Bacterial diether lipids as a novel proxy to reconstruct past changes in sedimentary oxygenation

The degree of oxygenation of the marine water column and sediments has a major impact on the dynamics of biogeochemical cycles and habitability for life. Reconstructing past changes in marine oxygenation through time is therefore crucial for our understanding of the evolution of Earth’s system and Life. Although a range of inorganic and organic proxies exist to reconstruct changes in water column and/or sediment oxygenation, each comes with its own limitations and additional proxies are needed to obtain a holistic understanding of ocean oxygenation through time. Here we explore the distribution of molecular fossils (biomarkers) in the oxic water column and a range of marine core top sediments from the South Atlantic that differ in their oxygenation state. Specifically, we focus on the distribution of bacterial diether glycerol (DEG) lipids. We show that the relative abundance of bacterial C33 DEGs is significantly higher in anoxic sediments compared to oxic sediments. They are absent from the water column, which is oxic at our sites. These results are consistent with the only known sources of C33 DEGs all being anaerobic bacteria. Based on the correlation between sedimentary oxygenation and relative abundance of C33 DEGs, we propose that the latter can serve as a novel proxy to reconstruct changes in sedimentary oxygenation, and by extension bottom water ventilation and deep-sea carbon storage. We tested this novel proxy against other proxies for bottom water ventilation/sedimentary anoxia in a marine sediment core from the Bering Sea that spans the last 27,000 years. We show that the diether proxy indicates that a mass of poorly ventilated bottom waters existed in the Bering Sea between 21 and 17 kyr, which is largely consistent with published data from the same core as well as those from across the Bering Sea and results from sites elsewhere. Our results highlight the potential for this biomarker proxy to reconstruct past changes in sedimentary oxygenation and by extension bottom water ventilation and carbon storage.