Paleoclimate changes over the past 13,000 years recorded by Chibuzhang Co sediments in the source region of the Yangtze River, China

摘要

Climate change characteristics in the Qinghai-Tibetan Plateau (QTP) exhibit spatial differences. These differences are due to different large circulation systems, comprising Asian summer monsoons and mid-latitude Westerlies. However, the paleoclimate pattern at the transition zone between the two systems existing since the Late Glacial remains unclear. We present a 13,000-year high-resolution record of climate change and monsoon effects in the source region of the Yangtze River at the modern limit of the Indian Summer Monsoon (ISM) influence, which lies in the interaction zone with the Westerlies. Multi-proxy records-such as grain size (GS), total inorganic carbon (TIC) and element geochemistry-suggest four stages of paleoclimate change since the Late Glacial: 1) relatively cold in the Late Glacial, 2) warm-wet conditions in the early Holocene, 3) cool-dry conditions in the mid- and late Holocene and 4) a warming and wetting trend over the last 2000 years. Correspondingly, the lake experienced a shallow-water phase in the Late Glacial and a deep-lake phase during the Holocene. The cold Younger Dryas (YD) event occurred from 12.7 to 11.0 cal. kyr BP in the core, with extremely low total organic carbon (TOC) and coarse GS. In contrast, the Holocene Megathermal occurred between 10.0 and 9.1 cal. kyr BP, as recorded by the TOC maxima, which had a finer grain size. In this study, the TIC (plus the XRF-Ca and Ca/Si ratio) reflected the combined effects of biochemical processes (e.g., algal photosynthesis) and hydrological changes (e.g., precipitation, evaporation and runoff). Comparisons between regional records and reconstructed paleoclimate indexes suggested that the studied area's effective moisture pattern mostly followed fluctuations in the ISM-dominated region. The monsoonal circulation drove the paleoclimate variations on the central QTP since the Late Glacial, mainly controlled by enhanced summer insolation until the mid-Holocene. Further, the effective moisture variation was influenced by the August insolation (increased evaporation), on which the Westerlies effect (related to intensely cold air) and the meltwater runoff were later superimposed. Recently, the hydroclimatic conditions have acquired a temperate-humid status in the study area, which likely approaches that in the early Holocene's late phase. Therefore, the paleoclimate pattern of the source region of the Yangtze River exhibits a transitional nature (phase) between two circulation regions during the Holocene.