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Volume 31 Issue 2
Apr.  2020
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Yi Ding, Daizhao Chen, Xiqiang Zhou, Taiyu Huang, Chuan Guo, Rumana Yeasmin. Paired δ13Ccarb-δ13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle. Journal of Earth Science, 2020, 31(2): 342-353. doi: 10.1007/s12583-018-0886-1
Citation: Yi Ding, Daizhao Chen, Xiqiang Zhou, Taiyu Huang, Chuan Guo, Rumana Yeasmin. Paired δ13Ccarb13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle. Journal of Earth Science, 2020, 31(2): 342-353. doi: 10.1007/s12583-018-0886-1

Paired δ13Ccarb13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle

doi: 10.1007/s12583-018-0886-1
Funds:

the National Natural Science Foundation of China 41472089

the National Natural Science Foundation of China U1663209

More Information
  • This study provides δ13C profiles from a lower-slope (Well ZK102) to basin (Bahuang Section) environment to better understand the temporal and spatial variability in δ13Ccarb13Corg of the Yangtze Block during the Late Ediacaran. Our new δ13C profiles together with the reported data suggest that the Upper Ediacaran successions from different depositional environments are generally bounded by negative δ13Ccarb and/or δ13Corg excursions in the underlying and overlying strata. Moreover, the Upper Ediacaran δ13Ccarb profiles generally can be subdivided into two positive excursions and an interjacent negative excursion, whereas the paired δ13Corg profiles from different depositional environments have individual variation trends. On the other hand, these data show a large surface-to-deep water δ13C gradient (~5‰ variation in δ13Ccarb, >10‰ variation in δ13Corg) which can be reasonably explained by the heterogeneity of the biological activities in the redox-stratified water column. Furthermore, the decoupled δ13Ccarb13Corg pattern with large δ13Corg perturbations at the lower slope precluded the existence of a large dissolved organic carbon reservoir at the Yangtze Block during the Late Ediacaran. Thus, the high d13Ccarb values in the Upper Ediacaran succession could be balanced by large amounts of buried organic carbon likely associated with high productivity.
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Paired δ13Ccarb13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle

doi: 10.1007/s12583-018-0886-1
Funds:

the National Natural Science Foundation of China 41472089

the National Natural Science Foundation of China U1663209

Abstract: This study provides δ13C profiles from a lower-slope (Well ZK102) to basin (Bahuang Section) environment to better understand the temporal and spatial variability in δ13Ccarb13Corg of the Yangtze Block during the Late Ediacaran. Our new δ13C profiles together with the reported data suggest that the Upper Ediacaran successions from different depositional environments are generally bounded by negative δ13Ccarb and/or δ13Corg excursions in the underlying and overlying strata. Moreover, the Upper Ediacaran δ13Ccarb profiles generally can be subdivided into two positive excursions and an interjacent negative excursion, whereas the paired δ13Corg profiles from different depositional environments have individual variation trends. On the other hand, these data show a large surface-to-deep water δ13C gradient (~5‰ variation in δ13Ccarb, >10‰ variation in δ13Corg) which can be reasonably explained by the heterogeneity of the biological activities in the redox-stratified water column. Furthermore, the decoupled δ13Ccarb13Corg pattern with large δ13Corg perturbations at the lower slope precluded the existence of a large dissolved organic carbon reservoir at the Yangtze Block during the Late Ediacaran. Thus, the high d13Ccarb values in the Upper Ediacaran succession could be balanced by large amounts of buried organic carbon likely associated with high productivity.

Yi Ding, Daizhao Chen, Xiqiang Zhou, Taiyu Huang, Chuan Guo, Rumana Yeasmin. Paired δ13Ccarb-δ13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle. Journal of Earth Science, 2020, 31(2): 342-353. doi: 10.1007/s12583-018-0886-1
Citation: Yi Ding, Daizhao Chen, Xiqiang Zhou, Taiyu Huang, Chuan Guo, Rumana Yeasmin. Paired δ13Ccarb13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle. Journal of Earth Science, 2020, 31(2): 342-353. doi: 10.1007/s12583-018-0886-1
  • The Ediacaran-Cambrian (E-C) transition is a critical period in geological history, as is recorded drastic changes in the chemical compositions of the atmosphere and ocean (Cui et al., 2016; Chang et al., 2012; Shen et al., 2010; McFadden et al., 2008; Fike et al., 2006), along with the sudden appearance and rapid evolution of metazoans (Mason et al., 2017; Zhang and Cui, 2016; Landing et al., 2013; Steiner et al., 2007). To decipher the co-evolutionary process of environment and life, extensive δ13C stratigraphic and paleoceanographic studies have been focused on the E-C (~635–521 Ma) succession of the Yangtze Block. However, previous works have mainly focused on the Lower Ediacaran Doushantuo Formation (~635–551 Ma) (Wang et al., 2016; Lu et al., 2013; Jiang et al., 2010, 2007; McFadden et al., 2008; Zhou and Xiao, 2007; Zhu et al., 2007) and Lower Cambrian strata (~542–521 Ma) (Guo et al., 2013, 2007; Ishikawa et al., 2013, 2008; Li et al., 2013; Jiang et al., 2012; Wang J G et al., 2012; Chen et al., 2009). As a critical transitional succession (~551-542 Ma), the Upper Ediacaran Dengying (coeval Liuchapo) Formation has not been thoroughly studied. Because of the widespread deposition of siliceous rock in the slope-basin setting (Fig. 1), previous studies of this succession have focused on either the δ13Ccarb13Corg (mainly δ13Ccarb) of the shallow-water carbonate strata (Cui et al., 2016; Wang et al., 2014; Kunimitsu et al., 2011; Zhou and Xiao, 2007; Zhu et al., 2007) or the δ13Corg of the deep- water chert strata (Guo et al., 2013, 2007; Wang J G et al., 2012). In this case, the spatial variation and chemostratigraphic correlation of δ13Ccarb13Corg across the platform-to-basin transect remain poorly constrained, preventing a thorough understanding of the carbon cycle and the paleoceanographic evolution of the Yangtze Block during the Late Ediacaran.

    Figure 1.  Paleogeographic map of South China during the Late Ediacaran (modified from Liu et al., 2016; Chen et al., 2015; Liu and Xu, 1994) and locations of the relevant sections. 1. Meiziwan, Meitan County, Guizhou Province; 2. Well ZK102, Songtao County, Guizhou Province; 3. Bahuang, Tongren City, Guizhou Province; 4. Longbizui, Guzhang County, Hunan Province; 5. Siduping, Zhangjiajie City, Hunan Province; 6. Tianping, Zhangjiajie City; 7. Zhangjiaxi, Zhangjiajie City; 8. Ganziping, Zhangjiajie City; 9. Yangjiaping, Shimen County, Hunan Province; 10. Jiulongwan, Zigui County, Hubei Province

    In this study, the Dengying Formation and coeval Liuchapo Formation from Well ZK102 and the Bahuang Section in the northeastern Guizhou Province were selected to establish the lithostratigraphic and chemostratigraphic (δ13Ccarb13Corg) frame- work of the slope-to-basin setting. Particularly in combination with the reported data, the temporal and spatial variability of δ13Ccarb13Corg was further reconstructed, in favor of a refined stratigraphic correlation of the shallow-water strata with the deep-water equivalents and a better understanding of paleoceanographic changes of the Yangtze Block during this time.

  • Following the Nantuo (Marinoan) Glaciation, it has been proposed that a rimmed carbonate platform developed on the passive margin of the Yangtze Block during the Early Ediacaran (Jiang et al., 2011). In this context, the Lower Ediacaran Doushantuo Formation consists mainly of carbonate in the shallow-water setting and alternating carbonate-black shale in the deep-water setting (Jiang et al., 2011; Zhu et al., 2007). Its upper boundary has been approximately constrained by a U-Pb zircon age of 551.1±0.7 Ma from the top of the Doushantuo Formation in the Three Gorges (Fig. 2; Condon et al., 2005).

    Figure 2.  Simplified lithostratigraphic framework of the Ediacaran–Lower Cambrian across the platform-to-basin transect

    Following this, particularly from the terminal Ediacaran to the Earliest Cambrian, the Yangtze Block experienced intense extension, leading to the fragmentation of the carbonate platform (Liu et al., 2016) and silica-rich hydrothermal activities along the platform margin (Chen et al., 2009). Under this condition, the Yangtze Block evolved into three detached carbonate platforms respectively in the Upper, Middle and Lower Yangtze areas, with the formation of intraplatform basins on (or between) these carbonate platforms during the E-C transition. Meanwhile, the surrounding slope-basin environment was covered by siliceous deposits (Fig. 1; Liu et al., 2016; Chen et al., 2015; Liu and Xu, 1994). On the carbonate platforms, the Upper Ediacaran Deng- ying Formation overlying the Doushantuo Formation is composed mainly of light grey dolomite which wedges out and shifts basinwards into the bedded chert of the coeval Liuchapo Formation (Fig. 2).

    During the terminal Ediacaran, the carbonate platform was subjected to platform-wide subaerial exposure, producing varying degrees of stratigraphic hiatus (Figs. 1, 2; Zhu et al., 2007, 2003). The overlying Yanjiahe Formation (and its correlative strata), composed of muddy limestone with black shale, chert and phosphorite interbeds, commonly developed in the intraplatform basins (Figs. 1, 2; Liu et al., 2016; Zhu et al., 2003). The presence of a prominent negative δ13Ccarb excursion and small shelly fossils (SSF) at the base of the Yanjiahe Formation constrains the Dengying/Yanjiahe boundary in approximate accordance with the E-C boundary (Landing et al., 2013; Jiang et al., 2012; Steiner et al., 2007; Zhu et al., 2007, 2003). From the platform margin to the foreslope, the corresponding Lower Cambrian strata are the bedded chert of the Liuchapo Formation and an overlying unnamed dolomite unit (Chen et al., 2009). The E-C boundary at the platform margin is well constrained by a U-Pb age of 542.1±5.0 Ma calibrated with an apparent negative δ13Corg excursion at the basal Liuchapo Formation from the Ganziping Section (Fig. 2; Chen et al., 2015). Meanwhile, in the lower slope-basin environment, the E-C boundary is constrained by a U-Pb age of 542.6±3.7 Ma in the upper Liuchapo Formation of the Bahuang Section, accompanied by a lithofacies change from bedded chert to shaley bedded chert (Fig. 2; Chen et al., 2015). Along with the subsequent transgression, the black shale of the Niutitang Formation (and its correlative strata) quickly covered the antecedent carbonate platforms and basins at ~522 Ma (Fig. 2; Chen et al., 2015; Wang X Q et al., 2012).

  • A borehole and a well-exposed section across the slope-to- basin transect were measured and sampled at an average spacing of ~50 cm (~20 cm for critical intervals). Detailed field investigation and microfacies analysis of thin sections were conducted to establish a reliable lithostratigraphic and sedimentological framework for geochemical studies. Then, both the carbonate and organic carbon isotope analyses were completed at the Laboratory for Stable Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences. All isotope values are reported in per mil relative to the international Vienna Peedee Belemnite (V-PDB) standard value.

    For the δ13Ccarb and δ18O analyses, 37 samples from the Well ZK102 were initially cut to expose fresh surfaces. To avoid veins, clay-rich laminae and diagenetic fabrics, sample powders were obtained from the fresh surfaces via a micro-drill. Approximately 250 μg of sample powders was reacted with anhydrous phosphoric acid at 70 ℃ for 200 s to extract CO2 and was analyzed using a Finnigan MAT-253 mass spectrometer. The analytical precision was better than 0.2‰ and 0.15‰ for the carbon and oxygen isotopes, respectively.

    Seventy-two samples from the Well ZK102 and the Bahuang Section were selected for δ13Corg analyses. They were initially cut to remove the weathered surface and veins, and then crushed into powders. Afterwards, the prepared sample powders were treated with 4 M hydrochloric acid solution (HCl) for 24 h to remove carbonates (at least two times). The decarbonated samples were repeatedly washed using distilled water to remove the HCl and dried in a heating oven at 50 ℃. Total organic carbon (TOC) and organic carbon isotopes were analyzed using a LECO CS-400 analyzer and a Finnigan MAT-253 mass spectrometer, respectively. The analytical precision was better than 0.2‰ for δ13Corg.

  • Based on detailed sedimentological investigation, the two studied Upper Ediacaran successions in northeastern Guizhou Province were interpreted to have been deposited in a lower slope (Well ZK102), and basinal environment (Bahuang Section), respectively.

  • Well ZK102 is in Daotuo Village, Songtao County. The uppermost Doushantuo Formation in this well is composed of 2.83 m of black calcareous shale. Above the Doushantuo Formation, the Dengying Formation is only 8.17 m in thickness, consisting mainly of laminated muddy dolomite (Figs. 3a, 3b). In general, the middle-upper Dengying Formation has increased clay content, marked by the alternation of mm-scale light dolomite and dark clay-rich laminae. These horizontal laminae are locally contorted or distorted, likely as a result of soft sediment deformation (Fig. 3a). Above the Dengying Formation is the Liuchapo Formation, which comprises 2.36 m of bedded chert. The overlying succession is 1.83 m of unnamed cherty dolomite unit, commonly displayed as dm-thick couplets of mudstone and wackestone. This wackestone is composed of mud-supported fine-grained intraclasts with a scoured base and normal graded bedding (Fig. 3c). The Niutitang Formation overlying the cherty dolomite of the unnamed unit consists of black cherty/phosphatic shale at its base.

    Figure 3.  Representative photographs (a)–(e) and a photomicrograph (f) from studied sections. (a) Muddy dolomite showing horizontal lamination locally with synsedimentary deformation. Coin for scale (2 cm in diameter). (b) Photomicrograph of the laminated muddy dolomite (plane polarized light). (c) Dolowackestone with normal graded bedding. (d) Pure thin-bedded chert. Hammer for scale (28 cm). (e) Shaley bedded chert at the upper Liuchapo Formation, overlain by black shale of the Niutitang Formation. Marker pen for scale (14 cm). (f) Bedded chert composed of microcrystalline silica with dark clay-rich laminae and aggregates (cross polarized light). (a)–(c) Well ZK102; (d)–(f) Bahuang Section

    The muddy dolomite of the Dengying Formation is considered to represent a low-energy deep water environment dominated by suspended sedimentation. Furthermore, the soft-sediment deformation could be gravity-related, further suggesting a slope environment. Compared to proximal gravity deposits composed of dolobreccia from the Tianping Section (Chen et al., 2009) or the Meiziwan Section to the west of the Well ZK102, the couplets of dolomudstone and dolowackestone with a scoured base and normal graded bedding are interpreted as distal turbidites, representing a depositional environment far from the carbonate platform. These features together suggest a lower slope environment for Well ZK102 during the E-C transition, comparable to the Siduping Section reported by Chen et al. (2009).

  • The Bahuang Section is located in the Bahuang area, Tongren City. Above the black shale of the Doushantuo Formation, the Liuchapo Formation comprises a 13.12 m of bedded chert (Fig. 3d), and a 4.15 m of shaley bedded chert that is overlain by black phosphoritic shale of the basal Niutitang Formation (Fig. 3e). The bedded chert is composed mainly of cryptocrystalline to microcrystalline silica with minor dark clay-rich laminae and aggregates. It is generally thin-bedded without synsedimentary deformation or the influx of granules. These features are indicative of a deep-water setting with a flat topography unaffected by gravity flow, representing a basinal environment such as the Longbizui Section reported by Chen et al. (2009).

  • Combined with sections from the western Hunan Province described by Chen et al. (2009), the stratigraphic and lithofacies changes of the E-C boundary successions from the Upper Yangtze Platform via the Nanhua Basin to the Middle Yangtze Platform are summarized in Fig. 4. In general, the Liuchapo Formation is present as a stratal wedge that is thicker in the basinal environment and gradually pinches out towards both the Upper and Middle Yangtze platforms (Fig. 4). However, the spatial distribution of the chert wedge of the Liuchapo Formation is slightly different between the Upper and Middle Yangtze areas; it widely extends from the lower slope to the platform margin on the middle Yangtze side but merely occurs at the lower slope on the Upper Yangtze side (Fig. 4). Under such circumstances, lithostratigraphic units of the E-C boundary successions are highly diachronous among different sections across the platform-basin transect, and their temporal distribution is further constrained by carbon isotope chemostratigraphy as discussed in the following section.

    Figure 4.  Detailed stratigraphic and lithofacies changes of the E-C boundary successions from the Upper Yangtze Platform via the Nanhua Basin to the Middle Yangtze Platform

  • The results of δ13Ccarb18O and δ13Corg-TOC are shown in Fig. 5. The δ13Ccarb values of the Dengying Formation range from -3‰ to +2.2‰ and arrange into two positive excursions and two negative excursions. A positive excursion (P1) with δ13Ccarb values changing from -3‰ to +2.2‰ occurs in the lower Dengying Formation. P1 is followed by a transient negative δ13Ccarb excursion (N1) with a nadir as low as -0.3‰. The second positive excursion (P2) occurs in the middle and upper Dengying Formation, and is characterized by an upward increase in δ13Ccarb from -0.3‰ to a stable plateau of approximately +1‰. The uppermost Dengying Formation records the onset of a negative excursion (N2) with a rapid decrease in δ13Ccarb values from 0.7‰ to -1.9‰.

    Figure 5.  Profiles of δ13Ccarb, δ13Corg, Δδ13C, TOC across the E-C transition at the Well ZK102 and the corresponding cross-plots of δ13Ccarb18O, δ13Corg-TOC. Open symbols represent the samples altered by diagenesis severely

    The samples for the organic carbon isotope study cover the uppermost Doushanto Formation to the basal Liuchapo Formation, with δ13Corg values ranging between -33.5‰ and -23.5‰. Three positive excursions and three negative excursions are present in the studied succession. The Dengying Formation starts with a positive excursion with δ13Corg values increasing from -30.2‰ to -23.5‰ (P1-org) in the lower part, followed by a negative excursion to -30.8‰ (N1-org) in the middle part and then a broad positive excursion with δ13Corg values returning to a stable plateau of approximately -26‰ (P2-org) in the middle-upper part. The upper Dengying Formation is characterized by a minor negative δ13Corg excursion to -31.6‰ (N2-org) and a sharp positive excursion with δ13Corg values returning to -26.4‰ (P3-org) upwards. A pronounced negative δ13Corg excursion starts at the uppermost Dengying Formation, reaching minimum values as low as -33.5‰ at the base of the Liuchapo Formation (N3-org).

    The Dengying Formation generally exhibits a decoupled δ13Ccarb13Corg pattern, with highly variable Δδ13C (Δδ13C= δ13Ccarb−δ13Corg) values ranging from 24.1‰ to 32.4‰. Generally, the δ13Corg values display a wider range of variations relative to δ13Ccarb, and thus strong inverse correlation between Δδ13C and δ13Corg is present. Notably, although a general decoupled pattern is present, the δ13Ccarb and δ13Corg show an approximately co-varying positive shift in the lower Dengying Formation and negative shift in the uppermost Dengying Formation.

  • The results of δ13Corg-TOC are shown in Fig. 6. The δ13Corg profile of this interval (~18 m) covers a wide range from -38.8‰ to -26.7‰, comprising three positive excursions and three negative excursions. The lower-middle Liuchapo Formation records a positive excursion (P1-org) marked by a progressive increase in δ13Corg values from -38.8‰ to -32.9‰ with apparent oscillation. Subsequently, δ13Corg values decrease to a nadir of -36.1‰ in the middle Liuchapo Formation (N1-org), followed by a positive shift towards -32.4‰ (P2-org). A minor negative shift to -34.6‰ (N2-org) followed by a sharp positive excursion (P3-org) up to -26.7‰ is found at the middle-upper part of the Liuchapo Formation. An abrupt decrease in δ13Corg from -26.7‰ to -35.4‰ occurs in the upper Liuchapo Formation (N3-org), concurrent with the lithofacies change from pure bedded chert to shaley bedded chert.

    Figure 6.  Profiles of δ13Corg, TOC across the E-C transition at the Bahuang Section and the corresponding cross-plot of δ13Corg-TOC

  • The primary carbon and oxygen isotopic compositions of marine carbonate are prone to alteration by post-depositional diagenesis. Because of the higher O/C ratio in diagenetic fluids compared to that of carbonate and the temperature dependence of oxygen isotope fractionation during carbonate mineral precipitation, the oxygen isotopic composition of marine carbonate is sensitive to post-depositional alteration relative to its carbon isotopic composition (Jacobsen and Kaufman, 1999). Furthermore, diagenetic fluids (e.g., meteoric water) generally have low δ18O values compared to seawaters and the δ18O values of carbonate minerals are inversely correlated to their formation temperatures which tend to increase during burial (Jacobsen and Kaufman, 1999). In this case, marine carbonates with δ18O values greater than -5‰ have been widely used to reconstruct the secular δ13Ccarb changes (Shen et al., 2010; Ishikawa et al., 2008). The δ18O values from the Dengying Formation mostly fall into a range from -5‰ to +1.7‰ except for six samples (Fig. 5), suggesting that the majority of the δ13Ccarb values reflect changes in seawater composition. The remaining six samples with δ18O values less than -5‰ and the correlation between δ13Ccarb and δ18O values suggest that their carbon and oxygen isotopic compositions might have been altered during diagenetic processes (Fig. 5; Jiang et al., 2012, 2007; Kunimitsu et al., 2011; Shen et al., 2010; Ishikawa et al., 2008). Because the carbon isotopic composition of marine carbonate is relatively resistant to diagenetic alteration, the three samples with δ18O values from -5‰ to -10‰ should retain the acceptable δ13Ccarb values as previous studies have proposed (Jacobsen and Kaufman, 1999). The last three samples with quite low δ18O values of < -10‰ are interpreted to have been severely altered by diagenesis and are labelled in the δ13Ccarb curve. Notably, the deletion of these three samples would not change the variation trend of the δ13Ccarb curve (Fig. 5).

    The primary δ13Corg signature of marine organic matter is potentially altered by the diagenetic process, migrating hydrocarbons and an influx of detrital organic carbon (Ishikawa et al., 2013; Jiang et al., 2012; Johnston et al., 2012; Wang J G et al., 2012; Chen et al., 2009). Diagenesis (thermal maturation) producing 12C-enriched hydrocarbons is considered to result in higher δ13Corg values in the residual organic carbon (Wang et al., 2014; Jiang et al., 2012; Guo et al., 2007; Des Marais et al., 1992). Guo et al. (2007) reported H/C values > 0.2 in kerogens near our study area, suggesting that the alteration (increase) in δ13Corg values should be less than < 3‰. Furthermore, this process is mainly controlled by diagenetic temperature, which should have exerted the same influences on our samples from such a short stratigraphic interval (~10 m). In this scenario, the stratigraphic variations in δ13Corg reaching up to ~10‰ in both the Bahuang Section and Well ZK102 are unable to be explained only by thermal alteration. Our samples are composed mainly of microcrystalline dolomite or silica without distinct recrystallization, further supporting that they were subjected to limited thermal alteration (Figs. 3b, 3f). The low porosity of the samples precludes the contamination of migrating hydrocarbons (Figs. 3b, 3f). On the other hand, diagenesis removes hydrocarbons from primary marine organic matter and thus can amplify the signature of migrating hydrocarbons and/or detrital organic carbon. This mechanism could significantly influence the organic-poor samples compared to those that are organic-rich, leading to a strong correlation of δ13Corg values with TOC contents (Ishikawa et al., 2013; Jiang et al., 2012; Johnston et al., 2012; Wang J G et al., 2012; Chen et al., 2009). In this study, the correlation of δ13Corg values with TOC contents (or lithology) is not obvious (R2=0.115 for the Bahuang Section, R2=0.025 for Well ZK102), further indicating that the marine δ13Corg signature is generally preserved in our samples (Figs. 5, 6).

  • Based on carbon isotope analysis as well as the latest radiometric ages, the correlation of Well ZK102 and Bahuang Section with other sections from the Yangtze Platform was reconstructed as shown in Fig. 7. Previous δ13Ccarb profiles reported from the Yangtze Platform consistently exhibit a positive excursion in the lower Dengying Formation (Wang et al., 2016, 2014; Kunimitsu et al., 2011; Jiang et al., 2007; Zhou and Xiao, 2007; Zhu et al., 2007; Condon et al., 2005), which was summarized as Ediacaran positive excursion 3 (EP3) by Zhou and Xiao (2007). It is accepted as a reliable chemostratigraphic marker immediately following the termination of the Shuram negative excursion at ~551 Ma. On the other hand, a short-lived negative excursion has been widely reported at the base of the Yanjiahe Formation (or its correlatives) on the Yangtze Platform (Ishikawa et al., 2013, 2008; Li et al., 2013; Jiang et al., 2012; Zhou and Xiao, 2007; Zhu et al., 2007). This notable excursion has been defined as Ediacaran negative excursion 4 (EN4; Zhou and Xiao, 2007).

    Figure 7.  Correlation of δ13Ccarb13Corg and Δδ13C profiles across the platform-to-basin transect of the Yangtze Block. Sources of data: Yangjiaping Section from Kunimitsu et al. (2011); Dengying Formation and Yanjiahe Formation of Jiulongwan Section from Wang et al. (2014) and Jiang et al. (2012), respectively; Longbizui Section from Wang J G et al. (2012). Zircon age marked in the Jiulongwan Section is from adjacent Jijiawan Section reported by Condon et al. (2005). Zircon age marked in the Bahuang Section is reported by Chen et al. (2015). See other legends in Fig. 3

    According to the criteria previously described, the Deng- ying Formation in Well ZK102, although of much thinner strata thickness, could be approximately correlated with that on the Yangtze Platform: its lower boundary (the Doushantuo/Dengying boundary) is marked by the onset of the positive δ13Ccarb excursion (P1) that is correlated with EP3; the negative δ13Ccarb excursion (N2) starting at the uppermost Dengying Formation should represent the onset of EN4 (Fig. 7). Because of the chert deposition across the E-C succession in the basinal environment, only δ13Corg data are available for stratigraphic correlation. In the Bahuang Section, a U-Pb age of 542.6±3.7 Ma (Chen et al., 2015) at the nadir of the negative δ13Corg excursion (N3-org) in the upper Liuchapo Formation suggests that N3-org should be equivalent to EN4 (Fig. 7). The overall decreasing trend of both δ13Ccarb and δ13Corg across the E-C boundary from different depositional environments (e.g., the Well ZK102; the Xiaotan Section from Li et al., 2013) implies that δ13Corg stratigraphy might be reliable for the identification of the E-C boundary in deep water chert strata and also supports the correlation of the N3-org in the Bahuang Section to EN4 on the Yangtze Platform. Similarly, the δ13Ccarb13Corg profiles from Well ZK102 and those of previously reported sections show a δ13Corg negative anomaly during the Shuram excursion and an increasing trend at the onset of the subsequent EP3 (Wang et al., 2016, 2014; Kunimitsu et al., 2011). Thus, the positive δ13Corg excursion (P1-org) starting at the basal Liuchapo Formation of the Bahuang Section might be approximately synchronous with the beginning of EP3 on the Yangtze Platform (Fig. 7). In this case, the lower-middle Liuchapo Formation in the basinal Bahuang Section could be time equivalent to the Dengying Formation on the Yangtze Platform (Fig. 7).

    Except for in an isolated depositional environment (e.g., the Yangjiaping Section; Fig. 7; Kunimitsu et al., 2011), the δ13Ccarb profiles of the Dengying Formation on the Yangtze Platform generally can be further subdivided into the EP3 in its lower part and a broad small positive δ13Ccarb excursion (defined as EI; Zhou and Xiao, 2007) in its middle-upper part, between which an unnamed small negative excursion occurs. A similar δ13Ccarb variation trend is present in the Dengying Formation in Well ZK102 suggesting that P1 and P2 should be correlated with EP3 and EI on the Yangtze Platform, respectively (Fig. 7). In contrast, the δ13Corg profiles of the Dengying Formation in Well ZK102 and the coeval Liuchapo Formation in the Bahuang Section are distinct from and are unable to be correlated with the other δ13Corg or δ13Ccarb profiles from the Yangtze Block (Cui et al., 2016; Wang X Q et al., 2014; Guo et al., 2013, 2007; Wang J G et al., 2012; Kunimitsu et al., 2011; Zhou and Xiao, 2007; Zhu et al., 2007), reflecting a greater heterogeneity of δ13Corg compared to that of the δ13Ccarb record (Fig. 7). Considering the presence of three positive δ13Corg excursions and interjacent two negative δ13Corg excursions in the Dengying (lower-middle Liuchapo) Formation in both Well ZK102 and the Bahuang Section, these minor δ13Corg excursions are supposed to be regionally correlatable in the Daotuo- Bahuang area (Fig. 7).

  • Our new δ13C profiles together with previously published data of the Dengying (coeval Liuchapo) Formation (Cui et al., 2016; Wang X Q et al., 2014; Guo et al., 2013, 2007; Wang J G et al., 2012; Kunimitsu et al., 2011; Zhou and Xiao, 2007; Zhu et al., 2007) indicate a large surface-to-deep water δ13C gradient (~5‰ variation in δ13Ccarb, > 10‰ variation in δ13Corg) in the Yangtze Block during the Late Ediacaran. Of these, the δ13Ccarb varies from ~0‰ to +7‰ in the interior of the Yangtze Platform (e.g., the Yangjiaping Section), from ~-2‰ to +5‰ in the margin of the intrashelf basin (e.g., the Jiulongwan Section), and from ~-3‰ to +2‰ in the lower slope (Well ZK102; Fig. 7). Meanwhile, the δ13Corg varies from ~-29‰ to -21‰ in the interior of the Yangtze Platform (e.g., the Yangjiaping Section), from ~-30‰ to -24‰ in the margin of the intrashelf basin (e.g., the Jiulongwan Section), from ~-32‰ to -23‰ in the lower slope (Well ZK102), and from ~-36‰ to -33‰ in the basinal environment (e.g., the Bahuang Section; Fig. 7).

    Carbon isotope gradients have been widely reported in modern and ancient stratified basins in which the depth- dependent carbon isotope record has been considered to be related to the heterogeneity of biological activities in the redox- stratified water column (Wang et al., 2016, 2014; Wakeham et al., 2012; Shen et al., 2011; Jiang et al., 2007; Hollander and Smith, 2001). Similarly, multiple geochemical proxies have verified a redox-stratified ocean in the Yangtze Block during the Late Ediacaran (Cui et al., 2016; Ling et al., 2013; Chang et al., 2012; Wang J G et al., 2012). Under such conditions, the combination of high primary productivity and a suitably low remineralization rate of organic matter in the euphotic zone, could cause the dissolved inorganic carbon (DIC) enrichment in 13C in shallow water, as recorded by the relatively high δ13Ccarb values of the carbonate succession on the Yangtze Platform (Fig. 8). Meanwhile, a mass of 13C-depleted organic matter should be exported from the euphotic zone towards deep water. From the oxycline/chemocline downward, the decomposition of the imported organic matter via chemoheterotrophic bacteria (e.g., sulfate-reducing bacteria) could release 13C-depleted inorganic carbon, and thus significantly decrease the δ13C value of the DIC below the chemocline (Fig. 8). As a result, the authigenic carbonates precipitating as carbonate crusts, nodules or thin cements in the deep water could inherit 13C-depleted composition from the DIC (Canfield and Kump, 2013; Schrag et al., 2013; Jiang et al., 2010). Considering that the dolomite from Well ZK102 commonly displays a mudstone texture with a slightly 13C-depleted isotope composition (~5‰ less than that of the platform carbonates), we infer that the majority of carbonate sediments settled on the slope might have been shed from a shallow-water carbonate factory and the ~5‰ isotopic difference was probably generated by the incorporation of the authigenic carbonate precipitation.

    Figure 8.  Models for the carbon cycle at the Yangtze Platform. See Fig. 1 for paleogeographic interpretation

    The primary δ13Corg values of TOC are dependent on the source of the organic matter, mainly including photoautotrophic, chemoautotrophic and heterotrophic biomass (Wang et al., 2016, 2014; Jiang et al., 2012, 2010; Wakeham et al., 2012; Hollander and Smith, 2001). In the euphotic zone, phytoplankton is the major producer utilizing 13C-enriched DIC, which could allow for the TOC (mainly photoautotrophic biomass) in the shallow water deposits with relatively high δ13Corg values (Fig. 8). On the other hand, the Δδ13C values induced by primary producers (photosynthetic carbon isotope fractionation) were demonstrated to be in the range from 23‰ to 32‰ (Hayes et al., 1999). Therefore, the generally high δ13Corg values and low Δδ13C values (approximately 28‰) from the Late Ediacaran Yangtze Platform indicate that the TOC should have largely originated from photosynthetic organic carbon in the oxic surface ocean. With an increase in water depth, chemoautotrophic organisms would replace the phytoplankton as the dominant producer (Wakeham et al., 2012; Hayes et al., 1999). Because the chemoautotrophic organisms fix carbon from the 13C-depleted DIC below the euphotic zone and the isotope fractionation associated with chemosynthesis is generally higher than that with photosynthesis (Hollander and Smith, 2001), the biomass produced by chemoautotrophs is significantly depleted in 13C relative to primary photosynthate (Fig. 8). In addition, methanotrophs assimilating carbon from methane could also supply organic matter with an extremely 13C-depleted isotopic composition (Jiang et al., 2012, 2010; Wakeham et al., 2012; Hollander and Smith, 2001). Thus, the lower 13Corg values from the contemporaneous lower slope (Well ZK102) might have resulted from an increased chemoautotrophic and/or methanotrophic biomass contribution to TOC. Hayes et al. (1999) demonstrated that Δδ13C values greater than 32‰ suggest a significant contribution of chemoautotrophic or/and methanotrophic biomass to TOC. However, the Δ13C values derived from the Well ZK102 are also approximately 28‰ similar to those from the Yangjiaping and Jiulongwan sections, suggesting the contribution of chemoautotrophic or/and methanotrophic biomass is also subordinate. Although Δ13C data cannot be derived from the chert strata, the presence of lower 13Corg values in the Bahuang and Longbizui (Wang J G et al., 2012) sections suggests that chemoautotrophic or methanotrophic organisms provided more 13C-depleted and/or larger numbers of biomass in the ultimately preserved TOC at these locations.

  • As previously mentioned, the existing δ13Ccarb profiles of the Upper Ediacaran successions of the Yangtze Block mostly possess the same variation trend (Cui et al., 2016; Wang et al., 2014; Kunimitsu et al., 2011; Zhou and Xiao, 2007; Zhu et al., 2007), whereas their paired δ13Corg profiles are diverse among different regions (Fig. 7; Wang et al., 2014; Guo et al., 2013, 2007; Wang J G et al., 2012; Kunimitsu et al., 2011). As a result, the Upper Ediacaran δ13Ccarb13Corg profiles generally show a decoupled pattern with a highly variable Δδ13C corresponding to the large spatial and temporal heterogeneity of δ13Corg (Fig. 7; Wang et al., 2014; Kunimitsu et al., 2011).

    It is generally accepted that DIC is globally homogeneous in the well-circulated ocean, while dissolved organic carbon (DOC) is more susceptible to local environmental influences (Maloof et al., 2010). Local factors such as the influx of terrigenous organic carbon, contribution of chemoautotrophic or/and methanotrophic biomass, buffering effect of a large DOC reservoir and variation in the carbon isotope fractionation factor could readily result in the δ13Corg record being independent from the isotope composition of the ocean DIC, leading to the a decoupling of δ13Ccarb and δ13Corg (Wang et al., 2016; Ishikawa et al., 2013; Jiang et al., 2012; Kunimitsu et al., 2011; McFadden et al., 2008). Thus, the large spatial and temporal heterogeneity of δ13Corg across the platform-to-basin transect could be attributed to the influence of local carbon cycle processes. As previously documented, the Late Ediacaran ocean of the Yangtze Block was redox stratified with a large surface-to-deep water δ13Corg gradient generated by the incorporation of chemoautotrophic (methanotrophic) organic matter into TOC below the chemocline (Fig. 8). In this scenario, the frequent δ13Corg perturbation with relatively constant δ13Ccarb recorded from the lower slope-basin (Well ZK102 and the Bahuang Section) should reflect the chemocline fluctuations that have controlled the relative contribution of photoautotrophic versus chemoautotrophic (methanotrophic) biomass to TOC.

    Previous studies have proposed the existence (or episodic growth) of a large DOC reservoir in the anoxic deep ocean during the Ediacaran perhaps lasting until the Earliest Cambrian (Wang et al., 2016; Ishikawa et al., 2013; Lu et al., 2013; MacDonald et al., 2013; Jiang et al., 2012, 2010; Shen et al., 2011; McFadden et al., 2008; Rothman et al., 2003). This large DOC reservoir could continuously provide organic matter with uniform δ13Corg values to deposits, generating a decoupled δ13Ccarb13Corg pattern with invariable δ13Corg values as have been observed in some intervals of the Ediacaran to Earliest Cambrian of the Yangtze Block (Wang et al., 2016; Jiang et al., 2012, 2010, 2007). However, the δ13Corg records in the Late Ediacaran successions of the slope-to-basinal environment show large spatial and temporal heterogeneity, which is inconsistent with the buffering effect of a large DOC reservoir. For this reason, we favor the interpretation that the large DOC pool had been oxidized prior to the Dengying period (Lu et al., 2013; Maloof et al., 2010; McFadden et al., 2008; Fike et al., 2006).

  • Considering the interjacent negative δ13Ccarb excursion is small (nadir of ~0) and even indistinguishable in some sections, the Upper Ediacaran could be generally considered as a broad positive δ13Ccarb excursion between two significant negative excursions (the Shuram negative excursion and EN4). The similarity in the δ13Ccarb variation trend of the Upper Ediacaran successions of the Yangtze Platform (Fig. 7) and other carbonate platforms worldwide suggests that these δ13Ccarb profiles record the variation in the global carbon cycle (Bowring et al., 2007; Jiang et al., 2007; Kaufman et al., 2006; Saylor et al., 1998). Because the existence of a large DOC reservoir was excluded as previously discussed, the carbon cycle during the Late Ediacaran as a whole should conform to a steady-state model which supposes that the input of carbon to the ocean is balanced by the deposition of carbonate and the burial of organic carbon (Canfield and Kump, 2013; Schrag et al., 2013; Des Marais et al., 1992). In this model, an increase in the input of 13C-rich carbon could theoretically cause this positive δ13Ccarb excursion. However, the isotopic composition of the carbon input has been approximately -5‰ in Earth history (Canfield and Kump, 2013; Schrag et al., 2013). Furthermore, the underlying strata were deposited during the large Shuram negative excursion, the weathering of which unlikely resulted in 13C-rich carbon. Thus, the persistently medium-high δ13Ccarb values in the Upper Ediacaran strata should have been mainly driven by the increased ratios of organic carbon burial or/and an increase in Δδ13C.

    It is noteworthy that the Late Ediacaran successions generally have higher δ13Corg values compared to those of the overlying or underlying strata, although they show large spatial and temporal heterogeneity (Fig. 7). In this case, the Δδ13C values mostly fluctuate between 25‰ and 32‰ within this positive excursion, and fall in a range of photosynthesis-induced carbon isotope fractionation and overlap with those from the overlying or underlying strata (Fig. 7). These features indicate the high δ13Ccarb and δ13Corg values during the Late Ediacaran should have been principally induced by the burial of large amounts of organic carbon likely associated with relatively high productivity. The accumulation of abundant organic carbon means that abundant oxygen was produced via photosynthesis and released into the atmosphere (Fig. 8; Maloof et al., 2010; Ishikawa et al., 2008). This is in accordance with the flourishing of the Late Ediacaran macroscopic multicellular animals along with the positive excursion (Mason et al., 2017; Zhang and Cui, 2016; Cai et al., 2010; Bowring et al., 2007; Jiang et al., 2007; Kaufman et al., 2006; Saylor et al., 1998). Moreover, previous geochemical studies have suggested that both shallow water and deep water had been progressively oxidized during the Late Ediacaran (Ling et al., 2013; Chang et al., 2012; McFadden et al., 2008; Fike et al., 2006), which was also likely linked to this process.

  • (1) In the northeastern Guizhou Province, the E-C transitional successions were deposited in a lower-slope (e.g., Well ZK102) to basin (e.g., the Bahuang Section) environment. Of these, the Liuchapo Formation occurs as a chert-dominated stratal wedge that is thicker in the basinal environment and gradually pinches out into carbonate strata towards the Upper Yangtze Platform.

    (2) The Dengying Formation in the Well ZK102 and the lower-middle Liuchapo Formation in the Bahuang Section are bounded by negative δ13Ccarb and/or δ13Corg excursions in the strata both below and above, and could be approximately time equivalent to the Dengying Formation of the Yangtze Platform. The two positive δ13Ccarb excursions in the Dengying Formation in Well ZK102 are likely correlated with those reported from the platform sections, whereas the paired three positive and interjacent two negative δ13Corg excursions might be merely correlatable in the Daotuo-Bahuang area.

    (3) A large surface-to-deep water δ13C gradient (~5‰ variation in δ13Ccarb, > 10‰ variation in δ13Corg) occurred in the redox-stratified ocean of the Yangtze Block during the Late Ediacaran. The export of organic matter downward and the main contribution of photoautotrophic biomass to TOC respectively resulted in the high δ13Ccarb and δ13Corg values in the shallow water carbonate platform. Meanwhile, the low δ13Ccarb and δ13Corg values recorded in the deep-water sections should have been caused by the anaerobic oxidation of the imported organic matter and increased chemoautotrophic (methanotrophic) biomass contribution to TOC below chemocline.

    (4) The decoupled δ13Ccarb13Corg pattern of the Dengying Formation in the lower slope is characterized by large δ13Corg perturbations with relatively constant δ13Ccarb, likely in response to chemocline fluctuations rather than the buffering effect of a large DOC reservoir. Based on a steady-state model, the positive δ13Ccarb excursion in the Upper Ediacaran succession should have been principally induced by the burial of large amounts of organic carbon likely associated with relatively high productivity.

  • This work was supported by the National Natural Science Foundation of China (Nos. 41472089, U1663209), and the Open Research Fund Program of Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Hunan University of Science and Technology. We would like to thank Wennian Han and Hongwei Li for carbon and oxygen isotope analysis at the Laboratory for Stable Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0886-1.

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