Deglacial environments in eastern Prydz Bay, East Antarctica

Quaternary Science Reviews 29 (2010) 2731e2740

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Deglacial environments in eastern Prydz Bay, East Antarctica Loïc Barbara a, *, Xavier Crosta a, Guillaume Massé b, Olivier Ther a a b

UMR-CNRS 5805 EPOC, Université Bordeaux1, Avenue des Facultés 33405 Talence Cedex, France UMR-CNRS 7159 LOCEAN, IPSL Université Pierre et Marie Curie, ParisVI, Jussieu 75252 Paris Cedex 05, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 September 2009 Received in revised form 10 June 2010 Accepted 14 June 2010

A high-resolution continuous record of diatom census counts and diatom specific biomarkers in sediment core NBP0101-JPC24 allows assessment of oceanographic and environmental conditions in eastern Prydz Bay during the deglaciation (11 100e9000 cal yr BP) at decadal timescale. Our study improves previous snapshots investigations based on resin-embedded thin sections and presents a new proxy that compliments the diatom census counts. Our results suggest that the ice sheet retreat over the core site is dated ate11 100 cal yr BP, setting the onset of local deglaciation and subsequent open marine conditions. The glacial retreat in Prydz Bay is due to global warming initiated at 18 cal ka BP and the regional development of the Prydz Bay cyclonic gyre. Our results further demonstrate that the deglaciation in eastern Prydz Bay can be separated in four phases: the first between 11 100 and 10 900 cal yr BP when the ice shelf was proximal and sea ice was almost perennial; the second and the third phases between 10 900e10 400 cal yr BP and 10 400e9900 cal yr BP, respectively, when the ice shelf retreated and seasonal sea ice cycle consequently developed promoting warmer water to pump into the bay within the gyre, which in turn forced the ice shelf recession and the yearly sea ice cycle establishment; and the fourth between 9900 and 9000 cal yr BP when Holocene condition were set with a recurrent seasonal sea ice cycle and a well established Prydz Bay gyre. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Transitions from glacial to interglacial periods are governed by the Earth’s orbit (Imbrie et al., 1992, 1993; Davis and Brewer, 2009) though complex processes and feedbacks in the global climate system involving the atmosphere, ocean, cryosphere and biosphere determine the timing and amplitude of the glaciale interglacial cycles (Imbrie et al., 1992; Alley and Clark, 1999). It appears important to understand the forcing mechanisms, the relationships between the components of the Earth’s climate system and their feedback on climate through the reconstruction of environmental conditions during deglaciations. In fact, this may help us understand what our environment will be in the near future, even though initial conditions were different during deglaciations than during the Late Holocene that represents the baseline of the recent warming. In that vein, it appears essential to document deglaciations, and particularly the last deglaciation for which high-resolution, well dated records exist, in the Southern Ocean and Antarctica because of their role on global climate through atmospheric circulation (Toggweiler and Russell, 2008),

* Corresponding author. Tel.: þ33 5 40 00 84 38; fax: þ33 5 56 84 08 48. E-mail address: [email protected] (L. Barbara). 0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.06.027

bottom and intermediate ocean circulations (Broecker, 1998; Keeling and Stephens, 2001; Parrenin and Paillard, 2003) and sea level rise (Clark et al., 2002). To date, studies focusing on the last deglaciation in the Southern Ocean have been performed on deep-sea cores from the Antarctic Polar Front region to document temperature changes and hydrographic front movements (Labracherie et al., 1989; Shemesh et al., 2002; Bianchi and Gersonde, 2004; Allen et al., 2005). Studies from Antarctic ice cores similarly focused on describing temperature fluctuations and timing of warming/cooling events which punctuated this climatic change (Jouzel et al., 2001; Stenni et al., 2001), whereas studies from glacial archives on land (moss banks, lake sediments and penguin rookeries) investigated the timing of deglaciation and subsequent post-glacial developments around Antarctica (Ingólfsson et al., 1998 and references therein). At the interface between the Southern Ocean and the Antarctic inlands lies the continental shelf where sites of very high sediment accumulation have been documented (Domack et al., 2001; Leventer et al., 2006). However, studies from the Antarctic continental shelf are either (1) at low resolution and/or cover only the onset of the ice sheet retreat (Domack et al., 1998; Taylor and McMinn, 2002; Heroy and Anderson, 2007), or (2) present highresolution snapshots of the last deglaciation from resin-embedded thin sections (Stickley et al., 2005; Maddison et al., 2006). Thus,

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investigations on environmental changes during the last deglaciation from the East Antarctic Margin are scarce, though this information is crucial for understanding the relationships between the ice-ocean-atmosphere systems and their impact on global warming. We here present a high-resolution record of environmental conditions for the interval 11 100e9000 years BP, which represents the deglaciation period in southeastern Prydz Bay. The timing of deglaciation in Prydz Bay, which represents the period of ice shelf retreat over the core site, occurred during the end of the warming phase of the Last Deglaciation (Jouzel et al., 2001), Environmental conditions were inferred from diatom census counts and diatom specific biomarkers, namely Highly Branched Isoprenoids (HBIs). A focus is done on the relationships between the ocean and the cryosphere.

relatively warm and salty waters of the Circumpolar Deep Water (CDW), initially transported as part of the Antarctic Circumpolar Current (ACC), that upwells at the shelf edge (Smith and Tréguer, 1994; Nunes Vaz and Lennon, 1996). A similar situation where CDW is involved in a cyclonic circulation is found in the Weddell and the Ross seas (Yabuki et al., 2006). Sea ice extends to approximately 58 S during the winter and generally retreats back to the continent during the summer though important iceberg fields can be present, especially in the eastern Prydz bay because of the inflow of EWD cold waters (Smith et al., 1984; Worby et al., 1998). Resultant surface water stratification favours sea ice formation, thus reducing the ice-free period in eastern Prydz Bay (e10e11 months of sea ice) compared to western Prydz Bay (e9e10 months of sea ice) (Smith et al., 1984).

2. Oceanographic settings

3. Material and methods

Prydz Bay is located in the Indian sector of the Southern Ocean, East Antarctic Margin, between approximately 70 E and 80 E (Fig. 1). The southernmost part of the bay is in contact with the Amery Ice Shelf while the northernmost part extends to the continental shelf edge. Prydz Bay physiography is characterised by an 800 m deep central depression, which is trenched by the Prydz Channel to the west and the Svenner channel to the east. The bay is bordered by the Fram Bank to the northwest and the Four Ladies Banks to the northeast. The banks form partial barriers to deep-water exchange between the bay and the deep ocean (Smith and Tréguer, 1994). The westward flowing Antarctic Coastal Current represents the principal surface inflow (<200 m) in the bay as part as of the winddriven East Wind Drift (EWD) (Fig. 1). It is characterised by cold, relatively fresh waters because of the presence of the West Ice Shelf, east of Prydz Bay. The Antarctic Coastal Current enters Prydz Bay by Cape Penck to the east and exits the bay via Cape Darnley to the west (Smith et al., 1984; Wong, 1994; Nunes Vaz and Lennon, 1996). Part of the Antarctic Coastal Current flowing through the bay is captured by the Prydz Bay gyre and recirculated into the bay. The hydrographic feature is mainly governed by a cyclonic gyre, centred on the mid- to western part of the bay, and fed by inflow of

3.1. Core description and stratigraphy The e17 m-long piston core NBP0101-JPC24 was collected in eastern Prydz Bay at 68 41.6370 S, 76 42.7120 E in 816 m water depth (Fig. 1). Core JPC24 appears to represent an uninterrupted sediment record (Leventer et al., 2006). It is composed of glacial diamicton (below 1580 cm) and diatom oozes (above 1558 cm). The deglacial interval in core JPC24 is represented in as ae7 m-long section (Fig. 2). The early deglaciation section is represented by a 22 cm-long interval of rhythmically laminated diatom ooze composed of 10 paired laminations (Leventer et al., 2006). Each couplet is composed of one orange/orangeebrown lamina of diatom ooze and one brown/blueegrey lamina of diatom-rich terrigenous silt and clay, which possibly represents an annual varved sequence. The rest of the deglaciation is represented by laminated diatom ooze with no varve connotation. This e7 m-long, non-perturbed section therefore provide a unique high-resolution sediment record of the transition from the last glacial to the Holocene. Six radiocarbon dates were measured on JPC24 sediments, from which four were obtained on carbonate material and two on decalcified organic matter (Leventer et al., 2006) (Fig. 2). A linear

Fig. 1. Bathymetric map of Prydz Bay showing the location of JPC24 core site along with detail of the oceanographic surface currents, i.e. the cyclonic gyre and the East Wind Drift (EWD), the modern ice flow lines of Amery Ice Shelf from Fink et al. (2006) and the position of nearby cores cited in text. Modern winter sea ice covers the whole area encompassed in the map while summer sea ice generally retreats to the continent (Smith et al., 1984). Bathymetric contours are reported in meters.

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Fig. 2. Age model and synthetic log of core JPC24. The table details the different steps taken to develop an age-model from raw 14C dates to calibrated ages. The graph depicts calibrated ages versus depth with sedimentation rates (cm/yr) calculated by linear interpolation between two consecutive ages. Circles represent ages obtained from decalcified bulk organic matter (decal. TOC), squares represent ages from carbonate material, and the star represents the age of the boundary between diamicton and diatom ooze calculated by linear extrapolation on the four carbonate ages (extrapolat.). Empty symbols represent calibrated ages while the filled symbol represents the corrected age from the diamicton. Radiocarbon ages were corrected using a local reservoir age of 1280 years (Leventer et al., 2006).

extrapolation was applied to the four 14C raw dates performed on carbonate material and covering the last deglaciation to obtain an uncorrected age for the transitional boundary between the diatom ooze and the diamicton. The raw radiocarbon and raw interpolated ages were then corrected from the local marine reservoir age of 1280 yr  200 yr estimated from the core top age in the kasten twin core that preserved the modern interface (Leventer et al., 2006). The local reservoir age agrees well with the mean marine reservoir age of 1300 years recommended for coastal zones from the East Antarctic Margin (Ingólfsson et al., 1998), but is different to the reservoir ages from nearby ODP site 740A (1750 14C yr BP, Domack et al., 1991) and GC29 (2493 yr BP, Taylor and McMinn, 2002). Possibly because of different dating techniques and/or lower sedimentation rates at the OPD and GC29 core sites. Finally, the ages were calibrated using the CALIB 5.0 software (Stuiver et al., 2005). 3.2. Diatom and HBIs analyses To reconstruct environmental conditions during the last deglaciation in eastern Prydz Bay, we sampled core JPC24 every 4 cm between the diamicton (e11 100 cal yr BP) and 1300 cm (e10 250 cal yr BP) and every 8 cm between 1300 cm and 900 cm (e9000 cal yr BP) (Fig. 2). Given the sedimentation rates, this sampling strategy allows us to obtain a e10 years resolution during the 11 100e10 250 cal yr BP period and ae20 years resolution between the 10 250e9000 cal yr BP period. Samples were split in two for diatom and HBI analyses. Diatom analyses, sediment treatment and slide preparation followed the technique described in Rathburn et al. (1997). At least,

300 hundred complete valves were counted at a magnification of 1250 using the protocol described in Crosta and Koç (2007). Diatoms were identified to species or species group level, and the relative abundance of each was determined as the fraction of diatom species against total diatom abundance in the sample. More than 60 species or species groups were recognized in core JPC24. Diatom species representing similar ecological preferences were subsequently grouped together to reconstruct past environmental conditions. When Chaetoceros Hyalochaete resting spores (CRS) are overwhelmingly abundant throughout a record, it is of common practice to assess the contribution of minor species on a CRS-free basis (Taylor et al., 2001; Allen et al., 2005). This is not the case in core JPC24 where CRS relative abundances are strongly variable throughout record (Fig. 3). Chaetoceros RS represent e50% of the diatom assemblage only during the early deglaciation, when very specific environmental conditions prevailed, while other diatom groups are dominant during the late deglaciation phases. To capture all the complexity of rapid changing environmental conditions, we did not exclude CRS from the diatom counts. In addition to the taxonomic identifications, we analyzed the hydrocarbon fraction of every sample for the presence of C25 Highly Branched Isoprenoids (HBIs). The presence of a mono-unsaturated isomer (IP25) of these diatom specific biomarkers (Belt et al., 2000a; Massé et al., 2004; Sinninghe Damste et al., 2004) have been recently used as a proxy for sea ice in the Northern Hemisphere (Massé et al., 2008; Vare et al., 2009). A range of isomers were also identified in Antarctic sea ice, phytoplankton and sediment samples (Johns et al., 1999). Particularly, a C25 HBI Diene was

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Fig. 3. Diatom groups and biomarker records in core JPC24 along with record of temperature over Antarctica between 11 100 cal yr BP and 9000 yr BP. (a) Relative abundances of the Chaetoceros Hyalochaete group, (b) relative abundances of the Fragilariopsis curta group, (c) relative abundances of the Thalasiosira antarctica group, (d) relative abundances of the Mixed Water Diatoms group, (e) absolute abundances of diatom valves per gram of dry sediment, (f) ratio of sea ice-dwelling diatom biomarker to open water diatom biomarker, (g) relative abundances of Diene, (h) relative abundances of Triene, (i) Southern Ocean SST are from Nielsen et al. (2004) and EPICA Dome C temperatures (Tsource) are from Stenni et al. (2010).

recognized in Antarctic sea ice and is synthesized by sea icedwelling diatoms while a C25 HBI Triene was identified in open waters and is synthesized by the diatom species that thrive both at the sea ice edge and in the permanent open ocean. Recently, it has

been proposed that the relative abundances of C25 HBI Diene (D) and Triene (T) could be used for sea ice cover reconstructions in modern and Holocene sediments from the Antarctic continental shelf (Massé et al., 2007). A higher D/T ratio indicates longer sea ice

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duration and reduced ice-free season, in agreement with observations of higher relative abundances of sea ice-related diatoms in the corresponding sediment samples. We therefore use the D/T ratio in this study to compliment the diatom assemblage analyses. HBI extraction and analysis followed the technique described by Belt et al. (2007). Briefly, an internal standard was added to the freeze-dried sediments, lipids were extracted using a Dichloromethane/Methanol mixture (3  3 ml; 2:1 v/v) to yield a total organic extract (TOE). TOE was purified using open column chromatography (silica), hydrocarbons were eluted with hexane (6 ml) and collected before analysis by Gas ChromatographyeMass Spectrometry (GCeMS). Relative abundances of HBI Diene and Triene were calculated on the basis of their individual GCeMS responses and those of the internal standard, together with the mass of sediment analysed. 4. Results and discussion 4.1. Timing of the deglaciation in eastern Prydz Bay The onset of the Last Deglaciation is dated ate18 cal ka BP when atmospheric temperatures began to rise (Jouzel et al., 2001). However in this study, “deglaciation” pertains to the period when the East Antarctic ice sheet, which extended out to the shelf break during the last glacial period, retreated southward of the core location. We therefore describe a local/regional deglaciation. The sedimentary boundary between glaciomarine clays and diatom oozes in core JPC24 is dated at 11 069  70 cal yr BP by linear extrapolation of the 4 carbonate 14C ages (Fig. 2). Initiation of the deglaciation in nearby cores, ODP site 740A and GC29 (Fig. 1), was dated from bulk organic carbon 14C ages at 10 700 14C yr BP (e12 400 cal yr BP) and 11 650 14C yr BP (e13 500 cal yr BP), respectively (Domack et al., 1991; Taylor and McMinn, 2002). These ages are older than what was calculated for core JPC24. It is however worth noting that 14C ages obtained on bulk organic carbon can be affected by remobilization and/or deposition of allochthonous recycled organic matter. Radiocarbon dates based on bulk organic carbon can be up to a thousand years older than carbonate 14C dates from the same core (Mollenhauer et al., 2003), though this value might be around 400 years in coastal sediments from the East Antarctic Margin (Costa et al., 2007). Cores ODP 740A and GC29 additionally have much lower sedimentation rates than core JPC24, thus hindering a clear identification of the timing of the local/regional deglaciation. Conversely, the record from core JPC43B from Iceberg Alley in western Prydz Bay, presenting a similar deglaciation sequence to core JPC24, places the onset of open ocean conditions at 11 400 cal yr BP based on carbonate 14C dates (Stickley et al., 2005; Leventer et al., 2006). In view of these data, deglaciation in coastal regions of Prydz Bay seemingly began between 11 400e11 000 cal yr BP, in agreement with glacier grounding-line retreat recorded in the Dumont d’Urville Trough (Denis et al., 2009a). The JPC24 interval studied here (1580 cme900 cm) was deposited between e11 100 cal yr BP and e9000 cal yr BP, which represents the period covering the onset of the ice shelf retreat from the core location, to an ice shelf position similar to that of today. Sediment accumulation rates in this interval fluctuate between 0.27 and 0.36 cm/yr (Fig. 2), which is twice than that for core ODP Site 740A (0.14e0.18 cm/yr; Domack et al., 1991) and ten times higher than for core GC29 (0.011e0.022 cm/yr; Taylor and McMinn, 2002). Diatom census counts and HBIs analysis were also conducted at much higher resolution than in previous studies. Our results thus represent the highest resolution continuous record of environmental conditions prevailing during deglaciation in eastern Prydz Bay.

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4.2. From proxy records to environmental reconstructions Diatoms are abundant and well preserved in JPC24 and no preservation difference is observable at the visual inspection level. Around sixty diatom species or species groups were recognized in core JPC24, of which just Chaetoceros Hyalochaete spp., Fragilariopsis curta, F cylindrus and Thalassiosira antarctica commonly account for more than 5% of the total diatom assemblages. Environmental reconstructions are here based on the most abundant species mentioned above and on diatom groups that associate species having similar ecological preferences. These reconstructions are refined with the D/T ratio record. The first diatom group, composed mainly of Chaetoceros Hyalochaete resting spores and few vegetative cells, is indicative of stratified, low salinity waters associated with glacial retreat and/or early spring sea-ice melt (Leventer, 1992; Crosta et al., 1997; Armand et al., 2005; Beans et al., 2008). The F. curta gp (F. curta gp), composed of F. curta, F. cylindrus and F. vanheurckii, is indicative of heavy sea ice cover with a well established yearly seasonal cycle (Leventer, 1992; Zielinski and Gersonde, 1997; Armand et al., 2005; Beans et al., 2008). The third diatom group combines the two forms of T. antarctica resting spores (Buffen et al., 2007), with the cool form (T1) being overwhelmingly abundant over the warm form (T2), and very few vegetative cells. This group gives insight on the length of the growing season in the seasonal sea ice zone (Taylor et al., 1997; Cunningham and Leventer, 1998; Pike et al., 2009). The fourth diatom group, composed of open ocean species thriving out off the continental shelf (Corethron criophilum, Rhizosolenoids lumping R. antennata var. semispina, R. antennata var. antenneta and Proboscia inermis, Chaetoceros Phaeoceros spp. and Fragilariopsis kerguelensis), indicates intrusions of warmer water masses and/or a mixed water column (Stickley et al., 2005; Denis et al., 2006; Maddison et al., 2006; Beans et al., 2008). This fourth group is referred hereafter as the mixed water diatom group (MWD). The down-core records of the four diatom groups mentioned above, associated with the D/T down-core record, allows us to split the deglaciation into 4 phases (Fig. 3). The limits between the four phases were placed where important shifts in both diatom relative abundances and HBIs occurred, thus separating very distinct data trends. The first phase covers thee11 100e10 900 cal yr BP period, and represents the earliest deglaciation stage at the core location. During this phase, Chaetoceros Hyalochaete gp (CRS) accounted for more than 50% of the total diatom assemblages and up to 80e90% at 11 020e11 010 cal yr BP (Fig. 3a). The highest CRS abundances occurred during the rhythmically laminated siliceous mud and ooze overlying the diamicton in JPC24 (Leventer et al., 2006). Similar rhythmical laminations were observed in the deglacial section of core JPC43B from Iceberg Alley, Mac. Robertson Shelf, western Prydz Bay (Stickley et al., 2005). Investigation of resinembedded thin sections identified the seasonal and long-term information preserved in the laminations, some of which represented a few centimetres of annual sedimentation. In these laminations, high abundances of Chaetoceros Hyalochaete spp. (mainly vegetative cells) were indicative of stratified, low salinity and nutrient-rich surface waters associated with melting sea ice or glacial runoff during the early spring. During Phase 1, the F. curta gp represented the second most abundant diatom group while the T. antarctica gp and the MWD gp were almost absent (Fig. 3b,c,d), thus confirming a cold, icy and stratified environment during the growing season. Throughout this first phase, the D/T ratio was low (Fig. 3f) with a Triene abundance one order of magnitude higher than the Diene (Fig. 3g,h). The low Diene abundance combined with the high abundance of F. curta gp argues for the presence of heavy sea ice cover, certainly perennial, with a very reduced

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seasonal sea ice cyclicity. However, the presence of Triene indicates the presence of open water conditions during the summer, possibly as cracks and leads or polynyas. Low salinity stratified surface waters generally prevail in the environments described above where Chaetoceros Hyalochaete are highly competitive and can reach extremely high abundances. Mass sedimentation events are frequent in such conditions. Presence of laminations additionally argues for sub-oxic conditions and, thus, enhanced preservation of the settling fluxes. As a result, extremely high absolute diatom abundances of e3000 millions of valves per gram of dry sediment are encountered during Phase 1 and are associated with the high abundances of CRS (Fig. 3e). The peak in Triene recorded during Phase 1 indicates that Triene synthesizing diatoms were also competitive in such conditions. Early deglaciation at the core site, and highest CRS relative abundances, occurred when atmospheric temperatures were e1.5  C above the preindustrial mean and e10  C above the mean glacial value (Jouzel et al., 2007; Stenni et al., 2010) and when Southern Ocean seasurface temperatures were 1e3  C above the pre-industrial mean (Bianchi and Gersonde, 2004; Anderson et al., 2009; Stenni et al., 2010) and e4e6  C above the mean glacial value (Brathauer and Abelman, 1999; Crosta et al., 2007; Stenni et al., 2010) (Fig. 3i). During the second phase, covering thee10 900e10 400 cal yr BP period, CRS relative abundances decreased gradually frome45%ee35% (Fig. 3a) while the F. curta gp increased drastically to represente30 50% of the total diatom assemblages (Fig. 3b). The T. antarctica gp remained low but with slightly increasing occurrences (Fig. 3c). Such assemblage composition was observed in Prydz Bay by Hemer and Harris (2003) in surface sediments adjacent to the ice shelf front, and was described by Taylor et al. (1997) as a coastal assemblage. It corresponds to a near ice shelf environment where surface water is stratified by glacier runoff and sea ice persists throughout the summer at moderateelow concentration. The MDW gp, however, is present as sporadic peaks (Fig. 3d), thus indicating periodic inflow of warmer surface or sub-surface waters. The D/T ratio is higher in the second phase than in the first (Fig. 3f) and HBI concentrations were also higher indicating greater overall synthesis of these biomarkers and especially the Diene, which is specific to sea ice-dwelling diatoms. Therefore, diatoms and biomarkers highlight heavy sea ice cover but with the initiation of a seasonal cycle that allowed for the development of sea ice-related and sea ice-dwelling diatoms. Thus, throughout the second phase, conditions apparently evolved from an initial coastal environment with stratified, low salinity surface waters supporting multiyear sea ice cover to a shelf environment characterised by episodic incursions of warmer waters and mixed water column, leading to a lengthening of the sea ice free condition and diatom growing season. As a result of the lengthening of the sea ice-free season and stratified, nutrientrich surface waters because of the proximity of the ice shelf, diatom productivity was high during Phase 2 with absolute abundances around 500 millions of valves per gram of dry sediment. Ice shelf retreat during Phase 2 is congruent to sea-surface temperatures as warm as during Phase 1, which provided part of the energy necessary to melt the ice shelf. Short-lived events in both diatom and D/T records are apparent in Phase 2, with a pluridecadal variability. Maximum abundances of the F. curta gp and of maximum D/T ratio values are observed at 10 830e10 870 cal yr BP and 10 600e10 700 cal yr BP in phase with cooler sea-surface temperatures within the accuracy of the age models (Fig. 3b,f and i). This shows a fast response of the ocean-cryospheric system to climate change. It is very tempting to see in this decadal variability the climate modes prevailing today at high southern latitudes (Yuan, 2004; Stammerjohn et al., 2008). During the third phase covering thee10 400e9900 cal yr BP, the relative abundances of the CRS and F. curta gp showed a gradual

decrease frome40% toe15% and from 25% to 20%, respectively (Fig. 3a, b). Conversely, the T. antarctica gp accounted for 40% of the diatom assemblages except for a hundred year-long event centred at 10 100 cal yr BP when relative abundances decreased abruptly to 20% (Fig. 3c). The MWD gp relative abundances increased slightly again to 5% in comparison to the second phase (Fig. 3d). These results indicate, again, stratified surface water during spring and reduced yearly sea ice duration with open water condition during the summer. Influx of warmer waters from the open ocean was also more important, probably during the summer season when sea ice was not present. Taylor and McMinn (2002) observed a similar diatom assemblage to that one in the nearby GC29 core, and interpreted it to represent a mixing between shelf diatoms produced in spring and oceanic diatoms transported by horizontal current during the sea ice-free season. During the third phase, the D/T ratio was generally lower than in the previous phase (Fig. 3f), further supporting reduced yearly sea ice persistence in relation to sustained warm climate and warm sea-surface temperatures (Fig. 3i) and more intense inflow of warmer waters by horizontal currents as part of the gyre. The D/T ratio, however, showed a large peak centred at 10 100 cal yr BP concomitant to the decrease in T. antarctica gp relative abundances and a drop in sea-surface temperatures inferred from EDC deuterium record (Fig. 3i) suggesting a brief return to icier conditions. Since 10 200 cal yr BP, the resolution is not sufficient to provide insight on the persistence of decadal climate variability. Though open water conditions during summer and longer growing season are inferred during Phase 3, diatom abundances were lower than during Phase 2 when sea ice persisted longer. Less stratified, nutrient-poorer surface waters due to the increasing distance of the ice shelf edge were more favourable to slow developing summer diatoms at the expense of fast-blooming Chaetoceros Hyalochaete. However, biogenic silica content in Antarctic continental shelf sediments generally depends on the occurrence of large, heavily silicified diatoms (Denis et al., 2009b). Biogenic silica content might therefore be greater during Phase 3 than Phase 2. The fourth phase covering thee9900e9000 cal yr BP is characterised by low CRS relative abundances, a large increase of the F. curta gp abundance, a decrease of T. antarctica gp occurrence (Fig. 3aec). Relative abundances of the MWD gp were also slightly higher than during the three previous phases (Fig. 3d). The D/T ratio presented near zero values (Fig. 3f). The D/T record seems, at face value, in disagreement with the sea ice-related diatom records. The diatom records indicate a reduction of the spring surface water stratification and a more dynamic and more established seasonal sea ice cycle in conjunction to slightly cooler sea-surface temperatures (Fig. 3i). The D/T ratio conversely argues for reduced sea ice persistence at the core site. One way to reconcile both records is to consider the combined action of (1) a strong reduction of surface water stratification because of the diminishing influence of the ice shelf, (2) a well-established sea ice cycle as a consequence of slightly cooler sea-surface temperatures (Fig. 3i) and, (3) more important and recurrent warmer water intrusions by a well developed gyre. These warmer waters probably brought in open ocean diatom species synthesising Triene during the summer season (Fig. 3h), which diluted the Diene synthesised by sea icedwelling diatoms during spring (Fig. 3g). This phase marks the onset of the Holocene period when the ice shelf influence gave way to oceanic forcing. Diatom absolute abundances oscillated between 100 and 1000 millions of valves per gram of dry sediment and follow the occurrence of the MDW group (Fig. 3e), certainly because intrusion of surface or sub-surface waters favoured the development of species adapted to mixed surface waters and low light levels (Beans et al., 2008). Most species of the MWD are part of the “shade flora” in

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reference to their ecological behaviour (Kemp et al., 2000). They usually thrive at the nutricline where they can make use of the large nutrient stocks at low light levels to achieve large biomass. These species may also be the main Triene synthesisers. 4.3. Atmosphere-cryosphere-ocean interactions during the deglaciation Recent studies described the role of eustatic sea level rise (Mackintosh et al., 2007) and ocean temperature warming (Williams et al., 2002) as the main mechanisms forcing the East Antarctic Ice Sheet retreat at the end of the last glacial period. These information, associated with the environmental reconstructions inferred from diatom and HBI records in core JPC24, give insight on the interactions between atmosphere-cryosphere-ocean in eastern Prydz Bay during the different phases of deglaciation mentioned above. However, the lack of data in western Prydz Bay does not allow the reconstruction of the calving front position to a larger scale or the identification of the ice origin. Indeed, though an Amery Ice/Lambert Glacier System origin is generally accepted (Domack et al., 1998), ice contribution from the Ingrid Christensen Coast is possible (O’Brien et al., 2007). During the first phase of regional deglaciation (11 100e10 900 cal yr BP), diatom and D/T records demonstrate a highly stratified, nutrient-rich and icy environment during summer. The relative absence of Diene further argues for heavy, pluri-annual sea

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ice cover. We suggest that the ice shelf margin was very close to JPC24 core site (Fig. 4). Melting and retreat of the ice shelf promoted iceberg calving and injection of cold, low salinity water, hence facilitating surface water freezing during winter. Additionally, iceberg presence constrained sea ice, preventing its melting and break-out during spring/summer, though cracks, leads and possibly polynyas may have existed due to wind activity. Indeed, Prydz Bay is where the strongest katabatic winds have been monitored (Xie et al., 2002). Additionally, the release of aeolian dust and allochthonous particles from the Antarctic bedrock locked into the ice shelf since the last glacial period may explain eutrophication and, therefore, the exceptionally high CRS abundance during the onset ice-shelf retreat (Stickley et al., 2005). Environmental conditions and biological production in Eastern Prydz Bay during early deglaciation were therefore strongly, and possibly solely, influenced by proximity to the ice shelf creating a glacial ice dominated environment. Because the bay was still partly covered by the ice shelf and perennial sea ice, the gyre was not very well developed and horizontal circulation was thus restricted to the outer bay (Fig. 4). During the second phase (10 900e10 400 cal yr BP), diatom assemblages and D/T ratio indicate low salinity, stratified surface waters, and multiyear sea ice persisted although open water within the sea ice cover was more important than during the first phase. We propose that the gradual retreat of the ice shelf due to rising sea level and warm ocean temperatures gave passage to the East Wind

Fig. 4. Schematic environmental condition patterns in Prydz Bay showing the potential Amery Ice Shelf edge, summer sea ice limit and surface currents during the four phases identified from the diatom and biomarker ratio records.

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Drift transporting both coastal cold waters and icebergs (Smith et al., 1984) (Fig. 4). Fresh water runoff from the ice shelf and iceberg favoured sea ice formation in winter and its persistence during spring/summer (Streten and Pike, 1984). However, the receding ice shelf limit allowed for the establishment of a sea ice environment in which sea ice-dwelling diatoms could produce. The more expanded cyclonic gyre, started to import warmer water into the central bay, but its influence was reduced and did not allowed sea ice to melt every summer. At the end of the second phase, the progressive ice shelf recession led to the southward migration of the EWD coastal current and to the reduction of glacial runoff to the core site. In addition, the influence of cyclonic gyre increased, bringing more open ocean warm water into the bay. It is possible that upwelling of CDW followed the same intensification as the cyclonic gyre. Warmer waters influenced seasonal sea ice melting and ice shelf retreat. It seems that episodic cooling events (Fig. 3i) induced simultaneous sea ice readvance (Fig. 3b,f) and reduction of horizontal circulation (Fig. 3d). Strong interactions between the atmosphere, the cryosphere and the ocean took place during the second phase of the deglaciation whereby warmer ocean temperatures influenced ice shelf recession that allowed for the sea ice seasonal cycle to establish and the gyre to develop, which in turn brought warmer waters into the bay that helped accelerate local deglaciation (Williams et al., 2002). During the third phase (10 400e9900 cal yr BP) diatom and D/T data indicate a reduction of the surface water stratification and yearly sea ice duration. We suggest that the impact of glacial runoff was weak and that instead the short seasonal surface water stratification resulted from sea ice melting during spring/summer. Progression of warmer waters carried by the cyclonic gyre and upwelling of CDW into internal regions of Prydz Bay promoted the ice shelf retreat and sea ice seasonal waning, and thus competed and constrained the EWD to the recessing ice shelf coast (Fig. 4). The trend towards greater influence of the warmer gyre at the detriment of the cold, fresh EWD induced a positive feedback on sea ice duration and ice shelf melting and on the lengthening of the growing season favouring summer diatoms at the expense of sea ice-related and sea ice-dwelling diatoms. During the fourth phase (9900e9000 cal yr BP), the ice shelf continued its retreat toward its modern position in agreement with studies of continental records that indicate gradual glacial recession untile8000 years BP in East Antarctica (Ingólfsson et al., 1998) (Fig. 4). The diatom assemblages and the D/T ratio record suggest that environmental conditions in eastern Prydz Bay were mainly controlled by the seasonal expression of the atmosphere-ocean-sea ice coupling. The decrease in CRS indicates that the ice shelf margin was far south of JPC24 core site with reduced influence on the regional marine environment. The southern position of the ice shelf margin and the seasonal sea ice free conditions induced more warm water intrusion through the gyre that in turn played on the cryosphere and on the biota by lengthening the growing season. Reduction of surface stratification may have also favoured upwelling of CDW that today inject nutrients to the continental shelf environments. The fourth period is here associated to the onset of the Holocene period when glacial ice influence was reduced, hence giving more importance to oceanic processes. Additionally, other climate forcing factors, which were tamed during deglaciation because of the overwhelming influence of the proximal ice sheet, could fully express (Renssen et al., 2005b, 2009; Crosta et al., 2007; Debret et al., 2009). 5. Conclusion This study highlights the initiation of deglaciation in southeastern Prydz Bay, East Antarctica, ate11 100 cal yr BP when the ice

shelf retreated to the south of the JPC24 core location. In addition, our results indicate four phases of deglaciation during which in the first three phases, oceanographic and environmental conditions were mainly controlled by Antarctic ice shelf recession: 1) During the first phase (11 100e10 900 cal yr BP) glacial ice dominated the area. Environmental conditions were mainly influenced by melt waters and icebergs from the nearby ice shelf, which resulted in a persistent sea ice cover. 2) The second period (10 900e10 400 cal yr BP) was characterized by the gradual retreat of the ice shelf allowing interactions between the atmosphere, cryosphere and ocean to take place and a seasonal sea ice cycle to establish. 3) The third period (10 400e9900 cal yr BP) was mainly marked by the interplay of water masses (dominance of warmer waters from the gyre relative to colder, low salinity waters from the East Wind Drift) which induced a positive feedback on ice shelf recession and seasonal sea ice melting. 4) Finally, during the fourth phase (9900e9000 cal yr BP) environmental conditions were mainly controlled by atmosphereocean-sea ice interactions. This period signals the end of deglaciation and the beginning of Holocene conditions in eastern Prydz Bay. Other high-resolution reconstructions of environmental conditions to decipher the interactions between the ice-ocean-atmosphere systems around Antarctica during the last deglaciation and/ or Holocene abrupt climate changes are required to give further insight on the feedbacks of the Antarctic cryosphere on future climate. Acknowledgments We thank Amy Leventer, Eugene Domack and Rob Dunbar for collaboration on the CHAOS material and fruitful discussions. We thank Catherine Stickley and an anonymous reviewer for helpful comments. Financial support for this study was provided by the projects TARDHOL through the national EVE-LEFE program (INSUCNRS) and European Research Council (ERC) ICEPROXY. This is an EPOC contribution N 1784. References Allen, C.S., Pike, J., Pudsey, C.J.a., Leventer, A., 2005. Submillennial variations in ocean conditions during deglaciation based on diatom assemblages from the southwest Atlantic. Paleoceanography 20 (2) doi: PA201210.1029/ 2004PA001055. Alley, R.B., Clark, P.U., 1999. The deglaciation of the northern hemisphere: a global perspective. Annual Review of Earth and Planetary Sciences, 149e182. Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B.E., Burckle, L.H., 2009. Wind-driven upwelling in the southern ocean and the deglacial rise in atmospheric CO2. Science 323 (5920), 1443e1448. Armand, L.K., Crosta, X., Romero, O., Pichon, J.J., 2005. The biogeography of major diatom taxa in Southern Ocean sediments: 1. Sea ice related species. Palaeogeography, Palaeoclimatology, Palaeoecology 223, 93e126. Beans, C., Hecq, J.H., Koubbi, P., Vallet, C., Wright, S., Goffart, A., 2008. A study of the diatom-dominated microplankton summer assemblages in coastal waters from Terre Ade? lie to the Mertz Glacier, East Antarctica (139 Ee145 E). Polar Biology 31 (9), 1101e1117. Belt, S.T., Allard, W.G., Masse, G., Robert, J.M., Rowland, S.J., 2000a. Highly branched isoprenoids (HBIs): identification of the most common and abundant sedimentary isomers. Geochimica et Cosmochimica Acta 64 (22), 3839e3851. Belt, S.T., Masse, G., Rowland, S.J., Poulin, M., Michel, C., LeBlanc, B., 2007. A novel chemical fossil of palaeo sea ice: IP25. Organic Geochemistry 38 (1), 16e27. Bianchi, C., Gersonde, R., 2004. Climate evolution at the last deglaciation: the role of the Southern Ocean. Earth and Planetary Science Letters 228 (3e4), 407e424. Brathauer, U., Abelman, A., 1999. Late Quaternary variations in sea surface temperatures and their relationship to orbital forcing recorded in the Southern Ocean (Atlantic sector). Paleoceanography 14 (2), 135e148. Broecker, W.S., 1998. Paleocean circulation during the last deglaciation: a bipolar seesaw? Paleoceanography 13 (2), 119e121.

L. Barbara et al. / Quaternary Science Reviews 29 (2010) 2731e2740 Buffen, A., Leventer, A., Rubin, A., Hutchins, T., 2007. Diatom assemblages in surface sediments of the northwestern Weddell Sea, Antarctic Peninsula. Marine Micropaleontology 62 (1), 7e30. Clark, P.U., Mitrovica, J.X., Milne, G.A., Tamisiea, M.E., 2002. Sea-level fingerprinting as a direct test for the source of global meltwater pulse IA. Science 295 (5564), 2438e2441. Costa, E., Dunbar, R.B., Kryc, K.A., Mucciarone, D.A., Brachfeld, S., Roark, E.B., Manley, P.L., Murray, R.W. and Leventer, A., 2007. Solar forcing and El Niño Southern Oscillation (ENSO) influence on productivity cycles interpreted from a late Holocene high-resolution marine sediment record, Adélie Drift, East Antarctic Margin., Online Proceedings of the 10th ISAES X, pp. 6p. Crosta, X., Debret, M., Courty, M.-A., Denis, D., Ther, O., 2007. Holocene long- and short-term climate changes off Adélie Land, East Antarctica. Geochemistry, Geophysics, Geosystems 8, Q11009. doi:10.129/2007GC001718. Crosta, X., Koç, N., 2007. Chapter eight diatoms: from micropaleontology to isotope geochemistry. Developments in Marine Geology, 327e369. Crosta, X., Pichon, J.J., Labracherie, M.,1997. Distribution of Chaetoceros resting spores in modern peri-Antarctic sediments. Marine Micropaleontology 29, 238e299. Cunningham, W., Leventer, A., 1998. Diatom assemblages in surface sediments of the Ross Sea: relationship to present oceanographic conditions. Antarctic Science 10 (2), 134e146. Davis, B.A.S., Brewer, S., 2009. Orbital forcing and role of the latitudinal insolation/ temperature gradient. Climate Dynamics 32 (2e3), 143e165. Debret, M., Sebag, D., Crosta, X., Massei, N., Petit, J.R., Chapron, E., BoutRoumazeilles, V., 2009. Evidence from wavelet analysis for a mid-Holocene transition in global climate forcing. Quaternary Science Reviews 28 (25e26), 2675e2688. Denis, D., Crosta, X., Schmidt, S., Carson, D.S., Ganeshram, R.S., Renssen, H., BoutRoumazeilles, V., Zaragosi, S., Martin, B., Cremer, M., Giraudeau, J., 2009a. Holocene glacier and deep-water dynamics, Adélie Land region, East Antarctica. Quaternary Science Reviews 28 (13e14), 1291e1303. Denis, D., Crosta, X., Schmidt, S., Carson, D.S., Ganeshram, R.S., Renssen, H., Crespin, J., Ther, O., Billy, I., Giraudeau, J., 2009b. Holocene productivity changes off Adélie Land (East Antarctica) on decadal to millennial timescales. Paleoceanography 24 PA320710.1029/2008PA001689. Denis, D., Crosta, X., Zaragosi, S., Martin, B., Mas, V., 2006. Seasonal and subseasonal climate changes recorded in laminated diatom ooze sediments, Adélie Land, East Antarctica. The Holocene 16 (8), 1143e1153. Domack, E., Jull, A.J.T., Nakao, S., 1991. Advance of East Antarctica outlet glaciers during the hypsithermal: implications for the volume state of the Antarctic ice sheet under global warming. Geology 19, 1059e1062. Domack, E., leventer, A., Dunbar, R.B., Taylor, F., Brachfeld, S., Sjunneskog, C., Party, O.L., 2001. Chronology of the Palmer deep site, Antarctic Peninsula: a Holocene palaeoenvironnemental reference for the circum-Antarctic. Holocene 11 (1), 1e9. Domack, E., O’Brien, P., Harris, P., Taylor, F., Quilty, P., DeSantis, L., Raker, B., 1998. Late Quaternary sediment facies in Prydz Bay, East Antarctica and their relationship to glacial advance onto the continental shelf. Antarctic Science 10, 236e246. Fink, D., McKelvey, B., Hambrey, M.J., Fabel, D., Brown, R., 2006. Pleistocene deglaciation chronology of the Amery Oasis and Radok Lake, northern Prince Charles Mountains, antarctica. Earth and Planetary Science Letters 243 (1e2), 229e243. Hemer, M.A., Harris, P.T., 2003. Sediment core from beneath the Amery Ice shelf, East Antarctica, suggests mid-Holocene ice-shelf retreat. Geology 31 (2), 127e130. Heroy, D.C., Anderson, J.B., 2007. Radiocarbon constraints on Antarctic Peninsula ice sheet retreat following the Last Glacial Maximum (LGM). Quaternary Science Reviews 26 (25e28), 3286e3297. Imbrie, J., Berger, A., Boyle, E.A., Clemens, S.C., Duffy, A., Howard, W.R., Kukla, G., Kutzbach, J., Martinson, D.G., McIntyre, A., Mix, A.C., Molfino, B., Morley, J.J., Peterson, L.C., Pisias, N.G., Prell, W.L., Raymo, M.E., Shackleton, N.J., Togweiller, J.R., 1993. On the structure and origin of major glaciation cycles. 2. The 1 00 000-year cycle. Paleoceanography 8 (6), 699e735. Imbrie, J., Boyle, E.A., Clemens, S.C., Duffy, A., Howard, W.R., Kukla, G., Kutzbach, J., Martinson, D.G., McIntyre, A., Mix, A.C., Molfino, B., Morley, J.J., Peterson, L.C., Pisias, N.G., Prell, W.L., Raymo, M.E., Shackleton, N.J., Togweiller, J.R., 1992. On the structure and origin of major glaciation cycles. 1. Linear responses to Milankovitch forcing. Paleoceanography 7 (6), 701e738. Ingólfsson, O., Hjort, C., Berkman, P.A., Björck, S., Colhoun, E., Goodwin, I.D., Hall, B., Hirakawa, K., Melles, M., Möller, P., Prentice, M.L., 1998. Antarctic glacial history since the Last Glacial Maximum: an overview of the record on land. Antarctic Science 10 (3), 326e344. Johns, L., Wraige, E.J., Belt, S.T., Lewis, C.A., Massé, G., Robert, J.-M., Rowland, S.J., 1999. Identification of C25 Highly branched isoprenoid (HBI) dienes in Antarctic sediments, sea-ice diatoms and laboratory cultures of diatoms. Organic Geochemistry 30, 1471e1475. Jouzel, J., Masson, V., Cattani, O., Falourd, S., Stievenard, M., Stenni, B., Longinelli, A., Johnsen, S.J., Stefenssen, J.P., Petit, J.R., Schwander, J., SOuchez, R., Barkov, N.I., 2001. A new 27 ky high resolution East Antarctic climate record. Geophysical Research Letters 28 (16), 3199e3202. Jouzel, J., Stievenard, M., Johnsen, S.J., Landais, A., Masson-Delmotte, V., Sveinbjornsdottir, A., Vimeux, F., von Grafenstein, U., White, J.W.C., 2007. The GRIP deuterium-excess record. Quaternary Science Reviews 26 (1e2), 1e17. Keeling, R.F., Stephens, B.B., 2001. Antarctic sea ice and the control of Pleistocene climate instability. Paleoceanography 16 (1), 112e131.

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Kemp, A.E.S., Pike, J., Pearce, R.B., Lange, C.B., 2000. The “Fall dump” - A new perspective on the role of a “shade flora” in the annual cycle of diatom production and export flux. Deep-Sea Research II 47, 2129e2154. Labracherie, M., Labeyrie, L.D., Duprat, J., Bard, E., Arnold, M., Pichon, J.J., Duplessy, J. C., 1989. The last deglaciation in the Southern Ocean. Paleoceanography 4 (6), 629e638. Leventer, A., 1992. Modern distribution of diatoms in sediments from the Georges V coast, Antarctica. Marine Micropaleontology 19, 315e332. Leventer, A., Domack, E., Dunbar, R., Pike, J., Stickley, C., Maddison, E., Brachfeld, S., Manley, P., McClennen, C., 2006. East Antarctic margin marine sediment record of deglaciation. GSA Today 16 (12), 4e10. Mackintosh, A., White, D., Fink, D., Gore, D.B., Pickard, J., Fanning, P.C., 2007. Exposure ages from mountain dipsticsk in Mac. Robertson Land, East Antarctica, indicate little change in ice-sheet thickness since the Last Glacial Maximum. Geology 35 (6), 551e554. Maddison, E.J., Pike, J., Leventer, A., Dunbar, R., Brachfeld, S., Domack, E.W., Manley, P., McClennen, C., 2006. Post-glacial seasonal diatom record of the Mertz glacier polynya, east Antarctica. Marine Micropaleontology 60 (1), 66e88. Massé, G., Belt, S., Rowland, S., Sicre, M.-A., Crosta, X., 2007. Highly Branched Isoprenoid Biomarkers as Indicators of Sea-ice Diatoms: Implications for Historical Sea-ice Records and Future Predictions. General Assembly of the European Geosciences Union, Vienna, Austria. 04001. Massé, G., Belt, S.T., Allard, W.G., Lewis, C.A., Wakeham, S.G., Rowland, S.J., 2004. Occurrence of novel monocyclic alkenes from diatoms in marine particulate matter and sediments. Organic Geochemistry 35 (7), 813e822. Massé, G., Rowland, S.J., Sicre, M.-A., Jacob, J., Jansen, E., Belt, S.T., 2008. Abrupt climate changes for Iceland during the last millennium: evidence from high resolution sea ice reconstructions. Earth and Planetary Science Letters 269 (3e4), 564e568. Mollenhauer, G., Eglinton, T.I., Ohkouchi, N., Schneider, R.R., Muller, P.J., Grootes, P.M., Rullkotter, J., 2003. Asynchronous alkenone and foraminifera records from the Benguela upwelling system. Geochimica et Cosmochimica Acta 67 (12), 2157e2171. Nielsen, S.H.H., Koç, N., Crosta, X., 2004. Holocene climate in the Atlantic sector of the Southern Ocean: controlled by insolation or oceanic circulation? Geology 32 (4), 317e320. Nunes Vaz, R.A., Lennon, G.W., 1996. Physical oceanography of the Prydz Bay region of Antarctic waters. Deep Sea Research Part I: Oceanographic Research Papers 43 (5), 603e641. O’Brien, P.E., Goodwin, I., Forsberg, C.F., Cooper, A.K., Whitehead, J., 2007. Late Neogene ice drainage changes in Prydz Bay, East Antarctica and the interaction of Antarctic ice sheet evolution and climate. Palaeogeography, Palaeoclimatology, Palaeoecology 245 (3e4), 390e410. Parrenin, F., Paillard, D., 2003. Amplitude and phase of glacial cycles from a conceptual model. EPSL 214, 243e250. Pike, J., Crosta, X., Maddison, E.J., Stickley, C.E., Denis, D., Barbara, L., Renssen, H., 2009. Observations on the relationship between the Antarctic coastal diatoms Thalassiosira antarctica Comber and Porosira glacialis (Grunow) Jørgensen and sea ice concentrations during the late Quaternary. Marine Micropaleontology 73, 14e25. Rathburn, A.E., Pichon, J.J., Ayress, M.A., De Deckker, P., 1997. Microfossil and stableisotope evidence for changes in Late Holocene palaeoproductivity and palaeoceanographic conditions in the Prydz Bay region of Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology 131 (3e4), 485e510. Renssen, H., Goosse, H., Fichefet, T., Massom-Delmotte, V., Koç, N., 2005b. Holocene climate evolution in the high-latitude Southern Hemisphere simulated by a coupled atmosphere-sea-ice-ocean-vegetation model. The Holocene 15 (7), 951e964. Renssen, H., Seppä, H., Heiri, O., Roche, D.M., Goosse, H., Fichefet, T., 2009. The spatial and temporal complexity of the Holocene thermal maximum. Nature Geoscience 2 (6), 411e414. Shemesh, A., Hodell, D., Crosta, X., Kanfoush, S., Charles, C., Guilderson, T., 2002. Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores. Paleoceanography 17 (4), 1056. doi:10.1029/ 2000PA000599. Sinninghe Damste, J.S., Rijpstra, W.I.C., Schouten, S., Fuerst, J.A., Jetten, M.S.M., Strous, M., 2004. The occurrence of hopanoids in planctomycetes: implications for the sedimentary biomarker record. Organic Geochemistry 35 (5), 561e566. Smith, N., Tréguer, P., 1994. Physical and chemical oceanography in the vicinity of Prydz bay, Antarctica. In “Southern Ocean Ecology: The BIOMASS Perspective”. Cambridge University Press, Cambridge. 25e45. Smith, N.R., Zhaoquian, D., Kerry, K.R., Wright, S., 1984. Water masses and circulation in the of Prydz Bay, Antarctica. Deep-Sea Research I 31 (9), 1121e1147. Stammerjohn, S.E., Martinson, D.G., Smith, R.C., Yuan, X., Rind, D., 2008. Trends in Antarctic annual sea ice retreat and advance and their relation to El NiñoeSouthern Oscillation and Southern Annular Mode variability. Journal of Geophysical Research C: Oceans 113 (3). Stenni, B., Masson-Delmotte, V., Johnsen, S., Jouzel, J., Longinelli, A., Monnin, E., Rothlisberger, R., Selmo, E., 2001. An oceanic cold reversal during the last deglaciation. Science 293 (5537), 2074e2077. Stenni, B., Masson-Delmotte, V., Selmo, E., Oerter, H., Meyer, H., Röthlisberger, R., Jouzel, J., Cattani, O., Falourd, S., Fischer, H., Hoffmann, G., Iacumin, P., Johnsen, S.J., Minster, B., Udisti, R., 2010. The deuterium excess records of EPICA Dome C and Dronning Maud land ice cores (East Antarctica). Quaternary Science Reviews 29 (1e2), 146e159.

2740

L. Barbara et al. / Quaternary Science Reviews 29 (2010) 2731e2740

Stickley, C.E., Pike, J., Leventer, A., Dunbar, R., Domack, E.W., Brachfeld, S., Manley, P., McClennan, C., 2005. Deglacial ocean and climate seasonality in laminated diatom sediments, Mac. Robertson Shelf, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology 227 (4), 290e310. Streten, N.A., Pike, D.J., 1984. Some observations of the sea-ice in the southwest India Ocean. Australia Meteorology Magazin 32, 195e206. Stuiver, M., Reimer, P.J. and Reimer, R.W., 2005. CALIB 5.0. [www program and documentation]. Taylor, F., McMinn, A., 2002. Late Quaternary diatom assemblages from Prydz bay, eastern Antarctica. Quaternary Research 57 (1), 151e161. Taylor, F., McMinn, A., Franklin, D., 1997. Distribution of diatoms in surface sediments of Prydz Bay, Antarctica. Marine Micropaleontology 32, 209e229. Taylor, F., Whitehead, J., Domack, E., 2001. Holocene paleoclimate change in the Antarctic Peninsula: evidence from the diatom, sedimentary and geochemical record. Marine Micropaleontology 41 (1e2), 25e43. Toggweiler, J.R., Russell, J., 2008. Ocean circulation in a warming climate. Nature 451 (7176), 286e288. Vare, L.L., Massé, G., Gregory, T.R., Smart, C.W., Belt, S.T., 2009. Sea ice variations in the central Canadian Arctic archipelago during the Holocene. Quaternary Science Reviews 28 (13e14), 1354e1366.

Williams, M.J.M., Warner, R.C., Budd, W.F., 2002. Sensitivity of the Amery ice shelf, Antarctica, to changes in the climate of the southern ocean. Journal of Climate 15 (19), 2740e2757. Wong, A., 1994. Structure and Dynamics of Prydz Bay, Antarctica, as Inferred from a Summer Hydrographic Data Set. University of Tasmania, Institute of Antarctic and Southern Ocean Studies, Hobart. Worby, A.P., Massom, R.A., Allison, I., Lytle, V.I., Heil, P., 1998. East Antarctic sea ice: a review of its structure, properties and drift. Antarctic Research Series 74, 41e67. Xie, S., Mei, S., Liu, K., Wei, L., 2002. Cyclone formation and development in the Antarctic Prydz bay. Acta Oceanologica Sinica 21 (1), 45e54. Yabuki, T., Suga, T., Hanawa, K., Matsuoka, K., Kiwada, H., Watanabe, T., 2006. Possible source of the Antarctic bottom water in the Prydz bay region. Journal of Oceanography 62 (5), 649e655. Yuan, X., 2004. ENSO-related impacts on Antarctic sea ice: a synthesis of phenomenon and mechanisms. Antarctic Science 16 (4), 415e425. Zielinski, U., Gersonde, R., 1997. Diatom distribution in Southern Ocean surface sediments (Atlantic Sector): implications for paleoenvironmental reconstructions. Paleogeography-Paleoclimatology-Paaleoceanography 129, 213e250.