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Project Progress

The Pearson group has been investigating the response of the marine nitrogen cycle to episodes of ocean anoxia in the Phanerozoic (specifically the Cretaceous), with the objective of focusing eventually on the Proterozoic. The nitrogen isotopic fractionation between biomass and chlorophyll was determined for a suite of prokaryotic and eukaryotic photosynthetic species (Higgins et al., 2011a). This included both marine and freshwater Cyanobacteria, purple sulfur bacteria, and purple non-sulfur bacteria. We now are following up on trying to find a mechanistic explanation for these isotopic fractionations. An undergraduate student, Leah Tsao, is doing a series of controlled chemostat experiments to look for pH effects on nitrogen isotope signatures in Cyanobacteria; while a graduate student, Aimee Gillespie, is completing our survey of photosynthetic groups by growing and measuring nitrogen isotopes in green sulfur bacteria and green non-sulfur bacteria.

Our paper about the role of eukaryotes vs. prokaryotes in Cretaceous Ocean Anoxic Events (OAEs) remains in review. Here we used measurements of the nitrogen isotopic composition of pigments to distinguish between different primary producers within surface ocean communities. Using the principles learned from our fractionation studies, we showed that eukaryotic phytoplankton remained an important component of the surface water primary producer community throughout the OAE (Higgins et al., 2011b). The results suggest that extreme negative excursions in nitrogen isotopic values of biomass are an indicator of deep ocean redox state – specifically, the balance between reduced (ammonium) and oxidized (nitrite, nitrate) N species. This suggests that nitrogen in ancient sediments can reveal the state of the global redox balance and nitrogen cycle.

This work is being complemented by an MIT study of organic matter degradation and diagenesis in and below the oxygen minimum zone off Central Chile. Here Julio Sepulveda, Benjamin Srain and Roger Summons are studying the occurrence of lipid biomarkers diagnostic of redox changes in the water column and preserved in Quaternary (last 2.5 Ma) sediments underlying the oxygen minimum zone. By comparing sediments from the current (Holocene) and last (Eemian) interglacial periods, we hope to evaluate the role of climate warming over ocean deoxygenation in upwelling regions.