Flux chamber measurements of natural geologic methane emissions across the United States

Natural geologic methane (CH4) emissions remain one of the most uncertain components of the global atmospheric CH4 budget. These emissions encompass the natural seepage of fossil CH4 from the earth to the atmosphere, including from offshore and onshore gas and oil seeps, diffuse microseepage, mud volcanoes, volcanic vents, and geothermal areas. The large uncertainty in this source stems from a disagreement between bottom-up extrapolation of flux measurements and top-down constraints from ice core 14C measurements. Due to this discrepancy, natural geologic sources could contribute 1-30% of the overall fossil CH4 budget. This large, unresolved uncertainty makes it critical to improve estimates of the magnitude of this source to improve the global budget of CH4, quantification of anthropogenic fossil CH4 emissions, and interpretation of the δ13C-CH4 budget. The largest contribution is thought to be from microseepage or the diffuse emission of CH4 over large areas primarily above hydrocarbon accumulations. However, the current dataset of measurements from microseepage is limited in number, geographic extent, and seepage intensity. Thus, more measurements and regional studies are needed to benchmark new and existing inventories of natural geologic CH4 emissions. To increase the number of CH4 flux measurements of microseepage, this project collected measurements across several hydrocarbon-bearing sedimentary basins across the United States: the Appalachian, Michigan, San Juan, Raton, and Denver-Julesburg basins. These basins ranged from low to high in their predicted level of seepage. Flux measurements were collected such that seepage hotspots were captured and a wide spatial coverage was achieved across the basins. The flux of CH4 from the ground to the atmosphere was measured using closed dynamic flux chambers connected to online, portable greenhouse gas analyzers (a Picarro G4302 and a Los Gatos Research Ultra-Portable Greenhouse Gas Analyzer (CH4/CO2)), which measured CH4 mole fraction. Each site or group of flux measurements (5-10 within ~50 m) was accompanied by measurements of air temperature, barometric pressure, soil temperature, and wind speed. For each measurement, soil moisture was measured and latitude and longitude coordinates were collected with GPS. The dataset includes measurements of both microseepage and miniseepage around seeps. The ~3,300 CH4 fluxes across the 344 sites sampled represent one of the largest datasets collected across the largest geographic extent with a main focus on microseepage. By using this dataset, the users agree to give fair credit to the dataset authors. This means reaching out to the authors early in the process to discuss whether co-authorship may be appropriate. This work was funded by NSF Award AGS-2039234 (to VVP and TSW) and the David and Lucille Packard Foundation Fellowship for Science and Engineering (to VVP). For more information on methods and data interpretation, see: Hall KR, Weber TS, Stock MP, Buursink ML, Piao H, Zhu M, Walter Anthony KM, Petrenko VV. 2026. New measurements indicate that natural geologic methane emissions from microseepage in the Michigan Basin are likely negligible. Elem Sci Anth 14(1): 00058. doi: 10.1525/elementa.2025.00058 Kazemi R, Schlageter W, Hmiel B, Weber TS, Murray LT, Petrenko VV. 2021. Investigating methane emissions from geologic microseepage in Western New York State, United States. Elementa: Science of the Anthropocene 9(1): 00066. doi: 10.1525/elementa.2020.00066 Scholer, M. R., Hall, K. R., Weber, T. S., Buursink, M. L., Zhu, M., Ihle, A. C., Hencmann, D., Smith, A. M., Anthony, K. M. W., and Petrenko, V. V.: Bottom-up characterization of geologic methane emissions in the San Juan Basin in the southwestern USA, Elem Sci Anth, 14, 00061, https://doi.org/10.1525/elementa.2025.00061, 2026.