Paleohydro-climatic changes revealed by anisotropy of magnetic susceptibility at the Heitugou section, Nihewan Basin, and its influences on human’s occupation

Abstract

Paleohydro-climatic changes in the habitats of early humans are closely related to their evolution and occupation of specific niches. In the Nihewan Basin of North China, a large number of archaeological sites dating back to the early Pleistocene have been discovered which record the occupation history of human ancestors in the region. However, thus far, studies of the history of paleohydro-climatic changes in the Nihewan Basin and their influences on human occupation remain insufficient. Here, we report the results of a systematic investigation of variations in the anisotropy of magnetic susceptibility (AMS) and magnetic susceptibility stratigraphy in the Heitugou (HTG) section to partially rectify this paucity of data and interpretations. The HTG section is located in the eastern Nihewan Basin where, in 2006, a Paleolithic site was discovered and excavated at the bottom of the sequence. By comparing detailed magnetic susceptibility sequences in the HTG section with those of the nearby Xiaochangliang and Dachangliang sections, we found that the sedimentation of the HTG section began about 1.9 Ma, indicating that the HTG archaeological site possibly records the earliest human occupation of middle-to-high latitude North China. Our studies of the AMS in the HTG section indicate that it is characterized by a primary sedimentary magnetic fabric without significant disturbance, demonstrating its potential for tracing regional paleo-hydrological changes. In most samples dating between 1.9 Ma and 0.4 Ma, the maximum susceptibility axes ( K max) cluster along the SE-NW vector, suggesting hydrodynamic changes tracable in the study area were mainly influenced by the Huliu River. In addition, a relatively weak K max direction biased towards the SW-NE axis was also observed, possibly generated by the predominant flow of the Sanggan River. These two rivers – the Huliu and Sanggan – determined the paleohydrological changes in the eastern Nihewan paleolake basin. Meanwhile, AMS studies also suggest multiple-phase hydrodynamic changes in the Nihewan Basin. Around 1.9 Ma, a thick layer of gravel was deposited in the study area due to the emergence of the Huliu River, or tectonic subsidence of the Nihewan Basin. Strengthened hydrodynamic changes and higher water-levels of the Nihewan paleolake may have driven early human ancestors to occupy higher topographic locations. Between 1.9–1.8 Ma, the paleolake expanded and the hydrodynamic regime weakened accordingly, which may correspond with the enhancement of the summer monsoon cycle in East Asia. Between 1.8–1.5 Ma, the sedimentary hydrodynamic regime strengthened noticeably early on, and subsequently weakened continuously. During the period from 1.5–1.3 Ma, as the East Asian summer monsoon regime intensified, warmer and wetter climatic conditions resulted in deepening of the Nihewan paleolake, which may have subsequently weakened influences from fluvial input and was associated with decreased sedimentary hydrodynamics. Between 1.3–0.9 Ma, high-frequency fluctuations in the AMS are noted, indicating a dramatic change in lacustrine hydrodynamics possibly attributable to the Mid-Pleistocene Transition (MPT) event. After 0.9 Ma, the Nihewan paleolake deepened again. Relatively high-amplitude fluctuations during this period reflect a cyclicity of 100 ka, possibly indicating that it was controlled by global ice volume. In concordance with other Paleolithic sites in the Nihewan Basin that have been chronometrically investigated, our studies at Heitugou reveal that paleohydro-climatic changes had a significant influence on early hominin activities in the Nihewan Basin, which shaped the spatio-temporal distribution of Paleolithic sites there. For example, during the early Pleistocene, early humans lived mostly on lower lake terraces in the eastern Nihewan Basin, when the lake level was still quite low. Subsequently, they moved to topographically higher sites due to gradually increasing lake levels. During the Middle and Late Pleistocene, when the Nihewan paleolake shrank, early humans moved toward the upper reaches of the Sanggan River, and were widely distributed along river courses across the Nihewan Basin during the Late Pleistocene in order to maintain home ranges near permanent water sources.