Impact of selective degradation on molecular isotope compositions in oxic and anoxic marine sediments

Abstract

One of the central assumptions of paleoenvironmental research using lipid biomarker isotope compositions in sedimentary archives is the isotopic stability of these compounds after deposition. If this indeed applies for open marine sediments with low sedimentation rates and strong biodegradation is, however, not well constrained. Here we study lipid biomarker isotope compositions from two sediment cores from the brine-filled Shaban Deep in the northern Red Sea that receive comparable water column flux due to close geographic proximity but are deposited under opposite anoxic (inside the brine) and oxic (outside the brine) sedimentary conditions. Despite almost complete degradation of some compounds in the oxic sediments, we show that the stable carbon isotope compositions of long-chain n-alkanes, alkenones and bulk organic matter (OM) remain stable among the two cores. The relative distribution of long-chain n-alkanes and alkenones is also preserved. Conversely, the stable hydrogen isotope composition of long-chain n-alkanes and bulk OM increases consistently during degradation (10‰ vs VSMOW for n-alkanes and 40‰ for of bulk OM) indicating the preferential degradation of labile bulk OM and n-alkane fractions that are isotopically lighter. For long-chain fatty acids we find shifts in relative distribution and variability of stable hydrogen and carbon isotope compositions indicating an overall marine sourcing of these compounds that contrasts their usual interpretation as plant derived markers. We conclude that isotope compositions of source-specific refractory molecules show only minor variations under strong degradation, while compounds with multiple sources can show strong variability potentially impeding their use as paleoenvironmental proxies. © 2021 Elsevier Ltd. All rights reserved.