Both climatic conditions and tectonic deformation are known to influence rates of erosion, although the response of erosion rates to changes in these controling mechanisms over time is still poorly understood. This question has been difficult to address due to a lack of reliable proxy data that can define past erosion rates that record rate changes over timescales comparable to significant changes in climate and tectonics. For example, when modern rates are assessed using the concentrations of cosmogenic nuclides in eroded sediments, those rates typically reflect erosion on millennial timescales, whereas erosion of mountain belts rates is typically assessed at million-year time scales that average across many climate cycles. By developing a ~10-million-year-long record of paleo-erosion rates in the Eastern Andes of Argentina, we will explore the trade-offs between climate and deformation as they modulate erosion. We will measure three different cosmogenic isotopes (10Be, 21Ne, and 26Al) in ancient river sediments that were deposited in the Andean foothills. This study will capitalize on a unique exposure that was inadvertently created by humans who, in about 1900, diverted part of a river into an irrigation ditch and triggered a massive erosional event that has cut a channel over 100 m deep and 10 km long in the past 100 years. In the midst of the jungle, this incision has revealed a pristine exposure of a ~7-km thickness sediments eroded from the Andes over the past 10 million years. Importantly, this exposure is ideal for cosmogenic studies of past erosion rates, and it offers superb age control via its already establish record of reversals of Earth?s magnetic field and volcanic ashes that can be precisely dated. Using three isotopes with different half-lives and cosmogenic production rates will allow significant reduction of uncertainties associated with sediment storage, recycling, and syn-depositional exposure, and will underpin a more robust history of erosion rates. The multi-isotope approach, paired with detrital zircon analyses, may also provide an opportunity to establish a stratigraphic record of sediment burial and remobilization within the catchment. When paired with regional records of climate and tectonic uplift, the resulting erosion and sediment transport records should provide valuable constraints for numerical models of landscape evolution. The project will also broaden the scope of applications for stable 21Ne and serve as an exploration of its ability to reliably record erosion rates beyond the limits imposed by natural decay of 10Be.
The relationship between rates erosion in mountain belts and the external factors that drive this erosion is key to our understanding of both modern and ancient geologic systems. For example, predicting climate-induced changes in erosion rates from mountain ranges can inform societal preparedness for the effects of global climate change. Likewise, studies of the long-term linkage between climate and erosion can help geologists understand the fate of ancient mountain belts and their role in Earth history. This project will represent a novel application of cosmogenic isotope dating: a technique that computes the residence time of rocks near the Earth?s surface by measuring rare isotopes produced when cosmic rays collide with rocks and minerals at the Earth?s surface. Success in addressing the many challenges associated with applying this type of dating to ancient sediments will help lay the foundation for asking similar questions in even older sediments, which record poorly understood periods of Earth?s much older history. The project will help to train the next generation of scientists by exposing them to cutting-edge isotope geochemistry techniques and by developing the skills necessary to interpret Earth?s sedimentary record. By pairing a senior PI with junior investigators, as well as graduate students with undergraduates in an international setting, the project will promote a continuum of training across a variety of skill sets and experience levels.
