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The Times of Sand: Applying Cosmogenic 21Ne to Examine the Brief Exposure Times of Quartz Sands throughout Sedimentary Cycles
  • Michal Ben Israel,
  • Ari Matmon,
  • Samuel Niedermann
Michal Ben Israel
Hebrew University of Jerusalem

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Ari Matmon
Hebrew Univ
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Samuel Niedermann
Deutsches GeoForschungsZentrum
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The sedimentary cycle of quartz sand is comprised of the weathering of source rock, followed by erosion, deposition, then burial, and in some cases, lithification. Over geological timescales, quartz sands undergo several sedimentary cycles, with sandstones retaining evidence of previous cycles in various mineralogical and geochemical traces. These sedimentary cycles can span across timescales of hundreds of millions of years, with some sands holding evidence of the earliest rocks in Earth’s history (e.g., Australian and Kalahari deserts). However, we have little knowledge of the timescales that sands spend exposed at the Earth’s surface and how these compare to the timescales of sedimentary cycles. Unlike the more commonly used cosmogenic nuclides, cosmogenic 21Ne is stable and can be used to study rates of surface processes in the deep geological past. We use cosmogenic 21Ne to quantify exposure times at the surface from Lower Cretaceous and Miocene outcrops (Kurnub and Hazeva formations) and one active fluvial system (Colorado River). Although not devoid of drawbacks and limitations (i.e., diffusion out of the quartz crystal and high levels of non-cosmogenic 21Ne is quartz), we are able to constrain exposure times at the surface to <106 years. Compared to the overall timescales of sedimentary cycles, evaluated using detrital zircon provenance data, exposure times at the surface are shorter by over an order of magnitude. These results demonstrate how in the context of sedimentary cycles, quartz sands spend the greater part of the time buried in fluvial systems sedimentary units (e.g., fluvial terraces, fans, and deltas) or at sedimentary basins. In comparison, the time that these sands spend at the surface during weathering, erosion, and deposition is strikingly brief.