Earth and Planetary Science Letters 259 (2007) 634–639www.elsevier.com/locate/epsl
Discussion
Comment on “Carbon-isotope record of the Early Jurassic (Toarcian)Oceanic Anoxic Event from fossil wood and marine carbonate(Lusitanian Basin, Portugal)” by Hesselbo S., Jenkyns H.C.,
Duarte L.V. and Oliveira L.C.V.
J.M. McArthur
Departmen of Earth Science, UCL, Gower Street, London WC1E 6BT, United Kingdom
Received 26 January 2007; received in revised form 10 April 2007Available online 8 May 2007
Editor: H. Elderfield
Hesselbo et al. (2006; hereinafter HJDO06) presentstable-isotopic profiles through lower Toarcian sedi-ments at Peniche, Portugal, and interpret them asevidence of a massive injection to the atmosphere ofisotopically-light CO2 derived from methane oxidation.The atmospheric inputs supposedly gave rise to negativeisotopic excursions in all carbon reservoirs. The paperfollows others proposing that environmental change inearly Toarcian time was driven by the release to theatmosphere of methane, either from marine clathrates(Hesselbo et al., 2000; Kemp et al., 2005), or fromorganic-rich shales that were baked by Karoo–Ferrarintrusions (McElwain et al., 2005).
The methane hypothesis was tested by van deSchootbrugge et al. (2005), who highlighted theexistence of sections that lacked a negative excursionin ä13C in one or more sample media: amongst them isthe section on the coast of Yorkshire, UK, used to erectthe methane hypothesis by Hesselbo et al. (2000), whichlacks a negative excursion in the δ13C of belemnitecalcite (McArthur et al., 2000; van de Schootbruggeet al., 2005). As the excursion is not seen in thesesections, it cannot represent a global event and so couldnot have resulted from methane release, which wouldhave influenced all reservoirs of carbon. New data arenow provided by HJDO06 for a section (Peniche,
Portugal) in which interesting isotopic variations areseen in δ13C that do nothing to explain why negativeexcursions are absent from many sections. In conse-quence, the methane hypothesis remains falsified.
In Section 3.5 of their paper, HJDO06 question thevalidity of the belemnite record used by van deSchootbrugge et al. (2005) to falsify the methanehypothesis, and do so on three grounds:
1] the belemnites of van de Schootbrugge et al. (2005)were too few in number to be convincing;
2] the stratigraphic positioning of the belemnites isuncertain in relation to the excursion in δ13C;
3] belemnites are unreliable recorders of δ13C inseawater.
With respect to 1], two belemnites of van deSchootbrugge et al.'s (2005) from Germany aresufficient for HJDO06 to assert (incorrectly) that a‘negative’ excursion exists in the belemnite record fromGermany. One sample of silicified Jet from Yorkshire issufficient for them to suggest that highly-altered wood(Jet) is a good sample medium, when it implies only thatsilicification post-dated jettification. The four belemnitesof van de Schootbrugge et al. (2005), with δ13C around+1 to +2‰, accepted by HJDO06 as being in the peak of
Fig. 1. Comparison of δ13C in belemnite calcite with δ13C in organic matter through the Toarcian of Yorkshire (modified from van de Schootbruggeet al., 2005). Data from Jones (1992) (via pers. comm. 1995), Sælen et al. (1996), Jenkyns and Clayton (1997), McArthur et al. (2000). Between A andB, δ13C of organic matter declines by 4‰ below a baseline of −26‰, whilst δ13C of belemnite calcite remains above +1‰. Between B and C, valuesof δ13C in organic matter are more than 4‰ below baseline. Between A and C, of more than 20 belemnites from four separate studies, none haveisotopic compositions less than +1‰ and none show a negative excursion. Except between C and D, the trend in δ13C of organic matter is decoupledfrom that of belemnite calcite, as it is decoupled higher in the section (see Fig. 2).
the negative excursion for δ13C(org), should thereforeconvince. In reality, the full record of δ13C in belemnitecalcite from Yorkshire is better populated than that, as isclear from Fig. 2 of van de Schootbrugge et al. (2005)and its reproduction, with additions, in Fig. 1. Through-out the section in Yorkshire, only one value of δ13C inbelemnite calcite is less positive than +1‰; such valuesare not characteristic of a negative excursion. A value of+1 to +2‰ can be regarded as normal for belemnite
calcite at that time (cf. Rosales et al., 2004 for latePliensbachian values).
The Yorkshire section shows a positive excursion inthe upper exaratum Subzone, and a hint of the commonpositive excursion in the uppermost tenuicostatum Zone(Fig. 2), as do some (not all) sections elsewhere (Fig. 3;Jenkyns et al., 2002 for a discussion). In Fig. 1, theupper part of the upper positive excursion, and the lowerpart of the lower positive excursion, are not shown
Fig. 2. Profile of δ13C in belemnite calcite through the upperPliensbachian and lower-to-middle Toarcian sediments of Yorkshire,UK, showing returns of δ13C to values considered normal for oceanicδ13C (large arrows) and a generalised trend through the data (greydotted line). Small arrows mark minor returns to lower δ13C thatinterrupt the general decline upsection to less positive values of δ13C.The trend to more negative δ13C through the Bornholm section ofHesselbo et al. (2000) is correlated by those authors, and HJDO06,using δ13C to the low exaratum Zone. It may equally well correlate tothe declines in δ13C that culminate either at A, in the mid-falciferumZone, or at B, at the P/T boundary.
because the profile is truncated stratigraphically. Thistruncation may give a false impression that the normalvalues of δ13C in belemnite calcite, seen around the9.5 m-level in Fig. 1, represent a minima rather than the
return to normal between two positive excursions.Reference to Fig. 2 removes this illusion. Fig. 2 alsosuggests an alternate correlation for the Bornholmsection of Hesselbo et al. (2000) and HJDO06, whichlacks ammonites: rather than correlate it to the lowerexaratum Subzone in Yorkshire using the δ13C profile,as did those authors, it can be correlated to thepronounced return of δ13C to normal values in the midfalciferum Zone (arrow A in Fig. 2), a level that occursabove the Toarcian OAE, or to the less-pronouncedreturn that occurs at the Pliensbachian–Toarcianboundary (arrow B in Fig. 2).
With regard to 2], HJDO06 imply that the strati-graphic placements of the belemnites of van deSchootbrugge et al. (2005) are imprecise, but do sowithout knowing how they were collected. The samplesderive from the Jet Rock (coincident with the exaratumSubzone) and underlying Grey Shales (Fig. 1). The JetRock, through which the excursion in δ13C(org) isexpressed, is 7 m thick, and contains 8 beds, of which 5are regional marker beds. The field guide of Hesselboand Jenkyns (1995), referring to the Yorkshire Toarcian,states that “Location in the section is facilitated byrecognizing ... the unmistakable named nodule bedswithin the Jet Rock”. The ease of stratigraphic placementin the section was demonstrated in HJDO06 by theirlocation of a sample of silicified Jet into the section atHawker Bottoms, with pinpoint accuracy (their Fig. 4),despite it being collected 10 km away. This accuracy isachievable because, as is made clear in Hesselbo andJenkyns (1995) and Howarth (1962), the sections enjoyclose to 100% exposure and include many laterally-continuous, prominent, marker-beds of concretions (thenamed nodule beds) that represent decreased rates ofsedimentation (McArthur et al., 2000; McArthur andWignall, in press). The sections have been documentedto the decimetre level by Hesselbo and Jenkyns (1995),Howarth (1962), and other authors. Kemp et al. (2005)recorded cm-spacing of samples and identification ofMilankovitch cyclicity (of uncertain periodicity) in theGrey Shales, strata that immediately underlie the earlyToarcian OAE. One study (that of Sælen et al., 1996)does record sample positions that are high by 1.83 m,but only because of a drafting error regarding thestratigraphic thickness of the Grey Shales of Howarth(1962, 1973): that error has been corrected here. Itseems that the matter of positional uncertainty isoverstated by HJDO06.
With respect to 3], the view that belemnites arenot good recorders of palaeo-oceanographic signalsis contrary to the view of the senior authors ofHJDO06 as presented in their summary of Mesozoic
Fig. 3. Profiles of δ13C through European sections. A positive C-isotope excursion is present in the tenuicostatum Zone in many sections, but it is variably developed. Some sections (a, c, d, f), but not(b) or (e), show a positive C-isotope excursion in the mid-to-upper falciferum Zone, after the putative OAE (renamed here RAE); it is notably absent from the profile of δ13C in organic matter inYorkshire (e) although strongly developed in belemnite calcite from the same section (d), thereby showing that carbon in organic matter and carbon in belemnite calcite in Yorkshire were sourced fromdifferent reservoirs. The steep return of δ13C to normal values around the mid-falciferum Zone (grey arrow) in Yorkshire (d), suggests an alternate level to HJDO06's for correlation to Yorkshire oftheir Danish section. Sources of profiles are: sections a and b, Hesselbo et al. (2000); section c from Jenkyns and Clayton (1997); section d rescaled from McArthur et al. (2000); section e from Cohenet al. (2004); section f from Fig. 3 of Jenkyns et al. (2002).
chemostratigraphy (Jenkyns et al., 2002) much of whichis much based on belemnite records. Some (not all)belemnite records are certainly ‘noisy’. That may bebecause belemnites are such good recorders of δ13C inseawater that they capture variations in δ13C that arehidden in records derived by analysis of bulk-rock,which averages δ13C in large numbers of specimens ofmixed species. HJDO06 observe that the δ13C inbelemnite calcite at Peniche tracks δ13C in bulk-carbonate. The latter is a bulk-measure made on materialcomprising around 30% of the rock, and includes acontribution from diagenetic cement. It might be betterto invert their conclusion and say that the belemniterecord at Peniche validates the trend of the bulk-carbonate record.
The list of factors (e.g. seasonal changes in watermass isotopic composition... etc.) that HJDO06 suggestmight influence δ13C of belemnite calcite would not beconfined to belemnites, but would affect most compo-nents sampled as bulk-carbonate. If δ13C in belemnitecalcite is invalid as a reliable palaeo-proxy because oneor more of the speculations of HJDO06 have weight, sois δ13C of bulk-rock. In reality, it would be unwise toreject either material as palaeo-proxies, given thesubstantial literature (e.g. Sælen et al., 1996; Jenkynset al., 2002; McArthur et al., in press) that attests to theirvalue.
Regarding the profile of δ13C in belemnites fromGermany, HJDO06's claim that it shows a weak negativeexcursion represents a false perception resulting from thesection's incompleteness, in a manner alluded to abovefor the Yorkshire profile. The ‘minimum’ in the Germanprofile represents a value normal for oceanic waters thatis bracketed by incompletely displayed positive excur-sions. Where the figured data to have encompassed agreater stratigraphic range, the positive excursion inδ13C in the tenuicostatum Zone, that is so commonelsewhere (Jenkyns et al., 2002; Fig. 2), would likely beseen. The value of around +2‰ seen at the very top ofthe semicelatum Subzone in Germany marks simply areturn to normal oceanic values around +2‰, before thepositive excursion seen in some (not all) sections post-OAE.
In their Section 3.5, HJDO06 write that it isnecessary to explain why belemnites do not exhibitstrongly negative carbon isotope values “when materi-als from all other coeval geological materials do.” (thiswriter's bolding). In their conclusion, they write that“The consistent shape of the excursion in wood, marineorganic matter, and marine carbonate in Europeansections studied to date, indicates an isotopic evolutionof the global shallow-ocean/biosphere/atmosphere car-
bon reservoir that is consistent with a massive injectionof isotopically light methane.”. In Fig. 3 are reproducedδ13C profiles from sections in Portugal and Spain(Hesselbo et al., 2000); Winterbourne Kingston andMochras, UK (Jenkyns and Clayton, 1997; Jenkynset al., 2002), and Yorkshire, UK (McArthur et al., 2000;Cohen et al., 2004; van de Schootbrugge et al., 2005).The statements of HJDO06 above are not entirely inagreement with the diagrams presented, and with thestatement in Hesselbo et al. (2000) that “The negativeexcursion is present, albeit very rarely, in Tethyanlimestones lacking organic enrichment.”
Concluding, the low TOC values for the sections atPeniche (HJDO06) also shows that the Early Toarcianwas not a time of universal deposition of organic-richrocks, and so the Toarcian OAE must now be regardedas a Regional Anoxic Event (RAE). Its previous statusas a global event may represent a skewed sampling ofthe stratigraphic record. The proposition that release ofmethane, from any source, influenced the carbon-isotopic composition of early Toarcian sediments (andso climates) remains a falsified hypothesis.
References
Cohen, A.S., Coe, A.L., Harding, S.M., Schwark, L., 2004. Osmiumisotope evidence for the regulation of atmospheric CO2 bycontinental weathering. Geology 32, 157–160.
Hesselbo, S.P., Jenkyns, H.C., 1995. A comparison of the Hettangianto Bajocian successions of Dorset and Yorkshire. In: Taylor, P.D.(Ed.), Field geology of the British Jurassic. Geological Society ofLondon, pp. 105–150.
Hesselbo, S.P., Gröcke, D.R., Jenkyns, H.C., Bjerrum, C.J., Ferrimond,P., Bell, H.C.M., Green, O.R., 2000. Massive dissociation of gashydrate during a Jurassic anoxic event. Nature 406, 392–395.
Hesselbo, S., Jenkyns, H.C., Duarte, L.V., Oliveira, L.C.V., 2006.Carbon-isotope record of the Early Jurassic (Toarcian) OceanicAnoxic Event from fossil wood and marine carbonate (LusitanianBasin, Portugal). Earth Planet. Sci. Letters 253, 455–470.
Howarth, M.K., 1962. The Jet Rock Series and the Alum Shale Series ofthe Yorkshire coast, Proc. Yorkshire Geol. Soc. 33, 381–422.
Howarth, M.K., 1973. The stratigraphy and ammonite fauna of the UpperLiassic Grey Shales of the Yorkshire coast. Bull. Br. Mus. (Nat. Hist.)Geol. 24, 235–277.
Jenkyns, H.C., Clayton, C.J., 1997. Lower Jurassic epicontinentalcarbonates and mudstones from England and Wales: chemostrati-graphic signals and the early Toarcian anoxic event. Sedimentol-ogy 44, 687–706.
Jenkyns, H.C., Jones, C.E., Gröcke, D.R., Hesselbo, S.P., Parkinson,D.N., 2002. Chemostratigraphy of the Jurassic System: applica-tions, limitations and implications for palaeoceanography. J. Geol.Soc. 159, 351–378.
Jones C.E., 1992. Strontium isotopes in Jurassic and Early CretaceousSeawater. Unpub D. Phil. thesis, University of Oxford.
Kemp, D.B., Coe, A.L., Cohen, A.S., Schwark, L., 2005. Astronom-ical pacing of methane release in the Early Jurassic period. Nature437, 396–399.
McArthur J.M., Wignall P., in press. Comment on “Waltham D., GröckeD.R. (2006). Non-uniqueness and interpretation of the seawater87Sr/86Sr curve. Geochim. Cosmochim. Acta, 70, 384–394”.
McArthur, J.M., Janssen, N.M.M., Reboulet, S., Leng, M.J., Thirlwall,M.F., van de Schootbrugge, B., in press for 2007. Early Cretaceousice-cap volume, palaeo-temperatures (Mg, ä18O), and isotopestratigraphy (ä13C, 87Sr/86Sr) from Tethyan belemnites. Palaeo-geogr. Paleoclimatol. Paleoecol. 248, 391–430.
McArthur, J.M., Donovan, D.T., Thirlwall, M.F., Fouke, B.W., Mattey,D., 2000. Strontium isotope profile of the early Toarcian (Jurassic)oceanic anoxic event, the duration of ammonite biozones, andbelemnite palaeotemperatures. Earth Planet. Sci. Lett. 179,269–285.
McElwain, J.C., Wade-Murphy, J., Hesselbo, S.P., 2005. Changes incarbon dioxide during an oceanic anoxic event linked to intrusioninto Gondwana coals. Nature 435, 479–482.
Rosales, I., Robles, S., Quesada, S., 2004. Elemental and oxygenisotope composition of Early Jurassic belemnites: salinity vs.temperature signals. Jour. Sed. Res. 74, 342–354.
Sælen, G., Doyle, P., Talbot, M.R., 1996. Stable isotope analyses ofbelemnite rostra from the Whitby Mudstone Fm., England: surfacewater conditions during deposition of a marine black shale. Palaios11, 97–117.
van de Schootbrugge, B., McArthur, J.M., Bailey, T.R., Rosenthal, Y.,Wright, J.D., Miller, K.G., 2005. The Toarcian oceanic anoxicevent (T-OAE): an assessment of global causes using belemniteC-isotope records. Paleoceanography 20, PA3008. doi:10.1029/2004PA001102.
Earth and Planetary Science Letters 259 (2007) 640–641www.elsevier.com/locate/epsl
Discussion
Reply to comment on “Carbon-isotope record of the Early Jurassic(Toarcian) oceanic anoxic event from fossil wood and marine
carbonate (Lusitanian Basin, Portugal)”
Stephen P. Hesselbo a,⁎, Hugh C. Jenkyns a, Luis V. Duarte b, Luiz C.V. Oliveira c
a Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UKb Departamento de Ciências da Terra, Centro de Geociências, Faculdade de Ciências e Tecnologia da Universidade de Coimbra,
3000-272 Coimbra, Portugalc Petrobras/Cenpes, Cidade Universitária, Ilha do Fundão, 21941-598, Rio de Janeiro, Brazil
Accepted 2 May 2007Available online 8 May 2007
Editor: H. Elderfield
McArthur's comment focuses on the origin of anegative carbon-isotope excursion reported from terres-trial and marine organic matter and marine carbonate inEarly Jurassic (Toarcian) strata from a number of localitiesin northern and western Europe. He argues that theabsence of a clear negative excursion in belemnite calcitefalsifies the hypothesis that the observed negativeexcursion in other materials originates from a methanesource. We note that McArthur telescopes the argumentthat we present: in our paper we firstly interpret the newdata as supporting the hypothesis that isotopically lightcarbon dominated the global ocean–atmosphere–bio-sphere carbon reservoirs during the Early ToarcianOceanic Anoxic Event (OAE), and secondly concludethat this phenomenon could be explained by a massiveinput of carbon from a methane source.
As we discuss in the paper, the belemnite δ13C valuescould only be regarded as falsifying this hypothesis wereit to be shown unambiguously that belemnite calcite more
DOI of original article: 10.1016/j.epsl.2006.11.009.⁎ Corresponding author.E-mail address: [email protected] (S.P. Hesselbo).
accurately recorded the isotopic characteristics of globalcarbon reservoir than all the other lithological constitu-ents. We have already noted the difficulty of accuratelylocating samples on extensive foreshore exposures wherethe dip is at a low angle and, because belemnites aretypically absent from strata displaying the peak of thenegative excursion, assignment of specimens to theselevels needs to be documented with particular care.Moreover, because belemnites must have been activeswimmers, inhabiting different geographic areas anddifferent levels in the water column, their carbon-isotoperatios cannot always be relied upon asmonitors of a globalocean reservoir, particularly at times of extreme oceanstratification. The coccolith-bearing bulk carbonate fromPeniche, on the other hand, is stratigraphically locatedwith centimetric accuracy; it cannot have been derivedfrom actively swimming organisms, and most likelyrecords the isotopic characteristics of the surface waters ofthe Lusitanian Basin.
McArthur is also mistaken in suggesting that the localabsence of the negative excursion in datasets from someother European sections somehow proves that theexcursion was not a global phenomenon. A well-knownproperty of the stratigraphic record is that it is commonlyextremely condensed or incomplete. Furthermore, the
641S.P. Hesselbo et al./ Earth and Planetary Science Letters 259 (2007) 640–641
sample resolution of the records in question is notablypoor, and there are no additional data to show to whatextent these records are affected by diagenesis. Absenceof a record of a process does not logically equate to thenon-occurrence of that process.
Perhaps most importantly, McArthur ignores theterrestrial carbon-isotope record. Localized atmosphericcarbon-isotope anomalies are conceivable, if onepostulates an intense persistent local source of isotopi-cally very light carbon, but close parallelism betweensurface marine and terrestrial isotope curves through theOAE would be extremely unlikely in these circum-stances. The reality of the atmospheric signal cannot bedoubted. Beyond drawing attention again to the newterrestrial data from Peniche, we here restate the finding,reported in Hesselbo et al. (2000), that samples of jetfrom the Jet Rock in Yorkshire that were subjected toextraction of marine bitumen using an organic solvent,still displayed the negative carbon-isotope excursion.
The new data from early-silicified jet ‘cores’ simplycorroborate this finding. Finally, the lowest values of thenegative excursion in terrestrial organic matter (δ13C=about −30‰) are isotopically much lighter than thelong-term background values for the Jurassic of thisregion (δ13C=about −25‰; e.g. Hesselbo et al., 2000,2003). The negative excursion in organic matter(whether terrestrial or marine) cannot be regarded as areturn to some kind of normality as McArthur suggests.
References
Hesselbo, S.P., Gröcke, D.R., Jenkyns, H.C., Bjerrum, C.J., Farrimond,P.L., Morgans-Bell, H.S., Green, O.R., 2000. Massive dissociationof gas hydrates during a Jurassic Oceanic anoxic event. Nature 406,392–395.
Hesselbo, S.P., Morgans-Bell, H.S., McElwain, J.C., Rees, P. McA.,Robinson, S.A., Ross, C.E., 2003. Carbon-cycle perturbation in theMiddle Jurassic and accompanying changes in the terrestrialpaleoenvironment. J. Geol. 111, 259–276.
