Search for: All records

Methane is a potent greenhouse gas that plays an important role in atmospheric chemistry and global warming. The current global methane budget has large uncertainties, and a better understanding of the budget would help to guide strategies for reducing anthropogenic emissions to fight climate change. Natural geologic methane emissions are a particularly poorly constrained source, with top-down estimates from 14C in ice cores suggesting much lower geologic emissions than bottom-up scaling of direct flux measurements. Our study aims to contribute to resolving this discrepancy through improved bottom-up characterization of geologic methane seepage in the San Juan Basin in southwestern Colorado and northwestern New Mexico, USA. We performed 983 new flux chamber measurements in this basin during summer 2022 and winter 2023 field campaigns. Our results, in combination with prior measurements, suggest that natural seepage in the San Juan Basin only occurs on or near the Fruitland coal outcrop. Specifically, our new measurements confirm previous measurements of seepage along the northwestern exposure of the Fruitland outcrop in Colorado (a known hydrodynamic overpressure region) and for the first time, identified seepage locations along the southernmost Fruitland outcrop exposure in New Mexico, in association with a coal cleat and a fault. Overall, seepage along the Fruitland coal outcrop is heterogeneously distributed, with both positive and negative (interpreted as microbial soil sink) methane fluxes. Features that are hypothesized to be predictive of seepage (e.g., faults) were not associated with positive methane fluxes in areas outside of the Fruitland outcrop. Our best estimate for total geologic methane seepage in the San Juan Basin from spatial interpolation and statistical upscaling is approximately 0.14 Tg CH4/yr, with a range from 0.029 to 0.48 Tg CH4/yr. This best-estimate value is lower than a previous bottom-up estimate from a gridded seepage inventory, but higher than a previous top-down estimate. 

The magnitude of natural geologic methane (CH4) emissions to the atmosphere (including emissions of fossil CH4 from offshore and onshore gas and oil seeps, diffuse microseepage, mud volcanoes, volcanic vents, and geothermal areas) is highly uncertain. The largest component of geologic emissions is thought to be microseepage, which is the diffuse flux of CH4 from soils across large areas of productive hydrocarbon basins. The accuracy of existing bottom-up estimates of microseepage is limited by low spatial coverage of published microseepage measurements. We present the first soil—atmosphere CH4 flux measurements from Michigan Basin, USA. Results from 335 measurements taken during summer and winter seasons across a large portion of the basin suggest microseepage is nonexistent in the sampled region. Even areas with predictive features for microseepage (e.g., underlying mature, organically rich source rocks, proven gas accumulations, faults, and lineaments) yield null or negative fluxes, suggesting that CH4 emissions from microseepage are negligible throughout our study region. A Monte Carlo method was used to place an upper limit on the regional-mean microseepage flux, in which synthetic patchy microseepage distributions were generated and tested against our observations to assess the impact of possible emission hot spots that were missed by our sampling strategy. Our analysis finds it is very unlikely that regional-mean emissions are as high as assumed in a previous global microseepage study. The observed lack of seepage may be explained by groundwater flow, active methanotrophy, glacial sediments, and bedded salt deposits, which could inhibit vertical gas migration and release to atmosphere. 

Oxyhydroxide phases in the (Al,Fe)OOH–MgSiO2(OH) system may form within oceanic lithosphere and transport hydrogen in their crystal structures into the lowermost mantle via cold, subducted slabs. In this work, we present new measurements of the seismic wavespeeds of the dense oxyhydroxide (Al,Fe)-phase H (Al0.84FeMg0.02Si0.06OOH) to 100 GPa constrained by nuclear resonant inelastic X-ray scattering, incorporating previous constraints on the equation of state of this phase. At 300 K and pressure greater than 70 GPa, (Al,Fe)-phase H exhibits high P-wave speeds (lnVp +12%) and low S-wave speeds (lnVs −7%) relative to the preliminary reference Earth model (PREM). Experimentally determined sound velocities are incorporated into a model of a hydrous metabasalt including (Al,Fe)-phase H and compared with the seismic wavespeeds of pyrolitic mantle along appropriate adiabats. Hydrous metabasalt may reproduce an anti-correlation of negative shear wave velocity and positive bulk sound velocity at the upper edges of large, low velocity provinces when compared to pyrolitic mantle but has similar wavespeeds to PREM in this region. Hydrous metabasalt with conceivable concentrations of (Al,Fe)-phase H can be distinguished from PREM in Vs at mid-mantle depths (1100–1700 km) and in V at shallower depths (750–1000 km). Subducted hydrous metabasalt could contribute to scattering of seismic waves across the depth interval of the post-stishovite transition, which may be affected by the formation of (Al,Fe)-phase H.