Dating Plio-Pleistocene glacial sediments using the cosmic-ray-produced radionuclides 10Be and 26Al

Abstract

We use the cosmic-ray-produced radionuclides 26 Al and 10 Be to date Plio-Pleistocene glacial sediment sequences. These two nuclides are produced in quartz at a fixed ratio, but have different decay constants. If a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic-ray flux, the 26 Al/ 10 Be ratio is related to the duration of burial. We first attempted to date pre-Wisconsinan tills by measuring 26 Al and 10 Be in fluvial sediments beneath them and applying the method of "burial dating," which previous authors have used to date river sediment carried into caves. This method, however, requires simplifying assumptions about the 26 Al and 10 Be concentrations in the sediment at the time of burial. We show that these assumptions are not valid for river sediment in glaciated regions. 26 Al and 10 Be analyses of such sediment do not provide accurate ages for these tills, although they do yield limiting ages in some cases. We overcome this difficulty by instead measuring 26 Al and 10 Be in quartz from paleosols that are buried by tills. We use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried, as well as the duration of burial. This technique provides a widely applicable improvement on other means of dating Plio-Pleistocene terrestrial glacial sediments, as well as a framework for applying cosmogenic-nuclide dating techniques in complicated stratigraphic settings. We apply it to pre-Wisconsinan glacial sediment sequences in southwest Minnesota and eastern South Dakota. Pre-Wisconsinan tills underlying the Minnesota River Valley were deposited 0.5 to 1.5 Ma, and tills beneath the Prairie Coteau in eastern South Dakota and adjacent Minnesota were deposited 1 to 2 Ma.introduction: purpose of this study

Quantifying regolith erosion rates with cosmogenic nuclides 10Be and 26Al in the McMurdo Dry Valleys, Antarctica

“…To remove pyroxene derived from the local dolerite, we used a heavy liquid separation, and the quartz was purified by repeated etching in 2% HF. We extracted beryllium and aluminum using standard methods [Stone, 2004] and measured 10 Be/ 9 Be and 27 Al/ 26 Al isotope ratios at the PRIME lab at Purdue University in West Lafayette, IN. The combined carrier and process blanks contained 1.67 ± 0.462 × 10 5 atoms 10 Be and 0 ± 2.04 × 10 5 atoms 26 Al.…”

“…We extracted beryllium and aluminum using standard methods [Stone, 2004] and measured 10 Be/ 9 Be and 27 Al/ 26 Al isotope ratios at the PRIME lab at Purdue University in West Lafayette, IN. The combined carrier and process blanks contained 1.67 ± 0.462 × 10 5 atoms 10 Be and 0 ± 2.04 × 10 5 atoms 26 Al. These are always less than 0.9% of the measured 10 Be atoms and 0.45% of the measured 26 Al atoms.…”

“…The combined carrier and process blanks contained 1.67 ± 0.462 × 10 5 atoms 10 Be and 0 ± 2.04 × 10 5 atoms 26 Al. These are always less than 0.9% of the measured 10 Be atoms and 0.45% of the measured 26 Al atoms. The beryllium measurements were originally standardized to the ICN standard, and we have restandardized them to the 07KNSTD [Nishiizumi et al, 2007].…”

“…For aluminum, the isotope ratios were referenced to the Nishiizumi [2004] standard. We determined the 10 Be and 26 Al production rates at the sample sites by using the rates of Stone [2000]. 10 Be production rates were further reduced by a factor of 1.106 following Nishiizumi et al [2007].…”

“…To do this, we will use the concentration of cosmogenic nuclides 10 Be and 26 Al in quartz sands from above and below two well-documented air fall ash deposits to examine the postdepositional geologic history of these surfaces. These deposits are the Arena Valley ash in Arena Valley ( 40 Ar/ 39 Ar age of 4.33 ± 0.07 Ma [Marchant et al, 1993a]) and the Hart ash in Wright Valley (K-Ar age of 3.9 ± 0.3 Ma ).…”