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

摘要

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 (13C) and oxygen isotopic (18O) 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, crevassesplay, and floodplain environments. 13C 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.