The evolution of life on Earth is marked by catastrophic extinction events, one of which occurred ca. 200 Ma at the transition from the Triassic Period to the Jurassic Period (Tr-J boundary), apparently contemporaneous with the eruption of the world's largest known continental igneous province, the Central Atlantic magmatic province. The temporal relationship of the Tr-J boundary and the province's volcanism is clarified by new multi-disciplinary (stratigraphic, palynologic, geochronologic, paleomagnetic, geochemical) data that demonstrate that development of the Central Atlantic magmatic province straddled the Tr-J boundary and thus may have had a causal relationship with the climatic crisis and biotic turnover demarcating the boundary

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Until now, descriptions of intracellular biomineralization of amorphous inclusions involving alkaline-earth metal (AEM) carbonates other than calcium have been confined exclusively to cyanobacteria (Couradeau et al., 2012). Here, we report the first evidence of the presence of intracellular amorphous granules of AEM carbonates (calcium, strontium, and barium) in unicellular eukaryotes. These inclusions, which we have named micropearls, show concentric and oscillatory zoning on a nanometric scale. They are widespread in certain eukaryote phytoplankters of Lake Geneva (Switzerland) and represent a previously unknown type of non-skeletal biomineralization, revealing an unexpected pathway in the geochemical cycle of AEMs. We have identified Tetraselmis cf. cordiformis (Chlorophyta, Prasinophyceae) as being responsible for the formation of one micropearl type containing strontium ([Ca,Sr]CO ), which we also found in a cultured strain of Tetraselmis cordiformis. A different flagellated eukaryotic cell forms barium-rich micropearls [(Ca,Ba)CO ]. The strontium and barium concentrations of both micropearl types are extremely high compared with the undersaturated water of Lake Geneva (the Ba/Ca ratio of the micropearls is up to 800,000 times higher than in the water). This can only be explained by a high biological pre-concentration of these elements. The particular characteristics of the micropearls, along with the presence of organic sulfur-containing compounds-associated with and surrounding the micropearls-strongly suggest the existence of a yet-unreported intracellular biomineralization pathway in eukaryotic micro-organisms.

The microbial role in the formation of the cortex of low‐Mg calcite freshwater ooids in western part of Lake Geneva in Switzerland has been suggested previously, but not demonstrated conclusively. Early work mostly concentrated in hypersaline milieus, and hence little is known about their genesis in freshwater environments. We designed an in situ experiment to mimic the natural process of low‐Mg calcite precipitation. A special device was placed in the ooid‐rich bank of the lake. It contained frosted glass (SiO2) slides, while quartz (SiO2) is the most abundant mineral composition of ooid nuclei that acted as artificial substrates to favour microbial colonization. Microscopic inspection of the slides revealed a clear seasonal pattern of carbonate precipitates, which were always closely associated with biofilms that developed on the surface of the frosted slides containing extracellular polymeric substance, coccoid and filamentous cyanobacteria, diatoms and heterotrophic bacteria. Carbonate precipitation peaks during early spring and late summer, and low‐Mg calcite crystals mostly occur in close association with filamentous and coccoid cyanobacteria (e.g. Tolypothrix, Oscillatoria and Synechococcus, Anacystis, respectively). Further scanning electron microscope inspection of the samples revealed low‐Mg calcite with crystal forms varying from anhedral to euhedral rhombohedra, depending on the seasons. Liquid cultures corroborate the in situ observations and demonstrate that under the same physicochemical conditions the absence of biofilms prevents the precipitation of low‐Mg calcite crystals. These results illustrate that biofilms play a substantial role in low‐Mg calcite ooid cortex formation. It further demonstrates the involvement of microbes in the early stages of ooid development. Combined with ongoing microbial cultures under laboratory‐controlled conditions, the outcome of our investigation favoured the hypothesis of external microbial precipitation of low‐Mg calcite as the main mechanism involved in the early stage of ooid formation in freshwater Lake Geneva.

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This study provides a reconstruction of the Late Permian and Triassic depositional history of the Arabian shelf in the northern United Arab Emirates based on facies analysis and foraminiferal biostratigraphy. The presented data show that sedimentation occurred in three major sequences. From the Late Permian to Olenekian carbonates and evaporites were deposited in restricted lagoons and tidal flats. After a hiatus, sedimentation resumed and continued until the Late Ladinian/Carnian, leading to the deposition of a carbonate platform dominated by peritidal dolostones. A period of shelf exposure and erosion, spanning from the Carnian to Norian, was followed by the third major sequence with sedimentation into the Early Jurassic. During this third depositional sequence sedimentation changed from pure carbonate into mixed carbonate–siliciclastic deposits. This transition reflects the global regression of the sea in the Late Triassic (Triasina hantkeni Zone) and the increased erosion of large parts of the Arabian hinterland. A comparison of the evolution of the Arabian shelf in the study area with chronostratigraphic reference schemes for the Arabian Plate reveals remarkable differences in the distribution of Middle and Upper Triassic sequences. These are most likely the result of poor biostratigraphic control on previously studied formations in the region

New investigations in the Nanpanjiang Basin indicate that the onset of the iconic microbialites associated with the Paleozoic-Mesozoic boundary was Early Triassic in age. Bathymetry (water agitation, oxygenation, light penetration) and clastic load are shown to have exerted a direct control on the growth of microbialites. Carbonate supersaturation is also required for the deposition of the microbialites. Bathymetric control is further corroborated by the topographic inheritance of a latest Permian pull-apart basin into Early Triassic times, with a distribution of basal Early Triassic microbialites (BETM) restricted to uplifted blocks. This control is also reflected by the accumulation of carbonaceous black shales in adjacent troughs. The geographically most extensive Nanpanjiang BETM bloomed on a large NW-SE trending uplifted block exceeding 12,000 km2 (Luolou Platform) centered on northwestern Guangxi. Post-Triassic displacements along the Youjiang Fault obscure the paleogeographic relation of BETM exposed west of this fault. Triassic foraminifers occur in the basal most BETM episode, which is locally bracketed by high-energy grainstones made of reworked Permian foraminifers. Therefore, the Permian-Triassic boundary (PTB) is within the unconformity that separates the Late Permian Heshan Fm. from the basal most BETM. Where accommodation space was sufficient, up to five event surfaces are associated with the unconformity. Microfacies analysis supports chemical dissolution but did not reveal evidence for subaerial erosion, although intercalated grainstone made of Permian foraminifers indicate reworking. Chemical dissolution and mechanical erosion both conceivably contributed to the genesis of the unconformity. The upward shift from tabulated to domical microbial build-ups is accompanied by accumulation of coquinoid lenses between domes, which indicates deepening of the Luolou Platform BETM. The main drowning resulting from both regional tectonic subsidence and a global sea-level rise led to the cessation of the BETM that were buried under predominant fine siliciclastics. Any concomitant change in sea water chemistry appears unlikely.

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