摘要
本文通过对太湖西山岛衙甪里孔(YLL1)沉积物的粒度、总有机碳含量(TOC)、质量磁化率、元素含量及沉积年代等进行测试分析,探讨了全新世以来太湖湖盆的演化历史。根据沉积特征,可将4m深的YLL1孔分为上、中、下三段明显不同的沉积层。中段(153~231cm)沉积物平均粒径为7.65φ,粘土级颗粒含量高达48.78%,对比上段(0~153cm,6.29φ,21.77%)和下段(231~400cm,6.91φ,30.88%)为全孔最细;并且中段粒径随深度变化很小,平均粒径标准偏差为0.13,明显低于上段的0.70和下段的0.38;中段TOC和质量磁化率平均值分别为0.78%和19.41×10^-8m^3/kg,为全孔最高;XRF扫描数据显示元素含量在三段中变化显著:中段Al、K、Fe等元素含量相对上段和下段为最高,而Si含量最低。YYL1孔沉积特征参数显示,近80cm厚的中段泥质层与上下段截然不同,是一段标志性沉积层。该标志层广泛存在于太湖及其周边地区的地层中,并且该层位含有大量的富铁质小球粒结核和菱铁矿结核,AMS14C显示该层位年龄约为7kaB.P.。研究表明,太湖YLL1孔沉积记录了湖盆环境的3个不同阶段:下段对应末次冰期后期11kaB.P.左右的风成硬黄土层沉积,此时太湖尚未完全形成;中段泥质沉积标志层代表太湖第一次大水面形成,时代在7kaB.P.左右;上段对应5kaB.P.左右以来的湖相沉积。
Taihu Lake is located in the Southeast of China (30°55'~31°33'N, 119°52'~120°36'E). The origin and evolution of Taihu Lake basin have always attracted the attention of Chinese geologists and remained unresolved, because it has huge flat basin (65 km in diameter) but very shallow water depth (less than 3 meters). The core YLL1 (400 cm in depth) is located in the lake bottom (31°07'34"N, 120°13'52"E) near the shore of Xishan island of Taihu Lake, which is close to Yaluli (YLL) Village in Xishan island, Suzhou City, Jiangsu Province. The analysis of grain size, total organic carbon (TOC), magnetic susceptibility, XRF scanning, and AMS14C dating were carried out for this four-meter-long core to understand the evolution of Taihu Lake basin. The lithology of the core is mainly composed of clay, silt and silty sand. The core YLL1 is divided into 400 small samples for further study with interval of 1 cm. 108 samples were selected for grain-size analyses. 76 samples were selected for TOC analyses, 67 samples for magnetic susceptibility analyses, and whole 400-cm-long core for XRF scanning analyses to get element content variation, 4 samples were selected for AMS14C dating measured by Beta Analytic Inc. USA. The results of grain size parameters, TOC, XRF data, and magnetic susceptibility data all indicate that three layers can be distinguished in the core YLL1 by divided at 153 cm and 231 cm depth:upper layer from 1~153 cm, middle layer from 153~231 cm, lower layer from 231~400 cm. The middle layer with average grain diameter of 7.65 φ and clay content of 48.77% has the most finest grain size compared with upper layer (6.29 φ, 21.77%) and lower layer (6.91 φ, 30.88%). The middle layer grain-size variation along the depth is very small, with the lowest grain-size standard deviation value of 0.13, compared with upper layer (0.70) and lower layer (0.38). The results show that the 80-cm depth middle mud layer is one landmark layer with significant different sedimentary characteristics, with a small and uniform median-grain-size value, higher TOC content, higher magnetic susceptibilities, and different XRF element content profiles, compared with upper and lower layer. This special landmark silty layer widely occur in Taihu lake area containing abundant siderite concretions and iron-rich spherules. AMS^14C dating from the middle layer suggests that the middle layer formed in 7 ka B. P. The sedimentary records of the core YLL1 of Taihu Lake show three different lake basin environments. The lower layer represents the hard loess layer with aeolian origin after the last ice age at 11 ka B. P. when Taihu Lake basin was not formed yet. The middle landmark layer represents the silty deposit layer at 7 ka B. P. when whole basin was full of water for the first time and the basin was formed. The upper layer represents the modern Taihu Lake deposit layer after 5 ka B. P.
作者
袁悦
李成龙
左书豪
盛雪芬
林春明
谢志东
Yuan Yue;Li Chenglong;Zuo Shuhao;Sheng Xuefeng;Lin Chunming;Xie Zhidong(School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,Jiangsu;Key Laboratory of Surficial Geochemistry,Ministry of Education,Nanjing University,Nanjing 210023,Jiangsu;State Key Laboratory for Mineral Deposits Research,Nanjing University,Nanjing 210023,Jiangsu)
出处
《第四纪研究》
CAS
CSCD
北大核心
2019年第5期1133-1147,共15页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40972031、41272057和41873010)资助.
关键词
太湖
全新世
粒度分析
沉积特征
湖盆演化
Taihu Lake
Holocene
grain size analysis
sedimentary characteristics
lake basin evolution
