Bahía Concepción is an excellent example of the extensional basins and accommodation zones that formed along the Baja California peninsula associated with the opening of the proto-Gulf of California during the Miocene. Its narrow (2-10 Km), elongate (40 Km) shape results from a half-graben controlled by northwest-southeast trending faults. The most prominent of these faults make up the Concepción fault zone, which runs along the peninsula forming the eastern margin of the bay. The prominent steep escarpment occurring along much of the western shore of Bahía Concepción suggests that this side of the bay is also bound by northwest-southeast-trending faults, which we have named the El Requesón fault zone. Some of these faults act as conduits, releasing hot fluids and gas. Hot springs associated with the Requesón fault zone have been documented in several locations along the western shoreline, and a former Moss Landing Marine Labs student, Diana Steller, found an area where geothermal fluids and gas bubbles are being released in the intertidal and subtidal (to 13m water depths). Fluids and gas flow through rock and soft sediment in a roughly linear fashion extending over 750m of coastline, trending along an onshore-offshore fault associated with the Requesón fault zone. Analyses of the geothermal fluids that are being released at sites along the

Requesón fault zone indicate that they are a mixture of local meteoric water and seawater, and are highly enriched in silica, manganese, and arsenic. The gas is rich in methane and carbon dioxide, with stable isotope ratios that suggest thermogenic sources. This hydrothermal venting is affecting the ecology of the area. Macrofaunal abundances and diversity appear to be enhanced in rocky habitats, particularly in fish and epifaunal filter-feeding invertebrate assemblages, while abundances and diversities of infaunal animals are lower in areas of active venting through soft sediment. Precipitates of iron oxyhydroxide, likely mediated by microbial activity, are forming in areas of active venting. These precipitates are heavily colonized by a community of morphologically diverse bacteria, forming yellow “mats” which are restricted to venting areas. Grazers and deposit feeders appear to be feeding on these mats, and stable isotope analyses of carbon and nitrogen have revealed that one particular organism, Holothuria inhabilis, a deposit-feeding holothurian (sea-cucumber) appears to be obtaining a substantial amount of carbon from the microbes within the mats. I hope to continue to work with this system, and to compare its biocomplexity with other shallow-water hydrothermal vent sites in Baja and Alta California.