Deconstructing the Bulk Signal: Compound-Specific Nitrogen Isotopes in Marine Sediment

Abstract:

Compound-specific nitrogen (N) isotopic analysis of amino acids (δ¹⁵N-AA) is an analytical technique with expanding applications in organic geochemistry. However, its applicability to marine sediments has only been explored in a highly productive, hypoxic margin setting. We use δ¹⁵N-AA measurements to provide insight into the source and preservation of particulate organic nitrogen preserved in sediment using a core top sample set in the equatorial Pacific that spans a range of depositional conditions, including surface productivity, sediment oxicity, accumulation rate, and water depth (378–4360m below sea level). These variables represent main drivers understood to determine the formation and preservation of bulk sedimentary N pool during N uptake and after initial deposition. Core top bulk nitrogen isotope values (δ¹⁵N_bulk) exhibit characteristic enrichment with decreasing surface nitrate concentration away from coastal and equatorial upwelling. We examine how this spatial pattern in δ¹⁵N_bulk then manifests in sedimentary δ¹⁵N-AA values. Additionally, we examine how δ¹⁵N-AA can inform interpretations of past changes in N utilization, water column N cycling, and post-depositional processes measured downcore in a suite of equatorial Pacific multicores (11.3ºN–8.5ºS, 81.2–161ºW). Our new δ¹⁵N_bulk records are regionally coherent with a priori data in the equatorial Pacific, recording changes in N cycling from the Last Glacial Maximum, deglaciation, and Late Holocene. Analyses of δ¹⁵N-AA in downcore samples preserve detailed information about changes in source nutrients, reconstruct changes in ecosystem structure, and elucidate the presence and importance of diagenetic alteration of bulk δ¹⁵N values. This is possible because δ¹⁵N-AA investigates the molecular-level constituents of most of the bulk sedimentary δ¹⁵N signal. Understanding depositional processes and biases in the bulk δ¹⁵N record informs interpretations of current records and future study site selection.