We investigated hydrothermal gas venting associated with a coastal fault zone along the western margin of Bahi??a Concepcio??n, B.C.S., Me??xico. Copious discharge of geothermal liquid (??? 90 ??C) and gas is occurring in the intertidal and shallow subtidal zones (to a depth of 13 m) through soft sediments and fractures in rocks along a ???750 m linear trend generally sub-parallel to an onshore fault near Punta Santa Barbara. Hydrothermal activity shows negative correlation with tidal height; temperatures in the area of hydrothermal activity were up to 11.3 ??C higher at low tide than at high tide (measured tidal range ??? 120 cm). Gas samples were collected using SCUBA and analyzed for chemical composition and stable isotope values. The main components of the gas are N2 (??? 53%; 534 mmol/mol), CO2 (??? 43%; 435 mmol/mol), and CH4 (??? 2.2%; 22 mmol/mol). The ??13C values of the CH4 (mean = - 34.3%), and the ratios of CH4 to C2H6 (mean = 89), indicate that the gas is thermogenic in origin. The carbon stable isotopes and the ??15N of the N2 in the gas (mean = 1.7%) suggest it may be partially derived from the thermal alteration of algal material in immature sedimentary organic matter. The He isotope ratios (3He/4He = 1.32 RA) indicate a significant mantle component (16.3%) in the gas. Here, we suggest the name El Requeso??n fault zone for the faults that likely formed as a result of extension in the region during the late Miocene, and are currently serving as conduits for the observed hydrothermal activity. ?? 2005 Elsevier B.V. All rights reserved.
Additional Publication Details
Gas geochemistry of a shallow submarine hydrothermal vent associated with the El Requeso??n fault zone, Bahi??a Concepcio??n, Baja California Sur, Me??xico