Dissertations / Theses on the topic 'Marine Ice sheet'

This study presents a stratigraphic investigation of the marine sediment core MD04-2822 from the Rockall Trough (56° 50.54' N, 11° 22.96' W; 2344 m water depth). This core is currently the only available high resolution record for the calibration of Late Quaternary sedimentary sequences of the British (Hebridean) margin. It therefore offers an unprecedented archive of changing sedimentological and climatological conditions for the last 175,000 years. The high resolution, multi-proxy records have enabled surface and deep water conditions within the Rockall Trough to be reconstructed. In addition, the fluctuating nature of ice-rafted debris (IRD) inputs to the MD04-2822 site allows a first order attempt of BIS dynamics for the entirety of the last glacial period (i.e. from the demise of the last interglacial to the decay of the Devensian/Weichselian ice sheet) as well as the majority of the penultimate (Saalian/MIS 6) glaciation. Sediment core MD04-2822 is ideally located to capture the dynamics of the British Ice Sheet (BIS) via a continuous record of IRD and fine-grained terrigenous inputs. Fundamental to this is the construction of a robust chronology. This was achieved via: the correlation of the benthic δ¹⁸O record to a global δ¹⁸O stack (SPECMAP); the correlation of the surface proxies (% N. pachyderma (sinistral) and XRF Ca) to the Greenland δ¹⁸O and Antarctic methane ice core records; and radiocarbon dating. This chronology was validated using both radiocarbon dating and tephra horizons. An evaluation of the event stratigraphy approach used in the construction of the MD04-2822 chronology is presented. The marine record provides a valuable archive of past ice sheet dynamics as much terrestrial evidence is removed or obscured by subsequent ice sheet oscillations MD04-2822 provides the first evidence for the expansion of the BIS onto the Hebridean Margin during MIS6 (thereby confirming previous long-range seismic correlations). The continuous sedimentation at MD04-2822 enabled the first insights into the early dynamics of the last BIS. Increases in IRD and fine grained terrigenous material delivered to the MD04-2822 at ca. 72 kyr represent the first significant delivery of material from the BIS across the continental shelf to the core site. The BIS would therefore have attained a marine calving margin by this time. A multi-proxy investigation of provenance was undertaken, however unequivocal provenance determinations remain problematic. The location of the core suggest the proximal BIS as the most likely source of terrigenous inputs. The expanded nature of the MD04-2822 sediments during the penultimate deglacial (Termination II) provides the first details of BIS dynamics for this period: the interplay of large inputs of freshwater from the decay of the Saalian (MIS 6) ice sheets (including the BIS) upon the surface and deep water circulation of the North Atlantic is investigated. In addition, sub-orbital climatic variability is documented at this location throughout the last interglacial (MIS 5e) and appears to be an intrinsic feature of both the N.E. Atlantic surface and deep water circulation of the last 175 kyr.

La calotte marine de l'Antarctique de l'Ouest présente la particularité d'être en grande partie en contact avec l'océan. Les dernières observations révèlent une accélération de sa perte de masse sur les dernières décennies, essentiellement provoquée par l'augmentation de la fonte sous les plateformes de glace flottante. En revanche, son évolution future reste très incertaine, du fait de notre mauvaise compréhension des processus physiques mis en jeu entre la calotte et l'océan.La dernière déglaciation (-21 000 - -11 000 ans), constitue l'un des changements climatiques majeurs les plus récents de notre histoire. Cette période est marquée par une augmentation des températures atmosphériques globales et la disparition des calottes nord-américaine et eurasienne. L'étude de la calotte marine de Barents-Kara (BKIS), qui couvrait les mers de Barents et de Kara au Dernier Maximum Glaciaire (DMG, -21 000 ans) et faisait partie intégrante de la calotte eurasienne, revêt un intérêt particulier en raison de ses caractéristiques communes avec l'Antarctique de l'Ouest actuel. Identifier les mécanismes responsables de son recul permet de fournir des informations pour mieux comprendre le comportement de l'Antarctique de l'Ouest dans des contextes climatiques actuel et futur.L'impact du climat sur l'évolution d'une calotte marine dépend de deux processus principaux : le bilan de masse de surface, influencé par les températures atmosphériques et précipitations, ainsi que la fonte sous la glace flottante, liée aux températures océaniques et la salinité. Pour identifier les mécanismes ayant initié la fonte de BKIS, j'ai utilisé le modèle de glace GRISLI2.0 afin d'analyser la réponse de cette calotte à des perturbations du climat au DMG. Cette étude a mis en évidence le rôle déterminant des températures atmosphériques dans le déclenchement de la fonte de la calotte via la fonte de surface, tandis que les températures océaniques n'ont eu qu'un impact limité malgré une grande partie de la calotte BKIS en contact avec l'océan. J'ai aussi identifié que la fonte totale BKIS pouvait être attribuée à une instabilité mécanique à la ligne d'échouage, provoquée par une diminution de l'épaisseur de glace dû à une augmentation de la fonte de surface. Afin de mieux comprendre l'impact des calottes sur le climat global, j'ai également réalisé la première simulation transitoire de la dernière déglaciation avec le modèle IPSL-CM5A2 en modifiant à certaines périodes clés la géométrie des calottes de glace donnée par la reconstruction GLAC-1D. Les simulations montrent une tendance du réchauffement en accord avec les reconstructions, notamment lors du MWP1A caractérisé par une augmentation abrupte des températures atmosphériques. A partir d'expériences de sensibilité, j'ai mis en évidence que les changements de géométrie des calottes glaciaires ont participé à l'augmentation des températures atmosphérique via les rétroactions température-altitude et l'effet d'albédo. Par ailleurs, j'ai aussi montré que la dynamique océanique a été notablement perturbée par les flux d'eau douce issus de la fonte des calottes. Ce phénomène a conduit à une atténuation de l'intensité de la circulation méridienne de retournement de l'Atlantique et à une réduction de sa profondeur de plongée, entraînant un ralentissement du réchauffement, principalement dans l'Atlantique Nord. De plus, les expériences IPSL-CM5A2 simulent toutes un arrêt de la circulation des eaux de fond antarctiques au début du MWP1A, entraînant un refroidissement significatif d'une centaine d'années dans la mer d'Amundsen, suivi d'une réactivation de cette même circulation. Ces travaux contribuent ainsi à une meilleure compréhension des mécanismes complexes régissant la dynamique des calottes glaciaires et de leur interaction avec le climat, tout en offrant des éléments de réponse pour anticiper les conséquences des changements climatiques actuels et futurs, notamment en ce qui concerne l'Antarctique de l'Ouest<br>The marine West Antarctic ice sheet is characterized by being largely in contact with the ocean. The latest observations reveal an acceleration in its mass loss over the last few decades, mainly due to increased melting under floating ice shelves. However, its future evolution remains highly uncertain, due to our poor understanding of the physical processes at play between the ice sheet and the ocean.The last deglaciation (21 ka-11 ka) is one of the most recent major climatic changes in our history. This period is marked by an increase in global atmospheric temperatures and the melting of the North American and Eurasian ice sheets. The study of the Barents-Kara Ice Sheet (BKIS), which covered the Barents and Kara Seas during the Last Glacial Maximum (LGM, 21 ka) and was an integral part of the Eurasian Ice Sheet, is of particular interest because of its common features with present-day West Antarctica. Identifying the mechanisms responsible for its retreat allows to provide information to better understand the West Antarctic behavior within under present and future climatic conditions.The impact of climate on the evolution of a marine ice sheet depends on two main processes: The surface mass balance, depending on atmospheric temperatures and precipitation, and melting under floating ice, related to oceanic temperatures and salinity. In order to identify the mechanisms triggering the BKIS retreat, I used the GRISLI2.0 ice-sheet model to analyse the ice-sheet response to climate perturbations at the LGM. This study highlighted the key role of atmospheric temperatures in triggering the melting of the ice sheet via surface melting, while ocean temperatures had only a limited impact despite a large part of BKIS being in contact with the ocean. I also identified that the total retreat of BKIS could be attributed to a mechanical instability at the grounding line, caused by a decrease in ice thickness resulting from an increase in surface melting.In order to better understand the impact of ice sheets on the global climate, I have also carried out the first transient simulation of the last deglaciation with the IPSL-CM5A2 model, modifying the geometry of the ice sheets provided by the GLAC-1D reconstruction at some key periods. The simulations show a warming trend in line with the reconstructions, particularly during MWP1A, which was characterised by an abrupt rise in atmospheric temperatures. Using sensitivity experiments, I have shown that changes in the ice sheet geometry have contributed to the increase in atmospheric temperatures via temperature-altitude feedbacks and the albedo effect. Moreover, I have shown that ocean dynamics have been significantly altered by freshwater fluxes from the melting ice sheets. This has led to a weakening of the strength of the Atlantic Meridional Overturning Circulation and a reduction of its deepening, resulting in a warming slowdown, mainly located in the North Atlantic Ocean. In addition, the IPSL-CM5A2 experiments all simulate a shutdown of the Antarctic bottom water circulation at the onset of MWP1A, leading to a significant cooling of about 100 years in the Amundsen Sea, followed by a restart of this circulation.This work is contributing to a better understanding of the complex mechanisms governing the dynamics of the ice sheets and their interaction with the climate, while also providing a basis for anticipating the consequences of current and future climate change, particularly in West Antarctica

The continental margin in the area west of Shetland was subjected to repeated and extensive ice sheet advances during the Late Pleistocene. Seabed imagery, seismic survey and borehole core data show the Late Devensian ice sheets expanded across the continental shelf three times, two of these advances reaching the shelf edge. On the inner shelf, where present-day water depths are generally less than 100m, only thin sediments from the last retreat phase and exposed rock surfaces remain, all other deposits from earlier phases having been removed by the last advance. On the mid to outer shelf elements of all three phases are preserved, including lodgement and deformation tills, melt-out and water-lain till sheets, in-filled hollows left by stagnant ice decaying in situ and a series of large recessional and terminal moraines. In addition, there is evidence of shallow troughs and overdeepend basins which indicate preferential ice-drainage pathways across the shelf which were formerly occupied by ice streams. At the shelf edge, a thick wedge of glacigenic sediment forms a transition from the till sheets and moraines of the shelf to debris flows composed of glacigenic sediments on the upper slope. Shelf-edge moraines show an architecture indicating floating ice in modern water depths over approximately 180m, suggesting the West Shetland ice sheet was no more than about 250m thick. The upper and middle slope is dominated by glacigenic debris flows which are focused in the slope areas below the proposed ice stream discharges at the shelf edge. The mid-to-lower slope has been subjected to contour current activity which has re-worked much of the glacigenic sediment in this position. The lower slope and floor of the Faroe-Shetland Channel are marked by either large debris flow lobes of glacigenic sediment or thin glacimarine muds deposited from suspension. A conceptual model of the glacigenic development of a passive continental margin based upon the West Shetland example shows the deposited sequence for both advance and retreat phases of a glacial cycle, and the actual preserved sequence which might be expected in the rock record. The model also shows that ice sheet buoyancy, thickness, and to a lesser extent, basin subsidence, are the most important factors in the deposition and preservation of a glacially-influenced marine sequence.

Ce travail présente une analyse des environnements sédimentaires marins profonds des mers nordiques à l’interface de l’Atlantique nord et de l’Arctique (mers du Groenland, de Norvège, de Barents et d’Islande) au cours du dernier million d’années. Il se base sur une base de données acoustiques (bathymétrie, imagerie multifaisceau) et sédimentologique (carottes calypso) issues de deux campagnes réalisées par le Shom. Les enregistrements sédimentaires ont montré une très grande variabilité des processus de sédimentation en jeu dans ces mers en fonction des périodes climatiques, avec notamment une sédimentation glaciomarine, gravitaire, contouritique, et hémipélagique. Ils ont également permis de se concentrer sur la chronologie des périodes de développement ou de retrait des calottes continentales périphériques (calotte du Groenland, calotte Fennoscandie et calotte de Barents et Svalbard), et du couvert de glace de mer. Une étude stratigraphique détaillée a été réalisée sur la base de différents outils (datations radiocarbones, géochimie isotopique, géochimie élémentaire, biostratigraphie sur microfossiles et nannofossiles calcaires, et analyses sédimentologiques). La reconstitution de l’historique d’évolution des apports sédimentaires et des processus responsables de ces apports depuis le Quaternaire moyen (début de la Mid-Pleistocene Transition, MPT) jusqu’à l’Holocène terminal, a permis de mieux caractériser l’impact des variations d’extension des calottes continentales sur la sédimentation des mers nordiques, mais aussi d’identifier les périodes de forte influence du couvert de glace Arctique (y compris calotte potentielle en maxima glaciaire) sur les mers nordiques, et les variations de l'influence des courants de surface et de fond dans la zone nord et la zone sud de ces bassins boréaux. Notamment, les variations d’influences des eaux de surface chaudes et salées de l’Atlantique nord ont pu être identifiées pour certaines périodes de temps<br>This study focuses on the study of the Middle Pleistocene to Late Quaternary sedimentation patterns and palaeoenvironmental conditions of the Nordic seas (Barents, Iceland, Norwegian and Greenland seas), which mark the transition between the North Atlantic and the Arctic oceans. It is based upon a compilation of acoustic data (bathymetry, multibeam imagery) and sedimentological data (calypso piston cores) retrieved during two cruises leaded by the Shom institute. Sedimentary records showed a large variability of the sedimentary processes at play in those seas, depending of the climatic stages and, thus, of extension or decay conditions of the surrounding ice-sheets. Glacimarine, contouritic, hemipelagic and gravity sedimentary facies are recorded in those sedimentary archives. High resolution stratigraphy was obtained using a combination of radiocarbon datings, XRF geochemistry, oxygen isotopic data and biostratigraphy. This allowed to investigate the sedimentary inputs and processes occurring in those seas from the Middle Quaternary (the beginning of the Mid-Pleistocene Transition) to the Late Holocene. It also allowed a better characterization of the variation in the boreal ice-sheet extension, and to identify periods of Atlantic Waters influence over the core sites

The East Antarctic Ice Sheet (EAIS) retains the largest volume of ice on the planet and has the capacity to raise global sea level by a substantial 52 m. Marine-based sectors of the EAIS are particularly susceptible to retreat and collapse and are currently losing mass at an unprecedented rate. Masked by kilometres of ice and shielded by extensive sea-ice proximal to the coast, central Wilkes Land (between 105-128°E) is one of the most poorly investigated regions of the EAIS. The Totten Glacier, situated in a trench at the Sabrina Coast of central Wilkes Land, drains the largest portion of the EAIS and has one of the highest thinning rates in East Antarctica. Complete melting of the ice drained by the Totten Glacier alone is anticipated to contribute 3.5 m to global sea-level rise. With large portions of the ice sheet in central Wilkes Land grounded below sea-level, on a retrograde slope steepening inland from the coast to the interior basins, this part of the EAIS is sensitive to ocean-forced retreat and marine ice sheet instability, rendering it an important region in the context of global climate change. Understanding the response of the ice sheet to past climate variation is integral for forecasting its future behaviour and for identifying those parts of the ice sheet that are most vulnerable to collapse and retreat in a warming climate. Two high priority objectives in Antarctic paleoclimate research are to determine the principal drivers of ice sheet retreat and to establish the timing of regional deglaciation over the Last Glacial Period-Holocene transition (from c. 25 ka). Thus far, the factors driving ice sheet retreat at the coast of central Wilkes Land over the Last Glacial Period- Holocene transition are not well understood and the timing of the last deglaciation is poorly constrained. The lack of physical samples and the absence of detailed paleoclimate and sediment provenance records from central Wilkes Land provides strong motivation for the research conducted in this thesis. The principal aims of this thesis are: 1. to constrain the timing of the last deglaciation in central Wilkes Land, 2. to characterise the nature of ice sheet retreat, and 3. to uncover more about the age and composition of the concealed subglacial basement rocks. To accomplish these aims, four marine sediment Kasten cores recovered by the RV Investigator from the upper continental slope of the Sabrina Coast, central Wilkes Land, were investigated. A broad range of measurements and analyses were conducted on each of the cores to reconstruct the deglacial history of the ice sheet in central Wilkes Land. This thesis presents the first detailed, multi-proxy paleoclimate and sediment provenance records for offshore central Wilkes Land. The research provides constraints on the timing of the last deglaciation and insights into the paleoenvironmental conditions of this important region over the Last Glacial Period- Holocene transition. The first research chapter explores the response of the ice sheet to climate change over the Last Glacial Period-Holocene transition and the timing of the onset of the last deglaciation. Multiple proxies were measured from all four of the cores to assess changes in processes on and above the continental rise associated with variation in climate and ice sheet configuration. Age models for each core were determined using bulk acid-insoluble organic matter radiocarbon ages. Primary biological productivity was reconstructed using biogenic silica concentrations, diatom abundances and Si/Al and Ba/Al ratios from x-ray fluorescence (XRF) measurements. Continental slope sedimentation was investigated using linear sedimentation rates, the iceberg-rafted debris flux and particle size. Current speed was qualitatively assessed using the calculated sortable silt percent. The last deglaciation was identified by the rise in biological productivity, an increase in current speed and changes to sedimentation on the continental slope associated with a retreating ice sheet and reducing sea-ice conditions at the coast. The results indicate that deglaciation at the coast of central Wilkes Land was possibly the earliest in East Antarctica, initiating at some time between 22.0 ± 3.2 ka and 19.2 ± 0.6 ka. Prompt retreat in the central Wilkes Land region suggests that this part of the ice sheet is highly sensitive to climate change and may pose a larger threat to future global sealevel rise than other regions of the EAIS. The second research chapter investigates a source-to-sink history of sediment transport at one of the core sites via detrital zircon, apatite, titanite and feldspar analysis. Multiple single-grain sediment provenance tracers were employed including U-Pb and Pb-Pb geochronology, rare earth element geochemistry and grain morphology. Results reveal a predominantly proximal source with U-Pb detrital zircon age signatures unique to the interpreted Mesoproterozoic basement rock terranes of central Wilkes Land: the Wilkes, Nuyina and Banzare provinces. A dominant c. 1200-1100 Ma age peak was consistent in the U-Pb age spectra for all minerals analysed. Pb-Pb compositions of detrital feldspar grains match those of feldspars from nearby rare coastal outcrop at Balaena Islets and Chick Island in central Wilkes Land, further supporting a local source. The rare earth element geochemistry indicated primarily felsic granitoid source rock compositions. Grain morphological analysis and the abundance of detrital feldspar in all samples indicated short-distance transport. Subtle temporal changes in sediment provenance are attributed to variation in climate and ice sheet configuration. Exceptionally high sedimentation rates during the glacial suggest the downslope redistribution of continental shelf sediments in gravity flows as the ice sheet advanced. Fluxes of meltwater principally fed by the Totten Glacier are deemed responsible for supplying detritus to the continental slope during the last deglaciation. A broad sediment provenance is interpreted during interglacial periods, with detritus delivered to the slope via multiple glaciers along the coast. The results from this chapter provide the first substantial offshore physical evidence for the age and composition of the concealed subglacial geology of central Wilkes Land and support geophysical interpretations of the basement rock terranes and two models that predict the erosion potential at the base of the ice sheet. The third research chapter provides Nd-Sr isotopic signatures from all four cores to establish the combined detrital Nd-Sr fingerprint, trace the source rock terranes and investigate spatial variability in sediment provenance along the upper continental slope of central Wilkes Land. Detrital εNd signatures were compared with whole rock εNd signatures from boreholes recovered from the conjugate region of southern Australia, and from rare outcrop at the coast of central Wilkes Land and in southern Australia. The εNd signal revealed that mafic rocks of the Haig Cave Supersuite (c. 1415-1390 Ma) of the Nuyina Province must be contributing to the Nd-isotopic signature. This finding supports geophysical interpretations of the subglacial geology in central Wilkes Land and suggests that the Totten Glacier, underlain by the Nuyina Province, likely had a major role in the supply of detritus to the continental slope, not only during the initial stages of the last deglaciation, but throughout the Last Glacial Period-Holocene transition. The εNd signature was spatially consistent, suggesting similar sediment provenance across the continental rise of central Wilkes Land. The 87Sr/86Sr ratios had been affected to come extent by the grain size distribution in each of the cores, providing more insight into the recent history of the ice sheet. The rich physical data and findings in this thesis can be used to inform future paleoclimate and sediment provenance studies of central Wilkes Land, paleoclimate models, plate tectonic reconstructions and ice sheet models that forecast the future response of this important part of the ice sheet to a warming climate.

Sub-Milankovitch climate oscillations are well documented phenomena in the Gulf of Mexico during Marine Isotope Stage (MIS) 3 and Termination I, however very little is known about equivalent events during older time intervals. Basin 4 is located on the northwest slope of the Gulf of Mexico and has provided a detailed record of late MIS 6 and Termination II. The results of this study show that the delta18O record of planktonic foraminifer G. ruber contains millennial-scale climate oscillations during MIS 6, a series of meltwater spikes, and a climate reversal during Termination II. Paired delta18O -- Mg/Ca data across these events reveal that the unusually large amplitudes in the delta 18O record cannot be explained by sea surface temperature (SST) or ice volume, but rather are a response to isotopically light glacial meltwater from the paleo-Mississippi river. This conclusion supports the studies of similar oscillations during Termination I and MIS 3.

Inaccessibility due to harsh weather conditions and perennial sea ice has left the Weddell Sea embayment (WSE) vastly under-studied in comparison to other regions of Antarctica. Yet understanding its deglacial history since the Last Glacial Maximum (LGM) is vital for understanding the dynamics and stability of the Antarctic ice sheet. Additionally, the debate continues as to the magnitude and timing of West Antarctic Ice Sheet (WAIS), Antarctic Peninsula Ice Sheet (APIS) and East Antarctic Ice Sheet (EAIS) advance during the LGM. Here we present geologic and geophysical evidence from the southern and eastern continental shelves of the WSE that show diachronous retreat by the WAIS and EMS. Detailed analysis of sediment cores display a retreat stratigraphy in the WSE with distal glacimarine sediments overlying proximal glacimarine deposits and till. These results, in combination with AMS radiocarbon ages, demonstrate that the grounding line of the EAIS was very near that of present day as early as 30,476 cal yr BP and indicate little, if any, advance of the EMS during the LGM. In contrast, multibeam swath bathymetry data show mega-scale glacial lineations, indicative of grounded, flowing ice in two troughs on the southern continental shelf, which drain ice from the WAIS. Although there are no radiocarbon ages to absolutely constrain the timing of this grounding event on the southern continental shelf, we interpret the lineations as LGM age based on their pristine nature. Further, there are similar geomorphic features on the western continental margin where the LGM timing of APIS advance has been demonstrated. Thus, during the LGM, the Antarctic ice sheets behaved independently in the WSE.

Zooplankton from inshore marine and marine-derived lacustrine Antarctic habitats were studied over two summers and the intervening winter from December 1993 to March 1995 at two sites in the Vestfold Hills region, East Antarctica. Particular emphasis was placed on the interaction between fast ice and the underlying water column, and the effect of this on the ecology of dominant copepod species. The overwintering strategies of commonly found copepods were investigated. The sea ice habitat was characterised by high abundance and low diversity of metazoans. Paralabidocera antarctica dominated the metazoan assemblage, reaching densities of up to 500,000 individuals `m^(-2)`.Other taxa present included Drescheriella glacialis, unidentified harpacticoids, Stephos longipes and Ctenocalanus citer. Horizontal patchiness of the sympagic biota varied as much on scales of less than one metre as it did at scales of several kilometres. Metazoan density was not clearly correlated with chlorophyll concentration, salinity or particulate organic carbon. The zooplankton assemblage at the inshore marine site was numerically dominated by Oncaea curvata and Oithona similis throughout the sampling period. Diversity was highest in the summer when the break-out of the fast ice, coupled with the phytoplankton bloom, encouraged the development of meroplanktonic larvae of benthic species. Other copepod species present included Paralabidocera antarctica, Calanoides acutus, Ctenocalanus citer, Stephos longipes, and unidentified harpacticoids. Grazing impact by the copepod assemblage on primary productivity during the 1994-5 summer was consistently low, ranging between 1 and 5%. The life cycle of Paralabidocera antarctica was strongly associated with the growth and development of ice algae. Lipid storage by this species was predominantly in the form of triacylglycerols, indicating that copepods were feeding throughout the year. In contrast, Oithona similis and Oncaea curvata predominantly stored wax esters, and their life cycles were not linked strongly to the summer phytoplankton bloom. A lacustrine population of Paralabidocera antarctica was also found to store triacylglycerols, suggesting that the copepods were able to graze throughout the year. This species, the only planktonic metazoan consumer present in the lake, reached abundances of up to 35,000 `m^(-3)`. The life cycle of this population had become much less tightly regulated than at the coastal site, and specimens were rarely found living within the lake ice. The lack of predators and competitors, along with measurable quantities of phytoplankton present in the lake throughout the year, has resulted in the decoupling of the life cycle of this population from the growth cycle of the ice algae.

Present-day observations near Antarctica’s ice sheets suggest anthropogenic warming is affecting ocean circulation, with implications for further ice sheet melt, and changes to global thermohaline circulation. The mechanisms for ocean-ice sheet-sea ice changes are uncertain, and it is unclear how they will respond to warmer than present climates. Sediment records on the Antarctic continental margin provide evidence of significant changes with respect to productivity, ice sheet size and ocean circulation, on glacial to interglacial timescales. In this thesis, I describe the integrated response of the Adélie region of East Antarctica to past orbital forcing driven climate, including during the warmer than present Last Interglacial (Marine Isotope Stage 5e) to provide clues to its future response. I have studied five marine sediment cores from the continental shelf and slope of the Antarctic margin to understand the changes in regional oceanography, regional ice sheet size, seasonal sea ice and primary productivity to deduce the relationships between these components, during glacial cycles over the past >240 k yrs. I use an integrated approach employing sedimentological (grain size, structure, Ice Rafted Debris/ IRD), micropaleontological (diatom data) and geochemical (biogenic silica and X-ray fluorescence/ XRF data) proxies. First, in Chapter 3, I describe the last deglacial retreat of the ice sheet, from the outer continental shelf of the Mertz Trough in the Adélie region. Three facies are identified in core TAN1302-68 based on sedimentological, geochemical, and biogenic changes. Facies III comprises a pebble structure, with high coarse sand to granule count and a geochemically and texturally homogenous matrix, suggesting an ice sheet covered this site. Overlying is Facies IIa, a massive interval with IRD (~1mm sized dispersed grains) and slight changes in Fe and Fe/Ti. Above it is Facies IIb, a laminated interval with IRD, and significant sedimentological (comprising decrease in coarse sand to granule counts in comparison to III; and an increase in very fine to fine sand in the sediment matrix to 39%) and geochemical (Si/Al, Ba/Ti, Fe, Ti, Zr/Rb) changes. Facies IIa is suggestive of an ice shelf distal environment, while the rapid changes in Facies IIb, suggests an ice shelf calving zone. At the top is Facies I, a massively bedded, geochemically homogenous sediment, with a high very fine to fine sand fraction (39%) and increased biogenic silica, Si/Al, and Ba/Ti suggesting open ocean setting with higher productivity and active bottom currents. Based on radiocarbon dating and sedimentation rates of ~2.3 cm/k yr, the ice sheet retreated over this core site at <14 k yr, while ice shelf calving occurred between ~12-8 k yr, during which time, at ~10 k yr, stronger bottom currents developed on the shelf. I suggest the strong bottom current is likely the commencement of Dense Shelf Water formation in the Adélie region. Secondly, in Chapter 4, I describe bottom currents, ice sheet dynamics and productivity on the Adélie continental slope, from the MIS 7 interglacial to the Holocene. The data is based on four cores: two cores from 2,600 m (TAN1302-58; TAN1302-30) and two cores from 3,000 m depth (TAN1302-44; TAN1302-39), collected from the WEGA and G channels. I characterise four different facies, which form a pattern down core. Using visual logs, productivity data (Si/Al and Ba/Ti, and biogenic silica) and IRD counts I associate these facies with interglacial (Facies 1), glacial (Facies 2), deglacial (glacial retreat; Facies 2A) and glaciation (glacial advance; Facies 1A) climates. I suggest sediments in three cores are deposited by contour currents (thermohaline induced bottom currents), based on evidence of traction structures, gradation, and coarsening, and consistency of biogenic silica and IRD data. I relate these characteristics to records of Adélie Antarctic Bottom Water down slope flow, based upon changes in sediment matrix texture, related to the decrease in very fine to fine sand content down slope. Facies 1 suggests MIS 7, MIS 5e and Holocene interglacial environments are fairly similar, comprising strong bottom currents (very fine to fine sand up to 43%), active ice sheet retreat (IRD is 2-15 grains/g), and low to high productivity (biogenic silica is 4-22%). Facies 2 suggest MIS 4-2 and MIS 6 glacials featured a stable ice sheet (IRD is 2-4 grains/g), lower productivity (biogenic silica is 2-11%), and generally reduced bottom current strength (very fine to fine sand is 0-10%). However, MIS 6 contains multiple traction structures, while MIS 4-2 contains locally increased sand and biogenic silica, suggesting a different source of bottom current during MIS 6, and a locally greater velocity of bottom current occurred at times during MIS 4-2. Facies 1A, suggests MIS 5/4 glaciation comprised low to moderate productivity (4-11%), and locally strong bottom currents, as evidenced by increased sand fraction and traction structures on the upper slope. Facies 2A suggests MIS 2/1 and MIS 6/5 deglacial comprised low to moderate productivity (2-10%) and generally slower bottom currents. The features of the glaciation and deglacial facies suggest stable ice sheets at these times. The fourth core (TAN1302-39) is influenced by turbidity current and debris flow deposits, with some influence of contourite deposits especially at its base, where carbonate facies is found. Lastly, in Chapter 5, I describe diatom assemblages from core TAN1302-44, from the base of the WEGA channel, studying the section from MIS 6/5 deglacial (~140 k yr) to the Holocene. I find diatom assemblages vary on glacial to interglacial timescale, according to the facies described in Chapter 4, suggesting diatoms reflect glacial cycles, likely in relation to regional sea ice and oceanographic changes. Using Principal Component Analysis, I have identified three main assemblages. PC1 comprises open ocean and seasonal sea ice species (Thalassiosira antarctica, Thalassiosira lentiginosa, Actinocyclus actinochilus, Asteromphalus hyalinus, Thalassiosira sp 2, Eucampia antarctica, and Fragilariopsis kerguelensis). PC1 is associated with the interglacial facies and suggests seasonal sea ice paleoenvironments and nutrients are similar between MIS 5e and Holocene. However, the unusual abundance of Thalassiosira antarctica resting spore and Thalassiosira lentiginosa (up to 40-60%), suggests some reworking by bottom currents and dissolution has affected the preservation of this assemblage. PC 2 comprises sea ice and coastal species (Fragilariopsis obliquecostata, Rhizosolenia styliformis, Asteromphalus parvulus and Chaetoceros dichaeta). It is associated with the glacial facies, but also with glaciation and deglacial facies, suggesting MIS 4-2 glacial, MIS 6/5 and MIS 2/1 deglacial and MIS 5/4 glaciation (especially MIS4-2 glacial) exhibit increased length of sea ice season relative to interglacials, but not a permanent sea ice cover. The gradual increase of PC 2, in the glaciation and throughout the glacial facies, suggests the sea ice season length gradually builds up with cooling, and rapidly disintegrates with warming. Lastly, PC 3 comprises warmer water/nutrient rich species (Thalassiothrix antarctica, Chaetoceros bulbosum and Thalassiosira oestrupii), which is associated with the deglacial and glaciation facies, suggesting increased upwelling of a water mass, which I infer is the Circumpolar Deep Water. To my knowledge, this study is among a few to present evidence of glacial Antarctic Bottom Water production from the Adélie Land region of East Antarctica. Furthermore, I present the first integrated sedimentological and XRF/ geochemical data set describing the timing and initial rate of ice sheet retreat from the Antarctic margin, from the Mertz Trough. This is the first study to describe contourite traction structures within the Quaternary glacial facies comprising ripples, mud off shoots, flaser bedding, pebble lined laminae, from the Adélie region and the Antarctic margin, and among just a few studies to describe the diatom assemblages for the last glacial cycle from the Antarctic continental margin.

Glacial isostatic adjustment (GIA) is the process by which the solid Earth responds to past and present-day changes in glaciers, ice caps, and ice sheets. This thesis focuses on vertical crustal motion of the Earth caused by GIA, which is influenced by several factors including lithosphere thickness, mantle viscosity profile, and changes to the thickness and extent of surface ice. The viscosity of the mantle beneath Antarctica is a poorly constrained quantity due to the rarity of relative sea-level and heat flow observations. Other methods for obtaining a better-constrained mantle viscosity model must be investigated to obtain more accurate GIA model predictions. The first section of this study uses seismic wave tomography to determine mantle viscosity. By calculating the deviation of the P- and S-wave velocities relative to a reference Earth model (PREM), the viscosity can be determined. For Antarctica mantle viscosities obtained from S20A (Ekstrom and Dziewonski, 1998) seismic tomography in the asthenosphere range from 1016 Pa∙s to 1023 Pa∙s, with smaller viscosities beneath West Antarctica and higher viscosities beneath East Antarctica. This agrees with viscosity expectations based on findings from the Basin and Range area of North America, which is an analogue to the West Antarctic Rift System. Section two compares bedrock elevations in Antarctica to crustal thicknesses, to infer mantle temperatures and draw conclusions about mantle viscosity. Data from CRUST 2.0 (Bassin et al., 2000), BEDMAP (Lythe and Vaughan, 2001) and specific studies of crustal thickness in Antarctica were examined. It was found that the regions of Antarctica that are expected to have low viscosities agree with the hot mantle trend found by Hyndman (2010) while the regions expected to have high viscosity are in better agreement with the trend for cold mantle. Bevis et al. (2009) described new GPS observations of crustal uplift in Antarctica and compared the results to GIA model predictions, including IJ05 (Ivins and James, 2005). Here, we have generated IJ05 predictions for a three layered mantle (viscosities ranging over more than four orders of magnitude) and compared them to the GPS observations using a χ2 measure of goodness-of-fit. The IJ05 predictions that agree best with the Bevis et al. observations have a χ2 of 16, less than the null hypothesis value of 42. These large values for the best-fit model indicate the need for model revisions and/or that uncertainties are too optimistic. Equally important, the mantle viscosities of the best-fit models are much higher than expected for West Antarctica. The smallest χ2 values are found for an asthenosphere viscosity of 1021 Pa•s, transition zone viscosity of 1023 Pa∙s and lower mantle viscosity of 2 x 1023 Pa∙s, whereas the expected viscosity of the asthenosphere beneath West Antarctica is probably less than 1020 Pa∙s. This suggests that revisions to the IJ05 ice sheet history are required. Simulated annealing was performed on the ice sheet history and it was found that changes to the recent ice load history have the strongest effect on GIA predictions.<br>Graduate