Nitrogen dynamics observed in high resolution: Understanding bulk sedimentary d15N using amino acid compound-specific nitrogen isotopes in the eastern tropical Pacific

Compound-specific nitrogen (N) isotopic analysis of amino acids (δ¹⁵N-AA) is a novel tool to study nitrogen cycling, with rapidly expanding applications in geochemistry and paleoceanography. However, its applicability to marine sediments has only just been explored in a highly productive, hypoxic margin setting. δ¹⁵N-AA can untangle major underlying drivers for changes in sedimentary bulk δ¹⁵N, representing a powerful new approach to study regions of complex N cycling with competing isotopic fractionation processes. For example, the δ¹⁵N-AA proxy indicates the δ¹⁵N value of source inorganic nitrogen (i.e. baseline δ¹⁵N), while simultaneously indicating trophic structure complexity in the overlying water column. Further, δ¹⁵N-AA patterns indicate effects of microbial resynthesis and degradation of organic nitrogen. Understanding depositional processes and their biases in the bulk δ¹⁵N record informs our understanding of N cycling globally. Here we use δ¹⁵N-AA to provide new insight into source and transformation of sedimentary organic nitrogen in diverse depositional conditions using a core top sample set in the eastern tropical Pacific (11.3ºN–8.5ºS, 81.2–110.6ºW). These sites vary in surface productivity, sediment oxicity, accumulation rate, and water depth (378–4,360m below sea level), parameters understood to determine the formation and preservation of bulk sedimentary N pool. Our core top bulk nitrogen isotope values (δ¹⁵N_bulk) exhibited characteristic enrichment with decreasing surface nitrate away from upwelled water masses, allowing us to examine how a nitrate utilization gradient is manifest in δ¹⁵N-AA values. Additionally, we examined a suite of multicores from a zone of diverse N cycling, the eastern tropical Pacific, where competing factors such as upwelling and denitrification influence δ¹⁵N-AA. Downcore δ¹⁵N-AA records from this region inform interpretations of past changes in N utilization, water column N cycling, and post-depositional processes.