Microfacies and carbon isotope records of

Palaeogeography, Palaeoclimatology, Palaeoecology 438 (2015) 96–112

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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Microfacies and carbon isotope records of Mississippian carbonates from the isolated Bama Platform of Youjiang Basin, South China: Possible responses to climate-driven upwelling Chao Liu a,b, Emilia Jarochowska b, Yuansheng Du a,⁎, Daniel Vachard c, Axel Munnecke b a b c

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 430074 Wuhan, PR China GeoZentrum Nordbayern, Fachgruppe Paläoumwelt, Universität Erlangen-Nürnberg, Loewenichstrasse 28, 91054 Erlangen, Germany 1 rue des Tilleuls, 59152 Gruson, France

a r t i c l e

i n f o

Article history: Received 9 April 2015 Received in revised form 26 July 2015 Accepted 27 July 2015 Available online 5 August 2015 Keywords: Late Paleozoic Ice Age Visean Euramerica Paleo-Tethys Point-counting Foraminiferal zones

a b s t r a c t Changes in Mississippian global paleogeography derived from the reconfiguration of the continents have been suggested to result in a change in oceanic circulation, carbon cycling, as well as in global cooling. Here, integrated δ13Ccarb chemostratigraphy and foraminiferal biostratigraphy across the Mississippian (late Tournaisian, Visean, early Serpukhovian; MFZ6–MFZ16 foraminiferal biozones) of the Gongchuan section located in the isolated Bama Platform in the Youjiang Basin, South China, are presented. The δ13Ccarb trend shows an abrupt decline during late Visean (Asbian-early Brigantian; MFZ13–14). This decline is also observed in subequatorial western Euramerica, whereas coeval sections in subequatorial eastern Euramerica show consistently elevated δ13C values across the entire Visean. The δ13C decline in western Euramerica and the South China Block (eastern Paleo-Tethys) coincides with a global regression with a suggested glacioeustatic origin, the onset of high-frequency climate and sea-level oscillations, and the closure of the Rheic seaway between Euramerica and Gondwana. We propose a model explaining the divergence of δ13C records resulting from the closure of the Rheic seaway and development of upwelling zones in the western margin of Euramerica and the eastern Paleo-Tethys realm. Quantitative microfacies analysis across the Mississippian succession in the Gongchuan section shows facies-independent disappearance of corals and increased proportion of cortoids and filter-feeding organisms at the onset of the Asbian δ13C decline, which may support an increase in nutrient level that can be expected as a result of upwelling. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Mississippian was one of the most pivotal time intervals in geological history with major changes in the Earth's paleogeography (e.g., Blakey, 2006; Franke, 2006), climate (the onset of the Late Paleozoic Ice Age in late Mississippian, e.g., Isbell et al., 2003; Fielding et al., 2008), carbon cycling (Saltzman et al., 2004), global sea level (a relatively high amplitude and rapid sea-level fall and the initiation of high-frequency cycles in late Visean, e.g., Smith and Read, 2000; Wright and Vanstone, 2001), as well as paleobiogeography (a distinct turnover of marine faunas during late Mississippian, e.g., Shen et al., 2006; Qiao and Shen, 2015). During the Early Carboniferous, Gondwana moved northward and converged with Euramerica to form the supercontinent of Pangea. It resulted in the closure of the Rheic gateway which has been proposed to have caused an abrupt reorganization of the oceanic circulation patterns (e.g., Saltzman, 2003; Reid et al., 2007). Transitions in paleoceanography and carbon cycling reflecting these geological events can be preserved in faunal assemblages and ⁎ Corresponding author. Tel.: +86 189 8612 7299. E-mail address: [email protected] (Y. Du).

http://dx.doi.org/10.1016/j.palaeo.2015.07.048 0031-0182/© 2015 Elsevier B.V. All rights reserved.

carbon isotopic compositions of marine carbonates (e.g., Mii et al., 1999; Reid et al., 2007), including platform-derived carbonates which have been suggested to be linked to the documented secular changes in the pelagic record (e.g., Saltzman, 2002, 2003; Krull et al., 2004). Mii et al. (1999) and Saltzman (2003) reported a significant divergence in carbon isotope trends between western and eastern Euramerica which began in the early Serpukhovian and continued across the Pennsylvanian. They attributed it to the contemporaneous closure of the Rheic gateway. However, the increasing number of studies of Visean carbon isotope stratigraphy (e.g., Buggisch et al., 2008; Qie et al., 2011) supplied new evidence in support of the similar divergence pattern in carbon isotope trends (abrupt decline in subequatorial western Euramerica and eastern Paleo-Tethys, whereas coeval sections in subequatorial eastern Euramerica show consistently elevated δ13C values) emerging during the Visean. That was accompanied by major changes in global sea level and climate (e.g., Wright and Vanstone, 2001). Nevertheless, the mechanisms beyond this divergence within subequatorial regions have not been previously discussed and remain poorly understood. Here, we integrated quantitative microfacies analysis, foraminiferal biostratigraphy, and carbon isotope chemostratigraphy in Mississippian

C. Liu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 438 (2015) 96–112

strata of an isolated carbonate platform of the Youjiang Basin, South China. The aims of the study are (1) to test the hypothesis of a systematic paleoceanographic pattern in the δ13C records across different areas of the Visean paleo-equatorial belt (Fig. 1); and (2) to propose a model of changes in low latitude oceanic circulation to explain this pattern. 2. Geological setting During the Mississippian, South China was located in a subequatorial position, south of the paleo-equator (Fig. 1A). The Youjiang Basin (or Nanpanjiang Basin or Dian–Qian–Gui Basin) was a large basin developed on a passive continental margin during Late Paleozoic to Middle Triassic. It was bordered to the west and north by the Yangtze Block and faced the Paleo-Tethys to the east and south (Fig. 1A, B) (Lehrmann, 1999). There has been a long-term controversy with regard to the nature of Youjiang Basin. Growing evidence – especially from cherts and volcanic rocks – demonstrated that the Youjiang Basin was a rift basin during the Devonian. It subsequently evolved into a passive continental margin basin from the Carboniferous to Early Triassic (Du et al., 2013; Huang et al., 2013), and into a foreland basin during the Middle Triassic, as indicated by detrital provenance analysis (Yang et al., 2012). Due to the eastward expansion of the Paleo-Tethys in the Devonian, two directional groups of faults (NW–SE and NE–SW) developed within the Youjiang Basin (Fig. 1C) as a result of extensional regime and increased subsidence. In parallel, a number of tectonic blocks broke up from the southern margin of the Yangtze Block, giving rise to a submarine landscape composed of multiple isolated carbonate platforms

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surrounded by deep-basin facies during the Carboniferous (Liu et al., 2014). Nearshore mixed siliciclastic–carbonate rocks were restricted to the southern margin of the Upper Yangtze Land (Fig. 1B). Except for the Great Bank of Guizhou and Chongzuo–Pingguo Platform (Fig. 1B; CPP, in southern Youjiang Basin) (Lehrmann et al., 1998, 2007), most of the isolated platforms in the Youjiang Basin were initiated during the Middle Devonian, and were progressively drowned and covered by clastics during the Late Permian to Early Triassic (Fig. 2) (Liu et al., 2014). The evolution of these platforms was primarily controlled by basin-wide tectonic, relative sea level, as well as the sedimentary environment (Liu et al., 2014). Carbonate deposits, which are described herein for the first time, developed in the interior of the Bama Platform, one of the isolated carbonate platforms in the Youjiang Basin (Fig. 1), where an about 500-m-thick succession of upper Tournaisian to lower Serpukhovian rocks is exposed (the biostratigraphy is discussed in Section 4.1). Regionally, the boundaries of the margin and slope of the Bama Platform are associated with synsedimentary faults (Figs. 1, 2). The Gongchuan section (N 23°39′58.69″, E 107°52′01.96″), which is situated in Gongchuan County, Guangxi, provides a continuous exposure along a newly opened mountain road (Fig. 3). From bottom to top, it comprises the upper Yaoyunling Formation and the Du'an Formation. The Yaoyunling Formation is mainly formed by dark thin- to thick-bedded wackestones to grainstones with nodular cherts; the Du'an Formation consists primarily of gray thick-bedded to massive skeletal and peloidal wackestones to grainstones. The uppermost part of this Formation is mostly dolomitized.

Fig. 1. Location map of the Bama isolated carbonate platform and the studied Gongchuan section, Youjiang Basin, South China. A: Global paleogeographic reconstruction for the Visean (modified from Blakey, 2006); yellow square represents the Antler foreland basin, red star the working area. NC — North China, SC — South China, In — Indochina. B: Paleogeographic map of South China and location of the Youjiang Basin and the Bama Platform (after Liu and Xu, 1994; Liu et al., 2014); yellow star represents the Longan section and the red star represents the Visean GSSP Pengchong section. CPP — Chongzuo–Pingguo Platform, JP — Jingxi Platform, GFP — Guangnan–Funing Platform, LP — Leye Platform, HP — Heshan Platform. C: Geological map of the Bama Platform shows location of Mississippian Gongchuan section studied. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Compiled from the Guangxi Bureau of Geology and Resources (1971).

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Fig. 2. Suggested model for evolution of the isolated platforms and the Youjiang Basin. Modified after Liu et al. (2014).

Fig. 3. Stratigraphy of the Mississippian strata in the Gongchuan section compared with global sea-level changes and emergence record from UK. Dashed lines in the column of “Foram Zones” represent uncertain boundaries. Samples in which foraminifers have been identified are marked in red. Global sea-level curve (brown line) is reproduced from Haq and Schutter (2008). The relative sea-level curve is based on microfacies analysis in this study (purple dashed line). Emergence records related to climate change of low-latitude sections in UK are shown after Vanstone (1996) and Wright and Vanstone (2001). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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3. Methodology We surveyed the upper Yaoyunling Formation and the Du'an Formation, and collected 70 unweathered samples for biostratigraphy, microfacies, and stable isotope analyses. Calcite veins and recrystallized areas were carefully avoided. For biostratigraphy and identification of depositional environments, 45 thin sections were produced. From among them, we selected 15 thin sections from different microfacies types to quantify the components by counting between 350 and 400 points per sample according to the method described by Flügel (2010). Sixteen types of components were recognized: corals, foraminifera, brachiopods, mollusks, ostracods, trilobites, bryozoans, crinoids, Tubiphytes, calcispheres, lithoclasts, peloids, aggregate grains, cortoids, matrix, and unidentified microfossils (e.g., algospongia Kamaena and Issinella). Point counting results (excluded the proportions of matrix and unidentified microfossils) were used to obtain sample ordination using detrended correspondence analysis (DCA) implemented in the Past3 software (Hammer et al., 2001). Carbon and oxygen isotope rations were measured from 63 powdered micrite samples using a Finnigan MAT 253 mass spectrometer coupled with a Kiel IV carbonate device at the Nanjing Institute of Geology and Paleontology. Results were calibrated to GBW-04405 laboratory standard (a Chinese National Material Standard from the Chinese National Standards Bureau, which is used as a reference standard and run every nine samples) of Nanjing Institute of Geology and Paleontology with the δ13Ccarb value of 0.57‰ and the δ18Ocarb value of − 8.49‰. Analytical error for δ13Ccarb and δ18Ocarb was better than 0.03‰ and 0.08‰ respectively.

4. Results 4.1. Biostratigraphy Fig. 4. Stratigraphic time table and correlation of foraminiferal zones and conodont zones for the lower to middle Mississippian based on Poty et al. (2006), Buggisch et al. (2008) and Hance et al. (2011).

The foraminiferal zonation of the Gongchuan section proposed in this paper follows the schemes of Poty et al. (2006) and Hance et al. (2011). Stratigraphic time table and correlation of foraminiferal zones and conodont zones for the lower to middle Mississippian are shown in Fig. 4.

Table 1 Foraminiferal zonation of the Mississippian in the Gongchuan section. Samples Foraminifera

Other microfossils

Foraminiferal zone and inferred age

GC72

Eostaffella sp.

Aoujgalia sp.

GC71

Endothyranopsis ex gr. plana, Euxinita? sp., Globoendothyra cf. antoninae, Pseudoendothyra sp., Plectomillerella? sp., Endothyra sp., Mediocris sp., Pseudoendothyra sp. Suleimanovella sp., Earlandia minor, Pseudoendothyra? sp., Eostaffella sp. Pseudoendothyra sp., Pojarkovella? sp. Earlandia vulgaris, Paraarchaediscus sp., Lituotubella sp., Endothyra sp., Mediocris sp., Eostaffella sp., Pojarkovella sp., Vissarionovella sp. Earlandia vulgaris, Lituotubella ex gr. glomospiroides, Pojarkovella sp., Asteroaoujgalia n. sp., Mediocris sp., Tetrataxis sp.,

–

MFZ16, earliest Serpukhovian MFZ16, earliest Serpukhovian MFZ15, V3c MFZ14, V3b gamma MFZ13, V3b beta MFZ12, V3b alpha/beta

GC70 GC61 GC51 GC49 GC48

GC42 GC35 GC24

GC18 GC17 GC13

Suleimanovella sp., Earlandia ex gr. elegans, Earlandia minor, Brunsia sp., Vissarionovella sp., Eoparastaffella sp., Eostaffella sp. primitive Pojarkovella sp., Brunsia sp., Endothyra sp., primitive Eostaffella sp., Mediocris sp., Neoparadainella sp., Suleimanovella sp. Suleimanovella sp., Inflatoendothyra sp., Urbanella cf. pseudoukrainica

Eoforschia sp., Uviella? sp., Praedainaella amenta, Endothyra aff. kosvensis, Suleimanovella suleimanovi, Earlandia elegans, Brunsia ex gr. pulchra, Septaglomospiranella sp., Septatournayella sp., Eblanaia? sp., Dainella sp. Spinoendothyra aff. analoga, Brunsia ex gr. pulchra, Urbanella sp., Endothyra sp., Praedainella sp.

– – – – Stacheoides sp., Calcisphaera laevis, Issinella sp., Luteotubulus licis, Kamaena sp., Asteroaoujgalia n. sp., Fasciella sp. – Calcisphaera sp. Kamaena sp.

– Kamaena sp. –

MFZ12, V2b

MFZ11B or MFZ12, V2a or V2b MFZ11B, Upper V2a/ late Arundian MFZ8 or MFZ9, Tournaisian–Visean boundary interval MFZ8, latest Tournaisian

MFZ6 or MFZ7, latest Tournaisian

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Fig. 5. Tournaisian–Visean foraminifers (scale bars = 0.500 mm). 1. Praedainella amenta (Ganelina, 1966) (bottom, left) and Endothyra aff. kosvensis Lipina, 1955 (right). Sample GC17. Latest Tournaisian (MFZ8). 2. Brunsiina n. sp. Axial section with the typical juvenarium. Sample GC17. Latest Tournaisian (MFZ8). 3. Eblanaia? sp. Sample GC17. Latest Tournaisian (MFZ8 zone). 4. Septaglomospiranella sp. Sample GC17. Latest Tournaisian (MFZ8). 5. Septatournayella sp. Subtransverse section (top, left) and axial section. Sample GC17. Latest Tournaisian (MFZ8). 6. Kamaena sp. (left), Septaglomospiranella sp. (center) and Eblanaia? sp. (right). Sample GC17. Latest Tournaisian (MFZ8). 7. Urbanella cf. pseudoukrainica Vdovenko, 1972. Axial section. Sample GC-24. Early Visean (MFZ9 zone). 8. Urbanella ex gr. pseudoukrainica Vdovenko, 1972 (axial section; left) and Suleimovella sp. (right). Sample GC24. Early Visean (MFZ9). 9. Endochernella? sp. Oblique section. Sample GC17. Latest Tournaisian (MFZ8). 10. Neoparadainella n. sp. 1. Transverse section showing the proloculus and the brunsiinoid juvenarium. Sample GC27. Early Visean (MFZ10/11). 11. Primitive Eostaffella sp. (left) and Neoparadainella n. sp. 2 (right). Sample GC34. Early Visean (MFZ11B). 12. Neoparadainella n. sp. 3. Sample GC38. Middle Visean (MFZ12). 13. Eostaffella sp. and Earlandia minor (Rauzer-Chernousova, 1948). Sample GC42. Middle Visean (MFZ12). 14. Vissarionovella sp. Subtransverse section. Sample GC42. Middle Visean (MFZ12). 15. Pseudolituotuba sp. Sample GC43. Middle Visean (MFZ12). 16. Laxoseptabrusiina n. sp. Transverse section. Sample GC47. Middle Visean (MFZ12). 17. Laxoseptabrusiina n. sp. Axial section. Sample GC47. Middle Visean (MFZ12).


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Foraminiferal biozones ranging from MFZ6 (upper Tournaisian) to MFZ16 (lowermost Serpukhovian) were identified (Table 1; Fig. 3). Most of the Tournaisian strata are absent or not exposed. The presence of the genera Brunsia and Endothyra in sample GC13 (Table 1) indicates a biozone no older than MFZ6 (Poty et al., 2006; Hance et al., 2011). As it lacks taxa characteristic for MFZ8, we attributed this level to MFZ6 or MFZ7. Eoparastaffella rotunda, an index taxon for MFZ8, was absent in our samples; instead, other significant taxa (e.g., Eoforschia sp., Uviella? sp., Eblanaia? sp.) were very abundant in GC17 and GC18 (Table 1; 1–6, 9 in Fig. 5), indicating that the level corresponds to MFZ8. Eoparastaffella simplex, an index taxon for MFZ9, was not identified, but the presence of evolved Inflatoendothyra spp. supports the position of sample GC24 within the Tournaisian–Visean boundary interval, as well as the presence of Urbanella pseudoukrainica (Table 1; 7–8 in Fig. 5). Additionally, due to the co-existence of Spinoendothyra ex gr. costifera and Endolaxina ex gr. laxa in sample GC20, the Tournaisian–Visean boundary is tentatively positioned at the sharp change from packstone–grainstone to mudstone between samples GC19 and GC20. The position of the boundary at this level is also supported by the carbon isotope trend (Fig. 3, see discussion in Section 4.3) with the Pengchong section (Fig. 1B, red star) (Gradstein et al., 2012). Due to the decrease in the abundance of foraminifera, MFZ9 and MFZ10 could not be well identified (Table 1; 10 in Fig. 5). The appearance of primitive Pojarkovella sp. (Table 1) in sample GC35 indicates an upper V2a age (MFZ11B). It is admitted in South China that only the upper part of MFZ11 is documented (Hance et al., 2011). MFZ12 in the Gongchuan section was undisputedly marked by the presence of Lituotubella glomospiroides and Pojarkovella sp. (6 in Fig. 6; 12–13 in Fig. 7). In the middle to upper part of the Gongchuan section, only a few foraminifers are present. No index species were found in samples from GC51 to GC70. In spite of this, the occurrence of Pseudoendothyra sp. (16 in Fig. 6; 14 in Fig. 7; 3–4, 6, 9–10 in Fig. 8) allows to infer that these samples fall within the interval of middle–late Asbian to Brigantian (upper MFZ13/ MFZ14/MFZ15) (Poty et al., 2006). Taking both lithofacies and foraminiferal assemblages into consideration, the boundary between MFZ12 and MFZ13 (also the boundary between Holkerian and Asbian) was determined between the samples GC49 and GC51 (Table 1; Fig. 3). Endothyranopsis ex gr. plana and Euxinita? sp. appear in sample GC71 in the absence of other important taxa (11–12 in Fig. 8). With regard to these two peculiar species, many authors regard them (especially the former species) as marker of the Serpukhovian (or Namurian) age (e.g., Cózar, 2004; Cózar et al., 2008). Based on the carbon isotope data in this study (see subsequent discussion), we prefer to put the boundary between the Visean and Serpukhovian at the level of sample GC71 (Fig. 3).

4.2. Microfacies Seven microfacies types were recognized in the studied interval of the Gongchuan section (Figs. 7–9). Among them, six (except for mf6, see Table 2) corresponded to SMF23, SMF25, SMF18, SMF10, SMF9, and SMF11 of the standard microfacies types (SMF) (Flügel, 2010).

Records of texture, structure, dominant components, interpretations as well as stratigraphic distribution are shown in Table 2 and Fig. 3. 4.2.1. mf1: homogeneous mudstone This facies consists of fine-grained mudstone with irregularly distributed voids or rosettes interpreted to result from the replacement of evaporite minerals by calcite (Fig. 9A). In addition, an accumulation of burrows appears within the only same bed (Fig. 9B; Fig. 10A). Interpretation: restricted upper intertidal to supratidal environment with low water energy. 4.2.2. mf2: laminated mudstone mf2 differs from mf1 in the presence of faint lamination (Fig. 9C). The lamination is mostly planar and partially disrupted by bioturbation. Microfossils, similarly as in mf1, are rare. Interpretation: restricted upper intertidal to supratidal environment. 4.2.3. mf3: bioclastic packstones with abundant foraminifera This microfacies is distinguished by the high abundance of foraminifera scattered in a micritic matrix, associated with sparse peloids, crinoids, and calcispheres (Fig. 9D). Foraminifera contribute 31–45% of the bioclasts. Common genera include Earlandia, Brunsia, Endothyra, and Eostaffella. Interpretation: restricted platform lagoonal environment. 4.2.4. mf4: bioclastic wacke–packstones with various abraded skeletal grains This facies is the most widely distributed in the Gongchuan section, and is distinguished by a higher diversity of skeletal grains than mf3. Common to abundant peloids (Fig. 9E), abraded crinoids, rugose corals, undetermined microproblematica, foraminifers, mollusks, calcispheres as well as ostracods are present within fine-grained matrix. Rugose corals are abundant in the lower part of the section but absent in the upper (Fig. 10B). Interpretation: open lagoonal environment. 4.2.5. mf5: bioclastic wacke–packstones with bioturbation The components of this facies are very similar to mf4. However, the sediment is commonly bioturbated, resulting in that some bioclasts are reworked (Fig. 9F). Interpretation: open lagoonal environment at or just below the wave base. 4.2.6. mf6: mudstone with calcispheres The only components of this facies are calcispheres and rare peloids. Calcispheres consist of two types: radiosphaerid calcispheres with more or less prominent radially arranged spines, and non-radiosphaerid calcispheres with a relatively smooth external surface (Fig. 9G). Most of them show micritization (Fig. 9G), and this has been proposed by Vachard (1994) and Berkyova and Munnecke (2010) to result from microboring, possibly linked to high nutrient levels. Interpretation: deep subtidal environment below the wave base. 4.2.7. mf7: bioclastic grainstones The facies consists of commonly abraded and micritized large bioclasts surrounded by sparry calcite cement (Fig. 9H). Brachiopods,

Fig. 6. Visean–early Serpukhovian foraminifers and incertae sedis algae (scale bars = 0.500 mm). 1. Asteroaoujgalia n. sp. Sample GC48. Middle Visean (MFZ12). 2. Fasciella kizilia Ivanova, 1973 and Mediocris sp. (top, right). Sample GC48. Middle Visean (MFZ12). 3. Tetrataxis sp. Subaxial section. Sample GC48. Middle Visean (MFZ12). 4. Luteotubulus licis (Malakhova, 1975). Longitudinal section with a bifurcation (left). Sample GC48. Middle Visean (MFZ12). 5. Luteotubulus licis (Malakhova, 1975). Typical oblique section. Sample GC48. Middle Visean (MFZ12). 6. Pojarkovella sp. Transverse section. Sample GC49. Middle Visean (MFZ12). 7. Mediocris sp. Axial section. Sample GC49. Middle Visean (MFZ12). 8. Vissarionovella sp. (left), Earlandia ex gr. minor (Rauzer-Chernousova, 1948) (center), and Paraarchaediscus sp. (right). Sample GC49. Middle Visean (MFZ12). 9. Eostaffella sp. Subaxial section. Sample GC49. Middle Visean (MFZ12). 10. Eostaffella cf. lilia Li, 1977. Oblique section. Sample GC55. Late Visean (MFZ13). 11. Eostaffella sp. 3. Oblique section. Sample GC61. Late Visean (MFZ14). 12. Eostaffella sp. 4. Subtransverse section. Sample GC-61. Late Visean (MFZ14). 13. Mediocris mediocris (Vissarionova, 1948). Axial section. Sample GC70. Latest Visean (MFZ15). 14. Eostaffella sp. 5. Subtransverse section. Sample GC-61. Late Visean (MFZ14). 15. Plectomillerella? sp. Subaxial section. Sample GC70. Latest Visean (MFZ15). 16. Pseudoendothyra sp. Axial section. Sample GC70. Latest Visean (MFZ15). 17. Endothyranopsis ex gr. plana Brazhnikova and Rostovceva, 1967. Oblique section. Sample GC71. Earliest Serpukhovian (MFZ16). 18. Globoendothyra cf. antoninae (Grozdilova and Lebedeva, 1954). Subtransverse section. Sample GC71. Earliest Serpukhovian (MFZ16). 19. Pseudoendothyra sp. Subtransverse section. Sample GC71. Earliest Serpukhovian (MFZ16). 20. Euxinita? sp. Oblique section. Sample GC71. Earliest Serpukhovian (MFZ16).


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Fig. 7. Tournaisian–Visean microfacies with foraminifers (Scale bars = 0.500 mm). 1. Bioclastic and peloidal wackestone with crinoid (top, right) and Septaglomospiranella (Neoseptaglomospiranella) sp. (center). Sample GC11. Late Tournaisian (mf5, SMF9; MFZ7). 2. Bioclastic and peloidal wackestone with Spinoendothyra aff. analoga (Malakhova, 1956) (center), Suleimanovella sp., Endothyra sp., and Praedainella sp. Sample GC13. Late Tournaisian (mf4, SMF10; MFZ7). 3. Bioclastic and peloidal wackestone with Brunsia sp. (center). Sample GC13. Late Tournaisian (mf4, SMF10; MFZ7). 4. Bioclastic wackestone with Eoforschia sp. and Endothyra spp. Sample GC18. Late Tournaisian (mf4, SMF10; MFZ8). 5. Bioclastic and peloidal wackestone with Spinoendothyra ex gr. costifera (Lipina, 1955) (bottom) and Endolaxina ex gr. laxa Conil & Lys, 1964 (top). Sample GC20. Early Visean (mf7, SMF11; MFZ9). 6. Bioclastic wackestone with Urbanella cf. pseudoukrainica Vdovenko, 1972. See detail 7 in Fig. 5. Sample GC24. Early Visean (mf4, SMF10; MFZ9). 7. Bioclastic wackestone with Florenella? sp. (left). Sample GC29. Early Visean (mf4, SMF10; MFZ10/11). 8. Bioclastic wackestone with Eostaffella sp. (right) and Neoparadainella n. sp. 2 (left). Sample GC34. Early Visean (mf4, SMF10; MFZ11B zone). 9. Bioclastic wackestone with Pojarkovella? sp. (top and center) and Brunsia sp. (bottom). Sample GC35. Early Visean (mf4, SMF10; MFZ11B). 10. Bioclastic wackestone with pojarkovellid gen. indet (white arrow). See detail 11 in Fig. 5. Sample GC35. Early Visean (mf4, SMF10; MFZ11B). 11. Bioclastic wackestone with Neoparadainella sp. 3 (see detail 12 in Fig. 5) and Suleimanovella sp. Sample GC35. Early Visean (mf4, SMF10; MFZ11B). 12. Bioclastic wackestone with Lituotubella ex gr. glomospiroides Rauzer-Chernousova, 1948, Calcisphaera pachysphaerica Pronina 1963, and crinoids. Sample GC48. Middle Visean (mf4, SMF10; MFZ12). 13. Bioclastic wackestone with Pojarkovella sp. (left) and Mediocris sp. (right). See detail 7-8 in Fig. 6. Sample GC49. Middle Visean (mf7, SMF11; MFZ12). 14. Bioclastic wackestone with Pseudoendothyra? sp. Sample GC51. Late Visean (mf5, SMF9; MFZ13).

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crinoids, bryozoans, gastropods, foraminifers, as well as rare peloids are present. In the upper part of the Gongchuan section, this facies contains brachiopod shell beds (Fig. 10C). The shells show preferred orientation parallel to bedding. Interpretation: very shallow sand shoal environment. 4.3. Carbon isotope Carbon and oxygen isotope data are reported in Table 3 and Fig. 3 and 11, and show no covariation (R2 b 0.002). The δ13Ccarb values all fall within published values for the Mississippian brachiopod shell calcite, whereas the oxygen isotope values are either within the range or lighter (Fig. 11). Generally, carbon isotope values in the Gongchuan section range from about 0 to +5‰ (Fig. 3; Fig. 11). The first short-lived positive excursion occurs within MFZ6 to MFZ7 with a peak of about +5‰ at the boundary between MFZ7 and MFZ8. A roughly steady trend (between + 3‰ and +4‰) appears through most of MFZ8 to MFZ12. A negative excursion is recorded from +4‰ to 0‰ within the upper part of the MFZ13– 14 interval, and above this level the δ13Ccarb values progressively rise up to about +3‰ until the lowermost Serpukhovian (MFZ16). 4.4. Detrended correspondence analysis (DCA) Sample ordination obtained from the point-counting results using DCA is shown in Fig. 12. The first DCA axis explains 55% of the variance and shows highly positive loadings of bryozoans, brachiopods, cortoids, and lithoclasts along with positive loadings of mollusks and crinoids. Undetermined microproblematica, aggregate grains, and corals have negative loadings on the first axis. The second DCA axis explains nearly 13% of the variance and is characterized by highly positive loadings of trilobites and corals, and negative loadings of aggregate grains and calcispheres. 5. Interpretation and discussion 5.1. Facies interpretation Microfacies types in the Mississippian strata of the Gongchuan section described above (Section 4.2) indicate depositional environments ranging from high-energy shallow sand shoals through back-bank low energy lagoons and tidal channels. They show how carbonate deposits in the Gongchuan section developed in response to fluctuations in the relative sea level (Fig. 3). The first DCA axis corresponds to the most influential factor to affect the distribution of carbonate components (Fig. 12). Low scores of peloids and calcispheres, which mainly occur in low energy environments, i.e., lagoonal and deep subtidal, as well as high scores of lithoclasts, brachiopods and crinoids, which predominantly appear in sand shoal facies, allow explaining the greatest variability of bioclasts by a water energy gradient. Nevertheless, corals, brachiopods, crinoids and cortoids, which show conspicuous distribution patterns across the studied interval, suggest that another environmental change took place in the recorded interval, independently of the sea-level oscillations.

105

Rugose and tabulate corals are very common in the upper Yaoyunling Formation and lower Du'an Formation, but absent in the upper part of the Gongchuan section starting from MFZ13 (Table 4 and field observations). In contrast, cortoids, crinoids, and brachiopods show an inverse trend (Fig. 10B–C, Table 4): their frequency is higher in the upper Du'an Formation (Table 4) in virtually all microfacies types. Although corals can strive in some extreme conditions (e.g., Atkinson et al., 1995), they are sensitive to excess nutrients (Hallock and Schlager, 1986; Hallock, 2001). Crinoids and brachiopods are filter- and suspension-feeders, whose distribution is largely controlled by food supply and nutrient availability (Fürsich and Hurst, 1974). Likewise, cortoids, which are formed by destructive micritization related to microboring (Flügel, 2010), are also associated with nutrient-rich conditions (Peterhänsel and Pratt, 2001; Woods, 2013). In addition, according to the observations on the relationship between nutrients and bioerosion reported in Peterhänsel and Pratt (2001), the gradual upward increase in the degree of micritic particles (primarily cortoids) originally consisting of bioclasts (e.g., crinoids, brachiopods) in the Gongchuan section suggests that microendoliths may have increased their abundance along with mesotrophic conditions expanding in late Visean. 5.2. Relative sea-level changes The interpretation of relative sea level fluctuations through the Mississippian in the Bama Platform was based on the succession of microfacies types (Fig. 3). To some extent, it shows synchronous changes with the global eustacy (Haq and Schutter, 2008), especially during the MFZ11–14 interval (Fig. 3). However, due to pervasive recrystallization of samples across the upper MFZ14 to MFZ15(?) interval, an interpretation of relative sea-level changes in this interval was not possible. A prominent relative sea-level drop in the late Visean (Asbian–early Brigantian; MFZ13–14?) was recorded in the Bama Platform (Fig. 3) and the entire Youjiang Basin (Shi et al., 2006). Actually, this relatively high amplitude and rapid drop in the sea level (10–50 m, Wright and Vanstone, 2001) was well documented worldwide at the same time level (late Visean) (e.g., Smith and Read, 2000; Wright and Vanstone, 2001; Rygel et al., 2008). It has been considered by some authors as the signal of the onset of Gondwana glaciation (e.g., Smith and Read, 2000; Wright and Vanstone, 2001). In parallel, it was also accompanied with the beginning of the high-frequency eustatic sea-level cycles (Haq and Schutter, 2008), which has been proposed as the result of waxing and waning of the continental ice sheets in Gondwana (e.g., Wright and Vanstone, 2001; Montañez and Poulsen, 2013). 5.3. Carbon isotope stratigraphy Our study has produced a carbon isotope curve of the Mississippian of the Bama Platform, South China (Fig. 3). The brief δ13C peak of +5‰ in the latest Tournaisian (MFZ8 and MFZ7 boundary interval corresponding to the Scaliognathus anchoralis conodont zone) by now, cannot be reliably correlated with other sections (Fig. 13), suggesting that it may represent a regional rather than global signal. Regionally, its occurrence is in correspondence with mixed deposits of carbonaceous shale and coal-bearing deposits of the Xiangbai Formation in South China. These deposits are restricted to attached platforms along the southern margin of the Upper

Fig. 8. Visean–early Serpukhovian microfacies with foraminifers (scale bars = 0.500 mm). 1. Bioclastic wackestone with Pojarkovella? sp. Sample GC51. Late Visean (mf5, SMF9; MFZ13). 2. Bioclastic wackestone with Pojarkovella? sp. Sample GC51. Late Visean (mf5, SMF9; MFZ13). 3. Bioclastic wackestone with Pseudoendothyra sp. Sample GC51. Late Visean (mf5, SMF9; MFZ13). 4. Bioclastic wackestone with numerous Pseudoendothyra? sp. Sample GC55. Late Visean (mf5, SMF18-For; MFZ13). 5. Bioclastic wackestone with crinoid (left), Mediocris sp., Brunsia sp., and Earlandia sp. Sample GC55. Late Visean (mf5, SMF18-For; MFZ13). 6. Bioclastic wackestone with Pseudoendothyra sp. Sample GC51. Late Visean (mf5, SMF9; MFZ13). 7. Bioclastic wackestone with Eostaffella sp. Sample GC55. Late Visean (mf5, SMF18-For; MFZ13). 8. Bioclastic wackestone with Earlandia minor (Rauzer-Chernousova, 1948), Brunsia irregularis (Möller, 1879), Eostaffella aff. lilia Li, 1977. Sample GC51. Late Visean (mf5, SMF9; MFZ13). 9. Bioclastic wackestone with Earlandia sp. and Pseudoendothyra sp. Sample GC61. Late Visean (mf4, SMF10; MFZ14). 10. Bioclastic wackestone with Pseudoendothyra sp. Sample GC70. Latest Visean (mf7, SMF11; MFZ15). 11. Bioclastic wackestone with Endothyranopsis ex gr. plana Brazhnikova in Brazhnikova et al., 1967. Sample GC71. Earliest Serpukhovian (mf4, SMF10; MFZ16). 12. Bioclastic wackestone with Euxinita? sp. Sample GC71. Earliest Serpukhovian (mf4, SMF10; MFZ16). 13. Bioclastic wackestone with Pseudoendothyra sp. Sample GC71. Earliest Serpukhovian (mf4, SMF10; MFZ16). 14. Bioclastic wackestone with Eostaffella sp. Sample GC72. Earliest Serpukhovian (mf7, SMF11; MFZ16). 15. Bioclastic wackestone with Eostaffella sp. Sample GC-72. Earliest Serpukhovian (mf7, SMF11; MFZ16). 16. Bioclastic wackestone with brachiopods. Sample GC-72. Earliest Serpukhovian (mf7, SMF11; MFZ16).

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Fig. 9. Typical microfacies with different components, textures, and structures. A — fine grained mudstone, with stellate pseudomorphs after evaporite minerals (in green circles), sample GC57. B — burrowed (green arrow) fine grained mudstone, sample GC58. C — faintly laminated fine-grained mudstone with mostly planar laminations (green arrow) and minor interrupted laminations (yellow arrow), sample GC82. D — wackestone with abundant foraminifers (F), sample GC55. E — bioclastic packstone with abundant peloids (P) and common foraminifers (F), upper sample GC13. F — bioclastic wackestone with bioturbation indicated by circular swirls (green arrow) of skeletal debris, sample GC51. G — mudstone with common calcispheres (C), sample GC31. H — grainstone with large bioclasts, sample GC72. Abbreviations: P — peloid, F — foraminifer, C — calcisphere, B — brachiopod, Bry — bryozoan, Cri — crinoid, Cd — cortoids. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Table 2 Texture, structure, dominant components, interpretations and distribution in samples of seven recognized microfacies types. Microfacies

Texture

Structure

Dominant components

Interpretation

Distribution in samples

mf1, SMF23

Mudstone

Bioturbation

FZ9A, supratidal

GC57, 58

mf2, SMF25

Mudstone

Laminated

No fossils, but with stellate evaporite minerals interpreted as evaporite pseudomorphs No fossils

GC78, 82

mf3, SMF18-For

Packstone

–

FZ9A, upper intertidal to supratidal FZ8, restrict platform lagoon

mf4, SMF10

Wackestone to packstone

–

mf5, SMF9

Wackestone to packstone Mudstone

Bioturbation –

Grainstone

–

mf6, calcisphere mudstone mf7, SMF11

Abundant forams; common peloids, crinoids, calcispheres Abundant to common peloids, crinoids; rugose and tabulate corals, cortoids (micritized i.e., caicispheres, crinoids), forams, undeterminate microproblematica and calcispheres are present Abundant to common peloids; rugose corals, forams, crinoids and calcispheres are present Only with abundant to common calcispheres Abundant to common cortoids (micritized i.e., calcispheres, brachiopods, crinoids, bryzoans), peloids, crinoids, brachiopods; rugose corals, forams, calcispheres and bryozoans are present

Yangtze Land (Shi et al., 2006; Wang et al., 2013). Therefore, we argue that a relative increase in local organic carbon burial led to this positive excursion in South China. With regard to the carbon isotopic characteristics during MFZ8 to MFZ11 (latest Ivorian to Moliniacian, or middle–late Osagean time), they correlate very well with the GSSP section for the base of the Visean Stage in the Pengchong section (Fig. 1B, red star) (Gradstein et al., 2012), although the foraminiferal assemblages differ, especially due to the absence or rarity of Eoparastaffella in our study area. Yet, the sharp negative excursion (− 1‰) occurring in the Longan platform (Fig. 1B,

GC17, 43, 55

FZ7, open platform lagoon

GC01, 03, 04, 06, 09, 10, 13, 15, 18, 19, 22, 27, 29, 33, 34, 35, 42, 44, 47, 48, 61, 62, 65, 66, 71

FZ7, open platform lagoon, just below the wave base FZ7, open platform lagoon, deep subtidal FZ6, sand shoal, very shallow water

GC07, 11, 16, 51 GC31, 38, 40 GC20, 24, 49, 69, 70, 72

yellow star) at the Tournaisian and Visean boundary is not recorded in the Bama platform (Qie et al., 2011). Within MFZ13 to MFZ14 (i.e., Asbian–early Brigantian, or early– middle Warnantian, or late Meramecian to early Chesterian), a decreasing trend (from +4‰ to 0‰) in carbon isotope appears (Fig. 3; Fig. 13). It cannot be explained by terrestrial influx to the Youjiang Basin because the Bama Platform was far from the Yangtze old land during that time (Fig. 1). It is noteworthy that similar negative excursions also exist in western Euramerica at Arrow Canyon, southern Nevada (Fig. 13; Saltzman, 2003), and on the North American Midcontinent (Fig. 13; Mii

Fig. 10. Outcrop photographs of the Gongchuan section. A, C–D, Du'an Formation. B, upper Yaoyunling Formation. A — numerous burrows within mudstone beds, generally filled with peloids (Fig. 5B), height = 276 m. B — dark wackestone with abundant small rugose corals, and common brachiopods, height = 56 m. C — brachiopod shell bed, note these shells show preferred orientation, height = 400 m. D — the uppermost strata of our Gongchuan section are dolomites without preserved biota, height = 430 m.

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Table 3 Carbon and oxygen isotope data of the Gongchuan section. Samples

Position in section (m)

δ13C (‰, VPDB)

δ18O (‰, VPDB)

GC-02 GC-03 GC-04 GC-05 GC-06 GC-07 GC-08 GC-09 GC-10 GC-11 GC-12 GC-13 GC-15 GC-16 GC-18 GC-19 GC-20 GC-21 GC-22 GC-23 GC-24 GC-25 GC-26 GC-27 GC-28 GC-29 GC-30 GC-31 GC-33 GC-34 GC-35 GC-36 GC-37 GC-38 GC-39 GC-40 GC-41 GC-43 GC-44 GC-47 GC-49 GC-50 GC-51 GC-52 GC-53 GC-54 GC-55 GC-56 GC-57 GC-61 GC-62 GC-63 GC-65 GC-66 GC-67 GC-69 GC-70 GC-71 GC-72 GC-76 GC-77 GC-81

0.00 2.44 7.37 9.22 12.93 14.58 15.16 19.46 19.97 22.91 25.50 28.67 36.51 40.47 45.58 54.71 56.99 61.59 67.44 79.30 87.89 103.20 105.97 114.62 119.01 124.93 128.88 131.04 140.55 143.18 145.05 147.58 151.00 157.63 162.57 176.21 179.28 181.66 182.81 196.21 210.92 226.04 227.20 235.69 243.78 249.96 253.07 264.31 275.75 299.97 310.33 325.22 380.93 383.93 392.02 405.69 407.91 415.28 422.73 442.56 487.96 496.24

1.2 1.0 3.6 3.5 1.5 3.1 3.5 3.7 3.9 4.4 4.2 4.3 4.9 3.9 3.5 2.6 3.0 2.9 3.4 3.7 3.6 3.6 3.7 3.5 3.3 3.1 3.7 2.7 3.0 3.3 3.3 3.0 3.4 3.7 3.8 3.5 2.9 3.2 3.0 2.0 3.7 3.9 3.6 3.9 3.4 2.7 1.8 0.2 −0.1 0.8 −0.1 1.6 2.1 2.5 1.9 1.9 1.8 2.2 2.7 1.2 0.9 2.6

−3.3 −3.4 −3.2 −3.1 −6.3 −5.0 −7.0 −7.0 −5.9 −4.9 −4.1 −5.1 −3.8 −4.3 −4.1 −8.1 −5.7 −7.0 −7.6 −4.4 −4.4 −4.6 −4.5 −4.1 −6.1 −6.4 −3.6 −7.9 −5.8 −5.5 −4.4 −5.9 −5.8 −4.8 −4.3 −5.2 −6.3 −4.2 −4.4 −5.1 −4.0 −3.7 −5.2 −4.8 −3.6 −4.1 −5.9 −4.0 −3.6 −4.8 −5.0 −4.6 −4.7 −4.1 −5.1 −5.3 −5.1 −4.3 −4.8 −5.3 −4.9 −3.7

et al., 1999). In contrast, slightly increasing and almost constant values (+3‰) sustain in eastern Euramerica represented by sections in the Russian Platform (Fig. 13; Bruckschen et al., 1999; Mii et al., 2001) and the Cantabrian Mountains, Spain (Fig. 13; Buggisch et al., 2008), respectively. There is a systematic divergence between eastward-facing continental margin of Euramerica and westward-facing margins of Euramerica, as well as eastern Paleo-Tethys (Fig. 13). In fact, similar divergences in carbon isotope records between eastern and western Euramerica occurred also during the late Kinderhookian (upper

Fig. 11. Carbon and oxygen isotope ratios from bulk-rock samples across the Mississippian strata in the Gongchuan section, compared with published value ranges for meteoric calcite cement (dashed), updip burial calcite cement (red line) and downdip burial calcite cement (green line) after Wynn and Read (2007). Blue line shows the range of Mississippian brachiopod shell calcite of North America (Popp, 1986; Mii et al., 1999). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

S. isosticha conodont Zone; see Fig. 3) (Saltzman et al., 2004) and Pennsylvanian times (Mii et al., 1999, 2001; Saltzman, 2003). The Gongchuan section examined here and the Arrow Canyon Range succession in Nevada lack evidence of exposure surfaces (Saltzman, 2003). Furthermore, the decreasing trend is also recorded in wellpreserved brachiopod shell calcite (Mii et al., 1999; Fig. 13). Thus, a meteoric diagenesis origin is less likely. Instead, the divergence in carbon isotope trends between westward- and eastward-facing paleocontinents during MFZ13 to MFZ14 may reflect a shift in the global carbon cycle associated with paleogeographic configuration and oceanic circulation patterns (see further discussion). Carbon isotope data across the Visean and Serpukhovian in the Gongchuan section is in accordance with data from South China (Longan section, Qie et al., 2011) and the Antler foreland basin (Batt et al., 2007, yellow square in Fig. 1A) . 5.4. Implications for global paleoceanography The divergence in carbon isotope values between western and eastern Euramerica during the Pennsylvanian has been interpreted by Saltzman (2003), Mii et al. (1999) and Mii et al. (2001) as resulting from the closure of the seaway between Euramerica and Gondwana. These authors proposed that the closure led to a reorganization of oceanic circulation and development of upwelling zones in the western coast of Pangea. There is no consensus on the precise timing of the initial closure of the Rheic seaway (e.g., Secor et al., 1986; Bozkurt et al., 2008; Nance et al., 2010). A middle to late Visean time has been proposed recently based on integrated geochronological studies (Jastrzębski et al., 2013) and paleobiogeographic analyses (brachiopods and ammonoids) (e.g., Korn et al., 2012; Qiao and Shen, 2014, 2015). A prominent global sea-level drop proposed to result from the onset of a glaciation (e.g., Wright and Vanstone, 2001), initiation of highfrequency cycles both in eustacy and climate (Fig. 3), as well as the beginning of a significant divergence in carbon isotope trends between subequatorial eastern and western Euramerica and eastern Paleo-Tethys


109

Fig. 12. Detrended correspondence analysis (DCA) results of point-counting in 15 thin sections across the Mississippian strata in the Gongchuan section. Red symbols represent components and blue — samples (Fig. 3 and Table 2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(South China), all took place within a short time accompanied with the closure of the seaway between Euramerica and Gondwana. Based on analogies with the well-documented closure of the Isthmus of Panama and restriction of the Indonesian Seaway (e.g., Cane and Molnar, 2001; Ravelo and Wara, 2004) and the influence of these events on meridional heat and vapor exchange with the closure of the Rheic seaway and associated changes with global oceanic circulation (e.g., Smith and Read, 2000; Saltzman, 2003), we propose how the closure of the Rheic seaway could have created zones of upwelling characterized by disparate carbon isotope signals. The closure of the Isthmus of Panama during the middle Pliocene uplift has been postulated to have increased atmospheric moisture content in the Northern Hemisphere, which triggered the positive-feedback mechanism leading to the development of Northern Hemisphere Glaciation (NHG; e.g., Haug and Tiedemann, 1998; Bartoli et al., 2005). The closure led to divergences in sea surface temperatures and carbon isotope trends between the North Atlantic and Pacific oceans (e.g., Ravelo et al., 2004; Fedorov et al., 2006; Butzin et al., 2011). Furthermore, this transition was also associated with intensification of zonal thermal gradient within the tropical Pacific and the onset of strong Walker circulation (e.g., Driscoll and Haug, 1998; Ravelo and Wara, 2004; McClymont and Rosell-Melé, 2005; Lee and Poulsen, 2006). It has been proposed that these processes were

linked with gradual shoaling of the thermocline (Cannariato and Ravelo, 1997). Modeling results of Philander and Fedorov (2003) showed that this shoaling across the Pliocene reached a threshold around 3 Ma, when the thermocline became sufficiently shallow for the winds to bring cold water to the surface in upwelling areas. We postulate that mechanisms similar to those operating during the Pliocene cooling could be responsible for the development of upwelling zones at the onset of the Late Paleozoic Ice Age (Fig. 14). According to this hypothesis, cooler sea surface temperatures are expected in low latitudes, particularly in eastern regions of both oceanic basins. These predictions are supported by evidence from faunal distribution of cool-water brachiopods (Waterhouse and Shi, 2010) and sedimentological, as well as mineral indicators of upwelling or cold waters from the west margin of Euramerica (Brandley and Krause, 1994, 1997). Oxygen isotope data of well-preserved brachiopod shells from US Midcontinent (Fig. 13; Mii et al., 1999) and western Paleo-Tethyan Russian Platform (Fig. 13; Mii et al., 2001) and SW Iberia (Spain) (Armendáriz et al., 2008), additionally, show noticeable divergence between eastern and western Euramerica, i.e., the surface ocean seawater of eastern Panthalassa is cooler than western Paleo-Tethys'. This could, in turn, explain the divergence in carbon isotope trends during late Visean (Asbian or early–middle Warnantian equivalent of late Meramecian to early

Table 4 Quantitative data (%) of dominant carbonate grains from point-counting analysis of selected samples from the Mississippian of the Gongchuan section. AG — aggregate grains, B — brachiopods, Ca — calcispheres, Co — corals, Cri — crinoids, F — foraminifera, Lt — lithoclasts, M — mollusks, O — ostracods, P — peloids, T — trilobites, Tu — Tubiphytes, Bry — bryozoans, Cd — cortoids, Mx — matrix, U — unidentified microfossils. Sample

AG

B

Ca

Co

Cri

F

Lt

M

O

P

T

Tu

Bry

Cd

Mx

U

GC03 GC04 GC06 GC13 GC15 GC17 GC20 GC43 GC47 GC49 GC51 GC55 GC65 GC69 GC72

1.58 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.27 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 1.38 0.00 0.00 0.82 0.00 0.00 0.00 2.13 21.40

0.53 0.40 0.25 1.23 4.61 1.11 0.46 4.51 1.34 2.19 0.00 3.65 0.85 1.60 0.00

0.80 12.40 4.00 3.80 0.00 0.00 8.00 0.00 1.20 1.30 0.00 0.00 0.00 0.00 0.00

3.69 2.30 4.83 0.98 1.36 4.46 8.72 5.92 1.88 21.66 6.46 2.25 21.53 14.13 16.30

0.26 0.40 0.76 3.44 1.63 10.03 3.44 5.07 5.64 9.32 3.62 11.52 3.68 1.33 0.30

0.00 0.00 0.00 0.74 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.33 1.60

0.00 0.00 0.00 0.00 0.81 0.00 0.00 0.00 0.54 1.65 0.00 0.28 1.42 1.60 0.80

0.00 0.00 0.25 0.00 0.81 0.00 0.69 0.00 0.00 0.27 0.00 0.00 0.28 0.00 0.00

24.54 32.00 0.51 19.88 4.88 11.98 16.29 6.48 24.43 15.63 3.10 2.81 4.25 5.60 1.90

0.00 0.00 0.25 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

6.07 4.60 0.00 0.00 0.00 0.00 0.23 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.60

2.11 0.40 0.00 2.94 0.27 0.56 3.90 1.69 4.03 11.24 1.81 5.62 18.13 47.70 68.80

51.45 40.50 88.38 66.51 85.09 67.69 57.36 61.41 59.87 34.82 60.47 74.44 58.07 32.80 20.60

6.07 7.30 0.76 3.44 0.81 3.90 3.44 16.62 4.83 11.79 20.70 4.78 9.92 36.00 35.20

110


Fig. 13. Mississippian carbon and oxygen isotope stratigraphy from eastern and western Euramerica, as well as eastern Paleo-Tethys. Black dots are values from bulk carbonates from the Bama Platform (this study), Cantabrian Mountains (Buggisch et al., 2008) and Arrow Canyon Range (Saltzman, 2003); blue symbols represent well preserved brachiopod shell calcite from the North American Midcontinent (Mii et al., 1999) and Russian Platform (Bruckschen et al., 1999; Mii et al., 2001). As the duration assigned to the Gnathodus bilineatus conodont Zone differs between America and Europe (Fig. 4), the precise correlation of the beginning of the δ13C decreases in late Visean (horizontal pink bar) between the Arrow Canyon section and the Bama Platform is not clear. Abbreviations: PT — Paleo-Tethys, Pa. — Panthalassa, Eu. — Euramerica, E. — Eastern, W. — Western. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 14. Possible model to explain the divergence in carbon isotope across the subequatorial realm. The explanations are shown in the text. Brown arrows represent the direction of the paleo-trade-winds; yellow arrows are surface currents and the blue are (intermediate) deep-water currents. 1—Arrow Canyon Range, southeastern Newada, 2—Midcontinent, North America, 3—Russian Platform, 4—Cantabrian Mountains, Spain. For abbreviations, see Fig. 1A.


Chesterian) across low-latitude (or subequatorial) regions (Fig. 14). We have used the modified paleogeographic reconstruction from Blakey (2006) (Fig. 1), which indicates that a relatively restricted setting was present during late Visean in the Paleo-Tethys realm. Before the closure of the Rheic seaway, the surface ocean waters could freely exchange between western and eastern Euramerica. Thus, there were no significant δ13C gradients across the subequatorial realm. The closure of the seaway would lead to a more intensive intermediate deep-water circulation, enhancing nutrient-enriched and δ13C-depleted upwelling on the west coast of Pangea (Mii et al., 2001; Saltzman, 2003). At the same time, in the Paleo-Tethys realm, a relatively restricted and isolated ocean–atmosphere system was probably formed. Thermocline shoaling, together with vigorous easterly winds, would lead to enhanced upwelling of intermediate deep water (enriched in nutrients, with lower δ13C values) towards the Youjiang Basin of South China (Fig. 14; eastern Paleo-Tethys) which includes the southern margin of the Bama Platform. Upwelling water depleted in δ13C mixed then with fresh surface waters, yielding a decreasing trend in the carbon isotope record in the Gongchuan section. An analogous mechanism was probably responsible for the nearly coeval δ13C decrease in the western margin of Euramerica. The analogy between changes in the oceanic circulation brought about by the closure of the Central American Seaway during Pliocene and the oceanographic consequences of the closure of the Rheic seaway, i.e., the development of a stronger zonal thermal gradient during the Visean across the subequatorial Paleo-Tethys, could be further tested using spatially resolved oxygen isotope records. 6. Conclusions Based on microfacies analysis, foraminiferal biostratigraphy, and δ13Ccarb chemostratigraphy across the Mississippian carbonate strata in the Gongchuan section located in the Bama Platform, South China, the following conclusions can be drawn: 1 Depositional environments range from high energy, shallow sand shoals through back-bank low energy lagoons and tidal channels. They are represented by SMF23, SMF25, SMF18, SMF10, SMF9, and SMF11 of the standard microfacies types (SMF; Flügel, 2010). 2 A biozonation based on foraminifers and rare algae is provided from late Tournaisian to early Serpukhovian. The section is assigned to the foraminiferal zones from MFZ6 to MFZ16. 3 A major sea-level drop is recorded in late Visean (Asbian–early Brigantian; MFZ13–14). It correlates with a regressive event which can be traced worldwide and which has been suggested to relate with the onset of Gondwana glaciation (Wright and Vanstone, 2001). 4 A systematic difference in carbon isotope values is present across the paleo-subequatorial realm in late Visean (Asbian–early Brigantian; MFZ13–14). Its occurrence is in accordance with the major sea-level drop and the onset of high-frequency cycles both in eustacy and climate. In our interpretation, these co-existing events could be closely tied to the closure of the Rheic seaway during middle–late Visean. The closure might give rise to reorganization of oceanic circulation, and subsequently play a vital role in exchanging heat and moisture between high-latitude Gondwana and the tropics, as well as in carbon cycling. 5 We propose a model explaining the divergence in δ13C values between western and eastern Euramerica, as well as eastern Paleo-Tethys (South China), based on analogies with changes in oceanic circulation induced by the closure of the Isthmus of Panama and restriction of the Indonesian Seaway. 6 A relatively restricted and isolated ocean–atmosphere system was probably formed after the closure of the Rheic gateway. Thermocline shoaling, together with vigorous easterly winds, would lead to enhanced upwelling of intermediate deep water (enriched in nutrients, with lower δ13C values) towards the Youjiang Basin of South China (eastern equatorial Paleo-Tethys), which includes the southern margin

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of the Bama Platform. An analogous mechanism was probably responsible for the nearly coeval δ13C decrease in the western margin of Euramerica. 7 Quantitative microfacies analysis across the Mississippian succession in the Gongchuan section shows facies-independent disappearance of corals and increased proportion of cortoids and filter-feeding organisms at the onset of the Asbian δ13C decline. It argues for an increase in nutrients which can be also expected as a result of climate-driven upwelling. Acknowledgments We are grateful to Jianghai Yang for guidance during the field works. Thanks also go to Xiaoming Chen in the Nanjing Institute of Geology and Paleontology for helping to measure stable isotopic compositions. This study was supported by the National Natural Science Foundation of China (41272120), the “111 Project” (Grant No. B08030), and the Natural Science Foundation of Guangxi, China (2010GXNSFA013009). References Armendáriz, M., Rosales, I., Quesada, C., 2008. 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