvery poorly dated. Although there are numerous large volcanoes in MarieByrd Land, each potentially preserving important evidence for past glacialconditions, most are essentially undissected and the only eruptive productsavailable for study are subaerial units. They thus yield minimal usefulenvironmental information other than maximum possible elevations for anyformer ice sheet(s) that may be hypothesized from other evidence. A glacialcover existed in Marie Byrd Land by 9–8Ma, at least. It was thin (tens ofmetres) and formed local icecaps on the large volcanoes. It was wet based atthe coast but may have been dry-based (polar) inland. A thicker ice sheet (atleast several hundred metres) existed by c. 7–6Ma, but its thermal regime isunknown. There is also evidence for at least one important episode ofoverriding by an unusually thick ice sheet in Pliocene or younger time(W1.5 km thick; o3.5Ma).

10.8.4. The Marine Record of the East Antarctic Ice Sheet

Marine deep-sea oxygen isotope records indicate that a period of ice-sheetinstability during the Early and Middle Miocene is superimposed on a long-term warming trend that culminated in the late Middle Miocene climaticoptimum from B17 to 15Ma (Miller et al., 1987; Zachos et al., 2001). Thisphase was followed by a gradual cooling with a major cooling eventoccurring at the Middle–Late Miocene boundaryB14Ma that continued forB1–2m.y. (Flower and Kennett, 1995).Long-term variation in the marine oxygen isotopic data (e.g. Hodell and

Venz, 1992; Kennett and Hodell, 1995; Zachos et al., 2001) is suggestive of awarming in the earliest Pliocene, culminating in the mid-Pliocene ClimaticOptimum at B3Ma. However, the oxygen isotope ratios do not allow majorchanges in ice volume on Antarctica. Obliquity-driven oscillations (41 kyr) in

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d18O of up to 0.6m, which dominate the Pliocene period, while capable ofproducing B20–30m sea-level changes (depending on the temperaturecontribution to the d18O signal), are also considered insufficient to causelarge-scale deglaciation of Antarctica without a significant increase in thetemperature of the deep ocean.

10.8.5. Marine Evidence for Antarctic Peninsula and West Antarctic Ice Sheets

From marine evidence, it is possible to deduce that significant ice-sheetgrowth in West Antarctica commenced during the Middle Miocene aroundca. 14Ma. In the Ross Sea and Weddell Sea regions, WAIS reached its fullyglacial configuration, comparable with today, during the Late Miocene.The APIS established its present size during the later part of theLate Miocene. In the western Bellingshausen and Amundsen seas, theWAIS may not have grown to its full size until the Early Pliocene.Throughout the Late Miocene and Pliocene, WAIS and APIS repeatedlyadvanced across the adjacent shelves, and during the Early Pliocene, WAISand APIS may have been more dynamic (Barker, 1995; Bart, 2001; Heppet al., 2006). Seismic profiles from the West Antarctic continental marginsuggest there was no prolonged deglaciation during the late Neogene, butthese records cannot be used to determine whether or not the WAIS andAPIS decreased drastically in size during short-term interglacial periods.Investigations on sedimentary records drilled at the ODP Leg 178 sites donot corroborate a significant ice volume reduction (Barker et al., 2002;Hillenbrand and Ehrmann, 2005).

10.8.6. Marine Records of the Southern Ocean

The widespread presence of hiatuses in Southern Ocean sedimentary cores isone indicator of the reorganization of ocean circulation patterns thatoccurred during the MMCT, which is believed to have been associated withthe expansion of the Antarctic Ice Sheet (e.g. Woodruff and Savin, 1989).Interbasinal offsets in benthic foraminiferal carbon isotopes suggest that theEarly Miocene Southern Ocean received warm saline deep water sourcedfrom low latitudes in the Tethyan region (e.g. Woodruff and Savin, 1989).The flow of this warm saline water mass likely ceased in the Middle Miocene,and the consequent reduction in meridional heat transport is a possibletrigger for the ice-sheet expansion in the Middle Miocene. Furthermore, anumber of Southern Ocean records reconstruct an increase in the strength of

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the ACC which may also have played an important role in the MMCT (e.g.Pagani et al., 2000).During the Late Miocene, Southern Ocean records indicate further high-

latitude cooling possibly associated with the establishment and/or growth ofthe WAIS and an intensification of the ACC (e.g. Kennett and Barker, 1990).By 6Ma, the relative flux of NADW into the Southern Ocean reachedmodern proportions, and subsequently intensified yet further, supporting thespeculation that the relative climatic warmth of the Early Pliocene was aresult of enhanced thermohaline overturn (e.g. Kwiek and Ravelo, 1999).During the Pliocene, biological productivity increased suggesting a reductionin sea-ice extent/coverage at this time (e.g. Grutzner et al., 2005; Hillenbrandand Cortese, 2006). Furthermore, silicoflagellate assemblages have been usedto pinpoint three intervals (3.7, 4.3–4.4 and 4.6–4.8Ma) within the Pliocenewhen sea surface temperatures in the Southern Ocean were roughly 51warmer than today (Whitehead and Bohaty, 2003).

10.8.7. Modelling Antarctic Middle Miocene to Pliocene Climates andAntarctic Ice Sheets

A number of palaeoclimate modelling studies have been conducted for theMiddle Miocene to Pliocene period (e.g. Ramstein et al., 2005). However,only a few of them have focused on reconstructing the climate andenvironments of Antarctica (see Haywood et al., 2002b; Francis et al., 2007;Hill et al., 2007). A limited number of studies, using a prescribed EAISwithin the climate model which is based on global sea-level estimates, suggestthat during warm phases of the Pliocene (specifically the mid-Pliocene),climatic conditions of Antarctica could have ameliorated sufficiently to allowtundra forms of vegetation to exist in deglaciated regions. Idealizedsensitivity experiments (e.g. Huybrechts, 1993), in which ice-sheet modelsare forced with different temperature and precipitation regimes, suggest thata surface temperature warming of between 17 and 201C is needed to generatethe ice-free corridor over the Pensacola and Wilkes subglacial basinshypothesized by Harwood (1983) and Harwood and Webb (1986) for thePliocene. For temperature rises of less than 51C, the models predict that theEAIS increased in size due to an increase in snowfall. More recent studies inwhich climatologies derived from globally evaluated climate modellingsimulations for the Pliocene that are used offline to drive an ice-sheet modelsuggest that the mean state for the Pliocene Greenland Ice Sheet and EAISwere, to some extent, smaller than today. For Antarctica, the model results

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clearly show that the Wilkes and Aurora Basins are the areas of the EAISthat are most susceptible to increased temperatures.Nevertheless, even in the most extreme scenario, the predicted EAIS still

covers all but the northernmost reaches of the Wilkes subglacial basin, whichis suggested to be a source of the diatoms found within the Sirius Groupsediments (Hill et al., 2007).

ACKNOWLEDGEMENTS

This chapter is a contribution to the SCAR programme Antarctic ClimateEvolution (ACE) and the BAS core science programme Greenhouse to Ice-house Evolution of the Antarctic Cryosphere and Palaeoenvironment(GEACEP). Acknowledgements for funding and logistical and technicalsupport with the field campaigns and drilling projects addressing MiddleMiocene and Pliocene are made to UK Natural Environment ResearchCouncil, the US Office of Polar Programs (NSF-OPP), Antarctica NewZealand, the Australian Antarctic Division, the Italian National AntarcticProgram (PNRA), the Alfred Wegener Institute for Polar and MarineResearch (AWI), Deutsche Forschungsgemeinschaft (DFG) and the OceanDrilling Program.

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