Microbialites shed light on past mass extinctions
Microbialites are created by communities of microorganisms that entrap and bind sedimentary particles to form mineral deposits. They possess a wide array of distinctive morphologies and are found in the geologic record as far back as 3.45 billion years ago. The goal of the project ET MICROBIALITES (Formation mechanisms of Early Triassic microbialites in the aftermath of the greatest mass extinction at the Permian-Triassic-Boundary) was to understand microbialite formation in periods of Earth’s history typified by changing environments and ecosystems. The example used was the end-Permian mass extinction event, the most fatal ever. Following this event more than 90 % of marine species became extinct, while microbialite forming microbes became major components of the marine ecosystem. Researchers conducted a unique multi-proxy study on carbonates and detrital sediments using well-preserved rock samples from Iran and Turkey. The aim was to understand carbonate formation processes and to reconstruct changing environments from the end of the Permian to the Early Triassic. Studies of rocks revealed that the microbial carbonate contained abundant lipid biomarkers (molecular fossils) indicating the presence of layered microbial mats on the sea floor. The biomarker evidence and carbon isotopes suggested that photosynthetic carbon dioxide (CO2) removal by cyanobacteria contributed significantly to carbonate precipitation. Moreover, up to 50 % of the rock is represented by sponge fossils (i.e. keratose sponge fabric). Microbes and sponges grew synergistically to form sponge-microbialite reefs in the immediate aftermath of the late Permian mass extinction. Within the sponge-microbialite reefs abundant fossils of oxygen-dependent metazoans, such as ostracods, bivalves, and gastropods, were found. This and the occurrence of abundant cyanobacteria indicated the presence of well-oxygenated environments. ET MICROBIALITES showed during the earliest Triassic metazoan reef ecosystems were not driven to extinction by the late Permian mass extinction and were able to persist, albeit in a reduced state. By understanding these processes scientists can better understand Earth’s history, which will help them to solve the environmental challenges of today and in the future.
Keywords
MICROBIALITES, Early Triassic, mass extinction, multi-proxy, biomarkers, layered microbial mats, sponges, reefs
