The influence of water transparency on fossil coralline algal assemblages: two examples from the Miocene of the Mediterranean

Crustose coralline algae are some of the most common components of Cenozoic shelf carbonates. Their distribution is strongly influenced by physical parameters, including the availability of light at the seafloor, which in turn is controlled by depth and water clarity. Consequently, corallines are used for paleoenvironmental interpretations and paleodepth reconstructions within the photic zone. Two different Miocene examples of coralline-based paleobathymetric reconstructions are presented: the early to late Miocene coralgal limestones of the Eratosthenes Seamount and the early Miocene heterozoan impure limestone of the Sommières Basin. The deeper settings of the Eratosthenes Seamount, in the Eastern Mediterranean, records a coralline flora dominated by Hapalidiales, a mixed assemblage (with Hapalidiales, Corallinales and Sporolithales), at intermediate water depth and a Corallinales-dominated assemblage in its shallow-most section, where also hermatypic corals are abundant. The Sommières Basin in SE France was a small Miocene embayment, connected to the Alpine Molasse Basin through a flooded valley. The coralline assemblage of the Sommières Basin is invariably dominated by Hapalidiales, but Corallinales are significantly more common closer to the reconstructed shoreline of the basin. While the Eratosthenes carbonates developed on an isolated platform, far from riverine influence and probably in oligotrophic water, the Sommières Basin was a semi-enclosed embayment with higher nutrient-supply, and consequently more turbid waters and compressed benthic zonation, testified by the detailed paleontological and sedimentological analyses. These results highlight that less obvious ecological factors, such as water turbidity controlled by nutrient abundance, can be identified, although indirectly, as an overarching driver of benthic communities.