Isotope Excursion Research Articles

Mapping amorphous SiO2 in Devonian shales and the possible link to marine productivity during incipient forest diversification

Silica cycling in the world’s oceans is not straightforward to evaluate on a geological time scale. With the rise of radiolarians and sponges from the early Cambrian onward, silica can have two depositional origins, continental weathering, and biogenic silica. It is critical to have a reliable method of differentiating amorphous silica and crystalline silica to truly understand biogeochemical and inorganic silica cycling. In this study, opal-A is mapped across the Western Canada Sedimentary Basin in the Late Devonian Duvernay Formation shales using longwave hyperspectral imaging alongside geochemical proxies that differentiate between crystalline and amorphous SiO2, during the expansion of the world’s early forests. Signaled by several carbon isotope excursions in the Frasnian, the punctata Event corresponds to the expansion of forests when vascular land plants develop seeds and deeper root networks, likely resulting in increased pedogenesis. Nutrients from thicker soil horizons entering the marine realm are linked to higher levels of primary productivity in oceans and subsequent oxygen starvation in deeper waters at this time. The results of this study reveal, for the first time, the spatial distribution of amorphous SiO2 across a sedimentary basin during this major shift in the terrestrial realm when forests expand and develop deeper root networks.

Enhanced magmatism played a dominant role in triggering the Miocene Climatic Optimum

The Miocene Climatic Optimum represents one of the major warming events during the Cenozoic and was accompanied by a positive stable carbon isotope excursion. Here, we report seawater osmium isotope data from Pacific Ocean sediments to determine if carbon dioxide emissions from coeval magmatism could have been the trigger for this event. Our data reveal a negative osmium isotope excursion from 0.80 to 0.72 between about 17 and 15.8 million years ago that can be explained by enhanced magmatism during this period. Simple mass balance calculations suggest an increase of 22‒45% in non-radiogenic osmium input from the mantle into the ocean during this period. Using osmium isotopic composition as a constraint for global carbon cycle modelling, we find that such an increase in magmatism is capable of elevating atmospheric carbon dioxide concentrations by 65‒140 ppmv and of causing a positive excursion in seawater stable carbon isotopes of 0.4‒0.7‰, consistent with geochemical observations. We conclude that it is conceivable that the enhanced magmatism played a dominant role in causing the Miocene Climatic Optimum.

Berriasian–Valanginian Geochronology and Carbon-Isotope Stratigraphy of the Yellow Cat Member, Cedar Mountain Formation, Eastern Utah, USA

The Early Cretaceous Yellow Cat Member of the terrestrial Cedar Mountain Formation in Utah, USA. has been interpreted as a “time-rich” unit because of its dinosaur fossils, prominent paleosols, and the results of preliminary chemostratigraphic and geochronologic studies. Herein, we refine prior interpretations with: (1) a new composite C-isotope chemostratigraphic profile from the well-known Utahraptor Ridge dinosaur site, which exhibits δ13C features tentatively interpreted as the Valanginian double-peak carbon isotope excursion (the so-called “Weissert Event”) and some unnamed Berriasian features; and (2) a new cryptotephra zircon eruption age of 135.10 ± 0.30/0.31/0.34 Ma (2σ) derived from the CA-ID-TIMS U-Pb analyses of zircons from a paleosol cryptotephra. Our interpretations of δ13C features on our chemostratigraphic profile, in the context of our new radiometric age, are compatible with at least one prior age model for the “Weissert Event” and the most recent revision of the Cretaceous time scale. Our results also support the interpretation that the Yellow Cat Member records a significant part of Early Cretaceous time.

Accelerated light carbon sequestration following late Paleocene-early Eocene carbon cycle perturbations

Carbon releases into the climate system produce global warming and ocean acidification events that can be reversed eventually by carbon sequestration. However, the underlying controls on the timescales of carbon removal, and their dependence on the amplitude of the initial perturbation, are poorly understood. Here, we assess a series of late Paleocene-early Eocene (LPEE) carbon cycle perturbations (∼56-52 Ma) of different amplitudes to constrain carbon removal timescales. Carbon isotope excursions (CIEs) and sedimentation patterns for the largest event, the Paleocene-Eocene Thermal Maximum (PETM), allow identification of a light carbon injection that appeared ∼85 kyr after the PETM onset. This CIE may have been triggered by orbital forcing of long (∼400 kyr) and short (∼100 kyr) eccentricity maxima. The various LPEE light carbon injections were followed by exponential carbon removal trends with half-life (t1/2) estimates of ∼6-26 kyr. These values are smaller than background estimates for the modern carbon cycle (t1/2>100 kyr), which reveals accelerated light carbon sequestration. We find that one estimated t1/2 period coincided temporally with ocean acidification recovery in different locations with contrasting paleo-water depths. This pattern indicates enhanced chemical weathering following LPEE CIEs; however, chemical weathering timescales are an order of magnitude longer than the observed t1/2 estimates. This reveals that several carbon processes were optimized during LPEE CIE recovery. Similar t1/2 estimates are obtained for light carbon injections of different sizes, which suggests that carbon removal was optimized to conditions induced by the initial perturbation. Temperature controls on oxygen solubility may have accelerated the oceanic biological pump in proportion to each LPEE carbon injection. This process may have caused accelerated carbon sequestration during LPEE CIE recovery and produced the short carbon removal timescales identified by t1/2 estimates of LPEE carbon cycle perturbations.

Deconstructing the Lomagundi-Jatuli Carbon Isotope Excursion

The early to mid-Paleoproterozoic Lomagundi-Jatuli Excursion (LJE) is ostensibly the largest magnitude (approximately +5 to +30‰), longest duration (ca. 130–250 million years) positive carbon isotope excursion measured in carbonate rocks in Earth history. The LJE has been attributed to large nutrient fluxes, an increase in the size of the biosphere, a reorganization of the global carbon cycle, and oxygenation of the atmosphere. However, significant debate remains about its genesis, synchroneity, global-versus-local extent, and role in atmospheric oxygenation. Here we review existing models and mechanisms suggested for the LJE and analyze a compilation of ∼9,400 δ13Ccarb and associated contextual data. These data call into question the interpretation of the LJE as a globally synchronous carbon isotope excursion and suggest that any model for the LJE must account for both the absence of a clearly defined initiation and termination of the excursion and a facies-dependent expression of 13C-enrichment. ▪The Lomagundi-Jatuli Excursion (LJE) continues to challenge current understandings of the carbon cycle.▪Understanding this excursion is critical for reconstructing biogeochemical cycles and atmospheric oxygenation through Earth history.▪Some evidence indicates local rather than global changes in δ13CDIC and raises the possibility of asynchronous, local excursions.▪Resolving whether the LJE was globally synchronous or asynchronous is essential for discriminating between different models.

Dynamic seawater redox status in the early Mesoproterozoic: Evidence from trace elements and sulfur isotopic compositions of carbonate associated sulfate in multiple sections

The early Mesoproterozoic (1.6–1.4 Ga) is a critical stage for oxygenation of Earth’s surface and early eukaryote evolution. However, the relationship between both remains uncertain. Here, we provide new carbonate associated sulfate (CAS) sulfur isotopic compositions (δ34SCAS) and trace elements from the 1.6 to 1.55 billion-year-old Gaoyuzhuang Formation in multiple sections within the Yanshan Basin, North China, to investigate the temporal and spatial variation of ocean redox conditions. New and collected δ34SCAS data from five sections (Pingquan, Kuancheng, Qianxi, Jixian and Gan'gou sections) reveal similar sulfur isotopic excursions, but the differences in absolute values indicate spatial heterogeneity of sulfur isotopic compositions, which may reflect marine low-sulfate concentrations and oceanic redox stratification. The most elevated δ34SCAS values appeared at the top of the Second Member of the Gaoyuzhuang Formation, followed by distinct negative excursions at the bottom of the Third Member, suggesting a dynamic sulfur cycle and seawater redox status during this period. Combined with previous reported carbon and sulfur isotopes and some redox proxies (e.g., Ce anomalies, manganese-rich deposit, I/(Mg + Ca) and Fe speciation), we suggest that at least two possible oxygenation events may have occurred in the Gaoyuzhuang Formation: (1) the bottom of the Second Member and (2) the bottom or middle part of the Third Member. Different sections show asynchronous timing of these events, indicating that the redox status was spatial heterogenous within the Yanshan Basin. The spatial heterogeneity may be controlled by facies and paleogeographic locations. Considering that decimeter‐scale multicellular eukaryotes are preserved in the middle part of the Third Member, we argue that oxic bottom water condition may be a critical factor for early eukaryote evolution.

Carbon isotopic composition and organic petrography of thermally metamorphosed Antarctic coal: Implications for evaluation of δ13Corg excursions in paleo-atmospheric reconstruction

Large, globally documented negative δ13C excursions that occur across the Permian-Triassic transition are thought to represent the isotopic fingerprint of considerable releases of isotopically light carbon into the atmosphere, with the largest of these excursions (as much as −22.2‰) measured in bulk organic matter in coals and carbonaceous shales from Antarctica. Previous studies have attributed the carbon isotopic excursion (CIE) at the Permian–Triassic boundary to the release of CH4 from organic-rich rocks intruded by dikes and sills associated with the Siberian Traps flood basalts (Russia) and Longwood-Bluff (New Zealand) intrusions. However, any assessment of paleoatmospheric conditions based on this or other δ13Corg excursions must consider the multiple controls on isotopic composition, including maceral content and post-depositional diagenetic and maturation processes. Permian-age coal seams and associated sediments in Antarctica were extensively altered after burial by localized high heat flow and, in some cases, by contact metamorphism associated with Jurassic dikes and sills. Levels of organic maturation reach anthracite, meta-anthracite, and even graphite; such intense thermal alteration had the potential to change the carbon isotopic composition of the bulk organic matter (δ13Corg). To evaluate the Permian–Triassic isotopic excursion in Antarctica, it is important to understand the inherent variability in δ13Corg in Permian-age coal and organic-rich sedimentary rocks prior to the CIE; thus, 128 samples representing pre-boundary coals and shales were selected from the United States Polar Rock Repository for analysis. Petrographic (maceral analysis and vitrinite reflectance), geochemical (C, H, N), and carbon isotopic analyses were performed. Based on the presence of coarse-grained circular and fine-grained lenticular mosaic in the anisotropic cokes found in samples in close proximity to intrusions, the rank of the samples prior to intrusion is estimated to have been medium volatile bituminous (∼1.2% Rmax). These highly altered coals and cokes can have random vitrinite reflectances (Rr) exceeding 6%. Subsequent to intrusion, alteration beyond the contact aureoles continued due to high heat flow, producing high rank coals with Rr > 2%. Values of δ13Corg range between −26.5 and –21.1‰ (5.4‰ difference). Significant relationships (at the 1 and 5% levels) exist between δ13Corg and the level of organic maturity, organic carbon (Corg) content (i.e., size of the carbon reservoir), and total inertinite content, suggesting variability due to these factors should be considered when assessing isotopic excursions and interpretation of Permian-Triassic atmospheric conditions. These findings highlight the importance of incorporating organic petrography into δ13Corg studies.

Astrochronology and carbon-isotope stratigraphy of the Fengcheng Formation, Junggar Basin: Terrestrial evidence for the Carboniferous-Permian Boundary

The Carboniferous–Permian Boundary (CPB) is a major stratigraphic boundary and a marker for global stratigraphic correlation. However, the delimitation and global correlation of the CPB is hindered by the erosion during the Late Paleozoic Ice Age. Several carbonate platforms show a negative carbon isotope excursion (NCIE) in response to the CPB, which has great potential for the delimitation of this boundary. However, the response of terrestrial profiles to the CPB is unclear, and the global synchroneity of this NCIE is controversial. We studied a high paleolatitude section to evaluating the feasibility of the NCIE for delimitating CPB. We used astrochronological analysis of the 340 m Gamma Ray log of the Fengcheng Formation in Mahu Sag, Junggar Basin, northwest China, where the CPB may have been recorded. We constructed a high-resolution chronostratigraphic framework for the Fengcheng Formation, which precisely defines the chronostratigraphic attribution of the Fengcheng Formation as the Gzhelian–Asselian, and confirms that the CPB is recorded in the Fengcheng Formation. In addition, a global correlation of carbon isotope profiles revealed a negative excursion in terrestrial carbon isotope profiles across the CPB, which is consistent with the characteristics of NCIEs in several marine profiles. The similarity of the NCIE records supports their global synchrony and confirms that they can be used to delimit the CPB in terrestrial sections. Combining the astronomical time scale and the carbon isotope profile, we constrain the position of the CPB to the depth range of 4694.84–4703.56 m, with 4698.9 m as a preliminary estimate of the CPB. This new astronomical timescale and the stratigraphic position of the CPB enable the revision of the chronostratigraphic attribution of the terrestrial strata in the Junggar Basin. Overall, our results contribute to studies of the Carboniferous–Permian global carbon cycle, high-latitude glaciation, eustatic fluctuations, global volcanism, and their potential relationships.

Ba stable isotope excursions induced by multiple hyperthermal events: A potential new index for transient global warming

Ba stable isotope excursions induced by multiple hyperthermal events: A potential new index for transient global warming

Carbonate�Corganic decoupling during the first Neoproterozoic carbon isotope excursion

<p><b>The Neoproterozoic Era (1000�C539 Ma) experienced repeated negative carbon isotope excursions, representing the largest negative excursions in Earth history and their origins remain enigmatic. One salient observation is that unlike younger Phanerozoic carbon isotope excursions that almost exclusively exhibit covariation in the carbon isotopes of carbonate and organic carbon, about half of Neoproterozoic excursions are expressed in carbonate carbon isotopes (��</b><sup><b>13</b></sup><b>C</b><sub><b>carb</b></sub><b>) but their organic carbon isotopes (��</b><sup><b>13</b></sup><b>C</b><sub><b>org</b></sub><b>) lack the same anomaly. Documenting which excursions exhibit carbonate�Corganic coupling or decoupling may provide a possible clue as to the enigmatic dynamics of the anomalous Neoproterozoic carbon cycle. We report here that a newly recognized and the first Neoproterozoic carbon isotope anomaly also exhibits decoupling. Thus, the ca. 940 Ma Majiatun anomaly now represents the oldest-known decoupled Neoproterozoic excursion��300 m.y. earlier than previously thought. Ruling out altogether or mostly merely localized artifacts such as diagenesis, changes in water depth, and contamination of detrital organic matter, we interpret this new evidence of carbonate�Corganic ��</b><sup><b>13</b></sup><b>C decoupling in the context of other anomalies through time. Although the remineralization of a large pool of dissolved organic carbon (DOC) was proposed to explain late Neoproterozoic ��</b><sup><b>13</b></sup><b>C decoupled anomalies, our earlier evidence of decoupling suggests this pool was much more ancient. Our new data also require the temporal oscillation between decoupling and coupling of the Neoproterozoic carbon cycle, implying a dynamic diminution of the large Precambrian DOC pool reflective of the rising but equally variable oxygenation of the oceans and rise of animal life at this time.</b></p>

Organic matter variations in the deep marginal basin of the Western Tethys and links to various environments in isotopic Albian–Cenomanian Boundary Interval

In the latest Albian–lowermost Cenomanian, the palaeoenvironments along the northern margin in the Carpathian domain of the Western Tethys were characterized by deep basins with floors below the calcium compensation depth, which were filled with flysch and hemipelagic sediments. On the other hand, the terrestrial areas surrounding the Western Tethys to the north were covered by a diverse flora of gymnosperms and experienced an increase in the diversity of angiosperms; the latter was characterized by a rapid spread in most terrestrial environments. The flysch/hemipelagic succession of the Silesian Basin (the Polish part of the Outer Carpathians) contains a sedimentological record of organic-rich accumulation that is related to the carbon isotope excursion, known as the Albian–Cenomanian Boundary Interval (ACBI); its older part corresponds to Oceanic Anoxic Event 1d (OAE1d). Organic matter (OM) records from these noncalcareous sediments are characterized by the occurrence of both terrigenous and marine organic compounds, and an increase in their accumulation intensity occurred during OAE1d, similar to other Western Tethyan areas and their northern surroundings. Quantitative analyses of the OM compounds using petrographic and chemical methods indicate that the majority were redeposited from land and estuarine environments; however, the OAE1d strata (Alternans Level) and the lower part of the post-OAE1d black shale succession are also enriched in OM of marine origin. This enrichment is interpreted as the result of enhanced surface water productivity, most likely related to redistribution of nutrients from coastal areas during the sea level rise of the KAl8 eustatic cycle. OAE1d was also the period in which the degradation of OM took place under reducing conditions in the bottom waters (although they were not anoxic), which was possibly caused by poor ventilation. A high proportion of terrigenous/estuarine OM compounds redeposited in the Silesian Basin is characteristic, especially for the post-ACBI black and dark grey shales. A large amount of this OM type has undergone relatively rapid degradation as it has settled from the shallows through a well‑oxygenated water column in the epi-, meso- and bathypelagic zones. Some terrigenous OM came from forest fires that occurred in the catchment area and supplied material to the Silesian Basin.

Clay hydroxyl isotopes show an enhanced hydrologic cycle during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event associated with a large injection of carbon into the ocean-atmosphere system, as evidenced by a diagnostic carbon isotope excursion (CIE). Evidence also suggests substantial hydrologic perturbations, but details have been hampered by a lack of appropriate proxies. To address this shortcoming, here we isolate and measure the isotopic composition of hydroxyl groups (OH−) in clay minerals from a highly expanded PETM section in the North Sea Basin, together with their bulk oxygen isotope composition. At this location, we show that hydroxyl O- and H-isotopes are less influenced than bulk values by clay compositional changes due to mixing and/or inherited signals and thus better track hydrologic variability. We find that clay OH− hydrogen-isotope values (δ2HOH) decrease slowly prior to the PETM and then abruptly by ∼8‰ at the CIE onset. Coincident with an increase in relative kaolinite content, this indicates increased rainfall and weathering and implies an enhanced hydrologic cycle response to global warming, particularly during the early stages of the PETM. Subsequently, δ2HOH returns to pre-PETM values well before the end of the CIE, suggesting hydrologic changes in the North Sea were short-lived relative to carbon-cycle perturbations.

A DOM regulation model for dolomite versus calcite precipitation in the Ediacaran ocean: Implications for the “dolomite problem”

The Ediacaran Period (∼635–539 Ma) witnessed the largest negative carbonate carbon isotope (δ13Ccarb) excursion in Earth history (to −12‰ VPDB), known as the Shuram Excursion (SE), which has been attributed to oxidation of a massive dissolved organic matter (DOM) reservoir, yielding highly 13C-depleted oceanic dissolved inorganic carbon (DIC). Limestone and dolostone are intimately interbedded in the Ediacaran Doushantuo Formation (South China) as well as in coeval successions globally, suggesting frequent shifts between calcite and dolomite precipitation in the Ediacaran ocean. In this study, we compiled paired Mg/(Mg + Ca) and δ13Ccarb data for 991 well-preserved (i.e., minimally altered) carbonate-rich samples from 17 SE-associated sections with a wide global distribution. δ13Ccarb compositions vary among the dolomite-dominated (−10‰ to +7‰), calcite-dominated (mainly in −10‰ to −5‰), and mineralogically mixed samples (with a bimodal distribution of −10‰ to −5‰ or +4‰ to +8‰). Inspired by the recent DOM catalyzation hypothesis and the highly-stratified redox model as well as the large DOM reservoir hypothesis for the Ediacaran ocean, we propose a “DOM regulation model” to account for controls on the precipitation of dolomite versus calcite in the Ediacaran ocean and, thus, for observed Mg/(Mg + Ca)-δ13Ccarb patterns. In this model, calcite was precipitated under low-DOM conditions, which existed in oxic surface waters and the nearshore part of a mid-depth DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, buffered by atmospheric CO2, whereas the latter had δ13CDIC decreasing from +4‰ to −10‰ due to increasing contributions of DOM oxidation. On the other hand, dolomite precipitation was catalyzed by high-DOM conditions, which existed in distal anoxic deep waters and the offshore part of the DOM-anaerobic oxidation zone. The former had δ13CDIC > +4‰, ultimately buffered by atmospheric CO2, whereas the latter had δ13CDIC increasing from −10‰ to +4‰ due to decreasing contributions of DOM oxidation. Our model is well supported by coexisting redox data. Our study highlights a possible link between massive dolomite formation and the existence of a large DOM reservoir in the Ediacaran ocean, providing an alternative perspective on the origin and widespread distribution of dolomite in deep-time marine systems (i.e., the “dolomite problem”).

More gaps than record! A new look at the Pliensbachian/Toarcian boundary event guided by coupled chemo-sequence stratigraphy

The Pliensbachian/Toarcian boundary (Pl/To) event precedes by ca. 1 Myr the onset of the Toarcian Oceanic Anoxic Event. It corresponds to a second order mass extinction associated with an outstanding collapse of shallow marine ecosystems at global scale. Yet, our knowledge about its exact driver(s) and unfolding is relatively ambiguous due to the numerous hiatuses present in the sedimentary record during this critical time interval. In this study, an integrated carbon isotope chemostratigraphy and sequence stratigraphy approach is applied to two case studies (the upper Pliensbachian in South-East France and the Pliensbachian–Toarcian transition in Morocco) to demonstrate how the major changes in sea-level and sedimentation supply accompanying the Pl/To event led to the formation of ubiquitous, often cryptic hiatal surfaces in the sedimentary record. Hence, as a consequence of strongly progradational stacking pattern during the latest Pliensbachian related to a global sea-level lowstand associated with cold greenhouse climate, proximal settings were characterized by bypass and/or erosion, inducing an incomplete record of the Spinatum chronozone in localities situated in the outer part of sedimentary basins. In the earliest Toarcian, the collapse of the neritic carbonate factory led to a halt of carbonate mud export into the basin, resulting in sediment starvation in most basins characterized by a carbonate-dominated sedimentation regime before the environmental perturbation. Only localities where vigorous siliciclastic sediment supply took over are likely to have a more complete sedimentary record of the immediate aftermath of the carbonate production collapse. This combination of causes explains the ubiquitous incompleteness of the record of the Pliensbachian/Toarcian transition in numerous European localities where the bulk of our current understanding about the Pl/To event derives from. A comparison between the two known most expanded and complete records of the Pliensbachian–Toarcian transition of the Llanbedr (Mochras Farm) core in Wales and Bou Oumardoul n'Imazighn section in Morocco shows that the onset of the environmental perturbations is associated with a positive carbon isotope excursion spanning the Pliensbachian/Toarcian boundary. This is followed by a negative carbon isotope excursion during the earliest Toarcian that coincides with the global collapse of neritic carbonate factory and an ample sea-level fall.

Mapping monstrosity: Malformed sporomorphs across the Smithian/Spathian boundary interval and beyond (Salt Range, Pakistan)

In the last 500 million years, Earth's biota experienced periods of crises with extinctions on a large scale and significant turnover events, one of them being the Permian-Triassic extinction event. The following Early Triassic was a critical time marked by a series of biological and environmental changes with a complex recovery pattern of marine faunas and ecosystems. Generally, animals and plants respond to stress by using different strategies that help them tolerate or recover from unfavourable environmental conditions. In this contribution, we quantitatively and qualitatively describe a variety of malformations of spores and pollen grains from the extensively studied Nammal section (Salt Range, Pakistan) across the Smithian/Spathian boundary interval (Early Triassic). High dominance of malformed sporomorphs is recorded throughout the studied interval, indicating stressful conditions for plants on the Indian margin during the Early Triassic. One of the highest abundances of malformed sporomorphs coincides with the spore spike and the negative carbon isotope excursion during the middle Smithian, emphasising this intense biotic stress. Either a cocktail of volcanic gases, acid rains, soil acidification, and heavy metal pollution as a consequence of a late pulse of the Siberian Trap large igneous province or climatic extremes might have been responsible for these malformations. Here teratology is used as a tool/means to assess the severity of biotic crises in the plant kingdom, indicating its potential value as a signal of ecological disturbance. • Fossil spores and pollen morphology as a proxy for ancient environmental pollution. • Sporomorphs identification schemes to quantify morphological change. • Evaluation of possible causes triggering morphological variation in sporomorphs.

No evidence for expansion of global ocean euxinia during the base Stairsian mass extinction event (Tremadocian, Early Ordovician)

A Tremadocian (Early Ordovician, base Stairsian North American Stage) mass extinction event is recorded globally in rocks from several ancient continents and is accompanied by a globally correlated positive carbon isotope excursion (CIE; the largest during the Early Ordovician). In this study, elemental concentrations and uranium isotope compositions (δ238U) were measured for carbonate samples from three sections (along a proximal-to-distal transect: Ibex area, Shingle Pass, Meiklejohn Peak, respectively) in the Great Basin to test the role of ocean anoxia/euxinia on the base Stairsian mass extinction event. Dysoxic/suboxic to oxic water columns and non-sulfidic porewaters are inferred for these sections based on multiple local redox proxies (e.g., Mo/Al, U/Al, Mo and U concentrations, Mo/U ratios).Carbonate δ238U data exhibit different δ238U trends in the three sections. The proximal Ibex area exhibits a small negative δ238U excursion (a magnitude of 0.2–0.3‰) and the Shingle Pass section has one sample with unusually low δ238U, in both cases within the middle of the CIE. Samples with the lowest δ238U in each section also have the highest Mn/Sr ratios, suggesting diagenetic alteration associated with adsorption of isotopically light U onto Mn oxides, followed by dissolution of Mn oxides by reducing porewaters and capture of the isotopically light U by sediments. The distal Shingle Pass and Meiklejohn Peak sections better preserve global ocean signals and the limited δ238U variations in these sections imply no significant changes in global seawater δ238U and thus no significant changes in the extent of global ocean euxinia. Given the non-sulfidic porewaters inferred for these carbonates, we apply a sediment–seawater δ238U offset of 0–0.27‰ (the 0.27 ± 0.14‰ isotopic offset observed for modern Bahamas carbonates with sulfidic porewaters is used as a maximum constraint) to the average carbonate δ238U from the two distal sections (−0.48 ± 0.09‰) to derive a coeval seawater δ238U of between −0.75 ± 0.09‰ and −0.48 ± 0.09‰. A three-sink U isotope mass balance model suggests 0.2–15.8% global euxinic seafloor area existed during deposition of these Tremadocian carbonates. Our study implies that a small biotic crisis during a small CIE was not associated with an expansion of global ocean euxinia, in contrast to major Phanerozoic mass extinctions that were commonly associated with significant expansion of ocean euxinia during large carbon perturbations (e.g., the Late Devonian). In addition, we propose that the post-SPICE Cambrian and Early Ordovician oceans were characterized by limited changes in ocean euxinia and only a few episodes of expanded ocean suboxia-anoxia during carbon perturbations. The “biomere” events within this period, including the base Stairsian event, were probably related to these episodes of expanded suboxic-anoxic oceans and/or other ecological stresses.

Facies Changes, Evolution of Biogenic Structures, and Carbon Isotope Stratigraphy of the Cambrian Series 2 to Miaolingian Transition on the Southern North China Craton

The Cambrian Series 2–Miaolingian transition is a pivotal period during Earth history, which witnessed the decline of biodiversity and the reduction in biomass, i.e., the redlichiid–olenellid trilobite extinction. The notable δ13C excursion (RECE) near the Cambrian Series 2–Miaolingian boundary in east Gondwana and China apparently corresponds with the redlichiid trilobite extinction. To better understand the causal mechanism of this biotic crisis, we report the carbon isotope stratigraphy and facies changes from Cambrian Series 2–Miaolingian transition of the Mantou Formation on the southern North China Craton. The carbon isotope excursions at the Cambrian Series 2–Miaolingian transition in the study area are 0.7‰ in the Chishanhe section and −0.2‰ in the Luoquan section, respectively, showing a weak negative excursion or even no negative excursion. The sedimentary environments in the study area gradually changed through time from a clastic tidal flat to a carbonate platform across the transition, which indicated a gradual rise in sea level, with anoxic conditions occurring predominantly before the RECE δ13C excursion. Microbially induced sedimentary structures and oncoids occurred widely at the top of Cambrian Series 2. Abundant metazoan trace fossils were preserved in the Miaolingian Series of the study area. The evolution of biogenic structures across the Cambrian Series 2–Miaolingian transition indicates the emergence of harsh environments associated with the proliferation of MISS and oncoids at the RECE horizon and the recovery of benthic metazoan fauna after the RECE biotic crisis.

Sedimentary model of the Ediacaran Doushantuo cap carbonates from the southeastern margin of the Yangtze Block: Evidence from carbon and oxygen isotopes and elemental geochemistry

Cap carbonate of the early Ediacaran period represents an abrupt climate shift from severe Marinoan glaciation to a post‐glacial warm environment. Some consensuses on the distinctive negative excursion of δ13Ccarb and the origin of its unique sedimentary structures have been reached over the past decades, but certain aspects such as the distribution and redox conditions of cap carbonate still remain controversial. To resolve these issues, we collected detailed samples of the Doushantuo cap carbonate from the Ganlong‐Fanyang section and the Poutoushang reservoir section in the Danzhai County, southeast Guizhou Province, respectively. We conducted carbon and oxygen isotope analysis and trace element analysis in this study. The Ganlong‐Fanyang section was possibly situated in a submarine highland because of its higher carbon isotope than that of the Potoushang section according to the carbon isotopic gradient. Negative carbon isotope excursions at the basal Doushantuo cap carbonate from the two sections correlate well with those from other equivalent cap carbonates and were corresponding to the first one (SN1 or EN1) of the carbon isotopic curve during the Ediacaran period, implying a remarkable global event after the post‐Marinoan glaciation. Geochemical features of trace elements and rare earth elements support an overall oxygen‐rich condition with syn‐depositional or pene‐contemporaneous hydrothermal activities and methane seeps during the deposition of cap carbonate. We reconstruct a new sedimentary model that the Doushantuo cap carbonates were mainly precipitated in relatively shallow‐ to deep‐water shelf environments without disturbance by abundant terrestrial materials, which may be on isolated platforms over the early developed grabens in a deep‐water basinal condition, rather than within the entire oceanic basin.

Molecular record of the SPICE event in well YD2, Tarim Basin: Insights from catalytic hydropyrolysis of kerogens

The Steptoean Positive Carbon Isotope Excursion (SPICE) event is extensively documented in globally distributed carbonate and organic-rich successions of slope and platform environments during the late Cambrian (ca. 497–494 Ma). Despite numerous studies, its regional manifestation remains controversial because of the major uncertainty of the redox conditions in sedimentary paleoenvironments and its effect on the changes in the biota. Biogeochemical processes that occurred during the SPICE event are important for assessing whether the excursion is associated with the co-evolution of organisms and environments. In this study, we investigated the changes in plankton and paleoenvironment at the molecular level by combining kerogen elemental analysis, catalytic hydropyrolysis (HyPy), and gas chromatography-mass spectrometry (GC-MS) analyses of saturates and aromatics with published carbon isotopic data. The three fluorene series, gammacerane/C30hopane (G/C30H) and pristine/phytane (Pr/Ph) ratios demonstrated that the shallow shelf seawater changed significantly from relatively reducing to oxidizing and back to reducing conditions during the SPICE interval coupled with a regression-transgression cycle. The phytoplankton during the rise of the SPICE were mainly organic-walled, such as cyanobacteria, various acritarchs, and algae, as evidenced by terpanes, hopanes, and steranes. Significant decreases in phytoplankton diversity and abundance coincided with the peak of the SPICE, corresponding to the most oxidized and hypersaline periods during the SPICE event in the eastern Tarim Basin. Plankton assemblages diversified significantly with increasing living space due to the rise of sea-level during the post-SPICE period. Therefore, this study supports the idea that plankton groups changed dramatically with the environment, and its revolution followed the SPICE into the Great Ordovician Biodiversification Event.

Tracking depositional and geochemical variations in the Cambrian North China Platform: Insights from sedimentology, geochemistry, and C-O isotopic records

The Cambrian carbonate sequence in North China is of significant global and regional scientific importance due to its continuous and thick exposure and the data on the major environmental changes and biological events of the early Phanerozoic, which can be gleaned from it. While the well-exposed Cambrian outcrops have been utilized to unravel the processes governing these events, few studies have focused on the outcrops' spatio-temporal depositional patterns and geochemical variations. Therefore, our study address, these issues by integrating fieldwork with petrographic and geochemical analyses. In the North China Platform (NCP), the Cambrian carbonate sequence is predominantly composed of six third-order depositional sequences (~370–400 m thick) that can be divided into nine microfacies, representing three primary microfacies associations, namely tidal flat, lagoon, and shoal. In contrast to the standard carbonate sequence stratigraphy model and many other examples of Cambrian platforms, the deposition of oolitic grainstones in the NCP are overlain by the deep-water calcareous mudstone and shale during a period of slow fall of sea-levels. Such a particular style shows a similar trend to the Schlager model of a type III sequence boundary that formed between a highstand systems tract (HST) and a transgressive system tract (TST). Scanning electron microscope (SEM) observations show an abundance of microbial fossils, a nanosphere, and remnants of extracellular polymeric substances (EPS) inside the dark micrite of normal regressive deposits (oolitic grainstone), suggesting the significant role of microbial activity in the formation of ooids. The δ18O isotopic values range from −4.15 to −11.75‰ VPDB, portraying pervasive diagenetic alterations with a variable degree of diagenesis at different localities, and both marine- and burial-related diagenesis. In contrast, the δ13C VPDB values have a very narrow range, from −2 to +2‰ VPDB, suggesting little to no influence of meteoric diagenesis. At the same time, the slight depletion of δ13C isotope values may reflect the role of microbial processes during the genesis of ooids. This study highlights the importance of microbial activity and marine-related diagenetic overprints during the formation of carbonate facies in the Cambrian Period and provides evidence that carbon isotopic excursions observed during different major environmental and biological events in the NCP are related to primary signals and not altered by intensive meteoric diagenesis.