川西泥炭汞同位素记录及其二维体系对工业革命以来大气汞沉降历史示踪研究

Mercury (Hg) is seemed as a global health concern due to its toxicity and ubiquitous presence in the environment. Mercury can transport long distance in the atmosphere, be persistent in the environment, be accumulated in the food web, and thus pose severe threat to human health. The sedimentary records of ombrotrophic peat can provide valuable information on the environmental changes during the process of geological history. With the development and application of multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS), accurate and precise measurements of isotope ratios of Hg have been made possible. Field sampling and laboratory analysis are used in this program to investigate atmospheric Hg deposition history since industrialization procession from the ombrotrophic peat in western Sichuan, China. Based on source analysis of Hg isotopes, the isotope labeling technique of the three element mixture model under the mass-dependent and mass-independent fractionation (MDF-MIF) system is proposed to mark the different sources of Hg. Therefore, the objective of this program is to reveal the impacts of industrialization on atmospheric Hg deposition and the isotopic composition of Hg emitted from both natural and anthropogenic sources based on high-precision Hg isotope compositions and concentrations. We hope this program could clarify the history of China's pollution of Hg, and provide a scientific basis for China to fulfill the “Minamata Convention on Hg”.

汞是全球性污染物，汞的生物地球化学循环受到科学家高度重视。泥炭能够准确记录人类向自然的排汞过程。随着多接收器等离子体质谱（MC-ICP-MS）技术的成熟与应用，现阶段建立的汞同位素体系明确了汞的二维同位素体系(MDF-MIF)可以作为汞污染源和生物地球化学反应及其发生程度的示踪剂。本研究以我国川西地区泥炭剖面中沉积的汞为研究对象，采取野外采样和实验室分析相结合的方法，利用汞同位素进行源解析的研究方法，提出MDF-MIF二维体系下三元混合模型的同位素标记技术标记不同来源的汞；结合泥炭汞浓度，辅以放射性元素定年技术，弄清工业革命前、近代工业革命及现代工业革命进程中的大气汞排放在我国境内的历史沉降记录。这对厘清我国区域与全球尺度大气汞源排放，量化西方发达国家工业革命时期在我国境内的大气汞沉降贡献，界定其大气汞排放历史的国际责任，对我国履行《水俣公约》、缓解国际压力具有重大现实意义。

选取川西地区西南部若尔盖典型泥炭地为研究对象，人工挖取泥炭剖面，通过室内实验手段结合分析测试技术，利用汞同位素MDF-MIF二维体系示踪汞污染源，判别沉积物中不同时期汞的来源，同时研究大气沉降和泥炭藓对沉积物汞的贡献，在此基础上对泥炭汞沉积特征、近现代汞累积速率，环境污染历史进行深入研究，旨在揭示若尔盖泥炭地汞累积对气候变化的响应，进而丰富和补充全球汞污染进程数据库。利用样品中汞的同位素比值分析汞累积随深度垂直空间分布规律和随时间变化情况，通过研究泥炭沉积物中的汞同位素组成，依据其MDF-MIF的二维特征，建立泥炭地汞污染源三元混合模型(自然源-近代人为源-当代人为源)，并采用若尔盖泥炭地各生态单元沉积物汞含量进行模型检验，示踪和量化环境介质中不同来源的汞。研究结果表明在泥炭剖面中，汞同位素比值δ202Hg在(-0.23‰ ± 0.11‰) ~ (-1.72‰ ± 0.18‰)范围内变化。通过研究发现，泥炭剖面汞同位素从下到上有2个比较明显的阶段：(1) 1900~1950年，泥炭中汞的同位素比值δ202Hg介于-0.22‰ ~ -0.68‰之间，平均值为-0.32‰ ± 0.16‰; (2) 1950~2000年，泥炭中汞的同位素比值δ202Hg变化明显，平均值为-0.89‰ ± 0.22‰。这也印证了不同源排放的大气汞可能具有不同的同位素组成。