Unusual Braarudosphaera bigelowii and Micrantholithus vesper enrichment in the Early Miocene sediments from the Slovenian Corridor, a seaway linking the Central Paratethys and the Mediterranean

During a stratigraphic survey of an Early Miocene sedimentary sequence outcropping in SE Slovenia an unusual enrichment of the calcareous nannoplankton assemblage with pentaliths – pentagonal nannoliths consisting of five crystal units and showing radial symmetry – was detected. Pentaliths were only abundant in a short interval, which was sampled in detail. The lithological composition of this interval was indistinguishable from that of the underlying and the overlying strata. The composition of nannoplankton assemblages and distribution patterns of different taxa were studied. Mineral composition, total calcite content and δ13C values of chosen sediment samples were analysed as well. The pentalith-enriched strata were deposited in the lower Early Miocene, corresponding to the Egerian stage of the Central Paratethys and the Aquitanian of the Mediterranean, in the Slovenian Corridor — a seaway linking the Central Paratethys and the Mediterranean. The presence of rare specimens of Discoaster druggii and Helicosphaera carteri allows stratigraphic correlation with the transition of NN1 — Triquetrorhabdulus carinatus Zone and NN2 — Discoaster druggii Zone of Martini [Martini, E., 1971. Standard Tertiary and Quarternary calcareous nannoplankton zonation. In: Farinacci, A. (Ed.), Proceedings of the II Planctonic Conference, Roma, 1970, Ed. Technoscienza, pp. 739–785]. Pentaliths belong to two species: Braarudosphaera bigelowii and Micrantholithus vesper. Both species display a wide range of morphological variation and most likely represent a group of sibling species. The pattern of distribution of both species displays obvious similarities throughout most of the studied interval. In the upper part of the studied interval B. bigelowii reaches maximum abundance, while M. vesper disappears. Since the ability of B. bigelowii to thrive in hyposaline water is well known, it appears that M. vesper tolerates reduced water salinity as well, but to a lesser extent than B. bigelowii. No significant paleoclimatic and paleogeographical changes are known in this particular region during the Early Miocene, however, the onset of far reaching extensional tectonic processes took place during this time. The enrichment of nannoplankton assemblage with pentaliths may represent a reflection of a short-lived episode of hyposaline conditions. Enhanced influx of freshwater and terrigenous material is indicated by a drop in 13C values, a rise in nannoplankton productivity and the presence of sand in some samples from the pentalith-enriched strata. The most likely causes of this episode are regional tectonic changes, associated with the onset of intensive tectonic activity in the western part of the Central Paratethys.