Geomorphic controls on mineral weathering, elemental transport, carbon cycling, and production of mineral surface area in a schist bedrock weathering profile, Piedmont Pennsylvania

Abstract

Association of organic carbon (OC) with mineral surface area is a key mechanism for protecting OC from microbial consumption, yet the development of mineral surface area by biogeochemical weathering processes and its role in controlling the organic carbon cycle and distribution within hillslopes have rarely been explicitly delineated. Addressing this significant knowledge gap is the goal of this study. The measurement of deep mineral surface area profiles in the Laurels Schist within the Christina River Basin Critical Zone Observatory, revealed abrupt transitions in mineral specific surface area at 3 meters. This depth did not coincide with the soil-weathered rock boundary or the water table, which is contrary to the present paradigm in delineating depth progression of weathering. This finding instead highlights the potential role of oxygen penetration as a critical process defining the vertical distribution of biogeochemical weathering process. The site’s schist bedrock is highly variable. Such heterogeneity has challenged earth scientists attempting to mark and determine the extent of chemical weathering in terrain underlain by sedimentary and meta-sedimentary bedrocks. To overcome this limitation, a new statistical model was developed to delineate the subsurface boundaries in heterogeneous rock types and to clearly assess and display weathering trends buried in elemental data. This comprehensive understanding of weathering trends and the genesis of mineral surface area facilitated the assessment of changes in OC and mineral surface area (OC/SA) in the hillslope soil transect. Minerals are found to hold more organic matter on their surface in response to soil mixing and soil erosion and deposition. Organic matter properties such as C/N, del15N, and del13C also respond to soil mixing as a function of soil profile depth and landscape position. A strong correlation between C/N and OC/SA indicates that both properties may provide a proxy for organic matter stability.