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Centuries of anthropogenic releases have resulted in a global legacy of mercury (Hg) contamination. Here we use a global model to quantify the impact of uncertainty in Hg atmospheric emissions and cycling on anthropogenic enrichment and discuss implications for future Hg levels. The plausibility of sensitivity simulations is evaluated against multiple independent lines of observation, including natural archives and direct measurements of present-day environmental Hg concentrations. It has been previously reported that pre-industrial enrichment recorded in sediment and peat disagree by more than a factor of 10. We find this difference is largely erroneous and caused by comparing peat and sediment against different reference time periods. After correcting this inconsistency, median enrichment in Hg accumulation since pre-industrial 1760 to 1880 is a factor of 4.3 for peat and 3.0 for sediment. Pre-industrial accumulation in peat and sediment is a factor of ∼ 5 greater than the precolonial era (3000 BC to 1550 AD). Model scenarios that omit atmospheric emissions of Hg from early mining are inconsistent with observational constraints on the present-day atmospheric, oceanic, and soil Hg reservoirs, as well as the magnitude of enrichment in archives. Future reductions in anthropogenic emissions will initiate a decline in atmospheric concentrations within 1 year, but stabilization of subsurface and deep ocean Hg levels requires aggressive controls. These findings are robust to the ranges of uncertainty in past emissions and Hg cycling.

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The role of atmospheric deposition of iron, nitrogen and phosphorus in supplying nutrients to marine systems has been described, individually, in previous works. Here we examine atmospheric dry deposition of all these nutrients simultaneously, using samples collected during two meridional transects of the Atlantic Ocean. We find that, in line with previous work, desert dust supplies excess iron to the water column. However, primary production promoted by aerosol nitrogen can be sufficient to consume all of the soluble aerosol iron input in some situations. Aerosol N:P is universally very high, so that aerosol is always deficient in P relative to phytoplankton requirements. Nitrogen fixation stimulated by any excess atmospheric iron supply and phytoplankton utilisation of atmospheric nutrient inputs will therefore tend to drive the ecosystem towards P limitation. This emphasises the need to study the biogeochemical impact of atmospheric nutrient deposition in an integrated manner.

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) over the NA (steeper than at Northern Hemispheric land sites) but no significant decline over the SA. Surface water Hg 0 measurements in the NA show a decline of À5.7% a À1 since 1999, and limited subsurface ocean data show an $80% decline from 1980 to present. We use a coupled global atmosphere-ocean model to show that the decline in NA atmospheric concentrations can be explained by decreasing oceanic evasion from the NA driven by declining subsurface water Hg concentrations. We speculate that this large historical decline of Hg in the NA Ocean could have been caused by decreasing Hg inputs from rivers and wastewater and by changes in the oxidant chemistry of the atmospheric marine boundary layer.

[1] We present results from a campaign in March 2009 to assess the current state of emissions from Masaya Volcano, Nicaragua. These results constitute one of the most comprehensive inventories to date of emissions from an active volcano and update the exceptional record of emissions from Masaya. Results from open-path Fourier transform infrared spectroscopy and filter packs demonstrate that, in terms of H 2 O, SO 2 , CO 2 , HCl, and HF (molar H 2 O/SO 2 = 63, CO 2 /SO 2 = 2.7, SO 2 /HCl = 1.7, SO 2 /HF = 8.8), the 2009 gas composition was highly comparable to that from the 1998 to 2000 period, indicating stability of the shallow magma system. This continuity extends to certain aerosol species (molar SO 2 /SO 4 2− = 190, Na + /SO 4 2− = 0.68, K + /SO 4 2− = 0.71, Ca 2+ /SO 4 2− = 1.6 × 10 −2 , Mg 2+ /SO 4 2− = 3.6 × 10 −3 ) and, to a lesser extent, the heavy halogens (i.e., molar HCl/HBr = 2.4 × 10 3 , HCl/HI = 5.0 × 10 4 ). In contrast to an earlier study at Masaya, we did not detect HNO 3 . SO 2 fluxes were low (690 Mg d −1 ), suggesting that Masaya was close to the minimum of its degassing cycle. By combining compositional results with the SO 2 flux, we estimate a total volatile flux of 14,000 Mg d −1 . This rate is consistent with 1−4 wt% volatile loss from a convective magma flux of 17,000-4000 kg s −1 . These results will allow for a better understanding of degassing processes at Masaya and other basaltic volcanoes.

[1] We report measurements of Hg, SO 2 , and halogens (HCl, HBr, HI) in volcanic gases from Masaya volcano, Nicaragua, and gaseous SO 2 and halogens from Telica volcano, Nicaragua. Mercury measurements were made with a Lumex 915+ portable mercury vapor analyzer and gold traps, while halogens, CO 2 and S species were monitored with a portable multi gas sensor and filter packs. Lumex Hg concentrations in the plume were consistently above background and ranged up to 350 ng m À3. Hg/SO 2 mass ratios measured with the real-time instruments ranged from 1.1 Â 10 À7 to 3.5 Â 10 À5 (mean 2 Â 10 À5