Grasslands expanded on the Indian sub-continent in the late Miocene. Dating is a key priority in order to compare the timing of the expansion between regions and to evaluate possible causes of the transition. Here we take a new look at old records from around Pakistan, India, and Nepal on land and update the age models to the current paleomagnetic timescale. These are then compared to new records from marine sediments of the Indus Fan. Based on microfossil appearance and paleomagnetic constraints, the timing of the C4 transition is found to be asynchronous across the Indian subcontinent, initiating in southern India at around 7.2 Ma, proceeding to northern Pakistan and India around 7.1 Ma, and reaching Nepal by about 6 Ma. This grassland expansion appears to post-date a collapse in atmospheric CO2 levels, but may have contributed to the collapse because grasses are much more efficient at sequestering CO2 than the forests and shrubs that preceded them.
Prof. Tauxe is a Distinguished Professor of Geophysics in the Geosciences Research Division at Scripps and a Member of the US National Academy of Sciences.