Characterising the deglacial history of the East Antarctic ice sheet in central Wilkes Land using marine sediment cores

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¬¨‚àûE) 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 ˜í¬µNd signatures were compared with whole rock ˜í¬µNd 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 ˜í¬µNd 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 ˜í¬µNd 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.