Phytoplankton dynamics in the eastern Mediterranean Sea during Marine Isotopic Stage 5e
Abstract Calcareous nannoplankton, diatoms and organic-walled dinoflagellate cysts from seven eastern Mediterranean sediment cores were investigated to clarify the climatic and paleoceanographic conditions that influenced the deposition of sapropel S5 during Marine Isotopic Stage 5e. Warming of surface waters during S5 deposition is indicated by the high abundance of the calcareous nannofossil “small” Gephyrocapsa group, the presence of the dinoflagellate species Spiniferites mirabilis, Spiniferites pachydermus, Lingulodinium machaerophorum and Polysphaeridium zoharyi, and the tropical–subtropical diatom Pseudosolenia calcar-avis, Chaetoceros resting spores, Rhizosolenids and Thalassionema group. Increased productivity accompanied S5 deposition as evidenced by a general decrease in abundance of calcareous nannofossil superficial species and a coeval increase of dinocysts and diatoms. The productivity increase is further supported by an increase of the deep dwelling calcareous nannofossil species Florisphaera profunda. Stratification of near-surface waters is recorded by the presence of the dinocyst species L. machaerophorum and P. zoharyi together with the co-occurrence of the diatom Chaetoceros resting-spore group, Thalassionema frauenfeldii, and Thalassionema nitzschioides var. parva, which reflect transport of neritic waters off-shore, and by the presence of some freshwater and transport-related diatom taxa (Cyclotella, Diatomella and Diploneis). Gradual shoaling of the nutricline and a gradual weakening of water stratification at the end of S5 deposition is indicated by an increase in the deep dwelling calcareous nannoplankton species Gladiolithus flabellatus, which suggests a less-deep niche compared to the deeper dwelling F. profunda. Time transgressive variations in the distribution of the calcareous nannofossil Emiliana huxleyi and changes in the abundances of Gephyrocapsa oceanica, small Gephyrocapsa, G. flabellatus, and of the dinocysts P. zoharyi and L. machaerophorum lead to paleocirculation considerations, as they always reflect the main current path. Moreover, the occurrence of upwelling-related diatom and dinocyst taxa, such as Chaetoceros resting spores and S. pachydermus, suggests the presence of a former gyre structure south of Crete that was active during S5 time.