Importance of Rare Earth Element Patterns in Discrimination Between Biotic and Abiotic Mineralization

In this research we determined the concentration of Rare Earth Elements and Yttrium (REE+Y) in the Calcare di Base (CdB). The shale normalized REE+Y patterns were used to prove the reliability of the REE as geochemical proxy to distinguish between biotic and abiotic precipitation. Shale normalized REE+Y patterns of the CdB show: (a) enrichment in heavy REE (NdSN/YbSN = 0.52, SD = 0.09), (b) negative Ce and positive La anomalies, (c) marine Y/Ho ratios (57.08, SD = 3.07), and (d) slightly positive Gd anomalies. These characteristics suggest that CdB incorporated REEs in equilibrium with oxygenated seawater. The CdB REE+Y patterns show also substantial differences in comparison to those of pure evaporitic sediments, such as gypsum. The Y and REE concentrations in gypsum samples are all in the low ppb range with lower concentrations for the HREE. The relative levels of the LREE increase from La to Sm and those of the MREE and HREE are depleted in a decreasing order from Sm to Lu. The CdB samples share the essential shale-normalised characteristics of microbial samples reported in literature for a large geological time span, confirming the microbialitic nature of these sediments. Moreover the differences between the Messinian CdB and gypsum REE+Y patterns strengthen the interpretation of a deposition not evaporitic.