EAST ASIAN MONSOON VARIATIONS DURING EARLY PLEISTOCENE: EVIDENCE FROM THE NIHEWAN BASIN, NORTH CHINA

The East Asian monsoon (EAM) is an important component of the global climate system. Long-term fluvio-lacustrine sequences from the Nihewan Basin, northern China, provide high-resolution records of paleomonsoon variability. An 82m-deep core was extracted from the Majuangou section, located in the eastern margin of the basin. The magnetostratigraphic results show that the core sediments cover an age range from ~2.14 to 1.0 Ma, confirming the previous dating results from the outcrops by Zhu et al. (2004).

Detailed records of the Asian summer monsoon (ASM) and the Asian winter monsoon (AWM) from the core are revealed, based on selected element-ratio pairs that are sensitive to paleo-environmental changes. Results suggest a gradual, long-trend intensification of the ASM since 2.14Ma ago, while a gradual long-trend diminishment of the AWM since 1.75 Ma ago, except for some relatively intense dry events. Results also suggest that the intensities of the AWM and the ASM closely coincide between ~1.75 and 1.3 Ma ago, whereas the other periods over ~2.14-1.75Ma and ~1.3-1.0 Ma were characterized by a strengthened ASM and a weakened AWM, except for some distinct dry events. Distinctive high aeolian inputs happened in L₂₇, L₂₅, L₂₃ and L₁₅, indicating the enhancement of the winter monsoon and aridification during those times.

Thus, the records of the AWM and the ASM from northern China show more complex patterns than those from loess-paleosol sequences during this period. The significant strengthening of the AWM and weakening of the ASM may associate with strong cooling in the North Atlantic, global ice volume and the responses of continent–ocean–atmosphere systems.