Antarctic Atmosphere-Ocean-Ice Interactions During Past Warm Climates: An Ice-Proximal Perspective

Graduation Year

2024

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

R. Mark Leckie, Ph.D.

Committee Member

Timothy R. Naish, Ph.D.

Keywords

geochemistry, Holocene, Miocene, paleoceanography, paleoclimate

Abstract

Antarctica’s ice sheets contain 58 m of sea level equivalent ice and thus exert a fundamental control on global sea level on decadal to million-year timescales. Interactions between Antarctica’s ice sheets, regional atmospheric circulation, and the Southern Ocean influence ice sheet mass balance, sea ice formation, and deep water production, which regulate Earth’s climate. At present, Antarctica’s fringing ice shelves and outlet glaciers, particularly in West Antarctica, are losing mass as warm ocean waters move landward across the continental shelves, melting the underside of ice shelves and eroding glacial grounding lines. In East Antarctica, net ice mass gain is occurring due to increased precipitation. Understanding of how Antarctica’s ice mass balance and global sea level will respond to ongoing warming is limited by relatively short observational time series data, where emerging trends can be difficult to identify.

Marine sediment cores from Antarctica’s continental margins can be used to extend observations of atmosphere-ocean-ice interactions back through time, which is useful for understanding forcings and feedbacks involved in Antarctica’s ice sheet evolution and for improving ocean-ice sheet models and sea level projections. Here, I reconstructed oceanographic conditions on Antarctica’s continental shelves during past warm periods using well-dated and high-resolution sediment cores from the western Antarctic Peninsula (WAP) and the central Ross Sea. My overarching hypothesis was that variability in Antarctica’s ice sheets is driven by atmosphere-ocean-ice interactions over the past ~16 million years, even as climate boundary conditions evolved. In one study (Chapter Two), I reconstructed ocean temperatures and sea ice presence over the WAP continental shelf over the last 700 years. In two other studies (Chapters Three and Four), I reconstructed surface ocean temperatures and meltwater input into the central Ross Sea during the late Early to Middle Miocene (~17 to 13.6 million years ago (Ma). In all chapters, I demonstrate the utility of geochemical data, including the TetraEther index of 86 carbon atoms (TEX86) paleothermometer (Chapters Two and Three), benthic foraminifer oxygen (18O) and carbon (13C) isotopes (Chapters Two and Four), and sedimentary x-ray fluorescence (XRF) scanning data (Chapter Four), as paleoenvironmental and paleoceanographic proxies.

Warming, sea ice decline, and ice mass loss has been observed along the WAP since the 1950’s, but it is not clear if these changes are beyond the range of natural variability. To assess recent environmental and oceanographic trends, I generated a 700-year time series of bottom water temperature and nutrients (benthic foraminifer 18O and 13C), sea surface temperature (TEX86), and sea ice presence from a well-dated and high-resolution marine sediment core collected from the WAP continental shelf. My results reveal deep ocean warming and sea ice decline since 1925  15 CE, forced by changes in atmospheric circulation. Deep ocean warming is unprecedented in the past 700 years, and increased nutrient content suggests that warming is associated with increased presence of warm, nutrient-rich Circumpolar Deep Water on the WAP shelf.

Studying past warm climate intervals can provide insights into how Antarctica’s ice sheets may respond with continued warming. The warmest climate interval of the past 40 million years occurred during the Miocene Climate Optimum (MCO; ~16.9-14.7 Ma), which immediately preceded the Middle Miocene Climate Transition (MMCT; 14.7-13.8 Ma), a major interval of cooling and Antarctic ice sheet growth. The MCO is considered an analog for Earth’s climate with continued warming. To determine the influence of ocean temperatures on Antarctica’s ice sheets during the MCO and MMCT, I used the TEX86-paleothermometer to reconstruct sea surface temperatures (SST) in the Ross Sea, Antarctica at International Ocean Discovery Program (IODP) Site U1521. Results reveal warm and variable SSTs (0-15C  2.2C) between ~16.35 and 15.94 Ma, associated with productive open-marine conditions and a retreated ice margin. Sometime between ~15.94 and 14.2 Ma, sediments at Site U1521 were eroded by glacial ice, which expanded across the relatively shallow Ross Sea shelf, suggesting regional ice advance during the late MCO/early MMCT. Glaciomarine sediments preserved above the unconformity at Site U1521 date to between ~14.2 and 13.6 Ma, indicating that ice retreated from its maximum extent during the MMCT. Reconstructed SSTs from mudstones within this interval reveal relatively warm interglacial conditions (9-15C  2.2C). Sometime after deposition of sediments from ~14.2-13.6 Ma, ice sheets advanced again resulting in another hiatus. Overall, my results indicate that ice advanced when ocean temperatures were relatively warm, supporting the hypothesis that poleward heat and moisture transport during the MCO and MMCT increased regional precipitation, initiating ice growth. My results challenge existing coupled ocean-ice sheet models, which struggle to grow ice under warm conditions and are not able to reproduce observed polar temperature amplification. Resolving data-model mismatches during past warm climate intervals, such as the MCO, is required to better predict ice sheet response to future climate change.

Sediments recovered from the Ross Sea continental shelf can be used in high-resolution paleoenvironmental and paleoceanographic studies. In my final study (Chapter 4), I assessed the potential of sedimentary XRF scanning data and benthic foraminifer δ18O and δ13C as paleoenvironmental and paleoceanographic proxies during the Early to Middle Miocene at two sites on the continental shelf of the Ross Sea: IODP Site U1521 and Deep Sea Drilling Project (DSDP) Site 273. The relative amounts of lithogenic and biogenic material delivered to Site U1521 is reflected in variations of titanium (Ti; a proxy for lithogenic sediment/grainsize) and bromine (Br; a proxy for biogenic silica), which may be paced by changes in Earth’s orbit during the MCO (16.35 to 15.94 Ma). Relatively low-resolution benthic foraminifer δ18O and δ13C records from Sites U1521 and 273 span the ~18 to 13.6 Ma interval and are interpreted to reflect changes in meltwater input, ice volume, and global carbon cycling. Both XRF and foraminifer isotopes show potential for future Early to Middle Miocene paleoenvironmental and paleoceanographic reconstructions.

Ice-proximal sediment records preserved on Antarctica’s continental shelves provide long-term context for cryosphere and Southern Ocean response to ongoing climate warming. The paleoceanographic proxy records presented in this dissertation reveal both long- and short-term atmosphere-ocean-ice interactions during past warm climate intervals. An improved understanding of polar amplification, temperature variability, and the associated cryosphere forcings and feedbacks under different boundary conditions will help to improve predictive models of the magnitude and rates of future global sea level rise.

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