Early Pleistocene pollen record from the western Chinese Loess Plateau and its implications for the evolution of the East Asian Summer Monsoon

Highlights: • The Early Pleistocene EASM evolution in different timescales was driven by Equatorial Pacific zonal thermal gradient and global temperature. • A more intense EASM during the Early Pleistocene (2.2-1.7 Ma) is suggested. • The evolution of the Early Pleistocene EASM over different timescales was driven by the Equatorial Pacific zonal thermal gradient and global temperatures. The East Asian Summer Monsoon (EASM) is the most important and active atmospheric circulation system in Asia. However, its evolution during the Early Pleistocene, and that evolution's associated drivers, remain controversial. Here, for the first time, a high resolution pollen record was obtained from a loess-paleosol sequence located in the Lanzhou Basin, western Chinese Loess Plateau (CLP), and used to reconstruct the evolution of EASM intensity during the Early Pleistocene (2.2–1.7 Ma). The type of vegetation indicated by analysis of the pollen assemblage is comparable to the forest-steppe vegetation type currently distributed on the eastern CLP. This area's present-day mean annual temperature (MAT) and mean annual precipitation (MAP) are ~6–10 °C and ~400–550 mm, respectively, implying the existence of a stronger EASM intensity during the 2.2–1.7 Ma period. When the auxiliary function of the diversity index was applied, the Early Pleistocene EASM, as recorded by the vegetation successions identified within the loess-paleosol sequence, showed a generally stronger intensity during the 2.2–2.06 Ma period, a relatively weak intensity between 2.06 Ma and 1.97 Ma, and then a gradual intensification from 1.97 Ma until 1.7 Ma. Further comparisons demonstrated that the zonal thermal gradient in the Equatorial Pacific may have played an important role in driving the evolution of the EASM over a tectonic timescale during the Early Pleistocene. In contrast, over a sub-orbital or orbital timescale, the EASM intensity appears to have been closely correlated to global temperature variations, given that the MATs of the Early Pleistocene were higher than today's. This would imply that both low-latitude forcing and global temperature change were important drivers influencing variations in precipitation in Northern China, against a background of ongoing global warming.