The distribution of man-made and naturally produced halocarbons in a double layer flow strait system

Abstract

The Bosphorus Strait, which connects the Black Sea and Mediterranean Sea via the Marmara Sea, is characterised by two distinct water masses. The upper layer consists of low density Black Sea water (sigma-t 10–11) 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 halocarbons together with basic hydrographic parameters in the strait. Concentrations of the transient tracers chlorofluorocarbons CFC-11 (CFCl 3), CFC-113 (CCl 2FCCIF 2) and carbon tetrachloride (CCl 4) were highest in the Cold Intermediate Black Sea Water (CIBSW), which is formed in the Black Sea during winter. This layer disappeared within the contraction region of the Bosphorus where a hydraulic jump occurs. The Marmara Sea deep water at the entrance to the Bosphorus Strait carried low amounts of CFCs indicating an equilibrium with the atmosphere 6–11 years earlier. Varying amounts of other halocarbons such as chloroform (CHCl 3), dibromomethane (CH 2Br 2), methyl iodide (CH 3I) and chloro-iodomethane (CH 2ClI) could be used for the identification of water masses of different origin. Around the Bosphorus-Marmara Junction (BMJ) in the Istanbul area, the upper water layer contained elevated levels of most of the halocarbons. This is attributed not only to contamination but also to natural production in the productive eutrophic waters of the region. Methyl iodide and chloro-iodomethane showed elevated concentrations of 0.7–0.9 ng/l and 0.4–0.6 ng/l, respectively, in the upper water layer 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 in the BMJ region and could be traced in a plume stretching northward along the pycnocline.