Widespread occurrence of high radium concentrations in bottom waters of the Eckernförde and Kiel bays (Western Baltic Sea): Are these related to submarine groundwater discharge?

The radium isotopes (²²⁴Ra, t_½ = 3.7 d;²²³Ra, t_½ =11.5 d; ²²⁸Ra, (t_½ = 5.7 y); ²²⁶Ra, t_½ =1600 y) are well established tracers for the detection and quantification of submarine groundwater discharge (SGD). In the Eckernförde Bay (Western Baltic Sea) SGD associated with pock marks have been observed at many locations on the seabed (~ 20-25m water depths). In order to investigate the spatial and temporal SGD variability we repeatedly measured between 2016 and 2020 the radium (²²⁴Ra, ²²³Ra, ²²⁸Ra, ²²⁶Ra) distribution in the water column of the Eckernförde Bay and the adjacent Kiel Bay. In general, the water-column radium profiles are characterized by relatively low radium concentrations in the upper water column (~ <15m water depth, salinity ~ 12-18) and significantly higher ones in deep waters (~ 20-25m water depth; salinity ~ 21-25). High radium occurs also in areas far off the coast where pock marks have previously not been reported. Monthly/bi-monthly measurements at the time-series station Boknis Eck (Eckernförde Bay) revealed that this high radium occurs only between spring and autumn, a period, where bottom waters have low or negligible oxygen content. This observation may indicate that processes other than SGD may contribute to seasonal changes in deep water radium. In the presentation possible other radium sources like e.g., diagenetic radium supply from anoxic sediments, sediment bioturbation and resuspension, advection of deep waters from the North Sea, are discussed in order to understand to what extent the radium distribution in the western Baltic Sea can still be interpreted as a tracer of SGD.