Zhang Guijie, Zhang Xiaolin, Zhou Changyong, Huang Jinyuan, Zhang Qiyue, Hu Shixue, Wen Wen, Huang Xiemin

1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. Chengdu Center, China Geological Survey, Chengdu 610081, China

Abstract: Previous studies have documented remarkable changes in the carbon cycle during the Early Triassic. To verify these trends and improve our understanding of carbon cycling during the Early-Middle Triassic, we investigate the new δ13Ccarb record for the Early-Middle Triassic in the Tianshengqiao section of South China. Our study shows that the δ13Ccarb values in the Tianshengqiao section have received minimal diagenetic alterations. The δ13Ccarb profile initially shows a negative excursion of 2.2‰ within the Member I of the Jialingjiang Formation, followed by a positive excursion of 3.8‰ in the lower part of the Member III of the Jialingjiang Formation. After that, δ13Ccarb displays a negative excursion of 2.5‰ in the Member III of the Jialingjiang Formation, followed by a positive excursion with a maximum of +5.0‰ at the base of the Member I of the Guanling Formation. Above the maximum value, the δ13Ccarb profile exhibits a third negative excursion and then recovers and stabilizes around +1‰ to +2‰ through the Member II of the Guanling Formation to the Yangliujing Formation. The observed trend of the δ13Ccarb profile in the Tianshengqiao section can be correlated with the global C-isotopic excursions derived from the coeval sections worldwide. The similarity of the Early-Middle Triassic δ13Ccarb profile to those from other sections worldwide is indicative of the near completeness of the stratigraphic interval of the Tianshengqiao section. Our results suggest that the δ13Ccarb profile in the Tianshengqiao section represents a global carbon cycle perturbation. The stratigraphic coincidence between the carbon cycle perturbation and the limited biotic recovery has been documented in the Tianshengqiao section, suggesting that the physical mechanism of the carbon cycle perturbation may have contributed to the prolonged biotic recovery.

Keywords: chemostratigraphic correlation; carbon cycle perturbation; prolonged biotic recovery; Early-Middle Triassic

1 Introduction

The end-Permian mass extinction represents the most devastating biotic crisis of the Phanerozoic, resulting in the loss of>80% of marine species on a global scale[1,2]. The subsequent biotic recovery was prolonged for nearly 4-8 million years until the beginning of the Middle Triassic[3]. The combinations of geochemical, sedimentological and paleontological studies show that the Early Triassic is characterized by harsh environmental conditions, including global warming[4], widespread development of oceanic anoxia[5-8], and intensified weathering[9,10]. The most substantial evidence for environmental disturbance during the Early Triassic is a series of large fluctuations in C-isotope, suggesting that significant perturbation of the global carbon cycle occurred during this time[11-15]. However, the physical mechanism of the carbon cycle disturbance and its potential relationship to the prolonged biotic recovery remain poorly understood.

The detailed understanding of the Early-Middle Triassic carbon cycling requires additional high-resolution studies at various paleogeographic settings. Here we report a new high-resolution δ13Ccarbrecord from the Early-Middle Triassic successions in the Tianshengqiao section in Shizong, Yunnan, South China. The Tianshengqiao section preserves the continuous marine successions with a stratigraphic thickness of ～1000 m from the Early Triassic to the Middle Triassic, allowing us to establish a high-resolution δ13Ccarbprofile. As a newly recognized outcrop, the biostratigraphic control in the Tianshengqiao section is relatively coarse. The C-isotopic chemostratigraphy can check the precise horizon of the critical stratigraphic boundary. Our data can provide new insights into the temporal evolution of carbon cycling and its possible link with the delayed biotic recovery.

2 Geological setting and stratigraphy

During the Early-Middle Triassic, the South China Block was situated in the eastern part of the Paleotethys Ocean. The Lower-Middle Triassic marine stratigraphic successions in South China are composed of various sedimentary facies. Five sedimentary provinces have been established for the Early Triassic marine sequences of South China, including the Lower Yangtze, Upper Yangtze, Youjiang, Cathaysia and Lijiang sedimentary provinces[16]. Of these, the Upper Yangtze sedimentary province preserves one of the most complete and best-studied Lower-Middle Triassic successions[16].

The Tianshengqiao section is located in Tianshengqiao village, approximately 20 km west of the Shizong town in Eastern Yunnan, South China. Tectonically, the Tianshengqiao section was situated in a shallow marine setting in the Upper Yangtze sedimentary province. The Lower-Middle Triassic successions in the Tianshengqiao section consist of the Jialingjiang, Guanling, and Yangliujing Formations in ascending order (Figure 1). The Jialingjiang Formation is subdivided into three members. The Member I primarily comprises the thin to medium bedded gray vermicular limestone containing gastropods and bivalves. The Member II is primarily composed of siltstone containing gastropods, bivalves, and plant debris fossils. The Member III is dominated by the thin to medium bedded vermicular limestone in the lower and middle parts and muddy limestone in the uppermost part. The Member III contains gastropods, bivalves, ammonoids, and crinoids. The occurrences of the ammonoidsTirolitesspinosusandMeekocerassp., the bivalveEntoliumdiscitesmicrotis, and the conodontNeospathodustriangularisin the Jialingjiang Formation indicate the Olenekian age[17,18]. Moreover, the conodonts assigned to thePachycladina-Parachirognathusassemblage occurred in the uppermost of the Member I of the Jialingjiang Formation, indicating deposition in the late Smithian[18].

Figure 1. Lithostratigraphy and chemostratigraphy of δ13Ccarb and δ18Ocarb in the Tianshengqiao section. The brown dashed line and grey intervals on the isotopic data represent the polynomial fit and confidence interval, respectively. Abbreviations: YLJ= Yangliujing.

The overlying Guanling Formation consists of two members. The Member I is primarily composed of muddy limestone interbedded with siltstone. This member contains gastropods, bivalves, crinoids, ammonoids, and plant debris fossils. The Member II is typified by limestone interbedded with muddy limestone. This member contains gastropods, bivalves, crinoids, and plant debris fossils. The occurrences of the bivalveCostatoriagoldfussiand the conodontNicoraellakockeliin the Guanling Formation indicate the Anisian age[19,20]. The Yangliujing Formation is dominated by dolostone.

3 Methods

Fresh rock samples were trimmed to remove weathered surfaces and visible veins, and then were cut into small pieces. Subsequently, the rock chips were individually powdered (<200 mesh) to avoid contamination. The analyses of δ13Ccarband δ18Ocarbwere performed at the China University of Geosciences in Wuhan. The powder samples (ca. 150 μg) were reacted with 102%-105% anhydrous phosphoric acid at 70℃ under vacuum in a Kiel IV carbonate device to liberate CO2. The generated CO2was cryogenically purified and then automatically introduced into a MAT 253 Mass Spectrometer for δ13Ccarband δ18Ocarbmeasurements. All the isotopic data are reported in units of per mille (‰) relative to the Vienna Peedee belemnite (VPDB) standard. Precision and calibration of the data were monitored by replicate analyses of a Chinese national standard GBW04416. The analytical precision was better than ±0.1‰ for δ13Ccarband δ18Ocarb.

4 Results

The samples from the Member I of the Jialingjiang Formation to the Yangliujing Formation were analyzed, except for the samples from the Member II of the Jialingjiang Formation, which are dominated by clastic rocks. Overall, the δ13Ccarbvalues in the Tianshengqiao section range from -2.8‰ to +5.0‰ (Table 1). The δ13Ccarbvalues initially display a negative excursion from -0.6‰ to -2.8‰ within the Member I of the Jialingjiang Formation, followed by a rise to +1‰ in the lower part of the Member III of the Jialingjiang Formation (Figure 1). The δ13Ccarbvalues show a second negative excursion from +1.0‰ to -1.5‰, followed by an increase to a maximum value of +5.0‰ at the base of the Member I of the Guanling Formation (Figure 1). Above the maximum value, the δ13Ccarbvalues display a third negative excursion and then recover and stabilize around +1‰ to +2‰ through the Member II of the Guanling Formation to the Yangliujing Formation (Figure 1).

Table 1. Geochemical data from the Tianshengqiao section.

5 Discussion

5.1 Evaluation of diagenetic influences

The primary C-isotopic signatures of carbonate sediments can be potentially influenced by diagenetic processes[21-26]. In order to discuss the stratigraphic and paleoenvironmental significance of the data, it is critical to evaluate the diagenetic effects on the primary isotopic signatures. The δ18O values of carbonate sediments are vulnerable to diagenetic modifications due to a large reservoir of oxygen within the diagenetic fluids and the susceptibility of δ18O values to diagenetic temperature[21]. It has been suggested that δ18Ocarbvalues lower than -10‰ may indicate significant diagenetic modifications[21]. In the present case, the δ18Ocarbvalues in the Tianshengqiao section range from -8.4‰ to +0.2‰, excluding the possibility of signi ficant diagenetic alterations (Figure 2).

Generally, theδ13Ccarbvalues are less susceptible to diagenetic alterations because most diagenetic fluids contain much lower carbon content compared to carbonate sediments. However, under the influence of diagenetic fluids which are enriched in organically derived carbon, both δ13Ccarband δ18Ocarbare likely to be altered[22,23]. Therefore, the positive covariation of δ13Ccarband δ18Ocarbis often used to identify diagenetic modifications in carbonate sediments[22,23]. A lack of significant positive correlation of δ13Ccarband δ18Ocarbeither for the individual intervals or the whole samples provides further evidence of minimal diagenetic modifications of the primary C-isotopic signatures (Figure 2).

Figure 3. Comparison of the C-isotopic profiles of the Tianshengqiao (this study), Guandao[11], Jinya[14], and Losar[14] sections within the biostratigraphic framework. Abbreviations: Frb = Flemingites rursiradiatus beds, Okb = Owenites koeneni beds, Amb = Anasibirites multiformis beds, TAb = Tirolitid n. gen. A. beds, T/Cb = Tirolites/Columbites beds, Pb = Procolumbites beds, Hb = Hellenites beds, HZ = Haugi Zone, Plb =Platycuccoceras beds.

5.2 Chemostratigraphic correlation with global trends

The Tianshengqiao section is constrained by the biozones of conodonts, as well as ammonoids and bivalves. The placement of the Olenekian stage is constrained by the occurrences of the ammonoidsTirolitesspinosusandMeekocerassp., the bivalveEntoliumdiscitesmicrotis, and the conodontNeospathodustriangularisin the Jialingjiang Formation[17,18]. Moreover, the placement of the upper Smithian is constrained by the occurrences of the conodonts assigned to thePachycladina-Parachirognathusassemblage in the uppermost of the Member I of the Jialingjiang Formation[18]. The placement of the Anisian stage is constrained by the occurrences of the bivalveCostatoriagoldfussiand the conodontNicoraellakockeliin the Guanling Formation[19,20]. Within the biostratigraphic framework, the δ13Ccarbprofile in the Tianshengqiao section can be correlated with those in the cotemporaneous sections globally[11-15].

The negative δ13Ccarbexcursion of 2.2‰ within the Member I of the Jialingjiang Formation can be correlated with the globally reported negative C-isotopic excursion that began at the early Smithian and reached a minimum value in the middle to late Smithian[11-15](Figure 3). This negative C-isotopic shift represents one of the most significant C-isotopic excursions in the Early Triassic. The following positive δ13Ccarbexcursion of 3.8‰ in the lower part of the Member III of the Jialingjiang Formation corresponds to the globally recognized positive C-isotopic excursion that began in the latest Smithian and peaked in the earliest Spathian[11-15](Figure 3). After that, a negative δ13Ccarbexcursion of 2.5‰ in the Member III of the Jialingjiang Formation can be correlated with the widely recognized negative C-isotopic excursion that began in the early Spathian and peaked in the middle Spathian[11-14](Figure 3). The following positive δ13Ccarbexcursion with a maximum of +5.0‰ at the base of the Member I of the Guanling Formation corresponds to the positive C-isotopic excursion that marks the Olenekian/Anisian boundary[11-15](Figure 3). Above the maximum value, the δ13Ccarbprofile in the Tianshengqiao section exhibits a third negative excursion, which can be correlated with the earliest Anisian negative C-isotopic excursion[11,14](Figure 3). Finally, the δ13Ccarbvalues stabilize through the Member II of the Guanling Formation to the Yangliujing Formation, which is consistent with the C-isotopic stability in the Middle Triassic[11](Figure 3).

Figure 4. Composite C-isotopic curve and general bioturbation intensity during the Early-Middle Triassic. The C-isotopic data is from this study (Tianshengqiao) and three typical sections worldwide including Guandao[11], Jinya[14] and Losar[14]. The general bioturbation intensity is from Luo et al.[35].

Each of these δ13Ccarbexcursions reported in the Tianshengqiao section is a unique event that can be correlated with those in the cotemporaneous sections globally. These C-isotopic events represent important chemostratigraphic markers that can be used to correlate the critical stratigraphic boundaries for the sections with inadequate biostratigraphic control. The similarity of the Early-Middle Triassic δ13Ccarbprofile with those from other sections globally is indicative of the near completeness of the stratigraphic interval of the Tianshengqiao section. The δ13Ccarbexcursions through the Early to Middle Triassic in the Tianshengqiao section correspond to the global perturbation of the C-isotopic compositions of seawater dissolved inorganic carbon (DIC).

5.3 Relationship of the carbon cycle perturbation to the prolonged biotic recovery

Multiple scenarios have been proposed to explain these C-isotopic fluctuations during the Early Triassic. For example, Payne and Kump[27]suggested that the Siberian Trap eruption provides a possible explanation for the repeated C-isotopic fluctuations. The negative C-isotope excursion during the Early Triassic was attributed to the release of isotopically depleted carbon during the Siberian Trap eruption[27]. The intervening positive C-isotope excursion was explained by increased productivity in response to the warming-induced anoxia and associated phosphate regeneration due to the Siberian Trap eruption[27]. However, the occurrence of sufficiently intense volcanism to affect the global biogeochemical cycle during the entire Early Triassic remains to be resolved[28]. Alternatively, Horacek et al.[12]suggested that episodic changes in the ocean’s circulation may have caused the C-isotopic fluctuations during the Early Triassic. The positive C-isotopic excursion was attributed to ocean stratification[11]. The negative C-isotopic excursion was ascribed to the overturning of a previously stratified ocean, resulting in the release of isotopically depleted DIC from the deep waters[12]. However, the possible mechanism of overturning remains poorly understood. Sexton et al.[29]proposed that an anoxic ocean would have promoted the buildup of13C-depleted dissolved organic carbon in the deep waters. The development of oceanic anoxia during the Early Triassic has been documented from different paleolatitudes globally[5-8]. Moreover, many studies demonstrate that the Early Triassic oceans were dominated by microbes[30,31]. The widespread development of anoxia and the proliferation of microbes during the Early Triassic may have facilitated the buildup of a large13C-depleted dissolved organic carbon pool in the deep waters. Expansion of anoxic deep waters into the oxygenated upper water column due to environmental perturbations could have induced the oxidative decay of the organic carbon, resulting in the negative C-isotopic excursion[32,33]. In addition, high nutrient availability in the immediate aftermath of the expansion of anoxic deep waters could have enhanced primary productivity, generating the ensuing positive C-isotopic excursion[32,33]. These fluctuations of redox conditions may explain the repeated negative and positive C-isotopic excursions during the Early Triassic. This scenario is supported by the ocean redox studies which demonstrated the temporal coincidence between the negative C-isotopic excursion and the ocean anoxia[5,8]. Further paleo-redox studies carried out in the framework of chemostratigraphy worldwide can test this scenario and improve our understanding of the mechanistic links between the ocean chemistry changes and the Early Triassic C-isotopic excursions.

Previous studies have tested the relationship between the carbon cycle perturbation and the limited biotic recovery during the Early Triassic[11,13]. These studies based on the body fossil records suggested that the physical mechanisms of the C-isotopic fluctuations may have contributed to the prolonged Early Triassic biotic recovery[11,13]. Body fossil records provide several metrics of the biotic recovery process, revealing low global biodiversity and poorly functioning ecosystems dominated by nektonic organisms but devoid of benthic groups during the Early Triassic[11,13]. Trace fossils serve as alternative evidence for the delayed biotic recovery during the Early Triassic. Trace fossils potentially provide more detailed records of both epifaunal and infaunal organisms, and their preservation would not have been influenced by environmental changes, thus facilitating the assessment of the biotic recovery processes[18,34,35]. The ichnological studies in the Tianshengqiao section showed that the Lower Triassic is marked by low overall ichnodiversity, suggesting prolonged recovery and sustained harsh environmental conditions until the early Middle Triassic[18,34,35]. Our study reveals that large C-isotopic fluctuations characterize the Lower Triassic in the Tianshengqiao section. The stratigraphic coincidence between the C-isotopic fluctuations and the ichnologically observed interval of limited biotic recovery in the Tianshengqiao section corroborates the causal connection between carbon cycling and biological re-diversification following the Earth’s most severe mass extinction (Figure 4).

6 Conclusions

A new high-resolution δ13Ccarbprofile in the Tianshengqiao section permits recognition of well-established negative and positive excursions in the Early-Middle Triassic successions. Overall, the δ13Ccarbprofile in the Tianshengqiao section can be comparable to those in the contemporaneous sections worldwide. The δ13Ccarbprofile in the Tianshengqiao section shows a negative excursion of 2.2‰ within the Member I of the Jialingjiang Formation, succeeded by an increase of 3.8‰ in the lower part of the Member III of the Jialingjiang Formation. After that, δ13Ccarbdisplays a negative shift of 2.5‰ in the Member III of the Jialingjiang Formation, followed by an increase with a maximum of +5.0‰ at the base of the Member I of the Guanling Formation. Above the maximum value, the δ13Ccarbprofile exhibits a third negative excursion and then recovers and stabilizes around +1‰ to +2‰ through the Member II of the Guanling Formation to the Yangliujing Formation. The new high-resolution δ13Ccarbrecord in the Tianshengqiao section provides additional evidence for the global instability of the Early Triassic carbon cycle. The stratigraphic coincidence between the C-isotopic fluctuations and the delayed biotic recovery has been documented in the Tianshengqiao section, suggesting a causal link between carbon cycle perturbation and prolonged biological recovery following the Earth’s most severe mass extinction event.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (41877319, 41703067, 41877318, 41772022), the USTC Research Funds of the Double First-Class Initiative (YD2080002005), and the Chinese Geological Survey (DD20190054).

Conflict of interest

The authors declare no conflict of interest.