Plain Language Summary

Membrane-spanning tetraether lipids of microbial organisms preserved in marine sediments, also known as “molecular fossils,” have been widely used as geochemical proxies to infer environmental changes in the past. Overly branched and sparsely branched tetraether lipids are new classes of organic compounds that have recently been identified and found to be abundant in marine waters where oxygen levels are low. Despite its potential as an oxygen indicator in past oceans, only a few studies have determined these lipids in modern marine sediments. Here, we analyzed these lipids from surface marine sediments collected from various locations with different oxygen levels in the water and found a strong link between the presence of these branched lipids and the amount of dissolved oxygen in deep oceans. This suggests that microbes in surface marine sediments are likely the main source of these new types of lipids. With new information from this work, we propose a new proxy that can track changes in deep ocean oxygen concentrations Earth’s history.