09:00 〜 09:15
[MIS03-01] Changes in C3, C4, and aquatic plant vegetation in the Indus River basin during the last 10.8 Ma
★Invited Papers
キーワード：インダス川、C4植物、中新世
Terrestrial plants are classified into C3 and C4 plants based on differences in photosynthetic circuits. C3 plants are suitable for cold, wet, and high pCO2 environments, while C4 plants are suitable for warm, dry, and low pCO2 environments. Therefore, the vegetation of C3/C4 plants responds sensitively to changes in the environment, and the proportion of C3/C4 plants varies significantly. C4 plants are known to have rapidly expanded their range at low latitudes after 8 Ma. Changes in temperatures, dry and wet environments, and CO2 concentration have been proposed as causes of this 8Ma C4 plant expansion event, but the causes have not been determined, and some have proposed that the causes differ from region to region.
In this study, we used ocean sediment cores covering the last 10.8 Ma drilled at Site U1457 in the Lakshmi Basin of the Arabian Sea during International Ocean Discovery Program (IODP) Expedition 355. Samples were freeze-dried and lipids were extracted using an Accelerated Solvent Extractor. The acidic fraction was methylated and used for the determination of δ13C of long-chain n-fatty acids from higher plants. Based on the results of these analyses, we reconstructed the vegetation changes in the Indus River Basin over the last 10.8 Ma.
The δ13C values of n-C32 long-chain fatty acids shifted from −34 to −22 ‰ from 10.8 to 6 Ma. In contrast, the δ13C of n-C24 mid-chain fatty acids remained almost constant at −23 to −22 ‰ during the same period. This large difference in δ13C values suggests that there is a contribution of aquatic vascular C3 plants to mid-chain n-fatty acids. before 6.3 Ma, the mean chain length of n-fatty acids was negatively correlated with δ13C values of long-chain n-fatty acids, suggesting that δ13C values reflect the relative abundance of terrestrial and aquatic C3 plants in the Indus River Basin and western India. After 5.8 Ma, the mean chain length changed, but the δ13C values were similar, suggesting that the δ13C values reflect the heavy δ13C values of aquatic C3 and C4 plants. Three endmembers model calculations suggest that terrestrial C3 plants were replaced by C4 plants from 9.7 to 6.3 Ma in the Indus River Basin. Isotopic studies suggest that about 70% of the river water in the Indus River originates from water vapor from the Mediterranean Sea, and the Mediterranean Sea and the Tethys Sea were reported to have shrunk during the late Miocene, which may have reduced water vapor transport to the Indus River basin. Aridification of the region during the late Miocene may have promoted the replacement of terrestrial C3 plants by C4 plants.
In this study, we used ocean sediment cores covering the last 10.8 Ma drilled at Site U1457 in the Lakshmi Basin of the Arabian Sea during International Ocean Discovery Program (IODP) Expedition 355. Samples were freeze-dried and lipids were extracted using an Accelerated Solvent Extractor. The acidic fraction was methylated and used for the determination of δ13C of long-chain n-fatty acids from higher plants. Based on the results of these analyses, we reconstructed the vegetation changes in the Indus River Basin over the last 10.8 Ma.
The δ13C values of n-C32 long-chain fatty acids shifted from −34 to −22 ‰ from 10.8 to 6 Ma. In contrast, the δ13C of n-C24 mid-chain fatty acids remained almost constant at −23 to −22 ‰ during the same period. This large difference in δ13C values suggests that there is a contribution of aquatic vascular C3 plants to mid-chain n-fatty acids. before 6.3 Ma, the mean chain length of n-fatty acids was negatively correlated with δ13C values of long-chain n-fatty acids, suggesting that δ13C values reflect the relative abundance of terrestrial and aquatic C3 plants in the Indus River Basin and western India. After 5.8 Ma, the mean chain length changed, but the δ13C values were similar, suggesting that the δ13C values reflect the heavy δ13C values of aquatic C3 and C4 plants. Three endmembers model calculations suggest that terrestrial C3 plants were replaced by C4 plants from 9.7 to 6.3 Ma in the Indus River Basin. Isotopic studies suggest that about 70% of the river water in the Indus River originates from water vapor from the Mediterranean Sea, and the Mediterranean Sea and the Tethys Sea were reported to have shrunk during the late Miocene, which may have reduced water vapor transport to the Indus River basin. Aridification of the region during the late Miocene may have promoted the replacement of terrestrial C3 plants by C4 plants.