when variations in the stable carbon isotope(denoted as d13C) in terrestrial plant leavesare taken into account. These factors sug-gest that the linkage between pCO2, globalwarming and the end-Triassic mass extinc-tion remains intact.

The stability of the atmospheric carbonreservoir across the Triassic–Jurassicboundary is inferred from pCO2 estimatesderived using measurements of the d13Ccomposition of pedogenic carbonates,ranging over 20 million years (between theLate Triassic and Early Jurassic), and apalaeosol pCO2 barometer4. A temporal resolution of 20 million years is, however,lower than in the study of fossil leaves3 andso is unlikely to be compatible with detect-ing the same transient carbon-cycle event.Moreover, a well-characterized feature ofthe palaeosol pCO2 barometer is its sensitiv-ity, not only to the concentration of CO2 inthe soil, as acknowledged by Tanner et al.2,but also to the isotope composition of terrestrial organic matter (d13COM)4,5. It istherefore necessary to measure d13COM

coexisting with soil carbonates4,5, but un-fortunately, under the very conditions inwhich palaeosol carbonates form (well-drained soils from arid and semi-aridregions), soil organic matter is rarely preserved, so this feature of the method isoften poorly constrained.

Tanner et al. calculate pCO2 by applying asingle d13COM value because they lackedorganic materials with a stratigraphic resolu-tion that corresponds to all of their soil samples. For the interval across theTriassic–Jurassic boundary, however, at leastone land-plant stable-carbon-isotope recordis available3, providing a means to accountfor natural variations in d13COM at this time.This negative isotopic excursion has sub-sequently been recorded in organic matterfrom the Queen Charlotte Islands in BritishColumbia6 and from St Audries Bay, UK7, as

well as in marine carbonates and organicmatter from Hungary8, indicating that itcould be a general geochemical fingerprint ofthe carbon-cycle perturbation across the Triassic–Jurassic boundary.

Calculation of palaeo-pCO2 levels usingthe mean isotopic ratios of Triassic andJurassic palaeosol carbonates determined by Tanner et al., but constrained usingstratigraphically detailed terrestrial-plantd13COM records from Jameson Land, eastGreenland3, indicates a substantial increaseacross the Triassic–Jurassic boundary (Fig. 1a). Within the margins of uncertaintyof the palaeosol approach, this increase inpCO2 is consistent with the original pCO2

reconstruction based on fossil stomata ofGinkgo cuticles3 (Fig. 1b) and equates to arise of around 1,000 p.p.m.v. The rise ismore strongly expressed if CO2-relatedglobal warming is allowed to influence thefractionation of soil CO2 in the reaction-diffusion model in a manner that is consis-tent with theory (Fig. 1a). Both the stomataland palaeosol reconstructions (Fig. 1) showa marked pCO2 increase that is broadly coincident with the emplacement of theCentral Atlantic Magmatic Province(CAMP) flood basalts9.

Neither the palaeosol nor the stomatalapproach to estimating pCO2 variations indeep time are certain, but both provide self-consistent results for the Palaeozoic4 era and the mass-extinction event across theTriassic–Jurassic boundary (Fig. 1). Suchself-consistency argues against stronglycoordinated ocean–atmosphere buffering ofCO2 from volcanic outgassing during theend of the Triassic, possibly because of thegeologically short duration (about 0.5Myr)10 and high intensity of CAMP activity.Precise dating of the proposed CO2 excur-sion is critical in this context.

Moreover, it is hard to envisage a mech-anism other than extreme, CO2-forced

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global warming that can explain the wide-spread selective floral extinctions andturnovers that are evident in the fossilrecord at this time. The proposed trans-gressive–regressive change in sea level2

would be unlikely to provoke such amarked response from the terrestrial biota.The mechanisms that drive mass extinctionin the marine and terrestrial realms warrant urgent investigation in the face ofthe current exponential rise in the Earth’satmospheric CO2 concentration.David BeerlingDepartment of Animal and Plant Sciences,University of Sheffield, Sheffield S10 2TN, UKe-mail: d.j.beerling@sheffield.ac.uk

1. Hallam, A. & Wignall, P. B. Mass Extinctions and their

Aftermath (Oxford Univ. Press, Oxford, 1997).

2. Tanner, L. H., Hubert, J. F., Coffey, B. P. & McInerney, D. P.

Nature 411, 675–677 (2001).

3. McElwain, J. C., Beerling, D. J. & Woodward, F. I. Science 285,

1386–1390 (1999).

4. Royer, D. L., Berner, R. A. & Beerling, D. J. Earth-Sci. Rev. 54,

349–392 (2001).

5. Cerling, T. E. Glob. Biogeochem.Cycles 6, 307–314 (1992).

6. Ward, P. D. et al. Science 292, 1148–1151 (2001).

7. Hesselbo, S. P. et al. Geology (in the press).

8. Pálfy, J. et al. Geology 29, 1047–1050 (2001).

9. Marzoli, A. et al. Science 284, 616–618 (1999).

10.Hames, W. E., Renne, P. R. & Ruppel, C. Geology 28,

859–862 (2000).

Triassic–Jurassicatmospheric CO2 spike

Iquestion the claim by Tanner et al.1 thatatmospheric CO2 levels remained constantacross the Triassic–Jurassic boundary

on the grounds of problems with strati-graphic completeness and contaminationwith atmospheric methane. Becausemethanogenic CH4 has a light isotope composition and oxidizes readily to CO2,methane–clathrate dissociation and oxida-tion events cannot be detected by palaeo-barometers that use the carbon-isotopecomposition of palaeosol carbonate.

The palaeosol isotopic data of Tanner etal. are not presented in the context of measured sections, nor is there any otherindication of stratigraphic completenessthat is suitable for identification of a transient greenhouse effect. It is there-fore unsurprising that the few sampledpalaeosols missed the brief CO2 greenhouseeffect inferred from the stomatal index offossil leaves in three beds within two continuous sequences in Sweden andGreenland2. The duration of the isotopicexcursion that occurred at the same time asthis transient CO2 greenhouse effect wasprobably no more than 500,000 years3. Thisslim target is easy to miss when examinedfrom a distance of 200 million years.

A more serious problem with isotopicpalaeobarometers of CO2 is the compromis-ing effect of massive dissociation events

Figure 1 Palaeo-atmospheric pCO2 changes across the

Triassic–Jurassic boundary 208 million years ago. a, Atmospheric

pCO2 changes were calculated using the carbon-isotope composi-

tion of North American fossil soils2 and land-plant organic-matter

records from Jameson Land, east Greenland3, using a standard

diffusion model5, where S(z ) (the concentration of C02 contributed

by biological respiration) is 5,000 p.p.m.v. The horizontal axis

represents the depth of successive fossil-plant-bearing beds of the

Cape Stewart formation from Jameson Land. pCO2 values were

calculated using a constant Triassic palaeosol carbonate value up

to the Triassic–Jurassic boundary and thereafter a constant Juras-

sic palaeosol value (both from ref. 2). Red line indicates the effects

of CO2-driven global warming on the temperature dependence of

soil CO2 fractionation4,5, with S(z )45,000 p.p.m.v. only. Upper and

lower broken lines denote the variability in these estimates by vary-

ing S(z ) between 3,000 and 7,000 p.p.m.v. b, Atmospheric pCO2

changes reconstructed from the stomatal characteristics of fossil

leaves from Jameson Land3. Vertical bars denote the upper and

lower range for any given depth calculated using this technique.

b

a

102030405060708090Depth (m)

0

500

1,000

1,500

2,000

2,500

Atm

osp

heric

pC

O2

(p.p

.m.v

.)

500

1,500

2,500

3,500

4,500

Triassic Jurassic200 Myr ago

Rhaetian Hettangian

© 2002 Macmillan Magazines Ltd© 2002 Macmillan Magazines Ltd

from methane hydrate reservoirs, becauseCH4 oxidizes in the atmosphere within7–24 years to CO2, which retains a very depleted carbon-isotope composition (typically 160‰, but as low as 1110‰d13Corg)

4. For example, unusually depletedcarbon-isotope values for Early Triassicorganic matter can be taken as indicationsof a methane-dissociation event5. An EarlyTriassic CO2 greenhouse effect is shown bythe very low stomatal index of fossil seedferns6, but pedogenic carbonate isotopicpalaeobarometers indicate low atmosphericCO2 levels in the Early Triassic7. The pedogenic–isotopic palaeobarometer wasnot designed for methanogenic isotopiccompositions, which compromise thispalaeobarometer during several green-house transients6, and perhaps also at theTriassic–Jurassic boundary.Gregory J. RetallackDepartment of Geological Sciences, University ofOregon, Eugene, Oregon 97403-1272, USAe-mail: gregr@darkwing.uoregon.edu

1. Tanner, L. H., Hubert, J. F., Coffey, B. P. & McInerney, D. P.

Nature 411, 675–677 (2001).

2. McElwain, J. C., Beerling, D. J. & Woodward, F. I. Science 285,

1386–1390 (1999).

3. Ward, P. D. et al. Science 292, 1148–1151 (2001).

4. Whiticar, M. J. in Atmospheric Methane (ed. Khalil, M. A. R.)

63–85 (Springer, Berlin, 2000).

5. Krull, E. S. & Retallack, G. J. Bull. Geol. Soc. Am. 112,

1459–1472 (2000).

6. Retallack, G. J. Nature 411, 287–290 (2001).

7. Ekart, D. D., Cerling, T. E., Montañez, I. P. & Tabor, N. J.

Am. J. Sci. 299, 805–827 (1999).

Tanner replies — Both Beerling and Retallack question our conclusion thatatmospheric CO2 levels remained relativelystable across the Triassic–Jurassic bound-ary, partly on the basis of insufficient stratigraphic resolution. However, thestratigraphy of the formations we studied iswell known1. The Lower Jurassic McCoyBrook Formation of the Fundy basin over-lies the North Mountain Basalt, which wasextruded during the initial stages of volcan-ism in the Central Atlantic MagmaticProvince (CAMP), and so this formationpost-dates the main eruptive episode.Palaeosols in this formation occur within10 m of the formation base; the age of thesepalaeosols is therefore constrained by thebasalt to within several hundred thousandyears of the Triassic–Jurassic boundary,which is located several metres below thebasalt. Because the duration of the erup-tions of CAMP volcanics, which occurredin several pulses, has been established asroughly 600,000 years2, palaeosol forma-tion in the McCoy Brook Formation is contemporaneous with the latter stages ofthe CAMP eruptions. These palaeosols may therefore be expected to record thecumulative effects of the eruptions.

Our palaeo-CO2 values are calculatedfrom the diffusion-reaction model, which

requires measurements or assumptions fora variety of factors that control soil CO2,not least the isotopic composition of plant-derived organic matter (d13COM)3,4. Ideally,organic matter in the palaeosol that con-tains the carbonate is used to obtain thisvalue because the d13C of C3 plants is knownto vary significantly in contemporaneoussoils from differing climatic regimes3.Unfortunately, the McCoy Brook Forma-tion, from which the Lower Jurassic car-bonate samples were obtained, lackswell-preserved plant material.

The only record of d13COM across theTriassic–Jurassic boundary available at thetime of our calculations provides data for locations in eastern Greenland andsouthern Sweden5, but the negative d13Cexcursion at the boundary shown in the eastern Greenland data, representedprimarily by a single data point, is notapparent in the southern Sweden data set.Moreover, the sediments that contain plantfossils at the eastern Greenland locationaccumulated under the influence of a sig-nificantly more humid climate than existedin the Fundy basin1, a fact that renders isotope data from this location inapplicableto the interpretation of data derived fromthe semi-arid palaeosols.

For these reasons, we chose not to usethese data, and instead assumed a singlevalue for both the Late Triassic and EarlyJurassic that is consistent with publishedmeasures of organic matter in Upper Trias-sic formations3 that were deposited underclimatic conditions similar to those of thestudied formations. This potential source ofinaccuracy may ultimately be resolved onlyby location and analysis of organic materialin the McCoy Brook Formation.

The fossil stomatal evidence of Beerlingand Retallack seems to indicate a several-fold increase in atmospheric pCO2 acrossthe Triassic–Jurassic boundary5,6. The useof stomatal indices for calculation ofpalaeo-CO2 levels is based on experimentsin which modern plants were grown atpCO2 values of up to twice present levels7,but calls for an extrapolation of the experi-mental pCO2 values to the much higherpalaeo-CO2 values interpreted for the past.The use of these indices also requires theassumption that the floral response to theseconditions was quantitatively the same 200 million years ago as it would be today.This approach may therefore also generateinaccuracies.

As mentioned by Beerling, data from themarine realm that indicate a significantnegative carbon-isotope excursion at theTriassic–Jurassic boundary shed new lighton the extinction problem8, demonstratingan intense but short-lived perturbation ofthe global carbon cycle of a magnitude thatis not consistent with volcanic outgassing.This is supported by a simple mass-balance

calculation, on the basis of the largest estimate of the volume and volatile contentof intrusions of the CAMP9, showing thatoutgassing during the eruptions produced5.6 21016 mol CO2 in total, an amount that is equivalent to that in the modernatmosphere.

This volume is insufficient to drive theisotopic excursion of marine carbonates8

and is unlikely to have affected the atmos-phere of the early Mesozoic era, given thehigh pCO2 of the Late Triassic1. Rapidrelease of seafloor methane hydrate, assuggested by Retallack, has been implicat-ed in other extinction events, and thisrelease may have been triggered by theeffects of CAMP volcanism. Such a release,which is consistent with the magnitude ofthe isotopic excursion, need not haveresulted in greatly increased atmosphericpCO2 as suggested by Retallack, as evi-dence exists that much of the methanereleased by this process would be oxidizedwithin the water column, resulting in abrief interval of ocean anoxia10 and wide-spread extinction.

Our conclusion stands: the isotopecompositions of pedogenic carbonates fail to indicate a substantial increase inatmospheric pCO2 as a result of the CAMPeruptions. We are all in agreement thatalthough existing methods of estimatingpalaeo-CO2 are inexact, their validity is notmutually exclusive, and also that the causeof the end-Triassic extinction event isuncertain, with environmental degrada-tion resulting from the CAMP volcanismprobably being involved. Acquisition ofnew data by both methods from otherlocations should resolve this uncertainty;better time resolution will constrain therelationships between the abrupt marineextinctions11, the possibly asynchronousfloral turnover12 and the duration of theCAMP eruptions, which may have lastedfor several million years13. Lawrence H. TannerDepartment of Geography and Geosciences,Bloomsburg University, Bloomsburg, Pennsylvania 17815, USAe-mail: lhtann@bloomu.edu

1. Tanner, L. H., Hubert, J. F., Coffey, B. P. & McInerney, D. P.

Nature 411, 675–677 (2001).

2. Olsen, P. E., Schlische, R. W. & Fedosh, M. S. The Continental

Jurassic (ed. Morales, M.) 11–22 (Mus. North. Arizona,

Flagstaff, Arizona, 1996).

3. Cerling, T. E. Spec. Publ. Int. Ass. Sedimentol. 27, 43–60 (1999).

4. Ekart, D. D., Cerling, T. E., Montanez, I. P. & Tabor, N. J.

Am. J. Sci. 299, 805–827 (1999).

5. McElwain, J. C., Beerling, D. J. & Woodward, F. I. Science 285,

1386–1390 (1999).

6. Retallack, G. J. Nature 411, 287–290 (2001).

7. Beerling, D. et al. J. Exp. Bot. 49, 1603–1607 (1998).

8. Pálfy, J. et al. Geology 29, 1047–1050 (2001).

9. McHone, J. G. Geol. Soc. Am. Abs. Prog. 32, 58–59 (2000).

10.Dickens, G. Geochem. Geophys. Geosys. 2,

2000GC000131 (2001).

11.Ward, P. D. et al. Science 292, 1148–1151 (2001).

12.Pálfy, J. et al. Geology 28, 39–42 (2000).

13.Marzoli, A. et al. Science 284, 616–618 (1999).

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