A Kungurian Oceanic Upwelling on Yangtze Platform: 13 Evidenced by Δ Corg and Authigenic Silica in the Lower Chihsia Formation of Enshi Section in South China
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Journal of Earth Science, Vol. 26, No. 2, p. 211–218, April 2015 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-015-0533-z A Kungurian Oceanic Upwelling on Yangtze Platform: 13 Evidenced by δ Corg and Authigenic Silica in the Lower Chihsia Formation of Enshi Section in South China Hao Yu1, Hengye Wei*2 1. Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University, Beijing 100871, China 2. College of Earth Sciences, East China Institute of Technology, Nanchang 330013, China ABSTRACT: The Late Paleozoic Ice Age across Carboniferous and Permian had a significant impact on the Kungurian (Upper Cisuralian series of Permian) Chihsia Formation in South China. This re- sulted in a unique interval with features such as the lack of reef in Chihsian limestone, widespread stinkstone and nodular/bedded chert. The Chihsia limestone (Kungurian stage) deposited during a time of cooling was resulted from oceanic upwelling. Here we present evidence for this upwelling using sev- eral geochemical analyses: bulk organic carbon isotope, biomarker molecular geochemical data, and authigenic silica of the stinkstone member in the lower Chihsia Formation of the Kuangurian stage from the Enshi Section in western Hubei Province, South China. The lower part of the stinkstone member shows a rapid organic carbon isotope excursion with a -3‰ shift triggered by the upwelling of 13C-depleted bottom water. The concurrent rapid increasing of authigenic silica content resulted from the enhanced supply of dissolved silica in the upwelling water mass. This upwelling at the Enshi Section also led to relative high TOC content, accounting for the widespread stinkstone in the lower Chihsia Formation during the Kungurian stage in Permian. KEY WORDS: Chihsia Formation, Enshi Section, organic carbon isotope, authigenic silica, upwelling. 0 INTRODUCTION the whole Late Paleozoic (Montañez and Poulsen, 2013) along The Late Paleozoic Ice Age (LPIA) lasted from the the eastern Panthalassic Ocean and on the lee side of the South Mid-Carboniferous (ca. 327 Ma) to the early Late Permian (ca. China Block. The sedimentary feature such as the glendonites 260 Ma) (Fielding et al., 2008) and is considered to have an in eastern Australian during Mid–Late Permian (Jones et al., important impact on the Phanerozoic Earth’s climate system 2006) and the trace elemental analysis of brachiopod in the (Frakes et al., 1992). Changes in vegetation during this period tropical region also suggested the Late Paleozoic upwelling led to an icehouse climate state (Gastaldo et al., 1996). Fielding (Powell et al., 2009). The Pangean phosphorites exhibited a et al. (2008) recognized eight discrete glacial intervals, termed record of Permian upwelling (Trappe, 1994). glaciations, during the LPIA. These glaciations are C1 to C4 in The Kungurian upwelling was inferred by the enrichment the Carboniferous and P1 to P4 in Permian, where the P3 glaci- in minerals such as widespread sepiolite (Yan et al., 2005), lack ation was located in the Middle and Late Kungurian stage in of reefs in the lower Chihsian Formation (Shi and Grunt, 2000) Early Permian. Mii et al. (2012) suggested that the paleoclimate and the associated chert nodules (Liu and Yan, 2007; Wang and fluctuated between warm and cool from Late Sakmarian to Jin, 1998; Lu and Qu, 1989). The reducing sediments in the Early Kungurian and that the Early Kungurian and Middle lower Chihsia Formation (Wei et al., 2012; Lu and Qu, 1989) Artinskian were associated with a weakened latitudinal tem- and the biogeographic distribution of brachiopods recorded the perature gradient. Therefore, during Kungurian, the interglacial cool-water upwelling systems in the Kungurian Chihsia Forma- to glacial transition interval should indicate oceanic upwelling tion of South China (Shi and Grunt, 2000; Shi, 1995). However, when the pole-to-equator temperature gradient was enhanced the Kungurian oceanic upwelling research still needs additional (e.g., Beauchamp and Baud, 2002). geochemical evidences. Here, we present bulk organic carbon 13 Upwelling during the LPIA was inferred from the ocean isotope (δ Corg) and authigenic silica (SiO2(auth)) data con- simulation in the Middle Permian (Winguth et al., 2002), Late strained by molecular geochemical data in the limestones of the Permian (Schoepfer et al., 2013; Kiehl and Shields, 2005) and Chihsia Formation at the Enshi Section in South China to show the evidence of this Kungurian upwelling. *Corresponding author: [email protected]; [email protected] © China University of Geosciences and Springer-Verlag Berlin 1 GEOLOGICAL SETTING Heidelberg 2015 The Enshi Section, the focus of this study, is located at the Tanjiaba Village, 5 km south of Enshi City, western Hubei Manuscript received June 18, 2014. Province in South China. The Enshi area became part of the Manuscript accepted January 15, 2015. intrashelf basin during the Middle and Late Permian (Wei and 13 Yu, H., Wei, H., Y., 2015. A Kungurian Oceanic Upwelling on Yangtze Platform Evidenced by δ Corg and Authigenic Silica in the Lower Chihsia Formation of Enshi Section in South China. Journal of Earth Science, 26(2): 211–218. doi: 10.1007/s12583-015-0533-z 212 Hao Yu and Hengye Wei Chen, 2011; Feng et al., 1997), which is equivalent to the 2-m-thick coal bed (Fig. 1) in ascending order. Above these two Xiakou-Lichuan bay of Yin et al. (2014). This intrashelf basin is siliciclastic successions also called Liangshan Formation (e.g., central-north of the South China Block, but was to the paleowest Tong and Shi, 2000), is a >120-m-thick Chihsia Formation of the South China Block during Permian (Algeo et al., 2013), limestone succession. We sampled the lower part of this lime- and thus was probably influenced by an eastern boundary current stone succession, in total ~20 m thick. This sampled interval, that was part of a circulation gyre within the Paleotethys Ocean also called stinkstone (Lu and Qu, 1989), is composed of (Kutzbach and Guetter, 1990). According to the paleomagnetic coarsely laminated marlstones or calcareous shale intercalated study (Ma and Zhang, 1986), the South China Block was located by thin-bed limestone or dolostone, locally bearing the black at 2.4°N during the time that the Chihsia Formation was depos- nodular chert in the marlstones/shales (Fig. 1). Grey ited, suggesting a low-latitude tropical climate. thick-bedded limestones were developed at the base and top of The Chihsia Formation is widely exposed across the South these marlstones/shales interval (Fig. 1). The so-called stink- China Block. The fusulinid and conodont biostratigraphic study stones smell like the bituminous odor, suggesting high organic in the Nanpanjiang Basin (Shen et al., 2007) suggest a latest carbon content. Artinskian through the entire Kungurian stage for the Chihsia The unconformity between Carboniferous karst limestone Formation in South China, indicating Early Permian, i.e., Ci- and lower Chihsian claystones represents the widespread Early suralian Epoch. However, the Chihsia Formation at Enshi Sec- Permian uplift and erosion for most of South China (Tong and tion unconformably overlies on Carboniferous karst limestones Shi, 2000). Therefore, the sampled stinkstone member in the (Fig. 1). The lowermost of Chihsia Formation consists of Chihsia Formation is reasonably Early–Middle Kungurian 5-m-thick ferrallitic claystones resulted from weathering and a stage in age. 13 Figure 1. The lithology and geochemical profiles of bulk organic carbon isotope (δ Corg), authigenic silica (SiO2(auth)) and total organic carbon (TOC) in the lower Chihsia Formation at the Enshi Section, western Hubei Province, South China. Note that the TOC data is from Wei et al. (2012). 13 A Kungurian Oceanic Upwelling on Yangtze Platform Evidenced by δ Corg and Authigenic Silica 213 2 METHODS 3 RESULTS AND DISCUSSION 13 A total of 31 samples were sampled in this 20-m-thick 3.1 Upwelling Evidenced from the δ Corg 13 study interval, with an average sampling-interval of 0.65 m. The bulk δ Corg values range from -29.04‰ to -26.22‰, The Enshi Section was a new road-cut section, and thus the with an average of -28.17‰ (Table 1 and Fig. 1). It shows a samples were very fresh. We clean the samples using distilled gradual negative excursion from ~-26‰ to -28.80‰ in the 13 water, then dried them and powdered to smaller than 200 mesh. lower part of stinkstones member, a persistent low δ Corg of 13 13 Sample splits (0.3 to 5 g) for bulk δ Corg analysis were -29‰ interrupted by several episodes of heavy δ Corg of treated with 6 N HCl for 24 h to remove carbonate. The solu- -27.8‰ in the middle part of stinkstones member, and a rapid tion was then retreated with excess 6 N HCl and allowed to sit positive excursion to -26.7‰ in the upper part of stinkstones 13 for 6 h to ensure there was no remaining carbonate. The decal- member (Fig. 1). This suggests a negative δ Corg excursion cified samples (30–100 mg)+CuO wire (1 g) were added to a event in the stinkstone member. 13 quartz tube and combusted at 500 °C for 1 h and 850 °C for 3 h. Diagenetic processes can affect the δ Corg values. Ther- The carbon isotope ratio of the generated CO2 was measured in a mal maturation of organic matter decreases the total organic Finnigan MAT-252 mass spectrometer. The isotopic ratio is carbon (TOC) composition of rocks and tends to shift residual reported in standard δ notation relative to the Vienna Peedee TOC to more 13C-rich values (Hayes et al., 1999; Popp et al., Belemnite (VPDB) standard. Analytical precision is better than 1997). However, this thermal process would not affect the 13 0.1‰. δ Corg trends (Des Marais et al., 1992) because the study in- Sample splits (0.5 g) for the major elements analyses were terval has similar thermal maturation level.