Cu isotopes and concentrations during weathering of black shale of the Marcellus Formation, Huntingdon County, Pennsylvania (USA)

The complex weathering processes which govern the production of soil from bedrock have proven difficult to understand for many lithologies. Weathering of black shale is of particular interest because it releases organic carbon and heavy metals as solutes and therefore impacts the health of terrestrial and aquatic ecosystems. To understand black shale weathering, a geochemical survey was initiated for soils developed on shales of the Marcellus Formation at a zero-order catchment at a satellite site of the Susquehanna/Shale Hills Critical Zone Observatory located in Jackson Corner, Pennsylvania. This formation is an organic- and metal-rich, carbonaceous shale that underlies much of New York, Pennsylvania, Ohio and West Virginia. In this paper, we focus on the effects of weathering on variations of Cu isotopes in the shale. Cu concentration data for soil were normalized using Ti concentrations to document the mobility of Cu relative to bedrock. At both the ridgetop and valley floor, depletion profiles for Cu are documented in the soils. The Cu in the soils is depleted in 65Cu (average δ65Cu = − 0.5‰ ± 0.2) compared to the parent material (average δ65Cu = 0.03‰ ± 0.15). Consistent with loss of Cu from soils, the pore waters contain 10 ppb Cu on average and are enriched in the heavy isotope (average value δ65Cu = 1.14‰ ± 0.44). Rayleigh fractionation models using the concentration and isotope data of the soils are consistent with pyrite weathering and loss of Cu from the ridgetop, but downslope transport and Cu re-precipitation at the valley floor.

► Cu isotope and concentrations used to identify weathering processes of black shales. ► Soils developed on black shales isotopically depleted and lower concentrations in comparison to parent rock. ► Isotopic mass balance exists between pore waters, soils and parent rocks. ► Data indicate pyrite weathering and loss of Cu from ridgetop, downslope transport and precipitation at valley floor.