Carbon isotope chemostratigraphy and implications of palaeoclimatic changes during the Cisuralian (Early Permian) in the southern Urals, Russia

Gondwana Research 21 (2012) 601–610

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Carbon isotope chemostratigraphy and implications of palaeoclimatic changes during the Cisuralian (Early Permian) in the southern Urals, Russia Jun Zeng a, Chang-qun Cao a, V.I. Davydov b, Shu-zhong Shen a,⁎ a b

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing, Jiangsu 210008, China Department of Geosciences, Boise State University, Boise, ID, 83725, USA

a r t i c l e

i n f o

Article history: Received 5 February 2011 Received in revised form 2 June 2011 Accepted 9 June 2011 Available online 17 June 2011 Handling Editor: M. Santosh Keywords: Carbon isotope Cisuralian Glaciation Isotope geochemistry Permian Urals

a b s t r a c t In order to meet the requirements for potential GSSPs in the Cisuralian Series (Early Permian), isotopic chemostratigraphy from the Carboniferous/Permian boundary to middle Artinskian using bulk carbonates was investigated under high-resolution biostratigraphical and new geochronologic constrains from three GSSP candidate sections at Usolka, Kondurovsky and Dal'ny Tulkas in the southern Urals, Russia. A gradually increasing trend in carbonate carbon isotope (δ13C) has been observed in the interval from the base of Asselian to early Sakmarian, which is generally consistent in timing with the increasing development of Glacial III or P1 from the latest Carboniferous to early Sakmarian (Early Permian) which prevailed in southern Gondwana. An excursion with double negative shifts in δ13C value is present around the Asselian/Sakmarian boundary in both the Usolka and Kondurovsky sections, which may have great potential to serve as chemostratigraphical marks for intercontinental correlation. The following highly positive excursion of δ13C in early Sakmarian indicates the maximium expansion of Glacial III or P1. The negative δ13C shift in the middle Sakmarian is possibly related to the quick collapse of Glacial III or P1 on the Gondwanaland. This negative shift is largely correlative with those documented in other areas of Russia, the North American Craton and South China, but further precise biostratigraphical and geochronologic constrains are neccessary to confirm this global signal. The late Sakmarian is characterized by a strong oscillation stage of δ13C, which probably indicates a complex climate transition marked by smaller alternating glacial–interglacial transitions during Glacial P2 superimposed on an overall warming trend. The sharp negative δ 13C shift around the Sakmarian/Artinskian boundary at the Dal'ny Tulkus section is difficult to interpret. This is followed by long-term low values (b−10‰) during the most part of Artinskian Stage. We suggest that the deeply depleted δ13C values in the Artinskian at the Dal'ny Tulkas section might result regionally from the enhanced input of organic carbon after the melt-out of ice sheets and the subsequent degradation and isotopic refractionation of the microbial chemosynthetic processes on the buried organic matter. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Carboniferous–Permian ice sheets have been well recognized as the largest glaciation that prevailed in the southern Gondwanan continent and peri-Gondwanan regions (Veevers and Powell, 1987; Isbell et al., 2003; Montañez et al., 2007; Fielding et al., 2008; Frank et al., 2008; López-Gamundí and Buatois, 2010). This glaciation has been previously documented as a single, long-lived ice sheet covering Gondwana (e.g., Veevers and Powell, 1987; Crowley and Baum, 1991; Francis, 1994; Hyde et al., 1999; Scotese et al., 1999). Recent detailed investigations suggest multiple episodes of glaciation and interglaciation (Isbell et al., 2003; Fielding et al., 2008; Korte et al., 2008), among which the episode from the latest Carboniferous to Early Permian, named as Glacial III (Isbell et al., 2003) or P1 (Fielding et al., 2008), is

⁎ Corresponding author. E-mail address: [email protected] (S.Z. Shen).

the largest one based on the sedimentological and palaeontological data in the high-latitude area of southern hemisphere. The evidence includes widespread distribution of diamictites containing typical cold-water brachiopod and the bivalve Eurydesma faunas from the Cisuralian (Early Permian) Asselian to Sakmarian Stage (Harrington, 1955; Shi and Waterhouse, 1991; Shi et al., 1996; Shen et al., 2000; Gonzalez, 2002; Stephenson et al., 2007; Clapham and James, 2008) spanning over approximately 8 million years (N299–291 Ma) as interpreted from zricon SHRIMP ages of volcanic horizons associated with Permian sediments in eastern Australia (Fielding et al., 2008). The glaciations might have lasted much later into the Middle (Guadalupian) or Late Permian (Lopingian) in eastern Australia (Fielding et al., 2008). In contrast, Carboniferous and Permian glacial deposits (e.g., diamictites) have been much less recorded in the northern hemisphere, except for a few in Siberia (Epshteyn, 1981; Chumakov, 1994; Raymond and Metz, 2004). However, marine and non-marine cyclothems were developed in the coeval rock suites in the tropic and subtropic areas of Euroamerican and Russian Platform

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.06.002

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in the northern hemisphere. They were interpreted to be caused by frequent eustatic fluctuations probably in response to the waxing and waning of the Carboniferous and Permian ice sheets on the Gondwanaland (Veevers and Powell, 1987), but this was debated by Isbell et al. (2003). Stable isotope profiles have been generally regarded as the most important proxy reflecting the changes of global carbon cycle and have been extensively applied in the field of chemostratigraphic correlations (e.g., Ishikawa et al., 2008; Le Guerroué and Cozzi, 2010; Jiang et al., 2011; Tang et al., 2011). With regard to geochemical correlation around the Permian/Carboniferous boundary in the southern Urals, pioneering work on carbon and oxygen isotopes based on bulk carbonates was carried out by Nelson and Ritter (1999), but was limited to the Carboniferous–Permian boundary at the Usolka Section. Subsequent isotopic investigations in this area mainly focused on the oxygen and carbon isotopic records based on brachiopod shells to discuss the isotopic responses to the changes of palaeo-temperature and ice volume during the Carboniferous and Permian glaciations and interglaciations (e.g., Mii et al., 1999, 2001; Brand, 2004; Korte et al., 2005a, b, 2008; Grossman et al., 2008). Although thick brachiopod shells with low Mg were suggested to be suitable materials to monitor the carbon and oxygen isotopes as temperature and ice volume proxies (e.g., Grossman et al., 2008), detailed data have not yet been available to reveal the isotopic chemostratigraphy for the Cisuralian Series in the southern Urals, Russia partly due to paucity of stratigraphic successions containing brachiopod shells for stable isotope study. In addition, the previous results were also controversial. Based on brachiopod shells from three sections in the southern Urals, Korte et al. (2005a) reported a decreasing trend in δ 18O value and relatively constant variations in δ 13C value from Asselian to Artinskian. They suggested a decline in intensity of the Permo-Carboniferous glaciations, and neither the waning of glaciations nor the vagaries of massive coal formation resulted in a visible disruption of the global carbon cycle. In contrast to the results of Korte et al. (2005a), two different trends, a decreasing δ 13C trend in North America and an increasing δ 13C trend in Russia from Asselian to Sakamarian were presented by Grossman et al. (2008) . They suggested a regional aridification exacerbated by restricted circulation in the epicontinental sea; but the relationship between isotope trends and glaciation was not evaluated. According to the international Permian timescale, the Permian System has been subdivided into three series and nine stages. These are the Cisuralian, the Guadalupian, and the Lopingian Series standardized respectively in the Urals, West Texas, and South China for the Lower, Middle and Upper Permian (Jin et al., 1997). Among these, all the GSSPs in the Guadalupian and Lopingian, as well as the GSSP of the Carboniferous/Permian boundary, have been established (Davydov et al., 1998; Glenister et al., 1999; Jin et al., 2001, 2006a, b; Lambert et al., 2007). However, biostratigraphical definitions and correlation marks for the potential candidates of GSSPs in the Cisuralian including the Sakmarian-base, Artinskian-base and Kungurian-base are still under study. Chuvashov et al. (2002a, b, c) proposed the Kondurovsky section, the Dal'ny Tulkas and the Mechetlino sections in southern Urals respectively for the candidates of the Sakmarian-base, Artinskian-base and Kungurian-base GSSPs. In order to meet the requirements of GSSPs for the Cisuralian Series, a field excursion to the Cisuralian GSSP sections in the southern Urals was organized by the Cisuralian Working Group from June 25 to July 4, 2007. The main goal of this field workshop was to investigate the biostratigraphic boundary definitions and chemostratigraphy for the potential GSSPs of the base of Sakmarian Stage (Kondurovsky section), the base of Artinskian Stage (Dal'ny Tulkas section) and the base of Kungurian Stage (Mechetlino section) and the auxiliary stratotype section of the Carboniferous–Permian boundary GSSP (Chuvashov et al., 2002a, b, c). Samples at the Mechetlino section were not collected because the section is mainly composed of heavily-weathered mudstone and this section has been

recently disregarded as the candidate of the Kungurian-base GSSP because of the poor conodont biostratigraphy (Henderson and Kotlyar, 2009). In this paper, we attempt to provide the carbon isotope variations based on the marine bulk carbonates from the Carboniferous/Permian boundary to middle Artinskian in the southern Urals and discuss the possible isotopic responses to palaeoclimatic changes, in particular the Early Permian glaciation/deglaciation in Gondwanaland. Our data are the first reported continuous carbon isotopes well constrained by high-resolution biostratigraphical and new geochronologic data in this area. 2. Geological setting and biostratigraphy The Usolka, Kondurovsky and Dal'ny Tulkas sections in the southern Urals contain continuous marine carbonates alternating with shale and mudstone deposited in the pre-Uralian foredeep (Fig. 1) from the Asselian to Artinskian stages. The Usolka section along the Usolka River is located approximately 120 km southeast away from Ufa. The exposed strata contain marine

Fig. 1. Main elements of the southern Pre-Uralian foredeep, highlighting the location of the studied sections: Kondurovsky section, Usolka section (U) and Dal'ny Tulkas section (DT) (after Davydov et al., 2002).

J. Zeng et al. / Gondwana Research 21 (2012) 601–610

deposits from the uppermost Carboniferous Kasimovian to the basal Artinskian, which was proposed as the GSSP candidate of the Carboniferous Gzhelian Stage (Chernykh et al., 2006; Davydov et al., 2008) and as a potential GSSP candidate of the Asselian/Sakmarian boundary (Henderson and Kotlyar, 2009). The ranges of significant conodont and fusulinid species and more than 70 tuff layers are distributed throughout the Kasimovian to Sakmarian stratigraphic interval (Davydov et al., 2002). The Carboniferous/Permian boundary was recognized by the first occurrence of the conodont species Streptognathodus isolatus (Chernykh and Reshetkova, 1987; Chernykh and Ritter, 1997) at 31.45 m above the base (Bed 16–3) with constrained radiometric age of 298.90 + 0.31/−0.15 Ma in this section (Ramezani et al., 2007) (Fig. 2). The Asselian/Sakmarian boundary in Bed 26 (at 54.15 m above the base) was defined by the first occurrence of the conodont species Sweetognathus merrilli (Chuvashov et al., 2002c; Chernykh, 2006). Forty-nine bulk carbonates were collected within the 32-meter interval spanning the entire Asselian and extended upward into the Sakmarian. The Kondurovsky section in Orenburg Province, Russia was previously proposed as the candidate of the Sakmarian-base GSSP

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(Chuvashov et al., 2002c; Davydov et al., 2005). The marine rock sets over 1200 m in thickness represent rapid carbonate-dominated deposits yielding conodont, fusulinacean and ammonoid fossils from late Asselian to lower Artinskian (Schiappa, 1999; Chuvashov et al., 2002c). The base of Sakmarian Stage was defined by the sweetognathid chronomorphocline, which exhibits the evolutionary change from Sweetognathus expansus to S. merrilli at 115 m [uppermost Bed 11 of Chuvashov et al. (1993) and Chernykh (2005, 2006)]. However, recent conodont study questioned the FAD of the index conodont S. merrilli in the designated bed (Henderson, 2009; Shen, 2009). Thus, the Permian Subcommission on Stratigraphy (SPS) decided to work on the Usolka section instead to redefine the Asselian/Sakmarian boundary (Henderson and Kotlyar, 2009) which especially contains better materials for the conodont genera Streptognathodus and Mesogondolella and many tuffaceous beds for radiometric isotopic dating (Schmitz and Davydov, in press). Fifty-nine bulk carbonates were collected for carbon isotope analysis in the Kondurovsky section. The Sakmarian/Artinskian boundary strata are represented in the Dal'ny Tulkas section. The exposed upper Sakmarian rocks are composed of dark-colored marl, argillite, and calcaerous mudstone,

Fig. 2. Carbon and oxygen isotope trends of the Usolka section. Geochronologic data for the Carboniferous/Permian boundary is from Ramezani et al. (2007). Stratigraphic log and fossil data are from Schmitz and Davydov (in press). Arrows indicate the first occurrences of the fossils in the section.

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or less commonly, detrital limestone with Sterlitamakian (late Sakmarian) fusulinids and organic-enriched materials (Chuvashov et al., 2002b). The early Artinskian deposits are marked by a member of breccia limestone (about 0.5 m thick), and the overlying Tyul'kas (Tulkas) Formation is composed of calcareous argillites and marls interbedded with calcareous mudstone and bioclastic limestone (Chuvashov et al., 1990). The breccia limestone is slightly above the base of Artinskian deposits with typical Artinskian fusulinids and the conodont species Mesogondolella bisselli and Sweetognathus whitei (Chuvashov et al., 2002b; Chernykh, 2005, 2006; Schmitz and Davydov, in press). Forty-eight carbonate samples were collected in the Dal'ny Tulkas section within 55 m interval covering the biostratigraphic boundary interval of Sakmarian/Artinskian stages. 3. Method and results All of micrite samples were checked to avoid calcite veins. 1 gram rock chip was powdered for carbonate carbon isotope (δ 13C) analysis. The stable isotopic analysis of the bulk carbonate uses the stand procedures of McCrea (1950). Rock powder was reacted with purified H3PO4 under vacuum at 50 °C for 12 h. The evolved CO2 was cryogenically extracted and sealed in vacuumed tubes for subsequent 13 12 C/ C ratio determination. Analyses of δ 13C were carried out using a Finnigan MAT 253 mass spectrometer at the Nanjing Institute of Geology and Palaeontology. Reproducibility was better than 0.03‰ for carbonate carbon isotope which was calibrated to GBW-04405 laboratory standard of Nanjing Institute of Geology and Palaeontology with the δ 13Ccarb value of 0.57‰ and the δ 18O value of −8.49‰. All data are presented in standard per mil (‰) notation relative to V-PDB. A gradually increasing trend in δ 13C from −4.8‰ at the base of Asselian upward to 4.2‰ within Bed 22 in the Sweetognathus expanus Zone occurs in the Usolka section. This is followed by an interval with high values around 4‰ from Beds 22 to 24 in the upper Asselian (Fig. 2). From Beds 25 to 26, two minor negative shifts in the δ 13C curve with the minimum values of 0.7‰ and 1.4‰ occur around the Asselian/Sakmarian boundary. The double negative δ 13C shifts are intervened by a positive recovery with the value of 3.2‰ in the S. merrelli Zone (Chernykh, 2006). Materials were not collected above the lower Sakmarian in the Usolka section because the strata in the middle Sakmarian and above are composed of heavily weathered mudstone with tectonic discordance. The δ 18O values present a gently decreasing trend from −1.9‰ to − 4.6‰ in the Asselian Stage, and then shift positively back to −2‰ in the early Sakmarian (Fig. 2). The double negative δ 13C shifts can be tracked in the lower part of the Kondurovsky section between 91 and 190 m, but with different minimum values of 0.3‰ and −1.6‰, respectively (Fig. 3). They are intervened by a positive recovery up to 5.5‰. The first negative shift is approximately around the first occurrence of the conodont Mesogondolella uralensis, which is consistent with that at the Usolka section (Figs. 2, 3). The second negative shift is in the Sweetognathus expanus and “S. merrelli” zones (Chuvashov et al., 2002c; Chernykh, 2006), which slightly higher than that at the Usolka section if the previouslyreported conodont biostratigraphy of Chuvashov et al. (2002c) can be confirmed. After the double negative shifts around the Sakmarian/Artinskian boundary, the δ 13C curve rebounds positively back to a much higher average value of 5.9‰ within the following 150 m (200–350 m), and constitutes a positive plateau from Bed 13 to Bed 16 in the lower part of the Kondurovsky section. The value rapidly plunges down to −3.9‰ from Bed 16 to Bed 17 (Fig. 3), and keeps distinct low levels with an average of −1.8‰ in the middle part of the Sakmarian (from Bed 16 to Bed 24). In the upper part of Sakmarian, the δ 13C curve generally shifts slightly back to an average value of 1.5‰, but oscillates frequently in the remaining part of the Sakmarian stage at the Kondurovsky section. The δ 18O curve presents a coeval and similar trend to the δ 13C profile. It starts with a positive plateau with the

maximum value of −0.2‰ in the early Sakmarian, and rapidly shifts down to the much negative value of −5.2‰ in the middle part of the Sakmarian Stage. Differing from the δ 13C profile in the late Sakmarian, the δ 18O curve reverses positively to values around −0.8‰ at the uppermost part of the Kondurovsky section (Fig. 3). In the Dan'ly Tulkas section, the δ 13C and δ 18O curves apparently present a concurrent trend and are characterized by a rapid and sharp drop around the Sakmarian/Artinskian boundary and a long-term deep depletion stage in the following Artinskian interval. According to the biostratigraphic data (Schiappa, 1999; Schmitz and Davydov, in press), the lower part of the Artinskian at the Dal'ny Tulkas section is approximately correlative with the uppermost interval above Bed 49 of the Kondurovsky section. We do not know whether the sharp negative shift of δ 13C is present or not in the Artinskian interval because the uppermost part of the section is not well exposed at the Kondurovsky section. The δ 13C values present a rapid and dramatic depletion from − 4.7‰ to − 11.7‰ around the Sakmarian/Artinskian boundary in the Dan'ly Tulkas section, and maintain a long-term highly negative level in the following part of the Artinskian Stage, except for one point with a value of − 2.2‰ in the very early Artinskian. These very negative values have been confirmed by three different labs in Nanjing analyzing the same samples. Similarly, the δ 18O values show a parallel trend with the δ 13C curve depleting from 1.1‰ to −2.2‰ in the early Sakmarian interval, and maintain a markedly negative level in the overlying Artinskian interval (Fig. 4). 4. Interpretation and discussion 4.1. Original signature or diagenesis In general, carbonate carbon isotope is relatively much resistant from meteoric diagenesis than oxygen isotopes (Marshall, 1992). To evaluate the effects of meteoric diagenesis, co-variant relationships of δ 13C vs δ 18O are plotted for the data obtained in this study (Fig. 5). All values of δ 18O are above −8‰ and without linear correlation to the values of δ 13C, which suggests no evident diagenetic effects on the original isotope compositions of carbonates (Kaufman and Knoll, 1995) analyzed in this study. The isotopic data of both δ 13C and δ 18O in the Usolka and Kondurovsky sections locate within the normal values above −5‰ in δ 13C in the area C in Fig. 5. However, the data from Dal'ny Tulkas have been separated into two areas (A and B in Fig. 5) with more lower δ 13C values and much greater δ 18O values in the late Sakmarian. The range of δ 13C values in the Dal'ny Tulkas section is apparently within the “forbidden zone” in the Phanerozioc carbonates (Knauth and Kennedy, 2009). Most of them vary abnormally in the range between − 10‰ and − 16‰ in the Artinskian Stage, and therefore were usually not used by previous studies. In contrast, the oxygen isotope values varied in the normal range between 0‰ and −4‰. Because the subsequent diagenesis would affect the original isotopic compositions of both δ 13C and δ 18O, or δ 18O alone (Beauchamp et al., 1987; Banner and Hanson, 1990). Thus it is difficult to account for these “forbidden” δ 13C values in the Dal'ny Tulkas section resulted from diagenesis, in the case of the normal range in δ 18O value in the bulk carbonates. Similar “forbidden” δ 13C values lower than −8‰ were also reported from bulk carbonates of the Carboniferous Gzhelian Stage in the same area (Nelson and Ritter, 1999), and the Permian–Triassic marine section in eastern Greenland (Twitchett et al., 2001) although no interpretation was presented by those authors. If these markedly negative values in δ 13C represent the original isotopic composition of the carbon cycle in the earth history, they cannot be correlated with the release of volcanogenic CO2 or change of biological productivity in terms of their magnitude and were probably derived from massive methane release from sea floor (e.g., Krull and Retallack, 2000; Retallack and Krull, 2006). However, according to the mechanism of massive methane release from sea-floor gas hydrate

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Fig. 3. Carbon and oxygen isotopic trends of the Kondurovsky section. Ammonite biostratigraphy is from Schiappa (1999) and Chuvashov et al. (2002c). Stratigraphic log and chronostratigraphy from Davydov et al. (2005).

reservoirs discussed by Payne et al. (2004) and Knoll et al. (2007), a long-term highly negative shift in carbonate δ 13C would require extended, alternating intervals of methane storage and release. It is, therefore, very difficult to account for this long-term and constant negative valley with the values below −10‰ in the Dal'ny Tulkas section using the scenario of the methane release from sea floor gas hydrates. The lithology in the Dal'ny Tulkas section is composed of

organic-enriched silty micrite interbedded with marly limestone and contains fusulinids, ammonoids and conodonts (Chuvashov et al., 2002b; Schmitz and Davydov, in press). We suggest that these abnormal depletions in δ 13C might have resulted from the enhanced organic carbon burial and subsequent isotopic refractionation by microbial chemosynthetic processes. Sedimentary microbes converting most of the dissolved methane to carbon dioxide was discussed

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Fig. 4. Carbon and oxygen isotopic trends of the Dal'ny Tulkus section. Stratigraphic log and biostratigraphic data are from Chuvashov et al. (2002b) and Davydov et al. (2005). Arrows indicate the first occurrences of the fossils at the section.

recently by Pohlman et al. (2011). In the restricted circulation in the epicontinental sea in the Artinskian in the southern Urals (Grossman et al., 2008), organic carbon burial can be enhanced and accumulated after the melt-out of glaciers. As the metabolic products of microbial chemosynthetic processes, the biogenetic carbon dioxide and/or methane cycles can greatly deplete the ambient dissolved inorganic (DIC) and organic carbon (DOC) compositions in the case of enhanced organic carbon burial. Thus, further studies to seek independent evidence are necessary.

Fig. 5. Crossplots of carbon and oxygen isotopes of the Kondurovsky section, Usolka section, and Dal'ny Tulkas section.

4.2. Chemostratigraphy and correlation The Cisuralian δ13C profile in the southern Urals provides potential chemostratigraphical correlation marks among different regions. The Asselian Stage is characterized by a gradually increasing trend in δ13C value, which ended within the Sweetognathus expansus Zone, and then followed by an interval with high δ13C values slightly below the Mesogondolella uralensis Zone. This gradually increasing trend of δ13C from −4.8‰ to 4.2‰ seems to succeed the rising trend from the latest Carboniferous Gzhelian Stage (Nelson and Ritter, 1999). It is also roughly comparable with the trends from North American Craton and other areas in the Russian Platform documented by Grossman et al. (2008). The double negative shifts of δ13C around the Asselian/Sakmarian boundary may have great potential as chemostratigraphicl marker for the correlation of the Asselian/Sakmarian boundary (Figs. 2, 3, 6). The first negative shift began at slightly above the first occurrence of the conodont Mesogondolella uralensis Zone, reached to the lowest level near the first occurrence of Sweetognathus merrelli at the Asselian/Sakmarian boundary. This negative shift is immediately followed by a positive recovery in the S. merrelli Zone and then by the next negative shift in the S. binodosus Zone (Fig. 2). The double negative shifts are basically confirmed at the Kondorovsky section in spite of different magnitudes (Fig. 3), and suggests that the Asselian/Sakmarian boundary interval between these two sections is generally correlative although further studies on conodont biostratigraphy are necessary (Figs. 2, 3, 6). Korte et al. (2005a) showed that the δ 13C values in the southern Urals are relatively constant through Asselian to Artinskian, with the running mean declining from 4.3‰ to 3.4‰, and suggested that no serious dislocation in the global carbon cycle arose during this time interval (Korte et al., 2005a, fig. 6). The double negative shifts around the Asselian/Sakmarian boundary, the positive plateau in the early Sakmarian and the sharp negative shift in the middle Sakamrian were not revealed by the data of Korte et al. (2005a). This may be due to the

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Fig. 6. δ13C and δ18O trends in the Cisuralian Series based on bulk carbonates of Usolka, Kondurovsky and Dal'ny Tulkas sections in the southern Ural. The glacial events follow Isbell et al. (2003) and Fielding et al. (2008). Stratigraphic columns scaled to the latest geochronologic ages (Ramezani et al., 2007; Schmitz and Davydov, in press). Arrows indicate the first occurrences of the fossils at the section.

availability of brachiopod materials or the limitations of available biostratigraphic constraints in their studied sections. Our data show similar trends with those reported by Grossman et al. (2008) based on integrated data from different regions. As plotted by Grossman et al. (2008), Figs. 5–7, δ 13C curve shows a minor increase of δ 13C values from the latest Carboniferous, followed by a relatively less pronounced positive plateau. A distinct negative shift occurred within the Sakmarian Stage, which is probably comparable with the sharp negative shift in the middle Sakmarian constructed based on our bulk carbonates at the Kondorovsky section. Grossman et al. (2008) do not have data for the most part of the lower Sakmarian in the Russian Platform, therefore, a comparison with our data is not possible. The δ 13C values around the Sakmarian/Artinskian boundary are about 4‰ followed by a negative shift in the early Artinskian. Although the δ 13C trend of Grossman et al. (2008) show some resemblance to ours, the magnitude of δ 13C fluctuations are very different from ours and the very negative values in the Artinskian at the Dal'ny Tulkus section were not identified by Grossman et al. (2008). Our Cisuralian δ 13C curve also shows some similarities with the curve established recently by Buggisch et al. (2011) based on bulk carbonates at the Luodian section in Guizhou Province, South China. According to the original data of Buggisch et al. (2011), the secular trend of δ 13C at the Luodian section shows a gradual increase from the latest Kasimovian to the Gzhelian Stage, a steady stage from the Gzhelian to middle Asselian followed by two negative shifts intervened by a positive peak up to 6‰ in late Asselian based on biostratigraphical subdivision of the Luodian section of Buggisch et al. (2011, fig. 7). The second negative shift is succeeded by a less pronounced positive shift about 2.5‰, then followed by a negative shift about 1.3‰ in the latest Asselian. The average value of δ 13C in the early Sakmarian at the Luodian section is about 1‰ lower than that in the interval from late Gzhelian to early Asselian. The late Sakmarian and Artinskian is characterized by an oscillation stage of δ 13C value with a fluctuation about 1‰, which is probably correlative with the

oscillation stage in the late Sakmarian at the Kondorovsky section in the southern Urals. If the carbon isotopic trend established by Buggisch et al. (2011) is chemostratigraphically correlative with the general pattern in the southern Urals established here, the Gzhelian Stage of Buggisch et al. (2011) at the Luodian section is partly equivalent to the Asselian at the Usolka section in the southern Urals. The Asselian/Sakmarian boundary at the Luodian section (Buggisch et al., 2011) is correlative with the horizon about 3 m higher than that at the Usolka and Kondorovsky sections in the southern Urals. Thus, further detailed biostratigraphical and conodont taxonomic work around the Carboniferous/Permian boundary and Lower Permian in the Loadian section is necessary to clarify this problem. In summary, the gradually increasing trend from the latest Carboniferous to late Asselian and the negative shift in the middle Sakmarian have been documented based on the data from North American Craton and other sections in the Russian Platform (Grossman et al., 2008) and probably South China (Buggisch et al., 2011), but with different magnitudes. The double negative shifts around the Asselian/Sakamrian boundary are present in both the Usolka and Kondorovsky sections and probably manifested in the Luodian section in South China as well. The highly negative values in the lower Artinskian have not been reported from anywhere so far, and these might reflect the enhanced organic carbon burial and subsequent microbial chemosynthetic processes. Further investigation is necessary to evaluate whether the excursion can be used as a potential chemostratigraphical marker of the Artinskian-base GSSP. 4.3. Implications for Cisuralian palaeoclimatic changes The Cisuralian Epoch is a critical transitional period when widespread glaciations developed and vanished on the Gondwanaland, and these should be imprinted in the stable isotopes (e.g., Frakes et al., 1992; Korte et al., 2008). This imprint is particularly evident in the Sakmarian Stage as indicated by the concurrent trends of δ13C and δ 18O in the three sections in the southern Urals (Fig. 6). Such concurrent trends of carbon

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and oxygen isotopes were suggested to be characteristic in the time intervals with cooling conditions, such as the end of Eocene Epoch (Coxall et al., 2005), the mid-Carboniferous (Mii et al., 2001), the Devonian/Mississippian boundary (Brand et al., 2004) and the Late Ordovician Hirnantian stage (Brenchley et al., 1995, 2003; Marshall et al., 1997). The reason for this trend has been associated with the onset of sequestration of large volumes of organic carbon (Brenchley et al., 1995; Marshall et al., 1997), and presumably glaciations, through changes in atmospheric CO2 levels (Mii et al., 2001). The concurrent trends of δ13C and δ 18O in the Cisuralian in the southern Urals, Russia are compatible with this scenario. The gradual positive shift in δ 13C value from early to late Asselian may be interpreted by enhanced terrestrial organic burial (Gastaldo et al., 1996) and lowered atmospheric CO2 levels (Mii et al., 2001). However, we interpret the gradual positive shift in δ 13C value to reflect the isotopic response to the long-term ice sheet growth because they temporally coincide with Glacial III of Isbell et al. (2003) or Glacial P1 of Fielding et al. (2008) in Gondwana and northern Siberia. A maximum expansion was attained in the early Sakmarian as indicated by the high plateau of δ 13C and δ 18O in the early Sakmarian (Fig. 6). This is consistent with the lower pCO2 (Montañez et al., 2007) and the relatively low sea level (Rygel et al., 2008) in this time interval. The overlying rapid negative shift of both δ 13C and δ 18O (about 350–410 m of the Kondurovsky section) in the middle-late Sakmarian generally coincided with the widespread collapse of ice sheets of Glacial III (Isbell et al., 2003) or P1 (Fielding et al., 2008) in the southern high-latitude areas due to rapid global warming in the middle-late Sakmarian (Fig. 6). Geochronologic data at the base of postglacial deposits in the southwest Karoo Basin, South Africa supports the middle Sakmarian age for the collapse of ice age (Bangert et al., 1999). It is worth mentioning that the positive trend revealed by Nelson and Ritter (1999) and this study, which is possibly related to the development of Glacial P1, began in the latest Carboniferous (N298.95 Ma ± 0.13) and ended in the early Sakmarian at about 294 Ma based on new high-precision U–Pb ages using ID-TIMS in the southern Urals (Fig. 6; Schmitz and Davydov, in press). The timings of both the beginning and ending of Glacial P1 in southern Urals are generally older than the timing of P1 between 299 and 291 Ma calibrated by the previous SHRIMP ages (Fielding et al., 2008). This discrepancy is highly likely due to different geochrologic technologies. As discussed by Korte et al. (2008) and Schmitz and Davydov (in press), the age model in Australia using SHRIMP is about 1% younger in addition to a lower precision than those using ID-TIMS (Schmitz and Davydov, in press). Therefore, it is apparent that the previous age control in the Gondwanan records requires significant improvement (Korte et al., 2008; Schmitz and Davydov, in press) using the same high-precision geochronologic technique to realize a precise temporal correlation between Gondwana and Urals. The melt-out of ice sheet provided large fresh water depleted in 18 O into the ocean and generated a negative excursion of oxygen isotope. The negative shift of carbon isotopes may be derived from the large oxidation of buried organic carbon during the warm period. This global warming event is well evidenced by the contemporary disappearance of the typical cold-water benthic faunas in the southern high-latitude area and the turnover of cold- to cool/warmwater biotic communities in the Gondwanan Realm (Harrington, 1955; Shi and Waterhouse, 1991; Shi et al., 1996; Shen et al., 2000; Shi and Grunt, 2000; Archbold, 2001; Gonzalez, 2002; Stephenson et al., 2007; Clapham and James, 2008). According to the quantitative data of Clapham and James (2008), communities in the Sakmarian in eastern Australia were dominated by the typical cold-water brachiopod Trigonotreta and the bivalve Eurydesma, whereas communities from the later greenhouse climate contained abundant productive brachiopods such as Terrakea and Echinalosia. It was also temporally

consistent with the terminal of early Sakmarian glacier subdivided by Fielding et al. (2008) based on the temperature trend from Australia (Korte et al., 2008), the widespread changes from glacial to postglacial facies in the southern Gondwanan and peri-Gondwanan regions (Fielding et al., 2008; Melvin et al., 2010) and the transgression related to the demise of the ice age (López-Gamundí and Buatois, 2010). Furthermore, the negative shift in the middle-late Sakmarian was relatively sharp suggesting that the deglaciation process was fast. The carbon and oxygen isotopes have a negative valley (between 410 and 575 m of the Kondurovsky section) indicating that the palaeoclimate was ameliorated by an interglacial stage during the middle Sakmarian (Fig. 6). The negative carbon isotope change may be caused by large organic matter oxidized into dissolved inorganic carbon when the climate becomes warm during the interglaciation. The following late Sakmarian Stage is complicated by a less pronounced positive shift with frequent oscillation in both carbon and oxygen isotopes (Fig. 6). This stage is coincident with a complex climate transition marked by smaller alternating glacial–interglacial transitions superimposed on an overall late Cisuralian warming trend during Glacial P2 (Frank et al., 2008). This complex palaeoclimate fluctuation was consistent with the latest Sakmarian–Artinskian highly variable benthic communities in eastern Australia, the changes observed in Artinskian tropical terrestrial ecosystems (Clapham and James, 2008) and probably the turnover of brachiopod communities in the Tarim Basin of Northwest China (Chen and Shi, 2003). The magnitude of carbon isotope shift less than that of Glacial P1 suggests that Glacial P2 was spatially less developed than Glacial P1. The organic carbon isotope from eastern Australia (Birgenheier et al., 2010) and the carbon isotope based on brachiopod shells in the North American Craton and Russian Platform also show large fluctuations during this time interval (Grossman et al., 2008). The high-value interval of δ 13C in the latest Sakmarian in the Dan'ly Tulkas section indicates, partly the occurrence of Glacial P2, which ended during the early Artinskian (Fielding et al., 2008). It is still unclear whether the record in the Urals suggests the earlier isotopic response of the Glacial P2 in the southern Urals than that in eastern Australia as suggested by Fielding et al. (2008) or whether it is a reflection of the scarcity of the samples and/or their preservation. 5. Conclusions Carbon and oxygen isotope analyses on bulk samples from the Cisuralian (Early Permian) in southern Ural lead to the following conclusion. (1) The Asselian Stage is characterized by a gradual positive trend succeeding the rising trend from the latest Carboniferous to late Asselian and culminated in two negative shifts around the Asselian/Sakmarian boundary at the Usolka and Kondorovsky sections. The double negative shifts may be served as potential chemostratigraphical marks for global correlation in this horizon (Fig. 6). (2) Carbon and oxygen isotopes in all the three sections show a concurrent trend in the Sakmarian Stage (Fig. 6) which might be an important feature in response to glaciation during the Sakmarian, similar to the mid-Carboniferous interval as studied by Mii et al. (2001) in the same area. The gradual positive shift of carbon and oxygen isotopes to the acme in the early Sakmarian is generally consistent with the maximum expansion of Glacial III (Isbell et al., 2003) and P1 (Fielding et al., 2008) in timing. The subsequent negative shift of carbon and oxygen isotopes in the middle-late Sakmarian might have resulted from the rapid collapse of ice age, and indicates that the deglaciation process was fast. The oscillation of carbon and oxygen isotopes in the late Sakmarian might indicate a complex climate transition marked by smaller alternating glacial–

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interglacial transitions during the beginning of Glacial P2 superimposed on an overall warming trend before the terminal turnover to an ice-free Earth in the Late Cisuralian. (3) The highly negative values of carbon isotope during the Artinskian Stage might have potentially resulted from the enhanced organic carbon burial and subsequent microbial chemosynthetic processes of biogenetic methane and carbon dioxide cycles; such low background values have not been reported in the coeval horizon anywhere in the world.

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