Compound-specific δ¹⁵N and chlorin preservation in surface sediments of the Peru Margin with implications for ancient bulk δ¹⁵N records

Understanding the processes that control the preservation of paleoceanographic proxies is of clear importance. Surface sediments from the Peru Margin oxygen-minimum zone are subject to lateral and downslope transport by bottom currents that decrease organic matter (OM) quality. Indicators of bulk OM quality (pyrolysis hydrogen index, pyrolysis S1+S2 and C/N) demonstrate significant degradation between 150 and 400m water depth, within the oxygen-minimum zone. Concentrations of the three most abundant chlorins (chlorophyllone, pheophytin and pyropheophytin) decrease from 750 to 150nmol gTOC^-1 from 150 to 400m water depth though the relative abundances of the chlorins in an individual sample do not change. This suggests that the three chlorins have similar reactivity over the ambient conditions. Values for δ¹⁵N of bulk sediments (δ¹⁵N_bulk) decrease by 3‰ from the inner shelf to the upper slope (1000m) but co-occurring compound-specific δ¹⁵N values (δ¹⁵N_chlorin) do not decrease downslope. The low variability of δ¹⁵N_chlorin values supports a single source for the chlorins, and demonstrates the recalcitrance of δ¹⁵N_chlorin values despite degradation. This set of observation raises questions about which type of OM fraction best records 'primary' signatures. We assess two possible models to guide our interpretation of these disparate datasets (1) that decreasing δ¹⁵N_bulk values are the result of degradation of a ¹⁵N-enriched fraction during downslope transport, and that δ¹⁵N_chlorin values reflect primary values; (2) that δ¹⁵N_bulk values are primary and that chlorins are derived from material transported from upslope. These data reaffirm that in active sedimentary environments such as the Eastern Tropical Pacific, transport of OM can significantly alter bulk geochemical parameters of OM integrity, but the impacts on the δ¹⁵N record of bulk sediments and chlorins are less clear, and require more study to be thoroughly understood.