Paleolimnology of recent sediments in yangcheng lake

Abstract

对阳澄湖两个短柱状样进行了地层年代以及各种环境指标分析.结果表明,湖底沉积物由校长期沉积间断所分割的3个岩性层组成:下部层C属再沉积的更新世晚期硬土层;中部层B为全新世沉积;上部层A为近100年以来的表层沉积.对层A的详细分析显示,本世纪以来,特别是70-80年代以来,金属元素、营养元素和色素的沉积通量明显提高,说明湖泊重金属污染和富营养化程度的不断加重.;Seven cores of recent sediments were obtained in Yangcheng I.ake, among which 102cm-core YCl and 55cm-core YC3 in the western part of the lake kept undisturbed. The core profile is divided into 3 lithological beds which are separated by 2 long-term sedimentation hiatus, namely upper bed A of sapropel, middle bed B of silty clay, and lower bed C of sticky silty clay (so called hard earth bed). ¹⁴C, ²¹⁰Pb, water content, grainsize? magnetic susceptibility, nutrient elements, metal elements (both total and liable facies), stable isotope of organic carbon and fossil pigments were analysed. Chronic stratigraphic study demonstrates that beds A, B and C belong respectively to the deposits of Late Pleistocene, Holocene and about recent 100 years. The environmental characteristics of every bed are described as follows. Bed C:Reworked fluvial sediments, in which lower values of TOC, TN, pigment content, CD/TC and heavy metals and higher values of C/N, δ¹³C, allogenic metals (Al, Na, Mg, Ca and Ti) manifest more intense erosion of catchment land, lower primary productivity and eu-trophic state. Bed B:Lacustrine sediments, of which every environmental proxy sits between beds A and C. Bed A:Lacustrine sediments, of which higher values of TOC, TN, pigment content, CD/TC and heary metals and lower values of C/N, δ¹³C and allogenic metals show enhanced primary productivity and eutrophication of the lake. Detailed analysis of the beds demonstrates slightly higher pollution coefficient of Cu, Co, Zn, Cr, Ni in 0-2cm on surficial sediments. In the accumulation rate profile, the accumulation of many heavy metals is increased gradually upwards, extensively since the 1970s. The accumulation rate of nutrient elements and pigments also indicate more intense lake eutrophication since the 1970s-1980s.