CHEMICAL WEATHERING RATES INFERRED FROM TASMANIAN LAKE SEDIMENTS

To evaluate the progress of chemical weathering in under temperate humid conditions, we have undertaken chemical and mineralogical analyses of sediment cores from Lake MacKenzie, Tasmania, Australia. Located within the Jurassic dolerites of the tectonically quiescent Tasmanian Highlands, the lake and its watershed were close to the ice margin at the last glacial maximum. The drainage basin was essentially cleared of weathered material at that time by glacial or periglacial processes. Since then, chemical weathering has led to development of shallow soil profiles and the accumulated lake sediments contain progressively more altered minerals. Because this region receives extremely low levels of aeolian dust, the Lake MacKenzie sediment record provides an archive of changing chemical weathering rates over the past 20,000 years.

To take advantage of this archive, we have undertaken elemental analyses by quantitative x-ray fluorescence (XRF) core scanning and clay mineral identification by x-ray diffraction (XRD). Our results indicate a decrease in chemical weathering rates within the first few thousand years after deglaciation, with relatively constant weathering thereafter. We will evaluate these rates in the context of chemical weathering determinations in contrasting climatic and tectonic settings.