| Summary: | 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, δ15N) of fixed N is sensitive to biogeochemical transformations that constitute the nitrogen cycle. Additionally, the δ15N of organisms increases with trophic level. δ15N 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 δ15N on million-year timescales. This dissertation focuses on million-year time scale nitrogen cycling and trophic ecology using biogenic mineral-bound organic matter δ15N. A foraminifera shell-bound organic matter δ15N (FB-δ15N) 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 δ15N 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 δ15N (δ15NEB) has a long-term method precision of 0.7 ‰ (1σ) and there is substantial evidence for preservation on million-year time scales. Modern shark tooth δ15NEB shows that despite significant intra- and inter-individual differences (up to 3 ‰), δ15NEB captures ocean inter- and intra-basin differences in fixed N δ15N. Modern bovid (cow and bison) δ15NEB shows a close correspondence to collagen δ15N, and reflects the dietary plant δ15N on the terrestrial landscape. Applying δ15NEB to the fossil record, the high δ15NEB of the ancient mega-tooth shark Otodus megalodon provides evidence for a remarkably high trophic position. A 100-million-year fossil shark δ15NEB record from the North Atlantic largely corroborates fixed N ...
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