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