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 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

- Project Outcome report. This collaborative project focused on trying to determine when and why some striking natural deposits of sand and gravel in Southern Baja California, Mexico, were laid down. This is important as these sediments represent the aftermath of huge flood events. If similar flood events occurred today, a tremendous amount of damage would be done, and many lives would be threatened. The historical record from written documents, extending back a few hundred years, tells us that large storms do occur in this region from time to time. However, the scale of these historic floods appears small in comparison to the events implied by the sediments that they left behind. This link has been confirmed by this research project, in particular, the tasks undertaken by our colleagues at the desert Research Institute, Reno, Nevada. The UCLA component of the research was focused at determining when these larger scale flood events occurred. Answering this question will help answer the following issues: Were the events concentrated into periods of flooding, with "quiet periods" between these? Was there a higher probability of flooding during periods when the climate was in a different state? Dating these events is not straight forward. The UCLA research component applied and developed luminescence dating to provide age estimates for the deposition of these sediments. This method relies on trapped electron populations that build up slowly in mineral grains – the individual grains of sand that form these deposits. The trapped electrons are removed in nature by exposure to daylight, and this occurs for some grains in the years and weeks before the flood event takes place. After deposition, trapped populations slowly rise again, until we collect the samples (without light exposure), and prepare selected sand grains in the laboratory at UCLA. Using experiments based on these samples, we have refined our methods, and can now achieve reliable age estimates for individual sand grains, using a complex sequence of measurements made by shining intense infra-red (IR) laser light onto each grain in turn, and determining the blue luminescence emission that represents the detrapping and recombination of specific electron populations. We have tested this approach against independent age control, and find good agreement, giving us confidence in our results. In summary, the highly technical requirements for this project, refining and developing luminescence dating using IR stimulation of a particular minerals group (potassium feldspar grains), has been successful, and we have now applied this to a large suite of samples from our target region in Mexico, in order to understand how the think sequence of storm sediments developed. We are able to recognise patterns in the clustering of the age estimates, and start to interpret what natural forces are driving the rises in frequency and intensity of these huge tropical cyclone storm events. The project has had, and will continue to have, broader impacts. Firstly, two graduate students and four undergraduate students at UCLA were able to benefit from training directly related to this project as they contributed to the preparation and measurement of the luminescence samples. Two of the undergraduates developed an educational outreach program for school students around the project themes that was undertaken in the Los Angeles region. Secondly, the age estimates themselves are contributing to an understanding of the driving forces of climate change and environmental response in the Pacific SW region of North America. The research outcomes are highlighting the possibility of huge storm events, the scale of water depth and sediment movement, and the threat to infrastructure and lives. Finally, the technical developments relating to luminescence dating that we made as part of this project will benefit a wide range of other projects and researchers, having the effect of increasing the reliability of sediment dating in situations where conventional approaches are not available.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Paul Cutler
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University of California Los Angeles
Los Angeles
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