An attempt to reconstruct past atmospheric carbon dioxide concentrations using carbon isotope ratios of long-chain fatty acids

Abstract

Atmospheric carbon dioxide (CO₂) is an essential factor in determining the Earth's climate. In predicting future warming, it is necessary to understand how atmospheric CO2 has affected past climate change. Past atmosphericCO₂ concentrations have been determined by analyzing gases in ice cores drilled through the Antarctic ice. In this study, we show that the carbon isotope ratios of plant-derived fatty acids in the Bay of Bengal sediments represent past atmospheric CO₂ concentration changes by comparing them with those in ice cores. Using this relationship, we have shown for the first time that atmospheric CO₂ concentration changes before ca 800,000 years ago can be determined with high precision and high temporal resolution. The results show that even before ca 800,000 years ago, the CO₂ concentration fluctuated almost synchronously with the volume of continental ice sheets. However, the CO₂ concentrations were unexpectedly not high during the warm period prior to 1 million years ago. It was also found that the CO₂ concentrations fluctuated earlier than the volumes of continental ice sheets before 1 million years ago but later than the ice volumes after ca 800,000 years ago.