Is Cyprideis agrigentina Decima a good paleosalinometer for the Messinian salinity crisis? Morphometrical and geochemical analyses from the Eraclea Minoa section (Sicily)

The living euryhaline species Cyprideis torosa (Jones) undergoes morphometric variations in size, noding and sieve-pore shape linked to the environmental salinity. In particular it is known that salinity values around 8–9 psu represent the osmoregulation threshold and also the turning point between smaller and greater valve dimensions and prevailingly noded against un-noded valves. The variation of the percentage of round-, elongate- and irregular-shaped sieve-pores on the valves has shown an empiric logarithmic correlation with the water salinity from 0 to 100 psu. Due to this ecologically cued polymorphism, C. torosa represents an invaluable paleosalinometer for the Quaternary brackish basins. In this paper we attempt to verify whether the ecophenotypical behavior of the post-evaporitic Messinian species Cyprideis agrigentina Decima was comparable with that of Cyprideis torosa. To reach this goal, three morphometric characters have been analyzed: 1) size variability; 2) noding and ornamentation; and 3) variability of the percentage of the sieve-pore shapes. The paleoenvironmental interpretation was made using synecological and geochemical approaches [stable isotopes, trace elements, Sr-isotopes and natural radioactivity (NRD)]. For this study, the 250 m-thick Messinian Lago-Mare succession of Eraclea Minoa (Agrigento, Sicily) was chosen for the presence of monotypic assemblages made only by C. agrigentina for around 70 m of thickness. The results of the morphometric analyses showed that: 1) size variations are not related to the salinity changes recognized both from synecological and geochemical analyses; 2) no noded specimens have been recovered along the Section; 3) the salinities calculated on the basis of the percentage of the sieve-pore shape are not correlated with the salinities inferred from the synecological and geochemical analyses. Thus, in this paper we conclude that Cyprideis agrigentina cannot be considered a paleosalinometer for the Messinian Salinity Crisis. There is a correlation of the δ13C with the percentages of sieve-pore shapes, linking them to the behavior of the dissolved inorganic carbon (DIC) and to the oxygen availability at the bottom of the basin