Assessing the marine geological cycle of cobalt from its interactions with particles in the Black Sea

The Black Seas is the world’s largest anoxic marine basin. Being extremely stable, the water column of this system is an ideal site to study the geochemical mechanisms affecting trace elements behavior in seawater across different oxygenation states, from the surface oxygenated layer to the deep euxinic waters. Understanding these processes in the modern Black Sea is also a pathway to assess the evolution of the ocean composition throughout geological timescales especially during the transition between the Proterozoic (euxinic) and the modern (oxic) ocean. Furthermore in the context of the spreading and intensification of oxygen deficient environments in coastal areas due to anthropogenic forcing, the study of heavy metals and key trace elements such as Co in the natural anoxic marine system of the Black Sea can provide insights into future environmental changes.

Here we report field measurements along the GEOTRACES-A04N section for cobalt (dissolved: dCo; particulate: pCo) and for others redox-sensitive trace elements (particulate Mn, particulate Fe). High surface dCo concentrations (> 500 pM) were measured probably induced by coastal inputs combined with low ventilation rate. Below the oxygenated waters, competitive redox-processes combined with a strong interaction with particles governed distribution of dCo. In the suboxic waters, the dCo concentrations dramatically decreased (as low as 28 pM) resulting from adsorption onto manganese oxides (MnOx). In the upper sulfidic layer, the reduction of MnOx released dCo leading to the highest dCo concentrations never encountered before in modern marine systems (up to 6.6 nM). In deeper waters, dCo sharply decreased with depth, likely due to sorption of dCo onto Fe and/or sulfide complexes and their subsequent precipitation in these sulfidic waters. This vertical gradient is then combined with complementary geochemical modeling in order to better figure how the cobalt cycle could have evolved across the ocean’s history.