The Early-Middle Pleistocene transition of Asian summer monsoon

Abstract

The global climate cyclicity transferred from an Early Pleistocene mode dominated by the obliquity periodicity to a new Middle Pleistocene state dominated by the eccentricity periodicity in the absence of any significant changes in orbital forcing, known as the Early–Middle Pleistocene transition (EMPT), is an unresolved issue of Milankovitch orbital theory. Here we contribute new insights into the orbital characteristic and dynamics of the Asian summer monsoon precipitation changes across the EMPT. Spectral analyses of precipitation over the past 1.6 Myr reconstructed from a new loess magnetic susceptibility (χ) stack suggest that the EMPT of the summer monsoon precipitation characterized by a shift from obliquity to eccentricity rhythm occurred over a ~640 kyr long-term period from ~1.24 to 0.6 Ma rather than quasi-instantaneously on a much shorter duration. The eccentricity cyclicity initiated as early as 1.24 Ma, subsequently intensified and became the dominated pacing from 1.1 Ma, while obliquity weakened at ~1.1 Ma and significantly weakened at ~0.7 Ma. These EMPT characteristics of the summer monsoon precipitation reflect complex impacts from global climate systems and regional climate changes in the Northern and Southern Hemispheres, and in both low- and high-latitude regions.