Vegetation and chemical weathering changes since the late MIS 3 in the mid-lower Yangtze River Valley: Evidence from pollen and geochemical records

The vegetation and chemical weathering history since the late Marine Isotope Stages 3 (MIS3) is still poorly studied in the Yangtze River Valley (YRV), central China, which hinders a better understanding of their relationship to climate and anthropogenic impacts. We here coupled pollen and geochemical proxy based on a fluvial sedimentary sequence from the YRV. The pollen data show that the past natural land-cover was dominated continuously by forest before 5 cal kyr BP, with arboreal pollen abundance larger than 60%. Nevertheless, the forest community underwent major changes in response to temperature fluctuations, from a evergreen and deciduous broadleaved mixed forest during the moderately warm late MIS 3, to a coniferous and broadleaved mixed forest during the cold MIS 2, and back to a evergreen and deciduous broadleaved mixed forest during the Holocene Thermal Maximum. Since ca. 5 cal kyr BP, the unprecedented increases in Poaceae and decreases in arboreal pollen abundances indicated that regional vegetation was strongly disturbed, along with the expansion of rice agriculture to the entire YRV. The geochemical data reveals more intense chemical weathering during the relatively cold periods (e.g., the LGM) than that during the relatively warm periods (e.g., the late MIS 3 and mid-Holocene), with a gradual weakening from the LGM to the early Holocene. The variations in chemical weathering were mainly controlled by regional precipitation changes. There was a “dipolar” mode of precipitation changes at millennial to orbital timescale between north China and the YRV during the last 30,000 years, which were predominantly regulated by the ENSO-like states in the tropical Pacific; an El Niño state resulted in more (less) precipitation in the YRV (northern China), while opposite condition occurred during a La Niña state. Severe deforestation by intense human activity also influenced the intensity of chemical weathering since ca. 5 cal kyr BP.