沉积集中效应对湖泊沉积记录的影响及我国中东部人为重金属大气沉降通量时空格局定量重建

Heavy metal pollution is one of the prominent environmental problems in China and global. The atmospheric emission of most anthropogenic metals in China ranks first in the world, which makes China facing with great public pressure and task in emission reduction. The mid-eastern China is the hotspot of atmospheric metal pollution. The anthropogenic metals in the atmosphere may be dispersed in large scale and deposited into the terrestrial ecosystem, which have produced profound influence on the soil and aquatic system where is far from emission sources. Due to the limitation of monitoring data in China, our knowledge on the historical characteristics of atmospheric metal pollution such as trend and the deposition flux especially in mid-eastern China is very limited. Lake sediment is one of the excellent archive used in reconstructing the past atmospheric heavy metal pollution. However, the anthropogenic metals in the lake sediment are influenced by the transfer of anthropogenic metals deposited in the terrestrial soil besides the direct atmospheric deposition, which makes the flux of anthropogenic metals recorded in the lake sediment generally higher than the real values of atmospheric deposition. Moreover, the metals in the lake may be mobilized influenced by the underwater topography and hydrodynamic force and concentrated in the lake center where the sedimentary flux of metals is usually higher than the mean of the whole lake. All these two factors, named as the sediment focusing effect, have great influence on the accuracy in reconstructed atmospheric deposition flux of anthropogenic metals using lake sediment. The methods such as regression analysis between accumulation rates of anthropogenic metal in each lakes and the catchment/lake area ratios, dividing the unsupported 210Pb inventory in the core to regional background, and the mass equilibrium method by comparing the monitoring data of deposition flux of atmospheric metals, metals in the trap and the water, were suggested to compensate the sediment focusing effect, however the comparative studies were seldom performed. In view of the limited understanding in atmospheric heavy metals pollution and deposition flux in mid-eastern China, the alpine lakes were selected. Anthropogenic metals in the sediment were differentiated from the detrital input combining the conservative element normalization and the mixing model of Pb isotope. The influence of sediment focusing effect on the accumulation of anthropogenic metals in sediment were discussed combining the above three methods taking the lakes from Arxan as an example. The accuracy and effective of the widely used unsupported 210Pb inventory approach in compensating the sediment focusing effect were evaluated and then were used in the other lakes from the mid-eastern China. The spatio-temporal variations of atmospheric deposition flux of anthropogenic metals were reconstructed and their correlations with the potential anthropogenic emissions were discussed. This study may deepen our understanding on the historical atmospheric metals pollution in mid-eastern China and their response and influence on atmospheric metal pollution and anthropogenic emission in regional, continental and even global scale.

重金属污染是我国面临的主要环境问题，大气沉降是地表生态系统中重金属污染的主要输入途径。与污染现状相比，目前对我国过去人为重金属大气沉降通量及其对地表环境影响的认识还十分有限。湖泊沉积是开展过去大气重金属变化研究的良好载体，然而流域二次输入与湖泊沉积再搬运(沉积集中效应)成为制约通过湖泊沉积记录开展人为重金属大气沉降定量重建的瓶颈。本项目针对以上问题，选择我国中东部地区典型山地湖泊，利用参比元素、铅同位素等方法区分沉积物中重金属人为与自然来源；通过沉积物210Pbex累积通量/区域背景比值法、流域/湖泊面积比值法、并结合大气沉降与沉积物捕获器等监测结果的比对，研究沉积集中效应对沉积物重金属累积的影响与矫正方法；在此基础上，结合沉积岩芯年代结果，重建近百年来我国中东部地区人为重金属大气沉降通量时空格局，探讨重金属污染与人类活动的关系，为全面评估区域大气环境质量演变及对地表环境影响提供科学依据。

重金属污染是我国面临的主要环境问题之一，大气输移与沉降是偏远地区表生环境中重金属污染的主要输入途径，湖泊沉积记录是开展大气重金属污染重建的主要途径。本项目针对我国在大气重金属污染历史过程及对偏远地区表生环境影响方面的研究不足，选择中东部五个典型地区（亚）高山湖泊，开展了百年-千年尺度重金属污染的对比研究，揭示了不同地区的大气重金属污染元素、历史过程及累积通量，解析了其来源特征。通过不同消解与评价模型的对比，进一步完善了重金属污染的评价方法。我国中东部地区大气重金属污染及历史过程与区域经济发展密切相关，不同地区典型大气重金属污染元素的种类存在明显差异，如东北地区为Hg、Cd、Pb、Sb、As和Zn，华北地区为Hg、Cd、Pb和As，秦岭地区为Cd、Hg和Pb，华南地区为Cd、Hg、Pb和Sb。东北及华北地区大气Hg污染开始于1880s，在秦岭地区还发现了18世纪的Hg污染，东北地区大气Hg沉积通量背景值为2.7-3.7µg/m2/a，与全球平均值相当；1950s年前大气Hg污染与全球释放密切相关。研究区其余重金属元素污染主要开始于1950s~1980s并呈逐渐加重态势，但区域上仍存在一定的差异，如东北地区大气重金属污染开始于1930s，明显早于其他地区。区域燃煤与有色金属冶炼对大气重金属污染贡献较大，而含铅汽油对中东部偏远地区大气Pb污染变化影响较弱，与欧美地区存在明显差异。探讨了流域人为/遗存输入与水下地形等因素对湖泊沉积记录的影响，明确了不同类型湖泊典型污染物累积的空间变化及主要影响因素。水下地形是影响小型深水湖泊人为重金属累积的主要因素，而大型湖泊沉积物中人为重金属的累积主要受到流域输入的影响；阐释了单一岩芯在重建目标污染物累积通量以及210Pbex通量比值法在沉积集中效应矫正方面的局限性。本项目研究结果为利用沉积记录开展大气重金属污染重建、科学评估大气污染对偏远地区表生环境的影响提供了数据支撑与科学参考。