sea ice seasonality during the holocene, adélie land, east antarctica

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Sea ice seasonality during the Holocene, Adélie Land, East Antarctica X. Crosta , D. Denis, O. Ther UMR-CNRS 5805 EPOC, Avenue des Facultés, 33405 Talence Cedex, France Received 13 March 2007; received in revised form 16 July 2007; accepted 4 October 2007 Abstract Thin sections of laminated cores from different Antarctic coastal areas have demonstrated the potential of diatom species to document climate change at the seasonal scale. Here we present the relative abundances of four diatom species and species groups (Fragilariopsis curta group as a proxy for yearly sea ice cover, F. kerguelensis as a proxy for summer sea-surface temperature, Chaetoceros Hyalochaete resting spores as a proxy for spring sea ice melting and the Thalassiosira antarctica group as a proxy for autumn sea ice formation) in core MD03-2601 retrieved off Adélie Land on the Antarctic continental shelf. These abundances were compared to surface temperatures and sea ice cover modelled over the last 9000 years. Both the marine records and the simulated climate demonstrated a cooler Early Holocene (90007700 years BP), a warmer Mid-Holocene (77004000 years BP) and a colder Late Holocene (40001000 years BP). Yearly sea ice cover followed an inverse pattern to temperatures with less sea ice during the Mid-Holocene Hypsithermal than during the Late Holocene Neoglacial. However, diatom census counts and model output indicate that sea ice spring melting happened earlier in the season, as expected, but that autumn sea ice formation also occurred earlier in the season during the Hypsithermal than during the colder Neoglacial, thereby following seasonal changes in local insolation. © 2007 Elsevier B.V. All rights reserved. Keywords: Southern Ocean; Sea ice; Holocene; Diatoms 1. Introduction Sea ice is one of the fastest reacting, most seasonal geophysical parameters on the Earth's surface of which the waxing and waning strongly influence the global climate. Sea ice acts as a blanket insulating the ocean from the atmosphere, hence limiting the exchange of energy, water vapour and gas (Wu et al., 1997). Sea ice presence strongly increases the albedo of the Southern Ocean (Ebert et al., 1995), especially during the spring and summer seasons. The sea ice seasonal cycle strongly influences deep water formation via brine injection during winter freezing and freshwater formation during the spring-summer meltback (Foldvik and Gammelsrod, 1988). Deep water formation in the Southern Ocean is believed to strongly impact on the global thermohaline circulation and global climate (Keeling and Stephens, 2001; Shin et al., 2003). Sea ice also affects biota by favouring diatoms over organic-walled micro-organisms (Wright and van den Enden, 2000) that export large amounts of carbon to the sea-bed (Vaillencourt et al., 2003). Given its important role in global climate control and the need to constrain and validate paleoclimatic models, Antarctic sea ice history has been actively investigated Available online at www.sciencedirect.com Marine Micropaleontology 66 (2008) 222 232 www.elsevier.com/locate/marmicro Corresponding author. E-mail address: [email protected] (X. Crosta). 0377-8398/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2007.10.001

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Page 1: Sea ice seasonality during the Holocene, Adélie Land, East Antarctica

Available online at www.sciencedirect.com

66 (2008) 222–232www.elsevier.com/locate/marmicro

Marine Micropaleontology

Sea ice seasonality during the Holocene,Adélie Land, East Antarctica

X. Crosta ⁎, D. Denis, O. Ther

UMR-CNRS 5805 EPOC, Avenue des Facultés, 33405 Talence Cedex, France

Received 13 March 2007; received in revised form 16 July 2007; accepted 4 October 2007

Abstract

Thin sections of laminated cores from different Antarctic coastal areas have demonstrated the potential of diatom species todocument climate change at the seasonal scale. Here we present the relative abundances of four diatom species and species groups(Fragilariopsis curta group as a proxy for yearly sea ice cover, F. kerguelensis as a proxy for summer sea-surface temperature,Chaetoceros Hyalochaete resting spores as a proxy for spring sea ice melting and the Thalassiosira antarctica group as aproxy for autumn sea ice formation) in core MD03-2601 retrieved off Adélie Land on the Antarctic continental shelf. Theseabundances were compared to surface temperatures and sea ice cover modelled over the last 9000 years. Both the marine recordsand the simulated climate demonstrated a cooler Early Holocene (9000–7700 years BP), a warmer Mid-Holocene (7700–4000 years BP) and a colder Late Holocene (4000–1000 years BP). Yearly sea ice cover followed an inverse pattern totemperatures with less sea ice during the Mid-Holocene Hypsithermal than during the Late Holocene Neoglacial. However, diatomcensus counts and model output indicate that sea ice spring melting happened earlier in the season, as expected, but that autumn seaice formation also occurred earlier in the season during the Hypsithermal than during the colder Neoglacial, thereby followingseasonal changes in local insolation.© 2007 Elsevier B.V. All rights reserved.

Keywords: Southern Ocean; Sea ice; Holocene; Diatoms

1. Introduction

Sea ice is one of the fastest reacting, most seasonalgeophysical parameters on the Earth's surface of whichthe waxing and waning strongly influence the globalclimate. Sea ice acts as a blanket insulating the oceanfrom the atmosphere, hence limiting the exchange ofenergy, water vapour and gas (Wu et al., 1997). Sea icepresence strongly increases the albedo of the SouthernOcean (Ebert et al., 1995), especially during the springand summer seasons. The sea ice seasonal cycle strongly

⁎ Corresponding author.E-mail address: [email protected] (X. Crosta).

0377-8398/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.marmicro.2007.10.001

influences deep water formation via brine injectionduring winter freezing and freshwater formation duringthe spring-summer meltback (Foldvik and Gammelsrod,1988). Deep water formation in the Southern Ocean isbelieved to strongly impact on the global thermohalinecirculation and global climate (Keeling and Stephens,2001; Shin et al., 2003). Sea ice also affects biota byfavouring diatoms over organic-walled micro-organisms(Wright and van den Enden, 2000) that export largeamounts of carbon to the sea-bed (Vaillencourt et al.,2003).

Given its important role in global climate control andthe need to constrain and validate paleoclimatic models,Antarctic sea ice history has been actively investigated

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in recent years, first focusing on the Last GlacialMaximum (Crosta et al., 1998; Gersonde et al., 2003,2005) and then on long records from the Atlantic(Gersonde and Zielinski, 2000; Stuut et al., 2004) andthe Indian sectors (Crosta et al., 2004) of the SouthernOcean. Sea ice evolution at transitions has also beeninvestigated (Bianchi and Gersonde, 2002, 2004). Thefocus is now on reconstructing Holocene changes in seaice cover (Leventer et al., 1996, 2006; Hodell et al.,2001; Barcena et al., 2002; Nielsen et al., 2004).However, all these studies only indicated whether therewas more or less sea ice in the past at a given location.These studies only give insight into sea ice extent andnot sea ice seasonality, understood here as the yearlytiming of waxing and waning. Seasonality obviouslyimpacts on phytoplankton productivity, Southern Oceanalbedo, deep water convection and gas-energy exchangeat the ocean-atmosphere interface, and thus is animportant parameter in climate dynamics and modelling.

The novel use of the resin embedding technique onhigh accumulation sediment cores allows the implicitstudy of diatom assemblages captured and quicklyburied at the sediment interface. This technique has beensuccessfully applied to several cores from MacRobertson Shelf (Stickley et al., 2005), Palmer Deep(Maddison et al., 2005) and the Dumont D'UrvilleTrough (Denis et al., 2006; Maddison et al., 2006). Thediatom species preserved in-situ and presented hereafterwere shown to have a strong potential to documentclimate change at the seasonal level. These studies havedemonstrated that the Fragilariopsis curta group is aproxy for yearly sea ice cover, F. kerguelensis is a proxyfor summer sea-surface temperature, ChaetocerosHyalochaete resting spores is a proxy for spring seaice melting and the Thalassiosira antarctica group is aproxy for autumn sea ice formation. We here build uponthe findings in thin sections and compare relativeabundances of the four diatom groups mentioned abovein core MD03-2601 retrieved off Adélie Land todocument sea-surface temperatures and sea ice season-ality at the precessional timescale during the Holocenein East Antarctica. Many hypotheses were proposed toexplain long-term climate evolution at high southernlatitudes during the Holocene (Masson et al., 2000;Nielsen et al., 2004; Renssen et al., 2005), each onecontradicting the others. The goal of our study was touse diatom assemblages to reconstruct sea ice season-ality during the Holocene and to infer what mechanismscan explain the Holocene long-term climate pattern. Toachieve our goal, we thoroughly compared our diatomrecords with model output of surface temperatures andsea ice extent for East Antarctica (Renssen et al., 2005).

Describing variations in diatom abundances at decadal-to-centennial scales and identifying their forcing factorsis beyond the scope of this paper.

2. Background

2.1. Seasonal diatom succession and sea ice

Studies of laminated sediments aroundAntarctica havedemonstrated the succession of annual couplets com-posed of one diatom ooze lamina representing the springproduction and one terrigenous lamina representing thesummer/autumn production (Pike et al., 2001; Leventeret al., 2002; Maddison et al., 2005; Stickley et al., 2005;Denis et al., 2006; Maddison et al., 2006). In thesecouplets, diatom succession is strongly dependent on theseasonal evolution of oceanographic and climatic condi-tions. More precisely off Adélie Land, cryophilic Fragi-lariopsis species such as F. curta and F. cylindrus appearfirst in early spring when sea ice is still present (Deniset al., 2006; Maddison, 2005; Maddison et al., 2006).Then, an assemblage of cryophilic Fragilariopsis speciesand Chaetoceros spp. (vegetative cells and spores)dominates the community during spring when sea icemelts and that surface water is well stratified. Atransitional assemblage dominated by Rhizosolenia spp.andCorethron criophilummay develop during late springto early summer. A mixed open water diatom assemblagecomposed of F. kerguelensis, large centrics and Chaeto-cerosHyalochaete spores then persists during the summerwhen sea ice has retreated and that surface waterswarmed. The summer/autumn period is dominated bycentric species such as T. antarctica, P. glacialis andHyalochaete spores. It is worth noting that F. rhombicareplaced the F. curta gp during the warmer mid-Holocenerelative to the colder Late Holocene (Denis et al., 2006).Many chains of F. rhombica were found in situ in thespring laminae during the Hypsithermal while few singlecells were encountered during the Neoglacial. It isbelieved that this finding is linked to a reduced com-petitivity of the F. curta gp when sea ice is less presentduring the spring/summer season (Armand et al., 2005).

Based on the ecological signal evidenced by in-vestigations of thin sections, the F. curta gp is a goodproxy for sea ice presence and concentration duringspring; Hyalochaete resting spores are a good proxy fordiatom productivity in well stratified waters and there-fore of the amount of meltwater; F. kerguelensis is agood proxy for sea-surface temperature during thesummer and possibly for the length of the growingseason; and the T. antarctica gp is a good proxy for seaice presence and concentration during autumn.

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2.2. The Holocene in the Southern Ocean

Holocene climate development in the Southern Oceanand on Antarctica is often reported as evolving from awarm Early Holocene between ∼11,000–9000 yr BP,passing through a cool event between 9000–8000 yr BP,followed by a warm Mid-Holocene between ∼8000–3000 yr BP, and terminating with a cold Late Holoceneuntil modern time (Cunningham et al., 1999;Masson et al.,2000; Hodell et al., 2001; Taylor et al., 2001; Lamy et al.,2002; Brachfeld et al., 2002; Presti et al., 2003; Nielsenet al., 2004; Masson-Delmotte et al., 2004). The timing ofthe different Holocene periods is variable depending on theproxy, the resolution of the study and the stratigraphy.Additionally, the climate phasing between West and EastAntarctica observed today (Stammerjohn and Smith, 1997;Liu et al., 2004) possibly persisted throughout theHolocene (Renssen et al., 2005). This phasing is especiallyevident for the transition between theMid-Holocene warmperiod and the Late Holocene cold period found at∼5000 yr BP in the east Antarctic sector (Hodell et al.,2001) and ∼3000 yr BP in the west Antarctic sector(Taylor et al., 2001; Brachfeld et al., 2002).

Holocene long-term changes at southern latitudeswere previously explained as a response to decreasing

Fig. 1. Map showing the location of core MD03-2601, the bathymetry aCircumpolar Deep Water, HSSW: High salinity Shelf Water, ACC: Antarcti

summer insolation at high northern latitudes (Nielsenet al., 2004) or to decreasing annual mean meridionalinsolation gradient in the Southern Hemisphere (Massonet al., 2000). Both mechanisms impact on heat transportby the thermohaline circulation to or from the SouthernOcean. The seasonal difference in insolation at a givenlatitude was also invoked (Lamy et al., 2002). Morerecently, a modelling experiment demonstrated thatlong-term climate trends in the Southern Ocean resultmost likely result from a delayed response to localorbital forcing with temperatures lagging seasonalinsolation by few months and winter storage of oceanicwarmth below the shallow summer mixed layer(Renssen et al., 2005). We show hereafter that ourdiatom records from the continental shelf off AdélieLand support the last hypothesis.

3. Material and methods

Piston core MD03-2601 was retrieved duringMD130 — Images X cruise (CADO-Coring AdélieDiatom Oozes) in February 2003 on board R.V. MarionDufresne II with the logistic support of the French IPEV(Institut Paul Emile Victor). Core MD03-2601 wasretrieved at 66°03.07′S, 138°33.43′E in 746 m water

nd the oceanography of the Adélie Land region. MCDW: Modifiedc Coastal Current.

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depth in one of several small depressions composing theD'Urville Trough (Fig. 1). The trough has an averagedepth of 600–700 m and consists of a series of smalldepressions reaching a maximum depth of 900–1000 m(De Santis et al., 2003). These depressions act as fun-nels, which focus the phytoplankton cells (mainlydiatoms, Wright and van den Enden, 2000) producedin the area, thereby increasing their preservation ef-ficiency. Core MD03-2601 is composed of diatomooze alternating between structureless greenish oozeand green to dark-green laminations of millimetre tocentimetre thickness.

Nine radiocarbon dates were performed on the humicfraction of bulk organic matter at the Leibniz Laboratory,Kiel, Germany. Raw 14C dates were calibrated to calendarages using Bard's polynome (Bard et al., 1998) afterapplying a 1300 year reservoir age effect (Ingólfssonet al., 1998). A linear regression was applied to the nine14C dates tominimize possible biases due to varying inputof old carbon. The correlation coefficient of the regressionis 0.98. The core covers the period from1000–10.000 yearsBP with an average sedimentation rate of 0.4 cm.year−1

(Table 1).Diatoms were identified every 4 to 8 cm providing a

10–20 year resolution. Diatom analysis, sedimenttreatment and slide preparation followed the techniquedescribed in Rathburn et al. (1997). Diatom countsfollowed Schrader and Gersonde (1978) and Laws(1983). Around 350 diatom valves were counted in eachsample at a magnification of ×1250. Diatoms wereidentified to species or species group level, and therelative abundance of each was determined as thefraction of diatom species against total diatom abun-dance in the sample. More details about slide prepara-

Table 1Accelerator mass spectrometry 14C ages from core MD03-2601

Depth (cmbsf) Raw ages (14C years BP) SD (years)

2 2350 70498 3235 30998 5175 601498 6135 351998 6310 1002498 7450 402998 8775 403498 9570 503998 10,855 45

Regression Coef

Y=24,461X+1000 R2=0.9795

Ages were calibrated to calendar ages using Bard's polynome (Bard et al.,1300 years (Ingólfsson et al., 1998).

tion, diatom identification and statistical treatment arefound in Crosta et al. (2004).

Total absolute diatom abundance (ADA) in terms ofthe number of frustules per gram dry weight wasdetermined for each sample using the followingequation:

ADA ¼ Nd=Nfð Þ⁎ Sp=Sf� �⁎

D= N⁎drVdr

� �� �⁎1=Wð Þ ð1Þ

where, Nd is the number of diatoms; Nf the number offields of view; Sp the area of the petri dish (cm2); Sf thearea of the microscope field of view (cm2); Ndr thenumber of drops; Vdr the volume of a drop (ml); D thedilution (ml); and W the dry weight of the sample (g).

4. Results

Around sixty diatom species or species groups wererecognized in core MD03-2601. Only the eight mostabundant species or species groups that bear a clearseasonal ecological signal as evidenced from thinsections are presented here (Fig. 2). Rare diatomspecies, benthic diatom species and ubiquist diatomsspecies were not taken into account.

The Fragilariopsis curta gp (F. curta gp) combinedFragilariopsis species having similar ecological prefer-ences (Armand et al., 2005) such as F. curta, F. cylindrusand F. vanheurckii. F. curta was the main species of thisgroup. The F. curta gp accounted for 10–20% during the10,000–9100 yr BP period, dropped to 5–10% duringthe 9100–8400 yr BP period and increased back to 10–20% during the 8300–7700 yr BP period (Fig. 2a).Relative abundances of the F. curta gp were ∼10% ofthe total diatom assemblage between 7700 yr BP and

Calibrated ages (years BP) Values used (years BP)

850–983 9161835–1906 18714238–4390 43145454–5537 54965585–5820 57036941–7027 69848328–8411 83699154–9263 920810,682–10,803 10,742

1998) after correction of raw 14C ages by a constant age reservoir of

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Fig. 2. Diatom records versus calendar ages in core MD03-2601. Relative abundances of diatom species and species groups are ranked from sea ice/cold water species [Fragilariopsis curta group (a), Fragilariopsis cryophilic group (b), and Porosira glacialis (c)] to open ocean species[Fragilariopsis kerguelensis (d), Fragilariopsis rhombica (e) and centric species group (f)] and to productivity-linked species [Hyalochaete restingspores (g) and Thalassiosira antarctica group (h)]. Cumulative relative abundance of the species mentioned above are presented in (i) and absoluteabundances of diatom valves per gram of dry sediment are presented in (j).

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4000 yr BP except for short events at 6000 yr BP and4500 yr BP. Relative abundances sharply increased at4000 yr BP to account for 30–40% of the siliceous florauntil 1000 yr BP.

Other cryophilic Fragilariopsis species (the F. cryo-philic gp),F. obliquecostata,F. ritscheri andF. sublinearis(Armand et al., 2005), followed a similar pattern observedunder the F. curta gp except during the 10,000–9000 yrBP period when they present low relative abundances(Fig. 2b). Abundances of the F. cryophilic gp were 3–6%during the 9000–4000 yr BP period after which theysharply increased accounting for 6–12% of the total dia-tom assemblage until 1000 yr BP.

Cold-water thriving Porosira species, P. glacialis andP. pseudodenticulata (Armand et al., 2005) represented2–6% of the total diatom assemblage during the 10,000–8700 yr BP period, less than 4% during the 8700–3800 yrBP period, and variable abundances of 4% and 8% duringthe 3800–1000 yr BP period (Fig. 2c).

Fragilariopsis kerguelensis, an open ocean diatomspecies (Crosta et al., 2005) presented an inverse patternto sea-ice and cold-water species presented above. Thisspecies revealed relative abundances of ∼9% during the10,000–9000 yr BP period, ∼12% during the 9000–7700 yr BP period and ∼18% during the 7700–4000 yrBP period (Fig. 2d). At 4000 yr BP, relative abundancesof F. kerguelensis dropped abruptly to account for ∼9%until 1000 yr BP.

Fragilariopsis rhombica, a diatom species thriving incold waters near the ice edge where unconsolidatedsea ice prevails during the winter and no sea ice per-sists during the summer (Armand et al., 2005), equallyrevealed a similar abundance pattern observed inF. kerguelensis data. F. rhombica accounts for ∼3% ofthe total assemblage during the 10,000–8700 yr BPperiod, ∼9% during the 8700–7700 yr BP period and∼15% during the 7700–4000 yr BP period (Fig. 2e).Relative abundances of F. rhombica continuously de-crease from 4000 yr BP to reach ∼6–9% of the as-semblage at 1000 yr BP.

The centric diatom group, composed of Azpeitiatabularis, Thalassiosira gracilis, T. lentiginosa, T. oliver-ana, T. oestrupii and T. tumida, is indicative of open oceanconditions during the growing season (Crosta et al., 2005).Relative abundances of this group are ∼3% during the10,000–8700 yr BP period, ∼6% during the 8700–4000 yr BP period and∼4% during the 4000–1000 yr BPperiod (Fig. 2f).

Two major diatoms species groups in core MD03-2601, the Chaetoceros Hyalochaete resting spores gp(CRS) and the T. antarctica group (rare T. antarcticavegetative cells and dominant T. antarctica resting

spores), present a different pattern over the Holocenethan the species mentioned above. In as much as theprecedent species showed sharp changes in their relativeabundances, the CRS and T. antarctica groups showed agradual decrease from ∼30% and ∼20% respectively at9000 yr BP to ∼15% and ∼5% respectively at 1000 yrBP (Fig. 2g and h). The 10,000–9000 yr BP periodis particularly unusual with very high abundances ofCRS (∼50–60%) and relatively low abundances of theT. antarctica gp (∼15%).

The mentioned diatom species and species groupsaccount for 70–80% of the total diatom assemblage(Fig. 2i). Other diatom species, such as Eucampiaantarctica, vegetative cells of Chaetoceros Hyalochaete,Pseudo-nitzschia species, Corethron criophilum, Tha-lassiothrix antarctica and Tricotoxon reinboldii presentedlow abundances and no clear Holocene trends. Ubiqui-tuous distributions of these species in the phytoplanktonand in surface sediments were considered of lowecological significance (Zielinski et al., 1997).

Absolute diatom abundances of ∼900 millions ofvalves per gram of dry sediment are encountered during the10,000–9000 yr BP period when the highest relativeabundances of CRS were found (Fig. 2j). Absoluteabundances are ∼400 millions of valves per gram of drysediment during the 9000–8000 yr BP period, ∼600–700millions of valves per gram of dry sediment during the8000–4000 yr BP period and∼400–500millions of valvesper gram of dry sediment during the 4000–1000 yr BPperiod. Short-lived maxima in absolute diatom abundanceswere encountered throughout the core and were generallyassociated with the elevated occurrence of CRS.

5. Discussion

The diatom records in core MD03-2601 fit well withprevious records of Holocene climate change at highsouthern latitudes (cf. Section 2.2). The different dia-tom species abundances vary concordantly througheach of the Holocene periods (Fig. 2). However, thediatom assemblage during the 10,000–9000 yr BPperiod is particularly unusual in that it was stronglydominated by CRS, revealed a slight increase in theF. curta and the P. glacialis groups and a strong de-crease in F. kerguelensis and F. rhombica abundances(Fig. 2). During this period, carbon and nitrogenisotopic values measured in the bulk organic matterare also lower than during the Mid-Holocene (data notshown). Diatom and isotopic data indicated a coolerclimate but more stratified surface waters, a result thatis at odds with a warmer climate inferred from Ant-arctic ice cores (Masson et al., 2000; Masson-Delmotte

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et al., 2004) and South Atlantic sediment cores (Hodellet al., 2001; Nielsen et al., 2004). It is possible that theimportant retreat of local glaciers (Ingólfsson et al.,1998) during the earliest part of the Holocene hadaffected the oceanographic conditions by stronglyenhancing surface water stratification. Such stratifica-tion may have promoted greater sea ice extent thanexpected but warmer conditions may have rapidlymelted sea ice in spring thus favouring the productionof Chaetoceros Hyalochaete species and eventually theformation of CRS. It is therefore difficult to infer seaice seasonality during the deglaciation until 9000 yr BP,which is confirmed by the lack of model output for thisperiod. We will hereafter focus on the 9000–1000 yrBP period.

Younger than 9000 yr BP, records of sea ice diatoms(Fig. 2a–c) and open ocean diatoms (Fig. 2d,e) showeda long-term inverse relationship. Open ocean diatomswere more abundant during the 9000–4000 yr BPperiod while sea ice diatoms were dominant during the4000–1000 yr BP period. We therefore estimate thatclimate conditions were warmer and that sea ice coverwas less extensive during the Mid-Holocene (Hyp-sithermal) than during the Late Holocene (Neoglacial).Replacing the relative abundances of F. curta andF. kerguelensis observed during the Hypsithermal andNeoglacial periods on previously documented distribu-tions in Southern Ocean surface sediments (Armandet al., 2005; Crosta et al., 2005) allowed us to estimate a∼1 °C cooling and ∼1 month increase in sea ice coverat the Hypsithermal–Neoglacial transition in phase withother studies. More quantitative estimates of sea-surfacetemperature and sea ice presence changes throughtransfer functions are not possible here because of areduced modern database around East Antarctica. In theSouth Atlantic where diatom-based transfer functionsallow quantitative estimates, sea-surface temperaturechanges were calculated to be around 1–2 °C during the9000–8000 yr BP cool event (Nielsen et al., 2004) andaround 2–3 °C over the Hypsithermal–Neoglacialtransition (Hodell et al., 2001; Nielsen et al., 2004).Temperature changes at EPICA DOME C core site were∼2 °C during the 9000–8000 yr BP cool event and∼1 °C over the Hypsithermal–Neoglacial transition at3000 yr BP (Masson-Delmotte et al., 2004). Sea ice wasclearly more extended during the cool event and thecolder Neoglacial than during the warmer Hyspithermal(Hodell et al., 2001; Nielsen et al., 2004).

Most of previous studies dealing with Holoceneclimate changes in the Southern Ocean (cf. Chapter 2.2)unfortunately do not go further than to suggest more orless sea ice cover and warmer or colder temperatures

during the Holocene. Investigations of thin sections haveshown that it may be possible to document the seasonalcycle at high southern latitudes (Cf. Chapter 2.1). Thesame investigation have also demonstrated strong var-iations in thickness of adjacent couplets, which wererelated to inter-annual changes of surface temperatures,sea ice cover and atmospheric conditions (Denis et al.,2006). Because we are here interested in the long-termevolution of temperature and sea ice seasonality, weapplied polynomial regressions to the diatom records todiscard decadal-to-centennial cyclicities. Holocenelong-term trends in the dominant species or speciesgroups were then compared to seasonal surface tem-peratures and sea ice area as estimated by an ocean-atmosphere-sea ice-vegetation model (Renssen et al.,2005).

Diatom species included in the F. curta gp arecommon components of the Antarctic pack ice (Leven-ter, 1998) and of the spring plankton within the sea iceedge and meltwater (Armand et al., 2005). This group iscommonly used to document the winter sea ice limit inthe Southern Ocean (Gersonde and Zielinski, 2000) andsea ice cover in coastal areas (Cremer et al., 2003). Thelong-term evolution of the F. curta gp in core MD03-2601 (Fig. 3a, grey solid line) indicated a gradualincrease in sea ice cover during the Holocene. This is inagreement with the modelled sea ice cover (Renssenet al., 2005) that similarly showed reduced sea ice coverduring the Mid-Holocene than during the Late Holocene(Fig. 3a, black line). Sea ice cover increased againduring the Early Holocene concomitantly with greaterabundances of sea ice diatoms. The Holocene sea icetrend in East Antarctica is seemingly related to surfacetemperatures during the growing season that showed aninversed pattern with higher temperatures during theMid-Holocene, which subsequently declined graduallyat ∼4000 yr BP towards pre-industrial values (Fig. 3b,black line). The long-term evolution of F. kerguelensis,a species that does not develop well when sea ice ispresent during the summer season (Crosta et al., 2005),followed the same pattern as observed under the mod-elled summer surface temperatures with elevated diatomrelative abundances during the warm Hypsithermal andreduced relative abundances during the Neoglacial andthe cool event (Fig. 3b, grey solid line).

The seasonal sea ice distribution in East Antarcticaduring the Holocene may, however, be more compli-cated than a simple pattern of less sea ice during theHypsithermal and more sea ice during the Neoglacial.Chaetoceros Hyalochaete species mainly thrive incoastal areas where they can achieve a very high biomass(Armand et al., 2005) due to their ability to bloom rapidly

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Fig. 3. Comparison of relative abundances of major diatom species and species groups in core MD03-2601 (grey dashed lines) versus calendar agesand modelled temperature and sea ice outputs (black lines) (Renssen et al., 2005). Holocene trends in diatom relative abundances (grey solid lines)were extracted from the raw percentages (grey dashed curves) through polynomial regressions. The Holocene trend of the F. curta gp is compared toanomalies of sea ice cover (a), the Holocene trend of F. kerguelensis is compared to anomalies of temperatures integrated over the growing season (b),the Holocene trend of Chaetoceros Hyalochaete resting spores is compared to anomalies of temperatures for the beginning of the austral spring (c),and the Holocene trend of the T. antarctica gp is compared to anomalies of temperatures for the beginning of the austral autumn (d). Note the reversedscale for the temperature anomalies in (d). Diatom relative abundances and model outputs are plotted on independent timescales.

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when conditions are favourable. High nutrient contentin surface waters, surface water stratification by sea icemeltwater and stabilization by low wind intensity ap-pear to be the most important factors for ChaetocerosHyalochaete blooms (Leventer, 1991; Leventer, 1998).Resting spores are formed within the vegetative cellswhen diatom blooms exhausted surface nutrients or

when vegetative cells sink out of the photic zone(Hargraves and French, 1983). Relative abundances ofCRS are therefore commonly used to track high pro-ductivity events at the receding sea-ice edge (Leventeret al., 1996; Denis et al., 2006). The Holocene long-termtrend of CRS relative abundances in core MD03-2601followed a gradual decrease since 9000 yr BP (Fig. 3c,

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grey solid line), indicating more stratified and morestabilized surface waters during the Early Holocene thanduring the Late Holocene. A similar long-term evolutionwas encountered in the Antarctic Peninsula with higherdiatom productivity during the Early Holocene than theLate Holocene (Taylor et al., 2001). Modelled Octobersurface temperatures (Renssen et al., 2005) indicated acomparable pattern with a gradual cooling of∼3 °C overthe last 9000 years (Fig. 3c, black line). Long-termsurface water stratification may result from warmerspring temperatures that led to earlier and more rapidsea ice break-up during the Hypsithermal, and whenassociated with fewer storm events (Denis et al., 2006),may have promoted Chaetoceros Hyalochaete bloomswhile limiting nutrient re-supply and therefore may havepromoted CRS formation. Early sea ice break-up andsurface water stratification may have also enhancedsurface water warming thus favouring F. rhombica pro-duction during spring andF. kerguelensis during summerat the expense of the F. curta gp. The spring coolingtowards pre-industrial times may have limited spring seaice waning thus reducing the injection of melt-water andsurface stratification. Less favourable conditions, asso-ciated with more windy conditions (Denis et al., 2006),may have reduced Chaetoceros Hyalochaete productionand CRS formation. Unsurprisingly, given knownecological preferences of the taxa concerned, greatersea ice presence during spring was however moreoptimal to F. curta production at the expense of F.rhombica and F. kerguelensis.

The T. antarctica gp in core MD03-2601 isdominated by spores of the cold form as described byTaylor et al. (2001). T. antarctica is a near-coastalspecies, for which production is related to open oceanconditions during autumn when sea ice re-advances andlimits other species competitiveness (Cunningham andLeventer, 1998; Armand et al., 2005). Relative abun-dances of T. antarctica are therefore commonly used totrack productivity events at the advancing sea ice edgeor in open ocean zones in the sea ice (Taylor et al., 2001;Denis et al., 2006). The Holocene long-term trend forthe T. antarctica gp in core MD03-2601 showed highrelative abundances during the 9000–7700 yr BPperiod, a gradual decrease from 7700 yr BP to 2000 yrBP and low relative abundances since 2000 yr BP(Fig. 3d, grey solid line). Given the ecological pre-ference of the species included in the T. antarctica gp,this pattern may indicate earlier late summer/earlyautumn sea ice readvance during the Early and Mid-Holocene than during the Late Holocene. This evolutioncorrelates well with modelled surface temperatures forthe month of April (Renssen et al., 2005). Autumn

surface temperatures were around −0.5 °C during the9000–7000 yr BP period and subsequently increased toreach 0.2 °C at 3000 yr BP (Fig. 3d, black line). Coolertemperatures during the Early and Mid-Holocene mayhave promoted water freezing and sea ice waxing earlyin the year, and therefore production of T. antarcticaspecimens at the expense of open ocean diatoms. Thewarming of surface temperatures towards pre-industrialtime may have conversely delayed sea ice autumnfreezing thus limiting production of T. antarctica duringthe Late Holocene.

Diatom records in core MD03-2601 off Adélie Land,East Antarctica, and model output (Renssen et al., 2005)both indicated warmer temperatures and less extensivesea ice cover during the Hypsithermal and cooler tem-peratures and greater sea ice cover during the Neogla-cial. Warmer temperatures and lesser sea ice coverduring the Hypsithermal may indicate early sea icebreak-up during spring and late sea ice freezing duringautumn. However, down-core records of ChaetocerosHyalochaete resting spores and T. antarctica gp incombination with model output of seasonal surfacetemperatures (Renssen et al., 2005) indicated that thiswas not the case. High relative percentages of these twodiatom group species during the Early and Mid-Holocene demonstrated early spring sea ice meltingbut, also, early autumn sea ice freezing in contrast to theLate Holocene observations. However, sea ice meltingduring spring was more rapid than sea ice freezingduring autumn as indicated by the amplitude of tem-perature changes in the model output. Spring and autumnsurface temperatures were 1–2 °C higher and 0.5 °Ccooler respectively during the Hypsithermal in compar-ison to the Neoglacial period. The diatom growing seasonwas therefore longer during the Hypsithermal than in theNeoglacial, but not as long as one may have believedfrom the relative abundances of the F. curta gp andF. kerguelensis. The diatom growth was also advanced inthe year, based on our observations, which is in phasewithgreater spring insolation and lower autumn insolation.

6. Conclusion

The goal of this study was to use diatom assemblagesto reconstruct sea ice seasonality during the Holoceneand to infer what mechanisms can explain the Holocenelong-term climate pattern.

Although sea ice was annually less present during thewarmer Hypsithermal than during the colder Neoglacial,diatom records in core MD03-2601 and model output(Renssen et al., 2005) both indicate a different sea iceseasonal cycle with the autumn freezing occurring

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earlier in the season than expected during the Hyp-sithermal. Diatom records and model output thereforesupport each other, thereby implying a strong localinsolation and local feedbacks control on Holoceneclimate evolution in the high-latitude Southern Hemi-sphere (Renssen et al., 2005).

It is believed that the use of some diatom species aspaleo-ecological and paleo-environmental indicators islargely under-estimated. We showed here that Chaeto-ceros Hyalochaete resting spores and the T. antarctica gpcan be used to document the seasonal cycle of sea ice inAntarctic coastal environments. Once diatom ecology ismarkedly improved and understood, diatom censuscounts will provide exceptional indications over modelsthat currently represent average climatic and environ-mental conditions. Seasonal sea ice conditions may bedocumented at very high resolution when sedimentationrates are large enough, thereby giving important infor-mation on the chaotic nature of climate evolution due tolocal ocean-atmosphere-cryopshere feedbacks. The cha-otic nature of sea ice and climate evolution may in returnbe very important in paleoclimate modelling studies todocument rapid changes in albedo and energy transfers.

Acknowledgments

We thank Hans Renssen for data and discussions. Wealso thank Amy Leventer, Leanne Armand and JennyPike for constructive discussions. Claire Allen andanother unknown reviewer provided helpful commentsthat improved the manuscript. Extended thanks areoffered to William Fletcher and Leanne Armand forreviewing the English. We personally thank people fromImages X (CADO) cruise and from NSF-fundedNBP0101cruise for data and suggestions concerningthe D'Urville Trough. Logistical support was providedby IPEV-TAAF. Financial support for this study wasprovided by the project TARDHOL through the nationalEVE-LEFE program (INSU-CNRS). This is an EPOCcontribution No.1683.

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