Response to Comment on 'Ice Core Perspective on Mercury Pollution during the Past 600 Years'

摘要

Zhang and Kang focus on our suggestion that current estimates of Hg emissions from Colonial and Gold Rush mining are erroneously high. They propose that comparison of the Mount Logan Hg_T flux record to modeled atmospheric Hg concentrations or atmospheric deposition, as shown in Figure 1 of their comment, would be more appropriate than our comparison to emission estimates. The global Hg model they highlight uses the same emission estimates to force a 7-reservoir box model, with exchanges between reservoirs governed by first-order rate constants that are constrained by limited modern-day observations. We chose to compare the Mount Logan HgT record with the emission estimates to avoid further uncertainty associated with the model. We note that the Mount Logan Hg_T record returns to background-level fluxes immediately following the Gold Rush, which is inconsistent with both the global Hg model and the assertion by Zhang and Kang that atmospheric Hg levels are cumulative over half a century. Even if the Mount Logan Hg_T record is compared with the global Hg model outputs, our suggestion of erroneously high anthropogenic Hg emission estimates from Colonial and Gold Rush mining remains, as shown in Figure 1 of Zhang and Kang. This suggestion is also made in a recent compilation of 22 lake sediment Hg records from around the world and in chemical modeling of the Colonial stiver ore mining process.