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Letters to Nature
Nature 338, 411 - 413 (30 March 1989); doi:10.1038/338411a0

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

J. W. Murray*, H. W. Jannasch, S. Honjo, R. F. Anderson, W. S. Reeburgh§, Z. Top, G. E. Friederich, L. A. Codispoti & E. Izdar£

* School of Oceanography, University of Washington, Seattle, Washington 98195, USA
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964, USA
§ Institute of Marine Sciences, University of Alaska, Fairbanks, Alaska 99701, USA
Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA
Monterey Bay Aquarium Research Institute, Pacific Grove, California 93950–4616, USA
£ Dokuz Eylul University, Institute of Marine Sciences and Technology, 35213 Izmir, Turkey

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.

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