Paleoenvironmental bottom water conditions in the early Zanclean of the Capo Rossello area (Agrigento, Sicily)

The Pliocene “Trubi” Formation outcropping in Southern Sicily, has been so far the object of numerous researches only on composite section. To obtain a continuous succession of unweathered sediments considered more suitable for paleomagnetic studied and cyclostratigraphical analyses, a continuous core drilling has been performed in the area East of Capo Rossello, between Punta di Maiata and Punta Grande. The bore-hole crossed the basal part of the Monte Narbone Formation, all the “Trubi” Formation and about 1 m of the underlying Messinian “Arenazzolo” Formation with a recovery of 99%. The complete drilling reached 155.26 m below ground surface. The present paper deals with the paleoecological bottom water changes during the Zanclean from M Pl 1 to the lower part of M Pl 3 Biozones by means of the detailed quantitative analysis of benthic foraminifers and ostracods. The assemblages are typical of lower epibathyal zone all along the succession with benthic foraminifers that document a depth-range from 600 to 800 m. The two faunal groups pointed out different colonization patterns. The benthic foraminifers show a quick re-colonization at the base of the succession, almost completed in the lower part of M Pl 2 Biozone, whereas the ostracods show a slow fluctuating re-colonizzation along M Pl 1 and M Pl 3 Biozone and a rapid increase only a few meters above the M Pl 2 – M Pl 3 boundary. Moreover some principl bioevents have been identified. Above the initial re-colonization at the base of M Pl 1 Biozone, the assemblages change rapidally in the upper part of the same biozone. This interval is characterized by the “Uvigerina pygmaea-peregrina Event”, among the benthic foraminifers, and by a remarkable reduction of the ostracods assemblages in both number of species and specimens, indicative of a clearly defined dysaerobic or kenoxic phase with bottom slowed circulation. These conditions quickly change at the base of the M Pl 2 Biozone with the beginning of improving bottom oxygenation characterized by the FOD of Agrenocythere pliocenica, typical psychrospheric ostracod, and of Cibicidoides robertsonianus, a benthic foraminifer characteristic of the NADW. A second change starts from the middle part of the M Pl 2 Biozone. In fact, the benthic foraminiferal co-dominant assemblages are characterized by species related to increased productivity and circulation. This change begins a few meters below the FO of Globorotalia puncticulata, a species which testifies a slight cooling. Therefore the benthic foraminifers seem to show a paleoenvironmental cooling-related bottom change, which occurs shortly before the FO of this planktic foraminifer. From the base of M Pl 2 upwards the ostracods testify the entrance of the psychrosphere which becomes stabilized within this biozone and a gradual improvement of bottom circulation. From M Pl 3 Biozone, after the paleoclimatic change, the assemblages are abundant and well diversified and indicate that the paleoceanographic conditions at the bottom improved and evolved to become more stable. Finally a probable correlation of the quantitative distribution of Planulina ariminensis (benthic foraminifer) with a maximum in the eccentricity astronomical record of 400 ky has been pointed out.