Abstract

Vegetation changes in the Indus River basin within the past 10.8 million years were investigated based on the analysis of n-fatty acids and their carbon isotopes in sediments from IODP Site U1457 in the Laxmi Basin of the Arabian Sea. The δ13C of long-chain n-C32 fatty acid shifted from −34 to −22 ‰ from 10 to 6.3 Ma, while the δ13C of mid-chain n-C24 fatty acid was nearly constant at around −23 to −22 ‰ over the same period. This large difference in the δ13C values suggests that the mid-chain fatty acids reflect the contribution of aquatic vascular C3 plants. Before 6.3 Ma, the average chain length of n-fatty acids and the δ13C values of long-chain fatty acids were negatively correlated, suggesting that the δ13C values reflected the relative abundance of terrestrial C3 versus aquatic C3 plants in the Indus River basin and western India. After 5.8 Ma, the average chain length was variable, but the δ13C values remained nearly the same, suggesting that the δ13C values reflected heavier δ13C values of both aquatic C3 and C4 plants. A three-end-member model calculation suggests that terrestrial C3 plants were replaced by C4 plants in the Indus River basin and western India from 9.7 or 8.2 to 6.3 Ma. Aridification in those areas during the late Miocene period may have driven the replacement of terrestrial C3 plants by C4 plants. An episodic increase in the abundance of terrestrial plants around 8 Ma is attributed to elevated precipitation by regionally enhanced moisture transport.

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