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History of Contemporary Anaerobic Conditions in Santa Monica Basin, California Continental Borderland

D. S. Gorsline

Box cores from the basin plain of Santa Monica basin, California borderland, in the presently anoxic basin central plain, indicate that this condition is a very recent event in the basin's sedimentary history. From radiometric dating and varve counts, it has been determined that the initiation of anoxic conditions commenced about 180 years ago and increased to the present. The rate of growth appears to have declined in the past few decades. The center of deepest thickness of laminated undisturbed sediments is offset to the northwest from the actual bathymetric deepest point, which suggests that the condition is related to deep-basin circulation. The period since about 1800 coincides with the development of western settlement and population growth in southern California, b ginning with the mission period and the establishment of the early pueblos. Rapid expansion of the zone during the 19th and early 20th centuries appears to parallel the growth of land development. There does not seem to be a climatic correlation and the only exceptional event at about the time of initiation of anoxic conditions is the Mission Quake of 1811. This event triggered large turbidity-current flows, but such large influxes of sediment and contained organic matter would not upset the oxygen demand balance for more than a few years given normal mixing and overturn of the deep basin waters. The continued growth of the zone with time suggests a continuing perturbation of the system and is most easily correlated with the anthropogenic effect of coastal land development and resulting ntroduction of oxygen-demanding wastes to the basin floor.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.