The carbon isotope excursion on GSSP candidate section of Lopingian–Guadalupian boundary

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Earth and Planetary Science Letters 220 (2004) 57^67 www.elsevier.com/locate/epsl

The carbon isotope excursion on GSSP candidate section of Lopingian^Guadalupian boundary Wei Wang  , Changqun Cao, Yue Wang Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, 39# East Beijing Rd, Nanjing 210008, PR China Received 26 May 2003; received in revised form 26 November 2003; accepted 7 January 2004

Abstract An isotopic stratigraphically well-documented outcrop in Penglaitan, Guangxi Province, South China, has been proposed as a candidate GSSP for the Lopingian^Guadalupian boundary. Correlatable outcrops from the west wing (Tieqiao section) and east wing (Penglaitan section) of the Laibin Syncline exhibit synchronous excursions in carbon isotopes. The isotopic excursions (N13 C) show the best placement of the boundary may lie at the base of Bed 6k which coincides with the Clarkina postbitteri conodont zone and with eustatic change. N13 C increases during the uppermost Guadalupian (Jinogondolella granti conodont zone from the top of 3c to the base of 6i). A N13 C peak value of 5x is located at the transition between these two conodont zones and is suggested as a proxy for the transition from transgression to regression. A gradual depletion of carbon isotopes occurs in the C. postbitteri zone from Bed 6e to 7b, and this gradual N13 C excursion also suggests the sequences around the Guadalupian^Lopingian boundary at the candidate section are conformable. During the middle-later C. postbitteri zone a 3.5x dramatic N13 C depletion is recorded at the Tieqiao section, but only a 2x depletion at the deeper facies Penglaitan section, synchronous with conodont zones that mark eustatic changes. 8 2004 Elsevier B.V. All rights reserved. Keywords: carbon isotopes; Lopingian; Guadalupian; GSSP; South China

1. Introduction The Late Permian mass extinction has been studied for several decades, but only recently have paleontologists demonstrated that two discrete episodes of extinction occur, the ¢rst at the

* Corresponding author. Tel.: +86-25-83282156; Fax: +86-25-83375200. E-mail addresses: [email protected] (W. Wang), [email protected] (W. Wang), [email protected] (C. Cao), [email protected] (Y. Wang).

end of the Guadalupian and the second at the end of the Changhsingian [1]. These events are separated by an interval of radiation and recovery [2]. Together these two events eliminated upwards of 90% of marine species diversity and perhaps 70% of terrestrial species [3]. The end-Changhsingian event has received considerable attention, but far less is known about the earlier, end-Guadalupian episode. This ¢rst event occurred near the end of the Guadalupian or Middle Permian stage, correlated with top of the Maokouan Formation in South China. Jin et al. [1] suggested that low-latitude faunas of marine shelf carbonate environ-

0012-821X / 04 / $ ^ see front matter 8 2004 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(04)00033-0

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ments were particularly a¡ected by this event; many fusulinids, echinoderms, brachiopods and bryozoans were among the victims. Hallam et al. [4] suggest a link between regression and the end-Guadalupian extinction, and the majority of studies of this crisis invoke marine habitat loss as a signi¢cant cause of the extinction [1,5,6]. Indeed, the end-Guadalupian event arguably provides one of the best demonstrations of a link between major marine regression and mass extinction. The boundary of the Guadalupian and Lopingian Series was typically suggested to coincide with global regression, and has been associated with an important mass extinction event [1,4]. For this reason we have analyzed the carbon isotopic changes at one of the most complete marine extinctions to develop a proxy for change in the carbon cycle and in biomass. Carbon isotopic excursions are a useful tool for understanding changes in biomass and ecosystem dynamics. N13 C excursions can also provide independent evidence to test the existence of a sedimentological gap which was proposed by Wang and Kuzor in Bed 6i [7]. Although well-documented sections across the Permo^Triassic boundary have been described worldwide [8,9], N13 C records are rare for the Guadalupian^Lopingian boundary sections. A regional survey of Guadalupian^Lopingian boundary successions shows that the Laibin Limestone represents a lowstand systems tract deposited on the slope during the Guadalupian^Lopingian boundary interval. This interval is probably characterized by an unconformity in all other shelf sections [10]. This welldeveloped carbonate sequence in Guangxi Province, South China provides a unique opportunity to integrate a detailed biostratigraphic record with carbon isotopic analysis.

2. Litho- and biostratigraphy and geographic setting Extensive surveys of marine sections over the past few decades demonstrate that only a few sections span the Guadalupian^Lopingian boundary. Those with a complete succession of pelagic fauna are particularly rare, and this interval is

probably characterized by an unconformity in all other shelf sections [10]. The sections of the Laibin Syncline in Guangxi Province, China appear to be unique among these sections in containing a complete and inter-regionally correlatable succession of pelagic conodont zones and other diverse, and inter-regionally correlatable Permian fossils. At the recommendation of the International Commission on Stratigraphy [11], the Lopingian Series is the uppermost sequence of the Permian System. The global stratotype section and point (GSSP) for the boundary of the series suggested by the Subcommission on Permian Stratigraphy [12] is located at the Penglaitan section based on its well-documented conodont biostratigraphy [10,13,14]. The International Guadalupian^Lopingian Boundary Working Group has de¢ned the boundary as the ¢rst appearance datum (FAD) of Clarkina postbitteri postbitteri at the base of Bed 6k at the Penglaitan section. This placement of the boundary has been disputed; however, Jin et al. [10] suggested the best options are the FAD of C. postbitteri hongshuiensis or C. postbitteri postbitteri based on an evolution lineage from Jinogondolella granti to C. postbitteri. Kuzur and Wang [7] recommended the FAD of Clarkina dukouensis is the best zone for the base of the Lopingian and also suggested there is a gap at Bed 6i around the boundary suggested by Jin et al. [10]. During the Permian, South China lay in the eastern Tethys [1]. Recent studies of Permian biostratigraphy, chemostratigraphy, sequence stratigraphy, and magnetostratigraphy have produced the most detailed record of the Guadalupian^ Lopingian marine sequence in South China [4,11, 13^15]. The two sections, named the Penglaitan section and the Tieqiao section, studied are in Laibin County (Fig. 1), midway between Guilin, one of the major tourist cities in China, and Nanning, the capital of Guangxi Province. This area is located in the Jiangnan Basin, which consistently subsided during the late Paleozoic and Early Triassic. This basin lies between the Yangtze and Cathaysian cratons, and extends southward into the Laibin area of eastern Guangxi [5]. The Penglaitan section, the GSSP candidate, and the Tie-

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Fig. 1. A map showing the locality of research sections in South China. The Hongshui River cuts the syncline from west to east and the Tieqiao section (west) and Penglaitan section (east) outcrops along the riverbank. The distance between these two sections is 15 km.

qiao section are located on the eastern and western slope of the Laibin Syncline, respectively (Fig. 2). The Hongshui River cuts the syncline from west to east and these sections outcrop along the riverbank. The Permian rocks are extensively exposed along the riverbank of Hongshui River from the lower Permian to the base of the Triassic. The Guadalupian Maokou Formation is 302 m in thickness and is divided into ¢ve members. The uppermost member, discussed here, is named the Laibin limestone. At the Tieqiao section, 11 m of Laibin limestone are composed of interbedded chert-limestone at the base, massive limestone in the middle part and thin-moderate bedded gray to dark-gray bioclastic limestone in its top part. It is overlain by the Heshan Formation, which is composed of black cherty limestone in the lower part representing a basinal facies. Comparable facies are found in the Penglaitan section, and its upper part is thicker than the Tieqiao section. The Laibin limestone at the Penglaitan section is about 8 m thick and contains a interbedded chert-dolomitic limestone in the base, a thinner

bedded carbonate in the middle, and massive limestone mixed with volcanic ash in the upper part. The overlying sequence is the Heshan Formation which is mostly composed of chert and lenticular limestone representing basinal facies (Fig. 3). Conodonts from the Laibin limestone in both the Tieqiao and the Penglaitan sections are exclusively dominated by Jinogondolella conodont species in the basal part (Bed 2 at these two sections), and by Clarkina species in the uppermost part (Bed 6i to Bed 8 at Tieqiao and Bed 6i-upper to 6k at Penglaitan).

3. Sampling protocol The phosphoric acid evolution and evolved CO2 were measured with a MAT 251 mass spectrometer (N13 C, all standardized by PDB, x) at the Nanjing Institute of Geology and Paleontology, and Nanjing Institute of Soil Sciences. Samples from the main sections were measured in Nanjing, and some samples from the Tieqiao sec-

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Fig. 2. Geological map showing the Laibin Syncline and the positions of sections. Marks are as follows: K1 : Lower Cretaceous; T1 : Lower Triassic; P3t : Talung Formation; P2m : Maokou Formation; P2q : Chihsia Formation; P1mp : Maping Formation.

tion were measured at the University of Tokyo (sample numbers C5 and C7). Further testing of the results was provided by analysis at the University of California, Santa Cruz (sample numbers LA11 and C8). The measurement has been evaluated in previous works [15]. Before measurement, all samples were screened rigorously in lab and ¢eld. Laboratory checking included testing for enhanced weathering of carbonates by determining Mn and Sr contents by ICP after pretreatment with dissolved HF. N18 O and 87 Sr/86 Sr were also analyzed to evaluate sample preservation [16^18]. Mn and Sr contents were low in the Maokou Formation : Mn contents 15^25 ppm and Sr contents 800^1000 ppm, except for an elevated Mn content of 1011^4661ppm and a Sr content slump to 288^922 ppm at a massive limestone with pink stromatolite in this interval at the Tieqiao section. This elevated Mn/Sr ratio around

the interval is considered with global regression [4,5,10], and HF possibly dissolve all clastics so that it represents higher Mn and lower Sr content. There is no evidence of high-temperature^pressure diagenetic alteration based on the H/C atomic ratio at the Tieqiao section, which is between 0.47 and 0.49 [19]. N18 O ranges between 34x and 37x at these two sections. A plot of N13 C vs. N18 O shows no statistical relationship, suggesting the N13 C values are relatively original (R2 value is 0.2 for the Penglaitan section and 0.4 for the Tieqiao section). 87 Sr/86 Sr at the Penglaitan section is 0.70671 from Bed 6b, which normalized by NBS987 (0.710235 P 0.000015) from two Laibin limestone samples near the Guadalupian^Lopingian boundary, pre-treated with acetic acid (measured at Nanjing University). To con¢rm the dramatic N13 C depletion in the Tieqiao section, one sample was measured at 0.70702 in the base of Bed 8a. This relatively low value also suggests carbon isotopes from these samples of the sections are well-preserved. All of this evidence suggests that the N13 C data (Table 1) from these sections have not been a¡ected by signi¢cant weathering or alteration [17,18].

4. Carbon isotopic excursions at the Guadalupian^ Lopingian boundary After Baud et al. [8] suspected N13 C was depleted around the Guadalupian^Lopingian boundary, we examined carbonate sections that had well-developed biostratigraphic information. The ¢rst N13 C depletion spanning the Guadalupian^ Lopingian boundary was found in studies at the Tieqiao section [26]. When the Penglaitan section was voted as a GSSP candidate, a higher-resolution carbon isotopic investigation was begun to compare it to the Tieqiao section. The carbon isotope results show a correlatable excursion between these two sections. The higher value of N13 C coincides with the transition from the J. granti zone to the beginning of the C. postbitteri hongshuiensis zone, and represents a depletion in the C. postbitteri postbitteri zone. At the end of C. postbitteri postbitteri, the two sections appeared to have a lower value that

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Fig. 3. Two correlatable sections of the Penglaitan section (right) and the Tieqiao section (left) represent N13 C excursions with well restricted biozones of conodonts. A paleoenvironmental report on the paleobiology and sedimentology suggested that the Penglaitan section is deeper than the Tieqiao section, which coincides with fauna habitat and N13 C excursion. The Tieqiao section suggests a shallow-water bioevent near the boundary of the Guadalupian^Lopingian.

presents as follows. At the Penglaitan section (Fig. 3), around the lower part of Laibin limestone, Bed 2 is divided into three units: 2a, 2b and 2c. Bed 2 is composed of black chert alternated with gray dolomitic limestone, contains brachiopods and overlies Bed 1 which is dominated by chert interbedded with gray dolomitic limestone. It is composed of thin beds of chert and pale gray limestone in Bed 2a and pale gray limestone in 2b and 2c. N13 C shows a lower value of 3.2x at Bed 2a and 3.5x at 2b which may relate to the dolomitic process. And then, N13 C gradually increases to 4.5x around the middle of Bed 3c. For this N13 C increasing progress, from the lower value of 3.2x near Bed 2 to Bed 3a, and as high as 4.5x at Bed 3c, it nearly just covers a conodont zone of Hindodus excavatus.

Bed 3 is massive pale gray limestone, and is divided into three units: 3a, 3b and 3c. The conodont zone of H. excavatus covers the upper part of 3a, 3b and lower part of 3c. From the end of the conodont zone, N13 C drops in Bed 3c from 4.5x at the base of the bed to 4x at the upper part. This interval appears around the J. granti conodont zone. N13 C increases in this conodont zone from the base of Bed 4a to Bed 6h. The details are as follows: N13 C increases from 4x at the beginning of the J. granti zone at the base of Bed 4a, to as high as 5.2x at the end of the zone near the base of Bed 6f. Around the end of the zone, from Bed 6c to the lower part of Bed 6h, N13 C shows a few £uctuations with a high value around 4.9^5.2x. An important N13 C negative excursion is near the C. postbitteri conodont

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Table 1 Stable isotope data Sample

Thickness (m)

87

Lithology

Sr/86 Sr

Stable isotope data of the Penglaitan section pl2a 14 micrite pl2b 14.5 micrite pl3a 15.2 micrite pl3b 15.4 micrite pl3c-1 16 micrite pl3c-2 17.5 micrite pl4a 18.2 micrite /w bioclastic strip pl4b 18.5 micrite pl4c 18.7 micrite pl4d 18.8 micrite pl4f 19.2 micrite pl4h 19.4 micrite /w bioclastic and volcanic ash strip pl4k 20.2 micrite pl5b 20.6 micrite /w bioclastic strip pl5c-1 20.8 micrite /w bioclastic strip pl5c-2 21 micrite /w bioclastic strip pl6a 21.2 micrite /w bioclastic strip pl6b-2 21.6 micrite /w bioclastic strip pl6c 21.7 micrite /w bioclastic strip 0.706712 P 18 pl6e 22.2 micrite /w bioclastic strip pl6f 22.3 micrite /w bioclastic strip pl6g 22.4 micrite /w bioclastic strip pl6h-1 22.5 micrite /w bioclastic strip pl6h-2 22.6 micrite /w bioclastic strip pl6i 22.7 micrite /w bioclastic strip pl6k 23 micrite /w bioclastic strip pl7b 23.3 micrite /w clastic spots pl7d 23.6 light gray micrite pl7-2 25.2 dark gray micrite pl7-3 27 dark gray micrite pl7-5(c-5) 27.3 dark gray micrite Stable isotope data of the Tieqiao section C12 703.8 mid-thin bed silicate /w micrite lens C10 703.6 mid-thin bed silicate /w micrite lens C9 703.55 mid-thin bed silicate /w micrite lens LA42 703.5 gray mid-bed micrite /w silicate strip C8 703.3 gray mid-bed micrite LA41 703.1 gray mid-bed micrite C7 702.9 gray mid-bed micrite LA38 702.7 gray mid-bed micrite LA37 702.3 gray mid-bed micrite C6 702.2 gray mid-bed bioclastic micrite LA36 702 gray mid-bed bioclastic micrite LA35 701.85 dark gray mid-bed bioclastic micrite C5 701.75 dark gray mid-bed bioclastic micrite LA33 701.6 dark gray mid-bed bioclastic micrite C4 701.5 dark gray mid-bed bioclastic micrite LA32 701.4 dark gray mid-bed bioclastic micrite LA27 701.15 dark gray mid-bed bioclastic micrite LA23 701 dark gray mid-bed bioclastic micrite

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N13 CPDB

N18 OPDB

3.262 3.563 3.734 4.187 4.447 4.252 3.762 4.502 4.241 4.068 4.017 4.309

37.02 35.699 36.322 35.811 34.889 34.429 34.446 33.643 35.209 35.639 35.22 34.61

4.49 4.783 4.817 4.861 4.813 4.668 4.957 5.249 4.991 5.173 5.201 4.576 4.761 4.396 3.807 3.983 4.275 4.739 4.599

34.141 34.672 35.422 34.732 34.505 35.086 34.127 33.256 34.179 33.61 33.253 35.025 33.138 32.443 35.931 33.936 34.457 33.095 35.032

2.63 2.72 2.8 0.20 0.19 30.39 2.78 3.57 2.90 3.5 3.55 3.39 2.86 3.73 3.28 3.71 3.65 3.52

35.96 35.89 35.85 38.30 37.61 37.22 35.75 35.02 34.22 34.78 33.84 34.23 35.63 35.77 35.56 34.27 36.05 34.68

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Table 1 (Continued). Sample

Thickness (m)

87

Lithology

C3 700.9 dark gray mid-bed bioclastic LA19 700.8 dark gray mid-bed bioclastic C2 700.6 dark gray mid-bed bioclastic LA15 700.51 dark gray mid-bed bioclastic LA13 700.4 dark gray mid-bed bioclastic LA12 700.25 dark gray mid-bed bioclastic LA11 700 dark gray mid-bed bioclastic LA10 699.92 dark gray mid-bed bioclastic C1(6b) 699.8 dark gray mid-bed bioclastic C-1 699 massive bed gray micrite C-2 698.25 massive bed gray micrite C-3 697 massive bed gray micrite C-4 696.5 massive bed gray micrite C-4-1 696.3 massive bed gray micrite C-4-2 696.2 massive bed gray micrite Mn/Sr ratio of GLB in Tieqiao section (dissolved by HF) Stratigraphy # Thickness Sr (m) (ppm) LA139 556.3 349.56 LA142 673.9 596.49 LA6 696.2 922.41 LA11 700 515.77 LA12 700.25 975.23 LA19 700.8 373.45 LA23 701 284.09 LA27 701.15 377.27 LA33 701.6 433.63 LA35 701.85 326.58 LA37 702.3 292.04 LA42 703.5 288.29 LA146 737.7 420.27 LA152 790.5 330.51 Mn/Sr ratio of GLB in Penglaitan section (dissolved by HF) P2a 0.15 615.04 P3c5 4.5 562.5 P4a 5.25 635.14 P4k 6.6 589.29 P5b 7.1 641.89 P5c 7.4 741.07 P6a 7.6 519.57 P6c 8.08 417.95 P6f 8.35 355.86 P6g 8.45 435.27 P6h 8.55 445.95 P6il 8.62 410.87 P6iu 8.75 573.2 P6ju 8.86 316.96 P6k 9 361.94 P7b 10 586.36 P7c 10.18 27.252 P7j 11.6 13.17

Sr/86 Sr

micrite micrite micrite micrite micrite micrite micrite micrite micrite

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N13 CPDB

N18 OPDB

2.62 1.96 2.04 1.96 2.06 1.49 1.57 2.30 2.62 3.28 3.37 3.03 2.57 4.13 3.27

35.97 36.92 37.29 36.19 35.31 38.16 37.35 37.86 36.64 35.18 34.43 36.11 37.57 38.2 37.54

Mn (ppm) 94.912 1177.6 1687.5 3220.7 3855.9 3535.4 3490.9 4661.4 4637.2 1047.3 1433.6 1011.3 145.05 57.839

Mn/Sr

3515.5 2008.9 2159.9 2654 5382.9 6022.3 4030.4 2686.4 2144.1 2435.3 2750 3271.7 3346.8 1082.6 3632.9 3765.9 108.78 18.75

5.72 3.57 3.40 4.50 8.39 8.13 7.76 6.43 6.03 5.59 6.17 7.96 5.84 3.42 10.04 6.42 3.99 1.42

0.27 1.97 1.83 6.24 3.95 9.47 12.29 12.36 10.69 3.21 4.91 3.51 0.35 0.17

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zone from the lower part of Bed 6h to Bed 7b. This depletion begins from Bed 6h and extends to Bed 7b as 3.8x, spanning the C. postbitteri hongshuiensis conodont zone. The lowest value of this negative excursion occurs around Bed 7b. The excursion from Bed 6h to Bed 7b is quite smooth, suggesting relatively continuous deposition [20]. Near the end of the conodont zone, N13 C goes from 3.8x to higher than 5x. This N13 C increase is from the end of C. postbitteri postbitteri and through the FAD of C. dukouensis. The Tieqiao section includes comparable N13 C excursions, N13 C is generally 1^2x lower when compared with the results from the Penglaitan section ; however, it represents a stronger negative £uctuation near the end of the C. postbitteri zone. A little before the FAD of H. excavatus in the Tieqiao section, N13 C increases from 2.5x to 3.5x, and then begins dropping around the FAD of J. granti from 3.5x to 1.2x near the upper part of the conodont zone. Similar to the results from the Penglaitan section, N13 C changes from 1.2x to 3.8x, from the middle of the J. granti conodont zone to the end of the zone. Comparatively, N13 C drops from as high as 3.8x to 30.5x around the middle to end of C. postbitteri and then increases to 2.8x during the end of the zone. The N13 C curves are roughly similar in these two sections, but some di¡erences occur, probably related to the di¡erence in facies. Nonetheless, the detailed conodont biostratigraphy reveals that the major shifts are synchronous between the two localities.

5. Discussion 5.1. Carbon isotopic excursion correlation of these two sections The carbon isotope results are broadly similar between the two sections, with a synchronous negative excursion near the C. postbitteri conodont zone at the Guadalupian^Lopingian boundary. However, the depletion is about 1.5x in the Penglaitan section and 3.5x in the Tieqiao section. Relatively, N13 C presents the heaviest value

in the FAD and/or early of the C. postbitteri conodont zone, that is 5.2x in the Penglaitan section and 3.5x in the Tieqiao section. The increase in both sections around the J. granti zone suggests a short lag in the Tieqiao section if we believe the FADs of the conodont zones are synchronous in these two sections. The increasing N13 C is at the FAD in the Penglaitan section, but begins in the early-middle part of the zone in the Tieqiao section. There are three possible reasons for this lag. (1) If the N13 C value shifts were uniform, then the FAD of the conodont zone in the Penglaitan section represents a slight time lag from the Tieqiao section, possibly because this biozone developed in a shallow-water shelf (Tieqiao section) to deep (Penglaitan section) and/or extended from a shallow-water shelf into a basin area. (2) If we believe the FAD of the conodont zone is primarily isochronous, the N13 C response to the environmental change is a small lag in the Tieqiao section. Normally, the N13 C response in shallow water is earlier than in deep water in modern oceans [21,22], and the above interpretation will con£ict with the reason for the Tieqiao lag. (3) The conodont correlation may be slightly o¡set, with its extension to the middle part of Bed 3c in the Penglaitan section rather than the top part of Bed 3c. That means the FAD of the conodont zone may be located just after the conodont zone of H. excavatus which is around at the middle of Bed 3c, except the upper part of the Bed 3c. N13 C increases in the lower part of the Laibin limestone in the Jinogondolella xuanhuaensis and H. excavatus conodont zones. The 1x o¡set range of N13 C of these two sections is similar, from 2.5x to 3.4x in the Tieqiao section and 3.5x to 4.5x in the Penglaitan section. For the upper part of the Laibin limestone the conodont zone and N13 C excursion are still synchronous, i.e. N13 C presents an increasing process near the end of the zone of C. postbitteri postbitteri in these two sections. 5.2. Depositional sequences around the boundary The N13 C excursion around the C. postbitteri conodont zone, especially from Bed 6e to 7b,

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shows a gradual depletion without any abrupt shift in the Penglaitan section. Following the suggestion of Holser et al. [16,17] a gradual change of N13 C pro¢le in a section generally suggests conformity of deposition. It is easy to understand that N13 C will present an abrupt shift if depositional gaps exist, since biomass, carbon cycle and paleoenvironmental change in general will continually develop and any obvious halt in deposition will stop N13 C records in sediments, producing an abrupt shift in the record. Around Bed 6e at the Penglaitan section the N13 C value of 5.2x may suggest a high primary productivity in the interval from the later J. granti conodont zone to the early C. postbitteri hongshuiensis zone. These two sections include a diverse array of bioclastics (crinoids, corals and bryozoans). The drop in N13 C present from the later J. granti to early C. postbitteri hongshuiensis zones may coincide with a gradual biomass reduction caused by sea level regression, which may coincide with enhancement of erosion and oxidation of organic matter, reduction of primary productivity, and/ or additional input of other sources of organic carbon etc. The gradual N13 C excursion suggests continual deposition in the Penglaitan section which is more complete and thicker than the Tieqiao section, especially around its upper part near the boundary [23,24]. The N13 C excursion at the Tieqiao section contains all characteristics of the Penglaitan section except the heaviest peak in the later J. granti to early C. postbitteri hongshuiensis zones, and a gradual drop in middle-later C. postbitteri zones. Possible reasons for the drop to 30.5x in the Tieqiao section and the higher value of 3.8x in the Penglaitan section are as follows. (1) Samples in the Tieqiao section may be more diagenetically altered than in the Penglaitan section, since the lower N18 O in the Tieqiao section is generally around 35x to 37x, and higher in the Penglaitan section for 33x to 35x. However, the lower 87 Sr/86 Sr 0.70702 does not support poor preservation of samples around Bed 8 in the Tieqiao section [20]. (2) The depositional environment di¡ers between these two sections, in that the Tieqiao section is shallower and will more easily be a¡ected by nearshore materials which may present lower N13 C and

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N18 O, or, like the N13 C and N18 O in the modern Paci¢c, the surface water layer is more easily changed than the deeper layer [21,22]. 5.3. Tieqiao depleting and possible bioevent in shallow-water marine environment at the end of the Guadalupian The excursion of N13 C in the Penglaitan sections represents a gradual drop with a di¡erence of 1.6x (from 5.2x to 3.6x), which is a characteristic of sequences with no gaps, and implied deeper than the Tieqiao section [10]. The Tieqiao section represents a very special dramatic depletion with a greater than 3x di¡erence (from 3.2x to 30.5x). As documented by lithostratigraphy and biostratigraphy, the Tieqiao section was deposited in a shallower environment than the Penglaitan section [10]. Generally, N13 C has a stronger response to carbon cycle events in shallow and/or near-shore environments than deeper ones based on a key of modern Paci¢c N13 C excursions [21,22], and the N13 C excursion is also stronger in the Tieqiao section as the depletion di¡erence is as high as 3x while in the Penglaitan section it is only 1.6x. This carbon cycle event is associated with a balance of carbon input, such as N13 C marked land and marine deposit weathering, degassing of volcanic areas, light carbon of organic matter from ocean basin, and carbon output, such as burial of organic material which links with primary productivity. Dramatic depletion of N13 C in a short interval mostly associates with dramatic biomass changes, except eustatic change which mostly spans a longer interval [17,18]. This dramatic drop in this very short interval, i.e. the N13 C shift at the Guadalupian^ Lopingian boundary, is normally synchronous with a dramatic drop of primary productivity. This dramatic depletion and interpretation has been well documented in sections spanning the Permo^Triassic boundary worldwide [8,9,25]. Considering that the Tieqiao section has been suggested to be a near-shore and/or shallow environment, and has a stronger depletion of N13 C than the Penglaitan section, this stronger depletion of N13 C in the Tieqiao section suggests the £uctuating carbon input/output or the event of

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carbon cycle is more severe around the ocean surface. It is comparable with modern ocean surface where the N13 C £uctuation is higher in surface environment than deeper. This N13 C excursion also suggests that the frequency of N13 C £uctuations is higher near the end of the Guadalupian compared with middle-later Guadalupian. These £uctuations of N13 C shift may the associate with £uctuation of the carbon cycle that links ecosystem, which may be a possible cause of the endGuadalupian mass extinction. In conclusion, two N13 C £uctuations which are biostratigraphically well dated at the Penglaitan and Tieqiao sections represent synchronous proxies of the carbon cycle and biomass change. The change in sea level is shown by the correlative change in N13 C which marks the carbon cycle, and the inferred habitat of the conodont zones. Compared with the gradual depletion of N13 C, the Penglaitan section is conformably deposited in this discussed interval where is located the GSSP candidate of the Lopingian^Guadalupian boundary. A possible ocean surface bioevent is exhibited in these two sections and magni¢ed in the Tieqiao section from its depletion of N13 C near the middlelater C. postbitteri.

[3]

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[7]

[8]

[9]

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[12]

Acknowledgements [13]

Funded by CAS Research Project (KZCX2SW-129), NFSC and Major Basic Research Projects of the Ministry of Science and Technology, China (G2000077700). We thank Professor Y.G. Jin of Nanjing Institute of Geology and Paleontology, CAS for his assistance in ¢eldwork and biostratigraphy. The authors appreciate Dr. Douglas Erwin of the Smithsonian Institute for his kind review and comments.[BOYLE]

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