A Reassessment of the Chronostratigraphy of Late Miocene C<sub>3</sub>–C<sub>4</sub> Transitions
Lisa Tauxe, Sarah J. Feakins
Abstract
Abstract Combining magnetostratigraphic and carbon isotopic data for the late Miocene can provide a temporal framework for an isotopic shift first documented in soil carbonate nodules of northern Pakistan. The shift has been interpreted as a change in vegetation from trees and shrubs (using the C 3 photosynthetic pathway) to grassland (using the C 4 pathway). The cause of the event has been hotly debated and its timing is close to a shift in carbon isotopes in the marine realm. Further understanding depends critically on chronology. Unfortunately, temporal calibration of the various records published over decades relied on different time scales. To address the lack of a consistent chronology, we have reevaluated the constraints for the carbon isotopic shifts recorded from the Indian subcontinent. These show a diachronous transition ranging in age from about 7.8 Ma in Pakistan to as late at 6 Ma in Nepal. The record from IODP Expedition 355 Site U1457, drilled on the Indus fan shows that the transition in peninsular India began at about 7.2 Ma. Similar records from the African margin saw an earlier shift to C 4 dominance starting around 10 Ma and those from Australia and South America transitioned later, during the Pliocene. The diachroneity around the globe does not invalidate pCO 2 as a driver, but is consistent with it being one of several drivers of the global C 4 expansion.