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Article

Carbon Isotope Stratigraphy across the Devonian–Carboniferous Boundary in the East Paleo-Tethys Realm, Tibet, China

by
Zhanhu Cai
1,*,
Haisheng Yi
2 and
Hong You
3
1
Department of Petroleum Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China
2
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
3
Department of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(9), 1144; https://doi.org/10.3390/min13091144
Submission received: 17 July 2023 / Revised: 15 August 2023 / Accepted: 16 August 2023 / Published: 30 August 2023
(This article belongs to the Special Issue Tectono-Magmatic Evolution and Metallogeny of Tethyan Orogenic Belts)
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Abstract

:
The Devonian–Carboniferous boundary is one of the most important turning points in geological history, marked by the Late Paleozoic Ice Age and Late Devonian extinction. This study investigates the carbon isotope stratigraphy across the Devonian–Carboniferous boundary in Lhasa block, Tibet, China, which was part of the Gondwana continent during that time. The carbon isotope curves show a significant negative excursion trend, consistent with those of the South China block and other regions on the Laurentia continent. This global negative shift may be attributed to the burial of significant amounts of 12C-rich organic matter in strata, a consequence of the Late Devonian extinction event. Based on the carbon isotope curve and stratigraphic data of the Lhasa block, this study determined, for the first time, the specific horizon of the Devonian–Carboniferous boundary in Tibet, which is located between grayish white bioclastic limestone and yellowish-brown sandy limestone in the upper part of the Chaguoluoma Formation (D1C1ĉ). These findings provide a new reference for the international stratigraphic community to reconsider the position of GSSP on the Devonian–Carboniferous boundary, as well as study the Late Devonian mass extinction and Late Paleozoic glaciation.

1. Introduction

The Devonian–Carboniferous boundary (DCB) represents a significant turning point in geological history, marked by two major events: the Late Paleozoic Ice Age (LPIA) and the Late Devonian mass extinction (LDME) [1,2,3]. The former induced a global positive carbon isotope excursion (CIE), as demonstrated by research on black shale deposits [4,5], whereas the latter is connected to a negative CIE [6,7,8].
Carbon isotope fluctuations have been extensively employed as supplementary indicators for stratigraphic division and correlation. The detection of CIE anomalies in black shale deposits [9] and their association with extinction events [6,7] have enhanced our comprehension of the interrelated processes among sedimentary, seawater, atmospheric, and biospheric domains in geological history [10,11,12]. In this context, CIEs in carbonate rocks are predominantly linked to the biosphere, with substantial negative CIEs coinciding with major geological boundaries and smaller oscillations associated with fossil zone alternations [13].
Brian Popp (1986) examined the δ13Ccarb curves of whole rocks and double shells in the Ordovician–Permian strata in North America and identified a positive deviation, characterized by a sudden increase in the early Carboniferous and a high value in the late Carboniferous–Permian [14,15,16].
At present, the negative drift trend of δ13Ccarb near the Devonian–Carboniferous boundary is ascribed to the global augmentation of organic carbon burial during the late Devonian Hangenbao event, a hypothesis widely accepted by scholars. Negative δ13Ccarb drifts in proximity to the Devonian–Carboniferous boundary have been documented in various profiles, such as the Exshaw profile in western Canada, the Oklahoma profile in the United States the, Rhenisch Schiefergebrige profile in Germany, the West Pomerania profile in northwestern Poland, the Moravia profile in the former Soviet Union, and the Hua Mu profile in southern China [17]. Correspondingly, negative δ13Corg migration has been observed at the Devonian–Carboniferous boundary in the Grüne Schneid section in Austria, the Kronhofgraben section in Italy, and the Hasselbachtal section in Germany, with migration amounts of 6.58‰, 0.83‰, and 4.58‰, respectively [18,19], offering additional evidence that the negative CIE resulted from an increase in buried organic matter.

2. Geological Setting

The formation and evolution of the Qinghai–Tibet Plateau stem from multiple collisions between various Gondwanan landmasses starting in the Early Paleozoic [20,21]. The Lhasa block extends from the Indus–Yarlung Zangbo suture zone in the south to the Bangong Lake–Nujiang suture zone in the north and is detached from Gondwana, subsequently colliding with the Qiangtang block during the Mesozoic era [20,21,22,23]. The Lhasa block is divided into two segments: the southern and northern Lhasa blocks, which are delineated by the central Sumdo Paleo–Tethyan suture [24,25] (Figure 1). The collision between the Lhasa block and the Qiangtang block in the northern region of the Lhasa block transpired in the Late Jurassic and persisted until the early Late Cretaceous [26]. This collision led to a 180 km shortening of the Lhasa block [27], resulting in an elongated strip approximately 2500 km in length in an east–west direction. The Lhasa block is the final terrain preceding the collision between the Indian and Eurasian plates [28].
The Xainza area, situated in the northern Lhasa block, is globally recognized as an exemplary section of Carboniferous Gondwana facies [29]. The strata outcrops of the region represent the most comprehensive in northern Tibet, barring those from the Triassic and Cambrian periods. The Paleozoic strata in the Xainza area exhibit well-developed, continuous exposure and possess an extensive array of paleontological fossils. Consequently, this area serves as an ideal location for studying and comparing Paleozoic strata both in China and worldwide [29].
In 1977–1978, Lunzhujiacuo and Cai Li established Devonian and Carboniferous strata from the first detachment of the Tibet Geological Bureau, with the Luogong Member acting as a transitional group between the two [30] (Figure 2). More recently, between 2000 and 2003, the Regional Geological Survey of Jilin Province produced significant advancements in a regional geological survey of Xainza and Duoba, determining the Paleozoic stratigraphic sequence in the Xainza area [31]. The Late Paleozoic strata in the Xainza area are classified into seven groups, listed in ascending order as follows: the Daerdong Formation (D1d), the Chaguoluoma Formation (D1C1ĉ), the Yongzhu Formation (C1–2y), the Laga Formation (C2P1l), the Angjie Formation (P1a), the Xiala Formation (P2x), and the Muijucuo Formation (P3m).
The Chaguoluoma Formation (D1C1ĉ) was initially identified by Xia Daixiang in 1979 within the eastern area of Chaguoluoma–Daerdong, situated in the northern region of Xainza County [30]. The lithology of the formation primarily consists of gray limestone and dolomite. Subsequent revisions by several scholars [29,31,32,33,34,35] have led the academic community to adopt the 1/250,000 Xainzatu scheme of the Jilin Provincial Geological Survey Institute for the division of the Chaguoluoma Formation. This thick carbonate stratum is distributed in Yongzhuqiao, Talma, and other areas, with the upper boundary being the Early Carboniferous coral Canadiphyllum? sp. and Zaphriphyllum? sp. and the lower boundary being the Pragian–Emsian conodonts Pandorinellina exigua philipi and Ozarkodina excavata excavata [31].
The Luogong section referred to in this study is located near the Luogong Bridge in Zhu Yong Township, Xainza County, Tibet. The collected rock samples are from the Chaguoluoma (D1C1ĉ) and Yongzhu Formations (C1–2y). The lower member of the Chaguoluoma Formation is grayish-white bioclastic limestone, while the upper member is yellowish-brown gravelly limestone. The lower member of the Yongzhu Formation is grayish-green siltstone, and the upper member is grayish-black calcareous shale mixed with grayish-black siltstone, which produces biological fossils such as ammonites and brachiopods.
The seventh layer of the Chaguoluoma Formation in the Pengga–Langma section—as measured by the Jilin Regional Geological Survey Institute in Yongzhu Township, Xainza County—is large gray oolitic limestone mixed with thin-layer glutenite sparry limestone [31]. The conodont Caudicriodus angustus angustus produced in this layer was identified as an important fossil from the Eifelian Stage of the Middle Devonian by Wang Chengyuan from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS) [31]. The fourth layer of conodont Ozarkodina excavata excavata and the first layer of conodont Pandorinellina exigua philipi were identified as originating from Pragian–Emsian-stage fossils by Wang Chengyuan. In addition, the corals Canadiphyllum? sp. and Zaphriphyllum? sp., collected from the upper part of the Chaguoluoma Formation on the east bank of the Guomangcuo Lake, were identified as Early Carboniferous by Liao Weihua, a researcher from NIGPAS [31]. The lithology of the topmost horizon of the Luogong section in the measured section from the Carboniferous to the Lower Permian is light-purple bioclastic limestone, and the collected brachiopod tracheid Syringothyris sp. fossils belong to the Tournaisian–Visean and were most developed within the Tournaisian stage [31,32]. In addition, the Shidian area in Yunnan Province [36], Beishan Mountain in Gansu Province [37], the Balikun area in Xinjiang [38], and other places have confirmed this, and all of them are considered to be of the early Tournaisian–Visean stage and were most developed within the Tournaisian stage. Rotaia subtrigona, a brachiopod fossil, is widely found in the late Tournaisian stage in North America, Kazakhstan, and China, and some can be found in Visean-stage strata [31]. Therefore, the age of the Chaguoluoma Formation in the Xainza area is Devonian–Early Carboniferous.
The stratum previously known as the Yongzhu Group has been reclassified as the Yongzhu Formation (C1–2y). The original site designated for the Yongzhu Group is situated in the Lashan area, precisely 4.5 km to the east of Yongzhu Township in Shenzha County, Tibet. In 1979, Xia Daixiang identified the lower strata of the former Yongzhu Group—which generated brachiopods such as Buxtonia sp. and Marginifera cf. vuenina and bivalves including Aviculopecten chuniukouensis—as originating in the Yongzhu Formation. The principal lithology of this formation is composed of an array of gray-to-gray-green shale, quartz sandstone, and a minor proportion of siltstone. Additionally, interspersed layers of gray limestone and calcareous sandstone can be observed.
The Yongzhu Formation can be predominantly found in two locations: adjacent to the Yongzhu Bridge and within the Talma syncline. This formation seamlessly integrates with the underlying purple-red bioclastic limestone formation of the Chaguoluoma Formation and the gray-white middle-layered sandy conglomerate formation of the Laga Formation situated above it.
In the Deriangma–Lashan area of Jilin Province, Xainza County, the conodont Gnathodus girtyi simplex Dunn was retrieved from the sixth layer of the gray-green shale in the Lower–Upper Carboniferous Yongzhu Formation section. The Gnathodus texanus Roundy assemblage was categorized as being of the late Early Carboniferous assemblage [31]. The International Commission on Chronostratigraphy acknowledges this fossil as the benchmark for demarcating the boundary between the Tournaisian and Visean epochs.
Gnathodus texanus Roundy, collected from the Shihuadong Formation of the Yudong section in Yunnan, has been accepted as the boundary marker between the Dunayian–Visean, defined by its first appearance in [32]. In the Moorefield Formation in northcentral Arkansas, USA, the abundant presence of the conodont Gnathodus texanus Roundy is considered to be indicative of the Visean epoch [33]. Currently, the base age of the Yongzhu Formation is recognized as Visean, while the age of the upper boundary remains a subject of contention.
The coral species Cyathaxonia sp. has been identified within a specific section as an archetypal specimen of cold-water habit corals. Its geochronological dating is traced back to the Kasimovian epoch of the Late Carboniferous era [31,34]. Contrastingly, Cyathaxonia sp. specimens extracted from the Dingjiazhai Formation in Yunnan have been designated as a Sakmarian–Artinskian fossils according to various studies [35,36]. In a similar vein, Cyathaxonia sp. fossils found within the Lublin Basin situated in eastern Poland are dated to the Moskvaian epoch [37]. As a result of these findings, it can be concluded that the conodont fauna located in the upper strata of the Yongzhu Formation is consistent with Upper Carboniferous Moskvaian fossils found in regions spanning South China, North China, North America, and Russia [38,39].
In conclusion, based on the available evidence, it is plausible to infer that the age of the Yongzhu Formation spans the Vissian–Kasimovian epochs.

3. Materials and Methods

The data were derived from field collection in the Luogong section (LGS) east of Yongzhuqiao in Xainza County, Tibet Autonomous Region (Figure 3). The Luogong profile is situated approximately 4 km from the northeastern part of Xia La Mountain, with an elevation of 5666 m above sea level. The total length of the profile spans 172.65 m and is divided into 15 distinct layers. The geological strata encompass both the Chaguoluoma and Yongzhu Formations.
The Luogong section, situated In the Xainza region, predominantly comprises a stratum of gray-white bioclastic limestone at its base. This limestone layer, thickly formed, is notably characterized by the visible presence of a multitude of crinoid stems, oolites, and a minor occurrence of spherulite fossils. Proceeding upward, the middle portion of the section features a composite set of gray-green siltstone and yellow-gray sandstone, interleaved with thin layers of gray-green siltstone mudstone. At the apex of the section, one finds a stratum of gray-black calcareous mudstone interspersed with thin-layered gray-black siltstone, approximately 1.00 m in thickness. Occasionally, a stratum of gray-black thin-layered bioclastic limestone, around 0.90 m in thickness, replete with brachiopods, corals, crinoid stems, and ammonite fossils, can be found (Figure 4).
Based on the unique sedimentary attributes of the Luogong section, one can classify it as a marine facies group. The lower part exhibits the features of a neritic carbonate platform sedimentary system, while the middle and upper parts signify the presence of a neritic continental shelf sedimentary system. The Chaguoluoma Formation showcases the development of a carbonate depositional system, mainly characterized by gray-white bioclastic limestone, topped with a layer of yellow-brown sandy limestone. The Yongzhu Formation, in contrast, represents the sedimentary system of the marine shelf, featuring a dominant lithology of gray-black calcareous shale. The formation begins with siltstone at the base and concludes with bioclastic limestone at the top.
A total of 136 samples were taken, from the Chaguoluoma Group (D1C1ĉ) to the Yongzhu Group (C1–2y), requiring that the test sample results reflected the original information of the geological events and were not strongly affected by diagenetic processes in the later period [40]. Therefore, when collecting samples, the weathered surface of the rock surface was removed, and about 500 g of rock on the fresh surface was collected.
These samples were screened based on rock sections, and 77 samples were selected for carbon and oxygen isotope testing. The lithology of 23 samples, consisting of micrite and bioclastic limestones, was analyzed for inorganic carbon and oxygen isotopes. For 63 samples, including 56 samples of argillaceous shale and mudstone and 7 samples of micrite and bioclastic limestones, organic carbon isotope analysis was conducted.
In this study, inorganic carbon and oxygen isotope tests were conducted at the Analytical Testing Research Center of the Beijing Institute of Geology, China Nuclear Industry. First, fresh carbonate rocks, without dikes or later alteration, were selected and ground into a 200-mesh powder. Second, co2 was prepared using the orthophosphoric acid method and tested with a MAT 253 gas isotope mass spectrometer. According to geological and mineral industry standard DZ/t 0184.17–1997, “Analysis Method of Isotope Geological Samples”, the error ranges of carbon and oxygen isotopes are +0.05‰ and +0.10‰, respectively.
Organic carbon and oxygen isotopes were tested at the Sichuan Coalfield Geology Bureau using a stable isotope ratio mass spectrometer (model: MAT253), manufactured in the United States. First, the fresh part of the sample was dried at a low temperature and ground into a 100-mesh powder, and carbonate rocks were removed by adding 10% hydrochloric acid in a PC centrifuge tube and glass beaker. Second, two analytical methods, namely, EA–IRMS online and offline preparation technology, were used for carbon dioxide gas preparation and mass spectrometry analysis. The whole process of the test conformed to geological and mineral industry standard DZ/T 0184.17-1997, “Isotope Geological Sample Analysis Method”. The error for organic carbon and oxygen isotopes was less than 0.2‰.

4. Results

4.1. Oxygen Isotope

Oxygen isotope tests were carried out on 23 carbonate rock samples from the Chaguoluoma Formation, and the δ18O values ranged from −14.9‰ to −8.60‰, with an average value of −13.24‰ (Figure 5). The δ18O values of Tournaisian-stage samples ranged from −12.40‰ to −8.60‰, with an average value of −10.05‰. For the carbonate sample, δ18O > −10‰ or −11‰ was a sign that the carbonate rocks did not suffer diagenesis [6,41,42]. The δ18O value of the carbonate rocks in the Chaguoluoma Formation had a minimum value of −14.95‰, a maximum value of −8.68‰, and an average value of −13.24‰, indicating that they underwent weak diagenesis.

4.2. Carbon Isotope

The LGS is positioned within the stratigraphic region encompassing Tibet and western Yunnan, a region that is integral to the Tethyan domain. Twenty-three carbonate rock samples from the Chaguoluoma Formation in the Luogong section were analyzed for inorganic carbon isotopes, with δ13Ccarb values ranging from −2.70‰ to 1.10‰ and an average value of 0.17‰ (Figure 6). The inorganic carbon isotope curve of the Chaguoluoma Formation is distributed in two sections, with an average deviation of 1.99‰. Fifty-five clastic rock samples from the Yongzhu Formation were tested for organic carbon isotopes, and the δ13Corg values ranged from −27.00‰ to −22.56‰, with an average value of −23.83‰ (Figure 7). The organic carbon isotope curve of this profile showed three-stage distribution characteristics: the distribution range of the δ13Corg value in Section I is −23.01‰ to −21.37‰, with an average value of −21.99‰; the distribution range of the δ13Corg value in Section Ⅱ is −27‰ to −26.4‰, with an average value of −26.67‰; and the δ13Corg value in Section Ⅲ ranges from −24.72‰ to −22.56‰, with an average value of −23.49‰. The curve shows a negative migration trend as a whole, in which the Tournaisian stage at the top of Section I and the Visean stage of Section Ⅱ show strong negative migration.

4.3. Lithology and Paleoenvironmental Evolution

An examination of the Luogong section in the stratigraphic sequence, from the base to the apex, reveals the existence of two primary sedimentary systems, namely, the carbonate platform and neritic shelf facies. The carbonate-platform facies, progressing from the base to the apex, can be categorized into open-platform subfacies, carbonate-platform-margin shoal subfacies, and carbonate-platform foreslope subfacies (Figure 4).
The basal stratum of the profile comprises a grayish-white bioclastic limestone. This layer is characterized by a predominance of crinoid stem biodebris, supported by fragments of crinoids. Crinoids, being typically benthic, reside in shallow sea environments with deep water bodies. The conservation environments of these crinoid stems tend to be tranquil waters, and their large-scale preservation suggests a location within open-platform facies or continental shelf facies in the low-energy environment of a carbonate sedimentary system.
The prevalence of Shanghai lily stems tapers off at this layer, supplanted by the occurrence of a substantial quantity of oolites, which are relatively homogenous in size. The growth in oolites is generally observed in high-energy environments, specifically, warm, shallow waters. According to Walther’s law and considering the sedimentary characteristics of the underlying layer, it can be inferred that this layer’s depositional environment corresponds to the shallow marine carbonate-platform-margin shoal subfacies.
Simultaneously, the underlying stratum, abundant in crinoid stem fossils, is designated as an open-platform facies. Overlying this layer, an abundance of encrusted biospheres can be observed within the limestone, with the sizes of these spheres typically ranging between 0.02 and 0.10 mm. Should they be well sorted, these could be mechanical-origin, fine-sand-level debris, indicating a moderately energetic water body environment.
The second layer of the profile, at the apex of the Chaguoluoma Formation, contains an internal stratum of clastic limestone. Given the poor sorting and rounding of the clastics, they are likely to be colluvial rock subfacies of the carbonate-platform foreslope. This sedimentary system in the Chaguoluoma Formation demonstrates the trend of a transgressive water body, progressively deepening from base to apex. The Late Devonian marked the zenith of Tibetan transgression [43].
The third to fifth strata, located in the middle and lower parts of the profile or the base of the Yongzhu Formation, are composed of gray-green and yellow-gray siltstones. They occur in thin or very thin layers, with a sand-to-mud ratio of 8:1, representing the depositional environment of the inland shelf subfacies close to the mainland. These are considered low-stand system tracts in the lower part of the Yongzhu Formation [43,44,45].
The sixth to fifteenth strata of the profile predominantly consist of gray-black shale, occurring in extremely thin layers and sporadically interspersed with siltstone. These strata exhibit numerous brachiopods, crinoid stems, corals, and solitary ammonite fossils, indicating the further deepening of the water environment. Here, the depositional environment corresponds to outer-shelf facies deposition, classified under the transgressive system tract.
During the Carboniferous, the Yongzhu Formation in the Shenza area exhibited a subsidence rate that outpaced the deposition rate, resulting in a non-compensatory evolutionary stage in the basin [45]. The primary phase of the Carboniferous was the transgressive system tract [43].
The expansive Paleo-Tethys Ocean passive continental-margin, carbonate platform evolved during the Devonian Gondwana. Throughout this period, the Gondwana continent principally migrated in a north–east direction, with the Carboniferous Laurasia colliding with the clockwise-rotating Gondwana continent to form Pangea. Concurrently, the Late Paleozoic Great Ice Age experienced continuous development in the latitude area, leading to a drop in temperature and a corresponding decrease in the global sea level. This change fostered the development of shallow marine clastic rock deposits along the northern margin of the Gondwana continent.

5. Discussion

5.1. Xainza Area of Tibet

The degree of original information preservation is the key to explaining the carbon and oxygen isotope values of samples. Generally, well-preserved isotopes indicate that there is no cathodoluminescence, with low manganese, low iron, low magnesium, and high strontium, but cathodoluminescence microscopy and trace elements have potential defects in evaluating potential diagenesis and alteration [46].
Carbonate sediments have a high sedimentation rate and obvious biological sedimentation. Their cement easily forms, dissolves, and undergoes metasomatism, and their diagenesis is diverse [47,48]. Therefore, the diagenesis of carbonate rocks has a significant impact on rock properties [49]. The influence of carbonate rock diagenesis on carbon and oxygen isotopes includes two aspects: ① the controlling effects of a sedimentary environment and a diagenetic environment on carbon and oxygen isotopes during their formation process; ② in the process of supergene and supergene diagenesis, diagenetic fluid and the diagenetic environment changed the original carbon and oxygen isotopes in sedimentary rocks [46].
There are three indexes used to judge that the carbon and oxygen isotopic compositions of carbonate rocks are not influenced by diagenesis.
① There is no positive correlation between δ13Ccarb and δ18O in the carbonate rocks [6,41]. If there is a positive correlation between them, it means that they are influenced by the diagenesis of fresh water in the atmosphere [50]. According to the correlation value, R2, the correlation degree can be divided into six levels, whereby |R2| < 0.3 indicates micro-correlation. As shown in Figure 8, the correlation coefficient, R2, between δ13Ccarb and δ18O in carbonate rocks in the Luogong section is 0.08, which is almost irrelevant. This shows that the carbon and oxygen isotopic compositions of carbonate rocks in the Chaguoluoma Formation of the Upper Devonian, the Angjie Formation, and the Xiala Formation of the Lower Permian studied in this paper are basically unaffected by the diagenesis of carbonate rocks.
② The δ18O of carbonate rocks is > −10‰ or −11‰ [6,41,42]. The δ18O value of carbonate rocks in the Luogong section ranges from −14.9‰ to −8.6‰, with an average of value −13.24‰, indicating that they are weakly influenced by diagenesis. For carbon isotopes, burial diagenesis is a closed system, which will not cause obvious changes in the δ13Ccarb value, so the carbon isotope value of the sample keeps its original features.
③ As shown in Figure 9, the δ13Ccarb and δ18O values of carbonate rocks in the study area are within the 95% effective region of the Phanerozoic carbon and oxygen isotope trend [16].
Graham et al. (1993) studied the origin of the earliest terrestrial plants from the Middle Ordovician to the Late Ordovician (470 million to 450 million years ago) [51]. The stratigraphic age studied in this paper is from the Late Devonian to the Middle Permian, so it is necessary to make a detailed study of the source of organic matter in the samples. Compared with the δ13Ccarb value of the sample, the influencing factors of the δ13Corg value of the sample have many complex characteristics, but it is less affected by late diagenesis, so the δ13Corg value of the sample retains its original information.
At present, the standard conodont Siphonodella praesulcata–Siphonodella sulcata fossils at the bottom boundary of the Carboniferous have not been found in the Chaguoluoma Formation in the Xainza area of Tibet, so the specific stratum of the Devonian–Carboniferous boundary of this formation has not been demarcated yet [30,31,34]. This group is considered an interdecadal stratigraphic group of the Devonian–Carboniferous, with the lower bound at the Emsian stage of the Early Devonian and the upper bound of the Tournaisian Stage in the Early Carboniferous [31]. According to the above research, the purple-red and yellow-brown sandy limestone in the Luogong section of the Chaguoluoma Formation at the bottom of the Luogong section belongs to the Tournaisian stage [31], and its δ13Ccarb curve has two negative offsets, which are 2.6‰ and 3.5‰. There are five reasons for the negative migration of inorganic carbon isotopes, discussed below.
① The productivity decreased [52]. ② The organic carbon burial decreased. ③ The input of volcanic material increased. ④ Methane was released from submarine clathrates or seafloor seeps. ⑤ There was a strong mix of seawater [53,54]. These two negative shifts at the top of the Chaguoluoma Formation may be due to a decrease in productivity caused by the mass extinction of organisms at the end of the Devonian [55,56]. Considering the broader context of the Tibetan transgression and the Paleozoic tectonics and climatic shifts (including the Late Paleozoic Great Ice Age), the negative shifts could be part of a broader geochemical signal, reflecting large-scale changes in climate, sea levels, and tectonic activity.
The negative shift in inorganic carbon isotopes within the internal stratum of the clastic limestone of the second layer provides valuable information about the paleoenvironmental conditions at the time of deposition. Careful interpretation, considering the sedimentary characteristics, broader geological context, and potential correlation with other geochemical and isotopic records, can provide insights into the complex interplay of tectonics, climate, oceanography, and sedimentation during this period of the Earth’s history. Such isotopic information, integrated with other geological data, can be a powerful tool for reconstructing ancient environmental conditions and understanding large-scale geological processes.

5.2. South China

This segment of the study encapsulates data from eight distinct sections located in the southern region of China. The paleogeography study of the South China stratigraphic region belongs to the Paleo-Tethyan tectonic domain [42]. Sections located in Guizhou and Guangxi Provinces, namely, the Muhua section, the Qilinzhai section, the Baihupo section, the Nanbiancun section, the Huangmao section, and the Long’an section, all belong to the Nanhua stratigraphic division within the South China stratigraphic area. These sections are characterized by a high level of biological community abundance and diversity [57,58]. Both the Nanbiancun section in Guangxi and the Muhua section in Guizhou were previously among the three candidate sections for the international Devonian–Carboniferous boundary stratotype.
The Huangmao section, the Nanbiancun section, and the Muhua section, all located in Guangxi, exhibit well-preserved conodont zones, with the Siphonodella praesulcata–Siphonodella sulcata evolutionary zone of normalized conodonts having developed at the bottom of the Carboniferous [57,58,59]. The strata of the Ganxi section in Sichuan belong to the Qinling–Longmenshan stratigraphic division of the South China stratigraphic region, representing neritic and shallow water sediments [60]. The Luogong section under study, located in the Himalayan stratigraphic division in the Tibet–Western Yunnan stratigraphic region, is distinguished by its abundant carbonate rock formations.
Currently, no normalized conodont Siphonodella praesulcata–Siphonodella sulcata evolutionary zone at the bottom of the Carboniferous has been discovered in the Chaguoluoma Formation in the Xainza area of Tibet. Nevertheless, this formation is considered a Devonian–Carboniferous cross-age stratigraphic group, with its upper boundary being the early Carboniferous Tournaisian stage [31,38]. The exact stratigraphic horizon of the Devonian–Carboniferous boundary has yet to be calibrated.
Near the Devonian–Carboniferous boundary, noticeable negative anomalies are present in the Huangmao section, the Long’an section, and the Muhua section in Guizhou, as well as the Ganxi section, the Nanbiancun section, and the Qilinzhai section in Guangxi [61]. The offsets of these sections are 17‰, 2.37‰, 12.6‰, 3.99‰, 1.03‰, and 1.11‰, respectively (Figure 10). However, offsets in the Nanbiancun and Baihupo sections do not reach 1‰.
The purplish-red and yellow-brown psammites of the Chaguoluoma Formation at the bottom of the Luogong section in the Xainza area of Tibet belong to the Tournaisian. The δ13Ccarb curve here displays two negative shifts, measuring 2.6‰ and 3.5‰. A positive shift of 2.1‰ occurred at the top of the Chaguoluoma Formation. The drift trends of the δ13Corg, δ18O, and δ13Ccarb curves at the top of the Chaguoluoma Formation suggest the beginning of the Carboniferous ice age.
The carbon isotope drift indicates significant geological stratigraphic correlation. The drift trend and amplitude of the carbon isotope curve near the Devonian–Carboniferous boundary in the Xainza area of Tibet bear a striking resemblance to other domestic Devonian–Carboniferous boundary profiles supported by standard conodont fossils. We hypothesized that the intersection of the gray-white bioclastic limestone and the yellow-brown inner clastic limestone in the lower part of the Luogong section in the Xainza area could represent the Devonian–Carboniferous boundary. This hypothesis warrants further investigation. The drift trends of the δ13C curves in the South China and Tibet–Western Yunnan strata within the Tethys tectonic domain generally align.
In the realm of geochemistry studies, a shift exceeding 1‰ in carbon isotope values is generally perceived as significant. This variation transcends the typical boundaries of measurement uncertainty, hinting at pronounced modifications in the underlying biogeochemical processes. It is important to note that interpreting these records requires careful consideration of all the potential influences, along with the local and global context.

5.3. Global

This segment of the research amasses data from six individual sections spanning Europe and the Americas. Thus far, carbon isotopes near the Devon–Carboniferous boundary have been studied in the United States, Canada, Belgium, the Czech Republic, Poland, Austria, and Germany, as well as China [18,63,64,65,66,67].
In the above sections, except the Alberta section in Canada, where there is no standard conodont Siphonodella Praesulcata–Siphonodella sulcata evolution pedigree, Siphonodella Sulcata appears to determine the Carboniferous bottom boundary for the first time, and other sections have biostratigraphic control horizons. The Alberta section in Canada, the Hasselbachtal section in Germany, the Grüne Schneid section in Austria, and the Namur section in Belgium all show the negative δ13Ccarb drifts at the Devonian–Carboniferous boundary, with offsets of 1.09‰, 4.73‰, 6.43‰, and 1.48‰, respectively (Figure 11).
The Grüne Schneid section in Austria, the Lowa section in the United States, and the Moravian Karst section in the Czech Republic also exhibit a negative drift trend in δ13Ccarb at the Devonian–Carboniferous boundary, although the offset is less than 1‰. The pronounced downward shift in δ13Ccarb can primarily be attributed to the augmentation of the quantity of organic carbon in the seabed. This proliferation fosters the development of an anoxic environment, subsequently leading to an increased concentration of organic matter within the underlying carbonate rock strata [64]. The strong negative drift of δ13Ccarb is thought to be caused by an increase in organic carbon burial. This increase could have led to mass extinction events, as well as changes in glaciers and sea levels [65]. Other extinction events in the Phanerozoic had similar factors.
In summary, the drift trend and amplitude of carbon isotope curves near the Devonian–Carboniferous boundary of the Luogong section in the Xainza area of Tibet are very similar to those of other profiles with standard conodonts in China and the rest of the world (Figure 12). Therefore, we consider the Devonian–Carboniferous boundary in the Xainza area to lie between the yellow-brown clastic limestone in the Luogong section and the lower gray-white bioclastic limestone.

6. Conclusions

1. The carbon isotopes of the Devonian–Carboniferous boundary in Tibet, South China; the United States; Canada; Germany; Austria; Belgium; and the Czech Republic show a significant negative migration trend. This is crucial for our understanding of global climate patterns and paleobiological shifts and aids in providing a more comprehensive understanding of the Earth’s history.
2. We consider the Devonian–Carboniferous boundary in the Xainza area to lie between the yellow-brown clastic limestone in the Luogong section and the lower gray-white bioclastic limestone. In conclusion, identifying the Devonian–Carboniferous boundary in the Xainza area of Tibet not only facilitates the exploration and development of oil and gas resources in the region but also provides valuable information for geological research. This contributes to our understanding of this specific area and, more broadly, our knowledge of geological periods.

Author Contributions

Conceptualization, Z.C. and H.Y. (Haisheng Yi); investigation, Z.C.; writing—original draft, Z.C., H.Y. (Haisheng Yi), and H.Y. (Hong You); writing—review and editing, Z.C., H.Y. (Haisheng Yi), H.Y. (Hong You). All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the project of “Geochemical characteristics of oil shale and its paleoclimatic significance” from Guangdong University of Petrochemical Technology (No. 519013) and Research on Reservoir Anatomy and Accumulation Model in Deepwater West Area of Qiongdongnan Basin from Shanghai Beikairui Energy Technology Co., ltd (No. H20230072).

Data Availability Statement

The research created experimental data that can be found in the tables and figures presented in this manuscript.

Acknowledgments

In closing, we would like to express our profound gratitude to all the co-authors for their invaluable contributions to this paper. Their collective expertise, commitment, and collaboration have been the driving force behind the realization of this research project. We would also like to extend our heartfelt appreciation to the editor-in-chief of the journal, whose astute guidance and unwavering support have been instrumental in shaping this work. Furthermore, we are immensely grateful to the anonymous reviewers for their insightful comments, constructive criticism, and invaluable suggestions, which have significantly enhanced the quality and rigor of our manuscript. The collective efforts of these individuals have been integral to the success of this endeavor, and we are truly indebted to them for their time, expertise, and dedication.

Conflicts of Interest

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

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Figure 1. Global paleoplate reconstruction map (390 Ma) [25].
Figure 1. Global paleoplate reconstruction map (390 Ma) [25].
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Figure 2. Late Paleozoic stratigraphic evolution in the Xainza area.
Figure 2. Late Paleozoic stratigraphic evolution in the Xainza area.
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Figure 3. Profile position, geological map, and lithostratigraphy of the study area. (A) Carboniferous stratigraphic division map of China: Junggar–Xingan Stratigraphic Region (I), North China–Tarim Stratigraphic Region (II), South China–Qiangtang Stratigraphic Region (III), and Himalayan Stratigraphic Region (IV); (B) structural outline map of the Qinghai–Tibet Plateau; (C) geological map of the study area in this paper; (D) stratigraphic framework of the research presented in this paper.
Figure 3. Profile position, geological map, and lithostratigraphy of the study area. (A) Carboniferous stratigraphic division map of China: Junggar–Xingan Stratigraphic Region (I), North China–Tarim Stratigraphic Region (II), South China–Qiangtang Stratigraphic Region (III), and Himalayan Stratigraphic Region (IV); (B) structural outline map of the Qinghai–Tibet Plateau; (C) geological map of the study area in this paper; (D) stratigraphic framework of the research presented in this paper.
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Figure 4. Sedimentary evolution of the Luogong section.
Figure 4. Sedimentary evolution of the Luogong section.
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Figure 5. Oxygen isotope curve of the Luogong section.
Figure 5. Oxygen isotope curve of the Luogong section.
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Figure 6. Inorganic carbon isotope curve of the Luogong section.
Figure 6. Inorganic carbon isotope curve of the Luogong section.
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Figure 7. Organic carbon isotope curve of the Luogong section.
Figure 7. Organic carbon isotope curve of the Luogong section.
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Figure 8. Correlation diagram of δ13Ccarb and δ18O values of carbonate rocks in the Luogong section.
Figure 8. Correlation diagram of δ13Ccarb and δ18O values of carbonate rocks in the Luogong section.
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Figure 9. Phanerozoic carbon and oxygen isotope distribution map [16]. (A) Phanerozoic δ13C trend based on 1564 brachiopod; (B) Phanerozoic δ18O trend based on 1654 brachiopod.
Figure 9. Phanerozoic carbon and oxygen isotope distribution map [16]. (A) Phanerozoic δ13C trend based on 1564 brachiopod; (B) Phanerozoic δ18O trend based on 1654 brachiopod.
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Figure 10. Comparison of carbon isotopes near the Devonian–Carboniferous boundary in China [46,58,59,60,61,62,63].
Figure 10. Comparison of carbon isotopes near the Devonian–Carboniferous boundary in China [46,58,59,60,61,62,63].
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Figure 11. Comparison of carbon isotopes near the Devonian–Carboniferous boundary [18,63,64,65,66,67].
Figure 11. Comparison of carbon isotopes near the Devonian–Carboniferous boundary [18,63,64,65,66,67].
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Figure 12. Comparison of carbon isotopes near the Devonian–Carboniferous boundary worldwide [18,46,59,61,62,64,67].
Figure 12. Comparison of carbon isotopes near the Devonian–Carboniferous boundary worldwide [18,46,59,61,62,64,67].
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Cai, Z.; Yi, H.; You, H. Carbon Isotope Stratigraphy across the Devonian–Carboniferous Boundary in the East Paleo-Tethys Realm, Tibet, China. Minerals 2023, 13, 1144. https://doi.org/10.3390/min13091144

AMA Style

Cai Z, Yi H, You H. Carbon Isotope Stratigraphy across the Devonian–Carboniferous Boundary in the East Paleo-Tethys Realm, Tibet, China. Minerals. 2023; 13(9):1144. https://doi.org/10.3390/min13091144

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Cai, Zhanhu, Haisheng Yi, and Hong You. 2023. "Carbon Isotope Stratigraphy across the Devonian–Carboniferous Boundary in the East Paleo-Tethys Realm, Tibet, China" Minerals 13, no. 9: 1144. https://doi.org/10.3390/min13091144

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