Expansion of C₄ grasses in the Late Miocene of Northern Pakistan: evidence from stable isotopes in paleosols

Stable-isotopic, clay-mineralogic, and bulk-chemical analyses were conducted on paleosols of the Neogene Siwalik sections in northern Pakistan in order to reconstruct floodplain environments over the past ∼ 17 Ma. The stable carbon isotopic composition of soil carbonate (mean δ¹³C (PDB) = -10.2%) and associated organic matter (mean δ¹³C (PDB) = -24.1%) in paleosols representing 17- ∼ 7.3 Ma reveal that floodplain vegetation was dominated by C₃ plants. At 7.3 Ma, a shift toward more positive carbon isotopic values began, signaling the gradual expansion of C₄ grasses onto the floodplain. From 6 Ma to present, carbon isotopic values for paleosol carbonate (mean δ¹³C (PDB) = +0.6%) and organic matter (mean δ¹³C (PDB) = -14.4%) are uniformly enriched in ¹³C, indicating the presence of nearly pure C₄ grassland. The scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1.0-1.25 m/yr or less for the entire 17 Ma of record. Paleosols in the lower portion of the section lack organic A horizons but have reddish B horizons often containing secondary iron-oxide nodules. Leaching depths of soil carbonate in these older paleosols are typically 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 the pre-7.3 Ma part of the record are dominantly calcareous Alfisols or Mollisols that once underlay nearly pure C₃ vegetation, perhaps trees and shrubs, while calcareous Mollisols underlying C₄ grassland dominate the upper part of the record. The carbon- and oxygen-isotopic trends in the paleosol record in Pakistan are also evident in the diet of fossil mammals, and in paleosols from Nepal, thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent-wide. Local effects, such as the development or intensification of the Asian Monsoon driven by uplift of the Tibetan Plateau, may have led to the expansion of C₄ grasses. If, however, the expansion of C₄ grasses proves globally synchronous, then a larger scale cause, such as a marked decrease in ρ{variant}CO₂, may be the driving mechanism.