Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Amelia E. Shevenell, Ph.D.

Committee Member

Brad E. Rosenheim, Ph.D.

Committee Member

Timothy M. Conway, Ph.D.

Committee Member

John M. Jaeger, Ph.D.

Committee Member

Leanne K. Armand, Ph.D.

Keywords

geochemistry, Holocene, paleoceanography, sediments

Abstract

The East Antarctic Ice Sheet (EAIS) contains ~53 meters of sea level equivalent (SLE) ice, and observations suggest it is sensitive to ongoing and past climate change. The EAIS has traditionally been considered insensitive to climate perturbations because it is largely grounded above sea level. However, aerogeophysical surveys, oceanographic observations, and models indicate that large areas of the EAIS are grounded below sea level and contain 19.2 m SLE. Marine-based parts of the EAIS are thought to be located on inland-sloping beds that drain through marine terminating outlet glaciers, indicating large areas of the EAIS may be more sensitive to ongoing climate change than previously thought. Many of Antarctica’s marine outlet glaciers and fringing ice shelves are losing mass as warm ocean waters move across continental shelves toward deep glacial grounding lines. Predictions of future ice sheet response to climate change are limited by the short time-series of observations and the complexity of associated forcings and feedbacks. To accurately predict future Antarctic cryosphere response to ongoing ocean warming, it is critical to understand how Antarctica’s ice sheets responded to past climate variations. Antarctic margin marine geological records provide a longer-term perspective (up to millions of years) on current Antarctic ice retreat than instrumental and ice core records. Ice-proximal geologic records are crucial for understanding past regional ice dynamics near sensitive Antarctic outlet glacier systems.

The Totten Glacier and Moscow University Ice Shelf systems on the Sabrina Coast, East Antarctica, drains a large, marine-based catchment system called the Aurora Subglacial Basin (ASB), which contains one eighth of the ice in East Antarctica. Oceanographic observations indicate that warm modified Circumpolar Deep Water (mCDW) flows across the continental shelf to access regional grounding lines, thus the ASB catchment might be susceptible to ice mass loss via ocean thermal forcing. Present estimates show lowering of Totten Glacier’s ice surface (1 m/yr) and the regional transfer of grounded ice to the Southern Ocean (100 G tons/yr). There is a significant amount of ice in the ASB catchment, so continued ocean and atmospheric warming could lead to contributions to global eustatic sea level rise from the ASB.

In order to put the present day oceanographic and glacial changes on the Sabrina Coast, East Antarctica into the context of natural variability, this dissertation establishes the region’s deglacial (16,500 years before present) to recent paleoceanographic history. All data included in this dissertation are from a suite of seven marine sediment cores collected from ~600 m water depth on the middle continental shelf, seaward of the Moscow University Ice Shelf system, where the Antarctic Coastal Current flows onshore and relatively warm deep water (0.3-0.5 °C) was observed.

To develop palaeoceanographic reconstructions, seven sediment cores were studied to determine their lithologic composition, geochemistry, micropaleontology, and chronology (Chapter 2). Microfossil assemblages (foraminifer and diatom), benthic and planktic foraminifer stable isotopes (δ18O, δ13C), and diatom assemblages were analyzed (Chapter 2). To address the deglacial to Holocene paleotemperature history of the Sabrina Coast, ocean temperature estimates were made by analyzing foraminifer Mg/Ca and archaeal membrane lipids from surface sediment samples, compared to regional upper ocean temperatures, and added to existing TEX86 and Mg/Ca to temperature calibrations (Chapter 3). To reconstruct Holocene upper ocean temperatures, TEX86 and foraminifer Mg/Ca were generated (Chapter 4).

Deglaciation of the middle continental shelf occurred ~16.5 ka, coincident with circum-Antarctic atmospheric warming, increasing concentrations of atmospheric CO2, and enhanced Southern Ocean upwelling after the end of the Last Glacial Maximum. Upper ocean paleotemperatures reconstructed using TEX86 are the first from the Sabrina Coast sector of the East Antarctic margin, and reveal that upper ocean temperatures were relatively warm immediately following deglaciation. Diatom and foraminifer assemblages indicate shelf water formation and seasonally open water conditions existed in the early to middle Holocene, coincident with relatively warm upper ocean temperatures. Early to middle Holocene conditions are inferred to be associated with local polynya dynamics and ocean current influence, which are driven by regional atmospheric forcing. Late Holocene sediments contain diatom assemblages that suggest increased offshore water mass influence, similar in timing and character to other Antarctic marine, terrestrial, and ice core records, suggesting that regional oceanography was influenced by forcings from lower latitudes. Upper ocean temperatures based on TEX86 and foraminifer Mg/Ca reveal confirm that the paleotemperature proxies produce upper ocean temperatures realistic for the Antarctic shelf setting, and warming of 2°C occurred over the last 1000 years.

The first deglacial to Holocene paleoenvironmental reconstruction for the Sabrina Coast, East Antarctica provides crucial information about timing of ice sheet retreat during the last deglaciation. Although present ocean temperatures do not exceed the range of reconstructed ocean temperatures since deglaciation, data suggest that present temperatures are approaching those of the middle Holocene, when Antarctic climate was generally warmer and ice extent was reduced. As such, this region likely contributed to global eustatic sea level in the past, and will likely under future warming scenarios.

Download

Share

COinS