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

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 (delta B-11(pr)) 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 mu g/g) and more positive delta B-11 values, from -12.1 parts per thousand to +9.4 parts per thousand, 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 delta B-11(pr), signature. The resulting delta E-11(pr) values range from +6.8 +/- 0.95 parts per thousand in loess layers to +17.3 +/- 3.32 parts per thousand 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 delta B-11(pr) 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 delta B-11(pr) 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 delta B-11(pr) in terms of the proportion of water vapor loss after multi-step fractionation processes, which is independent of temperature. The monsoon record based on delta B-11(pr) shows a significant shift at similar to 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. (C) 2021 Elsevier B.V. All rights reserved.