Late Oligocene to Late Miocene Antarctic Climate Reconstructions Using Molecular and Isotopic Biomarker Proxies

Abstract:

Major climate and environmental changes occurred during late Oligocene to the late Miocene when atmospheric CO₂ ranged between 500 and 300ppm, indicating threshold response of Antarctic ice sheets and climate to relatively modest CO₂ variations. This implies that the southern high latitudes are highly sensitive to feedbacks associated with changes in global ice sheet and sea-ice extent, as well as terrestrial and marine ecosystems. This study focuses on two key intervals during the evolution of the Antarctic Ice Sheet: (1) The Late Oligocene and the Oligocene/Miocene boundary, when the East Antarctic Ice Sheet expanded close to present day volume following an extended period of inferred warmth. (2) The Mid-Miocene Climate Optimum (MMCO ~17-15 Ma), a period of global warmth and moderately elevated CO₂ (350->500 ppm) which was subsequently followed by rapid cooling at 14-13.5 Ma. Reconstructions of climate and ice sheet variability, and thus an understanding of the various feedbacks that occurred during these intervals, are hampered by a lack of temperature and hydroclimate proxy data from the southern high latitudes.

We present proxy climate reconstructions using terrestrial and marine organic biomarkers that provide new insights into Antarctica’s climate evolution, using Antarctic drill cores and outcrop samples from a range of depositional settings. Bacterial ether-lipids have been analysed to determine terrestrial mean annual temperatures and soil pH (via the methylation and cyclisation indexes of branched tetraethers – MBT and CBT, respectively). Tetraether-lipids of crenarchaeota found in marine sediments sampled from continental shelves around Antarctica have been used to derive sea surface temperatures using the TEX₈₆ index. Compound specific stable isotopes on n-alkanes sourced from terrestrial plants have been analysed to investigate changes in the hydrological and carbon cycles.