An isochron method for burial dating with cosmogenic nuclides: Application to river incision in southern Africa

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The age of river terrace sediments can provide important information about river incision, tectonic uplift rates, and how rivers respond to climate change. However, the age of terrace gravels is usually difficult to determine in the absence of datable volcanic rocks. One method that has been used to date terrace gravels is cosmogenic nuclide burial dating, in which the rare nuclides aluminum-26 and beryllium-10 are measured in the mineral quartz by accelerator mass spectrometry. These two nuclides are produced by cosmic rays that originate in space and travel through the atmosphere, but are blocked as they travel through rock. If quartz grains are first exposed to cosmic rays near the ground surface, but are then buried within a deposit that is at least 10 meters deep, then the gradual radioactive decay of aluminum-26 and beryllium-10 offers a means to date deposition over the past 3-5 million years. HOwever, this technique requires great burial depths in order to minimize post-burial production by deeply penetrating cosmic rays. This project introduces and tests a new method that circumvents the problem of post-burial production by analyzing several individual pebbles from a shallower depth. Each pebble will have exactly the same post-burial production. This allows the use of an isochron method, in which a plot of aluminum-26 versus beryllium-10 concentrations yields a line whose slope depends on age. This study will test the isochron burial dating method at sites of known age, and by analyzing samples in stratigraphic sequence. The dating method will then be applied to a terrace sequence on the Sundays River in southern Africa, to determine uplift rates over the past 3 million years. Uplift rates in southern Africa remain very poorly constrained, and can provide important information on the interactions between erosion and uplift, as well as the role of Earth's mantle in supporting African topography. The development of a new dating method for gravels over the past 5 million years will open opportunities for determining erosion and uplift rates, and can be used for dating gravel deposits that contain stone tools or fossils. Results of the field investigations in souther Africa will help show when the climate became drier, and how rivers responded by changing their load or their incision rate. The results will also provide, for the first time, a dated record of river incision and tectonic uplift on the southern coast of South Africa, where rates and patterns of uplift have been debated for over 50 years. This work will also provide dates for terrace gravels that contain important assemblages of stone tools, and will provide a timescale for technological developments in the Earlier Stone Age. It is anticipated that the isochron method of cosmogenic nuclide burial dating developed in this study will receive wide applications in many fields studying earth and human history over the past 5 million years.