The phase lag between Indian Summer Monsoon and Northern Hemisphere summer insolation driven by ice volume and greenhouse gas

The Indian Summer Monsoon (ISM) is an important part of the global climate system, affecting the economic prosperity of heavily populated regions. On the orbital scale, the significant phase differences in the precession band between ISM records from marine sediment and speleothem δ18O records have furthered the debate about the driving mechanism(s) of the ISM. Thus, to examine the ISM variations on the orbital scale and their potential forcings, we conducted environmental magnetic and geochemical measurements on marine sediment from the Ninety-east Ridge in the south Bay of Bengal covering the past 260 k.y. Environmental magnetic results indicate that coarser (finer) magnetic grain size corresponds to arid (humid) periods. Magnetic grain size is associated with the intensification of chemical weathering, reflecting the pedogenic process in the floodplains influenced by the ISM precipitation. Therefore, combined with these five proxy records from the core CJ04-57 in the Ninety-east Ridge, we build a new multi-proxy record of the ISM stack. Our ISM stack exhibits a −68° (∼4.5 k.y.) lag with maximum Northern Hemisphere summer insolation (NHSI) but is nearly in phase with the global ice volume and greenhouse gas in the precession band. These results emphasize that ISM rainfall is further influenced by internal forcing mechanisms such as global ice volume and greenhouse gas besides NHSI.