THE GEOCHEMISTRY AND MINERALOGY OF SEDIMENTS FROM LAKE HOARE, ANTARCTICA

The McMurdo Dry Valleys, Antarctica, are the largest ice-free area on the continent, and are considered polar deserts with a mean annual temperature of ~ -20°C and perhaps only 3 cm/yr of precipitation. However, they contain a number of perennially ice-covered lakes, most of these closed basin, and many of them saline. We have analyzed the geochemistry and mineralogy of the top 15 cm of a sediment core from Lake Hoare, one of the freshest, and perhaps youngest of these lakes. Previous work has suggested that the sedimentation rate in Lake Hoare could be as low as 0.05-0.08 mm/yr, so that the best estimate of the sediment interval investigated is ~ 2000 years.

The mineralogy of these sediments is dominated by primary minerals such as quartz, feldspar (~ andesine/labradorite composition and orthoclase), pyroxenes (diopside, enstatite and augite composition), amphibole, and biotite, although SEM and geochemical spot analysis by EDX also indicate clay minerals and CaCO₃ are present. The Chemical Index of Alteration (CIA) has been calculated from the major oxide data and strongly suggests that little to no chemical weathering of the sediments has taken place. The CIA’s are similar to those found in nearby stream channel materials, as well as aeolian debris deposited on the lake ice. When normalized to upper continental crust composition, the bulk elemental geochemistry of these lake sediments also demonstrates little chemical weathering, and values similar to those of the stream channels and aeolian materials. In general, the data suggest that the bulk of the aluminosilicate material found in the surficial sediments of Lake Hoare represent sediment that is extremely immature, is locally sourced, has been produced by physical mechanisms in this harsh environment, and has undergone little to no chemical alteration. Results support previous work and indicates no major change in depositional dynamics over this time period.