末次冰消期以来泰国湾沉积物物源变迁的元素地球化学证据

Geochemical evidence for the sediment provenance evolution in the Gulf of Thailand since the last deglacial

本文对泰国湾西部T93柱状沉积物样品进行了常微量元素测试,探讨了沉积物物源和沉积环境变迁。T93柱状样全长381 cm,底层年代约14640 cal.a B.P.,间隔10 cm取样测试,样品平均年代间隔375 a。测试结果表明,末次冰消期以来泰国湾沉积物来源主要经历了4个阶段的变迁:1)14640~11700 cal.a B.P.期间,该阶段为低海平面时期的陆相沉积,沉积物为粘土质粉砂,Na、K、Ca等活跃的碱金属和碱土金属元素含量较低,Ti、Zr、Ba等含量高,表现出强烈的化学风化特征;δEu平均为0.60,(La/Yb)PAAS平均0.92,物质来源以马来半岛为主,但分异程度高,成分较复杂。2)11700~8000 cal.a B.P.期间,该阶段为海平面快速上升期的滨海沉积,沉积物主要为粘土质粉砂,粉砂组分含量开始增加,Mg和Mn元素含量大幅升高,Fe、Ti含量减少;δEu平均为0.58,(La/Yb)PAAS平均0.99,物质来源主要为马来半岛。3)8000~4000 cal.a B.P.期间,该阶段为高海平面时期的陆架沉积,沉积物包括粉砂和粘土质粉砂,Mg、Na、Ca含量明显增大,化学风化程度减弱;δEu平均为0.60,(La/Yb)PAAS平均1.08,沉积物主要来自中南半岛。4)4000 cal.a B.P.至今,该阶段为海平面稍有下降的稳定陆架沉积,沉积物主要为粉砂,沉积物元素总体稳定,现代沉积体系基本形成;δEu平均为0.62,(La/Yb)PAAS平均0.97,沉积物以马来半岛物质为主,中南半岛物质为辅,物源与现代一致。

A 381-centimeter long gravity core T93 located at 9.9261°N,100.6442°E was collected from the western Gulf of Thailand(5°~13°30'N,99°~106°E)by SEAFDEC in May 2012,with a sampling water depth of 59 m.A total of 38 samples were collected at intervals of 10 cm,and the bottom age is about 14640 a B.P.,so the sampling interval is about 375 a.Major and trace element content of<0.1 mm part of the samples were measured,and the evolution of sediments provenance and sedimentary environment since the Last Deglacial is discussed on the basis of AMS 14C dating,grain-size,color reflectance and element content data.The results show that the process of sedimentation can be divided into four stages:(1)From 14640 cal.a B.P.to 11700 cal.a B.P.,the sea level is basically low and the sediments composed of clayey silt is deposited in a terrestrial environment.The content of alkaline elements such as Na,K,Ca and Mg is extremely low,while the content of Fe,Ti and Ba is high,showing a strong chemical weathering intensity.The averageδEu is 0.60 and(La/Yb)PAAS is 0.92,respectively,indicating that the sediments are mainly from the Malay Peninsula and partly the Indo-China Peninsula and the South China Sea.(2)From 11700 cal.a B.P.to 8000 cal.a B.P.,the sea level rises rapidly and the western Gulf of Thailand is in a coastal environment.The content of silt and active elements show a slight increase.By contrast,the content of Fe,Ti,and Ba decrease.The averageδEu is 0.58 and(La/Yb)PAAS is 0.99,indicating the sediments are mainly from the Malay Peninsula.(3)From 8000 cal.a B.P.to 4000 cal.a B.P.,the sea level reaches the maximum.The western Gulf of Thailand become a shelf environment and the sediments consist of silt and clayey silt.The content of Na increases by 50%while the other elements change little,and the chemical weathering intensity becomes weaker.The averageδEu and(La/Yb)PAAS value is 0.60 and 1.08 respectively,indicating that the sediments mainly derive from the Indo-China Peninsula.(4)The last 4000 years,the elemental and grain-size composition of the sediments remain steady as the sea level slightly decreases.The averageδEu is 0.62 and(La/Yb)PAAS is 0.97,hence the sediments are mainly influenced by material from the Malay Peninsula,and partly affected by sediments from the Indo-China Peninsula.