The significance of iron-stained foraminifera off SE Trinidad, West Indies, western central Atlantic Ocean

Abstract

AbstractEleven samples of seafloor sediment were taken from water depths of 78–90 m within the Savonette Field, off SE Trinidad, western Atlantic Ocean. This surface sediment is relict, having been deposited during an early Holocene transgression. The samples yielded much iron-stained quartz and a rich assemblage of dead foraminifera, of which 75% of planktonic foraminifera were stained with iron, as was 66.5% of the calcareous benthonic foraminiferal assemblage. The fauna, both iron-stained and unstained, was dominated byCibicidoidesex. gr.pseudoungerianus, and is concluded, despite the proximity of the Orinoco Delta, to be equivalent to a relictCibicidoidesbiofacies in carbonate-rich areas of the Gulf of Mexico. Staining was by limonite and hematite. Differing percentages of calcareous benthonic species had been stained with iron, ANOVA revealing three groups of species within which the mean percentage of iron-stained specimens per sample did not differ: (a)Globocassidulina subglobosaandHanzawaia concentrica; (b)Amphistegina gibbosa, Cassidulina norcrossi australis Cibicioidesex. gr.pseudoungerianus,C. io,Elphidium translucensandQuinqueloculina lamarckiana; and (c)Eponides antillarumandE. repandus. It is concluded that species differ in their susceptibility to iron staining, and that planktonic foraminifera are more susceptible than most benthonic species. Although waters off northern South America are turbid and the photic zone only ~25 m deep, the relict assemblage contained 8.4% algal symbiont-bearing foraminifera (especiallyA. gibbosaandE. translucens) that would be limited to the photic zone. These are thought to have lived at a time early in the Holocene transgression when sequestration of sediment within the Orinoco delta rendered the water sufficiently clear for viable populations of symbiont-bearing foraminifera. Should iron-stained foraminifera prove to be restricted to transgressive systems tracts, this would make them a useful sequence stratigraphic tool.