COLLOID AND SOLUTE MASS REDISTRIBUTION QUANTIFIED ALONG A GRANITIC SOUTH AFRICAN CATENA

Soils and geomorphology are linked through transport processes that redistribute soil material across landscapes as well as within individual soil profiles. Water flow through soil redistributes both dissolved and suspended colloidal material at relatively slow rates, but over time such redistributions strongly influence soil chemistry, mineralogy, and texture. Determining the processes controlling mass redistribution across hillslopes is an important goal in soil geomorphology, but it is often difficult to identify colloidal versus solutional redistribution because of their slow rates, periodic nature, and a lack of unique natural tracers.

We traced upslope removal and downslope accumulation of colloidal material along a granitic South African catena using ratios of the low solubility, high field strength elements Ti and Zr. Quantification of colloidal material redistribution was accomplished by expanding the standard use of mixing equations to also address losses. Mass balance relationships implicit in mixing equations were used to determine redistributed colloidal mass and parent material mass relative to modern soil profiles. Redistributions of mass that could not be accounted for by colloidal transfers were assumed to result from dissolved transport.

The maximum mass loss of colloidal material from a soil horizon was 10% relative to the parent material from which the horizon formed. Integrated across whole soil profiles, maximum colloidal mass loss was 8% relative to parent material. These maxima occurred in the more strongly redox-influenced seepline zone located at midslope. Mass losses via solution were greater, with a maximum 48% loss from a soil horizon, and a 41% loss from a soil profile. Maximum accumulation of colloidal material relative to parent material was 25% for a downslope soil horizon and 12% for a downslope profile. No soil profile showed an overall mass gain of material transferred in solution, but mass gains via solution of up to 22% for individual soil horizons were suggestive of clay mineral neoformation. The results clarify the relative importance of dissolved versus suspended transport and provide unique insight into processes that shape soils and geomorphology.