钱塘江下切河谷SE2孔全新世沉积物黏土矿物特征及其意义*

通过X射线衍射系统分析了杭州湾地区SE2孔全新世沉积物的黏土矿物组成,结果显示研究层段黏土矿物主要由伊利石、绿泥石、高岭石和蒙脱石组成;伊利石结晶度较好,化学风化指数普遍大于0.5,表明以化学风化为主,且风化趋势自下而上呈递减趋势。通过对比中国东南部主要河流沉积物的黏土矿物组成,认为钱塘江下切河谷全新世沉积物的黏土矿物组成具有较好的物源指示意义: 全新世 Ⅰ 段(即古河口湾和河漫滩)沉积物主要来自钱塘江上游,特征黏土矿物为高岭石,河口外物质贡献不大;Ⅱ 段(即现代河口湾和近岸浅海)沉积物包含较多蒙脱石,表明不仅包括钱塘江上游物质,河口外长江物质也开始进入钱塘江河口。黏土矿物中,高岭石对气候有较好的指示作用:中全新世高岭石含量达到最高值,反映气候最为湿热,化学风化程度最高;晚全新世含量逐渐降低,反映气候逐渐回冷,化学风化强度降低。

Characteristics and its significance of clay minerals in the Holocene deposits of core SE2 in Qiantang River incised valley

Characteristics of clay minerals in the Holocene Qiantang River incised-valley fill was systematically analyzed by X-ray diffraction. Results show that clay minerals in the Holocene Qiantang River incised-valley fill mainly consist of illite,chlorite,kaolinite,and smectite;Illite has a good degree of crystallization;Chemical index of illite is generally higher than 0.5,indicating clay minerals had undergone strong chemical weathering and the degree decreased gradually from bottom to top. By comparing the clay mineral composition of main rivers in southeastern China,we suggest that the sediments of section Ⅰ(i.e. the paleo-estuary and fluvial)in the Qiantang River incised valley are mainly derived from the Qiantang River drainage basin characteristic of high amount of kaolinite;whereas the sediments of section Ⅱ(present-day estuary and shallow marine)comprise numerous Changjiang sediments with smectite as diagnostic mineral except the Qiantang River deposits. Additionally,kaolinite could act as a good indicator of climate: kaolinite content reached the highest value in the middle Holocene,indicating a humid and hot climate and the highest chemical weathering degree;in the late Holocene,with the decrease of kaolinite content,the climate gradually became colder with a gradually decreasing chemical weathering degree.