PROGRESSIVE VENTILATION OF THE OCEANS - POTENTIAL FOR RETURN TO ANOXIC CONDITIONS IN THE POST-PALEOZOIC

Abstract

After the ventilation of the residual anoxic layer in the late Paleozoic (Berry and Wilde~l978) a return to ephemeral anoxic conditions in the ocean is suggested by anoxic sediments found in the Hesozoic cores of the deep~sea drilling program (Schlanger and Jenkyns 1977, and Theide andVan Andel 1977) .A preliminary physi~ cal oceanographic model is presented to explain the development of oxygen depleted layers in mid~waters below the surface wind~mixed layer during non~glac:i.alclimates.The model shows the range of temperature, salinity and density values for hypothetical water masses for two climatically related oceanographic situations: Case A where bottom waters are formed at mid-latitudes at the surface salinity maxima, and Case B where bottom waters are produced at high latitudes but not by sea-ice formation as in the modern ocean.1~e hypothetical water masses are characterized by examples from the modern ocean and extrapolation to non-glacial times is made by eliminating water mases produced by or influenced by sea-ice formation in modern glacial times.The state of oxidation is made by plot the model water masses on a oxygen saturation diagram and comparing the relative oxygen capacity with modern conditions of zonal organic productivity.The model indicates for Case A (high latitude temperatures above 5C) two oxygen, depleted layers in the equatorial regions ( 1) from about 200m to the depth of completed oxidation of surface material separated by an oxygenated zone to (2) a deep depleted zone along the base of the pycnocline at 2900M. The deep depleted zone extend along the Case A pycnocline polarward toward the high latitude productivity maximum.For case B with a pycnocline at about 1500m the deep anoxic layer is not sustained.Considerations of density only, suggest that neutral stratification and the potential for overturn is enhanced for climates transitional between Case A and Case B where the density contrast between major water masses formed at high latitudes and mid~ latitudes is minimal or non-existent.