Calcareous nannofossils in extreme environments: The Messinian Salinity Crisis, Polemi Basin, Cyprus

The rapidly changing and extreme environmental conditions of the early Messinian Salinity Crisis are reflected in abrupt variations in nannofossil assemblages within the Messinian units (Kalavasos Formation) from the Polemi Basin. During the Messinian, sedimentary and microfossil data indicate that the Polemi Basin was a semi-enclosed, shallow water basin, subject to repeated influxes of marine and freshwater. This is supported by the absence of many open marine nannoplankton (e.g. Discoaster) and by the presence of neritic–littoral and freshwater diatoms. Whilst calcareous nannoplankton are known to occupy near-shore habitats, they are rarely preserved in such environments due to terrigenous and clastic influx. The shallow and eutrophic environments of the Messinian Polemi Basin therefore provide an unusual opportunity to investigate which extinct nannofossil taxa occupied marginal marine environments.Nannoplankton diversity (3 to 11 species) is low in comparison to the open ocean, and the assemblages are extremely uneven, with high dominance. One of five species, Reticulofenestra minuta, Reticulofenestra antarctica, Helicosphaera carteri, Umbilicosphaera jafari and Sphenolithus abies, was observed to dominate in all of the assemblages. These were broadly distributed marine species, but capable of opportunistic behaviour. Salinity and nutrient levels are thought to be the primary factors controlling the overall nature of the nannoplankton assemblages. Using the associated diatom and sedimentological evidence we infer the palaeoecology of key nannofossil taxa and speculate on the palaeoenvironments of the Messinian Polemi Basin. R. antarctica is thought to have dominated in normal salinity, mesotrophic, shallow water environments; H. carteri in shallow, brackish, eutrophic environments; U. jafari in hypersaline conditions; R. minuta in eutrophic conditions with an abnormal salinity from brackish to hypersaline and S. abies in mesotrophic, deeper and normal salinity environments.