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Seismic refraction tracks porosity generation and possible CO2 production at depth under a headwater catchment.
Gu, Xin; Mavko, Gary; Ma, Lisa; Oakley, David; Accardo, Natalie; Carr, Bradley J; Nyblade, Andrew A; Brantley, Susan L.
Afiliación
  • Gu X; Department of Geosciences, Pennsylvania State University, University Park, PA 16802; xug102@psu.edu sxb7@psu.edu.
  • Mavko G; Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802.
  • Ma L; Stanford Rock Physics Laboratory, Stanford University, Palo Alto, CA 94305.
  • Oakley D; Department of Geosciences, Pennsylvania State University, University Park, PA 16802.
  • Accardo N; Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802.
  • Carr BJ; Department of Geosciences, Pennsylvania State University, University Park, PA 16802.
  • Nyblade AA; Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071.
  • Brantley SL; Department of Geosciences, Pennsylvania State University, University Park, PA 16802.
Proc Natl Acad Sci U S A ; 117(32): 18991-18997, 2020 08 11.
Article en En | MEDLINE | ID: mdl-32719121
In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO2 produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.
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Texto completo: 1 Colección: 01-internacional Base de datos: MEDLINE Tipo de estudio: Prognostic_studies Idioma: En Revista: Proc Natl Acad Sci U S A Año: 2020 Tipo del documento: Article Pais de publicación: Estados Unidos
Texto completo
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Texto completo: 1 Colección: 01-internacional Base de datos: MEDLINE Tipo de estudio: Prognostic_studies Idioma: En Revista: Proc Natl Acad Sci U S A Año: 2020 Tipo del documento: Article Pais de publicación: Estados Unidos