Variation in humidity and the forcing mechanism in Asian monsoon-influenced regions indicated by hematite/goethite from Baxian Lake, southern China, since AD 800

Abstract

Understanding the potential mechanisms driving the precipitation pattern in the Asian summer monsoon (ASM) area is significant to reconstructing the environmental and hydrological conditions over the past 2000 years. However, robust and consistent conclusions have been hampered by the complex processes controlling the dynamics and diverse interconnected linkage of the Asian monsoon. Here, we present a reconstruction of variations in humidity since AD 800, based on the ratio of hematite to goethite (Hem/Goe) and other magnetic parameters in the sediments of Baxian Lake, southern China. The record indicates that a dramatic transition from dry to humid climate occurred during AD 800–950 and then returned to extreme drought during the second half of the Medieval Warm Period. The fluctuations between a relatively wet climate and weak drought were also demonstrated during the Little Ice Age (AD 1450–1800). Climate-influenced vegetation coverage may influence the flux of weathered detrital magnetic minerals in sediments, as it hinders the input of coarser detritus into Baxian Lake. The magnetic minerals in the sediments were not significantly altered during the post-depositional processes. Additionally, the Hem/Goe record indicates that an opposite precipitation pattern occurred in southern China when compared with precipitation records in northern China and India. We ascribe this result to the superimposition of land-ocean-atmosphere dynamics on the traditional model of the intertropical convergence zone (ITCZ) movement forced by the Atlantic Meridional Overturning Circulation. This is significant for enhancing our understanding of the relationship among the continent-ocean thermal gradient, ASM variation, ITCZ movement, El Niño Southern Oscillation-like events, and extreme climates in areas influenced by the ASM.