Benthic foraminiferal assemblages from a central Apennines early Middle Miocene section: preliminary paleoecological observations

Quantitative analyses of benthic foraminiferal assemblages were carried out on a Burdigalian - Langhian succession cropping out along the Adriatic coast (Cònero Riviera, Marche region, Italy). This succession, about 56 m thick, mainly consists of cyclically bedded mostly brown marls and more indurated marls, and subordinately of limestones. The lower part of the succession (0 - 23m) is characterized by the occurrence of 4 composite but apparently massive layers (“megabeds”). By integrating the calcareous plankton events with the magnetostratigraphic record the section results to range from C5Cn.1n to C5Bn.2n Subchrons (Gennari et al., 2009). The most abundant and/or paleoecologically significant benthic taxa are: Bolivina reticulata, B. spathulata, Bulimina costata, Cibicidoides dutemplei/subhaidingerii, C. incrassatus and C. pachyderma, Gyroidina spp., Hanzawaia boueana, Lenticulina spp., Melonis barleeanum, Siphonina reticulata and Uvigerina semiornata. The co-occurrence and the relative abundance of B. reticulata, C. dutemplei/subhaidingerii and H. boueana suggest a paleobathymetry ranging from the upper to the middle bathyal zones. The most remarkable changes recorded in the benthic foraminiferal assemblages are shown by the pattern of the Benthic Number and the abundance curves of B. reticulata, B. spathulata, B. costata, C. dutemplei/subhaidingerii, C. incrassatus, Gyroidina spp., Lenticulina spp., S. reticulata and U. semiornata. In particular significant variations are recorded around 30 m, about 7 m above the disappearance of the “megabeds”. These variations allow the distinction of the following two intervals: 0 – 30 m interval: general high values of BN; assemblage composition characterized by the predominance of B. spathulata and Lenticulina spp., dysoxic and suboxic species respectively (Kaiho, 1994), and the occurrence of B. costata, a high productivity typical species (Van der Zwaan, 1982; Russo et al., 2007), and of the suboxic species Gyroidina spp. (Kaiho, 1994) and U. semiornata (Borsetti et al., 1986); 30 – 56 m interval: decrease in the BN; disappearance of B. spathulata and B. costata, decrease of Gyroidina spp., Lenticulina spp. and U. semiornata; increase and appearance of the oxic species B. reticulata, and C. dutemplei/subhaidingerii, C. incrassatus and S. reticulata respectively (Van der Zwaan, 1982; Kaiho, 1994; Russo et al., 2007). In conclusion changes in the BN pattern and assemblages composition are indicative of bottom water masses characterized by dysoxic/suboxic conditions, maybe due to a relatively higher productivity during the deposition of the “megabeds” (0 - 30 m interval) and an increase of the bottom water oxygen content together with a relatively decrease of productivity in the upper part of the succession (30 - 56 m). References Borsetti A.M., Iaccarino S., Jorissen F.J., Poignant A., Sztrakos K., Van der Zwaan G.J., Verhallen P.J.J.M., 1986, The Neogene development of Uvigerina in the Mediterranean. In Van der Zwaan G.J., Jorissen F.J., Verhallen P.J.J.M. & Von Daniels C.H. (eds.), Atlantic-European Oligocene to Recent Uvigerina taxonomy, paleoecolgy and paleobiogeography. Utrecht Micropaleontological Bulletins, 35, 183-235. Gennari R., Sagnotti L., Cascella A., Iaccarino S.M., Turco E., 2009, Magnetostratigraphy and rock magnetism of La Vedova section (Conero Riviera, Italy). 13th Congress RCMNS – 2nd – 6th September 2009, Naples, Italy, Acta Naturalia de “Ateneo Parmense”, 45, 1/4, 10-11. Kaiho K., 1994, Benthic foraminiferal dissolved-oxygen index and dissolved-oxygen levels in the modern ocean. Geology, 22: 719-722, Boulder. Russo B., Curcio E. & Iaccarino S., 2007, Paleoecology and paleoceanography of a Langhian succession (Tremiti Islands, southern Adriatic Sea, Italy) based on benthic foraminifera. Boll. Soc. Paleont. It., 46(2-3): 107-124, Modena. Van der Zwaan G.J., 1982, Paleoecology of Late Miocene foraminifera. Utrecht Micropal. Bull., 25: 1-201 pp., Utrecht.