Abstract

A key societal service of the critical zone (CZ) is its regulation of the global carbon cycle through silicate mineral weathering. Recent research at the Jemez River Basin Critical Zone Observatory (JRB-CZO) has indicated that a deep groundwater reservoir likely resides in the fractured rhyolitic bedrock, and that longer residence time water, undergoing prolonged reaction with primary silicates, contributes to stream flow. However, the structure and composition of the deep CZ at the JRB-CZO has been poorly defined because of a lack of direct measurements. Furthermore, the role of complex geologic variability on mineral dissolution and transformation reactions in the JRB-CZO is poorly understood. To resolve the influence of spatial variability in regolith composition on pore water chemistry, we conducted a set of kinetic dissolution experiments on extracted core samples. Subsamples from cores collected from two deep-drilling sites (to 35 m, with differing geology) at the JRB-CZO were reacted in batch mixed reactors with DI water that was pre-equilibrated with the atmosphere. Samples were sacrificed at a range of time steps (1, 4, 24, 336, 1344, and ca. 3000 h) and solutions were analyzed for major and trace solutes. Initial results suggest that mineralogical differences in the two cores drive corresponding differences in reaction rates and the time evolution of solution chemistry. Core 1, with weathered regolith and higher overall mass fractions of secondary minerals (e.g., iron oxides and 2:1 clays) had lower ion release (i.e., aqueous concentrations) at all time steps compared to the less weathered regolith of Core 2, which had larger mass fractions of primary minerals (e.g., feldspars, cristobalite, and volcanic glass). Interestingly, zones of hydrothermally altered regolith (≥50 % zeolites and smectites) in Core 2 had higher ion release than weathered regolith in Core 1. These results suggest that the degree of antecedent weathering and mineral assemblage (both primary and secondary) in relatively organic-poor deep CZ exert primary control on aqueous geochemistry, and therefore regulate kinetics and thermodynamics on pore water evolution during transport within the deep critical zone.

Citation

Keifer, V.C., Moravec, B.G., Root, R., Amistadi, M.K., Chorover, J. (2018): Weathering in the Deep Critical Zone: Mineralogical Controls on Aqueous Geochemistry in the Deep Subsurface. Abstract H13J-1858 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.