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. Thus the negative 13 analyzed on fused glass pellets using a Phillips PW 1500 X-ray excursion and variable changes of δ Corg in this study (Fig. 1) fluorescence spectrometer. The precision of the major elements rule out the thermal maturation change of organic matter. Mi- 13 data is better than 3%. gration of hydrocarbons or contamination by detrital δ Corg 13 The authigenic fraction of element X was calculated as from rock weathering could also affect the δ Corg values [X]–[Al]×[X/Al]detrital, where the detrital X/Al ratio was based (Meyer et al., 2013). Examination of microfacies by thin sec- on average upper crustal concentrations (McLennan, 2001). tions did not observe the migration of hydrocarbons in the 13 Excess silica (SiO2(xs)) was calculated as SiO2(total)–SiO2(illite)– C-rich carbonate rock both at the base and top of the stink- SiO2(chlorite)=SiO2(total)–(m×K2O×2.36)–((Al2O3–m×K2O)×1.18) stone member, and thus also rule out the migration effect. The (c.f., Shen et al., 2013), on the assumption that most siliciclas- fresh samples and careful treatment in the lab make sure that tic silicon is present as the clay mineral illite and chlorite (e.g., contamination of modern organic carbon was minimized and 13 Hadjira et al., 2011). Where m is the slope of the Al2O3-K2O thus can not account for the large changes of δ Corg in this regression, the coefficients 2.36 and 1.18 represent the weight study. 13 ratios of SiO2/(0.5×Al2O3) in clay minerals of stoichiometric However, the crossplot between TOC vs. δ Corg (Fig. 2) composition having TO and TOT structures (i.e., illite and shows a negative correlation (R2=0.7). Since there is no chlorite), respectively. diagenetic effect, this negative relationship represents an envi- Eleven samples were prepared for analysis of saturated ronmental signal, instead of diagenetic signal. This negative 13 hydrocarbon compounds. Powdered samples (~120 g) were correlation, e.g., the lower the TOC, the heavier the δ Corg, can Soxhlet extracted using chloroform for approximately 72 h. be due to the proportion between marine organic matter and Asphaltenes were removed from the chloroform extracts by terrestrial organic matter during the depositional period precipitation with n-hexane followed by filtration. The (Meyers, 1997; Whiticar, 1996) and/or the oceanic conditions de-asphalted extracts were then separated into saturated, aro- changes such as the upwelling of anoxic alkalinity-charged, matic fractions and non-hydrocarbons by column chromatog- 13C-depleted deepwaters (Werne and Hollander, 2004; Kauf- raphy, using hexane, benzene and methanol, respectively. For man et al., 1997), volcanic CO2 input into the ocean (Korte and gas chromatography-mass spectrometry analyses, the saturated Kozur, 2010; Hansen, 2006; Grard et al., 2005; Berner, 2002) hydrocarbon fractions were performed using an Agilent 5973N and methane release from the seafloor (Korte et al., 2010; mass spectrometer equipped with a HP 6890 gas chromato- Svensen et al., 2009, 2004; Retallack and Jahren, 2008). Large graph at the Research Institute of Petroleum Exploration and greenhouse gases input from the volcanism and methane hy- Development, China National Petroleum Corporation. The drate seem unlikely even though there was a large scale extru- silica capillary column used was 60 m×0.25 mm in size, with sion of basaltic lavas in north-western Europe during the 0.25 μm film in thickness. The sample was injected with an Carboniferous–Permian transition (Heeremans et al., 1996; injection temperature of 300 °C. Helium was used as the carrier Olaussen et al., 1994). The possibility that volcanism and gas at 1 mL/min. The oven temperature was initially pro- methane impact on the 3‰-magnitude negative excursion of 13 grammed at 100 °C for 5 min, then was programmed to in- δ Corg in the Chihsian Formation, could be low. crease from 100 to 220 °C at 4 °C/min. Afterwards, it was pro- Biomarker analyses may be a useful tool for identification grammed to increase from 220 to 320 °C at 2 °C/min and to of organic matter origin in the sediments and enable a better remain at the highest temperature for 20 min. For GC-MS understanding of bulk organic carbon isotope (Fenton et al., analysis, the instrument was operated routinely in multiple ion 2007; Schwab and Spangenberg, 2004; Meyers, 1997). Our detection mode (MID) with a mass scan range of 50–560 m/z. n-alkanes distribution of saturated hydrocarbon fraction (Fig. 3)

The ion source was operated in the electron impact (EI) mode ranges from n-C15 to n-C29 with the peak of n-C17, suggesting at an electron energy of 70 eV and emission current of 200 μA. the main contribution of algae and bacteria (Schwab and

214 Hao Yu and Hengye Wei

Table 1 The major elements and bulk organic carbon isotope data in the lower Chihsia Formation at the Enshi Section, western Hubei Province, South China

13 Depth δ Corg SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI SiO2(auth) Sample (m) (‰) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) ES18- 10.56 -26.22 1.68 0.03 0.66 0.44 0.07 0.62 51.36 0.1 0.08 0.25 41.69 0.00 ES18 11.31 -27.01 2.95 0.03 0.56 0.52 0.02 1.51 51.39 0 0.01 0.05 41.48 2.13 ES19 11.96 -27.70 ES20 12.11 -27.55 29.26 0.14 2.69 1.22 0.02 6.36 31.59 0.06 0.1 0.05 27.92 24.40 ES21 12.84 -27.77 28.28 0.16 3.19 1.34 0.01 8.16 30.27 0.03 0.16 0.04 27.72 21.89 ES22 13.22 -27.71 4.33 0.05 0.89 0.54 0.01 2.35 50.56 0.01 0.02 0.31 40.26 2.95 ES23 13.92 -28.32 40.41 0.12 2.24 0.91 0.01 9.14 23.18 0.05 0.15 0.02 23.17 35.31 ES24 14.32 -28.57 46.71 0.1 1.76 0.98 0.01 18.08 14.03 0.08 0.14 0.01 17.69 42.34 ES25 14.52 -28.69 47.35 0.09 1.61 0.82 0.01 18.13 12.71 0.12 0.13 0.01 16.92 43.32 ES26+ 15.03 -28.57 ES27 15.12 43.37 0.08 1.5 0.6 0.01 12.09 20.85 0.05 0.09 0.02 21.09 40.12 ES28 15.73 -27.31 ES29 16.5 -28.60 41.4 0.07 1.26 0.61 0.01 16.26 19.43 0.1 0.08 0.02 20.40 38.60 ES30 17.55 -28.42 55.54 0.16 3.15 1.19 0.01 20.85 6.81 0.09 0.24 0.02 11.96 47.89 ES30+ 18.2 -28.76 ES31+ 19.12 -28.70 30.51 0.11 2.08 0.79 0.01 10.21 28.78 0.07 0.12 0.02 26.85 26.09 ES31 19.49 -28.38 ES32 20.15 -28.74 47.43 0.05 1.05 0.29 <0.01 20.11 13.53 0.13 0.08 0.01 15.66 44.88 ES33 20.69 -27.78 ES34- 21.19 -29.04 ES34 21.64 -28.94 39.56 0.04 0.56 0.27 <0.01 16.37 21.04 0.16 0.05 0.01 21.41 38.08 ES35 22.37 -27.97 ES36 23.02 -28.47 30.97 0.03 0.54 0.26 <0.01 12.72 28.77 0.05 0.03 0.02 26.12 29.84 ES37 23.77 -28.91 33.27 0.02 0.39 0.14 <0.01 15.6 25.5 0.06 0.03 0.01 24.40 32.32 ES37+ 24.52 -29.03 ES38 25.21 -28.66 ES39 25.83 -28.88 18.92 0.02 0.37 0.15 0.01 7.92 38.74 0.01 0.02 0.01 33.27 18.16 ES40 26.95 -28.83 31.33 0.03 0.47 0.19 <0.01 14.62 27.13 0.05 0.03 0.01 25.57 30.28 ES41 27.15 -27.87 9.351 0.02 0.33 0.17 0.01 2.42 47.76 0.05 0.01 0.01 39.21 8.80 ES42 28.02 -28.01 27.58 0.02 0.26 0.15 <0.01 13.29 30.76 0.02 0.01 0.01 27.46 27.11

LOI. Loss on ignition.

Spangenberg, 2004; Hunt, 1996). However, there is one sample (ES20) which shows double peaks of n-alkanes distribution at 13 n-C17 and n-C25 (Fig. 3). Even this, the δ Corg of this sample (ES20) only show a little difference (<0.22‰) from the over- lying two samples (ES21 and ES22, Figs. 1 and 3) which show

single peak at n-C17. Some samples (ES37, ES28) showing relative abundant from n-C19 to n-C25 but still having a 13 n-C17-peak display no difference of δ Corg compared to their surrounding samples (Figs. 1 and 3). The 13C-depleted samples and 13C-rich samples in the lower part of stinkstones member

(Fig. 1) have very similar pattern of n-alkanes distribution (Fig. 13 Figure 2. The crossplot of TOC vs. δ Corg in the lower 3). This suggests that the organic matter sources only had a Chihsia Formation at the Enshi Section, western Hubei small change and cannot account for the large-scale negative 13 Province, South China. excursion of δ Corg in this study. Enhanced upwelling carrying the reducing water masses

13 A Kungurian Oceanic Upwelling on Yangtze Platform Evidenced by δ Corg and Authigenic Silica 215

Figure 3. The n-alkanes distribution of saturated hydrocarbon fraction (m/z=218) in the lower Chihsia Formation at the Enshi Section, western Hubei Province, South China.

13 13 with C-depleted dissolved inorganic carbon (DIC) can make triggered the large-scale negative excursion of δ Corg in this the surface water DIC depleted in 13C (Werne and Hollander, study. 2004; Kaufman et al., 1997), and thus result in the 13C deple- tion of organic matter via photosynthesis (Walker et al., 2014). 3.2 Upwelling Evidenced from the SiO2(auth) The Kungurian stage experienced a change from an interglacial Low Al2O3 contents (less than 3.2wt.%, average=1.7%, to a glacial interval (Mii et al., 2012; Fielding et al., 2008), Table 1), combined with the high SiO2 contents (Table 1), indi- consistent with the coal bed and the stinkstone member, respec- cate that most of the SiO2 may not be derived from continental tively since the coal bed represents the warm and humid cli- detritus. After eliminating the continental silica, the excess mates (Hasiotis and Honey, 2000; Bohacs and Suter, 1997; silica may be mainly derived from the chert and sepiolite which Cecil, 1990). The transition to glaciation during the stinkstone contains Si and Mg elements (e.g., Yan et al., 2005). Observa- member deposition caused enhanced oceanic upwelling and tion during our fieldwork shows that chert bands were common

216 Hao Yu and Hengye Wei in the stinkstone member of the Chihsia Formation (Fig. 4), Paleotethys Ocean (Kutzbach and Guetter, 1990), which was indicating that the diagenetic chert contribute to the excess usually associated with strong upwelling along the South China silica. Several thin-bed dolomite layers were developed in the Block. This oceanic upwelling reasonably accounts for the stinkstone member (Fig. 5). Therefore, the relatively high Mg rapid increasing of SiO2(auth) in the lower part of stinkstone contents in the stinkstone member (Table 1) might be resulted member in the Chihsia Formation within the Early–Middle from these dolomite minerals. In addition, the sepiolite also Gungurian. contains Mg element (Yan et al., 2005). Therefore, we suggest that this excess silica is mainly derived from diagenetic chert 4 CONCLUSIONS (i.e., authigenic silica), and the sepiolite is a minor contribution The stinkstones in the lower Chihsia Formation at Enshi of excess silica. Section in South China recorded a 3‰-magnitude negative

The authigenic silica SiO2(auth) is additional evidence of excursion of bulk organic carbon isotope and a rapid increasing upwelling (e.g., Beauchamp and Baud, 2002). The SiO2(auth) of authigenic silica content, suggesting a cause of oceanic up- values in this study range from 0.00 wt.% to 44.88 wt.%, with welling in the western margin of PaleoTethys Ocean during the an average of 27.72 wt.% (Table 1 and Fig. 1). It shows a rapid Early-Middle Kungurian stage. This upwelling also accounts increasing to 40 wt.% from 0.00 wt.% in the lower part of for the widespread high organic carbon content sediments of stinkstones member in the Chihsia Formation and gradual de- the lower Chihsia Formation across the South China Block. creasing to 25 wt.% in the upper part of the stinkstone member (Fig. 1). Oceanic upwelling carries the cold dissolved sil- ACKNOWLEDGMENTS ica-charged water, hindering the silica dissolution and thus We thank Jiaxin Yan and Wei Wang for their constructive accounting for the supply of authigenic silica (Beauchamp and comments. Research by H. Y. Wei is supported by the National Baud, 2002). The Enshi Section was located near the paleowest Natural Science Foundation of China (No. 41302021), and by of South China Block and thus affected by the eastern boundary the Science and Technology Research Project of Jiangxi Prov- current that was part of a circulation gyre within the ince Education Department (No. GJJ13452). We also thank Allison Young for improving this paper. Research by Hao Yu is supported by the National Natural Science Foundation of China (No. 41290260) and by the Ministry of Education of China (No. 20120001110052).

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