Variation of summer monsoon intensity in the North China Plain and its response to abrupt climatic events during the early-middle Holocene

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Ningjinpo paleolake was the largest lake in the North China Plain during the Holocene, and its continuous sedimentary record has the potential to reconstruct the summer monsoon intensity and determine the limnological response to abrupt climatic events at a high resolution. Here we use AMS 14C dating and analyses of grain-size and major element contents, and the results show that: (1) During 11.5–10.0 cal kyr BP, the geochemical climate change index (C) (1.16), sediment accumulation rate (SAR) (0.007 cm/yr) and median particle diameter (Md) (20.6 μm) are the lowest within the section, while the geochemical drought index (0.0323), Fe/Mn ratio (0.0088) and clay content (17.3%) are the highest, which indicate that the North China Plain received less precipitation, and chemical weathering and erosional intensity in the Ningjinpo paleolake catchment were weak. (2) During 10.0–7.7 cal kyr BP, the C index (1.36), content of sand (19.1%) and silt (68.0%), Md (27.2 μm) and SAR (0.019 cm/yr) increase significantly. These characteristics indicate that the climate gradually became wetter, chemical weathering and erosion intensity increased, and there was increased runoff to the lake, which raised the lake level. (3) During 7.7–5.5 cal kyr BP, the C index (1.56), silt content (71.8%) and SAR (0.030 cm/yr) increase to the highest, while the Fe/Mn ratio (0.0068), sand content (14.4%) and Md (21.4 μm) decrease to their lowest levels. These suggest the climate was warm and humid, the runoff to the lake was large, chemical weathering in the catchment was strong, and the water level of the lake reached the maximum for the Holocene. (4) During 5.5–3.3 cal kyr BP, the C index (1.26) and SAR (0.011 cm/yr) decrease substantially, whereas the sand content (28.2%) and Md (18.3 μm) increase to the highest levels. These characteristics demonstrate that the climate gradually became arid, the runoff to Ningjinpo paleolake was reduced, the intensity of chemical weathering and erosion in the catchment weakened, and the lake level decreased, although the high frequency of floods in this period. (5) In addition to the globally-recorded abrupt climatic events at ~8.2 cal kyr BP, ~5.3 cal kyr BP and ~4.2 cal kyr BP, which were recorded in the northern monsoon region of China, an abrupt climatic event in the middle Holocene, at ~7.2 cal kyr BP, was also recorded. In the northern monsoon region of China, these events may have been amplified by unusually active ENSO since the middle Holocene.