Publication Date:
2017-04-04
Description:
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/RaN6) 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 (N3,000) 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.
Description:
Published
Description:
67-83
Description:
4.5. Studi sul degassamento naturale e sui gas petroliferi
Description:
JCR Journal
Description:
reserved
Keywords:
Salton Sea Geothermal System
;
hydrothermal seeps
;
gas and water geochemistry
;
flux measurements
;
mantle
;
03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases
;
04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
Permalink
