triassic–jurassic atmospheric co2 spike
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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: [email protected]
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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: [email protected]
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Am. J. Sci. 299, 805–827 (1999).
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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).
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11.Ward, P. D. et al. Science 292, 1148–1151 (2001).
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