BIOLOGIC AND GEOLOGIC CHARACTERISTICS OF COLD SEEPS IN MONTEREY BAY, CALIFORNIA

Cold seep communities discovered at three previously unknown sites bet ween 600 and 1000 m in Monterey Bay, California, are dominated by chem oautotrophic bacteria (Beggiatoa sp.) and vesicomyid clams (5 sp.). Ot her seep-associated fauna included galatheid crabs (Munidopsis sp.), v estimentiferan worms (Lamellibrachia barhami?), solemyid clams (Solemy a sp.), columbellid snails (Mitrella permodesta, Amphissa sp.), and py ropeltid limpets (Pyropelta sp.). More than 50 species of regional (i. e. non-seep) benthic fauna were also observed at seeps. Ratios of stab le carbon isotopes (delta(13)C) in clam tissues near - 36 parts per th ousand indicate sulfur-oxidizing chemosynthetic production, rather tha n non-seep food sources, as their principal trophic pathway. The ''Mt Crushmore'' cold seep site is located in a vertically faulted and frac tured region of the Pliocene Purisima Formation along the walls of Mon terey Canyon (similar to 635 m), where seepage appears to derive from sulfide-rich fluids within the Purisima Formation. The ''Clam Field'' cold seep site, also in Monterey Canyon (similar to 900 m) is located near outcrops in the hydrocarbon-bearing Monterey Formation. Chemosynt hetic communities were also found at an accretionary-like prism on the continental slope near 1000 m depth (Clam Flat site). Fluid flow at t he ''Clam Flat'' site is thought to represent dewatering of accretiona ry sediments by tectonic compression, or hydrocarbon formation at dept h, or both. Sulfide levels in pore waters were low at Mt Crushmore (ca 0.2 mM), and high at the two deeper sites (ca 7.0-11.0 mM). Methane w as not detected at the Mt Crushmore site, but ranged from 0.06 to 2.0 mM at the other sites. Copyright (C) 1996 Elsevier Science Ltd