Indus-wide C4 expansion between 7-6 Ma: an IODP Expedition 355 discovery
Tuesday, 15 December 2015: 14:40
2012 (Moscone West)
In April-May 2015, International Ocean Discovery Program Expedition 355 cored sediments from the Indus Fan (Site U1457, 67°55’E, 17°09’N, 3523 m water depth). Sediment recovery includes coarse silt to fine sand turbidite deposits spanning the late Miocene with a mineralogical composition indicating provenance predominantly from the Indus River catchment. We analyzed the carbon isotopic composition (δ13C values) of plant waxes and found a +9‰ shift in δ13C values of C28, C30 and C32 n-alkanoic acids. We infer that this carbon isotopic shift is the regional, offshore expression of the C4 expansion previously reported from the Siwalik Formation at the foreland of the Himalaya. Fortuitously the C4 expansion occurs at a time with frequent magnetic reversals into and out of Chrons C3B to C3A. Based on the shipboard age model for U1457, the isotopic shift occurs in C3Ar, an interval of 0.4 Myr with a sediment accumulation rate of 50 m/Myr. This allows us to securely date the Indus Fan transition to within 7-6 Ma on the geomagnetic polarity timescale. Independent of absolute age uncertainties, magnetostratigraphy allows us to report that the shifts were coincident in the Siwaliks and the broader region sampled by the Indus Fan. Based on the δ13C values of plant waxes at and after 6Ma (ca. -21‰) in the Indus Fan, the lowland Indus catchment must have been dominated by C4 grasslands.