Amino acid racemization in quaternary foraminifera from the southern ocean

Hunt, Scarlett E. (2023) Amino acid racemization in quaternary foraminifera from the southern ocean. Masters thesis, Northern Arizona University.

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Abstract

Accurate geochronology is fundamental to understanding Earth’s history. This is especially the case in polar regions, where past ice sheet dynamics are often interpreted from marine sediment records to better understand the effects of the changing climate, but where unavailability of sufficient suitable material for dating and/or abrupt discontinuities in sedimentation often pose challenges to conventional geochronological methods. Amino acid racemization (AAR) geochronology is a Quaternary dating method in which the concentrations of D- and L-forms of amino acids in carbonate fossils are measured to determine the time elapsed since death of the organism. AAR geochronology has been verified and calibrated using independently dated marine sediments from deep-sea sites in the Arctic, Pacific, and Atlantic Oceans, but few AAR data have been published from the Southern Ocean prior to this study. I tested the AAR method on two species of foraminifera, Neogloboquadrina pachyderma sinistral and Cibicidoides cf. wuellerstorfi, in ten sediment cores from the Southern Ocean. Specifically, I determined whether the extent of AAR, as measured by the ratio of the D- and L-forms of aspartic acid (Asp) and glutamic acid (Glu), (1) increases regularly downcore, (2) is similar for stratigraphically correlated samples from different cores around Antarctica, and (3) is consistent with the available geochronology when compared with D/L values published in studies from other oceans. A total of 58 foraminifera samples were analyzed for their amino acid composition and extent of AAR, each comprising an average of 11.6 subsamples. Of these samples, 43 had ≥ 50% of subsamples that passed established data-screening criteria, and of these accepted samples, 95% had Asp and Glu D/L values that increased monotonically downcore. The downcore trend is also documented by the significant slope (p < 0.05) of linear regressions of D/L versus depth. Fifteen of the accepted samples from five cores were correlated with each other based on their published marine isotope stage (MIS) assignments ranging from MIS 5 to MIS 17. Of these stratigraphically correlative samples, nearly all (93%) overlapped within one standard deviation of the standard error of the average D/L for each isotope stage for both amino acids. Using the published ages for the cores, the D/L values of samples analyzed in this study, which range in age from around 7 thousand to 1.1 million years, were generally similar to those in foraminifera samples of similar age from other cold-water deep-sea core sites. I also investigated possible causes of intra-sample variability but did not find any obvious correspondence between the extent of variability and the sample age, carbonate content, sedimentation rate, or amino acid composition. Nonetheless, the results show that the AAR is viable dating method using Quaternary foraminifera in Southern Ocean sediments. As such, my results provide further evidence that this method is reliable for dating sediments from cold-water sites. Presumably, this includes the central Arctic Ocean where previously published AAR data can now be used with greater confidence to revise the geochronology of sediment cores.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	amino acid racemization; foraminifera; geochronology; Southern Ocean
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	23 Jul 2026 21:43
Last Modified:	23 Jul 2026 21:43
URI:	https://openknowledge.nau.edu/id/eprint/6298

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