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