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Wright, James D.1, Benjamin S. Cramer2, Kenneth G. Miller1, Miriam E. Katz1 
(1) Rutgers University, Piscataway, NJ
(2) Institute of Geology and Paleontology, Tohoku University, Sendai, Japan

ABSTRACT: Orbital Cycles, Climate, and Diagenesis: High Resolution Records from OAE2, New Jersey Coastal Plain

The Cenomanian-Turonian section from Bass River State Park, NJ contains a 15 m thick interval representing Ocean Anoxic Event (OAE2). Our high resolution (~5cm) bulk sediment inorganic stable isotope record was used to define OAE2. The down-hole gamma log shows well-developed cycles throughout the OAE2 interval that we interpret as short eccentricity cycles, providing a chronology for the OAE2 event. Assuming a 95 kyr eccentricity cycle and that OAE2 is defined the interval of high 13C values, this event was ~700±100 kyr long. The dominant eccentricity forcing here suggests that monsoonal circulation controlled the regional and global carbon cycles as evidenced by sedimentary organic carbon and 13C, respectively. 
Our results show a series of negative 13C excursions, that typically vary from 3 to 5 per mil, but in two instances the excursions were >20 per mil. We initially interpreted these excursions as reflecting methane releases from gas hydrates. Closer examination shows that the 13C excursions were produced by seafloor diagenesis in the presence of elevated organic carbon levels. These transients are characterized by: 1) lack of corresponding decrease in organic carbon 13C values; 2) calcite infilling of foraminiferal shells; and 3) increased %CaCO3. The 13C value of the authigenic CaCO3 is ~-25 per mil, indicating that reducing organic-rich sediments supplied much of the CO2. We argue that increases in organic carbon flux raised alkalinity (through sulfate reduction) and resulted in authigenic precipitation of CaCO3 at Bass River. In our model, negative 13C excursions result from seafloor diagenesis that was forced by climate rather than hydrate dissociation.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.