CLIMATE CHANGE DURING THE LATE STAGES OF MIS 3 PERIOD INFERRED FROM THE LOESS RECORD IN THE UPPER HANJINAG RIVER VALLEY
Marine Isotope Stage 3 (MIS 3) is a relatively warm-wet period during the Last Glacial.This special climate change has also been recorded by different carriers (e.g.loess,stalagmites,lake sediments,ice cores) in different regions of China.But there are still some controversies surrounding the climate characteristics during MIS 3 recorded by Chinese loess profiles.The upper Hanjaing River valley located in the southern margin of Qinling Mountains,is more sensitive to the climate change.Recently,loess deposits were founded in the first to fourth river terraces.Several studies have shown that the loess profiles in the first terrace recorded a continuous climate evolution information since 55.0ka B.P.in the region.However,whether the climate change during MIS 3 has been recorded by the loess profiles in the upper Hanjiang River valley? Relevant researches have seldom been reported.In this paper,the Tuojiawan (TJW) profile (32°05′47″N,110°22′51″E;168m a.s.l.) at the first river terrace of the Yunxi reach of the upper Hanjiang River was studied in detail.This profile is 8m thick and consists of Holocene loess-paleosol (0~178cm) and Malan loess (178 ~ 818cm) of the Late Pleistocene.The stratigraphic sequence is quasi-complete,continuous,mostly undisturbed and well comparable with other loess profiles in the area,indicating that the TJW profile has also recorded a continuous climate change information since 55ka B.P.On the basis of optically stimulated luminescence (OSL) ages from 11 samples and the proxy indexes of magnetic susceptibility,grain-size and color variation from 347 samples in the profile,we can drawing the following results:(1) Two soil layers (228~ 260cm and 294 ~ 370cm) with dark reddish brown,clay-silt textures,dense and solid structure were found inserted in the Malan loess L1 layer during the field investigations.The experimental results show that their magnetic susceptibility (136.9 × 10-8 m3/kg and 161.3 × 10-8 m3/kg),clay (<5μm) contents (17.2% and 19.2%),a* values (5.6 and 5.7) are higher and sand (>50μm) contents (35.7% and 25.9%),L* (57.9 and 56.5),b* values (17.5 and 17.0) are lower than the loess L1.These suggest that they were two weak paleosol layers (L1-2-S1 and L1-2-S2),which have experienced a certain degree of pedogenic modification.(2) Samples from the two weak paleosol layers (L1-2-S1 and L1-2-S2) were OSL dated to 21.59±1.20ka and 27.26±1.44ka,suggesting the formation period of the two weak paleosol layers can be confined into approximately 28.5 ~21.0ka B.P.The forming time of weak paleosol layer L1-2-S2 (27.26±1.44ka) can approximately correspond to the late stages of MIS 3 period,suggesting that the warmer and wetter climate change during MIS 3 have also been reflected in the upper Hanjiang River valley.And the climate is not as warm and wet as that during the Mid-Holocene Climatic Optimum as indicated by weaker pedogenic intensity than the paleosol S0.(3) The relatively warm-wet and unstable climate during the late stages of MIS 3 period in the upper Hanjiang River valley is similar with the climate records from different carriers (e.g.loess,ice cores,stalagmites) in the world and considered to be regional responses to global climate change.Compared with the loess records in the Chinese Loess Plateau,the paleoclimate during MIS 3 recorded by different loess profiles are not as warm and wet as that of the Holocene.However,the rangeability and the warm-wet intervals of the paleoclimate during MIS 3 are significantly different in these loess profiles,presenting an obvious regional discrepancy.This paper plays an important role in better understanding the climate characteristics during MIS 3 in China.
loessthe Last Glacialclimate changeMIS 3upper Hanjiang River