DID NITROGEN LIMITATION CONTROL PRIMARY PRODUCTIVITY DURING GREENHOUSE CLIMATES? HOW CAN WE TELL? (Invited Presentation)

Studies of marine nutrient cycling during warm climate intervals have reached divergent conclusions about the role of nitrogen in controlling marine productivity. Some authors have inferred that fixed nitrogen availability limited productivity to relatively low levels during greenhouse climates; the low or negative δ¹⁵N values of sedimentary nitrogen in these intervals have been interpreted to reflect enhanced N fixation in response to widespread oligotrophy. Other studies have ascribed this observation to a relative increase in sedimentary rather than water-column denitrification, increased assimilation of ammonium rather than nitrate, or the use of alternative nitrogenase enzymes when trace metal concentrations are low. While these processes represent major reorganizations of the nitrogen cycle, they are not necessarily correlated with systematic changes in global primary productivity. If nitrogen availability is capable of limiting primary productivity over geologically significant timescales, it raises the question of whether nitrogen limitation is a typical “baseline” feature of greenhouse climates (i.e., it can persist over tens of millions of years with minimal ecological disruption), or if intervals of nitrogen limitation represent discrete biogeochemical crises.

The next step toward addressing these questions is to compile parallel datasets of nitrogen isotopes and paleoproductivity estimates, from a range of time periods and depositional environments. If low or negative δ¹⁵N values are consistently correlated with low productivity across a wide range of environments, it is likely that they indicate persistent oligotrophy, which may have steered the course of marine evolution during greenhouse intervals. Initial results have been compiled from several late Paleozoic and early Mesozoic settings where primary productivity can be estimated. In general, low δ¹⁵N values appear to correlate with low primary productivity, suggesting that N limitation plays a role in controlling productivity in greenhouse settings. Exceptions to this general pattern appear to occur as discrete events, and may represent the incursion of ammonium and other reduced species into the photic zone. However, additional work is needed to extend this analysis to stable “baseline” greenhouse intervals.