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Palaeogeography, Palaeoclimatology, P

Carbon isotopic evidence for chemocline upward excursionsduring the end-Permian event

Anthony Riccardi a,⁎, Lee R. Kump a, Michael A. Arthur a, Steven D'Hondt b

a Department of Geosciences and NASA Astrobiology Institute, The Pennsylvania State University, University Park, PA 16802, United Statesb Graduate School of Oceanography and NASA Astrobiology Institute, University of Rhode Island, Narragansett, RI 02882, United States

Received 1 March 2006; received in revised form 17 November 2006; accepted 24 November 2006

Abstract

A negative shift in marine inorganic carbon-isotope composition (δ13Ccarb) during the end-Permian mass extinction has beenused as evidence for several different extinction mechanisms. Changes to the δ13C of organic matter and the difference between itand δ13Ccarb (Δ13C=δ13Ccarb − δ13Corg) have been examined at few locations, with conflicting interpretations. We examine thechanges to both organic and inorganic carbon isotopes across the Permian–Triassic boundary at two marine sections from SouthChina (Meishan and Shangsi) and compare these to data from other previously published sections. Through these analyses, wedemonstrate that a decrease in Δ13C occurred during the extinction event throughout the Paleo-Tethys ocean. The extent andintensity of the decrease varies by location averaging a negative shift of ∼ 5‰. Several possibilities as to the cause of this shiftexist including Siberian trap volcanism, a change in the terrestrial/marine organic carbon input to the system, or a change in thedominant marine biota brought about through environmental changes (such as widespread ocean anoxia/euxinia). The decrease inΔ13C observed at many of these sections across the event horizon is here interpreted to represent a shift from algae/cyanobacteria toless fractionating phototrophic sulfur bacteria in marine shelf environments resulting from upward excursions of the chemocline.These chemocline upward excursions would release euxinic water to the photic zone allowing phototrophic sulfur bacteria tothrive. The limited available biomarker data are consistent with this interpretation.© 2006 Elsevier B.V. All rights reserved.

Keywords: Permian; Triassic; Mass extinction; Carbon-isotope

1. Introduction

The end-Permian mass extinction was the greatestmass extinction of the Phanerozoic, with an estimated lossof 90% of marine species (Erwin, 1993). The cause ofthe end-Permian extinction remains a subject of intensedebate, with mechanisms internal and external to thebiosphere being invoked. Many internal mechanismsfocus onwidespread ocean anoxia (Wignall andTwitchett,

⁎ Corresponding author. Tel.: +02 814 863 1274.E-mail address: ariccard@geosc.psu.edu (A. Riccardi).

0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2006.11.010

Please cite this article as: Riccardi, A. et al. Carbon isotopic evidencePalaeogeogr. Palaeoclimatol. Palaeoecol. (2007), doi:10.1016/j.palaeo.200

1996; Isozaki, 1997), possibly creating euxinic oceansleading to a buildup, and catastrophic outgassing of, H2S(Kump et al., 2005), or rapid global warming eitherthrough the destabilization of methane clathrates (Krulland Retallack, 2000) or from Siberian Trap volcanism(Renne and Basu, 1991). External mechanisms focus on abolide impact to the ocean (Becker and Poreda, 2001;Kaiho et al., 2001; Becker et al., 2004), but the evidencefor this remains highly controversial (Kerr, 2004; Koeberlet al., 2004). The main phase of the extinction appears tobe coincident with the eruption of the Siberian Traps(Mundil et al., 2004).

for chemocline upward excursions during the end-Permian event.6.11.010

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The above mechanisms have all been invoked toexplain the worldwide decrease in the δ13C of marinecarbonate (δ13Ccarb) seen at the event horizon of themass extinction. The event horizon is the bed where thelargest number of species became extinct and is distinctfrom the Permian–Triassic boundary, which is definedas the first occurrence of the conodont Hindeodusparvus (Yin, 1996). While many studies focus separate-ly on the changes to the inorganic and organic carbon-isotope records, few (e.g., Magaritz et al., 1992)examine the changes to Δ13C (δ13Ccarb − δ13Corg). Thechange in Δ13C during the end-Permian seems to varygeographically with some locations (South China: Caoet al., 2002; Slovenia: Schwab and Spangenberg, 2004)showing a decrease in Δ13C at the event horizon andothers (Carnic Alps:, Magaritz et al., 1992; Japan:Musashi, 2001) showing parallel excursions in bothorganic and inorganic carbon and thus no change inΔ13C.

Here we provide new carbonate and organic carbonisotope data from the well-studied Meishan and Shangsigeologic sections from South China (Fig. 1) and

Fig. 1. Modern (A) and paleogeographic (B) locations for all of the measuredfrom Wignall et al. (1995); 2: W. Slovenia; 3: Abadeh (Iran); 4: GartnerkofScotese (2002).

Please cite this article as: Riccardi, A. et al. Carbon isotopic evidencePalaeogeogr. Palaeoclimatol. Palaeoecol. (2007), doi:10.1016/j.palaeo.200

compare these to data from sections in Iran (Korteet al., 2004), Slovenia (Dolenec et al., 2004; Schwab andSpangenberg, 2004), Japan (Musashi et al., 2001),Austria (Magaritz et al., 1992; Wolbach et al., 1994),and two other south China sections: Heping and Taiping(Krull et al., 2004) (Fig. 1). We evaluate varioushypotheses in light of these observations, and concludethat the data are most consistent with a shift in thedominant mode of photosynthesis at these locations tothat carried out by non-oxygenic phototrophic greensulfur bacteria.

2. Materials and methods

A total of 105 samples were obtained from theMeishan and Shangsi sections located in southern China(Fig. 1). The samples were powdered and the inorganiccarbonate was dissolved individually in an automatedcommon acid bath system at 90 °C and analyzed on aFinnigan 252 Mass Spectrometer in the Penn State StableIsotope Biogeochemistry laboratory. A laboratory

sections are shown above. 1: South China sections (see insert modifiedel-1 core (Austria); 5: Kamura and Taho, Japan. Maps modified after

for chemocline upward excursions during the end-Permian event.6.11.010

Fig.2.

Carbonate

(closedsquares)

andorganiccarbon

(opensquares)

isotopic

data

fortheMeishan

andShangsisections,SouthernChina.Notethat

vertical

scales

differ.

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Please cite this article as: Riccardi, A. et al. Carbon isotopic evidence for chemocline upward excursions during the end-Permian event.Palaeogeogr. Palaeoclimatol. Palaeoecol. (2007), doi:10.1016/j.palaeo.2006.11.010

Fig. 3. Changes toΔ13C across the PTB, with the y-axis zero at the Permian–Triassic boundary. The biostratigraphy and isotope values for each section were modified after the published data (see textfor references). The Meishan and Shangsi data are from this study. The light grey boxes are inferred times where cyanobacterial photosynthesis dominated, while the dark grey boxes are periods whereeuxinic conditions may have existed.

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standard was run every seven samples and the calcitestandard NBS-19 was also analyzed at the beginning andend of each sample set. Subsets of the carbonate sampleswere decarbonated using hydrochloric acid, and theresidue was combusted on a CE Instruments NC2500elemental analyzer. The CO2 produced was flame sealedin glass vials and transferred to a Finnigan 252 MassSpectrometer fitted with a special attachment fortransferring the gas. The organic carbon isotopic ratiowas determined relative to calibrated internal laboratorystandards and referenced to V-PDB. The variationbetween replicate analyses was ∼ 0.02‰ for carbonateand organic carbon, with all data presented in standardper mille (‰) notation relative to V-PDB.

3. Results

For the Meishan section a negative excursion inδ13Corg (bed 24) precedes the decrease in δ13Ccarb (bed25), generating a substantial increase inΔ13C just belowthe event horizon (uppermost bed 24). The δ13Corg thenincreases while the δ13Ccarb decreases across the eventhorizon (main extinction interval), creating a markeddecrease in Δ13C (Fig. 3A). Our data for the Meishansection differ slightly from those previously published(Cao et al., 2002) for the interval below the boundary,but overlap exactly during the period leading up to theextinction interval and through the recovery. For theShangsi section, δ13Corg remains relatively constantuntil the latest Permian where it rapidly decreases, thenincreases before falling across the event horizon, whileδ13Ccarb increases (Fig. 2) leading up to the extinctionevent (bed 27), again creating an increase inΔ13C. Thenboth decrease, but with a temporal offset that creates adecline in Δ13C mirroring the one at Meishan (Fig. 3B).

Although other sections from the Tethyan Ocean donot show the pre-event rise in Δ13C unambiguously,they do exhibit a decrease in Δ13C across the extinctioninterval (Fig. 3). The simultaneity of these decreases atwidely separated sites argues against diagenetic alter-ation as the cause of the shift. Data from the WordieCreek formation in Greenland (Stemmerik et al., 2001)suggest a decrease in δ13Corg before the drop in δ

13Ccarb

but the data are insufficient to determine a change toΔ13C across the boundary, and because of this, are notused in this study. Other higher latitude sections werealso not considered due to the lack of both inorganic andorganic carbon isotopic data across the PTB. TheWestern Slovenian section also displays a sharp de-crease in Δ13C of around 5‰ but values remain aroundthe new lower value for the rest of the measured section(Fig. 3C). The lack of suitable paleontologic specimens

Please cite this article as: Riccardi, A. et al. Carbon isotopic evidencePalaeogeogr. Palaeoclimatol. Palaeoecol. (2007), doi:10.1016/j.palaeo.200

for more precise dating makes it difficult to determinehow much of the early Triassic is exposed in this sec-tion. The Abadeh (Iran) section also displays a decreaseacross the boundary after a slowly increasing trend andrecovers to pre-extinction values during the earlyTriassic (Fig. 3D). The other S. China sections (Hepingand Taiping) both exhibit a decrease across the eventhorizon (Fig. 3E, F). The Heping section displays aslight increase before the boundary similar to Meishanand Shangsi but this is not as evident at Taiping. TheGartnerkofel (Austria) core (Magaritz et al., 1992;Wolbach et al., 1994) displays a small Δ13C changewith a short-lived negative excursion that begins wellbefore the PTB (Fig. 3G). TheΔ13C values reach a peakjust before this decrease similar to the change atMeishan, but more spread out due to the higher rate ofsedimentation. Interestingly, the shallow-water sectionsKamura and Taho from Japan (Musashi et al., 2001),deposited in the open Panthalassic ocean, produce aconstant and intermediate value for Δ13C across theboundary (Fig. 3H).

4. Discussion

Changes in the organic and inorganic carbon-isotopecomposition of marine sediments need not be coincidentbecause of additional factors controlling δ13Corg (seebelow; Kump and Arthur, 1999). Indeed, the negativeshift in δ13Ccarb and δ13Corg are not coincident at manyof the sections. The use of δ13Corg is therefore unsuitablefor determining the location of the event horizon atsections where the paleontology is ambiguous or whereδ13Ccarb cannot be measured.

The variations in Δ13C reflect perturbations to thecarbon cycle, including changes in one or more of thefollowing factors: atmospheric pCO2 (which influencesphotosynthetic isotope discrimination) (Bidigare et al.,1997), and/or the dominant source of marine sedimen-tary organic matter (e.g., dominance by different marinebiota or terrestrial organic input) (Sackett and Thomp-son, 1962). We now evaluate existing hypotheses forend-Permian extinction in terms of these factors and theavailable data.

4.1. Siberian trap volcanism and atmospheric pCO2

Because photosynthetic isotope discrimination de-creases as CO2 availability falls (Bidigare et al., 1997),the decrease in Δ13C could have resulted from theremoval of CO2 from the atmosphere due to enhancedsilicate weathering on the continents, following SiberianTrap eruption as the large amount of basaltic material

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formed by the trap volcanism was eroded and weathered(Holser and Magaritz, 1987). A trend towards increasing87Sr/86Sr in marine sediment can be interpreted asevidence for increased continental weathering across theevent horizon, and the Siberian Traps would haveweathered relatively quickly (Grard et al., 2005).However a dominance of non-radiogenic basaltic weath-ering would not generate such a trend (Korte et al.,2003). Moreover, a preceding increase in Δ13C thatwould have been associated with increasing atmosphericpCO2 during massive Siberian trap eruption is notunambiguously and universally evident in the data.

4.2. Terrestrial organic matter input

A second possibility to explain theΔ13C patterns is achange in the ratio of the terrestrial to marine compo-nents of organic carbon delivered to sediments duringthe extinction period, as a result of the extinctions oraccompanying environmental change. The terrestrialorganic matter at this time would have been enriched in13C relative to marine organic matter, as opposed to thesituation in modern sediments (Arthur et al., 1985), ifatmospheric pCO2 were highly elevated over modernvalues as predicted by climate models (Kiehl andShields, 2005). Terrestrial δ13Corg values from areindeed less negative than contemporaneous marineδ13Corg, with an average between − 24‰ and − 22‰(Krull, 1999; Ward et al., 2005). A decrease in Δ13Cwould thus demand that the input of terrestrial organicmatter to marine environments would have to haveincreased across the main pulse of extinction.

In fact, slightly higher contents of terrestrial organicmatter during the PTB and in the lowermost Triassichave been inferred in two western Slovenian sections,based on compound-specific isotope measurements, se-quences that are otherwise dominated by marine organicmatter (Dolenec et al., 2004; Schwab and Spangenberg,2004). However, the increase in terrestrial organicmatter concentration in both Slovenian sections doesnot occur until after the initiation of the main decrease inΔ13C. In the Abadeh (Iran) section the organic materialis entirely marine in origin with no increased terrestrialcomponent across the P/Tr interval based on organicgeochemical analyses, but the decline in Δ13C is clearlydisplayed (Korte et al., 2004). Recent evidence for amore widespread change in the terrestrial/marineorganic matter input ratio has been suggested througha potential land plant biomarker (Sephton et al., 2005).The negative shift inΔ13C measured at Val Badia (Italy)is not as large (∼2‰) as the one seen in the majority ofsections presented here, and the short-duration pulse of

Please cite this article as: Riccardi, A. et al. Carbon isotopic evidencePalaeogeogr. Palaeoclimatol. Palaeoecol. (2007), doi:10.1016/j.palaeo.200

terrestrial matter described by Sephton et al. (2005)would have produced a less pronounced change than thelonger term shift in Δ13C being discussed here. Anoscillation between marine and terrestrial inputs fol-lowing the extinction in China has been suggested basedon a repeated series of rapid shifts in δ13Corg (Cao et al.,2002). However, this occurs after the measured decreasein Δ13C and the extinction. Sea-level rise has beendocumented at these sections during the latest Permian(Wignall and Hallam, 1993; Hallam and Wignall, 1999),which is difficult to reconcile with increased terrestrialinput given the increased distance to the source. Onbalance, the evidence for a terrestrial/marine organicmatter shift during the widespread drop in Δ13C acrossthe event horizon is weak.

4.3. Euxinia

A third possibility to explain the Δ13C decline is ashift from dominantly algal or cyanobacterial produc-tivity prior to the event to a bloom of phototrophic greensulfur bacteria around the boundary. Green sulfurbacteria utilize the reverse TCA cycle during incorpo-ration of carbon, which results in a smaller carbon-isotope fractionation than that associated with the morecommon Calvin cycle (van Breugel et al., 2005). In-creasing contribution of green sulfur bacterial biomassto the carbon flux would decreaseΔ13C across the eventhorizon and up to the PTB. If the apparent fractionationbetween CO2 and organic carbon for the green sulfurbacteria were −12.5‰, which is the less negative end ofthe range for potential discrimination (van Breugel et al.,2005), the input of green sulfur bacterial biomass wouldhave to have been approximately 30% of the totalorganic matter to create the Δ13C drop from 30 to 25‰.The increase of Δ13C to pre-extinction values in theearly Triassic may have been caused by a return tooxygenic photoautotrophy, due to the re-oxygenation asthe chemocline returned to depth.

Green sulfur bacteria may have become prevalent inthe photic zone through the late Permian, particularlyduring chemocline upward excursion (CUE) events(Kump et al., 2005), when euxinic water would havebeen present in the photic zone (Fig. 4). The instabilityof the chemocline separating the deep euxinic waterfrom oxygenated surface water (Kump et al., 2005)would have generated spatially and temporally variableCUE events. This variability may have created thedifferences in duration and severity of the decrease inΔ13C between the various measured sections. Someregions, perhaps represented by the Panthalassic Japa-nese sections (Musashi et al., 2001), might not exhibit a

for chemocline upward excursions during the end-Permian event.6.11.010

Fig. 4. Depiction of a CUE event, compared to changes in Δ13C at Shangsi. During a CUE event the euxinic water in the photic zone would allowgreen sulfur bacteria to thrive, lowering the fractionation factor creating a dip in Δ13C. Sometime during the lower Triassic the euxinic watersretreated and eventually the entire water column was oxygenated again.

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shift inΔ13C, if they were in stable oceanic regions. Theshift in Δ13C would therefore not have been a uniformglobal phenomenon, and would only be observed wherephotic zone euxinia was present for an extended periodof time.

This hypothesis makes a strong prediction, thatisorenieratane, a biomarker for green sulfur bacteria,should be found at all sites that exhibit a decreasingΔ13C trend, providing that favorable conditions forpreservation existed. Isorenieratane has been found atthe event horizon of Meishan and in the Hoeva-3 coreoff the coast of Australia (Grice et al., 2005). Thecontinued search for this biomarker at other PTBsections will help to determine the extent of CUEevents. Sedimentologic evidence for euxinia in the formof frambiodal pyrite grain distributions exists for themiddle to late Permian in Greenland (Nielsen and Shen,2004), and in the earliest Triassic (Wignall andTwitchett, 2002) and the PTB in India (Wignall et al.,2005). At the Heping and Taiping sections, lithologieschange from fossiliferous limestone to calcimicrobialframestone around the event horizon coincident with adecline in species abundance (Krull et al., 2004). Thewestern Slovenian section shows a change fromlimestone to a black algal packstone and higherconcentrations of extractable sulfur which has beensuggested to indicate deposition under euxinic condi-tions (Schwab and Spangenberg, 2004). The Abadehsection changes to stromatolitic facies in the latestPermian coincident with the decrease in Δ13C while thesediments grade from red to gray (Korte et al., 2004).These changes in sedimentary facies coincide with thedecrease in Δ13C and during the proposed euxinic

Please cite this article as: Riccardi, A. et al. Carbon isotopic evidencePalaeogeogr. Palaeoclimatol. Palaeoecol. (2007), doi:10.1016/j.palaeo.200

interval in Fig. 3. Pyrite isotopic composition from alow-sulfur siliceous Japanese section also suggests deepwater anoxia/euxinia (Kajiwara et al., 1994) as dochanges to the isotopic composition of carbonate-associated sulfate from the Suisi section in Italy(Newton et al., 2003).

5. Conclusion

The shift in inorganic carbon isotopes during the end-Permian extinction has been shown here to be ac-companied by a non-parallel change in organic carbonisotopes, documenting a change in the fractionationfactor between organic and inorganic carbon for thePaleo-Tethys ocean. CUE events provide a potentialtrigger for the extinction event through the release ofhydrogen sulfide to the surface waters and atmosphere(Kump et al., 2005). The asynchronous duration andonset of the shift in Δ13C would have easily beencreated by CUE events in different parts of the Paleo-Tethys ocean during the latest Permian. Widespreadphotic zone euxinia, (substantiated by biomarker (Griceet al., 2005) and sedimentary (Wignall and Twitchett,2002; Nielsen and Shen, 2004) evidence), would haveled to a significant change in the dominant marine biota,enough to alter the mean fractionation between inor-ganic and organic carbon at many but not all locationsduring the end-Permian extinction.

Acknowledgments

This study was funded through NASA cooperativeagreement, grant #NNA04CC06A, NSF grant #NSF

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EAR-0208119 and by the Penn State BiogeochemicalResearch Initiative for Education (BRIE) sponsored byNSF (IGERT) grant DGE-9972759. We would like tothank Dennis Walizer for his assistance with carbonateanalyses, Hiroshi Ohmoto for laboratory access throughthe Penn State Astrobiology Research Center, andTheresa Menotti for her assistance with sample prepa-ration. The authors would like to thank Finn Surlyk andtwo anonymous reviewers for comments which im-proved an earlier version of this manuscript. Samplecollection was funded by the NASA ExobiologyProgram (award NAG5–8814 to Sam Bowring andDouglas Erwin). SD thanks Bowring, Chang-qun Cao,Erwin, Yugan Jin, Mark Martin, Wei Wang and YueWang for their guidance and generosity in the field.

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