RESURGENCE OF METEORIC 10Be: RATES AND DATES FROM A NEW, OLD GEOCHRONOMETER

Although the very presence of soil within a landscape attests to faster net soil formation processes than the denudation processes over long timescales, long-term rates of soil production and weathering and erosion may also be determined by geochemical methods such as measurements of recalcitrant element abundances and terrestrial in situ cosmogenic nuclides, like Beryllium-10. These tools have led to nothing less than a revolution in our understanding of rates of landscape change and surface processes. Recently, applications of the fallout cosmogenic nuclide Beryllium-10 (¹⁰Be_met) coupled with stable Beryllium-9 (⁹Be) have seen resurgence because of the relatively high production rate of ¹⁰Be_met in the atmosphere and the sorption of both nuclides to fine grained particles. While meteoric ¹⁰Be percolates into the soil from an atmospheric origin, ⁹Be is present in silicate minerals. Dissolved ⁹Be is released via weathering and either adsorbed to clays and mineral surface coatings or remains in the dissolved phase in pore water. Together, this nuclide pair becomes a very powerful tool; the details, caveats and applications for this new technique are just beginning to be realized [1]. Using these coupled ⁹Be and ¹⁰Be_met concentrations, we can now track the movement of fine-grained sediments within suspended load over a hydrograph and we can normalize the concentrations for complicating factors, such as grain-size differences, via adsorbed ⁹Be. Because lakes and other depo-centers contain time series records of fine-grained sediment, we may be able to elucidate erosion patterns from landscapes that are now long-gone. We can understand the timing and impact of meteoric water percolation into saprolite, soils and rivers. We may even be able to trace the global silicate weathering cycle over the last 20 million years using time series ¹⁰Be/⁹Be ratios in ocean records (e.g. [2]) that act as faithful recorders of past ocean water chemistry for understanding ancient global weathering rates and processes.

[1] Willenbring & von Blanckenburg (2010a) Earth Sci. Rev. 98.

[2] Willenbring & von Blanckenburg (2010b) Nature 465.