The Baltic Sea is the largest anthropogenically induced hypoxic area on Earth. Hypoxic settings, also referred to as Dead Zones, are defined by bottom water dissolved oxygen (O2) concentrations less than 2 mg L–1. (1,2) The hydrography of the Baltic Sea makes it susceptible to hypoxia; North Atlantic seawater must pass two shallow sills before entering the semi-isolated basin. (3,4) Baltic seawater is brackish as a result, and has a near-permanent salt-gradient, or halocline, that prevents efficient mixing of surface and deep waters. Human activities have compounded these hydrographic effects since at least ∼1950. (5) Warming surface waters in the Baltic Sea negatively impact water column mixing and O2 solubility. (6) And high nutrient loading drives eutrophication, in turn promoting large amounts of bottom water O2 consumption during microbial respiration. (7) Deep basins are the locus of hypoxia in the central Baltic, most prominently those near the Islands of Bornholm, Fårö, and Gotland, as well as the Landsort Deep. Conditions are so reducing in some of these “deeps” that bottom waters are anoxic and rich in hydrogen sulfide (H2S), conditions referred to as “euxinic”. (8)

Our focus in this investigation is the post-transition metal thallium (Tl). Thallium has a high toxicity: it is the most toxic metal for mammals, with an estimated minimal lethal dose for humans of 10 mg/kg. (9) The bioavailability of Tl in nature therefore has important implications for human health. (10) Some anthropogenic activities are strong point sources of Tl. An estimated ∼2000 tons of Tl are released annually as atmospheric emissions during coal combustion, cement production, and pyrite roasting. (11,12) Due to its low melting point, Tl is volatilized during these high-temperature processes and thereafter, upon cooling, condenses on the surface of ash particles. (13) Ash produced during these anthropogenic processes can be highly enriched in Tl, by up to a factor of 10 compared to the raw materials. (12,14) Thallium can accumulate to very high abundances in environments near anthropogenic point sources, greatly increasing the risk of Tl poisoning to neighboring communities. (15)