WHEN WERE THE HILLS ALIVE? METEORIC BE-10 ABSENCE IN FRIIS HILLS SUGGESTS POLAR ARIDITY IN TAYLOR VALLEY SINCE THE MID-MIOCENE

Meteoric Beryllium-10 (¹⁰Be) measurements made on buried paleolacustrine sediments interbedded in the Friis Hills, Taylor Valley, Antarctica yield concentrations within the detection limit of the AMS measurement (i.e. "zero" concentration). This implies that sediments have been emplaced at least long enough to entirely decay an initial ¹⁰Be concentration ([¹⁰Be]_initial). Using independently estimated maximum and minimum till ages, and [¹⁰Be]_initial estimates in lake sediments from published data, we model a range of possible sediment ages. Modeled ages range from 10.7 to 18.7 Ma. The upper limit of the ¹⁰Be-decay dating method produces a possible lake sediment age range of 14-18.7 Ma, in agreement with dated ash layers stratigraphically above and below lake sediments. This range spans the Middle Miocene Climatic Optimum (beginning ~15.7 Ma) and Middle Miocene Climatic Transition (beginning ~13.8 Ma), which are periods of increased land and sea temperatures followed by intense cooling and ice sheet expansion, respectively. The absence of ¹⁰Be in buried sediments suggests no infiltration from ice melt or meteoric waters following their burial. This result requires that the onset of polar arid conditions likely began as early as Middle Miocene, and supports the case for a stable East Antarctic Ice Sheet since this time.

A single surface ¹⁰Be concentration (1.5x10⁷ atoms g^-1) collected from an exposed lacustrine bed yields an erosion rate slightly lower than, but still comparable to, regolith and bedrock erosion rates collected throughout the Dry Valleys. Assuming steady state conditions were met by the time of emplacement, the calculated surface erosion rate is ~110 cm Ma^-1.