Late Miocene environmental change in Nepal and the northern Indian subcontinent: Stable isotopic evidence from paleosols

Neogene sediments belonging to the Siwalik Group crop out in the Himalayan foothills along the length of southern Nepal. Carbon and oxygen isotopic analyses of Siwalik paleosols from four long Siwalik sections record major ecological changes over the past [approximately]11 m.y. The carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca. 7.0 Ma, marking the displacement of largely [C.sub.3] vegetation, probably semi-deciduous forest, by [C.sub.4] grasslands. By the beginning of the Pliocene, all the flood plains of major rivers in this region were dominated by monsoonal grasslands. The floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest Miocene. A similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of Pakistan, and in terrigenons organic matter from the Bengal Fan, showing that the floral shift was probably continentwide. The latest Miocene also witnessed an average change of [approximately]4% in the oxygen isotopic composition of soil carbonate, as observed previously in Pakistan. The reasons for this are unclear; if not diagenetic, a major environmental change is indicated, perhaps related to that driving the carbon isotopic shift. Recently described pollen and leaf fossils from the Surai Khola section show that evergreen forest was gradually displaced by semi-deciduous and dry deciduous forest between 11 and 6 Ma. The gradual nature of this floral shift, which culminated in the rapid expansion of [C.sub.4] grasses starting [approximately]7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pC[O.sub.2] alone (Cerling et al., 1993) but fits well with a gradual onset of monsoonal conditions in the late Miocene in the northern Indian subcontinent. Himalayan uplift, driving both monsoonal intensification and consumption of C[O.sub.2] through weathering, may be the common cause behind major late Miocene environmental change globally. However, the decline of effective moisture associated with monsoon development has probably slowed, not increased, the rate of consumption of C[O.sub.2] by chemical weathering of Himalayan sediments.