FORAMINIFERA ENCRUSTING PHOSPHORITIC HARDGROUNDS OF THE PERUVIAN UPWELLING ZONE - TAXONOMY, GEOCHEMISTRY, AND DISTRIBUTION

Phosphorite hardgrounds recovered in 1992 from the Peruvian margin at 120 and 13,5 degrees S during submersible operations were encrusted wi th foraminifera on their upper surfaces, The foraminiferal-encrusted p hosphorites occur in the upper and lower parts of the oxygen-minimum z one, where dissolved oxygen concentrations increase slightly and where bottom current velocities tend to wane, at 162 m and between 465 and 620 m water depth, under high productivity surface waters. One new gen us, Ammodiscellites, and two new species, A. prolixus and Hemisphaeram mina celata, are described. The foraminifera are mostly permanently af fixed agglutinated encrusters with hemispherical and tubular or serial tests which lack walls on the surface of attachment. Calcareous perfo rate or agglutinated adherent species with complete tests that are cem ented to the substrate around their apertures compose <1% of the assem blages. Maximum densities of 96.6 specimens per cm(2) were recorded in the assemblages, which exhibit strong dominance by A. prolixus from 4 65 to 517 m and by Placopsilina bradyi with Tholosina bulla from 538 t o 620 m water depth. Energy dispersive spectrometer (EDS) analyses of the two most abundant species show the tests to be constructed primari ly of siliciclastic grains, with illitic or other mixed-layer clays an important component of the finely textured Ammodiscellites. These com positions, which are in marked contrast to that of the carbonate fluor apatite substrate, suggest that the foraminifera are not actively invo lved in phosphorite precipitation.