Links between Catchment Erosion and Climate Investigated with Uranium Series Isotopes

Summary

To understand environmental responses to climate change and to human activity, we need to constrain the rates of processes that shape the Earth’s surface like the production of soils by physical and chemical weathering of rocks and the transport of sediments. These processes are controlled either directly or indirectly by climate change. Studying the history of the sediments deposited throughout the river and its palaeochannels can give insight to the history of climate change occurred. A new approach using uranium series isotopes has been devised and being used recently to address the questions presented above. In silt-size soil material, a (²³⁴U/²³⁸U) activity <1 can be observed because when ²³⁸U decays near the surface, its daughter nuclide ²³⁴Th can be ejected from soil grains, and subsequently decays into ²³⁴U. The measurement of this radioactive disequilibrium can be used to determine how much time has elapsed since the soil material was produced from the bedrock, i.e. a soil residence time. We have measured U-series isotope disequilibrium in soils from four different profiles at Frogs Hollow, a catchment area of the Murrumbidgee River in western New South Wales, which gave a top soil residence time of over a million year and a saprolite residence time of over a hundred thousand years. These ages are significantly different than the ages reported earlier at the same location using cosmogenic radio nuclide measurement. This difference may be due to the need of more developments in the new technique or the assumption of steady state thickness of the soil profile required by the cosmogenic radio nuclide measurement technique, which may not stand if the soil thickness is not steady.

Key words::

• Soil production
• comminution age
• U fractionation
• alpha recoil.