北部湾东北部2000年以来高分辨率粘土矿物记录及古环境意义

A HIGH-RESOLUTION CLAY MINERALOGICAL RECORD AND ITS PALEOENVIRONMENTAL SIGNIFICANCE IN THE NORTHEASTERN GULF OF TONKIN OVER THE PAST 2000 YEARS

北部湾东北部GC19-28孔260BC^1480A.D.粘土矿物学的高分辨率(平均分辨率为15年)分析表明,该孔的粘土矿物主要由蒙脱石(50%~61%)组成,含中等含量的伊利石(13%~18%)、高岭石(13%~18%)和绿泥石(11%~16%),在该时间序列上蒙脱石与其他3种粘土矿物变化趋势基本相反。物源区分析显示,GC19-28孔粘土矿物中的蒙脱石主要由红河提供,伊利石、高岭石和绿泥石主要来源于珠江。该孔高岭石/蒙脱石比值可以反映华南珠江流域季风降雨变化,比值增加表明珠江流域降雨增强;反之亦然。研究结果发现,GC19-28孔的4种粘土矿物整体表现2个阶段性变化:1)260BC^1200A.D.,4种粘土矿物的相对含量基本保持不变,高岭石/蒙脱石比值同样保持不变;2)从1200~1480A.D.,蒙脱石相对含量逐渐降低,伊利石、高岭石和绿泥石相对含量逐渐增加,高岭石/蒙脱石比值开始增加。结合贵州董哥洞石笋、广东湖光岩、西沙东岛湖泊沉积物和西太平洋暖池区海表古温度等高分辨率地质记录,推断华南珠江流域由中世纪暖期向小冰期转换过程中,降雨逐渐增加。

Clay mineralogical analyses have been performed on marine sediments from Core GC19-28 （20°17. 614＇N, 109°08. 295＇E, water depth 24m, core length 5.75m）collected in a tidal sand ridge at the northeastern Gulf of Tonkin at extremely high resolutions. Bulk sediment from the core consists mostly of dark greenish gray silty clay with slight bioturbation. Chronology of the core sediment is established based on 6 AMS 14C age points performed on shells of mollusks tested in the Beta Laboratory. It is noted that the core covers a time interval of ca. 1740 years spanning from 260BC to 1480A. D. A total number of 115 samples, with a depth interval of 5cm and a corresponding temporal resolution of about 15 years, are obtained for the clay mineralogical analyses. The clay mineral assemblages of Core GC19-28 show high content of smeetite （50% N 61% ）, moderate contents of illite （ 13% - 18%）, kaolinite （ 13% - 18%） and chlorite （ 11% - 16% ）. In general, the temporal oscillations of smectite are negatively correlated to the other three groups of clay. Through comparing the clay assemblages in this core to that of the potential sources, it is suggested that almost all smectite derives from the Red River, while other kinds of clay are provided by the Pearl River. The ratio of kaolinite/smectite at the Core GC19-28 is thus adopted as an indicator of the amount of precipitation in the Pearl River drainage basin, with high values corresponding to greater precipitation, and vice versa. There are two stages of clay mineral assemblages change on the time series, even though their overall fluctuations aren＇t startling. （1） During 260BC - 1200A.D., all kinds of clay mineral and the ratio of kaolinite/smectite nearly remain the same. （2） During 1200 - 1480A. D., the content of smectite gradually decreases; while illite, kaolinite and chlorite as well as the ratio ofkaolinite/smeetite gradually increase. Combined with other high-resolution records in surrounding areas, e.g. laminated stalagmites from Dongge cave, sediment cores in Lake Huguangyan situated in Guangdong Province, lake sediment records in Dongdao Island located in Xisha Island and sea surface temperature reconstructed from the West Pacific Warm Pool, indicating the Pearl River drainage basin receive more abundant precipitation since 1200A.D., a time interval that is concurrent with the transit from the Medieval Warm Period to the Little Ice Age.