Microbialites and micro-encrusters in shallow coral bioherms (Middle to Late Oxfordian, Swiss Jura mountains)

Abstract

Benthic microbial crusts (microbialites or microbolites) are an important component of Middle to Upper Oxfordian shallow-water coral bioherms in the Swiss Jura. They display stromatolitic (laminated), thrombolitic (clotted), and leiolitic (structureless) fabrics, which are distributed heterogeneously throughout the studied sections. The bioherms can be subdivided into coral-microbialite facies, microbialite-dominated facies, and sediment matrix. Macroscopic and microscopic study reveals that microbialitic encrustations commonly occur in two layers. The first one is directly in contact with the substrate and composed of leiolite (locally stromatolite) and a well-diversified micro-encruster fauna; the second one fills the remaining porosity partly or completely with thrombolite and low-diversity micro-encrusters. The growth of the first layer accompanies the growth of the coral reef and thus formed under the same environmental conditions. The second layer is the result of a moving encrustation front filling the remaining porosity (micro- and macrocavities) inside the reef, below the living surface. Both layers play an important role in early cementation. Phototrophic cyanobacteria probably intervene in the formation of the first encrustation zone, whereas heterotrophic bacteria associated to acidic, Ca2+-binding macromolecules in biofilms are thought to contribute to the thrombolite inside the reef body. When coral growth cannot take pace with microbialite development, the thrombolite from reaches the surface of the construction and finally covers the reef. The result is a thick interval of thrombolite, which can be interpreted as being related to an ecological crisis in coral-reef evolution. A semi-quantitative analysis of the relative abundance of microbialite types and associated micro-encrusters permits to better constrain the processes leading to a reef crisis. Four micro-encruster associations can be distinguished, and each follows an evolutionary trend in the studied section:Terebella-Tubiphytes dominated,Serpula-Berenicea dominated,Litho-codium dominated, andBacinella dominated. These trends are interpreted to reflect changes in environmental conditions. Bioerosion generally is at its maximum before and after abundant growth of microbialite. According to microbialite-bioerosion relationships and shifts in micro-encruster associations, we propose that the evolution towards a coral-reef crisis involves four main phases: (1) An oligotrophic to low mesotrophic phase when low water turbidity and good oxygenation allow phototrophic metabolisms. This leads to a maximum of coral diversity and development of light-dependent micro-encrusters. (2) A low-mesotrophic phase when increased turbidity and slack water circulation reduce the photic zone and favor heterotrophic micro- and macrofauna. Bioerosion through bivalves increases. (3) A high-mesotrophic phase when environmental conditions are so bad that only microbiatite can be produced. (4) A eutrophic phase when carbonate production is inhibited by high nutrient input and clay flocculation as a result of increased terrestrial run-off. The observed evolutionary trends are not directly linked to changes in bathymetry, but sea-level fluctuations played an important role in opening and closing the depositional environments on the shallow platform. Climatic changes contributed in modulating the influx of siliciclastics and nutrients, and the alkalinity of the water. Demise of coral reefs generally coincides with low sea level and humid climate. Sea-level and climatic fluctuations and, consequently, the crises in reef growth are linked to orbital cycles in the Milandkovitch frequency band.