Unexpected changes in the oxic/anoxic interface in the Black Sea

THE Black Sea is the largest anoxic marine basin in the world today1. Below the layer of oxygenated surface water, hydrogen sulphide builds up to concentrations as high as 425 μM in the deep water down to a maximum depth of 2,200 m (ref. 2). The hydrographic regime is characterized by low-salinity surface water of river origin overlying high-salinity deep water of Mediterranean origin1,3. A steep pycnocline, centred at about 50 m is the primary physical barrier to mixing and is the origin of the stability of the anoxic (oxygen/hydrogen sulphide) interface. Here we report new observations, however, that indicate dramatic changes in the oceanographic characteristics of the anoxic interface of the Black Sea over decadal or shorter timescales. The anoxic, sulphide-containing interface has moved up in the water column since the last US cruises in 1969 and 1975. In addition, a suboxic zone overlays the sulphide-containing deep water. The expected overlap of oxygen and sulphide was not present. We believe that these observations result from horizontal mixing or flushing events that inject denser, saltier water into the relevant part of the water column. It is possible that man-made reduction in freshwater inflow into the Black Sea could cause these changes, although natural variability cannot be discounted. © 1989 Nature Publishing Group

ARCHITECTURE OF LATE OROGENIC QUATERNARY BASINS IN NORTHEASTERN MEDITERRANEAN-SEA

In the northeastern Mediterranean Sea, Pliocene to Quaternary depocentres have formed in extensional basins bounded by splays of the East Anatolian Transform Fault. This tectonic regime is superimposed on a Miocene and older back-arc environment, that experienced late Miocene compression along the Misis-Kyrenia thrust, which now lies in the middle of the extensional zone. The thrust zone is now represented by a narrow horst that appears to be bounded by strike-slip faults. Pliocene-Quaternary extension took place on listric fault fans that are orthogonal to the bounding transform splays and sole at a Messinian evaporite horizon, and on some deeper-soling listric faults parallel to and near the bounding faults. The rapid extension has resulted in progressive landward migration of paleoshorelines and low depositional gradients. Glacio-eustatic fluctuations in shoreline positions strongly influenced sediment distribution. Most sediment dispersion was from deltaic plumes, with turbidites of minor significance. Depocentres landward of the maximum seaward extent of paleoshorelines were formed almost entirely by tectonic subsidence. Minor deep-water depocentres, controlled by halokinesis, accumulated mud turbidites during extreme low-stands of sea-level

Geochemistry and sedimentology of shelf and upper slope sediments of the south-central Black Sea

Eighty-five surface samples from oxic, suboxic and anoxic sites of the south-central Black Sea shelf consist predominantly of clayey silts and silty clays, reflecting the moderate-to-high energy, wave-dominated hydrological regime. Grain size data and net sediment transport patterns suggest that the western part of this shelf is an area of cast-directed active sediment transport. With the, exception of localized nearshore zones, the eastern shelf is by contrast an area of major sediment deposition. The sediment transport vectors are correlated with patterns of surface water circulation and storm-generated longshore currents

Unexpected changes in the oxic/anoxic interface in the Black Sea

THE Black Sea is the largest anoxic marine basin in the world today1. Below the layer of oxygenated surface water, hydrogen sulphide builds up to concentrations as high as 425 μM in the deep water down to a maximum depth of 2,200 m (ref. 2). The hydrographic regime is characterized by low-salinity surface water of river origin overlying high-salinity deep water of Mediterranean origin1,3. A steep pycnocline, centred at about 50 m is the primary physical barrier to mixing and is the origin of the stability of the anoxic (oxygen/hydrogen sulphide) interface. Here we report new observations, however, that indicate dramatic changes in the oceanographic characteristics of the anoxic interface of the Black Sea over decadal or shorter timescales. The anoxic, sulphide-containing interface has moved up in the water column since the last US cruises in 1969 and 1975. In addition, a suboxic zone overlays the sulphide-containing deep water. The expected overlap of oxygen and sulphide was not present. We believe that these observations result from horizontal mixing or flushing events that inject denser, saltier water into the relevant part of the water column. It is possible that man-made reduction in freshwater inflow into the Black Sea could cause these changes, although natural variability cannot be discounted. © 1989 Nature Publishing Group