Dancing Towards the End—Ecological Oscillations in Mediterranean Coral Reefs Prior to the Messinian Salinity Crisis (Calcare di Rosignano Formation, Acquabona, Tuscany, Italy)

The lower Messinian Calcare di Rosignano Formation (Tuscany, Italy, 43° N) preserves one of the youngest and northernmost examples of coral reefs in the Mediterranean. The outcropping succession of the Acquabona quarry consists of four main facies, namely, in ascending stratigraphic order: (1) coral boundstone, (2) coralline algal rudstone, (3) serpulid floatstone to packstone, and (4) peloidal packstone to grainstone. The succession displays a trend toward increasingly more shallow conditions and progressively more restricted water circulation. The coral reef displays a limited coral biodiversity and a remarkable abundance of heterotrophs, similar to modern coral reefs developed at the edges of the ecological niche of symbiont-bearing colonial corals. The widespread presence of coral colonies pervasively encrusted by coralline algae and benthic foraminifera suggests that short-term environmental perturbations caused temporary shutdowns of the coral-dominated carbonate factory. Moving upwards, there are fewer corals and more highly adaptable carbonate producers like coralline algae and serpulids. This suggests that the decline of corals had been caused by the conditions in the basin becoming more stressful, up to the collapse of the coral community. The overall succession indicates that coral-dominated ecosystems located at the edges of the coral zone are very sensitive; they can be affected even by minor perturbations and easily collapse if negative conditions persist.