Abstract

Shales, covering 25% of land surface, are an important lithology in linking the CO₂ drawdown and continental silicate weathering on the global scale. In this study, we aim to evaluate the potentials of shale weathering to consume CO₂ by investigating the elemental chemistry as well as stable isotopes (C, S and O) along the water flow paths in the well-studied Susquehanna Shale Hills Critical Zone Observatory (SSHO). We also try to determine the potentials of releasing CO₂ by quantifying the relative importance of sulfuric acid in carbonate dissolution and the decomposition
rates of ancient organic matter.

At SSHO, shallow soils are dominated by clay dissolution, and soil waters are low in dissolved inorganic carbon (DIC) concentrations controlled by equilibrium with soil pCO₂. Here CO₂ is produced primarily by oxidative decomposition of soil organic matter, and its concentrations vary seasonally and spatially. Ankerite, present in the Rose Hills bedrock, is depleted in soils and only remains at greater depths.   Weathering of ankerite contributes to much higher concentrations of DIC and divalent cations (Ca and Mg) in groundwaters, but groundwater chemistry evolves to different extents with respect to ankerite saturation because the depths to ankerite weathering fronts vary due to heterogeneity of the Rose Hill shales and landscape position. Consistently, the δ¹³C_DIC ratios of these groundwaters are indicative of mixing between DIC from ankerite and soil CO₂ endmembers. Hydrologically, the first-order stream is contributed by different proportions of groundwater and shallow soil waters as observed by major elemental chemistry, [DIC] and δ¹³C_DIC ratio of stream waters.

In addition to reacting with carbonic acid , shale can also react with sulfuric acid. The strong acidity derives from oxidative dissolution of pyrite at SSHO. Similar to ankerite, pyrite, at trace levels, is depleted from soils and is only present close to the ankerite weathering front at depth. Additionally, Pennsylvania receives high rates of acid deposition, loading significant amounts of sulfuric acid at the land surface. Thus at SSHO, the dissolution of the carbonate mineral ankerite by sulfuric acid may release CO₂, instead of consuming CO₂, and may be important in the mass balance of inorganic carbon. We collected preliminary S/O isotope data to evalute the sources of sulfuric acid and quantify its involvement in shale weathering.

The global inventory of organic carbon in sedimentary rocks such as shales is greater than all the other surface reservoirs combined. Although most of the ancient organic matter is relatively refractory, it may still be altered at Earth’s surface, which is important in the atmospheric CO₂ and O₂ levels on global scales. Our ongoing investigation is also focused on the degradation rates of this ancient organic matter during shale weathering and its contribution to CO₂ mass balance using C isotope analyses.

Citation

Jin, L., Ogrinc, N., Yesavage, T., Kaye, J.P., Brantley, S.L. (2012): Drawdown of atmospheric CO2 by gray shale weathering: insights from carbon, sulphur, and oxygen isotope systematics in the Susquehanna Shale Hills Critical Zone Observatory. AGU Annual Fall Conference Proceedings.

This Paper/Book acknowledges NSF CZO grant support.