Fluid origin, gas fluxes and plumbing system in the sediment-hosted Salton Sea Geothermal System (California, USA)

The Salton Sea Geothermal System (California) is an easily accessible setting for investigating the interactions of biotic and abiogenic geochemical processes in sediment-hosted hydrothermal systems. We present new temperature data and the molecular and isotopic composition of fluids seeping at the Davis-Schrimpf seep field during 2003–2008. Additionally, we show the first flux data for CO2 and CH4 released throughout the field from focused vents and diffuse soil degassing.The emitted gases are dominated by CO2 (~ 98%) and CH4 (~ 1.5%). By combining δ13CCO2 (as low as − 5.4‰) and δ13CCH4 (− 32‰ to − 17.6‰) with 3He/4He (R/Ra > 6) and δDCH4 values (− 216‰ to − 150‰), we suggest, in contrast to previous studies, that CO2 may have a significant Sub-Continental Mantle source, with minimal crustal contamination, and CH4 seems to be a mixture of high temperature pyrolitic (thermogenic) and abiogenic gas.Water seeps show that δD and δ18O increase proportionally with salinity (Total Dissolved Solids in g/L) ranging from 1–3 g/L (gryphons) to 145 g/L (hypersaline pools). In agreement with elemental analyses, the isotopic composition of the waters indicate a meteoric origin, modified by surface evaporation, with little or no evidence of deep fossil or magmatic components. Very high Cl/Br (> 3,000) measured at many seeping waters suggests that increased salinities result from dissolution of halite crusts near the seep sites.Gas flux measurements from 91 vents (pools and gryphons) give a conservative estimate of ~ 2,100 kg of CO2 and 11.5 kg of CH4 emitted per day. In addition soil degassing measured at 81 stations (20x20 m grid over 51,000 m2) revealed that 7,310 kg/d CO2 and 33 kg/d CH4 are pervasively released to the atmosphere. These results emphasise that diffuse gas emission from soil can be dominant (~ 75%) even in hydrothermal systems with large and vigorous gas venting. Sediment-hosted hydrothermal systems may represent an intermediate class of geologic methane sources for the atmosphere, with emission factors lower than those of sedimentary seepage in petroleum basins but higher than those of traditional geothermal-volcanic systems; on a global scale they may significantly contribute to the atmospheric methane budget.

Research highlights
► CO2 stable carbon and helium isotopic composition (R/Ra > 6) suggest a considerable Sub-Continental Mantle source
► CH4 likely to be a mixture of high temperature pyrolitic and abiogenic gas
► strong component of meteoric input in seeping waters
► emission of at least 3,400 t/y of CO2 over a 51,000 m2 both by venting and soil degassing
► soil degassing represents 75% of gas released in the analysed area