title: The variability of ocean circulation, productivity, and sea ice in the Adélie region, East Antarctica, over the last two glacial cycles
Creators: name: Pesjak, L
Publisher: name: University of Tasmania
Year: 2022

description: 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 &gt;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 &lt;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.