THE DISTRIBUTION OF MAN-MADE AND NATURALLY PRODUCED HALOCARBONS IN A DOUBLE-LAYER FLOW STRAIT SYSTEM

The Bosphorus Strait, which connects the Black Sea and Mediterranean S ea via the Marmara Sea, is characterised by two distinct water masses. The upper layer consists of low density Black Sea water (sigma-t 10-1 1) flowing southward, and it is underlain by high density water (sigma -t > 28) of Mediterranean origin flowing northward. The sharp density gradient between the two layers is due to the difference in salinities . Here we report measurements on a suite of low molecular weight haloc arbons together with basic hydrographic parameters in the strait. Conc entrations of the transient tracers chlorofluorocarbons CFC-11 (CFCl3, ), CFC-113 (CCl2FCClF2) and carbon tetrachloride (CCl4) were highest i n the Cold Intermediate Black Sea Water (CIBSW), which is formed in th e Black Sea during winter. This layer disappeared within the contracti on region of the Bosphorus where a hydraulic jump occurs. The Marmara Sea deep water at the entrance to the Bosphorus Strait carried low amo unts of CFCs indicating an equilibrium with the atmosphere 6-11 years earlier. Varying amounts of other halocarbons such as chloroform (CHCl 3), dibromomethane (CH2Br2), methyl iodide (CH3I) and chloro-iodometha ne (CH2ClI) could be used for the identification of water masses of di fferent origin. Around the Bosphorus-Marmara Junction (BMJ) in the Ist anbul area, the upper water layer contained elevated levels of most of the halocarbons. This is attributed not only to contamination but als o to natural production in the productive eutrophic waters of the regi on. Methyl iodide and chloro-iodomethane showed elevated concentration s of 0.7-0.9 ng/l and 0.4-0.6 ng/l, respectively, in the upper water l ayer of the BMJ region, and also in near-bottom water at the northern exit of Bosphorus (0.2 and 0.1 ng/l, respectively). Chloroform (23 ng/ l) and dibromomethane (10 ng/l)were also found within the pycnocline i n the BMJ region and could be traced in a plume stretching northward a long the pycnocline. (C) 1996 Elsevier Science Ltd.