USING STABLE MERCURY ISOTOPES TO EXAMINE MERCURY CYCLING IN A PEATLAND-UPLAND WATERSHED IN NORTHERN MINNESOTA (U.S.A.)

Stable mercury (Hg) isotopes undergo both mass dependent fractionations (MDF; e.g., δ²⁰²Hg) and mass independent fractionations (MIF; e.g., Δ¹⁹⁹Hg) during biotic, abiotic, and photochemical reactions, and are diagnostic of complex Hg cycling processes in natural ecosystems. In this study, we examined the variability of stable Hg isotopes in a relatively well characterized small peatland-upland watershed (S2) in Marcell Experimental Forest in northern Minnesota, U.S.A. Water samples at the watershed outlet were collected biweekly when the stream flowed in both 2014 and 2015. We also collected surface and subsurface runoff from upland soils, porewater from bog and lagg zones, bog cores, upland soil, foliage and litter, as well as invertebrates for determining total-Hg, methylmercury, and stable Hg isotope compositions. Preliminary data on exported water over time show a large range of δ²⁰²Hg (-2.1 to -1.3 permil; n=13) but a small range of Δ¹⁹⁹Hg (-0.35 to -0.1 permil; n=13), with Δ¹⁹⁹Hg resembling the values of bog/foliage samples. Samples with more positive δ²⁰²Hg in the exported water were collected during the spring snow melt event, and these δ²⁰²Hg values are similar to the values of surface and subsurface runoff from the upland (i.e., -1.4 to -1.2 permil; n=3). Sample collection is still in progress for the remainder of 2015 and analyses for the remaining samples are in progress. Overall, our preliminary results suggest that the variation of Hg isotopes are large in this small upland-peatland watershed and such variation can potentially offer new insight into complex Hg cycling in these ecosystems.