Silica-carbonate stromatolites related to coastal hydrothennal venting in Bahia Concepcion, Baja California Sur, Mexico

Submarine diffused seepage (from 5 to 15 in depth) and intertidal focused gasohydrothermal venting take place on the West shore of the Bahia Concepcion Bay, on Baja California, Mexico. The intertidal venting consists of a cluster of hot springs that occur a few meters offshore, with vent temperatures up to 62degreesC and a pH of 6.68. Two irregularly shaped patches of silica-carbonate hot spring deposits occur around the main intertidal vent areas. In addition, a fossil bed of silica-carbonate hot spring deposits of about 75 in long crops out along a cliff next to the active vent area. Both fossil and modem silica-carbonate deposits are finely laminated, and form columnar, bulbous and smooth undulating microstromatolites up to 10 cm thick. Noncrystalline opal-A is the only silica phase present in the modem and fossil hot spring deposits and occurs as microspheres up to 300 nm in diameter forming porous aggregates and irregular clusters, chains and spongy filament networks. The silica supersaturation state of the thermal fluid necessary for opal precipitation is achieved by cooling when it reaches the surface. The presence of preserved microbial remains (diatoms and possibly filamentous microbes) in both modem and fossil deposits reflects the biological activity around the hot springs. The biological activity constrains the fabrics and the textures of the deposit, and could mediate silica deposition. Calcite is the most abundant crystalline phase in the hot spring deposits and forms discontinuous horizons of subhedral bladed crystals within the silica aggregates. Calcite crystals are unusually enriched in C-13, With delta(13)C(V-PDB) values between +3.0parts per thousand and +9.3parts per thousand. The large C-13 enrichment is attributable to a geothermal CO2 degassing process, which yields calcite supersaturation. The delta(18)O(V-PDB) values in calcite, between -10.0parts per thousand and -6.6parts per thousand, indicate precipitation from a hot spring fluid that is a mixture of seawater and meteorically derived water. With the methods applied in this study, no indication of biogenic influence on calcite precipitation has been found. Minor amounts of barite occur in the fossil and modem hot spring deposits and precipitates when Ba2+-rich thermal water mixes with seawater. (C) 2004 Elsevier B.V. All rights reserved.