Preservation and degradation of ancient organic matter in mid-Miocene Antarctic permafrost

Abstract. The Antarctic environment is amongst the coldest and driest environments on Earth. The ultraxerous soils in the McMurdo Dry Valleys support exclusively microbial communities, however, 15 million years ago, a tundra ecosystem analogous to present-day southern Greenland occupied this region. The occurrence of ancient soil organic carbon combined with low accumulation of contemporary material makes it challenging to differentiate between ancient and modern organic processes. Here, we explore the additions of modern organic carbon, and the preservation and degradation of organics and lipid biomarkers, in a 1.4 m mid-Miocene age (∼14.5–14.3 Ma) permafrost soil column from Friis Hills. The total organic carbon is low throughout the soils (<1 wt %). The near-surface (upper 35 cm) dry permafrost has lower C:N ratios, higher δ13Corg values, higher proportion of branched fatty acids with an iso and anteiso configuration relative to n-fatty acids, lower phytol abundance and higher contributions of low-molecular weight homologues of n-alkanes, than the underlying icy permafrost, indicating higher contributions from bacteria-derived organic matter. Conversely, the icy permafrost contains higher molecular weight n-alkanes, n-fatty acids and n-alkanols, along with phytosterols (e.g. sitosterol and stigmasterol) and phytol (and its derivatives pristane and phytane) that are indicative of the contributions and preservation of higher-level plants. This implies that legacy mid-Miocene age carbon in the near-surface soils (ca. 35 cm) has been prone to microbial organic matter degradation during times when the permafrost thawed, likely during relatively warm intervals through the late Neogene (∼6.0 Ma) and sporadically during the Holocene (<1 %), when ground summer temperatures were ≥+2 °C (based on branched glycerol dialkyl glycerol tetraether (brGDGT) temperature reconstructions). Conversely, lipid biomarkers found deeper in the permafrost have been preserved for millions of years. These results suggest that ancient organics preserved in permafrost could underpin significant ecological changes in the McMurdo Dry Valleys under the current warming climate.