Doushantuo Research Articles

Problematic Ediacaran sail-shaped fossils from eastern Yunnan, China

ABSTRACT This study focuses on a rare sail-shaped macrofossil, Alienum velamenus gen. & sp. nov. from the Jiangchuan Biota in Yunnan Province, China. The new species presented here possesses an unique bipartite body consisting of a wide unbranched, asymmetrical sail-shaped anterior section and a small narrow tail-like posterior section. The macrofossil of the new species is preserved through carbonaceous compression. Its appearance is different from known macroalgae but similar to Vetulicolia from the early Cambrian Chengjiang Biota. The anterior section of the new species, which is broad and smooth, shows only indistinctly arranged suspected gill slits and a nearly horizontal pharyngeal groove, without significant segmented structures. Owing to lack of samples, the specific biological properties and taxa of the new species are still undefined. The discovery of Alienum velamenus indicates that the Jiangchuan Biota is an exceptional lagerstätte with great potential for the preservation of Precambrian early life through carbonaceous compression. Furthermore, the Jiangchuan Biota shows significant differences in species diversity from the Miaohe Biota of the Doushantuo Formation and the Shibantan Biota of the Dengying Formation in the Three Gorges area of Hubei Province.

Oceanic and Sedimentary Microbial Sulfur Cycling Controlled by Local Organic Matter Flux During the Ediacaran Shuram Excursion in the Three Gorges Area, South China.

The increased difference in the sulfur isotopic compositions of sedimentary sulfate (carbonate-associated sulfate: CAS) and sulfide (chromium-reducible sulfur: CRS) during the Ediacaran Shuram excursion is attributed to increased oceanic sulfate concentration in association with the oxidation of the global ocean and atmosphere. However, recent studies on the isotopic composition of pyrites have revealed that CRS in sediments has diverse origins of pyrites. These pyrites are formed either in the water column/shallow sediments, where the system is open with respect to sulfate, or in deep sediments, where the system is closed with respect to sulfate. The δ34S value of sulfate in the open system is equal to that of seawater; on the contrary, the δ34S value of sulfate in the closed system is higher than that of seawater. Therefore, obtaining the isotopic composition of pyrites formed in an open system, which most likely retain microbial sulfur isotope fractionation, is essential to reconstruct the paleo-oceanic sulfur cycle. In this study, we carried out multiple sulfur isotope analyses of CRS and mechanically separated pyrite grains (>100 μm) using a fluorination method, in addition to secondary ion mass spectrometry (SIMS) analyses of insitu δ34S values of pyrite grains in drill core samples of Member 3 of the Ediacaran Doushantuo Formation in the Three Gorges area, South China. The isotope fractionation of microbial sulfate reduction (MSR) in the limestone layers of the upper part of Member 3 was calculated to be 34ε = 55.7‰ and 33λ = 0.5129 from the δ34S and Δ33S' values of medium-sized pyrite grains ranging from 100 to 300 μm and the average δ34S and Δ33S' values of CAS. Model calculations revealed that the influence of sulfur disproportionation on the δ34S values of these medium-sized pyrite grains was insignificant. In contrast, within the dolostone layers of the middle part of Member 3, isotope fractionation was determined to be 34ε = 47.5‰. The 34ε value in the middle part of Member 3 was calculated from the average δ34S values of the rim of medium-sized pyrite grains and the average δ34S values of CAS. This observation revealed an increase in microbial sulfur isotope fractionation during the Shuram excursion at the drill core site. Furthermore, our investigation revealed correlations between δ34SCRS values and CRS concentrations and between CRS and TOC concentrations, implying that organic matter load to sediments controlled the δ34SCRS values rather than oceanic sulfate concentrations. However, these CRS and TOC concentrations are local parameters that can change only at the kilometer scale with local redox conditions and the intensity of primary production. Therefore, the decreasing δ34SCRS values likely resulted from local redox conditions and not from a global increase in the oceanic sulfate concentration.

A refined model for the mechanisms of Precambrian phosphorite formation

Despite the economic and scientific importance of Precambrian phosphorites, our understanding of the mechanism leading to their formation remains limited, including for the largest phosphogenic episode in the late Neoproterozoic. To improve our understanding of Precambrian phosphorite formation, we combined sedimentology, petrography, and elemental, and Fe-C isotopic analyses to study the two main phosphorite beds (the lower and upper phosphorite beds) in the Ediacaran Doushantuo Formation, Zhangcunping area, South China. The phosphorites consist mainly of granular textures characterized by densely packed grains, some of which are coated with secondary phosphate growth. However, there are notable differences in the mineralogy, microfossil assemblages, and elemental contents of the two beds. The lower phosphorites have no Ce anomaly, and relatively low Y/Ho ratios and positive δ56Fe values (0.04–0.30 ‰, average of 0.19 ‰). In contrast, the upper phosphorites have negative Ce anomalies and higher Y/Ho ratios and near-zero δ56Fe values (−0.29–0.19 ‰ (average of −0.01 ‰). These observations suggest that the lower phosphorites formed in anoxic-suboxic environments, whereas the upper phosphorites formed in relatively oxygenated environments. The δ13Ccarb values of the phosphorites range from −3.97 ‰ to 1.71 ‰ (average of −1.56 ‰), and are lighter than values in dolostones (−0.52 ‰ to 4.39 ‰, average of 2.02 ‰). This suggesting that formation of the Doushantuo phosphorites was influenced by degradation of organic matter in an ocean with high primary productivity. The lower phosphorites, which were also regulated by Fe redox pumping, have positive δ56Fe values, along with the presence of pyrite framboids and iron oxides, suggesting deposition near the Fe-redox boundary where extensive Fe cycling. The upper phosphorites show positive correlations between Mn and Fe, and Mn/Fe and P2O5, suggesting formation near the Mn boundary with extensive Mn cycling, primarily mediated by Mn redox pumping. Sedimentological observation indicate that primary phosphates were concentrated into granular phosphorites by winnowing processes following primary precipitation. Accordingly, we propose a refined model for Precambrian phosphorite formation in which degradation of organic matter, Fe and Mn pumping, and physical reworking of deposits co-evolve and interact within a dynamic Precambrian redox environment. Our model provides a reasonable explanation for the distribution of global phosphorite deposits throughout geological history.

Upwelling-driven high organic production in the late Ediacaran

The Ediacaran ocean (635-541 million years ago) is typically viewed as a stagnant, redox-stratified ocean with low phosphorus availability. However, this contradicts the high total organic carbon (up to 15 wt%) recorded in the upper Ediacaran Doushantuo Formation in South China. Here, we present a quantitative reconstruction of primary productivity and sediment total organic carbon using basin-scale simulations based on various redox-nutrient scenarios. Our results indicate that oceanic upwelling was the primary driver of enhanced primary productivity, with estimates over 2.5 times the modern ocean’s average, providing quantitative evidence for oceanic circulation in the Ediacaran Nanhua Basin. Our results also indicate the development of spatially linked but opposing redox trends, i.e., a ‘redox anticouple’, whereby the expansion of the oxic surface layer occurred concurrently with intensification of deep-water anoxia due to enhanced PP in surface and its consumption of oxidants in underlying waters. This model may explain the conflicting redox records across the Nanhua Basin. Our findings explain the unusually high rates of organic matter production and preservation in the low-pO2 Ediacaran ocean and provide a mechanism for its progressive oxygenation.

Dolostone-barite-phosphorite sequence in the basal Doushantuo Formation: Origin and implications for post-Marinoan ocean chemistry and phosphogenesis

Barite (BaSO4) represents a major component of many post-Marinoan cap dolostones worldwide, recording important information on climate upheavals during the terminal Neoproterozoic. However, its origins and relations with other lithofacies remain to be explored. Here we report new occurrences of barite in the basal Ediacaran Doushantuo Formation at a major phosphate mine in Weng'an, South China, and present results of detailed petrographic and geochemical investigations on the barites and related phases. At the outcrop scale, barite is mainly present as a 1–2 cm-thick layer at the contact of the cap dolostones and overlying phosphorites. Detailed petrographic observations reveal a lithofacies transition from dolostone through a mixture of barite, dolomite and apatite to barite and finally to granular phosphorite. The barites can be categorized into three generations in stratigraphic order, which are present as loose, randomly-oriented blades (B1), dense aggregates (B2), and radially aligned crystal fans (B3), respectively. B1 is dispersed in a matrix of apatite and dolomite, which has an average δ13Ccarb of −0.61‰. This, along with the common presence of organic matter (OM) in this barite, point to a possible source of Ba from organic degradation. B3 forms a centimeter-scale layer of barite fans that laterally extends over the entire mining pit, whereas B2 is locally distributed between B1 and B3, and occurs as relatively unoriented aggregates. δ34S values of B3 show a narrow range between +25.9‰ and +30.9‰, which is similar to that of contemporaneous seawater and indicate a seawater source of sulfate in barite. Based on the present investigation and a compilation of published data, we propose that B1 is diagenetic barite with a main source of Ba from degradation of organic matter and possibly also from dissolution of previous pelagic barite, whereas B2 and B3 represent seafloor precipitates that resulted from interactions between deep, anoxic Ba-rich waters and oxic, sulfate-rich surface waters during the post-glacial transgression. This episode of barite precipitation was immediately followed by massive accumulations of phosphorites in Weng'an and many areas of South China. The consecutive deposition of cap dolostone, barite and phosphorite signifies the transition from an alkalinity-rich, stagnant anoxic ocean to a dynamic marine regime featuring oxygenated surface waters and upwelling currents. These pH and redox changes likely facilitated the cycling and enrichment of phosphate in the shallow waters that was necessitated by the unprecedented accumulation of Ediacaran phosphorites in South China.

Nanoscale characterization of Ediacaran microfossils from lower Doushantuo formation chert nodules

Microfossils identification, especially in very old rocks, is extremely challenging because morphological and chemical signatures of microfossils are subtle and inevitably altered with aging. Chert nodules from the Doushantuo Formation in the Yangtze Gorges area of South China have captured significant interest due to their remarkable preservation of Ediacaran microfossils. To understand the taphonomic processes leading to an exceptional preservation of microbes in these rocks, we studied the morphological and chemical features of microfossils within the lower Doushantuo Formation chert nodules down to atomic scales via advanced microscopy techniques. Our results align with previous studies, confirming that most microfossils there are preserved by silicification. Further analysis of a representative filamentous microfossil, Polytrichoides lineatus, reveals that both the cell wall (or extracellular sheath) and cytoplasm are preserved by loose aggregates of subrounded or irregular quartz grains, along with patchy organic residuals, which are distinctive from the closely packed and angular-shaped quartz grains in the non-fossil matrix of the chert nodule. The cell wall or extracellular sheath likely provided narrower spaces and more nucleation sites for silicification, resulting in smaller quartz grains (i.e., 115 ± 42 nm) compared to those formed in the cytoplasm (i.e., 1150 ± 258 nm). The permeation and precipitation of quartz grains within the cell wall or extracellular sheath could contribute to an exceptional preservation of subcellular structures. This study offers valuable insights into the preservation of microbes in the Doushantuo Formation chert nodules and even older siliceous sedimentary rocks.

Statistical estimation of the early to middle Ediacaran ocean redox architecture in the Yangtze block of South China

Early to middle Ediacaran organic-rich black shales host several famous fossil biotas and provide key evidence for understanding the coevolution of multicellular organisms and palaeoceanic environment. The water column redox states play critical roles in organic-rich shale deposition. Amongst multiple geochemical redox indexes, iron-speciation chemistry (FeHR/FeT, Fepy/FeHR) in shales constitutes a reliable proxy to reconstruct the first-order redox framework of the ocean basin. However, iron-speciation is a local marine redox proxy, and only compiled data collected from multiple different sedimentary facies record statistically significant changes of oxidation states within a depositional basin. Here we compiled the spatiotemporal distribution of iron-speciation data in early to middle Ediacaran black shales of the Yangtze block in South China. New high-resolution analysis on two outcrops and one drill-core reflects overall ferruginous condition and dominant euxinic condition locally interrupted by oxic states, respectively, generally indicating pervasive anoxic depositional environment for the Member II shales of the Ediacaran Doushantuo Formation in the Lower Yangtze block. Compilation of fourteen Ediacaran sections from shallow-, slope- and deep-water facies demonstrates frequent spatiotemporal oscillation of the redox conditions throughout the Yangtze block. Widespread ferruginous states, accompanied with euxinic zones mainly focusing in lower slope facies, are detected during the early Ediacaran period, in contrast with more widespread euxinic settings in the middle Ediacaran period. Statistical data including new high-resolution results from the Lower Yangtze block show generally low enrichment for redox sensitive elements (RSEs, e.g., Mo, V and U) in the Member II shales probably due to their contemporary low seawater concentrations. An increase of RSEs concentrations occurs in the Member IV shales, which probably reflects more oxidized Earth surface environment during middle Ediacaran period. The increasing Earth oxygenation documents the occurrence of widespread euxinic states during deposition of the Member IV shales. Compilation of spatial distribution of TOC contents in black shales demonstrates their apparent relevance to spatial distribution of the euxinic rather than ferruginous states, which is likely due to the favorable role of sulfurization process on burial and preservation of organic matters. The euxinic states may also facilitate the exceptional preservation of the soft-bodied Ediacaran fossils.

New materials of acanthomorphic acritarchs from the Ediacaran Weng'an Biota (South China)

Abstract The Weng'an Biota from the Ediacaran Doushantuo Formation in Guizhou Province, southwestern China, is known for its three-dimensionally phosphatized acritarchs, multicellular algae, and embryo-like animal fossils. Among these diverse microfossils, acanthomorphic acritarchs have played a significant role in the biostratigraphic subdivision and correlation of the lower-middle Ediacaran System. However, most previous studies on the biostratigraphy of the Doushantuo Formation in the Weng'an area have focused on large acanthomorphic acritarchs (LAAs, vesicle diameter >200 μm), whereas the smaller acanthomorphic acritarchs (SAAs, vesicle diameter <100 μm) from the Weng'an Biota have been largely overlooked. In this study, we examined >500 thin sections and discovered a large number of well-preserved, small (<100 μm) and medium-sized acanthomorphic acritarchs (MAAs, vesicle diameter ranging 100–200 μm). In total, we have identified SAAs in four genera and six species (Tanarium conoideum Kolosova, 1991, emend. Moczydłowska et al., 1993; Tanarium elegans Liu et al., 2014; Mengeosphaera membranifera Shang, Liu, and Moczydłowska, 2019; Mengeosphaera minima Liu et al., 2014; Estrella recta Liu and Moczydłowska, 2019; Variomargosphaeridium gracile Xiao et al., 2014), as well as two types of MAAs (Tanarium tuberosum Moczydłowska, Vidal, and Rudavskaya, 1993, emend. Moczydłowska, 2015; Weissiella cf. W. grandistella Vorob'eva, Sergeev, and Knoll, 2009, emend. Liu and Moczydłowska, 2019). This updated acritarch assemblage of the Weng'an Biota is valuable for correlating the Ediacaran Doushantuo Formation between the Weng'an and Yangtze Gorges areas. It also serves as a tool to test the proposed acritarch biozones in Ediacaran formations of South China and other localities, including Australia, Siberia, and the East European Platform.

Developmental biology of Spiralicellula and the Ediacaran origin of crown metazoans.

The early Ediacaran Weng'an biota (Doushantuo Formation, South China) provides a rare window onto the period of Earth history in which molecular timescales have inferred the initial phase of crown-metazoan diversification. Interpretation of the embryo-like fossils that dominate the biota remains contentious because they are morphologically simple and so difficult to constrain phylogenetically. Spiralicellula from the Weng'an biota is distinguished by spiral internal bodies, allied through development to Megasphaera or Helicoforamina and interpreted variously as metazoan embryos, encysting protists, or chlorophycean green algae. Here we show, using X-ray microtomography, that Spiralicellula has a single-layered outer envelope and no more than 32 internal cells, often preserving a nucleus and yolk granules. There is no correlation between the extent of spiral development and the number of component cells; rather, the spiral developed with each palintomic stage, associated with cell disaggregation and reorientation. Evidence for envelope thinning and cell loss was observed in all developmental stages, reflecting non-deterministic shedding of gametes or amoebae. The developmental biology of Spiralicellula is similar to Megasphaera and Helicoforamina, which otherwise exhibit more rounds of palintomy. We reject a crown-metazoan affinity for Spiralicellula and all other components of the Weng'an biota, diminishing the probability of crown-metazoan diversification before the early Ediacaran.

Extreme organic matter enrichment in the Ediacaran Doushantuo formation: New constraints from the NE Sichuan basin, China

The Yangtze Block is a highly prospective area for Ediacaran natural gas exploration in recent years, especially the Doushantuo Formation, a vital source rock for organic-rich hydrocarbon and a shale gas reservoir in the region. However, understanding of the Ediacaran organic-rich source rocks systematically has been limited due to the heterogeneous accumulation of organic matter and confusing paleoclimatological and paleoceanographic conditions. Such a scenario is not conducive to further exploration of the deep Ediacaran natural gas resources or understanding the evolution of early Earth. In this study, based on two new well drillings in the northeastern Sichuan Basin, the integrated sedimentological and geochemical investigations were focused on the Ediacaran middle−upper Doushantuo Formation (DST) fine-grained rocks interval. Results show this set of black organic-rich shales (total organic carbon up to 13.97%) deposited under episodic warming climate during an otherwise protracted cooling trend. These frequent climatic fluctuations intensified hydrological circulation, which together with the concurrent hydrothermal activity and the Neoproterozoic Oxidation Event, led to eutrophication of surface seawater. Under these circumstances, a substantial enhancement in primary productivity driven by algae blooming, subsequently resulted in consume excess dissolved oxygen in seawater during the degradation stage and anoxic-euxinic bottom water expansion. Consequently, the high paleoproductivity controlled the extreme organic matter enrichment during the Doushantuo time and formed the DST 4 organic-rich shales in the Chengkou and Nanhua subbasins. In contrast, although the DST 3 shales still deposited under a high productivity condition, the shallower sedimentary environments with anoxic-oxic bottom water conditions resulted in greater variation in total organic carbon contents.

Stratigraphic evidence for a major unconformity within the Ediacaran System

The subdivision and correlation of the Ediacaran System within the geologic timescale is desirable, but challenging. The Ediacaran Doushantuo Formation (∼635–550 Ma) in South China is one of the most important sedimentary successions for studying the co-evolution of Earth's environment and life at that time. However, its stratigraphic continuity is confusing and deserves scrutiny. In this study we evaluate stratigraphic continuity of the Doushantuo Formation from a key section deposited in an alleged deep-water “basinal” setting. Using litho-, chemo-, and chrono-stratigraphic data anchored by two new CA-ID-TIMS U-Pb zircon dates of 629.7 Ma and 556.4 Ma from two tuff layers, we reveal that this section contains a cryptic subaerial unconformity with a >44 Myr gap in the sedimentary record. The unconformity occurs immediately prior to the EN3/Shuram δ13C excursion in the mid-Ediacaran, and can be traced regionally and may be correlative to other mid-Ediacaran unconformities globally. We suggest that its development is related to glacio-eustatic sea-level changes during the mid-Ediacaran. This finding provides a critical stratigraphic boundary for future subdivision and correlation of the Ediacaran System in South China, and we advocate for a reconsideration of the completeness of the Ediacaran stratigraphic record in this area.

Assessing chemostratigraphic and biostratigraphic correlation of Ediacaran phosphorites (Bocaina, Khesen and Doushantuo Formations): Diachronic, local signals of the Neoproterozoic phosphogenic-taphonomic event

The Neoproterozoic phosphogenic event records biotic and biogeochemical changes that seem to indicate intrinsic relationships between phosphogenesis and phosphatic fossil preservation. However, chemo-litho and biostratigraphic correlations conducted on fossiliferous, phosphatic successions of the Doushantuo Formation, South China, evidence a highly heterogeneous record, featuring diachronic depositional ages, variable taxonomic diversity and inconsistent, “noisy” δ13C records. Similar detailed chemo-litho and biostratigraphic correlations targeted in high-grade phosphorite strata have not been performed along Ediacaran units elsewhere. Considering the significance and scarcity of Ediacaran high-grade phosphorite strata, establishing a comprehensive correlation framework within and between those units is fundamental to understanding Ediacaran biogeochemical and biotic evolution. This work aims to integrate new δ13C isotope stratigraphy, and existing Doushantuo-Pertatataka acritarch assemblages and geochronological records from Bocaina (Brazil), Khesen (Mongolia) and Doushantuo (China) Formations. Phosphate rich strata in those units feature inconsistent δ13C signatures characterized by negative values showing large bed-to-bed variations, possibly reflecting local modification of original marine signals by phosphogenesis, suggesting caution against the use in global/regional chemostratigraphic correlation. The record of phosphatized acritarch assemblages shows contrastive taxonomic diversities, and a small number of common taxa including Megasphaera. Whereas the presence of biozonal Doushantuo Formation taxa on Khsesen Fm, resonates with growing views of strong environmental resilience and large temporal ranges of acanthomorph acritarchs throughout the Ediacaran-early Cambrian, the scarcity of similar taxa in the Bocaina Fm may question the reliability of Doushantuo Fm acritarch biozones in other paleogeographic or paleoenvironmental scenarios. Alternatively, the low taxonomic diversity recorded in Bocaina and Khesen Fms may also reflect the effects of the late Ediacaran decline in acritarch diversity. Further, combined evidence of selective paleoenvironmental phosphatization, local geochemical signals, narrow time spans and heterogeneous taxonomic diversity found in phosphatized DPK acanthomorph assemblages indicate the local nature of Ediacaran phosphatic fossil preservation events. Finally, evidence for diachronic preservation between high-grade phosphatic strata and phosphatized microfossils show non-causal relationships between Ediacaran phosphogenesis and phosphatic fossil preservation, highlighting the punctuated, intrinsic local nature of those events.

Integrated Stratigraphy and Mineralogy of the Doushantuo Formation in Weng’an, South China, and Implications for Ediacaran Phosphogenesis

Integrated Stratigraphy and Mineralogy of the Doushantuo Formation in Weng’an, South China, and Implications for Ediacaran Phosphogenesis

Study on the material source and enrichment mechanism of REE-rich phosphorite in Zhijin, Guizhou

Rare earth element (REE)-rich phosphorite in the Guizhou region mainly exists in the Doushantuo Formation and Gezhongwu Formation in early Cambrian strata, which are some of the important strata containing phosphorite resources in China. The early Cambrian Zhijin phosphorite in Guizhou Province, China, has high rare earth element and yttrium (REY) contents of up to 2500 ppm, with heavy REY (HREY) contents accounting for ~ 30% of the total REY contents. However, the specific controlling source and environment of phosphorite (especially the REEs in Zhijin phosphorite) are still unsolved. Through field geological investigations; mineralogical, geochemical, Sr–Nd isotope analyses; and tectonic characteristics, the material source, sedimentary environment and seawater dynamics of REEs in phosphorite are studied. It is considered that the REEs enriched in the Zhijin phosphorite are mainly affected by precipitation from hydrothermal fluid. Moreover, from the late Ediacaran to the early Cambrian, the depositional environment from the bottom to the top of the water tended to be hypoxic, and the activity of hot water fluid tended to be strong. The change in redox conditions is closely related to the rise and fall of sea level. Combined with the tectonic background, these results show that the weakly oxidized environment may be an important factor controlling the enrichment of REEs. The enrichment of REEs may be closely related to volcanic hydrothermal activity, later diagenesis and seawater dynamics.

A New Negative Carbon Isotope Interval Caused by Manganese Redox Cycling After the Shuram Excursion

AbstractSeveral negative C isotope excursions (CIEs) occurred at the end of the Neoproterozoic era which have been generally attributed to the oxidation of organic carbon using sulfate as the terminal electron acceptor and the subsequent release of 13C‐depleted dissolved inorganic C (DIC). Based on new analyses from the Doushantuo Formation in South China, we observe a negative C isotope excursion right after the well‐known Shuram excursion. This excursion is equivalent to the ∼550 Ma negative CIE which is globally expressed within several continental margins. However, the origins of this CIE in the termination of Ediacaran remain unresolved. Here, we hypothesize that this post‐Shuram negative CIE was caused by a localized manganese cycling that began with the oxidation of hydrothermal Mn(II) in a water column to insoluble Mn(IV)‐oxide, followed by accumulation of Mn(IV)‐oxide to the seafloor and its subsequent dissolution via Mn(IV) reduction leading to the release of dissolved Mn(II) and 13C‐depleted DIC into ambient seawaters. This ultimately led to the precipitation of particulate Mn(II)‐carbonate characterized by low δ13Ccarb values ranging from −11.1‰ to −2.8‰. The presence of microbial fabrics in association with the Mn(II)‐carbonate further suggests that Mn(II)‐carbonate precipitation took place at the seafloor in shallow sun‐lit waters rather than in the deeper sediment pile, which archived ambient seawater C isotopic signal. Although most Ediacaran negative CIEs were generally attributed to sulfate reduction, our findings suggest that at a local level, Mn cycling can also lead to negative CIE in the Neoproterozoic, and potentially at other times in Earth's history.

A local rather than a global oxidation in the early Ediacaran deep ocean: Evidence from the Doushantuo Formation in East Guizhou Province, South China

After the end of the Cryogenian Snowball Earth glaciations, the Ediacaran Ocean experienced rapid oxidation and a subsequent increase in marine sulfate concentration. This led to a significant negative excursion in pyrite sulfur isotope values, particularly observed in the Doushantuo Formation of the slope facies in the South China. However, the extent of this oxidation event remains unclear. In order to address this, we carried out geochemical analysis of the Doushantuo Formation in the Cenpiaokou section of the Jiangkou County, Guizhou Province. In the Cenpiaokou section, the pyrite is mainly euhedral-subhedral and framboidal in crystal morphology. The pyrite contents are highest at the base of the Doushantuo Formation and decrease towards the middle and upper parts. Similarly, the proportion of framboidal pyrite also decreases upward, ranging from 93 % at the bottom to 9 % in the upper part of the Doushantuo Formation. Additionally, the sulfur isotope of pyrite (δ34S) shows significant differences compared to other slope facies sections. There is no negative excursion in δ34S at the base of the Doushantuo Formation. The lower part of the Doushantuo Formation exhibits frequent oscillations with generally high δ34S values (+6.6 ‰ ∼ +37.3 ‰), while the upper part shows a decreasing trend. The organic carbon isotope (δ13Corg) displays an inverse correlation with δ34S, with stable values in the lower part and gradually heavier values in the upper part. The nitrogen isotopes (δ15N) range from -1.6 ‰ to +2.1 ‰, suggesting a generally anoxic state with strong nitrogen fixation at the Cenpiaokou section. These findings suggest that the deep ocean oxidation during the early Ediacaran was not a global event, but rather a regional event, with anoxia still dominant overall.

Stratigraphic framework and sedimentary evolution during the Cryogenian-Ediacaran transition in northeastern Sichuan Basin, South China

The northeastern Sichuan area in the northern Yangtze margin has unique Ediacaran geological records, especially the Doushantuo Formation (DST), and become a hot research area in recent years. However, the Cryogenian-Ediacaran (C-E) boundary has not been precisely identified, which restricts the in-depth study of geological information during this crucial transitional period and is unfavorable for a systematic and complete understanding of the Yangtze Block and even the global paleogeographic pattern. This study conducted stratigraphy, sedimentology, and chronostratigraphy to establish the stratigraphic framework and sedimentary evolution of the C-E transition strata in northeastern Sichuan. The results showed that the Ediacaran sediments, without the cap dolomite, unconformably overlaid the Cryogenian sediments in the studied area. The Member II of the DST, characterized by 50–160 m of red-green sandstone (approximately equivalent to the original Chengkou “Guanyinya Formation”), directly overlaid the Cryogenian sediments and displayed a 623 ± 2.3 Ma maximum depositional age from the detrital zircon U–Pb dating. Regional stratigraphic correlations indicate that the C-E transition strata in northeastern Sichuan had a consistent lithological association and sedimentary sequence characteristics but differed from the Three Gorges. Typically, the upper Nantuo massive glacial diamictites transition to the icebergs rafted lonestone-bearing mudstones at the top, then change upward to DST barrier coast sandstones. The proposed DST of the northeastern Sichuan Basin was divided into three lithostratigraphic members without the regional Member I cap dolomite: (i) Member II purple-red, gray-green sandstone strata, (ii) Member III black mudstone strata, and (iii) Member IV P–Mn bearing strata. During the C-E transition, the study area experienced (i) the global deglaciation stage in the terminal Marinoan glaciation and (ii) the filling-leveling up stage with clastic rocks in the early Ediacaran. Overall, the early Ediacaran of northeastern Sichuan succeeded the paleogeographic features of the late Cryogenian.

Authigenic clay mineral constraints on spatiotemporal evolution of restricted, evaporitic conditions during deposition of the Ediacaran Doushantuo Formation

The carbonate-rich shelf facies of the Ediacaran Doushantuo Formation (South China) have produced a broad array of microfossil and isotope proxy records which underpin much of our understanding of environmental change and biospheric evolution during this key time period. Recent reports of locally abundant authigenic, Mg-rich saponite clay in the Yangtze Gorges Area hint at potentially widespread evaporitic conditions in a lagoonal setting. Despite the implications for interpretation of proxy records and the environmental setting of early metazoans, the spatiotemporal extent of restricted, evaporitic conditions across the Yangtze Block remains largely unexplored. Here we use mineralogical and petrographic techniques to document the spatial and stratigraphic distribution of authigenic clay minerals across seven sites, representing a transect from shallow proximal shelf to deep basinal settings. Our results demonstrate the widespread and persistent occurrence of authigenic saponite in proximal shelf settings, whereas talc is identified at more distal shelf sites, consistent with Mg-clay authigenesis in an evaporitic lagoon with spatially variable detrital aluminosilicate input. Slope and basinal sites contain no Mg-clays and are instead characterized by abundant authigenic illite, consistent with an open marine setting. Stratigraphically, illite was found in the basal cap carbonate and uppermost Doushantuo Formation, with Mg-clays only present in the interval in-between, suggesting gradual development of an extensive carbonate rimmed lagoon over the Yangtze shelf that restricted seawater exchange, followed by stronger marine influence due to erosion of the carbonate rim and subsequent marine transgression. We conclude that restricted, evaporitic conditions were much more expansive than previously assumed, potentially providing favorable conditions for the Doushantuo Biota. Further, the widespread presence of authigenic clays across the shelf supports the case for enhanced reverse weathering before the advent of biosilicification.

Nitrogen isotopic compositions of organic-rich shales (∼560 Ma) in the Chengkou region, South China: Implications for a stable and relatively large nitrate reservoir of the late Ediacaran ocean

The late Ediacaran period is a critical transitional period in Earth's history, marked by the evolution of the Ediacara biota (∼574 to 538.8 Ma). The marine redox state and nutrient level, having a close relationship with bioevolution, can be reflected by sedimentary nitrogen isotopes (δ15N). Doushantuo Formation Member IV, a widespread organic-rich interval during this critical period of South China, may provide important clues for nitrogen (N) cycling. However, high-resolution δ15N research on Member IV is still lacking. In this study, a drill core from the Chengkou sub-basin, with the well-developed Doushantuo Member IV, was carefully studied base on its N isotopic composition. The implications for the N cycling characteristics are discussed using a combination of proxies for bottom water redox conditions and basin openness. The total nitrogen (TN) concentrations (up to 0.79 wt%) of these organic-rich samples are substantially higher than those of other counterpart profiles. The TOC content shows a positive linear relationship with TN concentration, and a significant positive TN-intercept (∼0.21 wt%) represents the addition of inorganic N produced by the remineralization of sinking organic N. The high TN concentrations may also be due to the flourishing and enhanced burial of primary producers. The δ15N value in the Doushantuo Member IV progressively decreases from ∼8.2‰ to ∼3.2‰ (from bottom to top), a result of the changes in basin openness and the enhancement of N2-fixation. Compilation shows the δ15N and δ13Corg values are approximately covariant, and both have significant temporal and spatial variations at this member of South China, indicating a strong heterogeneity in the water column redox environments and changes in the type of primary producers. A mode δ15N value of 4‰ indicates a stable and relatively large nitrate reservoir in the late Ediacaran ocean.

Rare earth element enrichment in sedimentary phosphorites formed during the Precambrian–Cambrian transition, Southwest China

Numerous sedimentary phosphorites in Southwest China were formed around the Precambrian–Cambrian transition (PC–C), including the upper Ediacaran Doushantuo Formation and lower Cambrian Gezhongwu Formation. The Gezhongwu phosphorites in Zhijin exhibit marked rare earth element (REE) enrichment (>1000 ppm), and may represent new REE resources. Although the main characteristics of the Gezhongwu phosphorites have been well constrained, the REE enrichment mechanisms remain unclear. We undertook a comparative study of three typical sedimentary phosphorites with variable REE contents formed at the PC–C transition in central Guizhou Province, Southwest China. These include sections A and B of the Doushantuo phosphorites (560 ± 8 Ma) from the Weng’an area (i.e., WA-A and WA-B), and the Gezhongwu phosphorites (527 ± 24 Ma) from the Zhijin area (ZJ). The phosphorites were investigated with state-of-the-art macroscale to nanoscale analytical techniques. In contrast to the extraordinary REE enrichment in the ZJ phosphorites (average ΣREE = 1157 ppm), the phosphorites in WA-A (average ΣREE = 234 ppm) and WA-B (average ΣREE = 114 ppm) are REE-poor. Elemental mapping by laser ablation–inductively coupled plasma–mass spectrometry, along with transmission electron microscopy analyses, showed the REEs in the studied phosphorites are hosted in nanoscale francolites. The 87Sr/86Sr and Y/Ho ratios of the francolite grains indicate that greater terrigenous input may have led to more REE enrichment in the WA-A than WA-B phosphorites, but this cannot explain the extraordinary REE enrichment in the ZJ phosphorites. The F/P2O5 values of the francolite grains in the ZJ phosphorites (∼0.097) are higher than those in the WA-A (∼0.084) and WA-B (∼0.084) phosphorites, and the grain size of the francolite in the ZJ phosphorites (∼89.9 nm) is larger than those in the WA-A (∼56.6 nm) and WA-B (∼57.4 nm) phosphorites, indicative of more intense reworking of the ZJ than WA phosphorites during early diagenesis. A plot of Nd concentration versus Ce/Ce* reveals that lower sedimentation rates characterized the ZJ phosphorites. Therefore, intense sedimentary reworking during early diagenesis resulted in more REEs being sequestered by the marine phosphates from seawater and pore waters at a lower sedimentation rate, which resulted in the extraordinary REE enrichment in the ZJ phosphorites. Our findings highlight the multiple factors that controlled formation of sedimentary phosphorites around the PC–C transition (especially the intense reworking and redox conditions of the overlying seawater), and provide further insights into REE enrichment in sedimentary phosphorites worldwide.