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Frequency distributions were determined for more than 250 species of benthic foraminifera from 121 core-top samples taken on the Nazca plate, in the Peru-Chile Trench, and on the Peru shelf and slope. According to their species content, the benthic faunas may be regarded as continental margin, plate-bathyal, or trench and plate-abyssal faunas.

About two-thirds of the species from the Peru Margin live in depths shallower than middle bathyal, for which there is no comparable environment on the plate. The displacement of these shallow-dwelling species downslope serves to differentiate the lower-bathyal continental-slope assemblages from those of the plate. Five margin assemblages were recognized on the basis of the shallowest occurrences of species and modes of their percentage distributions. The shallowest assemblages—outer shelf and upper bathyal—are characterized by species of Bolivina living under low-O2 conditions. Many of the species of these assemblages extend northward only as far as Central America. In contrast, assemblages in upper and lower middle-bathyal and lower-bathyal depths are more widespread along the west coast of the Americas; therefore, wider comparison of their distribution and wider potential use of them as paleobathymetric indices are permitted. One species, Bolivina costata, was found in all samples from the margin and in some samples from the eastern extremity of the plate; this suggests its potential use as a paleoecologic indicator of plate-boundary conditions.

The predominantly calcareous plate-bathyal populations occupy the sea floor between 2,600- and 4,100-m water depth. About one-fourth of the species of this group occur also in each of the other two groups. Q-mode factor analysis indicates the presence of three principal and two subordinate assemblages. As in other oceans, the deepest calcareous assemblage is dominated by Nuttallides umbonifera, which occupies a zone associated with the Antarctic Bottom Water Mass in which there is intensive solution of calcium carbonate. This zone of dissolution occurs between about 3,700 and 4,100 m in the study area. The Epistominella exigua Assemblage dominates the shallower parts of the plate between 2,600 and 3,700 m, except for the sea floor beneath the equatorial region of high productivity, where the Gyroidina turgida Assemblage predominates.

The trench and plate-abyssal faunas consist entirely of agglutinated species that occur deeper than 4,100-m water depth, the regional CCD (carbonate compensation depth). Half of the species are restricted to the abyss. Q-mode factor analysis indicates one principal and two subordinate assemblages. The Reophax dentaliniformis Assemblage occupies the Peru-Chile Trench, which is the principal path for the movement of Antarctic Bottom Water to the north. The distribution of the assemblage is also coincident with a high-productivity belt associated with the Peru Current. Two distinctive agglutinated species, Ammomarginulina sp. and Saccammina tubulata, are restricted to the trench.

Water-mass and substrate preferences of the benthic species may be used to interpret the paleoenvironments of sedimentary sequences in deep-sea cores as well as the origins of sedimentary rocks forming the continental margin. Because this is the first documentation of modern benthic foraminifera in the area, all pertinent species are illustrated.

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