Quantifying Changes in Erosion and Relief with Detrital Apatite (U-Th)/He Thermochronlogy and Cosmogenic Nuclides

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

PROJECT SUMMARY Quantifying changes in erosion and relief with detrital apatite (U-Th)/He thermochronlogy and cosmogenic nuclides Technical Description: This study performs a detailed study of the distribution and rates of catchment erosion in the southern Sierra Nevada of California using integrated detrital apatite (U-Th)/He thermochronometry and cosmogenic nuclides in sediment. Our first line of research proposes that detrital apatite (U-Th)/He cooling ages can act as sediment tracers, and thus be used to test whether a suite of steep catchments, in order of increasing size and complexity, are eroding uniformly or are dominated by point source erosion processes. Our second line of research compares the distribution of cooling ages from modern river sediments with those from older deposits, specifically river sediments preserved in caves, to investigate temporal changes in relief and catchment hypsometry. We use a 3D thermal model to account for the influence of the topographic bending of isotherms on interpreted detrital grain-age distributions from sediment. In addition, we propose to use cosmogenic 10Be and 26Al concentrations in river and cave sediments to investigate spatial and temporal changes in catchment-average erosion rates. Detrital apatite (U-Th)/He thermochronometry and cosmogenic nuclides are easily integrated tools because they utilize different minerals from the same bag of river sand. By developing and integrating these tools in detrital settings, and by exploiting a wealth of previous (U-Th)/He data and a set of well-dated caves, we investigate the topographic evolution of the southern Sierra Nevada, California, from the earliest Pliocene to today. Our integrated thermochronometric and cosmogenic approach is readily applied to other settings. For example, our development of the detrital apatite (U-Th)/He approach will determine the suitability of this technique for older deposits (e.g., sedimentary basins) for quantifying paleorelief and paleoerosion rates in orogenic belts around the world. Broader Significance: This project addresses fundamental problems in evolution of the Earth's surface in a mountain range of keen interest to geologists and laypersons alike. Our research should help answer how steep mountain valleys in the Sierra Nevada erode, the rates at which they erode and produce sediment, and how the distribution of elevation within valleys has evolved in light of climate change (e.g. repeated glaciations) over the last 2 million years. Furthermore, the geochemical tools we will develop should prove valuable in the overall effort of quantifying topography and erosion rate changes in mountain belts over longer timescales (>2 million years). Broader significance to the scientific community include development of a new application of a geochemical tool (apatite (U-Th)/He thermochronometry) and integration of this technique with other more conventional geochemical tools (e.g. cosmogenic isotopes). Career development and training associated with this project includes training and preparation of a postdoctoral scientist for a career in academia. The study will also facilitate at least one undergraduate student completing a senior thesis associated with this project. Public outreach and K-12 education will occur in the form of (1) education of National Park Service interpreters at Sequoia-Kings Canyon National Parks charged with teaching park visitors about the geology and landscapes of the southern Sierra Nevada, and (2) involvement of an elementary school teacher to development an awareness of current research topics in the Earth sciences as well as new lesson plans and course materials.