Alluvial fan formation in desert environments is still a debated subject, and application of conceptual models developed for fan formation in specific regions to areas with different climate settings generally lacks robust quantitative support. This international collaborative project aims to input data into these conceptual models by testing alternate hypotheses on the formation of Late Quaternary alluvial fans in the southern Baja California peninsula in Mexico. The central hypothesis is that tropical cyclones have performed most of the geomorphic work during this period and that seasonal high intensity precipitation has been coupled to rapid weathering of bedrock to generate cyclic alluvial fan aggradation in response to millennial-scale climate variability. Combined cosmogenic nuclide depth profile age determination and optically stimulated luminescence depth profile dating will be supported with detailed analyses of sedimentology and soil development of the sedimentary units in the Late Pleistocene-Holocene. These techniques are aimed to determine simultaneously sediment production on the hillslopes and delivery and accumulation rates in the alluvial units. The results will yield a clear picture of when the alluvial units were built, when the alluvial surfaces stabilized, how fast sediment was deposited, and how long the sediment was stored in the hillslopes before transport and deposition.

This project (supported by NSF's programs in Geomorphology and Land Use Dynamics, P2C2, and EPSCoR, and Office of International Science and Engineering) will produce datasets and knowledge that are of interest to a larger scientific audience and to the broader public. First, we will test a widely used model of alluvial fan aggradation for the deserts of southwestern North America. Second, we will test how the relevance of tropical cyclones has evolved over millennial timescales in this area and help to analyze tropical cyclone effects in other arid and semi-arid regions of the world (in particular, we will link with studies that assess activity of Eastern Pacific tropical cyclones over the last centuries and their effects on ecosystems and human population in the rapidly growing areas of southwestern U.S. and northwestern Mexico to improve hazard characterization of these storms). Third, our more precise correlation of alluvial fans surfaces normally used as paleoseismological markers will improve determination of earthquake recurrence intervals and of local and regional fault slip rates, increasing our understanding of fault kinematics not only in neighboring areas in Baja California but also in all arid southwestern North America and other regions where alluvial surfaces are used to infer earthquake recurrence rates.

Project Report

This project sought to understand timing and style of sediment transport and deposition through alluvial fans during the last 120,000 years (120 ka, Late Pleistocene and Holocene periods) in the Sonoran Desert of southern Baja California, Mexico. Knowing the evolution of these variables has implications for the hydrologic and geomorphic response that landscapes of arid and semiarid regions in southwestern North America have under climate variation. We were looking especifically to observe a potential response of this landscape to tropical climate variation. The research was performed with a multiple techniques approach, including historical documentation research, geomorphological, sedimentological, and geochronological analysis and comparison with paleoclimate records. Geographically, we focused on San Jose and La Paz basins, separated about 100 km. They hold a succession of comparable alluvial deposits, being located in one of the areas most affected today by Eastern Pacific tropical cyclones. Intellectual Merit Our investigation of the historical record of effects on the landscape of tropical cyclones led us to conclude that since 1697 these storms are the main responsible for deposition of up to 3-4 m thick sediment packages along most present-day channels. The amount of sediment deposited in these historical storms is nevertheless an order of magnitude smaller than the amount of sediment in the larger alluvial sediment piles that conform the landscape in the San Jose and La Paz basins. At geological timescales (thousands to tens of thousands of years), thick sediment units (up to 30-60 m thick) were deposited in a rapid succession of events, each related to an individual storm similar to the historical events. The geochronological study in our project aimed to date these event sequences. Luminescence dating (UCLA) and cosmogenic surface exposure profile dating (DRI) techniques formed the core of our methods. Results indicate that sediment deposition was accomplished mainly in thousand-year cycles of deposition that ended when the upper sediment surface stabilized and a soil formed on it. A break in time ensued with relatve landform stability, then a new cycle started with deposition covering older sediments. The main conclusion refers to the periodicity of the cyles of deposition observed in Baja California. We conclude that cyclicity shown by sediment package deposition correlates with a precessional (~21 thousand year cycles) signal since ~55-60 ka. Summer insolation peaked in the Tropical Pacific at times when the Baja California sediment packages were deposited, suggesting a connection between insolation cycles and storm activity of the Tropical Pacific. Based on our sedimentological analyses, we conclude that potentially more intense or frequent tropical cyclones affected the region at these times. Broader Impacts The record supports the idea that Eastern Pacific tropical storms were at some time much powerful than during the present, to the point of dominating water and sediment transport and deposition in the area. The effects observed in landscape processes could extend to the north, towards the present-day highly populated regions of Southern California and Arizona. The results highlight the need for a more comprehensive study of the past geomorphic effects of tropical cyclones in these regions. We used soil development as one of the main distinguishing features when correlating sediment units from catchment to catchment and then from one basin to the other. In this context, we confirmed that deep, well-developed soils can form here within 50-100 thousand years. These results, combined with ongoing work by the research group in other North American deserts, has implications for development of chronologies based on soil development, and used in paleoclimate reconstructions and in paleoseismicity assessment using alluvial surfaces. The project allowed training of several undergradate and graduate students throughout its duration, mostly in fieldwork and in the preparation and analysis of geochronology samples at DRI and UCLA. It allowed also mentoring of the PI, a postdoctoral fellow at DRI at the beginning of the project, through the development of collaborative ties with several key institutes and researchers working in the subject. The PI became Faculty at DRI in 2012. A well-based collaboration strenghtened between US and Mexican institutes working in the region, allowing e.g., participation of the co-PIs in outreach and dissemination activities organized in Mexico.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Paul Cutler
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University of Nevada Desert Research Institute
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