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A remarkably aerially extensive (∼2,000 km2) unit of carbonate microbialites occurs in many Triassic–Jurassic boundary interval outcrops of the southwestern United Kingdom and captures petrographic evidence that could link them to the end-Triassic extinction event. The bioherms—known regionally as the Cotham Marble—occur as discrete ∼20-cm-thick, decimeter- to meter-scale mounds, and display at least five growth phases that alternate between laminated and dendritic mesofabrics. Cross sections parallel to bedding through the dendritic phases expose a reticulate dendritic framework separated by polygonal spaces (∼1–3 cm diameter), characteristic of “tubestone” microbialites. Microscopically, the dendrolites contain evenly distributed rod to filamentous putative microfossils (∼2 µm diameter and ∼10 µm in length) in a matrix of micrite and contain higher total organic carbon than the surrounding matrix. Round to ellipsoidal spar-filled regions (∼200 µm in diameter) within the dendrolites (previously interpreted as serpulid worm tubes) likely resulted from the production of gas bubbles within rapidly lithifying mats or are a two-dimensional artifact of evenly spaced three-dimensional branching within the mats. The fill between the dendrolites of the first layer contains abundant phycoma clusters of the green algal prasinophyte Tasmanites, commonly considered a “disaster taxon.” The cyclic phases represent episodic and laterally extensive environmental change within shallow water coastal environments during a marine transgression. Collectively, the presence of microbial micrite in a shallow marine setting, the marked lateral extent of the bioherms, and the abundance of Tasmanites suggest the Cotham Marble microbialites formed during the high pCO2 and relatively warmer conditions associated with the events of the end-Triassic mass extinction.
A paleontological reconnaissance survey on Cretaceous and Paleogene terrestrial units along the Yukon River drainage through much of east-central Alaska has provided new chronostratigraphic constraints, paleoclimatological data, and the first information on local biodiversity within an ancient, high-latitude ecosystem. The studied unnamed rock unit is most notable for its historic economic gold placer deposits, but our survey documents its relevance as a source rock for Mesozoic terrestrial vertebrates, invertebrates, and associated flora. Specifically, new U-Pb ages from detrital zircons combined with ichnological data are indicative of a Late Cretaceous age for at least the lower section of the studied rock unit, previously considered to be representative of nearly exclusively Paleogene deposition. Further, the results of our survey show that this sedimentary rock unit preserves the first record of dinosaurs in the vast east-central Alaska region. Lastly, paleobotanical data, when compared to correlative rock units, support previous interpretations that the Late Cretaceous continental ecosystem of Alaska was heterogeneous in nature and seasonal.
During the Furongian (late Cambrian) and Early Ordovician, maze-like (maceriate) microbialites flourished in both Laurentia and Gondwana. The maze-like microbialites are characterized by centimeter- to decimeter-scale branching, complex structures. However, organisms responsible for the formation of maze-like structures are poorly known. In order to understand formational processes of maze-like microbialites, this study focuses on the Furongian microbialites of the North China Platform in which microbial components and siliceous sponges co-occur. The maze-like structures consist of microbial components such as microstromatolites, Girvanella, and Renalcis-like forms, as well as sponge spicule networks, whereas lime mud and bioclasts occupy the space between the structures. The maze-like structures developed on a relatively flat seafloor, forming low synoptic relief (<1 cm) above the sediment surface. Continuous growth of maze-like structures with balanced deposition of sediments led to meter-scale bioherms and biostromes, under the control of both microbes and siliceous sponges. This study suggests that siliceous sponges may have played an important role in the construction of maze-like structures between the end-Cambrian Series 2 extinction and the Great Ordovician Biodiversification Event.
Relative abundances of planktonic foraminiferal species from two cores from the subtropical, southwestern Atlantic indicate changes in oceanic conditions during the last glacial–interglacial cycle. During interglacial intervals (biozone X or Marine Isotopic Stage 5 (MIS-5) and biozone Z or MIS-1), the relative abundance of the intermediate water-dwelling species of menardiform plexus was high, whereas those of deep-dwelling species Globorotalia truncatulinoides and Globorotalia inflata were relatively low, suggesting high temperature conditions (≥22°C) and/or increased upper-water stratification during this period of time. In contrast, the absence of menardiform plexus and high abundances of cold-water species (G. truncatulinoides and G. inflata) during the glacial interval (biozone Y [MIS-4, MIS-3, and MIS-2]) suggest cold-water conditions (≤22°C) and/or a reduction of upper-water stratification. Two intervals of moderate temperature and/or low salinity during the last glaciation, however, are suggested by the increase in abundance of Pulleniatina plexus. Foraminiferal fauna have suggested a difference of 1 to 2°C between the last glacial interval and the late Holocene. Millennial-scale paleoceanographic events have been identified during the last interglacial interval (MIS-5), suggesting that warm conditions and/or a stratified water column were replaced by short intervals of cooler water and reduced upper-water stratification, as indicated by changes in abundances of menardiform plexus and G. truncatulinoides. The short interval during which menardiform plexus disappeared during the Holocene suggests a temperature decrease and can be related to the widespread stadial event in the Holocene at 8.2 ka.