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This synthesis of organic geochemical studies from Leg 64 was designed to answer fundamental questions about the source, deposition, preservation, and alteration of organic matter in the Gulf of California. The answers to these questions help to determine the extent and degree of influence not only of paleoenvironmental factors, but also of geothermal processes; understanding both these features was among the main objectives of drilling on this leg. The organic matter is mostly marine, and was deposited as pelagic sediment. Deep in the Guaymas Basin at Sites 477, 478, and 481, turbidites containing terrigenous organic matter are abundant. Total organic carbon (TOC) is most abundant at Sites 479 and 480 in the oxygen-minimum zone on the continental margin. Deeper-water sites contain much less organic matter, and generally their organic contents decrease with depth downhole in response to diagenetic processes. There are local exceptions to this general rule, for instance (1) deep in Hole 476, where there are some fossil(?) lagoonal deposits, rich in organic matter, (2) deep in Hole 478, where laminated sediments rich in organic matter represent a period of stagnation of bottom water in the Guaymas Basin, and (3) 15-20 meters sub-bottom throughout the Gulf, probably in response to some basinwide productivity event in the Holocene transgression. Despite high heat flow, most of the sediments drilled are immature, except close to sills and in Guaymas Basin Hole 477, where the heat flow reaches 20 HFU. Thermogenic methane is widespread as a minor component of gases near the spreading center in the Guaymas Basin, and is a major component of gas deep in Hole 477. Small amounts of light (gasoline-range) hydrocarbons are being generated within the deeper parts of the drilled section in the Guaymas Basin, close to sills throughout the Gulf, and on the Guaymas margin (Site 479). Migration of these light hydrocarbons is most apparent near sills.

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Sources, Preservation, and Maturation of Organic Matter in Pliocene and Quaternary Sediments of the Gulf of California: A Synthesis of Organic Geochemical Studies from Deep Sea Drilling Project Leg 64