On the rise of the upper boundary of the anoxic zone in the density field of the Black Sea in 1989-2000

This paper provides an analysis of the variability of the position of the u pper boundary of the Black Sea anoxic zone in the density field in the year s 1989-2000. The study is based on archived data of observations on the dis tribution and variability of hydrogen sulfide, ammonium, dissolved manganes e, and methane acquired in 20 expeditions of the research vessels of the Sh irshov Institute of Oceanology, Russian Academy of Sciences, in the northwe stern part of the Black Sea. The results obtained allow one to conclude tha t, during the period 1989-1998, the position of the upper boundary of the a noxic zone was steady; the conventional density values at the points of dep letion of hydrogen sulfide and ammonium were 16.16-16.20 and 15.90-16.00 c. u., respectively, which agrees with the data of other scientists. In the ye ars 1999-2000, a rise of the upper boundary of the anoxic zone occurred; it s new boundary stabilized itself at isopycnals of 16.11 c.u. for hydrogen s ulfide and 15.80 c.u. for ammonium. Thus, magnitude of the rise was approxi mately 0.05-0.10 c.u. (about 5-10 m). Over both of the periods distinguishe d, the calculated vertical gradient values were constant and comprised 0.60 muM/m for hydrogen sulfide, 0.25 muM/m for ammonium, 0.15 muM/m for dissol ved manganese, and 0.025 muM/m for methane. This phenomenon may have been c aused by climatic or anthropogenic factors. The warm winters of the years 1 998-2000 might result in a distortion of the balance between the fresh wate rs of a riverine origin and the saline waters supplied via the Bosporus, as well as in a disturbance in the formation of the oxygen-rich cold intermed iate layer.