Article

Stratigraphy and paleogeography of the Ediacaran Doushantuo Formation (ca. 635–551 Ma) in South China

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Abstract

The Ediacaran Doushantuo Formation (ca. 635–551Ma) in South China contains exceptionally well-preserved fossils of multicellular eukaryotes including early animals, and it is one of the most intensively investigated Ediacaran units in the world. Various stratigraphic methods including litho-, chemo-, bio-, and sequence-stratigraphy have been applied to establish a stratigraphic framework for the Doushantuo Formation, but so far regional correlation across the basin relies heavily on two distinctive marker beds, the cap carbonate at the base and the organic-rich black shale at the top of the Doushantuo Formation. The majority of the Doushantuo Formation in the Yangtze platform was deposited on a rimmed carbonate shelf, with a shelf margin shoal complex that restricted the shelf lagoon from the open ocean. Large facies variations are observed in the shallow margins of the shelf lagoon and in the shelf margin-to-slope transition, where depositional environments were near the chemocline of the stratified, anoxic/euxinic shelf lagoon and of the broader Nanhua basin, respectively. Chemocline instability in the shelf lagoon and in the Nanhua basin caused local geochemical cycling, resulting in significant variations in carbon and sulfur isotopes and in redox-sensitive elemental concentrations. Most benthic eukaryotic fossils (including animal fossils) of the Doushantuo Formation have been found from the shallow margins of the shelf lagoon and from the shelf margin–slope transition, but rarely from deep-water environments that may have been below the chemocline for most of the Doushantuo time, implying the sensitivity of eukaryotes to paleogeographically controlled chemocline fluctuations.

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... The studied carbonate samples were collected from the Ediacaran Doushantuo Fm. and Dengying Fm. in the Jiulongwan and Gaojiaxi sections, Yangtze Gorges Area, South China (Fig. 1). The Jiulongwan section was deposited in a shallow-water (near wave-base) lagoonal environment during the Ediacaran (Jiang et al., 2011). The Doushantuo Fm. in Jiulongwan can be divided into four members. ...
... Moreover, the 87 Sr/ 86 Sr ratios in marginal areas or restricted basins are usually higher than the open ocean due to higher riverine and submarine groundwater influence (Peucker-Ehrenbrink and Fiske, 2019). These local processes could have been even more pronounced in a restricted lagoonal environment like Yangtze Gorge area (Jiang et al., 2011). The sedimentary evidence of exposed platform (Xiao and Knoll, 1999;McFadden et al., 2008;Lu et al., 2013) and the fact that all the isotope actions occur in a condensed carbonate and siliciclastic interval during lowstand sea-level are all consistent with enhanced freshwater inputs. ...
... Despite difference in origin, both mechanisms require a terrestrial fluid resource to satisfactorily explain the negative δ 13 C, δ 18 O and δ 44/40 Ca excursions within member III of the Doushantuo Fm. at the Jiulongwan section. This is consistent with the decreasing magnitude of δ 13 C isotope excursions as in coeval deeper water sections in South China ( Fig. 4; Jiang et al., 2007Jiang et al., , 2011Lu et al., 2013;Li et al., 2017). Indeed, both mechanisms can explain our observations of δ 44/40 Ca variations and further distinction between them may need more detailed investigation from different localities. ...
Article
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The Ediacaran Shuram excursion marks one of the largest carbon isotope excursions in Earth's history, but its origin remains enigmatic. In this study, we examine a carbonate section from Yangtze Gorges Area, South China that archives the Ediacaran Shuram excursion (i.e., "DOUNCE" event in South China), and report its carbon (δ 13 C), oxygen (δ 18 O), calcium (δ 44/40 Ca) together with published radiogenic strontium (87 Sr/ 86 Sr) isotope values. We find that the negative δ 13 C excursion coincides with decreases of δ 18 O and δ 44/40 Ca values, along with a positive shift in 87 Sr/ 86 Sr ratios. Two different but internally-coherent mechanisms can attribute to our observations-extensive diagenetic alteration under different fluid-buffered systems and coastal processes, and we speculate that both of them may have played important roles in shaping the observed isotope patterns. Using a coupled CO -Sr-Ca model, we find that these two processes both require an isotopically (13 C, 18 O, 44 Ca) depleted exogenous fluid source (meteoric water or groundwater), potentially generated by oxidation of litho-spheric organic carbon. Our hypothesis is consistent with the unique hydrogeological condition in Yangtze Gorge area during the middle Ediacaran. Further, we use an organic matter and pyrite oxidation model to provide a new conservative constraint for middle Ediacaran oxygen partial pressure (pO 2). A 10-20% present atmospheric level (PAL) of oxygen concentration is estimated if the Shuram excursion can be more confidently linked to meteoric water or groundwater impacts.
... Previous sedimentological research suggests deposition of the Doushantuo Formation in two stages. The first stage witnessed deposition of the couplet of cap carbonate and overlying black shale of the lower Doushantuo Formation in an open shelf/ramp depositional environment, whereas the second stage is typical of a rimmed carbonate shelf with a shelf-margin barrier separating the intra-shelf lagoon from open ocean settings (Jiang et al. 2011). The Yangtze Gorges area accommodated the most complete and continuous outcrops of the Doushantuo Formation exposed at the Jiulongwan, Huajipo and other sections (Jiang et al. 2011). ...
... The first stage witnessed deposition of the couplet of cap carbonate and overlying black shale of the lower Doushantuo Formation in an open shelf/ramp depositional environment, whereas the second stage is typical of a rimmed carbonate shelf with a shelf-margin barrier separating the intra-shelf lagoon from open ocean settings (Jiang et al. 2011). The Yangtze Gorges area accommodated the most complete and continuous outcrops of the Doushantuo Formation exposed at the Jiulongwan, Huajipo and other sections (Jiang et al. 2011). The Doushantuo Formation is best exposed at the intra-shelf basin Jiulongwan section (Jiang et al. 2011). ...
... The Yangtze Gorges area accommodated the most complete and continuous outcrops of the Doushantuo Formation exposed at the Jiulongwan, Huajipo and other sections (Jiang et al. 2011). The Doushantuo Formation is best exposed at the intra-shelf basin Jiulongwan section (Jiang et al. 2011). Therein, the Doushantuo Formation has a conformable contact with the underlying Nantuo Formation (Fig. 2a, b), and is composed of c. 160 m thick mixed shale and carbonate. ...
Article
Ediacaran cap dolostone atop Marinoan glacial deposits contains complex sedimentary structures with extremely negative δ ¹³ C carb values in close association with oscillations in palaeoclimatic and oceanographic proxy records. However, the precise geological, geochronological and geochemical context of the cap dolostone is not clarified, which hampers us from correctly interpreting the extremely negative δ ¹³ C carb values and their causal relationships with the Snowball Earth hypothesis. In this study, we conducted detailed in situ geochronological and geochemical analyses on the calcite within the cap dolostone from the Ediacaran Doushantuo Formation in South China in order to define its formation and relationship to the Snowball Earth hypothesis. Petrographic observations show that formation of dolomite pre-dates precipitation of calcite and pyrite, which pre-dates quartz cementation in the basal cap carbonate. Calcite cement within the cap dolostone yielded a U–Pb age of 636.5 ± 7.4/17.8 Ma (2σ, MSWD = 1.6, n = 36/40), which is within uncertainty of a published dolomite U–Pb age of 632 ± 17 Ma (recalculated as 629.3 ± 16.7/22.9 Ma). These age constraints negate the possibility that the calcite cement was formed by late Ediacaran or Cambrian hydrothermal activity. The rare earth element distribution patterns suggest a dominant seawater origin overprinted by subsequent early Ediacaran hydrothermal activity. The combined age, petrographic and geochemical data suggest oxidization of methane clathrates in response to complicated interplay between eustasy and isostatic rebound and hydrothermal fluids.
... The Ediacaran Doushantuo Formation in the Yangtze block of South China overlies the Nantuo diamictite and begins with a c. 635 Ma Marinoan cap carbonate (Fig. 2) (Condon et al. 2005;Zhu et al. 2007b;Jiang et al. 2011). Stratigraphically overlying the Doushantuo Formation is the dolostone-dominated Dengying Formation. ...
... Deposition of the Doushantuo Formation in the Yangtze block can largely be divided into two stages, beginning with an open ramp shelf that gradually transitioned into a rimmed shelf protecting an intra-shelf basin (Jiang et al. 2011;Cui et al. 2015). Stratigraphic data and palaeogeographic reconstructions indicate an increase in water depth from proximal intertidal environments in the west and NW to distal deep basinal settings in the SE. ...
... Stratigraphic data and palaeogeographic reconstructions indicate an increase in water depth from proximal intertidal environments in the west and NW to distal deep basinal settings in the SE. Three platform facies belts have been proposed, including a proximal inner shelf dominated by peritidal carbonates, an intra-shelf basin containing mixed carbonates and shales, and a rimmed outer shelf shoal complex consisting of carbonates and phosphorites ( Fig. 2) (Jiang et al. 2011). A more nuanced depositional model has also been proposed for the Doushantuo Formation, featuring two depositional mega-sequences and multiple stages of deposition (Zhu et al. 2007b;2013;2021). ...
Conference Paper
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Compared with the Phanerozoic strata, sulfate minerals are notably rare in the Precambrian record largely due to lower concentrations of sulfate in dominantly anoxic oceans. Here, we present a compilation of sulfate minerals, including diagenetic barite (BaSO4), pseudomorphs of gypsum (CaSO4·2H2O) and anhydrite (CaSO4), and celestine (SrSO4) that are stratigraphically associated with the Ediacaran Shuram excursion (SE) — arguably the largest negative carbon isotope excursion in Earth history. In this study, we investigated 15 SE-equivalent sections including: the upper Doushantuo Formation in South China, the Shuram Formation in Oman, the Wonoka Formation in Australia, the Krol Group in India, the Nama Group in southern Namibia, the Rainstrom Member in western USA, the upper Clemente Formation in Mexico, and the Alyanchskaya Formation in Siberia. All of the studied successions reveal the presence of sulfate minerals and/or concentration enrichment in carbonate-associated sulfate ([CAS]). The sulfur isotope (δ34S) trends of both sulfate and sulfide during the SE show progressively decreasing values, suggesting an increasing sulfur reservoir coincident with independent U isotope evidence for oxygenation of the oceans. To test the hypothesis of an increased oceanic sulfate level, we conducted a petrographic search for evidence of sulfate-bearing minerals, including barite, evaluated stratigraphic trends of barium concentration ([Ba]) in five SE-equivalent sections, with a primary focus on South China. Where [Ba] data are available, it reveals considerable enrichments relative to pre- and post-SE intervals. Based on our compiled observations, we propose that elevated seawater sulfate concentrations during the SE — triggered by enhanced oxidative weathering of terrestial pyrite or evaporite dissolution — faciliated the formation of sulfate minerals as primary precipitates and secondary authigenic cements. At the same time, a larger influx of dissolved Ba to the Ediacaran basins further faciliated barite deposition. Enhanced seawater sulfate concentrations would also stimulate microbial sulfate reduction and anaerobic oxidation of organic matter (including methane) in the sulfate-methane transition zone (SMTZ). Our study highlights the dynamic interplay of biogeochemical C, S, and Ba cycles in increasingly oxygenated Ediacaran surface environments.
... The first hypothesis invokes a large 13 C-enriched authigenic carbonate reservoir, and suggests that the SE results from a global increase (Jiang et al., 2011). Red dots indicate the locations of the Jiulongwan (intra-shelf) and Zhongling/Yangjiaping (outer-shelf shoal) sections. ...
... The Ediacaran Doushantuo Formation in the Yangtze block of South China overlies the Nantuo diamictite and begins with a ca. 635 Ma Marinoan cap carbonate ( Fig. 2) (Condon et al., 2005;Jiang et al., 2011). Stratigraphically above the Doushantuo Formation is the dolostone-dominated Dengying Formation. ...
... Deposition of the Doushantuo Formation has been broadly divided into two stages: an open ramp shelf followed by a rimmed shelf protecting an intra-shelf basin (Jiang et al., 2011;Cui et al., 2015). In addition, two depositional mega-sequences with multiple sub-stages of deposition have also been proposed for the Doushantuo Formation (Zhu et al., 2007b;Zhu et al., 2013;Zhu et al., 2022). ...
Article
The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (δ¹³Ccarb) values from +6‰ down to ca. –10‰, representing the largest δ¹³Ccarb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable δ¹³Ccarb profiles among correlative sections. This inconsistent expression of the SE challenges the conventional view of a homogeneous marine dissolved inorganic carbon (DIC) reservoir. A potential process that could explain δ¹³Ccarb variability is local mineralization of isotopically distinct authigenic carbonates near the sediment–water interface during early diagenesis. However, a direct test of such authigenic carbonates is still limited. Here, following a recent study on the SE in an intra-shelf environment, we revisited an outer-shelf section, identified and analyzed μm-scale, syn-depositional authigenic calcite cements via integrated cathodoluminescence (CL), micro-X-ray fluorescence (μXRF), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and secondary ion mass spectrometry (SIMS). Our new SIMS results reveal remarkable micron-scale heterogeneity of δ¹³Ccarb in authigenic calcite cements, including extremely negative values down to –37.5‰ (VPDB). We interpret these calcite cements as methane-derived authigenic calcite (MDAC) resulting from microbial sulfate reduction (MSR) and anaerobic oxidation of methane (AOM) during early diagenesis. Based on the new results, we propose that the heterogeneous SE in South China — manifest on micrometer, centimeter, and basinal scales — was modulated by methane oxidation under variable local redox and water depth conditions. The SE, therefore, was coupled with different degrees of methane oxidation in individual basins, and globally triggered by enhanced seawater sulfate during an atmospheric oxygenation event. In light of this study, the potential role of redox variability in methane oxidation during the SE may have been underestimated. Our study demonstrates that integrated SIMS-SEM analysis can distinguish different generations of isotopically distinct carbonates otherwise undetected by conventional analysis, and is thus an effective approach to assess the origin and diagenetic history of δ¹³Ccarb anomalies in the sedimentary record.
... Over the past three decades, diverse microfossils have been recovered from chert nodules and phosphorites of the Ediacaran Doushantuo Formation deposited in different paleogeographic and sedimentary environments of South China (Fig. 1A-B;Jiang et al., 2011;Muscente et al., 2015). The best-known and most-studied Doushantuo microfossils are the silicified microfossils from the Yangtze Gorges area in western Hubei Province (e.g., Liu and Moczydłowska, 2019;Liu et al., 2014a;Ouyang et al., 2021;Xiao et al., 2012) and phosphatized microfossils from the Weng'an area in central Guizhou Province (e.g., Xiao et al., 2014;Yuan and Hofmann, 1998;Zhang et al., 1998a). ...
... Currently available micropaleontological data of the Doushantuo Formation mostly come from cherts and phosphorites in strata deposited in the intra-shelf, shelf margin, and upper slope facies (Fig. 1A-B;Jiang et al., 2011;Muscente et al., 2015). In comparison, Doushantuo strata deposited in an inner shelf setting have not been as thoroughly explored for microfossils. ...
... This "barren zone" represents a significant gap hampering a full view of biostratigraphic zonation. To fill these knowledge gaps and to achieve a better Jiang et al. (2011). The Doushantuo Formation at eastern Shennongjia area was deposited in an inner shelf setting (Gu et al., 2021). ...
Article
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Microfossils of the Ediacaran Doushantuo Formation in South China provide an important window onto the rapid diversification of marine eukaryotes after the terminal Cryogenian global glaciation. They also offer key data in the biostratigraphic subdivision and correlation of the lower–middle Ediacaran System. Previously published Doushantuo microfossils in South China were mostly from intra-shelf facies in the Yangtze Gorges area, shelf margin facies in the Weng’an area, and upper slope facies in the Zhangjiajie area, whereas paleontological data from shallow-water inner shelf facies have been rarely documented. In addition, previous data from South China leave a “barren zone” associated with the negative δ¹³Ccarb excursion EN2 that impedes biostratigraphic correlation. To address these environmental and stratigraphic gaps, we conducted an integrated chemostratigraphic and biostratigraphic analysis of the Doushantuo Formation at the inner shelf Lianhuacun section in the eastern Shennongjia area of Hubei Provence, South China. The Lianhuacun fossils are preserved in chert nodules/bands of the lower Doushantuo Formation, precisely in a chemostratigraphic interval identified as EN2, indicating that the so-called “barren zone” is likely a taphonomic artifact. A total of 33 genera and 82 species are identified, including a new genus (Duospinosphaera gen. nov.) and seven new species (Duospinosphaera shennongjiaensis sp. nov., D. biformis sp. nov., Jixiania retorta sp. nov., Mengeosphaera mamma sp. nov., Sinosphaera exilis sp. nov., Tanarium columnatum sp. nov., and Weissiella concentrica sp. nov.). The Lianhuacun acanthomorphic acritarchs can be broadly assigned to the third microfossil assemblage zone (i.e., the Tanarium conoideum – Cavaspina basiconica Assemblage Zone) recognized on the basis of the Weng’an biota that is correlated with upper Member II of Doushantuo Formation in the Yangtze Gorges area, consistent with chemostratigraphic correlation between the Shennongjia and Yangtze Gorges areas. Thus, the so-called “barren zone” may be a poorly preserved part of the third microfossil assemblage zone. Alternatively, it may represent a new and yet unnamed microfossil assemblage zone between the third and the overlying Tanarium pycnacanthum – Ceratosphaeridium glaberosum Assemblage Zone. Regardless, the new data presented here fill important gaps in the geographical, paleoenvironmental, and stratigraphic distributions of Ediacaran acanthomorphic acritarchs in South China. As such, they facilitate integrative chemostratigraphic and biostratigraphic correlation of Ediacaran strata at regional and global scales.
... The studied section (Wuhe section, GPS coordinate: 26°4 5 0 56 00 N, 108°25 0 01 00 E) is located in the Jianhe County, Guizhou Province, South China (Fig. 1). Paleogeographic reconstruction indicates that the strata at the Wuhe section were deposited in the lower slope environment of the Ediacaran Yangtze Platform (Jiang et al., 2007(Jiang et al., , 2011. The samples were collected in the autumn of 2009 when low water level of the Qingshui River exposed the entire section. ...
... The Doushantuo Formation overlies the Nantuo (Marinoan) glacial diamictites and has a thickness of $120 m. It was subdivided into four members that roughly correlate with those in the Yangtze Gorges area (Jiang et al., 2011;Sahoo et al., 2016). Member I is a 2.3-m-thick cap carbonate that serves as a marker bed for regional stratigraphic correlation (Jiang et al., 2006(Jiang et al., , 2011. ...
... It was subdivided into four members that roughly correlate with those in the Yangtze Gorges area (Jiang et al., 2011;Sahoo et al., 2016). Member I is a 2.3-m-thick cap carbonate that serves as a marker bed for regional stratigraphic correlation (Jiang et al., 2006(Jiang et al., , 2011. The age of the cap carbonate has been well constrained at $ 635 Ma based on two Jiang et al. (2011). ...
Article
Pulses of the Ediacaran ocean oxygenation were inferred from strong enrichments of redox-sensitive elements (RSEs; particularly Mo, V, U, Re) and negative pyrite sulfur isotopes (δ³⁴Spy) in black shales of the Doushantuo Formation in South China. These oceanic oxygenation events (OOEs) have been challenged by the lack of comparable RSE enrichments in correlative strata of northwestern Canada. Here we report four positive chromium isotope (δ⁵³Cr) excursions with peak values (+0.79 ± 0.03‰ to +1.45 ± 0.06 ‰; 2SD) close to the average δ⁵³Cr of the modern ocean (+1.0 ± 0.3‰) at the intervals of OOEs, which are separated by low δ⁵³Cr values close to that of the bulk silicate Earth (BSE, −0.124 ± 0.101‰). The positive δ⁵³Cr excursions could be explained by episodic input of oxygenated water from the open ocean to the restricted Nanhua basin, or pulses of ocean oxygenation during the Ediacaran-early Cambrian. The two interpretations can explain the majority of the geochemical data available from the Wuhe section, but both have limitations. The first interpretation implies that the open ocean was more oxygenated perhaps during the entire Ediacaran-early Cambrian period, which is not supported by the currently available data from other Ediacaran successions. The latter interpretation has difficulties to explain the thallium isotope (ε²⁰⁵Tl) data from the same section, pending confirmation from other successions globally. The data from the Wuhe section highlight the complexity of interpreting the redox history of the global ocean from geochemical results of a single section. Nonetheless, the synchronous changes of multiproxy records—including positive δ⁵³Cr excursions, RSE peaks, negative δ³⁴Spy and molybdenum isotope (δ⁹⁸Mo) excursions—at the same stratigraphic intervals do support a more oxygenated ocean during these times, regardless of the degree of restriction of the Nanhua basin from the open ocean.
... 635-551 Ma (Condon et al., 2005) on the Neoproterozoic Yangtze Platform in South China. The outcrops are patchy but are found over an area of 1.6 million km 2 (Jiang et al., 2011). The formation unconformably overlies the Nantuo Formation, a glaciogenic formation that has been correlated with the Marinoan glaciation (e.g., Yan et al., 2020). ...
... The formation unconformably overlies the Nantuo Formation, a glaciogenic formation that has been correlated with the Marinoan glaciation (e.g., Yan et al., 2020). The Doushantuo Formation is complex; it varies in thickness from 40 to 300 m with rapid facies changes between measured sections (Jiang et al., 2011). It has been subdivided into four members. ...
... It has been subdivided into four members. At the well-studied Jiulongwan section, Member I is a 3-to-5-meter-thick limestone and dolostone, interpreted as a cap carbonate (Jiang et al., 2006); Member II is a 71meter-thick dolostone interbedded with mudstone and contains microfossiliferous chert nodules in the dolostone (Jiang et al., 2011); Member III is 65 m-thick and predominantly composed of dolostone with microfossiliferous chert (Jiang et al., 2011); and Member IV is ca. 10 mthick and primarily composed of a black shale . The sample examined in this study comes from Member II at Wangfenggang (30 • 51 ′ 20 ′′ N; 111 • 9 ′ 13 ′′ E) in the Yangtze Gorges region of Yicheng County, Hubei, China (Fig. 1). ...
Article
Raman and Fourier-transform infrared (FTIR) microspectroscopic analyses were performed to examine the chemical signatures and their spatial distribution of organic-walled microfossils together with organic matter (OM) in a black chert nodule from the Ediacaran (635–551 Ma) Doushantuo Formation in China. Raman spectral parameters (I-1350/1600 values) reveal that the acritarchs, bundled filaments, and OM have different degrees of structural order, and that there is no significant difference in I-1350/1600 values among the different portions of individual acritarchs (i.e., inner structure, inner membrane, and outer wall). The IR mapping reveals portion-specific chemical signatures within the individual acritarch specimens. The inner membrane of Tianzhushania contains aliphatic C-H bonds. The acritarchs, including Tianzhushania, show chemically four types of inner structures: (1) aromatic C-H-rich type, (2) aliphatic C-H-rich type, (3) aromatic/aliphatic C-H-poor type, or (4) heterogeneous. All of the examined acritarchs have aromatic C-H rich outer wall, irrespective of the presence/absence of processes. These features support a eukaryotic cyst origin, although it is difficult to determine whether the acritarchs correspond to the cysts of Protista or Metazoa. The bundled filaments also consist of an aromatic C-H bond, but its origin is uncertain. The OM shows a high degree of variation in Raman spectral I-1350/1600 values, as does the IR spectral intensity ratio of asymmetric aliphatic CH3/CH2, alongside a heterogeneous spatial distribution of aromatic and aliphatic C-H bands. This suggests the contribution of various precursors derived from dead microbial cell debris and extracellular organic compounds. Overall, these results confirm the presence of diverse microorganisms in the Doushantuo microbiota. Furthermore, the spatial distribution of organic functional groups in individual microfossils could provide clues about the taxonomy of microfossils of unknown origin.
... The Miaohe biota refers to the fossil assemblage preserved in a siliceous black shale unit known as the Miaohe Member of the Ediacaran Doushantuo Formation ( Figures 1C,D) which crops out in the Hubei and Guizhou provinces. Traditionally, the Miaohe Member is considered the equivalent of the uppermost black shale member of the Ediacaran Doushantuo Formation (Jiang et al., 2011;Xiao et al., 2017;Zhou et al., 2017). The Doushantuo Formation, as mentioned above, is a lithostratigraphic equivalent of the Lantian Formation. ...
... Its age is constrained to 551 Ma (TIMS U-Pb dating, Condon et al., 2005). This black shale member is interpreted as transgressive sediments deposited during sea-level rise (Jiang et al., 2007;Jiang et al., 2011), which may represent a restricted intra-shelf basin setting in the Yangtze Gorges area (Zhu et al., 2013) at low paleolatitudes (Li et al., 2008), and probably represent a continental slope environment in the Wenghui area (Jiang et al., 2011). The surface sediment of the Wenghui area was considered to be soupy (Wang and Wang, 2008). ...
... Its age is constrained to 551 Ma (TIMS U-Pb dating, Condon et al., 2005). This black shale member is interpreted as transgressive sediments deposited during sea-level rise (Jiang et al., 2007;Jiang et al., 2011), which may represent a restricted intra-shelf basin setting in the Yangtze Gorges area (Zhu et al., 2013) at low paleolatitudes (Li et al., 2008), and probably represent a continental slope environment in the Wenghui area (Jiang et al., 2011). The surface sediment of the Wenghui area was considered to be soupy (Wang and Wang, 2008). ...
Article
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Holdfast morphologies and attachment strategies of benthic macroalgae are somewhat flexible and controlled by both the substrate condition and species. Six forms (tapered base, globose holdfast, composite globose holdfast, discoidal holdfast, rhizoids and horizontal rhizomes) of attachment structures of Ediacaran benthic macroalgae are recognized from the early Ediacaran Lantian biota and late Ediacaran Miaohe biota in South China based on functional morphology. Each form is considered either adapted to firm substrates that dominate the Precambrian seafloor, or soft substrates that are more common in the Phanerozoic. The results show a diversification in both holdfast morphology and attachment strategies of macroalgae during the Ediacaran Period. In the early Ediacaran Lantian biota, none of the benthic macroalgae is adapted to soft substrates, while in the late Ediacaran Miaohe biota, a considerable number (41%) of species are adapted to relatively soft substrates. This shift might be an adaptive response to the diversification of macroalgae and a changing substrate condition during the Ediacaran Period: the decline of microbial mats and increase of water content in the sediments in the Ediacaran.
... Distinctive sedimentary structures such as low-angle cross-laminae, giant wave ripples, and sheet-crack cements are common in these cap dolomites and are considered to result from a series of petrological, sedimentological, biological, mineralogical, and geochemical changes caused by the abrupt global warming postdated global glaciation. Over the past decades, numerous studies have focused on the cap dolomite within the YC (Jiang et al., 2003a(Jiang et al., , 2003b(Jiang et al., , 2006a(Jiang et al., , 2011Zhu et al., 2007aZhu et al., , 2007bZhu et al., , 2013Zhou et al., 2010, Liu et al., 2014a. However, important questions related to the sedimentary facies interpretation and the formation model for the cap dolomites remain controversial. ...
... However, important questions related to the sedimentary facies interpretation and the formation model for the cap dolomites remain controversial. One of the popular and traditional views is that the cap dolomites represent a spatial-temporal transition of sedimentary facies from a shelf to a slope environment, and then to a basin (Hoffman et al., 1998;Hoffman and Schrag, 2002;Jiang et al., 2011). ...
... Furthermore, the dynamic formation mechanism of the sedimentary structures of the cap dolomite, reported to originate from deep-water deposition in previous studies (Hoffman et al., 1998;Hoffman and Schrag, 2002;Jiang et al., 2011), is open to further discussion. Carbonate tidal flats may experience water-escape and liquification in response to the release of pore fluid pressure (Corkeron, 2007) or gas expansion (Kennedy et al., 2008). ...
Article
Ediacaran cap dolomites are strong evidence for the glaciation during the Neoproterozoic. Stratigraphic-sedimentological studies combined with δ¹³C and δ¹⁸O isotope analyses are used for defining the processes of post-glacier environmental changes and sea-level rise caused by glacier melting and the reconstruction of the depositional environments for the Doushantuo cap dolomites in Shennongjia, northern Yangtze Craton (YC). Like many other cap dolomites in the YC and worldwide, those in Shennongjia exhibit strongly negative δ¹³Ccarb excursions, ranging from −7.3 to −2.5 ‰ (averagely from −5.4 to −4.1 ‰). A laminated clast-free calcareous siltstone commonly occurs between the cap dolomite and the underlying diamictites, indicating a low-energy, shallow-water depositional environment, wherein alkaline conditions developed before the deposition of the cap dolomite. The most prominent features of the cap dolomites in Shennongjia are the very thin thickness and monotonous sedimentary structures. The maximum thickness of the cap dolomites from the six sections in Shennongjia is 1.8 m, and the thinnest cap dolomite is 0.4 m with simple laminations. Sedimentary structures such as low-angle cross-bedding, giant wave ripples, sheet-crack cement, and tepee breccias are rare in Shennongjia except in the area near the Three Gorges. These evidences, therefore, point to the deposition of the Shennongjia cap dolomite in restricted shallow-water tidal environments. The space–time distribution of sedimentary facies in Shennongjia and the southeastern areas to Shennongjia indicate that the Ediacaran cap dolomite was formed on a ramp. Accordingly, we propose a sedimentary facies model for the formation of the cap dolomite, covering the inner and middle ramp facies. This model explains the thickness variations and distributions of the cap dolomites at both the local and regional scale, including their sedimentary structures, compositions, and barite occurrences. The model successfully illustrates why the cap dolomites in the Shennongjia area share a unique set of sedimentary features. Finally, the cap dolomite is interpreted as a diachronous deposit (bottom to top), tracking glacioeustatic flooding and recording ocean-wide changes over the time following deglaciation, and a correlation and cause of negative δ¹³Ccarb excursions in cap dolomites worldwide, particularly in those in the Yangtze Carton, are further discussed.
... The Nanhua Basin, initially forming as an intracratonic basin between the Yangtze and Cathaysia blocks, may have evolved into a passive continentalmargin basin during the Ediacaran (Jiang et al., 2003), although crustal separation was probably never achieved . Paleogeographic studies indicate that water depths increased from northwest to southeast in modern coordinates along with a change in lithology from carbonate to shale (Fig. 1B) (Jiang et al., 2011). ...
... The transect investigated here consists of three sections deposited at different water depths on the Yangtze Block (Zhu et al., 2007b;Jiang et al., 2011), including the ZCP section (drillcore) in proximal inner shelf, the JLW section (outcrop) in an intrashelf basin, and the SDP section (outcrop) in upper slope facies. The ZCP and JLW sections were collected in the Three Gorges area, ca. ...
... The JLW section (30 • 47 ′ 49 ′′ N, 111 • 03 ′ 08 ′′ E) is the most well-studied Ediacaran succession in South China, in which the Doushantuo Formation is overlain by the Dengying Formation (Jiang et al., 2011). In the intrashelf area, e.g., JLW, the Doushantuo Formation can be divided into four members, which are described in ascending order (Fig. 1C). ...
Article
The Ediacaran Period (~635-539 Ma) witnessed the largest negative inorganic carbon isotope (δ 13 C carb) excursion in Earth's history (i.e., the Shuram Excursion), which is characterized by decoupling from the organic carbon isotope (δ 13 C org) record. The cause(s) of this event remains highly debated. Here, we report a major (~8-9-Myr-long) episode of strong barium (Ba) accumulation during the Shuram Excursion in the form of barite, as recorded in the Ediacaran Doushantuo Formation (~635-551 Ma) of South China. The inner-shelf Zhang-cunping section exhibits minimal Ba enrichment, while the slope Siduping section shows maximal Ba enrichment with the intrashelf basinal Jiulongwan section in the middle. The Siduping section contains ~5 μm-diameter, ellipsoidal barite crystals of marine origin; and the Jiulongwan section contains large (>50 μm), euhedral barite crystals and cements that are partly replaced by pyrite, pointing to a diagenetic origin with barite formation within the sulfate-methane transition zone. The barite δ 34 S is ~10‰ higher than δ 34 S of carbonate-associated sulfate at Jiulongwan in contrast to similar values of these two components at Siduping, suggesting a limited influence of methane oxidation, if any, on the formation of Shuram Excursion at Jiulongwan. The Ce/Ce* exhibits a lateral gradient among study sections which is reverse to the Ba enrichment, supporting the hypothesis that local surface-water productivity controlled dissolved oxygen levels in the Ediacaran surface ocean. Based on these findings, we attribute the highest Ba enrichment at Siduping to oceanic upwelling which enhanced local marine productivity. Furthermore, we propose that episodic oceanic upwelling in the Ediacaran shelf regions likely transported phosphorus and dissolved organic carbon (DOC) to shallow waters, increasing their productivity and facilitating the oxidation of DOC, contributing to the largest negative carbon isotope excursion in Earth's history.
... The Neoproterozoic (<900-580 Ma) is an important stage in the evolution of the marine environment and life on the earth (Jiang et al., 2011;Li et al., 2013;Zhou et al., 2019). During this period, two important glacial events (Sturtian and Marinoan "snowball events") and multiple intermittent oxygen-increasing events occurred globally, exerting critical impacts on the evolution of the marine environment, marine life, and sedimentation (Hoffman et al., 1998;Jiang et al., 2011;Wang et al., 2019;Wang et al., 2020b). ...
... The Neoproterozoic (<900-580 Ma) is an important stage in the evolution of the marine environment and life on the earth (Jiang et al., 2011;Li et al., 2013;Zhou et al., 2019). During this period, two important glacial events (Sturtian and Marinoan "snowball events") and multiple intermittent oxygen-increasing events occurred globally, exerting critical impacts on the evolution of the marine environment, marine life, and sedimentation (Hoffman et al., 1998;Jiang et al., 2011;Wang et al., 2019;Wang et al., 2020b). Under the influence of global snowball events, the sea-level changes, and extensional tectonic background, Doushantuo Formation sediments dominated by black shales and carbonates were deposited during the Early Ediacaran ( Fig. 1), overlying on the tillites of the Nantuo Formation (Fig. 2) Dengying Formation carbonates were widely developed on platformslope facies Wang et al., 2020b). ...
... In the Sichuan Basin and surrounding areas, the types and characteristics of the Doushantuo Formation sediments are driven by facies (Jiang et al., 2011). The facies include shallow-water carbonate platforms, mixed shelf, shallow-water, and deep-water shelf, and rift troughs around the platform (Fig. 1). ...
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Several commercial Precambrian petroleum systems have been discovered worldwide. The microorganisms found in the Precambrian successions play a significant role in the development of high-quality hydrocarbon source rocks and microbialites with a high reservoir porosity. Yet, it remains unclear regarding issue of environments in which microbial activities can lead to the development of organic-rich hydrocarbon source rocks or reservoirs, and how the microbial source rock and reservoir constitute an effective hydrocarbon source-reservoir assemblage system. During the Edicaran Doushantuo Formation period, high-quality black shale source rocks were widely developed, and the organic matter in the source rocks was mainly derived from microbes including bacteria and algae. The thickness of the source rocks is up to 200 m, and the total organic carbon (TOC) value is ∼ 17.9%. During the Dengying Formation period, thick-layered microbialites, including microbial stromatolite, thrombolite, and botryoidal dolomite were developed extensively, with the abundant primary framework and secondary dissolution pores. During the Precambrian evolution, the transition of the seawater environment controlled the development of microbial source rocks or microbialite reservoirs. In a deep, euxinic reducing seawater condition, microbial organic matter was well preserved and accumulated in fine-grain sediments, thus forming the high-quality microbial source rocks. Meanwhile, under such a reducing condition, a large amount of layered, nodular, granular, and strawberry pyrites were precipitated due to the effect of BSR reaction with extremely positive δ³⁴S values (up to 39.0‰). Meanwhile, in a shallow turbulent high-energy oxidized seawater condition, especially in the intermittent “Dolomite Sea” environment, microbial dolomitization promoted the massive development of microbial dolomite mound/shoal frameworks, constituting the matrix for the development of large-scale microbialite reservoirs. During the later burial evolution processes, the oil and gas generated from the Doushantuo Formation source rocks migrated and accumulated into the overlying Dengying Formation microbialite reservoirs, which constituted an effective hydrocarbon source-reservoir assemblage system. Overall, the areas surrounding the Mianyang, west side of the Chengkou, and northwest side of the western Hubei troughs are considered to be the potentially favorable areas with microbial source rock-reservoir assemblage system. The microbial source-reservoir assemblages in these Precambrian strata worldwide are worth further exploring.
... Continental crust S o u th Q in li n g O c e a n 947 ± 14 Ma Huashan ophiolite (Shi et al., 2011) 866-858 Ma I-type granite (Xu et al., 2016) 871 ± 7 Ma calcalkaline mafic dikes (Xu et al., 2016) 877 ± 9 Ma I-type granite (Liao et al., 2016) 864 ± 2 Ma gabbro 868-858 Ma granitic gneiss 941-884 Ma diorite (Hu et al., 2016) Future Xiaomoling Complex 885 ± 4 Ma diorite (Yan et al., 2014) N S C o n ti n e n ta l a r c 925-859 Ma diorite Liu et al., 2011) D o u li n g b lo c k Y a n g tz e c ra to n 820-815 Ma amphibolite facies peak metamorphism (Hu et al., 2013(Hu et al., , 2019 821-813 Ma granite (Zhang et al., 2009) 826 ± 13 Ma granite (Zhao et al., 2013b) 820 ± 17 Ma diabase (Hu et al., 2015) 812 ± 13 Ma gabbro (Liu and Zhao, 2019) (Zhao et al., 2013a) 847-841 Ma calc-alkaline basalt and rhyolite (Shi et al., 2014,This study) 843 ± 5 Ma diorite (Hu et al., 2013(Hu et al., , 2019 794 ± 11 Ma postcollisional granite 783 ± 5 Ma postcollisional granitoids 788 ± 28 Ma postcollisional rhyolite (This study) ...
... Therefore, we infer that the detached subducting oceanic slab into the lower mantle induced thermal instability and asthenosphere mantle upwelling (Sutherland et al., 2010;, resulted in the late Neoproterozoic magmatism in a continental rift setting in the northern margin of the Yangtze craton (Fig. 13f). The continuous continental rifting led to the breakup of Rodinia, the opening of new oceans, and the deposition of the Sinian strata (e.g., Doushantuo and Dengying Formations) in a neritic to bathyal environment unconformably overlying on basement complexes (Jiang et al., 2003(Jiang et al., , 2011Huang et al., 2020). ...
Article
The tectonic mechanism responsible for the Neoproterozoic magma generation at the northern margin of the Yangtze craton is crucial to understanding the assembly and breakup of the Rodinia supercontinent. Here, we present a systematic study of the petrology, whole-rock geochemistry and geochronology of mafic and felsic volcanic rocks from the Yaolinghe Group, which outcrops mainly in the Shangnan area (north) and the Wudang area (south) of the South Qinling orogenic belt along the northern margin of Yangtze craton. Basalts from the Shangnan area are tholeiitic and calc-alkaline and indicated an arc environment. They have a LA-ICP-MS zircon U-Pb age of 847 ± 5 Ma with positive εHf(t) values of + 6.93 to + 8.79 and TDM1 model ages of 1130–1053 Ma. The rhyolites from this area exhibit geochemical characteristics similar to S-type granites and have a zircon U-Pb age of 841 ± 7 Ma, with a wide range of εHf(t) values of + 1.02 to − 11.40 and TDM2 model ages of 2453–1674 Ma. This is different from the rhyolites from the Wudang area, which reveal geochemical affinities similar to A2-type granites, with a zircon U-Pb age of 788 ± 28 Ma, uniform negative εHf(t) values of − 8.75 to − 0.15 and TDM2 model ages of 2214–1663 Ma. Both rhyolites were sourced from partial melting of Paleoproterozoic crust with minor input of mantle material, but were formed in an arc setting and a post-collision extension setting, respectively. The 663–644 Ma mafic volcanic rocks from the Wudang area are enriched in Th, U, Nb and Ta, similar to oceanic island basalts and have positive zircon εHf(t) values ranging from + 9.24 to + 4.89 with TDM1 model ages peaking at 1018 Ma. They were derived from partial melting of an asthenospheric mantle source that had been modified by slab-derived melts and erupted during late Neoproterozoic continental rifting. Combining our results with existing data, we suggest that the South Qinling orogenic belt resulted from bidirectional subduction of the South Qinling Ocean between the Douling block and the Yangtze craton at ca. 950–750 Ma, accompanied by the formation of a trench-arc-basin system along the Douling block and an Andean-type continental arc belt at the northern margin of the Yangtze craton. After accretion, the amalgamated Douling block and Yangtze craton were affected by oceanic subduction of the Mirovoi Ocean at ca. 750–680 Ma, and subsequent continental rifting at ca. 680–620 Ma. The model further poses that the Yangtze craton was assembled to the periphery of the Rodinia supercontinent.
... Table 1 shows stratigraphic end-members of Member IV (described in Supplementary Information), representing the chemo-stratigraphic trends for the evolution of the measured C and N geochemical parameters. Proximal sections include Xiangerwan, Zhimaping, and Qinglinkou, which are located in the shelf lagoon paleo-environment 25 . Distal sections include Taoying (upper slope), Xiajiaomeng (lower slope), and Fengtan (basin) 25 (see Fig. S1a-c). ...
... Proximal sections include Xiangerwan, Zhimaping, and Qinglinkou, which are located in the shelf lagoon paleo-environment 25 . Distal sections include Taoying (upper slope), Xiajiaomeng (lower slope), and Fengtan (basin) 25 (see Fig. S1a-c). The proposed stratigraphic correlation of Member IV of these sections is based on new and published data from the studied and nearby sections that constrain the lateral variability and isotopic expression of the Shuram excursion in South China 5,26-28 . ...
Article
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Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history. However, the main biogeochemical controls that ultimately drove this recovery have yet to be elucidated. Here, we report new carbon and nitrogen isotope and concentration data from the Nanhua Basin (South China), where δ ¹³ C values of carbonates (δ ¹³ C carb ) rise from − 7‰ to −1‰ and δ ¹⁵ N values decrease from +5.4‰ to +2.3‰. These trends are proposed to arise from a new equilibrium in the C and N cycles where primary production overcomes secondary production as the main source of organic matter in sediments. The enhanced primary production is supported by the coexisting Raman spectral data, which reveal a systematic difference in kerogen structure between depositional environments. Our new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and suggest that blooms of oxygenic phototrophs modulated the recovery from the most negative δ ¹³ C carb excursion in Earth history.
... The Ediacaran Doushantuo Formation (~635-551 Ma) in the Yangtze Block largely comprises both carbonate and siliciclastic rock, which overlies the Cryogenian Nantuo Formation and underlies the Ediacaran Dengying/Liuchapo formations (Condon et al., 2005). A passive continental margin in a low-middle palaeolatitude position has been inferred on the basis of stratigraphic pattern, thickness variation and palaeogeographic reconstruction (Jiang et al., 2011;Li et al., 2013). The Doushantuo Formation was deposited on a rimmed carbonate shelf with a shelf marginal shoal complex that restricted an intrashelf basin from the open ocean ( Fig. 1C; Zhu et al., 2013). ...
... We studied the samples from the member II of the Doushantuo Formation measured in eight sections at Jiulongwan, Chenjiayuanzi, Northern Xiaofenghe, Jiuqunao, Qinglinkou, Miaohe, Liaojiagou and Wuzhishan (Fig. 1B;. The Doushantuo Formation at these sections in this study was deposited on the inner shelf near the proximal side of intrashelf basin on the Ediacaran Yangtze Block ( Fig. 1D; Jiang et al., 2011). ...
Article
Morphologically diverse multicellular algae preserved with anatomical details are recovered from the chert nodules in the member II of the early Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China. They are taxonomically identified as Archaeophycus yunnanensis, Sarcinophycus papilloformis, Thallophyca ramosa, Thallophyca sp., Thallophycoides phloeatus, Wengania exquisita, W. globosa, W. minuta and two unnamed species. An individual growth of spheroidal thallus is evidenced by a series of specimens of Wengania minuta. Archaeophycus yunnanensis and Sarcinophycus papilloformis are possibly green algae, while the others are interpreted as red algae. The radiation of multicellular algae likely enhanced the efficiency of the biological pump, influencing the ocean ecology and environment during the early Ediacaran.
... The Tamengo Formation contains allochemical and bioclastic constituents typical of Ediacaran carbonate ramps (e.g., Sumner and Grotzinger, 1993;Narbonne et al., 1994;Walter et al., 1995;Grotzinger and James, 2000;Gaucher et al., 2003;Jiang et al., 2003b;Lindsay et al., 2005;Jiang et al., 2011;Xiao et al., 2016;Vaziri and Laflamme, 2018;Warren et al., 2019;Álvaro et al., 2020). On the other hand, the detail facies analysis presented here allows for unraveling more specific characteristics of its depositional environment. ...
... The Tamengo Formation yields lithofacies like those of the Doushantuo and Dengying formations, which form an extensive, well preserved, and well-studied Ediacaran carbonate platforms system onto the Yangtze block, in South China. There, the sediments of the carbonate ramp are preserved with lateral continuity, from the inner shelf to the basin (Jiang et al., 2011;Ding et al., 2019;Ding et al., 2021). This indicates that Ediacaran carbonate platforms were heterogeneous and the presence of subtidal stromatolites, thrombolites, or even metazoan reefs is not necessarily a prerogative. ...
Article
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The Ediacaran is a period characterized by the diversification of early animals and extensive neritic carbonate deposits. These deposits are still not well understood in terms of facies and carbon isotope composition (δ13C). In this study we focus on the Tamengo Formation, in southwestern Brazil, which constitutes one of the most continuous and well-preserved sedimentary record of the late Ediacaran in South America. We present new detailed lithofacies and stable isotopes data from two representative sections (Corcal and Laginha) and revise the paleoenvironmental and stratigraphic interpretation of the Tamengo Formation. The Corcal section consists of neritic deposits including shallow-water limestone beds, alternated with shale and subordinate marl beds. These facies yield specimens of the Ediacaran fossils Cloudina lucianoi and Corumbella werneri. On the other hand, the Laginha section shows more heterogeneous facies, such as impure carbonates, breccias, marls, and subordinate mudstone beds, as well as no evidence of Corumbella werneri. The stable carbon isotope record is also different between the two sections, despite belonging to the same unit. The Corcal section displays higher and more homogeneous δ13C values, consistent with those of Ediacaran successions worldwide. The Laginha section, instead, displays more variable δ13C values, which suggest the influence of local and post depositional processes. The difference between the two sections was attributed to the different distance from the shore. We propose that the difference is due to topographic variations of the continental platform, which, at the Laginha site, was steeper and controlled by extensional faults. Therefore, the Corcal section is a better reference for the Tamengo Formation, whereas the Laginha is more particular and influenced by local factors. Besides, the lithofacies associations of the Tamengo Formation are like those of the Doushantuo and Dengying formatios, in South China, with no significant biogenic carbonate buildups, and different from those of other important Ediacaran units, such as the Nama Group in Nmibia and the Buah Formation in Oman. Our work highlights the complexity and heterogeneity of Ediacaran carbonate platforms and of their carbon isotopic composition. In addition, we characterize the Corcal section as a possible reference for the Ediacaran in South America.
... The spectrum of interpretation is between a sedimentary-exhalative model, i.e., metal supply from submarine hydrothermal activity (Coveney and Chen 1991;Coveney et al. 1994;Lott et al. 1999;Steiner et al. 2001;Emsbo et al. 2005;Jiang et al. 2007;Pašava et al. 2008;Carignan 2011, Och et al. 2013;Jia et al. 2018;Han et al. 2020;Fu et al. 2021) and a seawater model, i.e., metal supply from average seawater under anoxic/euxinic conditions Lehmann et al. 2007Lehmann et al. , 2016Xu et al. 2012Xu et al. , 2013Yin et al. 2017;Pagès et al. 2018). A mixed seawater (Mo) and Fig. 1 The variety of Early Cambrian stratiform ore deposits on the margin of the Yangtze platform within its Early Cambrian geotectonic and paleogeographic setting (from Frei et al. 2020, based on Wang et al. 1985;Jiang et al. 2011;Zhao and Cawood 2012). The area between the Yangtze carbonate platform (very light blue) and the deep-sea basin (blue) is mapped as "starved marginal shallow Yangtze sea" or "protected basin" (Wang et al. 1985) or as "shelf lagoon" (Jiang et al. 2011) and shown in light blue. ...
... A mixed seawater (Mo) and Fig. 1 The variety of Early Cambrian stratiform ore deposits on the margin of the Yangtze platform within its Early Cambrian geotectonic and paleogeographic setting (from Frei et al. 2020, based on Wang et al. 1985;Jiang et al. 2011;Zhao and Cawood 2012). The area between the Yangtze carbonate platform (very light blue) and the deep-sea basin (blue) is mapped as "starved marginal shallow Yangtze sea" or "protected basin" (Wang et al. 1985) or as "shelf lagoon" (Jiang et al. 2011) and shown in light blue. This area has a broad spectrum of paleo-environments with deposition of black shale, phosphorite, sapropelite, V and Mo-Ni shale, stratiform barite, and very strong lateral changes in stratigraphic thickness, including a chain of paleoislands, the "Jiagnan islands" (grey shaded; only schematically shown). ...
Article
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Early Cambrian black shales on the Yangtze Platform host a regionally distributed highly metalliferous sulfide-rich carbonaceous unit which has been the subject of extensive debate. This marker unit, with a few centimeters or tens of centimeters in thickness, displays extreme enrichment in Mo and Ni (wt% range), and in a broad spectrum of other metals such as As, Au, PGE, Re, Cu, Zn, Cd, Ag, Sb, Se, Tl, and Hg, and occurs discontinuously along the western passive margin of the Yangtze Platform. It grades laterally in stratigraphically equivalent meter-thick vanadium-rich shale and tens-of-meter-thick sapropelite (combustible shale). New Cu and Zn isotope data, combined with existing Cd, Cr, Ni, Mo, Hg, and Se isotope and other chemical data, allow to attempt an integrated view on the formation of this intriguing unit of hyper-enriched metalliferous black shale. The authigenic Cu enrichment in the 1000-ppm range has produced no or little Cu isotope fractionation (0.03 ± 0.26 ‰ δ ⁶⁵ Cu) beyond the lithogenic background. Heavy zinc and cadmium isotope enrichment in the sulfidic samples (1.11 ± 0.18 ‰ δ ⁶⁶ Zn, 0.31 ± 0.10 ‰ δ ¹¹⁴ Cd) is controlled by sulfide fractionation and contrasts with V-rich and barren shale (0.60 ± 0.18 ‰ δ ⁶⁶ Zn, 0.00 ± 0.14 ‰ δ ¹¹⁴ Cd). The distinctly negative Ni isotope composition of the metalliferous unit (−0.84 ± 0.05 ‰ δ ⁶⁰ Ni) with Ni in the percent range has been interpreted as due to hydrothermal activity related to the leaching of mafic rocks and their sulfides. Sorption processes (Fe-oxyhydroxides) and redox cycling in the water column and the bottom sediment with microbial activity could be an alternative interpretation. The extreme metal enrichment can be understood as due to a process chain, from high biological productivity in the oxic photic zone to sulfate reduction in the deeper sulfidic water column and upper sediment layer. Key to the metal enrichment seems to be extremely low clastic sedimentation and advanced carbon destruction by anaerobic oxidation. Hydrothermal input of basinal brines along the rifted margin of the Yangtze Platform was likely a part of this scenario.
... We collected samples from two Ediacaran-Cambrian shallow-marine carbonate-dominated successions in South China (Figure 1), the Gaojiaxi-Yanjiahe section in the Yangtze Gorges area, Hubei Province, China, which records an intrashelf basin paleoenvironment (subtidal shale-carbonate facies) from the late Ediacaran to the early Cambrian, and the Xiaotan section, NE Yunnan Province, China, which records an inner shelf paleoenvironment (peritidal carbonate-dominated facies) from the late Ediacaran to the Cambrian (Jiang et al., 2011). Previous studies have included systematic characterization of the biostratigraphy and carbon isotope chemostratigraphy of these two sections, and suggest that they can be correlated, at the sub-stage level, with other, broadly contemporaneous successions globally, including in Morocco, Mongolia and Siberia (e.g., Bowyer et al., 2022;Jiang et al., 2012;Li et al., 2013;Maloof et al., 2010). ...
... As seen in modern Bahamian carbonates , and as recently proposed for other carbonates inferred to have been deposited in the late Neoproterozoic (Busch et al., 2022;Hoffman & Lamothe, 2019), carbonate sediments deposited in deeper (i.e., subtidal) settings may be more likely to experience sediment-buffered diagenesis and therefore document primary geochemical signatures. The deposition of limestones in the Shibantan and Dahai members coincides with transgressive system tracts and represent deeper marine (subtidal) depositional settings relative to dolostone intervals (interpreted to record peritidal settings) in these successions (Jiang et al., 2011;Li et al., 2013). These paleoenvironmental differences-and associated differences in preservation potential, with a greater potential for sediment-buffered diagenesis in more subtidal settings-may provide some explanation for why the Shibantan Member and intervals of the Dahai Member are characterized by the most prominent (and diagnostic) indicators of predominantly aragonitic deposition. ...
Article
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The primary mineralogy of marine carbonates has varied over geological time in concert with the secular evolution of global climate and seawater chemistry. Here, we employed a multi-proxy geochemical and petrographic approach, including measuring the Ca isotope (δ 44 Ca) and Sr content of Ediacaran-Cambrian carbonates, to provide new insights into the timing of the transition from a "dolomite-aragonite sea" to a "calcite sea" across the Ediacaran-Cambrian transition. We find robust evidence for the persistence of an aragonite sea well into Cambrian Age 2 (and potentially up through Age 4). However, together with an updated Accepted Article petrographic compilation of abiotic carbonate precipitates (i.e., ooids and cements), these new δ 44 Ca and Sr data provide further evidence that there was a protracted transition from aragonite seas to calcite seas in the Cambrian. We propose that this transition was mediated, in part, by changes in seawater Mg/Ca ratios potentially regulated by the global marine redox state and extents of authigenic clay precipitation.
... During the late Neoproterozoic, with the reorganization of Laurentia and Gondwana, South China developed into a series of platforms on a passive continental margin (Wang and Li, 2003;Zhao et al., 2011). An extensive transgression flooded most of the Yangtze region after the Marinoan glaciation (Nantuo Formation) (Jiang et al., 2011;Xiao et al., 2020aXiao et al., , 2020b, resulting in the widespread deposition of Ediacaran sequences and the formation of a sedimentary basin (Jiang et al., 2003(Jiang et al., , 2011Shi et al., 2018Shi et al., , 2020aShi et al., , 2020b. The Ediacaran strata (635− 541 Ma) in the upper Yangtze region consist of the older Doushantuo (635− 550 Ma) and younger Dengying (550− 541 Ma) Formations (Condon et al., 2005;Yang et al., 2016;Xiao et al., 2020aXiao et al., , 2020b. ...
... During the late Neoproterozoic, with the reorganization of Laurentia and Gondwana, South China developed into a series of platforms on a passive continental margin (Wang and Li, 2003;Zhao et al., 2011). An extensive transgression flooded most of the Yangtze region after the Marinoan glaciation (Nantuo Formation) (Jiang et al., 2011;Xiao et al., 2020aXiao et al., , 2020b, resulting in the widespread deposition of Ediacaran sequences and the formation of a sedimentary basin (Jiang et al., 2003(Jiang et al., , 2011Shi et al., 2018Shi et al., , 2020aShi et al., , 2020b. The Ediacaran strata (635− 541 Ma) in the upper Yangtze region consist of the older Doushantuo (635− 550 Ma) and younger Dengying (550− 541 Ma) Formations (Condon et al., 2005;Yang et al., 2016;Xiao et al., 2020aXiao et al., , 2020b. ...
Article
Dolomitic microbialites were developed extensively during the Neoproterozoic and record widespread oxygenation of the oceans and the accompanying rise of animal life. Primary precipitation of dolomite cements has now been recognized in both Cryogenian and Ediacaran periods, although the origin of matrix dolomite is debated. The terminal Ediacaran Dengying Formation (551–541 Ma) in the Upper Yangtze Platform of China contains massive dolomite. Particularly delicate microbial structures are preserved in the second member of the formation (Z2dn²). This study aimed to elucidate the origin of matrix dolomite in these microbialites, the main mesoscale features of which are stromatolites, thrombolites and composite microbialites. The stromatolites typically have planar and low-relief planar–domal forms, with distinct laminated microstructure alternated with diffuse laminated microstructures. The thrombolites comprise a variety of mesoclot structures and cavities, which are rimmed by isopachous fringes of fibrous cement and filled by late sparry cement. The composite microbialites have clotted and laminated textures of various styles that are intermingled at the sub-centimeter scale. These microbialites are inferred to have been deposited in a subtidal to intertidal setting. Abundant nanoscale dolomite biomorphs were observed with scanning electron microscopy, including single nanoscale spheroids and aggregates of nanometer filaments with granulated, micro-rose shaped, micro-sized foliage-like, and trumpet-like forms, most of which are presumably of biogenic origin. These nanoscale microbial textures act as archives of ancient microbial activity that may have produced abundant extracellular polymeric substance, providing sites for the nucleation and maturation of dolomite. This process may have been facilitated by Mn–S redox cycling and resulted in precipitation of primary ferroan dolomite in the matrix.
... The successions of the Doushantuo Formation studied in this research-including the Zhangcunping (ZCP), Xiangerwan (XEW), Xiaofenghe (XFH), Jiulongwan (JLW), Huajipo (HJP), Tianjiayuanzi (TJYZ), Beidoushan (BDS), Datang (DT), Mukong (MK), Daping (DP), Songlin (SL), and Wenghui (WH) sections-represent deposition in the inner shelf, shelf lagoon, outer shelf, and slope facies. The thickness of the Doushantuo Formation varies from ~ 90 m at the inner-shelf Zhangcunping section, to ~ 150 m at the shelf-lagoon Jiulongwan section, to ~ 40 m at the outer-shelf Beidoushan section (Jiang et al., 2011;Jiang et al., 2008;Wang et al., 2016;Xiao et al., 2012;Xiao et al., 2014a;Zhou et al., 2017a). Facies variations of the Doushantuo Formation were described by Jiang et al. (2011) and are summarized below, with a focus on the sections studied in this research. ...
... The thickness of the Doushantuo Formation varies from ~ 90 m at the inner-shelf Zhangcunping section, to ~ 150 m at the shelf-lagoon Jiulongwan section, to ~ 40 m at the outer-shelf Beidoushan section (Jiang et al., 2011;Jiang et al., 2008;Wang et al., 2016;Xiao et al., 2012;Xiao et al., 2014a;Zhou et al., 2017a). Facies variations of the Doushantuo Formation were described by Jiang et al. (2011) and are summarized below, with a focus on the sections studied in this research. ...
Article
Cryogenian snowball Earth glaciations may have had disastrous impacts on the biosphere, particularly the terrestrial ecosystem. However, how the terrestrial ecosystem responded to and recovered from these glaciations remains poorly understood. Speleothems offer important insights into terrestrial life because their formation is critically dependent on soil CO2 derived from microbial respiration. Here we report the wide distribution of miniature paleo-speleothems from the ∼ 635 Ma Doushantuo cap dolostone in South China and assess their implications for the recovery of terrestrial life after the terminal Cryogenian Marinoan glaciation. The ∼ 3-m-thick cap dolostone was deposited during the initial transgression following deglaciation but subsequently experienced subaerial exposure due to post-glacial rebound, which resulted in the development of extensive sheet-cavities in the cap dolostone and a widespread karstic surface atop the cap dolostone. The sheet-cavities were filled with multiple generations of minerals, including isopachous dolomite interpreted to have formed in the phreatic zone, speleothems consisting of fibrous calcite interpreted to have formed in the vadose zone, as well as later phases (i.e., isopachous radial chalcedony, crystalline quartz, and blocky calcite). The Doushantuo speleothems are millimeter-to-centimeter in size and include gravitational speleothems (stalactites and stalagmites) and non-gravitational speleothems (helictites, moonmilk, botryoids, and flat crusts). Some of them were secondarily silicified by hydrothermal fluids before a renewed transgression in which ∼ 632 Ma shales overlying the cap dolostone were deposited. The wide distribution of Doushantuo speleothems and the preservation of microfossils in these speleothems indicate the presence of an active soil-microbial ecosystem in the earliest Ediacaran Period and the rapid recovery of terrestrial life after the Marinoan snowball Earth.
... The successions of the Doushantuo Formation studied in this research-including the Zhangcunping (ZCP), Xiangerwan (XEW), Xiaofenghe (XFH), Jiulongwan (JLW), Huajipo (HJP), Tianjiayuanzi (TJYZ), Beidoushan (BDS), Datang (DT), Mukong (MK), Daping (DP), Songlin (SL), and Wenghui (WH) sections-represent deposition in the inner shelf, shelf lagoon, outer shelf, and slope facies. The thickness of the Doushantuo Formation varies from ~ 90 m at the inner-shelf Zhangcunping section, to ~ 150 m at the shelf-lagoon Jiulongwan section, to ~ 40 m at the outer-shelf Beidoushan section (Jiang et al., 2011;Jiang et al., 2008;Wang et al., 2016;Xiao et al., 2012;Xiao et al., 2014a;Zhou et al., 2017a). Facies variations of the Doushantuo Formation were described by Jiang et al. (2011) and are summarized below, with a focus on the sections studied in this research. ...
... The thickness of the Doushantuo Formation varies from ~ 90 m at the inner-shelf Zhangcunping section, to ~ 150 m at the shelf-lagoon Jiulongwan section, to ~ 40 m at the outer-shelf Beidoushan section (Jiang et al., 2011;Jiang et al., 2008;Wang et al., 2016;Xiao et al., 2012;Xiao et al., 2014a;Zhou et al., 2017a). Facies variations of the Doushantuo Formation were described by Jiang et al. (2011) and are summarized below, with a focus on the sections studied in this research. ...
Article
Cryogenian snowball Earth glaciations may have had disastrous impacts on the biosphere, particularly the terrestrial ecosystem. However, how the terrestrial ecosystem responded to and recovered from these glaciations remains poorly understood. Speleothems offer important insights into terrestrial life because their formation is critically dependent on soil CO2 derived from microbial respiration. Here we report the wide distribution of miniature paleo-speleothems from the ∼ 635 Ma Doushantuo cap dolostone in South China and assess their implications for the recovery of terrestrial life after the terminal Cryogenian Marinoan glaciation. The ∼ 3-m-thick cap dolostone was deposited during the initial transgression following deglaciation but subsequently experienced subaerial exposure due to post-glacial rebound, which resulted in the development of extensive sheet-cavities in the cap dolostone and a widespread karstic surface atop the cap dolostone. The sheet-cavities were filled with multiple generations of minerals, including isopachous dolomite interpreted to have formed in the phreatic zone, speleothems consisting of fibrous calcite interpreted to have formed in the vadose zone, as well as later phases (i.e., isopachous radial chalcedony, crystalline quartz, and blocky calcite). The Doushantuo speleothems are millimeter-to-centimeter in size and include gravitational speleothems (stalactites and stalagmites) and non-gravitational speleothems (helictites, moonmilk, botryoids, and flat crusts). Some of them were secondarily silicified by hydrothermal fluids before a renewed transgression in which ∼ 632 Ma shales overlying the cap dolostone were deposited. The wide distribution of Doushantuo speleothems and the preservation of microfossils in these speleothems indicate the presence of an active soil-microbial ecosystem in the earliest Ediacaran Period and the rapid recovery of terrestrial life after the Marinoan snowball Earth.
... The Ediacaran Doushantuo Formation (~635-551 Ma) in the Yangtze Block largely comprises both carbonate and siliciclastic rock, which overlies the Cryogenian Nantuo Formation and underlies the Ediacaran Dengying/Liuchapo formations (Condon et al., 2005). A passive continental margin in a low-middle palaeolatitude position has been inferred on the basis of stratigraphic pattern, thickness variation and palaeogeographic reconstruction (Jiang et al., 2011;Li et al., 2013). The Doushantuo Formation was deposited on a rimmed carbonate shelf with a shelf marginal shoal complex that restricted an intrashelf basin from the open ocean ( Fig. 1C; Zhu et al., 2013). ...
... We studied the samples from the member II of the Doushantuo Formation measured in eight sections at Jiulongwan, Chenjiayuanzi, Northern Xiaofenghe, Jiuqunao, Qinglinkou, Miaohe, Liaojiagou and Wuzhishan (Fig. 1B;. The Doushantuo Formation at these sections in this study was deposited on the inner shelf near the proximal side of intrashelf basin on the Ediacaran Yangtze Block ( Fig. 1D; Jiang et al., 2011). ...
Article
The origin and early evolution of animal development remain among the many deep, unresolved problems in evolutionary biology. As a compelling case for the existence of pre-Cambrian animals, the Ediacaran embryo-like fossils (EELFs) from the Weng'an Biota (approx. 609 Myr old, Doushantuo Formation, South China) have great potential to cast light on the origin and early evolution of animal development. However, their biological implications can be fully realized only when their phylogenetic positions are correctly established, and unfortunately, this is the key problem under debate. As a significant feature of developmental biology, the cell division pattern (CDP) characterized by the dynamic spatial arrangement of cells and associated developmental mechanisms is critical to reassess these hypotheses and evaluate the diversity of the EELFs; however, their phylogenetic implications have not been fully realized. Additionally, the scarcity of fossil specimens representing late developmental stages with cell differentiation accounts for much of this debate too. Here, we reconstructed a large number of EELFs using submicron resolution X-ray tomographic microscopy and focused on the CDPs and associated developmental mechanisms as well as features of cell differentiation. Four types of CDPs and specimens with cell differentiation were identified. Contrary to the prevailing view, our results together with recent studies suggest that the diversity and complexity of developmental mechanisms documented by the EELFs are much higher than is often claimed. The diverse CDPs and associated development features including palintomic cleavage, maternal nutrition, asymmetric cell divisions, symmetry breaking, establishment of polarity or axis, spatial cell migration and differentiation constrain some, if not all, EELFs as total-group metazoans. This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.
... The Wenghui biota can be correlated paleontologically with another Ediacaran macrobiota, the Miaohe biota (e.g., Xiao, 1991, 1992;Ding et al., 1992Ding et al., , 1996Chen et al., 1994Chen et al., , 2000Steiner, 1994;Yuan et al., 1995;Xiao et al., 2002;Ye et al., 2019) from the upper Doushantuo black shales at Miaohe, Zigui, western Hubei, South China, which shares many common species (see Wang et al., , 2016aWang et al., , 2020aYe et al., 2019). The lithological succession of the Doushantuo Formation in northeastern Guizhou is similar to that of western Hubei (e.g., Qin et al., 1984;Wang et al., 1987;Liu and Xu, 1994;Zhu et al., 2007;Jiang et al., 2011;Wang et al., 2012). The macrofossil-bearing black shales of the upper Doushantuo Formation at both the Wenghui and Miaohe areas were dated to ca. 560-551 Ma (see Condon et al., 2005;Kendall et al., 2015;Li et al., 2015;Wang et al., 2017Wang et al., , 2020aWang and Wang, 2018). ...
Article
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With differentiated tissues and organs, a high-level eukaryotic macroalga Lanceaphyton xiaojiangensis n. gen. n. sp. lived on the middle–late Ediacaran (ca. 560–551 Ma) seafloor in South China. Its body had a pith (perhaps mechanical tissue) and outer tissue (perhaps epidermis and/or cortex). The lance-like macroalga consists of an unbranching thallus that grew over the sediment surface for sunlight and a holdfast grown into sediments to keep the thallus fixed on the seafloor. The pithy stipe (lower thallus) might have served to support the upper pithless thallus for photosynthesis. The holdfast is composed of a tapering pithy rhizome growing down into the sediments, with many filamentous pithless rhizoids dispersedly growing within the sediments. With the differentiated tissues and organs, especially the pith accounting for about half of the width of the rhizome and stipe, Lanceaphyton n. gen. was a high-level eukaryotic macroalga, similar to phaeophytes in morphological features, but further research is needed on its microstructural details. The pithy macroalga shows that the macroalgal pith had emerged in the Ediacaran. UUID: http://zoobank.org/bc924c5c-84e4-4170-9ca1-caee0d56c6d5 .
... There are extremely well-preserved fossils of multicellular eukaryotes including early animals in the Ediacaran strata of Yangtze Block, and negative δ 13 C excursions, particularly the correlated Shuram excursion recorded in Member 3 of the Doushantuo Formation. This greatly appeal to many scientists, so that numerous studies including geochronology, geochemistry, paleobiology, stratigraphy, and paleomagnetism have been carried out in the last few decades (e.g., Condon et al., 2005;Jiang et al., 2011;Zhang et al., 2005Zhang et al., , 2015bZhou et al., 2017). ...
Chapter
China has three major Precambrian cratons, named the north China craton (NCC), the south China craton (SCC), and the Tarim craton. Each craton has some billion years of geological history. The authors of this chapter favor an NCC-N Australia connection for the interval between ~1.80 and ~1.32 Ga in the supercontinent Nuna/Columbia and an NCC-NW Laurentia connection between ~1.11 and 0.78 Ga in Rodinia, based on comprehensive analyses of geological and paleomagnetic data. High-quality paleomagnetic data indicate that the SCC had experienced a long journey from the north polar regions to the Equator between ~0.8 Ga and the early Cambrian and it was unlikely once located between Australian cratons and Laurentia in the center of Rodinia in that period. The SCC’s apparent polar wander path may contain a component of true polar wander that needs more precise age constraints. Two possible paleogeographic models, either by placing the Tarim craton against NW Australia or placing it in the center of Rodinia to be the link between Laurentia and the Australia-Mawson craton, were reviewed.
... Sedimentary sequences of Ediacaran system, namely, the postglacial Doushantuo Formation and overlying Dengying Formation, are widespread occurrence in a passive continental margin on the southeastern side of the Yangtze Block (Fig. 1A). The Doushantuo Formation in South China has been widely studied to understand the paleoenvironmental condition and accompanied biological evolution after the Marinoan glaciation (Jiang et al., 2011;Lv et al., 2019). The studied Gaoyan deposit, which is situated about 10 km west from Chengkou city, occurred in the top part of the Doushantuo Formation of Ediacaran System as part of the south-western limb of the Chengkou synclinorium of the Dabashan fold belt ( Fig. 1B; Fig. 2; Fan and Yang, 1999). ...
Article
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The stratiform Mn-carbonate ore from the Gaoyan deposit occurs within terrigenous-sedimentary rocks of the upper part of the Ediacaran Doushantuo Formation and is characterized by rhodochrosite spherulites. Here we examine the effect of depositional environment and hydrothermal contribution based on new petrographical, mineralogical and geochemical data to shed light on major controls on formation of sedimentary Mn mineralization. The significant geochemical characteristics such as high Mn/Fe ratios (average 32.07), high Ba content (average 1940.9 ppm), positive Eu anomalies (average 1.38) and low REE contents (average 51 ppm) reveal a hydrothermal source. Meanwhile, 87Sr/86Sr ratios of Mn ores are substantially higher than those of carbonate wall rocks and contemporaneous seawater, indicating mixing of radiogenic Sr from the Mn-rich hydrothermal extraction from underlying sediments. This explanation is supported by a positive linear correlation between 87Sr/86Sr ratios and Eu anomalies. The slightly positive Ce anomaly of most Mn carbonate ore samples is mirrored in the negative Ce anomaly of black shale, while different Ce anomalies provide evidence that Mn redox cycling took place across a sub-oxic to anoxic/euxinic water interface. The euxinic conditions, as evidenced by the co-occurrence of fine-grained disseminated pyrite to framboidal aggregates (average diameters <5 μm) with rhodochrosite, can account for the high Mn/Fe ratios via ongoing Fe-sulfide precipitation. Calcite introduction to metal-rich hydrothermal waters with a lower pH relative to surface waters, together with Mn-oxide reduction and sulfate reduction, triggers substantial increases in rhodochrosite saturation and precipitation near the water-sediment interface. Subsequently, increasing Ca2+ in early diagenesis resulted in the precipitation of kutnohorite rims. Altogether, we develop a hydrothermal sedimentary-early diagenesis model of the Gaoyan Mn mineralization with rhodochrosite spherulites as the hydrothermal sedimentary mineral phases, and kutnohorite forming during early diagenesis. Slightly light carbon isotope compositions (∼–6.1‰) of the manganese carbonate ores than those of normal dolostones (∼–4.8‰) suggest a limited carbon source from oxidation of organic material by methanogenesis processes during burial diagenesis. This model supports the model for direct Mn-carbonate precipitation from redox-stratified water columns and means that sedimentary Mn mineralization may be mainly controlled by hydrothermal contribution, rather than seawater redox conditions. This means exploration strategies that focus on oxic zone alone as an indication of prospectivity may be ineffective.
... It is proposed that deposition of the Nantuo Formation was coincident with the waning stage in the rifting of the South China Craton that consists of the Yangtze and Cathaysia blocks . The Nantuo Formation conformably underlies the Doushantuo cap carbonate, and variably overlies sandstone of the Tonian Liantuo Formation with a significant depositional gap in the shelf, black shale of the Datangpo Formation in the slope, and siltstone/sandstone of the Fulu Formation in the basin (Jiang et al., 2011;Lan et al., 2015;Yu et al., 2016;Zhou et al., 2019;Ning et al., 2021). In the shelf settings, the Nantuo Formation is mainly composed of massive diamictite or conglomerates intercalated with sandstone and siltstone units (Lang et al., 2018a;Shen et al., 2021). ...
Article
Termination of the Marinoan global glaciation (650-635 million years ago, Ma) was immediately followed by the diversification of eukaryotes and possible occurrence of animals, suggesting the potential linkage between biological evolution and global glaciation. It is proposed that the post-glacial eukaryote evolution might have been triggered by environmental stress imposed by the extreme pan-glacial condition. However, how life survived through the global glaciation remains speculative. Neither is known about the habitability when the ocean was completely or mostly frozen. In this study, we explore the marine carbon and sulfur biogeochemical cycles during the Nantuo (Marinoan) glaciation (650-635 Ma) in South China. Both organic carbon (δ¹³Corg) and pyrite sulfur isotopes (δ³⁴Spy) show significant stratigraphic variations, suggesting active biogeochemical cycles in the pan-glacial condition. By coupling both δ¹³Corg and δ³⁴Spy data, we develop numerical models to constrain marine productivity and redox conditions during the Nantuo glaciation. The marine primary productivity showed a sharp decline in the onset of glaciation and recovered right before the cap carbonate precipitation. An episodic recovery of primary productivity was observed in the non-glacial interval between the two glacial episodes. In contrast, the marine productivity was still suppressed in initial deglaciation, during which the continental weathering was intense, presumably indicating high atmospheric CO2 level and low seawater pH value. The final recovery of marine primary productivity occurred in the topmost of Nantuo Formation, when the weathering intensity was already low. Therefore, the active biogeochemical cycle implies a sustained habitability, warranting the survivorship in the pan-glacial ocean, while the persistently low and delayed recovery of marine productivity might have imposed environmental stress for tens of million years, paving the way for the evolution of animals.
... Field occurrence of pyrite nodules. The Doushantuo Formation in the South China Block directly overlies the Nantuo glacial diamictite and consists of as much as 250 m of carbonates, siltstones, and shale [10][11][12][13] . In a well-exposed field section in Taoying, eastern Guizhou ( Fig. 1), an about 1.4 m light-grey cap dolostones directly overlies a dark-grey Nantuo glacial diamictite. ...
Article
Full-text available
Pyrite nodules up to 20 cm in diameter are found at the top of the Marinoan (~ 635 Ma) Nantuo glacial diamictite as well as in the cap dolostones and shale/siltstones in the lower Doushantuo Formation in eastern Guizhou, southern China. Field occurrences, petrography, and stable sulfur isotopic compositions of pyrite nodules were studied from a section at Taoying, eastern Guizhou, China. Pyrite δ34S values from different nodules varied from 7.3 to 60.5‰ at different stratigraphic levels. No stratigraphic trend existed for the δ34S, supporting the scenario of pyrite formation in sediments before the precipitation of the cap dolostone. Pyrite δ34S values were also homogeneous within individual nodules at a 0.3 to 1 cm sampling scale, but were more heterogeneous at a 2 mm sampling scale. Homogeneity was not expected from the particular model for pyrite nodule formation in a largely closed or semi-closed environment. Thus, differential cementation and compaction of the pyrite-bearing sediments may have produced the nodular shape of the pyrite deposit.
... The late Neoproterozoic strata in this region consist of the Liantuo, Nantuo, Doushantuo, and Dengying formations in ascending order (Fig. 1C). In the study area, the Doushantuo Formation was deposited from the inner shelf of the Ediacaran Yangtze Platform (Jiang et al., 2011;Fig. 1A). ...
Article
The Shuram carbon isotope excursion (CIE) is the largest negative CIE in geological history that may have recorded dramatic changes in Earth's surface environments, yet its time, duration, and global synchroneity have been debated. In South China, the Shuram CIE has been correlated with the negative CIE of the upper Doushantuo Formation (named as EN3 or DOUNCE). It is previously suggested that the Shuram CIE in South China stratigraphically consists of the upper Member 3 and Member 4, with an ending age of ca. 551 Ma. Recent studies, however, demonstrated that the EN3 or DOUNCE shifts to near zero or positive carbon isotope (δ¹³C) values within carbonate rocks of the Doushantuo Member 3. To better understand the duration of this CIE and its spatial variability, we have conducted a high-resolution cyclostratigraphic study using magnetic susceptibility (MS) in two sections: the Zhengjiatang section that preserves a complete EN3 in carbonate rocks of the Doushantuo Member 3 and the Wuhe-Gaojiaxi (WG) drillcore in which the EN3 does not shift to positive δ¹³C values until the end of the Member 4 black shales. In both sections, power spectral analyses of the MS series of the carbonate rocks reveal periodicities that match well with those of the Milankovitch cycles at 570 Ma. The duration of the Shuram CIE is estimated to be 6.1 ± 0.2 Myr by counting the interpreted 405-kyr sedimentary cycles observed in the Zhengjiatang section. Taking the newly reported ReOs age of 567.3 ± 3.0 Ma as the approximate termination age, the Shuram CIE in the best-preserved section in South China is constrained between ca. 573.4 and 567.3 Ma. Regional correlation, however, shows a ≥ 1 Myr stratigraphic hiatus or unconformity at the top of the Doushantuo Member 3 that separates the Shuram CIE from overlying post-Shuram ẟ¹³C “excursions”. Similar phenomena may exist in the Shuram CIE documented from stratigraphic successions in Oman, Australia, and the Death Valley region of western United States. The results invoke a reevaluation of the stratigraphic range of the Shuram CIE and suggest that the geochemical records documented from the black shales of the Doushantuo Member 4 and its globally correlative intervals may record paleoceanographic conditions millions of years after the Shuram CIE. Comparison of astrochronologically calibrated δ¹³C profiles reveals significant difference in the shape and amplitude of the Shuram CIE among the sections in South China and globally, which may have been caused by isotopic departure in local depositional environments and/or diagenetic overprints.
... Unlike some well-known Ediacaran carbonate sequences, such as the Doushantuo Fm. in South China (Jiang et al., 2011), the Khatyspyt Fm. in Siberia (Nagovitsin et al., 2015) or the Ara Group in Oman (Amthor et al., 2005), for which marine conditions are obvious, the Ediacaran deposits on the western EEC comprise chiefly siliciclastic rocks of the Volyn and Valdai series, the latter including the Redkino and Kotlin regional stages (e.g. Paszkowski et al., 2019), for which the depositional setting is not that clear in many areas. ...
Article
The Ediacaran sedimentary rocks on the East European Craton (EEC) comprise chiefly siliciclastic deposits of the Volyn and Valdai series that have commonly been regarded as deposits in a shallow epicontinental sea on Baltica. However, an intermittent fresh-water sedimentary setting was also indicated for some part of the paleocontinent. Surprisingly, although Baltica was positioned in low latitudes during the Ediacaran period, no carbonate sequences have formed on the EEC. Systematic and detailed mineralogical, petrographic and stable isotope evaluation of carbonate minerals occurring in the Ediacaran sedimentary rocks of the western EEC stretching from the Arkhangelsk area at the White Sea to the Ukraine-Moldova borderland near the Black Sea show that continuously marine or sea water-influenced depositional conditions existed only in the marginal parts of the area: Podillya, Moldova, and the Arkhangelsk area. In contrast, a fully continental depositional setting stretched in the central part (St. Petersburg area, Lithuania, E Belarus) with intermittent periods of marine or brackish-water conditions only in E Belarus. SW Belarus and Volyn were influenced by marine or brackish environments. Highly ¹³C-enriched early-diagenetic calcite from Poddillya and pedogenic siderite from the St. Petersburg area attest for bacterial methanogenesis operating during the Kotlinian period both in marine and continental environments, respectively. This shows that not only marine, but also terrestrial biosphere was already very productive at the end of the Ediacaran. The δ¹⁸O composition of meteoric, marine hypersaline, burial, and hydrothermal fluids was reconstructed by integration of mineralogical, petrographic and stable isotope data for various types of Ediacaran carbonates from the western EEC. Moreover, the range of δ¹⁸O values for Ediacaran marine carbonate was constrained by setting the lower limit at −5‰, which is the maximum value for pedogenic siderite, and the upper limit at +2‰, which is the minimum value for carbonates associated with saline fluids.
... 我国华南广泛发育有新元古代至寒武纪的海侵黑色页岩, 例 如埃迪卡拉系陡山沱组 [42] , 及寒武系下部的牛蹄塘组、石岩 头组等 [43] . 尤其是, 寒武系下部的牛蹄塘组(局部地区称水井 沱组、郭家坝组等)与下伏地层呈突变(图1(a))或不整合接触 关系(图1(c), (d)), 含有直径可达1 m的灰岩结核(图1(b)), 是很 典型的海侵黑色页岩 [43] . ...
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The marine refractory dissolved organic carbon (RDOC) pool, derived from the microbial carbon pump (MCP), holds a tremendous amount of carbon, as much as that in today’s atmosphere; hence, an understanding of this pool is crucial for investigating carbon cycling and climate change at the global scale. The size of the RDOC pool dynamically evolves over time. In the geological past, particularly during the Neoproterozoic to Cambrian transition, the marine RDOC pool might have reached a hundred to a thousand times its present size. However, the patterns of the reduction in the RDOC pool through time and the means by which RDOC was transferred into the geological reservoir have been poorly known to science. Black shales, which are the source rocks of oil and gas reservoirs and are enriched in many rare metals, are economically important sedimentary rocks. Black shale is characterized by the enrichment of clay sediments and organic matter (total organic carbon (TOC) > 1%). At least a dozen depositional models have been proposed to interpret various types of black shales based on the depositional environments and three major controls on organic content, i.e., the production and supply of organic matter, the demand and consumption of oxygen, and the rate of sedimentation. Organo-clay interactions and potential organic sources of the bulk RDOC in ancient oceans have not been fully considered. Clay minerals, e.g., smectite and illite, are well known for their capability of adsorption and preservation of dissolved organic matter in significant quantities. Organic compounds adsorbed by clays are present dominantly within interlayer sites and thus are difficult to be decomposed or used by microbes. Accordingly, here, I propose a hypothetical mechanism of RDOC deposition and preservation, which also functions as an alternative model for the origin of transgressive black shales that always abruptly contact the underlying rock unit. The proposed mechanism is outlined as follows: During global-scale sea level rise driven by crustal subsidence or climate change, upwelling currents transport RDOC-rich water bodies from deep oceans to clay-rich shallow sea depositional environments, where clay sediments mix well with the RDOC; the mixture triggers the interplay between clay minerals and organic matter, and thus leads to large amounts of RDOC being absorbed by clay sediments; after deposition, the clay sediments are able to absorb organic matter from pore water; finally, the organic-rich transgressive clay sediments form black shales through diagenesis. Experiments indicate clays are capable of absorbing as much as 12% or more dissolved organic matter, far more than the minimal 1% TOC in black shales. Therefore, it is reasonable to assume that the RDOC is an important organic source in transgressive black shales. The new model provides a potential link between ancient marine RDOC and black shales; hence, it might offer a mechanism for transferring marine RDOC into the geological carbon reservoir and thereby present an additional path of marine carbon cycling. Unlike the previous model that invoked primary production as the major organic source, the present model emphasizes the contribution of RDOC to the organic matter of black shales. However, it does not refute the contributions of any other organic source or any other model for transgressive black shales. To test this hypothesis, future work may include enhanced experimental studies of the interplay between clay minerals and organic compounds under different settings of temperature, pressure, and redox state, establish a set of geological criteria to recognize RDOC from organic-rich rocks, e.g., black shales; and quantitatively model the evolution of RDOC over geological time. Each of these tests would provide critical information for understanding the global carbon cycles of the past, present, and future. 惰性溶解有机碳是现代海洋最主要的有机碳储库,在古海洋中可能有更大的容量,因此是海洋科学、碳循环、全球变化等领域关注的热点。黑色页岩不仅是油气资源的母岩,而且富含多种稀有金属,是地质学与矿产资源勘查领域的重点研究对象。黑色页岩有机质含量高,主要由黏土矿物组成。黏土矿物最显著的特点是具有很大的活性表面,可以大量吸附溶解在水中的有机化合物,并有助于有机化合物的长期保存。因此,提出海洋惰性有机碳的沉积与保存机制,同时也是一种海侵黑色页岩沉积模型。在海侵过程中,惰性溶解有机化合物可以与黏土沉积物充分混合并被吸附后沉积,经成岩作用后形成海侵黑色页岩,因此进入地质储库。本文提出的机制或模型为惰性有机碳从海洋储库到地质储库的转移提供一种可能的途径,对研究黑色页岩有机碳的来源有一定的启示,但目前仅代表一种假说或者猜测,需要开展广泛的研究工作去验证。
... In addition, the post-Shuram subaerial exposure surface and associated paleokarst in Northwest Canada (Fig. 2H) provides a local minimum zero water depth constraint at the Shuram CIE's termination. Other locations not examined in this study also record a similar sequence stratigraphic trend to those described herein (e.g., Grotzinger et al., 2011), with a sequence boundary separating pre-and syn-Shuram strata observed in member 3 of the Doushantuo Fm in South China and the Nikol'skaya Fm in Siberia (e.g., Jiang et al., 2011;Melezhik et al., 2009). ...
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... The Ediacaran succession in South China was deposited on a passive continental margin on the Yangtze Platform (Jiang et al., 2007a;Jiang et al., 2011). Thereafter, the Cambrian stratigraphic sequence was sedimented during the unparalleled emergence of organisms (Steiner et al., 2001;Marshall, 2006;Zhu et al., 2007;Fox, 2016). ...
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“Old phosphorite” deposits (Ediacaran to early Cambrian) located in Central Guizhou, South China, are hosted in carbonate and represent the earliest phosphogenesis. These old phosphorites, as well as Pliocene to Pleistocene and modern phosphate sediments (recent phosphorite) have been extensively studied. However, the possibility of recent phosphorite being the incipient stage of long-time phosphorite remains uncertain. Taking Weng’an (Ediacaran) and Zhijin (early Cambrian) phosphorites as examples, this study conducted in situ geochemical and mineralogical analyses of phosphate minerals (old apatites). Analogies between recent and old apatites were determined by comparing geochemistry based on rare earth elements (REE) and yttrium (REE+Y). Old phosphorite could be divided into authigenic grains and biodetritus, both comprising closely accumulated apatite nanocrystals. REE+Y concentrations (∑REE+Y) were much higher in the outer rim than in the inner core of biodetritus, whereas the authigenic grains showed relatively homogeneous REE+Y distributions. Three REE+Y patterns were observed: (1) “left-inclined” type, with no Ce anomalies; (2) shale-like pattern, with weak MREE enrichment and slightly negative Ce anomalies; (3) “hat-shaped” pattern, with notable MREE enrichment, evident HREE depletion, and remarkable negative Ce anomalies. Positive La, Gd, and Y anomalies, as well as MREE enrichment increased from type (1) to type (3), which can be explained by increasing degrees of oxidation. The REE+Y distributions of old apatites were similar to those of recent apatites, but exhibited MREE enrichment and HREE depletion and old authigenic apatites had no Ce anomalies. Furthermore, old apatite compositions lie in the diagenetic area on Y/Ho vs SmN/YdN plots. These characteristics indicate that old phosphorite could have been subjected to long-term diagenetic modification after burial, during which apatites recrystallized and absorbed REE+Y. Hence, it can be concluded that recent apatites could serve as predecessors of major ancient phosphorite, especially biogenic phosphorites. Globally, typical old phosphorites have quite variable REE+Y compositions, suggesting that the phosphorites experienced different depositional conditions and diagenetic alteration despite their contemporaneous formation. This study provides new data and insight on the diagenesis of old phosphorite and improves our deeper understanding of phosphogenesis in paleo- and modern environments.
... Cyclostratigraphic analysis shows that the duration of the Datangpo Formation is about 9.8 million years (Bao et al., 2018). Regionally, the non-glacial Datangpo Formation varies greatly from a few meters to several hundred meters in thickness (Jiang et al., 2011;Zhou et al., 2016), which may be related to the palaeogeographic differences caused by basin extension Wang and Li, 2003;Wang et al., 2015). Generally, the Datangpo Formation can be divided into three intervals in lithology: the basal Mn-carbonate with Mn-bearing shale in the IntervalI, organic-rich black shale in the IntervalII, and organic-poor gray shale and siltstone in the Interval III. ...
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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.
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Glacial diamictites of the Cryogenian Nantuo Formation of the Yangtze Craton of South China record major environmental transitions during the Marinoan glaciation. Although the provenance of the siliciclastic materials in the Nantuo Formation have been constrained via detrital zircon analysis, the source areas for the abundant carbonate clasts within the formation have yet to be determined. Given that the Mesoproterozoic Shennongjia Group is the only major extant pre-Cryogenian sedimentary carbonate succession in South China, it has been proposed as a source of the Nantuo dolostone clasts in the northern Yangtze Craton and may have been a major supplier of Cryogenian carbonate clasts across the entire Yangtze Craton, but this speculative provenance has not been tested or formally investigated. To test this hypothesis, δ¹³C and δ¹⁸O values of 18 clasts collected near Yichang, Hubei Province were analyzed along with petrographic examination to determine lithology, diagenetic alteration, and relationship to the Shennongjia Group carbonates. Our data show that the δ¹⁸O and δ¹³C values of these clasts are distinct from and thus unlikely sourced from the Shennongjia Group. Therefore, the Shennongjia Group cannot be the sole source of Nantuo carbonate clasts. Instead, we propose the Nantuo carbonate clasts in the study area were derived from either (1) pre-Marinoan carbonate successions in South China that were lost due to erosion or (2) another craton (e.g., the Indian Craton) adjacent to the Yangtze Craton during the Cryogenian. Given that paleogeographic and detrital zircon provenance analysis suggest separation between the Yangtze Craton and the Indian Craton in the Cryogenian Period, we favor the former origin, and that the Shennongjia Group is not the exclusive source of carbonate clasts from the Nantuo Formation.
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A comprehensive study of the lithology, mineralogy, major element and trace element geochemistry of the organic-rich shales in the Lower-Middle Cambrian Niutitang and Wunitang Formations in South China was performed to assess the depositional environments and the mechanisms for organic matter enrichment. Results show that the Lower Cambrian Niutitang shales were mainly deposited under anoxic conditions with high paleo-productivity, while the overlying Wunitang shales were deposited under dysoxic conditions with lower paleo-productivity. Both these shales were deposited in a moderately restricted deep-water basin. This paleogeographic setting was conducive to limited water interchange between the outer shelf and open ocean, resulting in a high flux of nutrients from shelf upwelling. These nutrients were produced from upwelling and hydrothermal vents and were carried by oceanic currents, enhancing blooms of primary producers within the surface waters. Organic matter enrichment of the Niutitang and Wunitang shales was mainly controlled by redox conditions and paleo-productivity. The intermittent renewal of connections to open ocean water during sea level rise weakened the restricted conditions in the depositional area, which significantly accelerated the consumption of organic matter.
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The advent of animal biomineralization at the terminal Ediacaran remarkably changed the marine ecological system. Changed seawater Mg/Ca ratio is considered as an important external trigger for this transition, but the intrinsic relationship between seawater Mg/Ca variation and animal biomineralization is not well constrained due to the lack of reliable archives. To constrain the coeval seawater chemistry and explore its role in the origin of metazoan biomineralization, we systematically conducted petrographical and geochemical investigations on dolostones from the terminal Ediacaran (∼551 Ma) Dengying Formation of the Yangtze Block in South China. The well-preserved sedimentary fabrics, growth zones as well as cathodoluminescent bands in microbialites and fibrous dolomite cements in the Algal Dolomite Member of the Dengying Formation indicate these dolomites were formed during syn-sedimentary diagenesis. The distribution patterns of rare earth elements in these dolomite components documented relatively anoxic shallow water conditions. Furthermore, negative correlations between O and Mg isotope compositions in well-preserved dolomite components indicate the variable mixture of freshwater with seawater in the shallow platform environment. The pervasive influence of freshwater on seawater indicates an enhanced riverine input, and it should be dominated by silicate weathering flux due to the heavier Mg isotopic compositions in freshwater endmember. Enhanced silicate weathering flux at the basal Dengying Formation may be related to the abundant erosions and exposures caused by the significant changes in Earth’s surface tectonic and climatic conditions at the late Ediacaran. Consequently, the increased silicate weathering flux would deliver more Mg cations and net alkalinity to the oceans than the mixed weathering flux, which probably facilitated the subsequent skeletonization of early animals.
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The Ediacaran Doushantuo Formation in South China documents profound biological and environmental changes after the Neoproterozoic global glaciations. However, since the geological records of these changes are scattered in various lithofacies, the establishment of the co-evolutionary pattern of life and environment largely relies on regional stratigraphic correlation. A negative δ¹³C excursion in the upper Doushantuo Formation in the shelf-margin facies has been correlated with different δ¹³C chemostratigraphic features in the Doushantuo Formation in the shelf-lagoon facies: either with EN3 (widely regarded as equivalent to the Shuram excursion in Oman) or with EN2, which occur respectively in the upper and middle Doushantuo Formation in the Yangtze Gorges area of South China. Here we report new litho-, chemo-, and biostratigraphic data of the Douoshantuo Formation at the Caojunba section in shelf-margin facies, where the lithostratigraphic sequence and carbon isotopic profile resemble those at other shelf-margin sections, e.g., the Yangjiaping and Zhongling sections. Diverse microfossils including nine genera and eighteen species of acanthomorphic acritarchs are found from the upper Doushantuo Formation at Caojunba, which represent to date the first report of the upper Doushantuo acritarch assemblage from the shelf-margin facies in South China. The newly recovered microfossil assemblage includes Hocosphaeridium anozos, H. scaberfacium, Tanarium conoideum, and clustered leiospheres, and these taxa support the stratigraphic correlation of upper Doushantuo Formation between shelf-margin and shelf-lagoon facies. This correlation indicates that the interval with negative δ¹³C values in the upper Doushantuo Formation of shelf-margin facies is likely equivalent to EN3. This correlation also suggests that the δ¹³C profiles of the EN3/Shuram excursion interval in different facies in South China have various expressions (e.g., stratigraphic range, magnitude), making its regional and continental correlations more challenging. On the other hand, the discovery of acritarch assemblage characteristic of the upper Doushantuo Formation in the Yangtze Gorges area from shelf-margin facies confirms their potential as an independent stratigraphic tool in the subdivision and correlation of Ediacaran strata.
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The evolution of the atmospheric oxygen content through Earth’s history is a key issue in paleoclimatic and paleoenvironmental research. There were at least two oxygenation events in the Precambrian that involved fundamental changes in both biotic innovation and the surface environment. However, a large dissolved organic carbon (DOC) pool maintained in deep oceans during the Neoproterozoic may have extended the time interval between the two oxygenation events. To test the DOC hypothesis, we conducted detailed micro-drilled analyses of carbonate carbon isotopes (δ¹³Ccarb) of a long Ediacaran drill core (the Wangji drill core), for which whole-rock δ¹³Ccarb and organic carbon isotope (δ¹³Corg) records were available. The micro-drilled δ¹³Ccarb values obtained in this study are consistent with whole-rock δ¹³Ccarb results, precluding the influence of severe authigenic carbonate incorporation. Importantly, the multiple negative δ¹³Ccarb excursions in the Wangji drill core were likely linked with upwelling events, during which DOC was supplied to the surface water and oxidized. Using box models, we estimate that ~3.6 × 10¹⁹ mol and ~2.0 × 10¹⁹ mol DOC were converted to bicarbonate during two negative δ¹³Ccarb excursions spanning millions of years. The estimations are approximately 1000 times the modern marine DOC reservoir. Our results support a relatively high oxidation capacity (elevated atmospheric pO2 and/or oceanic [\begin{document}${\rm{SO}}_4^{2 - }$\end{document}]) of the Earth’s surface during the early Ediacaran Period.
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Termination of the Marinoan global glaciation (~650–635 Ma) was followed by the diversification of eukaryotes (e.g., early animals) and oxygenation of deep oceans in the early Ediacaran Period. Previous studies indicate the recovery of marine primary productivity immediately before the cap carbonate precipitation but after melting of the Marinoan global glaciation. Pyrite concretions from the topmost Nantuo Formation in the Yangtze Block, South China, decrease in abundance from shelf to basin facies, while their high positive S isotope values argued the development of oceanic euxinia, which required ample supply of organic matter, presumably reflecting the recovery of marine productivity. However, pyrite contents and pyrite S isotopes alone cannot uniquely constrain the extent and spatial distribution of surface ocean organic matter production. In this study, we conducted in situ analyses of pyrite Fe (δ⁵⁶Fepy) and S isotopes (δ³⁴Spy) of pyrite concretions collected from four sections of the Nantuo Formation, spanning from shelf to basin environments. Pyrite from different facies has distinct δ³⁴Spy and δ⁵⁶Fepy values. In general, δ³⁴Spy values display an increasing trend from shelf to basin, whereas δ⁵⁶Fepy values demonstrate an opposite trend with the lowest value in the basin sections. The opposite δ³⁴Spy and δ⁵⁶Fepy shelf-basin gradients strongly argue against the hydrothermal origin, yet verify the diagenetic precipitation of Nantuo pyrite in sediment porewater. Numerical models were developed to quantify the microbial sulfate reduction (MSR), microbial iron reduction (MIR) and pyrite precipitation processes. The modeling results provide an explanation for the shelf-basin decrease of δ⁵⁶Fepy values, indicating that both the fraction of MIR (fMIR) and the fraction of Fe²⁺ consumed by pyrite precipitation (fpy) increase from shelf to basin sections. In addition, high δ³⁴Spy values and pyrite contents in the basin sections also imply more intense MSR and higher supplies of H2S from sulfidic seawater. Both MSR and MIR were fueled by organic matter, thus suggesting that the surface ocean primary productivity displayed an unusual increasing trend toward the open ocean, which was different from the higher productivity in modern near-shore regions. We speculate that the reversed shelf-basin gradient of surface ocean primary productivity might be attributed to high P concentration in the post-glacial ocean. Terrestrial riverine P supply, on the contrary, might have diluted seawater P concentration in the near-shore regions. Therefore, our result indicates that the recovery of marine productivity in the aftermath of the Marinoan global glaciation may be not controlled by the availability of nutrients, instead, the immediate recovery of productivity might have been prohibited by, e.g., low seawater pH at a high atmospheric pCO2 level in the initial melting of the Marinoan global glaciation. The marine primary productivity did not recover until a substantive rise of seawater pH, during which deglacial intense continental weathering delivered abundant P into the ocean and accordingly elevated the seawater P concentration.
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The processes that formed the ultra-large and high-grade Kaiyang phosphate deposit, which is located in Guizhou Province, China, remain unclear. Here, we present the results of a petrological and mineralogical investigation of the compact and white lamination-bearing phosphorite within the deposit. These white lamination-bearing phosphorites contain many silicified collophane breccias that are surrounded by collophane cements. Electron probe microanalysis indicates that the phosphorite within the deposit is enriched in P2O5, SiO2, K2O, and Al2O3 and is depleted in FeO, F, MgO, SO3, and Na2O compared with the more common clastic phosphorite rocks. In addition, the coated-grain phosphorite located beneath this layer contains a cement that was gravitationally transported and was formed by leaching. These new data indicate that the deposit represents a crust formed as a result of palaeo-weathering. The widespread presence of this type of phosphatic rock in the Kaiyang area indicates that palaeo-weathering played a key role in the formation of the phosphorite deposits in this region.
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Ultra‐deep Cambrian System in the Tarim Basin is an important field for petroleum exploration, while fine division of the Cambrian strata is still controversial. In recent years, carbon isotope stratigraphy of the Cambrian System has been established and widely used. Here we report an integrated profile of carbonate and organic carbon isotopic values (δ13Ccarb and δ13Corg) from cuttings of the Tadong2 Well in the eastern Tarim Basin. Three carbon isotope anomalies of BACE, ROECE and SPICE were recognized on δ13Ccarb profile. Three apogees and a nadir on δ13Corg profile and the onset of ROECE on δ13Ccarb profile were suggested as boundaries of present four series of the Cambrian System. Suggested boundaries are easily identifiable on gamma logging profile and consistent with previous division scheme based on biostratigraphy evidence in outcrop sections. Abnormal carbon cycle perturbations and organic carbon burials during the BACE and SPICE events might be related to the reduction and expansion of a huge dissolved organic carbon reservoir in deep ocean of the ancient Tarim Basin.
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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.
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Four carbonate carbon isotope (δ13Ccarb) excursions are recognized in the Ediacaran Doushantuo Formation in South China, the genesis of which remains disputed. Whereas three of these δ13Ccarb excursions possibly record secular biogeochemical variations, the other one, namely Weng’an negative carbonate carbon isotope excursion (WANCE) with an age of ca. 620 Ma occurs mainly within the northern Yangtze Platform. In this study, a SIMS U-Pb age of ca. 620 Ma was documented from continental rift volcanism within the adjacent South Qinling terrane. Its temporal overlap with WANCE suggests a possible causal link. Volcanism-induced seafloor uplift may have prompted DOC oxidation in surficial oxygenated oceans, inducing the occurrence of WANCE.
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Silica‐phosphatic nodules are abundant in black shales of the Yanjiahe Formation in Three‐Gorges Area of South China, which is correlated to the Fortunian Stage, Terreneuvian Series, Cambrian System. The nodules are rich in small shelly fossils, and hence attract attentions of numerous paleontologists and sedimentary geologists. However, the genesis of the nodules and the preservation of small shelly fossils are poorly understood. Here we analyze morphological, structural, mineralogical, and chemical features of the nodules in multiscale using a combination of micro‐X‐ray fluorescence spectroscopy, X‐ray diffraction, scanning electron microscopy and confocal laser Raman spectrometer. Results reveal that nodules are concentric in chemical and mineralogical compositions, comprising a silica‐phosphatic core encrusted sequentially by a phosphatic zone, a siliceous zone, and a very thin pyrite outer rim. The black shales hosting the nodules demonstrate a laminated texture of alternating clayey and silty laminae, which were respectively deposited in sulfidic/less sulfidic, high/low production, intense/weak chemical weathering conditions. The phosphogenesis of the nodules resulted in the phosphatization of small shelly fossils, which prevented the fossils from being dissolved during diagenesis, whereas the silica encrustation sealed the fossils within the nodules and thus protected the fossils from alterations of deep burial and surface weathering.
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The Ediacaran Period (ca. 635–538 Ma) was a critical interval in Earth’s history, bridging the Proterozoic and the Phanerozoic Earth systems. It witnessed the diversification of eukaryotic phytoplanktons in the early stage, the evolution of macroscopic multicellular lives in the late stage, and frequent oscillations of carbonate carbon isotopes of large magnitudes, presumably reflecting large perturbations of the global carbon cycle. It has been proposed that a large dissolved organic carbon (DOC) pool, hundreds to thousands times larger than the present one, was developed in the Ediacaran deep ocean. The wax and wane of the marine DOC pool might be responsible for the secular variation of carbon isotopes of marine dissolved inorganic carbon (DIC) pool, which was recorded in the carbonate carbon isotope chemostratigraphy. However, neither the extent nor the evolutionary history of marine DOC pool in the Ediacaran ocean has been well constrained. In this study, we use Germanium/silica (Ge/Si) ratios of chert nodules from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, to trace the evolution of marine DOC pool. These chert nodules yield abundant microfossils, i.e. acanthomorphic acritarchs, and were precipitated in the early diagenetic replacement of calcareous silty/muddy sediments. Most chert nodules show positive correlations between Al contents and Ge/Si ratios, which can be resolved by a binary mixing between a silica (quartz) endmember and an Al-rich silicate (clay) endmember. On the one hand, the Ge/Si ratios of silica component ((Ge/Si)SiO2) are comparable to the modern seawater composition, suggesting the normal seawater rather than hydrothermal fluids was the major source of Si. On the other, the Ge/Si ratios of the clay component ((Ge/Si)clay) are one to two orders of magnitudes higher than that of normal marine clays. The Ge-enrichment of the clay component could be attributed to the extensive chelation of Ge by organic ligands and massive formation of organic-Ge-Clay complex at high DOC concentration. This observation supports the presence of a large DOC pool in the Ediacaran ocean. In addition, there is an increase of (Ge/Si)clay from the Member II to Member III of the Doushantuo Formation, suggesting an increase of DOC concentration. The expansion of marine DOC pool is coincident with the reduction of the sizes of the upper acanthomorphic acritarch assemblage in the Member III. We suggest that the increase of marine DOC pool might be attributed to the reduction of sizes of particulate organic matter, which favored DOC generation and accumulation in the Ediacaran seawater.
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Oceanic redox changes may be a key environmental trigger for the “Cambrian explosion”. However, a hypothesized predominantly oxygenated early Cambrian ocean, inferred from high sedimentary molybdenum (Mo) isotope ratios, conflicts with other evidence for euxinic continental margins. Here, we demonstrate that seawater Mo isotope ratios like those of today’s predominantly well-oxygenated oceans can also occur in less oxygenated early Cambrian oceans. New and existing Mo isotope data from early Cambrian black shales in South China span a range of greater than 3‰, which can only be explained by multi-stage fractionation of Mo isotopes. The lowest Mo isotopes ratios likely reflect a Fe-Mn (oxyhydr)oxide shuttle that preferentially transferred the lighter Mo isotopes into weakly euxinic bottom waters where Fe-Mn (oxyhydr)oxide dissolution released isotopically light Mo that was then scavenged by organic matters and sulfide minerals. We use a novel isotope mass balance model to show that Fe-Mn (oxyhydr)oxide shuttle operating on euxinic continental margins would preferentially bury lighter-mass Mo isotopes to such an extent that early Cambrian seawater can become enriched in heavier Mo isotopes at levels comparable to modern seawater. Hence, widespread ocean oxygenation may not have occurred during the early Cambrian as previously thought. Based on this paradigm shift, reconsideration of sedimentary Mo isotope temporal trends suggests a stepwise and protracted oxidation of the oceans until the Late Devonian. Sporadic intervals with Mo isotope ratios spanning over 3‰ and often accompanied by dramatic change in biodiversity (e.g., Ediacaran-Cambrian and Permian-Triassic transitions) probably had euxinic continental margins. Links between fossil records and marine redox structures disclose the importance of spatiotemporal fluctuations in continental margin euxinia on biodiversity changes.
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The Ediacaran–Cambrian (E–C) transition was a key interval in geological history. Chert was widely distributed in the Yangtze Block at the end of the Ediacaran and may have recorded important information about the coevolution of earth systems in this period. Five lithofacies of chert were identified in the northern margin of the Yangtze Platform. Breccias with crustiform chert were formed by hydrothermal fracturing of dolomite to form breccias in vents, which were immediately immersed and crusted by a silicon-rich fluid. This silicon-rich fluid was deposited directly onto the inner walls of the conduits to form chert linings. The silicon-rich fluid filled the water-fractured fissures to form spider-veinlet chert and then mixed with residual seawater in primary pores of the bedding to form void-filling cherts. Finally, the dolomite far away from the conduits was replaced by the silicon-rich fluid, forming chert bands. In the vents, there are typical hydrothermal mineral assemblages of quartz, pyrite, barite, Fe–Mn metal microparticles, and saddle dolomite and these assemblages contain abnormally high-temperature inclusions. The rare-earth element (REE-Y) patterns of void-filling chert, crustiform chert, and saddle dolomite show that they originated from normal seawater influenced by a low-temperature hydrothermal fluid, silicon-rich hydrothermal fluid, and high-temperature hydrothermal fluid, respectively. The trace element ratios and Si isotope contents of chert also show that deposition was mainly caused by hydrothermal solutions. All these data indicate that the various types of cherts at the top of the Dengying Formation near the synsedimentary fault zone constitute typical siliceous sinter. There was massive upward migration of silicon-rich hydrothermal fluids from depth to the seafloor during the E–C transition, which resulted in abnormally silicon-rich seawater and a widespread distribution of various cherts in the Yangtze Block. The identification of siliceous sinter in synsedimentary faults indicates an intense extension of the northern margin or even of the entire Yangtze Block during the E–C transition. This may be related to the combined effects of slab pull on the western side and rollback on the northeastern side of the Yangtze Block during the convergence of Gondwana.
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The Ediacaran-Cambrian (E-C) succession in South China records remarkable oceanic, biological and geochemical variations, but it was not well defined geochronologically, which hinders the interpretation of the spatio-temporal seawater chemical architecture during the time E-C interval. This study presents two Re-Os isochron ages of 520.2 ± 6.1 and 561.7 ± 8.5 Ma for the barite-rich black shales from the top Liuchapo and Doushantuo formations respectively in Tianzhu County, Guizhou Province. In combination with existing age data, the two new Re-Os isochron ages suggest that the Liuchapo Formation was deposited between 550 and 520 Ma. Moreover, like the polymetallic Ni-Mo-PGE layers of shelf margin (or platform) facies and V-rich horizons of transitional (or shelf slope) to deep-water facies, the barite deposits were likely formed due to differential mineralization. The timing offset likely resulted from differential elemental concentration related to certain local factors (i.e., hydrothermal fluids, seawater redox and biological activity). The isochron-derived initial 187Os/188Os ratios of the top Liuchapo Formation (0.902 ± 0.048) and the Doushantuo Formation (0.740 ± 0.042) fall within the range of continental weathering flux (1.54) and oceanic crust (0.126), implying the involvement of marine hydrothermal fluids. Moreover, their difference of initial 187Os/188Os ratios may reflect variations of continental weathering intensity and uplift magnitude.
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Barite in basal Ediacaran cap carbonates in South China shows distinct, non-mass-dependent depletion in O-17, a signal most likely reflecting an extremely high pCO(2) atmosphere during Marinoan glacial meltdown. The precise geological context of the barite within the cap carbonate was not defined, however, and such information is crucial to an accurate interpretation of the anomalous O-17 signal. Based on an extensive field survey of Marinoan cap carbonates in South China and detailed sedimentological, petrographic, and isotope data, we propose here a unified sequence of events that followed the deposition of the Nantuo diamictite: the cap dolostone was first deposited and later uplifted due to isostatic rebound, undergoing karstic dissolution in both shallow platform and transitional facies of the Yangtze Block; subsequent transgression initiated a consistent sequence of mineral deposition on karstic surfaces or within cavities. The O-17-depleted barite, either visible on outcrops or identifiable in thin sections, is among the first minerals precipitated when the transgression flooded the karstified cap dolostone. The calcite with extremely negative delta C-13 values occurs at the last stage of the cavity filling, well after the deposition of cap dolostone and the O-17-depleted barite, suggesting that the signal of methane hydrate was registered much later in the cap dolostone. A similar recognition of karstic dissolution and subsequent barite deposits in cap dolostones in northwest Africa and northwest Canada suggest that deposition of the O-17-depleted barites may be a global event, recording an atmospheric-biological-hydrological condition shortly (within 0-1.6 m.y.) after the initial meltdown of the Marinoan snowball Earth.
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Much of the controversy in the microbial vs. non-microbial phosphorite debate results from a lack of rigor in defining the specific levels at which microbial activity intervenes in phosphogenesis. A clear distinction between the processes that control the sedimentary transformations of particulate phosphorus fractions and the processes that directly participate in the apatite precipitation reaction seems to be necessary in order to reconcile the conflicting views. Microbial processes are the primary driving force behind the transformation of deposited particulate phosphorus into dissolved phosphate, a critical first step in the formation of apatite. The precipitation of apatite itself, however, is widely controlled by kinetic factors. Depending on the initial degree of supersaturation of pore solutions, different mechanisms of the precipitation reaction may prevail in sediments. In most continental margin sediments, low to moderate concentrations of dissolved phosphate may result in a direct nucleation of apatite crystals, leading to a slow formation of dispersed apatite in the sediment column. In phosphogenic sediments, high localized rates of dissolved phosphate generation promote the fast nucleation of a metastable precursor of apatite. This creates a large number of crystallization sites in the sediment, and results in the rapid formation of phosphatic bodies. The transitory fixation and release of phosphate by microbial communities may be responsible, in part, for the elevated concentrations of dissolved pore phosphate observed in close proximity to the water-sediment interface at sites of present-day phosphorite formation (e. g. sediments on the Peru-Chile shelf). Localized pulses of high supersaturation enhance the likelihood of rapid apatite deposition close to the water-sediment interface. Microbial redox processes may further help focusing the rapid precipitation of apatite in the interface environment by telescoping chemical gradients and associated pH shifts in the surface sediment layer. There is no convincing evidence of other important microbial effects being directly involved in the formation of apatite in marine sediments, for instance intra-cellular precipitation of calcium phosphate. A close association of benthic microbial activity and the formation of apatite can be widely traced in ancient phosphogenic environments. The organic matter of fossil phosphorites preserves a clear molecular signature of the intense degradation of sedimentary organic matter at the seafloor, and points to liberation of phosphorus from organic compounds as a predominant microbial process involved in apatite formation. Ultrastructure analysis of microbial fabrics preserved in phosphorites permits the identification of abundant and di¬ verse benthic microbial communities. Extracellular precipitation of apatite was a common mechanism of microbiota preservation, although no direct cellular control of apatite precipitation can be inferred. The development of localized phosphatic bodies in many ancient phosphorites is likely to reflect the spatial association between the activity of individualized benthic microbial communities and the precipitation of apatite. Stromatolitic phosphorites provide an extreme example of such an association.
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Phosphatic sediments form not only beneath upwelling systems, but also on oceanic islands, atop seamounts, guyots and plateaus, and on some shelves, slopes and in epeiric environments that may not have experienced upwelling at all. We evaluate the formation of phosphorites and the sources of phosphorus in each of these settings. We define primary phosphogenesis, which produces primary phosphorite, as the process of carbonate fluorapatite precipitation within sediments, at the sediment/water interface and/or during diagenesis. Primary phosphogenesis is distinct from the hydraulic and biological reworking processes by which phosphatic grains are concentrated into granular phosphorite. Non-transported phosphorites may occur in any or all of the principal phosphorite settings. Granular phosphorites, which we believe are derived from primary phosphorites, are accumulations which developed on ancient shelves and in epeiric seas and appear to dominate phosphorite occurrences in the Phanerozoic record. These are variously cemented phosphorite siltstones, sandstones and conglomerates composed chiefly of phosphatic particles, many of which show some evidence of transport and deposition under the influence of bottom currents. Current activity plays an important role for many phosphorite deposits, not only for concentrating dispersed authigenic precipitates, but also for prolonging phosphate precipitation at critical shallow burial levels. In some cases, redeposition as phosphorite turbidites and tempestites has also been important. We summarize 13 important topics which require future research.
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This research illustrates application of a fluid-inclusion technique for quantifying water depth of ancient carbonate platforms. Jurassic limestones of Monte Kumeta, Italy, were cemented with submarine calcite during a transition to carbonate platform termination. The calcite cements contain fluid inclusions consisting of Jurassic seawater and immiscible gas bubbles trapped during the growth and penecontemporaneous recrystallization of the cements. Crushing analysis indicates that gas bubbles are under pressures indicative of entrapment in water depths of 23 112 m. Assuming simple deepening and acknowledging chronostratigraphic errors, rates of relative rise in sea level were initially less than 7 m/m.y. followed by a rate of at least 33 m/m.y. These slow rates are evidence that the platform's demise was caused by an environmental perturbation other than rapid sea-level rise. The facies transitions and regional studies indicate that the perturbation resulted from nutrient excess or eutrophication in shallow water followed by deepening into ephemeral dysoxic waters at depths perhaps as shallow as 23 m.
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Glacial deposits of Sturtian and Marinoan age occur in the well-studied Neoproterozoic successions of northern Namibia, South Australia, and northwestern Canada. In all three regions, the Marinoan glaciation is presaged by a large negative δ13C anomaly, and the cap carbonates to both glacial units share a suite of unique sedimentological, stratigraphic, and geochemical features. These global chronostratigraphic markers are the bases of a new correlation scheme for the Neoproterozoic that corroborates radiometric data that indicate that there were three glacial epochs between ca. 750 and 580 Ma. Intraregional correlation of Neoproterozoic successions in the present-day North Atlantic region suggests that glacial diamictite pairs in the Polarisbreen Group in northeastern Svalbard and the Tillite Group in eastern Greenland were deposited during the Marinoan glaciation, whereas the younger of a pair of glacials (Mortensnes Formation) in the Vestertana Group of northern Norway was deposited during the third (Gaskiers) Neoproterozoic glaciation. Gaskiers-aged glacial deposits are neither globally distributed nor overlain by a widespread cap carbonate but are associated with an extremely negative δ13C anomaly. The chronology developed here provides the framework for a new, high-resolution model carbon-isotope record for the Neoproterozoic comprising new δ13C (carbonate) data from Svalbard (Akademikerbreen Group) and Namibia (Otavi Group) and data in the literature from Svalbard, Namibia, and Oman. A new U-Pb zircon age of 760 ± 1 Ma from an ash bed in the Ombombo Subgroup in Namibia provides the oldest direct time-calibration point in the compilation, but the time scale of this preliminary δ13C record remains poorly constrained.
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Abstract Sedimentary rocks deposited during the Ediacaran period (∼630–542 Ma) contain carbonates whose carbon isotopic ratios show a marked negative excursion consisting of a precipitous drop from +5‰ to −12‰, followed by a sub-linear recovery to positive δ13C values. Isotopic ages (U/Pb) and thermal subsidence modelling are combined to constrain the excursion in time and indicate an onset at ∼600 Ma, and duration of recovery of approximately 50 Myr. The excursion is widely recognized in Oman and has potential correlatives in Ediacaran strata elsewhere, and may thus represent a characteristic feature of the Ediacaran period. The amplitude of this carbon isotope excursion far exceeds those of other Neoproterozoic anomalies. The isotopic trend of negative excursion and long-term recovery spanned at least one short-lived glacial episode (at 580 Ma), but appears unrelated to glaciation, which indicates that negative anomalies in the Neoproterozoic marine carbon isotope record are not directly or uniquely linked to ice ages. Terra Nova, 18, 147–153, 2006
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A sample from the top of the Banxi Group (early Neoproterozoic) in Hunan Province (South China) was dated by the SHRIMP zircon U-Pb method. The resulting age of 725 ± 10 Ma probably solves the debate between the synglacial and the preglacial correlation of the Liantuo Formation and indicates that the Liantuo Formation most likely correlates with the preglacial Banxi Group. Since the Banxi (Danzhou) Group is upwardly transitional to the glacial Jiangkou Group, the age becomes a new maximum age constraint on the onset of the Jiangkou glaciation, which therefore most likely correlates with the Sturtian glaciation.
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A possible global drop in marine carbon isotope values to as low as -12 parts per thousand Peedee belemnite (PDB), recorded in the Ediacaran Shurarn Formation of Oman, has been attributed to the non-steady-state oxidation of oceanic dissolved organic carbon (DOC) resulting from the rise in atmospheric oxygen to near modern values at the end of the Precambrian. Geologic constraints indicate that the excursion lasted between 25 and 50 m.y., requiring a DOC pool thousands of times to 10,000 times the modern inventory to conform with carbon isotope mass balance calculations for a -12 parts per thousand excursion. At the consequent rates of DOC oxidation, oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of similar to 800 k.y. Oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the Shuram excursion. Furthermore, a DOC-driven excursion does not explain strong covariation between the carbon and oxygen isotope record. These indicators show that negative isotope excursions recorded in the Shuram and other Ediacaran sections are unlikely to represent a global ocean signal.
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Two distinctive volcanic ash beds were found in the terminal Proterozoic Doushantuo Formation in south China. The lower ash bed, ˜2.5 m above the cap carbonate at the base of the Doushantuo, yields a U-Pb zircon age of 621 ± 7 Ma, providing the closest upper limit for a correlative of the Marinoan glaciation. The upper ash bed, near the Doushantuo-Dengying boundary, yields a U-Pb zircon age of 555.2 ± 6.1 Ma, providing for the first time a direct age determination for a prominent negative delta13C excursion (
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There is widespread interest in the Neoproterozoic period of the Earth's history (1000 to 542 Ma) because of unprecedented δ13C fluctuations to < − 10‰ PDB through thick (> 1000 m) succession of stratigraphically complex sedimentary rocks deposited during tens of millions of years. In contrast, Phanerozoic large negative C-isotope excursions have been interpreted as the result of diagenetic fluid mixing during carbonate stabilization and burial and are less enigmatic due to the excellent biostratigraphic control on their timing and duration.
Chapter
Sequence boundaries were originally defined as bounding surfaces of conformably stratified units. Within this descriptive definition, two types of boundaries were specifically recognized and widely used—the boundary between highstand and lowstand systems tracts and that between highstand systems tract and shelf-margin wedge. A third type of boundary, the flooding surface between a highstand tract and an overlying transgressive tract, is increasingly used as a sequence boundary, particularly in carbonates. It should be formally recognized as a specific type of sequence boundary because it is prone to developing long submarine hiatuses, occurs commonly in the geologic record, particularly on drowned carbonate platforms, tends to be a pronounced reflector and unconformity in seismic images, and represents a logical third combination in the interplay of subsidence and eustasy—one in which the rate of subsidence exceeds even the most rapid fall of sea level during a particular eustatic cycle. Including type 3 unconformities as sequence boundaries has two advantages: (1) the general definition of sequence boundary remains descriptive and thus workable even where the boundary must be drawn solely on geometric grounds without supporting evidence for sea-level fall and exposure, and (2) the designation of boundary types 1 and 2 can be restricted to surfaces with reliable evidence for relative fall in sea level. On drowned reefs and carbonate platforms, the hiatuses associated with type 3 unconformities frequently exceed 10 m.y., occasionally even 100 m.y. Relicts of pelagic cover in hardgrounds, along with marine diagenesis, attest to the submarine, erosional nature of these hiatuses. On rapidly flooded siliciclastic shelves, the effects are similar but less pronounced. The mechanism behind this persistent and powerful marine erosion is amplification of sluggish oceanic tides by the interaction with sharp topography. This process continues and expands the effects of marine shoreface erosion at greater water depth.
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Much of the controversy in the microbial vs. non-microbial phosphorite debate results from a lack of rigor in defining the specific levels at which microbial activity intervenes in phosphogenesis. A clear distinction between the processes that control the sedimentary transformation of particulate phosphorus fractions and the processes that directly participate in the apatite precipitation reaction seems to be necessary in order to reconcile the conflicting views. The development of localised phosphatic bodies in many ancient phosphorites is likely to reflect the spatial association between the activity of individualised benthic microbial communities and the precipitation of apatite. Stromatolitic phosphorites provide an extreme example of such an association. -from Authors