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Geological Society, London, Special Publications; 1991; v. 58; p. 65-82;
DOI: 10.1144/GSL.SP.1991.058.01.05
© 1991 Geological Society of London

Modern Shelf Anoxia

Biofacial patterns in river-induced shelf anoxia

G. J. Van Der Zwaan & F. J. Jorissen

University of Utrecht, Department of Geology, Budapestlaan 4, 3508 TA Utrecht, The Netherlands

Three areas were studied to assess the effects of riverine input on the marine benthic ecosystem: the Adriatic Sea (with the Po river), the Orinoco-Paria shelf (with the Orinoco and Essequibo rivers) and the northern Gulf of Mexico (with the Mississippi and Rio Grande rivers). In all these areas, the interplay of river-discharge and subsequent transport by surface-water currents has resulted in the deposition of a zone with finegrained, organic-rich sediments parallel to the coast. The foraminiferal associations occupying these mud-belts indicate the occurrence of regular, widespread anoxic or dysoxic conditions at the sediment-water interface. Across the mud belt, benthic foraminifera display a very characteristic zonation, which is essentially the same in all areas studied. In the centre of the zone, low diversity and the presence of opportunistic species indicate a degree of stress. A model which describes the microhabitat occupation within these muddy zones is discussed in some detail. We conclude that the width of the shelf, together with the force of the long-shore current carrying the riverine products into the basin, are essential in determining the occurrence of dysoxic or anoxic conditions while the discharge volume of rivers is a subordinate factor. The chance of anoxia appears to be highest during periods of high sea level, leading to large shelf areas. A final conclusion is that benthic formainifera are excellent tools for determining the occurrence and degree of marine eutrophication induced by the input of polluted river waters.





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