Daniel J. Lehrmann

Ocean anoxia played a key role in the end-Permian mass extinction, the most severe biotic crisis in Earth’s history, yet reconstructions of oxygenation through this event have lacked a focus on deep-water oxygen loss. Thallium isotopes, which track deep-ocean oxygenation via manganese oxide burial, offer a potential solution but have only been applied to siliciclastic sections, limiting spatial and temporal coverage. Here, we present a combined carbonate-shale Tl isotope record, spanning four geographically distant regions and providing a near-continuous record of global-scale redox change during the extinction interval. Both lithologies record deep-marine anoxia beginning 1 million years before the extinction, demonstrating that carbonate- and shale-hosted Tl isotopes preserve consistent redox signals. Late Permian oxygen depletion likely remained largely constrained to the deep ocean and was distinct from the shelf- and slope-depth euxinia of the Early Triassic that was coupled with climate warming and central to the loss of animal diversity.

This study used systematic coring, hyperspectral scans, carbonate/siliciclastic analysis, and stable isotope measurements to evaluate environmental controls on large microbial buildup evolution. Buildups initiated on a discontinuous transgressive lag of flat-pebble conglomerate, controlling their spatial distribution and flat-based geometry. The consistent three-phase buildup architecture indicates allogenic forcing by fluctuations in sea level and siliciclastic influx. The initiation of each phase began in a pure carbonate system, with high-energy skeletal-oolitic grainstone interbuildup deposition. Depositional moats formed in the grainstone adjacent to buildups during phases 1 and 2. Buildups developed up to 5.7 m of synoptic relief during phases 1 and 3 prior to being onlapped by siliciclastic sediment. Phase 2 of the buildups intertongued with heterolithic interbuildup sediment, developing only 30 cm of synoptic relief. Lateral variability in phase 2 architecture indicates autogenic controls causing localized differences in microbialite growth rates versus interbuildup accumulation. Phases 1 and 3 display a dense calcitic calcimicrobial rind that surrounds and encloses the top of the buildups. The interior of buildups is composed of decimeter-scale stromatolitic columns, each encased by a calcimicrite rind and exhibiting mottled internal texture due to bioturbation and dolomitization. Interiors of buildups entrapped more carbonate sediment, creating greater porosity, and thus were destructively dolomitized. In contrast, the calcimicrite rinds of columns and the thick rinds surrounding buildups formed as biochemical carbonate precipitates, with minimal porosity, and, therefore, escaped dolomitization. The δ18O and δ13C values of the column and buildup rinds are consistent with late Cambrian seawater, supporting an origin as a precipitate and age as Sunwaptan.

Triassic strata of the Yangtze Platform at Guanling contain a dolomitized interior, undolomitized margin, and partially dolomitized slope to basin margin. Dolomitized microbial laminate caps of peritidal cycles and massive dolomite with associated evaporite nodules and solution collapse breccias are consistent with penecontemporaneous tidal flat and evaporative dolomitization in the platform interior. The preferential dolomitization of the slope and basin margin (up to 7 km basinward of the margin), dolomitization along fractures, and selective dolomitization of the matrix in slope breccia that diminishes toward the margin are interpreted to have resulted from the incursion of basin-derived fluids during burial. Integrated analysis of fluid-inclusion microthermometry, oxygen, carbon, and strontium isotopes, trace element geochemistry, U-Pb age dates of carbonate phases, and burial history support the recrystallization of interior dolomite and slope to basin-margin dolomitization by brines at high temperatures during burial. The Yangtze Platform at Guanling provides an excellent example of widespread stratiform dolomitization resulting from the superposition of multiple mechanisms, including penecontemporaneous dolomitization by evaporative seawater brines, high-temperature dolomitization of the slope and basin margin by basinal brines, and high-temperature recrystallization of dolomite by brines during burial. This study provides an example that suggests that widespread stratiform dolomite may result from superposed Earth surface and high-temperature burial dolomitization processes and provides a valuable analog for other carbonate platforms in which the margin remains undolomitized while the interior and basin margin are dolomitized. Similar mechanisms likely contributed to the widespread dolomitization of platforms across the Nanpanjiang and Sichuan basins.

Several new foraminiferal taxa are described from the Changhsingian carbonates of southern Turkey, and their evolutionary relationships are discussed within the middle to late Permian time frame. Comprising Retroseptellina, Septoglobivalvulina, and Paraglobivalvulinoides, Retroseptellininae n. subfam. originated in the Wordian with thin and dense microgranular walls and became diverse and abundant in Changhsingian strata. Paraglobivalvulina? intermedia n. sp. appeared in the late Capitanian, survived into the Changhsingian, and gave way to completely involute tests of Paraglobivalvulininae. From the class Miliolata, Midiellidae n. fam., consisting of Midiella and Pseudomidiella, is characterized by sigmoidal coiling, and Pseudomidiella sahini n. sp. is probably the youngest known Changhsingian descendant. Glomomidiellopsis? okayi n. sp., which is interpreted as an evolutionary link between Capitanian Hemigordiopsis and Lopingian Glomomidiellopsis, survived into the Changhsingian. In the class Nodosariata, from the fully coiled Robuloides lineage of Robuloididae, Robuloides lata n. sp. and Plectorobuloides taurica n. gen. n. sp. most likely originated from R. lens in the Changhsingian. The R. acutus lineage, characterized by the reduction of laterally thickened hyaline wall and the appearance of evolute coiling, yielded Robuloides? rettorii n. sp. and Pseudorobuloides reicheli n. gen. n. sp. Calvezina anatolica n. sp. and Eomarginulinella galinae n. sp. are interpreted to have evolved from weakly coiled lineages in Robuloididae, whereas Pseudocryptomorphina amplimuralis n. gen. n. sp. is a poorly understood taxon and requires further study. Robustopachyphloia farinacciae n. sp. is interpreted as a descendant of some species within the genus Pachyphloia. The presence of canal-like pores in the wall of some Pachyphloia specimens is suggestive of a new morphological structure in the evolutionary history of the Changhsingian foraminifera.

The tempo of biotic recovery following extinction reflects the time scales of evolutionary processes and the long-term consequences of degraded ecosystems, but recovery patterns are poorly resolved. In this study, we investigated the tempo of biotic recovery by evaluating metazoan-algal reef assembly following the end-Permian mass extinction. We combined satellite imagery analysis, field mapping, biostratigraphy, and quantitative petrography to assess recovery in the oldest-known and most stratigraphically extensive Lower to Middle Triassic platform-margin reef. The reef occurs in upper Spathian (upper Lower Triassic) to upper Anisian (lower Middle Triassic) strata of the Great Bank of Guizhou (GBG) isolated carbonate platform in south China. Previous work suggests that metazoan-algal reefs were absent for 8−10 m.y. following extinction but were biologically diverse from their Pelsonian (middle Anisian) initiation. This pattern implies that reefs can reassemble rapidly (<< 1 m.y.) when environmental conditions are favorable. In contrast, our analyses indicate that calcareous sponges, calcareous algae, and early scleractinian corals occurred progressively in the GBG reef and that biotic recovery metrics increased gradually. Unlike nonreef ecosystems, biodiverse metazoan-algal reefs were delayed until the late Pelsonian or early Illyrian, postdating broader marine ecosystem recovery and isotopic evidence for carbon-cycle stabilization by 2−4 m.y. Our findings suggest that reef and nonreef ecosystems differ in their recovery pattern and tempo. Reef recovery from severe environmental perturbation can require several million years, even after hospitable conditions return, highlighting the importance of modern reef conservation.

Keichousaurus huiis the most abundant and representative species in the Xingyi biota in South China.K. huihas been studied in many aspects, including its functional morphology, osteology, ontogeny, allometric growth, sexual dimorphism, and reproduction. Previous studies have assumed thatKeichousaurus’ anteriorly curved teeth were used to impale prey and to tear and swallow either the whole or partially fragmented prey. Prey items have been assumed to be small fish or soft-bodied invertebrates, such as squid and shrimp, that were also present in the Xingyi biota. However, there has been no direct evidence for the anatomy of the digestive tract or the diet of theKeichousaurus. There are few reports of the soft tissue preservation ofK. hui. In this study, we present relatively complete preservation of the digestive tract with food remains in several well-preserved specimens ofK. hui. By comparing with modern reptiles, we reconstructed the internal distribution and organ composition of the digestive tract ofK. hui.Through the analysis of the alimentary canal remnants by microscope and X-ray computed microtomography, we infer thatK. huiwas a frequently piscivorous species that usually swallowed its prey whole.

Ooid size in natural systems can be predicted based on the expected equilibrium between precipitation and abrasion rates. Therefore, ooid size distributions may enable quantitative inferences about past ocean and climate conditions from sedimentology that complement insights from geochemistry. A few previous attempts have been made to compile information on ooid sizes across the Phanerozoic, but no systematic ooid size dataset is available and equilibrium models of ooid growth have not been explored comprehensively. In this study, convolutional neural network‐based segmentation was used to automate ooid size measurement and provide a systematic sampling of ooid sizes spanning the Phanerozoic. This data set is coupled with Monte Carlo simulations to quantitatively explore the interplay between three testable physicochemical parameters (long‐term average carbonate saturation state, bed shear velocity and intermittency of transport) in determining equilibrium ooid size. The numerical model shows that a typical‐sized ooid (≤1000 µm) can grow under a wide range of parameter combinations whereas giant ooids (>2000 µm) can only form under a more restricted set of circumstances. Furthermore, the model predicts that the formation of giant ooids is common when conditions favourable for rapid CaCO₃ precipitation are achieved. This finding is consistent with the naturally rare occurrences of giant ooids, which developed under exceptional environmental conditions and are primarily associated with intervals of bimineralic seas and elevated sea‐surface temperature (greenhouse or transitional climate). Whereas global controls could help explain secular trends in ooid size, significant spatial variability of ooid size in several time intervals further demonstrates the importance of local environmental parameters in the growth of ooids. Together, spatial and temporal variation in ooid size distributions can complement geochemical proxies in quantitatively reconstructing ancient tropical marine environments.

Ocean anoxia was an important kill mechanism in the end‐Permian mass extinction and uranium isotope data are among the most powerful tools for quantifying the global extent and duration of ocean deoxygenation due to the dependence of uranium isotope fractionation on bottom‐water redox conditions. Although coherent stratigraphic variation in uranium isotope ratios (δ²³⁸U) and uranium concentrations ([U]) indicative of prolonged deoxygenation beginning coincident with the extinction is well established, the precise extent of anoxia and associated uncertainty have yet to be quantified. Uncertainty arises from both noise in the data and imprecise knowledge of key parameters within the uranium cycle. In this study, we use the Monte Carlo method to explore a range of scenarios and their implications for the uranium cycle across the Permian‐Triassic boundary and through the first 1.7 million years of the Triassic. We then compare model predictions against measured data using principal component analysis to identify model runs and associated parameter values most compatible with trends in the observed data. The best‐fitting models indicate a pronounced increase in the extent of seafloor anoxia across the Permian/Triassic transition, reaching 18% of the seafloor (95% CI: [11%, 47%]), lasting anywhere from 20 kyr to 1.2 Myr. There is an inverse relationship between the extent and duration of anoxia in the set of best‐fitting models. This initial pulse of pronounced anoxia is followed by a prolonged aftermath, which continues through the remainder of the study interval, of less extensive, yet still expanded, anoxia covering 7.8% of the seafloor (95% CI: [1.6%, 48.9%]). Both expanded and protracted anoxia are required to fit existing data, with no indication of full re‐oxygenation during the study interval.

The Great Bank of Guizhou (GBG) is a latest Permian to earliest Late Triassic isolated carbonate platform in the Nanpanjiang Basin of southern China. The GBG is exposed in cross-section by exhumation and erosion along a syncline, providing an essentially continuous record of platform initiation, development and drowning across platform to basin palaeoenvironments. Platform evolution spanned the Palaeozoic to Mesozoic transition associated with physical, chemical and biological evolution of marine environments and ecosystems, including the end-Permian extinction and Triassic biotic recovery. Consequently, the GBG records the influence of late Palaeozoic to early Mesozoic carbonate factory evolution on the stratigraphic architecture of an isolated carbonate platform, facilitating examination of the close relationship among ocean environments, marine ecosystems and carbonate depositional systems.