| 1. |
- Silva-Tamayo, Juan Carlos, et al.
(author)
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Global perturbation of the marine calcium cycle during the Permian-Triassic transition
- 2018
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In: Geological Society of America Bulletin. - 0016-7606. ; 130:7-8, s. 1323-1338
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Journal article (peer-reviewed)abstract
- A negative shift in the calcium isotopiccomposition of marine carbonate rocksspanning the end-Permian extinction horizonin South China has been used to arguefor an ocean acidification event coincidentwith mass extinction. This interpretationhas proven controversial, both because theexcursion has not been demonstrated acrossmultiple, widely separated localities, and becausemodeling results of coupled carbon andcalcium isotope records illustrate that calciumcycle imbalances alone cannot accountfor the full magnitude of the isotope excursion.Here, we further test potential controlson the Permian-Triassic calcium isotoperecord by measuring calcium isotope ratiosfrom shallow-marine carbonate successionsspanning the Permian-Triassic boundary inTurkey, Italy, and Oman. All measured sectionsdisplay negative shifts in δ44/40Ca of upto 0.6‰. Consistency in the direction, magnitude,and timing of the calcium isotope excursionacross these widely separated localitiesimplies a primary and global δ44/40Ca signature.Based on the results of a coupled boxmodel of the geological carbon and calciumcycles, we interpret the excursion to reflect aseries of consequences arising from volcanicCO2 release, including a temporary decreasein seawater δ44/40Ca due to short-lived oceanacidification and a more protracted increasein calcium isotope fractionation associatedwith a shift toward more primary aragonitein the sediment and, potentially, subsequentlyelevated carbonate saturation statescaused by the persistence of elevated CO2delivery from volcanism. Locally, changingbalances between aragonite and calcite productionare sufficient to account for the calciumisotope excursions, but this effect alonedoes not explain the globally observed negativeexcursion in the δ13C values of carbonatesediments and organic matter as well. Onlya carbon release event and related geochemicalconsequences are consistent both withcalcium and carbon isotope data. The carbonrelease scenario can also account for oxygenisotope evidence for dramatic and protractedglobal warming as well as paleontologicalevidencefor the preferential extinction ofmarine animals most susceptible to acidification,warming, and anoxia.
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| 2. |
- Foster, William J., et al.
(author)
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Suppressed competitive exclusion enabled the proliferation of Permian/Triassic boundary microbialites
- 2020
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In: Depositional Record. - : Wiley. - 2055-4877. ; 6:1, s. 62-74
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Journal article (peer-reviewed)abstract
- During the earliest Triassic microbial mats flourished in the photic zones of marginal seas, generating widespread microbialites. It has been suggested that anoxic conditions in shallow marine environments, linked to the end-Permian mass extinction, limited mat-inhibiting metazoans allowing for this microbialite expansion. The presence of a diverse suite of proxies indicating oxygenated shallow sea-water conditions (metazoan fossils, biomarkers and redox proxies) from microbialite successions have, however, challenged the inference of anoxic conditions. Here, the distribution and faunal composition of Griesbachian microbialites from China, Iran, Turkey, Armenia, Slovenia and Hungary are investigated to determine the factors that allowed microbialite-forming microbial mats to flourish following the end-Permian crisis. The results presented here show that Neotethyan microbial buildups record a unique faunal association due to the presence of keratose sponges, while the Palaeotethyan buildups have a higher proportion of molluscs and the foraminifera Earlandia. The distribution of the faunal components within the microbial fabrics suggests that, except for the keratose sponges and some microconchids, most of the metazoans were transported into the microbial framework via wave currents. The presence of both microbialites and metazoan associations were limited to oxygenated settings, suggesting that a factor other than anoxia resulted in a relaxation of ecological constraints following the mass extinction event. It is inferred that the end-Permian mass extinction event decreased the diversity and abundance of metazoans to the point of significantly reducing competition, allowing photosynthesis-based microbial mats to flourish in shallow water settings and resulting in the formation of widespread microbialites.
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