Prevailing anoxia in the Kungurian (Permian) of South China: Possible response to divergent climate trends between the tropics and Gondwana

Gondwana Research 49 (2017) 81–93

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Prevailing anoxia in the Kungurian (Permian) of South China: Possible response to divergent climate trends between the tropics and Gondwana Chao Liu a,b,c, Emilia Jarochowska c, Yuansheng Du a,⁎, Axel Munnecke c, Xianduo Dai b a b c

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China School of Earth Sciences, China University of Geosciences, Wuhan 430074, China GeoZentrum Nordbayern, Fachgruppe Paläoumwelt, Universität Erlangen-Nürnberg, Loewenichstrasse 28, 91054 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 8 December 2016 Received in revised form 4 May 2017 Accepted 21 May 2017 Available online 29 May 2017 Handling Editor: I. Somerville Keywords: Early–Middle Permian Icehouse–greenhouse climate transition Primary productivity Ocean stagnation

a b s t r a c t The Kungurian ocean-climate system has received little attention, but a new compilation of geochemical and paleoclimatic proxies suggest more complex climate dynamics during the Late Paleozoic icehouse–greenhouse transition than previously considered. Here, integrated carbon isotope stratigraphical, sedimentological, and geochemical data across two Early–Middle Permian successions in the Youjiang Basin, South China, is presented. These proxies indicate widespread anoxia below or near the water-sediment interface in South China during the Kungurian. High primary productivity, high sedimentation rate, and a relative sea-level rise are here proposed as being responsible for this process in the late Kungurian, but cannot account for the remaining anoxia at cycle boundaries. We put forward a hypothesis that divergent climate trends between the tropics and Gondwana might have played an important role in ocean stagnation and prevailing O2-deficient conditions in and probably outside South China during the Kungurian. The termination of anoxia in this region during the latest Kungurian is attributed to an intensification of oceanic circulation, a rise in atmospheric pO2 concentrations, a sea-level drop, depressed primary productivity, and low sedimentation rates. This study calls for further high-resolution sedimentological and geochemical investigations on the paleotropics outside South China, in order to elucidate the icehouse-greenhouse transition at the global scale. © 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Late Paleozoic Ice Age (LPIA) was the most severe glaciation in the Phanerozoic (Raymond and Metz, 2004). The timing of its onset (Smith and Read, 2000; Jones and Fielding, 2004; Fielding and Frank, 2015; Liu et al., 2015), development (Isbell, 2003; Fielding et al., 2008; Isbell et al., 2012; Davydov et al., 2016; Isbell et al., 2016), and demise (Montañez et al., 2007; Yang et al., 2014) has been constrained precisely. Little attention has been, however, paid to the icehouse–greenhouse climate transition (Nakazawa et al., 2015; Liu et al., 2017). The Kungurian is one of the most pivotal intervals during this transition. It was previously proposed that the third phase of the Permian glaciation (P3) in eastern Australia began in the late Kungurian (Fielding et al., 2008). However, the latest high-precision U-Pb geochronological calibration moved the base of the P3 to the early Roadian (Metcalfe et al., 2015). The Kungurian was essentially ice-free, and numerous major changes, such as a substantial decrease in atmospheric pCO2 concentrations (Royer, 2006; Montañez et al., 2007), a decline in the areal extent of coal forests in the tropical realm (Cleal and Thomas, 2005), a slow-

⁎ Corresponding author. E-mail address: [email protected] (Y. Du).

down in the development and turnover in the evolution of reefs (Flügel and Kiessling, 2002; Kiessling, 2002; Weidlich, 2002), and divergent trends in latitudinal diversity gradient in brachiopod genera between low and higher latitudes (Powell, 2007), took place during the Kungurian, indicating significant climate perturbations (Fig. 1). The character of these perturbations has not been elucidated, let alone the mechanisms behind them. The Kungurian organic-rich Chihsia Formation of South China has long been interpreted as being deposited in O2-dificient conditions, as evidenced by multiple proxies (e.g., Yan and Liu, 2007; Wei et al., 2012; Liu et al., 2014). These researchers attributed it to high primary productivity, but the underlying triggers of oxygen depletion have not been constrained. Here, we integrated carbon isotope stratigraphical, sedimentological, and normalized geochemical data from two Early– Middle Permian transitional successions in the Youjiang Basin, South China. The aims of our study are (1) to clarify the roles of the relative sea level, sedimentation rate and primary paleoproductivity in anoxia below or near the water-sediment interface prevailing in the Kungurian of South China; (2) to propose a hypothesis on the mechanism behind this anoxia; and (3) to discuss the causes for a complete demise of anoxia in the Kungurian of South China. Our study sheds new light on complex climate dynamics during the Late Paleozoic icehouse–greenhouse transition.

http://dx.doi.org/10.1016/j.gr.2017.05.011 1342-937X/© 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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Fig. 2. Paleogeographical reconstructions of South China during the Early–Middle Permian transition (A), location map of the studied Tongle and Wuzhuan sections (B; modified from Wang and Jin, 2000; Liu et al., 2017), and Permian stratigraphical frame of different depositional settings in the Youjiang Basin (C). The yellow band marks the study interval.

2. Geological setting and stratigraphy During the Early–Middle Permian transition the Yangtze region was situated within the eastern equatorial Paleo-Tethys (Wang and Jin, 2000; Blakey, 2011). We concentrated on the Bama Platform here, a representative isolated carbonate platform in the Youjiang Basin (Fig. 2A). The tectonic evolution of the Youjiang Basin and isolated platforms located within the basin have been reviewed by Liu et al. (2015). We investigated two new sections from different depositional settings in the northern part of the Bama Platform, apart from the shallow-water Gongchuan (GC) section we studied earlier in the same basin (ca. 100 km southwards) (Fig. 2B; Liu et al., 2017). The Tongle section (TL; N 24°36′34″, E 107°28′32″) is exposed on a platform slope and basin margin position; the Wuzhuan section (WZ; N 24°21′58″, E 107°20′42″) was predominately deposited in restricted inner platform facies. The Early–Middle Permian strata are assigned to the Sidazhai Formation in the TL section and the Chihsia Formation in the WZ section (Fig. 2C). The Sidazhai Fm. is ca. 580 m and mainly formed by thin-bedded black to dark grey micrite limestones (Fig. 3A) with abundant chert beds or nodules (Fig. 3B). The Chihsia Fm. is ca. 160 m and composed of fossiliferous grey limestones and dolostones (Fig. 3C), which are punctuated by two subaerial exposure surfaces. We constrained the base of the Kungurian by the same means as described in Liu et al. (2017). This level corresponds to an unconformity in the TL section (Fig. 3D). The top of the Kungurian is identified by a sharp drop following a positive peak in the carbon isotope records, which can be correlated precisely with the conodont-based Tieqiao section (TQ;

the TQ section is about 200 km east of the Bama Platform) (Tierney, 2010; Sun et al., 2017). The placement of this level is also confirmed by the appearance of Maklaya pamirica (a typical Roadian fusuline species) slightly above the Kungurian/Roadian boundary in both studied sections (Angiolini et al., 2015; Liu et al., 2017). 3. Methods 3.1. Sedimentological analyses To construct depositional models and the chronological framework, and employ them to trace the relative sea-level (RSL) development of the TL and WZ sections, we investigated the Cisuralian–Guadalupian transitional strata and collected 150 fresh samples (79 from the TL Section and 71 from the WZ Section), in association with field observations. We followed the methodology described in Liu et al. (2017), in which lithofacies were distinguished based on thin sections, following Dunham's (1962) classification. 3.2. Time scales and linear sedimentation rate We assumed that the high frequency transgressive-regressive cycles recognizable in the successions are of third order and, within the same stage, have equal durations. Kungurian cycles are here designated K1 through K7 and Roadian cycles – R1 through R3 (Fig. 4). On account of the R1 cycles spanning the Kungurian/Roadian boundary, with the maximum flooding surface roughly corresponding to this boundary

Fig. 1. Summary diagram illustrating crucial changes in the Earth system during the Early–Middle Permian. The timescale is adopted from Gradstein et al. (2012). (1) Permian glacial intervals of eastern Australia according to Fielding et al. (2008) and Metcalfe et al. (2015). (2) Atmospheric pCO2 variations adapted from Montañez et al. (2007) (solid blue curve), Royer (2006) (brown dots), and Tabor et al. (2004) (vertical bar); pO2 variations adapted from Berner (2006). (3) Areal extent of lycopsid wetland forests in the tropics from Cleal and Thomas (2005). (4–5) Reef development, evolution, and crisis adapted from Kiessling (2002), Flügel and Kiessling (2002), and Weidlich (2002). (6) Latitudinal diversity gradient of brachiopod genera from Powell (2007). Road. and Word. are abbreviations for Roadian and Wordian, respectively.

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Fig. 3. Field photographs of the studied sections. A, a typical facies transition showing deepening-upward within the K3 cycle in the Tongle section. B, slump structures in chert-rich micrite limestones deposited in the lower slope to the toe of the slope facies, the K5 cycle in the Tongle section. C, overview of the Artinskian/Kungurian transition in the Wuzhuan section. D, the base of the Kungurian corresponds to an unconformity in the Tongle section, indicating a major relative sea-level drop at the Artinskian/Kungurian boundary in South China.

(similarly as in the case of the Roadian/Wordian boundary), we assigned seven cycles and a half to the Kungurian and three to the Roadian (Liu et al., 2017; this study). In this case, the durations of K1– K7, R1, and R2–R3 cycles are ~933, ~1050, and ~1167 kyr, respectively, in accordance with the international geological timescale (Gradstein et al., 2012). These durations were applied to calculate the average linear sedimentation rate (LSR) of each cycle as: LSR = thickness / duration, where the LSR is linear sedimentation rates in unit of cm × kyr−1. (Fig. 4; Appendix 1).

3.3. Geochemical analyses Carbon and oxygen isotope ratios were measured on powdered bulk carbonate samples using a Finnigan MAT 253 mass spectrometer at the Nanjing Institute of Geology and Palaeontology. Analytical error for δ13Ccarb and δ18Ocarb was better than 0.03‰ and 0.08‰, respectively. Total organic carbon (TOC) was determined by an Analytik Jena Multi EA 4000 C-S analyzer at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). Analytical precision was better than ± 0.1%. Major, trace, and redoxsensitive metal elements (e.g. Ni, Cu, Cr, Zn, and Cd) were analyzed at ALS Chemex (Guangzhou) Co Ltd. at Guangzhou, China. The concentration of major elements was measured by wave length-dispersive X-ray fluorescence (XRF) using a Philips PW2404 spectrometer and trace elements content was determined with a Perkin Elmer Elan 9000 inductively coupled plasma mass spectrometry (ICP-MS). Redox-sensitive metal elements were measured by a Varian VISTA inductively-coupled plasma-optical emission spectroscopy (ICP-OES) after digestion with HNO3–HF–HCl–HClO4. Analytical precision based on replicate analyses was better than ± 2% for major elements (detection limit is 0.01%),

and ±5% for trace and redox-sensitive metal elements (detection limits were 0.5–1 ppm for most elements).

3.4. Flux calculations of paleoproductivity and paleoredox proxies The utility of TOC, organic phosphorous (P), and biogenic barium (Babio) concentrations as potential proxies for marine paleoproductivity has been widely discussed and accepted (Dymond et al., 1992; Van Cappellen and Ingall, 1994; Föllmi, 1996; Tribovillard et al., 2006; Algeo and Ingall, 2007; Paytan and Griffith, 2007), and recently reviewed and improved by Schoepfer et al. (2015) and Shen et al. (2015). Here, we apply the mass accumulation rate (MAR) of these three proxies (denoted as OCAR, PAR, and BaAR) instead of the simple raw data (cf. Algeo et al., 2013; Shen et al., 2015), in association with the enrichment factor of selected trace metals (see below) and empirical formulae (PP18/21) from Cenozoic marine systems (Schoepfer et al., 2015), to evaluate the Early–Middle Permian transitional paleomarine primary productivity. We calculated the non-detrital/excess fraction of total Ba (i.e., Baxs; it is assumed to be equivalent to biogenic Ba here) as: Babio = Bat − Al × (Ba/Al)d, where Bat, Babio, and Al represent the total barium, biogenic barium, and aluminum concentrations of a sample, respectively. The detrital Ba/Al ratio (i.e., (Ba/Al)d) of the studied sections was estimated by the same means as described in Shen et al. (2015). Accordingly, the (Ba/Al)d values are 23 and 28 for the Tongle and Wuzhuan sections, with both values within the ranges reported from the upper Permian in South China (Shen et al., 2015). For paleoproductivity, the mass accumulation rates (MAR) of different proxies (TOC, P, and Babio), i.e. OCAR, PAR, and BaAR, respectively, were calculated as: XAR = Concentration X × ρ × LSR, where X is the proxy of interest (TOC, P, and Babio), ρ is bulk density of a sample (we

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where (X/Al)sample and (X/Al)PASS represent the values of one trace metal element of interest normalized to aluminum concentration of a sample and that of in the PASS, respectively. Although Al-normalization for marine deposits with a detrital fraction lower than 3–5% has some potential pitfalls (Van der Weijden, 2002; Tribovillard et al., 2006), they are not expected to affect our data substantially for three reasons: 1) the Al/Ti values in both sections are in accordance with those in the PASS ((Al/Ti)PASS = 14) (Appendix 2; Taylor and McLennan, 1985), indicating that considerable excess Al did not occur, 2) cross-plots of BaAl and V-Al show no obvious covariation (Appendix 2), implying that the trace metals of interests are predominately authigenic, as Ba and V are susceptive to detrital contamination and, 3) the stratigraphical variability patterns of non-normalized trace metals are highly coincident. TMEF values were subsequently used to calculate MARs of individual trace metal enrichment factors (TMEFAR; XEFAR = XEF × ρ × LSR), treated here as a proxy for paleoredox variations in this study. Raw data of TOC and major and trace elements, and calculated trace metal enrichment factor (TMEF) values are provided in Appendix 2. Fig. 4. Linear sedimentation rate variations against time or depositional cycles identified in the Tongle, Gongchuan, and Wuzhuan sections across the Kungurian through Roadian. Data of the Gongchuan section is from Liu et al. (2017).

4. Results 4.1. Depositional models, RSLs, and correlations

−3

assumed that all the samples share the same value of 2.6 g × cm ), and LSR is the linear sedimentation rate, as described earlier. Primary productivity estimates (i.e., empirical formulae of PP18/21) were calculated as:   PP18 ¼ 1000  104:10  TOC =ðρ  LSRÞ0:54  0:43 PP21 ¼ 108:55  OCAR Redox-sensitive trace metals generally become enriched in marine sediments under reducing conditions. U, V, Cu, Zn, Ni, and sometimes Cr are therefore used as indicators for paleoredox conditions (Algeo and Maynard, 2004; Brumsack, 2006; Tribovillard et al., 2006; Eldrett et al., 2014). Especially Cu and Ni are employed as markers of the original presence of organic matter, even if it is partially or totally lost after deposition (Tribovillard et al., 2006). We calculated trace metal enrichment factors (TMEF) normalized to aluminum and Post-Archean average Australian shale (PASS) (Taylor and McLennan, 1985; Tribovillard et al., 2006), which were calculated as: XEF = (X/Al)sample / (X/Al)PASS,

Fifteen rimmed-platform lithofacies were identified within the WZ and TL sections (see detailed descriptions and interpretations in Appendix 3). The bioclastic shoal, back-shoal open platform and tidal flat facies dominate the inner-platform WZ section, while the deeper-water TL section was deposited primarily in the upper and lower slope to basin (or basin margin) settings (Fig. 5). Lithofacies stacking patterns enabled us to reconstruct RSL fluctuations (cf. Liu et al., 2017). Reconstructed RSL curves from the GC, WZ and TL sections can be closely correlated throughout the Early–Middle Permian transition (Fig. 6). Due to multiple subaerial exposure events in the shallower WZ section, the K1–3 and even the bases of the K4 and K7 cycles are presumably incomplete (Fig. 6). 4.2. Carbon isotopic curves and correlations Analytical results and a cross-plot of carbon and oxygen isotope data from the TL and WZ sections are given in Appendix 4. They show no obvious covariation, as is also the case for the isotope data collected from the GC section (Liu et al., 2017). At the TL section, the δ13C values span the range of approximately 4‰, from + 2.1‰ to + 6.2‰. A positive peak of +6.1‰ is present in the latest Artinskian, followed by a sharp decrease by about 3‰ at the Artinskian/Kungurian boundary. After a

Fig. 5. A general rimmed-platform depositional model showing distributions of different lithofacies (“w” and “t”) identified in the Wuzhuan and Tongle sections. Detailed lithofacies descriptions are provided in Appendix 3.

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Fig. 6. Detailed correlations in the carbon isotope stratigraphy and reconstructed relative sea-level patterns based on lithofacies across the Gongchuan (Liu et al., 2017), Tongle, and Wuzhuan sections. Abbreviations: Artins. — Artinskian; Sta.-Ha. — Staffella-Hayasakina assemblage Zone; M. pamirica — Maklaya pamirica Zone; Ski.-Para. — Skinnerella-Parafusulina Zone.

short-term oscillation around +4.5‰, a prominent negative excursion (down to + 2.1‰) occurs at the K2/K3 boundary, with a recovery to +4.8‰ in the middle of the K3 cyclothem. Above, a general decline follows until the base of the K5 cycle. A long-term rise to +6.2‰, subsequently, occupies the remaining cycles in the Kungurian, i.e. it extends into the R1 cycle, near the Kungurian/Roadian boundary. From then on, although with multiple disturbances, carbon isotope values exhibit a descending trend until the maximum flooding surface of the R3 cycle. At the WZ section, the δ13C curve displays generally parallel trends with the TL section, but the values are in a narrower range (+2.0‰ to +4.6‰). Furthermore, the consistent steady values around +4.0‰ within the K1–K3 cycles and the unexpected decline in the K6 cycle seem to be affected by meteoric diagenesis, related to two exposure events (Fig. 6). Liu et al. (2017) have reviewed and discussed the Kungurian δ13C stratigraphy in low latitudes. Integrating δ13C data from this study and the biostratigraphically well controlled TQ section shows comparable δ13C patterns during the Cisuralian–Guadalupian transition (Tierney, 2010; Sun et al., 2017). 4.3. Linear sedimentation rates (LSRs) The LSRs of the deeper-water TL Section are roughly within the range of 1–2 cm × kyr− 1 within the study interval (Fig. 4). The shallower-water WZ and GC sections exhibit more variable ranges (from b 1 to about 4 cm × kyr−1). Although the LSR values are comparable for both WZ and GC sections, they are lower for the former section, pointing to more constrained accommodation space. It is worth mentioning that, below the K6 cycle, the TL section exhibits an opposite LSR trend and higher values, when compared with the WZ and GC sections, but from starting at the K6 cycle, all sections share the same trend, with LSR distribution patterns reversed (Fig. 4), i.e. the GC and WZ sections having higher accumulation rates. 4.4. TOC concentrations At the TL section, TOC values range from 0.3 to 7.0‰ (Fig. 7). A generally rising trend reaching 4.4‰ with fluctuations is present from the uppermost Artinskian to the K2 cycle, but it instantaneously drops by almost 4.0‰ at the K2/K3 boundary. Above, until the R1 cycle spanning

the Roadian/Kungurian boundary, a long-term ascending trend reaching 7.0‰ appears, with lower values found near the cycle boundaries (except for the K5/K6). This trend is terminated in coincidence with a RSL fall at the R1/R2 boundary. Above, only limited values around 2‰ are present within the study interval. TOC concentrations of the shallow-water WZ section are, in comparison, relatively steady at low levels below the K7 cycle, with abrupt and transient positive peaks reaching about 4‰ in the upper K3 and K6 cycles. Yet, similar trends and amplitudes with that of the TL Section develop during the K7 to R2 cycles. The slower decline during the R1/R2 transition is attributed to a temporarily higher sedimentation rate. 4.5. MARs of TOC, organic P, and Babio, and the paleoproductivity estimate (PP18/21) In this section, the MARs of TOC, organic P, and Babio (i.e. OCAR, PAR, and BaAR) are reported in mg × cm−2 × kyr−1 for the first two, and μg × cm−2 × kyr−1 for BaAR. The paleoproductivity estimate of PP18/21 is provided in mg × cm−2 × kyr−1. At the TL section, OCAR shows variations from 1.1 to 30.7 and is generally in line with the TOC trend (Fig. 7). Its maximum decrease (from 30.7 to 3.8) is recorded near the K7/R1 boundary, predating that of the TOC (R1/R2 boundary). BaAR ranges from 1.5 to 26.2. A decline from 18.9 to 8.1 in the uppermost Artinskian is followed by an increasing trend reaching a peak of 26.2 at the K2/K3 boundary. Above, another positive spike reaching 15.8 is found in the lower part of the K7 cycle, after a return to background values and multiple fluctuations. Near the K7/R1 boundary a prominent negative excursion occurs, with values around 10 in the Roadian. PAR displays values in the range of 0.10 to 0.78. Positive PAR excursions are mainly distributed in the lower K1 and K2 cycles, at the K4/K5 boundary, and within the K7 cycle. A sharp decline to background values (about 0.20) appears at the K7/R1 boundary. PP18/21 values, which are based primarily on OCAR, are within the range of 103 to 104. They consequently show similar trends either with TOC or with OCAR. At the WZ section, P concentrations for most samples are lower than the detection limit (Appendix 2). Within the K1–K6 cycles, both OCAR and BaAR exhibit flat trends with negligible disturbances (Fig. 8). OCAR begins to increase abruptly in the middle part of the K7 cycle and reaches its maximum of 55.4 in the upper part, but it goes back to

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Fig. 7. Integrated carbon isotope values, TOC contents, as well as paleoproductivity and paleoredox proxies across the Tongle section. The units of PP18/21, OCAR, PAR, and BaAR are provided in the text. Note that the scales of the top and bottom within the same column sometimes represent two distinct proxies.

background values at the K7/R1 boundary. Above, only a minor increase is found near the R1/R2 boundary. In contrast, BaAR reaches a spike of 52.3 earlier in the lower K7 cycle, and declines to a trough of 5.1 near the K7/R1 boundary. From then on, an increasing trend is present. Because PP18 is far beyond the productivity values for modern marine environments (103–105) (Longhurst et al., 1995), we only show PP21 here. PP21 values are within the range of 103 to 104, and display the same trend as OCAR (Fig. 8).

and near (or at) the K1/K2, K2/K3, K3/K4, and K6/K7 boundaries. At the WZ section, however, the K1–K6 cycles show no variability, with only slight enrichments near the Artinskian/Kungurian, K3/K4, and K6/K7 boundaries (Fig. 8). Above, apparent enrichments are found in the K7, middle R1 and R2 cycles. 5. Discussion

4.6. Mass accumulation rates of trace metal enrichment factors (TMEFAR)

5.1. Paleoredox and paleoproductivity status in the studied sections during the Kungurian

At the TL section, the MARs of enrichment factor of trace metals (i.e., U, V, Cu, Ni, Zn, and Cr) exert coincident enrichment patterns, although at different orders of magnitude (10–104) (Fig. 7). Prominent enrichments are found in the uppermost Artinskian, lower K1 and K7 cycles,

5.1.1. Paleoredox status: evidence from trace metal and lithofacies Multi-proxy trace metal (i.e., U, V, Cr, Zn, Cu, and Ni) show largely synchronous enrichment patterns (Figs. 7–8), allowing us to exclude primary and secondary overprints (e.g. remobilization during

Fig. 8. Integrated carbon isotope values, TOC contents, as well as paleoproductivity and paleoredox proxies across the Wuzhuan section. The units of PP21, OCAR, and BaAR are provided in the text. Trace metal element concentrations below the detection limit were excluded, so the XEFAR curves are not successive. Note that the scales of the top and bottom within the second last column represent two distinct proxies.

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diagenesis). Their enrichment in the sediment commonly results from oxygen-deficient conditions during deposition (Calvert and Pedersen, 1993; Brumsack, 2006; Tribovillard et al., 2006; Eldrett et al., 2014). Simultaneous enrichment in Zn, Ni, and Cu points to sulfate-reducing conditions prevailing at least in the sediment (i.e., below the water -sediment interface; Tribovillard et al., 2006), as a result of the tendency of these elements to be incorporated into pyrite after the decay of organic matter (Morse and Luther, 1999). At the deeper-water TL section, (sulfate-) reducing conditions in the sediment were present in the lowermost K1 cycle, around the K1/ K2, K2/K3, K3/K4, and K6/K7 cycle boundaries, as well as within the K7 cycle (Fig. 7). We attribute the sparse and faint trace metal enrichment signal below the K7 cycle at the WZ section to oxygenated shallow-water environments (Figs. 5–6). Within the K7 cycle, similar trace metal enrichment patterns with that of the TL section were present, indicating reducing conditions in the sediment. In general, (sulfate-) anoxic conditions below the water-sediment interface prevailed at least in the upper slope settings during the early Kungurian (K1–3) and in both deep lower slope–basin and shallow shoal environments during the Late Kungurian (K6–7). Under such conditions, oxygen deficiency is anticipated in the bottom water because of rapid consumption of excess O2 near the water-sediment interface. It is confirmed by the appearances of laminated, unbioturbated mudstones with abundant pyrite (Appendix 3-3H) in the TL section. Such laminated facies tends to indicate anoxia in the water column (Bond et al., 2004; Flügel, 2010), although its occurrence could not be confirmed in all anoxic intervals because of limited sampling resolution.

5.1.2. Paleoproductivity status The TL and WZ sections were little affected by terrestrial input within the study interval, as evidenced by very low aluminum and titanium concentrations (Appendix 2), indicating that the total P content and TOC were predominately of biogenic origin. The application of the P and Babio proxies for reconstruction of primary productivity is limited to oxic–suboxic conditions, as a result of intense P recycling and possibly barite dissolution and Ba migration under sulfate-reducing conditions (Ingall and Jahnke, 1994; Föllmi, 1996; Torres et al., 1996; Benitez-Nelson, 2000; Mort et al., 2007). Export productivity and subsequent final burial flux of organic matter (OM) have long been suggested to be generally proportional to surface-water productivity (e.g., Tribovillard et al., 2006), which impelled researchers to apply TOC measured from ancient sediments to reconstruct primary productivity (Kuypers et al., 2002; Algeo et al., 2011, 2013; Shen et al., 2015). However, preservation status of OM largely depends on redox conditions and sedimentation rate near the sediment-water interface (Canfield, 1994; Arthur et al., 1998; Arndt et al., 2013). At high rates of deposition, for instance, OM degradation can even be equally efficient under reducing and oxic conditions (Canfield, 1994). Fortunately, Ni and Cu have the property of being mainly delivered to the sediments in association with OM (Tribovillard et al., 2006). Thus, enrichment in Ni and Cu may be a more reliable indicator of primary productivity than both OCAR and empirical formulae derived from it (PP18/21) under certain conditions (e.g., high sedimentation rate). During anoxic intervals in the Kungurian (except for the K7 cycle) at the TL section, prominent enrichments in Ni and Cu are present regardless of BaAR and PAR variations (Fig. 7). However, it does not seem to imply high primary productivity levels,

Fig. 9. Integrated diagram illustrating the evolution of stable carbon and oxygen isotopes, selected organisms, and sedimentary records both in low and higher latitudes across the Early– Middle Permian transition. (1) The compound carbon isotope stratigraphy is based on this study (TL and WZ), Liu et al. (2017) (GC), and Tierney (2010) (TQ). (2) The three-point weighted running average curve of detrended δ18Obrachiopod values is adapted from Montañez et al. (2007). (3) The relative species diversity in fusulinids of the North American shelf is taken from Davydov (2014) and Davydov et al. (2016). (4) The conodont zonation and changes in the diversity/fauna associations of the Tieqiao section, South China, from Sun et al. (2017). The black dot and triangle are Neostreptognathodus prayi and Sweetognathus adjunctus zones. Abbreviations: Sw. asy. — Sweetognathus (Sw.) asymmetrica n. sp. Zone, Sw. guizhou. — Sw. guizhouensis Zone, Sw. iranic. — Sw. iranicus Zone, Sw. s. — Sw. subsymmetricus Zone, Sw. h. — Sw. hanzhongensis Zone, J. nankingensis — Jinogondolella nankingensis Zone. (5–6) The carbon and oxygen isotope data of well-preserved brachiopods from Australia adapted from Mii et al. (2012, 2013). The higher oxygen isotope values from western Australia within the grey shaded area were interpreted as being influenced by high salinity by Mii et al. (2013). The levels of the occurrences of glendonite are adapted from Eyles et al. (1998) and Jones et al. (2006). (7) The species diversity in brachiopods and bivalves of eastern Australia and New Zealand are adapted from Waterhouse and Shi (2013). (8) The changes in brachiopod associations and sedimentary records of northern peri-Gondwana adapted from Shen et al. (2002), Zhang et al. (2013) and Shen et al. (2016). B, Q, and T are abbreviations for the Baoshan, Qiangtang, and Tengchong blocks.

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indicated by depressed OCAR values. High primary productivity is usually concomitant with high OM burial rates under O2-depleted conditions as a result of enhanced preservation of OM under such settings (Emerson et al., 1985; Canfield, 1994; Kuypers et al., 2002; März et al., 2016). Although evidence from experiments and surface sediments revealed no significant differences in the preservation of OM under varying O2 concentrations (Calvert and Pedersen, 1992a; Lee, 1992), it is unclear whether this observation can be applied to ancient sediments (Canfield, 1994). Within the K4–6 cycles, rare synchronous increases in OCAR (or PP18/21), BaAR, and PAR point to a lack of increases in primary productivity. In contrast, within the K7 cycle of both TL and WZ sections all proxies (i.e., OCAR, PP18/21, BaAR, PAR, Cu, and Ni) exhibit substantial enrichments (Figs. 7–8), indicating very high primary productivity levels in surface waters. This condition, however, was abruptly terminated during the latest Kungurian near the K7/R1 boundary (see further discussion). 5.2. Possible triggers for the prevailing anoxia of South China 5.2.1. Roles of relative sea level, sedimentation rate and productivity Rapid sea-level rise commonly results in O2-deficient conditions near the water-sediment interface (i.e., in the bottom water and sediment) (Jenkyns, 1991; Schlanger and Jenkyns, 2007; Bond and Wignall, 2008). This mechanism might explain reducing conditions at the base of the K1 cycle and within the K7 cycle in the deeper-water TL section (Fig. 7). The remaining anoxic phases, nevertheless, only roughly correspond to cycle boundaries, i.e. RSL drops. This, in association with the fact that anoxia below the water-sediment interface developed in parallel in persistent shallow-water environments within the K7 cycle at the WZ section (Figs. 6, 8), implies that other factors, apart from sea-level changes, contributed to the paleoredox evolution

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with the Youjiang Basin. High sedimentation rate and productivity both can promote increased export and burial rates, which in turn prevents aerobic bioturbation and OM decay (Ingall and Van Cappellen, 1990; Canfield, 1994; Kuypers et al., 2002; Arndt et al., 2013). In addition, under such circumstances O2-depletion is always concomitant near the water-sediment interface because of rapid consumption of excess O2 in the process of deposition (Calvert and Pedersen, 1992b). The last phase of anoxia within the K7 cycle is here proposed to have been generated in such settings (Figs. 4, 7–8). The Chihsia Formation of South China is commonly characterized as organic-rich and has long been interpreted as deposited in predominantly oxygen-depleted conditions, based on both geochemical and sedimentological investigations (e.g., Yan and Liu, 2007; Wei et al., 2012; Liu et al., 2014). Consequently, O 2 -deficient conditions prevailed in South China during the Kungurian. Although they have been widely interpreted as resulting from high productivity (e.g., Yan and Liu, 2007; Wei et al., 2012), it cannot account for all intervals of O2-deficiency, as shown in this study. The Kungurian conodont record from the TQ section, South China (Fig. 9; Sun et al., 2017) and fusulinid species diversity from the North American shelf (Fig. 9; Davydov, 2014; Davydov et al., 2016) both show a closer correspondence with redox dynamics in this study than with the negative anomaly in tropical seawater temperature (Montañez et al., 2007). This might indicate that O2-deficient environments in low latitudes during the early (K1–3) and late (K6–7) Kungurian expanded in shelf areas also outside South China, affecting benthic fauna in other regions. This needs to be tested by further sedimentological and geochemical research in North America and other low-latitude regions. Altogether, the above observations compel us to explore more powerful controlling factors in addition to sea-level changes, sedimentation rate and productivity.

Fig. 10. Stratigraphical correlations of well-studied and constrained formations across Gondwana and South China (this study) during the late Artinskian through Roadian. (1) Eastern Australian data adapted from Fielding et al. (2008) and Metcalfe et al. (2015). (2) Western Australian data adapted from Mory and Backhouse (1997), Eyles et al. (2002, 2006), and Mory et al. (2008). (3) Antarctic data adapted from Isbell and Cúneo (1996) and Isbell et al. (1999, 2008). (4) South American data adapted from Santos et al. (2006), Iannuzzi et al. (2010), Holz et al. (2010), and Limarino et al. (2014). (5) South African data adapted from Turner (1999), Catuneanu et al. (2005), Césari (2007), Wopfner and Jin (2009), and Ruckwied et al. (2014). (6) Indian data adapted from Gupta (1999), Ray and Chakraborty (2002), and Mukhopadhyay et al. (2010). (7) Northern peri-Gondwanan (Baoshan, Tengchong, and Qiangtang blocks) data adapted from Wang et al. (2001), Shen et al. (2002), Zhang et al. (2013), and Shen et al. (2016). (8) Data from South China (this study).

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5.2.2. Global patterns: divergent climate trends between the tropics and Gondwana Paleoclimate reconstructions for the early Permian have been impeded by the lack of precise constrains on Gondwanan stratigraphy (e.g., Retallack et al., 1998; Isbell et al., 1999; Isbell, 2003; Fielding et al., 2008). Permian palynology-based biostratigraphy has emerged as the most important tool in terrestrial correlations in Gondwana. It has been revised to some extent thanks to the significantly advancing improvements in U-Pb geochronological calibration around the studied interval (Turner, 1999; Santos et al., 2006; Césari, 2007; Mori et al., 2012; Metcalfe et al., 2015; Nicoll et al., 2015). This, combined with biostratigraphic data derived from marine index taxa (e.g., conodont, fusulinid, and brachiopod) (Shen et al., 2002; Waterhouse and Shi, 2013; Zhang et al., 2013), encouraged us to attempt to correlate well studied and

biostratigraphically constrained Gondwanan formations of both terrestrial and marine realms with those in South China from the late Artinskian through Roadian (Fig. 10). We compiled classical as well as less widely used potential climate indicators (cf. Rees et al., 2002; Tabor and Poulsen, 2008; Boucot et al., 2013), such as occurrences of coal, plants in coal, transgression, dropstones, glendonite, evaporites, bauxite, kaolinite, and changes in marine biotic assemblages. These occurrences are plotted on reconstructed paleogeographic maps from the late Artinskian to Kungurian (Fig. 11). A prominent warming climate trend during the Kungurian emerges from this compilation in both terrestrial and marine settings. This warming trend, on the other hand, is independently reinforced by a decline in oxygen isotope values of well-preserved brachiopods from Australia (Fig. 9; Mii et al. 2012, 2013) and an increasing trend in brachiopod and bivalve species

Fig. 11. Reconstructed paleogeographic maps (Mollweide projection with 45° longitude lines) of the Southern Hemisphere exhibiting classic and potential climate indicators across Gondwana during the Late Artinskian to Kungurian. Geographic maps are modified from Rees et al. (2002) and Zhang et al. (2013). Most climate indicators are collected from the references provided in the Fig. 10, and the remaining adapted from Oelofsen (1987), Dickins (1996), Powell (2007), Clapham and James (2008), Srivastava et al. (2011), and Waterhouse and Shi (2013).

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diversity (Fig. 9; Waterhouse and Shi, 2013) with an increasing proportion of greenhouse-type associations (Clapham and James, 2008) from eastern Australia and New Zealand. At the same time, the tropical realm simultaneously underwent climate cooling, indicated by a negative seawater temperature anomaly (Montañez et al., 2007). Moreover, the coincident drop in brachiopod generic diversity (Fig. 1; Powell, 2007) and depressed reef development (Fig. 1; Flügel and Kiessling, 2002; Kiessling, 2002; Weidlich, 2002) could be largely linked to falling seawater temperatures (Rohde, 1992; Roy et al., 1998), although O2-deficiency should be also considered as a driver. We propose a hypothesis that divergent climate trends between the tropics and Gondwana were present during the Kungurian. Albeit the mechanisms behind this divergence are unclear, it would be expected to decrease the latitudinal gradient in sea surface temperature (SST). Further consequences would include a weaker ocean circulation mode (i.e., ocean stagnation) (Hotinski et al., 2001; Kiehl and Shields, 2005) and expansion of oxygen minimum zones (OMZ) (Karstensen et al., 2008; Keeling et al., 2010). The widespread anoxia in South China and probably other low-latitude regions are then interpreted as resulting from ocean stagnation and expansion of OMZs during the Kungurian. The presence of glendonite in the Pebbley Beach Formation near the Artinskian/Kungurian boundary and in the Roadian Wandrawandian Siltstone in eastern Australia (Eyles et al., 1998; Frank et al., 2008) reflect probably normal ocean circulation before and after the Kungurian stagnation, considering the glendonite formation was interpreted to be triggered by cold upwelling waters (Jones et al., 2006). Although an alternative mechanism (methane oxidization) of glendonite formation have been also proposed (Teichert and Luppold, 2013), the carbon isotope values of the Australian glendonites discussed in this fragment are generally in line with those of the host organic-matter-rich sediments (Frank et al., 2008), excluding a methane oxidization explanation. 5.3. The demise of anoxia during the latest Kungurian (K7/R1 boundary) in South China The demise of anoxia in the latest Kungurian coincided with a RSL drop and a sharp decrease in paleoproductivity and sedimentation rate in South China (Figs. 6–8). Even though two reducing pulses persisted into the Roadian at the WZ section, they were probably linked to transgressions (Fig. 8). Besides, the enrichment in trace metals might represent signals of restricted water mass, evidenced by deposition above the K7 cycle taking place predominately in a back-shoal setting. The substantial burial of organic carbon within the K7 cycle is also mirrored by a prominent increase in δ13C (Fig. 9; Kump and Arthur, 1999). This rising trend can be traced outside of the South China in the Orogrande Basin (Koch and Frank, 2012) and Venezuelan Andes (Laya et al., 2013), presumably indicating a signal of tropic-wide high primary productivity. Both high productivity and ocean deoxygenation (i.e., expansion of OMZs) in the tropical realm would underpin a higher atmospheric pO2 levels by intense photosynthesis and depressed OM decomposition. This is in line with the reconstructed pO2 curve based on combined modelling study (Fig. 1; Berner, 2006). However, as the latitudinal gradient in SST recovered following the gradual rebound in tropical seawater temperature anomaly (Montañez et al., 2007), vigorous ocean circulation, in combination with high atmospheric pO2 levels, depressed productivity, a sea-level drop (this study; Haq and Schutter, 2008) and low sedimentation rates, all are proposed here to have contributed to a complete demise of anoxia in South China during the latest Kungurian. 6. Conclusions Based on a detailed carbon isotope stratigraphical, sedimentological, and geochemical study across two Early–Middle Permian transitional successions in the Youjiang Basin, South China, the following conclusions can be drawn:

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1) The third-order depositional cycles in the Kungurian can be traced across different sedimentary facies in the Youjiang Basin. This enables us to apply them to build a basin-wide, high-resolution stratigraphic framework and further calculate linear sedimentation rates; 2) Widespread anoxia below or near the water-sediment interface occurred in South China during the Kungurian. High primary productivity, high sedimentation rate, and a relative sea-level rise were responsible for the late Kungurian anoxia (K7); 3) We propose a hypothesis that divergent climate trends between the tropics and Gondwana might have played an important role in ocean stagnation and prevailing O2-deficiency in and probably out of South China during the Kungurian; 4) The complete demise of the prevailing anoxia resulted from a couple of coincident events during the latest Kungurian (K7/R1 boundary), including vigorous ocean circulation, high atmospheric pO2 concentrations, a sea-level drop, depressed primary productivity and low sedimentation rates.

Acknowledgements We thank the Editor Ian Somerville (University College Dublin) and an anonymous reviewer for their constructive comments. Many colleagues helped with field and lab work, including Gang Lu (Guilin), Jianghai Yang, Rong Chai, Xin Yu, Yaguan Zhang, Zihu Zhang (Wuhan), Xiaoming Chen, and Yuping Wu (Nanjing). This study is supported by the Natural Science Foundation of China (Grant No. 41272120), the “111 Project” of China University of Geosciences (Wuhan), and a scholarship from Chinese Scholarship Council (CSC). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gr.2017.05.011.

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