Barium (Ba) is an important proxy element yet its cycling in the ocean is not fully understood. More than 60 years ago, its interaction with the biological system was associated to the biologically mediated precipitation of barite (BaSO4) in the water column. Several mechanisms for barite precipitation in seawater that is undersaturated relative to barite have been suggested, including saturation in micro-environments and direct and indirect microbial interactions. Recently, a wealth of high-resolution water column dissolved Ba profiles has become available, mostly showing moderate depletions in surface water - even in the oligotrophic Mediterranean - and enrichments in deep water. These suggest removal of Ba from surface water, partial release in deep water and potential storage in the sediment. However, quantitatively there are large differences in hitherto reported contributions, complicating an evaluation of sources and sinks. Potential sources for Ba are from rivers and groundwater systems and from dissolution of dust, as well as from mid-ocean ridge hydrothermal systems.

An advanced and preferably integrated level of knowledge and understanding is therefore required and will contribute to an improved comprehension of Ba cycling and the utility of Ba proxies for reconstructing past ocean export productivity, circulation, and preservation. In particular, the detection of Ba isotope fractionation has been indicated as a potential tool to distinguish prevailing processes. The recent capability to detect such isotopic differences together with the now available large set of dissolved Ba data should provide new insights in the Ba cycling and processes.

We welcome contributions on processes and interactions related to Ba and its isotopes, from dissolved Ba in riverine, groundwater, estuarine, hydrothermal, and oceanic shallow/deep water systems to particulate/dissolved natural or experimental exchange/dissolution processes including microbiological interactions.

• EGU: European Geosciences Union -