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- 0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S0012-821X(01)00398-3 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 repre- sent 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 is- land 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]. EPSL 5904 10-8-01 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 11 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 EPSL 5904 10-8-01 12 M. Musashi et al. / Earth and Planetary Science Letters 191 (2001) 9^20 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), 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 tenta- tively 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.