Cosmogenicnuclide burial dating of PlioPleistocene sediments an introduction

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Cosmogenic-nuclide burial dating of
Plio-Pleistocene sediments -- an
introduction Greg Balco, John Stone University
of Washington Carrie Jennings Patterson Minnesota
Geological Survey Acknowledgements DOSECC,
Fannie and John Hertz Foundation, Geological
Society of America, NSF EAR-0207844
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Useful range of radiocarbon dating
Lava Creek ash -- 0.61 Myr
Bishop ash -- 0.74 Myr
Brunhes/Matuyama reversal -- 0.78 Myr
millions of years B.P.
Mesa Falls ash -- 1.2 Myr
Huckleberry Ridge ash - 2.0 Myr
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Burial dating 1 production of 26Al, 10Be

• Nuclides produced in quartz grains by cosmic-ray
  bombardment of Si, O nuclei
• Production rate variable with altitude, latitude
• Production ratio fixed
• R26/10 6.1

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Burial dating 2 sediment burial

• Cosmic-ray flux decreases exponentially with
  depth below the surface.
• If a previously exposed target (rock surface,
  sediment, whatever) is buried, nuclide production
  ceases -- nuclide inventory decays to whatever
  level is supported by the (much lower) production
  rate at depth.

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Burial dating 3 the 26Al/10Be ratio
t1/2 (26Al) 0.705 Myr t1/2 (10Be) 1.5 Myr
exposure
burial
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Initial surface exposure
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Initial exposure, followed by deep burial
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Initial exposure, followed by shallow burial
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Applications --
1. simplest -- surface exposure followed by deep
burial
Fluvial sediment in caves -- cf. work of Darryl
Granger et al.
Fluvial sediment deposited in deep lakes
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Applications --
2. More complicated, but more versatile --
surface exposure followed by shallow burial, a
sequence of burial, or gradual burial
till
paleosol
Accretionary alluvial deposits -- gradual burial
Glacial deposits -- sequence of instantaneous
burials by thick tills
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Review of critical assumptions --

• So in order to get the right burial age --
• Initial nuclide concentrations on simple exposure
  line
• 2. Continuous burial from the event that were
  trying to date until the present -- no
  re-exposure (i.e., sample from drillcore)
• 3. Some information about the burial history
  that can be used to select a simplifying model.
  Single instantaneous burial? Gradual burial at
  constant accumulation rate? Series of progressive
  burials?
• Summary the geologic context of the samples is
  really important.

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AMS measurement and error
Accelerator mass spectrometer
Precision 10Be - 3 26Al - 5 26Al/10Be -
6 Age in years - 8-10
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Example Fisher Valley, Utah
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Results --Fisher Valley, Utah
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Example - early/middle Pleistocene glacial
sequence, SW Minnesota
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Results -- hole UMRB-1, SW Minnesota
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Whoops! -- results from modern fluvial sediment
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Summary --

• This technique works pretty well for dating
  otherwise difficult Pleistocene sediments.
  Especially applicable to material in drillcore --
  guaranteed no re-exposure. We expect learning
  quite a lot about the timing of Pleistocene
  glaciations, in particular, as the technique
  develops.
• The geologic/geomorphic context, the sediment
  source, and the burial history of whatever you
  want to date are really important.