Doushantuo Research Articles

Evidence for high-frequency oxygenation of Ediacaran shelf seafloor during early evolution of complex life

Increasing oxygenation of the early Ediacaran Ocean is thought to have been responsible for the emergence of early animals. Although geochemical studies have suggested periods of oceanic oxygenation in the Ediacaran, direct evidence for seafloor oxygenation has been lacking. Here, we report frequent occurrences of distinctive, sub-millimetric, and early diagenetic pyrite-marcasite rosettes in phosphorites from the lower Ediacaran Doushantuo Formation (Weng’an, South China). They typically consist of a nucleus of framboidal pyrite, a cortex of radiating marcasite blades intergrown with quartz, and a rim of second-generation pyrite, recording partial oxidative dissolution of pyrite and co-precipitation of marcasite and quartz. This inference is further supported by near-zero carbon isotope values of the host dolostone, similarly low sulfur isotope values for pyrite and marcasite, and evident Fe-isotope fractionation between marcasite and pyrite. Collectively, our findings reveal intermittent bottom-water and porewater oxygenation events, providing direct evidence of high-frequency oxygenation of Ediacaran continental shelves.

Trace element evidence for diverse origins of superheavy pyrite in Neoproterozoic sedimentary strata

Sedimentary pyrite has long been used as an archive of marine environments in Earth history. To capture reliable paleoenvironmental signals, however, we need to first evaluate pyrite in sedimentary strata as it can be altered and masked by later diagenetic and/or hydrothermal processes. Here, we trained two supervised machine learning algorithms on a large LA-ICP-MS pyrite trace element database to distinguish pyrite of different origins. The analysis validates that two models built on the co-behavior of 12 trace elements (Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Au, Tl, and Pb) can be used to accurately predict pyrite origins. Further statistical analysis suggests four trace element clusters behaving differently among sedimentary (syngenetic and early diagenetic), synsedimentary hydrothermal (syngenetic hydrothermal), and post-sedimentary hydrothermal (epigenetic hydrothermal) pyrite, which is probably driven by chemical and physical properties of source fluids, interactions between elements, competition among coprecipitating minerals, and pyrite growth rate. Armed with this initial success and aided by new LA-ICP-MS trace element data from 9 samples, we then demonstrated the efficacy of this approach in identifying the origins of pyrite from two Neoproterozoic sedimentary successions in South China. The first set of samples contain isotopically superheavy pyrite (i.e., whose bulk-sample δ34S values greater than those of contemporaneous seawater sulfate and whose origins remain controversial) from the Cryogenian Tiesi’ao and Datangpo formations. The second set of samples contain pyritic rims (associated with fossiliferous chert nodules and thought to be critical in exceptional fossil preservation) from the Ediacaran Doushantuo Formation. For the superheavy pyrite, the models consistently show high confidence levels (mostly > 80 % probability) in identifying its genesis type, and three out of four samples were given sedimentary origins. For the pyritic nodule rims, the models suggest that early diagenetic pyrite was subsequently altered by hydrothermal fluids and therefore shows mixed signals. The study highlights the importance of pyrite trace elements in deciphering and distinguishing the origins of pyrite in sedimentary strata.

Geochemical properties, mechanism of formation, and source of solid bitumen in the Ediacaran Dengying Formation from the central to northern Sichuan Basin, China

Several large gas reservoirs, including the Weiyuan and Anyue gas fields, have been discovered in the Ediacaran Dengying Formation in the Sichuan Basin, indicating vast opportunities for deep-petroleum exploration. Understanding the origin and evolution process of these gas reservoirs is crucial for further petroleum exploration and development. Although the burial history of the Sichuan Basin and its surrounding areas is basically dominated by Paleozoic burial and preservation, and Meso-Cenozoic uplift and erosion. However, the source of the Dengying Formation natural gas in the Sichuan Basin is still a subject of controversy between the Ediacaran and Cambrian source rocks due to high thermal maturity, secondary alteration, and multistage tectonic activities. This study aims to examine the investigation on the properties and genetics of Dengying solid bitumens that existed in the central to northern Sichuan Basin, in order to provide insights into the source and evolution history of the Dengying paleo-oil reservoirs. For this purpose, petrologic, elemental, and organic geochemical analyses were used to investigate the properties, origin, and formation mechanism of solid bitumens. The results indicate that these solid bitumens have suffered intense thermal alteration (EqVRo 2.1%–5.3%), and are classified as pyrobitumens. Solid bitumens in the Gaoshiti bulge and the north slope area were subjected to tectonic thermal events (e.g., Emeishan igneous province) and relatively strong thermochemical sulfate reduction (TSR) alteration. These solid bitumens were assumed to be pyrolysis products of light (condensate) oil. In contrast, solid bitumens in the Weiyuan bulge and the Micangshan uplift area were not influenced by tectonic thermal events but experienced weak TSR alteration, and originated from either normal or heavy crude oil. Moreover, a systematic bitumen-source correlation was carried out using δ13Corg values, bound steranes, and trace element distributions. Results reveal that the solid bitumens in the central Sichuan paleo-uplift were mainly derived from the lower Cambrian Maidiping Formation, those in the north slope area originated from the lower Cambrian Qiongzhusi Formation, while those in the Micangshan area were sourced from the Ediacaran Doushantuo Formation. Structural and burial history analysis implies that the differences in the source of the Dengying paleo-oil reservoir (i.e., solid bitumen) between these study areas are attributed to the distribution of source rocks and their contact relationship with reservoirs. This research successfully obtained insights into the source and genetic mechanism of paleo-oil reservoirs by analyzing the properties of high-maturity solid bitumens, which can guide further exploration and development of deep petroleum resources.

Sedimentary sequence and evolutionary characteristics of the rift system of Neoproterozoic Nanhua System and Doushantuo Formation: A case study from the northeastern margin of Sichuan Basin

Neoproterozoic strata are widely developed in the Upper Yangtze region of South China, among which the Sinian Dengying Formation has been discovered with numerous large gas fields in the Sichuan Basin, and hence they have become an essential domain for natural gas exploration and development. The early‐to‐middle Neoproterozoic Nanhua System and Doushantuo Formation are characterized by rifting deposits in the northeastern margin of Sichuan Basin, but their depositional succession and sequence architectures are still unclear, which has largely restricted our understanding of the resource potential of these strata. In this paper, based on the outcrops of the Neoproterozoic Nanhua System and Doushantuo Formation in the northeastern margin of Sichuan Basin, we describe the stratigraphic distribution and sedimentary evolutionary characteristics, and establish the regional stratigraphic framework, sedimentary cycle and evolutionary sequence of the Nanhua System and Doushantuo Formation. The results show that the lithology of the Nanhua System and Doushantuo Formation is primarily composed of conglomerate, conglomeratic sandstone, medium‐fine sandstone, siltstone, mudstone, shale and tillite; a variety of sedimentary facies including alluvial fan, river, delta, coastal shore, shelf, basin and tillite were developed; five third‐order sequences could be identified, representing multiple cycles of marine transgression and regression. The climate during the deposition of the Nanhua System and Doushantuo Formation underwent from a warm and humid pre‐glacial period through cold and arid multiple glacial periods to a warm and humid interglacial period, which consequently formed a sedimentary sequence of alluvial fan and fluvial delta to tillite during the early period, shelf basin to tillite during the middle period and slope basin to delta facies during the late period. In terms of a plan view, the depositional pattern is composed of alluvial fans and deltas at the proximal part of the rift, deep‐water shelf to slope at the middle part of the rift and slope to basin facies at the central part of the rift. Finally, we propose that large‐scale distribution of high‐quality source rocks developed in the interglacial Datangpo and Doushantuo formations, while fluvial‐deltaic reservoirs occurred in the Gucheng, Nantuo and Doushantuo formations, thus showing favourable resource potential and exploration prospects within the Sichuan Basin.

Geochemical evolution of the Doushantuo economic phosphorite in Central Guizhou, South China: Quantified multi-stage metallogenic process

Economic phosphorite from Ediacaran Doushantuo Formation is the first true phosphorite giant and represents the onset of the Precambrian-Cambrian global phosphogenic event. The Doushantuo phosphorite may have deposited during the early-middle Ediacaran (ca. 632–574 Ma) according to the latest radiochronological data. Petrographically, in central Guizhou, it can be subdivided into two types of lithofacies: pristine phosphorite (micritic phosphorite, phosphatic shale, banded phosphorites) and reworked phosphorite (intraclastic phosphorite, vuggy intraclastic phosphorite, unconsolidated phosphorite). Geochemical data show that chemical/biochemical enrichment is the most effective process to condense the phosphatic sediments and obviously improve the grade of P2O5 (can reach 38.8 wt%). Hydrodynamic reworking may not play an important role in the further enrichment of P2O5 content but effectively got rid of SiO2 and Al2O3 components in phosphorite by the removal of lighter terrigenous particles and clays. Mass change calculation shows that Ca and Mg are heavily depleted in the chemical leaching process, implying it is a key process to upgrading the quality of phosphorite by separating the carbonate constituents from phosphorite. Therefore, a multi-stage metallogenic model of chemical/biochemical enrichment, hydrodynamic reworking, and chemical leaching is proposed, quantified the composition changes and elucidated the constrained factors in the formation of high-graded phosphorite. Such a metallogenic process effectively produced high-quality phosphorite deposits and bring great benefits to metallurgy.

Lithium isotope composition of Ediacaran dolostones from the Nuccaleena and Doushantuo formations

The end of the Cryogenian glaciations undoubtedly affected the chemistry of the Neoproterozoic oceans, with potential consequences for the evolution of life; the duration and extent of this influence are poorly constrained. Lithium (Li) isotopes in carbonates can be used to investigate past weathering events and riverine input into the oceans. Here, we report the Li isotope (δ7Li) composition of the Ediacaran cap dolostones (Marinoan) from the Nuccaleena Formation, South Australia and the Doushantuo Formation, South China to investigate changes in weathering during the aftermath of the Marinoan glaciation. The origin of dolomite formation is still hotly debated, and the inability to precipitate dolomite at ambient temperatures (the ‘dolomite problem’) has not yet been resolved. The dominant hypothesis for the presence of marine dolomite is that it is of secondary origin, owing to diagenetic replacement of calcium carbonate, but a second hypothesis is that it forms as a result of primary marine deposition; both are plausible hypotheses. Using carbon isotopes (δ13C) and ratios of manganese and strontium (Mn/Sr), we suggest that diagenesis may not have significantly altered the δ7Li composition of the primary dolomite, but this cannot be discounted without further diagenetic modelling. As a result, we cannot infer the δ7Li composition of the Ediacaran oceans, as further work must be done to address the impact of diagenesis on the δ7Li values. Nevertheless, this dataset contributes to a Li isotope chemostratigraphic record of the Proterozoic, which is a key element to understanding the emergence of complex life.

Recurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran

The Ediacaran Period (~635–539 Ma) is marked by the emergence and diversification of complex metazoans linked to ocean redox changes, but the processes and mechanism of the redox evolution in the Ediacaran ocean are intensely debated. Here we use mercury isotope compositions from multiple black shale sections of the Doushantuo Formation in South China to reconstruct Ediacaran oceanic redox conditions. Mercury isotopes show compelling evidence for recurrent and spatially dynamic photic zone euxinia (PZE) on the continental margin of South China during time intervals coincident with previously identified ocean oxygenation events. We suggest that PZE was driven by increased availability of sulfate and nutrients from a transiently oxygenated ocean, but PZE may have also initiated negative feedbacks that inhibited oxygen production by promoting anoxygenic photosynthesis and limiting the habitable space for eukaryotes, hence abating the long-term rise of oxygen and restricting the Ediacaran expansion of macroscopic oxygen-demanding animals.

Dating Precambrian sedimentary carbonate strata by in situ U–Pb isotopes of dolomite

Precambrian sedimentary carbonate succession ages are challenging due to the lack of suitable fossil records and mineral chronometers and techniques. Here we present the first result for in situ LA-ICP-MS U–Pb dolomite dating from the Dengying Formation in the Mayuan area, Shaanxi Province, South China. The sample yields a date of 565.0 ± 5.5 Ma (2σ, MSWD = 2.8, n = 47). This age agrees well with regional Ediacaran successions in the Yangtze Block. The new ages are considered alongside regional correlations of Ediacaran successions in South China and help to re-evaluate the chronostratigraphic framework of the Dengying and Doushantuo Formation. It also opens a new window for dating those blind sedimentary carbonate strata, particularly during the Precambrian era.

Generation Time and Accumulation of Lower Paleozoic Petroleum in Sichuan and Tarim Basins Determined by Re–Os Isotopic Dating

With the targets of petroleum exploration transferred to the deep and ancient strata, abundant oil and gas resources have been found in Lower Paleozoic and older strata in central and western China. Due to complex evolutionary processes including multiple episodes of hydrocarbon accumulation and ubiquitously accompanied by secondary alterations, significant uncertainties remain concerning the generation time and accumulation processes of these revealed petroleum sources. In this paper, relative pure Re and Os elements existing in the asphaltene fractions of Lower Cambrian solid bitumen collected from the Guangyuan area, western Sichuan Basin, SW China and Middle–Lower Ordovician heavy oils in the Aiding area of the Tahe oilfield in the Tarim Basin, NW China were successfully obtained by sample pretreatments, and Re–Os isotopic analysis was subsequently carried out for the dating of these. The Re–Os isotopic composition indicates a generation time of Guangyuan bitumen to between 572 Ma and 559 Ma, corresponding to the late Sinian period of the Neoproterozoic era. By the means of Re–Os isochron aging, initial 187Os/188Os ratios, and carbon isotopic compositions, the Lower Cambrian bitumen is supposed to originate from source rocks of the Doushantuo Formation in the Sinian strata and subsequently migrated into the reservoirs of the Dengying Formation. This previously reserved petroleum was transformed into its present bitumen state by the destruction of reservoirs caused by tectonic uplift. The Re–Os dating results of Middle–Lower Ordovician heavy oil of Tarim Basin suggest that it was formed between 450 Ma to 436 Ma, corresponding to the Late Ordovician–Early Silurian system, and the generated petroleum likely migrate into the Middle–Lower Ordovician karst reservoirs to form early oil reservoirs. With tectonic uplift, these oil reservoirs were degraded and reformed to the heavy-oil reservoirs of today.

Distinct REY sources recorded in the Doushantuo phosphorite

During the Ediacaran period, massive marine phosphorites were deposited worldwide, which could represent a global sink of rare earth elements and yttrium (REY) in oceans. In the Doushantuo Formation of South China, the REY source has been preliminarily constrained as the mixed sources of seawater and terrigenous matter in these phosphorites. However, the REY composition of both seawater and terrigenous endmembers is still unclear; therefore, it’s difficult to identify the contributions of seawater and terrigenous sourced REY. In this work, REY and other trace elements of bulk phosphorites, in situ carbonates, and apatite grains were analyzed from a drill core (Fangmashan) of the Doushantuo Formation in South China. Combined with mineralogical observations, the REY composition of seawater and terrigenous endmembers is well-constrained. Along the investigated Fangmashan drill core, all in situ carbonates (calcites and dolomites) show a modern seawater-like REY pattern, indicating that the REY characteristics of the Doushantuo seawater could be similar to those of modern oxic seawater without significant changes. Alternatively, apatites from the lower and upper phosphorites showed distinct REY patterns. One type of apatite in the upper phosphorites showed a modern seawater-like REY pattern, indicating authigenic REE enrichment from the seawater column. However, another type of apatite in the upper phosphorites with an REE pattern that deviated from seawater showed high ΣREY (∼691.16 ± 309.78) and low Y/Ho (∼40.52 ± 4.05), probably reflecting mixed REY sources and/or the effects of early diagenesis. It is noted that most apatites in the lower phosphorites are obviously enriched in light REY (LREY: La - Nd) compared to heavy REY (HREY: Ho - Lu and Y), which may represent a significant contribution of the terrestrial REY source. This study reveals that the REY composition of paleo-seawater could be similar to modern seawater as early as the Neoproterozoic and supports a significant contribution to understanding the difference in the REY composition of phosphorites in geological history.

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Distinguishing geobiological signatures from organic matter in the Ediacaran chert nodules

The Doushantuo Formation (DST) in South China preserved significant bio- and chemostratigraphic records that are critical to the study of Ediacaran life and environment. Abundant microfossils and organic matter (Corg) were preserved in the DST nodules. We investigated the mineralogy, petrography and the structural characteristics of Corg in three different types (Type I: silica-rich; Type II: pyrite-rich; Type III: barite-rich) of DST nodules, in order to better understand the geobiological processes during the formation of nodules and the taphonomic details of Corg preserved therein. According to Raman spectral parameters, the Corg experienced low-grade metamorphism, and represents obvious heterogeneities of structural order and H:C in the center, rim and the host rock, respectively. Furthermore, the structural characteristics of Corg show distinct bias to various mineral matrices (silica, pyrite, goethite and barite) where they were preserved, probably due to differential mineral-organic interactions during the formation of different nodules. These mm- and μm-scale structural heterogeneities of Corg reflect the nature of precursor organism, which were subsequently affected by various post-depositional processes. Our study proposes the possible formation processes of three types of DST nodule, suggests the geobiological influences on the Corg preserved therein, and sheds light on the geobiological signatures from the DST using Raman-based approaches.

Metal elements potential for hydrocarbon–source correlation

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the BaN–NiN–CuN ternary diagram, VN–NiN–MoN ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE)N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the VN–NiN–MoN ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the VN–NiN–MoN ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions.

A refined recycled carbonatite model for the δ13C negative anomaly in the upper Doushantuo Formation

The upper Doushantuo Formation (terminated at 551 Ma) in the Yangtze platform keeps a large carbonate δ13C (carbon isotope composition) negative anomaly, which has been proposed to be related to the Shuram Excursion (SE). However it is still controversial whether such anomaly records the carbon isotope variation of seawater caused by oxidizing dissolved organic carbon (DOC) or the input of a local exotic carbon reservoir. Here, we conducted further chemical and mineralogical studies of samples from the 13C-depleted strata of the Doushantuo Formation at Jiulongwan. The 13C-depleted strata are featured by flat REY patterns and constant δ13Ccarb (δ13C of carbonate) but variable δ13Corg (δ13C of organic carbon), which disagree with the prediction of DOC oxidation model. Euhedral quartz grains separated from these strata show bluish-violet catholuminescence indicating a high temperature origin. Carbonate inclusions are found in some of these quartz grains. Above chemical and mineralogical features of the 13C-depleted strata can be explained by the previously suggested recycled carbonatite model. However, zircons separated from the 13C-depleted carbonates record not only a young age of 567.17 ± 3.57 M, but also two old age clusters of 640 ∼ 960 Ma and 1760 ∼ 2160 Ma. The young age of 567.17 ± 3.57 Ma may record the carbonatite magmatic activity. The age spectrums of old zircons imply that the recycled carbonatite is more likely to be transported and deposited in the intra-shelf area with other clastics through weathering.

Multiple ocean oxygenation events during the Ediacaran Period: Mo isotope evidence from the Nanhua Basin, South China

The Ediacaran Period (ca. 635–539 Ma) was an eventful interval in Earth history, during which a succession of biological and environmental changes, including episodic ocean oxygenation events (OOEs), paved the way for the Cambrian radiations of animal life. To better understand the evolution of ocean redox conditions and to estimate the extent of seafloor oxygenation during this period, we analysed molybdenum (Mo) isotope compositions and redox sensitive element (RSE) concentrations from a continuous, mid-slope section of the Doushantuo Formation (ca. 635–560 Ma) of the Nanhua Basin on the South China Craton. Alongside an updated compilation of published Mo isotope and RSE data, our new data show that three OOEs occurred within a generally anoxic Ediacaran ocean, with the last, particularly extensive event occurring during deposition of Doushantuo Member IV. Here we show how the global balance of redox-related Mo sinks shifted dynamically in response to these transient OOEs, which correlate well with the first appearance of Ediacaran fossil groups and so may have triggered or stimulated biotic innovations and radiations. Moreover, the spatial distribution of the Mo data from multiple sections supports episodic expansion of a euxinic wedge on the slopes of the Nanhua Basin, consistent with pyrite burial as a potential cause of the OOEs.

Studies on geochemical characteristics and biomineralization of Cambrian phosphorites, Zhijin, Guizhou Province, China.

Phosphate rocks, an important ore resource in Guizhou Province, China, are mainly hosted within the Sinian Doushantuo Formation and the Cambrian Meishucun Formation. In addition, the phosphate rocks of the Cambrian Meishucun Formation are rich in biological fossils. Although numerous studies investigating the genesis of phosphate deposits have been performed, the relationship between biological activity and the formation of phosphate deposits in the lower Cambrian Meishucun Formation has not been convincingly explained. This study focuses on the biological fossil assemblage, the characteristics of phosphorus, and the relationship between biological and phosphorus enrichment of the lower Cambrian phosphorites. The primary objectives of our study are to analyze the role of organisms in the formation of phosphorites, restore the phosphorus-formation environment of the Cambrian Meishucun Formation, and construct a sedimentary model of the phosphorites in the Meishucun Formation. The results indicate that there is a significant positive correlation between biological activity and the deposition of phosphorites, that is, the higher the degree of biological enrichment and differentiation, the stronger the deposition. The geochemical analysis of several profiles in the Zhijin phosphorite block shows that the phosphorite block was deposited in an oxygen-rich environment and was affected by a high-temperature hydrothermal fluid. Upwelling ocean currents supplied abundant phosphorus and other nutrients, which provided the conditions for small shells and algae to flourish. Biochemical activity was a crucial factor in the deposition of the phosphorite.

The high-frequency sea-level change in the aftermath of the Marinoan snowball Earth: Evidence from the Doushantuo formation in the northern margin of the Yangtze Craton, South China

The rapid rise of glacioeustatic change is the most extreme paleoenvironment alteration in the aftermath of Snowball Earth. Although geologists conducted a lot of multi-subdiscipline research on this issue previously, there still exists the potential room for further discussions of the process in detail. For decades, the practice proved that the Fischer plot is a simple and robust tool to illustrate the fluctuations of accommodation patterns v.s. cycle sets or strata depth which could be interpreted as relative sea-level changes. This research simulates the Fischer plot to unravel the sea-level change in the aftermath of Marinoan glaciation by measuring the lower Ediacaran Doushantuo Formation in Shennongjia, South China. The result shows that 131 fifth-order cycles and nine third-order cycles help us propose a completed second-order cycle variation of ice melting-forced sea-level change, i.e., (1) early high-frequency and slow to rapid stepwise rising, and (2) followed by a stable decrease in the latter. In addition, the vertical sedimentary facies of the lower Doushantuo Formation display, in ascending order, (1) intertidal carbonate rock, (2) subtidal clastics with turbidite, and (3) intertidal lagoon fine clastics, indicating the process of the relative waxing and waning of sea-level. Such interpretation of the Fischer plot and the sedimentary facies’ vertical evolution is beneficial for studying the high-frequency sea-level change and paleogeographic reconstruction of post-Marinoan deglaciation.

Hydrocarbon accumulation conditions and exploration targets of the Sinian Dengying Formation in the Southeastern Sichuan Basin

No breakthrough has been made in petroleum exploration of the Sinian Dengying Formation in the southeastern Sichuan Basin as the major factors controlling the hydrocarbon accumulation of the Dengying Formation have not been well established, including the development period and distribution of the platform margin, the effectiveness of the source-reservoir combination and the reliability of the Cambrian pre-salt structural trap. Based on the interpretation of seismic data and hydrocarbon-source correlation, the distribution of the platform margin of the Sinian Dengying Formation in the southeastern Sichuan Basin was mapped. Moreover, the gas source rocks of the Dengying Formation were clarified according to geochemical characteristics of fluid inclusions, and the pre-salt structures were further confirmed by seismic forward modeling and experiments of prestack depth migration. Results showed that the sedimentary facies of the 4th member of Dengying Formation in the southeastern Sichuan Basin was platform margin of gentle slope type. The platform margin reservoir of the Dengying Formation and the Lower Cambrian source rocks constitute a favorable source - reservoir combination. The carbon isotope characteristics of gas in fluid inclusions indicate that some of the oil/gas in the Dengying Formation were contributed by the Doushantuo Formation source rocks. As the difference in seismic velocities between carbonate rocks and gypsum/salt beds may cause a reduction in the amplitude of pre-salt structures, the low-amplitude structures identified on current seismic profiles are underestimated instead of overestimated. Accordingly, the Sinian structural traps are confirmed to develop in the deep pre-salt sequences in the low-steep anticline belt, southeastern Sichuan Basin. On the basis of the above new understanding, we propose two types of hydrocarbon migration modes for the Sinian Dengying Formation in southeastern Sichuan Basin, including lateral migration of hydrocarbons generated from the Cambrian Qiongzusi Formation and vertical migration of hydrocarbons derived from the Sinian Doushantuo Formation. Lithologic traps at the platform margin and pre-salt structural traps within the platform are favorable targets for natural gas exploration.

Redox heterogeneity of the Ediacaran ocean constrained by chromium isotopes

Ocean oxygenation during the Ediacaran Period (635–541 Ma) is thought to be a potential stimulus for the emergence and diversification of early animals. However, the atmospheric and oceanic redox states during this period remain controversial. The chromium (Cr) stable isotopic system is an emerging proxy for constraining the redox states of the ocean and atmosphere over the geological time. Here, we present systematic Cr isotopic (δ53Cr) data for two Ediacaran successions in the Yangtze Block of South China: the Wangji section deposited in shallow water and the Lantian section deposited in deep water. The positively fractionated δ53Cr values at the Wangji section (up to 1.15‰ for carbonates and up to 1.11‰ for shales) imply that atmospheric oxygen had risen to a relatively high level to allow the oxidative weathering of Cr(III) during the Ediacaran. Additionally, a peak of δ53Cr values is closely correlated to the δ13C positive excursion in the upper Doushantuo Formation, likely because of the enhanced biological productivity at that time. In contrast, positively fractionated δ53Cr values are suppressed (from −0.11‰ to 0.05‰) in the shales at the Lantian section, indicating that the contemporary deep water might have still been extensively anoxic, such that the terrestrial Cr(VI) with positive δ53Cr values would have been quantitatively reduced and deposited before being transferred to the deeper settings. A comparison of these two sections of different depositional environments and their δ53Cr profiles suggests that the redox conditions of the Ediacaran ocean were highly stratified and heterogeneous.

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”).