MIS 3时期是末次冰期中气候相对温湿的特殊时段,在汉江流域黄土研究中鲜有对这一时期气候变化记录的报道。对汉江上游郧西段庹家湾(TJW)剖面(厚约8m)的野外特征、磁化率、粒度和色度进行了分析,用OSL方法进行了断代。结果表明:1)马兰黄土L_1中深度228~260cm和294~370cm呈暗红棕色、粘土-粉砂质地、比较紧实坚硬的特征,其磁化率、<5mum粘粒含量和红度a~*值明显偏高,而>50mum粗颗粒含量、亮度L*和黄度b*值偏低,属于有明显成壤特征的弱古土壤(L_(1-2-S_1)和L_(1-2-S_2))。2)OSL测年显示L_(1-2-S_1)和L_(1-2-S_2)这两层弱古土壤形成时间介于28.5~21.0ka B.P.,暗示汉江地区这一时期发生两次气候较温湿事件,且有明显冷暖波动;其中弱古土壤L_(1-2-S_2)的形成时间(27.261.44ka)与MIS 3晚期阶段大致相当,说明MIS 3晚期气候较温湿变化在汉江流域亦有表现;且成壤强度变化特征指示其气候温湿程度弱于中全新世大暖期。3)TJW剖面弱古土壤L_(1-2-S_2)记录的MIS 3晚期气候较温湿事件是该区域对全球气候变化的具体响应。对比研究发现,不同地区黄土剖面均记录了MIS 3时期气候发生增温增湿变化特征,其温湿程度均弱于全新世时期,但不同剖面记录的气候变化幅度及冷暖变化阶段存在明显差异。TJW剖面记录的MIS 3时期气候较温湿事件主要发生在其晚期阶段。
英文摘要:
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. Oka 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°05'47N, 110°22'51E; 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 L_1 layer during the field investigations. The experimental results show that their magnetic susceptibility (136.9 * 10~(-8) m~3/kg and 161.3 * 10~(-8) m~3/kg), clay (<5mum) contents (17.2% and 19.2%),a* values (5.6 and 5.7) are higher and sand (>50mum) contents (35.7% and 25.9%), L* (57.9 and 56.5), b * values (17.5 and 17.0) are lower than the loess L_1. These suggest that they were two weak paleosol layers (L_(1-2-S_1) and L_(1-2-S_2)), which have experienced a certain degree of pedogenic modification. (2) Samples from the two weak paleosol layers (L_(1-2-S_1) and L_(1-2-S_2)) were OSL dated to 21.591.20ka and 27.26 1.44ka, suggesting the formation period of the two weak paleosol layers can be confined into approximately 28.5 ~ 21. Oka B.P. The forming time of weak paleosol layer L_(1-2-S_2) (27.261.44ka) 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 S_0. (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.