Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic matter (NOM) represents a large fraction of mercury in the runoff. We present a method to measure Hg stable isotope signatures of colloidal Hg, mainly complexed by high molecular weight or colloidal natural organic matter (NOM) in natural waters based on pre-enrichment by ultrafiltration, followed by freeze-drying and combustion. We report that Hg associated with high molecular weight NOM in the boreal forest runoff has very similar Hg isotope signatures as compared to the organic soil horizons of the catchment area. The mass-independent fractionation (MIF) signatures (Δ¹⁹⁹Hg and Δ²⁰⁰Hg) measured in soils and runoff were in agreement with typical values reported for atmospheric gaseous elemental mercury (Hg⁰) and distinctly different from reported Hg isotope signatures in precipitation. We therefore suggest that most Hg in the boreal terrestrial ecosystem originated from the deposition of Hg⁰ through foliar uptake rather than precipitation. Using a mixing model we calculated the contribution of soil horizons to the Hg in the runoff. At moderate to high flow runoff conditions, that prevailed during sampling, the uppermost part of the organic horizon (Oe/He) contributed 50–70% of the Hg in the runoff, while the underlying more humified organic Oa/Ha and the mineral soil horizons displayed a lower mobility of Hg. The good agreement of the Hg isotope results with other source tracing approaches using radiocarbon signatures and Hg:C ratios provides additional support for the strong coupling between Hg and NOM. The exploratory results from this study illustrate the potential of Hg stable isotopes to trace the source of Hg from atmospheric deposition through the terrestrial ecosystem to soil runoff, and provide a basis for more in-depth studies investigating the mobility of Hg in terrestrial ecosystems using Hg isotope signatures.

中文翻译：

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汞稳定同位素在北方森林径流中与汞结合的天然有机物的源示踪

陆地径流是水生生态系统中汞（Hg）的主要来源。在北方森林流域，例如瑞典北部的一个流域，与天然有机物（NOM）结合的汞占径流中汞的很大一部分。我们提出了一种方法，通过超滤预富集，然后冷冻干燥和燃烧，来测量天然水中主要由高分子量或胶体天然有机物（NOM）络合的胶体Hg的Hg稳定同位素特征。我们报告说，与集水区有机土壤层相比，北方森林径流中与高分子量NOM相关的Hg具有非常相似的Hg同位素特征。质独立分馏（MIF）签名（Δ ¹⁹⁹ Hg和Δ在土壤和径流中测得的²⁰⁰ Hg与报告的大气气态元素汞（Hg ⁰）的典型值一致，并且与降水中报告的Hg同位素特征明显不同。因此，我们建议北方陆地生态系统中的大多数Hg源自Hg ⁰的沉积通过叶面吸收而不是降水。使用混合模型，我们计算了土壤层位对径流中汞的贡献。在采样过程中普遍存在的中等到高流量径流条件下，有机层的最上层（Oe / He）贡献了径流中汞的50-70％，而底层的有机质Oa / Ha和矿物质土壤则更为腐化。地平线显示出较低的汞迁移率。汞同位素与其他使用放射性碳特征和汞的源追踪方法的一致性很好：C比率为Hg和NOM之间的强耦合提供了额外的支持。这项研究的探索性结果表明，汞稳定同位素具有追踪从大气沉积到陆地生态系统到土壤径流的汞来源的潜力，并为开展更深入的研究以研究汞在陆地生态系统中的迁移性提供了基础同位素签名。