The O2-deficient zones (ODZs) of the eastern tropical Pacific and Indian Oceans have an outsized importance in the global cycles of nutrients and carbon, and they are expected to respond dynamically to changes in ocean oxygen, circulation, and productivity in coming years. Although we have known for decades that O2-limited environments tend to bury large amounts of organic matter (OM), we still do not fully understand why, which means that we cannot yet predict how these changes in ODZ size and intensity will impact carbon burial. Intriguingly, growing evidence indicates that large sinking particles in ODZs can host microbial sulfate reduction (MSR) in diffusively limited internal microenvironments. Particle-hosted MSR generates sulfide that can react with (or ‘sulfurize’) OM, making it more resistant to microbial breakdown. This mechanism – particle-hosted OM sulfurization – may link water column anoxia with enhanced sedimentary OM preservation and thereby help answer fundamental, unresolved questions about the causes of elevated marine carbon burial. Currently, however, we lack the information to assess the scale or significance of particle-hosted sulfur cycling in ODZs, or to estimate its effects on marine sedimentary OM burial.

To address this gap, I propose an integrated research and education program centered on an expedition to the Eastern Tropical North Pacific ODZ, where we will systematically investigate sources and transformations of organic S in sinking particles, suspended particles, and shallow sediments from three regions of the continental margin. Sinking particle material will be collected using a set of float-tethered net traps, which will be tested during a short training cruise to an anoxic fjord. At sea, we will use the traps to measure rates of MSR using both in-situ 33S-sulfate and shipboard 35S-sulfate experiments. We will collect shallow sediment multicores from sites with a range of bottom water redox conditions to explore the relationships between local redox structure, particle flux, and sedimentary organic S burial using multiphase isotopic analyses and X-ray absorption spectroscopy. During the second half of this project, we will conduct sulfurization experiments on ODZ materials under a range of reaction conditions and compare the products with sediments and ancient shales. Together, these results will allow us to make the first quantitative estimates of the scale of OM sulfurization in ODZs, its contribution to sedimentary OM burial, and its sensitivities to environmental change.