PHOSPHORUS GEOCHEMISTRY DURING OCEAN ANOXIC EVENTS: RESULTS FROM DEMERARA RISE

Cretaceous ocean anoxic events (OAEs) are characterized by enhanced burial and preservation of organic matter. While the cause and effects of OAEs is debatable, a common explanation for the occurrence of these OAEs is either higher rates of primary productivity, possibly related to increased nutrient availability, or increased ocean stratification. Because phosphorus is an important limiting nutrient to biological productivity in the oceans on geologic time scales, phosphorus availability, utilization, and burial is strongly linked to carbon burial in the oceans. Records of detailed phosphorus geochemistry across OAEs may shed light on the role of phosphorus in the production of these organic rich deposits. To further investigate the role of phosphorus during OAEs, samples from Demerara Rise (ODP Leg 207) in the tropical western Atlantic were analyzed using a sequential extraction for phosphorus (SEDEX) following the method of Ruttenberg (1992) with modifications by Anderson and Delaney (2000). Samples from Site 1258 (3192 m) and Site 1260 (2549 m) were selected consisting of predominantly laminated black shales which span the Cenomanian/Turonian boundary (OAE2). SEDEX quantifies the oxide-associated, authigenic, detrital and organic phosphorus content of sediments. The extraction and analysis of Leg 207 samples is currently ongoing. The sedimentary distribution of phosphorus along with total reactive phosphorus concentrations and fluxes will be compared with organic carbon, carbon isotope, and trace metal data to determine the contribution of phosphorus, based on burial records, to the formation of OAEs.