Catchment-wide denudation rates from the Murrumbidgee River, Murray-Darling Basin, SE Australia, using in situ cosmogenic ¹⁰Be

The Murray-Darling Basin (MDB) is a large inland basin in southeast Australia (draining one seventh of the continent), and is by far the most important agricultural centre of the country. Sustainable use of the basin resources is a significant national agenda where the balance between two contrasting aspects - ecological preservation and industrial/agricultural development - has been long-debated. To evaluate the impact of human land use, it is required to understand the natural response of the basin to various external forcing such as climate and CO₂ fluctuation. This study is the first part of a larger investigation on the behavior of different river systems within MDB in response to past climate changes. Here, we present a study of the Murrumbidgee River catchment for the estimate of catchment-wide denudation rates using in situ cosmogenic-nuclide measurement in sediments. The Murrumbidgee River is among the longest (length approximately 1485 km) of the MDB rivers. Four stages of paleo-channel systems were identified and dated previously by thermoluminescence. The paleo-denudation rates were calculated from the measured ¹⁰Be corrected for post-depositional nuclide production. Results show remarkably constant nature of catchment-wide denudation rate (12-15 m/Ma) for the last 100 ka. The average rate of 14 + or - 1 m/Ma (n = 8) is much slower than the global mean ¹⁰Be-based basin denudation rate of 218 + or - 35 m/Ma (n = 1149). At such low rate, time-averaging of denudation rate estimate is c. 50-70 ka, and our data integrate over the entire glacial or interglacial period. Overall, our results suggest that catchment-wide denudation in the Murrumbidgee River catchment has not been changed dramatically for the last two glacial-interglacial cycles. The result here is somewhat in contrast to U-series comminution ages of the same deposits, where interglacial sediments exhibit relatively longer residence time (c. 100-500 ka) over sediments deposited during the glacial periods (<50 ka), which implies changes in weathering or erosion regimes between the two periods. Finally, response to certain climatic conditions may have been significantly different between rivers within the MDB, and our ongoing project will address this issue by comparing temporal variations in fluvial activity, denudation rates, and sediment residence times between different catchments within the basin.