对汉江上游谷地弥陀寺黄土剖面的地层序列和常量元素进行了研究,用光释光测年法(OSL)对地层进行了断代。结果显示:剖面中Na_2O、Mg_O、Ca_O表现淋失,Fe_2O_3、K_2O和AI_2O_3则相对富集,其中在古土壤中这些元素表现淋失或富集程度更为强烈;剖面具有斜长石风化为主的初级化学风化程度,以Ca、Na流失严重为特征;从马兰黄土L_1古土壤S_0全新世黄土Lo,风化由弱变强再变弱。这些信息揭示了汉江上游地区晚更新世以来的环境尤其是气候的变化规律:18 000?11 500aBP气候以干冷为主,进入全新世后逐渐转暖,8500~3 100 a BP为全新世大暖期,气候温湿,3 100 a BP后气候又转凉,呈现出由冷干转暖暖湿干凉的演变过程,同时弥陀寺黄土对6 000?5 000 a BP期间发生的气候波动冷事件也有记录响应。
英文摘要:
Stratigraphic sequence and constant elements were studied in Mituosi' s loess profile,Yun County,Hubei Province, China. And optically stimulated luminescence (OSL) was carried out on dating the loess sediments. The aim is to discuss the chemical weathering characteristics and the palaeoenvironmental change based on comprehensive approaches,such as Chemical Index of Alteration ( CIA ),A-CN-K diagram,Na/K and Fe/Mg. Results showed that the major composition of Mituosi' s loess section was made up with Si02 and AI_2O_3. Na_2O,Mg_O,Ca_O were performed leaching in the profile,while Fe_2O_3,K20 and AI_2O_3 were enriched. Moreover these elements behaved leaching or enriching more intensely in the palaeosol. The leaching of Na,Mg and Ca is due to the weathering of plagioclase and pyroxene. While Fe and Al s relative enrichment is because of leaching of other elements. The K enrichment is likely to be its ion adsorpted by relevant clays. These elements geochemical variations could reflect the pedogenic modification of loess in Mituosi. The CIA (65.41) and A-CN-K diagram showed the profile experienced the primary chemical weathering processes in which Ca,Na loss was serious,but almost no leaching of K. And it was transited to middle stage of chemical weathering. The weathering products mainly are montmorillonite and illite. It has not reac- hed kaolinite and gibbsite-based degree of weathering .The CIA,Na/K and Fe/Mg s curves all indicated the weathering was from weak to strong degrees and then weakened,which from Malan loess to palaeosol then to Holon-cene loess. This information revealed the palaeoenvironment,especially the palaeoclimate change since the late Pleistocene. The climate was characterized by cold and dry climate at about the late Pleistocene 18 000-11 500 a BP ; Then that came to the Holocene, it was becoming warm. And the Holocene warm period was during 8 500-3 100 a BP, after about 3 100 a BP. The climate turned cool again. All above indicated the climate was "cold-drywarmwarm-wetcool-dry". Meanwhile the climatic cold fluctuation events at about 6 000-5 000 a BP. have been recorded and responded by Mituosi' s loess.