Persistence of modern Porites holobiont functionality since the Late Miocene: Insights from Mediterranean coral sclerochronologies (Tortonian, Crete, Greece)

The growth and demise of Upper Miocene reefs in the Mediterranean region corresponded with climate fluctuations driven by insolation changes in the Milankovitch frequency band. Until now, however, quantitative, high-resolution records linking orbital insolation changes with hydroclimatic seasonality and interannual environmental variability have been scarce. Here, exceptionally well-preserved, massive reef corals ( Porites , Tarbellastraea ) from a Tortonian (8.2 Ma) reef in Crete (eastern Mediterranean) were used to construct 13 chronologies that collectively span 224 years of environmental seasonality and the corresponding growth responses of the corals. Overall, calcification rates were low, with a minimum during the cool seasons, coeval with minimal zooxanthellae photosynthesis. When compared to unstressed extant Porites undergoing temperature-stimulated growth in oligotrophic, Indo-Pacific reef settings today, Tortonian calcification properties are consistent with a mean annual surface temperature of ~24 °C, while geochemical records imply a mean seasonality of ~10 °C. Moreover, the geochemical records document activations of ephemeral streams during winter, modulated by North Atlantic Oscillation-type, multiannual hydroclimatic variability such as prevailing in eastern Mediterranean regions of today. Remarkably, upregulation of the carbonate value of the coral's calcifying fluid was intensified in winter in order to compensate for reduced DIC of the ambient waters. These results fit the high subtropical paleolatitude of the reef systems in the eastern Mediterranean region and an insolation regime similar to the present-day. Our findings imply that some key aspects of the modern Porites holobiont functionality, such as its skeletal growth potential, have existed largely unchanged for at least 8.2 Ma.