COUPLED PALYNOLOGICAL AND CARBON ISOTOPIC ANALYSES OF NOTHOFAGUS TO QUANTIFY NEOGENE ANTARCTIC CLIMATE CHANGE

Changes in the plant fossil record show that the Antarctic climate deteriorated throughout the Neogene, culminating in the demise of tundra and extinction of most plant species on the continent. This event is believed to have happened by ~13.85 Ma in the Dry Valleys (Lewis et al., 2008) and after 12.8 Ma in the Antarctic Peninsula (Anderson et al., 2011) in response to progressively cooler and drier conditions. However, quantifying these changes (e.g., increased aridity) is difficult due to the scarcity of outcrops and associated plant fossils.

This study is designed to gather such information on two well-dated Neogene sections, one in the Antarctic Peninsula (SHALDRIL; Eocene-Miocene) and the other in the Ross Sea region (ANDRILL; Miocene-Pliocene). Preliminary palynological analyses of these localities show a dominance of Nothofagus pollen through most of the Early and Middle Miocene (Warny et al., 2009; Warny and Askin, in press). Modern species of this genus, the Southern beech, are adapted to cool, temperate conditions and are found today in Southern Hemisphere regions such as Patagonia and New Zealand.

Because fossil Nothofagus pollen is sufficiently abundant in these cores and comparison with extant material growing under known climatic conditions can be done, we propose that this genus is the best candidate for a new technique in carbon isotopic analysis. Indeed, data from fossil Nothofagus pollen is used to reconstruct variations in moisture availability which influences plant water-use efficiency (Seibt et al., 2008) and address hypotheses concerning the timing and cause of Neogene Antarctic climate deterioration (e.g. development of the Antarctic Circumpolar Current (Anderson et al., 2011); declining levels of CO₂ (DeConto & Pollard, 2003)).

We report δ¹³C values from small quantities of modern and fossil Nothofagus pollen grains obtained using a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer (Nelson et al., 2008). We also compare δ¹³C values from modern leaf and pollen tissues to calibrate interpretation of data from fossil grains. Preliminary results show a small range in Eocene-aged Nothofagus grains (~-25.1 to -23.7‰). These fossil grains have higher δ¹³C values than the modern grains analyzed which range from ~-27.2 to -25.7‰.