2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 8
Presentation Time: 10:00 AM


JOO, Young Ji, SAGEMAN, Bradley B. and HURTGEN, Matthew T., Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208, yjoo@earth.northwestern.edu

Cretaceous oceans experienced short-lived episodes of widespread anoxia, termed oceanic anoxic events (OAEs). Because OAEs were likely driven by high primary productivity in surface waters, analysis of changes in nutrient cycles, as well as bottom water redox conditions that influence these cycles, can help to evaluate mechanistic hypotheses for such events. This study seeks to test a new hypothesis linking the carbon, sulfur and phosphorous cycles that is supported by sulfur isotope data from the study interval. The hypothesis proposes that oceanic sulfate levels remained low following the early Cretaceous deposition of (South Atlantic) evaporites. Upon this background, a significant increase in submarine and subaerial volcanic activity in the Cenomanian increased atmospheric pCO2 levels, metallic micronutrients, and marine sulfate levels. The combination of decreasing bottom water O2 and increasing bacterial sulfate reduction led to a major shift in the recycling rate for phosphorus. The onset of OAE2 followed a major shift from P burial to P recycling, and thus a significant increase in bioavailable P. Global bioproduction may have become N-limited, but proliferation of N-fixing cyanobacteria, suggested by other studies, may have helped shift the system to a hyper-eutrophic state. Focusing on a section from the Western Interior seaway in which the onset of OAE2 is expanded (Aristocrat Angus core, Denver Basin), this study evaluates trends in organic carbon concentration and isotopic composition, sulfur isotopes, and selected phases of phosphorus (using the SEDEX technique) to test whether trends in P burial behave as predicted.