Evolution of C4 plants and controlling factors: Insight from n-alkane isotopic values of NW Indian Siwalik paleosols

The δ13C 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 C3-C4 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% C4 plants at ca. 11 Ma. The existence of C4 plants at ca.11 Ma indicate an early appearance of C4 plants, compared with the published data from the Siwalik regions. The Ranital section shows a patchy occurrence of C4 plants between 11.3 Ma and 7.4 Ma and the Jabbarkhad section shows (5.2-3.0 Ma) a gradual increase in C4 plant abundance. The C4 plant abundance fluctuated between 83% and 0% (5.8-1.3 Ma) in the Haripur Khol section of the Subathu sub-basin. Comparison of C4 plant abundance from the four sections shows that each section was characterised by a distinct evolutionary pattern. The δD 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 δDC29 and δ13CC29 values varied in different sections. It was also observed that the C4 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 C4 plants in the Siwalik floodplain.