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Abstract: 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 1–3g/L (gryphons) to 145g/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 ~2,100kg of CO2 and 11.5kg of CH4 emitted per day. In addition soil degassing measured at 81 stations (20x20 m grid over 51,000m2) revealed that 7,310kg/d CO2 and 33kg/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. [Copyright &y& Elsevier]

Geologic hydrocarbon seepage is considered to be the dominant natural source of atmospheric methane in terrestrial and shallowwater areas; in deep-water areas, in contrast, hydrocarbon seepage is expected to have no atmospheric impact because the gas is typically consumed throughout the water column. Here, we present evidence for a sudden expulsion of a reservoir-size quantity of methane from a deep-water seep during the Pliocene, resulting from natural reservoir overpressure. Combining three-dimensional seismic data, borehole data and fluid-flow modeling, we estimate that 18-27 of the 23-31 Tg of methane released at the seafloor could have reached the atmosphere over 39-241 days. This emission is ~10% and ~28% of present-day, annual natural and petroleum- industry methane emissions, respectively. While no such ultraseepage events have been documented in modern times and their frequency is unknown, seismic data suggest they were not rare in the past and may potentially occur at present in critically pressurized reservoirs. This neglected phenomenon can influence decadal changes in atmospheric methane. [ABSTRACT FROM AUTHOR]

Abstract: GEMS (Gamma Energy Marine Spectrometer) is a prototype of an autonomous radioactivity sensor for underwater measurements, developed in the framework for a development of a submarine telescope for neutrino detection (KM3NeT Design Study Project). The spectrometer is highly sensitive to gamma rays produced by 40K decays but it can detect other natural (e.g., 238U,232Th) and anthropogenic radio-nuclides (e.g., 137Cs). GEMS was firstly tested and calibrated in the laboratory using known sources and it was successfully deployed for a long-term (6 months) monitoring at a depth of 3200m in the Ionian Sea (Capo Passero, offshore Eastern Sicily). The instrument recorded data for the whole deployment period within the expected specifications. This monitoring provided, for the first time, a continuous time-series of radioactivity in deep-sea. [ABSTRACT FROM AUTHOR]

Earth's hydrocarbon degassing through gas-oil seeps, mud volcanoes and diffuse microseepage is a major natural source of methane (CH 4 ) to the atmosphere. While carbon dioxide degassing is typically associated with extensional tectonics, volcanoes, and geothermal areas, CH 4 seepage mostly occurs in petroleum-bearing sedimentary basins, but the role of tectonics in degassing is known only for some case studies at local scale. Here, we perform a global scale geospatial analysis to assess how the presence of hydrocarbon fields, basin geodynamics and the type of faults control CH 4 seepage. Combining georeferenced data of global inventories of onshore seeps, faults, sedimentary basins, petroleum fields and heat flow, we find that hydrocarbon seeps prevail in petroleum fields within convergent basins with heat flow ≤ 98 mW m -2 , and along any type of brittle tectonic structure, mostly in reverse fault settings. Areas potentially hosting additional seeps and microseepage are identified through a global seepage favourability model.

The accelerated increase in global methane (CH 4 ) in the atmosphere, accompanied by a decrease in its 13 C/ 12 C isotopic ratio (δ 13 C CH4 ) from -47.1‰ to -47.3‰ observed since 2008, has been attributed to increased emissions from wetlands and cattle, as well as from shale gas and shale oil developments. To date both explanations have relied on poorly constrained δ 13 C CH4 source signatures. We use a dataset of δ 13 C CH4 from >1600 produced shale gas samples from regions that account for >97% of global shale gas production to constrain the contribution of shale gas emissions to observed atmospheric increases in the global methane burden. We find that US shale gas extracted since 2008 has volume-weighted-average δ 13 C CH4 of -39.6‰. The average δ 13 C CH4 weighted by US basin-level measured emissions in 2015 was -41.8‰. Therefore, emission increases from shale gas would contribute to an opposite atmospheric δ 13 C CH4 signal in the observed decrease since 2008 (while noting that the global isotopic trend is the net of all dynamic source and sink processes). This observation strongly suggests that changing emissions of other (isotopically-lighter) CH 4 source terms is dominating the increase in global CH 4 emissions. Although production of shale gas has increased rapidly since 2008, and CH 4 emissions associated with this increased production are expected to have increased overall in that timeframe, the simultaneously-observed increase in global atmospheric CH 4 is not dominated by emissions from shale gas and shale oil developments.

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today 1 . A number of different measurements of methane show evidence of transient, locally elevated methane concentrations and seasonal variations in background methane concentrations 2-5 . These measurements, however, are difficult to reconcile with our current understanding of the chemistry and physics of the Martian atmosphere 6,7 , which-given methane's lifetime of several centuries-predicts an even, well mixed distribution of methane 1,6,8 . Here we report highly sensitive measurements of the atmosphere of Mars in an attempt to detect methane, using the ACS and NOMAD instruments onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter from April to August 2018. We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections 2,4 . We suggest that reconciliation between the present findings and the background methane concentrations found in the Gale crater 4 would require an unknown process that can rapidly remove or sequester methane from the lower atmosphere before it spreads globally.

Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions to the total atmospheric methane budget. However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage, which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage, we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories. Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.