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- Friesenbichler, Evelyn, et al.
(författare)
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Sponge-microbial build-ups from the lowermost Triassic Chanakhchi section in southern Armenia: Microfacies and stable carbon isotopes
- 2018
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182. ; 490, s. 653-672
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Tidskriftsartikel (refereegranskat)abstract
- The end-Permian mass extinction was the most severe biotic crisis in Earth's history. In its direct aftermathmicrobial communities colonized some of the space left vacant after the severe decline of skeletal metazoans.The Permian-Triassic boundary microbialites were peculiarly abundant on low-latitude shallow-marine carbonateshelves of central Tethyan continents. Armenia features particularly well preserved and diverse basalTriassic sponge-microbial build-ups (BTSMBs), which were not studied in detail to date. Here, the Chanakhchisection in southern Armenia is described petrographically and by means of stable isotope analyses. TheArmenian BTSMBs formed in a distally open marine setting on a pelagic carbonate ramp in the course of twophases of microbial growth during the Induan (Lower Triassic). The BTSMBs are represented by predominantlythrombolitic but also dendrolitic and digitate stromatolite biostromes and mounds that vary in height between5 cm to 12 m. The digitate stromatolites are associated with calcium carbonate crystal fans (CCFs). Microfaciesanalyses revealed that the BTSMBs exhibit a number of different growth forms and internal fabrics. The formationof CCFs was apparently not devoid of biological influence and took place above the sediment surface. Theabundance of sponges in the BTSMBs reveals that ecologically complex metazoan-microbial reefs have beenpresent already early after the end-Permian mass extinction. However, the formation of biostromes and moundsdid not depend on sponges or other metazoans. BTSMBs that formed during the second microbial growth phaserevealed similar δ13C-values like the surrounding sediment. In contrast, the δ13Cmicrobialite and δ13Csediment valuesfrom the BTSMBs and CCFs of the first growth phase show a difference of up to +2.3‰, suggesting a significantinfluence of photoautotrophy during microbially induced carbonate precipitation.
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| 2. |
- Heindel, Katrin, et al.
(författare)
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The formation of microbial-metazoan bioherms and biostromes following the latest Permian mass extinction
- 2018
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Ingår i: Gondwana Research. - : Elsevier BV. - 1342-937X. ; 61, s. 187-202
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Tidskriftsartikel (refereegranskat)abstract
- After the latest Permian mass extinction event, microbial mats filled the ecological niche previously occupied by metazoan reefs, resulting in widespread microbialites. This study focuses on the lipid biomarker (molecular fossil) and invertebrate fossil records from Neotethyan platform margin sections to understand microbial-metazoan bioherm formation. Here, we find that early Griesbachian thrombolitic and stromatolitic microbialites from Çürük Dag (Turkey) and Kuh e Surmeh (Iran) contain abundant lipid biomarkers, representing input from cyanobacteria, anoxygenic phototrophic bacteria, sulfate-reducing bacteria, and halophilic archaea. The biomarker inventory suggests that the microbialites were constructed by cyanobacteria-dominated microbial mats. Biomarkers of halophilic archaea are interpreted to reflect input from the water column, suggesting that the Neotethys experienced at least episodically hypersaline conditions. We also demonstrate that bacteria, possible keratose sponges (up to 50% of the carbonate is represented by the possible sponges), and microconchids lived synergistically to form microbial-metazoan bioherms in the immediate aftermath of the extinction along the western margin of the Neotethys. Abundant fossils of oxygen-dependent invertebrates (i.e. microconchids, bivalves, gastropods, brachiopods, and ostracods) and foraminifers were also found within these bioherms. The presence of invertebrates in conjunction with abundant molecular fossils of cyanobacteria indicates an oxygenated water column. Even though the presence of the biomarker isorenieratane in microbialites may considered as evidence for euxinic conditions in the water column, its absence in the background sediments rather points to a source organism belonging to the mat community. The new finding of bioherms built in part by metazoans suggests that reef ecosystems underwent a major turnover across the extinction event, and shortens the ‘metazoan reef gap’ to just the uppermost Changhsingian. During the Early Triassic, therefore, reefal ecosystems were able to recover in oxygenated settings since the earliest Griesbachian, albeit in an impoverished state.
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