A 20 Ma pollen record of East-Asian summer monsoon evolution from Guyuan, Ningxia, China

Pollen analysis was conducted on the fluviolacustrine sediments from the east side of the Liupan Mountains, a relatively monsoon-sensitive region, and provided for the first time a continuous evolutionary history of vegetation and climate in East China during the Neogene. The pollen record spanning the last 20 Ma indicates that the vegetation and thus East-Asian summer monsoon evolution can be divided into three stages: 20.13–14.25 Ma, 14.25–11.35 Ma and 11.35–0.08 Ma, in spite of general dominance of steppe throughout the Neogene. During the period 20.13–14.25 Ma, the pollen assemblage was characterized by high abundance of Artemisia and small amounts of temperate to subtropical deciduous broad-leafed trees such as Juglans, Carya, Quercus and Betula, suggesting that the East-Asian summer monsoon was generally strong. The interval from 14.25 to 11.35 Ma was marked by a significant decline in the East-Asian summer monsoon, as indicated by Artemisia gradually replaced by Humulus and halophytic Chenopodiaceae. Nitraria and Ephedra also reached high values in this period. Since 11.35 Ma ago, percentages of halophytes dominated by Chenopodiaceae and Tamaricaceae maintained a high mean value of about 40%, revealing a weak intensity of East-Asian summer monsoon. The general weakening of the monsoon circulation during the Neogene and the significant monsoon decline during the late Middle Miocene in particular might link with the global cooling trend probably through two mechanisms. Cooling and ice-sheet expansion over the polar region caused an increase in meridional temperature gradients leading to the southward retreat of the monsoon circulation. The global cooling may have also reduced the amount of water vapor held in the atmosphere, and led to both additional cooling and further weakening of the East-Asian summer monsoon. The cause for a short-lived resumption during the period 1.0–0.8 Ma is under investigation.