Compound-specific nitrogen isotopes of equatorial Pacific sedimentary record

Abstract:

Compound specific nitrogen isotopic analysis of amino acids (δ¹⁵N-AA) is a technique that is widely used in regional ecology and food web studies, with newly expanding applications in organic geochemistry. However, its applicability to marine sediment has been minimally examined. This study is one of the first δ¹⁵N-AA applications into the paleorecord of marine sediment. We explore how δ¹⁵N-AA measurements provide insights into past changes in water column N cycling and N utilization, and into post-depositional processes that impact sedimentary N. This is possible because δ¹⁵N-AA investigates the molecular-level basis of the bulk sedimentary δ¹⁵N signal, revealing possible diagenetic alteration of sedimentary organic matter. Our goal was is to investigate the extent of alteration (vs. preservation) of individual sedimentary amino acid δ¹⁵N values from surface nitrate δ¹⁵N across a wide range of depositional environments. The δ¹⁵N of bulk sediment differs from that of the surface nitrate δ¹⁵N signal because of water column processes or more often because of alteration of the signal during initial sedimentation. To investigate this alteration we compare δ¹⁵N-AA to bulk δ¹⁵N measurements in a suite of equatorial Pacific core tops (378–4360 m below sea level) across contrasting oceanographic and sedimentary depositional conditions (e.g. high vs. low productivity, hypoxic vs. oxic bottom waters). To examine down core diagenetic alteration of the sediment record, we present δ¹⁵N-AA and bulk δ¹⁵N of selected deeper depths to observe 1) if diagenetic shift is coherently resolved by both types of measurements and 2) if select individual δ¹⁵N-AA values remain representative of the surface organic δ¹⁵N signal. We hypothesize that compound specific analysis (δ¹⁵N-AA) will provide a molecular level assessment of mechanism for diagenetic changes in bulk organic δ¹⁵N values while also preserving detailed information about planktonic ecosystem structure.