Collaborative Research: A multiple-technique approach for deconvolving tropical cyclone effects on Late Quaternary geomorphic change in arid southwestern North America

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The Eastern Pacific Ocean in the Northern Hemisphere is one of the most prolific regions on Earth in terms of generation of intense tropical cyclones that make landfall on the coasts of arid southwestern North America and dramatically enhance runoff, flooding, and associated erosion and sedimentation. Recent research indicates that these large-scale events have altered the hydrological and ecosystem balances over historical and geological timescales (decades to thousands of years). This project will assess how far north along the coast of southwestern North America these storms have occurred in the geologic past, and how far inland they controlled geomorphic change events through rain and erosion. The specific goal is to determine if previously documented periods when tropical cyclones dominated runoff and sediment deposition across large alluvial fan systems of the southern Baja California peninsula can be detected in central Baja California, the southern California deserts, and even as far north as the southern Arizona Sonoran Desert. Data will test whether dissipating cyclones were drivers of alluvial fan sedimentation over thousands of years, and if a temporal correlation can be established to a period of transition between a milder, humid paleoclimate to the current arid climate. For the first time, a tropical cyclone landfall chronology covering the last few millennia will be developed for the Pacific coast of southwestern North America. This research has the potential to inform large-storm prediction scenarios for southern California and northwestern Mexico, which is relevant to hazards management for communities in need of risk assessment of rare and extreme events. The project will contribute to the training of the next generation of earth scientists using a tiered approach, with field-based collaboration of postdoctoral fellows, graduate and undergraduate students. This approach has been proven successful for inclusion of underrepresented minorities, enhanced also with planned research alongside Mexican collaborators and students. The project will provide unique broader educational experiences for grade-school students in Indiana and Arizona, through the use of technology to connect fieldwork and classrooms.This project will compare a recently established alluvial fan chronology in southern Baja California, with newly-obtained alluvial fan and paleotempestological records. A Holocene paleotempestological record of overwash deposits in the Pacific coastal Vizcaino Desert will be developed for the first time. The inferred tropical storm activity will be compared with inland alluvial fan deposition in this area and in the northern Sonoran Desert, enabling discrimination of signals from different moisture sources, based on observed coastal and alluvial sedimentology, stratigraphy, and specific proxy records. Effects of different sources of moisture that drive sedimentation will be assessed by probing different time periods, and compared to independent paleoclimatic proxies. Bayesian analysis of luminescence, cosmogenic, and radiocarbon geochronology will improve age control precision. The coupled alluvial and coastal record at orbital timescales will help to understand linkages between Quaternary alluvial sedimentation and hydroclimatic variability in the region, and will increase our understanding of basic principles of alluvial fan aggradation in response to change in arid hydroclimates. The project will test effects of millennial- and orbital-scale shifts in tropical circulation on landscape evolution of the region, which are, in turn, critical to test models of occurrence and effects of hydrological extremes and associated landscape changing events.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.