青藏高原共和盆地风成沉积记录的全新世有效湿度变化研究

Study on Holocene moisture changes and its formation mechanisms in the monsoonal boundary region, northwestern China, is of great scientific significance in the earth sciences. Researches show that in this region there still exist significant differences regarding the optimal moisture period occurred in the early-middle Holocene or the middle Holocene. Moreover, the direct geological evidences from the main sediments of the arid region are still inadequate. The Gonghe Basin in the northeastern Qinghai-Tibet Plateau, a typically alpine, arid and semi-arid inland intermountain basin (desert), is located at the convergence of the Asian summer monsoon, winter monsoon, and the westerlies. Conducting the study of the regional palaeoclimatic change and dynamic mechanism has the importantly scientific significance for recognizing the spatial-temporal evolution of summer monsoon. In this study, we will systemically investigate the characteristics of aeolian deposits on the basis of the aeolian sand-palaeosol sequences in the Muge Sandy, Gonghe Basin. The high-precision chronology study of OSL and AMS14C will be performed to establish the reliable stratigraphic framework chronology. The chemical elements (i.e., the contents of the stable and soluble elements and their ratios) and the pollen parameters (i.e., the content of the different species and characteristic parameters) are taken as the proxies representing the regional effective moisture change. By combining with the variation characteristics of carbonate, organic matter and grain size, the history of the regional effective moisture change during the Holocene will be synthesized through a comprehensive analysis of multi-sedimentary sequences. A regional-scale comparative study will be carried out to understand the internal relationship and the driving mechanisms of the effective moisture change between the Gonghe Basin and the Asian summer monsoonal evolution.

中国西北季风边缘区全新世湿度变化过程及其成因机制是地球科学研究的重要内容之一，研究显示季风边缘区湿度最优期发生于早中全新世还是中全新世仍存在显著的分歧，而且尚缺少来自干旱区主体沉积的直接地质学证据。青藏高原东北缘的共和盆地地处亚洲冬夏季风与西风的交汇区域，属典型的高寒干旱-半干旱内陆山间盆地(沙漠)，开展古气候变化及动力机制研究对认识夏季风的时空演变具有重要的科学意义。本研究以共和盆地木格滩风成砂-古土壤序列为基础，系统调查区域风沙沉积的组成特征，拟进行高精度的OSL和AMS14C年代学研究，建立可信的地层年代学框架；以化学元素(稳定和易溶性元素含量及其比值)和孢粉学参数(不同种属含量及特征参数)为有效湿度变化的指标，结合碳酸盐、有机质和粒度的变化特征，通过多个沉积序列的综合分析，集成区域全新世有效湿度变化历史；开展区域尺度的对比研究，认识该区有效湿度变化与夏季风演变的内在联系及其成因机制

中国北方季风边缘区全新世有效湿度变化及其对东亚季风的响应是地球系统科学研究的重大科学问题，其对理解干旱-半干旱地区现代地表过程和未来环境变化趋势有着重要的科学意义。研究表明季风边缘区中东部湿度变化主要遵循 “季风模式”，而西北部则同时受到季风与西风的交互影响，其有效湿度变化呈现典型的区域特色。本研究以青海共和盆地表土沉积和全新世木格滩风沙沉积序列为研究对象，侧重表土沉积现代过程和全新世有效湿度变化2个方面进行研究。① 共和盆地不同类型表土沉积地化元素和粒度分析发现，表土沉积主要地化组成总体相似，以Si、Al和Fe为主，化学风化条件类同，均为寒冷干燥气候条件下的低等化学风化；粒度组成以中粗沙为主，不同类型表土沉积细颗粒含量差异显著；表土沙漠沙主要来源为LGM以来古沙翻新，部分为山麓冲洪积物质堆积。② 以木格滩8个风成沉积序列为基础，采用OSL和AMS14C断代技术和粒度、地化元素、孢粉、有机质、碳酸盐等指标分析方法，发现共和盆地在晚冰期-早全新世9/8 ka之前，地层主要堆积风成沙，为干旱多风的流动沙地环境，有效湿度为全新世最低；中全新世(9/8-4/3 ka左右)普遍发育沙质古土壤，植被以蒿藜为主，沙地总体上处于固定-半固定状态，有效湿度显著提高；晚全新世(~4/3 ka)以来，频繁交替的古土壤-风沙沉积序列和地化元素反映区域有效湿度有所降低，但较早全新世高。总体上，早中全新世区域有效湿度由干旱-相对湿润的转变受控于夏季风强度及其降水条件，而晚全新世有效湿度的变化不仅与季风降水-蒸发平衡有关以外，西风携带的水汽也不可忽视。 这一初步研究结论为增进理解季风-西风过渡区气候环境变化及其相互作用提供新的地质证据。