Long-term rates of denudation in the Dry Valleys, Transantarctic Mountains, southern Victoria Land, Antarctica based on in-situ-produced cosmogenic [formula omitted

Abstract

Concentrations of cosmogenic 21 Ne ( 21 Ne c ) measured in quartz have been used to estimate long-term rates of denudation for contrasting landscape components in the Dry Valleys area of the Transantarctic Mountains, southern Victoria Land, Antarctica. Samples of Beacon Supergroup sandstones and granitic basement were collected from two contrasting landscape elements—low-relief, high-elevation surfaces and rectilinear slopes—to assess variations in rates of denudation with topographic position. The sample sites for rectilinear slopes were selected because of proximity to 40 Ar/ 39 Ar-dated lavas and ash-avalanche deposits. All 21 Ne/ 20 Ne ratios are significantly greater than the atmospheric value, and concentrations of 21 Ne c were calculated from the measured 21 Ne values assuming an atmospheric composition for the trapped component. Apparent exposure ages calculated from the concentrations of 21 Ne c , assuming no denudation since exposure, range from 3.78–4.66 Ma for samples from the high-elevation plateau surface, and 0.61–2.48 Ma for samples from the rectilinear slopes. Exposure ages for 3 He c were 2 to 42 times lower than those derived from the abundances of 21 Ne c because of preferential diffusive loss of 3 He c from the quartz lattice; concentrations of 3 He c were, therefore, not used in the calculation of rates of denudation. We interpret the abundances of 21 Ne c as reflecting variations in the rates of denudation rather than exposure age in view of independent evidence for prolonged exposure (>15 Ma) of bedrock surfaces at the sample sites. Calculated maximum rates of denudation range from 0.26–1.02 m Ma −1 for the rectilinear slopes, down to only 0.133–0.164 m Ma −1 for the high-elevation surface sites. These rates are comparable to other estimates of denudation rates for the Dry Valleys derived from analyses of cosmogenic isotopes, but are around two orders of magnitude lower than the long-term mean rate over the past ∼50 Ma estimated from fission-track thermochronology. Combined with the preservation of volcanic deposits dating back to the mid-Miocene, these 21 Ne c data demonstrate that only minimal modification of the landscape has occurred in the Dry Valleys over at least the past ∼15 Ma. This significant conclusion supports the view that the East Antarctic Ice Sheet has been essentially stable over this period rather than experiencing major fluctuations as late as the Pliocene, as has previously been suggested.