Shift of the Dominant Orbital Periodicity of the East Asian Summer Monsoon Linked to the Intensification of Northern Hemisphere Glaciation at 2.7 Ma

The intensification of Northern Hemisphere Glaciation (iNHG) with the onset of glacial-interglacial cycles at ∼2.7 Ma had a profound impact on the global climate system. However, there have been few systemic assessments of the response of orbital-scale East Asian summer monsoon (EASM) variability to the iNHG, partly due to controversies regarding the interpretation of the dominant orbital rhythms of the EASM. Here, we present grain size and other proxy records from mainly silt-sized lacustrine-fluvial deposits in northern China. The results shows that the EASM underwent stepwise weakening at ∼2.7 and ∼1.8 Ma, coincident with two major cooling steps of the global climate and that its dominant orbital periodicity changed from ∼41 kyr during ∼3.6–2.7 Ma, to ∼100 kyr during ∼2.7–1.8 Ma. However, these findings are inconsistent with the strengthening of the 41-kyr cyclicity in marine δ¹⁸O records after the iNHG, whereas they are consistent with northern high-latitude sea surface temperature records that bear the imprint of local climate signals when the Arctic ice sheets were of limited extent during the Early Pleistocene. We propose that the dominant ∼41-kyr cyclicity during the Late Pliocene resulted from obliquity-induced changes in the meridional insolation gradient, or in heat and moisture transport from low latitudes; whereas the dominant ∼100-kyr cyclicity during ∼2.7–1.8 Ma reflects an increased response to northern high-latitude forcing. Our findings, combined with previous studies, highlight the importance of eccentricity in modulating the EASM and other key climate system components prior to the Mid-Pleistocene Transition during the course of Northern Hemisphere Glaciation.