High latitude Albian climate variability: Palynological evidence for long-term drying in a...

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High latitude Albian climate variability: Palynological evidence for long-term dryingin a greenhouse world

Barbara E. Wagstaff a,⁎, Stephen J. Gallagher a, Martin S. Norvick a, David J. Cantrill b, Malcolm W. Wallace a

a School of Earth Sciences, The University of Melbourne, Victoria 3010, Australiab National Herbarium of Victoria, Royal Botanic Gardens Melbourne, South Yarra, Victoria 3141, Australia

a b s t r a c ta r t i c l e i n f o

Article history:Received 5 March 2013Received in revised form 30 May 2013Accepted 12 June 2013Available online 22 June 2013

Keywords:AlbianPalynologyPalaeoclimateGippsland BasinAustralia

Detailed “Quaternary-style” quantitative spore–pollen counts, with a pollen sum based on totalnon-angiosperm seed plants, from a southern high palaeolatitude (60–65°) terrestrial sequence in Gippsland,southeast Australia has revealed strong vegetation and climate variability during the Albian. This variability ismore pronounced than previously suggested in global Early Cretaceous vegetation and climate reconstruc-tions. The quantitative spore–pollen record shows drying throughout the Albian based on upward decreasingtotal ferns and variation in podocarp and total Alisporites/Vitreisporites pollen. This record suggests that al-though global climate during the Albian is considered to have been warm, stable and equable, regional factorssuch as water availability and continentality were the main drivers of this vegetation change.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Presence–absence data and (semi-quantitative) acme spore–pollenzones are typically used to date Early Cretaceous strata in Australia(Helby et al., 1987). Reconstructions of past vegetation and climate inthese basins using pollen and spore records are usually broad summariesof the difference between regions (on a basin scale) and over long pe-riods (10s millions of years) (e.g. Dettmann, 1994). “Quaternary-style”palynomorph analyses to reconstruct climate history (Moore andWebb, 1983) of Cretaceous strata are rare (Wagstaff et al., 2006). InAustralia, Burger (1974, 1980) used detailed quantitative palynomorphassemblage data (spores, pollen, dinoflagellates and acritarchs) to corre-late Early Cretaceous strata in the Surat Basin (Fig. 1). Using these data,Burger (1980) reconstructed the palaeogeography of the region and clas-sified some taxa into coastal (humid) and inland (dry) species and alsointerpreted a cool-temperate climate. Dettmann (1986) used quantita-tive data from six Aptian samples from the Leongatha borehole in theGippsland Basin in southeast Australia (Fig. 1) to interpret a cool climatewith high humidity and seasonality and the presence of amosaic of plantcommunity associations that were regionally distributed.

In this paper, we present the first detailed “Quaternary-style”palynomorph assemblage analyses of high palaeolatitude Albian terres-trial strata from theGippsland Basin in southeastAustralia (Fig. 1). Thesedata reveal considerable climate variability at 60 to 65°S palaeolatitude

reflecting a more dynamic Albian climate than previously interpreted(Clarke and Jenkyns, 1999).

2. Depositional setting

This study concentrates on two hydrocarbon exploration wells(Wombat-1 and Wombat-3) with strata of Albian age (the StrzeleckiGroup) in the Gippsland Basin, Victoria, Australia (Fig. 2). The StrzeleckiGroup was deposited as a series of fluvial systems in a high latitude riftvalley between Australia and Antarctica (Wagstaff et al., 2012). In agross sense wireline log data for the wells in this study showed thatthe sedimentary style was braided (low sinuosity) fluvial throughoutthe section examined. The source of the sediments and the directionof current flow in the Bass Strait basins has long been a matter for con-jecture. The current consensus is that the rivers flow from the east(O'Sullivan et al., 2000) and that the volcanogenic sediments aresourced from contemporaneous volcanism (Duddy, 2003) from a volca-nic arc along the eastern margin of the continent (Bryan et al., 1997) orfrom volcanic centres within the main rift (Duddy, 2003).

3. Albian vegetation of Victoria

Dettmann (1981) described the palynological record of the region asdominated by podocarpacean saccate grains, including those attribut-able to Podocarpidites spp. and Microcachrydites antarcticus with minorgenera such as Araucariacites, Classopollis and Cycadopites. The Albianmega and microflora records show that the forests were dominated bypodocarps andaraucarians,with rare ginkgoes and thefirst angiospermscoinciding with the disappearance of bennettitaleans, taeniopterids and

Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 501–511

⁎ Corresponding author at: School of Earth Sciences, The University ofMelbourne, Victoria3100, Australia. Tel.: +61 3 8344 6537; fax: +61 3 8344 7761.

E-mail address: [email protected] (B.E. Wagstaff).

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sphenopsids (Dettmann et al., 1992). In Australia these non-conifer seedplant groups are thought to have declined during the Albian as angio-sperms increased (McLoughlin et al., 2010). In Victoria osmundaceousfernswere common in the understorey and in general fern communitieswere less diverse and lycopod sporesweremore common than in basinsto the north (Dettmann et al., 1992). By the mid-Albian the angiospermflora had a higher diversity (Dettmann, 1994) and included a widerrange of pollen morphologies (when tricolpate and tricolporoidatetypes appeared and joined the taxa: Clavatipollenites and Asteropollis).

4. Palaeoenvironmental interpretation of spore–pollen records

The first assumption when making an interpretation of a spore–pollen diagram is that the pollen assemblage reflects the surroundingvegetation. The second assumption is that the vegetation reflects en-vironmental conditions (Moore and Webb, 1983). There are manyfactors that can influence the presence of a particular plant in thelandscape (e.g. soil, altitude). However, the overriding factor thatcontrols regional vegetation is climate. To aid in the understanding

of the methods used in this study, the basis for palaeoenvironmentalanalyses using spores and pollen is reviewed below.

There are two main ways that Quaternary pollen distribution isinterpreted. (i) Pollen and spores can be used to reconstruct past veg-etation and (ii) assemblages can be used to make ecologic interpreta-tions of palaeovegetation in terms of past environments, in particular,climate (Macphail and McQueen, 1983). Assigning pollen or spores toa parent-plant is a key part of this interpretation (Faegri and Iverson,1975; Moore and Webb, 1983) and is fairly straightforward in such“near modern” floral assemblages.

Distinguishing the source of the flora is essential in palaeoenvi-ronmental interpretation. Two main source areas are:

• Regional: These are plants that define the vegetation of the regionat any time and are controlled by climate. Regional pollen is sourcedfrom more than several hundred metres from the site of deposition(Traverse, 1988).

• Local: These are plants growing on (e.g. a bog) or in (e.g. aquaticplants in a lake) or near (e.g. reeds around a lake) the site of

Fig. 1. Australian Mesozoic sedimentary basins.Palaeolatitudes derived from Li and Powell (2001).

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deposition. Traverse (1988) defines local plants as those that growwithin 20 m of the site of deposition. Therefore, at any site thereis an over-representation of spores and pollen from local plants.Such plants are controlled by the site hydrology and as such cannotbe used to reconstruct the climate regime of an area.

5. Pollen and spores: taphonomic factors in a fluvial system

Muller (1959) analysed the distribution of palynomorphs in themodern sediment of the Orinoco Delta in eastern Venezuela. Thiswork was the first to show the correspondence between the sourcevegetation and resultant palynomorph distribution in sediment wasreliable and could be used as a basis for palynological reconstructionsof past environments. The strata in Wombat-1 andWombat-3 are flu-vial and sorting of the palynomorphs is likely to have happened.Flume experiments show that once a pollen grain is in suspension(i.e. wetted) it behaves in a similar way to other small particles(such a silt or mud grains) (Brush and Brush, 1972). Studies ofspore–pollen assemblages in modern fluvial settings show they cor-respond well to the vegetation in the drainage basin (Chmura et al.,1999). Catto (1985) showed that the variation in upper and middlestory produced pollen in braided fluvial silt reflects changes in theirabundance in the source vegetation. Moss et al. (2005) analysed tworiver catchments in north eastern Australia and demonstrated thatpalynomorph assemblages in sediment from both rivers reflected thevariation in the vegetation cover, and climate difference between catch-ments. However, pollen produced by understory and groundcover plantsis likely to be overrepresented in river sediment as it is transported intothe system by surface runoff (Holmes, 1990). Chmura et al. (1999) alsosuggested that surface runoff from riverbanks and islands introduce pol-len from local plantswith the amount being dependent on river geomor-phology. These authors suggested that braided river channels would beexposed to more local pollen than a meandering river that has a singlemain channel. A major problem in fluvial deposited pollen assemblagesis degraded grains. These may be sourced during times of peak flowfrom riverbanks and bars where they have been exposed to aerobic

conditions and sediment erosion since the last period of low river stage(Chmura and Liu, 1990).

6. Methods

TheWombat-1 (38°21′15″S, 147°09′32″E) andWombat-3 (38°21′33″S, 147°08′57″E) wells are in theWombat gas field (Fig. 2). The only sam-ples available from thewells are cutting samples. Although these samplesare not ideal for fine resolution pollen studies (in a Quaternary sense thiswould be at time intervals of a century or less e.g. Newnham and Lowe(2000)), as the cuttings in any horizon may be sourced from above andbelow the actual recorded depth. With careful consideration of potentialcontamination, quantitative analyses of spore pollen data from cuttingscan be used in palaeoenvironmental analyses. For example, Oligo-Miocene quantitative analyses of spore/pollen assemblages in cuttingsamples from the subsurface of the Murray Basin in southeast Australiahave been used in palaeoecological interpretations (Martin, 1984). Quan-titative palynological analyses from Oligocene upper deltaic cutting sam-ples have been used for palaeonvironmental analyses (Rull, 2003).Cuttings also have the advantage of giving a smoothed record of the (ca.3 mG.Holdgate pers. comm.November 2012) interval analysed allowinglong-term variability to be interpreted in thick stratigraphic sequencesthat may span millions of years. All samples were selected from mud-dominated (fluvial overbank deposits) sections in the well, guided bygamma log data, to minimise facies-controlled variability of the spore–pollen assemblage. Core Laboratories Australia Pty Ltd. in WesternAustralia processed all samples from Wombat-1 and Wombat-3 usingstandard palynological processing, involving acid digestion, heavy liquidseparation, oxidationwith nitric acid and 10 μmsieving. Before undertak-ing the spore–pollen counts an achievable and statistically reliable pollensum had to be determined. Moore and Webb (1983) consider 150 treepollen grains as the standard count needed to investigate forest historyin the Quaternary of Europe. While Faegri and Iverson (1975) concludedthat there actually no “correct” pollen total, as each circumstancemay re-quire different counts to address particular research questions. For exam-ple a total of sixteen pollen taxa has been selected as a standard pollen

Fig. 2. The location of Wombat-1 and Wombat-3 exploration wells in the Wombat tight gas field in Petroleum Exploration Permit area 157, Gippsland Basin, Victoria, Australia.

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sum to reconstruct regional Quaternary vegetation and climate history ofsoutheast Australia (D'Costa and Kershaw, 1997). Since the purpose ofthe current study was to investigate the changes in vegetation relatedto climate, itwas decided to count the non-angiosperm seed plant speciesas the pollen sum total and all other taxa were counted against this sum.Early Cretaceous non-angiosperm seed plants from Victoria can

be divided into a number of broad taxa groups. These include conifers(in the families Araucariaceae, Cheirolepidaceae and Podocarpaceae),Cycadales, Ginkgoales, Bennettitales, Pentoxylales and Corysptospermales(McLoughlin et al., 2002). Modern studies show that individual pollenproducing plants may in fact be over or under represented consistentlydue to the nature of pollen production or dispersal of a particular spe-cies. Investigations of the production and dispersal habits of the livingrelatives of some of these taxa indicate that they can vary from beingover represented (e.g. Cupressaceae (Dodson, 1983)), well represented(e.g. Podocarpaceae (Macphail and McQueen, 1983)) to underrepre-sented (e.g. Araucariaceae (Kershaw, 1976)). However, as stated byMacphail (1979) in special circumstances, even very low percentagesof poorly dispersed pollen types may be diagnostic of a particular envi-ronment. Therefore using the pollen produced by these plants as thepollen sum is based on the inference that changes in the ratio of thetree taxa is primarily driven by large-scale climate factors.

Although the ability of spore-producing plant to live in any partic-ular area (i.e. its presence) is also controlled by the climate, changesin the abundance of spore-producing plants at any site are more likelyto have been driven by local hydroseral successions. Also in modernfluvial systems the riparian vegetation (i.e. local vegetation) will beoverrepresented and in an Early Cretaceous sense this riparian vege-tation will primarily be spore-producing plants. It is generally accept-ed that spores are dispersed only small distances with large sporesnot dispersing as far as small ones (Tryon, 1986) and most sporeslanding close to the parent plant (Wolf et al., 2001). Peck et al.(1990) also showed that spore distribution is overwhelmingly local.

In this study, the palynomorph yield was low and this limited thepollen sum (non-angiosperm seed plant taxa) to 100 grains, resultingin a total palynomorph count per slide of ≥200 grains. Pollen dia-grams (Figs. 3 and 4) were constructed using the sum (regional =non-angiosperm seed plant) taxa to calculate the percentage data.Using the formula below each individual pollen and spore taxon isexpressed as a percentage of the non-angiosperm seed plant species(pollen sum).

% of a taxon in a sample

¼ Number of the taxon counted in the sampleNumber of non� angiosperm seed plant taxa counted in the sample pollen sumð Þ � 100:

Percentage data for all the wells were calculated and the datawere plotted using WellPlot 4 (Paz Software). To facilitate illustrationand interpretation some the spore-pollen taxa were grouped (seeTable 1 for composition of taxa groups). Some taxa are not shownas they were rare or showed minor variability. All taxa are shown inthe diagrams as percentages; however total palynomorphs countedare shown with a log scale, as values are sometimes very high. A listof all taxa is provided in the online Supplementary data (Table 2)with their possible botanical affinities. Angiosperm pollen is not plot-ted on the diagrams, as their abundance was low to very low. It issuspected that the rarity of angiosperm pollen was primarily due toa 10 μm-sieving step in the preparation of the samples.

The palaeogeography of Australia at two snapshots within the EarlyCretaceous is shown in their reconstructed palinspastic positionswithinGondwana (Fig. 5). The Cretaceous environments of deposition wereextracted as ESRI Shapefiles from the AGSO Palaeogeographic Atlas ofAustralia (available from http://www.ga.gov.au, Langford et al., 2002).The Shapefiles were imported into the Earthbyte GPlates software (ver-sion 1.2.0, available from http://www.earthbyte.org, Muller et al., 2008)and rotated to the relevant million year age (see below) using the rota-tion files downloaded from the samewebsite. Then, the two reconstruc-tions were exported as Shapefiles for the environments of deposition,static plate polygons and present day coastlines (at their position inthe Cretaceous). These three Shapefiles were loaded into ArcGIS, sym-bolized, annotated and displayed in a South Polar stereographic projec-tion (projected coordinate system WGS 1984 USGS Transantarctic

Table 1Spore–pollen groups used to create Figs. 3 and 4.

Spore-pollen group in figures 3and 4

Taxa included in group Comments

NON-ANGIOSPERM SEED PLANTSTotal Alisporites/Vitreisporites Alisporites grandis

Alisporites similisVitreisporites spp.

These taxa are also shown asindividual columns to show howthey are contributing to thechange in the total.

Araucariacites australis Araucariacites australisOther ?Araucariaceae Callialsporites segmentatus

Callialsporites trilobatusClassopollis chateaunovi Classopollis chateaunoviCycadopites spp. Cycad opites spp.Taxodiaceapollenites hiatus Taxodiaceapollenites hiatusBisaccate Podocarpaceae Podocarpidites ellipticus

Podocarpidites multisemusTrisaccate Podocarpaceae Microcachryidites antarcticus

Trichotomosulcites subgranulatusThese species are also shownas individual columns to showhow they are contributing to thechange in trisaccatePodocarpaceae.

BRYOPHYTIC SPORESBryophytes Aequitriradites burgerii

Aequitriradites spinulosusAequitriradites verrucosusAequitriradites sp. Dettmann 1963Cingutriletes clavusCoptospora paradoxaForaminisporis asymmetricusForaminisporis dailyiForaminisporis wonthaggiensisStereisporites antiquasporitesStereisporites pocockiiTriporoletes laevigatusTriporoletes radiatusTriporoletes reticulatusTriporoletes simplex

LYCOPHYTIC SPORESLycophytes Ceratosporites equalis

Densoisporites velatusFoveosporites canalisLeptolepidites majorLeptolepidites pudensLeptolepidites verrucatusNeoraistrickia truncataPerotriletes linearisPerotriletes majusRetitriletes austroclavatiditesRetitriletes circulomensisRetitriletes eminulusRetitriletes facetusRetitriletes nodosusRetitriletes radiatusRetitriletes reticulumsporitesRetitriletes spp.Sestrosporites pseudoalveolatus

FILICEAN SPORESBaculatisporites/Osmundacidites Baculatisporites comaumensis

Osmundacidites mollisOsmundacidites welmannii

Cicatricosisporites/Ruffordiaspora Cicatricosisisporites hughesiiRuffordiaspora australiensisRuffordiaspora ludbrookiaeRuffordiaspora sp. Dettmann &Clifford 1992

Crybelosporites striatus Crybelosporites striatusCyathidites australis/minor Cyathidites australis

Cyathidites minorAll other Cyathidites spp. Cyathidites asper

Cyathidites punctatusCyathidites sp.Dictyophyllidites crenatus Dictyophyllidites pectinaeformisDictyophyllidites sp.

Gleicheniidites senonicus Gleicheniidites senonicusAll other fern spores Balmeisporites holodictyus

Biretisporites eneabbaensisBiretisporites potonieiBiretisportites spectabilisConcavissimsporites penolaensisDictyophyllidites crenatusDictyophyllidites pectinaeformisContignisorites spp.Dictyophyllidites sp.Dictyotosporites complexDictyotosporites speciosusFoveotriletes parviretusIschyosporites craterisIschyosporites punctatusKlukisporites scaberisLaevigatosporites spp.Lycopodiacidites asperatusPilosisporites grandisPilosisporites notensisPilosisporites parvispinosusRetispora triquetraTriletes cf. T. tuberculiformisTrilobosporites tribotrysTrilobosporites trioreticulosus



Fig. 3. Spore–pollen distributions through the section in Wombat-1. The diagram was constructed using the pollen sum (regional = non-angiosperm seed plant = 100%) taxa to calculate the percentage data. Each pollen and spore taxon isexpressed as a percentage of this pollen sum.

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Fig. 4. Spore–pollen distributions through the section in Wombat-3. The diagram was constructed using the pollen sum (regional = non-angiosperm seed plant = 100%) taxa to calculate the percentage data. Each pollen and spore taxon isexpressed as a percentage of this pollen sum.

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Mountains). Please note that GPlates will only reconstruct rigid platesand will not represent extended or deformed margins.

Choice of the specific time for each of the two Gondwana reconstruc-tions is based ultimately on the Australian spore-pollen zonation of Helbyet al. (1987) as updated by Partridge (2006). In this casewe are interestedin the upper Crybelosporites striatus zone and the upper Coptosporaparadoxa zone, which correspond to the AGSO Palaeogeographic Atlasof Australia Cretaceous timeslices K5 and K7 respectively. Correlation ofthe timeslices and the biozones is taken from stratigraphic charts pub-lished by Geoscience Australia (Shafik et al., 1998) based on the Youngand Laurie (1996) timescale. The absolute Ma age of these two biozoneintervals has since been modified. Using Partridge (2006) and Gradsteinet al. (2004), the best estimate for a snapshot of upper Crybelosporitesstriatus zone and timeslice K5 is 110 Ma, and for the upper Coptosporaparadoxa zone and K7 is 104 Ma.

7. Results

7.1. Changes in spore-pollen abundances

For the ease of discussion the pollen diagrams (Figs. 3, 4) havebeen divided into their spore-pollen biostratigraphic zone/subzone(Wagstaff et al., 2012) and the changes in the vegetation assemblageare discussed using this framework.

7.1.1. Crybelosporites striatus zoneThis zone is best represented near the base of Wombat-1 between

1848 m and 1981.2 m (T.D.). The two lowermost samples in Wombat-3are also tentatively placed in this zone. The interval is characterised byhigh values of total Alisporites/Vitreisporites pollen (30–45%) with totaltrisaccate podocarpaceous conifers varying between 20 and 30%. Highervalues of Araucariacites australis (10% in this section inWombat-1) corre-spond to decreases in all the podocarpaceous taxa and Classopollischateaunovi. Callialisporites spp. (?Araucariaceae) are very rarely recordedand Taxodiaceapollenites hiatus is rare in this zone. The total fern sporeabundance decreases through the zone, with the decrease largely beingexpressed by a reduction in the schizaeaceous fern species that areincluded in Cicatricosisporites/Ruffordiaspora totals and Cyatheaceae/Dicksoniaceae sourced ferns (i.e. Cyathidites australis/minor). All otherspore groups are rare or show no discernible patterns in the zone.

7.1.2. Trilobosporites trioreticulosus subzoneTotal Alisporites/Vitreisporites species are abundant in this interval,

reaching up to 50%. In Wombat-1 their distribution varies markedlywith values as low as 10%. The averaged total abundance of thesetaxa is similar to the underlying zone. However when their valuesare low they are replaced in the flora by higher values of trisaccateand bisaccate podocarps. Both of these podocarp groups are typicallycommon (20 to 40%) and bisaccate and trisaccate gymnospermsappear to have a slight negative correlation with each other. InWombat-3 total bryophytes, lycophytes and the Osmundaceae typeferns (Baculatisporites/Osmundacidites spp.) increase in abundancenear the base of the zone, thereafter decreasing up-section to thetop of the zone. The abundance of spores attributable to the Schizaeaceae(Cicatricosisporites/Ruffordiaspora spp.) and Cyatheaceae/Dicksoniaceae(Cyathidites australis/minor) decrease up-section continuing the trendfrom the previous zone (although the latter taxa decline gradually,while the decline of the former is stepwise).

7.1.3. Pilosisporites grandis subzoneThe abundance of pollen produced by Alisporites grandis and

Alisporites similis decreases significantly at the base of this interval.This event partly defines the base of the subzone (Wagstaff et al.,2012). The decrease in these pollen types is associated with an equiva-lent increase in trisaccate podocarpaceous grains and all podocarp-sourced pollen. Cycadopites spp. disappear in this zone (they are rare

at the base in Wombat-1). Taxodiaceapollenites hiatus is rare in thissubzone in Wombat-1, but shows a more consistent presence inWombat-3. The decrease in total ferns that occurred in the older intervalcontinues in this interval again controlled by decreases in the familiesSchizaeaceae (Cicatricososiporites/Ruffordiaspora spp.) and Cyatheaceae/Dicksoniaceae (Cyathidites australis/minor). The water fern producedspore Crybelosporites striatus was shown on the diagrams as it wassuspected that itmay have show somepattern in relation to the sedimen-tary facies. However, values for this taxon were consistently low, exceptfor two samples with values approaching 10% in this subzone inWombat-1 at approximately 1650 m. The top of the P. grandis subzoneis not well defined in the Wombat wells and the uppermost samplesmay lie in this subzone or belong to the overlying Cicatricosisporitescuneiformis subzone and are therefore not included in the interpretation.

8. Discussion

Detailed quantitative palynological analyses of two southern highpalaeolatitude Albian sections in southeast Australia (Wombat-1 and 3)has revealed strong variability in spore-pollen distribution that we inter-pret to reflect long term climate change.

The upward reduction in total Alisporites grandis and Alisporitessimilis pollen and total fern spores (particularly those attributable tothe Schizaeaceae and Cyatheaceae/Dicksoniaceae) abundance is par-ticularly marked. The decrease in Alisporites grandis and Alisporitessimilis pollen coincides with the global decline in abundance and di-versity of the group of plants (McLoughlin et al., 2002, 2008, 2010)associated with the pollen types. McLoughlin et al. (2010) interpretedthe decline in non-coniferous gymnosperms such as seed ferns andgingkoes as being related to the rise in angiosperms. It has been suggestedthat there was a diverse and abundant fern flora at this time and that thepattern of changes in the abundance of fern species associated with thisdecline is unknown (McLoughlin et al., 2010). However, Nagalingum etal. (2002) interpreted Australia-wide data to indicate a decline in theabundance of spores attributable to Cyatheaceae/Dicksoniaceae andOsmundaceae during the Albian associated with an increase in theSchizaeaceae and Gleicheniaceae. Lupia et al. (2000) examined globalchanges in floral abundances through the Cretaceous and noted that al-though the ferns, conifers and “other gymnosperms” declined at lowpalaeolatitudes, as a response to angiosperm diversity increase, this wasnot the case at high palaeolatitudes where they still comprised a majorfloral component.

There is no change in depositional style in the well sections to ex-plain the change in Alisporites spp. (given a tentative affinity with theseed fern order Corystospermales by McLoughlin et al. (2002)) abun-dance especially near the T. trioreticulosus/P. grandis subzonal bound-ary. Seed ferns in Neocomian strata in the Gippsland Basin areinterpreted to have required moderate moisture and a flood basinsetting (McLoughlin et al., 2002). Seed fern megafossils in late Albianstrata on Alexander Island in Antarctica are interpreted to haveformed an understory in a braid plain fluvial setting (Falcon-Lang etal., 2001). Based on these interpretations the relative abundance ofseed ferns should not decrease up section in the Albian strata studied,as braided (low sinuosity) fluvial environments typify the full sequenceof thesewells. McLoughlin et al. (2002) described a podocarp-seed fernassociation in Neocomian floodbasin settings. Similarly, podocarps andseed ferns dominate braid plain environments on Alexander Island(Falcon-Lang et al., 2001). Pollen grains attributable to Podocarpaceae(trisaccate conifer pollen types) increase up section in this study espe-cially in the sand-dominated sections. This increase is probably due totheir preference for disturbed braid plain environments where theyhave a role in forest regeneration on flood prone semi-infertile sub-strates (Falcon-Lang et al., 2001).

Other non-angiosperm seed plants in the pollen flora, apart fromthe apparent disappearance from the record of Cycadopites spp.,show no significant change in their distribution. Araucariaceae,



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1,800Kilometers

Fig. 5. Palaeogeography of Australia at two time slices: a) 110 Ma and b) 104 Ma. For methodology for creating the palaeogeographies, see text. (Note that the overlap of the Northand South Island New Zealand are an artefact of the plotting programme GPlates that only reconstructs rigid plates and will not represent extended or deformed margins.)



ginkgoes and Cycadophytes are present in late Albian meanderingfluvial settings on Alexander Island and are interpreted asrepresenting a forest of Araucariaceae and Podocarpaceae with asub-canopy of ginkgoes and Cycadophytes along river channel levees(Falcon-Lang et al., 2001). The loss of pollen of Cycadopites spp. (at-tributable to Cycadales/Gingkoales/Benettitales/Pentoxylales) is notregarded as evidence of an extinction based on this one study.McLoughlin et al. (2010) suggest that a reduction in plants withthese affinities may have been due to their inability to competewith the better-adapted flowering plants in their preferred disturbedstreamside and levee habitats.

Spores attributable to bryophytes and lycophytes do not showmuchvariability in this study. The presence of liverworts may indicate boggyconditions (Howe and Cantrill, 2001), although some hepatics wouldhave been able to withstand seasonally dry conditions (Dettmann,1986). Lycophytes required humid conditions (Dettmann, 1986) andtheir high water requirements meant that they grew on perennially orseasonally wet substrates in abandoned channels and wet floodbasinsettings (McLoughlin et al., 2002). As these spore-producing plantsare relatively uncommon we interpret this to mean that they formeda minor part of the vegetation, living adjacent to river channels andwithin abandoned channels.

The fact that braided (low sinuosity) fluvial environments remainconstant through the well should have resulted in high numbers oflocally sourced palynomorphs (Chmura et al., 1999). In this case,however, there is decrease in total ferns through the section. Asschizaeaceous ferns (Cicatricosisporites/Ruffordiaspora spp.) inhabitedmoist environments (Dettmann and Clifford, 1992) the reduction inthis fern group is significant. Crybelosporites striatus may have beenaquatic similar to modern Marsileaceae (Cookson and Dettmann,1958). However, this taxon is too rare to interpret its significance.All the remaining fern taxa are likely to have inhabited perenniallyor seasonally wet substrates, such as abandoned channels and wetflood basin settings (McLoughlin et al., 2002).

In general, the climate was considered to be warm and equableduring the Cretaceous (Spicer and Corfield, 1992). However, therewas some climate variability. For example Frakes et al. (1992) provid-ed evidence of the Cretaceous climate switching from cool to warmmode at around 105 Ma (Frakes et al., 1992) when ice-rafted deposi-tion ceased at high southern and northern high palaeolatitudes. ThisAlbian warming was associated with high precipitation at mid tohigh latitudes (Ufnar et al., 2004) with suggested rainfalls of6000 mm/year in the northern high mid-latitudes (White et al.,2000). Recent investigations have suggested that CO2 levels of1000 ppm may have occurred for short periods during the Albian(Royer, 2006) and that Cretaceous warm climates were punctuatedby brief (b2 myr) cooler periods.

Southern hemisphere δ18O data (Clarke and Jenkyns, 1999) sug-gest warming from the Aptian to Albian that continued into theearly Turonian. Stable isotope data from DSDP site 463 in the PacificOcean show that the early-middle Albian Southern Ocean was wellmixed with the cooler temperatures at low latitudes due to increasedpoleward heat transport warming the high latitudes (Price and Hart,2002). By the Cenomanian, the ocean temperature gradient wasmore sluggish and the reduced poleward heat transport led to polarcooling (with limited polar ice) while the tropics remained warm(Price and Hart, 2002).

Our data indicate an upward drying from the period from ca. 110to 103.5 Ma reflecting increasingly cooler conditions through theAlbian. This pattern is clearly at odds with the global climatic trends.We suggest this pattern is a palaeogeographically controlled and re-flects Australia's high palaeolatitude position in Gondwana (Fig. 5).The Early Cretaceous strata of the Gippsland Basin were connectedto Antarctica via a rift valley, in the absence of the Tasman Sea tothe east (Li and Powell, 2001). This palaeogeography would haveresulted in a continental climate in the absence of anymarine influence.

In northern Australia there is stratigraphic evidence for the contrac-tion of the inland seas from the latest Aptian onwards (Dettmann et al.,1992). Indeed, the Aptian saw the maximum post-Palaeozoic floodingof the Australian continent. The Aptianmarine transgression is often at-tributed to Australia drifting over a detached subducted slab, whichresulted in a geoid anomaly causing depression andflooding of a low re-lief landscape (Gurnis et al., 1998). Albian shoreline reconstructions(Fig. 5) show a regression and contraction of the large shallow inlandseas during the early to middle Albian that had flooded the continentin the Aptian. Regressive conditions continued into the latest Albianand by the Cenomanian the sea had retreated from the continental inte-rior (Frakes et al., 1987). Burger (1980) documented the onset of re-gressive conditions in detail from the Surat Basin during the Albianwhen marine environments were replaced by brackish conditionsfollowed by fluvial deposition.

The eastern margin of Australia was also land-locked through theAlbian, with a possible marine incursion in the Cenomanian (Frakes etal., 1987). The nature of this incursion on the southern margin of thecontinent is a matter of conjecture; Frakes et al. (1987) interpreted alate Albian marine incursion from the inland basins in the northfollowed by a Cenomanian incursion from the west. However theexact position of the shoreline is unknown. The disappearance ofinland seaways to the north left the Gippsland Basin in the middleof large continental area with reduced atmospheric moistureavailability.

Work by Hay (2008, 2011) on Cretaceous seasonal variation in windcirculation patterns suggests that in southern hemisphere Cretaceoussummer a lowpressure over Antarctica, with clockwise inwardly spiral-lingwinds, resulted in awesterly airstream across Australia. Hay (2008)describes these southern hemisphere summer winds as light and vari-able. However, unlike today, we can assume the presence of landalong the southern continental margin rather than ocean would proba-bly have resulted in a drying of any moisture the winds would havecontained by the time they reached the Gippsland Basin. Hay (2011)described a high sitting over Antarctica in the Cretaceous southernhemisphere winter reconstruction. This would have resulted inanticlockwise outwardly spiralling winds that drew air across theAustralian continent where the inland seas were being reduced in area.

Walker et al. (1995) in discussing the flow regimes of large riversystems in arid and semi-arid regions note that many such systemsare allogenic and arise in well-watered areas and flow for long dis-tances through arid and semi-arid country that contributes little run-off. Plant species that occur in these environments have opportunisticand flexible life history strategies as adaptations to unpredictable re-gimes where the flood pulse has a complex character (Walker et al.,1995). McLoughlin et al. (2010) postulated that although it could bethe rise of better-adapted angiosperms that caused the reductionand eventual demise of the seed ferns and plants allied to theCycadophyta/Ginkgophyta, climate might have also have played arole. The authors (McLoughlin et al., 2010) noted that the reductionof these taxa and their replacement by the angiosperms was coinci-dent with the substantial rise in temperature through the Albian toCenomanian. The current study implies that there may have been achange in the water budget of the basins that also played a role inthis floral change.

9. Conclusions

This study of exploration wells (Wombat-1 and Wombat-3) in theGippsland Basin, Victoria, Australia shows quantitative palynologicalanalysis of cuttings samples can be used to reconstruct past climates.Cuttings provide a smoothed sample and reduce the effect of faciescontrolled variability on the floral assemblages.

Adopting a “Quaternary-style” counting methodology (with a de-fined pollen sum), in particular when spores dominate palynologicalcounts can minimise the effects of the swamping of the spore–pollen



assemblage by the local flora. This “Quaternary-style” counting re-vealed variability in the non-angiosperm seed plants and distinctivetrends in spore abundances.

The reduction in Alisporites grandis and A. similis cannot be explainedin terms of the change in sedimentary style as they are thought to forman association with the podocarps on braid plains (Falcon-Lang et al.,2001;McLoughlin et al., 2002). However it is possible that thepodocarps,particularly the trisaccate forms that increase up section, were bettersuited to the disturbed environments. Other non-angiosperm seedplants show no strong patterns, except for Cycadopites. There is a reduc-tion up section of total ferns, in particular, Cyathidites australis/minor andschizaeaceous ferns (Cicatricosisporites/Ruffordiaspora spp.) through theAlbian in the Gippsland Basin.

Globally, the Albian temperature increases (Clarke and Jenkyns,1999) in the absence of permanent ice and high CO2 (Royer, 2006)and rainfall was high at mid to high latitudes (White et al., 2000;Ufnar et al., 2004). The drying climate of the Gippsland Basin through-out the Albian was caused by the gradual retreat of large interior seasto the north. This study shows that regional palaeogeography shouldbe considered when assessing spore–pollen records when they appearto contradict the global climate pattern.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.palaeo.2013.06.018.

Acknowledgements

We would like to thank Lakes Oil N.L. for providing access to geo-logical data on the wells and some of the samples from Wombat-3.The Department of Primary Industries allowed sampling of the wellsand we would particularly like to thank Terry Smith for providinghelp with accessing the cuttings. We would like to thank two anony-mous reviewers for their helpful comments to improve the text. ARCLinkage Grant LP0989203 funded this research with industry partnersLakes Oil N.L., Nexus Energy and Geotrack International.

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