[Houghton Lecture] Ann Pearson (Harvard): Records of episodic anoxia in the marine nitrogen cycle

Abstract:
The sensitivity of the marine nitrogen cycle to past biogeochemical disturbances and future anthropogenic warming depends on redox conditions. The relative balance of ocean overturning, oxygenation, and export production determines whether the ocean’s inventory of “fixed” N increases or decreases. Such changes are recorded in the stable nitrogen isotope compositions of marine sediments, and in their assemblages of organic biomarker molecules.  Sediments of both the Mesozoic and the Plio-Pleistocene record episodes of elevated organic carbon burial, deposited in association with anoxic bottom waters and bearing signatures of N-cycling that are markedly different from the modern ocean. Using molecular-level and bulk stable isotope ratio distributions of the organic nitrogen and carbon pools, in tandem with a simple water column overturning model, we reconstruct scenarios for these events.  Mesozoic ocean anoxic events (OAEs) appear largely to reflect the intersection of a greatly increased nutrient supply and export production with only moderately restricted circulation, while Plio-Pleistocene sapropels of the Eastern Mediterranean developed primarily through enhanced water-column stratification of a largely oligotrophic system. Differences between these scenarios are reflected in diagnostic patterns of lipid biomarker distributions, and the bulk N-isotope signatures of the resulting sediments can be simulated by varying the fractional contributions of nitrification, denitrification, and anammox.  In all cases, eukaryotic algae dominate export production, indicating that efficient export may be a stable property of recent oceans, despite large-scale biogeochemical changes.

Houghton Lecture Series

Supported by the Houghton Fund, Houghton Lecturers are distinguished visitors from outside MIT invited by the EAPS Program in Atmospheres, Oceans and Climate (PAOC) to spend a period of time, ranging from a week to several months, as scientists-in-residence.

Contact: kbauer@mit.edu