The CO₂ consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory

Shale covers about 25% of the land surface, and is therefore an important rock type that consumes CO₂ during weathering. We evaluated the potential of gray shale to take up CO₂ from the atmosphere by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ¹³C_DIC) along water flow paths in a well-characterized critical zone observatory (Susquehanna Shale Hills catchment). In this catchment, chemical weathering in shallow soils is dominated by clay transformation as no carbonates are present, and soil pore waters are characterized by low DIC and pH. In shallow soil porewaters, the DIC, dominated by dissolved CO₂, is in chemical and isotopic equilibrium with CO₂ in the soil atmosphere where pCO₂ varies seasonally to as high as 40 times that of the atmosphere. The degradation of ancient organic matter is negligible in contributing to soil CO₂. The chemistry of groundwater varies along different flowpaths as soil pore water recharges to the water table and then dissolves ankerite or secondary calcite under the valley floor. Weathering of carbonate leads to much higher concentrations of DIC (~2500μmol/L) and divalent cations (Ca²⁺ and Mg²⁺) in groundwaters than soil waters. The depth to the ankerite weathering front is hypothesized to be roughly coincident with the water table but it varies due to heterogeneities in the protolith composition. Groundwater chemistry therefore shows different saturation indices with respect to ankerite depending upon location along the valley. The δ¹³C_DIC values of these groundwaters document mixing between the ankerite and soil CO₂. The major element concentrations, DIC, and δ¹³C_DIC in the first-order stream incising the valley of the catchment are derived from groundwater and soil waters in proportions that vary both spatially and temporally. The CO₂ degassed slightly in the stream but little evidence of C isotopic equilibration with the atmosphere is observed, due to the short length of the stream and short contact time with air.The ankerite reaction front also lies close to the pyrite dissolution front. Pyrite oxidation in bedrock likely released sulfuric acid and played a minor role in the ankerite dissolution, shifting groundwater δ¹³C_DIC slightly above the expected mixing values. At the catchment scale, the stream SO₄^2- is also dominantly derived from wet deposition, as stream has δ³⁴S_SO4 values around 3‰, well within the range of acid deposition.