CHEMICAL WEATHERING ACROSS A PASSIVE MARGIN ESCARPMENT, SOUTHEASTERN AUSTRALIA

The Great Escarpment of southeastern Australia is an abrupt topographic boundary between the coastal lowlands and the tablelands of southeastern New South Wales, Australia. Differences in rainfall and temperature have likely created differences in the intensity of chemical weathering in granitic saprolite across the escarpment. Topographic unroofing and physical weathering rates, calculated from U/Th(He) low temperature thermochronometry and from 10Be and 26Al cosmogenic nuclides, set the boundary conditions for chemical weathering rates and influence chemical weathering intensities. Here we present initial results from a study of chemical weathering rates on soil-mantled hillslopes and their coupling to climate, deep exhumation rates, and soil production rates both above and below the escarpment.

We used a truck-mounted drill rig with dry rotary augers to core and sample several saprolite weathering profiles to refusal on hillslopes at three sites: 'FH,' on the upland plateau, 'NR,' at the immediate base of the escarpment, and 'SN,' 10 km seaward of the escarpment. The samples were analyzed for major oxides and trace elements by x-ray fluorescence spectrometry and for mineralogy by x-ray diffraction. We used geochemical mass balance calculations to determine chemical weathering rates and weathering indices to examine the chemical weathering intensities. The rate of chemical weathering represents 32-34% of total weathering at all sites. The intensity of weathering in cores varies between sites: the mean and standard deviation of Chemical Index of Alternation values at FH is 63.4±2.1 (n=36) while at NR it is 68.7±3.3 (n=16). The fresh granodiorite at FH and NR has CIA values of 59. The upland site, FH, exhibits a lower chemical weathering intensity despite having a land-surface age almost three times that of the NR. The faster physical weathering rates at the base of the escarpment may correspond to faster chemical weathering rates. We should find a link between elevation, rainfall, and chemical weathering rates and intensities. Our investigation supports our previous work in northern California on the inverse relationship between the intensity of chemical weathering in saprolite and the rate of soil production.