Trace and Rare Earth Elements in Microbial Deposits of Upper Jurassic Coral and Sponge reefs: Palaeoenvironmental Implications

Carbonate microbial deposits (microbialites) are abundantly represented throughout the Precambrian and Phanerozic, and developed in a wide range of palaeogeographic settings. These biologically induced carbonates can be used as paleoceanographic proxies, because they are characterized by a rapid lithification and their development well-above the sea floor reduced the contamination by porewaters. Moreover, rare earth elements (REE) are incorporated with uniform partition coefficients from seawater suggesting no biological fractionation occurred (Webb and Kamber, 2000). Major and trace elements (REE and Y, Zr, U, Th) were analyzed on 17 microbialites of Upper Jurassic coral and sponge reefs located on the northwestern Tethys and eastern Atlantic Ocean margins. According to the different palaeogeographic position of bioconstructions, microbialites were collected in lagoon, external-ramp, and deep-shelf to epicontinental basin settings (France). The very low content of siliciclastic material (SiO2 between 0.1 to 7%) recorded in microbialites of thrombolitic fabric does not significantly disturb the REE patterns. In contrast, lateral and intra-reef sediments, and to a lesser degree stromatolitic microbial deposits, are more subject to terrigenous contamination. Major discrepancies of both Ce and heavy REE concentrations recorded in microbialites from different sites hence reflect changes in seawater chemistry. The cerium anomalies in the range of 0 to 0.15 (negative) suggest the formation of microbial deposits in these coral and sponge reefs occurred both in oxic and suboxic seawaters. The microbialitic development, usually linked to a change in environmental conditions, would be induced by an increase in nutrient content rather than a high alkalinity. Microbialites collected in lagoon and deep-shelf settings have REE patterns similar to those of the actual seawater. These REE results confirm the use of microbialites in palaeoenvironmental reconstruction and Nd isotopes analyses in progress will place additional constraints on the marine chemistry. Webb and Kamber, GCA 64, 1557-1565 (2000)