ET MICROBIALITES · Formation mechanisms of Early Triassic microbialites in the aftermath of the greatest mass extinction (Permian-Triassic-Boundary)

Formation mechanisms of Early Triassic microbialites in the aftermath of the greatest mass extinction (Permian-Triassic-Boundary)

The ultimate goal of this project was to understand the modes of reefal microbial carbonate formation (microbialites) in periods of the Earth history typified by changing environments and ecosystems. We used the examples of the end-Permian mass extinction, the most fatal ever, and the period of recovery during the Early Triassic in the aftermath of the extinction. Due to the lack of metazoan reef builders (about 90% of the marine species went extinct), microbial communities fostered reefal microbialite (mound) formation on the margins of Early Triassic ocean platforms. To unravel carbonate formation processes and to reconstruct changing environments from the end-Permian to the Early Triassic, a unique multiple proxy study was pursued on carbonates and detrital sediments (control samples) from Iran and Turkey. The approach included organic geochemical (lipid biomarkers), rare earth elements, stable carbon, oxygen and sulphur isotopes, and sedimentological as well as petrographic methods. Additional studies on fossils (molluscs, ostracods, sponges and others) helped to shed light on the role of grazing organisms across the Permian-Triassic-Boundary and to reconstruct changing environmental conditions. An interdisciplinary cooperation with microbiologists allowed learning from modern microbial ecology and allowed to use that knowledge for the interpretation of the past. In an Iranian Early Triassic section (Late Dienerian to Early Smithian) on the southwestern margin of the Neotethys Ocean, we discovered finely laminated calcareous shales that are interbedded with laterally extending carbonate beds mostly composed of calyx-shaped carbonate crystal fans. The calyxes consist of fibrous crystals that apparently formed within the subseafloor in soft, water-saturated sediment by displacive growth. Rare earth element and yttrium patterns indicate that calyx-shaped crystals precipitated from anoxic pore waters. Biomarker patterns of the carbonate beds reveal major input of lipids from prokaryotes that typically occur within layered benthic microbial mats (cyanobacteria, anoxygenic phototrophic bacteria, sulphate-reducing bacteria, and methanogenic archaea). Along their length, the fibrous crystals of the calyxes reveal a trend of increasing δ13Ccarb values (from 2.7 to 3.3‰), suggesting that archaeal methanogenesis affected the pool of dissolved carbonate. Apart from the biomarkers of benthic prokaryotes, molecular fossils of halophilic, most likely planktic, archaea were detected in the carbonate beds. The former presence of a benthic microbial mat with cyanobacteria underlain by anoxygenic phototrophic bacteria, sulphate-reducing bacteria, and methanogenic archaea is in accordance with studies on living microbial mats in hypersaline, low-oxygen marine environments. The Early Triassic subseafloor seems to have been a sink for carbon, driven by biologically-induced formation of secondary, diagenetic carbonate (Heindel et al., 2015). In order to better constrain the benthic microbial and metazoan communities and the formation mechanisms of Early Triassic (Griesbachian) reefal microbialite mounds, well preserved rocks from Iran and Turkey were studied. Based on petrography and lipid biomarkers, we demonstrated that microbes and keratose sponges grew synergistically to form microbialite-metazoan reefs in the immediate aftermath of the late Permian mass extinction along the western margin of the Neotethys. The microbial carbonate contains abundant biomarkers representing input from cyanobacteria, halophilic archaea, anoxygenic phototrophs and sulphate-reducing bacteria, indicating the presence of a layered microbial mat on the seafloor, in which carbonate precipitation and, thus, mound formation was induced. The biomarker evidence and compound-specific carbon isotopes suggest that photosynthetic CO2 removal by cyanobacteria contributed significantly to carbonate precipitation. Moreover, up to 50% of the carbonate represents fossils of sponges (keratose sponge fabric). Abundant fossils of oxygen-dependent metazoans, i.e. bivalves, gastropods, microconchids, ostracods and foraminifera, were found within the clotted and keratose sponge fabric, with the shelly fauna being more abundant in the keratose sponge fabric than in the clotted microbialites. In the absence of sponges, stromatolitic buildups formed, whereas microbial mats in association with sponges apparently formed reefal bioherms with a thrombolitic fabric. The occurrence of metazoans in conjunction with abundant lipid biomarkers of cyanobacteria indicate an at least local presence of well-oxygenated environments. Based on our new findings, the earliest Triassic marine benthic communities on the northern and western platform margins of the Neotethys were more diverse than previously suggested. The recognition of reefal structures built at least in part by metazoans during the earliest Triassic (Griesbachian) suggests that metazoan reef ecosystems were not driven to extinction by the late Permian mass extinction and were able to persist, albeit in a depauperate state (Heindel et al., in prep; Richoz et al, in prep). contact: Katrin Heindel (katrin.heindel@fau.de) University of Erlangen-Nürnberg (FAU) - GeoZentrum Nordbayern Palaeontology Loewenichstraße 28 D-91054 Erlangen Jörn Peckmann (joern.peckmann@uni-hamburg.de) University of Hamburg Institute for Geology University of Hamburg Bundesstraße 55 D-20146 Hamburg homepages: https://geologie.univie.ac.at/sedimentology-stratigraphy/current-projects/et-microbialites/ https://geologie.univie.ac.at/sedimentology-stratigraphy/academic-staff/katrin-heindel/ https://geologie.univie.ac.at/home/news/news-singleview/article/katrin-heindel-has-been-awarded-a-marie-curie-fellowship-of-the-european-union/?tx_ttnews[backPid]=122050&cHash=e508525a2fc95c9f3a9b808c7fe058bb

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