Please join us for the second talk of the Chow Series Lectures by Professor Daniel Sigman of Princeton University, titled "Nitrogen in the ocean: Dynamic flows but a stable reservoir".
A half-hour reception with Dr. Sigman will immediately follow the talk!
Abstract: The ocean’s internal cycling of biologically available nitrogen (or “fixed N”) reflects nutrient cycling in general; for example, it is often directly comparable to phosphorus. In contrast, the input/output budget of fixed N is distinct from the budgets of most other nutrients in that its dominant inputs and outputs are biological processes. The major input is oceanic biological N2 fixation, in which nitrogen gas (N2) is converted to ammonia (NH3), mostly by a special N2 fixing group of cyanobacteria at the ocean surface. Fixed N is lost from the ocean mostly by denitrification, the conversion of nitrate (NO3-) to N2, which occurs when organic matter is decomposed by bacteria under low O2 conditions. Given these biological input/output terms, the ocean’s fixed N budget provides an important test case for the role of biologically controlled feedbacks in the stabilization of Earth’s environmental conditions.
Beginning with the work of Alfred Redfield, it has long been argued that N2 fixation provides the key sensitivity in the N budget that allows the ocean to maintain a stable reservoir of fixed N. Specifically, any deficit in N relative to phosphorus (P) in surface waters, relative to phytoplankton requirements, should induce N2 fixation; this will tend to restore the N-to-P ratio to a given value determined by phytoplankton requirements. In recent decades, however, it has been suggested that the strength of this feedback is restricted by the lack of iron available to N2 fixers, and the N-to-P ratio required by the phytoplankton community has been observed to covary as a function of surface ocean conditions. These findings have been interpreted diversely by the oceanographic community, leading to uncertainty as to the importance of the N2 fixation feedback and the potential variability of the oceanic fixed N reservoir.
Here, I review my work with the N stable isotopes, which documents the operation of the N2 fixation feedback in the modern ocean and in the past ocean over the ice age cycles of the last million years. Inferences regarding N2 fixation can also be extracted from our reconstruction of ocean N isotope change over the millions of years of the early Cenozoic Era, extending back to the Age of the Dinosaurs. These data indicate that, over decades within ocean basins as well as over thousands or millions of years on the regional to global scale, the N2 fixation feedback has operated in the face of dramatic changes in denitrification to maintain a stable reservoir of fixed N in the ocean. This finding conforms with the notion that the Earth system has the feedbacks it requires to maintain and stabilize the biological fertility of the ocean. If so, we must ask why this is the case. Is there a mechanism by which a sustainable global biosphere has evolved?
The cost of the series is supported by the Chow Fund.