Stable Carbon Isotope Signature in Mid-Panthalassa Shallow-Water Carbonates Across the Permo^Triassic Boundary: Evidence for 13C-Depleted Superocean

Earth and Planetary Science Letters 191 (2001) 9^20 www.elsevier.com/locate/epsl

Stable carbon isotope signature in mid-Panthalassa shallow-water carbonates across the Permo^Triassic boundary: evidence for 13C-depleted superocean

Masaaki Musashi a;*, Yukio Isozaki b, Toshio Koike c, Rob Kreulen a;1

a Department of Geochemistry, Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands b Department of Earth Science and Astronomy, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan c Department of Earth Science, Yokohama National University, Hodogaya, Yokohama 240, Japan Received 30 March 2001; accepted 12 June 2001

Abstract

The Jurassic accretionary complex in southwest Japan contains exotic blocks of the Permo^Triassic limestone primarily deposited on ancient mid-oceanic seamounts in an ancient Pacific Ocean or superocean Panthalassa. This 13 13 study examines stable carbon isotope compositions (N Ccarb and N Corg) of such open-ocean shallow-water limestone across the Permo^Triassic boundary (PTB) at Kamura and Taho in southwest Japan. The results show an almost 13 identical secular change in N Ccarb values with a remarkable negative spike across the PTB in both sections. This confirms for the first time that the mid-Panthalassa shallow-water carbonates are bio- and chemo-stratigraphically 13 correlated not with previously studied PTB sections from the peripheries of Pangea. The negative shift in N Ccarb occurs 13 13 13 13 parallel to that of N Corg in both sections, and the difference (v C=N Ccarb3N Corg) remains nearly constant throughout the sections. This implies that the 13C-depleted water should have developed widely, probably in a global extent, throughout the superocean Panthalassa across the PTB. These findings suggest that a large input of 12C-enriched carbon into the ocean^atmosphere system has occurred and may have caused a global environment change probably relating to the greatest mass extinction in the Phanerozoic. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: carbon; C-13/C-12; carbonates; pelagic environment; world ocean; Permian^Triassic boundary

1. Introduction * Corresponding author. Present address: Faculty of Sys- tems Science and Technology, Akita Prefectural University, 84-4 Tsuchiya-Ebinokuchi, Honjyo, Akita 015-0055, Japan. Across the Permo^Triassic boundary (PTB) ca. Tel.: +81-184-27-2166; Fax: +81-184-27-2189. 251 Ma, the largest mass extinction in the Phaner- E-mail addresses: [email protected] (M. Musashi), ozoic occurred in which up to 96% of marine in- [email protected] (Y. Isozaki), vertebrate species became extinct [1]. Although [email protected] (T. Koike). several hypotheses including sea-level change, 1 Present adress: ISOLAB, Zuiderlingedijk 97, 4211 BB temperature change, seawater salinity change, an- Spijk (Lingewaal), The Netherlands. oxia, hypercapnia etc. were proposed, the sub-

EPSL 5904 10-8-01 10 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 stantial cause of the PTB catastrophe has not In this study, we analyzed the N13C values of 13 been identi¢ed yet (e.g., [2,3]). the bulk carbonate (N Ccarb vs. PDB) of mid-oce- Stable carbon isotope study has had a strong anic shallow-water limestones in two sections at impact on the study of major mass extinction Kamura and Taho in southwest Japan (see Fig. 1) events since the 1970s (e.g., [4^6]). As a negative in order to check carbon isotope signatures relat- 13 shift of N Ccarb implies that the lighter carbon ing to the mass extinction at the PTB. These two isotope (12C) is enriched in sediments but depleted sections cover the Changhsingian (Late Permian) in seawater, such a shift is regarded as a proxy for to Griesbachian^Dienerian (Early Triassic) inter- reconstructing paleoclimatic changes of lost val with clearly documented PTB by paleontolog- oceans [7]. Concerning the PTB, Holser and his ical data [22] (see Fig. 2). In this paper, we present colleagues [8] in the 1980s started to analyze sta- the ¢rst result of N13C measurements for the mid- 13 ble carbon isotopic compositions (N Ccarb vs. Pee- oceanic shallow-water PTB carbonates from the dee belemnite (PDB)) of carbonates spanning lost superocean, and discuss the implications for across the PTB. A clear short-period negative the PTB event. shift across the PTB was detected by Holser and Magaritz [9], Holser et al. [8] and Baud et al. [10] in various parts of the world, such as Austria^ 2. Materials Italy, Transcaucasia, China etc. Similar results were later added from other sections (e.g., The two study sections of the PTB limestone at [11,12]). All of these data suggest that chemostra- Kamura in central Kyushu and at Taho in west- tigraphic correlation of PTB using carbon iso- ern Shikoku occur in the Jurassic accretionary topes is useful and that a remarkable change has complex belt called the Chichibu belt, southwest occurred in biological productivity across the Japan (Fig. 1). Previous biostratigraphic studies PTB. All these studied PTB sections, nevertheless, using fusulinids, corals, pelycipods, ammonoids, represent ancient continental shelf sediments de- conodonts and other fossils (e.g., [23^27]) clari¢ed posited on and around the supercontinent Pan- that the limestone at Kamura spans from the mid- gea. There were no data available from the wide Permian to Late Triassic, and that the limestones superocean Panthalassa until the deep-sea chert at Taho from the latest Permian to the Late Tri- spanning across the PTB was found in Japan assic. Recently, Koike [22] ¢rst con¢rmed that [13^15]. these two sections contain the Griesbachian (the In the Jurassic accretionary complex in south- earliest Triassic) interval by recognizing the Hin- west Japan, fragments of ancient open-ocean (pe- deodus parvus and Isarcicella isarcica (conodont) lagic) biogenic sediments are contained as exotic blocks [16]. These include deep-sea bedded cherts and shallow-water limestones. The cherts represent ancient pelagic sediments deposited on mid- oceanic sea-£oor [17], while the limestones with- out coarse-grained terrigenous clastics represent ancient atoll or carbonate buildup developed on top of mid-oceanic seamount [18,19]. The pil- lowed basaltic greenstones underlying limestones have a characteristic geochemistry of oceanic island basalt a¤nity (e.g., [20,21]). The limestones often occur as hundred meter thick, sometimes kilometer long, exotic block within the Jurassic mudstone matrix. These allochthonous limestones range in age from Carboniferous to Triassic, and Fig. 1. Index map of the study sections. Distribution of the some of them preserve the PTB interval. Jurassic accretionary complex is after Isozaki [16].

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Fig. 2. Stratigraphic columns of the Kamura and Taho sections, after [22,24^27]. Not to scale.

Zones (Fig. 2). At both sections, the Griesbachian samples from the Taho section were carefully limestone conformably overlies the Changhsin- chosen through the screening test for diagenetic gian (Late Permian) dolomite, which is character- alteration using geochemical parameters (see Ap- ized by fusulinids and smaller foraminifers of the pendix). The stratigraphic horizons of the study Paleofusulina Zone. These consist mainly of bio- samples are displayed in Figs. 2 and 3. The ana- clastic carbonates, and completely exclude terrige- lyzed samples include 11 from the Changhsingian nous clastics, such as coarse quartzo-feldspathic and 10 from the Griesbachian^Dienerian for the grains, suggesting that their origin was in mid- Kamura section, and seven from the Changhsin- oceanic carbonate buildups remote from conti- gian and 15 from the Griesbachian^Dienerian for nental areas. On the basis of the stratigraphic dis- the Taho section. The results of chemical analysis tribution of index fusulinid and conodont fossils, by inductively coupled plasma atomic emission the PTB horizon is tentatively referred to the spectrometer (ICP-AES) and those of mineral lithologic boundary between the white dolomite analysis by X-ray di¡ractometry (XRD) for these and dark gray limestone in both sections (Fig samples are summarized in Table 1. 3). For details of litho- and biostratigraphy of Chemistry and mineralogy of these samples these sections, see [19,22,24^27]. change across the biostratigraphically docu- For the chemical and isotopic analyses, 21 fresh mented PTB (see Table 1 and Fig. 3). In both rock samples from the Kamura section and 22 the Kamura and Taho sections, the latest

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Fig. 3. Detailed columns of the study sections across the PTB at Kamura and Taho, showing the stratigraphic distribution of index fossils (fusulinids and conodonts) and horizons of samples for stable carbon isotope analysis. The highest horizon of the Per- mian fusulinid (Sta¡ella sp.) is just 1 cm below the dolomite/limestone contact (A. Ota and Y. Isozaki, unpublished data), while that of the lowest of the Triassic conodonts (H. parvus) is 50 cm above it [22] in the Kamura section. The PTB horizon is tentatively referred to the lithologic boundary between the white dolomite and dark gray limestone in both sections.

Changhsingian is represented by a gray to white and Sr show negative evidence for the fatal dia- dolomitic limestone which is enriched in Mg, Mn, genesis in these carbonates, judging from [28] (see and Fe, but depleted in Ca and Sr. In contrast, Appendix). the Griesbachian^Dienerian interval is composed of a dark gray or black micritic limestone, bearing calcite as a solo component, which is depleted in 3. Analytical procedures Mg, Mn and Fe, but enriched in Ca and Sr. In particular, the Griesbachian black limestone con- Minerals in the limestone were identi¢ed by tains the highest amount of total organic carbon powder XRD. Contents of Mg, Ca, Fe, Mn and (TOC) with a Corg value of 0.08%, although the Sr were measured by ICP-AES (Perkin Elmer) Corg values of the rest are normally less than with an analytical error of 2%. The content of 0.01% in both sections (Table 2). organic carbon (Corg) was obtained by measuring Geochemical parameters such as Mg, Mn, Fe, the volume of CO2 gas which was converted from

EPSL 5904 10-8-01 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 13

Table 1 Results on chemical and mineralogical analysis of limestone samples examined in this study Name Distancea Constituents Atomic ratio XRDb Mg Ca Mn Fe Sr Mg/Ca (m) (mg/g) (mg/g) (Wg/g) (Wg/g) (Wg/g) Kamura section 599 8.00 32.8 353 606 433 216 0.156 CD 600 6.80 4.4 391 94 28 377 0.019 C 601 6.50 22.1 365 197 183 279 0.102 CD 602 5.00 16.5 380 57 85 372 0.073 CD 604 3.80 31.4 368 135 127 300 0.143 CD 605 3.30 2.7 404 41 u.d.l. 494 0.011 C 608 2.70 2.4 406 7 u.d.l. 828 0.010 C 609 1.90 2.2 408 8 u.d.l. 1137 0.009 C 611 1.00 2.2 412 6 u.d.l. 1195 0.009 C 612c 0.00 ^ ^ ^ ^ ^ ^ C 612.5 30.32 112.0 251 304 97 154 0.748 D 612.8 31.04 103.5 255 28 39 195 0.681 D 613 32.00 28.9 375 101 u.d.l. 297 0.130 CD 615 33.84 1.7 405 14 u.d.l. 422 0.007 C 616 34.20 2.2 405 15 u.d.l. 384 0.009 C 655 310.04 2.2 406 44 u.d.l. 457 0.009 C 654 311.04 2.2 407 22 u.d.l. 483 0.009 C 653 311.76 1.9 408 9 u.d.l. 358 0.008 C 652 313.84 1.9 408 25 u.d.l. 434 0.008 C 651 314.24 1.7 410 14 u.d.l. 500 0.007 C 650 315.20 1.7 410 15 u.d.l. 517 0.007 C Taho section Q1 6.745 67.2 275 125 466 412 0.402 D R3 6.000 84.9 248 133 175 328 0.565 D S2 5.025 10.3 341 181 994 437 0.050 CD T1 3.625 82.0 263 650 599 337 0.515 D U3 2.875 45.3 305 290 565 534 0.246 D A5 2.025 11.1 351 124 134 595 0.052 CD B4 1.900 12.2 349 76 365 568 0.057 CD C4 1.525 5.8 370 51 u.d.l. 529 0.026 C D3 1.050 12.3 354 111 u.d.l. 460 0.057 CD E4 0.900 5.4 364 59 u.d.l. 561 0.024 C F4 0.425 35.3 308 159 u.d.l. 472 0.189 CD G4 0.300 2.1 369 62 u.d.l. 862 0.009 C H3 0.150 1.7 369 55 u.d.l. 1274 0.008 C I1 0.050 9.7 355 76 u.d.l. 753 0.045 CD I3 0.000 60.3 290 68 u.d.l. 378 0.343 D J2 30.075 87.2 261 65 u.d.l. 322 0.551 D K3 30.375 84.5 251 41 u.d.l. 240 0.556 D L2 30.475 87.4 257 59 u.d.l. 308 0.560 D M4 31.550 105.2 238 52 191 209 0.730 D N3 31.700 108.5 234 45 u.d.l. 177 0.764 D O3 31.925 105.0 231 44 u.d.l. 201 0.748 D P45 32.475 102.9 232 45 u.d.l. 233 0.733 D u.d.l.: under detection limit of Fe content analysis by ICP-AES. The limit was ca. 20 Wg/g. aThe distance is the length in sequence from the PTB (see Fig. 3). bThe minerals dominating in a sample detected by XRD are indicated as follows: C, calcite; D, dolomite; or CD, both minerals. cChemical analysis of this sample was not performed.

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Table 2 Analytical results on amount of organic carbon and the carbon isotope compositions in organic and inorganic carbon

a b 13 13 c 13 e Name Distance Corg N Corg N Cx (x) Rdolomite v C (m) (%) (x) x = calcite x = dolomite x = carbd (x) Kamura section 599 8.00 0.025 325.6 2.1 3.0 2.4 0.29 28.0 600 6.80 0.015 325.5 1.8 1.8 27.3 601 6.50 0.014 325.5 1.9 2.3 2.0 0.20 27.5 602 5.00 0.004 324.8 1.9 2.6 2.0 0.11 26.8 604 3.80 0.014 326.2 1.8 2.5 2.0 0.33 28.2 605 3.30 0.017 326.8 1.7 1.7 28.5 608 2.70 0.010 325.9 1.5 1.5 27.4 609 1.90 0.011 326.4 0.7 0.7 27.1 611 1.00 0.081 326.7 0.7 0.7 27.4 612f 0.00 ^ ^ 1.0 1.0 612.5 30.32 0.005 324.6 0.6 2.5 2.3 0.87 26.9 612.8 31.04 0.003 325.3 1.3 2.7 2.4 0.81 27.7 613 32.00 0.005 325.2 0.9 2.4 1.6 0.43 26.8 615 33.84 0.022 325.1 2.6 2.6 27.7 616 34.20 0.020 325.2 2.6 2.6 27.8 655 310.04 0.022 325.3 2.4 2.4 27.7 654 311.04 0.006 326.0 2.5 2.5 28.5 653 311.76 0.003 324.2 2.0 2.0 26.2 652 313.84 0.011 324.9 2.5 2.5 27.4 651 314.24 0.011 324.5 3.1 3.1 27.6 650 315.20 0.010 322.9 3.4 3.4 26.3 Taho section Q1 6.745 0.057 325.6 2.4 2.4 28.0 R3 6.100 0.073 325.6 S2 6.000 0.049 325.7 2.5 2.5 28.2 T1 5.025 0.022 325.5 U3 3.625 0.077 325.4 2.2 2.6 2.4 0.37 27.8 A5 2.875 0.011 325.5 2.5 2.5 28.0 B4 2.025 0.024 326.2 2.5 2.5 28.7 C4 1.900 0.010 326.3 D3 1.525 0.005 326.5 2.4 2.4 28.9 E4 1.050 0.019 326.3 F4 0.900 0.024 325.9 2.2 3.2 2.5 0.30 28.4 G4 0.425 0.013 327.9 1.9 1.9 29.7 H3 0.300 0.020 327.1 1.7 1.7 28.8 I1 0.150 0.021 326.4 1.9 1.9 28.3 I3 0.050 0.012 325.3 2.8 3.0 2.9 0.53 28.2 J2 0.000 0.008 325.9 2.0 2.9 2.6 0.73 28.5 K3 30.075 0.004 325.5 2.2 3.2 2.9 0.74 28.4 L2 30.375 0.013 325.8 2.6 3.0 2.9 0.72 28.7 M4 30.475 0.034 325.6 N3 31.550 0.019 325.7 2.7 3.4 3.3 0.84 29.0 O3 31.700 0.022 325.5 P45 31.925 0.024 325.5 2.7 3.5 3.2 0.64 28.7 aThe distance is the length in vertical sequence from the PTB (see Fig. 3). bThe notation expresses the amount of TOC extracted from limestone (see text). cThe N13C values of carbonates are normalized to the PDB value. d 13 The N Ccarb, which made a correction with the Rdolomite, expresses the value of a bulk carbonate (see text). e 13 13 13 v C=N Ccarb3N Corg. f 13 The Corg and N Corg values were not measured, because of the lower amount of this sample.

EPSL 5904 10-8-01 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 15 the residue extracted from the decalci¢ed sample. 4.1. Kamura section The gas was also used for isotope analysis of the 13 13 organic carbon (N Corg). Recovery yield on Corg In this section, N Corg values range from 326.8 13 and systematic error of N Corg analysis through a to 322.9x. The value gradually decreases from whole process were regularly checked by process- 322.9 to 325.3x in Changhsingian samples ing laboratory standards such as NBS-21 and (650^612.8, hereafter see Table 2). The value of NBS-22. To obtain the carbon isotope composi- sample 612 located above the PTB becomes mini- 13 tion of carbonates (N Ccarb), a sample was dis- mum and is 2 x lighter than that of sample solved with 100% HPO3 to extract inorganic car- 612.5 located directly underneath the PTB. Once bon as CO2. When a sample was composed of increasing to 325.9x in sample 608, the value calcite and dolomite, the proportion of the becomes nearly constant at 326x in the Gries- amount of calcite to that of dolomite composed bachian^Dienerian samples (608^599), while in 13 as Pdolomite/Pcalcite, where Pcalcite is the partial pres- calcite and dolomite in the carbonates, N Ccalcite sure of CO2 gas from calcite and Pdolomite is the values range between +0.6 and +3.4x, and 13 partial pressure of that from dolomite. The ex- N Cdolomite values range narrowly between +2.4 13 tracted gases were introduced into a mass spec- and +3.0x. The N Ccalcite value gradually de- trometer, and the N13C values of the calcite and creases from +3.4 to +2.6x between 650 and dolomite were measured, separately. The N13C 615, drops down to +0.6x between 613 and values were normalized to those of the standard, 609 across the PTB, and then increases up to PDB. +1.5x to stay at around +1.8x between 609 13 These analyses were performed at the laborato- and 599. The N Cdolomite value appears to change ry of the Faculty of Earth Sciences, Utrecht Uni- less signi¢cantly between the Griesbachian^Dien- versity, The Netherlands. The carbon isotopic erian and Changhsingian samples. Throughout 13 compositions were measured by a VG SIRA 24 these sections, the N Cdolomite values are higher 13 EM mass spectrometer with a dual-inlet double- than the N Ccalcite values. 13 collector system. All analyses on both Corg and Combining Eq. 1 with Eq. 2, N Ccarb values of N13C were carried out in multiplicate. Overall an- the bulk carbonates are calculated for the Early alytical errors were normally better than þ 0.1x Triassic (599, 601^604) and Late Permian (612.5^ (2c). 613) samples. Here, the proportion of dolomite (Rdolomite) obtained from Eq. 1 varies from 11 to 87%. The higher values are found in samples 4. Results 612.5 and 612.8 situated direct below the PTB 13 (Table 2). Along with the N Corg values, the 13 13 13 The analytical results on N Corg and N Ccarb of N Ccarb values show secular changes in Fig. 4A. 13 the Kamura and Taho sections are summarized in From 650 to 612.5 toward the PTB, the N Ccarb Table 2 and Fig. 4A,B. The N13C values of both values gradually decrease from +3.4 to +2.3x 13 calcite and dolomite are shown together with the with decreasing N Corg values. Between 612.5 13 corrected value for a bulk carbonate's carbon and 611 across the PTB, both the N Ccarb and 13 13 (N Ccarb). The correction was made to calculate N Corg values drop by a magnitude of ca. 2x. 13 13 a mean from N C values of the calcite (N Ccalcite) As the amount of sample 612 located on the PTB 13 13 and dolomite (N Cdolomite) in bulk carbonates, as was very small, only N Ccarb analysis was per- follows: formed and the value was +1.0x. These negative excursions, which started at the PTB, last up to Rdolomite Pdolomite= Pdolomite Pcalcite 1 13 609 in the Early Triassic. From 609, the N Ccarb values gradually increase to +2.0x with increas- 13 13 N Ccarb 13R N C 13 13 dolomite calcite ing N Corg values, and the N Ccarb value becomes nearly invariant at around +2.0x. The di¡erence 13 13 13 13 RdolomiteN Cdolomite 2 (v C=N Ccarb3N Corg) is constant at 28 þ 2x,

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13 13 Fig. 4. Secular changes in N Corg and N Ccarb values across the PTB in the Kamura (A) and Taho (B) sections. A marked negative shift of N13C of ca. 2x is identi¢ed across the PTB in both the Kamura and Taho sections. Note the parallel secular change 13 13 between the N Corg and N Ccarb values in both sections. which agrees with the value of 28 þ 3x obtained range between +2.6 and +3.5x, are always high- 13 in the Austrian limestones [29]. er than their N Ccalcite values. The Rdolomite value calculated from Eq. 1 varies from 30 to 84%, and 4.2. Taho section the highest is found in sample N3 located at 1.7 m below the PTB. A comparison in pro¢les between 13 13 13 In this section, N Corg values range between N Ccarb values obtained from Eq. 2 and N Corg 325.3x and 327.9x with an arithmetic mean values is shown in Fig. 4B. Between the Changh- 13 13 of 325.9x, and are nearly constant throughout singian and PTB, the N Ccarb and N Corg values this section. Among these samples, two (G4, are both nearly constant at +3x and 326x, 327.9x; H3, 327.1x) show negative anomaly respectively. Across the PTB between K3 and 13 13 (see Table 2). While N Ccalcite values range be- G4, the N Corg value drops drastically and step- 13 13 tween +1.7 and +2.8x, the lowest N Ccalcite val- wise from 325.5 to 327.9x. The N Ccarb value ue is found in sample H3 (0.15 m above PTB) synchronously shifts from +2.9 to +1.7x be- 13 where the N Corg value is also very light tween K3 and H3. These sharp shifts occur within (327.1x). Eight samples (U3, F4, I3, J2, K3, 0.43 m between I3 and F4 above the PTB in this 13 L2, N3, and P45) are composed of dolomite and section. The magnitude of the negative N Ccarb 13 calcite. Their N Cdolomite values, which narrowly shift is about 1.8x, which is nearly equal to

EPSL 5904 10-8-01 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 17 that of the shift in the Kamura section, 1.6x. cludes the superocean domain covering nearly 13 Beyond G4, N Ccarb values become nearly con- two thirds of the Earth's surface at the time. 13 stant at +2.5x, whereas N Corg values stay at The magnitude of the negative deviation of ca. 325.5x. In addition, the v13C value is nearly N13C(ca.2x) seems similar in every PTB sec- constant at 28 þ 1x, which is equivalent to the tion. This fact strongly suggests the global ubiq- value (28 þ 3x) reported by Magaritz et al. [29]. uity of carbon behavior. In addition, a chronolog- ical study using the U^Pb method [30] recently clari¢ed that the negative excursion across the 5. Discussion PTB lasted for ca. 160 000 years in south China, and probably also in other areas in the world. On 5.1. Global chemostratigraphic correlation of PTB the geological time scale, this duration is quite horizon short. These facts suggest that the global £ux of carbon isotopes may have been rapidly disturbed 13 13 The secular changes of N Corg and N Ccarb val- by an usual forcing across the PTB. ues of the two limestone sections show a remark- ably similar pattern, as seen in Fig. 4A,B. A sharp 5.2. Parallel negative shifts of N13C values negative excursion of N13C values is detected ex- actly in the basal part of the Triassic in both The second signi¢cant ¢nding in this study is sections. This indicates that the two sections are the parallel negative shifts of N13C values between properly correlated with each other not only by organic carbon and carbonate carbon across the 13 litho- and biostratigraphy, but also by chemostra- PTB, although a N Corg value of sample 612 in tigraphy. Because the two study sections are at the Kamura section was not obtained (see Fig. present separated from each other physiographi- 4A,B). The di¡erence (v13C) remains nearly con- cally by more than 100 km, it is reasonable to stant throughout the sections in both Kamura assume that the two sections were derived from and Taho. Such parallelism between organic and two distinct ancient seamounts, rather than frag- carbonate carbon has already been reported from mented from one large seamount. This suggests many PTB sections of the world, e.g., in Austria the isotope chemistry of the shallow seawater by Magaritz et al. [29]. The present result indi- around those two paleoseamounts was quite ho- cates that the carbon isotope behavior in western mogeneous. According to the paleogeographic re- Panthalassa was common with that around Pan- construction [16,21], these paleoseamounts were gea. Thus, parallelism in N13C values has a global located in the western half of the superocean Pan- context, and the magnitude of the negative devia- 13 13 13 thalassa. Therefore, the secular changes in N C tion of N Corg from N Ccarb is globally consistent detected in this study are the ¢rst regional infor- across the PTB. mation on the surface environment of western The samples we examined originated from a Panthalassa across the PTB. mid-oceanic shallow-water environment, where a The negative shift in N13C across the PTB imply biological pump usually may have made carbon that the two Japanese sections are chemostrati- isotopes kinetically £uctuate between organic and graphically correlated with other well-studied sec- inorganic carbon. In such a normal condition, the tions in the world, such as those in the southern lighter isotope (12C) tends to enrich in organics, Alps (Austria^Italy), Transcaucasia, south China, while the heavier one (13C) tends to concentrate in western Canada, and so on [8,10^12]. All of these seawater, with which carbonates are in equilibri- PTB sections, which were formed on and around um. The mechanism to hold the v13C value the supercontinent Pangea, show a sharp negative steady, however, cannot be explained by the nor- shift of N13C at the very base of the Lower Tri- mal biological pump function, because there is no assic. Thus, the negative spike of N13C across the carbon reservoir that counterbalances the negative PTB is a truly useful marker for chemostrati- £uctuation in organic carbon. This suggests that graphic correlation in a global context that in- the gross negative trend of N13C developed in shal-

EPSL 5904 10-8-01 18 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 low water, regardless of the biological pump. Fur- tween the shallow-water limestone of mid-Pan- 13 thermore, the N Ccarb values during the negative thalassa origin and continental shelf sequences 13 excursion of N Corg are likely to deviate nega- of Pangea. 13 13 tively from the range of N Ccarb values o¡ the 2. The parallel negative shift between N Corg and 13 13 interval of the excursion. These facts may favor N Ccarb values implies that C-depleted water the interpretation that the global parallelism re- may have developed widely throughout the sulted from the extensive development of extraor- superocean across the PTB. A large input of dinarily 12C-enriched seawater in equilibrium with light-weighing carbon into the seawater^at- carbonates. Consequently, the N13C values of all mosphere system may have been related to carbon compounds in the carbon circulation sys- the global environment change and the greatest tem then may have become depleted across the mass extinction in the Phanerozoic. PTB. The 12C-enriched seawater may have appeared in the superocean when the Earth's surface was Acknowledgements polluted by a great amount of 12C-rich carbon at the time of the PTB as suggested by Isozaki [31]. We thank D.H. Erwin and an anonymous re- A candidate for this pollutant, for instance, is gas viewer for thoughtful comments and E. Boyle for hydrate, as Kvenvolden [32] and Erwin [2] sug- manuscript handling. We appreciate J.L. Kirsch- gested. Because the N13C value of methane hy- vink, H. Visscher, R.D. Schuiling, and P. Van drate is reported to be as low as 365x, and Cappellen for their encouragement of this re- the burial amount at present is estimated more search. We also thank A.E. van Dijk and A. than 10 000 gigatoms of carbon [2], this pollutant van Leeuw-Tolboom for their helpful assistance could have changed the N13C value of seawater, if in isotope analysis, and A. Ota for letting us use an extensive amount of storage of the hydrate unpublished data.[EB] existed in the Permian. Yet, the possible mechanism to release the gas hydrate remains as a mys- tery. Appendix. Geochemical parameters The change in carbon behavior across the PTB presented here is likely related to a certain unusu- In order to evaluate the degree of diagenesis by al global phenomenon that coincided with the meteoric water in the calcite phase (i.e., [33]), we PTB mass extinction in both terrestrial and ma- used geochemical parameters such as Mg, Mn, rine environments [2,14]; nonetheless the cause Fe, and Sr with comparison to a cathode-lumines- and mechanism of the PTB event need to be clari- cence study by Mii et al. [28], who performed ¢ed yet. trace element analyses in the non-luminescent spots of the calcite shells of brachiopods. Their results, which were 860 þ 1180 Wg/g for Mg, 190 6. Conclusions Wg/g for Mn, 270 þ 70 Wg/g for Fe, and 960 þ 210 Wg/g for Sr, were applied to distinguish the lumi- 13 13 The secular change of N Corg and N Ccarb nescent part from the non-luminescent and across the PTB (251 Ma) in a mid-superocean slightly luminescent parts of the shells, and they shallow-water environment was documented, an- were used as the critical values to distinguish un- alyzing limestone samples from two separate areas altered from altered limestone. In the case of cal- in southwest Japan. cite in the Changhsingian sparitic limestone in the Kamura section and in the Early Triassic micritic 13 13 1. The secular change of N Corg and N Ccarb is limestone in both sections, the calcite contains a characterized by a marked negative shift of ca. trace amount of Mn, the content of which is less 3x across the PTB. This proves the accurate than the lowest e¡ective minimum concentration bio- and chemostratigraphic correlation be- for Mn-activated luminescence. The Fe content in

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