Mercury in the Black Sea: New Insights From Measurements and Numerical Modeling

Abstract

Redox conditions and organic matter control marine methylmercury (MeHg) production. The Black Sea is the world's largest and deepest anoxic basin and is thus ideal to study Hg species along the extended redox gradient. Here we present new dissolved Hg and MeHg data from the 2013 GEOTRACES MEDBlack cruise (GN04_leg2) that we integrated into a numerical 1‐D model, to track the fate and dynamics of Hg and MeHg. Contrary to a previous study, our new data show highest MeHg concentrations in the permanently anoxic waters. Observed MeHg/Hg percentage (range 9–57%) in the anoxic waters is comparable to other subsurface maxima in oxic open‐ocean waters. With the modeling we tested for various Hg methylation and demethylation scenarios along the redox gradient. The results show that Hg methylation must occur in the anoxic waters. The model was then used to simulate the time evolution (1850–2050) of Hg species in the Black Sea. Our findings quantify (1) inputs and outputs of Hg T (~31 and ~28 kmol yr −1 ) and MeHg T (~5 and ~4 kmol yr −1 ) to the basin, (2) the extent of net demethylation occurring in oxic (~1 kmol yr −1 ) and suboxic water (~6 kmol yr −1 ), (3) and the net Hg methylation in the anoxic waters of the Black Sea (~11 kmol yr −1 ). The model was also used to estimate the amount of anthropogenic Hg (85–93%) in the Black Sea.

Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea

“…It is close to the particulate Hg:Fe spm fraction of 7 × 10 −5 mol mol −1 found in an Atlantic Ocean hypothermal vent plume (Bowman et al, ), which supports our interpretation. For the Black Sea, where only dissolved THg has been collected, no Hg II peaks above the dissolution zone similar to what we observe in the Baltic Sea have been reported (Lamborg et al, ; Rosati et al, ). This supports our hypothesis that the peaks above the Fe (III) dissolution zone in the Baltic Sea are caused by the aggregation of particulate, not dissolved, THg.…”

“…On the other hand, data available on MeHg concentrations from hypoxic and anoxic waters in coastal seas present contradictory profiles. Malcolm et al () did not find elevated MeHg concentrations in unfiltered hypoxic waters of the Arabian Sea, while, for the Black Sea, a dissolved MeHg peak (>1 pM) was reported in the hypoxic layer by Lamborg et al () but in the anoxic layer by Rosati et al (). In the Baltic Sea, elevated MeHg concentrations have been measured in both filtered (0.2‐μm filter) and unfiltered hypoxic water, but highest concentrations have been found in anoxic water (Kuss et al, ; Soerensen et al, ).…”

HR3DHG version 1: modeling the spatiotemporal dynamics of mercury in the Augusta Bay (southern Italy)

“…The investigation of the biogeochemical dynamics of Hg species in the marine environment addresses the need to accurately model sources and pathways of this priority contaminant within and among the different abiotic and biotic compartments of the aquatic ecosystem (Driscoll et al, 2013;Batrakova et al, 2014). Over the last few years some theo-retical studies have offered innovative tools to reproduce the mass balance and the dynamics of [Hg] in the marine environment by means of biogeochemical models based on interconnected zero-dimensional boxes representing water or sediment compartments: among these are the River MERLIN-Expo model (Ciffroy, 2015) and the WASP (Water Analysis Simulation Program) model (Melaku Canu et al, 2015;Canu and Rosati, 2017;Rosati et al, 2018). In particular, the River MERLIN-Expo model (Ciffroy, 2015) has been used to reproduce the spatiotemporal distribution of inorganic and organic contaminants in the 1D domain of rivers and to calculate the mass balance for each of them.…”

“…Although the model is able to describe many of the physical and chemical processes involved in fresh water and sediment, it specifically targets environments characterized by (i) nearly homogeneous water bodies and (ii) limited variations in landscape geometry. The WASP models have been used to simulate the Hg cycle within aquatic ecosystems characterized by wellmixed water layers and homogeneous sediment layers coupled through the boundary conditions at the water-sediment interface (Melaku Canu et al, 2015;Canu and Rosati, 2017;Rosati et al, 2018). In particular, a WASP model applied to a 1D domain and calibrated by using experimental data for dissolved Hg and MeHg allowed us to explore [Hg] dynamics in the Black Sea (Rosati et al, 2018).…”

“…The WASP models have been used to simulate the Hg cycle within aquatic ecosystems characterized by wellmixed water layers and homogeneous sediment layers coupled through the boundary conditions at the water-sediment interface (Melaku Canu et al, 2015;Canu and Rosati, 2017;Rosati et al, 2018). In particular, a WASP model applied to a 1D domain and calibrated by using experimental data for dissolved Hg and MeHg allowed us to explore [Hg] dynamics in the Black Sea (Rosati et al, 2018). Similarly, the box model approach has been adopted in a 2D configuration (Melaku Canu et al, 2015;Canu and Rosati, 2017) to calculate Hg mass balance in the coastal areas of the Marano-Grado lagoon (northern Italy), where heterogeneous spatial distributions of Hg species have been observed experimentally.…”