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BIGFOOT: BIG-storm FOOTprint on California and future hazards

 

 

Estimates of Catchment-Scale Erosion Rates

Historical descriptions and offshore sedimentary records indicate that big storm ("atmospheric river") events in the western Transverse Range (WRT), southern California, might have a significant impact on the surficial processes and rates that shape the onshore landscape.  However, little evidence of big storm events has been documented in the onshore sedimentary record of the WTR, and the rates of different surficial processes shaping the geomorphology of the WTR landscape remain poorly defined.  Further complicating the climatic signal of big storms on surficial processes and geomorphology in the WRT is the tectonic signal imparted to the landscape by Neogene and Quaternary transpressional folding and faulting associated with the San Andreas Fault system.  In this task we aim to use the concentration of the cosmogenic radionuclide beryllium-10 (Be-10) in order to

  1. quantify the rates of regolith production across the WRT landscape over the past 10^3 to 10^5 years,
  2. determine the hillslope lowering rates associated with specific transport processes over the past 10^3 to 10^5 years,
  3. determine the depositional ages of sediments thought to be associated with big storms (e.g., deep seated landslides, alluvial fan deposits, fluvial terraces, etc.),
  4. determine the paleoerosion rates of sediment, stored in lacustrine, alluvial fan, and/or fluvial deposits thought to be associated with big storms,
  5. provide constraints on rock uplift rates by measuring the absolute ages of marine and fluvial terraces and/or catchment-averaged erosion rates.

The primary task is to document the spatial and temporal distributions of catchment-averaged hillslope erosion rates related to big storm events in the WRT region.  The concentration of Be-10 is proportional to the amount of time that rock and sediment spends at or near the Earth's surface (or inversely proportional to the erosion rate of the landscape) (Figure 10).  Recent catchment-averaged erosion rates will be determined by measuring the concentrations of in situ Be-10 in quartz from the sand and gravel sized fraction of modern stream sediments.  Paleoerosion rates will be determined from the concentrations of in situ Be-10 in quartz from sand and gravel sized sediments in dated alluvial and fluvial deposits, and from the concentrations of meteoric Be-10 in the silt sized fractions of dated intervals of lacustrine sediment.  Catchment-averaged erosion rates determined by these methods will be a function not only of the influence of big storms on the landscape, but the underlying tectonic forcing, lithologic resistance to erosion, soil production rate and properties (e.g., moisture content, cohesiveness), and morphometry of the landscape (e.g., hillslope gradient, channel gradient).

 Theoretical landscape contribution of sediments used to derive catchment-scale erosion rates.

Theoretical landscape contribution of sediments used to derive catchment-scale erosion rates.

   

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