Cosmogenic-nuclide burial dating

Depth profiles use a number of samples (ideally >5) through a sedimentary unit to provide a single surface exposure age based on cosmogenic nuclide concentration at different depths (Darvill, 2013). Sediment burial can be dated by the radioactive decay of cosmogenic nuclide minerals (10Be and 26Al), provided that the sediments were exposed prior to burial (Granger and Muzikar, 2001).

This method can be applied over million-year timescales, and can be essential for dating older glacial deposits where moraines are severely degraded (Darvill et al., 2015), or older outwash fans where boulders have weathered away (Ivy-Ochs et al., 2013). It is straightforward if sediment burial is deep enough to prevent the ongoing formation of cosmogenic nuclides, but more complex if the sediment is insufficiently shielded.

Simple burial dating has been used to date outwash fans and give surface exposure ages in Patagonia. A modelled nuclide attenuation profile through several depth samples will yield a most probable age for surface exposure as well as average inheritance (nuclide concentrations deeper than the penetration of cosmic ray reactions) and surface erosion (projecting measured nuclide concentrations to the modelled surface according to the expected attenuation curve) in the unit (Hein et al., 2009; Hidy et al., 2010; Marrero et al., 2016a).

Exposure ages from depth profiles can be strengthened using single exposure ages from cobbles on the unit surface, to provide a check on the modelled exposure age and surface inflation/deflation (Cogez et al., 2018; Darvill et al., 2015; Hein et al., 2017, 2011, 2009). It is also possible to reconstruct more complicated depositional histories using multiple cosmogenic nuclides (Balco and Rovey, 2008; Granger and Muzikar, 2001; Häuselmann et al., 2007; Hein et al., 2009). The depth-profile method may be particularly useful for degraded moraines, fans, or other landforms deposited prior to the last glacial cycle (Cogez et al., 2018; Darvill et al., 2015; Hein et al., 2017, 2010; Ivy-Ochs et al., 2013).

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