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

Abstract

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 fl oodplains using carbon (δ 13 C) and oxygen isotopic (δ 18 O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, northcentral Pakistan. Revised, high-resolution magnetostratigraphy and a new 40 Ar/ 39 Ar 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 fi ve 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 fl uvial sediments: channel fi ll, crevasse-splay, and fl oodplain environments. δ 13 C 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 fi rst invading the drier portions of the system (fl oodplain 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 (~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 defi ned 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.δ 18 O values as well as δ 13 C 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 δ 13 C and δ 18 O 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 δ 18 O values. We interpret the trend toward heavier soil carbonate δ 18 O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.

Ecological changes in Miocene mammalian record show impact of prolonged climatic forcing

“…Although fossil plants are extremely rare from the Miocene deposits of the Potwar Plateau, stable isotopes from paleosol carbonates throughout the sequence provide evidence for vegetation composition on the flood plain. A large shift in carbon and oxygen isotope values during the late Miocene, initially documented in this Potwar Siwalik sequence, records changes in vegetation composition and structure linked to tectonic uplift and onset of the South Asian monsoon system (12)(13)(14). Isotopic analysis of mammalian tooth enamel independently documents changes in vegetation and precipitation sources and correlates well with the record from paleosol carbonates (15)(16)(17).…”

“…By 6.0 Ma, all herbivores sampled ate a mixed C 3 -C 4 or pure C 4 diet (␦ 13 C values Ͼ Ϫ7.5‰), and paleosol carbonates indicate that C 4 vegetation was widespread. Lateral variation in ␦ 13 C values from younger paleosols document areas of C 3 vegetation that remained on the flood plain in specific depositional environments until at least 4.5 Ma (14).…”

“…The Miocene flood basin was traversed by large rivers-on the scale of major tributaries to the modern Indus and Ganges-and smaller flood plain channels (18)(19)(20). Paleosol sequences of variable maturity indicate the presence of forest, woodland, and grassland vegetation that was stable for decades to thousands of years (12,14). Although shallow lacustrine sediments imply the presence of year-round water, the high frequency of pedogenic carbonate nodules signifies high seasonal evapotranspiration.…”

The integration of quantitative genetics, paleontology, and neontology reveals genetic underpinnings of primate dental evolution

“…The dramatic evolution of these two GP dental traits across the Old World higher primates shows a remarkable correlation with global climate and vegetation change as reflected in decreasing δ 18 O and δ 13 C values following the Middle Miocene Climatic Optimum (34,35). Paleobotanical and faunal evidence suggests that the largely mosaic forests and woodlands of Oligocene Africa transitioned in the Middle Miocene to more heterogeneous landscapes (36,37).…”

River-margin habitat of Ardipithecus ramidus at Aramis, Ethiopia 4.4 million years ago

“…In any case, our sedimentological data strongly argue for the presence of sizable rivers at 4.4 Myr in the study area. 11,14,15 . In one study at Gona (70 km north of Aramis; Fig.…”