Dating Alluvial Landforms to Understand the Role of Climate in California's Water Supply

Alan Hidy | 19-LW-036


Understanding variations in fresh water supplied to regions in the past is important for preparing for future droughts. Where landscapes have an abundance of erodible material, such as California and much of the U.S. southwest, alluvial landforms (e.g., river terraces and alluvial fans) are likely emplaced during prolonged periods of intense rainfall. Thus, their deposits record periods of increased water availability, and timing of their accumulation provides insight into climate or other mechanisms that control water supplies. Unfortunately, dating methods applicable to alluvial deposits over timescales critical for capturing first-order climate change (104 – 105 years) are limited, and consist primarily of cosmogenic exposure and burial dating techniques that are complicated by poor preservation quality over these times. To improve age-dating capabilities for these deposits, a cosmogenic nuclide depth profile simulator was developed that can incorporate diverse soils geomorphology and landform data into age models as well as aid the sample collection process. The simulator is broadly applicable since alluvial deposits are globally common geomorphic features that occur in a variety of climatic and tectonic settings. This approach is applied in parallel with other dating strategies for validation and to investigate optimal sampling strategies. At one site, a commonly applied exposure dating strategy is compared with high quality optically stimulated luminescence burial ages; the results demonstrate the potential pitfalls of applying simple exposure dating strategies without observation-based geologic constraint on shielding history. Results from alluvial deposits dated at eight sites broadly distributed in the U.S. southwest were highly correlative—consistent with the hypothesis that these landforms primarily form in response to regional climate drivers rather than tectonic or other local factors.

Mission Impact

Our multi-nuclide dating tool represents a new capability for modeling sediment deposition ages that supports the earth and atmospheric science, and nuclear, chemical, and isotopic S&T core competencies by expanding applications for the unique measurement capabilities at Lawrence Livermore National Laboratory's Center for Accelerator Mass Spectrometry (CAMS). Methodologies developed here can also be applied to date fault-dissected deposits for better quantification of long-term slip-rate estimates for active faults. This would improve seismic hazard estimates and support R&D mission directives that address the security of critical energy infrastructure. Furthermore, this work supports Department of Defense objectives for predicting soil and terrain conditions for military activities (e.g., mobility of military vehicles, generation of dust hazards, location of water resources, and improvised landing zones for aircraft). Finally, this project fostered the establishment of a CAMS-compatible terrestrial cosmogenic nuclides (TCN) sample prep lab at the Desert Research Institute (DRI) with a strategic relationship where CAMS serves as their key isotopic development facility.

Publications and Presentations

McCarroll, N. R. et al. "Chronostratigraphy of talus flatirons and piedmont alluvium along the Book Cliffs, Utah-Testing models of dryland escarpment evolution," Quaternary Science Reviews 274 (2021): 107286. (LLNL-JRNL-815462).

Tanksi, N. M. et al. "Investigating the roles of salt tectonics and hillslope mass movements in forming of the Cataract Canyon knickzone, Utah using chronostratigraphy of Colorado River terraces." In Geological Society of America General Meeting (2021). (LLNL-ABS-825995).

Porter, T. J. et al. "A continuous record of climate and vegetation change from Late Miocene lake Fort Yukon in continental Alaska." In American Geophysical Union Fall meeting (2021). (LLNL-ABS-825996).