MODELING SMALL MINERAL PARTICLE LOSSES ALONG SLOPES OF THE SUSQUEHANNA SHALE HILLS CRITICAL ZONE OBSERVATORY

Small mineral particles suspended in soil solution are termed colloids and their redistribution is an important process in the development of soils. The geochemistry, mineralogy and surface properties of colloids are distinct from bulk soil material and thus their physical gain or loss drives changes in soil characteristics. Gain and loss of material in true solution is a separate process involving mineral dissolution/precipitation and driven primarily by chemistry. Effects of both processes can be distinguished and quantified by the recently developed dual-phase mass balance (DPMB) model . Published application of the model has been limited to granite-derived soils on a gentle (≤ 5%) slope under semi-arid climate in South Africa. Here the model is applied to shale-derived soils on a ~21% slope under temperate humid climate at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO).

In granite-derived soils, the preferential partitioning of Ti relative to Zr into colloids as igneous minerals weather makes the Ti/Zr ratio an effective tracer of colloid redistribution. In contrast, shale-derived soils arise from material that has already experienced continental weathering, transport and deposition, and the Ti/Zr ratio did not distinguish colloids. Instead, select elements associated with clays (Al, Ga, Rb) were used in the numerators of tracer ratios and Zr and Hf in the denominators, yielding six different tracers for colloid redistribution. Colloid losses dominated soil development from a mass balance perspective at SSHCZO, ranging from −68 ± 7% to −15 ± 5% relative to starting parent material. Solution losses were predictably smaller considering parent material previously exposed to continental weathering and ranged from -7 ± 2% to a possible gain of 6 ± 1%. By comparison, colloid losses on the granite-derived soils were much smaller (maximum −14 ± 4%) and solution losses were larger (maximum −49% ± 5%). The gentle slope of the granite-derived soils also allowed mineral colloids to accumulate at its base, while the lack of an accumulation zone at SSHCZO may be attributable to a steeper slope. Colloidal versus solutional redistribution of individual elements showed further contrasts controlled by parent material. These results illustrate the insights into soil processes possible with the DPMB model.