Direct Observations of Aluminosilicate Weathering in the Hyporheic Zone of an Antarctic Dry Valley Stream

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Geochimica et Cosmochimica Acta

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This study focused on chemical weathering and bacterial ecology in the hyporheic zone of Green Creek, a McMurdo Dry Valley (Antarctica) stream. An in situ microcosm approach was used to observe dissolution features on the basal-plane surface of muscovite mica. Four mica chips were buried in December 1999 and dug up 39 d later. Atomic force microscopy (AFM) of the basal-plane surfaces revealed small, anhedral ∼10-Å-deep etch pits covering ∼4% of the surfaces, from which an approximate basal-plane dissolution rate of 8.3 × 10−18 mol muscovite cm−2 s−1 was calculated (on the basis of the geometric surface area) for the study period. This is an integrated initial dissolution rate on a fresh surface exposed for a relatively brief period over the austral summer and should not be compared directly to other long-term field rates. The observation of weathering features on mica agrees with previous stream- and watershed-scale studies in the Dry Valleys, which have demonstrated that weathering occurs where liquid water is present, despite the cold temperatures.

AFM imaging of mica surfaces revealed biofilms including numerous small (μm long), rounded, oblong bacteria. The AFM observations agreed well with X-ray photoelectron microscopy results showing increased organic C and N. Bacteriologic analysis of the hyporheic zone sediments also revealed