Datangpo Formation Research Articles

Empirical Reconstruction of Earth‐Moon and Solar System Dynamical Parameters for the Past 2.5 Billion Years From Cyclostratigraphy

AbstractThe chaotic behavior of the Solar System prevents the prediction of the precise evolution of the planetary secular frequencies. Here we use four cyclostratigraphic sequences, ∼259 Ma Wujiaping Formation, ∼455 Ma Pingliang Formation, ∼655 Ma Datangpo Formation and ∼2,465 Ma Dales Gorge Member, and Bayesian inversion to provide powerful geological constraints to this problem. The inversion provides constraining estimates for the Solar System fundamental secular frequencies g1, g2, g3, g4, and g5, and the Earth's precession rate k from 105.26 ± 2.68 (2σ) arcseconds/year at ∼2,465 Ma to 55.86 ± 2.60 (2σ) arcseconds/year at ∼259 Ma. Stochastic monotone interpolation enables the reconstruction of Earth‐Moon distance from 2.5 billion years to the present. Calculation of tidal dissipation supports the ocean model before 1.4 Ga, but is outside the error range after ∼1.8 Ga, suggesting lower tidal dissipation in the early Proterozoic Eon, and a possible high value in the early Paleozoic.

Enigmatic super-heavy pyrite formation: Novel mechanistic insights from the aftermath of the Sturtian Snowball Earth

It is not well understood how, in the immediate aftermath of the Sturtian Snowball Earth, marine sulfur cycling resulted in a global distribution of sedimentary pyrite with δ34S values higher than coeval seawater. Here, we analyze the quadruple sulfur isotope systematics of organic-bound sulfur (OS) from the lowermost post-Sturtian Datangpo Formation, South China, and identify two generations of OS formation, each sampling an isotopically distinct sulfate reservoir (δ34S ≈ 26‰ and 52–93‰) that differentially impacted its respective, co-occurring pyrite. Combining several lines of geochemical evidence, we argue that the first OS generation was the product of a sulfate-impoverished meltwater-influenced setting, with OS preservation being the result of resistance to acid hydrolysis. However, the second OS generation was sourced from H2S produced in sediments during early diagenesis via microbial reduction of a 34S-enriched sulfate pool derived from overlying euxinic or ferruginous seawater. This is the first ancient marine data set where all observed pyrite is more enriched in 34S than its associated OS. Our proposed origin may be applied to global superheavy pyrite (SHP) immediately after the Sturtian and is comparable to processes linked to freshwater-to-marine transitions during rising sea level in the wake of recent glaciation.

Geochemical characteristics of organic-rich intervals within the Cryogenian non-glacial Datangpo Formation in southeastern Yangtze Block-implications for paleoenvironment and its control on organic matter accumulation

To understand the mechanism of organic matter enrichment in the organic-rich intervals of the Cryogenian non-glacial strata, we report high-resolution geochemical data of the well-developed non-glacial Datangpo Formation at a section in the southeastern margin of the Yangtze Block, including iron (Fe) speciation, sulfur and organic carbon isotopic compositions and elemental contents. The organic-rich intervals are mainly distributed in the basal manganese (Mn) carbonate and Mn-bearing shales (intervalⅠ), and overlying black shales (intervalⅡ). The paleo-redox reconstruction shows a transition from ferruginous-dominated watermass during deposition of the IntervalⅠto euxinic-dominated watermass during deposition of the IntervalⅡ, which provides good conditions for the preservation of organic matter. A variety of geochemical indicators demonstrate that during deposition of the IntervalⅠ, elevated primary productivity in the surface water driven by hydrothermal activities and improved post-glacial basinal watermass exchange, coupled with some organic carbon influx from the dissolved organic carbon reservoir, collectively promoted the enrichment of organic matter. For the interval Ⅱ, enhanced chemical weathering accompanied by persistently warm climatic conditions exerted a major control on the organic matter enrichment by importing abundant nutrients, resulting in enhanced surface-water primary productivity. In addition, favorable sedimentation rates also made positive contributions to the organic-rich black shales in the intervalⅡ. Based on a combined analysis of paleoclimates, hydrological conditions and hydrothermal activities, this study puts forward organic matter enrichment models for the organic-rich intervals deposited during the Cryogenian non-glacial period.

Low marine sulfate levels during the initiation of the Cryogenian Marinoan glaciation

The microbial sulfate reduction plays a significant role in regulating marine organic matter degradation. Thus the variation of ancient marine sulfate levels is closely coupled with the changes in global organic carbon burials and Earth’s surface redox evolution in geological time. It is widely assumed that seawater sulfate concentration was deficient during the anoxic Cryogenian Marinoan glaciation (ca.650–635 Ma) which represents the most extreme icehouse condition in Earth’s history. However, such a low seawater sulfate level has not been explicitly confirmed. To address this, here we carried out pyrite content and sulfur isotope (δ34Spy) analysis of the Cryogenian non-glacial (the Datangpo Formation) and the Marinoan glacial successions (the Nantuo Formation) in South China. The Datangpo and Nantuo Formation show relatively high values of δ34Spy, ranging from +19.9 ‰ to +32.9 ‰, and low values of pyrite content (mean = 0.72 wt%). These pyrites are dominated by euhedral crystals with a rare occurrence of framboids, indicating an early diagenetic origin, i.e., pyrite formed in sediments with sulfate supplied from seawater. Using and a one-dimensional diffusion–advection-reaction (DAR) model, we calculate marine sulfate levels. The modeling results show that the seawater sulfate concentration is <1 mM during the transition from the top Datangpo to the basal Nantuo Formation, suggesting a shrink of the marine sulfate reservoir at the onset of Marinoan glaciation. We propose that the low marine sulfate concentration was resulted from a decrease in continental chemical weathering. This study provides a quantitative constraint on marine sulfate levels during the Cryogenian snowball Earth event, and it may shed new light on our understanding of the marine biogeochemical changes in the Neoproterozoic.

Nitrogen isotope evidence for oxygenated upper ocean during the Cryogenian interglacial period

The Cryogenian interglacial period have witnessed dramatic changes in climate, oceanic environment and biological evolution. The nitrogen isotopic composition, as an important biogeochemical proxy, has the potential to track both the nutrient cycling and redox conditions in the past. However, nitrogen isotopic data during this interglacial time is rather limited. Here, we present integrated data for nitrogen isotopes (δ15N), as well as organic carbon isotopes (δ13Corg), iron (Fe) speciation, pyrite morphology and trace elements from the Cryogenian interglacial Datangpo Formation derived from a drill core from South China to figure out the nitrogen cycling and coeval redox states. The results reveal a gradual oxygenation bottom waters along with deepening O2-H2S chemocline from euxinic black shale in the lower unit (LU), to oxic siltstone in the upper unit (UU), which is consistent with the redox variation inferred in previous studies. Sedimentary δ15N values constantly remain positive (mean value> + 4‰) throughout the Datangpo Formation and display an increase from the LU to the UU. We interpreted the positive δ15N values in the LU as a result of denitrification related to the development of euxinic conditions. The further increase towards the UU associated with limited nutrient supply may indicate the existence of a stable nitrate reservoir in the upper water column, which was consistent with an increased oxygenation during the interglacial time. The elevated O2 levels and bioavailable nitrogen in seawater may have further contributed to the appearance of the earliest metazoan. In addition, our data lay good foundation for further global δ15N correlations among other interglacial successions.

Manganese Oxide Minerals from the Xiangtan Manganese Deposit in South China and Their Application in Formaldehyde Removal

Because of the nano-scale tunnel constructed by the active Mn-O octahedron in cryptomelane, cryptomelane-type manganese oxides have high activity in the oxidation of several volatile organic compounds (VOCs). Natural cryptomelane, in the form of supergene oxide manganese ore, carpets much of South China. In the lower part of the Datangpo Formation of Nanhua System on the southeastern Yangtze Platform, cryptomelane is one of the major manganese oxides in black shale of the Xiangtan manganese deposit in this deposit. Formaldehyde is a dominant indoor pollutant among volatile organic compounds (VOCs), and applications of synthetic cryptomelane have been reported to eliminate it. To study the removal capacity of naturally outcropping cryptomelane, representative samples of manganese oxide (the primary mineral component of cryptomelane) from the Xiangtan Mn deposit were analyzed in this study. The chemical composition, crystal structure and micromorphology of the manganese oxide minerals were explored using ICP-AES, XRD, EPMA, SEM and HR-TEM techniques. Fine-grained and poorly crystalline, these minerals consist primarily of cryptomelane, along with minor amounts of pyrolusite, hollandite, lithiophorite, limonite and quartz. Natural cryptomelane is a monoclinic crystal, and its cell parameters are refined. The results of catalytic tests revealed that natural cryptomelane has obvious catalytic activity in the oxidation of formaldehyde in a static environment under room temperature. This study may provide a natural mineral material as an inexpensive and efficient catalyst for the purification of formaldehyde in industrial or indoor air treatment.

Neoproterozoic extensional basins and its control on the distribution of hydrocarbon source rocks in the Yangtze Craton, South China

• Neoproterozoic source rocks in the Yangtze Craton. • Structural and sedimentary patterns of the extensional basins evaluated. • The source rocks are related to the extensional basins, climate and life evolution. • Favorable areas for the source rock distribution are predicted. Hydrocarbon exploration in the Neoproterozoic extensional basins in the Yangtze Craton of South China has made prominent progresses in the past decades. However, the spatial distribution of source rocks across the craton still remains unclear. Here, we use outcrop observations, borehole data, and reflective seismic profiles to investigate the prototype, distribution and sediment infilling of these basins in the Yangtze Craton during the periods of source rock deposition. The basin prototype is categorized based on its tectonic origin and structural-sedimentary feature as follows. (1) During the deposition of the Datangpo Formation, rift basins extensively developed, band are characterized by a series of graben or half graben structures that consist of basin-controlling normal faults. The source rocks in the Datangpo Formation were deposited as the inner shelf basin or deep shelf facies, obtaining high thickness (i.e., 200 m) in the Wuling rift basin. (2) During the deposition of the Doushantuo Formation, the intracratonic rift basin and passive continental margin basin formed. These basins were controlled by syn-depositional faults that inherited the existing rift fault structures. The source rocks of the Doushantuo Formation were deposited as shelf lagoon, slope to deep-water basin facies. (3) During the deposition of the Dengying Formation, the intraplatform rift basin and passive continental margin basin developed with influence of regional tectonic extension. Not only are these basins affected by the preexisting faults, but also by the newly developed syn-depositional faults. The argillaceous source rocks of the Dengying Formation deposited as slope and deep-water basin facies. Further in-depth analyses revealed that a range of mechanisms may have contributed to the formation and distribution of source rocks in the Yangtze Craton, such as the break-up of the Rodinia Supercontinent, paleaolatitudes, glacial-interglacial geological events, basin extension and sedimentary environments. By integrating the distribution of the source rocks and the late tectonic reworking, this study predicts several targets for the source rock distribution in the Yangtze Craton, including in the central Sichuan, northeastern Sichuan, southern Shanxi, western Hubei, northeastern Guizhou-western Hunan, and the northeastern Guangxi-central Hunan areas.

Paired U and Mo isotope evidence for pervasive anoxia in the Cryogenian early interglacial ocean

The delayed diversification of early animals until the middle Ediacaran Period, despite their likely origin in the Cryogenian Period or earlier, has been attributed to low levels of dissolved O2 in global seawater. To provide more robust constraints on the extent of global-ocean oxygenation during the mid-Cryogenian Period, we analyzed paired U (δ238U) and Mo (δ98Mo) isotope compositions of organic-rich shale in the basal Datangpo Formation from two separate drillcores in South China. This unit yielded an average δ238U of −0.39 ± 0.23‰ (2SD; relative to standard CRM145), which is significantly lower than the average δ238U (+0.02 ± 0.12‰) of euxinic sediments of the modern restricted Black Sea and well below a modeled δ238U of +0.2‰ for modern unrestricted euxinic facies (e.g., Saanich Inlet). The U/Al ratios of our samples indicate that ~ 40% of U was derived from detrital materials (δ238Udetrital = ~−0.3‰), suggesting that the δ238U of authigenic U is even lower than the measured bulk-sample values. Taking into consideration the 0.4–0.8‰ enrichment of 238U in euxinic sediments relative to seawater, the δ238U values of the Datangpo shales correspond to contemporaneous seawater δ238U of −1.20‰ to −0.73‰. Owing to mass balance considerations, these low seawater δ238U values are indicative of widespread development of anoxia in the Cryogenian interglacial ocean (~658–654 Ma). These shales yield mostly low δ98Mo values (+0.56 ± 0.46‰), although the youngest samples have values as high as +1.21 ± 0.23‰ (relative to NIST SRM 3134 = +0.25‰), all of which are significantly lower than modern seawater δ98Mo (+2.34 ± 0.10‰). The Datangpo shales either directly record the global seawater δ98Mo signature or provide a minimum estimate thereof (in the event of local influences). The consistently low δ98Mo values of the Datangpo shales thus confirm widespread anoxia in the Cryogenian interglacial ocean. These findings based on paired δ238U-δ98Mo analyses thus provide key information regarding global-ocean redox conditions during the interval immediately preceding the rise of macroscopic and morphologically complex animals

Separation of Fe from Mn in the Cryogenian Sedimentary Mn Deposit, South China: Insights from Ore Mineral Chemistry and S Isotopes from the Dawu Deposit

Manganese and Fe have similar geochemical properties in the supergene environment. Separation of Mn and Fe is an important process for the formation of high-grade sedimentary manganese deposits. Large-scale manganese carbonate deposits (total reserves of approximately 700 Mt) were formed during the interglacial of the Sturtian and Marinoan in South China. The orebodies are hosted in the black rock series at the basal Datangpo Formation of the Cryogenian period. The Fe contents in ores range from 1.15 to 7.18 wt.%, with an average of 2.80 wt.%, and the average Mn/Fe ratio is 8.9, indicating a complete separation of Mn and Fe during the formation of manganese ores. Here, we present element data of manganese carbonates and sulfur isotopes of pyrite from the Dawu deposit, Guizhou, China, aiming to investigate the separation mechanism of Mn and Fe and the ore genesis. The Fe in ores mainly occurs as carbonate (FeCO3) and pyrite (FeS2). The Mn, Ca, Mg and Fe exist in the form of isomorphic substitutions in manganese carbonate. The contents of FeCO3 in manganese carbonates are similar in different deposits, with averages of 2.6–2.8 wt.%. The whole-rock Fe and S contents have an obvious positive correlation (R = 0.69), indicating that the difference of whole-rock Fe content mainly comes from the pyrite content. The δ34SV-CDT of pyrite varies from 40.0 to 48.3‰, indicating that the pyrite formed in a restricted basin where sulfate supply was insufficient and the sulfate concentrations were extremely low. Additionally, the whole-rock Fe content is negatively correlated with the δ34S values of the whole-rock and pyrite, with correlation coefficients of −0.78 and −0.83, respectively. Two stages of separations of Mn and Fe might have occurred during the mineralization processes. The reduced seawater became oxidized gradually after the Sturtian glaciation, and Fe2+ was oxidized and precipitated before Mn2+, which resulted in the first-stage separation of Mn and Fe. The residual Mn-rich and Fe-poor seawater flowed into the restricted rift basin. Mn and Fe were then precipitated in sediments as oxyhydroxide as the seawater was oxidized. At the early stage of diagenesis, organic matter was oxidized, and manganese oxyhydroxide was reduced, forming the manganese carbonate. H2S was insufficient in the restricted basin due to the extremely low sulfate concentration. The Fe2+ was re-released due to the lack of H2S, resulting in the second-stage separation of Mn and Fe. Finally, the manganese carbonate deposit with low Fe and very high δ34S was formed in the restricted basin after the Sturtian glaciation.

Cracking the superheavy pyrite enigma: possible roles of volatile organosulfur compound emission.

The global deposition of superheavy pyrite (pyrite isotopically heavier than coeval seawater sulfate in the Neoproterozoic Era and particularly in the Cryogenian Period) defies explanation using the canonical marine sulfur cycle system. Here we report petrographic and sulfur isotopic data (δ34Spy) of superheavy pyrite from the Cryogenian Datangpo Formation (660–650 Ma) in South China. Our data indicate a syndepositional/early diagenetic origin of the Datangpo superheavy pyrite, with 34S-enriched H2S supplied from sulfidic (H2S rich) seawater. Instructed by a novel sulfur-cycling model, we propose that the emission of 34S-depleted volatile organosulfur compounds (VOSC) that were generated via sulfide methylation may have contributed to the formation of 34S-enriched sulfidic seawater and superheavy pyrite. The global emission of VOSC may be attributed to enhanced organic matter production after the Sturtian glaciation in the context of widespread sulfidic conditions. These findings demonstrate that VOSC cycling is an important component of the sulfur cycle in Proterozoic oceans.

Oceanic water chemistry evolution and its implications for post-glacial black shale formation: Insights from the Cryogenian Datangpo Formation, South China

The Cryogenian Nanhua Basin in South China comprises one of the best-preserved Neoproterozoic glacial-interglacial sedimentary fills. The Datangpo Formation (ca. 663–654 Ma) was deposited after the Sturtian glaciation and is characterised by extensive organic-rich black shales and interbedded manganese deposits. Trace element data suggest overall anoxic waters with varying degrees of euxinia. These anoxic and sulfidic conditions, as inferred by high ratios of FeHR/FeT and Fepy/FeT and significant enrichments of molybdenum, developed due to high organic carbon loading, resulting in the formation of the black shales. The high organic matter flux was likely the result of increased nutrient supply from continental weathering, upwelling nutrient-rich deep seawater, and seawater exchange due to elevated sea levels after deglaciation, which resulted in a temporary connection with the open sea. An episode of oxygenation during deposition of the manganese-bearing interval shortly after deglaciation is indicated by a negative excursion in the iron speciation data, and generally low enrichments of trace elements. Muted sulfate reduction rates, and thus inferred less euxinic conditions, occurred due to decreased organic matter loading during deposition of the upper-part of the black shales. The lower marine primary productivity was likely a result of suppressed bio-essential elements due to progressively weaker chemical weathering under cooler climatic conditions. Superheavy pyrite with δ34Spy up to +61‰ for the Datangpo Formation is consistent with a compilation of published data. We propose that multiple processes contributed to the superheavy pyrite formation, and suggest a new, more accurate way of recalibration of the sulfate reservoir in sedimentary basins with similar geological and/or geochemical settings. This dataset and model serves as a reference for better understanding the formation of post-glacial organic-rich black shales, including the temporal evolution of oceanic conditions after low-latitude glaciations.

Geochemistry and molybdenum isotopes of the basal Datangpo Formation: Implications for ocean-redox conditions and organic matter accumulation during the Cryogenian interglaciation

The basal Datangpo Formation in South China deposited in the Cryogenian interglacial period has gained increasing interest with the well development of organic-rich black shale and its hosted manganese (Mn) deposits (Mn carbonate), which contain important clues to the paleoclimate and ocean redox of this critical interval in Earth's history. The depositional environment of Mn deposits is still uncertain, as suggested by different suites of redox-indicating parameters. In addition, the Mn deposits are notable for their enrichment in organic matter (OM), despite the fact that the diagenetic origin for the Mn deposits would result in large consumption of OM. To gain insight into these issues, we present the geochemical and molybdenum (Mo) isotopic data of black shale and Mn carbonate samples from a new section. The Datangpo Mn carbonates are characterized by low Mo content (mostly <3.2 ppm) and slightly light Mo isotopic composition (mostly from +0.07‰ to +0.52‰), suggesting that Mo, once adsorbed in Mn-oxide particles, was released in suboxic or anoxic (non-sulfidic) pore water due to Mn-oxide reduction and then diffused back into the water column. Hence the extremely light Mo isotopic signatures might be rarely preserved in sediments. The Mn carbonate formation accompanied by OM re-mineralization within sediments may have caused lower total organic carbon (TOC) content relative to that in black shales. However, elevated nutrient input related with hydrothermal fluid activity probably enhanced primary productivity during the Mn deposition period. It is evidenced by: (1) the Mn-TOC-total inorganic carbon covariation displayed by the Datangpo section and (2) a positive correlation between TOC and Mn content when Mn content is higher than 10% in a large compiled dataset. A compilation of Mo concentration and Mo isotope data suggests that widespread anoxia prevailed during the Cryogenian interglaciation. Instead, Neoproterozoic ocean oxygenation was postponed to the Ediacaran.

Development and distribution rules of the main Neoproterozoic source and reservoir strata in the Yangtze Block, Southern China

In recent years, significant breakthroughs in gas exploration have been made in Neoproterozoic strata in the Yangtze Block, in southern China, as the Anyue super gas field was discovered in the central Sichuan region. The potential of Neoproterozoic exploration in other parts of the Yangtze Block remains a question. Defining the development and distribution rules of Neoproterozoic source and reservoir strata is an important reference for the next step of deep-zone oil and gas explorations in the Yangtze Block. Based on field surveys and sample collections, the analyses of many predecessor field sections and drilling logging data of the Neoproterozoic in the Yangtze Block, source-rock series tests and analyses, as well as analyses of reservoir thin sections, and the distribution rules of the source and reservoir strata were determined by industrial mapping methods. Three sets of source rocks are found in the Neoproterozoic in Yangtze Block, namely, the Datangpo Formation, Doushantuo Formation, and the third member of the Dengying Formation. From top to bottom, the thermal evolutions in the three sets of source rocks show an increasing trend, with the Dengying Formation reaching the stage of over-maturation, while that of Doushantuo and Datangpo formations enter thermal metamorphism stages. The source rocks of the Datangpo Formation are relatively limited in distribution, displaying a northeastward rift style, whereas those of the Doushantuo Formation are distributed widely with two thickness centers. The Neoproterozoic reservoirs in the Yangtze Block developed in the Dengying Formation, with the carbonate reservoir spaces mainly comprising caves, dissolved bores, or pores and cracks. The thickness center of the dolomite reservoirs in Dengying Formation are mainly located in the Middle and Upper Yangtze carbonate platform marginal zones, which are distributed in the eastern Sichuan Basin, and in the Mianyang–Changning rifting platform margin belts in the central Sichuan Basin and the northern Hubei areas. Based on the typical reservoir anatomy, the hydrocarbon accumulation rules of the Neoproterozoic in the Yangtze Block are summarized as “near the rift trough or the passive continental margin hydrocarbon generation centers, adjacent to Karst bioherm-beach reservoir, and stable structural zones afterwards”.

Geochronological constraints on Neoproterozoic rifting and onset of the Marinoan glaciation from the Kingston Peak Formation in Death Valley, California (USA)

AbstractDeath Valley (California, USA) hosts iconic Cryogenian snowball Earth deposits, but the lack of direct geochronological constraints has permitted a variety of correlations and age models. Here, we report two precise zircon U-Pb isotope dilution–thermal ionization mass spectrometry dates for the Kingston Peak Formation: a volcanic eruptive age of 705.44 ± 0.28 Ma from the synglacial Limekiln Spring Member, and a maximum depositional age of 651.69 ± 0.64 Ma from the nonglacial Thorndike submember, which is below the Wildrose diamictite. These dates confirm that the Limekiln Spring and Surprise Members were deposited during the Sturtian glaciation, while the Wildrose submember is a Marinoan glacial deposit, and the overlying Sentinel Peak Member of the Noonday Formation is a Marinoan cap carbonate. Additionally, the age from the Thorndike submember supersedes existing radioisotopic ages from the Datangpo Formation in South China as the youngest constraint on the onset of the Marinoan glaciation, demonstrating that the Cryogenian nonglacial interlude lasted for at least 9 m.y. and the Marinoan glaciation was &amp;lt;17 m.y. long. Cryogenian glaciation in western Laurentia occurred against the backdrop of ∼85 m.y. of episodic rift-related subsidence and magmatism within laterally discontinuous, fault-bound basins.

The Cryogenian Nanhuan System (South China) during the interglacial-glacial transition: geochemistry, sedimentary provenance, and tectonic setting

ABSTRACT Samples of the Nanhuan System during the interglacial-glacial transition in the Shennongjia area, northern Yangtze block, were analysed via major and trace elements, clay mineralogy, and sulphur isotopes to infer provenance, paleoweathering intensity of the source rocks, tectonic setting, and marine chemistry. The magmatogenic sedimentary rocks are typical of acidic igneous sources, whereas the remaining samples show a sedimentary trend. The tectonic discrimination of a passive continental margin was determined via major element diagram. The samples showing subduction-affinity geochemistry may represent provenance virtually entirely comprising old recycled material. Low chemical index of alteration values and high illite contents of the sedimentary deposits reveal that the basin is characterized by physical erosion owing to the combined effects of tectonic movement and long-term cool and dry climatic conditions. Extreme δ34S enrichment in the Datangpo Formation indicates that this Cryogenian formation was a restricted rift basin with limited exchange with the open ocean. The δ34Ssulfide and illite values increase rapidly at the end of the Sturtian glaciation followed by a sharp decline at the end of inter-glacial silt deposition. Low oceanic sulphate concentrations and intense bacterial reduction of sulphate may account for the 34S enrichment in this restricted deep-water basin.

Evolution of paleo-weathering during the late Neoproterozoic in South China: Implications for paleoclimatic conditions and organic carbon burial

The late Neoproterozoic was a key transition time for Earth, involving turbulent fluctuations of the atmosphere-ocean system including Snowball Earth events, supercontinent reorganisation, and revolution of the nutrient cycle, all of which could have potentially triggered the development of increasingly complex life forms. To further investigate the temporal variations of paleoclimatic conditions and to better understand the burial of organic carbon during this interval, comprehensive geochemical and mineralogical analyses were performed on upper Neoproterozoic samples from the Cryogenian Datangpo, the Marinoan-age Nantuo, and the Ediacaran Doushantuo formations in South China. Geochemical compositions indicate that sedimentary rocks from the Doushantuo Formation have a provenance from granodiorite-like rocks, and the Datangpo samples likely have a mixed provenance of basalt and granite. The chemical index of alteration (CIA) values of black shales from the Datangpo and Doushantuo formations are in a similar range (65–85), indicating moderate chemical weathering under a relatively warm and humid climate. However, conspicuous low Rb/Sr ratios and enrichment of chemically immature minerals for the uppermost Doushantuo samples, are indicative of elevated physical erosion during the terminal Ediacaran. Furthermore, a negative correlation between corrected CIA (CIAcorr) and total organic carbon (TOC), and a positive correlation between organic carbon isotopic compositions (δ13Corg) and TOC, imply that supercontinent-related nutrient supply and concomitantly elevated marine primary productivity played an important role in organic carbon burial during the Ediacaran-Cambrian transition. In contrast, elevated chemical weathering under increased temperatures and intensity of the hydrological cycle occurred during the deposition of post-glacial shales from the Datangpo Formation and Member II of the Doushantuo Formation. Enhanced primary productivity fostered by chemical weathering-released nutrients and sequestration of organic carbon by pedogenic clay minerals generated under such climatic conditions, and probably redox condition variations, are inferred to have been the main control on organic carbon burial during this interval.

Refining the termination age of the Cryogenian Sturtian glaciation in South China

Zhou et al. (2004) reported a zircon U-Pb isotope dilution – thermal ionization mass spectrometry (ID-TIMS) age of 662.9 ± 4.3 Ma from the basal Datangpo Formation of the Cryogenian System in South China. It was the first zircon U-Pb age to constrain both the termination of Sturtian glaciation and the onset of Marinoan glaciation, despite its much larger uncertainty expected for modern ID-TIMS analysis. In this study, we carried out U-Pb chemical abrasion – isotope dilution – isotope ratio mass spectrometry (CA-ID-IRMS) analysis on the same sample, yielding a precise 206Pb/238U date of 659.96 ± 0.46 Ma. Bayesian eruption age estimate indicates that the ash layer was most likely deposited at 659.23 ± 1.24 Ma. The present study suggests that the Sturtian glaciation terminated before ∼660 Ma. Together with previously reported radiometric dates, the precise U-Pb age presented herein helps refine the current Cryogenian chronology.

Enhanced chemical weathering triggered an expansion of euxinic seawater in the aftermath of the Sturtian glaciation

The Cryogenian Period comprised two episodes of global glaciation (Sturtian and Marinoan glaciations) separated by a non-glacial interval, which was characterized by early radiations of eukaryotic algae and putative metazoans. Geochemical data indicate that the non-glacial interval might be marked by a transient marine oxygenation, nevertheless oceanic redox conditions varied both in time and space. Further, the links between non-glacial climate and marine redox variations are not well constrained. Here we present high-resolution lithium isotope (δ7Li), Fe speciation and trace element (Mo and U) data for clastic sedimentary rocks from the Cryogenian interglacial Datangpo Formation, South China, in order to track the evolution of continental chemical silicate weathering and driving factors behind marine redox variability during the Cryogenian non-glacial interval. A significantly negative δ7Li excursion of ∼−5‰ is observed in the basal Datangpo Formation, suggesting a dramatic increase in chemical silicate weathering intensity in the aftermath of the Sturtian glaciation. Expansion and contraction of anoxic-sulfidic conditions, as demonstrated by Fe speciation and trace element (Mo and U) data, mirror changes in silicate weathering intensity. Our study provides evidence that greater nutrient and sulfate availability, due to high silicate weathering intensity associated with increased exposure of fresh rocks and a warm climate, facilitated the spread of euxinic waters over the continental margins of the otherwise ferruginous Cryogenian ocean.

Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling in Cryogenian Seawater

The behaviour of bioavailable trace metals and their stable isotopes in the modern oceans is controlled by uptake into phototrophic organisms and adsorption on and incorporation into marine authigenic minerals. Among other bioessential metals, Cd and its stable isotopes have recently been used in carbonate lithologies as novel tracer for changes in the paleo primary productivity and (bio)geochemical cycling. However, many marine sediments that were deposited during geologically highly relevant episodes and which, thus, urgently require study for a better understanding of the paleo environment are rather composed of a mixture of organic matter (OM), and detrital and authigenic minerals. In this study, we present Cd concentrations and their isotopic compositions as well as trace metal concentrations from sequential leachates of OM-rich shales of the Cryogenian basal Datangpo Formation, Yangtze Platform (South China). Our study shows variable distribution of conservative and bioavailable trace metals as well as Cd isotope compositions between sequential leachates of carbonate, OM, sulphide, and silicate phases. We show that the Cd isotope compositions obtained from OM leachates can be used to calculate the ambient Cryogenian surface seawater of the restricted Nanhua Basin by applying mass balance calculations. By contrast, early diagenetic Mn carbonates and sulphides incorporated the residual Cd from dissolved organic matter that was in isotopic equilibrium with deep/pore waters of the Nanhua Basin. Our model suggests that the Cd isotopic composition of surface seawater at that time reached values of modern oxygenated surface oceans. However, the deep water Cd isotope composition was substantially heavier than that of modern fully oxygenated oceans and rather resembles deep waters with abundant sulphide precipitation typical for modern oxygen minimum zones. This argues for incomplete recycling of Cd and other bioavailable metals shortly after the Sturtian glaciation in the redox stratified Cryogenian Nanhua Basin. Our study highlights the importance of sequential leaching procedures when dealing with impure authigenic sediments such as OM-rich carbonates, mudstones, or shales to achieve reliable trace metal concentrations and Cd isotope compositions as proxies for (bio)geochemical metal cycling in past aquatic systems.

Subtle Cr isotope signals track the variably anoxic Cryogenian interglacial period with voluminous manganese accumulation and decrease in biodiversity

Earth’s atmosphere experienced a step of profound oxygenation during the Neoproterozoic era, accompanied by diversification of animals. However, during the Cryogenian period (720–635 million years ago) Earth experienced its most severe glaciations which likely impacted marine ecosystems and multicellular life in the oceans. In particular, large volumes of Mn and Fe accumulated during the interglacial intervals of the Cryogenian glaciations, indicating large anoxic marine metal reservoirs. Here we present chromium isotope-, rare earth element-, and redox-sensitive trace element data of sedimentary rocks from the interglacial Datangpo Formation deposited between the Sturtian and Marinoan glaciations in South China, in an attempt to investigate the oxidation state of the oceans and atmosphere. Both the Cr isotope and trace element data indicate mainly anoxic water conditions with cryptic oxic surface water incursions after the Sturtian glaciation. Glacial-fed manganese precipitated as manganese carbonate in anoxic basins, and the non-fractionated δ53Cr record of −0.10 ± 0.06‰ identifies anoxic conditions with a cryptic component of slightly fractionated Cr isotope composition in manganese ore, in line with distinctly fractionated Mo isotope composition. Both the manganese carbonate ore and the black shales exhibit very low redox-sensitive element concentrations. Our study demonstrates that the oxygenation of the seawater, and inferably of the atmosphere, at the beginning of the Cryogenian interglacial interval was much subdued. The post-glacial rebound then allowed the Ediacaran biological diversity.