Understanding the Modern and Past Mercury Biogeochemical Cycle Using Hg Stable Isotopes

我的研究计划的长期目标是利用汞同位素来提高我们对汞生物地球化学循环的理解，并开发汞同位素作为过去环境变化的代理。汞是一种分布在全球的金属，可在水生食物网中生物积累，导致人类和野生动物接触危险。汞通常通过自然和人为来源以其还原形式气态单质汞（GEM）排放到大气中。GEM相对稳定，具有较长的停留时间（约1年），这使得它可以在全球分布。最终，GEM被氧化并迅速沉积到陆地和水生系统中（数小时至数周）。汞循环的主要复杂性之一是，在沉积后，汞可以还原为GEM并排放回大气。这导致主要来源占大气汞排放量的不到一半（30%人为，10%自然），60%的大气输入是主要来自土壤和海洋的汞再排放。许多关键的知识差距仍然阻碍着我们对现代和过去汞循环的理解，并使预测排放和气候变化如何影响汞循环变得具有挑战性。例如，在地方、区域和全球尺度上评估不同人为源与再排放的相对贡献是一项巨大的挑战，而且往往依赖于没有足够数据约束的模式。此外，最近对土壤间GEM交换的评估强调，特别是在森林生态系统中仍然存在很大的不确定性。******汞稳定同位素地球化学是一种快速发展的研究汞的工具，主要是由汞同位素的大质量独立分馏（MIF）的发现所推动的。我发现项目将集中在三个目标,令人兴奋的新领域汞同位素可能使大影响Hg领域的研究:(1)的开发和应用新方法测量Hg大气汞浓度和同位素组成物种,(2)使用汞同位素量化和理解汞的来源和再活化土壤,和(3)开发和测试的汞和汞同位素作为代理古火山排放。目标1和目标2将通过急需的实验研究、用于汞收集和同位素分析的采样器的新开发以及实地工作来验证和应用新技术，推进对大气和土壤中汞源的建模和评估。第三个目标是研究大气中汞的主要自然来源、火山活动和火山汞物种的同位素特征，以及这些特征如何在地质记录中保存下来。这一地球成因汞源的同位素特征将有助于我们对现代汞循环的理解，并将为汞同位素作为古代火山排放的可靠示踪剂建立基线。

The long-term goal of my research program is to use Hg isotopes to improve our understanding of the Hg biogeochemical cycle and to develop Hg isotopes as a proxy of past environmental changes. Mercury is a globally distributed metal that bioaccumulates in aquatic food webs leading to dangerous exposure in humans and wildlife. Mercury is often emitted to the atmosphere in its reduced form, gaseous elemental Hg (GEM), by both natural and anthropogenic sources. GEM is relatively stable and has a long residence time (~1 yr), which allows it to be distributed globally. Eventually GEM is oxidized and deposited to terrestrial and aquatic systems quickly (hours to weeks). One of the major complexities of the Hg cycle is that after deposition, Hg can be reduced back to GEM and emitted back to the atmosphere. This results in primary sources making up less than half of the Hg emissions to the atmosphere (30% anthropogenic, 10% natural) with 60% of inputs to the atmosphere being Hg re-emissions dominantly from soils and the ocean. Many key knowledge gaps still hinder our understanding of both the modern and past Hg cycle and make it challenging to predict how changes in emissions and climate will affect the Hg cycle. For example, assessing the relative contributions of different anthropogenic sources versus re-emissions on local, regional and global scales is a huge challenge and is often dependent on models without enough constraints from data. Also, a recent assessment of GEM exchange to and from soils highlighted that very large uncertainties still exist especially in forested ecosystems. ****** A rapidly growing tool to study Hg is Hg stable isotope geochemistry, which was largely driven by the discovery of large mass independent fractionation (MIF) of Hg isotopes. My Discovery program will focus on three goals that are exciting new areas where Hg isotopes are likely to make large impacts on the field of Hg research: (1) the development and application of new methods to measure Hg concentrations and isotopic compositions of atmospheric Hg species, (2) the use of Hg isotopes to quantify and understand sources and remobilization of Hg in soils, and (3) the development and testing of Hg and Hg isotopes as a proxy of ancient volcanic emissions. Objectives 1 and 2 will advance modelling and assessment of sources of Hg to the atmosphere and to soils through much needed experimental studies, novel development of samplers for collection and isotopic analysis of Hg, and fieldwork to verify and apply new techniques. The third objective will investigate a primary natural source of Hg to the atmosphere, volcanism, and the isotopic signatures of volcanic Hg species along with how these signatures are preserved in geological records. The isotopic characterization of this geogenic source of Hg will contribute to our understanding of the modern Hg cycle and will establish the baseline for Hg isotopes as a reliable tracer of ancient volcanic emissions.