Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Carbon and sulfur isotopic anomalies across the Ordovician–Silurian boundary on the Yangtze Platform, South China

Detian Yan a,c, Daizhao Chen b,⁎, Qingchen Wang c, Jianguo Wang b, Zhuozhuo Wang b a Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China b Key Laboratory of Petroleum Geophysics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China c State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info abstract

13 Article history: Abundance of organic matter, carbon isotopic compositions of organic matter (δ Corg) and sulfur isotopic Received 27 February 2008 34 compositions of iron sulfide (δ Ssulfide) across the Ordovician–Silurian (O–S) boundary was analysed from Received in revised form 22 November 2008 two sections (Wangjiawan, western Hubei; Nanbazi, northern Guizhou) on the Yangtze Platform, South Accepted 23 December 2008 China. The organic abundance across the O–S boundary at these two sections is generally high, except for the Hirnantian interval. At Wangjiawan section in western Hubei, the δ13C values vary from about −30.8‰ Keywords: org VPDB in the mid-Ashgill to −27.6‰ in the upper Hirnantian interval, which abruptly return to the pre- Stable isotopes δ13 − ‰ − ‰ Carbon–sulfur isotope Hirnantian values. At Nanbazi section in northern Guizhou, Corg values vary from 30.5 to 26.6 from Mass extinctions the mid-Ashgill to the upper Hirnantian horizons, which shift negatively to the low spike at −29.2‰ in the Climate change lowermost Rhuddanian, then increase slightly to relatively persistent values around −28‰. Similar variation 34 Anoxia patterns of δ Ssulfide values are unravelled at the two sections, showing a positive excursion from the mid- Ordovician–Silurian boundary Ashgill to the upper Hirnantian, then a sharp negative shift in the lowermost Rhuddanian, although with Yangtze Platform 34 different background values of δ Ssulfide from these two sections. These data, together with those from other areas, demonstrate large climatic fluctuations from warming to cooling, then to warming, oceanic changes from and anoxic to oxygenated, to anoxic water columns, during the O–S transition. Climatic fluctuations, together with multiple oceanic anoxia and sea-level fluctuations, were likely responsible for the stepwise massive demise of the O–S biotic crisis. © 2009 Elsevier B.V. All rights reserved.

1. Introduction corroborated in the Hirnantian shales as well (e.g., Melchin and Holmden, 2006). By analogy to carbon cycles, sulfur reservoir sizes The Ordovician–Silurian (O–S) transition was a critical interval in and redox variations within the sedimentary sulfur cycle account for geological history, during which profound biotic, climatic and tectonic the observed variations in δ34S values (Burdett et al., 1989; Strauss, changes occurred (Brenchley et al., 1994, 2003; Kump et al., 1999; 1997). Until now, a few comparable sulfur isotopic studies across Wang et al., 1997; Finney et al., 1999; Chen et al., 2004); two phases of several critical boundaries were conducted (Geldsetzer et al., 1987; extinction events, coincided with the major sea-level changes, were Kajiwara et al., 1994; Joachimski et al., 2001; Newton et al., 2004; Fike, identified (Brenchley, 1988). Several hypotheses, including bolide 2007; Gill et al., 2007; Riccardi et al., 2007). In this paper, we present impact (Wilde et al., 1986; Goodfellow et al., 1992), productivity new carbon and sulfur isotopic chemostratigraphic data across the O– collapse (Brenchley et al.,1994) and weathering processes (Kump et al., S boundary, which provide useful clues to understand the tremendous 1999), have been proposed to be responsible for this particular geologic climatic, oceanic changes and relevant biotic crisis during this critical event. The ultimate cause, however, is still highly controversial. transition. Extensive studies revealed a large-magnitude positive carbon excursion (up to 7‰ PDB locally) in the Hirnantian stage (N. 2. Geological setting extraordinarius–N. ojsuensis zone) of the latest Ordovician (e.g., Brenchley et al., 1994, 2003; Marshall et al., 1997; Finney et al., South China block appears to have been operated as a distinct plate 1999; Kump et al., 1999; Kaljo et al., 2004). An organic-carbon isotopic during the mid-Paleozoic, but it was still attached to the margin of excursion of similar to somewhat larger magnitudes has been Gondwana during the Late Ordovician to Early Silurian (Fig. 1A) (Metcalfe, 1994). One of the most important features of this tectonic unit in the Ordovician was that the Yangtze Platform, covered by a ⁎ Corresponding author. broad epeiric sea, was bordered southeast by the deep Pearl River Sea, E-mail address: [email protected] (D. Chen). such that it was likely connected more or less to the open ocean

0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.12.016 D. Yan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39 33

Fig. 1. Late Ordovician paleogeography of the South China blocks. (A) Location of South China on the reconstructed global paleogeographic map (after Scotese and McKerrow,1990). (B) Proposed Late Ordovician paleogeography of China, after Mu et al. (1981). (C) Details of the Yangtze Platform, after Wang et al. (1997). Section localities: 1 — Wangjiawan; 2 — Nanbazi.

(Fig. 1). Regional palaeogeographical reorganization during the O–S Hirantian fauna. Exact descriptions of these two sections (Wangjiawan transition was coincided with eustatic sea-level changes (Zhang et al., and Nanbazi) on their stratigraphy and palaeogeography can be seen 2000; Chen et al., 2004). The Yangtze Platform was divided into the in Wang et al. (1993a,b), Chen et al. (1999), Rong et al. (1999) and Su western Upper Yangtze Platform and the eastern Lower Yangtze et al. (2003). Platform, which were separated by the Jiujiang Strait (Mu et al., 1981). As sea level dropped, the Yangtze Platform, especially the Upper 3. Analytical methods Yangtze Platform, was largely enclosed (Wang et al., 1997; Chen et al., 2004). Fresh samples were extracted and crushed into powders less than The graptolitic black shale facies predominated over the Yangtze 200 mesh in size, ready for analysis of total organic carbon content 13 34 Platform during the O–S transition. Mu et al. (1981) first identified the (TOC), δ Corg and δ Ssulfide ratios. TOC was measured by High TOC II Ashgillian (Wufeng Formation) graptolite zones in this region. The analyzer. 13 Hirnantian Substage defined here spans, in ascending order, from the Sample splits (300 mg to 1.5 g) for δ Corg analysis were firstly N. extraordinarius–N. ojsuensis zone to the N. persculptus zone (Rong dissolved with 5 N HCl in a centrifuge beaker to remove carbonates et al., 2000). Under this circumstance, the lowermost part of the through multiple acidification (at least two times) and subsequent Longmaxi Formation that bears fossil elements of the N. persculptus drying in the heating oven, and repeatedly rinsed with deionized zone is placed in the uppermost Hirnantian Substage, above which is water to neutrality. The decalcified samples (30 to 110 mg)+CuO wire the proposed boundary demarcating the Ordovician and Silurian (1 g) were added to a quartz tube, and combusted at 500 °C for 1 h and (Chen et al., 2000). In this paper, the biostratigraphic nomenclatures of 850 °C for another 3 h. Isotopic ratios were analyzed using

Chen et al. (2000) are adopted. cryogenically purified CO2 on a Finnigan MAT-253 mass spectrometer, Biostratigratigraphically well-constrained sections from two dif- and reported in standand δ-notation relative to Vienna Peedee 13 ferent localities in the Yangtze basin were chosen for chemostrati- Belmnite (VPDB) standard. Analytical precision for δ Corg is better graphic studies. They are the Wangjiawan section at Yichang, Hubei than±0.06‰. 34 province, and the Nanbazi section at Tongzi, Guizhou province. Rong The sample split (1–2g)forδ Spy was treated with 100 ml CrCl2+ et al. (1999) and Chen et al. (1999) have suggested that the GSSP 10 ml alcohol in the vessel. The hydrogen sulfide was immediately candidate sections of the Hirnantian Substage can be established at purged by the N2 stream, and was trapped and collected in the zinc Wangjiawan, Yichang, Hubei province, and Honghuayuan at Tongzi, acetate (ZnAc) solution, in which 2% AgNO3 +6NNH4OH solutions were Guizhou province. The better-preserved Nanbazi section that suffered added to precipitate Ag2S, then it was mixed with V2O5 +pure quartz less weathering, ~500 m southeast of Honghuayuan, is thus selected sands, and combusted up to 1050 °C in the presence of copper turnings for the chemostratigraphic study in view of same depositional under vacuum to convert to SO2 gases. The purified SO2 was used to successions. The depositional successions are dominated by black analyze the sulfur isotopic ratio in the Finnigan Delta-S mass spectro- shales intercalated with an argillaceous limestone horizon bearing meter. Sulfur isotopic ratios are expressed as standard δ-notation 34 D. Yan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39

Fig. 2. Carbon-sulfur isotopic chemostratigraphic profiles across the O–S boundary at Wangjiawan, Yichang, Hubei province, South China. Biostratigraphic framework is based on Chen et al. (2000) and Su et al. (2003).

Fig. 3. Carbon-sulfur isotopic chemostratigraphic profiles across the O–S boundary at Nanbazi, Tongzi, Guizhou province, South China. Biostratigraphic framework is based on Chen et al. (2000) and Su et al. (2003). See Fig. 2 for rock legends. D. Yan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39 35

13 relative to Canon Diablo Troilite (CDT) standard. The analytical precision lowed by a drastic drop in δ Corg values of about 2.6‰ to the trough is better than ±0.2‰. at −29.2‰ in the uppermost Hirnantian, which bounce back slightly to about −28‰ in the C. vesiculosuc zone and generally persist 34 4. Results upwards. Although a systematic offset of δ Ssulfide value is apparent from the two sections: relatively negative (avg. −13.5‰)at 34 Forty-one and thirty-seven samples were analysed for TOC Wangjiawan and positive (avg. 1.3‰) at Nanbazi, positive δ Ssulfide 13 34 abundance, δ Corg and δ Ssulfide values across the O–S boundary excursions, starting from the P. pacificus zone and reaching the peak succession at Wangjiawan and Nanbazi sections, respectively (Figs. 2 in the N. extraordinarius zone (7.1‰ at Wangjiawan, 24.6‰ at and 3; Tables 1 and 2). Nanbazi), are apparent both at these two sections (Figs. 2 and 3). 34 At Wangjiawan, the TOC abundance varies generally between 0.35 Going upwards, δ Ssulfide values decrease sharply and reach the and 9.32 wt.%, with the lowest abundance in the upper Hirnantian. trough (−27.5‰ at Wangjiawan, −19.2‰ at Nanbazi) in the lower 13 δ Corg values vary commonly between −30.8 and −27.6‰ showing a Rhuddanian, which increase slightly upwards (Figs. 2 and 3). SEM positive excursion starting from the lower P. pacicicus zone and observations revealed that framboidal pyrite is the common and reaching the maximum (−27.6‰) in the uppermost part of the N. major sulfur carrier in these rocks; these framboidal aggregates, extraordinarius zone in the upper Hirnantian at this locality. Going generally 2 to 5 μm in diameter, consist of cubic pyrite microcrystals 13 upwards, the δ Corg values decrease rapidly from the N. persculptus smaller than 1 μm(Fig. 4). zone in the uppermost Hirnantian through the P. acuminatus zone. 13 Further upwards, the δ Corg values display relatively consistent 5. Discussion values between −30.1 and −30.0‰ from the O. vesiculosus through the C. cyphus zones. Our data derived from two studied sections as described above 13 At Nanbazi, the TOC contents are highly variable, but are gene- clearly show a positive δ Corg excursion (“heavy-C event”), which is 13 rally low in the Hirnantian horizon as well (Fig. 3; Table 2). The δ Corg followed by a rapid drop and a subsequent bounce across the O–S 34 values in this section show a positive excursion from −30.5‰ in the boundary. A parallel variation in δ Ssulfide values is well demonstrated D. complexus zone to the zenith of −26.6‰ near the top of the at the two sections as well (Figs. 2 and 3). The positive excursions in 13 34 N. extraordinarius zone of the upper Hirnantian (Fig. 3); this is fol- δ Corg and δ Ssulfide values are estimated to have lasted for about

Table 1 Isotopic compositions of organic carbon and pyrite sulfur from Wangjiawan section, western Hubei, South China

13 34 Sample Log height Graptolite zone Lithology TOC δ Corg δ Ssulfide No. (m) (%) (‰, VPDB) (‰, CDT) 10 0.05 D. complexus Shale 2.4 −30.74 −16.36 10+ 0.2 D. complexus Shale 2.45 −30.79 −17.28 11 0.35 D. complexus Shale 2.58 −30.83 −18.72 12 0.6 D. complexus Shale 4.48 −30.61 −17.7 13 0.85 P. pacificus Shale 4.2 −30.69 −22.34 14− 1.05 P. pacificus Shale 4.19 −30.28 −22.51 14 1.15 P. pacificus Shale 4.2 −29.88 −23.61 15 1.4 P. pacificus Shale 3.05 −30.45 −18.7 16 1.55 P. pacificus Shale 3.64 −29.62 −19.16 17 1.7 P. pacificus Shale 4.73 −30.72 −18.73 18 1.85 P. pacificus Shale 5.31 −30.31 −17.15 19 2 P. pacificus Shale 2.8 −29.31 −11.8 19+ 2.08 P. pacificus Shale 7.18 −29.44 −8.01 20 2.15 P. pacificus Shale 9.32 −30.01 −7.33 21 2.3 N. extraordinarius–N. ojsuensis Carbonaceous mudstone 7.2 −29.64 −1.18 22 2.4 N. extraordinarius–N. ojsuensis Carbonaceous mudstone 4.81 −29.15 −9.52 23 2.55 N. extraordinarius–N. ojsuensis Carbonaceous mudstone 4.15 −29.22 −1.93 24 2.65 N. extraordinarius–N. ojsuensis Carbonaceous mudstone 3.73 −29.4 −19.35 25 2.75 N. extraordinarius–N. ojsuensis Carbonaceous mudstone 2.93 −29.33 7.11 26 2.8 N. extraordinarius−N. ojsuensis Carbonaceous mudstone 1.74 −29.09 −16.06 27 2.85 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.35 −28.68 1.48 27+ 2.9 N. extraordinarius−N. ojsuensis Argillaceous limestone 0.6 −27.72 6.22 28 2.93 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.62 −27.64 −1.74 29 3 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.55 −29.21 5.34 30 3.02 N. persculptus Shale 6.65 −28.94 −14.1 31 3.07 N. persculptus Shale 7.72 −29.04 −18.87 32 3.15 N. persculptus Shale 7.65 −30.17 −21.88 33 3.25 N. persculptus Shale 7.69 −30.01 −17.67 34 3.35 A. ascensus Shale 4.98 −30.29 −17.02 35 3.45 A. ascensus Shale 5.76 −30.33 −16.83 36 3.6 P. acuminatus Shale 4.71 −30.48 −19.12 37 3.8 P. acuminatus Shale 2.86 −28.96 −27.46 38 4 C. vesiculosus Shale 2.68 −30.13 −22.32 39 4.2 C. vesiculosus Shale 3.97 −30.16 −21.18 40 4.4 C. vesiculosus Shale 3.45 −29.45 −21.85 41 4.65 C. cyphus Mudstone 4.05 −30.13 −15.92 42 4.95 C. cyphus Mudstone 4.98 −30.07 −17.96 43 5.2 C. cyphus Mudstone 4.57 −30.01 −13.06 44 5.5 C. cyphus Silty mudstone 4.98 −30.06 −4.75 44+ 5.65 C. cyphus Silty mudstone 5.07 −29.98 −7.15 45 5.8 C. cyphus Silty mudstone 5.18 −30.01 −9.06 36 D. Yan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39

Table 2 Isotopic compositions of organic carbon and pyrite sulfur from Nanbazi section, northern Guizhou, South China

13 34 Sample Log height Graptolite zone Lithology TOC δ Corg δ Ssulfide No. (m) (wt.%) (‰, VPDB) (‰, CDT) 6 0.3 D. complexus Shale 5.72 −30.48 −13.52 7 0.6 D. complexus Shale 4.74 −30.46 −6.29 81.15D. complexus Shale 5.06 −30.42 −11.21 9 1.95 D. complexus Shale 4.95 −30.29 – 10 2.85 D. complexus Shale 4.21 −30.26 – 11 3.75 P. pacificus Shale 4.74 −30.29 −15.83 12 4.55 P. pacificus Shale 5.25 −29.94 −13.02 13 5.25 P. pacificus Shale 5.25 −29.87 – 14 5.75 P. pacificus Shale 6.63 −29.74 −11.01 15 6.25 P. pacificus Shale 10.42 −29.12 0.12 16 6.55 P. pacificus Shale 10.21 −28.9 0.93 17 6.8 N. extraordinarius–N. ojsuensis Shale 6.62 −28.8 8.98 18 6.95 N. extraordinarius–N. ojsuensis Argillaceous limestone 4.21 −28.69 3.21 19 7.15 N. extraordinarius–N. ojsuensis Argillaceous limestone 1.98 −28.7 5.98 20 7.5 N. extraordinarius–N. ojsuensis Argillaceous limestone 3.02 −28.82 16.58 21 8.05 N. extraordinarius–N. ojsuensis Argillaceous limestone 1.23 −28.58 14.78 22 8.45 N. extraordinarius–N. ojsuensis Argillaceous limestone 1.34 −28.47 17.6 23 9 N. extraordinarius–N. ojsuensis Mudstone 1.57 −28.39 11.01 24 9.5 N. extraordinarius–N. ojsuensis Mudstone 2.09 −28.5 20.79 25 10.05 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.38 −28.08 10.37 26 10.25 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.76 −28.09 9.37 27− 10.55 N. extraordinarius–N. ojsuensis Mudstone 1.76 −28 18.33 27 10.8 N. extraordinarius–N. ojsuensis Mudstone 0.51 −27.1 20.45 28 11.1 N. extraordinarius–N. ojsuensis Mudstone 0.56 −28.08 24.59 29 11.6 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.26 −26.85 14.21 30 11.9 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.14 −27.17 13.21 31 12.2 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.12 −26.63 9.24 32 13.25 N. extraordinarius–N. ojsuensis Argillaceous limestone 0.09 −27.48 4.66 33 13.5 ? Mudstone 8.45 −28.99 −15.79 34 13.85 ? Mudstone 2.14 −29.23 −18.7 35 14.25 ? Mudstone 1.52 −29.05 −19.21 36 14.45 C. vesiculosus Silty mudstone 0.47 −28.25 −15.32 37 14.95 C. vesiculosus Silty mudstone 0.6 −28.34 – 38 15.45 C. vesiculosus Silty mudstone 0.51 −28.03 −13.17 39 16.45 C. cyphus Silty mudstone 0.37 −28.02 −5.14 40 17.6 C. cyphus Silty mudstone 1.31 −28.29 −11.37 41 18.6 C. cyphus Silty mudstone 0.51 −28.17 −12.21

1 m.y., given that each graptolite zone lasted for 0.5 m.y. in duration to preferential removal of 12C at least from surface waters. The low (Mu et al., 1981; Brenchley et al., 1994). abundance of organic carbon within this excursion horizon (Figs. 2 13 The positive δ Corg excursion and subsequent variations across and 3) excludes the possibility of high productivity during the the O–S boundary reported here are unlikely of local signatures, since deposition. On the other hand, the high abundance of organic carbon 13 13 this positive excursion both in δ Ccarb and δ Corg values associated below the Hirnantian horizon points to an enhanced burial of organic with the Hirnantian interval has been widely reported from a number matter likely in much more anoxic water columns (Yan et al., 2008), of sections around the world, including Siljan area, central Sweden during which the warm climate (high pCO2 level) could have (Marshall and Middleton, 1990), Mackenzie Mountains (Wang et al., accelerated photosynthetic carbon fixation of marine phytoplankton, 13 1993a,b) and Anticosti Island, Canada (Long, 1993), Scotland (Under- leading to lower values of δ Corg (Freeman and Hayes, 1992; Rau et al., wood et al., 1997), South China (Wang et al., 1997), Nevada, USA 1997) as observed (Figs. 2 and 3). The progressive increase in burial of (Finney et al., 1999; Kump et al., 1999), and Estonia and Latvia organic matter could have led to substantial removal of 12C from the (Brenchley et al., 2003; Kaljo et al., 2004). Although this δ13C excursion carbon reservoir, resulting in the climate cooling (even glaciation) and is widespread, the timing of the peak may not be completely a positive excursion of δ13C(Kump and Arthur, 1999) during the synchronous between different regions (Melchin and Holmden, Hirnantian. During the Hirnantian glaciation, the low pCO2 level, on 13 2006). It is also interesting to note that a δ Corg excursion of similar the other hand, could have decreased photosynthetic carbon fixation 13 12 to somewhat larger magnitude than the δ Ccarb excursion is present of marine phytoplankton, through which less C would be incorpo- 13 (Melchin and Holmden, 2006). rated into the organic carbon, leading to the heavy δ Corg values The abrupt shift in carbon isotope values indicates a significant (Freeman and Hayes, 1992; Rau et al., 1997). The subsequent rapid rise change in the relative carbon fluxes between ocean water, organic in sea-level, and recurrences of oceanic anoxia and warm climate, matter and sediments. Any model of carbon cycle must be consistent probably triggered by unidentified forcing, resumed the scenarios as with the temporal relationships among carbon cycle, sea-level fall, those prior to Hirnantian glaciation, in which the organic matter was and temperature change identified here (Brenchley et al., 2003). Kump massively accumulated and buried in parallel with a sharp negative 13 13 et al. (1999) suggested that the positive δ C excursion was the result excursion of δ Corg as observed (Figs. 2 and 3) as a result of the of increased carbonate-platform weathering during glacioeustatic acceleration of carbon fixation of marine phytoplankton under a high sea-level lowstand. Whether or not the changes in weathering pCO2 level. 34 processes could trigger the glaciation event, however, is still highly The δ Ssulfide values of sulfides precipitated on the earth surface controversial (Melchin and Holmden, 2006). In an alternative model, are dependent on the fractionation of bacterial sulphate reduction in Brenchley et al. (1994, 2003) proposed that the positive δ13C shift was water media, which is commonly related to the oxygen level, sulphate caused by enhanced burial of carbon or enhanced productivity leading and iron concentrations of water columns (Canfield and Thamdrup, D. Yan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39 37

Fig. 4. Scanning electron microscope (SEM) photomicrographs of pyrite from studied samples: (A) cluster of well developed octahedral crystals in sample 5 from Wangjiawan; (B) framboidal pyrite made up of uniform microcrystals in sample 23 from Wangjiawan; (C) pyrite framboids in sample 38 from Wangjiawan; (D) pyrite framboids in sample 18 from Nanbazi.

34 34 1994; Habicht et al., 1998; Strauss, 1999). The offset of δ Ssulfide values pattern with the anoxic periods, and heavy δ Ssulfide pattern with between Wangjiawan and Nanbazi (Figs. 2 and 3) may reflect the ventilated periods, suggests that the oxygen level of bottom waters differences in depositional setting. Paleogeographically, the Yangtze could have played a major role in controlling the fractionation of Platform was a sort of epeiric shelf during the O–S transition, on which bacterial sulphate reduction in water–sediment interface. During numerous sub-depressions and sub-highs were present (Mu et al., the oceanic anoxia, the bacterial sulphate reduction was greatly 1981; Chen et al., 2004). The Nanbazi was located in a sub-depression enhanced, through which 32S was increasingly incorporated into the in the innermost part of the shelf, while Wangjiawan was located on sulfides precipitating from bottom waters, leading to the light 34 the outer part of the shelf facing the northern oceanic basin (Fig. 1); δ Ssulfide values. On the contrary, during the ventilated period, i.e., therefore, the Nanbazi was more restricted in water circulation, in during the mid-early Hirnantian interval, the bacterial sulphate which the progressive bacterial sulphate reduction could have reduction could have substantially decreased in the bottom water, 34 resulted in the depletion of sulfate in the water column and increased leading to the heavy δ Ssulfide values due to less incorporation of influences of sulphate sulfur isotopes upon the sulfide sulfur isotope 32Sintothesulfides (Canfield and Thamdrup, 1994; Habicht and 34 compositions, thereby leading to heavier δ Ssulfide values (Canfield Canfield, 1996). and Thamdrup, 1994; Habicht and Canfield, 1997) as demonstrated in this study. In addition, a relatively high oxygen level in water columns 6. Implications for the Late Ordovician extinction at Nanbazi due to its proximity to the source area and shallower water, which could have depressed the bacterial sulphate reduction as well, The latest Ordovician biotic crisis was marked by two-phase 34 may also contribute, more or less, to the heavy δ Ssulfide values there. extinction events (Brenchley et al., 1995). The first strike occurred 34 The similar patterns in δ Ssulfide changes: a positive excursion during the glacial period (or a eustatic sea-level fall) coincident with followed by a rapid negative shift during the O–S transition, both at the positive shift in δ13C values of carbonates and organic carbon; the Nanbazi and Wangjiawan (Figs. 2 and 3) could be a reflection of similar second wave of extinctions occurred during the subsequent rise in oceanic environmental changes, likely in response to changes in sea-level marked by a large negative shift in δ13C values (Marshall 13 34 oxygen level in water columns that were further controlled by eustatic et al., 1997). Our δ Corg and δ Ssulfide isotopic data also provide a sea-level fluctuations and climate changes. Studies on C–S–Fe relation- good constraint on the timing of the two-phase extinctions and ship of organic-rich sediments spanning O–S transition revealed a relevant oceanic and climatic changes. The rapid shift from warm to stratified anoxic water column on the shelf during the mid-Ashgill cold climate accompanied with eustatic sea-level fall in the beginning interval to an oxygenated water condition during the mid-early of Hirnantian age could have exerted severe ecological stresses on Hirnantian, which was followed by a rapid recurrence of anoxic those organisms, both benthic and planktonic dwellers, who had been water column during the latest Hirnantian–earliest Rhuddanian adapting to tropical warm waters, leading to their massive demise. 34 interval (Yan et al., 2008). The temporal coincidence of light δ Ssulfide The subsequent sharp rise in sea-level and return of warm climate 38 D. Yan et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 274 (2009) 32–39 accompanied with oceanic anoxia gave another detrimental strike for Gill, B.C., Lyons, T.W., Saltzman, M.R., 2007. Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir. Palaeogeogr. those organisms, especially the benthic habitats, i.e., the Hirnantian Palaeoclimatol. Palaeoecol. 256, 156–173. fauna, that had been adapting to cool temperature and ventilated Goodfellow, W.D., Nowlan, G.S., McCracken, A.D., Lenz, A.C., Gregoire, D.C., 1992. shallow-water conditions, resulting in the second wave of severe Geochemical anomalies near the Ordovician–Silurian boundary, Northern Yukon Territory, Canada. Hist. Biol. 6, 1–23. declines of these particular fauna. In general, the organisms can Habicht, K.S., Canfield, D.E., 1996. Sulphur isotope fractionation in modern microbial gradually get accustomed to slow long-term climatic changes, but mats and the evolution of the sulphur cycle. Nature 382, 342–343. they cannot readily adjust their physiology to frequent short-term Habicht, K.S., Canfield, D.E., 1997. Sulfur isotope fractionation during bacterial sulfate climatic changes, which could inhibit the origination of new species reduction in organic-rich sediments. Geochim. Cosmochim. Acta 61 (24), 5351–5361. and their radiation (Clarke, 1993). Moreover, the oceanic anoxia is also Habicht, K.S., Canfield, D.E., Rethmeier, J., 1998. Sulfur isotope fractionation during harmful for the biotic colonization mainly because of the presence of bacterial reduction and disproportionation of thiosulfate and sulfite. Geochim. – abundant toxic H S in the water columns (Kump et al., 2005; Algeo Cosmochim. Acta 62 (15), 2585 2595. 2 Joachimski, M.M., Henning, C.O., Pancost, R.D., Strauss, H., Freeman, K.H., Littke, R., et al., 2007). In view of Fe depletion in the anoxic water bodies during Damste, J.S.S., Racki, G., 2001. Water column anoxia, enhanced productivity and δ13 δ34 – the O–S transition (Yan et al., 2008), significant amounts of H2S gases concomitant changes in C and S across the Frasnian Famennian boundary. produced through bacterial sulphate reduction during the oxidation of Chem. Geol. 175, 109–131. fi Kajiwara, Y., Yamakita, S., Ishida, K., Ishiga, H., Imai, A., 1994. Development of a largely organic matter could not be fully sequestrated to form sul des (mainly anoxic stratified ocean and its temporary massive mixing at the Permian/Triassic pyrite), so that they would be released into the photic zone and even boundary supported by the sulfur isotopic record. Palaeogeogr. Palaeoclimat. atmosphere, resulting in the photic euxinia (Kump et al., 2005); this Palaeoecol. 111, 367–379. Kaljo, D., Hints, L., Martma, T., Nõlvak, J., Oraspõld, A., 2004. Late Ordovician carbon scenario would be much more harmful, both for the benthic and isotope trend in Estonia, its significance in stratigraphy and environmental analysis. planktonic fauna. Therefore, the frequent large-scale climate fluctua- Palaeogeogr. Palaeoclimat. Palaeoecol. 210, 165–185. tions, together with multiple oceanic anoxia (particularly photic Kump, L.R., Arthur, M.A., 1999. Interpreting carbon-isotope excursions: carbonates and fl organic matter. Chem. Geol. 161, 181–198. euxinia) and sea-level uctuations, may account for the stepwise Kump, L.R., Arthur, M.A., Patzkowsky, M.E., Gibbs, M.T., Pinkus, D.S., Sheehan, P.M.,1999. – massive demises during the O S transition. A weathering hypothesis for glaciation at high atmospheric pco2 during the Late Ordocician. Palaeogeogr. Palaeoclimat. Palaeoecol. 152, 173–187. Acknowledgements Kump, L.R., Pavlov, A., Arthur, M.A., 2005. Massive release of hydrogen sulphide to the surface ocean and atmosphere during intervals of oceanic anoxia. Geology 33, 397–400. We thank Fusong Zhang and Lianjun Feng (IGGCAS) for access to Long, D.G.F., 1993. Oxygen and carbon isotopes and event stratigraphy neat the – the stable isotope laboratory and assistance in stable isotope analysis. Ordovician Silurian boundary, Anticosti Island, Quebec. Palaeogeogr. Palaeoclimat. Palaeoecol. 104, 49–59. This research was supported by the National Key Basic Research Marshall, J.D., Middleton, P.D., 1990. Changes in marine isotopic composition and the Project (973 Project) through grant 2005CB422101 and NSFC-SINOPEC Late Ordovician glaciation. J. Geol. Soc., Lond. 147, 1–4. United Foundation through grant 40839907. Thanks to Ben Gill, Marshall, J.D., Brenchley, P.J., Mason, P., Wolff, G.A., Astini, R.A., Hints, L., Meidla, T., 1997. 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