Peritidal Carbonate Cycles Induced by Carbonate Productivity Variations: A Conceptual Model for an Isolated Early Triassic Greenhouse Platform in South China

Eustasy has commonly been invoked to explain peritidal carbonate cyclicity, but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal. We propose that peritidal cycles on an Early Triassic isolated carbonate platform in Guizhou, South China, were formed by hierarchical carbonate productivity variations. Most of the 149 shallowing-upward cycles are typically terminated by flooding over intertidal facies and contain rare supratidal facies and no prolonged subaerial exposure. Low-diversity benthos in the platform interior during the post-end-Permian biotic recovery were sensitive to environmental perturbations, which caused variations in benthic sediment productivity in the subtidal carbonate factory. The perturbations may be driven by changes in salinity and degree of eutrophication, or repeated platform mini-drowning by anoxic and/or CO2-charged deep water upwelled onto the banktop. They were modulated by Milankovitch orbitally-driven climatic and oceanographic factors as suggested by the hierarchical stacking pattern and spectral signals of these cycles. A one-dimensional conceptual model shows that hierarchical productivity variations alone may generate hierarchical peritidal carbonate cycles under conditions of constant subsidence and no sea-level fluctuation

Controls on Microbial and Oolitic Carbonate Sedimentation and Stratigraphic Cyclicity Within a Mixed Carbonate-Siliciclastic System: Upper Cambrian Wilberns Formation, Llano Uplift, Mason County, Texas, USA

The upper Cambrian Wilberns Formation in central Texas records deposition on a low-gradient shelf within a mixed carbonate–siliciclastic tidal-flat system that changes offshore to subtidal shelf and open-marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE–SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low-frequency and intermediate-frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea-level fluctuation. Random facies stacking and the lack of metre-scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms

The Role of Carbonate Factories and Sea Water Chemistry on Basin-Wide Ramp to High-Relief Carbonate Platform Evolution: Triassic, Nanpanjiang Basin, South China

The end-Permian extinction and its aftermath altered carbonate factories globally for millions of years, but its impact on platform geometries remains poorly understood. Here, the evolution in architecture and composition of two exceptionally exposed platforms in the Nanpanjiang Basin are constrained and compared with geochemical proxies to evaluate controls on platform geometries. Geochemical proxies indicate elevated siliciclastic and nutrient fluxes in the basal Triassic, at the Induan—Olenekian boundary and in the uppermost Olenekian. Cerium/Ce* shifts from high Ce/Ce* values and a lack of Ce anomaly indicating anoxia during the Lower Triassic to a negative Ce anomaly indicating oxygenation in the latest Olenekian and Anisian. Uranium and Mo depletion represents widespread anoxia in the world\u27s oceans in the Early Triassic with progressive oxygenation in the Anisian. Carbonate factories shifted from skeletal in the Late Permian to abiotic and microbial in the Early Triassic before returning to skeletal systems in the Middle Triassic, Anisian coincident with declining anoxia. Margin facies shifted to oolitic grainstone in the Early Triassic with development of giant ooids and extensive marine cements. Anisian margins, in contrast, are boundstone with a diverse skeletal component. The shift in platform architecture from ramp to steep, high-relief, flat-topped profiles is decoupled from carbonate compositions having occurred in the Olenekian prior to the onset of basin oxygenation and biotic stabilisation of the margins. A basin-wide synchronous shift from ramp to high-relief platforms points to a combination of high subsidence rate and basin starvation coupled with high rates of abiotic and microbial carbonate accumulation and marine cement stabilisation of oolitic margins as the primary causes for margin up-building. High sea water carbonate saturation resulting from a lack of skeletal sinks for precipitation, and basin anoxia promoting an expanded depth of carbonate supersaturation, probably contributed to marine cement stabilisation of margins that stimulated the shift from ramp to high-relief platform architecture

Proliferation of \u3cem\u3eChondrodonta\u3c/em\u3e as a Proxy of Environmental Instability at the Onset of OAE1a: Insights from Shallow-Water Limestones of the Apulia Carbonate Platform

Chondrodonta is an opportunistic, oyster-like bivalve, common in shallow-water carbonates of the Cretaceous Tethyan Realm. Despite its high abundance and widespread geographic distribution, the precise relationship between the early Aptian proliferation and environmental perturbations resulting from the Oceanic Anoxic Event 1a (OAE1a), has not been investigated. Stratigraphic and geochemical analyses of the lower Aptian Chondrodonta bedsets within the inner platform limestones of the Apulia Carbonate Platform (Gargano Promontory, southern Italy) are conducted to assess the environmental controls on the Chondrodonta proliferation and its timing and causal relationship to OAE1a. Chondrodonta occurs with sparse to common individuals within requieniid rudist floatstone–rudstones, forms monospecific biostromes during the early phase of stressed environmental conditions and then rapidly disappears at the peak of OAE1a. It proliferates in dysoxic seawater with relatively increased trophic sources, which correlate to increasing nutrient levels in the nearby pelagic realm. Chondrodonta-rich beds are associated worldwide with the onset of OAE1a and occur in a transitional context between a stable and a strongly stressed environment, where the opportunistic behaviour of Chondrodonta is rather efficient. Increasing nutrient load and unstable environmental conditions right below the peak of OAE1a created an environmental ‘window’ favourable for Chondrodonta to proliferate, outplaying the less tolerant benthos (for example, rudists). The occurrence, duration and position of the environmental window were controlled by local palaeogeographic and hydrodynamic settings (i.e. low energy, decreased seawater oxygenation and circulation). Further increase in inhospitable conditions, leading to OAE1a, constituted an upper threshold for Chondrodonta and allowed mesotrophic taxa like Bacinella–Lithocodium and orbitolinids to dominate the benthic communities. The present study suggests that the proliferation of Chondrodonta in shallow-water platform carbonates can be used as proxy for the initial phase of ecological stress related to OAE1a

Milankovitch Climatic Signals in Lower Triassic (Olenekian) Peritidal Carbonate Successions, Nanpanjiang Basin, South China

Meter-scale peritidal carbonate strata from an isolated platform in the Nanpanjiang Basin of South China were tested for the presence of periodic or quasi-periodic climatic signals. These signals provide information on factors controlling cyclic sedimentation in the equatorial eastern Tethys during the Early Triassic greenhouse period. Parasequences are composed of shallowing-upward successions of subtidal facies, including skeletal packstone and grainstone, calcimicrobial boundstone, and oolitic packstone and grainstone, intertidal laminated lutite and ribbon rock, and rare supratidal microbial laminite. Parasequence stacking patterns display three orders of cyclicity, suggesting hierarchical stratigraphic relationships. Gamma analysis of Kominz and Bond [Earth Planet. Sci. Lett. 48 (1990) 233-244] was used to estimate facies-dependent thickness-time conversion factors (i.e. γs), and to construct γ-tuned and γ-untuned time series for two stratigraphic sections. Spectra of these time series indicate the presence of quasi-periodic signals, with prominent short-eccentricity (94.9-131.2 kyr), short-obliquity (35.8 kyr), and long-precessional index (21.2 kyr) peaks, and minor long-eccentricity (412.9 kyr), long-obliquity (45.3 kyr), short-precessional index (17.7 kyr), and constructional-tone (9.7 kyr) peaks when calibrated on the γ-tuned spectra. Thus, Milankovitch climatic forcing probably greatly influenced sedimentation. The calibration indicates that the subtidal facies has a sedimentation rate of 24.6-30.7 cm/kyr and the intertidal-supratidal facies has a rate of 2.7-6.0 cm/kyr. The estimated duration of deposition of the two sections is 1139-1423 kyr, and corresponds to a stratigraphic completeness of 32-92%. The completeness is much greater than that of icehouse stratigraphic records. We speculate that variations in carbonate productivity and environmental conditions driven by Milankovitch climatic forcing, combined with low-amplitude sea-level fluctuations, were likely major controls on cyclic sedimentation. Furthermore, evolutional spectra of the two sections show that dominant Milankovitch climatic forcings varied from short-eccentricity, obliquity, to long-precessional index during the course of sedimentation. Some spectral differences between the two sections suggest variations in depositional/recording mechanisms of Milankovitch climatic signals between platform interior and windward platform margin

The Pattern and Timing of Biotic Recovery from the End-Permian Extinction on the Great Bank of Guizhou, Guizhou Province, China

Microfacies analysis and point Counts of thin sections from 608 hand samples were used to track changes in the abundance and diversity of fossil grains through the extended recovery interval following end-Permian mass extinction on the Great Bank of Guizhou (GBG)-an isolated Late Permian to Late Triassic carbonate platform in south China. Exposure of a two-dimensional cross-section of the platform permits the comparison of faunal patterns along an environmental gradient front shallow to deep water The diverse Late Permian biota was dominated by calcareous sponges, crinoids, articulate brachiopods, foraminifera, and calcareous algae. In contrast, Early Triassic communities were dominated by mollusks, with increasing abundance of crinoids beginning in the Spathian. Increase in the diversity and abundance of fossils 071 the GBG was confined to a brief interval near the Spathian-Anisian boundary and concentrated along the platform margin. Later Middle Triassic diversification, the return of calcareous algae and calcareous sponges, and the appearance of scleractinian corals did not substantially alter the mollusk-crinoid-Tubiphytes assemblage before the end of the Middle Triassic. The low abundance of skeletal grains in Lower Triassic strata implies: (1) similarities in the relative contributions of micrite, microbialites, and oolites to Neoproterozoic carbonates result, at least in part, from the temporary removal of skeletal sinks for calcium carbonate; and (2) animals with hard skeletons remained at low abundance from the time of the end-Permian extinction through much of the Early Triassic.Earth and Planetary SciencesOrganismic and Evolutionary Biolog

Facies Selectivity of Benthic Invertebrates in a Permian/Triassic Boundary Microbialite Succession: Implications for the Microbialite Refuge Hypothesis

Thrombolite and stromatolite habitats are becoming increasingly recognized as important refuges for invertebrates during Phanerozoic Oceanic Anoxic Events (OAEs); it is posited that oxygenic photosynthesis by cyanobacteria in these microbialites provided a refuge from anoxic conditions (i.e., the “microbialite refuge” hypothesis). Here, we test this hypothesis by investigating the distribution of ~34, 500 benthic invertebrate fossils found in ~100 samples from a microbialite succession that developed following the latest Permian mass extinction event on the Great Bank of Guizhou (South China), representing microbial (stromatolites and thrombolites) and non‐microbial facies. The stromatolites were the least taxonomically diverse facies, and the thrombolites also recorded significantly lower diversities when compared to the non‐microbial facies. Based on the distribution and ornamentation of the bioclasts within the thrombolites and stromatolites, the bioclasts are inferred to have been transported and concentrated in the non‐microbial fabrics, that is, cavities around the microbial framework. Therefore, many of the identified metazoans from the post‐extinction microbialites are not observed to have been living within a microbial mat. Furthermore, the lifestyle of many of the taxa identified from the microbialites was not suited for, or even amenable to, life within a benthic microbial mat. The high diversity of oxygen‐dependent metazoans in the non‐microbial facies on the Great Bank of Guizhou, and inferences from geochemical records, suggests that the microbialites and benthic communities developed in oxygenated environments, which disproves that the microbes were the source of the oxygenation. Instead, we posit that microbialite successions represent a taphonomic window for exceptional preservation of the biota, similar to a Konzentrat‐Lagerstätte, which has allowed for diverse fossil assemblages to be preserved during intervals of poor preservation

Large Perturbations of the Carbon Cycle During Recovery from the End-Permian Extinction

High-resolution carbon isotope measurements of multiple stratigraphic sections in south China demonstrate that the pronounced carbon isotopic excursion at the Permian-Triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the Early Triassic before ending abruptly early in the Middle Triassic. The unusual behavior of the carbon cycle coincides with the delayed recovery from end-Permian extinction recorded by fossils, suggesting a direct relationship between Earth system function and biological rediversification in the aftermath of Earth's most devastating mass extinction.Organismic and Evolutionary Biolog

Fully Automated Carbonate Petrography Using Deep Convolutional Neural Networks

Carbonate rocks are important archives of past ocean conditions as well as hosts of economic resources such as hydrocarbons, water, and minerals. Geologists typically perform compositional analysis of grain, matrix, cement and pore types in order to interpret depositional environments, diagenetic modification, and reservoir quality of carbonate strata. Such information can be obtained primarily from petrographic analysis, a task that is costly, labor-intensive, and requires in-depth knowledge of carbonate petrology and micropaleontology. Recent studies have leveraged machine learning-based image analysis, including Deep Convolutional Neural Networks (DCNN), to automate description, classification and interpretation of thin sections, subsurface core images and seismic facies, which would accelerate data acquisition and reproducibility for these tasks. In carbonate rocks, this approach has been applied primarily to recognize carbonate lithofacies, and no attempt has been made to individually identify and quantify various types of carbonate grains, matrix, and cement. In this study, the applicability and performance of DCNN-based object detection and image classification approaches are assessed with respect to carbonate compositional analysis. The training data comprised of more than 13,000 individually labelled objects from nearly 4000 carbonate petrographic images. The dataset is grouped into six and nine different classes for the image classification and object detection tasks, respectively. Even with a small and relatively imbalanced training set, the DCNN was able to achieve an F1 score of 92% for image classification and 84% mean precision for object detection by combining one-cycle policy, class weight, and label mixup-smoothing methods. This study highlights the inefficiency of image classification as an approach to replicating human description and classification of carbonate petrography. By contrast, DCNN-based object detection appears capable of approaching human speed and accuracy in the area of carbonate petrography because it is able to individually locate and identify different carbonate components with greater cost-efficiency, speed, and reproducibility than conventional (human) petrographic analysis