Isotopic evidence for biogenic precipitation as a principal mineralization process in coastal gasohydrothermal vents, Punta Mita, Mexico

Mineral deposition in shallow submarine gasohydrothermal vents may be triggered by microbial activity according to isotopic data obtained for sulphides and carbonates. Submarine gasohydrothermal vents in Punta Mita, at the western coast of Mexico, discharge a mixture of water and gas (mainly nitrogen and methane) at a temperature of 85 degrees C. Algal mats cover the areas where thermal fluids are being discharged. The main minerals deposited due to the hydrothermal activity are calcite and pyrite. Moreover, barite, carbonate-hydroxylapatite, cinnabar and Tl-sulphide are actively depositing. Calcite is deposited as fine-scale laminated tufa-like aggregates with interlayered pyrite in thin layers. Cinnabar and Tl-sulphide are present within pyrite layers. In Punta Mita vents, almost all delta(34)S values measured in pyrite range from - 13.3 parts per thousand to - 4.9 parts per thousand, and delta(13)C values in calcite vary from 0 parts per thousand to -39 parts per thousand. These results agree with the hypothesis that calcite and pyrite were deposited by mediation of microbial activity, caused by the coupled reactions of vent methane oxidation and marine sulphate reduction, and suggest that biomineralization prevails over abiotic precipitation processes in such coastal gasohydrothermal vents. The C-14 ages obtained from carbonates, of approximately 40,000 years, may represent the minimum age for the methane that generates the carbonates in the vents. (c) 2005 Elsevier B.V. All rights reserved.