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 Himal ayan foothills along the length of southern Nepal. Carbon and oxygen i sotopic analyses of Siwalik paleosols from four long Siwalik sections record major ecological changes over the past similar to 11 m.y. The c arbon isotopic composition of both soil carbonate and organic matter s hifts dramatically starting ca. 7.0 Ma, marking the displacement of la rgely C-3 vegetation, probably semi-deciduous forest, by C-4 grassland s. By the beginning of the Pliocene, all the food plains of major rive rs in this region were dominated by monsoonal grasslands. The floral s hift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the l atest Miocene. A similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of Pakistan, and in terrigenous o rganic matter from the Bengal Fan, showing that the floral shift was p robably continentwide. The latest Miocene also witnessed an average ch ange of similar to 4 parts per thousand in the oxygen isotopic composi tion of soil carbonate, as observed previously in Pakistan. The rea so ns for this are unclear; if not diagenetic, a major environmental chan ge is indicated, perhaps related to that driving the carbon isotopic s hift. Recently described pollen and leaf fossils from the Surai Khola section show that evergreen forest was gradually displaced by semi-dec iduous and dry deciduous forest between 11 and 6 Ma. The gradual natur e of this floral shift, which culminated in the rapid expansion of C-4 grasses starting similar to 7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pCO(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 m onsoonal intensification and consumption of CO2 through weathering, ma y be the common cause behind major late Miocene environmental change g lobally. However, the decline of effective moisture associated with mo nsoon development has probably slowed, not increased, the rate of cons umption of CO2 by chemical weathering of Himalayan sediments.