汉江一级阶地抬升以来北亚热带风尘堆积、发育与东亚季风演变的耦合关系——以郧县段汉江上游谷地为例

The Coupling Relationship of Pathogenesis and East Asian Monsoon Variability since 18 000 a BP in the Upper Reaches of Hanjiang River Valley of China

风成黄土对东亚冬夏季风优势期转换及起迄时间起到良好指示作用,探究风尘黄土堆积、发育特征与东亚季风演变的耦合关系十分必要。本文通过研究汉江一级阶地上覆黄土-古土壤序列的构型、磁化率、烧失量、粒度、元素组成特征,分析汉江一级阶地抬升以来黄土-古土壤序列的成因及成壤特征,探究东亚冬夏季风的演化序列。结果表明:(1)汉江上游地区黄土-古土壤序列的地层构型为表层土(MS)-近代黄土(L0)-古土壤(S0)-过渡层(Lt)-马兰黄土(L1),磁化率、粒度、烧失量、元素组成等特征指示各地层成壤强度特征为S0】L0/Lt】L1。(2)成壤强度指示18 000aBP以来,东亚冬夏季风演替特征为末次盛冰期冬季风强烈→全新世早期冬季风减弱→全新世中期夏季风主导→全新世晚期夏季风减弱、冬季风强盛,其演变滞后于低纬太阳辐射变化2 000~3 000a。(3)该风尘堆积物理化性质对6 000~5 000aBP的气候恶化期有所响应。总而言之,汉江上游一级阶地上覆黄土为风尘堆积物,指示18 000aBP以来东亚季风的变迁规律及季风异常事件,能够体现北亚热带对全球气候变化及极端事件的响应。

As the accumulation of dust storm,aeolian loess indicates the conversion of East Asian Summer Monsoon,which is necessary to explore the coupling relationship of the loess accumulation &pathogenesis characteristics and East Asian Monsoon variability.Pedogenic modification characteristics and the structure of soil profiles were observed in the field survey process in the Hanjiang River valley,and magnetic susceptibility,particle-size distribution,mineral assemblage,major elements trace and OSL age were analyzed in laboratory.The research tries to use loess-like as ancient inductor to induct the transformation of East Asian Monsoon.(1)The loess sediments profile structure is MS-L0-S0-Lt-L1-AD in the Hanjiang River valley. The pathogenesis strength's sequence shows S0>L0/Lt>L1,the elements features(Al,Fe,Na,K,Si)are consistent with the Loess Plateau.(2)East Asian Monsoon and solar radiation exists aphase difference of 2 000-3 000 a.The East Asian Monsoon succession characteristic in northern subtropics was as following: strong winter wind(Last Glacial Maximum)→weak winter monsoon(early Holocene)→dominant summer monsoon(Mid-Holocene)→weak summer monsoon(the late Holocene)(3)Hanjiang River valley loess recorded cold and dry events happened during 5 000-6 000 a BP.