Evolution of C-4 plants and controlling factors: Insight from n-alkane isotopic values of NW Indian Siwalik paleosols

摘要

The delta C-13 values of long chain n-alkanes from paleosol samples (n = 74) of four NW Indian Siwalik sections have been used to reconstruct the relative abundance of C-3-C-4 plants, after isotopic characterization of modern plants from the Ganges floodplain considered equivalent to the past Siwalik floodplain. The values from the Naladkhad (11.6-8.9 Ma) and Ranital (11.3-7.4 Ma) sections of the Kangra sub-basin indicate the presence of ca. 20% C-4 plants at ca. 11 Ma. The existence of C-4 plants at ca. 11 Ma indicate an early appearance of C-4 plants, compared with the published data from the Siwalik regions. The Ranital section shows a patchy occurrence of C-4 plants between 11.3 Ma and 7.4 Ma and the Jabbarkhad section shows (5.2-3.0 Ma) a gradual increase in C-4 plant abundance. The C-4 plant abundance fluctuated between 83% and 0% (5.8-1.3 Ma) in the Haripur Khol section of the Subathu sub-basin. Comparison of C-4 plant abundance from the four sections shows that each section was characterised by a distinct evolutionary pattern. The delta D values of the same n-alkanes from the Kangra and Subathu sub-basins indicate two episodes of Indian summer monsoon intensification at ca. 9 Ma and ca. 4 Ma. The correlation between delta D-C29 and delta C-13(C29) values varied in different sections. It was also observed that the C-4 plant abundance variation could be linked to the variable channel/overbank ratio of the studied sections. This observation suggests that Indian summer monsoonal rainfall, type of overbank sediments and moisture content controlled the abundance of C-4 plants in the Siwalik floodplain. (C) 2017 Elsevier Ltd. All rights reserved.