Miocene-Pliocene Rainfall seasonality over East Asia

Under certain climate regimes, rainfall occurs in specific seasons instead of year-round, and changes in seasonal rainfall distributions can have a significant effect on regional water availability. The water cycle over East Asia is dominated by the East Asian monsoon system, which is controlled by multiple factors, including the Hadley cell, the westerly jet, and Tibetan Plateau¹. In this study, to better understand the evolution of East Asian hydroclimate under global warming, we looked at the \(\delta^{18}O\), \(\Delta_{47}\), and \(\Delta^{‘17}O\) of carbonate nodules from the Red Clay formation on the Chinese Loess Plateau, spanning 8-2.5 million years ago.

The Miocene-Pliocene Red Clay formation on the Chinese Loess Plateau (Location: Shilou, Shanxi).

Soil carbonates are generally formed in oxygen isotope equilibrium with ambient soil waters, the latter of which inherit the \(\delta^{18}O\) of rainfall but can be strongly affected by subsequent processes such as evaporation. With the help of soil-carbonate \(\Delta^{‘17}O\)², we can parse out the effect of evaporation and reconstruct changes in rainfall-\(\delta^{18}O\), which contain information of regional hydrological cycle and large-scale air circulation patterns.

Meridional positions of the westerly jet during the summer. Red and black crosses indicate the mean jet positions from CESM2 simulations based on Pliocene and pre-industrial conditions.

Our records suggest increases in rainfall-\(\delta^{18}O\) during warmer climates such as the early Pliocene, which cannot be explained by changes in rainfall amount. Instead, we propose that rainfall seasons switched from summer to spring under warmer climates, as a result of more poleward position of the westerly jet and thus an early onset of Meiyu in northern China. In this respect, northern China is likely going to experience more spring rainfall and summer drought as global warming proceeds.

Related Publications:

1. Da J, Zhang Y G, Li G, et al. Aridity-driven decoupling of δ13C between pedogenic carbonate and soil organic matter[J]. Geology, 2020, 48(10): 981-985.
2. Da J, Li G K, Ji J. Seasonal changes in the formation time of pedogenic carbonates on the Chinese Loess Plateau during Quaternary glacial cycles[J]. Quaternary Science Reviews, 2023, 305: 108008.