Snapshot of cooling and drying before onset of Antarctic Glaciation

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The late Eocene to Oligocene transition (EOT) witnessed a major ice advance on Antarctica. Because of the paucity of accessible outcrops and difficult drilling logistics, little is known about hydrological conditions in the Antarctic Peninsula during the late Eocene prior to the major ice advance. Here we study hydrological conditions with proxy evidence from marine sediment core NBP0602A-3C, adjacent to the tip of the Antarctic Peninsula, with sediments dated to 35.9 ± 1.1 Ma providing a snapshot of conditions prior to the EOT. We reconstruct conditions during the latest Eocene based on plant leaf wax hydrogen isotopic evidence (δDwax) paired with previously-published evidence from pollen assemblages, and compared to climate model experiments simulating conditions before, during and after the transition. The pollen from late Eocene sediments of NBP0602A-3C indicate a Nothofagidites (southern beech) dominated landscape. In the same sediments, δDwax values average -202 ± 7‰ (1σ, n = 22) for the C28 n-alkanoic acid. Uncertainty around the appropriate net fractionation between water and wax (εwax/w) provides the largest degree of uncertainty in paleoprecipitation δD reconstructions and we therefore use two reasonable fractionations -100 and -60‰ to constrain the likely range of average precipitation δD values to between -113 and -151‰. Model experiments confirm that isotopic compositions show low sensitivity to temperature changes at the tip of the Antarctic Peninsula, in comparison to much larger changes in the continental interior. Across the interval sampled, a decline in pollen abundance from 105 to 103 gdw -1 represents cooling and drying towards more tundra-like conditions. The 30‰ trend in δDwax values towards more positive values can best be explained by smaller fractionations as the vegetation shifted from forest to tundra, with greater water stress. Model results are broadly consistent with the data and quantify the cooling and drying across the full EOT from ca. 7 to 2°C and 700 to 600 mma -1 with an isotopic shift in δD of only about -15‰ at the tip of the Antarctic Peninsula. Pollen and leaf wax proxies reconstruct vegetation and hydrological conditions prior to the transition, and model experiments through the EOT provide proxy-groundtruthed quantification of terrestrial transformations. © 2014 Elsevier B.V.

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Earth and Planetary Science Letters

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