Carbon cycling and mineral weathering in temperate forested watersheds: An integrated study of solution and soil chemistries

The interactions between carbon transformations and mineral weathering on both the plot and watershed scale were explored in this dissertation. The mid-continent U.S. hosts permeable Pleistocene glacial drift deposits that forge a close coupling between groundwaters and surface waters and allow investigation into weathering processes occurring at relatively shallow depths. Additionally, because glacial drift commonly contains carbonate minerals, the deposits serve to control watershed pH values and generate globally significant weathering fluxes of inorganic carbon. Along a climate gradient from northern to southern Michigan, higher inorganic carbon fluxes were observed in southern Michigan accompanied by higher groundwater pCO2 values that suggest landscape-level differences in the carbon cycle across the region. A plot manipulation in which trees were grown under different treatments of atmospheric CO2 and soil fertility showed that carbonate weathering increased under both increased atmospheric CO2 and in more fertile soils. The weathering response was likely the result of increases in soil respiration and accordingly soil pCO2. Silicate mineral weathering values in the soil, which are commonly thought to slow weathering reactions. Importantly, the charge on organic ligands present in dissolved organic carbon increases with increasing pH, potentially enhancing feldspar dissolution reactions. These results were investigated further at the watershed scale in the contrasting geochemical environments of the carbonate-rich Cheboygan and carbonate-poor, organic-rich Tahquamenon watersheds. Weathering processes and fluxes were observed through examination of soil solutions, stream, and groundwater chemistry. Interestingly, soil solution and stream chemistries indicate that the well-buffered Cheboygan watershed generates higher fluxes of weathering-derived Na (Na*) than the Tahquamenon. A statistically significant though small positive correlation between Na* and pH across the watersheds tends support to the hypothesis that organic ligands may be more effective in moderate- than low-pH environments. The studies comprising this dissertation demonstrate that the carbon cycle is dynamically linked to mineral weathering and suggest that anthropogenic perturbations to the carbon cycle may impact continental weathering fluxes.