Variability of East Asian Summer Monsoon precipitation during the Early Pleistocene based on pollen analysis from the western Loess Plateau, China

Abstract

Understanding Early Pleistocene variability in precipitation associated with the East Asian Summer Monsoon (EASM) may help improve understanding of the effects of future global warming on this system. In this study, a quantitative EASM precipitation record with ∼2.7 kyr resolution was reconstructed for the Early Pleistocene (∼2.2–1.7 Ma) based on a pollen record from a loess-paleosol sequence in Lanzhou Basin, on the western Chinese Loess Plateau (CLP). The results show that the average annual precipitation (Pann) value was ∼505 mm, almost ∼1.6 times as much as today (∼310 mm), implying much stronger EASM intensity from ∼2.2–1.7 Ma. The EASM experienced a period of stronger intensity from ∼2.2 to 2.07 Ma, with Pann up to ∼525 mm and vegetation dominated by forest-steppe, as well as a period of weaker intensity between ∼2.07 and 1.99 Ma with lower average Pann (∼481 mm) and steppe dominated vegetation. A gradual intensification followed from ∼1.99 to 1.7 Ma, with increasing Pann (∼503 mm) accompanied by a significant increase in arboreal abundance. Our analysis suggests that the west to east temperature gradients across the Equatorial Pacific region influenced the Early Pleistocene EASM evolution over geological timescales. This finding differs from the traditional view that high-latitude ice sheet fluctuations were the primary driving force of Quaternary EASM. On orbital timescales, the Pann exhibits a dominant ∼41 kyr obliquity periodicity between ∼2.2–1.99 Ma and a ∼18 kyr precession periodicity between ∼1.99–1.7 Ma, indicating that Northern Hemisphere ice sheet and low-latitude solar insolation were the dominant drivers of EASM precipitation cycles in these two periods.