Peatlands are sources of the bioaccumulating neurotoxin methylmercury (MeHg) and linked to adverse health outcomes, yet the impact of climate change and reductions in atmospheric pollutants on mercury (Hg) export from peatlands are highly uncertain. Here, we present the response in annual flow-weighted concentrations (FWC) and yields of total-Hg (THg) and MeHg to cleaner air and climate change using an unprecedented hydroclimatic (55-years; streamflow, air temperature, precipitation, regional and peatland water tables), depositional chemistry (21-years; Hg and major ions concentration and total mass), and streamwater chemistry (~17-years; THg, MeHg, major ions, total organic carbon, and pH) datasets from a reference peatland catchment in the north central USA. Over the hydroclimatic record, annual mean air temperature increased by ~1.8 ℃, decreasing baseflow and, subsequently, the efficiency that precipitation was converted to streamwater runoff (runoff ratio). Concurrently, precipitation-based deposition of sulfate and Hg declined, where wet Hg deposition rates declined to near pre-industrial levels. Annual MeHg FWC was positively correlated mean annual air temperatures (p=0.03, r=0.51), annual runoff ratio (p<0.0001, r=0.76), and wet Hg deposition concentration (p<0.0001, r=0.79). Over the study period, decreasing wet Hg deposition concentration and annual runoff ratios counterbalanced increased peatland MeHg production due to higher air temperatures, leading to an overall decline in streamwater MeHg FWC. Climate change and cleaner air were responsible for 0.51 and 0.32 of the variability in MeHg FWC, respectively. Streamwater MeHg export may continue to decrease only if declines in runoff ratio and wet Hg deposition concentration persistently outpace increased air temperature.

In upland soils in humid climates, mineral stabilization of organic matter (OM) on millennial scales is often driven by the abundance of poorly crystalline, metastable chemical weathering products. Studies of volcanic ash soils have demonstrated that these metastable materials transform into increasingly crystalline minerals at advanced stages of weathering, so that the overall affinity of mineral surfaces for OM declines with time. However, the abundance of clay-sized (<2 𝜇m diameter) particles tends to increase with weathering, enhancing soil specific surface area (SSA) and potentially compensating for the loss of mineral affinity for OM. As a first step towards understanding the net effects of these simultaneous transformations on OM stabilization, we compared the coverage of SSA by OM in A and B horizons of ash-derived soils sampled along an elevation gradient in Veracruz, Mexico. N2 adsorption isotherms and Brunauer–Emmett–Teller (BET) theory were used to estimate SSA of bulk soil versus samples from which OM had been removed via combustion (muffling) and chemical oxidation (bleaching). In addition to comparing the effectiveness of the OM removal treatments, we characterized the extent to which the treatments altered the mineral matrix and introduced errors into the estimates of mineral SSA. Pore size distribution was estimated via density functional theory as a complement to the BET analysis. N2-accessible SSA ranged from 9 to 105 m2 g-1 after removal of OM, with muffling yielding higher values than bleaching for most samples. The probable loss of SSA associated with mineral transformations (e.g., of Fe oxides) at high temperatures during muffling was evidently offset by the more thorough removal of OM by that treatment. Although SSA tended to increase with weathering status, relative coverage of SSA by OM was relatively consistent across profiles and tended to be greater on average in A horizons (bleaching: 45% SSA covered, muffling: 51%) than in B horizons (bleaching: 28%, muffling: 34%). The apparent lack of OM coverage of SSA in the B horizon of the most weathered soil (0% of 60 m2 g-1 covered) underscores the overall importance of mineral reactivity in determining OM stabilization. Future work will extend these analyses to examine land-use effects on SSA coverage by OM.