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Isotopic hysteresis in detrital cosmogenic nuclide-derived denudation rate studies (Invited)
American Geophysical Union, Fall Meeting 2013 (2013)
  • Patrick Belmont, Utah State University
  • J Willenbring
  • N M Gasparini
  • B T Crosby
  • G Y Brocard
In equilibrium landscapes, the concentration of beryllium-10 (10Be) from fluvially transported material is expected to quantitatively reflect basin-wide denudation rates. No isotopic time-dependent path is considered because the concentrations reflect an integrated measurement over a sufficiently long period of time to be static. However, the responses of landscapes to changing conditions are often addressed with cosmogenic nuclides in transient landscapes to identify and quantify the primary topographic and climatic controls on erosion. With the advent of techniques that allow event-scale measurement of cosmogenic nuclide concentrations over the course of a flood wave, in the case of meteoric 10Be, or over the course of an uplift wave, in the case of in situ-produced 10Be, we can now evaluate the how the isotope changes and what the 'mean' denudation rate from a single time means. Meteoric 10Be concentrations can be extracted from, and measured in, milligram-sized sediment samples. This attribute enables us to measure suspended sediment through a hydrograph. Here, we give a case study in an agricultural setting. The meteoric 10Be concentration in river sediment changes with the source areas of the fine sediment and fluxes of material supplied to the stream. The average concentration from the couplet of the rising and falling limbs of the hydrograph can differ from the concentration of the sediment that is preserved in depo-centers. Using traditional in situ-produced 10Be, the timescale of the perturbation must be sufficiently long to change the isotopic composition of the bedload, but also for the landscape to respond to the forcing factor. Here, we give an example from a transient landscape where a wave of uplift moves through the basin and a wave of incision follows in its wake. In this setting, 10Be from detrital quartz is derived from both the incising, adjusting lowland and the unadjusted, relict upland, and the integrated 10Be concentrations still provide a denudation rate averaged across the two domains. Because field samples using in situ-produced 10Be can only provide a snapshot of the current upstream-averaged erosion rate, we employ a numerical landscape evolution model to explore how 10Be derived denudation rates vary over time and space during long-term transient adjustment. Model results suggest that the longitudinal pattern of mean erosion rates is generated by the river's progressive dilution of low-volume, high-concentration detritus from relict uplands by the integration of high-volume, low-concentration detritus from adjusting lowlands. The proportion of these materials in any detrital sample depends on what fraction of the upstream area remains unadjusted. In such cases, the history of the basin and the amount of time passed since uplift wave will change the observed concentrations through a delayed response. Finally, we consider two implications from these two case studies. First, how can we best interpret paleoerosion rates from depo-centers derived from meteoric or in situ-produced 10Be concentrations? Second, can we use mean 10Be concentrations to understand forcing factors in transient landscapes without a landscape evolution model?
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Citation Information
Patrick Belmont, J Willenbring, N M Gasparini, B T Crosby, et al.. "Isotopic hysteresis in detrital cosmogenic nuclide-derived denudation rate studies (Invited)" American Geophysical Union, Fall Meeting 2013 (2013)
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