The East Asian monsoon variability in the Nihewan Basin, northern China, during the Early Pleistocene: A grain size end-member modelling analysis

Abstract

The Pleistocene sediments of the Nihewan Basin in northern China preserve a detailed terrestrial sediment archive for reconstructing the palaeoenvironment and palaeoclimate changes during early times of hominin occupation in E Asia, following the earliest locations outside of Africa. In this study, we investigate the composite 86.2-m long NH-T section of Dachangliang which was formed with an astronomically tuned age between ca. 1.66 and 0.78 Ma. Parameterized grain size end-member modelling analysis is applied for the first time in the Nihewan Basin to unravel the characteristics of sediment sources, transportation dynamics, and climatic signals in the region. The grain-size distributions of the NH-T sediment samples are attributed to a mixture of four distinct end members (EMs 1-4). EM 1 represents a global atmospheric fine silt component (mode at 7.9 μm) which probably resulted from high-altitude westerly transport from distal sources. EM 2, a medium silt component (mode at 27.6 μm) was probably transported by the low-level westerly winds during the spring from a relatively proximal source in comparison to EM 1. EM 3, a coarse silt component (mode at 59.9 μm), represents short distance suspended materials that were blown out by N or NW winds of the East Asian winter monsoon (EAWM). EM 4 is a fine sand component (mode at 221.1 μm), probably representing fluvial deposits carried by overland flow. Therefore, the temporal fluctuations in the abundances of EMs 1-3 are used to infer the history of the East Asian summer monsoon (EASM) (EM 1), of dust-storm outbreaks during springtime (EM 2), and of the EAWM (EM 3). The NH-T climate record shows an overall increase in EM 3 peaks, accompanied by a decrease in EM 1 minima from ca. 1.45 to 0.82 Ma, indicating a long-term aridification and cooling trend in the Nihewan Basin. At ca. 1.25 Ma, the EM patterns transition from short-frequency to longer fluctuations, apparently coinciding with the onset of the Mid-Pleistocene transition (MPT). Periods of stronger EASM with more frequently warm and wet conditions probably occurred in the basin between ca. 1.66-1.62, 1.52-1.25 and after 0.82 Ma. Intensified EAWM conditions probably prevailed in the basin during the intervening periods from ca. 1.62-1.52 and the MPT 1.25-0.82 Ma. The inferred warmer and wetter conditions likely supported hominin activities in the Nihewan Basin, in contrast to mostly colder and drier conditions. Nevertheless, the higher number of discovered Palaeolithic sites and recorded lithic artifacts from the first half of the MPT apparently reflects the successful adaptability of hominins to the prevailing cold climate. The relatively consistent patterns observed between the variations of a median grain-size stack of the Chinese Loess Plateau (CLP MD) and EM 3, and of the magnetic susceptibility stack of the CLP (CLP MS) and EM 1, indicate the climatic sensitivity of these EMs in response to the long-term glacial/interglacial oscillations previously inferred from the CLP. However, significant differences in the CLP MD and CLP MS on the one hand side and in the trends of the EMs 3 and 1 on the other suggest that regional climate conditions varied between the CLP and the Nihewan Basin. Further research is required to explore such regional climate differences and their possibly underlying factors in the Early Pleistocene.