The role of hydroclimate in the late-Miocene expansion of C4 grasses in India

Abstract:

The dramatic expansion of C₄ grasslands in northern India, Pakistan and and Nepal during the late Miocene has been well documented from carbon isotope changes in paleosols and marine sediments. This expansion is part of a trend in the late Miocene towards increasing tropical C₄ savannah vegetation worldwide. The roles that evolution, global boundary conditions, and local factors played in this trend remain uncertain. The expansion of C₄ grasses in India has been variously ascribed to uplift of the Tibetan plateau, changes in the Indian monsoon, declining atmospheric CO₂ levels, and global cooling. However, the ultimate cause of this expansion remains unknown.

Here we investigate the role that hydrologic change may have played in driving the expansion of C₄ grasslands in India. We analyzed the molecular distribution of leaf-wax n-alkanes in the Bengal Fan and their carbon and hydrogen isotopic composition to determine the history of vegetation, aridity and hydroclimate associated with the expansion. Ocean Drilling Program sites 717 and 718 on the distal Bengal Fan preserve lipid molecules from terrestrial vegetation transported to the core site by deep-sea turbidites. We found that changes in the molecular distribution and hydrogen isotopic composition of terrestrial leaf wax n-alkanes precede changes in the carbon isotopic composition of these molecules. These data suggest that vegetation and hydroclimate changes preceded the expansion of C₄ grasses, implying a more nuanced ecological and environmental trajectory. The hydrogen isotope data indicate a substantial enrichment in the δD of source water prior to the C₄ transition. Some combination of increasing aridity and rainfall δD could lead to the observed δD changes. Comparison with soil carbonate δ¹⁸O suggests that changes in rainfall δD is the more likely cause, reflecting the moisture source, rainfall amount, or seasonality of rainfall. Overall, the timing of changes in molecular distribution and δD values compared to the C₄ expansion suggests that the vegetation transition was complex and that hydroclimate played a major role in late-Miocene vegetation change in India.