Stable carbon isotope signature in mid-Panthalassashallow-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 Netherlandsb 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 limestoneprimarily deposited on ancient mid-oceanic seamounts in an ancient Pacific Ocean or superocean Panthalassa. Thisstudy examines stable carbon isotope compositions (N13Ccarb and N13Corg) of such open-ocean shallow-water limestoneacross the Permo^Triassic boundary (PTB) at Kamura and Taho in southwest Japan. The results show an almostidentical secular change in N13Ccarb values with a remarkable negative spike across the PTB in both sections. Thisconfirms for the first time that the mid-Panthalassa shallow-water carbonates are bio- and chemo-stratigraphicallycorrelated not with previously studied PTB sections from the peripheries of Pangea. The negative shift in N13Ccarb occursparallel to that of N13Corg in both sections, and the difference (v13C = N13Ccarb3N13Corg) remains nearly constantthroughout the sections. This implies that the 13C-depleted water should have developed widely, probably in a globalextent, throughout the superocean Panthalassa across the PTB. These findings suggest that a large input of12C-enriched carbon into the ocean^atmosphere system has occurred and may have caused a global environmentchange probably relating to the greatest mass extinction in the Phanerozoic. ß 2001 Elsevier Science B.V. All rightsreserved.

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

1. Introduction

Across the Permo^Triassic boundary (PTB) ca.251 Ma, the largest mass extinction in the Phaner-ozoic occurred in which up to 96% of marine in-vertebrate species became extinct [1]. Althoughseveral hypotheses including sea-level change,temperature change, seawater salinity change, an-oxia, hypercapnia etc. were proposed, the sub-

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 3 9 8 - 3

* Corresponding author. Present address: Faculty of Sys-tems Science and Technology, Akita Prefectural University,84-4 Tsuchiya-Ebinokuchi, Honjyo, Akita 015-0055, Japan.Tel. : +81-184-27-2166; Fax: +81-184-27-2189.

E-mail addresses: mmusashi@akita-pu.ac.jp (M. Musashi),isozaki@chianti.c.u-tokyo.ac.jp (Y. Isozaki),koike@ed.ynu.ac.jp (T. Koike).

1 Present adress: ISOLAB, Zuiderlingedijk 97, 4211 BBSpijk (Lingewaal), The Netherlands.

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stantial cause of the PTB catastrophe has notbeen identi¢ed yet (e.g., [2,3]).

Stable carbon isotope study has had a strongimpact on the study of major mass extinctionevents since the 1970s (e.g., [4^6]). As a negativeshift of N13Ccarb implies that the lighter carbonisotope (12C) is enriched in sediments but depletedin seawater, such a shift is regarded as a proxy forreconstructing paleoclimatic changes of lostoceans [7]. Concerning the PTB, Holser and hiscolleagues [8] in the 1980s started to analyze sta-ble carbon isotopic compositions (N13Ccarb vs. Pee-dee belemnite (PDB)) of carbonates spanningacross the PTB. A clear short-period negativeshift across the PTB was detected by Holser andMagaritz [9], Holser et al. [8] and Baud et al. [10]in various parts of the world, such as Austria^Italy, Transcaucasia, China etc. Similar resultswere later added from other sections (e.g.,[11,12]). All of these data suggest that chemostra-tigraphic correlation of PTB using carbon iso-topes is useful and that a remarkable change hasoccurred in biological productivity across thePTB. All these studied PTB sections, nevertheless,represent ancient continental shelf sediments de-posited on and around the supercontinent Pan-gea. There were no data available from the widesuperocean Panthalassa until the deep-sea chertspanning across the PTB was found in Japan[13^15].

In the Jurassic accretionary complex in south-west Japan, fragments of ancient open-ocean (pe-lagic) biogenic sediments are contained as exoticblocks [16]. These include deep-sea bedded chertsand shallow-water limestones. The cherts repre-sent ancient pelagic sediments deposited on mid-oceanic sea-£oor [17], while the limestones with-out coarse-grained terrigenous clastics representancient atoll or carbonate buildup developed ontop of mid-oceanic seamount [18,19]. The pil-lowed basaltic greenstones underlying limestoneshave a characteristic geochemistry of oceanic is-land basalt a¤nity (e.g., [20,21]). The limestonesoften occur as hundred meter thick, sometimeskilometer long, exotic block within the Jurassicmudstone matrix. These allochthonous limestonesrange in age from Carboniferous to Triassic, andsome of them preserve the PTB interval.

In this study, we analyzed the N13C values ofthe bulk carbonate (N13Ccarb vs. PDB) of mid-oce-anic shallow-water limestones in two sections atKamura and Taho in southwest Japan (see Fig. 1)in order to check carbon isotope signatures relat-ing to the mass extinction at the PTB. These twosections cover the Changhsingian (Late Permian)to Griesbachian^Dienerian (Early Triassic) inter-val with clearly documented PTB by paleontolog-ical data [22] (see Fig. 2). In this paper, we presentthe ¢rst result of N13C measurements for the mid-oceanic shallow-water PTB carbonates from thelost superocean, and discuss the implications forthe PTB event.

2. Materials

The two study sections of the PTB limestone atKamura in central Kyushu and at Taho in west-ern Shikoku occur in the Jurassic accretionarycomplex belt called the Chichibu belt, southwestJapan (Fig. 1). Previous biostratigraphic studiesusing fusulinids, corals, pelycipods, ammonoids,conodonts and other fossils (e.g., [23^27]) clari¢edthat the limestone at Kamura spans from the mid-Permian to Late Triassic, and that the limestonesat Taho from the latest Permian to the Late Tri-assic. Recently, Koike [22] ¢rst con¢rmed thatthese two sections contain the Griesbachian (theearliest Triassic) interval by recognizing the Hin-deodus parvus and Isarcicella isarcica (conodont)

Fig. 1. Index map of the study sections. Distribution of theJurassic accretionary complex is after Isozaki [16].

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Zones (Fig. 2). At both sections, the Griesbachianlimestone conformably overlies the Changhsin-gian (Late Permian) dolomite, which is character-ized by fusulinids and smaller foraminifers of thePaleofusulina Zone. These consist mainly of bio-clastic carbonates, and completely exclude terrige-nous clastics, such as coarse quartzo-feldspathicgrains, suggesting that their origin was in mid-oceanic carbonate buildups remote from conti-nental areas. On the basis of the stratigraphic dis-tribution of index fusulinid and conodont fossils,the PTB horizon is tentatively referred to thelithologic boundary between the white dolomiteand dark gray limestone in both sections (Fig3). For details of litho- and biostratigraphy ofthese sections, see [19,22,24^27].

For the chemical and isotopic analyses, 21 freshrock samples from the Kamura section and 22

samples from the Taho section were carefullychosen through the screening test for diageneticalteration using geochemical parameters (see Ap-pendix). The stratigraphic horizons of the studysamples are displayed in Figs. 2 and 3. The ana-lyzed samples include 11 from the Changhsingianand 10 from the Griesbachian^Dienerian for theKamura section, and seven from the Changhsin-gian and 15 from the Griesbachian^Dienerian forthe Taho section. The results of chemical analysisby inductively coupled plasma atomic emissionspectrometer (ICP-AES) and those of mineralanalysis by X-ray di¡ractometry (XRD) for thesesamples are summarized in Table 1.

Chemistry and mineralogy of these sampleschange across the biostratigraphically docu-mented PTB (see Table 1 and Fig. 3). In boththe Kamura and Taho sections, the latest

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

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Changhsingian is represented by a gray to whitedolomitic limestone which is enriched in Mg, Mn,and Fe, but depleted in Ca and Sr. In contrast,the Griesbachian^Dienerian interval is composedof a dark gray or black micritic limestone, bearingcalcite as a solo component, which is depleted inMg, Mn and Fe, but enriched in Ca and Sr. Inparticular, the Griesbachian black limestone con-tains the highest amount of total organic carbon(TOC) with a Corg value of 0.08%, although theCorg values of the rest are normally less than0.01% in both sections (Table 2).

Geochemical parameters such as Mg, Mn, Fe,

and Sr show negative evidence for the fatal dia-genesis in these carbonates, judging from [28] (seeAppendix).

3. Analytical procedures

Minerals in the limestone were identi¢ed bypowder XRD. Contents of Mg, Ca, Fe, Mn andSr were measured by ICP-AES (Perkin Elmer)with an analytical error of 2%. The content oforganic carbon (Corg) was obtained by measuringthe volume of CO2 gas which was converted from

Fig. 3. Detailed columns of the study sections across the PTB at Kamura and Taho, showing the stratigraphic distribution of in-dex 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), whilethat of the lowest of the Triassic conodonts (H. parvus) is 50 cm above it [22] in the Kamura section. The PTB horizon is tenta-tively referred to the lithologic boundary between the white dolomite and dark gray limestone in both sections.

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Table 1Results 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 section599 8.00 32.8 353 606 433 216 0.156 CD600 6.80 4.4 391 94 28 377 0.019 C601 6.50 22.1 365 197 183 279 0.102 CD602 5.00 16.5 380 57 85 372 0.073 CD604 3.80 31.4 368 135 127 300 0.143 CD605 3.30 2.7 404 41 u.d.l. 494 0.011 C608 2.70 2.4 406 7 u.d.l. 828 0.010 C609 1.90 2.2 408 8 u.d.l. 1137 0.009 C611 1.00 2.2 412 6 u.d.l. 1195 0.009 C612c 0.00 ^ ^ ^ ^ ^ ^ C612.5 30.32 112.0 251 304 97 154 0.748 D612.8 31.04 103.5 255 28 39 195 0.681 D613 32.00 28.9 375 101 u.d.l. 297 0.130 CD615 33.84 1.7 405 14 u.d.l. 422 0.007 C616 34.20 2.2 405 15 u.d.l. 384 0.009 C655 310.04 2.2 406 44 u.d.l. 457 0.009 C654 311.04 2.2 407 22 u.d.l. 483 0.009 C653 311.76 1.9 408 9 u.d.l. 358 0.008 C652 313.84 1.9 408 25 u.d.l. 434 0.008 C651 314.24 1.7 410 14 u.d.l. 500 0.007 C650 315.20 1.7 410 15 u.d.l. 517 0.007 CTaho sectionQ1 6.745 67.2 275 125 466 412 0.402 DR3 6.000 84.9 248 133 175 328 0.565 DS2 5.025 10.3 341 181 994 437 0.050 CDT1 3.625 82.0 263 650 599 337 0.515 DU3 2.875 45.3 305 290 565 534 0.246 DA5 2.025 11.1 351 124 134 595 0.052 CDB4 1.900 12.2 349 76 365 568 0.057 CDC4 1.525 5.8 370 51 u.d.l. 529 0.026 CD3 1.050 12.3 354 111 u.d.l. 460 0.057 CDE4 0.900 5.4 364 59 u.d.l. 561 0.024 CF4 0.425 35.3 308 159 u.d.l. 472 0.189 CDG4 0.300 2.1 369 62 u.d.l. 862 0.009 CH3 0.150 1.7 369 55 u.d.l. 1274 0.008 CI1 0.050 9.7 355 76 u.d.l. 753 0.045 CDI3 0.000 60.3 290 68 u.d.l. 378 0.343 DJ2 30.075 87.2 261 65 u.d.l. 322 0.551 DK3 30.375 84.5 251 41 u.d.l. 240 0.556 DL2 30.475 87.4 257 59 u.d.l. 308 0.560 DM4 31.550 105.2 238 52 191 209 0.730 DN3 31.700 108.5 234 45 u.d.l. 177 0.764 DO3 31.925 105.0 231 44 u.d.l. 201 0.748 DP45 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 2Analytical results on amount of organic carbon and the carbon isotope compositions in organic and inorganic carbon

Name Distancea Corgb N13Corg N13Cx

c (x) Rdolomite v13Ce

(m) (%) (x) x = calcite x = dolomite x = carbd (x)

Kamura section599 8.00 0.025 325.6 2.1 3.0 2.4 0.29 28.0600 6.80 0.015 325.5 1.8 1.8 27.3601 6.50 0.014 325.5 1.9 2.3 2.0 0.20 27.5602 5.00 0.004 324.8 1.9 2.6 2.0 0.11 26.8604 3.80 0.014 326.2 1.8 2.5 2.0 0.33 28.2605 3.30 0.017 326.8 1.7 1.7 28.5608 2.70 0.010 325.9 1.5 1.5 27.4609 1.90 0.011 326.4 0.7 0.7 27.1611 1.00 0.081 326.7 0.7 0.7 27.4612f 0.00 ^ ^ 1.0 1.0612.5 30.32 0.005 324.6 0.6 2.5 2.3 0.87 26.9612.8 31.04 0.003 325.3 1.3 2.7 2.4 0.81 27.7613 32.00 0.005 325.2 0.9 2.4 1.6 0.43 26.8615 33.84 0.022 325.1 2.6 2.6 27.7616 34.20 0.020 325.2 2.6 2.6 27.8655 310.04 0.022 325.3 2.4 2.4 27.7654 311.04 0.006 326.0 2.5 2.5 28.5653 311.76 0.003 324.2 2.0 2.0 26.2652 313.84 0.011 324.9 2.5 2.5 27.4651 314.24 0.011 324.5 3.1 3.1 27.6650 315.20 0.010 322.9 3.4 3.4 26.3Taho sectionQ1 6.745 0.057 325.6 2.4 2.4 28.0R3 6.100 0.073 325.6S2 6.000 0.049 325.7 2.5 2.5 28.2T1 5.025 0.022 325.5U3 3.625 0.077 325.4 2.2 2.6 2.4 0.37 27.8A5 2.875 0.011 325.5 2.5 2.5 28.0B4 2.025 0.024 326.2 2.5 2.5 28.7C4 1.900 0.010 326.3D3 1.525 0.005 326.5 2.4 2.4 28.9E4 1.050 0.019 326.3F4 0.900 0.024 325.9 2.2 3.2 2.5 0.30 28.4G4 0.425 0.013 327.9 1.9 1.9 29.7H3 0.300 0.020 327.1 1.7 1.7 28.8I1 0.150 0.021 326.4 1.9 1.9 28.3I3 0.050 0.012 325.3 2.8 3.0 2.9 0.53 28.2J2 0.000 0.008 325.9 2.0 2.9 2.6 0.73 28.5K3 30.075 0.004 325.5 2.2 3.2 2.9 0.74 28.4L2 30.375 0.013 325.8 2.6 3.0 2.9 0.72 28.7M4 30.475 0.034 325.6N3 31.550 0.019 325.7 2.7 3.4 3.3 0.84 29.0O3 31.700 0.022 325.5P45 31.925 0.024 325.5 2.7 3.5 3.2 0.64 28.7aThe 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.dThe N13Ccarb, which made a correction with the Rdolomite, expresses the value of a bulk carbonate (see text).ev13C = N13Ccarb3N13Corg.f The Corg and N13Corg values were not measured, because of the lower amount of this sample.

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the residue extracted from the decalci¢ed sample.The gas was also used for isotope analysis of theorganic carbon (N13Corg). Recovery yield on Corg

and systematic error of N13Corg analysis through awhole process were regularly checked by process-ing laboratory standards such as NBS-21 andNBS-22. To obtain the carbon isotope composi-tion of carbonates (N13Ccarb), a sample was dis-solved with 100% HPO3 to extract inorganic car-bon as CO2. When a sample was composed ofcalcite and dolomite, the proportion of theamount of calcite to that of dolomite composedas Pdolomite/Pcalcite, where Pcalcite is the partial pres-sure of CO2 gas from calcite and Pdolomite is thepartial pressure of that from dolomite. The ex-tracted gases were introduced into a mass spec-trometer, and the N13C values of the calcite anddolomite were measured, separately. The N13Cvalues were normalized to those of the standard,PDB.

These analyses were performed at the laborato-ry of the Faculty of Earth Sciences, Utrecht Uni-versity, The Netherlands. The carbon isotopiccompositions were measured by a VG SIRA 24EM mass spectrometer with a dual-inlet double-collector system. All analyses on both Corg andN13C were carried out in multiplicate. Overall an-alytical errors were normally better than þ 0.1x(2c).

4. Results

The analytical results on N13Corg and N13Ccarb ofthe Kamura and Taho sections are summarized inTable 2 and Fig. 4A,B. The N13C values of bothcalcite and dolomite are shown together with thecorrected value for a bulk carbonate's carbon(N13Ccarb). The correction was made to calculatea mean from N13C values of the calcite (N13Ccalcite)and dolomite (N13Cdolomite) in bulk carbonates, asfollows:

Rdolomite � Pdolomite=�Pdolomite � Pcalcite� �1�

N13Ccarb � �13Rdolomite�N13Ccalcite�

RdolomiteN13Cdolomite �2�

4.1. Kamura section

In this section, N13Corg values range from 326.8to 322.9x. The value gradually decreases from322.9 to 325.3x in Changhsingian samples(650^612.8, hereafter see Table 2). The value ofsample 612 located above the PTB becomes mini-mum and is 2 x lighter than that of sample612.5 located directly underneath the PTB. Onceincreasing to 325.9x in sample 608, the valuebecomes nearly constant at 326x in the Gries-bachian^Dienerian samples (608^599), while incalcite and dolomite in the carbonates, N13Ccalcite

values range between +0.6 and +3.4x, andN13Cdolomite values range narrowly between +2.4and +3.0x. The N13Ccalcite value gradually de-creases from +3.4 to +2.6x between 650 and615, drops down to +0.6x between 613 and609 across the PTB, and then increases up to+1.5x to stay at around +1.8x between 609and 599. The N13Cdolomite value appears to changeless signi¢cantly between the Griesbachian^Dien-erian and Changhsingian samples. Throughoutthese sections, the N13Cdolomite values are higherthan the N13Ccalcite values.

Combining Eq. 1 with Eq. 2, N13Ccarb values ofthe bulk carbonates are calculated for the EarlyTriassic (599, 601^604) and Late Permian (612.5^613) samples. Here, the proportion of dolomite(Rdolomite) obtained from Eq. 1 varies from 11 to87%. The higher values are found in samples612.5 and 612.8 situated direct below the PTB(Table 2). Along with the N13Corg values, theN13Ccarb values show secular changes in Fig. 4A.From 650 to 612.5 toward the PTB, the N13Ccarb

values gradually decrease from +3.4 to +2.3xwith decreasing N13Corg values. Between 612.5and 611 across the PTB, both the N13Ccarb andN13Corg values drop by a magnitude of ca. 2x.As the amount of sample 612 located on the PTBwas very small, only N13Ccarb analysis was per-formed and the value was +1.0x. These negativeexcursions, which started at the PTB, last up to609 in the Early Triassic. From 609, the N13Ccarb

values gradually increase to +2.0x with increas-ing N13Corg values, and the N13Ccarb value becomesnearly invariant at around +2.0x. The di¡erence(v13C = N13Ccarb3N13Corg) is constant at 28 þ 2x,

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which agrees with the value of 28 þ 3x obtainedin the Austrian limestones [29].

4.2. Taho section

In this section, N13Corg values range between325.3x and 327.9x with an arithmetic meanof 325.9x, and are nearly constant throughoutthis section. Among these samples, two (G4,327.9x ; H3, 327.1x) show negative anomaly(see Table 2). While N13Ccalcite values range be-tween +1.7 and +2.8x, the lowest N13Ccalcite val-ue is found in sample H3 (0.15 m above PTB)where the N13Corg value is also very light(327.1x). Eight samples (U3, F4, I3, J2, K3,L2, N3, and P45) are composed of dolomite andcalcite. Their N13Cdolomite values, which narrowly

range between +2.6 and +3.5x, are always high-er than their N13Ccalcite values. The Rdolomite valuecalculated from Eq. 1 varies from 30 to 84%, andthe highest is found in sample N3 located at 1.7 mbelow the PTB. A comparison in pro¢les betweenN13Ccarb values obtained from Eq. 2 and N13Corg

values is shown in Fig. 4B. Between the Changh-singian and PTB, the N13Ccarb and N13Corg valuesare both nearly constant at +3x and 326x,respectively. Across the PTB between K3 andG4, the N13Corg value drops drastically and step-wise from 325.5 to 327.9x. The N13Ccarb valuesynchronously shifts from +2.9 to +1.7x be-tween K3 and H3. These sharp shifts occur within0.43 m between I3 and F4 above the PTB in thissection. The magnitude of the negative N13Ccarb

shift is about 1.8x, which is nearly equal to

Fig. 4. Secular changes in N13Corg and N13Ccarb values across the PTB in the Kamura (A) and Taho (B) sections. A marked nega-tive shift of N13C of ca. 2x is identi¢ed across the PTB in both the Kamura and Taho sections. Note the parallel secular changebetween the N13Corg and N13Ccarb values in both sections.

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that of the shift in the Kamura section, 1.6x.Beyond G4, N13Ccarb values become nearly con-stant at +2.5x, whereas N13Corg values stay atca. 325.5x. In addition, the v13C value is nearlyconstant at 28 þ 1x, which is equivalent to thevalue (28 þ 3x) reported by Magaritz et al. [29].

5. Discussion

5.1. Global chemostratigraphic correlation of PTBhorizon

The secular changes of N13Corg and N13Ccarb val-ues of the two limestone sections show a remark-ably similar pattern, as seen in Fig. 4A,B. A sharpnegative excursion of N13C values is detected ex-actly in the basal part of the Triassic in bothsections. This indicates that the two sections areproperly correlated with each other not only bylitho- and biostratigraphy, but also by chemostra-tigraphy. Because the two study sections are atpresent separated from each other physiographi-cally by more than 100 km, it is reasonable toassume that the two sections were derived fromtwo distinct ancient seamounts, rather than frag-mented from one large seamount. This suggeststhe isotope chemistry of the shallow seawateraround those two paleoseamounts was quite ho-mogeneous. According to the paleogeographic re-construction [16,21], these paleoseamounts werelocated in the western half of the superocean Pan-thalassa. Therefore, the secular changes in N13Cdetected in this study are the ¢rst regional infor-mation on the surface environment of westernPanthalassa across the PTB.

The negative shift in N13C across the PTB implythat the two Japanese sections are chemostrati-graphically correlated with other well-studied sec-tions in the world, such as those in the southernAlps (Austria^Italy), Transcaucasia, south China,western Canada, and so on [8,10^12]. All of thesePTB sections, which were formed on and aroundthe supercontinent Pangea, show a sharp negativeshift of N13C at the very base of the Lower Tri-assic. Thus, the negative spike of N13C across thePTB is a truly useful marker for chemostrati-graphic correlation in a global context that in-

cludes the superocean domain covering nearlytwo thirds of the Earth's surface at the time.

The magnitude of the negative deviation ofN13C (ca. 2x) seems similar in every PTB sec-tion. This fact strongly suggests the global ubiq-uity of carbon behavior. In addition, a chronolog-ical study using the U^Pb method [30] recentlyclari¢ed that the negative excursion across thePTB lasted for ca. 160 000 years in south China,and probably also in other areas in the world. Onthe geological time scale, this duration is quiteshort. These facts suggest that the global £ux ofcarbon isotopes may have been rapidly disturbedby an usual forcing across the PTB.

5.2. Parallel negative shifts of N13C values

The second signi¢cant ¢nding in this study isthe parallel negative shifts of N13C values betweenorganic carbon and carbonate carbon across thePTB, although a N13Corg value of sample 612 inthe Kamura section was not obtained (see Fig.4A,B). The di¡erence (v13C) remains nearly con-stant throughout the sections in both Kamuraand Taho. Such parallelism between organic andcarbonate carbon has already been reported frommany PTB sections of the world, e.g., in Austriaby Magaritz et al. [29]. The present result indi-cates that the carbon isotope behavior in westernPanthalassa was common with that around Pan-gea. Thus, parallelism in N13C values has a globalcontext, and the magnitude of the negative devia-tion of N13Corg from N13Ccarb is globally consistentacross the PTB.

The samples we examined originated from amid-oceanic shallow-water environment, where abiological pump usually may have made carbonisotopes kinetically £uctuate between organic andinorganic carbon. In such a normal condition, thelighter isotope (12C) tends to enrich in organics,while the heavier one (13C) tends to concentrate inseawater, with which carbonates are in equilibri-um. The mechanism to hold the v13C valuesteady, however, cannot be explained by the nor-mal biological pump function, because there is nocarbon reservoir that counterbalances the negative£uctuation in organic carbon. This suggests thatthe gross negative trend of N13C developed in shal-

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low water, regardless of the biological pump. Fur-thermore, the N13Ccarb values during the negativeexcursion of N13Corg are likely to deviate nega-tively from the range of N13Ccarb values o¡ theinterval of the excursion. These facts may favorthe interpretation that the global parallelism re-sulted from the extensive development of extraor-dinarily 12C-enriched seawater in equilibrium withcarbonates. Consequently, the N13C values of allcarbon compounds in the carbon circulation sys-tem then may have become depleted across thePTB.

The 12C-enriched seawater may have appearedin the superocean when the Earth's surface waspolluted by a great amount of 12C-rich carbon atthe time of the PTB as suggested by Isozaki [31].A candidate for this pollutant, for instance, is gashydrate, as Kvenvolden [32] and Erwin [2] sug-gested. Because the N13C value of methane hy-drate is reported to be as low as 365x, andthe burial amount at present is estimated morethan 10 000 gigatoms of carbon [2], this pollutantcould have changed the N13C value of seawater, ifan extensive amount of storage of the hydrateexisted in the Permian. Yet, the possible mecha-nism to release the gas hydrate remains as a mys-tery.

The change in carbon behavior across the PTBpresented here is likely related to a certain unusu-al global phenomenon that coincided with thePTB mass extinction in both terrestrial and ma-rine environments [2,14]; nonetheless the causeand mechanism of the PTB event need to be clari-¢ed yet.

6. Conclusions

The secular change of N13Corg and N13Ccarb

across the PTB (251 Ma) in a mid-superoceanshallow-water environment was documented, an-alyzing limestone samples from two separate areasin southwest Japan.

1. The secular change of N13Corg and N13Ccarb ischaracterized by a marked negative shift of ca.3x across the PTB. This proves the accuratebio- and chemostratigraphic correlation be-

tween the shallow-water limestone of mid-Pan-thalassa origin and continental shelf sequencesof Pangea.

2. The parallel negative shift between N13Corg andN13Ccarb values implies that 13C-depleted watermay have developed widely throughout thesuperocean across the PTB. A large input oflight-weighing carbon into the seawater^at-mosphere system may have been related tothe global environment change and the greatestmass extinction in the Phanerozoic.

Acknowledgements

We thank D.H. Erwin and an anonymous re-viewer for thoughtful comments and E. Boyle formanuscript handling. We appreciate J.L. Kirsch-vink, H. Visscher, R.D. Schuiling, and P. VanCappellen for their encouragement of this re-search. We also thank A.E. van Dijk and A.van Leeuw-Tolboom for their helpful assistancein isotope analysis, and A. Ota for letting us useunpublished data.[EB]

Appendix. Geochemical parameters

In order to evaluate the degree of diagenesis bymeteoric water in the calcite phase (i.e., [33]), weused geochemical parameters such as Mg, Mn,Fe, and Sr with comparison to a cathode-lumines-cence study by Mii et al. [28], who performedtrace element analyses in the non-luminescentspots of the calcite shells of brachiopods. Theirresults, which were 860 þ 1180 Wg/g for Mg, 190Wg/g for Mn, 270 þ 70 Wg/g for Fe, and 960 þ 210Wg/g for Sr, were applied to distinguish the lumi-nescent part from the non-luminescent andslightly luminescent parts of the shells, and theywere used as the critical values to distinguish un-altered from altered limestone. In the case of cal-cite in the Changhsingian sparitic limestone in theKamura section and in the Early Triassic micriticlimestone in both sections, the calcite contains atrace amount of Mn, the content of which is lessthan the lowest e¡ective minimum concentrationfor Mn-activated luminescence. The Fe content in

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the calcite is less than 28 Wg/g, which is nearly thedetection limit of ICP-AES. In addition, Mg andSr contents are nearly below the criteria shownabove. As a result, as far as whole-rock analysisis concerned, these geochemical parameters safelyneglect fatal diagenesis for the calcite phase ana-lyzed in this study.

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