Benthic foraminifera as proxies of organic matter flux and bottom water oxygenation? A case history from the northern Arabian Sea

Benthic foraminifera have been quantified in two sediment cores from a topographic high (Murray Ridge) in the northern Arabian Sea. One core is from a station within the present-day Oxygen Minimum Zone (OMZ), the other from a station below the OMZ. Changes in the intensity of the OMZ, linked to monsoonal climate variability, had a strong impact on benthic foraminiferal composition and accumulation rates. At the shallower OMZ station (920 m) an intensification of the OMZ resulted in lower diversities and high dominances of low oxygen tolerant species. The deeper station (1470 m) was far less affected, though at least once during the last 225 000 years the benthic faunal composition and accumulation rates show evidence for a deepening of the base of the OMZ to a level close to or even below 1470 m. We suggest that percentages of miliolids can be used for rapid reconstruction of periods of increased ventilation in the northern Arabian Sea and are a promising proxy for tracing changes in bottom and pore water oxygenation in general. OMZ conditions are characterised by high abundances of deep infaunal species and increased burial efficiency of organic matter. The proportion of deep infaunal species appears to be a measure for the anoxic burial efficiency of organic matter and not a direct measure for the arriving flux at the seabed. Results, furthermore, show that benthic foraminiferal accumulation rates cannot be used as a proxy for surface water productivity under the prevalence of severe dysoxia. (C) 2000 Elsevier Science B.V. All rights reserved.