EXPANSION OF C-4 GRASSES IN THE LATE MIOCENE OF NORTHERN PAKISTAN - EVIDENCE FROM STABLE ISOTOPES IN PALEOSOLS

Stable-isotopic, clay-mineralogic, and bulk-chemical analyses were con ducted on paleosols of the Neogene Siwalik sections in northern Pakist an in order to reconstruct floodplain environments over the past simil ar to 17 Ma. The stable carbon isotopic composition of soil carbonate (mean delta(13)C (PDB)=-10.2 parts per thousand) and associated organi c matter (mean delta(13)C (PDB)=-24.1 parts per thousand) in paleosols representing 17-similar to 7.3 Ma reveal that floodplain vegetation w as dominated by C-3 plants. At 7.3 Ma, a shift toward more positive ca rbon isotopic values began, signaling the gradual expansion of C-4 gra sses onto the floodplain. From 6 Ma to present, carbon isotopic values for paleosol carbonate (mean delta(13)C (PDB)=+0.6 parts per thousand ) and organic matter (mean delta(13)C (PDB)=-14.4 parts per thousand) are uniformly enriched in C-13, indicating the presence of nearly pure C-4 grassland. The scarcity of kaolinite and abundance of smectite an d pedogenic carbonate in most paleosols suggest that rainfall in the r egion remained 1.0-1.25 m/yr or less for the entire 17 Ma of record. P aleosols in the lower portion of the section lack organic A horizons b ut have reddish B horizons often containing secondary iron-oxide nodul es. Leaching depths of soil carbonate in these older paleosols are typ ically greater than those in the Plio-Pleistocene part of the section, where organic A horizons are common, and B horizons are markedly more yellow. The combined evidence suggests that the mature paleosols in t he pre-7.3 Ma part of the record are dominantly calcareous Alfisols or Mollisols that once underlay nearly pure C-3 vegetation, perhaps tree s and shrubs, while calcareous Mollisols underlying C-4 grassland domi nate the upper part of the record.The carbon- and oxygen-isotopic tren ds in the paleosol record in Pakistan are also evident in the diet of fossil mammals, and in paleosols from Nepal, thus demonstrating that t hese paleoenvironmental changes in floodplain vegetation may be contin ent-wide. Local effects, such as the development or intensification of the Asian Monsoon driven by uplift of the Tibetan Plateau, may have l ed to the expansion of C-4 grasses. If, however, the expansion of C-4 grasses proves globally synchronous, then a larger scale cause, such a s a marked decrease in pCO(2), may be the driving mechanism.