The Role of Reactive Phosphorus Regeneration in Cretaceous Oceanic Anoxic Events
Previous studies have hypothesized that during mid-Cretaceous Oceanic Anoxic Events (OAEs) a positive feedback loop was created between increased primary productivity, anoxic benthic waters, and the preferential release of phosphorus (P) from sediments. These studies cite unusually high organic carbon (Corg):organic P ratios in sediments as evidence of P release. However, diagenetic processes in the sediments convert organic P to less labile forms, such as authigenic P. Therefore, total reactive P is a more accurate measure of all organic P burial. To evaluate the evidence for this proposed positive feedback loop during OAEs, P was measured in Corg-rich black shales corresponding to OAE 2 (93.5 Ma) samples collected on Ocean Drilling Program (ODP) Leg 207 from Site 1258 (Demerara Rise off the coast of Suriname). The SEDEX method of extraction was used to measure each type of P (reactive: organic, authigenic, and oxide-bound, and nonreactive: detrital) separately. The average relative (%) standard deviation for total P (reactive + nonreactive) was 3.62%.
Preliminary results show the average total P ranged from 9.57-4050 µmol/g, with an average of 298 µmol/g. Average total reactive P was 36.9 µmol/g (range 5.74-80.0 µmol/g), while the average organic P was only 3.47 µmol/g (range 0.36-17.8 µmol/g), which suggests that previous studies relying on Corg:organic P ratios may have overinflated the actual ratio of Corg:all organic (total reactive) P. These results may indicate that some other process besides preferential P regeneration may have been responsible for the high deposition of Corg during OAEs. To verify, Corg:total reactive P will be determined for these samples. In addition, an unusually high concentration of detrital P in some samples, which ranged from 0.69-3980 µmol/g, may provide evidence for increased terrestrial weathering during OAE 2.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009