Non-rigid cryptic sponges in oyster patch reefs (Lower Kimmeridgian, Langenberg/Oker, Germany)

Abstract

Two patch reefs which predominantely consist of the oyster Nanogyra nana (Sowerby 1822) are exposed in Lower Kimmeridigian strata of the Langenberg hillrange, central Germany. Left oyster valves making up the framework of the reefs formed small abundant cavities that were inhabited by a unique sponge community. The excellent preservation of non-rigid sponges was related to early organomineralization within the decaying sponge tissue. As a process of sponge taphonomy, different types of microbially induced carbonates precipitated preserving spicule aggregates. Organomineralization within sponge soft tissues is especially favored within the Langenberg patch reefs due to the closed or semi-closed system conditions within the cavities. The delta (13)C values of in situ formed microbialites reveal that carbonate precipitation was in equilibrium with Jurassic seawater. The carbon of the microbialites does not derive from the bacterial remineralization of organic matter, but is of a marine source. Likewise, organomineralization is probably related to bacterial EPS or decaying sponge tissues providing an organic matrix for initial carbonate precipitation. Biomarker analyses revealed, that the patch reef microbialites contain terminally branched fatty acids (iso- and anteiso- pentadecanoic acid) in significant concentrations. These fatty acids, like hopanoid hydrocarbons, are most likely of a bacterial source. This is in agreement with sulfate-reducing bacteria remineralizing the decaying sponges as further indicated by the occurrence of framboidal pyrite in sponge microbialites.