CLAY MINERAL FORMATION IN THE SUBGLACIAL ENVIRONMENT OF EAST ANTARCTICA - IMPLICATIONS FOR CHEMICAL WEATHERING AND BIOGEOCHEMICAL CYCLES

In order to determine the degree of chemical weathering occurring in the subglacial environment of East Antarctica, we compared the mineralogy of freshly emerging subglacial sediments to that of cobble sized clasts representative of the underlying bedrock. 12 rock samples and 15 fine sediment samples from Mt. Achernar Moraine, central Transantarctic Mountains were pulverized with a corundum standard, allowing for quantitative mineralogical assessments through powder x-ray diffraction. The underlying bedrock consists of Jurassic clastic sedimentary rock interbedded with dolerite sills. The dolerites were found to consist of primarily labradorite feldspars and pyroxenes, with a smaller quantity of sodic feldspars and quartz. The sedimentary rocks primarily contain albite, orthoclase, quartz, illite and chlorite.

Despite the fact that ~60% of the rock clasts at the surface of the moraine are dolorite, the feldspars and pyroxenes in the fine sediment suggest an ~65% contribution of sedimentary rock to the fine glacial sediment. The glacial sediment is enriched in illite and quartz compared to rock, is totally depleted of chlorite and has abundant smectite and kaolinite. As smectite and kaolinite have only trace abundance in the underlying rock, we infer that these minerals formed in the subglacial environment. The enrichment of quartz, despite its relative lack of friability, suggests loss of feldspars and pyroxenes to clays. Illite may have been enriched by mechanical processes. Chlorite appears to have nearly entirely altered to kaolinite.

We conclude that a substantial subglacial chemical alteration has occurred at this site. If the subglacial alteration occurring at this site is broadly representative of East Antarctica and of continental glaciation generally, then chemical alteration to clays on weathering continental surfaces continues to occur in Earth’s cold periods.