Research articleMarine Late Triassic-Jurassic carbon-isotope excursion and biological extinction records: New evidence from the Qiangtang Basin, eastern Tethys
Graphical abstract
Introduction
The mass extinction event of the Late Triassic remains the least understood of the five Phanerozoic mass extinction events (Sepkoski Jr., 1996; Raup and Sepkoski Jr, 1982; Tanner et al., 2004), compared to the other four mass extinction events, including marine and terrestrial assemblages. It is estimated that about 47% of genera and 80% of species vanished within a time-span of about 600 ka to 8.3 Ma (Ward et al., 2001). In the late Rhaetian, about 48% of invertebrate species became extinct, mainly including brachiopods, bivalves, cephalopods, and gastropods (Sepkoski Jr., 1986). The exact time and cause(s) for the Late Triassic biological extinction have been controversial, mainly because of the lack of continuous marine sections. Europe is currently the most well-studied area, where 92% of species and 42% of genera of marine biota went extinct in the Late Triassic (Deng et al., 2005). In order to explain the cause(s) of the extinction of some many of the earth's organisms, many hypotheses and feedback effects of the environment have been put forward and discussed intensely (Korte and Kozur, 2011; McElwain et al., 1999). It is speculated that the extinctions of marine and terrestrial organisms were triggered by some global events. There are four main hypotheses about the reasons for the extinction of the Late Triassic: large-scale volcanic activity with accompanying events (Hesselbo et al., 2002; Hautmann, 2012; McRoberts et al., 2012), rapid sea-level oscillations accompanied by oceanic anoxic (Hallam and Wignall, 1999), changes in climate and ocean environment (McElwain et al., 1999; Pálfy et al., 2001), and asteroid impact events (Olsen et al., 2002).
Marzoli et al. (2004) found that the magma emplacement age of basalts within the Central Atlantic Magmatic Province (CAMP) was 199 ± 2.4 Ma. Schaltegger et al. (2008) determined that the UPb age in the Triassic-Jurassic boundary is 201 Ma, a result which is supported by data from Marzoli et al. (2011) and Schoene et al. (2010). Schaltegger et al. (2008) demonstrated that the overflow of the large continental flood basalts from the CAMP corresponds to the Triassic-Jurassic boundary. Many studies have shown that inorganic and organic carbon isotope anomalies are often linked to environmental catastrophic events and biological extinction events (Holser et al., 1989; Dickens et al., 1995; Gröcke et al., 1999). The mass extinction at the Triassic-Jurassic (T-J) boundary seems to be bound up with a disturbance of the carbon cycle. Two distinct negative anomalies are present in the carbon isotope curve, usually divided into two different types, “initial” and “main”, which have been confirmed by the organic carbon isotope data from sections across the T-J boundary worldwide including the United States (Guex et al., 2004; Ward et al., 2007), Spain (Gomez et al., 2007), and British Columbia (Wignall et al., 2007) and inorganic carbonate carbon isotope curves from Italy (Galli et al., 2005), Hungary (Pálfy et al., 2007), and Slovakia (Michalik et al., 2007). Many studies have found that biological crises, environmental changes, and global carbon reservoir perturbations are associated with suddenly increased CO2 emissions from CAMP (Hautmann, 2012; McRoberts et al., 2012) and associated effects from the emission of methane hydrates (Marzoli et al., 2004). This sudden and intense carbon-isotope anomaly during the transition of the T-J is caused by the rapid release of a mass of light carbon isotopes into the air and seawater (Galli et al., 2005; Pálfy et al., 2001). Beerling and Berner (2002) and McElwain et al. (1999) confirmed a huge rise in carbon dioxide (about 8000–9000 Gt of carbon from carbon dioxide) and methane emissions (about 5000 Gt of carbon from methane) and an increase in ambient temperature (3–4 °C).
During the transition of the T-J, most of the sediments in China are non-marine facies, and thus the distribution of marine successions is very limited. The T-J boundary is mainly studied in terrestrial sequences, but knowledge of the marine background has many shortcomings. The best continental T-J boundary stratigraphy in north China is developed in the Zhunggar Basin of Xinjiang (Deng et al., 2011; Lu and Deng, 2009; Lu and Deng, 2005); while the continental T-J boundary stratigraphy in southern China has been studied most systematically in the Sichuan Basin (Deng et al., 2013; Cheng et al., 2004). Because the lithology and facies of the non-marine sections vary greatly laterally and isecologicaly complex, the study of non-marine boundary strata has failed to reach the accuracy of marine strata, and it is difficult to compare over large areas.
The Gemige section is located in the Nyalam area in southern Tibet and has a rare continuous marine section spanning the Triassic and Jurassic. Yin et al. (2006) made a large regional division and comparison in the Gemige section. However, this area was located in the southern hemisphere during the Triassic-Jurassic transition (Fig. 1b).
Although the marine T-J transition has been well documented in south of the equator (Kürschner et al., 2007), the west Tethys, and the Boreal realms (Galli et al., 2005; Pálfy et al., 2007), no records of the T-J transition have been mentioned from open sea environments in the eastern Tethys. Therefore, it is unclear if the T-J isotope anomaly and correlated extinction events impacted the eastern Tethys in a parallel fashion to the western Tethys and Boreal realms.
Qiangtang Basin is a Mesozoic residual basin, with an area of 180,000 km2, located in the northern part of the Qinghai-Tibet Plateau. Prior to the Cretaceous, the only marine succession was found in the basin, especially the distribution of sequences spanning the T-J boundary is the most extensive and most complete. Thereby, this basin is an ideal area to study the marine T-J transition in the eastern Tethys. For the first time, we put forward a high-resolution carbon isotope record, geochemistry, palaeontologic and lithofacies data from the Wenquan section of the Qiangtang Basin. The goals of the present study are to (1) determine the location of the marine T-J boundary in the study area, (2) characterize the carbonate carbon-isotope curve, sedimentary petrology and paleo-ocean variation spanning the T-J transition, and (3) obtain valuable data about the biological crisis that occurred in the Late Triassic and discuss the possible mechanisms for the mass extinction.
Section snippets
Geological setting
The Qinghai-Tibet Plateau consists of a series of terrestrial blocks from south to north, namely the Himalaya fold-thrust belt, the Lhasa block, the Qiangtang block, and the Songpan-Ganzi flysch complex (Wang et al., 2009a; Fig. 1a). In addition, the Qinghai-Tibet Plateau has developed three structural zones in the east-west direction, namely the Yarlung Tsangpo suture, the Bangong Lake-Nujiang River suture, and the Hoh Xil-Jinsha River suture, which are the boundary of the above-mentioned
Materials and methods
Sixty-one samples (30–50 cm intervals), each with a volume of 1000 cm3, were collected from the Wenquan section (Fig. 3). They were used for carbonate C-isotope (δ13Ccarb), geochemical analysis, lithology, composition, and structure analysis. The study area and section are shown in Fig. 1(c).
The samples were ground into powder, and 300 mg of powder was filtered out using a sieve of <150 mesh for δ13Ccarb analysis. The powder was treated with 100% H3PO4 for 24 h at 50 °C to collect the CO2
Previous biostratigraphic and chronological data
Wang et al. (2006) first recognized ammonites in the upper micrite of Wenquan section. Four ammonites belonging to the same genus with relatively well-preserved, whose convoluted shell, compressed form, wide and shallow umbilicus, slightly convex flank, non-tuberculate ribs, and wide L-shaped lobe coincide with the Late Toarcian Genus Dumortieria (Kovács, 2010). These ammonites were found in the strata above 26.2 m in the columnar profile (Fig. 3), corresponding to the Meneghinii Zone, Levesquei
Conclusion
We exhibit the first high-resolution carbonate C-isotope record of the marine T-J transition in eastern Tethys, from the Wenquan section in northern Tibet. This isotope curve records the classic evolutionary model at the boundary with an initial negative CIE in the lower grainstone beds, which is separated from a second main negative CIE in the upper micritic limestone beds by a positive excursion. It corresponds well with the global GSSP profile spanning the T-J boundary interval, strongly
Declaration of competing interest
None.
Acknowledgments
This work was supported by the second Tibetan Plateau Scientific Expedition and Research Program (STEP) (grant number 2019QZKK2704) and the National Natural Science Foundation of China (grant number 91955204).
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