--> Patterns of Organic Matter Preservation on the California Continental Margin: Holocene to Glacial Contrasts, by W. E. Dean and J. V. Gardner; #90986 (1994).

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Abstract: Patterns of Organic Matter Preservation on the California Continental Margin: Holocene to Glacial Contrasts

Walter E. Dean, James V. Gardner

Samples of bioturbated sediments from cores collected within the present oxygen-minimum zone (OMZ) on the upper continental slope (600-1000 m) off California show a gradual increase in the percentage and mass accumulation rate (MAR) of organic matter, and in the marine character (hydrogen richness and isotopic composition) of that organic matter, since the last glacial maximum (20 Ka). Even more striking, however, is the increase in the MAR of organic matter within the last interglacial (oxygen-isotope stage 3, ca 25-40 Ka) within laminated sediments in cores collected within the OMZ from as far north as the Oregon-California border to as far south as Santa Barbara. These upper Pleistocene laminated sediments contain more abundant hydrogen-rich (Type H) organic matter than even the la e Holocene bioturbated sediments. Concentrations of several trace elements indicative of anoxic conditions (e.g., Cd and Mo) also are higher in the laminated sediments. The stable carbon-isotopic composition of the organic matter does not change with time between bioturbated and laminated sediments, indicating that the greater abundance of type II organic matter in the laminated sediments is not due to a change in source but rather represents a greater degree of preservation of organic matter. The presence of abundant well preserved organic matter, as well as lack of bioturbation and the presence of elevated concentrations of Cd and Mo all support the theory that the OMZ in the northeastern Pacific Ocean was more intense, in fact, anoxic, during the late Pleistocene in response to increa ed coastal upwelling generating higher organic productivity.

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