HR: 1340h
AN: B43D-0306    [Abstracts]
TI: Chemical and Physical Weathering of Granites in a Semi-Arid Savanna
AU: * khomo, l
EM: lesegok@gmail.com
AF: university of the witwatersrand, 1 jan smuts avenue biology building PO wits, Johannesburg, 2050
AU: Hartshorn, A
EM: tony@geog.ucsb.edu
AF: university of california, department of geography 3611 ellison hall, santa barbara, ca 93106
AU: Chadwick, o
EM: oac@geog.ucsb.edu
AF: university of california, department of geography 3611 ellison hall, santa barbara, ca 93106
AU: kurtz, a
EM: kurtz@bu.edu
AF: boston university, department of earth sciences, boston, 02215
AU: heimsath, a
EM: Arjun.Heimsath@dartmouth.edu
AF: dartmouth college, department of earth sciences, hanover, 03755
AU: rogers, k
EM: kevinr@gecko.biol.wits.ac.za
AF: university of the witwatersrand, 1 jan smuts avenue biology building PO wits, Johannesburg, 2050
AB: The catena concept describes soil properties on hillslopes and implies a hydrological mass redistribution process that has been applied differently in different parts of the Earth. In tectonically active regions, it is mostly used to describe the redistribution of mass by overland flow leading to thickening soil mantles downslope. This application is somewhat different from its initial and still popular usage in tectonically inactive areas of Africa, where it defines long-term soil property differentiation along hillslopes as controlled by internal soil hydrology as opposed to overland flow. Many ecologists have found the "African" catena concept to be useful as an organizing principal for savanna studies, but there has been little recent research on catenas per se in Africa. Elsewhere however, there is a growing body of research that places the concept ever more strongly into a landscape evolution context. Here, we apply these new approaches to catenas in a South African savanna underlain by a heterogeneous suite of Basement granites straddling a gradient in effective precipitation. We constrain the weathering extent of hilly terrains formed on these oldrocks by calculating element losses with solid-phase mass-balance calculations augmented by cosmogenic (26Al/10Be) derived rates of landscape denudation. We test the efficacy of Ti, Zr and Nb as immobile elements to benchmark chemical losses and gains in these semi-arid weathering environments. We also trace and quantify the abundance of the host minerals for these elements (Ti = rutile and ilmenite, Nb = columbite and Zr = zircon and baddleyite) in a variety of rocks in the basement complex. This analysis provides the boundary conditions for assigning immobile elements to parent materials required for the mass balance calculations. We calculate total denudation using the cosmogenic isotopes and then partition it into chemical and physical loss vectors using the mass balance calculations for representative watersheds along the effective precipitation gradient. Preliminary results suggest that these semi-arid landscapes erode at a slow rate and the upper portions of the catenas are highly weathered with a predominance of quartz as existing primary minerals. The catenas appear to be some of the oldest and most highly evolved yet studied.
DE: 0400 BIOGEOSCIENCES
SC: Biogeosciences [B]
MN: Fall Meeting 2005