Global and Planetary Change 76 (2011) 22–32

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Petrography and carbonate isotope stratigraphy from MIS AND-1B core, Antarctica:Evidence of the early Pliocene warming event

G. Scopelliti, A. Bellanca ⁎, R. NeriDipartimento di Chimica e Fisica della Terra ed Applicazioni (CFTA), Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy

⁎ Corresponding author. Tel.: +39 091 23861637; faE-mail address: bellanca@unipa.it (A. Bellanca).

0921-8181/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.gloplacha.2010.11.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 June 2010Accepted 23 November 2010Available online 29 November 2010

Keywords:stable isotopepalaeoclimateearly PlioceneANDRILLAntarctica

A large portion of ANDRILL (ANtarctic geological DRILLing) core AND-1B recovered in the Western Ross Seaand spanning the early Pliocene has been investigated in order to obtain a detailed carbonate isotope recordfrom Antarctic margin sediments through the early Pliocene warming event. Petrographic observations andmineralogical analyses reveal the authigenic nature of the carbonate and small proportions of Fe and Mgincorporated within the calcite lattice. High productivity conditions testified by ~80 m-thick diatomiteinterval (383 to 460 mbsf) well fit with the composite nature of the authigenic carbonate generallycharacterizing organic matter-rich sediments. As is known, sediments from the Polar Region are generallypoor in carbonate. Although in the investigated portion of AND-1B core the carbonate seldom exceeds 5% incontent, an automated Carbonate Preparation Device was used to obtain a high-resolution stable isotopedataset. Paleoenvironmental conditions characterized by high organic matter flux are supported by negativeδ13C values suggesting a contribution of isotopically light biogenic CO2 during the carbonate precipitation. Asto δ18O, even if melting glaciers are thought to be responsible for depletion in 18O composition, the isotoperecord exhibits long- and short-term trends. Analysis of the long-term trend constrains the Pliocene warmingclimax in an interval between 400–450 mbsf highlighting that most of the event is not documented because ofa 800 kyr hiatus. The short-term trend documents the influence of obliquity controlling the annual insolation,but also that of precession-linked cyclicity seldom documented at high latitude.

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1. Introduction

Both East and West Antarctic ice sheets (EAIS and WAIS) havebeen acting as key components in the global climate regime sincethe late Eocene (Lawver et al., 1992; Fitzgerald, 1999; Exon et al.,2000; O'Brien et al., 2000; Naish et al., 2009). The AIS haveinfluenced the albedo and strongly controlled the ocean thermoha-line circulation and eustatic sea levels, their absolute volumerepresenting a major source for the global oxygen isotopic record(Barker and Cooper, 1997; Thiede and Tiedemann, 1998; Barker etal., 1999). The compilation of benthic oxygen isotope data describesa general cooling trend for the global climate during the Neogene(Kennet and Hodell, 1993; Zachos et al., 2001). Within this generaltrend, early Pliocene (4.5 to 3.0 Ma) warm climate conditions areindicated by global surface temperatures ~3 °C higher than today(Ravelo et al., 2004; and references therein). Boundary conditionssimilar to those existing at present, including first-order oceancirculation patterns, Earth's continental configuration, reducedNorthern Hemisphere ice coverage, and atmospheric carbon dioxide

concentration about 30% higher than pre-anthropogenic values(Ravelo et al., 2004), make this period a significant natural studycase to assess the consequence of the increased levels ofatmospheric carbon dioxide foreseen for the century end. However,what is known about climate cycles during the early-middlePliocene has been mostly derived from oxygen isotopic recordsfrom low and mid latitude regions (e.g. Hodell and Venz, 1992;Shackleton et al., 1995; Zachos et al., 2001; Lisiecki and Raymo,2005; Naish and Wilson, 2009). Stable isotope datasets from PolarRegions and, in particular from Antarctic margin sediment records,have been notoriously difficult to access also because carbonatepreservation in the sediments is often poor.

In the present work attention is mainly focused on the carbonatecarbon and oxygen isotope record from a portion of AND-1B corerecovered in the Ross Sea by the Antarctic geological drillingprogram, ANDRILL. Mineralogic and petrographic investigationswere preliminarily performed on selected samples with the aim tocharacterize the core lithologies and to investigate the nature ofcarbonate. The core portion object of this study (347–522 mbsf)includes 80 m-thick diatomite strata recording the early Plioceneinterglacial period. In AND-1B core, warm climate conditions havebeen characterized by high surface water productivity, minimalsummer sea ice and air temperatures above freezing connected to

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West Antarctic Ice Sheet (WAIS) collapse (Naish et al., 2009). Thesame authors highlighted unconformity-bounded glacimarine cyclesproviding ice-proximal evidence for ~40 kyr oscillations in PlioceneWAIS. The main aim of this work was to provide a detailed stableisotope curve in marginal sediments from southern high latitude andto contribute to better constrain timing and extent of the WAISresponse to early Pliocene warmer conditions highlighting theresponse to the astronomical forcing.

2. Geological setting

The present day McMurdo Ice Shelf (MIS) forms the northwesternpart of Ross Ice Shelf (Fig. 1A) where it has been pinned by the RossIsland for the last ~10 kyr and nourished by ice flowing from WestAntarctic Ice Sheet (WAIS) and East Antarctic Ice Sheet (EAIS) outletglaciers in the southern Transantarctic Mountains (Hughes, 1977;Denton and Hughes, 2000; Mosola and Anderson, 2006).

TheMIS drill sitewas located above a flexuralmoat basin adjacent tothe Ross Island (Fig. 1A and B) that formed in response to Quaternaryvolcanic loading of the crust by volcanoes of the Ross Island Massif,being superimposed on more regional subsidence associated withNeogene extension of the Terror Rift (Horgan et al., 2005; Naish et al.,2006; Fieldinget al., 2008). Thisflexuralmoatbasin is part of theVictoriaLand Basin, a long-lived Cenozoic rift basin with a multi-phase historyincluding extensive outcrops of the Cenozoic alkalic Erebus VolcanicProvince (Ross Island, White Island, Black Island, Minna Bluff and theMount Discovery–Mount Morning peninsula) (Kyle, 1990; Esser et al.,2004; Fielding et al., 2008). High rates of tectonic subsidence due to aunique combination of regional rift extension and flexure of the crust bylocal volcanic islands provided the accommodation space for preserva-tion of the thick silici- and volcani-clastic filling that records theinteraction among the bodies of ice present in the region (Ross Ice Shelf,EAIS and WAIS) as they responded to the climate evolution. Thispeculiar settingmade possible to recover a single, 1284.87 m-deep, drillcore (AND-1B) fromthebathymetric anddepocentral axis of themoat in943 m of water from an ice shelf platform (Naish et al., 2007).

3. Lithostratigrafic and chronostratigraphic features

The investigated portion of MIS AND-1B core covers the intervalbetween 522 and 347 metres below seafloor (mbsf) (Fig. 2). Accordingto the lithostratigraphic interpretationproposedbyKrissek et al. (2007),from 522 to 460 mbsf the sediments consist of massive diamictite withvariable amounts of volcanic glass (subglacial to grounding zone)alternating with stratified diamictite, sandstone and mudstone withdispersed clasts (grounding-line to distal glacimarine); a thin diatomiteinterval occurs between 510 and 504 mbsf. At 460 mbsf, the glacialfacies pass upwards into an 84 m-thick interval of stratified diatomite(open ocean) indicating an extended period of high-productivity openwater over the site. At 377 mbsf, diatomites pass to a 13 m-thick intervalof intercalated diatomite, mudstone and diamictite that is followed, upto the topcore, by clast-rich to clast-poor muddy diamictite.

A preliminary age model for the upper 700 m of the whole MISAND-1B core (Wilson et al., 2007) was based on diatom biostra-tigraphy, magnetostratigraphy, 40Ar/39Ar ages on volcanic material,87Sr/86Sr ages of calcareous fossil material and erosion surfacesidentified from physical appearance and facies relationships recog-nized in the AND-1B drill core. The same authors correlate it with theGeomagnetic Polarity Time Scale (GPTS; Ogg and Smith, 2004). Thispreliminary age model was improved from Naish et al. (2009). Inparticular, biostratigraphic analyses of the diatomite between 460 and383 mbsf evidenced a “hiatus” of ~800 kyr at 440 mbsf. Thechronostratigraphic reconstruction of the studied core portionshows that the investigated sediments span the early-middlePliocene, in particular Zanclean/Piacenzian Planes (Fig. 2).

4. Material and methods

Selected samples of different lithology were manually powdered inan agate mortar and investigated in order to definemineral assemblageby powder X-ray diffraction (XRD) using a Philips PW14 1373 with Cu-Kα radiationfiltered by amonochromator crystal. Scanning interval andspeedwas 15−60° 2ϑ and 2° 2ϑ/min, respectively. Corrections for peakpositions due sampling preparation errors were performed by checkingposition of 100 intensity peak of quartz. Thin sections were obtained bystandard methods and observed under polarized-light microscope.Scanning ElectronMicroscope (SEM) investigationswere performed onpolished, carbon-coated thin sections, using a LEO 440with EDS systemOXFORD ISIS 300 Link and Si (Li) PENTAFET detector.

A total of 134 samples containing carbonatewere analyzed for stableC andO isotope composition. Prior to powdering, sampleswere carefullyexamined in order to rule out areas of high carbonate cementation andcarbonate veins associated with faults. CO2 was obtained by classicalphosphoric acid – calcium carbonate reactionmethod by an automatedCarbonate Preparation Device (Thermo Scientific GasBench II). The 44-mass peak area was used to determinate the CaCO3 content. Thecarbonate isotopic compositions (δ18O and δ13C) were measured byThermo Scientific Delta VAdvantage continuousflow isotope ratiomassspectrometer. Results are expressed in delta (δ) notation relative to theV-PDB standard. Precision of the carbon and oxygen isotope ratios forduplicate analyses improved to 0.1 and 0.2‰, respectively.

5. Results

5.1. Mineralogy and petrography

Petrology of diamictites, sandstones, and mudstones from AND-1Bcore was studied by Pompilio et al. (2007) in terms of natureand provenance of sand fraction and clasts. These authorsdescribe authigenic carbonate occurring as cement growth withinthe matrix in sandstones and diamictites or filling of vesicles involcanic clasts.

For this paper, petrographic investigations were focused ondiatomite samples. In thin section, diatomites show a clayey matrixand lamination marked by oriented diatoms and rare clasts, mainlyquartz and feldspar (Fig. 3A). Diatom tests (Fig. 3B) are constituted byopal-A as confirmed by diffractograms showing the characteristicbroad scattering band in the range 21.5–24 °2ϑ. Thin sectionobservations also document the presence of scattered organic matter(Fig. 3C) and pyrite as micrometric dispersed crystals (Fig. 3A) or inform of framboids often inside of diatom tests (Fig. 3D). Under theSEM, the diatom tests appear well preserved without evidence ofmineral phase precipitation connected with late diagenesis (Fig. 4Aand B). Pyrite framboids (4−10 μm in diameter) locally fill diatomtests (Figs. 4D and 5A), as seen in thin section. In SEM, carbonateappears as dispersed anhedral crystals b10 μm in size (Fig. 4C).Carbonate in form of sparry, radiaxial fibrous, and spherulitic cementswas never observed. EDS spectra indicate the composite nature ofcarbonate displaying Mg and Fe peaks (Fig. 5B). XRD analysis showsthe main diffraction peak of the carbonate phase (d104) displacedtoward higher angles (~29.8 °2ϑ), confirming the presence of calcitewith small contents of incorporated ions. In samples from 425.60,426.66, 437.10 and 449.26 mbsf, XRD analysis shows the presence ofcarbonate with composition varying between ankerite and dolomite.

5.2. Carbonate content

The CaCO3 distribution for AND-1B core shows fluctuationsbetween 0 and 11% with lower values in correspondence of thediatomite intervals (Fig. 6) where CaCO3 content does not exceed 5%.The obtained percentages rule out the possibility that the sampledcarbonate is referable to intense cementation processes.

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Fig. 2. Litho- and chrono-stratigraphy of theMIS AND-1Bportion between522 and347 mbsf. Time scale fromOgg and Smith (2004);magnetostratigraphic data fromWilson et al. (2007).

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5.3. Carbon and oxygen isotope records

Bulk carbonate carbon and oxygen isotope compositions for theAND-1B core are listed in Table 1. From the core bottom to the top(504 mbsf) of the first diatomite interval, δ13C values widely

Fig. 1. (A) Drill site MIS AND-1B (star) in the southeastern corner of the McMurdo Ice Shelf (A-A’ showing the wedge-shaped basin geometry of Victoria Land Basin and target strata of

fluctuate between –2.52 and –18.32‰, with lower values insidethis diatomite interval (Fig. 6). At the passage with the overlyingshale and siltstone, C-isotope values rise abruptly to –1.60‰ and,from this depth to 470 mbsf, δ13C values show high frequencyoscillations around a mean value of –5.73‰. From 470 mbsf, just

image from USGS. International Polar Year, 2007-2008). (B) Interpreted seismic sectiondrill cores AND-1B, SMS and CIROS-1 (redrawn from Harwood et al., 2006).

Fig. 3. Photomicrographs of AND-1B diatomite samples in plane polarized light. (A) lamination marked by oriented diatom tests; arrows indicate dispersed microcrystals of pyrite;(B) diatom test; (C) organic matter permeating the matrix; (D) pyrite framboids inside of diatom test.

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11 m below the base of the thicker diatomite interval, to 438 mbsf(in proximity of the hiatus at ~440 mbsf), the values delineate anupcore increasing trend from –8.82 to –4.24‰ (Fig. 6). Abovecarbonate-free diatomite strata, very negative values of δ13C (–12.77‰)increase in a fewmetres up to –7.45‰, then fluctuating around a meanvalue of –8.52‰ up to 383 mbsf. The top of the diatomite interval ismarked by a new increase between 383 and 381 mbsf. Moderateoscillations around a mean value of –4.45‰ persist until the core top(Fig. 6).

The oxygen isotope record shows an overall increasing trendthroughout the investigated AND-1B core portion passing from –

13.82‰ at the bottom to –7.43‰ at the top (Fig. 6). Inside of this

Fig. 4. SEM images (SE) of AND-1B diatomite samples. (A) siliceous tests; (B) detail of A showpyrite in a diatomite sample. Stars point to EDS measured spots (see Fig. 5).

trend, some large excursions are evident. An abrupt increase of δ18Ovalues up to –0.95‰ is observable in correspondence of the deeperdiatomite interval at 510-504 mbsf. Immediately above this interval,values return to –12.11‰. From this depth, the O-isotope recordexhibits an upcore increasing trend until to 470 mbsf from where amean value of –8.51‰ decreases of ~2‰ in 20 mmarking the passageto the thicker diatomite interval. Then, after the hiatus and carbonate-free diatomite strata, the O-isotope composition fluctuates around amean value of –10.40‰ for 25 m. From 413 to 403 mbsf, the record ischaracterized by a further increase to a δ18O mean value of –8.48‰,which is maintained until the top of the stratigraphic sequence(Fig. 6).

ing good preservation of diatom tests; (C) early diagenetic carbonate; (D) framboids of

Fig. 5. EDS spectra of pyrite (A) and carbonate (B) in AND-1B samples.

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Comparable δ18O and δ13C values were obtained by Pompilioet al. (2007) for three samples from the same interval of the AND-1Bcore.

6. Discussion

6.1. Bulk carbonate 18O and 13C depletion

Both carbon and oxygen isotope compositions of bulk carbonatefrom AND-1B stratigraphic succession are characterized by valuesmore negative than those expected for a normalmarine origin (Fig. 6).Carbonate isotope records should be used with caution in derivingpaleoenvironmental information as recrystallization at higher tem-peratures associated with greater burial depths and/or sediment–hydrothermal fluid interaction can significantly change primarygeochemical signals. Petrographic observations of the analyzedsamples showed that carbonate is present essentially as fine cementconsisting of micrometric anhedral crystals whereas sparry, blocky orpoikilotopic calcite cements documented in ancient rocks andcommonly interpreted as emplaced during late burial (e.g. Franket al., 2008) are not apparent. In addition, opal-A nature of silicaphases in diatomite levels and the excellent preservation of structuraldetails of siliceous tests, without any signal of diagenetic overprintfrom late diagenetic cements, argue against extensive flux ofdiagenetic pore fluids which would sustain the process of silicamaturation. Few data of pore water geochemistry (elementalcomposition and δ18O values) concerning the interval of AND-1Bhere investigated were presented by Pompilio et al. (2007). Thesedata are similar to those reported by Frank et al. (2010) for pore watersamples from depths above ~120 mbsf in AND-2A core andinterpreted as close to the local seawater composition at temperaturesno lower than -2.3 °C. On this basis, the obtained isotopic data can beplausibly regarded as indicative of real trends in the evolution of theisotopic composition of precipitating fluids.

It is well established that marine authigenic carbonates charac-terized by the presence of various bivalent ions in the lattice, such asare documented here, form preferentially in organic matter-richsediments which encountered, during or shortly after their deposi-tion, anoxic conditions inducing Mn/Fe oxy-hydroxide remobilization(Gautier, 1982; Wada et al., 1982; Wada and Okada, 1983; Tassé andHesse, 1984; von Rad and Botz, 1987). On the other hand, stableisotope compositions of authigenic carbonates can reveal microbialdegradation processes that affected depositional/early-diageneticenvironments in which they precipitated and that are commonlytriggered by a succession of more and more reducing environmentsdepending on availability and exhaustion of various oxiding agents(Irwin, 1980; Pisciotto and Mahoney, 1981; Kelts and McKenzie,1982).

The negative δ13C values of AND-1B bulk carbonate suggest aformation with contribution of isotopically lighter biogenic CO2

with respect to a normal marine signature. The wide oscillations ofC-isotope values registered from the core lower portionup to 470 mbsfmay be related to CO2 produced by degradation of organic matter indifferent fluctuating reduction zones (von Rad and Botz, 1987). Thestrongly negative values measured for the first diatomite level will bediscussed below. From470 mbsf to the top of the sequence, the carbonisotope profile shows long-term fluctuations. Moderately negativeδ13C values (mean value –5‰) measured between 470 and 438 mbsfand from 380 to 347 mbsf suggest that organic matter microbialoxidation played a role in the authigenic carbonate formation. In thediatomite interval between 440 and 377 mbsf, where the occurrenceof micron-sized pyrite framboids indicates anoxic sulphidic condi-tions, the carbon isotopic composition shows a persistent verynegative signal (mean value –8.5‰) suggesting that 12C-enrichedCO2 derived from bacterial degradation of organic matter duringsulphate reduction was a significant CO2 source.

As to the 18O depleted compositions of AND-1B bulk carbonate, themeasured values are in the range found by Aghib et al. (2003) andBellanca et al. (2005) for carbonates of Antarctic sediments from theCRP-3 core (Fig. 1). In accordance with these authors, the oxygenisotopic composition of bulk carbonate was controlled by fluidsentering the sedimentary system whose composition was primarilycontrolled by a mixing between marine waters and meteoric watersfrom melting glaciers.

More problematic is the interpretation of large shifts to the lowerδ13C values (down to –18,32‰) and higher δ18O (up to –0.95‰)measured in correspondence of the thin diatomite interval at 504−510mbsf (Fig. 6). Very negative δ13C valuesmay point tomethane as amajor source for the bulk carbonate carbon. Methane oxidationimplies increase of carbonate alkalinity in pore waters, which triggersprecipitation of isotopically depleted carbonates (Whiticar and Faber,1986). Methane oxidation in correspondence of fluid seepage isreported to induce formation of 13C-depleted authigenic carbonates inancient and recent marine sediments (Aloisi et al., 2002). To explainthe concomitant relatively high δ18O values, gas hydrate destabiliza-tion may be invoked as a possible source for 18O-enriched fluids.During gas hydrate formation, 18O-rich water molecules enterpreferentially into the solid phase and, therefore, gas hydratedissociation induces 18O-enrichment of the interstitial solutions(Hesse and Harrison, 1981; Pierre et al., 2000). The anomaly in thecarbon and oxygen isotope profiles over the deeper diatomite intervalis here interpreted as possibly due to episodic expulsion of free gas

Fig. 6. Depth profiles of CaCO3 percentage and stable carbon and oxygen isotope composition for the AND-1B core between 522 and 347 mbsf. Solid line represents the main trend inδ18O (see text); lithologies as in Fig. 2.

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similarly to what found in actual areas of cold seeps. In these areas,dissociation of gas hydrate yields a net production of CO2 depleted inδ13C and enriched in δ18O (Sassen et al., 1999; Díaz-del-Río et al.,2003). The values relative to this interval are considered as producedby an episodic event and, for this reason, are left out of the discussionin Section 6.2.

6.2. Long- and short-term oxygen isotope trend and implications forearly Pliocene palaeoclimatology

A long-term trend in the O-isotope record is characterized byincreasing values from the bottom to the top of the core. This isapparently in contrast with palaeoclimate remarks. The core portionbetween 347 and 520 mbsf was deposited during the Zanclean-Piacenzian boundary. The 80 m-thick diatomite interval marks openmarine conditions testifying the retreat of Antarctic ice sheets inresponse to the early Pliocene global climate warming (Naish et al.,2009; Pollard and De Conto, 2009; Winter et al., 2010). Following thisassumption it should be awaited a decrease in δ18O values, at leastinside the diatomite interval, as a consequence of warm conditionsand of lighter seawater isotope compositions related to enhancedrunoff of local melt water. Actually, the O-isotope record of AND-1Bcore shows that a ~2‰ negative shift at 460 mbsf marks the onset of

diatomite deposition (Fig. 6). Then, although a complete isotopestratigraphy is lacking because of carbonate-free levels and also for ahiatus of ~800 kyr, δ18O values are low over a ~45 m-thick diatomite-rich interval and unexpectedly increase again at ~400 mbsf. It is heresupposed that this trend would indicate that the record of the earlyPliocene warming climax is confined to the 400–450 mbsf intervaland that most of this event is not documented because of the hiatusand of the carbonate absence. This result is consistent with that of astudy on the circum-Antarctica warming events recorded between 4and 3.5 Ma in sediments from the Prydz Bay and Antarctic Peninsula(Escutia et al., 2009). These authors conclude that a prolonged early-middle Pliocene warm period was superimposed on a cooling trend asrecorded by the decrease in sea surface temperatures (SSTs) andincrease in terrigenous sediment supply starting between 3.7−3.6 Ma. They also postulate that, although the start of a coolingtrend is recorded at about 3.7–3.6 Ma, relatively warm conditionsprevailed until 3.5 Ma. These data well fit with the isotope record hereproposed in which low δ18O values occur for about 35 m above thehiatus dated 3.596 Ma.

With the aim to confirm this hypothesis with temporal constrains,attention has been focused to shorter-term oscillations modulatingthe long-term oxygen isotope trend. In a recent work, Naish et al.(2009) well describe obliquity cyclic variations for the upper 600 m of

Table 1Carbon and oxygen isotope data (δ‰ relative to V-PDB standard) for samples fromAND-1B core.

mbsf δ13C(‰)

δ18O(‰)

mbsf δ3C(‰)

δ18O(‰)

mbsf δ13C(‰)

δ18O(‰)

347,00 -5,69 -7,43 393,00 -10,34 -8,13 473,00 -7,67 -9,58348,04 -3,83 -7,65 394,00 -10,52 -7,32 474,03 -5,41 -9,86349,01 -2,61 -7,81 395,00 -10,21 -8,60 475,03 -5,01 -10,50349,99 -5,91 -6,90 396,00 -10,18 -10,54 476,01 -4,68 -10,39351,01 -4,41 -10,18 397,01 -9,61 -7,54 477,00 -3,21 -11,45352,05 -3,20 -9,06 399,02 -9,97 -10,05 478,00 -4,95 -10,47353,07 -3,50 -10,45 400,01 -7,65 -9,16 479,00 -12,42 -9,11354,04 -3,78 -9,81 401,00 -7,71 -5,70 480,00 -4,08 -10,70355,02 -8,38 -6,88 402,00 -8,04 -7,92 481,00 -3,73 -11,33355,99 -8,59 -7,09 403,00 -8,29 -7,44 482,00 -5,44 -10,41357,01 -6,37 -6,36 405,00 -9,43 -7,57 483,02 -5,82 -10,93358,02 -3,02 -9,56 410,00 -6,87 -9,26 484,02 -5,03 -11,76358,91 -4,60 -9,26 411,00 -7,87 -9,77 486,02 -3,40 -11,46360,04 -0,20 -12,13 412,00 -8,52 -9,16 487,00 -6,03 -11,18361,08 -3,81 -9,29 413,06 -8,75 -10,03 488,00 -4,84 -12,03362,00 -7,84 -7,53 414,02 -6,65 -12,13 489,00 -4,62 -12,29364,09 -6,09 -7,55 415,01 -8,14 -9,80 490,00 -5,31 -12,05365,06 -4,27 -8,38 416,00 -7,12 -12,02 491,00 -5,37 -11,93366,02 -5,97 -9,32 417,02 -9,64 -10,91 492,00 -5,16 -11,27367,09 -5,18 -8,58 418,01 -9,65 -11,18 494,04 -8,89 -11,51368,04 -3,83 -10,98 420,00 -9,12 -9,63 495,00 -2,70 -12,69369,00 -3,46 -9,44 421,00 -7,45 -8,87 496,04 -4,24 -11,38370,00 -2,60 -9,31 422,00 -9,96 -10,69 497,04 -5,31 -11,78371,00 -2,08 -9,66 423,00 -10,31 -10,90 498,00 -7,51 -11,72372,00 -4,59 -8,08 426,05 -12,77 -9,46 499,00 -10,52 -10,48373,00 -4,73 -7,35 438,01 -4,24 -10,74 500,01 -1,99 -13,14374,01 -4,92 -4,64 439,01 -3,05 -12,32 502,01 -1,98 -9,36375,01 -2,69 -6,25 440,04 -4,41 -6,97 503,02 -1,60 -12,11376,02 -3,11 -5,48 450,00 -3,61 -10,51 506,00 -16,66 -9,04377,00 -4,73 -8,71 452,00 -5,78 -9,80 508,01 -18,32 -0,95378,01 -4,36 -4,92 455,03 -6,80 -9,54 510,00 -2,52 -5,92379,00 -4,19 -10,88 456,00 -4,18 -10,27 511,02 -2,79 -11,14380,00 -4,39 -9,95 457,00 -6,05 -10,89 512,01 -14,51 -9,11381,00 -3,56 -10,55 459,00 -3,77 -9,43 513,00 -7,55 -9,29382,00 -4,58 -8,94 461,02 -6,27 -10,05 514,00 -8,70 -10,52383,00 -7,93 -7,60 462,00 -6,66 -8,41 515,00 -14,80 -8,07384,00 -7,28 -8,22 463,00 -6,98 -9,06 516,01 -10,31 -9,29385,00 -7,30 -9,28 464,02 -6,67 -8,39 517,00 -3,14 -13,43386,00 -5,66 -9,51 465,01 -7,21 -8,79 518,00 -3,40 -14,13387,00 -7,56 -9,64 466,00 -7,23 -8,33 519,03 -8,25 -9,78388,00 -8,25 -7,88 467,00 -5,90 -9,83 520,06 -3,26 -13,42389,00 -9,18 -8,10 469,00 -6,79 -10,68 521,00 -3,28 -13,06390,01 -9,25 -8,84 470,00 -8,82 -7,23 522,01 -2,59 -13,82391,00 -7,24 -9,50 471,00 -11,18 -7,63392,01 -10,19 -9,09 472,01 -2,83 -12,58

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the AND-1B core. These authors emphasized that the cyclostrati-graphic interpretation throughout the early- to mid-Pliocene intervalwas made hard primarily for the presence of the large marinediatomite interval between 438 and 376 mbsf. Nevertheless Naishet al. (2009) found, inside this interval, four or five main orbital cyclesthat are identified by N2-mm-diameter iceberg-rafted debris (IBRD)interpreted as reflecting oscillations of local outlet glaciers. In thepresent paper, the O-isotope record is proposed as a key tool toimprove the cyclostratigraphic interpretation inside the large diato-mite interval. At this purpose, being the core portion between 522-347 mbsf well temporarily constrained by magnetostratigraphic data(Wilson et al., 2007), the δ18O record has been correlated to benthicδ18O LR04 stack (Lisiecki and Raymo, 2005), modelled Antarctic icevolume and obliquity from Naish et al. (2009) (Fig. 7). Remarkably, astrong relation betweenmaxima in δ18O from AND-1B bulk carbonateand those in benthic O-isotope composition is evident. This corre-spondence well fastens the boundaries of obliquity cycles in theinvestigated portion of core, and overall inside the large diatomiteinterval. On this basis, the 35 m-thick diatomite interval overlying thehiatus and characterized by lower δ18O values corresponds to the

isotopic stages Mg11, Mg 9 and Mg 7 documented by Escutia et al.(2009) as some of the events of extreme warmth during the earlyPliocene in both East and West Antarctica. The other events, theisotopic stages Gi5 and Gi1, are missing in AND-1B core because of thehiatus at ~440 mbsf.

To verify the possible influence of others astronomical periodi-cities on the isotope record, power-spectral analyses have beenapplied. Power spectrumwas estimated in the depth domain and theninterpreted on the basis of a sedimentation rate of 0.358 m/kyr. Thisrate is calculated from the magnetostratigraphic data between 520and 460 mbsf, the only portion not showing any important hiatus. Thepower spectra for δ18O shows peaks respectively at 36.56, 15.06, 9.14,4.57, and 3.88 m that, normalized to the estimated sedimentationrate, define periodicities at 102, 42, 26, 13 and 11 kyr (Fig. 8A). Thefirst three peaks are noticeably referable to eccentricity, obliquity andprecession cyclicity and, even if the last is shortly above theconfidence interval, it is often identifiable inside of obliquity cyclesin the O-isotope record (arrow in Fig. 7).

The identification of precession cyclicity in the geological record athigh latitude is peculiar considering that, while great importance hasbeen attributed to the annual insolation controlled by obliquity (e.g.Young and Bradley, 1984), the effect of seasonal insolation modulatedby precession is seldom documented.

The obtained cyclostratigraphic results offer the opportunity towell define the sedimentation rates in the portions above and belowthe hiatus by the construction of an age-depth profile (Fig. 8B). In thisdiagram magnetostratigraphic data are reported from Wilson et al.(2007; open rhombs in figure) and the age of AND-1B samples as fromthe obliquity and precession cycle boundaries. Besides the decrease inslope at 455 mbsf due to an erosional contact, the sedimentation rateincreases from 0.217 m/kyr throughout the diatomite interval abovethe hiatus to 0.331 m/kyr in the diamictites, sandstones, andmudstones below the hiatus.

7. Conclusion

Results from the study on the 347–522 mbsf AND-1B core portion,recording the early Pliocene interglacial period, can be summarized bythe following points:

1. The carbonate content seldom exceeds 5%. Mineralogical analysesand EDS spectra give evidence of incorporation of bivalent ionswithin the calcite lattice. SEM observations document theauthigenic nature of the carbonate and rule out late diagenesisprocesses that can affect the oxygen isotope composition.

2. High surface productivity, characterizing the diatomite intervals, issupported by petrographic observations showing organic matterpermeating the matrix; consistently, negative δ13C values suggestthat organic matter bacterial degradation played a role in theauthigenic carbonate formation.

3. The carbonate oxygen isotope record highlights that the early-middle Pliocene warm period is superimposed on a cooling trendand confined between 400–450 mbsf. At short scale it is possible torecognize cycles modulated by obliquity that correspond tofluctuations in the curves of modelled Antarctic ice volume andbenthic δ18O stack. Interestingly, the δ18O dataset shows alsoeffects of seasonal insolation related to precession that are seldomdocumented for high latitude.

Acknowledgements

This study was supported by the Italian Antarctic ResearchProgram (PNRA). ANDRILL-MIS Science Team, the Chief ScientistsTim Naish and Ross Powell are thanked for their encouragement andassistance. The ANDRILL project is a multinational collaborationbetween the Antarctic programmes of Germany, Italy, New Zealand

Fig. 7. Integration of the O-isotope record for AND-1B core between 522-347 mbsf (this work) with curves of benthic δ18O LR04 stack (Lisiecki and Raymo, 2005), modelled Antarcticice volume, and obliquity as reported in Naish et al. (2009; see text); arrows indicate boundary in precession cyclicity; lithologies as in Fig. 2.

30 G. Scopelliti et al. / Global and Planetary Change 76 (2011) 22–32

and the United States. Antarctica New Zealand is the project operatorand developed the drilling system in collaboration with Alex Pyne atVictoria University of Wellington and Webster Drilling and Enter-prises Ltd. Antarctica New Zealand supported the drilling team atScott Base; Raytheon Polar Services Corporation supported the scienceteam at McMurdo Station and the Crary Science and EngineeringLaboratory. The ANDRILL ScienceManagement Office at the Universityof Nebraska–Lincoln provided science planning and operationalsupport. Scientific studies have been jointly supported by the USNational Science Foundation, NZ Foundation for Research, Science andTechnology and the Royal Society of NZ Marsden Fund, the ItalianAntarctic Research Programme, the German Research Foundation(DFG) and the Alfred Wegener Institute Polar and Marine Research.Comments by C.R. Fielding and an anonymous reviewer on a previousversion of this manuscript were extremely helpful.

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