Different orbital rhythms in the Asian summer monsoon records from North and South China during the Pleistocene

摘要

Here we construct a Pleistocene astronomical timescale for the Nihewan fluvio-lacustrine sediments (North China), via tuning a stacked summer monsoon index generated from grain size and low-field magnetic susceptibility records to orbital obliquity and precession. Combining the summer monsoon records retrieved from the Nihewan and the Chinese loess deposits in North China on the one hand, and those from the stalagmites and the marine deposits in South China on the other, the Asian summer monsoon records from North and South China appear to show different orbital rhythms during the Pleistocene. The monsoon records from both the Nihewan Basin and Chinese Loess Plateau are equally characterized by dominant obliquity (41 kyr) before -0.9 Ma and dominant eccentricity (100 kyr) after this time, closely following the marine δ~(18)O record. In contrast, the δ~(18)O record of stalagmites from South China (Wang et al., 2001, 2008b; Cheng et al., 2009) and the iron oxide proxy record from the South China Sea (Zhang et al., 2007,2009; Ao et al., 2011), which are considered as a proxy indicator of the Asian summer monsoon intensity in South China, reveal a dominant cydicity of precession (23 kyr) over the past 1.8 Myr, closely following the solar insolation curve instead. We further present a possible interpretation of the different orbital rhythms in the Asian summer monsoon records from North and South China. The orbital rhythm in the summer monsoon records from North China is mainly modulated by the migration of the Asian summer monsoon due to changes in sea level and Northern Hemisphere ice volume during glacial-interglacial cycles. Strong summer monsoon may have reached North China mainly during interglacial periods. During glacial periods when the sea level was low and Northern Hemisphere ice volume was large, the southeasterly migration of summer monsoon would make North China beyond the reach of the summer monsoon. Thus the summer monsoon records from North China primarily show a cyclic oscillation similar to the glacial-interglacial climate cycles as indicated by cycle-by-cycle correlation between monsoon and marine oxygen isotope records. However, the summer monsoon always prevailed over South China during both glacial and interglacial periods. Therefore, the orbital-scale variability of the summer monsoon in South China shows a direct response to the orbital variations in the low-latitude summer insolation, without significant influence from the migration of monsoon during glacial-interglacial cycles.