湖北庹家湾剖面释光测年与地层年代学及记录的55ka BP气候变化

摘要

With the Tuojiawan sediment profile as study objectives,we analyzed their macroscopic structure,sedimentological characteristics and physico-chemical properties,and obtained their formation time using OSL dating.Based on the field observation and indoor laboratory data,this study also established the stratigraphic sequence of the loess-palaeosol in the Upper Reaches of Hanjiang River valley.OSL datingbased stratigraphic chronology study reconstructed the change history of the Upper Reaches of Hangjiang River since the last 55 ka BP.The results suggest that four sequences developed in this area.Terrace Ⅰ of Hangjiang River formed around 55 ka BP,immediately followed by continuous eolian sediments.The stratigraphic palaeosol-loess sequence of the Tuojiawan profile from the bottom to the top includes:loess L1-L3 (55.0～28.5 ka BP)→ palaeosol L1-S2 (28.5～24.5 ka BP)→ L1-L2 (24.5～23.0 ka BP)→ palaeosol L1-S1(23.0～21.0 ka BP)→ L1-L1(21.0～11.5 ka BP)→transitional loess Lt (11.5～8.5 ka BP)→ palaeosol S0 (8.5～3.0 ka BP)→recent loess L0 (3.0～0 ka BP).The Malan loess records four warm-wet periods occurring at 41.0～44.5 ka BP,34.0～37.5 ka BP,24.5～28.5 ka BP and 21.0～24.5 ka BP.Among them,the last two periods gave rise to the formation of palaeosol L1-S2 (370～296 cm) and paleosol L1-S1 (276～230 cm).During the megathermal period (8.5～3.0 ka BP),the upper Hangjiang river was not always the most warm-moist climate since the last glaciation,but intermittently,and relatively transient,relatively cool and wet climate occurred at 5～6 ka BP.%以湖北庹家湾剖面为研究对象,对其宏观组构、沉积学及理化性质(磁化率、粒度、色度、元素组成等)进行了测试,用OSL方法进行了测年断代.根据野外和室内实验数据,建立了汉江Ⅰ级阶地上黄土—古土壤的地层序列,并以OSL测年数据为依据进行了地层年代学研究,利用气候替代指标重建了汉江上游55 ka BP以来气候变化的轨迹.认为:汉江Ⅰ级阶地在55 ka BP前后形成,阶地形成后即刻开始接受连续的风尘堆积;其地层从下向上依次分为L1-L3 (55.0～28.5 ka BP)→L1-S2(28.5～24.5 ka BP)→L1-L2 (24.5～23.0 ka BP)→L1-S1(23.0～21.0 ka BP)→L1-L1 (21.0～11.5 ka BP)→Lt (11.5～8.5 ka BP)→S0 (8.5～3.0 ka BP)→L0 (3.0～0 kaBP);马兰黄土中记录了44.5～41.0 ka BP、37.5～34.0 ka BP、28.5～24.5 ka BP、23.0～21.0 ka BP四个暖湿气候阶段,其中28.5～24.5 ka BP和23.0～21.0 ka BP比较暖湿,形成了古土壤层L1-S2和L1-S1;庹家湾剖面记录的气候事件具有区域性特征,可与深海同位素曲线MIS-3进行良好比较,反映汉江地区MIS-3的后期暖湿程度较高;全新世大暖期(8.5～3.0 ka BP)在汉江上游地区并非—直表现为暖湿的特征,在5～6 ka BP出现短暂的相对凉湿的气候阶段.