Development and application of biogenic mineral-bound nitrogen isotope measurements to the million-year timescale

Abstract

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 δ15N changes observed in FB-δ15N.

This dissertation expands our understanding of marine nitrogen cycling and the trophic position of ancient sharks. This work establishes new mineral-bound δ15N tools and initiates mineral-bound N isotope investigation of the million-year geologic record.