Abstract: Increased Rates of Organic Carbon Accumulation Under Anoxic Conditions: Evidence from the Santa Monica Basin

David J. Hollander, Stephen M. Monk, Lisa M. Pratt, J. M. Hayes

A comparison of two box cores taken 2 km apart in oxygenated and anoxic bottom water provide a test of whether benthic conditions influence rates of organic carbon accumulation. Over the past (approx.) 300 years there has been an evolution in oxygen contents of bottom water in the Santa Monica basin. Prior to 1600 A.D., oxygenated conditions prevailed throughout the basin, and bioturbated sediments accumulated in both the basin depocenter (910 m) and periphery (820 m). Since 1600 there has been gradual shallowing of the anoxic boundary from water depths of 910 to 860 m. Finely laminated sediments in the depocenter testify to the generally anoxic character of the central bottom waters. In contrast, sediments on the periphery remain bioturbated. In-situ measurements of oxygen contents i bottom water are consistent with this interpretation of sediment fabrics.

For strata deposited prior to 1600, organic carbon contents are uniformly about 2 wt.%, but organic carbon accumulation rates are 10% higher in the peripheral basin than in the depocenter (0.75 and 0.68 mg C/cm²/yr. respectively). These results indicate that hydrologic processes did not concentrate small particles of organic matter in the depocenter. After 1600, organic carbon contents of sediments in the depocenter increased to 5.5 wt.% while those

in peripheral settings increased to 2.8 wt.%. Organic carbon accumulation rates in the depocenter are presently 30% higher than in the periphery (1.35 and 0.9 mgC/cm²/yr, respectively). Rates of primary productivity in the surface waters are equivalent for the two sites. Thus, bottom-water and sediment anoxia appear responsible for the observed increases in content and accumulation rate of organic carbon at the basin depocenter.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994