A total of 118 carbonate samples of the Ediacaran Dengying Formation were selected from Well PT1 for strontium isotopic (46 samples) and elemental analyses (100 samples). Additionally, according to the lithology and paleontological characteristics of Well PT1, the carbonate samples belong to the Deng–2 Formation. Because the overlying layer of the Deng–2 Formation of Well PT1 is the Cambrian Yanjiahe Formation, it can be concluded that the Deng–3 and Deng–4 formations have been eroded.

About 100 mg (to 0.1 mg precision) of carbonate rock materials were weighted into Savillex 7.5 ml Teflon-PFA vials and then were dissolved on a hotplate at 80°C using 2.0 ml of 0.2 M HCl for 4 h. The sample solution was cooled at room temperature for 1 h before centrifugation for 8 min at 5,000 rpm. Then, the sample solution was loaded onto the preconditioned resin column with 2 ml of AG50W × 12 (200–400 mesh) for the separation of Sr from the sample matrix. After rinsing four times with 0.5 ml of 2.5 M HCl, the column was washed with 7 ml of 5 M HCl. Afterward, the Sr fraction was stripped with 3.5 ml of 5 M HCl, and the Sr fraction was evaporated to dryness and was ready for the TIMS analysis. The Sr isotopic measurements were performed on a Thermo Fisher Triton Plus multicollector thermal ionization mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The mass fractionation of Sr was corrected using an exponential law with ⁸⁸Sr/⁸⁶Sr = 8.375209. The international standard sample NBS–987 was used to evaluate instrument stability during the period of data collection. During this time, the measured average value of NBS987 was ⁸⁷Sr/⁸⁶Sr = 0.710,245 ± 0.000015, which is in good agreement with the reported values (Li et al., 2016; Li et al., 2019a).

All samples were crushed into fine powders greater than 200 mesh size for elemental experiments. The major and trace element concentrations were analyzed with a PANalytical MagiX PRO wavelength-dispersive X-ray fluorescence (XRF) spectrometer and Inductively Coupled Plasma Mass Spectrometer (ICP-MS), respectively, at the Northwest Branch of China Petroleum Exploration and Development Research Institute.

The ⁸⁷Sr/⁸⁶Sr values of the 48 measured samples range from 0.708881 to 0.710167, with a mean value of 0.709242 (Figure 2A, 3A). Vertically, the ⁸⁷Sr/⁸⁶Sr curve fluctuates frequently. There is a slow downward trend of the ⁸⁷Sr/⁸⁶Sr curve at the depth of 6,234–5,966 m, and the ⁸⁷Sr/⁸⁶Sr value decreases from 0.709300 to 0.708900. Then, the ⁸⁷Sr/⁸⁶Sr value increases rapidly, and remains stable at ∼0.709400 (5,956–5,796 m). Finally, the ⁸⁷Sr/⁸⁶Sr curve shows a rapid drop at the depth of 5,796–5,711 m with values decreasing from 0.709500 to 0.708900.

The Sr contents are in a low level (∼100 ppm) at the depth of 6,276–5,912 m and 5,866–5,726 m, and remain relatively higher (400–500 ppm) at the depth of 5,906–5,876 m (Figure 2B). The Mn/Sr values initially decrease from ∼5 to ∼2 at the depth of 6,276–6,206 m, and then remain stable at ∼2 in the depth of 6,206–5,952 m (Figure 2C). Subsequently, there are large fluctuations at the depth of 5,952–5,866 m, and the Mn/Sr values range from ∼0.5 to ∼4. Finally, the Mn/Sr value decreases from ∼5 to ∼1.5 at the depth of 5,866–5,726 m. The CaO + MgO values share a similar trend of the Mn/Sr values (Figure 2D). The redox sensitive trace elements (RSTEs), including V, Cr, Co, and Ni, share a similar trend: initially stable at low values and then increase at the depth of 5,932–5,866 m. Finally, there is a significant increase in the trend of RSTEs at the depth of 5,800–5,700 m (Figures 3B–E). The Al₂O₃ values are at a low level (<2%), and the fluctuation of the Al₂O₃ curve is small on the whole (Figure 3F). At the depth of 6,276–5,916 m and 5,866–5,834 m, the Al₂O₃ content is almost invariable with the values of ∼0.1%. By contrast, the Al₂O₃ value is relatively higher at ∼0.5% and at the depth of 5,906–5,876 m. The Al₂O₃ curve fluctuates relatively intensively at the depth of 5,814–5,726 m, and shows higher values of ∼0.5–2%. The TiO₂ values are at a low level (∼100 ppm), and the trend of the curve is similar to that of the Al₂O₃ curve (Figure 3G).

Diagenetic processes can change the primary geochemical signatures of marine carbonates, further affecting the analysis of the coeval seawater (Kaufman et al., 1991; Derry et al., 1994). Elemental proxies have been widely used to evaluate the influence of later alteration, especially the diagenesis, on the geochemical records of carbonates (Derry et al., 1994; Kaufman and Knoll, 1995). Although the traditional view is that critical values of proxies can exclude the altered samples (Derry et al., 1994), recent studies have shown that the comprehensive analysis of stable isotopes and elements rather than on the use of a fixed value is a more reasonable way to determine the diagenesis (Loyd et al., 2012; Li et al., 2013a; Schiffbauer et al., 2017). In the present study, the inner relationship and trend of multi-elemental proxies were analyzed to evaluate the diagenesis.

The loss of Sr and Na and the enrichment of Fe and Mn occur during deposition, especially under the influence of the atmospheric water cycle (Derry et al., 1994; Kaufman and Knoll, 1995; Azmy et al., 2011; Azmy et al., 2014). Therefore, the Mn/Sr ratio can be used to determine whether geochemical records of carbonates represent the original composition of seawater (Derry et al., 1994). Generally, as diagenesis becomes more severe, the Mn/Sr ratio increases and samples that have maintained the original isotopic compositions of the seawater usually have an Mn/Sr value <5 (Derry et al., 1994; Zhang et al., 2020). In addition to the sample PT1-27 (5,842 m, Mn/Sr = 5.13), Mn/Sr values of the remaining samples are less than 5, indicating that these samples may maintain the primary geochemical signatures (Figure 2C).

The Sr concentration of aragonite is <7,740 ppm in modern seawater, that of high-magnesium (Mg) calcite is 400–5,000 ppm, and that of protodolomite is 245–600 ppm (Baker and Burns, 1985). As shown in Figure 2B, based on the Sr content, samples can be divided into two groups: group 1 (5,912–6,276 m and 5,726–5,872 m), having relatively low Sr contents (<200 ppm) and group 2 (5,876–5,906 m), having relatively high Sr contents (>350 ppm). Diagenesis can reduce the Sr content of carbonate rocks to below 155 ppm (Javanbakht et al., 2018), so the Sr concentration of 155 ppm may be a critical value for excluding carbonates altered by diagenesis (Ren et al., 2019). Therefore, group 1 seems to have undergone the diagenesis, and group 2 may maintain the primary geochemical signatures. However, the Sr content of group 1 is extremely stable in intervals with different dolomite contents, indicating that the diagenesis had no prominent effect on the Sr content (Figure 2B). Actually, the Sr content of the coeval carbonates in the Tarim Basin (Ediacaran Chigebrak Formation) (∼150 ppm) and the upper Yangtze Xiaotan section (Ediacaran Dengying Formation Baiyanshao Member) (<100 ppm) is also low (Li et al., 2013a; Zhang et al., 2020), indicating that Sr content is not the result of the diagenesis, but represents the original seawater composition, which further proves that a fixed critical value is not effective in some cases (Schiffbauer et al., 2017).

The samples with high purity (CaO + MgO concentrations) represent the purist carbonates which contain original ⁸⁷Sr/⁸⁶Sr signature (Li et al., 2013a). The (CaO + MgO) values of all samples are more than 40%, and most of the samples are more than 50%, indicating that the Dengying Formation carbonates of Well PT1 were unaffected by the diagenesis (Figure 2D). Additionally, the Rb/Sr ratio in cleaned carbonates has generally lower values than altered carbonates. In the present study, in addition to the sample PT1-27 (5,842 m, Rb/Sr = 0.16), the Rb/Sr values of the remaining samples are in a low level (Figure 4).

On the other hand, the relative relationship between these proxies was also analyzed to further exclude the altered samples (Burdett et al., 1990; Frank et al., 1997; Li et al., 2013a; Zhang et al., 2020). The location of samples in cross-plots roughly consisted with the division of samples which is based on the Sr content (Groups 1 and 2) (Figure 2B; Figure 4). Although there are good relationships between Mn/Sr and Sr (Figure 4B), CaO + MgO and Sr (Figure 4C), Mn/Sr and CaO + MgO (Figure 4E), and Rb/Sr and CaO + MgO (Figure 4F), which is presented by high coefficient of determination (R ²), the location of group 2 is clearly far from the digenetic trend, indicating that these carbonates were unaffected by the diagenesis. By contrast, one sample (PT1–27, 5,842 m) deviates from the main cluster of group 1 and has a diagenetic trend in relative to other samples, indicating that it has been altered by the diagenesis (Figures 4B–E). The cross-plots of Rb/Sr vs Sr, CaO + MgO vs Sr, Rb/Sr vs Mn/Sr, and Rb/Sr vs CaO + MgO indicates that the trend and location of group 1 show no diagenetic processes (Figures 4A,C,D,F). However, the diagenetic trend of group 1 is shown in the cross-plots of Mn/Sr vs Sr and Mn/Sr vs CaO + MgO. The inconsistency can be attributed to the low Sr content (Li et al., 2013a; Zhang et al., 2020). The low Sr content contributed to the high Mn/Sr ratios, further showing a diagenetic trend. Due to the low Sr content of seawater in the Sichuan Basin during the Ediacaran (Li et al., 2013a; Zhang et al., 2020), the use of Mn/Sr and Sr contents are obstructed. Therefore, it can be concluded that Rb/Sr and CaO + MgO values are more applicative proxies for determining the diagenesis in the present study.

Biological evolution may be related to the fluctuations of the oxygen level in the ocean–atmosphere system, which shows the significance of reconstructing the redox state of seawaters (Hardisty et al., 2013; Liu et al., 2019; Wei et al., 2020; Dodd et al., 2021). Although there are various proxies for reconstructing redox condition, such as iodine (Hardisty et al., 2017; Hardisty et al., 2020), molybdenum isotope (Siebert et al., 2003), and chromium isotope (Gueguen et al., 2016), the redox-sensitive trace elements (RSTEs) are the widely used and reliable proxies (Tribovillard et al., 2006; Algeo and Tribovillard, 2009). However, the use of bimetal RSTEs, especially U/Th, V/Cr, Ni/Co, and V/(V + Ni), remains controversial (Algeo and Li, 2020; Algeo and Liu, 2020). For example, V may precipitate in the form of a stable sulfide if hydrogen sulfide is present, and Ni is related to not only the redox but also the paleoproductivity (Tribovillard et al., 2006), which may mask the true seawater environment. On the other hand, the ratio of the two elements will eliminate the true enrichment degree of the RSTEs. Therefore, in the present study, the contents of RSTEs were applied in the reconstruction of redox conditions instead of bimetal RSTEs. RSTEs tend to be more soluble in water column under a more oxic environment and enter into sediments under a more anoxic environment (Tribovillard et al., 2006; Algeo and Tribovillard, 2009; Tribovillard et al., 2012).

Although the low contents of RSTEs shows that the marine environment was oxic as a whole during the latest Ediacaran (Figures 3B–E), two significant fluctuations of RSTEs indicate that there were two anoxic events. The dynamic redox condition is also supported by the δ²³⁸U data (Wood et al., 2015; Zhang et al., 2018). The Fe speciation data from Newfoundland in Canada, Ce anomaly data from the Nama Group in Namibia, and Fe-S-C data from south China suggest an anoxic environment in deep water settings during the late Ediacaran (Darroch et al., 2015; Wood et al., 2015; Tostevin et al., 2016). Thus, the redox condition inferred by the contents of RSTEs indicates that the minimum oxygen zone existed in the Sichuan Basin during the Ediacaran, and there were two expansions of anoxic seawater (Figures 3B–E). Furthermore, the previous study has shown that there was an extensive marine anoxia during the terminal Ediacaran, which resulted in the decline in the Ediacaran biota from ∼550 Ma (Zhang et al., 2018). Thus, fluctuations of the oxygen content may be closely related to the decline and eventual disappearance of the Ediacaran biota (Shen et al., 2008; Laflamme et al., 2013; Zhang et al., 2018).

The immobile elements, such as aluminum, zirconium, and thorium, are unaffected by weathering and diagenetic processes, and, thus, are regarded as effective proxies for the terrigenous debris input (Taylor and McLennan, 1985; Zhang et al., 2000; Tribovillard et al., 2006). Al and Ti are principally derived from aluminosilicate clay minerals, which are carried to oceans by terrigenous influx (Hayashi et al., 1997; Tribovillard et al., 2006). However, Al should not be used in cases where marine carbonates are characterized by a low detrital fraction, because excess Al may have been scavenged as hydroxides coating biogenic particles (Kryc et al., 2003). In the present study, the consistent trend of Al₂O₃ and TiO₂ suggests that it is feasible to use Al₂O₃ to represent the terrigenous detrital input (Figures 3F,G). According to the vertical trend of Al₂O₃ and TiO₂ values, the terrigenous detrital input was generally stable (Figures 2B,C). However, there were two episodes of high terrigenous detrital input, which can roughly correspond to two relatively anoxic intervals (Figure 3), indicating that there is a close relationship between the terrigenous detrital flux and redox condition. The terrigenous detrital flux is one of the sources of marine nutrients which can supply the organisms (Chang et al., 2019). In addition, the oxygen rise can stimulate biological diversification (Knoll and Carroll, 1999; Chen et al., 2015), and bioturbation and bioirrigation can affect the oxygen exchange between the surface water and the water column (Boyle et al., 2014). Therefore, during intervals of low terrigenous detrital flux, the low input of nutrients (Figure 3), such as phosphorus, indicate that less organisms demanded less oxygen, leading to the depletion of the RSTEs due to the oxic environment (Figure 3). By contrast, increased nutrients supply stimulated marine productivity during intervals of high terrigenous detrital flux (Figure 3), which led to high oxygen demand on a short time scale (10⁴ years), and this process would have tend to increase the ocean oxygenation on a long time scale (10⁶ years) (Fike et al., 2006; Lenton et al., 2014; Cui et al., 2016; Zhang et al., 2018), resulting in the enrichment of RSTEs due to the anoxic environment.

On the other hand, there is a significant increase in the Sr content at the depth of 5,906–5,876 m, which corresponds to the interval of the first episode of high terrigenous detrital flux and anoxic environment (Figures 2B, 3). In addition, the Sr content of this interval resemble that of normal seawater, while that of 6,276–5,912 m and 5,866–5,726 m is significantly lower than that of normal seawater. However, the effect of diagenesis on the Sr content has been excluded, and thus the interval of low Sr content may indicate a local process. The position of Well PT1 was located in the shallow carbonate platform (Figure 1B), which includes lagoonal facies, restricted platform, and tidal flat, which also suggest that low Sr content may be the result of local processes. Thus, the first episode of high terrigenous detrital flux and anoxic expansion may be a local event, while the second episode may be a global event.

Strontium in oceans has two main sources (Palmer and Edmond, 1989): 1) high-value strontium isotope from continental weathered rocks (global mean value of ⁸⁷Sr/⁸⁶Sr is 0.7119) (Peucker-Ehrenbrink and Miller, 2006) and 2) low-value strontium isotope supplied by hydrothermal exchange of mid-oceanic ridge and hydrothermal alteration of seafloor basalt (global mean value of ⁸⁷Sr/⁸⁶Sr is 0.7035) (Hofmann, 1997). Additionally, the diagenesis can result in elevated ⁸⁷Sr/⁸⁶Sr values, which can obscure the real ⁸⁷Sr/⁸⁶Sr signature (Derry et al., 1994). For example, the global strontium isotope composition of oceans during the Cambrian–Toyonian is 0.708,853–0.709,667 (Zhang et al., 2022), while the ⁸⁷Sr/⁸⁶Sr value of coeval altered carbonates exceed 0.710,300 (Fu et al., 2020). Due to the lack of high-precision ⁸⁷Sr/⁸⁶Sr records, the strontium isotope composition of the Ediacaran seawater has not been well-limited (Halverson et al., 2007; Sawaki et al., 2010a; Guacaneme et al., 2021). Therefore, the identification of the diagenesis and global comparison of ⁸⁷Sr/⁸⁶Sr values during the interval presented by the Ediacaran Dengying Formation are necessary.

Although altered samples have been excluded, the influence of the diagenesis on the strontium isotope composition is still evaluated in order to avoid possible interferences. As shown in the cross-plots of ⁸⁷Sr/⁸⁶Sr vs Sr, ⁸⁷Sr/⁸⁶Sr vs Rb/Sr, and ⁸⁷Sr/⁸⁶Sr vs CaO + MgO, there is no obvious correlation between these proxies and ⁸⁷Sr/⁸⁶Sr (Figures 5–D), which is presented by the low values of the coefficient of determination (R ²), indicating that the diagenesis unaffected the strontium isotope composition. The ⁸⁷Sr/⁸⁶Sr values and Mn/Sr ratios show a positive relationship, which can be attributed to the low Sr content of the Ediacaran seawater as previously mentioned (Li et al., 2013a; Zhang et al., 2020). Thus, our high ⁸⁷Sr/⁸⁶Sr values can be used to restrict the sedimentary environment of the Sichuan Basin during the interval presented by the Deng–2 Formation.

The strontium isotope values of the latest Ediacaran (551–542 Ma) in the present study (Figure 2A) are high in both long and short scales (Figures 6A,B) (Derry et al., 1994; Veizer et al., 1999; Halverson et al., 2007; Cui et al., 2020). Moreover, the global correlation of the late Ediacaran ⁸⁷Sr/⁸⁶Sr records show that in addition to the extremely high ⁸⁷Sr/⁸⁶Sr values of the Xiaotan section in south China (0.710,007–0.712,326) which has been attributed to the diagenesis (Li et al., 2013a), the ⁸⁷Sr/⁸⁶Sr values of Well PT1 are significantly higher than that of other coeval records (Figure 2A), such as the low ⁸⁷Sr/⁸⁶Sr values of the Tsagaan Oloom Formation in southwest Mongolia (0.70772–0.70869) (Brasier et al., 1996), the Sete Lagoas Formation in the Bambuí foreland Basin (0.707493–0.708663) (Guacaneme et al., 2021), and Bambuí Group in southern São Francisco (0.707332–0.708878) (Paula-Santos et al., 2015), and moderate ⁸⁷Sr/⁸⁶Sr values of the Chigebrak Formation in the Tarim Basin (0.708464–0.708994) (Zhang et al., 2020), unit-4 layer in central Iberia (0.70845–0.70875) (Valladares et al., 2006) and the Dengying Formation in Gorges area, South China (0.70835–0.70875) (Sawaki et al., 2010b; Wei et al., 2019) (Figure 6C). These low ⁸⁷Sr/⁸⁶Sr values (<0.7085) have been interpretated as the result of the local process rather than the global signature, while moderate ⁸⁷Sr/⁸⁶Sr values (0.7085–0.7090) have been regarded as the global signature (Paula-Santos et al., 2015; Guacaneme et al., 2021). During the Ediacaran, the Gondwana assembly and related marginal orogenesis caused paleogeographic changes, which led to marine isolation and unique geochemical characteristics (Li et al., 2008; Li et al., 2013c; Wei et al., 2019). A modern case also indicates the importance of tectonic activities on the geochemical characteristics: the uplifts of the Himalaya and Tibetan Plateau increased the continental weathering rates, leading to more input of radiogenic Sr isotope to oceans (Richter et al., 1992). Furthermore, the seawaters of near-shore basins were commonly flooded with continental freshwater with little chance to exchange with global seawaters (Frimmel, 2009).

In the case of excluding the effect of the diagenesis, the evidently high ⁸⁷Sr/⁸⁶Sr values of this study may be also the result of local processes (Figure 2A), which is consistent with the analyses of elements. Combined with the paleogeographic information (Figure 1), it can be concluded that the sedimentary environment of the Sichuan Basin may be restricted during the latest Ediacaran, such as lagoonal facies and restricted platform (Figure 1B). The high ⁸⁷Sr/⁸⁶Sr values also suggest the relatively high terrigenous detrital input from continental weathering (Peucker-Ehrenbrink and Miller, 2006; Zhao et al., 2009), and reduced contribution of oceanic hydrothermal sources to ⁸⁷Sr/⁸⁶Sr values (Hofmann, 1997). During the late Ediacaran, the assembly of Gondwana was still in process, indicating a period of intense tectonic activities. Thus, the widespread continental collision caused high topographic landscape, indicating intense continental weathering (Richter et al., 1992; Li et al., 2008; Li et al., 2013c). In addition, when the sea level falls, a large continental area is exposed for weathering, resulting in the increased supply of terrigenous material to the ocean (Palmer and Edmond, 1989; Hofmann, 1997). Therefore, the intense continental weathering and low sea level resulted in the increased supply of terrigenous strontium to the Sichuan Basin during the Ediacaran, further leading to high ⁸⁷Sr/⁸⁶Sr values (Figure 2A).

On the other hand, the Three Gorges area was also located in the upper Yangtze Platform (Sawaki et al., 2010b; Zhang et al., 2018; Wei et al., 2019), so carbonates of Well PT1 and the Three Gorges area should share similar ⁸⁷Sr/⁸⁶Sr values due to the homogeneity of strontium isotope composition (Jacobsen and Kaufman, 1999; Paula-Santos et al., 2015). However, the strontium isotope composition of the Three Gorges area (0.70835–0.70875) is similar with the extensive oceans, while that of well PT1 is higher than that of the extensive oceans. The paleogeographic position of PT1 and Three Gorges area shows that the position of Three Gorges area was closer to extensive oceans than that of Well PT1 (Figure 1A) (Zhang et al., 2018). Therefore, we inferred that there was a high submarine in the Sichuan Basin, restricting the exchange of water between the seawater of Sichuan Basin and the extensive oceans, which resembles the case of the late Miocene Mediterranean marine basin (Schildgen et al., 2014). In this case, due to the existence of the high submarine, the input of the terrigenous debris into the location of Well PT1 was more than that of Three Gorges area, which led to more radiogenic Sr in the seawater of the location of Well PT1. Thus, the ⁸⁷Sr/⁸⁶Sr record of the Three Gorges area of south China can represent the global strontium isotope composition (Valladares et al., 2006; Sawaki et al., 2010b; Wei et al., 2019), while that in the Sichuan Basin was a local process.

The vertical fluctuations of the ⁸⁷Sr/⁸⁶Sr values may be fundamentally controlled by the Gondwana assembly (Li et al., 2008; Li et al., 2013c). The Gondwana assembly controlled the continental exposure area and continental weathering intensity, further affecting the terrigenous detrital input and driving the restriction degree of the Sichuan Basin by controlling tectonic uplifts. According to the vertical trends of elements and ⁸⁷Sr/⁸⁶Sr values (Figure 2A), the sedimentary environment of the Sichuan Basin during the latest Ediacaran can be divided into four stages. During stage 1, the terrigenous detrital input was stable, which presented stable TiO₂, Al₂O₃, and Sr contents. Low Sr contents and high ⁸⁷Sr/⁸⁶Sr values indicate the restricted exchange with extensive oceans, and thus the Sichuan Basin is a restricted platform (Figure 7A). The long-term oxic environment and nutrients supply during stage 1 provided the prerequisite for the emergence of aerobic organisms in stage 2. During stage 2, the Gondwana assembly resulted in intense tectonic activity and low sea level (more exposed area), further leading to the increased terrigenous detrital input, which is presented by high ⁸⁷Sr/⁸⁶Sr values (0.7093–0.7095) (Figure 7B). The increased terrigenous detrital input stimulated the emergence of aerobic organisms, which consumed the previously stored oxygen, leading to the first episode of anoxic environment. During stage 3, the Gondwana assembly was still in process (Li et al., 2008; Li et al., 2013c), causing the continuous increase of the terrigenous detrital input. However, the ⁸⁷Sr/⁸⁶Sr and Sr values decreased, which may have resulted from global transgression. The rising sea level led to the mixture of the seawater of the Sichuan Basin and extensive oceans, further resulting in the decline of the ⁸⁷Sr/⁸⁶Sr values (Figure 7C). The high sea level provided upwellings which carried enough nutrients, stimulating the paleoproductivity. Thus, the increased paleoproductivity consumed oxygen and nutrients, furthering resulting in the second episode of anoxic environment in stage 4 (Figure 7D).