CHEMICAL WEATHERING AND MINERAL REACTION TEXTURES IN A POLAR DESERT- EXAMPLES FROM THE MCMURDO DRY VALLEYS, ANTARCTICA

The McMurdo Dry Valleys (MDV) are the largest ice free area (4500 km²) in Antarctica. Situated perpendicular to the Ross Sea coast of southern Victoria Land, the MDV are characterized by hyper-arid landscapes made up of coarse soils derived from glacial tills, glaciers, and closed-basin lakes. Annual precipitation for the MDV is less than 5 cm weq that falls as snow. Liquid water only flows during a 4-12 week period of the austral summer when ephemeral streams connect the glaciers to the endorheic lakes through well-defined stream channels. Although physical weathering is expected to be the dominate alteration process, evidence of chemical weathering of both carbonates and silicate minerals has been previously documented, primarily in the hyporheic zones of stream channels.

Samples of soil, stream sediments, hyporheic zone sediments, and lake bottom sediments were collected and analyzed by scanning electron microscopy (SEM) for evidence of chemical alteration and secondary mineral precipitation to better understand the processes controlling the geochemistry of lakes and streams in these areas. Sediments collected from stream beds and hypohreic zones were comprised primarily of angular to slightly rounded or euhedral silicate mineral grains. Etch pits and other crystallographically controlled dissolution features were observed on silicate minerals, however their distribution was sparse, indicating that the rates of chemical alteration are slow. In contrast, vesicular volcanic ash, presumably derived from nearby active volcano Mount Erebus, was extensively altered, exhibiting scalloped dissolution pits, or almost complete alteration to clay. Although volcanic ash comprises only a small fraction of the soils and sediments, << 0.1%, it contributes disproportionately to the total weathering flux.

Preliminary analysis of Lake Fryxell sediments show sediment is dominated by chemically and biologically formed precipitates. Calcium carbonate phases are abundant, though morphology varies as a function of depth within the sediments- surface carbonates are comprised of elongate crystals and biologically mediated mineral precipitates, whereas deeper in the sediments calcium carbonate is found predominately as silt sized rhombs. These sediment data will be discussed in terms of lake history.