A 16-Ma record of paleodiet using carbon and oxygen isotopes in fossil teeth from Pakistan

Abstract

The Siwalik Sequence of northern Pakistan contains a 16-Ma record of paleosol carbonate and fossil teeth from which a record of paleovegetation can potentially be reconstructed and compared. The carbon isotopic composition of paleosol carbonate and organic matter from Siwalik strata reflects a major paleoecological change on the floodplains of major rivers beginning ∼ 7.3 Ma ago. By 6 Ma C₃-dominated plant communities, probably composed of mostly trees and shrubs, were displaced by nearly continuous C₄ grassland. We find that the carbon isotopic ratios in herbivore tooth enamel reflect this dramatic ecologic shift. Carbonate in enamel older than 7 Ma averages −11‰ in δ¹³C_PDB, consistent with a largely C₃ diet. Enamel from the Plio-Pleistocene averages +1.9‰ in δ¹³C, similar to the value displayed by modern C₄ grazers. Analysis of post-burial carbonate cements, and the concordance with isotopic evidence from paleosols argues strongly against major isotopic alteration of the enamel, while coexisting bone may have been altered early in burial. This study confirms that enamel apatite is useful for paleodietary reconstruction much further back in the geologic record than was previously thought.