The structure and rate of late Miocene expansion of C₄ plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan

This study uses stable isotope variation within individual Mio-Pliocene paleosols to investigate subkilometer-scale phytogeography of late Miocene vegetation change in southeast Asia between ca. 8.1 and 5 Ma, a time interval that coincides with dramatic global vegetation change. We examine trends through time in the distribution of low-latitude grasses (C₄ plants) and forest (C₃ plants) on Indo-Gangetic floodplains using carbon (δ¹³C) and oxygen isotopic (δ¹⁸O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, north-central Pakistan. Revised, high-resolution magnetostratigraphy and a new ⁴⁰Ar/³⁹Ar date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C₃ versus C₄ plants at five stratigraphic levels, R11 (8.0 Ma), R15 (6.74-6.78 Ma), R23 (5.78 Ma), R29 (4.8-4.9 Ma), and upper boundary tuff (UBT; 2.4 Ma), using detailed sampling of paleosols traceable laterally over hundreds of meters. Paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments: channel fill, crevasse-splay, and floodplain environments. δ¹³C results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C₃ or C₄, respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C₄ grasses first invading the drier portions of the system (floodplain surfaces) and C₃ plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C₄ plants are modest (∼2% per 100,000 yr) compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the Quaternary. Earlier research documented a sharply defined C₃ to C₄ transition in Pakistan between 8.1 and 5.0 Ma, based on vertical sampling, but this higher-resolution study reveals a more gradual transition between 8.0 and 4.5 Ma in which C₃ and C₄ plants occupied different subenvironments of the Siwalik alluvial plain. δ ¹⁸O values as well as δ¹³C values of soil carbonate increase up section at Rohtas, similar to isotope trends in other paleosol records from the region. Spatially, however, there is no correlation between δ¹³C and δ¹⁸O values at most stratigraphic levels. This implies that the changes in soil hydrology brought about by the shift from forest to grassland (i.e., an increase in average soil evaporation) did not produce the shift through time in δ¹⁸O values. We interpret the trend toward heavier soil carbonate δ¹⁸O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.