The structure and rate of late Miocene expansion of C-4 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 (C-4 plants) and forest (C-3 plants) on Indo-Gangetic floodplains using carbon (delta C-13) and oxygen isotopic (delta C-18) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, north-central Pakistan. Revised, high-resolution magnetostratigraphy and a new Ar-40/Ar-39 date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C-3 versus C-4 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. delta C-13 results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C-3 or C-4, respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C-4 grasses first invading the drier portions of the system (floodplain surfaces) and C-3 plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C-4 plants are modest (similar to 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 C-3 to C-4 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 C-3 and C-4 plants occupied different subenvironments of the Siwalik alluvial plain. delta O-18 values as well as delta C-13 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 delta C-13 and delta O-18 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 delta O-18 values. We interpret the trend toward heavier soil carbonate delta O-18 values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma