Thallium dynamics in the southern North Sea

In open ocean waters thallium (Tl) belongs to the group of conservative elements, even though deviations from this trend have been observed in NW German coastal waters. Here, we report on tidal, seasonal and spatial dynamics of Tl along with Mo and Mn in the water column of a backbarrier tidal flat close to the island of Spiekeroog, the Jade system (Inner Jade and Jade Bay) and the adjacent offshore region. Dissolved thallium (Tl-diss) displays strong tidal and seasonal variations (similar to 25-60 pM) unrelated to salinity. In all study areas, Tl-diss clearly deviates from conservative behavior. In general, Tl-diss is low during low tide (with a loss of up to 50%) and inversely related to Mn-diss, except in summer. The tidal Tl variations as well as the loss of Tl in the water column may be due to Tl removal from pore waters in reducing sediments and drainage of Tl-free but Mn-rich pore waters into the water column during low tide. The negative Tl anomaly can be traced offshore for more than 40 km to the island of Helgoland. The redox chemistry of Tl is not well studied, and Tl removal from pore waters was previously suggested to only occur under anoxic/sulfidic conditions. By contrast, our preliminary pore water results suggest that Tl could be removed already under slightly reducing (suboxic) conditions, likely along with microbially induced Mn reduction in the sediments. Therefore, this study supports the biological involvement in the aqueous cycling of Tl. We propose the use of Tl-diss next to Mn-diss as valuable indicator of suboxic or anoxic pore water discharge to the coastal realm. (C) 2018 Elsevier Ltd. All rights reserved.