Recent foraminiferal assemblages near Ponza Island (Central Tyrrhenian Sea, Italy)

Ponza Island is located in the western sector of the Pontine volcanic Archipelago, on a structural high of the continental Tyrrhenian shelf. This preliminary study is comprised in a broad interdisciplinary project, aimed to know and describe the sea bottom of the Pontine Archipelago. On the whole, 22 grab samples, collected for micropalaeontological analyses East to Ponza during the cruise of spring 2001, have been studied. Sampling stations range from 20 to 380 m water depth and are located on four transects, having the NW/SE direction. Among these, one transect comprises 12 stations, whereas the others are constituted by 2 to 4 stations. A total of 300 benthic foraminifera with well-preserved tests from each sample were separated and classified, from >125 micron size fraction. All benthic foraminifera with broken and old tests (tests with yellow/brown walls and filled by glauconite) were interpreted as reworked and/or displaced. These tests were collected and counted; planktonic foraminifera were contemporaneously counted and identified. Moreover, all the other bioclastic components of sediment (bivalves, bryozoans, coralline algae, gastropods, ostracods, pteropods, spines of echinoids and spicules of sponges) were counted in each aliquot of samples, in order to better describe the bioclastic facies. In absence of significant sources of terrestrial sediments, organic fraction is generally prevalent; benthic foraminifera always dominate bioclastic component, except in two samples. A total of 199 species, belonging to 91 genera, have been recognised. The data set, containing relative abundance of 16 common taxa of benthic foraminifera, was used to perform the multivariate analysis (Hierarchical Cluster Analysis and Principal Cluster Analysis). The output of Q-mode HCA singles out three main clusters, corresponding to three distinct foraminiferal assemblages: 1) Rosalina bradyi and Asterigerinata mamilla assemblage (depth: 20-87 m, inner continental shelf with Posidonia oceanica meadows), with Lobatula lobatula, Spirillina vivipara, Tretomphalus concinnus and miliolids. This biofacies corresponds to the infralittoral-upper circalittoral zone and is dominated by benthic foraminiferal tests with, sometimes, rhodoliths. High values of broken tests may be related to action of wave motion. P/(P+B) ratio is very low; 2) Cassidulina carinata assemblage (depth: 79-202 m, outer continental shelf and upper slope), comprising Bolivina difformis and Textularia bocki. This biofacies corresponds to the circalittoral zone and presents high percentage of old glauconitic of shallow water taxa. They are probably related to sea level stillstand conditions during Late Quaternary. The P/(P+B) ratio increases and, among planktonic species, Globigerinoides ruber, Globigerina bulloides, Globorotalia inflata and G. truncatulinoides are present; 3) Uvigerina mediterranea assemblage (depth: 250 380 m, continental slope), with Bulimina marginata, Cassidulina crassa, Hyalinea balthica and Melonis pompilioides. This biofacies corresponds to the upper epibathyal zone, and is characterised by bioclastic sediment with abundant benthic and planktonic foraminifera, pteropods and spicules of sponges. P/(P+B) ratio shows the highest values.