| Abstract |
Water-rock interactions involving belowground respiratory CO2 in groundwater constitute an understudied portion of the critical zone, which limits our understanding of global carbon cycles. For example, basalt weathering is estimated to sequester approximately 0.21 petagrams of carbon per year (PgC yr-1), which is comparable to other systems (e.g. total terrestrial CO2 sink: 2.9±1 PgC yr-1, riverine fluxes: 0.78 PgC yr-1), however these basalt weathering estimates do not account for processes in deep groundwater. At the Reynolds Creek Experimental Watershed (RCEW) and Critical Zone Observatory (CZO), located south of the western Snake River Plain in the Intermountain West of the United States, we trace critical zone reactions in conjunction with groundwater flow and residence times in semi-arid weathered silicate aquifers using hydrochemical and geophysical techniques aimed to address these knowledge gaps. Using data from 10 groundwater monitoring wells in the Summit area, 6 springs across the watershed and 2 irrigation pond samples, multiple findings were published and are briefly listed herein. (1) Most sampled groundwater is from fractured basalt aquifers (87Sr/86Sr 0.70445 – 0.70545), though 3 springs are influenced by granitic aquifers (87Sr/86Sr 0.70616 – 0.70690). (2) Groundwater from monitoring wells is more affected by evaporation than springs, with less evaporation in older, deeper groundwater (as low as 10%) as compared to younger, more shallow groundwater (as high as 54%), though evapoconcentration occurs in all wells. Furthermore, evaporation of shallow groundwater likely occurred subaerially, whereas evaporation of deeper groundwater likely occurred predominantly in the unsaturated zone. (3) Shallow groundwater samples are generally less mixed and more recent (10 to 70 3H yrs BP) than deeper groundwater samples (1469 to 6350 14C yrs BP). (4) A previously unknown blind hydrothermal system is present, with cool (as low as 11.8oC) groundwater over warm (up to 28.6oC) groundwater, as detected by down-hole measurements, showing that mixing is limited between shallow and deeper fracture zones. (5) Most groundwater initially evolves under open system conditions (in contact with belowground respiratory CO2), followed by continued water-rock reactions under closed system conditions, as indicated by pH, dissolved inorganic carbon (DIC), and δ13CDIC isotope values. (6) Silicate weathering (including clay formation) and secondary calcite precipitation drive carbon sequestration, as determined by geochemical modeling, possibly sequestering 9% of belowground respiratory CO2 as calcite. Analyses of multiple parameters for each sample has resulted in an extensive dataset, and improved our knowledge of water-rock interactions in RCEW-CZO. Application of these results increases our understanding of the effects of critical zone groundwater-rock interactions on earth systems. |
| Authors |
Melissa E. Schlegel  , Kathleen A. Lohse , Jennifer Souza , Sara R. Warix , Erin Murray , Sarah E. Godsey , M. S. Seyfried , Mark D. Schmitz , Bruce P. Finney  , Zane K. Cram
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