Dissertation for the degree of Doctor of Philosophy in Quaternary Geologypresented at Uppsala University in 2002-03-27

AbstractSjunneskog, C. 2002. Diatom and Sedimentological Investigations on WestAntarctic Shelf Sediment. Acta Universitatis Upsaliensis. ComprehensiveSummaries of Uppsala Dissertations from the Faculty of Science and Technology692. 27 pp. Uppsala. ISBN 91-554-5256-6

Climate and environmental change following the retreat of the last glacial ice sheetin the Antarctic Peninsula has been interpreted, employing diatom abundance,relative abundance of Chaetoceros resting spores and diatom assemblages asproxies. These together with sedimentological data and radiocarbon dating, suggestfour major events that can be further subdivided.

Deglaciation ~13.2-11.5 kyr BP with ice shelf breakup and strong surfacewater stratification from melting ice.Climate reversal ~11.5-9.0 kyr BP with turbulent water masses.Climate optimum ~9.0-4.0 kyr BP with intrusions of northern ´warm` watermasses.Neoglacial ~4.0 kyr BP-present with extended periods of sea ice cover andincreased storm frequency.

One aspect of climate change is the stability of marine based ice sheets, and theinteraction with underlying sediment. A pilot study on characterizing sedimentinfluenced by past ice streaming (Ross Sea) was performed using diatom, textureand chemical analysis. The results show that:

Diamictons are chemically and texturally well homogenized, whereas diatomassemblages suggest different degrees of stratigraphic mixing and reworkingrelated to mode of glacial sediment transport.Mud appears in different stratigraphic sections deposited in sub-ice shelf or iceedge environment, or through winnowing by currents. This is evident throughstratigraphically-diverse diatom assemblages and texture. Most sedimentcharacterized as mud is enriched in zinc (Zn).Hemipelagic diatomaceous muds are enriched in barium (Ba) and the diatomassemblage is dominated by typical neritic post-glacial species.

Keywords: Antarctica, climate change, diatom abundance, geochemistry

Charlotte Sjunneskog, Department of Earth Sciences, Villav. 16, 75236, Uppsala,Sweden

© Charlotte Sjunneskog

ISSN 1104-232XISBN 91-554-5256-6

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PREFACE

This thesis is based on the following papers, which will be referred to in the text bytheir Roman numerals.

I Sjunneskog C. and R. Scherer, 1997, High-resolution diatom record in bioturbatedantarctic sediments, Antarctic Journal of the United States, 32, 5, 34-36.

II Domack E., A. Leventer, R. Dunbar, F. Taylor, S. Brachfeld, C. Sjunneskog andODP Leg 178 Scientific Party, 2001, Chronology of the Palmer Deep site,Antarctic Peninsula: a Holocene palaeoenvironmental reference for the circum-Antarctic, The Holocene 11,1, 1-9.

III Sjunneskog C., and F. Taylor, 2002, Postglacial marine diatom record of thePalmer Deep, Antarctic Peninsula (ODP Leg 178, Site 1098) I: Total diatomabundance, Paleoceanography, 2002.

IV Taylor F. and C. Sjunneskog, 2002, Postglacial marine diatom record of thePalmer Deep, Antarctic Peninsula (ODP Leg 178, Site 1098) II: Diatomassemblages, Paleoceanography, 2002.

V Sjunneskog C., R. P. Scherer and A. Aldahan, Major and trace elementgeochemistry of sediment facies of the Ross Sea (Antarctica), submitted.

VI Sjunnskog C. and R. P. Scherer, Mixed diatom assemblages in glacigenicsediment from the Central Ross Sea, Antarctica, submitted.

The publishers of The Holocene and Paleoceanography kindly gave permission toreproduce papers II, III and IV.

My contribution to the publications:I was responsible for all analyses, compiling of data and writing of Paper I. Paper II isa collaboration, by multiple authors, through the Ocean Drilling Program (ODP). Mycontribution is mainly selection of samples and discussion, models were run byBrachfeld and Taylor. Papers III and IV both authors contributed equally to analyses,discussion and writing. F. Taylor is responsible for the statistical analyses in PaperIV. I carried out all diatom analyses, compiling of data and writing Papers V and VI.

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TABLE OF CONTENTS

AbstractPreface 3Table of contents 4Introduction 5

Antarctica, brief overview 5The investigation areas 8

Antarctic Peninsula 8Ross Sea 8

Aims of the studies 8Materials and analytical methods 9

Results and discussion 11Post-glacial climate and environmental change 11

Diatom records 11Notes on ecology and diatom assemblages 14Obtaining a chronology 14

Comparing different climate proxies 15Final remarks 17

Recognizing sediment units, Ross Sea 18Geochemistry 18Texture 18Diatom distribution 18

Sediment facies distribution 20Diatomaceous mud 20Mud 20Diamicton 22Final remarks 22

Meeting the objectives 23Implication for future work 24

Acknowledgements 24References 25

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INTRODUCTION

This thesis assesses questions regarding climate change in West Antarctica during thelate Pleistocene and Holocene, and how to recognize glacigenic and glacial marinesediment facies. The methods employed are diatom analyses complemented by chemicaland textural analyses. Marine sediment cores were collected from two different areaswith distinctly different climate and glacial regimes, the Antarctic Peninsula and theRoss Sea (Fig. 1). The Antarctic Peninsula is recognized as an area sensitive to climatechange, situated in the transition between different climate zones: maritime, continental,temperate glacial, and polar (Griffith and Anderson, 1989; Domack and McClennen,1996; Ingólfsson et al., 1998; Anderson, 1999; Smith et al., 1999). The main emphasis ofthis investigation is on using the diatom record to infer past climate and environmentalchange. The topic of radiocarbon dating and the uncertanties involved is also discussed.In the Ross Sea, where the sediment stratigraphy is highly influenced by past ice streamsand ice sheet dynamics (Shipp et al., 1999), the topics include the complexity ofreworked glacial sediment facies, how and if diatom and chemical records can provideinformation on sediment sources, depositional precesses, and the interaction betweensediment and ice streams and ice sheets. Recent studies of sub-ice-shelf and sub-ice-stream sediment emphasize the importance of sediment properties on ice streaming andice stream behaviour (Engelhardt and Kamb, 1998; Tulaczyk et al., 1998), and thedemand for understanding sediment properties increases. Occurrence of diatoms in subice-stream sediment shows the potential use for diatoms as sediment tracers (Scherer1991; Tulaczyk et al., 1998).

This work is largely based on the use of diatoms, their environmental preferences,preservation status, age relation, and their properties as sedimentary paricles. Diatomsare single- celled algae, plants with a silica shell, living in the euphotic zone in wet andmoist environments. They occur in the geologic record from the Cretaceous and havebeen extensively used as biostratigraphic markers in the marine sediment record (e. g.Gombos, 1977; Baldauf and Barron, 1991; Harwood and Maruyama, 1992; Bohaty et al.,1998; Scherer et al., 2000). Many species have specific requirements for substrate,temperature, pH, salinity and nutrients, and the value of diatoms as indicators of climatechange is recognized widely in paleoecological studies (e. g. Jousé et al., 1963; Kozlova,1966; Burckle, 1972; Truesdale and Kellogg, 1979; DeFelice and Wise, 1981; Pichon etal. 1987; Leventer et al., 1993, 1996).

Antarctica, brief overview

The Antarctic continent is covered by ice, about 98% of the continent, hence there is alimited knowledge of the Antarctic bedrock geology. On a large scale the Antarcticcontinent consists of two major continetal blocks, East and West Antarctica, separated bythe Transantarctic Mountains. East Antarctica is a stable Precambrian continental craton,

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whith an ice sheet grounded mainly above sea level. West Antarctica is a youngerarchipelago with active volcanism and the West Antarctic Ice Sheet (WAIS) is largelymarine based (Anderson, 1999). Because it is marine-based, the stability of the WAIS isreduced, and it is expected that the West and East Antarctic ice sheets (WAIS and EAIS)respond differently to climate change. The interest in studying climate change in theAntarctic realm stems from the potential impact changes in growth and decay of theAntarctic Ice Sheet have on global climate. Changes in ice sheet volume, ice shelf extentand sesonal sea-ice cover, all have an impact on ocean circulation, albedo and sea level(Blachon and Shaw, 1995; Bentley, 1999). In Antarctic today we can also find similarsettings, and analogs to that of the Wisconsin / Weichselian Laurentide ice sheets (Stokesand Clark, 2001). Several episodes of growing and waning of ice sheets have beenrecorded in the sediment. The most prominent warming events in Quaternary time areMarine Isotope Stage (MIS) 11 (Mid-Bruhnes event) when expectedly large portions ofthe WAIS disappered (Scherer et al., 1998; Scherer, in press).

Ice streams, narrow bands of fast moving ice bounded by slow moving ice, drain interiorice and play an important role in mass balance of the ice sheet. They also feed ice shelvesfringing the continent, influencing the primary productivity and the ecology. Ice streamsterminating in confined embayments form large ice shelves, whereas smaller ice shelvesare fed by local ice streams. The most extensive ice shelves in West Antarctica are theRoss Ice Shelf (Ross Sea) and the Ronne-Filchner Ice Shelf (Weddell Sea) bothreceiving ice from East and West Antarctica (Figs. 1A and 1C). During the last decadessome of the smaller ice shelves have disintegrated, including the George V and parts ofthe Larsen ice shelf (Doake and Vaughan, 1991; Skvarca, 1993; Domack et al., 1995;Rott et al., 1996; Vaughan and Doake, 1996).

The continental shelf surrounding West Antarctica is wide and deep compared to othercontinents. The shelf is tilted inward toward the continent due to glacial overburden andthe tectonic setting. The average depth is 500 m, and the inner shelf and its basins can beas deep as 1200 m (Anderson, 1999). The large scale morphology of the shelf is highlyinfluenced by past ice sheet and ice stream erosion whereas the position of the heads ofthe ice streams in the interior is influenced by bedrock structure (Bell et al., 1998). Pre-exsisting troughs on the continental shelf act as channels, repeatedly eroded by icestreams (Anderson 1999, Shipp et al., 1999). Ice sheets and ice streams are responsiblefor mass transport of sediment out towards the shelf-break where prograding sequensesare formed (Hays et al., 1975; Barker et al., 1999). Late Pleistocene sediments drapeslodgement till or bedrock. The wide shelf is an area of high primary productivity,upwelling of nutrient-rich water combined with meltwater from seasonal sea ice form anenvironment where diatoms thrive. As a consequence, some of the richest diatomaceousoozes in the world can be found in Antarctic shelf sediment (Jousé et al., 1971) withbiogenic silica content up to 75% of the total sediment (Gersonde and Wefer, 1987).Basins that are not occupied by ice streams at present may act as sediment traps,accumulating thick sediment deposits (Kirby et al., 1998; Rebesco et al., 1998).

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Fig. 1. Map of Antarctica showing places mentioned in the text. B shows the Antarctic Peninsulaand C shows the Ross Sea Embayment. JR Is= James Ross Island, GS= Gerlache Strait, LF=Lallemand Fjord, CIR= Crary Ice Rise and RISP= Ross Ice Shelf Project.

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The investigation areas

Antarctic PeninsulaThe Antarctic Peninsula is a part of West Antarctica that protrudes into the DrakePassage. The Peninsula is bordered to the east by the Weddell Sea and the Ronne-Filchner ice shelf, and to the west by the Gerlach and Bransfield Straits and the PalmerArchipelago (Figs. 1A and 1B). The climate regime along the Western AntarcticPeninsula is sub-polar and more humid compared to the eastern side and the remainingpart of the continent. In the northern part, glaciers terminate on land whereas in thesouthern part small ice shelves exist. Due to the sheltering effect of the archipelagoprimary productivity is high as reflected by high sedimentation rate of post-glacialhemipelagic mud. Over the last decades, meteorological records have revealedsystematic warming (Harangozo et al., 1994; King, 1994; Stark, 1994; Smith et al.,1999) which is suggested to be one of the reasons that ice shelves are retreating (Skvarca,1993; Domack et al., 1995; Rott et al., 1996). The continental shelf in Bransfield Straitand the Palmer Archipelago is incised by tectonic troughs and basins (Kirby et al., 1998;Rebesco et al., 1998). Seismic investigations of the Palmer Deep basins show they areinfilled with a 'transparent' facies, interpreted as postglacial hemipelagic mud (Rebesco etal., 1998). Palmer Deep basins were mapped and cored by ODP Leg 178 (Barker et al.,1999), and advanced piston coring (APC) allowed recovery of 50 m of hemipelagic mud.

Ross SeaThe Ross Sea embayment is a different setting from the Antarctic Peninsula with the vastRoss Ice Shelf fed by ice streams from both East and West Antarctica as the dominatiangfeature (Fig. 1C). The extent of the Ross Ice Shelf has varied over time, as has the pathof past ice streams. Ice streams originate inland of the Ross Sea, where sub-glacialmarine basins are present (Bell et al., 1998) and are providing sediment transported andre-distributed through glacial processes, resulting in a complex stratigraphy. The onsetand maintenance of ice streams depends on a number of factors amongst which is thesubstrate. Several theories have been put forward regarding the ice – substrateinteraction. The main ones include subglacial deformation of till, to uncertain depth, icesliding on top of the sediment bed and plowing or pushing sediment in front of the ice orby an uneven subsurface of the ice (e.g., Tulazyk et al., 1998; 2001). The sediment covervaries from very thin in the inner part, where bedrock is exposed, to considerablethickness toward the shelf break, where sediment bedforms are developed. Thesebedforms, including drumlins and lineations, are related to past ice streaming (Shipp etal., 1999). The general stratigraphy recovered in piston cores is a post-glacialhemipelagic mud overlying muds and diamictons (Domack et al., 1999; Licht, 1999).

Aims of the studies

There are two main topics adressed in this thesis.

The first topic is to investigate post-glacial climate development in the AntarcticPeninsula area and evaluate different aspects of using the diatom record for this purpose.

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One working hypothesis is that diatom abundance in the sediment reflects changes insurface water properties and climate, including wind stress and temperature, as has beenproposed by Scherer (1992) and Leventer et al., (1996). Not only the diatom abundance,but the relative abundance of Chaetoceros resting spores (rs), the dominant species, theratio between Chaetoceros rs and 'vegetative cells' (referred to as ratio hereafter), and theratio between different species have been proposed to reflect different climatic conditions(Leventer et al., 1996; Crosta et al., 1997). Chaetoceros spp. (hyalochateae) are knownto form large blooms in connection with surface water stratification and sea ice melt,warming and access to abundant nutrients. At the end of the bloom, resting spores areformed, which may accumulate in profusion on the sea floor (Estrada and Delgado,1990). The assumption was also made that a refined climate signal could be obtainedfrom the diatom assemblage excluding the Chaetoceros rs, the so called non-Chaetoceros assemblage, since the Chaetoceros rs are likely to reflect only a short periodof the growth season.

The second topic is to characterize glacigenic and glacial marine sediment facies(diamicton, mud and diatomaceous mud) in the Ross Sea by using diatom data incombination with chemistry. Diamicton in the Ross Sea is typically derived from varyingsources including marine sediment components of different ages as well as bedrock ofdifferent composition (Anderson, 1999). It is problematic to distinguish till from glacial-marine diamicton, and to determine the sedimentary processes involved in theirdeposition (Tulaczyk et al., 1998; Domack et al., 1999; Licht, 1999). It has beensuggested that the sediment properties such as water saturation, pore space and grain sizeare linked to ice streaming (Alley et al., 1989; Tulaczyk et al., 1998). The argument forusing diatoms is that they are abundant in the sediment and they are age- andenvironmental specific. The aim here, as a piece in the puzzle of recognizing past icestream behaviour, is to find criteria to characterize sediment units that are similar intexture, but derived from different sources or deposited through different processes.

The objectives of this investigation are to:a) evaluate whether diatom abundance can be used as a proxy for paleo-productivity

and climate change for the post-glacial period in the Antarctic Peninsulab) evaluate whether diatom assemblages can reveal a detailed climatic record in the

Antarctic Peninsulac) test the use of geochemical parameters to characterize sediment units in the Ross Sead) characterize sediment units by using diatoms as sedimentary tracers and

environmental indicators in the Ross Sea.

Materials and analytical methods

The cores investigated in the Antarctic Peninsula were collected by the Deep Freezeproject in 1986 (Bransfield and Gerlache straits) and by ODP Leg 178, Site 1098 A, Band C (Palmer Deep) in 1998 (for location see Table 1). Samples for diatom analyses ofthe Deep Freeze core were collected at the Florida State University core repository. Core1098 C was sampled for radiocarbon dating on retrieval on deck. Samples for diatomanalyses were collected at 20 cm spacing from cores 1098 A and B at the Bremen core

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repository. The cores were described first onboard ship, followed by a detaileddescription at the Bremen sampling party. The three parallel cores, ~40-45 meters inlength, consist mainly of olive-green diatomaceous mud and diatomaceous ooze withdiscrete lamination of different colors. In cores A and C diamicton was collected at thebottom of the column. A composite depth scale, meters composite depth (mcd), wasconstructed based on magnetic susceptibility (MS) (Acton et al., 2001).

The cores from the Ross Sea were collected by Nathaniel B. Palmer (NBP) cruises 94-01and 95-01 (for location see Table 1). The cores were sampled for chemical and diatomanalyses at the Florida State University core repository in 2000.

Tabel 1. Location of coring sitesLat. S / long. W Cruise and core64°59.7 / 63°20.0 Deep Freeze 1986, 86-7964°37.8 / 62°52.0 Deep Freeze 1986, 86-8262°57.5 / 55°29.0 Deep Freeze 1986, 86-762°09.4 / 56°09.0 Deep Freeze 1986, 86-964°51.7 / 64°12.5 ODP Leg 178 Site 1098 ABC75.165° S/ 178.518° W NBP 94-01 TC/PC3175.455° S/ 179.615° E NBP 94-01 TC3376.453° S/ 179.087° W NBP 95-01 TC/PC1176.725° S/ 178.63° W NBP 95-01 TC1376.943° S/ 179.823° W NBP 95-01 TC1677.473° S/ 179.537° E NBP 95-01 TC1874.473° S/ 173.512° E NBP 95-01 KC3982.22° S/ 68.38° W RISP J-9

Diatom microscope slides were prepared for quantative analyses of diatom abundancefollowing the method of Scherer (1994). The method includes freeze drying of thesediment, weighing of an aliqout, which is then oxidized (H2O2) and allowed to settleonto a cover slip in a settling chamber. The diatom abundance is calculated from theequation:

Abundance = ((A x B) / (C x D)) / E

A= number of specimens counted, B= area of settling chamber, C= number of field of view in microscope,D= area of field of view, E= mass of sample.

For the Antarctic Peninsula and climate change project, diatom abundance wascalculated by counting complete valves or valves that had more than 50% of the areaintact. The aim was to count at least 500 valves per slide. One set of counts was analyzedincluding Chaetoceros resting spores and one set excluding the Chaetoceros rs (non-Chaetoceros count). Diatom abundance is given as valves per gram dry sediment (v/gds).The non-Chaetoceros data were analyzed using Bray-Curtis cluster analysis and thestudent Newman-Keuls multiple range test (SNK) in association with a single analysis of

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variance (ANOVA) following the method outlined in Taylor et al. (1997). Down-corecluster analysis was conducted using the method implemented by Whitehead (1996).

For the Ross Sea project, microscope slides were prepared using the same method asabove. Because of the different approach, compared to the Peninsula, diatom abundanceis calculated by counting complete diatom valves and fragments large enough to identifyto species or genus level. The technique of counting fragments introduces an error byrisk of counting the same frustule more than once, therefore the ´diatom abundance` isnot an absolute count, it is a means of obtaining an objective measure for comparingbetween samples. Furthermore, diatom fragments, 2-5 m are counted to assess a´preservation` signal (increased fragmentation results in increased surface area andincreased susceptibility to dissolution). Selected diamicton and mud samples were sieved(20 m sieve) in order to concentrate time stratigraphic markers.

Samples for geochemical analyses were sieved through 175 m mesh and washed withdistilled H2O. The dried sample was analyzed for 10 major and 17 trace elements byinductively coupled plasma atomic emission spectroscopy (ICP-AES) and massspectrometry (ICP-MS). The relative error of 1 standard deviation is < ± 3% for majorelements and < ± 5-20% for trace elements. A CILAS Granulometre 715 (laser based)was used for matrix grain size determination on the same fraction as the chemicalanalyses. Both chemistry and grain size data were reduced by using Canoco 3.1 softwarefor principal component analyses and graphical presentation (PCA; ter Braak andSmilauer, 1998).

RESULTS AND DISCUSSION

Post-glacial climate and environmental change, Antarctic Peninsula

Diatom records (Papers I, III and IV)The overall sediment diatom assemblage is dominated by Chaetoceros rs. In surface andsub-recent sediment the diatom abundance ranges from 107 to 109 (v/gds), Chaetocerosrs contribute 80-90% to the assemblage. The changes in diatom abundance show thesame pattern over long distances along the Peninsula, indicating that the diatomabundance is not a localized climate proxy (Papers I and III; Leventer et al., 1996). Theresults also show that despite bioturbation a high resolution is maintained in the diatomrecord (Paper I, Leventer et al., 1996).

The downcore diatom abundance in the Palmer Deep, Site 1098, varies over severalorders of magnitude, from 105 to 109 v/gds (Fig. 2). The lowest diatom abundance isobtained in the transition from glacial diamict to hemipelagic deposition whereas thehighest abundance is encountered in laminae and laminated sequences of the sedimentcolumn. Laminae have been observed in many Antarctic shelf sediment cores, wherethey are considered likely to form during single settling events from massive blooms(Pudsey, 1990; Jordan et al., 1991; Leventer et al., 1993, 1996). The downcorevariability in diatom abundance, relative abundance of Chaetoceros rs, and ratio between

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Chaetoceros rs and ´veg. cells` indicate five events, related to climate and environmentalchange (Fig. 2). The events are listed below starting from the bottom of the record.

Fig. 2. The figure shows diatom abundance, in valves/gds x106, and downcore distribution ofdiatom assemblages at Site 1098. Diatom assemblages unrelated to each other or the otherdefined assemblages are referred to as outliers. Climate events are given according tosedimentological parameters in Paper II.

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1) The lowermost occurrence of diatoms at ~47.5 mcd is the first indication of openwater; low relative abundance of Chaetoceros rs and a low ratio indicate unstableclimatic conditions and weak surface water stratification.

2) Average to low diatom abundance, 5x108 v/gds at ~43-25 mcd, and high relativeabundance of Chaetoceros rs. (90%) indicate relatively stable climate conditionsduring a period of the season, the ratio is low suggesting continued weak surfacewater stratification.

3) High average diatom abundance, ~109 v/gds and high ratio are indicative of stablewater conditions and strong stratification from ~25 to 10 mcd. A low relativeabundance of Chaetoceros rs suggests other factors to influence the diatomcomposition.

4) Average diatom abundance and average ratio suggest 'normal' conditions from ~10to 4 mcd; a continued low Chaetoceros rs abundance indicates unstable conditions.

5) The lowest average diatom abundance, 1x108 v/gds, is encountered in the core top,4-0 mcd together with a low ratio indicate unstable conditions and weakstratification. The high relative abundance of Chaetoceos rs, ~75%, suggest highproductivity during a short interval of the season.

As the extreme high abundance of Chaetoceros rs is likely to represent only a short periodof time, a second approach was applied to analyze the diatom record representing theremaining part of the growth season, the so called ´non-Chaetoceros` assemblage (PaperIV). It has been suggested that distribution of different morphotypes of the same speciesmay indicate environmental stress (Fryxell, 1994; Taylor et al., 2001). In the non-Chaetoceros count it was decided to differentiate between morphotypes of T. antarctica(T1 and T2 (Plate 2)) and E. antarctica (var. recta, var. antarctica, summer or wintergrowth stage and ´end of chain`) (Plates 3a-c). Statistical treatment of the non-Chaetocerosassemblage resulted in five different cluser groups, each dominated and characterized byspecific species and interpreted as climate events (Fig. 2, Table 2).

Table 2. Cluster groups from the non-Chaetoceros assemblageClusterGroup

Characteristic species Dominating species Interpreted climate

1 Cocconeis spp. Fragilariopsis curta,F. kerguelensis,T. antarctica

Increased wind and sea iceduration.

2 F. kerguelensis,F. separanda,Eucampia antarctica var. antarctica

F. curta,F. kerguelensis

Warm, open water productivity.

3 Rhizosolenia spp.E. antarctica var recta,F ritscherii

T. antarctica,F. curta

Turbulent water, ice breakup,and late season productivity.

4 Corethron criophilum,F. cylindrus,Pseudo-nitzschia turgiduloides

F. curta,F. cylindrus

Early season productivity,varying conditions.

5 Thalassiosira antarctica T. antarctica, F.curta Ice breakup and cold conditions.

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Notes on ecology and assemblagesCluster groups 2 through 4, from the non-Chaetoceros assemblage, comprise species andspecies combinations that are not common today

Certain diatom species are known to form massive blooms with subsequent formation oflaminae in the sediment record. The most common species to form laminae in presentday Antarctic shelf sediment are Chaetoceros rs, Corethron criophilum, andRhizosolenia spp. In our investigation we encountered laminae with high relativeabundance of E. antarctica var. antarctica. The presence of these is rather unusual.Cluster group 4 is encountered in sediment sections with C. criophilum dominatedlaminae.

The finding of distinctly different distribution of the two varieties of E. anatrctica, var.antarctica and var. recta, supports the hypothesis that Eucampia is not wholly sea iceassociated (Zielinski and Gersonde, 1997), but E. antarctica var. antarctica is a sub-polar form associated with open water, and var. recta is a polar form associated with seaice patterns (Fryxell and Prasad, 1990; Kaczmarska et al., 1993).

The ecology of Fragilariopsis vanheurckii, significantly abundant in cluster group 5, isnot well established. This species is rarely reported from present day diatomassemblages. Based on the occurrence of F. vanheurckii, together with sea ice and coldwater associated species, it was considered that F. vanheurckii is indicative of sea ice.

Obtaining a chronology (Paper II)A well constrained chronology is essential when comparing different climate records andclimate proxies. A number of challenges are involved in radiocarbon dating of Antarcticshelf sediment including the absence of terrestrial macrofossils, calcite shells(foraminifera) are usually not preserved in the sediment, the concentration of organiccarbon is low, ~1% (Barker et al., 1999), and the marine reservoir effect is highlyvariable as is the age of surface sediments (e. g. Licht et al., 1996; Andrews et al., 1999;Domack et al., 1999; Licht, 1999). Accordingly, carefully selected bulk samples are themost common way, and sometimes the only way, to obtain radiocarbon dates.

The chronology developed for the Palmer Deep is based on bulk sediment samples, theacid insoluble fraction. Foraminifera have been used in a few instances when possible.The marine reservoir effect has been set to 1260 years based on a high number of surfacesamples and from calcite of present day organisms. The age model for the upper 25meters is a third-order polynomial with the best fit to both the obtained dates andsediment properties. Linear trends are used between turbidites in the the lower part of thecore. The sediment properties and the dates suggest a near continous deposition since 13cal. ka BP with the exception a hiatus of ~1000 years in connection with turbidite 2 (at~33 mcd). This is the longest and best constrained dated sediment record from theAntarctic shelf at present, and the data set correlates well with shorter cores collectedfrom the same area (Leventer et al., 1996).

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Comparing different climate proxies

The timescale and the nomenclature developed in Paper II is used throughout thediscussion for consistancy. The climate events are interpreted as deglaciation followedby a climate reversal, a bifurcated climatic optimum and a late Holocene period ofunstable and cooling conditions, referred to as Neoglacial (Fig. 2). In the two diatomdata sets (Papers III and IV) and sediment data (Paper II) the same major climate eventsare recognized, but the exact timing between these do not precisely correspond (Fig. 3).The difference in timing of the events can partly be explained by 1) the presence ofunusual diatom assemblages that are difficult to interpret, 2) diatom productivity andpreservation is not only dependent on climate but also sediment chemistry, etc., and 3)transitional periods and gradual changes.

At ca 13.2 cal. kyr BP the first signs of diatom productivity appear in the sediment recordat the transition from diamicton to silty mud. The deglaciation phase shows a transitionfrom glacial setting to ice breakup, melting, and finally open water conditions. In thisphase, a section of cyclic lamination indicate ice melting and strong stratification of thewater column (McAndrews et al., submitted). In the Ross Sea, open water was presentalong the Victoria Land coast, where the oldest abandoned penguin rookeries date to thisperiod (Baroni and Orombelli, 1994) (Fig. 3).

The following climate event is the ’climate reversal’, ca 11.5-9 cal. kyr BP. Theinterpretation of this interval as a climate reversal is based on sedimentological data(Paper II). The diatom data from this phase might be interpreted in different ways; asdeglaciation of Gerlache Strait, a change in surface water circulation, or limitation ofmeltwater supply. The interpretation as a change in surface water circulation anddeglaciation of Gerlache Strait is favored in the diatom record. Further south, along theAntarctic Peninsula, the Lallemand Fjord is still in a deglaciation stage (Shevenell et al.,1996; Taylor et al., 2001), as is Terra Nova Bay in the Ross Sea (Cunningham et al.,1999) (Fig. 3).

The climatic optimum, ca 9-4 cal. kyr BP, is distinct in all three Palmer Deep records(Papers II, III and IV). Both diatom abundance and diatom assemblages indicate a warmperiod interrupted by a cooler period. An intrusion of northern surface water is evidentthrough the presence of the variety of Eucampia antarctica associated with subpolarwater masses, in the early stage (Optimum I in Fig. 5). This species occurs in distinctlaminae (Barkoukis and Leventer, in press). The interval of high abundance of E.antarctica var. antarctica gradually disappears and a short period of reduced primaryproductivity takes place. A second peak in productivity then grades into the Neoglacial.This brief interruption in high primary productivity is not visible in the magneticsusceptibility (MS) record (Paper II). A bifurcation of the climate optimum is also seenin the Lallemand Fjord diatom record (Taylor et al., 2001). Other circum-Antarcticrecords show a warm period that corresponds to the later stage (optimum II) in thePalmer Deep (Fig. 3; Birnie, 1990; Björck et al., 1991; Baroni and Orombelli 1994;Björck et al., 1996; Shevenell et al., 1996; Cunningham et al., 1999).

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) Pap

er IV

, (2)

Pap

er II

I, (3

) Pap

er II

, (4)

Tay

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t al.,

200

1, (5

) She

vene

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al.,

199

6, (6

) Cun

ning

ham

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l., 1

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Bar

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, 199

4, (8

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n et

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(10)

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(11)

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(12)

Birn

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.

17

The Neoglacial and late Holocene, ca 4 cal. kyr BP to present, is also recognized in allthree Palmer Deep records. The diatom records indicate unstable climate conditions untilthe late Holocene when a steady decrease in diatom abundance and a continuous recordof sea ice diatom assemblage dominates. There is no evidence of the little ice age (LIA)or the warming over the last decades in the diatom records, although recognized in thesediment record (Paper II). Most of the circum-Antarctic records suggest a coolingperiod during the early Neoglacial (Fig. 3; Birnie, 1990; Björck et al., 1991; Baroni andOrombelli 1994; Björck et al., 1996; Shevenell et al., 1996; Cunningham et al., 1999;Taylor et al., 2001). Still, some records indicate a warming trend over the last thousandyears, when the Palmer Deep was experiencing a cooling (Björck et al., 1991; Björck etal., 1996; Yoon et al., 2000).

Final remarksSince the end of the last glacial several climatic shifts are recognized in the Palmer Deepsediment record. The findings compliment ice core and lake sediment records from theAntarctic Peninsula, which add to the increasing amount of evidence that the Holocenehas been a period of rapid and variable climate fluctuation. Comparing different climaticrecords obtained from West Antarctica and the South Shetland Islands show that theclimate records vary greatly between sites, and that there is variation between marine andterrestrial cores. The marine records are generally in good accordance with each other.One of the major difficulties encountered when trying to compare records is the validityof radiocarbon dates.

Some major climate events such as the Younger Dryas cold event/Antarctic ColdReversal (YD/ACR) and the Little Ice Age (LIA) are not recognized in the diatom recordof Palmer Deep. A halting of the warming at the end of the last glacial is recognized inAntarctic ice-core records, (Blunier et al., 1997; Steig et al., 1998). In Palmer Deep it isspeculated that there may have been intrusions of open ocean water in the deglaciationphase, at a time that corresponds to the end of ACR/YD, but this is a question that cannotbe resolved based on only one sediment core record. Results from pollen investigationsof lake sediment from Southern Chile lead to the conclusion that there was little or nocooling in the Southern Pacific during the Younger Dryas chronozone (Bennett et al.,2000). The LIA and recent warming cannot be identified in the diatom records fromPalmer Deep, although evidence for the Little Ice Age has been identified previously inmarine sediments from the Palmer Deep (Domack and Mayewski, 1999; Paper II) andLallemand Fjord (Shevenell et al., 1996).

In the few records spanning the deglaciation (Lallemand Fjord and Ross Sea) Eucampiaantarctica var recta has been an important and common species in the transition betweenglacial and glaci-marine sediment (Cunningham et al., 1999; Taylor et al., 2001). In thePalmer Deep, E. antarctica var. recta is more common in the climate reversal,suggesting that different oceanic and physical settings prevailed during deglaciation atthese sites.

18

Recognizing sediment units, Ross Sea

The shelf sediment from the western Antarctic Peninsula is almost devoid of reworkeddiatoms from older strata. The grounded ice sheet of the last glacial scraped off nearly allsoft sediments, previously deposited. The shelf sediments of the Ross Sea (Figs. 1 and 4)are more suited to study the transition from glacial to interglacial and the interactionbetween sediment and ice sheet.

Geochemisty (Paper V)The results from geochemistry are compared to Continental Crust Average (CCA) andPost Archean Average Shale (PAAS) standards and evaluated through concentration,ratio, excess and linear correlation between elements. Silica (SiO2) is the most abundantelement, with the concentration above CCA and PAAS. The amount of SiO2 does notvary significantly downcore or between cores, despite a variation in sediment texture anddiatom abundance. Silica is derived from multiple sources which include biogenic silica,crystalline quartz, and silicate minerals. The silica excess indicates that the production ofbiogenic silica has been high through time and that it is an important constituent of thesediment and its properties. This compares favorably with the suggestion of Tulaczyk etal. (1998) that even diatom-poor subglacial diamictons from Upstream B are derivedfrom marine-dominated sediments.

The ratio of aluminum (Al) and titanium (Ti), indicating terrigenous input, is in the rangeof CCA and PAAS standards, still it is found that for example Al and iron (Fe) oxideconcentrations are below the average standard concentrations. Similar trends were alsonoted by Frakes (1975). The results suggest that the high silica concentration has adiluting effect, resulting in low concentration of other major and trace elements. Despitethe dilution effect, some trace elements stand out with high concentrations and excessvalues, especially barium (Ba) and zinc (Zn) both of which can be related to diatomprimary productivity (Paytane and Kastner, 1996; Pfeifer et al., 2001). The correlationcoefficient between elements for the total data set is low with a few exceptions, Al and Tiand Al and Fe show correlation above 0.8. Using the data from each core separately forcorrelation show that in KC39 (WRS) there is high corelation values between mostelements, whereas cores from the central Ross Sea have weak correlation values.

Texture (Paper V)The grain size data confirm the fine-grained nature of the sediment, grains >128 m arerare, and the fraction < 4 m always exceeds 12%, with an average of 27%. Most samplescharacterized as diatomaceous mud cluster along the 16- 32 and 32-64µm vectors inPCA plots, a size range that includes most common diatom valves. Samples collectedfrom 'gray mud' are widely scattered in size range. TC18 is unique among these cores,where all samples have a grain size distribution of nearly 100% <16 m. Core sectionsdominated by diamicton show a well homogenized and mixed grain size distribution.

Diatom distribution (Paper VI)Robust taxa, most likely to survive glacial transport, for example Paralia spp. andStephanopyxis spp., are the most ubiquious and abundant in Ross Sea sediment. These

19

Fig. 4. Map of the Ross Sea with coring sites, including core logs according to Domack et al.,(1999). Bathymetry and elevation is in meters, core depth in centimeters.

.

20

include extant species, but in the Ross Sea they represent reworking from Teritary strata.Paralia spp. for example, provide between 1 and 30 % of the diatom assemblage indiatomaceous mud, and 10 to 55% in the diamictons. The occurrence of ebridians andchrysophyte cysts, non-diatom siliceous microfossils, vary greatly between sites. Allsediments include presence of diatoms characteristic of the Quaternary and post-glacialtime (e.g. Thalassiosira antarctica, T. gracilis, and Fragilariopsis curta), althoughspecimens are rare in diamict units.

Reduction of the data by PCA, is used for both chemical and diatom data as a mean ofcomparing and presenting data sets. Three major groups and one subgroup are identifiedfor diatom data, referred to as G1a (post-glacial), G1b (post-glacial), G2 (mixed openocean assemblage) and G3 (mixed coastal assemblage) (Fig. 5). The chemical data isdivided into three groups, one group characterized by Ba, the second by Zn and the thirdcomprises a mixture of elements and is unspecific (Fig. 5).

Sediment facies distribution (Papers V and VI)

Diatomaceous mudDiatomaceous mud, defined and C-14 dated as post-glacial (Domack et al., 1999; Licht1999), is clearly separated from mud and diamicton through chemical, texture anddiatom analyses. Holocene diatomaceous mud is typically enriched in Ba, and texturallythe grains size range of 16-64 m is characteristic. The diatom abundance is in the samerange as other post-glacial high productivity areas of the West Antarctic and the RossSea (Paper II; Leventer et al., 1993; Leventer et al., 1996). The diatom assemblage isdominated by a typical post-glacial flora (G1) and Ba (Fig. 5). Post-glacial deposition atsites located in the Challenger Basin (Fig. 4) show reduced diatom abundance, mixeddiatom assemblages, and reduced concentration of Ba, indicating bottom transport andwinnowing.

MudMud in the Ross Sea cores is a loosely defined sediment type, varying in color and siltcontent, often referred to as mud, gray mud (used here) or transitional mud. Gray mudsequences generally display reduced diatom abundance, similar to the transition betweenglacial and interglacial (deglaciation) in Site 1098 (Paper III). Gray mud samples arecommonly enriched in Zn and corresponds to all diatom assemblages, probablydepending on mode of deposition (Fig. 5). The elevated concentration of Zn is related torelease of Zn during decay of diatoms and a subsequent adsorption to clay.

Both TC33 and TC18 include extended sequences of mud but there are few similaritiesin texture, diatom flora and abundance (Fig. 5), interpreted as a result of differentdepositional processes. TC33 is collected from the outer continental shelf area. Thediatom assemblage sucession shows a period of sediment mixing (G3) followed by aperiod of sea ice break-up (G1b), and finally with open ocean hemipelagic mud at the top(G1a) (Fig. 5). This succession is interpreted as reflecting the retreat of an ice shelf at asite not affected by grounded ice, a result that corresponds to the suggestion of Shipp etal., (1999). TC 18, on the other hand, comprises reworked diatom assemblages,

21

Fig.

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22

dominated by fragmented diatom valves. The setting is thought to represent adeglaciation/liftoff phase (Domack et al., 1999). The result suggests rapid deposition,perhaps through a sediment laden melt-water plume.

DiamictonDiamicton is by definition a mixed sediment facies of unknown genesis. Chemically, thediamicton appears as thoroughly homogenized, there is no difference between sites. Themicrofossil record reveals a complex distribution of assemblages at different sites,diatom groups G1b, G2 and G3 are all corresponding to sediment units with 'unspecific'chemistry (Fig. 5).

The Pennell Bank (site PC31) (Fig. 1) has a higher diatom and fragment abundancecompared to remaining sites. The diatom assemblage is mixed coastal (G3) with a highabundance of Miocene stratigraphic markers and little stratigraphic mixing. Similarrecords have been described from Crary Ice Rise (CIR), a pinning point at present, andfrom other sub-ice shelf sites (Scherer et al., 1988; Harwood et al., 1989). The resultssuggest that the Pennell Bank was a sub ice-shelf site during the last glaciation,experiencing predominantly local reworking.

The data from Challenger Basin indicate pervasive reworking through highly damageddiatom valves, low diatom abundance and strong stratigraphic mixing. Still there is aclear shift in diatom assemblages between inner shelf sites (TC13 through 18, Fig. 4)with G2 open-ocean type, and the outer shelf sites (TC/PC11 and TC33, Fig. 4)displaying a G3 coastal type (Fig. 5). The change in assemblages does not appear to berelated to sea floor morphology, although the cores might not be deep enough to reflectthe sediment creating the bed-forms.

These different diamicton sediment units may have formed during the same broad timeinterval, as they all contain late Quaternary diatom species. The differences seen betweensites suggest that the sediment is derived from different source areas, or the mode andlength of transport had altered the assemblages, and that exposure to corrosive agentsmight have varied through time.

Final remarksThe broad-brush approach used here shows that microfossil data from the reworkedsediment facies of the Ross Sea, together with chemical data, provide a wealth ofinformation that can be used to distinguish sediment units. Mud appears to be depositedin several different settings; as hemipelagic mud recognized in core tops in theChallenger Basin, in the transition between sub ice-sheet/grounding line deposition (e.g.TC18) and finally as a deposition close to the ice shelf edge (e. g. TC33). Diatomassemblages, different diatom fragment abundances and chemistry can distinguish thesesettings.

Based on diatom assemblages and abundance, three major groups of diamicton aredefined. A diamicton characterized by well-preserved diatoms with relatively unmixedage stratigraphic markers is interpreted as deposited by slow moving ice (PC31). A

23

second group is characterized by thorough reworking, and an assemblage of robustcoastal taxa, possibly a result of sub-glacial transport (TC/PC11). The last group istypically enriched in open ocean taxa, and a variety of stratigraphic ages (TC13, TC16).The chemical data indicate that the CRS receives sediment from diverse sources whereasthe WRS sediment has a more well defined source.

MEETING THE OBJECTIVES

a) Using diatom abundance and the relative abundance of Chaetoceros rs and the ratioof Chaetoceros rs versus 'vegetative cells' as proxies for climate change proved tobe useful.

Changes in diatom abundance are consistent with changes in sediment propertiesand diatom assemblages.

The relative abundance of Chaetoceros rs showed, rather surprisingly, thatextensive periods of high relative abundance coincide with periods of reducedproductivity and the lowest Chaetoceros rs abundance is encountered in theclimatic optimum. The conclusion is that relative abundance cannot be used,without complementary data, as a climate proxy.

The ratio of Chaetoceros rs versus 'vegetative cells' is highest in the climaticoptimum, an indication of strong surface water stratification; the lower ratios arefound during periods of turbulent water masses. These results agree with previousinvestigations, although the so-called 'vegetative cells' might be Chaetoceors rs of adifferent morphology.

b) Obtaining a detailed climate and environmental record from the non-Chaetocerosassemblage was successful. The non-Chaetoceros record reveals detailscorroborating diatom abundance and sedimentological data. Limited knowledge ofecology and distribution, as well as unusual assemblages, made interpretationdifficult.

The differentiation between different varieties of Eucampia antarctica proved to beuseful, giving information of ecology and separating sediment units.

c) The test of using chemical parameters to characterize different sediment unitsproved useful to a certain degree but needs to be refined. We now know whichparameters are the best indicators and what to add for future projects.

d) Diatom assemblages proved to be a very useful tool for distinguishing betweensediment units. Several diatom assemblage were identified and a spatial distributionrecognized.

24

Implications and future work

It is shown that diatom data provide useful information of changing marine conditions onthe shelf in relation to melting of sea ice, ice shelf breakup, and turbulent water masses.This thesis shows only one core spanning the entire post-glacial period. If more data arecompiled, meaningful comparisons with ice core records can be made to obtain a morecomplete post-glacial climate history. It also may help in future comparisons between theSouthern and Northern hemispheres, and for recognizing possible leads and lags ofglaciation.

During the work with the Palmer Deep diatom data we recognized the need to gain moreinformation and knowledge regarding diatom distribution and ecology in the Antarctic.Correlation between different proxies is a difficult task requiring a well constrainedchronology. All these are questions that are continously addressed.

A pilot attempt to characterize glacial and glacialmarine sediment facies of the Ross Seaby using diatom data together with geochemical data clearly show the potential of theseparameters. The broad approach proved useful for distinguishing between mixedsediment units in relation to deposition and ice stream-sediment interaction, but there isroom for refinement in analyses and treatment of data.

Future work in progress includes comparing diatom data from the Ross Sea with resultsfrom controlled experiments shearing diatomaceous sediments (simulation of icestreaming). As there is an apparent difference in sedimentological parameters betweenthe Western Ross Sea, the Central Ross Sea and the Pennell Bank, similar investigationshould be performed on cores from Eastern Ross Sea to identify changes across thewhole Ross Sea Embayment. Chemical investigations should be extended to includebiogenic silica (although methods are not optimized for fossil diatoms) and organiccarbon.

ACKNOWLEDGEMENTS

My supervisor, Reed Scherer, introduced me to the challenging world of diatoms,Antarctic geology and to the Ocean Drilling Program (ODP). I started off my work withhis never-ending enthusiasm and the support from late Lars-König Königsson andGonzalo Vidal. Ala Aldahan, who became assistant supervisor, has provided a lot ofsupport and showed a lot of patience when most needed. Financial support was given bymy supervisors, NFR and Ymer-80.

Meeting fellow students and researchers at conferences, workshops, participation inODP, and visits at different universities has been a great stimulation and inspiration and Iwant to thank all of you. Especially I want to thank Fiona Taylor, Amy Leventer, EugeneDomack and all the other members of the 'Palmer Deep group' for comments onmanuscripts, discussions and collaboration, Olafur Ingólfsson for inviting me toINSTAAR, Kathy Licht, Julia Smith Wellner and Anjana Khatwa for fruitfuldiscussions. I am grateful to the crew and scientists on ODP Leg 178 who provided me

25

with research material. Melissa Lencewski has been a great help and a very generousfriend during my visits at NIU. Ross Powell and Leanne Armand have been helpful withdiscussions and comments on manuscripts.

I want to thank Keith Bennett and the staff at Quaternary Geology program. The support,encouragement and friendship given by colleagues and friends at the department havebeen invaluable, and you receive my greatest gratitude.

Finally, I want to thank my family and friends from 'outside of the academic world',your care, understanding and link to reality have been very important.

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