Warm climate under high CO2 level in the early Pleistocene based on a leaf fossil assemblage in central Japan

The atmospheric levels of CO2, a greenhouse gas, are closely related to global change, and investigation of historic CO2 levels can help achieve a better understanding of the global climate change in the deep past. While stomatal frequency of fossil leaves has been recurrently used for palaeo-CO2 reconstruction, simultaneous investigation of both palaeo-temperature and palaeo-CO2 using the same fossil assemblage has been rarely performed. In this study, based on the stomatal analyses of Quercus gilva from a leaf bed in the Sayama Formation (1.66-1.55 Ma) in central Japan, we estimated the palaeo-CO2 values in an interglacial stage (MIS 57 or 55) at 36.41 ± 2.58 pa, which is generally higher than the previously reported data from early Pleistocene. To understand the climate under this high CO2 level, we also calculated the mean annual temperature (MAT) based on the same leaf assemblage using the leaf margin analysis (LMA) approach. The calculated MAT was 11.0 °C, suggesting a relatively warm climate during the interglacial stage. Our results revealed a warm environment under high CO2 level during the early Pleistocene, which demonstrates the vital role of CO2 in controlling the early Pleistocene interglacial temperature.