South Asian Summer Monsoon precipitation variability during late Pliocene: Role of Indonesian Throughflow

Understanding the response of the South Asian Summer Monsoon (SASM) to high atmospheric pCO(2) is required to predict future SASM changes under the current global warming scenario. The mid-Piacenzian Warm Period (mPWP), 3.0 to 3.3 Ma (million years ago) during the late Pliocene, provides us with such an opportunity as it had atmospheric pCO(2) comparable to the present. To reconstruct SASM variability during this period, we studied multiple proxies of terrigenous input (Ti), elasticity (Ti/Ca), chemical weathering (K/Al, Mg/Al), and productivity (Ba/Al, Ca-excess) in sediment collected during IODP Expedition 355 (Site U1456) spanning the mid to late Piacenzian (3.3-2.6 Ma). We find that the SASM intensified (weakened) during interglacials (glacials) of the mPWP and beyond in close correspondence with the global climate. However, we note that the SASM shifted to a higher mean state at 2.95 Ma. It could be because of the tectonically induced reorganization of the Indonesian Throughflow (ITF), which enhanced the cross-equatorial pressure gradient by changing the source of water to the southern equatorial Indian Ocean. This enhanced pressure gradient resulted in the intensification of the SASM during the late Piacenzian. During the northern hemisphere glaciation after 2.95 Ma, the SASM shows a long-term decline over and above the glacial-interglacial variability. The spectral and wavelet transform analysis indicates the control of the earth's orbital parameters, especially eccentricity, over the SASM variations during the late Piacenzian.

Automated summary

This study highlights the intensification and weakening of the South Asian Summer Monsoon during interglacials and glacials of the mid-Piacenzian Warm Period, and a shift to a higher mean state at 2.95 million years ago possibly due to tectonic induced reorganization of the Indonesian Throughflow. Orbital parameters like eccentricity are found to have control over the SASM variations during the late Piacenzian according to spectral and wavelet transform analysis.