Rare earth and trace elements of microbialites in Upper Jurassic coral- and sponge-microbialite reefs
Major, trace (Y, Zr, Pb, Th and U), rare earth element (REE) and Sm–Nd isotope analyses were carried out on microbialites and detrital carbonate sediments collected in Upper Jurassic coral- and sponge-microbialite reef settings. Selected bioconstructions developed in various palaeogeographic settings and depositional environments on northwestern Tethys and eastern Atlantic margins: pure carbonate lagoon, carbonate-dominated deep shelf to epicontinental basin and mixed carbonate-siliciclastic shallow ramp and lagoon. The proximity to terrigenous sources directly controlled REE patterns of carbonate samples. Lateral and intra-reef sediments, and to a lesser degree stromatolitic microbial crusts, are more susceptible to contamination by terrigenous material than are microbialites of thrombolitic fabric. In mixed carbonate-siliciclastic settings, microbialites display a flat shale-normalized REE pattern. In pure carbonate lagoonal settings, microbialites have shale-normalized REE patterns similar to those of modern seawaters with a negative Ce anomaly [(Ce/Ceâ) up to 0.14], positive La and Gd anomalies, and light REE depletion relative to heavy REE. These data imply that similar REE chemistry than modern seawater already occurred on the northwestern Tethys margin at the Upper Jurassic. Strong negative Ce anomalies (< 0.4) imply microbialite formation in well-oxygenated seawaters both in coral reefs of pure carbonate lagoonal environments (Pagny-sur-Meuse) and sponge bioherms of deep-shelf settings (Plettenberg). Absence of positive Ce anomalies rules out an influence of alkaline and high-pH seawaters. Thus, REE patterns and Nd isotopic signatures, which are in perfect agreement with existing results for Tethysian seawater, confirm that microbialites can be used as proxies of palaeoenvironmental seawater chemical characteristics.