Resolving Antarctic Margin Paleoceanography for Two Different Timescales and Archives

Graduation Year

2024

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Brad E. Rosenheim, Ph.D.

Committee Member

Amelia E. Shevenell, Ph.D.

Committee Member

Don P. Chambers, Ph.D.

Committee Member

Amy Leventer, Ph.D.

Committee Member

Claus-Dieter Hillenbrand, Ph.D.

Keywords

Antarctica, Deglaciation, Isotope geochemistry, Radiocarbon, Ramped Pyrolysis, Stylasterid corals

Abstract

Recent decades have seen increased rates of ice mass loss from Antarctic ice sheets, through thinning of many ice shelves around the periphery of the continent. The most vulnerable are the glacial systems that are marine-terminating and grounded below sea level, a characteristic that exposes these systems to sea level rise and warming oceans. Enhanced basal melting under the influence of warm ocean water masses can lead to thinning and retreat of glaciers, and ultimately to the acceleration of grounded ice loss. To contextualize current ice mass loss and contribute to accurate model projections for future retreat scenarios, it is vital to understand Antarctic cryosphere and Southern Ocean interactions over a longer period of time than instrumental and observational records allow. Requirements for such an understanding are robust paleoceanographic archives and accurate chronologic control.

To extend records of oceanographic change to the centennial or millennial scales, deep-sea coral archives can be analyzed, as their coral skeletons incorporate geochemical signals of the water masses in which they live. Acting as a living sediment trap affixed to the seafloor, in many regions around the Antarctic margin, corals have great potential to extend records of paleoceanography. There remains a cautious approach to deep-sea coral utilization concerning the ubiquitous family, Stylasteridae. These corals can precipitate both polymorphs of calcium carbonate which can not only complicate the environmental geochemical signals, but complicate large-scale growth patterns. To understand how these corals calcify, and demonstrate their utility for paleoceanography, I employ stable carbon and oxygen isotopes to map the coral growth patterns and establish a route for the most accurate sampling plan to reduce complicating “vital effects”. Additionally, I establish radiocarbon records from the same coral taxon that support a significant change in water mass presence over a large span of the Antarctic continental margin.

Looking further back in time, I address the constraining the timing of the most recent deglaciation from the Last Glacial Maximum (~25 ka) to contextualize future melt. The challenge here lies in the accuracy of radiocarbon-based chronologies of marine sediment records from the Antarctic continental shelf. Considering a low presence of carbonate fossils for radiocarbon (14C) dating, researchers rely on bulk organic carbon that consists of a mixture of pre-aged detrital organic material that can bias ages older than that of true deposition. In the latter half of this dissertation, I employ advanced 14C preparation methods that are aimed at separating the sources of organic carbon within a bulk sediment. I will compare the 14C ages produced by these methods to determine under which sediment conditions one method might perform better than another. I am able to describe the biological content of sediments that can affect the results of the different 14C dating methods and provide guidance on when to employ each method. Such an understanding is vital to accurately reconstructing the timing of Antarctic deglaciation, and thus more fully understanding the potential timing and sensitivity for future loss of Antarctic ice.

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