Antarctic climate and vegetation during the Neogene: a geochemical and modelling approach

During the mid- to late Neogene (20 - 2.5 million years ago), episodic retreat of the Antarctic Ice Sheet (AIS) coincided with periods of higher-than-present atmospheric CO2, indicating ice sheet sensitivity to climatic conditions similar to those projected for the coming decades. Understanding Antarctic climate and vegetation during such a period of AIS retreat is crucial for our fundamental understanding of high latitude environments in warmer-than-present climate scenarios. This thesis presents a detailed geochemical study of sediments and plant fossils from the terrestrial Sirius Group of Oliver Bluffs, Transantarctic Mountains, located at 85 °S today and during the Neogene.

Biomarker analysis of the sediments show strong evidence for a warmer Antarctica, where summer temperatures reached 5 °C. These relatively favourable conditions sup- ported a low diversity mixed vegetation. In contrast to the macrofossil record, there is geochemical evidence for conifers, suggesting that Antarctic vegetation was strongly controlled by local environmental variability. The warmer conditions are associated with a dynamic carbon cycle, evidenced by anomalously high and variable atmospheric 13C and possibly linked to atmospheric CO2 levels. Precipitation isotopes are reconstructed from plant compound isotope analysis of the fossils, and indicate markedly different hydrological cycling. This result is supported by climate modelling experiments which suggest that Antarctic hydrological cycling is most strongly governed by the extent of the ice sheet rather than by greenhouse gas radiative forcing. This thesis presents a new approach to exploring Antarctic climate and vegetation and provides important novel information on this crucial region of the world.