title: Stable light isotope biogeochemistry of hydrothermal systems.
abstract: The stable isotopic composition of the elements O, H, S and C in minerals and other chemical species can indicate the existence, extent, conditions and the processes (including biological activity) of hydrothermal systems. Hydrothermal alteration of the 18O/16O and D/H values of minerals can be used to detect fossil systems and delineate their areal extent. Water-rock interactions create isotopic signatures which indicate fluid composition, temperature, water-rock ratios, etc. The 18O/16O values of silica and carbonate deposits tend to increase with declining temperature and thus help to map thermal gradients. Measurements of D/H values can help to decipher the origin(s) of hydrothermal fluids. The 34S/32S and 13C/12C values of fluids and minerals reflect the origin of the S and C as well as oxygen fugacities and key redox processes. For example, a wide range of 34S/32S values which are consistent with equilibration below 100 degrees C between sulfide and sulfate can be attributed to sulfur metabolizing bacteria. Depending on its magnitude, the difference in the 13C/12C value of CO2 and carbonates versus organic carbon might be attributed either to equilibrium at hydrothermal temperatures or, if the difference exceeds 1% (10/1000), to organic biosynthesis. Along the thermal gradients of thermal spring outflows, the 13C/12C value of carbonates and 13C-depleted microbial organic carbon increases, principally due to the outgassing of relatively 13C-depleted CO2.
authors.affiliation: Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Des Marais
mesh_terms: D001669:Biochemical Phenomena; D001671:Biochemistry; D002244:Carbon / Q000737:chemistry; D002247:Carbon Isotopes; D002254:Carbonates / Q000737:chemistry; D003903:Deuterium / Q000737:chemistry; D004777:Environment; D019015:Geologic Sediments / Q000737:chemistry; D055593:Geological Phenomena; D005844:Geology; D006358:Hot Temperature; D006859:Hydrogen / Q000737:chemistry; D007554:Isotopes; D008903:Minerals / Q000737:chemistry; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry; D010103:Oxygen Isotopes; D012822:Silicon Dioxide / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry; D013460:Sulfur Isotopes; D014867:Water / Q000737:chemistry
chemical_list: D002247:Carbon Isotopes; D002254:Carbonates; D007554:Isotopes; D008903:Minerals; D010087:Oxides; D010103:Oxygen Isotopes; D013460:Sulfur Isotopes; D014867:Water; D013455:Sulfur; D002244:Carbon; D012822:Silicon Dioxide; D006859:Hydrogen; D003903:Deuterium; D010100:Oxygen