Amino acid geochronology of foraminifera from the central Arctic Ocean and Nordic Seas

Smith, Joshua Martin (2022) Amino acid geochronology of foraminifera from the central Arctic Ocean and Nordic Seas. Masters thesis, Northern Arizona University.

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Abstract

Accurate geochronology of Arctic Ocean Quaternary sediments is needed in order to properly investigate paleoclimatic and palaeoceanographic changes that have occurred in the region. Common Quaternary marine geochronologic methods such as oxygen isotope and paleomagnetic reversal stratigraphy are challenging to use in the Arctic Ocean, however. The extent of amino acid racemization (AAR) in foraminifera is a promising geochronological tool in the Arctic Ocean, although previous studies have highlighted anomalously high rates of racemization there compared to those from other oceans. Additionally, high intrasample variability limits the accuracy of the method as applied to foraminifera. To address these issues, this thesis presents data from AAR analysis conducted on Neogloboquadrina pachyderma samples ranging in age back to 780 ka from five sediment cores from the central Arctic Ocean and five cores from the Nordic Seas. The Nordic Seas samples were analyzed to further evaluate the validity of the AAR method as applied to cold-water sites where the cores are well dated using conventional methods. The central Arctic Ocean samples were analyzed to test the reproducibility of the apparent high rate of racemization in the region. The subsample rejection rate and coefficient of variation (CV) values for the samples were used to quantify intrasample variability. Scanning electron microscopy (SEM) imaging was conducted on Nordic Seas and central Arctic Ocean N. pachyderma tests to see if intrasample variability was associated with visibly altered tests. Neogloboquadrina pachyderma were extracted from a total of 86 samples and subdivided into 784 subsamples, most of which comprised 8 to 10 individual foraminifera tests. Subsample rejection rates and CV values for samples were comparable to those of previous AAR studies on foraminifera, with 27% of the subsamples rejected based on conventional data-screening procedures. SEM analysis of the tests did not detect widespread visual evidence of incomplete cleaning or alteration features that could readily be associated with high intrasample variability, though it did identify a sample with overall poor test preservation. All examined tests exhibit thick calcite encrustations, but their effect on intrasample variability is uncertain. The utility of the AAR geochronology method is reaffirmed for independently dated cold-water sites using the Nordic Seas data. Further, the apparently high rate of racemization was reproduced in the central Arctic Ocean samples. While the cause for this apparent high rate is unclear, my preferred explanation is that the currently accepted ages for the central Arctic Ocean samples are too young. An alternative age model extending back to 400 ka is proposed based on a simple linear sedimentation rate as indicated by AAR-derived ages using an existing AAR age equation from multiple cold-water sites globally.

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:	Arctic Ocean; Geochronology; Quaternary sediments; Amino acid racemization; Foraminifera; Neogloboquadrina pachyderma
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:	14 Jun 2023 16:29
Last Modified:	14 Jun 2023 16:29
URI:	https://openknowledge.nau.edu/id/eprint/6020

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