Prolonged South Asian Monsoon variability and weakened denitrification during Mid-Pleistocene Transition

Abstract

During Mid-Pleistocene Transition (MPT, 0.68 to 1.16 million years ago, Ma), the earth's climate cycle shifted from 41 kyr (thousand years) to quasi-100 kyr cyclicity. The South Asian Monsoon (SAM) variability and its forcing factors during MPT are not fully understood. We present a multiproxy record of productivity and denitrification related to SAM variability from the eastern Arabian Sea (IODP Expedition 355, Site 1457) during MPT. The denitrification and productivity vary distinctly during the early (1.19 Ma to 0.95 Ma), mid (0.95 Ma to 0.79 Ma), and late (0.79 Ma to 0.68 Ma) MPT. The long-term SAM variability followed the prolonged glacial-interglacial cycle as it shifted to quasi-100 kyr periodicity at 0.95 Ma. The SAM was strong (weak) during the interglacials (glacials) of MPT. During early MPT, the SAM intensity was high during one of the glacial periods (MIS 30) due to precessional and CO2 forcing. At mid-MPT, the denitrification anomalously decreases, possibly, due to enhanced oxygen solubility during the extended glacial period. During the various phases of MPT, both the dynamic (precession, interhemispheric insolation gradient) and the thermodynamic (CO2 variability) effects govern the SAM.