USING COSMOGENIC NUCLIDES TO UNDERSTAND THE CONNECTION BETWEEN EROSION RATES, ENVIRONMENTAL FACTORS, AND LANDSCAPE RESPONSE (Invited Presentation)

At the turn of the 20th century, many streams in the semi-arid southwestern US, rapidly entrenched into their alluvial valleys forming deep (5-30 m), steep-walled arroyo channels. Prehistoric entrenchment and aggradation events in southern Utah were reconstructed using stratigraphic descriptions, radiocarbon and optically stimulated luminescence dating, and geomorphic analysis. Stratigraphic records indicate that episodic entrenchment (4-5 events) interrupted largely aggradational conditions with shallow channels and high rates of floodplain accretion during most of the mid-late Holocene. We use radionuclide Beryllium-10 concentrations to estimate catchment average erosion rates and investigate how spatial and temporal variability in sediment supply/erosion rate influenced arroyo cut-fill dynamics.

Samples for ¹⁰Be catchment average erosion rates were collected from catchments in the Grand Staircase region of the Colorado Plateau, in south-central Utah. The region exhibits colorful cliffs and benches that are formed from alternating layers of sedimentary rocks. Holocene and Pleistocene alluvium fill valley-bottoms and form colluvial wedges at the base of cliffs. Modern and paleo catchment-average erosion rates are used to understand relations among climate change, lithology, sediment supply, and landscape response.

Modern erosion rates span two orders of magnitude, are correlated with slope and elevation, and vary with rock type. Modern versus Holocene erosion rates do not show significant or systematic differences. Importantly, discrete modern flood deposits from the same reach show systematic differences in erosion rates based on the color of the deposit, which in this setting, can be linked to source rock and distance of transport. Initial ¹⁰Be results from discrete flood deposits suggest samples from these ephemeral streams with flashy discharge may not represent catchment-averaged erosion rates from the contributing area. Overall, ¹⁰Be results suggest that these catchments have high erosion rates (40-1000 mm/ka). Resultant high sediment yields and observed high aggradation rates likely produce slope/concavity related threshold conditions that result in arroyo cut-fill dynamics.