Testing the antiquity of McMurdo Dry Valley soil surfaces with atmospheric 10BE

Abstract

Surficial deposits in the McMurdo Dry Valleys provide a valuable record of Cenozoic climate history. The global temperatures in earlier Cenozoic times have been warmer than today and understanding Antarctic climate during these periods may help to provide scenarios for further changes in the region resulting from global warming. One problem in the Dry Valleys is that many of theses surficial deposits cannot be reliably dated. A new dating method utilizing atmospherically produced l0Be offered a possible solution to the problem of age control for surficial deposits in the Dry Valleys. However, the method lacked verification through an independent test against a surficial deposit of known age. The main goal of this thesis is to test ages obtained from the new 10Be method against known ages of surficial deposits. For this purpose, soil profiles in the lower Wright Valley that contain the Hart ash, previously dated to 3.9±0.3 Ma (Hall et al., 1993), were chosen. The profiles were sampled vertically and the samples analysed for adsorbed l0Be and 9Bc, soluble anions (Cl-, F-, SO42-,NO3-) and cations (Ca2+, K+, Mg2+, Na+, Sr2+), and grain-size. Class shards in the volcanic ash were also analysed for major (electron microprobe) and trace element composition (laser-ion coupled plasma-mass spectroscopy). The analyses showed that the Hart ash consisted of two chemically distinct, unmixed ashes, of which only one had features indicating it was a primary airfall deposit. A diffusive model of 10Be migration into a soil showed that 10Be cannot be used for dating in the way that Graham et al. (2002) envisaged. The model also showed that near steady-state conditions are reached in a few hundred Thousand years, making it problematic under most conditions to use the 10Be inventory of a deposit for dating. Comparison of 10Be surface concentrations in the lower Wright Valley with samples from Table Mt, and Beacon Valley further suggest that the 10Be input rate into surficial deposits increases towards the East Antarctic Ice Sheet. However, in the central Dry Valleys 10Be input rates are well below the input rates on the East Antarctic Ice Sheet. This variation may be caused by the microclimate of the Dry Valleys. The special setting in H5, which contains a paleosol covered by in situ ash, allowed erosion rates for this profile to be obtained. The calculated maximum erosion rate of the paleosol prior to ash deposition (3.9±0.3 Ma: Hall et al., 1993) is 0.54 m Myr-1, and for the ash it is today in the range of 0.02 mMyr-1. Prior to the Hart ash deposition, increased precipitation in the wanner Early Pliocene climate may be the reason for the observed higher erosion rates at that time compared to the calculated recent rate.