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: from as long as millions of years to as short as years and from the Cretaceous Era to the last century.
Bio:
Prof. Sigman, the Dusenbury professor of Geological and Geophysical Sciences at Princeton University, earned his Bachelor’s degree in Geology from Stanford University in 1991. He completed his PhD in Oceanography through the MIT/WHOI joint program in 1997.
He studies the cycles of biologically important elements and their interaction with changing environmental conditions through the course of Earth history. He has pioneered innovative techniques for analyzing nitrogen isotopes in the environment. His research has significantly advanced the understanding of nitrogen cycling in the ocean, the role of carbon dioxide in driving ice age cycles, and the ocean’s response to climate change.
He has authored over 200 peer-reviewed papers, including ~40 in Nature and Science. His work has been cited more than 39,000 times, with a h-index of 94. He has trained over 30 PhD students and postdoctoral scholars in his laboratory, with more than half now holding faculty positions at institutions worldwide.
He has been recognized with numerous prestigious awards, including the James B. Macelwane Medal from the American Geophysical Union (AGU), the MacArthur Foundation “Genius Grant,” and the Science Innovation Award from the European Association of Geochemistry. He is a fellow of AGU and Geochemical Society.
