Combining a New Method in Carbon Isotopic Analysis with Standard Palynological Methods 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 the 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., aridity) is difficult due to the scarcity of outcrops and associated plant fossils.

This study is designed to gather information on two well-dated sections, one in the Antarctic Peninsula (SHALDRIL) that dates from Eocene to Miocene and the other in the Ross Sea region (ANDRILL) that dates from Miocene to Pliocene. Previous 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 are adapted to cool, temperate conditions and are found today in Southern Hemisphere regions.

Due to the nature of the technique, a suitable abundance of palynomorphs is required. We propose that this genus is the best candidate for a new technique in carbon isotopic analysis due to its abundance in these cores and availability of extant material growing under known climatic conditions. Nothofagus also makes for a particularly rewarding genus to study due to its poor fruit dispersal and profusion throughout the fossil record (Hill and Dettmann, 1996).

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) and 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‰.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California