Author: Emma Ruth Kast

Publisher:

Published: 2020

Total Pages: 0

ISBN-13:

As an essential nutrient required by all organisms, biologically available nitrogen (?fixed N?) plays an integral role in biogeochemical cycling at Earth?s surface. The nitrogen isotope ratio (15N/14N, ?15N) of fixed N is sensitive to biogeochemical transformations that constitute the nitrogen cycle. Additionally, the ?15N of organisms increases with trophic level. ?15N can be a powerful tool for reconstructing the N cycle and the trophic level of organisms, yet poor preservation of N in the geologic record have limited our ability to apply ?15N on million-year timescales. This dissertation focuses on million-year time scale nitrogen cycling and trophic ecology using biogenic mineral-bound organic matter ?15N.A foraminifera shell-bound organic matter ?15N (FB-?15N) record is presented for the early Cenozoic (66 million years ago until present). There is a large decrease (by ~15 ? in the Pacific and by ~ 6 ? in the Atlantic) between 57 and 50 million years ago, before Eocene global cooling. This indicates expanded ocean suboxia in the Paleocene, followed by its tectonically driven demise.Mineral-bound ?15N methods are adapted for use with tooth enamel and enameloid, abundant fossils that are widely used in geochemical and paleontological studies of the past. Enamel(oid)-bound organic matter ?15N (?15NEB) has a long-term method precision of 0.7 ? (1?) and there is substantial evidence for preservation on million-year time scales. Modern shark tooth ?15NEB shows that despite significant intra- and inter-individual differences (up to 3 ?), ?15NEB captures ocean inter- and intra-basin differences in fixed N ?15N. Modern bovid (cow and bison) ?15NEB shows a close correspondence to collagen ?15N, and reflects the dietary plant ?15N on the terrestrial landscape.Applying ?15NEB to the fossil record, the high ?15NEB of the ancient mega-tooth shark Otodus megalodon provides evidence for a remarkably high trophic position. A 100-million-year fossil shark ?15NEB record from the North Atlantic largely corroborates fixed N ?15N changes observed in FB-?15N.This dissertation expands our understanding of marine nitrogen cycling and the trophic position of ancient sharks. This work establishes new mineral-bound ?15N tools and initiates mineral-bound N isotope investigation of the million-year geologic record.