Quantifying the contribution of coralline species in rhodoliths as a tool for paleobathymetric reconstructions

Coralline algae are a powerful proxy of paleo-bathymetry because their distribution is controlled by light penetration, along the gradient of water depth. Although they are among the most abundant marine carbonate producers, especially in Cenozoic carbonates, they are relatively underexploited in paleoecological reconstructions, because of time-consuming material preparation, difficulties in identification, and nomenclatural issues. As a consequence, coralline algae have been reported in open nomenclature in a large number of recent scientific contributions. However, dealing exclusively with supraspecific taxa is a strong limitation to coralline potential as paleoecological and paleobiogeographic tool. In the early Miocene Sommi`eres Basin (southern France), rhodoliths were sampled along an obvious bathymetric gradient, reconstructed on the basis of sedimentological proxies. A group of distinctive and easily recognizable coralline species was studied along the paleo-depth gradient, and the contribution of each species was quantified by a new method based on the digitalization of thin sections. The association revealed a clear pattern when analyzed at the rank of species: Mesophyllum roveretoi and Lithothamnion ponzonense peak at middle depth; Phymatholithon sp. A dominates in deep-water rhodoliths, while Lithophyllum sp. A and Spongites fruticulosus are preferentially distributed in shallow water. Sporolithon sp. A is restricted to shallow water, contrarily to most of the modern records of Sporolithon. Nonetheless, shallow-water species of Sporolithon also occur in present-day oceans, especially at the transition between the tropics and the temperate realm, which is a climatic situation probably very close to that of the early Burdigalian Sommi`eres Basin. These results confirm that species distribution is very sensitive to environmental variations and is much more informative than the distribution based on higher taxonomic ranks would do. The digital technique employed is a user-friendly method for the quantification of coralline algae in thin section and is especially effective for the quantification of thin encrusting species.