[M-IS01] Evolution and variability of Asian Monsoon and its linkage with Cenozoic global cooling

[MIS01-P02] Two step expansion of C4 plants in northwestern India at 10 Ma and 7.5 Ma: Evidence of leaf wax carbon isotopes in IODP Site U1457 sediments

Kenta Suzuki², *Masanobu Yamamoto^1,2, Osamu Seki^2,3 (1.Faculty of Environmental Earth Science, Hokkaido University, 2.Graduate School of Environmental Sceince, Hokkaido University, 3.Institute of Low Temperature Science, Hokkaido University)

Keywords:C-4 plants, Miocene, India

Earlier studies indicated that C4 plants started to expand globally around 8 Ma (e.g., Cerling et al., 1997). This phenomenon is attributed to the decrease of atmospheric CO₂ (Cerling et al., 1997), cooling (Zhang et al., 2003) and aridification (Huang et al., 2001).

In this study, we generated a record of the stable carbon isotope (δ¹³C) of higher plants during the last 10.6 m.y. Long-chain fatty acids (>26 carbons) were analyzed for 90 samples between 0 to 1000 m in Site U1457, Laxmi Basin, Arabian Sea.
The δ¹³C of C_{26, 28, 30} fatty acids was –29 ‰ at 10.5 Ma, dropped at 10 Ma to –34 ‰, recovered to –28 ‰ by 9 Ma, dropped again at 8 Ma to –33 ‰, abruptly increased to –26 ‰ at 7.5 Ma, gradually increased to –23 ‰ by ~6 Ma, and constantly high after 6 Ma. This result indicates that the proportion of C4 plants around 10 Ma and 8 Ma was significantly low, and C4 plants expanded two times at 10 Ma and 7.5 Ma. The first retreat of C4 plants around 10 Ma corresponded to a wet period in eastern Europe (Böhme et al., 2007), suggesting that higher precipitation increased C3 plants. The second retreat of C4 plants around 8 Ma is unexplained, but the recovery of C4 plants at 7.5 Ma coincided with global cooling, suggesting a drastic decrease of atmospheric carbon dioxide concentration may have induced C4 plant expansion.