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Project Progress

We have continued research on several fronts:

1. We have been investigating nitrogen isotope systematics in anaerobic environments and at oxic/anoxic interfaces using the Black Sea and Fayetteville-Green Lake as modern environments for establishing the relative importance of water-column processes (nitrogen fixation, ammonia and nitrate assimilation, denitrification) for determining particulate N fluxes and N isotopic values. We have also examined the sedimentary record of N fluxes and isotope values and have attempted to document the effects of diagenesis. In addition we have analyzed black shales deposited in ancient anoxic marine basins (Cretaceous, Devonian) for comparison. Ultimately, we hope to demonstrate the fidelity of sedimentary nitrogen as a record of nitrogen cycling in ancient environments for application to interpreting Precambrian paleo-nutrient cycles. Thus far it appears that nitrogen fixation is an important process in black-shale forming environments; most black shales have nitrogen isotope values near 0 permil.
   
2. We have also continued to examine the record of carbon and sulfur cycling in anoxic basins and at times of widespread oceanic oxygen deficiency. We have pursued the record of sulfur isotopic values in sulfate incorporated into carbonates in the Neoproterozoic, near the Permo-Triassic (P/T) boundary and in the Cretaceous, the most recent episode of widespread oceanic oxygen deficiency. There are substantial sulfur isotope variations in the Neoproterozoic, associated with inferred low oceanic sulfate concentrations. The Permo-Triassic boundary event is accompanied by a large increase in δ³⁴S, possibly associated with widespread anoxia and sulfur degassing accompanying the Siberian Trap volcanism. We are investigating sulfide toxicity as an extinction mechanism for the Permo-Triassic event. Our work on carbon cycling and organic C burial in black-shale forming basins continues and we have examined the interplay of sedimentation rate, anoxia, and phosphate cycling, driven by sea level changes, in the origin of organic C rich strata.
   
3. We have explored biotic extinction records and proposed mechanisms, including extraterrestrial impacts and eruption of large igneous provinces. Our analysis suggests that not all large impacts or volcanic events cause substantial extinctions and that terrain is an important factor. The Cretaceous/Tertiary (K/T) impactor and the Siberian Traps, in the case of the Permo-Triassic event, had more profound effects on global environment because they impacted or erupted through carbonate and sulfur rich rocks, thereby increasing the volatile output.