Abstract

Chemical weathering of silicate minerals via the carbonic acid reaction pathway regulates global climate on geological timescales. However, strong acids are also key dissolution agents that drive silicate and carbonate weathering. In order to assess the potentials of silicate weathering on CO2 consumption, it is crucial to separate carbonic acid versus sulfuric acid reaction pathways, and also to separate the contribution of stream-dissolved inorganic carbon (DIC) from silicate versus carbonate dissoution. Here we address these two questions using C and S isotopes at the well-studied Susquehanna Shale Hills Critical Zone Observatory (SSHO).

In shallow soils of SSHO, clay dissolution dominates. Here soil waters are charaterized by low [DIC], which is controlled by equilibrium with soil pCO_². Carbonate minerals, in this Rose Hill Shale formation, are depleted in soils and have only been observed in few bedrock boreholes, i.e. at > 23m depth at ridges and > 2m depth under the valley. Indeed, some groundwaters have much higher [DIC], [Mg] and [Ca], presumably due to ankerite dissolution. Accompanied by the transition from silicate weathering in shallow soils to carbonate weathering below the water table, the source of sulfate shifts with depth from atmospheric deposition to pyrite dissolution. Apparently, the weathering fronts of ankerite and pyrite are at almost the same depth. The δ¹³C_DIC values of these groundwaters indicate C mixing equally from ankerite and soil CO₂, with only slight modification by the sulfuric acid pathway.

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. Interestingly, groundwaters along the valley floor at the outlet of the first-order catchment are influenced by carbonate dissolution but also show S isotope signatures indicative of anthropogenic sulfate in wet precipitation. This provides another line of evidence that at least some of the carbonate we observe at shallow depths in the valley floor may be secondary. Indeed, C isotopes of some of the shallow carbonates differ from those in Rose Hill bedrock.

Comparison between groundwater and soil water chemistry shows that at SSHO most DIC derives from the dissolution of carbonate minerals, i.e., primary ankerite or secondary carbonate. Sulfate derives almost entirely from atmospheric deposition in soil waters and some groundwater near the outlet; however, its source shifts to pyrite dissolution in groundwaters from ridges and headwater areas. Overall, in this catchment underlain by grey shale, the sulfuric acid pathway is insignicant due to the low pyrite content in comparison to ankerite or secondary carbonate.

Citation

Jin, L., Ogrinc, N., Yesavage, T., Hasenmueller, E.A., Ma, L., Kaye, J.P., Brantley, S.L. (2013): Using C and S isotopes to elucidate carbonic versus sulfuric acid reaction pathways during shale weathering in the Susquehanna Shale Hills Critical Zone Observatory . Abstract H54A-05 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..

This Paper/Book acknowledges NSF CZO grant support.