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Journal of Asian Earth Sciences 32 (2008) 22–33

The Paleozoic minimum of 87Sr/86Sr ratio in the Capitanian(Permian) mid-oceanic carbonates: A critical turning point

in the Late Paleozoic

Tomomi Kani a,*, Mamiko Fukui a, Yukio Isozaki b, Susumu Nohda a,c

a Department of Earth Sciences, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japanb Department of Earth Science and Astronomy, The University of Tokyo, Meguro, Tokyo 153-8902, Japan

c Earth Dynamic System Research Center, Department of Earth Sciences, National Cheng Kung University, Tainan 701, Taiwan

Received 22 May 2007; received in revised form 31 August 2007; accepted 3 October 2007

Abstract

The secular change in 87Sr/86Sr ratio of the Paleozoic recorded the lowest value below 0.7070 in Late Middle Permian, i.e., the Perm-ian minimum, suggesting that a major reform in oceanography occurred before the Paleozoic–Mesozoic transition. We have detected thecorrelative lowest 87Sr/86Sr value (0.706914 ± 0.000012) from the Middle-Upper Permian paleo-atoll limestone in Japan, formed prima-rily on an ancient seamount in mid-Panthalassa. The stratigraphic horizon of the minimum was constrained to the Capitanian interval(265.8–260.4 Ma), Middle Permian, by fusulines. As the mid-oceanic data represent the global average, the present study confirmed thechemostratigraphical utility of the ‘‘Capitanian minimum’’ of 87Sr/86Sr ratio in global correlation. After the long-term (nearly 280 millionyears) decrease throughout the Paleozoic since the Cambrian, a remarkable turnover of seawater 87Sr/86Sr values appeared in the Capit-anian immediately before the Guadalupian–Lopingian (Middle-Late Permian) boundary. A major global change likely appeared in theCapitanian to change the Sr-isotope balance in seawater from a mantle flux-dominated to a continental flux-dominated regime. The ini-tial rifting of Pangea probably started in the Capitanian, and the rifting-induced new connection of many intra-supercontinental drainagesystems to the superocean may have caused the overturn in global Sr isotopic trend.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Permian; Capitanian; Sr-isotope; Seamount; Limestone; Panthalassa

1. Introduction

The greatest mass extinction of the Phanerozoicoccurred at the end of the Paleozoic era, terminatingnearly 90% of marine invertebrate species (e.g., Sepkoski,1984; Erwin, 1993; Hallam and Wignall, 1997). This masskilling was in fact composed of two independent massextinctions occurred in a relatively short period of time;i.e., at the Middle-Late Permian (Guadalupian–Lopin-gian) boundary (G–LB) and at the Permian–Triassicboundary (P–TB) (Stanley and Yang, 1994; Jin et al.,1994; Isozaki and Ota, 2001; Ota and Isozaki, 2006).

1367-9120/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2007.10.007

* Corresponding author. Tel./fax: +81 96 342 3472.E-mail address: kani@sci.kumamoto-u.ac.jp (T. Kani).

Recently more emphasis is given to the G–LB event (Iso-zaki, 2007a,b) because the onset timing of a unique, glo-bal oxygen-depletion in the superocean (the P–TBsuperanoxia; Isozaki, 1997a) apparently coincides withthe G–LB extinction. Various possible kill mechanismswere proposed, however, their causes have not yet beenclarified (e.g., Erwin, 2006). The appearance of significantglobal environmental changes related to the two extinc-tions were suggested by several geochemical proxies mea-sured from ancient marine sediments, such as stableisotope ratio of carbon and sulfur (e.g., Holser et al.,1989; Kajiwara et al., 1994; Wang et al., 2004), U/Thratio (Wignall and Twitchett, 1996), rare earth elementabundance pattern (Musashino, 1990; Kato et al., 2002),and Moessbauer spectroscopy (Matsuo et al., 2003).

T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33 23

The initial ratio of strontium isotope (87Sr/86Sr) ofancient marine carbonates is regarded as another primegeochemical proxy for global chemostratigraphic correla-tion, dating, and paleoenvironmental analyses (e.g., Veizeret al., 1999; McArthur and Howarth, 2004). The isotopiccomposition of marine 87Sr/86Sr is driven essentially bythe two contrasting fluxes; i.e., the flux of the less radio-genic ‘‘mantle Sr’’ derived from hydrothermal activities inoceanic volcanism mostly along mid-oceanic ridges, andthe riverine flux of the more radiogenic ‘‘continental Sr’’(Peterman et al., 1970). During the Phanerozoic, 87Sr/86Srratio in carbonates recorded a long-term pattern of riseand fall as simplified in Fig. 1 (compiled from Denisonand Koepnick, 1995; Martin and MacDougall, 1995; Veiz-er et al., 1999; Korte et al., 2003, 2006).

The Permian period is characterized as an interval oflow 87Sr/86Sr ratios, including the Paleozoic minimumaround 0.7069–0.7068 in the Middle Permian (e.g., Burkeet al., 1982; Denison et al., 1994; Martin and MacDougall,1995; Veizer et al., 1999; Korte et al., 2003, 2006; McAr-thur and Howarth, 2004). After the long-term Paleozoicdecreasing trend for nearly 280 million years since theCambrian, the extremely low 87Sr/86Sr ratio appeared inthe Middle Permian (Guadalupian). This ‘‘Paleozoic mini-mum’’ or ‘‘Permian minimum’’ marks one of the two mostsignificant trough in oceanic Sr-isotope curve in the Phan-erozoic, besides the other in the Jurassic (Fig. 1). ThePermian minimum, therefore, appears likely to serve as agood marker in the chemostratigraphical correlation of aglobal scale.

Previous works pointed out that the Permian minimumoccurred at a certain time in the Middle-Upper Guadalu-pian, however, the data are too fragmentary to constrainits precise stratigraphic horizon. In addition, the latestamendment in the Permian chronostratigraphic subdivi-sion and mutual correlation among biozones of variousfossil groups (e.g., Wardlaw et al., 2004) requires re-evalu-ation of the stratigraphic level of the Paleozoic minimum of87Sr/86Sr ratio.

"Permian minimum"

0.7070

0.7090

0.7085

0.7080

0.7075

CretaceousJuraTriasPermCarbDevSilOrdCam

050100150200250300350400450500Numerical age (Ma)

87S

r / 86

Sr

Paleozoic MesozoicCenozoic

Fig. 1. The Phanerozoic secular change of Sr-isotope ratio (87Sr/86Sr). Thecurve modified from Fig. 7.1 of McArthur and Howarth (2004).

Mid-oceanic limestone primarily deposited on sea-mounts is composed of pure carbonates free of terrigenousclastic components due to the great distance from conti-nents. Thus they likely provide the best material to docu-ment averaged Sr-isotopic signature of the global seawater.

In order to determine the precise stratigraphic horizon/range of the 87Sr/86Sr minimum of the Paleozoic and tomake a global chemostratigraphical correlation, we ana-lyzed the fusuline-tuned Wordian, Capitanian (Guadalu-pian), and Wuchiapingian (Lower Lopingian; UpperPermian) limestones in Kyushu, Japan, that were depositedprimarily on an ancient mid-oceanic seamount in mid-Pan-thalassa (Isozaki and Ota, 2001; Ota and Isozaki, 2006).This paper reports the secular change in 87Sr/86Sr initialratio for the first time from mid-Panthalassan shallow mar-ine carbonates of the Upper Guadalupian to Lower Lopin-gian age. On the basis of the present and previous data, wediscuss the geological implications of the Permian Sr-iso-tope curve with respect to the Paleozoic minimum in theCapitanian.

2. Geologic setting

The Permian and Triassic limestone at Kamura (Tak-achiho town, Miyazaki prefecture; Fig. 2) in Kyushu formsa part of an ancient mid-oceanic atoll complex primarilydeveloped on a mid-oceanic paleo-seamount (Sano andNakashima, 1997; Isozaki and Ota, 2001; Ota and Isozaki,2006). This limestone, like many other Permian limestonesin Japan, occurs as allochthonous blocks incorporated inthe Middle-Upper Jurassic disorganized mudstone/sand-stone of the Jurassic accretionary complex in the Chichibubelt, southwest Japan (Isozaki, 1997b). The Late Paleozoicto early Mesozoic subduction polarity beneath the Asianblocks requires that the seamount was located to the eastsomewhere in the middle of the superocean Panthalassa.The limestone blocks in the Kamura area in central Kyu-shu retain parts of the primary mid-oceanic stratigraphy(ca. 135 m in thickness) and ranges in age from the Wor-dian (middle Guadalupian) to Norian (Upper Triassic)with several sedimentary breaks in the Triassic part (Kam-be, 1963; Kanmera and Nakazawa, 1973; Watanabe et al.,1979; Koike, 1996; Ota and Isozaki, 2006).

The Guadalupian part, called the Iwato Formation,consists of over 70 m-thick, dark gray to black bioclasticlimestone with a typical Tethyan shallow marine fauna thatincludes fusulines, smaller foraminifera, large-shelledbivalves, gastropods, brachiopods, rugose corals, and cal-careous algae. Fusulines are the most abundant, and theyprovide a basis for subdividing the Iwato Formation intofour biostratigraphic units; i.e., the Neoschwagerina Zone,Yabeina Zone, Lepidolina Zone, and a barren interval, inascending order (Ota and Isozaki, 2006; Isozaki et al.,2007b). The overlying Lopingian part, called the MitaiFormation, consists of nearly 40 m-thick, light gray bio-clastic dolomitic limestone with fusulines, smaller foramini-fers, and calcareous algae. The Mitai Formation is

35 N

130

E

135

EN

Mino-Tanba belt

Chichibu belt

Jurassic (latest Triassic to earliest Cretaceous)accretionary complex

300 km

Kamura

Akiyoshi

mid-oceanicridge

oceanic plate

accretionary complexes

volcanic arc

seamount

limestone

Fig. 2. Index map of the studied sections in the Kamura area in Kyushu. The Kamura area belongs to the Chichibu belt in Southwest Japan thatextensively exposes the Jurassic accretionary complex (upper). A simplified diagram showing the tectonic setting of the Permian Kamura seamount withina ridge-subduction system (lower; from Isozaki et al., 2007).

24 T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33

subdivided into two fusuline zones; the Codonofusiella-Rei-

chelina Zone and Palaeofusulina Zone (Kanmera and Nak-azawa, 1973; Ota and Isozaki, 2006). All these Tethyanfusuline assemblages and associated fossils (rugose coralsand large-shelled bivalves of family Alatoconchidae; Iso-zaki, 2006) indicate that the seamount was located in alow-latitude domain in the superocean Panthalassa undera tropical climate.

We studied three sections in the Kamura area; i.e., Sec-tions 1–3, from east to west (Fig. 3). Section 1 at southeastof Saraito village (32o45 01200N, 131o20 05500E) is composedof 57 m-thick limestone that belongs to the NeoschwagerinaZone and Yabeina Zone (Kambe, 1963; Isozaki, 2006). Sec-tion 2 at south of Shioinouso (32o44 05800N, 131o20 00200E),35 m thick, comprises the upper Iwato Formation andthe lower Mitai Formation, spanning across the Guadalu-pian–Lopingian boundary (G–LB). The Iwato Formationconsists of the Lepidolina Zone and a barren interval(Ota and Isozaki, 2006). Section 3 at northeast of Hijiriga-wa (32o45 00500N, 131o19 05200E) is 8 m thick that consists ofthe Yabeina Zone (Murata et al., 2003; Isozaki et al.,2007b).

The Neoschwagerina Zone is correlated with the Wor-dian (Middle Guadalupian) of Texas and with the Murga-bian in Transcaucasia, while the Yabeina Zone, Lepidolina

Zone, and most of the barren interval are correlated with

the Capitanian (Upper Guadalupian) of Texas and withthe Midian in Transcaucasia (e.g., Wardlaw et al., 2004).The Codonofusiella-Reichelina Zone corresponds to theWuchiapingian (Lower Lopingian) in South China. Fordetails of fusuline biostratigraphy and age assignment,refer to Ota and Isozaki (2006), Isozaki (2006), and Isozakiet al. (2007b). Thus, Section 1 represents the lowest amongthe three sections, while Section 2 the highest, and Section3 the middle, respectively (Fig. 3). A slight stratigraphicgap may exist between Sections 3 and 2; however, the sim-ilarity in lithofacies suggests that the possible gaps are con-siderably small, if at all. Likewise, the same fauna andlithofacies indicate that a possible gap between Sections 1and 3 is much smaller or even absent.

3. Sr-isotope analysis

In general, conodonts and brachiopod shells are rela-tively resistant to diagenesis (e.g., Popp et al., 1986; Ruppelet al., 1996) thus are often preferred for 87Sr/86Sr measure-ments. On the other hand, fine-grained micritic limestone(or lime mudstone) is another reliable material that retainsprimary 87Sr/86Sr signature, as demonstrated in a goodagreement among the coeval conodonts, brachiopods,and limestones of the Permian age (Popp et al., 1986; Den-ison et al., 1994; Denison and Koepnick, 1995; Martin and

main extinction ofGuadalupian fauna

radiation of Lopingian fauna

Section 1

Section 2

Section 3

131 20'E

32 45'N

1 km

Shioinouso

Hijirigawa

SaraitoN

Section 1Saraito

(55 m) Section 3Hijirigawa

(8 m)

Section 2Shioinouso

(35 m)

large fusulines (Verbeekinidae)

large bivalves (Alatoconchidae)

black limestone

black calcareous mudstone

260.4 Ma

265.8 Ma

Lopi

ng.

Gua

dalu

pian

Wor

dian

Cap

itani

anW

uch.

Neo

schw

ager

ina

Z.

Yabe

ina

Zon

eL

epid

olin

a Z

.ba

rren

inte

rval

C -

R. Z

.

dark gray limestone

light gray limestone

Fig. 3. Locality map and general stratigraphy of the studied sections in the Kamura area in Kyushu. Locality map and the three studied sections in theKamura and their stratigraphic relations Fossil data are from Isozaki (2006), Ota and Isozaki (2006) and Isozaki et al. (2007a,b). (1:25,000 topographicmap sheet ‘Mitai (Sobosan)’ published by the Geographical Survey Institute of Japan).

T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33 25

MacDougall, 1995). Owing to the facies-related scarcity ofbrachiopods and conodonts in the Permian limestone in

Kamura, we analyzed fine-grained micritic part of lime-stone in bulk.

26 T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33

We collected more than 50 samples of fine-grained lime-stone (lime mudstone composed of pure carbonates withscarce terrigenous components) from the above-describedthree sections in the Kamura area (Sections 1–3) andselected 32 vein-free fresh ones for Sr-isotope measure-ments. Sample horizons are shown in Fig. 3. The samplenumbers of Section 2 are common with those used in Iso-zaki et al. (2007a,b). Samples A1 and B1 from Section 1belong to the Neoschwagerina Zone, and Samples B37-40from Section 1 and all samples from Section 3 to the Yabe-

ina Zone, respectively. In Section 2, Samples KM1.8-4, 1.8,1.85, 4-0, 4-2 to the Lepidolina Zone, samples KM 4-4, 4-8,4-9, Core 1, Core 5, 4-b, Core 6, 4-d to the barren interval,and Samples KM 5-1, 5-7b, 6-12, 7-14, 15 to the Codono-fusiella-Reichelina Zone, respectively.

For Sr-isotope measurements, 40–50 mg of handpickedspecimens from each sample was dissolved in 5 ml of 2 Msuprapure hydrochloric acid. For two pilot samples(KM1.8 and KM 4-9), micritic limestone are dissolved sep-arately in two fractions for comparison; i.e., one with 2 Mhydrochloric acid, and the other with 1 M acetic acid, inorder to check if there is isotopic contamination fromdissolved silicate clastic phase. Sr was extracted in 1 mlmicro-columns filled with 100 ll Sr Spec resin (ElChromIndustries). The column was rinsed with 3 ml of 3 MHNO3, and Sr was eluted with 1 ml of warm 0.05 MHNO3. Separated Sr was loaded on single Ta filamentswith 1 N H3PO4. The 87Sr/86Sr ratio was measured by themass spectrometer (Finnigan MAT 262) at the Faculty ofScience, Kumamoto University, with a reproducibility of1 · 10�5. Recent average value of standard NIST SRM987 are 87Sr/86Sr = 0.710267 ± 14 (2SD; n = 24). Our labo-ratory blanks were <500 pg.

In the Carboniferous and Permian carbonates in Japan,strontium concentrations are varied from 300 to 3400 ppm,and Rb/Sr ratios are always very low (<0.01) (e.g., Fuji-nuki, 1968). The very low Rb/Sr ratios make the measured87Sr/86Sr ratios of carbonates close to the initial ratios ofPermian.

4. Results and discussion

4.1. 87Sr/86Sr minimum

Table 1 and Fig. 4 show the measurements of 87Sr/86Srratio and their stratigraphic position in the studied sectionsin the Kamura area. The 87Sr/86Sr values vary in 0.706915–0.707561. As to the pilot sample KM1.8, 87Sr/86Sr values ofa fraction dissolved by 2 M HCl and the other by 1 M ace-tic acid are 0.707282 ± 0.000024 and 0.707256 ± 0.000020,respectively (Table 1), and those of KM 4-9 are0.706929 ± 0.000019 and 0.706969 ± 0.000042, respec-tively. In both cases, the two fractions show more or lessthe same values within the measurement error ranges, sug-gesting that the limestone of the study sections do not con-tain significant amount of terrigenous silicate phase whichmight affect original Sr-isotope signature in carbonates.

The measured values are clearly lower than most of theknown Phanerozoic values ranging between 0.7075 and0.7085 (e.g., Veizer et al., 1999; McArthur and Howarth,2004; Fig. 1) and they show an overall symmetric patternalong stratigraphy. This pattern likely represents a uniquesecular change in Sr-isotope composition of ambient sea-water around the mid-Panthalassan Permian seamount ofKamura. In the Guadalupian Iwato Formation, the Wor-dian Neoschwagerina Zone is characterized by relativelyhigh values of 0.707232–0.707335, while the CapitanianYabeina Zone, the Lepidolina Zone, and the barren intervalby very low values of 0.706914–0.707260, 0.707049–0.707282, and 0.706929–0.707561, respectively. A slightjump in 87Sr/86Sr values from the top of Section 3 to thebottom of Section 2 suggests that a small stratigraphicgap may exist, probably within the increasing intervalimmediately above the minimum (Fig. 3). In turn, the top-most barren interval of the Iwato Formation and the over-lying Codonofusiella-Reichelina Zone of the WuchiapingianMitai Formation is characterized again by relatively highvalues of 0.707228–0.707418.

Extremely low 87Sr/86Sr values (<0.7070) were measuredat seven horizons; four of them (HJ 0, 2a, 2b, 3) in theYabeina Zone in Section 3, and the rest three (KM 4-9,Core 1, Core 5) in the barren interval in Section 2. In par-ticular, the lowest values among all the measurements inKamura, 0.706914 ± 0.000016 and 0.706915 ± 0.000009,were detected in Samples HJ 0 and HJ 2b from the upperpart of the Yabeina Zone at Section 3. Other nine sampleswith relatively low 87Sr/86Sr values (between 0.7070 and0.7071) are restricted in the Yabeina Zone in Section 1, plusin the Lepidolina Zone and the barren interval in Section 2.

Thus in terms of 87Sr/86Sr values, the upper Guadalu-pian to lower Lopingian limestone in Kamura is subdi-vided into the following five chemostratigraphic segmentsin ascending order (Fig. 4); i.e., Segment 1 with relativelyhigh 87Sr/86Sr values over 0.7072 (the Neoschwagerina

Zone and the lower Yabeina Zone), Segment 2 with low87Sr/86Sr values lower than 0.7071 (the upper Yabeina

Zone), Segment 3 with relatively high values of 0.7072–0.7073 (the lower Lepidolina Zone), Segment 4 with low87Sr/86Sr values lower than 0.7071 (the upper Lepidolina

Zone and the lower barren interval), and Segment 5 withrelatively high 87Sr/86Sr values over 0.7072 (the uppermostbarren interval and the Codonofusiella-Reichelina Zone).

The interval from Segment 2 to segment 4, more than30 m in thickness, is totally characterized by extremelylow 87Sr/86Sr values. The symmetric pattern in 87Sr/86Srstratigraphy, centered by the minimum (0.7069–0.7070),marks a unique turning point from a decreasing to increas-ing trend (Fig. 4). In detail, there are two negative peaks;i.e., one in the upper Yabeina Zone and the other in thebarren interval. Particularly interesting is the former withextremely low values (the upper half of Segment 2) thatis restricted to the 1.5 m-thick (Fig. 4).

As mid-oceanic paleo-atoll carbonates formed withoutreceiving direct terrigenous flux from continents, they likely

Table 1Analytical results of 87Sr/86Sr ratio of the Guadalupian to Lopingain (Middle-Upper Permian) limestone in Kamura, Japan

Sample Age Fusuline zone 87Sr/86Sr 2r

Section 2

KM 15 Wuchiapingian Codonofusiella-Reichelina 0.707387 0.000013KM 7-14 Wuchiapingian Codonofusiella-Reichelina 0.707418 0.000011KM 6-12 Wuchiapingian Codonofusiella-Reichelina 0.707288 0.000025KM 5-7b Wuchiapingian Codonofusiella-Reichelina 0.707389 0.000017KM 5-1 Wuchiapingian Codonofusiella-Reichelina 0.707228 0.000013KM 4-d Capitanian Barren interval 0.707230 0.000015Core 6 Capitanian Barren interval 0.707181* 0.000028KM 4-b Capitanian Barren interval 0.707370 0.000059Core 5 Capitanian Barren interval 0.706958* 0.000032Core 1 Capitanian Barren interval 0.706930* 0.000034KM 4-9 Capitanian Barren interval 0.706929 0.000019

0.706969* 0.000042KM 4-8 Capitanian Barren interval 0.707036 0.000011KM 4-4 Capitanian Barren interval 0.707053 0.000012KM 4-2 Capitanian Lepidolina 0.707051 0.000014KM 4-0 Capitanian Lepidolina 0.707049 0.000018KM1.85 Capitanian Lepidolina 0.707250* 0.000039KM1.8 Capitanian Lepidolina 0.707282 0.000024

0.707256* 0.000020KM1.8-4 Capitanian Lepidolina 0.707239 0.000140

Section 3

HJ 0 Capitanian Yabeina 0.706914 0.000016HJ 1 Capitanian Yabeina 0.707003 0.000009HJ 2a Capitanian Yabeina 0.706940 0.000009HJ 2b Capitanian Yabeina 0.706915 0.000009HJ 3 Capitanian Yabeina 0.706996 0.000021HJ 4 Capitanian Yabeina 0.707019 0.000014HJ 5 Capitanian Yabeina 0.707063 0.000010HJ 7 Capitanian Yabeina 0.707047 0.000020

Section 1

SR B40 Capitanian Yabeina 0.707080 0.000028SR B39 Capitanian Yabeina 0.707094 0.000020SR B38 Capitanian Yabeina 0.707215 0.000010SR B37 Capitanian Yabeina 0.707260 0.000015SR B1 Wordian Neoschwageria 0.707232 0.000019SR A1 Wordian Neoschwageria 0.707335 0.000018

Each measurement was corrected for mass fractionation by using 86Sr/88Sr = 0.1194.The measurement ratios were normalized to a nominal value of 0.710240 for the standard NIST SRM 987.

* Samples were dissolved in 1 M acetic acid.

T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33 27

recorded genuine Sr isotopic composition of ambient sea-water that represented the Panthalassan average, thus theglobal average. The present dataset apparently confirmsthat the global seawater had an extremely low 87Sr/86Srratio in the latest part of the Guadalupian, probablyreflecting more dominant influence of ‘‘mantle Sr’’ ratherthan ‘‘continental Sr’’.

4.2.2. Correlation

The extremely low 87Sr/86Sr ratios, lower than 0.7070,were recognized solely from the Middle Permian in thePaleozoic (Martin and Macdougall, 1995; Veizer et al.,1999; McArthur and Howarth, 2004; Fig. 1). The 87Sr/86Srminimum detected in Kamura evidently corresponds tothat in the Permian often called the ‘‘Permian minimum’’or ‘‘Paleozoic minimum’’ (e.g., Nishioka et al., 1991; Den-

ison et al., 1994; Martin and Macdougall, 1995; Joneset al., 1995; Korte et al., 2006).

The Phanerozoic secular change of seawater 87Sr/86Srinitial ratio was compiled by many previous workers(e.g., Burke et al., 1982; Veizer et al., 1999; McArthurand Howarth, 2004; Fig. 1). Particularly for the Permianto Triassic periods, Martin and Macdougall (1995), Deni-son and Koepnick (1995), and Korte et al. (2006) showedmore detailed summaries. All these compilations uniformlydemonstrated that the Permian minimum occurred in theupper Middle or lower Upper Permian, commonly in theupper Guadalupian. However, the resolution of the Mid-dle-Upper Permian chronostratigraphy and the relevantinternational correlation scheme have been greatlyenhanced during the last decade, including the detailedstratigraphy in the stratotype sections in South Chinaand West Texas particularly around the Guadalupian/

Cod

onof

usie

lla-

Rei

chel

ina

Zon

eFu

sulin

e Zo

ne

Lep

idol

ina

Zon

e

Wuc

hiap

ingi

anC

apita

nian

barr

en in

terv

alN

eosc

hwag

erin

a Z

one

Wor

dian

Yabe

ina

Zon

e

Gua

dalu

pian

Lopi

ngia

n

5

8

20

10

15

30

25

Sect

ion

3

0

5

35

40

70

Stra

tigra

phic

level

(m)

0.70

68

0.70

69

0.70

70

0.70

71

0.70

72

0.70

73

0.70

74

0.70

75

87Sr/86Sr

HJ 0HJ 1

SR B39

HJ 2a,2bHJ 4HJ 5

HJ 7

HJ 3

KM1.8-4

KM 4-dKM 5-1

KM 4-9KM 4-8

KM 4-4

KM 4-2

KM 5-7b

KM 6-12

KM 7-14

KM 15

SR A1

SR B1

Segm

ent 1

Seg.

3Se

gmen

t 5

Sam

ple N

o.

Age

G-L B.

65~ 20 m covered

35

30

25

20

SR B37SR B38

SR B40

40

Segm

ent 2

Segm

ent 4

Sect

ion

1Se

ctio

n 2

0

KM1.8KM1.85KM 4-0

Core 1

KM 4-bCore 5

Core 6

Fig. 4. The secular change in 87Sr/86Sr values in the three sections in Kamura. Note the interval of extremely low in 87Sr/86Sr values in the Yabeina Zoneand the Lepidolina Zone (fusuline) of the lower-middle Capitanian.

28 T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33

Lopingian (Middle/Upper Permian) boundary (e.g., Jinet al., 1994; Wilde et al., 1999; Wardlaw et al., 2004). Thus,

we must re-evaluate the horizon of the Permian minimumin the latest stratigraphic scheme.

T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33 29

Table 2 lists previously reported extremely low 87Sr/86Srvalues lower than 0.7070 from the Guadalupian rocks bio-stratigraphically controlled by conodonts and fusulines(Nishioka et al., 1991; Denison et al., 1994; Jones et al.,1995; Martin and Macdougall, 1995; Korte et al., 2006; thisstudy). Regardless of sample material ranging from micriticcalcite of lime mudstone (in bulk), brachiopod shells (cal-cite), to conodont (fluoro-apatite), their 87Sr/86Sr valuesstay in the range of 0.7068–0.7070. To date, there is novalue lower than 0.7068 reported from the Paleozoic rocks(Fig. 1). Most of the measurements lower than 0.7070 werefrom West Texas/New Mexico located on the western mar-gin of the supercontinent Pangea during the Permian time.Two more measurements were reported from Akiyoshi inJapan (Nishioka et al., 1991).

The 15 samples from West Texas/New Mexico withextremely low values (Table 2) were collected from theWordian to Capitanian rocks of the Altuda Formation inthe Del Norte Mountains, the Capitan Formation in DarkCanyon, plus the Lamar and Reef Trail members of theBell Canyon Formation in the Guadalupe Mountains;i.e., the stratotype area both of the Guadalupian andCapitanian (Denison et al., 1994; Martin and Macdougall,1995; Korte et al., 2006). The Wordian samples belong tothe Jinogondolella asserrata (conodont) Zone, the lower-middle Capitanian ones to the J. postserrata Zone, andthe upper Capitanian ones to the J. shannoni Zone plus

Table 2List of extremely low 87Sr/86Sr values (<0.7070) previously reported from the

No. Material Age Biozone

Texas/New Mexico/Utah, USA

1 Conodont Wordian Jas2 Brachiopod Wordian Jas3 Conodont Capitanian Jp4 Conodont Capitanian Jp5 Bulk ms Capitanian Jp6 Bulk ms Capitanian Jp7 Brachiopod Capitanian Jp8 Brachiopod Capitanian Jp9 Brachiopod Capitanian Js

10 Brachiopod Capitanian Jal11 Brachiopod Capitanian Jal12 Aragonite Capitanian13 Aragonite Capitanian14 Aragonite Capitanian15 Aragonite Capitanian

Akiyoshi, Japan

16 Bulk limestone Capitanian L17 Bulk limestone Capitanian L

Kamura, Japan

18 Bulk limestone Capitanian Y19 Bulk limestone Capitanian Y20 Bulk limestone Capitanian Y21 Bulk limestone Capitanian Y22 Bulk limestone Capitanian L

Data are compiled from (1) Martin and Macdougall (1995); (2) Denison and Ket al. (1991); (6) this study.Jas, Jinogondolella asserata Zone; Jp, J. postserrata Zone; Js, J. shannoni Zone

J. altudaensis–J. crofti Zone, respectively (Korte et al.,2006). Three 87Sr/86Sr values of conodonts, six of brachio-pod shells, and two of lime mudstone concordantly rangein 0.70680–0.706978. In contrast, numerous measurementsfor the Leonardian/Kungrian conodonts and limestone aregreater than 0.7073, and those for the immediately youngerLopingian ones are likewise over 0.7071, respectively (Mar-tin and Macdougall, 1995; Korte et al., 2006). This contrastsuggests that the interval of the extremely low 87Sr/86Sr val-ues (<0.7070) is constrained to Wordian and lower-middleCapitanian in the stratotype area. The lowest value 0.70680was recorded in the lime mudstone of the Lamar Memberthat occupied the middle one-third of the type Capitanianinterval.

Similarly low 87Sr/86Sr values were reported from thetopmost part of the Akiyoshi limestone in Japan (Nishiokaet al., 1991). This limestone also represents an accreted sea-mount complex that docked to the Japan (=South Chinaoff east Pangea) margin much earlier in the Late Permian(Fig. 2) than the Kamura seamount accreted in the Juras-sic, therefore, the Akiyoshi seamount recorded oceanicinformation of a slightly different domain even within thewestern Panthalassa, much closer to South China. Thetwo micrite samples with low values (0.7068) were collectedfrom the Lepidolina Zone in the topmost Akiyoshi lime-stone Group that is currently correlated with middle Capit-anian. On the other hand, the Lower Permian to Wordian

biostratigraphically controlled Wordian–Capitanian (Guadalupian) rocks

87Sr/86Sr initial ratio 2r Ref.

0.706914 0.000022 (1)0.706948 0.000032 (1)0.706913 0.000022 (1)0.706913 0.000022 (2)0.70684 0.00005 (2)0.70680 0.00005 (2)0.706844 0.000007 (2)0.706932 0.000007 (3)0.706904 0.000010 (3)0.706888 0.000010 (3)0.706857 0.000007 (3)0.706896 0.000011 (4)0.706876 0.000013 (4)0.706887 0.000010 (4)0.706877 0.000010 (4)

0.7068 0.00002 (5)0.7068 0.00002 (5)

0.706914 0.000016 (6)0.706915 0.000009 (6)0.706940 0.000009 (6)0.706996 0.000021 (6)0.706929 0.000019 (6)

oepnick (1995); (3) Korte et al. (2006); (4) Jones et al. (1995); (5) Nishioka

; Jalc, J. altudaensis–J. crofti Zone; L, Lepidolina Zone; Y, Yabeina Zone.

30 T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33

samples from the Akiyoshi Group, below the Lepidolina

Zone, are all characterized by higher values over 0.7072.No data are available for the Upper Permian from Akiyo-shi, owing to the absence of strata.

Thus, the 87Sr/86Sr minimum interval in Kamura (Seg-ments 2–4) is fairly correlated with the Capitanian rocksin the stratotype in Texas and with those deposited onthe Akiyoshi seamount. The Permian Kamura and Akiyo-shi seamounts with paleo-atoll limestone existed in the wes-tern Panthalassa, almost on the opposite side of the globewith respect to Texas located on the western margin ofPangea. The documentation of the contemporary isotopicsignal in marine carbonates in the two separated areas onthe globe proves not only the global homogeneity of87Sr/86Sr ratio in the Permian seawater but also the chemo-stratigraphical utility of the 87Sr/86Sr minimum in theCapitanian as a prime horizon in global correlation.

In general, mid-oceanic sedimentary rocks represent thebest archive of average seawater chemistry of globaloceans, as they are free from local effects (e.g., basin geom-etries and/or river runoffs affecting Sr signatures). Thepresent data from the Kamura area clarified that the lowest87Sr/86Sr ratios around 0.70690 are restricted to the Capit-anian in the mid-Panthalassan limestone. As the lowermostpart of the Yabeina Zone, as well as the uppermost part ofthe barren interval, has relatively higher values between0.7072 and 0.7074, the claimed low 87Sr/86Sr interval inKamura excludes the lowermost and the uppermost partof the Capitanian. According to Gradstein et al. (2004),the entire Capitanian ranges for ca. 5.4 million years from265.8 to 260.4 Ma. Therefore, the 87Sr/86Sr minimum inseawater persisted over the major part of Capitanian, fornearly 5 million years, in the mid-Panthalassan domain.As we obtained more strict age constraints, here we pro-pose a name ‘‘Capitanian minimum’’ for the key signalrather than previously used ‘‘Paleozoic minimum’’ or‘‘Permian minimum’’. In contrast, the very low 87Sr/86Srratio detected exceptionally in the Wordian in West Texasmay be explained by local effects reflecting tectonics and/orvolcanism; however, this needs further check.

4.3. Omen of the Pangean breakup?

The present study confirms the occurrence of the Sr min-imum in the Capitanian that represents one of the most sig-nificant signals in the Phanerozoic secular change in oceanchemistry. Despite of minor fluctuations, the Paleozoic eraas a whole experienced a long-term decrease in seawater87Sr/86Sr composition (Fig. 1). After the Capitanian mini-mum, the secular trend changed its course to the overallincrease toward the Early Mesozoic (e.g., Veizer et al.,1999; Korte et al., 2006; McArthur and Howarth, 2004)(Fig. 5). Thus the Capitanian world was at a critical condi-tion that bordered the Paleozoic and Mesozoic oceano-graphic regimes.

The very low 87Sr/86Sr values in marine carbonates (ca.0.7069), relatively closer to the value of oceanic crust/

upper mantle (0.703), generally indicate that hydrother-mal activity along mid-oceanic ridges was high enoughto propel an influential amount of less radiogenic Sr withrespect to riverine/groundwater discharge from continentsreplete with more radiogenic Sr. The extremely low87Sr/86Sr values are usually regarded as a consequenceof a unique tectonic setting with extremely high activityof mid-oceanic ridges that was coupled with a sea-levelrise and resultant suppression of on-land erosion/weather-ing rate. As to the Middle-Late Permian, however, thereis no supporting geological evidence neither for increasedproduction of oceanic crusts nor for the long-term high-stand suppressing continental erosion/weathering. Insteadthe lowest sea-level of the Paleozoic occurred in the Mid-dle-Late Permian (Hallam and Wignall, 1999; Tong andShi, 2000) (Fig. 6). This is supported by the latest carbonisotope study at the same Kamura section (Isozaki et al.,2007a,b) that suggests the appearance of a transient coolperiod in Panthalassa and associated sea-level drop in theCapitanian. Under a low sea-level condition, erosion/weathering rate on continents normally increases to shedvoluminous amounts of terrigenous clastics enriched inradiogenic Sr into oceans. This apparently contradictswith the above-mentioned general explanation for tecton-ics-driven Sr fluctuation.

Nonetheless, the suppression in continental Sr flux tooceans may have been the case for the end-Paleozoic.The origin of the Permian minimum of 87Sr/86Sr valuesin the Capitanian is likely explained by the initial riftingof Pangea (Isozaki, 2007b) considerably before the finalbreakup in the Triassic–Jurassic. The supercontinentalassembly of Pangea continued throughout the Paleozoic.By closing multiple oceans, all the major continental blocksmerged together by the mid-Carboniferous. This assemblymay have shut down the direct connection of many pre-existing continental rivers to Panthalassa, confining majorintra-supercontinental drainage systems with no escape.Such a unique tectonic setting during the Late Carbonifer-ous to Middle Permian, regardless of mid-oceanic ridgeactivity, may have suppressed the riverine input of terrrig-enous clastics replete with radiogenic Sr into the coeval sea-water until new drainage systems formed by the followingcontinental breakup. The total amount of groundwater dis-charges to oceans also decreased by loosing long shore-lines. The subsurface groundwater as another‘‘continental flux’’ discharges significantly contributes tothe global seawater Sr isotopic balance and the totalamount of groundwater flux also depend on the totallength of coastline (Chaudhuri and Clauer, 1986; Basuet al., 1999, 2001; Dowling et al., 2003). When Pangea ini-tially started to rift and to attenuate the Pangean crust inthe later part of the Permian, new major river systemsshould have appeared to open direct links to Panthalassaagain and supplied large amounts of radiogenic Sr-loadingterrigenous clastics previously stored within Pangea. It wasin the Triassic and Early Jurassic when mid-oceanic ridgesfirst appear to open a real oceanic domain (Atlantic

C-R.Z.

WuchiapingianCapitanianWordian

0.7068

0.7070

0.7071

0.7072

0.7073

0.7074

0.7075

87S

r / 8

6 Sr

Fusuline Zone

0.7069

0.7074

0.7076

0.7080

87S

r / 8

6 Sr

0.7070

0.7072

0.7078

87Sr/86Sr curve for well preserved Permian brachiopod shells after Korte et al. (2006)

GuadalupianCisuralian Lopingian0.7068

Permian270.6 Ma

87Sr/86Sr curve for Kamura paleo-atoll limestone

Neoschwagerina Zone Yabeina Z. Lepidolina Z. barren interval

260.4 Ma

Permianminimum

Permian minimum

Fig. 5. Chemostratigraphic correlation in Sr-isotope ratio between the Kamura paleo-atoll limestone (this study; upper) and the collected Permianbrachiopod analyses (lower; Fig. 5 of Korte et al., 2006). Note that the Permian minimum is mutually correlated.

Permian

WuchiapingianCapitanianWordian

Guadalupian Lopingian

extinction ofGuadalupian fusulines

radiation of Lopimgian fusulines

Permian minimum

0.7074

0.7068

0.7070

0.7072

sea-level drop sea-level rise

seaw

ater

87S

r/86

Sr

260.4 Ma

Pangean initial rifting

265.8 Ma

Fig. 6. A schematic correlation diagram showing a possible causal linkamong the Pangean initial rifting, seawater chemistry (87Sr/86Sr ratio)change, and the Guadalupian–Lopingian boundary mass extinction.

T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33 31

Ocean), and accordingly 87Sr/86Sr values started todecrease again (Fig. 1).

5. Conclusions

The Middle-Upper Permian paleo-atoll limestone inJapan derived primarily from mid-Panthalassa recordedthe minimum 87Sr/86Sr values (ca. 0.7069) in the Phanero-zoic. The stratigraphic horizon of the minimum was pre-cisely constrained to the Capitanian (Late MiddlePermian) immediately before the G–L boundary. The‘‘Capitanian minimum’’ horizon can serve as a useful mar-ker for further chemostratigraphic correlation of the Perm-ian. The sharp turning point in 87Sr/86Sr values likely hasrecorded the timing of the initial rifting of Pangea that con-nected many intra-supercontinental drainage systemsdirectly to the superocean. This apparent proximity in tim-ing between the Sr minimum and the mass extinction of theGuadalupian fauna may suggest a possible causal linkbetween the initial breakup of Pangea and the G–LBextinction.

Acknowledgements

We are grateful to Bor-ming Jahn who gave us valuablecomments to polish up the manuscript. S. Yamasaki, C.Hisanabe, T. Nii, S. Sutani helped us in fieldwork, and S.Ogata provided local conveniences. This research was

32 T. Kani et al. / Journal of Asian Earth Sciences 32 (2008) 22–33

partly supported by Grant-in-Aid of the Japan Society ofPromoting Sciences (No. 16204040 to Y.I.).

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