Variations of the East Asian monsoon over the past 800 kyr constrained by the boron isotope composition of paleo-rainwater inferred from loess-paleosol deposits in NE China

Abstract

Based on a study of the boron (B) isotope geochemistry of the loess-paleosol sequence of the Niuyangzigou section (NYZG) in Northeast (NE) China, the boron isotopic composition of paleo-rainwater (δ11Bpr) is used for the first time as a potential proxy for tracing changes in the East Asian summer monsoon (EASM) since the mid-Pleistocene. The acid-soluble phase of loess in NE China, which is characteristically a sandy loess, has lower boron concentrations (0.26 to 0.89 μg/g) and more positive δ11B values, from −12.1‰ to +9.4‰, compared to the loess of the Chinese Loess Plateau (CLP). According to the mass balance of boron accumulation from three distinct sources, a mathematical inversion calculation is used to quantify the proportion of boron from each source and to capture the original δ11Bpr signature. The resulting δ11Bpr values range from +6.8 ± 0.95‰ in loess layers to +17.3 ± 3.32‰ in paleosol layers, and the boron accumulated in loess-paleosol layers is shown to be mainly from atmospheric precipitation and silicate weathering. Synchronous variations of reconstructed δ11Bpr and other loess-based proxies, both in NE China and in the CLP, are observed over a large spatial range and on several temporal scales. The linkage of δ11Bpr to the intensity of the EASM is attributed to the kinetic boron isotope fractionation associated with vapor-precipitate phase separation along the transport path of moisture-bearing air masses. A Rayleigh fractionation model is proposed to interpret δ11Bpr in terms of the proportion of water vapor loss after multi-step fractionation processes, which is independent of temperature. The monsoon record based on δ11Bpr shows a significant shift at ∼430 ka, implying an abrupt weakening of the EASM after the Mid-Brunhes Event (MBE), which provides evidence for regional disparities in monsoon evolution in response to the MBE. The ice volume in Northern Hemisphere is a possible factor controlling changes in the EASM in NE China over the past 800 kyr.