226Ra in the Black Sea and Sea of Marmara

Abstract

Water column distributions of 226Ra were determined at stations in the Sea of Marmara and the Black Sea as part of the 1988 Joint U.S.—Turkish Black Sea Expedition. Black Sea surface water 226Ra concentrations were a factor of three to four lower than measurements made 20 years earlier. The most likely cause is increased removal of 226Ra and Ba [35] due to increased surface biological activity; a secondary effect is decreased fluvial discharge and related dimunition of inputs by desorption from fluvial suspended sediments. The amount of 226Ra missing from the surface waters of the Black Sea over this period is accounted for in the high-porosity surficial “fluff” sediment layer. Throughout the Black Sea, depth profiles of 226Ra exhibited pronounced maxima of approximately 25 dpm/100 L at about σ θ = 16.2–16.3 , in the vicinity of a bacterial maximum, but slightly shallower than the total dissolved Mn and Fe maxima ( σ θ = 16.4–16.5 ) reported by Lewis and Landing [38]. While the 226Ra maximum may, in part, be linked to the cycling of Mn and Fe oxyhydroxides near the O 2H 2S interface, its distribution appears to be more plausibly explained as a result of the microbial breakdown of particulate organic matter and the subsequent release and partial dissolution of associated barite in this region. A simple steady-state two-☐ model has been used to obtain a semiquantitative understanding of the behavior of 226Ra in the Black Sea. By incorporating reasonable estimates for the input and removal of 226Ra in the Black Sea, an excellent agreement between predicted and observed (1988) 226Ra concentrations was achieved. The model suggests that the dominant variables controlling the distribution of 226Ra in the Black Sea are riverine input and cycling with Ba.