The Late Ordovician (Katian-Hirnantian stage) to early Silurian (early Rhuddanian) was an important interval in Earth history, during which the environment, biodiversity, and plate tectonics changed considerably1,2,3. The Late Ordovician mass extinction (LOME) was the first of the five considerable Phanerozoic extinction events, along with the Hirnantian glaciation event, major sea level changes, and the extensive deposition of organic-rich shales2,3. It comprised two discrete pulses: the first pulse started during or just before the Metabolograptus extraordinarius graptolite biozone4,5,6,7,8 and mainly affected benthic, nektonic and planktonic species9,10. The second pulse, initiating at the start of the M. perculptus biozone, is associated with the extinction of a distinctive fossil assemblage known as the Hirnantia Fauna, including diverse and abundant brachiopods and trilobites11,12. Hirnantia Fauna has been interpreted to be mainly cool-water benthic fauna that began to spread rapidly and widely after the first extinction event but suddenly became extinct during the second extinction event6,8,11,12. During this short time interval, approximately 85% of marine animal species and 25% of animal families went extinct13,14.

The past decade has witnessed research efforts toward understanding the context and nature of the LOME, and yet a consensus has not been reached on the causes of Earth's second most severe biotic catastrophe. A number of possible extinction mechanisms are associated with changes in the global climate and ocean chemistry. Numerous analyses have demonstrated that the Hirnantian glaciation was a relatively short but intense climatic event marked by an abrupt glacio-eustatic sea-level change, that was broadly coincident with the LOME15. These large fluctuations are recorded as major lithofacies changes in the South China Craton, where shelly limestones of the syn-glacial Guanyinqiao Formation are sandwiched between the graptolitic black shales of the pre-glacial Wufeng Formation and the post-glacial Longmaxi Formation and are known as deep-water graptolitic black shales6,7,12. Anoxia was widespread and expanded repeatedly in the Late Ordovician–early Silurian ocean during the deposition of black shales, and these events are commonly discussed as the preferential extinction of pelagic species, such as graptolites, as well as key benthic taxa, such as trilobites16,17. A stratified anoxic water column has also been confirmed based on geochemical evidence, such as high redox-sensitive trace element abundances (Mo, U, and V), heavy sulfur isotopic records (δ34Sδ), Fe speciation analysis, and distribution of pyrite framboid diameters, mainly from South China17,18,19,20. There is an apparent temporal correlation between large igneous province (LIP) eruptions and at least half of the major extinctions of the Phanerozoic. The LOME has not been correlated with a recorded LIP event, until recently, when Derakhshi21 reported a series of voluminous volcanic events of the Middle Ordovician–Silurian from northern Iran. These erosional and deformed remnants (volcanic rocks) distributed over a length of ca. 1700 km, identified as the newly proposed Alborz LIP, have a high potential to be the main cause of the LOME. Before the recorded LIP event had been discovered, many researchers postulated volcanism as the trigger for the LOME as a primary driver of global environmental and climate changes. Volcanism could lead to eutrophication, increased amounts of CO2 in the atmosphere, and as a consequence, increased anoxia22,23,24,25,26. However, the causes of the far-reaching catastrophic biological and environmental changes during the Late Ordovician to early Silurian remain controversial.

Carbon isotopes represent an important tool for correlating the Ordovician system, both at global27,28 and regional scales29,30. Previous studies have revealed the Hirnantian isotope carbon excursion (HICE), which is characterized by significant δ13C-enrichments of both sedimentary carbonates (δ13Ccarb) and organic carbon (δ13Corg) that is broadly recorded in Ordovician–Silurian sections (e.g., Refs.31,32,33,34,35,36). The HICE was associated with a mass extinction, extensive glaciation of Gondwana, and a global sea-level low stand (e.g., Refs.37,38,39). The positive δ13C excursions have been interpreted to reflect the enhanced burial of organic carbon driven by expanded anoxic conditions18,29. However, both shallow and deep water columns have indicated increasing oxygenation during the Hirnantian glaciation based on redox-sensitive elements, iron speciation, nitrogen, molybdenum, and uranium isotope data (e.g., Refs.40,41,42,43). Therefore, the possible origin of the positive δ13C excursions and its potential relationships to environmental changes during the Hirnantian Stage require further investigation.

The temporal relationship between glaciation, marine anoxia, and the δ13C curve is clearly a subject of debate, as is the relationship between palaeoclimate, redox changes, and the O–S biotic crisis. We examine these topics here based on the Xindi No. 2 drill core section in the Sichuan Basin and aim to show the relationship between palaeoclimate, anoxia, and δ13C trends. A combined graptolite biostratigraphic and carbon isotope study is used to generate a framework. This framework, combined with mineralogy and diverse elemental data, provide new insights into the provenance, palaeoclimate, redox conditions, and palaeoenvironmental change during the Late Ordovician crisis interval.

Geological setting

The South China Block was an isolated microcontinent located off the northwestern margin of Gondwana near the equator in the Late Ordovician44,45. At that time, the South China Block comprised two subblocks, the Yangtze Block in the northwest and the Cathaysia Block in the southeast. The Yangtze Block formed a shallow carbonate platform during the Cambrian period46,47,48,49. During the Late Ordovician Kwangsian Orogeny, the Cathaysia Block collided with the Yangtze Block, resulting in a compression effect in the northwest-southeast orientation, which caused a relative rise in sea level and a partial uplift of the craton edge (e.g., Chuanzhong, Xuefeng, and Qianzhong uplifts; Fig. 1). As a result, the Yangtze carbonate platform evolved into a siliceous clastic shelf basin, and the Yangtze Sea was largely isolated from the open sea, forming an anoxic, stagnant, low-energy seawater environment, which eventually led to the deposition of thick and rich organic shales in this area50,51,52,53.

Figure 1
figure 1

Geological setting. (A) Late Ordovician (445 Ma) palaeogeography with the location of South China (adapted from Torsvik and Cocks45). Red circle represents the Xindi-No. 2 well. (B) Simplified palaeogeographic map of the Yangtze Shelf Sea during the Ordovician–Silurian transition, showing the well localities.

The Xindi No. 2 well is located in Daguan District, Zhaotong City, Yunnan Province, in the southwestern Sichuan Basin. The strata across the Ordovician–Silurian boundary of the Xindi No. 2 well are the Wufeng, Guanyinqiao, and Longmaxi formations, in ascending order. The Wufeng Formation is mainly composed of dark-grey, carbonaceous shales with interlayers of siliceous shales and mudstone, containing abundant and diverse graptolite faunas and radiolarians (Fig. 2A,B,D). The overlying Guanyinqiao Formation is composed of carbonaceous marls, which contain cold-water adapted Hirnantia Fauna6 (Fig. 2C). The Longmaxi Formation, generally much thicker than the other two units, is subdivided into two parts. The lower part is composed of black, graptolite-rich, carbonaceous shales, siliceous shales, and carbonaceous mudstone. The upper part is composed of grey, green mudstone with fewer fossils. K-bentonite beds have been observed in the Wufeng Formation(nine beds) and in the lower part of the Longmaxi Formation (seven beds).

Figure 2
figure 2

Sedimentary features of the Wufeng-Longmaxi formations in Xindi No. 2 well, Sichuan Basin, South China. (A) Pyrite occurs as girdles in the black shales of the Longmaxi Formation (B) Graptolites developed in the Wufeng Formation (C) A fragile piece of brachiopods (yellow arrow) in sample XD2P-B30. (D) Radiolarians developed in black shales of the Wufeng Formation (sample XD2P-B21). (E) Detrital quartz, moderately sorted and moderately rounded, is present in the black shales of the Longmaxi Formation (sample XD2P-B34). (F) SEM image of framboidal pyrite clusters.

Samples and methods

A total of 54 samples were collected from the Xindi No. 2 well. Twenty-one samples were obtained from black shales of the Wufeng Formation with an average interval of 0.5 m, seven carbonaceous marls from Guanyinqiao Formation were collected with an average interval of 0.1 m and 26 black shales of the Longmaxi Formation were taken with an average interval of 0.6 m. The above sampling location is given in Fig. 3. Samples were coarsely crushed through a steel jaw crusher and then powdered in an agate mill down to a grain size smaller than 200 mesh for geochemical analysis.

Figure 3
figure 3

Biostratigraphic data, δ13Corg profiles, and CIA values for the Xindi No. 2 well. The biostratigraphy is based on Zhang et al.54. HICE Hirnantian isotopic curve excursion.

Fifty-four samples of major oxides including Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, SiO2 and TiO2 were measured by a PW4400 X-ray fluorescence spectrometer at the National Research Center of Geoanalysis, China. The sample preparation for major elements was conducted using Chinese National Standard GB/T 14506.28-2010. The analytical uncertainty was generally less than 5%. A total of 54 samples of trace element compositions (including rare earth elements [REEs]) were measured using a PE300D inductively coupled plasma mass spectrometer, strictly followed by Chinese National standard GB/T 14506.30-2010.

A Nova Nano SEM450 scanning electron microscope (15 kV and 10–10 A) was applied to study the morphology and distribution of typical minerals in ten selected samples in the laboratory of the Beijing Institute of Uranium Geology. The analytical procedures were followed by the Chinese National standard GB/T 18295-2001 and GB/T 17359-2013. The mineral compositions of the whole-rock and clay minerals of seventeen samples were determined using the ZJ207 Bruker D8 advance automated powder diffractometer equipped with Ni-filtered Cu–Ka radiation (40 kV accelerating voltage and 40 mA beam current) at Chongqing Mineral Resources Supervision and Test Center, China. The X-ray diffraction (XRD) pattern was scanned from 3° to 45° with a step size of 0.02° (2θ), and the humidity was controlled at 39% RH during the determination of the XRD pattern.

Fifty-four samples of stable organic carbon isotope measurements were performed in the laboratory of the Beijing Institute of Uranium Geology. Approximately 200 mg of the samples were placed in a container containing ~ 250 ml 1 N HCl solution to remove the carbonate minerals. After standing for 48 h, the material was filtered through a vacuum pump and rinsed with deionized water to remove excess acid and chlorine. The remaining precipitate was dried, separated from the filter, and stored in 20 ml glass vials. The δ13C values of the organics separated from the decarbonated samples were analyse on a Thermo Fisher MAT-253 mass spectrometer. The Chinese national standard GB/T 18340.2-2010 was used to monitor analytical accuracy. Analytical precision was better than ± 0.1‰ for δ13Corg based on replicate analyses of standards and samples.

The graptolites from the Xindi No. 2 well were collected and identified by Zhang et al.54 for each sample. Nine graptolite biozones are recognized in the Xindi No. 2 well, including Dicellograptus, complanatus, D. complexus, P. pacificusM. extraordinarius, A. ascensusP. acuminatus, C. vesiculosus, C. cyphus, D. triangulatus, L. convolutes, and S. sedgwickii biozones. However, Metabolograptus extraordinarius (Sobolevskaya) and Metabolograptus persculptus (Salter) have not been found. The appearance of Normalograptus mirnyensis (Obut and Sobtolevskaya) at the depth of 2070.73 m confirmed that the M. perculptus biozone has been reached. The graptolite biozonation, thus, could be correlated with other Wufeng, Guanyinqiao, and Longmaxi Formation data.



The XRD analysis results are illustrated in Table 1 and Fig. 4. No significant qualitative difference was found in the mineralogical composition of samples from the Wufeng- Longmaxi formations. The samples are composed mainly of quartz (Fig. 2E, 22.1–71.3%, average of 39.1%), clay minerals (11.2–38.4%, average of 20.6%), calcite (4–28%, average of 17.7%) and dolomite (2.3–27.2%, average of 14.3%). Moreover, minor albite (1.6–6%, average of 3.6%), pyrite (0.5–5.9%, average of 3.3%) and microcline (0.4–3.4%, average of 1.3%) occur in the samples. The mixed layers of illite/smectite (43.0–65.0% average of 53.6%) and illite (22–40%, average of 30.8%) are the dominant constituents of the clay mineral contents of the Wufeng-Longmaxi formations. In addition, small numbers of minerals, including pyrite, molybdenite, and ilmenite, were detected simultaneously by XRD and scanning electron microscopy (SEM). Pyrite is mainly present as typical framboids and single euhedral pyrites in the studied samples (Fig. 2F).

Table 1 Mineral compositions of the shales and carbonaceous marls from the Wufeng-Longmaxi formations in the Xindi No. 2 well (XRD).
Figure 4
figure 4

XRD patterns of studied samples XD2P-B18 and XD2P-B35 (the marks in the figure are eigenvalues and corresponding minerals).


Major elements

The major element concentrations of samples from the Xindi No. 2 well are listed in Table 2. For the Wufeng and Longmaxi formations, SiO2 (20.79–81.98% and 39.9–75.11%, respectively), CaO (1.85–33.58% and 4.48–21.8%, respectively) and Al2O3 (1.83–14.6% and 3.39–20.55%, respectively) are the most abundant oxides.

Table 2 Analytical data of major elements (wt%) and organic carbon isotopes (‰) in shales and carbonaceous marls from the Wufeng-Longmaxi formations in the Xindi No. 2 well.

According to XRD analysis data, the mineral compositions of shales from the Wufeng and Longmaxi formations are mainly composed of quartz, clays, calcite, and dolomite, which is consistent with the major element compositions (Table 2). Based on the XRD analysis data, the mineral compositions of shales from the Wufeng and Longmaxi formations are dominated by quartz, clays, calcite, and dolomite, which is consistent with the chemical compositions (Table 2). MgO (0.93–6.2% and 1.13–5.62%, respectively), K2O (0.54–4.27% and 1.00–5.36%, respectively), Fe2O3 (0.06–4.54% and 0.05–8.05%, respectively), and FeO (0.47–2.34% and 0.57–1.90%, respectively) are the second most abundant oxides in the Wufeng and Longmaxi formations. While other oxides including MnO, Na2O, P2O5, and TiO2 contents are both lower than 1.0%.

The SiO2 content in the carbonaceous marl samples from the Guanyinqiao Formation is relatively high (36.66–58.11%, average of 48.22%). CaO (2.28–18.64%) and Al2O3 (6.15–14.39%) are the second-most abundant oxides, with averages of 13.09% and 8.51%, respectively. The contents of K2O and MgO are 1.81–4.12% and 0.91–3.7%, respectively. The Fe2O3 content of carbonaceous marls (0.85–12.96%) is relatively higher than the FeO content (0.83–1.26%). The contents of other oxides, such as MnO, Na2O, P2O5, and TiO2, are less than 1.0%.

The enrichment factor(EF) was applied by using aluminium as the detritus index to estimate the enrichment degree of elements in each sample, and the EF value was determined by using the formula: EFX = molar[(X/Al)sample/(X/Al)average shale], where X represents the element in Table 5. EFX > 1 reflects the relative enrichment of element X, while EFX < 1 indicates that element X is depleted compared to average shale55,56. The major element EFs of black shales and carbonaceous marls from the Wufeng-Longmaxi formations are shown in Table 5 and Fig. 5. The enrichment of CaO in the above three formations can be attributed to the existence of calcite (4.0–28%, average of 17.7%) and dolomite (2.3–27.2%,average of 14.3%) according to XRD analysis. The relatively high concentration of magnesium is also related to the dolomite content57. As shown in Fig. 5, the K contents of the sediments from the Wufeng, Guanyinqiao, and Longmaxi formations have relatively high K/Al ratios owing to higher illite contents58. Titanium, a diagenetically stable constituent of marine sediments, has an EF less than 1.0. The strong correlation between Ti and Al may also be because these two elements are dominated by detrital sources, but not autogenous enrichment55.

Figure 5
figure 5

Enrichment factors of the major elements from the Wufeng-Longmaxi formations in the Xindi No. 2 well. The horizontal line is the EF of average shale, which is equal to 1 to highlight the enrichment or depletion of elements.

The chemical index of alteration (CIA) was applied to quantify the degree of chemical weathering59,60. The CIA can be calculated using molecular proportions, from the equation CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100, where CaO* represents the concentration of CaO in silicate minerals only61. In this case, the CaO content must be corrected for the presence of carbonates (calcite, dolomite) and apatite. In this study, the available P2O5 data was initially used for phosphate correction of CaO by the following formula: CaO* = mole CaO − mole P2O5 × 10/3. If the remaining number of moles was less than that of Na2O, the CaO value was adopted as CaO*. Otherwise, the CaO value was assumed to be equal to Na2O62.

Rare earth elements

REE analysis and related parameters are listed in Table 3 and shown as chondrite-normalized models in Fig. 6. The samples from all three formations were normalized to chondrite values and the Eu anomaly was calculated according to McLennan63, i.e., Eu/Eu* = (Eucn)/[(Smsn × Gdcn)1/2], in which the subscript cn denotes normalization of the REE to chondrite values63.

Table 3 Analytical data of rare earth elements (REEs) of the shales and carbonaceous marls from the Wufeng-Longmaxi formations in the Xindi No. 2 well.
Figure 6
figure 6

Chondrite-normalized REE distribution patterns of samples from the Wufeng-Longmaxi formations in the Xindi No. 2 well (normalization values are after Taylor and McLennan64).

The total rare earth element contents (∑REEs) from the Wufeng, Guanyinqiao and Longmaxi formations vary significantly from 40.89 to 310.23 ppm, with average values of 133.16 ppm, 111.52 ppm and 113 ppm (Table 3), which are all lower than the average value of Post-Archean Australian Shale (PAAS) (184.77 ppm)63. Similar REE distribution curves are shown for all three formations, including relatively light rare earth element (LREE) enrichment, flat heavy rare earth element (HREE) patterns, and negative Eu anomalies (Fig. 6). The contents of LREEs (53.91–266.07 ppm) from the above three formations are significantly higher than those of HREEs (7.77–44.16 ppm). Similar conclusions can be drawn based on the ratios of Lacn/Ybcn, where Lacn/Ybcn < 1(> 1) shows HREE enrichment (depletion) compared to LREEs. The ratios Lacn/Ybcn from the three formations change from 3.25 to 16.32, with an average of 8.17 (Table 3). All samples from the Wufeng-Longmaxi formations exhibit negative Eu anomalies, varying from 0.46 to 0.97, with a median of 0.68, which is similar to the Eu anomalies of the PAAS (δEu = 0.66).

Trace elements

Analytical results of trace elements are given in Table 4. The elements Ba (with an average of 1444.48 μg/g, 1933.71 μg/g, and 2175.54 μg/g, respectively), Sr (with an average of 323.51 μg/g, 263.43 μg/g, and 228.98 μg/g, respectively), V (with an average of 138.60 μg/g, 135.63 μg/g, and 183.81 μg/g, respectively), Zn (with an average of 348.87 μg/g, 47.37 μg/g, and 127.71 μg/g, respectively) in Wufeng, Guanyinqiao and Longmaxi formations are the most abundant. Ni (with averages of 52.81 μg/g, 104.26 μg/g, 79.63 μg/g, respectively) and Zr (with averages of 85.83 μg/g, 139.23 μg/g, and 68.27 μg/g, respectively) are the second most abundant elements.

Table 4 Analytical data of some trace elements from the Wufeng-Longmaxi formations in the Xindi No. 2 well.

The average contents of all other elements are generally below 50 μg/g. The EFs for the selected trace elements of the three formations are shown in Fig. 5B and Table 5. Some trace elements in the Wufeng, Guanyinqiao, and Longmaxi formations show significant enrichment, including Ba (EF = 4.95, 6.45 and 8.57, respectively), U (EF = 2.64, 4.26 and 9.92, respectively), V (EF = 2.12, 2.05 and 3.23, respectively), Mo (EF = 4.64, 13.76 and 12.84, respectively), Hf (EF = 1.78, 2.88 and 1.91, respectively), Ni (EF = 1.54, 3.01 and 2.67, respectively), and Th (EF = 1.71, 1.73 and 1.79, respectively). Other elements Ho (EF = 0.92, 0.98 and 0.92 respectively), Ta (EF = 0.73, 0.74 and 0.66, respectively), Cr (EF = 1.09, 0.80 and 0.84, respectively) and Co (EF = 0.87, 1.10 and 0.90, respectively) have average EFs less than or approximately equal to 1.0 (Fig. 7). Most of the remaining trace elements in these three formations show slight enrichment. The trace element variation patterns in all three formations are similar (Fig. 7).

Table 5 Average concentrations of all elements normalized to Al from the Wufeng-Longmaxi formations in the Xindi No. 2 well.
Figure 7
figure 7

The EFs of trace elements relative to average shale in the Xindi No. 2 well. The horizontal line is the EF of average shale, which is equal to 1 to highlight the enrichment or depletion of elements.

Organic carbon isotopes

Fifty-four samples were analysed for δ13Corg values across the Ordovician–Silurian boundary succession at the Xindi No. 2 well (Table 2). The δ13Corg values commonly vary between − 27.8 and − 29.7‰ in Dicellograptus complanatusParaorthograptus pacificus biozones, showing a positive excursion starting from the upper P. pacificus zone and reaching the maximum (− 25.3‰) in the uppermost part of the Metabolograptus. persculptus zone in the uppermost Hirnantian. Going upwards, the δ13Corg values decrease rapidly from − 26.3 to − 29.3‰ in the lower part of the Akidograptus. ascensus zone in the early Rhuddanian stage. Farther up, the δ13Corg values are relatively consistent between − 28.5 and − 30.1‰ from the A.ascensus through the Cystograptus. veiculosus biozones.


Provenance and tectonic setting

In general, some heavy minerals, such as zircon, are enriched gradually during sedimentary recycling62. According to Taylor et al.64, the Th/Sc ratios reflect bulk source compositions, because Sc and Th are transferred quantitatively from the source to the sediment. McLennan63 suggests that Zr/Sc ratios can be used as tracers for zircon or heavy mineral concentrations, as zirconium is mainly concentrated in zircons, in which less resistant minerals are preferentially destroyed.

Therefore, the Th/Sc and Zr/Sc ratios in this study and their bivariate plots were used to derive their sediment recycling degrees and mineral composition varieties63,64. The Th/Sc and Zr/Sc ratios of these three formations fluctuated from 0.69 to 4.39 (average of 1.34) and from 5.64 to 43.51 (average of 9.76, Table 4). Both of these ratios indicate a lower fractionation, and the composition is close to PAAS. These data combined with the Th/Sc-Zr/Sc bivariate diagram (Fig. 8) indicate that during the deposition of the Wufeng-Longmaxi formations, sedimentary recycling was minor, and few clastic components came from older sediments. Furthermore, the sedimentary recycling of the studied samples is low, indicating that these geochemical data can be used to identify the provenance.

Figure 8
figure 8

Th/Sc–Zr/Sc bivariate diagram of shales and carbonaceous marls to show their sediment recycling (McLennan et al.62).

Geochemical data of detrital sediments have stable geochemical properties during weathering, transportation, and diagenesis; therefore, they can provide reliable information on provenance65,66. To infer the provenance of sedimentary rocks, several authors have proposed major-based (e.g., Al2O3, TiO2, and K2O) discrimination diagrams in various studies of unknown basins67,68. According to Hayashi et al.69, the Al2O3/TiO2 ratio change in igneous rocks is as follows: (a) the values of Al2O3/TiO2 in mafic igneous rocks vary from 3 to 8, (b) the values of Al2O3/TiO2 in intermediate igneous rocks fluctuate from 8 to 21, and (c) the values of Al2O3/TiO2 in felsic rocks vary from 21 to 70. The Al2O3/TiO2 ratio of the shales in the Wufeng Formation varies from 13.65 to 41.11 with an average of 22.11, the Al2O3/TiO2 ratio of the carbonaceous marls in the Guanyinqiao Formation varies from 18.12 to 41.11 with a median of 19.22, and the Al2O3/TiO2 ratio of the shales in the Longmaxi Formation varies from 20.375 to 60.44 (average of 23.59). These values suggest that the source rocks of the above formation are mostly between intermediate and felsic igneous rocks (Fig. 9A).

Figure 9
figure 9

Provenance identification plots for the studied samples. (A) Al2O3 vs. TiO2 (after Hayashi et al.69); (B) La/Th vs. Hf (after Floyd and Leveridge67); (C) La/Yb vs ∑REE (after Roser and Korsch68).

REE distributions in sedimentary rocks have played a vital role. The stable and unaffected properties during weathering, erosion, and early diagenesis make them valuable to provenance, leading to their special utility in tracing the source of sedimentary rocks70,71. The Eu anomalies in sediments are generally considered to have been inherited from the source rocks72,73,74. Generally, the LREE/HREE ratio of mafic rocks is low, and there is no Eu anomaly, while the LREE/HREE ratio of felsic rocks is usually high, and the Eu anomaly is significant75,76. The normalized abundance and pattern of chondrites indicate that all samples are characterized by LREE enrichment, HREE deficits, and distinctly negative Eu anomalies (0.46–0.92; average of 0.6). All of these results suggest that felsic source rocks are the major source rocks for the Wufeng-Longmaxi formations.

In addition, several stable elements (e.g., La, Th, Hf, Yb, Zr, REEs) have been used to deduce the provenance of sedimentary rocks due to their immobility during sedimentation76,77,78. On the bivariate diagram of La/Th-Hf67, most samples plot in or near the felsic source field (Fig. 9B). In addition, the bivariate diagram of La/Yb vs. ∑REE reflects that the Wufeng-Longmaxi samples mainly plot in the granite source rocks (Fig. 9C). Overall, the provenance discrimination diagrams reveal that felsic (granitic) source rocks are the major source rocks for the studied Wufeng-Longmaxi formations.

Numerous studies have shown that the geochemical characteristics of detrital rocks are significantly controlled by the plate tectonic setting of the source area, therefore, the tectonic setting of the ancient terrains has been widely identified by using major-, trace- and rare earth element-based discrimination diagrams79,80,81. Bhatia and Crook70 proposed trace element-based discrimination diagrams to differentiate four tectonic settings: continental island arc, oceanic island arc, active continental margins, and passive margins. As shown in Fig. 10, most of the studied samples fall within or adjacent to the continental island arc and active continental margin domain in diagrams of La–Th–Sc, Th–Co–Zr/10, and Th–Sc–Zr/10.

Figure 10
figure 10

La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10 plots of the shales and carbonaceous marls from the Wufeng-Longmaxi formations in the Xindi No. 2 well for tectonic discrimination. Dotted lines represent the dominant fields for sedimentary rocks from various tectonic settings (Bhatia and Crook70): (A) oceanic island arc; (B) continental island arc; (C) active continental margin; (D) passive margin.

To increase the success rate of identifying the tectonic setting, Verma and Armstrong-Altrin82 used common oxides (SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, P2O5, and MnO) to develop two multidimensional diagrams based on the loge-ratio transformation of major oxides to discriminate arc, continental rift, and collisional settings. All the major oxides must be adjusted to 100% after excluding the loss on ignition (LOI) and regarded as (X)adj, where X represents the major oxides. These diagrams can be divided into two types based on the difference in (SiO2)adj values, low-silica type (36%-63%), and high-silica type (63–95%). More details about the calculated equations for these two types of sediments are described in Verma and Armstrong-Altrin82. Recently, these diagrams have been successfully applied to discriminate the tectonic setting of older sedimentary basins83,84,85,86. In the present study, the (SiO2)adj contents of the Wufeng-Longmaxi formations vary from 32.47 to 87.15, which can be classified as low-silica samples (n = 41) and high-silica samples (n = 13). For the high-silica diagram (Fig. 11A), eleven high-silica samples from the Wufeng and Longmaxi formations plot in the collision field, with two samples from the Guanyinqiao Formation (XD2P-B31 and XD2P-B29) plotting in the arc field. On the low-silica diagram (Fig. 11B), all forty-one low-silica samples plot in the collision field. All the discriminant-function diagrams above reflect that the sediments of the Wufeng and Longmaxi formations mainly originated from a collisional setting, while the Guanyinqiao Formation may be derived from rift and collisional settings.

Figure 11
figure 11

New discriminant function multidimensional diagram proposed by Verma and Armstrong-Altrin82. (A) High-silica clastic sediments from three tectonic settings (arc, continental rift and collision). (B) Low-silica clastic sediments from three tectonic settings (arc, continental rift, and collisional).

Palaeoweathering indices and palaeoclimate implications

The intensity of chemical weathering is controlled mainly by the source rock composition, during weathering, climatic conditions, and rates of tectonic uplift of the source region (e.g., Refs.87,88). During chemical weathering, alkaline metal elements such as Ca, Na, and K are largely removed from source rocks, while the concentrations of Al, Si, and Ba increase in the residue89,90. The degree of weathering can be quantified by using mobile and immobile element oxides such as Na2O, CaO, K2O, and Al2O3. The ternary diagram of Al2O3 − (CaO* + Na2O) − K2O (A–CN–K) (molecular proportions91) has been widely used to evaluate the differences in chemical composition related to chemical weathering, which can also be used to analyse the weathering history and palaeoclimate92,93,94,95,96. As shown in Fig. 12, the samples of the three formations merged above the plagioclase-K-feldspar join, showed a narrow linear trend and were close to the muscovite and illite fields, which are consistent with the XRD results (Table 1). The weathering trend is parallel to the A-CN borderline and does not show any tilt to the K apex, indicating that the chemical weathering conditions are relatively stable, and excluding the effect of potassium salt metasomatism during diagenesis, which corrects CIA values for further analysis.

Among the different indices of weathering, the CIA is widely used in recent studies to quantify the degree of weathering97,98,99. Young and Nesbitt100 proposed that the degree of chemical weathering is related to the climatic conditions in the source area, and is considered a method to study palaeoclimate changes. This discovery also makes CIA indicators widely used to reconstruct palaeoclimate conditions20,97,98,99.

Figure 12
figure 12

Al2O3–CaO* + Na2O–K2O ternary diagram for shales and carbonaceous marls of the Wufeng-Longmaxi Formations in the Xindi No. 2 well (after Nesbitt and Young91).

The CIA values of the Wufeng-Longmaxi formation samples were calculated according to the method of McLennan62, and the results are listed in Table 2. Across the Ordovician–Silurian boundary, the CIA values of black shales from the Katian stage (from D. complanatus through P. pacificus biozones) are relatively consistent (Fig. 3), ranging from 67.48 to 73.37 (average of 69.72). In the lower M.extraordinarius zone (early Hirnantian), the CIA values, although they apparently decrease to a low, show a more fluctuating pattern in the M. extraordinarius-M. persculptus zone, varying between 58.30 and 68.74 (average of 62.62). At the top of the M. persculptus zone, they reached 64.66. Farther up, the CIA values return to persistently high values, from 63.47 to 72.85 (average of 67.77) in the Rhuddanian stage (from the A. ascensus through C.vesiculosus biozones).

In general, a higher CIA value (80–100) indicates tropical and humid climate conditions with strong chemical weathering, a medium CIA value (60–80) indicates moderate chemical weathering accompanied by warm and humid climate conditions, and a lower CIA value (50–60) indicates the palaeoclimate conditions of weak chemical weathering with a cold and arid climate60,61,97. The lower CIA value (67.48–73.37) of the lower Wufeng Formation (D. complanatusM. extraordinarius biozones) indicates a moderate degree of chemical weathering, which may reflect the warm and humid climatic conditions in the source area. Farther up, the CIA values during the Hirnantian stage (M. extraordinariusM. persculptus biozones) first decreased from 69.23 to 61.51, then suddenly increased to 68.74, and then still showed a fluctuating pattern (58.30–64.66). These results indicate that the climatic conditions were mainly cold and dry before deposition, and the sediments experienced weak to moderate weathering, which is consistent with widespread Hirnantian glaciation101,102,103. However, based on the data fluctuations, there may also be short-term pulses of climate warming during this interval. In summary, during the Hirnantian stage, the climate fluctuated rather than was persistently cold and arid.

A similar scenario has been reported in previous studies97,104,105, including approximately five glacial-interglacial cycles during the glacial period with an estimated duration of ~ 50013 to ~ 1.0-Myr106. As shown in Fig. 3, our fluctuating but relatively low CIA values correlate with a pronounced δ13C positive excursion, which represents a match with the global δ13C Hirnantian excursion (HICE)107,108. Following this minimum, the CIA values increased above the Early Rhuddanian (at the bottom of the A. ascensus biozone), indicating that chemical weathering gradually increased, but compared to the pre-glaciation sediments (lower part of the Wufeng Formation) the CIA values remained relatively low, which means that the post-glacial climatic conditions remained relatively cool.

Palaeoredox conditions

Many trace elements, including U, V, Mo, Ni, and Co, show that their oxidation state and solubility change along with the variations in the redox state of the sedimentary environment; thus, they have been widely used to reconstruct palaeoredox conditions of sedimentary rocks and marine sediments109,110,111,112,113. Among such elements, uranium (U) and molybdenum (Mo) have been extensively studied due to differences in their geochemical behaviour, and their covariation is considered to be related to specific redox conditions and processes in marine depositional systems114. The absorption of authigenic uranium by marine sediments starts under suboxic conditions, while the enrichment of authigenic molybdenum requires the presence of H2S (i.e., euxinic conditions)115,116,117,118. Moreover, aqueous U is completely unaffected through the particulate Mn/Fe-oxyhydroxide shuttle, but aqueous Mo may be enhanced during this process119,120. Based on the above differences, the autogenous U–Mo covariance mode can be used to track the redox conditions, which are determined by its EF (see definition in “Major element”). In addition, some redox indices (U/Th, V/Cr, Ni/Co, and V/V + Ni) have been widely used to derive information on the palaeo-oxygen level of depositional environments121,122,123,124,125. All four indices were calculated and reported as stratigraphic variations in Fig. 14.

All palaeoredox proxies indicate significant changes in redox conditions during the Late Ordovician to early Silurian, which can be identified as five distinct parts (Figs. 13, 14). For part I, during the accumulation of lower Wufeng formation (D. complantus, D. complexus, and lower part of P. Pacificus graptolite zones), all of the samples exhibit little or no enrichment for Mo and U, with (Mo/U)auth ratios ranging from 0.05 to 1.33 with a median of 0.13, which are mostly between 0.1 and 0.3 times those of seawater (Fig. 13). The U/Th, V/Cr and Ni/Co values range from 0.16 to 0.83 (average of 0.35), 1.36–6.05 (average of 2.48) and 3.48–7.79 (average of 6.24), respectively, pointing to oxic to suboxic bottom water (Fig. 14). The above data suggest fluctuations between oxic and suboxic conditions.

Figure 13
figure 13

Plot of MoEF vs. UEF from the studied samples normalized using shale data56. The solid line shows the Mo–U molar ratio equal to the seawater value (1 × SW). Dashed lines indicate Mo–U molar ratios equal to fractions of seawater values110,114 (3 × SW, 0.3 × SW).

Figure 14
figure 14

Stratigraphic variations in palaeoredox proxies for the Xindi No. 2 well.

For part II, during the upper Wufeng Formation (upper part of the P. Pacificus graptolite zone), most samples display elevated Mo enrichment, ranging from 5.36 to 18.1 ppm, with an average of 11.19 ppm. (Mo/U)EF ratios mostly fluctuate between 0.3 and 1 times seawater, plotting closer to the anoxic end (Fig. 14). The U/Th, V/Cr, and Ni/Co values increase, varying between 0.56 and 2.30 (average of 1.00), 2.64–6.15 (average of 3.84), and 4.85–9.74 (average of 7.25), indicating fluctuations between suboxic and anoxic bottom waters. All of the above data reflect at least intermittent anoxic conditions below the Guanyinqiao Formation.

For part III, during the accumulation of Guanyinqiao Formation (M. extraordinarius and M. persculptus graptolite zones), most palaeo-redox proxies show first an increase and then a decrease, including Mo and U contents and U/Th, Ni/Co, V/Cr, V/V + Ni ratios. Most samples show low concentrations of Mo (< 10 ppm), but much higher Mo contents (32.60 and 71.80 ppm) occurred at the base of the Guanyinqiao Formation. (Mo/U)EF ratios are generally less than that of seawater (0.3 × SW to 1 SW; Fig. 13), suggesting suboxic bottom water conditions. The U/Th, V/Cr, and V/V + Ni ratios indicate oxic to suboxic conditions (Fig. 14). Although most of the samples point to anoxic conditions, the Ni/Co ratio still shows a trend of increasing first and decreased afterwards. Thus, careful should be taken when applying these criteria to interpret the redox conditions.

For part IV, during the lower Rhuddanian Stage (A. ascensus-C. veiculosus graptolite zones), most palaeo-redox indices show similar variation patterns to that of part III, which increase first and then decrease. Most samples exhibit high average Mo concentrations of 49.67 ppm (and as large as 111 ppm), with (Mo/U)EF ratios mostly approximately 1 time that of seawater (Fig. 13), suggesting a euxinic condition. Similar conclusions were found in the interpretations of U/Th, V/Cr, Ni/Co, and V/V + Ni ratios (Fig. 14), which indicate persistent strong euxinic bottom water during the deposition of the lower part of the Longmaxi Formation.

For part V, during the middle to upper Rhuddanian Stage (upper part of A. ascensus–C. veiculosus graptolite zone), most proxies remain at a steady level compared to those in part IV. The V/Cr, Ni/Co and U/Th ratios range from 3.2 to 7.31 (average of 4.75), 6.84–12.47 (average of 9.2), and 0.38–2.09 (average of 1.35), respectively, reflecting basic anoxic bottom waters. The Mo concentrations maintain a high level (average of 44.1 ppm), with (Mo/U)EF ratios mostly approximately 1 times that of seawater (Fig. 13), revealing euxinic water conditions. The above data suggest that euxinia may have been maintained during the deposition of the lower Longmaxi Formation.

Relationship of δ13Corg variations to environmental changes and possible causes of carbon isotope excursions during the O–S interval

Our data that are derived from Xindi No. 2 well as described above clearly show water column redox variations pre-and-post Hirnantian glaciation, which can be summarized as two pulses of deepening degrees of anoxia. A parallel variation in δ13Corg values is also well documented in the Xindi No. 2 well. The positive δ13Corg excursion recognized as the HICE was initiated after the first pause of the Hirnantian glaciation from the M.extraordinarius zone to the lower M. persculptus zone (Figs. 12 and 14), followed by subsequent redox condition variations from increasing anoxia to sudden oxic-suboxic conditions. The δ13Corg values recorded above the HICE exhibit a − 3.2‰ negative excursion after the second pause of the Hirnantian glaciation from the upper M. persculptus zone to the lower A. ascensus zone, accompanied by rapid variations from oxic-suboxic to strong euxinic water conditions. These two abrupt shifts display strong covariations between δ13Corg and redox conditions, indicating a significant change in the relative carbon fluxes between ocean water, organic matter, and sediments.

In order to explore the spatial variation in the marine carbon isotopes during the Late Ordovician–early Silurian, we compared the lateral and vertical change in δ13Corg values across proximal to distal areas on the Yangtze Shelf Sea (Fig. 15). Three sections were selected for geochemical analyses20,42, along with the Xindi No. 2 well in this study, and each section represents a lateral change from the inner- to mid-shelf. All sections are well established by graptolite biozonation, which can be used as stratigraphic correlation. As is shown in Fig. 15, the positive δ13Corg excursions characterize the Hirnantian interval across all four sections, but by comparing the positive shift, the records appear to be diachronous. In the shallow-water setting (NBZ) recorded by Yan et al.40, the increase in δ13Corg occurs in the Metabolograptus extraordinarius biozone, while this excursion can be observed in the Metabolograptus persculptus biozone in a deeper water setting from the inner-shelf (SH section) to the mid-shelf (QL section), as well in the Xindi No. 2 well. The offset of δ13Corg values vary across all four sections, with a larger offset of ~ 3.7‰ in the Xindi No. 2 well, and of ~ 2.6‰ in the NBZ section, while relatively smaller offset values are present in the SH and QL sections (of ~ 1.2‰ and ~ 1.1‰). We suggest that this δ13Corg gradient might be explained by the different oceanic environments between shallow- and deep-water settings. SH and QL were under anoxic-euxinic conditions during Hirnantian glaciation according to Fe speciation and Mo concentrations20, while the NBZ and Xindi No. 2 well have been established as oxic and oxic-suboxic during the glaciation by Zhou et al.12 and by palaeoredox proxies in this study (Fig. 14). These spatial differences in water column redox conditions were probably due to differences in depositional water depths, with shallower waters associated with more oxic conditions.

Figure 15
figure 15

General trends of δ13Corg, climate change, and redox conditions in the Xindi No. 2 well (this study), NBZ42, SH, and QL20 sections. Graptolite zones: D. cn.—Dicellograptus complanatus; D. cx.—Dicellograptus complexus; P. pacificus—Paraorthograptus pacificus; M. e.—Metabolograptus extraordinarius; M. p.—Metabolograptus persculptus; A. a.—Akidograptus ascensus.

Positive excursions (both δ13Ccarb and δ13Corg) during the Hirnantian stage have been reported from sections of the Yangtze Platform (e.g. Refs.3,11,17,30,126,127) as well as from different regions worldwide(e.g. Refs.13,14,108,128). Proposed geological processes that may cause positive δ13C excursions include the following: (1) enhanced marine productivity and organic matter burial13,15,129,130, (2) increased carbonate-platform weathering128,131, (3) dissolved inorganic carbon (DIC) with high δ13C132, and (4) sea-level eustacy and shoaling of the marine chemocline38,127,133. The positive δ13C excursion has been interpreted as the result of increased organic carbon burial, which lowered atmospheric pCO2, leading to a consequent 13C enrichment108,128,134,135. Due to the fact that photosynthetic carbon fixation of marine phytoplankton favours 12C relative to the carbon reservoir, the progressive increase in the burial of organic matter could have led to substantial enrichment of 13C in seawater, leading to a positive excursion of δ13C (e.g. Refs.15,128,129,136. Based on these theories, the HICE is inferred to be indicative of a major carbon burial event during the Late Ordovician. However, this organic carbon burial causal model is yet to be reconciled with the marked lithofacies change, as the shelly limestone of the Guanyinqiao Formation is sandwiched between the pre-glaciation of the Wufeng Formation and the post-glaciation of the Longmaxi Formation black shales. Furthermore, data collected from recent studies have shown low TOC values during the Hinantian interval both on the Yangtze Platform (e.g. Refs.30,43) and from global strata (e.g. Refs.19,137,138,139). The carbonate weathering model proposed by Kump et al.128 linked the δ13C excursion, especially that of δ13Ccarb, to the enhanced carbonate platform weathering during glacioeustatic sea-level lowstand. The strong positive δ13Corg excursion from the Xindi No. 2 well coincident with the glacial Guanyinqiao Formation may reflect increased carbonate weathering during the Hirnantian glaciation. Although there may exhibit short-term pulses of climate warming during the Hirnantian glaciation (Fig. 12), which could accelerate carbonate platform weathering, high-resolution studies are still required to verify this conclusion. Apart from organic carbon burial and carbonate platform weathering, other factors, including dissolved inorganic carbon (δ13CDIC) and glacio-eustatic sea-level change, cannot be excluded. Dissolved inorganic carbon (DIC) with high δ13C values, which were possibly generated by an authigenic carbonate precipitation mechanism, may play an important but poorly understood role in the acknowledgment of the Hirnantian excursion. According to Ahm et al.19, glacio-eustatic restriction of shallow epeiric seas may increase both lateral- and vertical gradients in δ13CDIC from the shallow shelf to the deep basin. The lateral gradient exhibits higher seawater δ13C values in shelf-proximal than in shelf-distal environments due to increased photosynthesis and carbonate weathering, which can be observed in the Canada Arctic and Baltic basins133,140 as well in the Yangtze shelf sea127,141. This theory may explain such a high δ13Corg peak (~ − 25.3‰) in the Xindi No. 2 well, as its locality is closer to the shelf-proximal unit compared to previous studies (Fig. 15). Generally, organic matter is produced by photosynthetic organisms in surface water, as a result of the uptake of 12C, and is removed as sedimentary organic matter in deep water, causing even higher 13C values in proximal settings142. A surface- to deep-water gradient has been recorded in the modern Black Sea, which is a typical example of a bottom-sulfurized and stagnant ocean, where the δ13Corg of photosynthate in surface waters is ~ 4‰ heavier than the δ13Corg of sedimentary organic matter in sulfidic deep waters143. It is worth noting that the Xindi No. 2 well was characterized by the highest δ13Corg value among all four sections (Fig. 15), which can be explained by two factors. On the one hand, the increasing circulation of organic carbon fluxes, either by photosynthetic activity or by dissolved O2, contributed to the preservation of the heavier δ13Corg under more oxic water relative to those deposited under anoxia water conditions. On the other hand, increasing carbonate weathering exposed by the glacio-eustatic sea-level fall may have further accelerated carbon input to the proximal setting, contributing to the positive δ13Corg excursion. Thus, our high values of δ13Corg might be associated with increased photosynthetic activity and carbonate weathering when the sediments of the Xindi No. 2 well were deposited under oxic conditions in a shelf-proximal setting.

Environmental change in South China during the Hirnantian glaciation

The LOME was marked by two-phase extinction events144. The first strike occurred in the M. extraordinarius–N. ojsuensis biozone (~ 445 Ma) coinciding with the onset of the Hirnantian glaciation and the positive shift in δ13C values of carbonates and organic carbon. The second wave of extinctions occurred in the early M. persculptus Zone (~ 444 Ma) coinciding with the subsequent abrupt rise in sea-level marked by a large negative shift in δ13C values1,6,11,20,129,145. Our δ13Corg isotopic data provide a good constraint on the timing of the two-phase extinctions, combined with the aforementioned studies, which can also provide the relevant evolution of the temporal changes in climatic and ocean redox conditions (Figs. 8 and 15). From the late Katian (Late Ordovician), the Yangtze Block constantly collided with the Cathaysia Block due to Caledonian movement146,147,148,149,150, resulting in northwest–southeast compression that caused the constantly expanding uplifts (i.e., Chengdu, Dianqian, and Jiangnan-Xuefeng uplifts) around the Sichuan basin and a deepening of the inner Yangtze Sea151,152,153,154,155. During the deposition of the lower part of the Wufeng Formation, the Yangtze Sea evolved into a semi-isolated shelf sea, and gradually developed suboxic to anoxic bottom waters, which is consistent with all palaeoredox proxies (Mo-auth/U-auth, U/Th, V/Cr, and Ni/Co ratios). The climate of the Upper Yangtze basin during this time was generally warmer and more humid, consistent with CIA values from 67.48 to 73.37 (average of 69.72).

In the early Hirnantian, glaciation started on Gondwana, and its initial phase was related to the beginning of a cool climate. With the continuous formation of ice sheets, the global sea level began to fall, and the climate became colder and drier, which is consistent with the decline in CIA values (from 69.23 to 61.51) at the bottom of the Normalograptus. extraordinarius biozone (upper part of the Wufeng Formation). As temperatures decreased and eustatic sea-level dropped, redox conditions varied from anoxic to oxic-suboxic, which is consistent with the geochemical evidence discussed in our previous chapter. The overlying Guanyinqiao Formation thus deposited carbonaceous marls with abundant and diverse shelly fauna. During the early Hirnantian period, the oceanic redox evolution of the entire Yangtze Sea was mainly caused by sea level decline, and the sea level decline itself was the result of rapid global cooling. This result highlights the close relationship between the redox evolution of the Late Ordovician water column and climate change at that time26,40,156. These conclusions are supported by the δ13Corg evidence in the Upper Hirnantian samples (Fig. 15).

In the late Hirnantian stage (M. persculptus biozone), glaciation ended, marked by an abrupt eustatic sea level rise. The climate started getting warmer and moister, which is consistent with the increase in the CIA values in the upper part of the Guanyinqiao Formation (from 58.30 to 62.60). As temperatures increased and sea level rose, the water-column redox rapidly returned to euxinia, coincident with the rising ratios (Mo-auth/U-auth, U/Th, V/Cr, Ni/Co, and V/V + Ni ratios) and Mo concentrations. In the early Rudanian stage, sea level rise and widespread euxinia jointly promoted black shale deposition in the Longmaxi Formation.

These data, together with previous research from other areas in the Yangtze Sea, indicate significant climatic fluctuations, from warming to cooling at the end of the late Katian stage, and then back to a warm climate at the end of the late Hirnantian, possibly fluctuate during these two-time intervals. Correspondingly, the marine redox system changed from anoxic to oxygenated, and then suddenly evolved into euxinic bottom waters, consistent with two pulses of the LOME. Climatic fluctuations, coupled with oceanic anoxia, were likely responsible for the gradual end of the Late Ordovician biotic crisis.


This study aims to reconstruct the depositional history of the Late Ordovician-early Silurian boundary successions in the Xindi No. 2 well deposited on the Yangtze shelf sea and provides new insights into possible drivers of perturbations of the carbon isotope records and possible causes of the LOME. Sediments from the Wufeng-Longmaxi formations were mainly derived from the collisional settings, presumably from active continental margins based on the ternary diagrams and multidimensional diagrams. The provenance of shales and carbonaceous marls from these three formations are mainly from felsic igneous rocks and show weak sediment recycling. The CIA values of the studied samples indicate that the intensity of chemical weathering in the source area was weak to moderate. Lower Wufeng and Longmaxi formations indicate a warm and humid climate during deposition. In contrast, the CIA value suddenly began to decline in the upper part of the Wufeng Formation (Hirnantian stage) and continued to decline until the Guanyinqiao Formation, and data fluctuations during this period indicate that the cold and arid climate was interrupted by brief warm climates. The palaeoredox indices (Mo concentrations and Moauth/Uauth, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician- early Silurian indicate two cycles of water column euxinia. The first cycle occurred during the accumulation of the Wufeng Formation, with bottom waters evolving from oxic-suboxic (part I) to suboxic-anoxic (part II). Most samples show relatively low concentrations of redox-sensitive trace elements during the Guanyinqiao Formation (part III), pointing to oxic-suboxic conditions. The water column transitioned from oxic to euxinic in the late Hirnantian (at the base of the Longmaxi Formation, part IV). Our δ13Corg data are comparable to previously reported records and exhibit a strong correlation between the HICE, climate change, and redox conditions. We suggest that the variations in δ13C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions; (2) increased carbonate weathering exposed by the glacio-eustatic sea-level. In addition, our high values of δ13Corg compared to the other three previous sections might indicate a more shelf-proximal setting during the deposition of the Xindi No. 2 well. The δ13Corg isotopic data effectively constrain the timing of the LOME and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.