A research team from the University of California at Davis, University of Oregon, and Western Washington University is conducting a multi-disciplinary examination of a remarkably complete and well-exposed archive of late Pliocene to modern paleo-erosion rates preserved in the Fish Creek - Vallecito basin in southern California. Erosion rates in the source are determined from cosmogenic nuclide concentrations in detrital quartz, with corrections for minimal post-depositional nuclide in-growth during rapid burial and exhumation of basinal sediments. Relief production in the source areas are assessed by comparison of (U-Th)/He cooling ages in modern bedrock exposures to detrital minerals in basinal sediments. Depositional ages and sedimentation rates are calibrated with high-precision magnetic reversal and paleointensity stratigraphy. These data yield a high-resolution time series of cosmogenic nuclide in-growth of alluvial sediments, which record important changes in catchment-averaged erosion rates through time. Temporal changes in source-area erosion rate are compared to the stratigraphic record of changes in facies architecture and sediment accumulation rates in the basin, allowing assessment of the process-response functions that link erosional forcing to stratigraphic outcomes. Using these approaches, the team is testing three hypotheses for tectonic - versus climate driven changes in erosion of the bedrock source and progradation of sediment derived from this source into the adjacent basin: (1) a shift into cooler and more temporally variable climate conditions at the onset of northern hemisphere glaciation approximately 2.5 to 3.0 million years ago caused increased erosion and production of locally derived sediment; (2) an increase in tectonically driven uplift led to relief production and enhanced erosion; and (3) a reduction in tectonic subsidence rate drove progradation of locally derived sediments irrespective of changes in source-area erosion rate. Data collected during a 1-year pilot study show that erosion rates decreased by 40% since 2.8 Ma. This preliminary result is most consistent with either a climatic or subsidence-rate driver of progradation, though an uplift-driven model cannot be ruled out at this time. This new study will further analyze this record and replicate this signal from a second sediment source-area.
It is well documented that rates of bedrock erosion and sediment production are closely linked to climate forcing and tectonic construction of topographic relief, though the exact nature and feedback dynamics of those controls remain incompletely understood. Detrital cosmogenic nuclide concentrations in modern stream sediments are widely used for quantifying short-term catchment erosion rates, and for assessing the role of tectonic forcing, lithology, and erosion processes on sediment production rates. While this method is common in studies of modern sediment-catchment systems, its application to ancient sediments is relatively rare, and its use in continuous sedimentary sequences is rarer still. This study is charting new ground through the application of these methods to older sediments. If successful, geologists will have new tools to distinguish the role of climate versus tectonics in landscape evolution.
