Introduction

The late Cambrian SPICE (Steptoean positive carbon isotope excursion) event is a significant positive shift in δ13Ccarb values of approximately 5‰ between 497 and 494 Ma1. It was initially recognized in North America2, and later was confirmed as a global paleoceanographic phenomenon3, observed globally within carbonate successions from diverse paleogeographic settings4,5,6,7,8,9,10,11,12,13 (Fig. 1a). The SPICE event is not only concurrent with a major perturbation of the global carbon cycle14,15,16 but also reflects profound biogeochemical and ecological shifts within the ocean-carbonate system17,18,19 during a critical interval that precedes the Great Ordovician Biodiversification Event20,21,22. The onset of the SPICE event, often associated with the appearance of the trilobite Glyptagnostus reticulatus23,24, has been utilized as a reliable chronostratigraphic marker9,24. However, its application in precise global correlation has been contested due to reported facies-dependence and time-transgressive characteristics of the isotope signal5,9,25.

Fig. 1: Illustrative paleogeographic map of the Tarim Basin showing the distribution of sedimentary facies in the region.
Fig. 1: Illustrative paleogeographic map of the Tarim Basin showing the distribution of sedimentary facies in the region.The alternative text for this image may have been generated using AI.
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a Schematic global paleogeographic map at about 500 Ma of the distribution of SPICE event identified worldwide during the late Cambrian (modified from Pulsipher et al.9). Locations where the SPICE event has been documented are indicated by colored circles, color-coded by interpreted water depth. Schematic SPICE event with different variation patterns is shown in the upper middle part. Uncertain paleogeographic locations (Tarim Basin and Kazakhstania) are denoted by two question marks. b Simplified sedimentary facies framework in the Tarim Basin during the late Cambrian (modified from Chen et al.4). XEBLK and YEDS represent the Xiaoerbulake and Yaerdangshan outcrop sections, respectively. The abbreviations also include Shutan 1 (ST1), Luntan 1 (LT1), Yingtan 1 (YT1), Tadong 2 (TD2), Zhongshen 1 (ZS1), and Yingdong 2 (YD2).

Recent studies have amplified the discussion regarding the uniformity of the SPICE event’s magnitude, stratigraphic thickness, and timing, suggesting that regional sedimentary conditions may have significantly modulated the carbon isotope record1,9,19. Such variability poses a challenge to the previously accepted paradigm of a globally synchronous anoxic event driving the SPICE signal9,13,19,25,26, prompting new lines of inquiry into localized controls over carbon cycle perturbations19. As the SPICE event coincided with critical evolutionary and ecological transitions15,22,27,28, it is imperative to assess the influence of depositional environments on the expression of this geochemical anomaly.

Examination of the constraints imposed by biogeochemical processes, paleoenvironmental conditions, and local sedimentary factors is crucial for resolving debates on the synchroneity and magnitude of the SPICE event. In the Tarim Basin (Fig. 1b), NW China, substantial occurrences of Miaolingian-Furongian strata (Supplementary Fig. S1) offer an ideal opportunity to examine a variable sedimentary response to the SPICE event within distinct depositional facies during this pivotal period in Earth’s history. Earlier research in the Tarim Basin focused predominantly on the variations in δ13Ccarb, with limited attention to corresponding fluctuations in δ13Corg and their potential coupling4,22,29,30. Furthermore, the disparity in SPICE responses across different sedimentary environments has not been adequately addressed4,29, prompting the need for a comprehensive analysis that integrates both carbonate and organic carbon isotopic data.

This study aims to clarify the relationship between sedimentary facies and the SPICE event within the Tarim Basin by presenting new paired δ18Ocarb, δ13Ccarb and δ13Corg data from the Furongian Xiaqiulitag Formation, derived from two strategically chosen wells—Luntan 1 (LT1) and Shutan 1 (ST1). By extending the analysis to include eight δ13Ccarb and six δ13Corg sections across a range of platform-basin environments along a paleodepth gradient (for details see Supplementary Geological Background), we offer a nuanced perspective on the differential expression of the SPICE event. Through detailed lithostratigraphic and chemostratigraphic analyses, we aim to contribute to the understanding of local variation and the impact of depositional conditions on the SPICE signature. This investigation underscores the importance of regional studies for refining our understanding of the late Cambrian carbon cycle dynamics and the implications for the chemostratigraphic utility of the SPICE event.

Results

The comprehensive isotope analyses of the samples from two distinct depositional settings in the Tarim Basin—the platform facies well ST1 and the slope facies well LT1—reveal a distinct carbon isotope signature of the SPICE event. Both wells demonstrate a positive carbon isotope excursion within the carbonate fraction (δ13Ccarb), rising from –1.36‰ to +0.44‰ in ST1 and from –1.63‰ to +0.52‰ in LT1, with an amplitude of approximately +2‰ (Supplementary Tables S1 and S2; Fig. 2). Although these amplitudes are modest compared to the +4–5‰ shifts observed globally, they remain consistent with those recorded in the North China region7,9,25, confirming the presence of the SPICE event within the Tarim Basin.

Fig. 2: Composite lithostratigraphic and chemostratigraphic profiles of the upper Cambrian to Lower Ordovician from two sections located in the Tarim Basin.
Fig. 2: Composite lithostratigraphic and chemostratigraphic profiles of the upper Cambrian to Lower Ordovician from two sections located in the Tarim Basin.The alternative text for this image may have been generated using AI.
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a The platform facies from well ST1 (550–1650 m). b The slope facies from well LT1 (6400–7900 m). Symbols used in both panels denote specific lithological units and geochemical markers for easy reference.

Organic carbon isotopes (δ13Corg) also exhibit a notable positive shift across the same stratigraphic interval, from –29.84‰ to –26.55‰ in ST1 and from –29.82‰ to –26.24‰ in LT1, with an amplitude of about +3.5‰ (Fig. 2). These δ13Corg shifts align with global observations of stable carbon isotopic variations in sedimentary organic matter corresponding to the SPICE event15,23,31,32. Moreover, a parallel minor dip in both δ13Corg and δ13Ccarb was documented in the two wells, preceding the SPICE event. Specifically, in well ST1, there was a decline in δ13Corg from −28.72‰ to −29.84‰, and in δ13Ccarb from −1.36‰ to −1.95‰. Similarly, well LT1 exhibited a decrease in δ13Corg (from −27.83‰ to −29.82‰) and δ13Ccarb (from −0.38‰ to −1.64‰) values.

Intriguingly, these shifts of δ13Corg and δ13Ccarb within the Tarim Basin wells not only synchronize with other regions but also display unique intra-basinal differences between the platform and slope facies. For instance, in the slope facies well LT1, peak δ13Corg occurs noticeably later than the δ13Ccarb peak, and the stratigraphic thickness associated with the δ13Corg positive excursion is significantly larger than that for the δ13Ccarb shift (Fig. 2b). In contrast, in the platform facies well ST1 there is consistency in the timing and length of the δ13Corg and δ13Ccarb excursions (Fig. 2a).

The oxygen isotopic composition of the carbonates (δ18Ocarb) associated with the SPICE event have distinct patterns (Fig. 2). In well ST1, the δ18Ocarb transiently becomes less negative at the onset of the SPICE event and then reverts to baseline levels pre-event. The entire SPICE event duration correlates with heightened δ18Ocarb values, mirroring the synchronous trends detected in well LT1 (Fig. 2). Subsequently, in well ST1 there is a pronounced and gradual decline in δ18Ocarb from –5.52‰ at the peak of the SPICE event, to –9.49‰ after the SPICE event. Conversely, in well LT1 well there is a swift drop in δ18Ocarb to –9.15‰ from –4.62‰, concurrent with the event’s termination as indicated by δ13Ccarb. Total organic carbon (TOC) values in well ST1 show negligible change throughout the event, whereas in well LT1 there is a substantial reduction in TOC concurrent with the onset of the SPICE event (Fig. 2).

Discussion

Climatic transition evidenced by oxygen isotope profiles

Diagenetic assessments indicate that carbon and oxygen isotopes reflect changes in the primary marine sedimentary environment within the studied interval (for details see Supplementary evaluation of diagenetic alteration of the carbon and oxygen isotopic data, Fig. S2). Despite some potential alteration, congruent trends in oxygen isotopic compositions in many sections (Fig. 3) remain a valuable indicator of changes in seawater temperature33. The broad but distinct geochemical patterns during the SPICE event align it with a significant climatic episode, interpreted as a global cooling event34. In an analysis of the δ18O of phosphatic brachiopods, Elrick et al.33 documented a synchronous negative shift in δ18O values during the SPICE event across three North American profiles despite potential alteration due to early diagenesis, consistent with the δ18Ocarb trends observed in the equivalent interval in the YEDS section30. In contrast, data from the four wells (LT1, ST1, ZS1, and YT1) in the Tarim Basin, spanning both platform and slope facies carbonate stratigraphy, reveal a notable correlation with the SPICE event (Fig. 3). The δ18Ocarb values remain consistently elevated throughout the SPICE event before experiencing a pronounced drop in the subsequent period.

Fig. 3: Correlation of oxygen isotope data associated with the SPICE event in four drilling wells (ST1, LT1, ZS1, and YT1) in the Tarim Basin.
Fig. 3: Correlation of oxygen isotope data associated with the SPICE event in four drilling wells (ST1, LT1, ZS1, and YT1) in the Tarim Basin.The alternative text for this image may have been generated using AI.
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Oxygen isotope data for the wells ZS1 and YT1 are from our unpublished results and Chen et al.4, respectively. The blue arrow shows the peak of SPICE event, the pale blue to pink shading shows the overall extent of the SPICE event.

Elevated δ18Ocarb values, ranging approximately from –6‰ to –5‰, mark the onset and peak of the SPICE event (Fig. 3). This isotopic enrichment is indicative of substantial environmental changes, which are consistent with cooler oceanic conditions, a shift in the global hydrological cycle, or a reduction in 18O-depleted meteoric water entering the ocean system33,35,36. However, the falling trend and termination of the event are associated with a gradual or rapid decrease in δ18Ocarb values to between approximately −9‰ and −8‰. Furthermore, the δ18Ocarb variations observed in these wells paralleled those detected during the SPICE event in wells TD229 as well as YD222 in the Tarim Basin, the Wangcun section in South China37, and the North American TE-1 as well as 12EE cores in Missouri, USA19. This significant change signals a dramatic shift, likely representative of a rapid warming trend (Fig. 3) or an increased contribution from 18O-depleted water, which may be attributed to heightened weathering38 or volcanic emissions39.

The evolution curves of the δ18Ocarb values across the SPICE interval (Fig. 3) points to a multifaceted interplay of oceanographic and atmospheric processes driving these changes9. The resolution of the δ18Ocarb records associated with the SPICE event enables a finer understanding of the temporal dynamics of environmental change during the late Cambrian33. This isotopic signal not only delineates a probable climate tipping point from cooling to warming (Fig. 3), but also provides a valuable framework for interpreting the post-SPICE environmental recovery40. These findings suggest a climatic link to the SPICE event, potentially tied to glacial activity or other contemporaneous climatic factors, underscoring the utility of δ18Ocarb as a proxy for climate influences on biogeochemical cycles.

Biotic extinction and turnover reflected by synchronous carbon isotope changes

Recent studies have identified synchronous changes in both organic and inorganic carbon isotopes, corresponding with the SPICE event in multiple sections of the Tarim Basin4,22,29,30,41 and its adjacent plates, hinting at widespread environmental and oceanographic changes during this time7,23. Our research has extended this work by examining wells LT1 and ST1 within the Tarim Basin’s central-western region, offering a comprehensive evaluation of the paired δ13Ccarb and δ13Corg records from the Miaolingian-Furongian and conducting regional correlations to deepen our understanding of the SPICE event (Fig. 4 and S3–S4).

Fig. 4: Correlation and comparison of synchronous positive δ13Ccarb and δ13Corg excursions from different sections with various sedimentary facies in the Tarim Basin.
Fig. 4: Correlation and comparison of synchronous positive δ13Ccarb and δ13Corg excursions from different sections with various sedimentary facies in the Tarim Basin.The alternative text for this image may have been generated using AI.
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a Inferred paleo-oceanic environments (modified from Wang et al.29) and approximate locations of the wells as well as outcrop sections involved in this study, including wells ST1 and LT1 (this study), well TD229, wells ZS1 and YD222, and the Yaerdangshan (YEDS) section30. b Correlation of synchronous but varying magnitude positive carbon isotope excursions during the SPICE event. ① represents the phase preceding the SPICE event, corresponding to a significant extinction event; ② denotes the phase occurring during the SPICE event, correlating with the second phase of plankton revolution. Fm = formation.

The δ13Ccarb data reveal a positive shift across all depositional settings during the SPICE event, with the magnitude of this shift being greater in basin areas compared to platform areas (Fig. 4b). This trend aligns with global isotopic patterns which are attributed to the increasing influence of atmospheric CO2 over oceanic dissolved inorganic carbon (DIC) with decreasing depth29,42. The distinct isotopic signature in the eastern Tarim Basin, particularly in well TD2 and the Yaerdangshan (YEDS) section, suggests a significant sequestration of organic carbon, leading to pronounced isotopic fractionation within the DIC reservoir of ancient seawater. The positive isotope excursions observed across different depositional settings within the Tarim Basin are consistent with the global SPICE event signature12, allowing for meaningful interregional comparisons (Fig. 4).

In comparison to the δ13Ccarb data, δ13Corg profiles from various wells have a relatively similar positive excursion22,30, indicating an isotopic enrichment concurrent with the SPICE event (Fig. 4b). The baseline δ13Corg values at the onset of the SPICE event increases with decreasing water depth, highlighting a depositional facies-related pattern. The magnitude of the SPICE event varied across the depositional facies, with shallower waters displaying a more pronounced shift. However, the δ13Corg data from well TD2 deviate from this pattern29, suggesting the influence of post-diagenetic processes or hydrothermal alteration on the isotopic fractionation43. The discrepancy between δ13Ccarb and δ13Corg in certain profiles suggests differential responses to environmental changes, possibly hinting at biological influences preceding carbonate isotope shifts. Moreover, post-event, a distinct positive anomaly in δ13Corg across certain profiles raises questions about subsequent changes in the sedimentary environment (Fig. 4b), warranting further investigation.

During the SPICE event in the Tarim Basin, significant changes in both δ13Ccarb and δ13Corg across various depositional settings underscore biotic extinction and turnover possibly tied to depth-dependent biological and geochemical processes. Notably, a minor negative δ13Ccarb and δ13Corg shift precedes the SPICE event in shallow platform and slope facies (Fig. 4b), aligning with roughly 75% of global sections as compiled by Pulsipher et al.9. The initial negative δ13Ccarb and δ13Corg excursion (Stage I) could be indicative of the end-Marjuman (end-Miaolingian) extinction1,14,18,19,24,27,44, potentially linked to upwelling of deeper 12C-enriched and anoxic waters into shallower areas (Fig. 5a). In contrast, the absence of this feature in basin facies (well TD2 and the YEDS section) may indicate that the biotic extinction occurred exclusively in shallow water areas. Subsequently, the SPICE event saw a replacement of shallow-water plankton by those from deeper waters19, representing the biotic turnover (i.e., a plankton revolution featured with a CO2 concentrating mechanisms (CCM) induced in vivo) during stage II (Fig. 5b–d).

Fig. 5: Biogeochemical responses of biotic extinction and turnover related to the late Cambrian SPICE event.
Fig. 5: Biogeochemical responses of biotic extinction and turnover related to the late Cambrian SPICE event.The alternative text for this image may have been generated using AI.
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a Schematic facies model at the end-Marjuman (end-Miaolingian) extinction prior to the SPICE event (Stage I), corresponding to the initial negative δ13C shifts pre-SPICE. The extinction of plankton in shallow water regions was potentially linked to a diminished habitat area which was triggered by sea-level regression, or the upwelling of deep waters that were anoxic27 and enriched in 12C. The 12C enrichment of these waters could stem from the oxidation of methane, released from the decomposition of hydrates, along with other organic substances, including dissolved organic carbon, into CO2 during the upwelling process55. Stage II: Schematic facies model illustrating the role of development of a CO2 concentrating mechanism (CCM) by phytoplankton in constraining the different responses of the SPICE event in various sedimentary environments during the plankton revolution. b Ranges of δ13Ccarb values from the SPICE onset to the maximum (SPICE rising limb; left y axis) and the corresponding stratigraphic thickness over which the rising limb of the SPICE is expressed (right y axis). c The differences of the east-west sedimentary facies in the Tarim Basin during the SPICE event and the resulting variations in marine biochemical and redox conditions. PZE: photic zone euxinia. d The CCM was utilized by phytoplankton under the situations of a decrease in pCO2 and increase in pO2 during the SPICE event22.

Furthermore, the distinct double-peak features observed in slope facies wells LT1 and YD2 (Fig. 4b) might indicate the interplay between shallow and deep-water organisms, potentially related to the expansion of deep-water plankton into shallow-water areas during the SPICE event19 (Fig. 5c). Therefore, the observed variations in δ13Corg and δ13Ccarb during the SPICE event reflect complex interactions of environmental changes, biological responses, and geochemical processes. This highlights the need for further research to fully understand the implications of these findings on the reconstruction of past marine ecosystems and their response to the global event.

Constraints on the variable responses to SPICE event across diverse depositional facies

Investigations into the late Cambrian SPICE event have revealed its complex expression in various depositional facies, both regionally within the Tarim Basin (Fig. 4 and S3–S4) and globally12,19,23. Schiffbauer et al.19 explained the transgressive and sedimentary facies-controlled relationship of the SPICE event9 within the entire Missouri shelf basin, and hypothesized that the positive excursion in δ13C reflected local changes in sea level during marine transgression. Furthermore, Li et al.23 studied the various sedimentary facies late Cambrian in South China, and presented high-resolution synchronous positive shifts in δ13Ccarb and δ13Corg across three profiles, from the continental shelf margin to the slope-basin transition zone. These results indicated noticeable paleodepth-related gradients in δ13Ccarb and δ13Corg, with more depleted 13C with increasing water depth. The trend of δ13C variation with depositional water depth in South China is opposite to that controlled by sedimentary facies in the central Missouri shelf basin23, where 13C is more enriched with increasing water depth, although the paleoenvironmental systems there were overall shallower water19.

This study presents an analysis of wells ST1 and LT1, which both exhibit a synchronous positive shift in δ13Ccarb and δ13Corg during the SPICE event, aligning with similar shifts detected in other wells and sections within the basin4,22,29,30,41. The different response of SPICE across varying sedimentary facies, however, underscores the need to explore the controlling factors of this variability. While Schmid et al.12 reported uniform positive shifts across various facies in the Amadeus Basin, Australia, with variations in peak values attributed to chemical gradients in seawater and the mixing of marine DIC, the Tarim Basin presents a more intricate picture. Here, the timing, magnitude, and stratigraphic thickness of the isotopic shifts differ among depositional environments (Figs. 4 and 5b), suggesting that the sedimentary response to SPICE is modulated by factors including paleoceanographic conditions, redox states, and biological productivity9,36,45 (Fig. 5b–d).

The late Cambrian SPICE event serves as a pivotal chemostratigraphic marker for understanding the sedimentary and biogeochemical processes of the Early Paleozoic22,40. Comprehensive analysis of the Tarim Basin reveals insights into the behavior of stable carbon isotopes within various depositional facies during the SPICE event (Fig. 5b). Notably, a consistent positive shift in δ13Corg, ranging between 2‰ and 3‰, is observed across different sedimentary facies within the basin. This finding suggests that primary productivity, likely driven by flourishing of phytoplankton communities18,27,28,46,47, played a significant role in controlling the organic carbon isotope record during the SPICE event (Fig. 5c). In contrast to δ13Corg, the magnitude of the δ13Ccarb excursion is significantly different between the deepwater basin and shallow water platform depositional environments within the Tarim Basin, with a more substantial positive shift in δ13Ccarb in the deepwater basin (Fig. 5b). This pattern aligns with observations from Missouri basin9,19, but is the opposite of that seen in the South China basin23, which were explained by sedimentary facies and redox depth-gradient, respectively. The comparatively lower magnitude of the δ13Ccarb shift within the shallow platform facies of the Tarim Basin correlates well with the North China platform6,25, suggesting a regional consistency in the response of shallow-water carbonate settings to the SPICE event.

Comparative analyses with South China, Australia, and North America show that the magnitude of δ13Corg shifts during the SPICE event are consistent across these regions, and do not exhibit a clear control by sedimentary environment. This further supports the hypothesis that δ13Corg primarily reflects changes in primary productivity rather than sedimentary setting30. Aside from the wells ST1 and the ZS1, as well as regions in North America where changes in δ13Corg and δ13Ccarb are almost entirely synchronous, sections in South China and Australia have δ13Corg peaks occurring before δ13Ccarb15,23. Paired δ13Corg and δ13Ccarb records from multiple sections (LT1, TD2 and YEDS) within the Tarim Basin reveal a diachronous nature of δ13Corg relative to the δ13Ccarb-defined SPICE interval (Fig. 4b). An example of this is well LT1, which has a delayed peak in δ13Corg relative to δ13Ccarb. The δ13Corg values not only predate but also outlast the δ13Ccarb values, suggesting that δ13Corg captures the protracted impact of primary productivity on the carbon cycle. These observations imply that δ13Corg, as an indicator controlled by primary productivity, exhibits a more dominant influence over the carbon cycle compared to δ13Ccarb, which is more constrained by sedimentary environmental conditions.

In addition to these findings, the SPICE event is thought be intricately linked to the plankton revolution22,28,47, marking the emergence of new phytoplankton groups which likely contributed to primary productivity. Such changes would have had cascading effects on marine ecosystems, influencing global carbon cycling. Furthermore, this planktonic diversification may have set the stage for the Great Ordovician Biodiversification Event20,21, as the increased availability of organic matter would have supported a wider range of marine life and ecological interactions. Consequently, δ13Corg could provide a more direct signal of ecosystem response during the SPICE event, potentially linked to changes in atmospheric CO2 levels35, oceanic nutrient availability27, and the evolution of planktonic communities capable of efficient carbon fixation strategies22.

Moreover, the evolution of the atmospheric partial pressures of CO2 (pCO2) and O2 (pO2) during the late Cambrian likely played a pivotal role in shaping biological responses15,20,48 and, consequently, the sedimentary record of the SPICE event. An increase in pO215 and decrease in pCO233,35,49,50 during the SPICE event could have fostered the development of plankton capable of exploiting CCMs for photosynthesis and organic matter synthesis by the uptake of bicarbonate (HCO3–) (Fig. 5d), which would have impacted the marine DIC reservoir22. Because HCO3– is more enriched in 13C than CO251, the organic matter synthesized via CCMs during the SPICE has elevated δ13Corg values.

The resultant variations in the stable carbon isotope compositions of different sedimentary facies reflect these biogeochemical changes. However, the differential magnitude of δ13Ccarb excursions between shallow water platform and deepwater basin facies indicates a strong influence of the redox conditions on δ13Ccarb record (Fig. 5c). In the platform regions characterized by well-oxygenated waters29, a significant fraction of the organic matter produced would have been oxidized, contributing to relatively lower δ13C value in the local seawater DIC pool27. Conversely, in the deepwater basinal regions, the likelihood of photic zone anoxia or euxinia14,20 would have promoted the preservation of organic matter due to reduced rates of oxidation, resulting in a higher δ13C value. This interpretation is supported by the relative enrichment of 13C in deepwater basinal facies compared to shallower platform facies within the Tarim Basin, consistent with global δ13Ccarb trends observed in similar depositional environments during the late Cambrian52. The variations in δ13Ccarb across varying depositional environments exhibit different characteristics, which may also suggest varying contributions of CCMs under different environmental conditions (Fig. 5b–d). Additionally, cyanobacteria and algae possess functionally similar but genetically unrelated CCMs, and the differences in the biological contributions of these organisms may also lead to variations in isotope excursion magnitudes.

The SPICE event represents a pivotal juncture in the Earth’s early biosphere evolution22, with profound implications for comprehension of primary productivity, depositional regimes, and carbon cycle dynamics during this critical period27,53,54. This research elucidates the influences of biotic and abiotic factors on the stable carbon isotope record during the global SPICE biogeochemical event. The onset of the SPICE corresponds to a biotic extinction event, stemming from substantial environmental oscillations. The different expressions of the SPICE event across various depositional facies are governed by the interplay between sedimentary environments and the proliferation of specific biotic assemblages which were adept at exploiting CCMs. The ecological transition observed during the climatic cooling of the SPICE event represents an episode of plankton revolution that preceded the Great Ordovician Biodiversification Event.

Methods

The two wells ST1 and LT1, for which detailed logs and surveys exist, were accessed from the PetroChina Tarim Oilfield Company Core Library, and are situated in the platform (ST1) and slope (LT1) facies. To investigate the sedimentary geochemical record of the Miaolingian-Furongian in the Tarim Basin, in relation to the SPICE event, a total of 139 rock samples were collected from the Miaolingian-Furongian sections of the two wells. Samples were taken more densely around the preliminary stratigraphic boundaries identified in the well completion reports. Since core samples from most drilled wells are extremely limited, the samples collected in this study were all cuttings. For the ST1 well, samples were taken from 579 to 1622 m depth, with a total of 68 samples collected and an average sampling interval of approximately 15 m. In the LT1 well, the sampled section ranged from 6430 to 7866 m depth, with a total of 71 samples collected and an average sampling interval of about 20 m.

Screening and pretreatment

To ensure the quality of the cuttings, according to Wang et al.22, all cuttings were carefully screened in a first step to prevent contamination from collapsed overlying strata or crystalline particles from the drilling fluid. Subsequently, all samples were ground to a powder finer than 200 mesh to homogenize them, allowing for a consistent analysis of the bulk material. The rock powder was analysed for total organic carbon (TOC) content, and carbonate rock carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb). Before analysis of the carbon isotopic composition of the kerogen (δ13Corg), all samples were treated with acids to remove the carbonate and silicate minerals, following the kerogen preparation methods.

Approximately 10 g of bulk rock powder was placed in a 50 ml centrifuge tube, and was acidified multiple times with a 50% hydrochloric acid (HCl) solution. This was followed by treatment with a mixture of analytical-grade hydrofluoric acid (HF) and the aforementioned 50% HCl solution at a ratio of 3:1 (v/v). The samples were heated in a water bath at 80 °C to ensure full reaction with the acid solution, repeating the process 3 to 5 times with each reaction lasting at least 4 h. During this time, the mixture was stirred every 1 to 2 h to facilitate the complete removal of minerals (mainly carbonates and silicates) from the rock samples. After each acid treatment, the insoluble residue was separated from the reaction solution by centrifugation. The supernatant was discarded, and the residue was rinsed repeatedly with deionized (Milli-Q) water until it was neutral. The final insoluble component was then dried in a vacuum oven at 60 °C and ground into a fine powder of kerogen.

Total organic carbon content analysis

Approximately 100 mg of bulk rock powder sample was treated with 15% HCl and allowed to react overnight to ensure the complete dissolution of carbonates. Subsequently, the residual solid material was thoroughly rinsed with ultrapure (deionized, 18 MΩ) water until the solution pH neared neutrality. The decarbonated sample was then dried at 60 °C in preparation for further analyses. TOC content was quantitatively determined using an Eltra CS-800 Carbon Sulfur Analyzer at the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

Carbonate carbon and oxygen isotope analyses

The isotopic analyses of the carbonates (δ13Ccarb and δ18Ocarb) were measured at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Before analysis, the powdered carbonate was reacted with 100% phosphoric acid (H3PO4) at a controlled temperature, typically around 70 °C, to release CO2 gas from the carbonate matrix. This acid digestion process was performed using an automated carbonate preparation device (Gas Bench II), which was interfaced with a MAT-253 Mass Spectrometer (MS). The CO2 gas produced from the reaction was then purified to remove any contaminants and introduced into the MAT-253 MS.

The isotopic ratios of carbon (13C/12C) and oxygen (18O/16O) in the CO2 were measured relative to known international standards, the Vienna Pee Dee Belemnite (V-PDB) for carbon and the Vienna Standard Mean Ocean Water (V-SMOW) for oxygen. These ratios were then expressed as δ13Ccarb and δ18Ocarb values in per mil (‰) deviations from the standards. Precision and calibration of data were monitored through routine analysis of international carbonate standards (NBS–19 with δ13Ccarb of 1.95‰ and δ18Ocarb of –2.20‰), and internal laboratory standards (GBW-04405 with δ13Ccarb of 0.57‰ and δ18Ocarb of −8.49‰). These standards were typically run at the beginning and end of each analytical session and intermittently between samples to monitor instrument stability and performance. Analyses were reproducible to better than ±0.1‰ for carbon and ±0.2‰ for oxygen, based on analysis of replicates and internal laboratory standards.

Organic matter carbon isotope analyses

For isotope determination of organic carbon phases, an appropriate amount of kerogen powder (based on an approximate pure carbon content of 35 μg) was weighed into a tin capsule, sealed, and placed in the autosampler tray of a Flash 2000 Elemental Analyzer (EA), which was coupled with a Delta V Advantage Stable Isotope Ratio Mass Spectrometer (IRMS) for analysis at State Key Laboratory of Organic Geochemistry, Chinese Academy of Sciences. The combustion furnace temperature was set at 1450 °C, with helium gas as the carrier at a flow rate of 110 ml/min.

An analysis of blanks and three internal laboratory standards was performed prior to analyzing the first sample. After every ten samples, two standard samples were analyzed to ensure data accuracy and calibration, which were monitored through routine analysis of laboratory standards. To ensure that the experimental results fell within an acceptable analytical error range, each sample was analyzed at least twice. If the two analyses were within the error range, an average value was used. Stable carbon isotope composition (δ13Corg) was calculated in per mil (‰) deviations relative to the Vienna Pee Dee Belemnite (V-PDB) standard. The standard deviation for δ13Corg was less than ±0.13‰.