[M-IS02] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS02-05] Monsoon seasonality affecting Indian vegetation over the last 1.46 million years

*Masanobu Yamamoto¹, Steven C Clemens² (1.Faculty of Environmental Earth Science, Hokkaido University, 2.Earth, Environmental and Planetary Sciences, Brown University)

Keywords:Savannah, Wildfires, Monsoon

Monsoon is seasonally-reversed winds between continent and ocean and brings seasonal cycles in precipitation, resulting in characteristic landscapes such as savannah. Wildfires are essential in forming savannahs and are related to carbon sequestration in the soil. Here we report a 1.46 million year-long record of wildfires and grass Poaceae abundance in the savannah vegetation in east India based on the analysis of pyrogenic polycyclic aromatic hydrocarbons (PAHs), pentacyclic triterpene methyl ethers (PTMEs), and long-chain n-alkanes (LCNA) in sediments from Site U1446, the Bay of Bengal. The concentrations of PAHs and PTMEs normalized by long-chain n-alkane concentration showed Milankovitch cycles and were higher when the Indian summer monsoon precipitation (sea water δ¹⁸O; Clemens et al., 2021) was higher, which is attributed to the higher rainfall supporting grass growth in the rainy season and causing larger fires by accumulated dried-up plants in the dry season. This implies that changes in monsoon seasonality regulated seasonal grass biomass and fire activity. On the other hand, the leaf wax δ¹³C record indicates that the C₃/C₄ vegetation ratio has primarily reflected atmospheric CO₂ concentration rather than precipitation (Yamamoto et al., 2022). These findings suggest that the expansion of C₄ vegetation was independent of the expansion of grassland by responding to CO₂.