Reconstructing 15 Myr of environmental change in the McMurdo Dry Valleys through permafrost geochemistry

<p><b>The McMurdo Dry Valleys of Antarctica are the largest ice-free region in Antarctica. Valley downcutting by major outlet glaciers and post-glacial uplift since the mid-Miocene have resulted in predominantly younger surficial sediments in the low elevation, coastal areas and significantly older sediments in high elevation, inland areas. The hyper-arid conditions that prevail in the high elevations (> 1000 m a.s.l.) of the McMurdo Dry Valleys have protected these surfaces from alteration and weathering, and provide important sediment records of paleoenvironments dating back to the early Miocene. The Friis Hills (77°45’S, 161°30’E, 1200–1500 m a.s.l.) are a 12 km-wide inselberg situated at the head of Taylor Valley. This unique location allowed Miocene-age sediments to be preserved and protected from subsequent ice sheet expansions. Permafrost within these sediments is potentially the oldest on Earth. </b></p><p>As sediments accumulate in periglacial environments, permafrost aggrades with minimal lag time and potentially preserves sediments, organic material and ground ice. The 2016 Friis Hills Drilling Project retrieved a ∼50 m thick permafrost sequence, which not only consists of an archive of Antarctic environmental changes from approximately 14–15 Ma but also records the paleoenvironmental changes of the Neogene and provides insight on the modern hyper-arid environment. The main objective of this project is to understand the unique geochemical characteristics of these permafrost cores and document 15 Myr of change in the upper elevations of the McMurdo Dry Valleys. </p><p>Paleoenvironmental reconstructions of interglacial periods suggest a tundra-like environment in the high elevations of continental Antarctica through the mid-Miocene. Plants such as lichens, liverworts, mosses, grasses and sedges, dicots and Nothofagaceae occupied the Friis Hills during the mid-Miocene. The δ13C signal of C3 plants (-25.5 ± 0.7 ‰ VPDB) corresponds to a semi-arid environment with a mean annual precipitation ranging from 300 to 850 mm yr-1. The unusually high δ15N reflects an ecosystem with up to three trophic levels, supported by the presence of insect fragments, feathers barbs (birds) and tardigrades fragments within the sediment. The deep ice lenses and their meteoric signature suggest a near-saturated active layer during the mid-Miocene. Temperature reconstructions based on the corrected δ18O value of the deep ground ice and change in paleogeography imply that the mid-Miocene (11.1–13.9 Ma) was ∼6 to 12°C warmer. These paleoenvironmental conditions are comparable to those found in the modern Arctic, such as in west Greenland. </p><p>A dominant trend of literature suggests that the high elevations of the McMurdo Dry Valleys have remained under a hyper-arid polar climate since ∼13.8 Ma. However, the presence of 10Bemet in the upper section of the Friis Hills and Table Mountain cores provides evidence for the translocation of clays, which is only possible under a warmer and wetter climate. The 10Bemet concentrations imply that these conditions were present until ∼6.0 Ma at Friis Hills and Table Mountain, consequently challenging the idea that the upper McMurdo Dry Valleys have remained frozen under hyper-arid climate since the mid-Miocene climate transition. Hence, this finding supports the hypothesis that the Miocene has undergone progressive cooling with onset of polar aridity between 7 and 5.4 Ma. The erosion-corrected paleo-active layer depth suggests mean annual air temperatures ranging from -12 to -9°C ∼6.0 Ma. In other words, this thesis shows that the upper McMurdo Dry Valleys have been frozen under hyperarid conditions only since ∼6 Ma and not for 14 Myr as previously thought. </p><p>The ground ice in the uppermost 1 m originates from the modern freezing of evaporated snowmelt and the presence of high salt content which allows unfrozen water in the near-surface. The conformity of dry permafrost samples to biological ratios suggests that the modern environment is regulated by biochemical processes and the current pool of organic carbon in the dry permafrost appears to be in equilibrium with a modern climate and ecosystem. These findings not only characterize the paleoenvironmental changes of continental Antarctica through the late Miocene but also provide a better understanding of the modern ultraxerous conditions of the McMurdo Dry Valleys.</p>