HR: 1340h
AN: H43C-0383    [Abstracts]
TI: Investigating an Inverted Soil Column in Northern Tanzania: Could Intense Groundwater Weathering be the Culprit?
AU: * Little, M G
EM: mglittle@rice.edu
AF: Rice University, Deparment of Earth Sciences MS 126 6100 Main Street, Houston, TX 77005 United States
AU: Lee, C A
EM: ctlee@rice.edu
AF: Rice University, Deparment of Earth Sciences MS 126 6100 Main Street, Houston, TX 77005 United States
AB: Weathering of silicate rocks permanently sequesters a significant amount CO$_{2}$ on our planet (Berner et al., 1983; Dessert et al., 2003; Gaillardet et al., 1999). Therefore, the investigation of soils and their conjugate protoliths have implications for a wide range of disciplines from soil to atmospheric sciences. This study investigates soil formed in Northern Tanzania on the southern slope of the dormant volcano Mt. Kilimanjaro. Our sample site is in the Machame region at an elevation of ~1640 m where the phonotephrite to basaltic bedrock has been dated at 0.4 to 0.5 million years (Evernden and Curtis, 1965). We determined bulk elemental concentrations of soil and bedrock samples from this region using an ICP-MS and XRF. From initial investigations into the bulk soil and bedrock chemistry using a novel mass balance method, we were able to investigate the relative mobility of a suite of elements. Relative abundances of Ta, Nb, Hf, and Zr are constant and therefore these elements are immobile. In contrast, Ti, an element commonly thought to be immobile, is clearly not immobile in our samples. The entire soil column appears to be highly depleted in Si and Ca but enriched in Al. These features indicate extensive weathering and indeed some samples approach bauxite compositions. Surprisingly, however, weathering versus depth is reversed. Si and Ca have been removed by 60 and 70 % respectively from the upper 2 meters, but below 2 m, they have been removed by 95 and 99 %. This means that the soil is more weathered at depth than in the shallowest 2 meters. We believe that ground water weathering is responsible for this inverted soil profile and may increase CO$_{2}$ consumption estimates by 10-30 % for similarly affected basalts.
DE: 1886 Weathering (1625)
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting