Cosmogenic noble gas studies in the oldest landscape on earth: surface exposure ages of the Dry Valleys, Antarctica

摘要

Extraordinarily high surface exposure ages have been determined for Sirius Group tillites of Mt. Fleming and Mt. Feather as well as at localities in the Inner Dry Valleys using cosmogenic helium and neon. Ages of 10 Ma at Mt. Fleming, 5.3 Ma at Mt. Feather and 6.5 Ma at Insel Mountain are among the highest nominal exposure ages published so far. These values are minimal ages as they are based on the assumption of zero erosion and uplift. The Mt. Feather sample independently confirms the pre-Pliocene age of the Sirius Group sediments in the Dry Valleys as previously determined at Mt. Fleming. The Insel Mountain samples provide evidence for a landscape formation of the Inner Dry Valleys not later than Late Miocene time. Assuming conservatively low values of 2.5 cm Ma~(-1) for erosion rate and 50 m Ma~(-1) for uplift rate we infer that the Sirius Group tillites at Mt. Fleming were deposited earlier than 20 Ma ago. This indicates that the overriding of the Dry Valleys block of the Transantarctic Mountains by the East Antarctic Ice Sheet occurred not later than the Early Miocene. Maximum long-term erosion rates in the Inner Dry Valleys must be < 15 cm Ma~(-1) down to altitudes < 1000 m. Since such low erosion rates require permanently cold and hyperarid conditions, the response of Antarctica to the Pliocene warm climate episode must have been small. Cosmogenic nuclide data from both the Inner Dry Valleys and the Sirius Group sediment localities support the hypothesis of a stable East Antarctic Ice Sheet since at least Late Miocene time, implying that the climate of Antarctica was decoupled from that of lower southern latitudes. We present also new elemental ~(21)Ne production rates of P_(21)(Mg) = 196 atoms g~(-1) yr~(-1) and P_(21)(Al) = 55 atoms g~(-1) yr~(-1) at sea level and high geomagnetic latitude. These figures are consistent with a ~3He production rate of P_3 = 110 atoms g~(-1) yr~(-1), similar to previously published values. This consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 Ma. Cosmogenic Ne in quartz and pyroxene has a (~(22)Ne/~(21)Ne)_(cos) ratio of 1.266 +- 0.040 and 1.159 +- 0.040, respectively.