Sean Des Roches
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1st Class Honours
B.Sc. (Honours) Thesis
Testing a new grain-‐size dependent isochron cosmogenic nuclide burial dating method: Eastern Cordillera, Colombian Andes
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Terrestrial cosmogenic nuclide burial dating a powerful tool by which one can determine the timing of the burial of a layer of sediment or rock. A recently developed 26Al/10Be isochron burial dating approach uses samples with differing TCN concentrations collected from depth profiles in buried sediment. However, the use of this isochron burial dating method is dependent on finding a buried paleosol, or any surface that was exposed for a sufficient period of time (depending on duration of decay during burial) and then subsequently buried. In regions of high relief, which are prone to landslides, there may be an alternative methodology for isochron burial dating of sediments lacking paleosols. We evaluate here a new method of 26Al/10Be isochron burial dating based on the previously observed relationship between fluvial sediment grain size and TCN concentration in landslide-‐prone catchments. There may be a sufficient range in TCN concentration across the different grain sizes (150 to 2000 um) that an isochron curve can be precisely defined.
Fine sand to granular gravel fractions were extracted from five 3 kg sediment samples previously collected 112 m below an incised river terrace in the Eastern Cordillera of the Colombian Andes (4.979 N, 72.825 W, 686 m elevation above sea level). This site is ideal to test the new technique because its ongoing tectonic activity has generated high relief, landslides, and high erosion rates (therefore low TCN concentrations to test the method’s limit).
Pure quartz from six different grain size fractions was extracted, cleaned, dissolved, and converted to Al2O3 and BeO AMS targets at the Dalhousie Geochronology Center. Calculations of the results from the AMS (Accelerator Mass Spectrometer) at Lawrence Livermore National Lab gave 26Al concentrations ranging from 2.79 to 4.19 X 104 atoms/g (±21% 1-‐sigma) and 10Be concentrations ranging from 4.08 to 8.14 X 103 (±4-‐8% 1-‐sigma) across various grain-‐size fractions. The measured values were too low and had too little variation to be able to define an isochron. With these results we were not able to test the effectiveness of a grain-‐ size dependent isochron method.
We attribute the low measured AMS values in part to low initial TCN concentrations, which are the result of rapid erosion in the catchment area where the samples originated. Aluminum and beryllium may have also been lost during steps within the chemical preparation of the samples owing to the much larger quartz masses used than usual and to additional chemical isolation procedures that were used on the samples. Calculated paleo-‐erosion rates confirm high erosion rates for the catchment, which are 2.59 mm yr-‐1 (±25% 1-‐sigma) based on 10Be and 0.97 mm yr-‐1 (±33% 1-‐sigma) based on 26Al, which are consistent with other rapidly eroding tectonically active orogens.
Keywords: Cosmogenic Isochron Grain-‐Size Beryllium Aluminum Andes Colombia
Pages: 59
Supervisor: John Gosse