Distribution of Foraminifera and Sediments, Peru-Chile Trench Area

ABSTRACT Nineteen trawl samples and 13 trigger cores were collected between depths of 179 and 6,250 m. in the Peru-Chile trench area off the western coast of South America. Sediments are mainly olive-green silt, clay, and colloidal material; however, four cores contain significant amounts of either sand-size 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 exhibit definitive trends with either water depth or distance from shore. Calcium carbonate content decreases sharply below 3,500 m., reflecting reduced quantities of calcareous Foraminifera in the trench. Deeper than 1,500 m., radiolarians are commonly more than twice as abundant as Foraminifera. Foraminifera larger than 0.5 mm. were concentrated in the trawl samples and below 1,000 m. are dominantly arenaceous. Among smaller Foraminifera, calcareous forms predominate down to 2,000 m.; at greater depths calcareous- arenaceous ratios fluctuate greatly. Planktonic foraminiferal tests 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 usually is between 1–10 mm. The zonation is: TRAWLSMeters 179Valvulineria infiala Group878Cibicides wuellerstorfi and Reophax scorpiurus Groups1,171Cyclammina cartellata Group1,863Alveolophragmium subglobomm and Reophax nodulosus Groups2,489Hormosina ovicula Group3,149Plantspirinoides bucculenta Group3,404Recurvoides turbinatus-Bathysiphon GroupCORESMeters 796Epistominella pacifica smillii Group1,171Bulitnina rostrata Group1,932Eponides tumiduhis Group2,498Nonion pompilioides Group3,257Stilostomella antillea Group Estimates of the total volumes of material caught by each trawl range from 2 to about 43 kg., dry weight.