Nutrient influence on fossil carbonate factories: Evidence from SEDEX extractions on Burdigalian limestones (Miocene, NW Italy and S France)

Different Miocene shallow-water carbonate successions have been examined to test the potential of phosphorus sequential extraction (SEDEX) in studying palaeo-nutrient levels, and to investigate nutrient influence on carbonate-factories during this epoch. The coralline-algal-dominated sequence of the Pietra da Cantoni Group (NW Italy) and the bryozoan-dominated sequence of the Sommières Basin (S France), have been targeted for this study. Actually, albeit they both developed during the Burdigalian, in the very same latitudinal zone, they present remarkably different carbonate factories. Palaeontological analyses have investigated the distribution of the major groups of carbonate-producing organisms throughout the successions. These results have been compared with those of geochemical analyses on phosphorus and organic matter abundance, testifying a clear relationship between the skeletal assemblage and the availability of nutrients. As expected on the basis of modern oceanography and benthic ecology, bioavailable phosphorus concentration was higher in the bryozoan-dominated basin than in the coralline-algal-dominated basin. This result strongly supports SEDEX sequential extraction as a viable instrument to investigate the nutrient supply in the geological record. This also proves that the early-Miocene Mediterranean carbonate-factories were deeply controlled by nutrient influence, with patterns similar to those observed in present-day oceans. Moderate nutrient-concentrations resulted in coralline-algal dominance, while high nutrient-concentrations produced carbonate factories dominated by heterotrophs suspension-feeders like bryozoans. The euphotic-zone producers, which dominate in classical tropical associations, were probably restricted to areas with low nutrient-concentrations.