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Geological Society, London, Special Publications; 1992; v. 64; p. 151-166;
DOI: 10.1144/GSL.SP.1992.064.01.10
© 1992 Geological Society of London

Modern Upwelling Systems and Palaeo-Upwelling Criteria

Variations in the benthic foraminiferal fauna of the Arabian Sea: a response to changes in upwelling intensity?

J. O. R. Hermelin

Deep Sea Geology Division, University of Stockholm, S-106 91 Stockholm, Sweden

Results from ODP Leg 117 show that abundance and species composition of the benthic foraminiferal fauna in the northwestern Arabian Sea vary extensively through time. Studies of the Recent faunas from the region show that the distribution of benthic foraminifers is strongly correlated with the depositional environments present, of which some are associated with the OMZ (oxygen minimum zone). The OMZ is a result of strong seasonal upwelling caused by monsoonal winds and the Recent benthic foraminifers that thrive in this specific environment form a characteristic assemblage. Assuming that this relation between this benthic foraminiferal assemblage and OMZ/upwelling conditions were present in the geological past the relative abundance of the species forming that assemblage can be used to reconstruct past changes in the intensity of the monsoonal upwelling and changes in the water circulation pattern in this area.

A deepening of the lower boundary of the OMZ occurred between 6.5 and 3.0 Ma with a maximum at 5.0 Ma (= more intense upwelling ?). After 5.0 Ma, mixing of more oxygenated outflow water originating in the Red Sea weakened the severity of the OMZ in the interval 200 to 500 m in this part of the Arabian Sea. This mixing became more intense with time, as reflected in the benthic foraminiferal fauna, and today there is a pronounced intercalation of oxygenated water at this water depth and a low percentage of the assemblage associated with the OMZ.





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