CHANGES IN NITROGEN AND PHOSPHORUS CYCLING DURING MARINE REDOX EVENTS

The middle Devonian Marcellus Shale in the U.S. Appalachian Basin, the global Cenomanian-Turonian OAE2, and the Holocene Black Sea exhibit similar characteristics in the rapid onset of anoxic to euxinic conditions in each unit, as inferred from patterns of organic carbon and associated trace metal and sulfur enrichment. These marine redox events are related initially to rapid eustatic sea level rise or basin subsidence. The imprints of these relative sea level changes on inferred water column processes and sedimentary expression differ among these examples, but impacts on the nitrogen and phosphorus cycles are similar. In all cases, there is evidence for more effective cycling of phosphorus and a significant role for nitrogen fixation fueling apparent high primary productivity in surface waters. Development of widespread deepwater anoxia leads to eutrophication by release of substantial amounts of soluble phosphorus. Under these anoxic conditions, fixed nitrogen inventories in deepwater masses decrease as the result of bacterial denitrification and annamox reactions. Fixed nitrogen is replaced by increased rates of nitrogen fixation by planktonic cyanobacteria in surface waters, or, possibly, by nitrogen fixation near the top of the chemocline by sulfide-oxidizing chlorobiaceae. Prevalence of nitrogen fixation produces decreases in nitrogen isotope ratios in deposited organic matter—a signal that is commonly observed in association with these marine redox events. The increase in nitrogen isotope values expected during initial rapid denitrification is rarely seen, possibly because of the rapid ecological changes in surface waters.