## Materials and Methods
### CV-MC-ICP-MS Measurements  
The mercury isotope ratio is expressed using the following delta notation:  
\[\delta^{\mathrm{x}}\mathrm{Hg}\left(\mathrm{\%_{o}}\right)=\left[\frac{ \left(\frac{\mathrm{x}\mathrm{Hg}}{\mathrm{196}\mathrm{Hg}}\right)_{\mathrm{ sample}}}{\left(\frac{\mathrm{x}\mathrm{Hg}}{\mathrm{196}\mathrm{Hg}}\right)_{\mathrm{ reference}}}-1\right]\times 1000 \tag{1}\]  
where x and the bracketed isotope ratios with sample and reference stand for the mass of the mercury isotope and the mercury isotope ratios of the mass x to the mass 198 for the sample and reference materials, respectively. The standard reference material 3133 (or SRM 3133; NIST, Gaithersburg, MD, USA) was used for the reference material. The following is a brief description of the stable mercury isotope measurement using a CV (HGX-200, Teledyne CETAC Technologies, Omaha, NE, USA), followed by an MC-ICP-MS (Neptune Plus, Thermo Fisher Scientific GmbH, Bremen, Germany). A sample solution and a 5% (_w/w_) SnCl\({}_{2}\) in 10% (_v/v_) hydrochloric acid solution, which instantaneously reduces Hg\({}^{2+}\) in a solution to GEM, were introduced into the CV by a peristaltic pump (Perimax, Spectec GmbH, Erding, Germany) under a flow rate of 0.58 mL min\({}^{-1}\). Thallium standard reference material (SRM 997, NIST, Gaithersburg, MD, USA), of which the isotope ratio of \({}^{203}\)Tl to \({}^{205}\)Tl is known, was also introduced into the CV in order to correct artificial mass-dependent isotope fractionations as they occurred on the surfaces of the mercury and thallium at the ICP. This injection was made by introducing the dried thallium chloride aerosols generated from the hydrochloric acid/thallium (SRM 997, NIST, Gaithersburg, MD, USA) solution using an aerosol generator (Aridus II, Teledyne CETAC Technologies, Gaithersburg, NE, USA), which was operated with a 100 \(\mu\)L min\({}^{-1}\) PFA nebulizer (SP820A, Teledyne CETAC Technologies) under 383 K for the spray chamber and a desolvator, with 0.40 mL min\({}^{-1}\) and 2.9 L min\({}^{-1}\) argon flow for the nebulizer and desolvator, respectively. Argon carrier gas for the CV (known as the \"counter flow\" for a straight gas-liquid separator) flowed into the CV from one end at 0.56 mL min\({}^{-1}\) and flowed out from the other end so that all gases and aerosols inside the CV, including the sample GEM and the generated thallium aerosols, were carried into the MC-ICP-MS system. The MC-ICP-MS was tuned on a daily basis using a 10 ng g\({}^{-1}\) solution of SRM 3133 (NIST, Gaithersburg, MD, USA) prepared in 5% inversed aqua regia (2:1 nitric acid/hydrochloric acid) prior to the measurements. A sample was preanalyzed by the CV-MC-ICP-MS to determine the concentration. When the concentration was higher than 10 ng g\({}^{-1}\), the sample was diluted to 10 ng g\({}^{-1}\)--a concentration equivalent to the reference solution. When the concentration was below 10 ng g\({}^{-1}\), the concentration of the reference solution was diluted to an identical concentration of the sample. The typical sensitivity of the instruments under the conditions above was 0.19 V min\(\cdot\)g ng\({}^{-1}\) mL\({}^{-1}\) for \({}^{202}\)Hg. This value is low compared to other reported values (e.g., 1 V min\(\cdot\)g ng\({}^{-1}\) mL\({}^{-1}\)). The variation in the sensitivity was very likely caused by the use of different instruments (i.e., CV-MC-ICP-MS). For a sample or reference run, the operation software for the MC-ICP-MS was programmed to preinject a sample solution for 3 min for flushing, to measure the isotope ratios in 50 cycles and one block, and then to average out the measurement results. A 10 ng g\({}^{-1}\) standard solution of SRM 3133 in 5% inversed aqua regia was measured before and after the sample run (i.e., the bracketing method), and the average of those reference isotope ratios was used as the reference isotope ratio in the calculation of Equation (1).