Environmental Engineering Reference
In-Depth Information
BIOGEOCHEMISTRY OF THE BLACK SEA
ANOXIC ZONE WITH A REFERENCE TO
SULPHUR CYCLE
Lev N. Neretin
1,2
, Igor I. Volkov
3
, Alexander G. Rozanov
3
, Tatyana P. Demidova
3
,
Anastasiya S. Falina
3
1
Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany
2
Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany
3
P.P.Shirshov Institute of Oceanology of Russian Academy of Sciences, 37 Nakhimovsky prosp.,
117997 Moscow, Russia
Abstract
The Black Sea is the largest anoxic basin on earth. Hydrogen sulphide inventory
in the sea is about 4,600 Tg. This review discusses the evolution and contem-
porary physical and chemical characteristics of the Black Sea anoxic zone. We
present hydrogen sulphide concentrations at different depths in the sea and dis-
cuss mechanisms of physical mixing in the anoxic interior. Special emphasis
is given to recently discovered bottom convective layer located at depths be-
low 1700-1750 m and concentrations of dissolved sulphide and other chemical
species in this zone. The mechanism of double diffusion driven by geothermal
heat flux is the main mixing process there. Mesoscale physical dynamics asso-
ciated with the Main Rim Current has important imlications for the dissolved
sulphide spatial distribution and, in general, for ventilation of the anoxic interior.
We suggest that, together with the Bosporus influx, the nearshore convergence
zone supplies dissolved oxygen into the anoxic zone and leads to the formation
of inorganic sulphur intermediates. A detailed analysis of total alkalinity and its
components in the Black Sea and other euxinic basins shows unambigously that
sulfate reduction is the main process of organic matter anaerobic mineraliza-
tion in the water column. This result is confirmed by the sulphur budget, whose
components are discussed at the end.
Keywords:
Black Sea, anoxic zone, evolution, sulphur cycling, alkalinity
1. HISTORY OF THE BLACK SEA ANOXIA
During the past three million years, the Black Sea experienced at least
eight marine flooding events with the last Pleistocene/Holocene transition of
the highest magnitude [78]. During the Last Glacial Maximum, also called
Neoeuxinian epoch, which lasted from ca. 17 to 11 and possibly 9 ka BP (Before
Present) [87], the Black Sea was a freshwater lake with a minimum water levels
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