阿拉斯加巴罗AB-67钻孔岩芯的沉积环境分析

ANALYSIS OF SEDIMENTARY ENVIRONMENT OF AB 67 CORE AT BARROW, ALASKA

巴罗ＡＢ－６７钻孔岩芯的４８个样品被进行了２５个项目的粒度分析和４４个项目的地球化学分析。本文对这些数据进行了Ｑ型因子分析，并得到了４个主因子。研究表明，这４个因子分别代表了沉积环境的四项指标，它们分别为沉积速率、降水、海平面和温度。从岩芯顶部５个样品与巴罗气象台观测记录确定了古温度和古降水的转换函数，从２１０Ｐｂ测年和几个环境事件确定了沉积速率的转换函数。从沉积相的变化识别出了海平面因子。重建的温度和降水的曲线表明，１６、１７世纪巴罗地区的环境比现在更为寒冷、干旱。１８、１９世纪温度升高而降水变幅很大，２０世纪的温度和降水波动都很剧烈，重建的温度曲线在岩芯的微体古生物变化组合上得到了很好的印证。

The purpose of this study is to reconstruct the record of recent environmental change at Barrow, Alaska through temporal inverse deduction using a sedimentary core. In this field of study, the traditional approach is qualitative, taking grain size, its statistical feature, geochemical composition and its combination as indexes of sedimentary environment to indicate environmental change. In recent years, the use of Q mode factor analysis and transfer function techniques has facilitated the quantification of paleoenvironment research. Q mode factor analysis has been used to study several undetermined factors, revealing certain results that are widely known. Changes of sea level, sedimentary rate, temperature and precipitation must cause changes in sedimentary grain size and chemical composition in the lagoon sedimentary environment. The causal relationship may not be strictly linear, but before nonlinear technique can be practically used, we will analyze grain size and geochemical composition by Q mode factor analysis to extract sedimentary environment factors. Factor identification is the key to Q mode factor analysis. Here, the identification was done according to: (1) the cause and effect relationship in sedimentary modeling; (2) the qualitative feature of environmental changes that are already known; (3) the degree of correlation between recent meteorological data and the factors identified. In this paper, we will identify temperature, precipitation, sedimentation speed and sea level as the major factors affecting environmental change. Defining transfer function is important in paleoenvironment quantitative research. The temperature and precipitation transfer function is obtained from linear transformation of these factors, and the two unknown constants are obtained by linear regression of 11 years of running mean climatic data from Barrow Meteorological Station. Determining sedimentary rate is very important for establishing sedimentary temporal model. The transfer function of sedimentary rate is from linear transformation of its factors, and the two constants needed are determined by 210 Pb timing and recognition of several environment events. The transfer function of sea level change is obtained from linear transformation of the sea level factor, and the two constants are determined by the time when marine sediment began and by the present ocean depth. Research on sea level change is only exploratory here because, first, there are no recent observed sea level data to compare with, and second, it remains to be further studied what effect sea level change had on shore zone sediment prior to marine transgression. Based on the analysis of the sedimentary environment of AB 67 core at Barrow, some conclusions are reached: 1) From the reconstructed sedimentary rate curve, we have determined the sedimentary period of each layer in the AB 67 core. Sedimentation in the bottom layer began in the latter half of the 16th century. 2) From our study of sea level change, we get the sea level change curve of Elson Lagoon. According to the grain size, geochemical composition and micropoleontology combination, marine deposit begins at the depth of 20 cm, which corresponds to the last half of the 18th century. This is earlier than the date estimated by 210 Pb. The mean uplift speed of sea level is 10 mm·a 1 over the last 200 years. 3) The trends of temperature and precipitation show that the environment in the 16th and the 17th centuries was colder and dryer than that of today, that temperature rose and precipitation fluctuated greatly in the 18th and the 19th centuries, and that both temperature and precipitation fluctuated greatly in the 20th century. The pattern of temperature fluctuation is well confirmed by the realignment of microponotoly in the AB 67 core.