Tarbellastraea (Scleractinia): A new stable isotope archive for Late Miocene paleoenvironments in the Mediterranean

Abstract

Geochemical proxy records of sea surface temperature (SST) or sea surface salinity (SSS) variability on intra- and interannual time-scales in corals from geological periods older than Pleistocene are extremely rare due to pervasive diagenetic alteration of coralline aragonite. Very recently, however, stable isotope data ( δ 18O, δ 13C) from specimens of Porites of Late Miocene age (10 Ma) have been shown to preserve original environmental signatures. In this paper we describe new finds of the zooxanthellate corals Porites and Tarbellastraea in exceptional aragonite preservation from the island of Crete in sediments of Tortonian (∼ 9 Ma) and Early Messinian (∼ 7 Ma) age. Systematic, comparative stable isotope analysis of massive Tarbellastraea and Porites sampled from the same beds and localities reveal identical stable isotope fractionation patterns in both genera. Therefore, extinct Tarbellastraea represents an additional environmental archive fully compatible and mutually exchangeable with Porites. Provided that seasonal variations in δ 18O reflect SST changes only, seasonal SST contrasts of 7.3 °C for the Tortonian and 4.8 °C for the Early Messinian are inferred, implying warmer summer and cooler winter SSTs during the Tortonian than during the Messinian. However, reduced δ 18O seasonality (1.1‰ in the Tortonian and 0.7‰ in the Messinian) and slightly less negative mean δ 18O in Messinian corals (− 2.4‰) compared to Tortonian specimens (− 2.7‰) may not necessarily indicate a long-term fall in SSTs, but changes in surface water δ 18O, i.e. global ice build-up or enhanced evaporation during summer or increased precipitation/river discharge during winter and changes in insolation. On the other hand, coral communities of Tortonian and Messinian age in central Crete are identical, and compatible annual extension rates indicate similar average SSTs during the two investigated time periods. In addition, lithological and paleobotanical data from Central Crete document a change from humid to dry climatic conditions during the Late Miocene. Therefore, a likely explanation for the observed shift in coral mean δ 18O and reduced δ 18O seasonality from the Tortonian to the Early Messinian is a change in ambient seawater δ 18O caused by a change in the hydrological balance towards high evaporation/high salinity during summer.