Distribution of foraminifera and sediments, Peru-Chile trench area

Abstract

Eighteen trawl samples, one Petersen grab, and thirteen Phleger cores were collected between depths of 179 and 6250 m in the Peru-Chile Trench area off the west coast of South America. Sediments are mainly olive-green silt, clay and colloidal material; however, four cores contain significant amounts of either sand-sized foraminifera or shale fragments, and one of these cores is mainly white volcanic ash. Values for organic carbon and nitrogen are much higher in the bathyal than in the abyssal zone. Sediment grain sizes do not exihibit definitive trends with either depth or distance from shore.Calcium carbonate contents decrease sharply below 3500 m, reflecting reduced quantities of calcareous foraminifera in the Trench. Radiolarians are usually more than twice as abundant as foraminifera deeper than 1500 m. Foraminifera larger than 0·5 mm were concentrated in the trawl samples and are mainly arenaceous types below 1000 m. Among the smaller foraminifera, calcareous forms predominate down to 2000 m; at greater depths calcarenus-arenaceous ratios fluctuate extremely. Tests of planktonic foraminifera are most abundant in the bathyal zone.Bathymetric foraminiferal zonation is based upon upper limits of occurrence for both the larger live foraminifera from the trawls and the smaller foraminifera from the cores. Maximum size of the larger foraminifera is usually between 1 and 10 mm. The zonation is: TrawlsCores1. 179 M Valvulineria inflata Group2. 878 M Cibicides wuellerstorfi and796 M Epistominella pacifica smithi GroupReophax scorpiurus Groups1171 M Cyclammina cancellata Group1171 M Uvigerina peregrina dirupta Group4 1863 M Alveolophragmium subgloobosum and1932 M Eponides tumidulus-UvigerinaReophax nodulosus GroupsHispida Group5. 2489 M. Hormosina ovicula Group2489 M Nonion pompilioides Group6. 3149 M Planispirinoides bucculenta GroupM Stilonstomella antieea Group7. 3404 M Recurvoides turbinatus-BathysiphonGroupEstimates of the total volumes of material caught by each trawl range 2 to about 43 kg, dry wt. Species restricted to deep water appear to have evolved from depth-tolerant ancestors.