Daniel Sigman - SMSP Spring Seminar Series 2026

Daniel Sigman

Abstract:  In the thermocline “shadow zones” of the Pacific Ocean’s subtropical gyres, water residence time is decades-long, and sinking organic matter drives respiration that completely consumes the dissolved oxygen (O2) supplied by the circulation. This produces O2-deficient zones (ODZs) in both the eastern tropical North and South Pacific. The ODZs are, on the one hand, inhospitable to many organisms, affecting habitats, migrations, and feeding behaviors. On the other hand, the ODZs host unique biota that perform important biogeochemical processes, which, in turn, influence the biological productivity of the global ocean, the carbon cycle, and ocean-atmosphere fluxes of greenhouse gases. One of these biogeochemical processes in the ODZs is “water column denitrification” (WCD), in which microbial communities oxidize organic matter for energy by reducing nitrate (NO3-) to N2. WCD is a major driver of oceanic fixed N loss, and it produces nitrous oxide (N2O), a potent greenhouse gas, as a side-product. The extents of the ODZ and the rates of WCD are, thus, important in their own right. Moreover, their histories can provide needed insight into tropical ocean dynamics and their future under anthropogenic global warming.

In this talk, I will provide an overview on my group’s and collaborators’ progress in using the nitrogen isotopic composition of fossil-bound organic matter to reconstruct changes in WCD and, thus, the Pacific ODZs, over Earth’s history. I will present findings from a broad range of time scales and periods in Earth history. The overarching finding is that warmer climates yield smaller Pacific oxygen-deficient zones.