COARSE ALLUVIUM COSMOGENIC CHRONOMETRY (Invited Presentation)

Stream terraces and alluvial fans are targets for establishing tectonic strain rates, paleo-flood discharge and frequency, incision histories, paleo-erosion rates of catchments, and landscape evolution over hundreds to millions of years. Terrestrial cosmogenic nuclides (TCN) have been used to date alluvium for decades. Exposure dating strategies have employed surface boulders and cobbles, pebble pavements, amalgamated subsurface clasts and sands, and swaths vs. discrete subsurface sample intervals. Burial dating strategies, including isochron burial dating, determine the age since alluvium was effectively shielded. TCN dating is directly impacted by complexities in cosmic particle exposure of the sample media prior to, during, and after deposition, including mass depth changes caused by aggradation or burial, soil development, or other shielding factors. TCN exposure dating of coarse facies in alluvial fans and low-order stream sediments has been particularly challenging, owing in large part to bed-to-bed variability in provenance which violates assumptions of homogeneous inheritance for depth profile dating. This presentation will review the results of experiments and strategies used to minimize the impact of these challenges to TCN dating of alluvium, with a focus on coarse and poorly-sorted alluvium. Advantages of drawing key constraints from soils geomorphology will be a recurrent theme for sample site selection and adjustment for aggradation and erosion history. A strategy to estimate time-varying erosion or aggradation of a surface by combining the saturation concentration of a short-lived TCN with the soil constraints is highlighted. The ‘turkey baster’ method for bulk density and impact of changes in bulk-density with time are evaluated. Experiments revealing the benefit of choosing highly eroded catchments when other options exist along a bajada, why sub-surface amalgamated clast samples should be avoided, and considerations when using TCN calculators are introduced to motivate continued developments in this chronometry.