Seasonal changes of 234Th scavenging in surface water across the western Black Sea: an implication of the cyclonic circulation patterns

Two seasonal surveys of 234Th activity in surface water were carried out in November 1998 and June 1999 along the transect Sevastopol (Ukraine) – Istanbul (Turkey) crossing the entire western Black Sea between the SW Crimean Peninsula and the Bosphorus Strait entry. A steady-state model of 234Th/238U equilibrium in seawater was used to calculate the net removal flux of 234Th by particles. The results obtained from central stations of the transect were in good agreement with those observed seasonally in 1992–1994 within the western cyclonic gyre of the abyssal Black Sea, showing that in June 1999 the sampling was conducted during the peak summer bloom of coccolithophorids, while in November 1998 the samples were collected three weeks after the autumn phytoplankton bloom. The 234Th scavenging rates and the total mass fluxes were thus much higher in the June sampling period than in November at the stations located in the western cyclonic gyre and at the northern margin of the abyssal Black Sea basin. In contrast, the considerable seasonal differences were not found at the location near the Bosphorus Strait entry, suggesting a larger lateral input of particulate matter transported to this area from the western and northwestern shelf due to cyclonic circulation of the surface waters.