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Article

Carbon Isotope and Sterane Records of Biological Diversity in the Fortunian Stage of the Early Cambrian Tarim Basin, Northwest China

by
Wenhao Li
1,2,*,
Yifan Chen
1,2 and
Longwei Wang
1,2
1
National Key Laboratory of Deep Oil and Gas, China University of Petroleum (East China), Qingdao 266580, China
2
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(5), 1530; https://doi.org/10.3390/pr13051530
Submission received: 8 April 2025 / Revised: 10 May 2025 / Accepted: 13 May 2025 / Published: 16 May 2025
(This article belongs to the Special Issue Recent Advances in Hydrocarbon Production Processes from Geoenergy)
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Abstract

:
Carbon isotope of the kerogen (δ13Corg), steranes/hopanes (S/H), and C28/C29 sterane ratios in the source rocks from the SARK section at the Early Cambrian Yurtus Formation in the Fortunian Stage in the Tarim Basin of Northwest China reveal a positive excursion that is associated with biological diversity. The enrichment of vanadium/(vanadium + nickel) (V/(V + Ni)) ratios (0.64~0.99, averaging 0.87) for the Yurtus Formation of the Fortunian Stage provide evidence for predominant anoxic bottom water conditions. A sharply decreased V/(V + Ni) ratio in the middle Yurtus Formation suggests enhanced oxygen content of the water column in this interval. However, the total organic carbon (TOC) values in the sedimentary rocks show a marked increase in the middle Yurtus Formation, which is due to the enhanced productivity suggested by a positive δ13Corg increase of ~2.0‰ and enhanced S/H and C28/C29 sterane ratios. We suggest that the enhanced oxygen content may have contributed to the biological diversity during the Fortunian Stage in the Tarim Basin. The δ13Corg excursion first reported here associated with biological diversity can be correlated with that in South China and possibly elsewhere in this interval.

1. Introduction

The δ13C excursions and biomarkers are commonly used to discuss the environment and its control on the well-known “Cambrian explosion” [1,2,3], which is characterized by the appearance of skeletal metazoan [4,5,6]. It is reported that δ13C excursions may have been associated with the biological diversity events at the Nemakit-Daldynian Stage in Siberia and South China [7,8]. A positive organic carbon isotope shift reported by Chen et al. [9] has been found in the upper Liuchapo Formation, which is correlated to the positive isotopic excursion in the upper Xiaowaitoushan Member (peak S2) of the Laolin section and the Nemakit-Daldynian Stage in Siberia [10,11]. In this interval, carbon isotope and fossil data in feature D in the Mongolian section also record this Cambrian diversification [12].
The biological diversity event at the Nemakit-Daldynian Stage in South China has been paid extensive attention. However, the biological diversity in this interval when Yurtus black shales were developed in the Tarim Basin of northwest China has been poorly understood mainly due to the lack of research, although many scholars have investigated the black shales in the Early Cambrian Yurtus Formation [13,14,15,16]. The Yurtus black shales are mainly developed in an anoxic environment [17,18,19], and the high primary productivity, anoxic environment, upwelling, and hydrothermal fluids may have contributed to the formation of the Yurtus black shales [13,19,20,21,22]. Li et al. [14] reported that some crude oils and Cambrian source rocks in the basin have a high content of C28 regular steranes with C28/C29 sterane ratios greater than 0.50. However, Grantham and Wakefield found that the ratio of the C28/C29 sterane is less than 0.5 for the Lower Paleozoic oils by analyzing 400 oil samples corresponding to 600 Ma of evolution covering the entire Phanerozoic and believed that the increase in C28 steranes might be related to the increased diversification of phytoplanktonic assemblages [23].
In this study, we report the co-evolution of carbon isotope of the kerogen (δ13Corg), steranes/hopanes (S/H), and C28/C29 sterane ratios (C28/C29) from the SARK section to record the change to the evolution and diversification of algal assemblages in the Early Cambrian. Our results may hold important implications for the Cambrian explosion during the Fortunian Stage. This biodiversification event is not only the first found in northwest China but also has enhanced its global correlation.

2. Geological Settings and Samples

2.1. Geological Settings

The Tarim Block assembled in Australia formed part of the Rodinia supercontinent during the Neoproterozoic [19,24,25]. With the collapse of the Rodinia supercontinent in the Late Neoproterozoic, it was disrupted from the supercontinent [19,24,25,26]. In the early Cambrian, the Tarim Block rotated and rifted apart from northwestern Australia, then drifted from the high-latitude region to near the equator [27].
The Tarim Block is composed of a Precambrian basement and a Late Neoproterozoic to Cambrian cover series [25]. Cambrian outcrops are mainly in the northwestern Tarim Basin, especially in the Wushi–Keping area [25]. The Lower Cambrian Yurtus Formation unconformably overlies the Ediacaran Qegebulake Formation in this region [19,28]. The samples numbered N1 to N13 in the Lower Cambrian Yurtus Formation were collected from the SARK section in the Tarim Basin, NW China (Figure 1 and Figure 2). The Yurtus Formation in the Tarim Basin is at the Fortunian Stage to Stage 2 (Figure 2). Based on the small shelly fossils and acritarch assemblage data, the Siliceous shales intercalated with thin dolomites, black shales intercalated with some straticulate siliceous shales were developed at the Fortunian Stage [29], while dolomites were mainly developed at the Stage 2 (Figure 2). The total organic carbon (TOC) values of the black shales in the Yurtus Formation in the Tarim Basin are high, the average of which can reach up to >6% [13].

2.2. Sample Pre-Treatment and Analytical Methods

In this study, the source rock samples were measured using a Leco CS230 analyzer (LECO Corporation, USA) for TOC analysis. The samples were milled in an agate mortar and pestle for 200-mesh particles, and some details are in Li et al. [30]. The experimental conditions and procedure of the trace elements are listed in Li et al. [30].
The carbon isotopic compositions of the kerogens were measured using a Finnigan DeltaPlus XL IRMS instrument connected to a CE flash1112 EA via a ConfloIII interface (Finnigan MAT, Bremen, Germany). The samples were converted to CO2 through high-temperature combustion using helium as the carrier gas, and then separated by chromatographic column and entered into a mass spectrometer for analysis. The δ13C value was reported relative to Vienna PeeDee Belemnite (VPDB). Each sample was analyzed at least twice, and the deviations for the results were no more than 0.3‰. The average for the two runs was accepted as the final carbon isotopic result for the sample. The analytical precision of these analyses is better than 0.2‰.
Before the analysis for gas chromatography–mass spectrometry (GC-MS), the samples were crushed into 80 meshes and then extracted in a Soxhlet apparatus with dichloromethane (CH2Cl2) for 24 h. The extracts were evaporated, handled with n-hexane, and then separated through column chromatography into the saturated, aromatic, and polar fractions, respectively. The saturated and aromatic fractions were analyzed by gas chromatography–mass spectrometry (GC-MS). The experimental conditions were listed in Zhan et al. [31]. The trace elements data were measured using an inductively coupled plasma mass spectrometer (ICP-MS). The experimental conditions of these analyses were reported by Li et al. [30].

3. Results

3.1. Total Organic Carbon and Redox Conditions

The TOC values are stable from 0.93~3.37% (samples from N1 to N6), with an average of 1.85%, in the lower to middle Yurtus Formation, which increase to 1.61~13.17% (samples from N7 to N13), with an average of 7.88%, from the middle to upper Yurtus Formation (Table 1, Figure 2). A significant increase in the TOC value in the sample N7 has occurred (Table 1, C1 in Figure 2).
The vanadium/(vanadium + nickel) (V/(V + Ni)) ratios can be used to infer redox conditions. When the V/(V + Ni) ratio > 0.60, it may suggest anoxic bottom water conditions [32,33,34]. The V/(V + Ni) ratios are in the range from 0.64 to 0.99, with an average of 0.87, indicating predominant anoxic bottom water conditions. The V/(V + Ni) ratios are stable from 0.90 to 0.99 in the samples N1~N6, and the ratio sharply decreased to 0.64 in the sample N7 (Table 1, C1 in Figure 2), suggesting the increase in oxygen content in the water column in this interval. Iron speciation and Mo isotope data have also supported the similar conclusion that the paleo-environmental conditions of the source rocks of the Lower Cambrian Yurtus Formation is a predominant anoxic bottom water condition, but transient oxic conditions may have existed [19]. The V/(V + Ni) ratio shows a decreasing trend from the bottom to the top, which is not coordinated with the change in the TOC value. Thus, the enhanced TOC values in the middle to upper Yurtus Formation (especially in sample N7) have no correlation with the redox conditions.

3.2. Biomarkers Related to Biological Diversity

The ratio of steranes to hopanes can record the relative contribution of eukaryotic and bacterial lipids to marine sediments [35]. Brocks et al. [36] discussed the relative contribution of eukaryotic and bacterial lipids to sedimentary organic matter from 850 Myr ago to the present estimated by the sterane/hopane (S/H) ratio and believed that S/H ratios in 820 to 720 Myr old sediments (0.003 to 0.300, 0.06 ± 0.10 (mean ± s.d.)) are three orders of magnitude lower than in typical Phanerozoic marine sediments (~0.5 to >2, 0.75 ± 1.1). The S/H ratios in the marine sediments range from 0.29 to 0.47, with an average of 0.37 (Table 1), suggesting that the bacterial still had a major role. A positive S/H ratio increase of ~0.1 has been observed in the middle Yurtus Formation (C1 in Figure 2), which is associated with biological diversity. The increase in dissolved oxygen content in the water column suggested by V/(V + Ni) promoted the algal blooms (C1 in Figure 2).
The C28/C29 ratios range from 0.46 to 0.95, with an average of 0.66 (Table 1). This result does not match the results reported by Grantham and Wakefield [23] that oils generated from the Lower Paleozoic and older marine source rocks have relatively lower C28/C29 regular sterane values (less than 0.5). However, there is a great increase in the C28/C29 ratio from <0.55 to >0.70 at the Devonian/Carboniferous boundary relating to the enhanced prasinophytes [37]. Considering that C28 steranes are associated with the increased diversification of phytoplanktonic assemblages [23], the high C28/C29 ratio in our study may imply the presence of biological diversity in the early Cambrian in the Tarim Basin. A positive C28/C29 ratio increase of ~0.2 in the middle Yurtus Formation (C1 in Figure 2) suggests the algal blooms due to enhanced photosynthesis.

3.3. δ13Corg Chemostratigraphy and Its Global Correlation Related to Biological Diversity

The δ13Corg values from the SARK section range from −36.67‰ to −34.77‰, with a mean value of −35.57‰ (Table 1). The δ13Corg shows a stable low value from the bottom to the middle Yurtus Formation (Samples from N1 to N6 in Figure 2). A positive excursion of the δ13Corg is exhibited toward the Anabarites–Protohertzina biozone in the middle Yurtus Formation during the Fortunian Stage (C1 in Figure 2), which is associated with biological diversity in the Tarim Basin.
The similarity positive excursions of the δ13C at the Anabarites–Protohertzina biozone also are reported in Siberia and South China (Figure 3). Brasier et al. [7] believed that positive δ13Ccarb excursions up to 3‰ in the Lower Cambrian of Siberia at the Nemakit-Daldynian Stage indicate the Cambrian explosion. Microfossils including macroscopic algae, conotubular life forms, and presumed metazoans have also been reported in the interval to record this biological diversity event [8]. Further studies on detailed fossil information during the Fortunian Stage will enhance the understanding of the globally biological diversity event in this interval.

4. Discussion

A positive TOC increase of ~10% in the middle Yurtus Formation is exhibited (C1 in Figure 2), which is caused by phytoplankton blooms. A positive δ13Corg increase of ~2‰ occurred in the middle Yurtus Formation shortly after the biological diversity (C1 in Figure 2). The increase of ~0.2 and ~0.1, respectively, in C28/C29 and S/H of the marine sediments in the middle Yurtus Formation also (C1 in Figure 2) supported the event of biological diversity. We believe the oxygen increase in the water column (supported by the V/(V + Ni) ratio) is the environmental trigger for Cambrian biological diversity in this study. Although the redox conditions are mainly anoxic in the Fortunian Stage, the oxygen content is close to suboxic suggested by the V/(V + Ni) ratio shortly after the biological diversity (C1 in Figure 2). A similar point has also been reported that short-lived oxic conditions may have existed in the Lower Cambrian Yurtus Formation in the northwest Tarim Basin [19]. The V/(V + Ni) ratios in the marine sediments from the middle to upper Yurtus Formation fluctuate frequently (Figure 2), suggesting changeable redox conditions, which may trigger the small shelly animals to flourish at the Tommotian Stage.
We noticed that the positive δ13Corg excursion at the Fortunian Stage is with global correlation. This positive δ13Corg excursion event commonly occurred in South China, and Laolin and Songtao sections from the Yangtze Platform [11,38], Longbizui section from Western Hunan [39], and Ganziping section from Dayong of Hunan Province [9] record this excursion. The δ13Corg excursion ranges in the above sections (except the Ganziping section with a range over 7.0‰) are close to the section in this study. δ13Ccarb data in the sections from Mongolian and Siberia also indicated the Cambrian diversification in this interval [7,12].

5. Conclusions

The Yurtus source rocks in the Fortunian Stage in the Tarim Basin of Northwest China were mainly deposited in anoxic conditions. Although the oxygen content is increasing, a positive TOC increase of ~10% in the middle Yurtus Formation has occurred, which is attributed to phytoplankton blooms. A positive C isotope excursion and increase in the S/H and C28/C29 ratios in this interval record the biological diversity. The enhanced oxygen content may have contributed to the biological diversity. The δ13Corg excursion first reported here associated with biological diversity event in the Fortunian Stage can be correlated with that in South China and possibly elsewhere in this interval. Further studies on detailed fossil information during the Fortunian Stage in the Tarim Basin will improve our understanding of biological evolution.

Author Contributions

W.L. contributed to the design of the study, wrote the manuscript, and was the principal author of the manuscript. Y.C. contributed to the methodology. L.W. contributed to the discussion of the results and manuscript refinement. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the National Natural Science Foundation of China (42472193) and the Natural Science Foundation of Shandong Province (ZR2022YQ30).

Data Availability Statement

All the data analyzed during this study are included in Table 1 of this article, which has been uploaded.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Location of the SARK section in the Tarim Basin, NW China.
Figure 1. Location of the SARK section in the Tarim Basin, NW China.
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Figure 2. Total organic carbon (TOC), V/(V + Ni), S/H, C28/C29, and δ13Corg values for Early Cambrian sediments in the Yurtus Formation at the SARK section, Tarim Basin. Note: A-P = Anabarites–Protohertzina; L-P = Lapworthella–Paragloborilus; S/H = steranes/hopanes (C27–C29 steranes/C27–C35 hopanes); C28/C29 = C28/C29 steranes.
Figure 2. Total organic carbon (TOC), V/(V + Ni), S/H, C28/C29, and δ13Corg values for Early Cambrian sediments in the Yurtus Formation at the SARK section, Tarim Basin. Note: A-P = Anabarites–Protohertzina; L-P = Lapworthella–Paragloborilus; S/H = steranes/hopanes (C27–C29 steranes/C27–C35 hopanes); C28/C29 = C28/C29 steranes.
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Figure 3. A correlation of the δ13C profiles among representative sections in the world in the Early Cambrian [7,9].
Figure 3. A correlation of the δ13C profiles among representative sections in the world in the Early Cambrian [7,9].
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Table 1. Distribution of several organic geochemical parameters of Early Cambrian sediments in the Yurtus Formation at the SARK section, Tarim Basin. Note: S/H = steranes/hopanes (C27~C29 steranes/C27~C35 hopanes); C28/C29 = C28/C29 steranes.
Table 1. Distribution of several organic geochemical parameters of Early Cambrian sediments in the Yurtus Formation at the SARK section, Tarim Basin. Note: S/H = steranes/hopanes (C27~C29 steranes/C27~C35 hopanes); C28/C29 = C28/C29 steranes.
SampleTOC (%)V (ppm)Ni (ppm)V/(V + Ni)S/HC28/C29δ13Corg (‰)
N13.3731511.10.970.350.64−36.20
N21.8710261160.900.350.58−35.67
N31.2544140.00.920.360.67−36.21
N41.0339121.40.950.390.95−35.84
N52.65261725.00.990.410.59−36.40
N60.9324927.90.900.370.46−36.67
N711.0016149130.640.470.65−34.92
N81.6114067.70.670.350.62−35.24
N94.1516419.40.890.290.66−34.91
N108.279031690.840.370.69−34.77
N1113.1759357.10.910.290.80−35.37
N1210.4342132.10.930.380.67−35.01
N136.5539385.90.820.420.64−35.26
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Li, W.; Chen, Y.; Wang, L. Carbon Isotope and Sterane Records of Biological Diversity in the Fortunian Stage of the Early Cambrian Tarim Basin, Northwest China. Processes 2025, 13, 1530. https://doi.org/10.3390/pr13051530

AMA Style

Li W, Chen Y, Wang L. Carbon Isotope and Sterane Records of Biological Diversity in the Fortunian Stage of the Early Cambrian Tarim Basin, Northwest China. Processes. 2025; 13(5):1530. https://doi.org/10.3390/pr13051530

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Li, Wenhao, Yifan Chen, and Longwei Wang. 2025. "Carbon Isotope and Sterane Records of Biological Diversity in the Fortunian Stage of the Early Cambrian Tarim Basin, Northwest China" Processes 13, no. 5: 1530. https://doi.org/10.3390/pr13051530

APA Style

Li, W., Chen, Y., & Wang, L. (2025). Carbon Isotope and Sterane Records of Biological Diversity in the Fortunian Stage of the Early Cambrian Tarim Basin, Northwest China. Processes, 13(5), 1530. https://doi.org/10.3390/pr13051530

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