In 2005, Science presented 125 scientific conundrums urgently needed to be solved in the coming future, including "What causes ice ages?". This issue was closely related to onset of the 100-kyr glacial cycles since the mid-Pleistocene transition (MPT).

The MPT was widely recognized as a shift in paleoclimatic periodicity from 41- to 100-kyr cycles, which largely reflected integrated changes in global ice volume, sea level, and ocean temperature from the marine realm. However, much less was known about monsoon-induced terrestrial vegetation change across the MPT.

Recently, a research team led by Prof. SUN Youbin from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences suggested that Chinese loess documented a unique MPT reflecting terrestrial vegetation changes from a dominant 23-kyr periodicity before 1.2 Ma to combined 100, 41, and 23-kyr cycles after 0.7 Ma. Their findings were published in Nature Communications.

This study proposed a new concept of diverse manifestations of the Mid-Pleistocene climate transition, induced by varied sensitivity of Earth climate system to astronomical forcing under different ice/CO₂ boundary conditions. These new findings provided novel insights into our understanding of monsoon variability and dynamics from the natural past to the anthropogenic future.

Based on paleomagnetic results, ²⁶Al/¹⁰Be burial dating and pedostratigrahic comparison of a 430-m high-quality loess core, the researchers constructed a reliable age model for the loess-paleosol sequences.

The loess δ¹³C_IC record (a sensitive summer monsoon proxy) exhibited a significantly shift of the coupled monsoon-vegetation changes from dominant precession cycles before 1.2 Ma to combined 23, 41 and 100-kyr cycles after 0.7 Ma, very different from the conventional MPT.

Model simulations revealed that the MPT transition likely reflected decreased sensitivity of monsoonal hydroclimate to 21-kyr insolation forcing as the Northern Hemisphere became increasingly glaciated through the MPT. The proxy-model comparison suggested varied responses of temperature and precipitation to astronomical and ice/CO₂ forcing.

Variations of monsoon proxy, solar radiation, ice volume and wavelet spectra over the past 1.7 Ma. (Image by IEE)