The 3.6-Ma aridity and westerlies history over midlatitude Asia linked with global climatic cooling

Xiaomin Fang,Zhisheng An,Steven C Clemens,Jinbo Zan,Zhengguo Shi,Shengli Yang,Wenxia Han

doi:10.1073/pnas.1922710117

Abstract

Midlatitude Asia (MLA), strongly influenced by westerlies-controlled climate, is a key source of global atmospheric dust, and plays a significant role in Earth's climate system . However, it remains unclear how the westerlies, MLA aridity, and dust flux from this region evolved over time. Here, we report a unique high-resolution eolian dust record covering the past 3.6 Ma, retrieved from the thickest loess borehole sequence (671 m) recovered to date, at the southern margin of the Taklimakan desert in the MLA interior. The results show that eolian dust accumulation, which is closely related to aridity and the westerlies, indicates existence of a dry climate, desert area, and stable land surface, promoting continuous loess deposition since at least ∼3.6 Ma. This region experienced long-term stepwise drying at ∼2.7, 1.1, and 0.5 Ma, coeval with a dominant periodicity shift from 41-ka cyclicity to 100-ka cyclicity between 1.1 Ma and 0.5 Ma. These features match well with global ice volume variability both in the time and frequency domains (including the Mid-Pleistocene Transition), highlighting global cooling-forced aridity and westerlies climate changes on these timescales. Numerical modeling demonstrates that global cooling can dry MLA and intensify the westerlies, which facilitates dust emission and transport, providing an interpretive framework. Increased dust may have promoted positive feedbacks (e.g., decreasing atmospheric CO2 concentrations and modulating radiation budgets), contributing to further cooling. Unraveling the long-term evolution of MLA aridity and westerlies climate is an indispensable component of the unfolding mystery of global climate change.

Highlights

Midlatitude Asia (MLA), strongly influenced by westerlies-controlled climate, is a key source of global atmospheric dust, and plays a significant role in Earth’s climate system
The Tarim basin (560,000 km2) in northwest China (Fig. 1B), containing the world’s largest active dune field, the Taklimakan desert, provides nearly two-thirds of the total dust generated in the MLA interior [1], with approximately half of the
Few studies have been carried out on the loess deposits from the lower terraces in the southern Tarim basin, due to the scarcity of good outcrops, the remote location, and the sparse attention that has been paid to the dust emission history of the major contributor of the MLA dust budget

Summary

Introduction

Midlatitude Asia (MLA), strongly influenced by westerlies-controlled climate, is a key source of global atmospheric dust, and plays a significant role in Earth’s climate system. The astronomically tuned time series of high-resolution coarse size fraction (>30 μm) and the related flux records of the loess core reveal a long-term increasing trend with obvious stepwise increases at ∼2.7, 1.1, and 0.5 Ma, suggesting enhancements of the westerlies and drying climates at those times.

Results

Conclusion