|
Discussion |
1 Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT (e-mail: j.mcarthur{at}ucl.ac.uk)
2 Department of Earth Sciences, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes MK7 6AA, UK (e-mail: a.s.cohen{at}open.ac.uk)
3 Neftex Petroleum Consultants Ltd, 115BD Milton Park, Abingdon OX14 4SA, UK
4 2 Eastfield Court, Southwell NG25 0NU, UK (e-mail: bill_robinbailey{at}hotmail.com)
5 15 Stratton Terrace, Truro TR1 3EW, UK (e-mail: d.g.smith{at}talktalk.net)
R. J. Bailey and D. G. Smith write: Cohen et al. (2007) review the major changes in ocean chemistry in the Early Jurassic (Toarcian) and Late Palaeocene–Early Eocene, the latter corresponding to the Palaeocene–Eocene Temperature Maximum (PETM). These changes involved global carbon isotope excursions (CIEs), temperature increases, marine anoxia and biospheric crises. Their preferred explanation is the massive thermal dissociation of methane hydrates, releasing isotopically light carbon.
What is problematical is their view that the isotopic data series from the Toarcian and Palaeocene–Eocene intervals (fig. 1a and b) record not only the amounts of change in the carbon isotopes, but also the rates of change. In practice, this claim is reliable only if the samples analysed derive from intervals characterized by unbroken accumulation at an unchanging rate. For the Toarcian, its justification lies in an earlier paper (Kemp et al. 2005) and involves: (1) radiometric dates that place the Toarcian CIE within a c. 2.2 Ma interval; (2) the assumption that this interval was characterized by continuous accumulation at a constant rate; (3) spectral analysis of lithological data series suggesting 81 cm cycles in carbon isotope and biogenic carbonate abundance in the laminated, unbioturbated, sediments accumulated during the CIE; (4) use of the estimated 2.2 Ma steady rate of accumulation to calibrate the cycles, and the duration of the CIE, in terms of a 21.46 ka periodicity; (5) identification of this periodicity with precession-related, c. 21 ka, M-orbital forcing of terrestrial insolation, which is presumed to have triggered the abrupt shifts in 
13C. (The origin of this solution of the quasi-period of Early Jurassic precession is not indicated.)
The concordance between the time calibration of the spectrally determined cycles and the estimated period of Early Jurassic orbital precession, somewhat circularly, supports the assumed constant rate of accumulation during the Toarcian CIEs in the Yorkshire sections. It also discounts likely uncertainties in arriving at the 90–95% probabilities (Kemp et al. 2005) that the ‘cycles’ are real; that is, not chance outcomes of the spectral analyses (Vaughan 2005).
A millennial-scale calibration derived in this way yields the ‘accurate’ time scale (Cohen et al. 2007, p. 1104) that is the basis for the suggestion that the important shifts in isotopic abundance were both very abrupt and orbitally forced. If the key assumptions concerning continuity and rate of accumulation are dubious, then both these aspects of the interpretation are doubtful.
For example, the shifts logged A–D (fig. 1b, Cohen et al. 2007) could equally plausibly be read as evidence of local hiatuses in accumulation. Hiatuses would, in fact, remove the need to invoke a succession of catastrophically abrupt (<1 ka) and globally lethal releases of isotopically light methane into the Early Jurassic environment. Instead, the intervals between hiatuses (horizons A–D, fig. 1b) could be taken to provide snapshots of a more broadly fluctuating Toarcian isotopic signal, with some superimposed higher frequency ‘noise’.
Arguably, the profound environmental changes evidenced by the Toarcian and Palaeocene–Eocene CIEs are likely to have been accompanied by some, possibly protracted, breaks in deeper marine accumulation; and they are unlikely to have been accomplished without significant changes in rates of accumulation. In fact, a break of some 55 ka facilitates the inter-oceanic correlation of PETM records. These, too, are otherwise assumed to be unbroken; but the orbital time calibration ‘tunes’ changes in the rate of accumulation (Rohl et al. 2000).
Objective proof that isotopic shifts (e.g. A–D, Cohen et al. 2007, fig. 1b) represent large-scale, very short-term, global events will be difficult to provide. The broad similarities in the two CIEs (fig. 1a and b) underline the fact that such proof will require time correlations with millennial-scale precision. Unfortunately, such correlations depend on time calibrations that must either (1) adopt unprovable assumptions concerning the continuity and unchanging rate of accumulation (Toarcian, Kemp et al. 2005) or (2) ‘tune’ irregular lithological or isotopic fluctuations to the mean periodicity of contemporary orbital precession (Toarcian, Cohen et al. 2007; PETM, Rohl et al. 2000). This latter approach, of course, encounters the problem that the periodic sedimentary effects of M-forcing under conditions of fluctuating accumulation rate will be indistinguishable from aperiodic, but similarly irregular, stratigraphic repetition of facies.
The worldwide physical, chemical and biotic changes associated with the PETM and Toarcian CIEs are well established. The rates of change, and their pacing by orbital precession, are not. It is debatable whether the latter, relatively weak, influence on global climate (Wunsch 2004), even where superimposed on longer climatic trends (Cohen et al. 2007) is capable of repeatedly triggering massive and rapid thermal dissociation of methane hydrates. Are the stocks of hydrate replenished during the supposed c. 20 ka intervals between these global episodes; or does each relate to a different sub-seafloor P–T environment, with spasmodic reduction of the total hydrate stock?
The release of methane, with its associated feedbacks, provides a powerful explanation of the CIEs; but it should be borne in mind that the stratigraphically rapid shifts in isotopic abundance may only reflect the imperfections in a record of a more broadly varying 13C. The parallels between the shifts and the current rapid anthropogenic release of greenhouse gases thus remain questionable.
26 November 2007
A. S. Cohen, A. L. Coe & D. B. Kemp reply: Bailey and Smith comment on our interpretation of the cyclostratigraphy of the early Toarcian event in Yorkshire, and on the cyclostratigraphy of the PETM as established by others. Our interpretation of the Yorkshire cyclostratigraphy was originally presented and discussed in earlier studies (Kemp et al. 2005; Kemp 2006), as is made clear in the text (Cohen et al. 2007). It was in an earlier publication (Kemp et al. 2005), and in a comment and reply that arose from that work (Kemp et al. 2006; Wignall et al. 2006a), that nearly all the issues raised by Bailey and Smith were addressed (in particular, the comments made about hiatuses, stratigraphic completeness and hydrate replenishment). Nevertheless, we take this opportunity to reiterate our position on two points made by Bailey and Smith.
(1) Bailey and Smith suggest that the carbon isotope excursions during both the PETM and early Toarcian events would have been accompanied by significant changes in sediment accumulation rates. Although this may be true for globally averaged sedimentation rates, local sedimentation rates in the Yorkshire basin during the Toarcian, or in the deep ocean at the PETM, will be governed by local conditions. For the Toarcian section in Yorkshire, the fact that regular cycles in depth, as proven mathematically (Kemp et al. 2005; Kemp 2006), are observed across the abrupt carbon isotope shifts precludes the possibility that significant changes in accumulation rate occurred. If accumulation rates slowed sufficiently to generate the abrupt shifts in the early Toarcian data, then cycles across these shifts would not have the same wavelength as cycles through parts of the succession with no abrupt shifts; thus, the mathematically proven regularity of the cycles attests to a relatively constant sedimentation rate. Theoretically, entire cycles could be missing from the Yorkshire section (although one wonders about the likelihood that exact integer multiples of cycle(s) would be absent). However, there is no sedimentological evidence for any missing strata across the abrupt shifts, a fact discussed at length by Kemp (2006). Cyclostratigraphy represents the only well-established tool that can discern and quantify the short-term changes in accumulation rate that are suspected by Bailey and Smith (see Weedon 2003).
A further major obstacle to Bailey and Smith's suggestion that the abrupt negative shifts in 13Corg in the Toarcian section documented by Kemp et al. (2005) were the result of hiatuses is that shifts of similar brevity (and sometimes similar magnitude as well) have been documented in high-resolution 13Ccarb records from the exaratum Subzone at other European locations (Jenkyns et al. 2001; Hermoso 2007; Hesselbo et al. 2007a; Suan et al. 2008b). For the reasons elaborated in our review (Cohen et al. 2007), we believe that the sudden introduction of isotopically ‘light’ carbon is the only readily conceivable mechanism that would cause very different parts of the carbon cycle, from widely differing locations, to show such similar responses. The chance of obtaining robust cyclostratigraphic results and a concurrence of carbon isotope patterns from the fortuitous occurrence of hiatuses seems impossibly remote.
(2) Bailey and Smith question whether it is possible to obtain objective proof that isotopic shifts such as those that we highlighted in figure 1 of Cohen et al. (2007) represent large-scale, very short-term, global events. Objective proof that the isotopic shifts during both the early Toarcian and PETM were short-term events is in fact provided by the cyclostratigraphic interpretations that we have cited in our paper (Cohen et al. 2007). The unique utility and proven veracity of the cyclostratigraphic method is well established (see, for example, the text by Weedon 2003), and is precisely why astronomical chronologies now constrain nearly a fifth of the entire Phanerozoic time scale (Gradstein et al. 2005).
J. M. McArthur writes: In their recent review paper, Cohen et al. (2007) compare the Palaeocene–Eocene thermal maximum (PETM) event with events in the early Toarcian. They conclude that the isotopic profiles of 13C across the PETM and the early Toarcian are similar and can be attributed to release of marine methane hydrate to the atmosphere, thereby causing global warming. That climate change across the PETM may have been caused by release of sediment-hosted methane is a hypothesis (but not a proven fact) that is not disputed here, despite growing doubts about its validity (Higgins & Schrag 2006). That such a release of methane is not a robust explanation for the Toarcian isotopic profiles has been the subject of several recent publications (e.g. Wignall et al. 2006a), and the evidence presented in these publications warrants further consideration than is afforded in the review by Cohen et al. (2007). Other relevant discussions (e.g. Newton & Bottrell 2007) may have been published too late to be mentioned in the review by Cohen et al. (2007), so they are summarized here.
The Toarcian. In regard to Toarcian events, the main points made by Cohen et al. (2007) are as follows.
(1) That the early Toarcian was a time of global deposition of organic-rich strata. Organic-rich (i.e. black) shales appear to be common in the Toarcian, but it is not clear whether this is because they have been sought assiduously for their novelty, or because they really do represent a high proportion of all Toarcian strata. Hesselbo et al. (2007a, and references therein) and Osete et al. (2007) have documented the accumulation in many localities in Portugal, and two in Spain, of Early Toarcian strata that were oxic, organic-poor, and carbonate-rich (see also McArthur 2007a, b).
(2) That the early Toarcian ‘negative excursion’ in 13C is present in all carbon reservoirs. That it is not was mentioned briefly by Hesselbo et al. (2000), and discussed and illustrated at length by van de Schootbrugge et al. (2005). Yorkshire is the ‘type locality’ for the excursion in 13Corg, yet belemnite calcite through that interval shows no negative excursion in 13Ccarb (van de Schootbrugge et al. 2005; McArthur 2007b). The absence of a negative excursion in sections elsewhere cannot always be due to the absence of relevant strata, because some sections lacking the excursion are explicitly stated to preserve the appropriate ammonite Zone (Falciferum Zone; now renamed the Serpentinum Chronozone; Page 2003) and organic-rich sediment (e.g. the Winterbourne Kingston borehole section in the southern UK; Jenkyns & Clayton 1997).
(3) That the C-isotopic profiles across the PETM and the early Toarcian ‘negative excursion’ are similar. The negative excursion seen in all profiles of 13Ccarb through PETM sections is unlike the flat profile seen in the 13Ccarb record from Yorkshire (compare Zachos et al. 2007 with McArthur 2007b). In any event, passing similarities or dissimilarities carry no genetic information, as Louis-Schmid et al. (2007) observed when they stated that the shape of negative isotopic profiles in 13C ‘cannot be used as an argument when determining their origin if they have not been shown to represent a global perturbation of the carbon reservoir’. The PETM appears to have been a global event. The Toarcian negative excursion in 13Corg has not been shown to be correlative globally by any means other than the circular use of C-isotope stratigraphy between Denmark and Yorkshire (Newton & Bottrell 2007). Few of the localities displayed in figure 3 of Cohen et al. (2007) have been profiled for 13C. According to Wignall et al. (2005, 2006a), the deposition of Toarcian black shales may have been diachronous.
(4) That the rise in 87Sr/86Sr across the exaratum Subzone is unusually rapid. Changes in the slope of the 87Sr/86Sr profile through the late Pliensbachian and Toarcian strata of Yorkshire are not unusually rapid in time; they simply reflect changes in sedimentation rate, a matter discussed by McArthur et al. (2000) and McArthur & Wignall (2007). The relation between sedimentation rate and 87Sr/86Sr in seawater is made more graphic in figure 11 of McArthur et al. (2007).
(5) That the duration of the ‘negative isotope excursion’ in 13Corg is around 300 ka on the evidence of cyclostratigraphy. The figure of 300 ka was deduced by cyclostratigraphic analysis of strata in Yorkshire that were below the ‘negative excursion’, not through it. The resulting power spectrum had a single frequency (Kemp et al. 2005) that was assumed, not proven, to have a period of 21 ka. Cyclostratigraphic studies by Suan et al. (2008b) of the Lower Toarcian Posidonia Shale of Germany, and its correlative equivalent in Peniche, Portugal, yielded a signal, with one to four frequencies, that is consistent with it being coupled to orbital cycles; a duration of 900 ka was deduced for the interval. This estimate is three times longer than that accepted by Cohen et al. (2007) for the same interval, but is close to the 1.1 Ma estimated by McArthur et al. (2000) on the basis of Sr-isotope stratigraphy through the equivalent interval in Yorkshire.
More generally, Toarcian data presented by Cohen et al. (2007) and some others are capable of interpretations different from those accepted in their review. For example, two levels (both at concretionary horizons) in the lower exaratum Subzone have high 187Os/188Os values (Cohen et al. 2004) compared with adjacent strata. Cohen et al. (2004) suggested that these high values represent a 400–800% increase in weathering. No other evidence exists to support this interpretation, which is not persuasive, given the absence of the sediment that such a change should have produced; the isotopic anomalies are found in sections that are condensed (McArthur et al. 2000; McArthur & Wignall 2007). The restricted-basin model (Küspert 1982; Sælen et al. 1996) provides a ready explanation for these anomalies because the model, especially as developed by Sælen et al. (1996), requires that surface water freshened as restriction developed. The value of 187Os/188Os in local seawater would have increased as salinity decreased because the concentrations of Os in seawater and river water are approximately equal today, and were probably so during Toarcian time, whereas the 187Os/188Os value of modern river water is much higher than it is in modern or, presumably, Toarcian seawater (Peuker-Ehrenbrink & Ravizza 2000; Cohen & Coe 2004).
To conclude, Toarcian events should not be evaluated without considering the alternative views given in papers cited here; Newton & Bottrell (2007) in particular touched on other complications regarding Toarcian events that are worthy of consideration, such as circular reasoning in correlating C-isotope profiles between supposedly coeval Toarcian sections. Although the restricted basin model for deposition of Toarcian black shales by no means explains all of their features, it has fewer drawbacks than does the methane hypothesis, which is contrary to many published facts and interpretations that are not discussed in the review by Cohen et al. (2007).
12 December 2007
A. S. Cohen, A. L. Coe and D. B. Kemp reply: In our review paper (Cohen et al. 2007), we presented a wide range of information that pertains to the striking carbon isotope excursions (CIEs) that occurred at the Palaeocene–Eocene thermal maximum (PETM) and during the Toarcian (Early Jurassic). We noted a number of important characteristics that were shared by the two events, which included the magnitude of the CIEs and their similar profiles and durations that involved rapid and repeated decreases in 13C followed by a more gradual recovery. For both events, there is a large body of evidence for rapid global temperature increases, for substantial increases in the rates of continental weathering, for widespread carbonate dissolution, and for significant changes in species distributions on land and in the oceans. We also noted, as many others have done, that the CIEs and the associated environmental changes occurred at the same time as the emplacement of two Large Igneous Provinces (LIPs): the Karoo–Ferrar LIP for the Toarcian event, and the North Atlantic LIP for the PETM. A further crucial similarity between the two events was the contemporaneous deposition of organic-rich sediments in many marine localities, an observation that, for the Toarcian, has resulted in its recognition as an Oceanic Anoxic Event (OAE) (Jenkyns 1988; Erba 2004).
On the basis of our detailed comparison, we concluded that the two CIEs and the associated events probably arose as the consequences of some or all of a similar set of causes. We described and discussed a total of six different causative mechanisms that have been suggested for the two CIEs. Our comparison led us to conclude that the most likely explanation for the CIEs during both the Toarcian and at the PETM, and also for the wide-ranging environmental changes associated with them, involved the sudden and repeated destabilization of large quantities of methane hydrate consequent upon LIP emplacement.
In his comment, McArthur questions the validity and significance of much of the wide variety of evidence from the literature that we have assembled for our comparison of the Toarcian and PETM CIEs. Disagreement with the interpretations that we, and others, have presented for the Toarcian was not only discussed in our paper (Cohen et al. 2007), but has already been published (van de Schootbrugge et al. 2005), mostly as comments on other original work (Wignall et al. 2006b; McArthur 2007a,b; McArthur & Wignall 2007). In essence, McArthur's view of the Toarcian CIE appears to us to be rooted in two beliefs that most evidence (and all emerging evidence of which we are aware) shows are highly questionable. The first belief is that the Toarcian CIE was a local event that was caused by overturn in a stratified, marginal basin; the second is that the rate of increase in the seawater 87Sr/86Sr during the deposition of the Toarcian exaratum Subzone, as calculated independently from an assumption that the laminations are annual (Hallam 1997; Cope 1998) and from time series analysis (Kemp et al. 2005; Kemp 2006), was impossibly high, and therefore that this interval in Yorkshire is greatly condensed. However, the arguments that McArthur makes both here and elsewhere are inconsistent with published observations, and many pertinent observations that do not concur with his view have been ignored. Our replies to the specific points raised by McArthur are as follows.
(1) McArthur casts doubt upon the long-standing and extensive evidence indicating that the Toarcian was a time of global deposition of organic-rich strata (e.g. Jenkyns 1988; Erba 2004; Hesselbo et al. 2007a), thereby implying that the OAE was not an OAE at all. However, McArthur presents no new evidence to support this suggestion. In mentioning correctly that carbonate sedimentation continued during the Toarcian CIE in some Portuguese and Spanish sections, McArthur crucially fails to note that ‘the Toarcian episode is marked by a pronounced decrease in carbonate’ (Erba 2004), and that evidence for the intensification of reducing conditions during the CIE (e.g. a highly impoverished fauna, laminated strata, an increase in TOC and the diminution of bioturbation) is frequently found (Jenkyns 1988; Hesselbo et al. 2000; Schouten et al. 2000; Kemp et al. 2005; Hesselbo et al. 2007a). The persistence of carbonate-rich sedimentation will, of course, have continued in some places across the CIEs during both the Toarcian and the PETM (Erba 2004; Hesselbo et al. 2007a; Woodfine et al. 2008) as a result of local depositional conditions, but this does not invalidate the fact that the expansion of reducing conditions during the Toarcian CIE (e.g. Pearce et al. 2008) led to the widespread accumulation of organic-rich lithologies.
(2) McArthur's assertion that the Toarcian CIE is not present in all carbon reservoirs of Toarcian age is based either on low-resolution data where the excursion has been missed (e.g. the belemnite data of van de Schootbrugge et al. 2005), on data from sections whose stratigraphic position is uncertain (i.e. they are not from the Toarcian exaratum Subzone; Wignall et al. 2006b) or on data from samples that may have been altered diagenetically. The problems that have been introduced into any interpretation of the Toarcian CIE by the sparse, multi-species belemnite 13C data reported by van de Schootbrugge et al. (2005) have already been discussed at length elsewhere (Hesselbo et al. 2007a, b). In contrast, a series of new, high-resolution 13Ccarb datasets show unequivocally that the Toarcian CIE was recorded within bulk carbonate samples and samples of carbonate macro- and micro-fossils (brachiopods and coccoliths) from the exaratum Subzone (Hermoso 2007; Hesselbo et al. 2007a; Suan et al. 2008a); these observations demonstrate that the negative 13C excursion was present throughout the entire water column. Finally, we noted in our review (Cohen et al. 2007) that the records of the Toarcian CIE in some carbonate samples may well have been compromised by dissolution following seawater acidification, in exactly the same manner as has been documented for carbonate-based records from the PETM.
(3) The similarity between the Toarcian and PETM CIEs is strikingly obvious where high-resolution data are used (e.g. Cohen et al. 2007, fig. 1). However, using low-resolution belemnite data that do not record a marked negative 13C excursion (van de Schootbrugge et al. 2005), McArthur claims that the 13C profile for the Toarcian CIE was flat, thereby casting doubt upon the global reach of the Toarcian CIE and upon the similarities between the Toarcian and PETM events. In an earlier round of comment and discussion (Hesselbo et al. 2007a, b; McArthur 2007), Hesselbo et al. (2007a, b) have drawn attention to the major flaws in McArthur's arguments that arise from using the sparse belemnite 13C data. Additionally, both in his present comment and elsewhere (e.g. Wignall et al. 2006b; McArthur 2007b), McArthur has ignored the significance of the 13C excursion that is found in fossil wood from a number of locations (Hesselbo et al. 2000; McElwain et al. 2005; Hesselbo et al. 2007a, b). The fossil wood data demonstrate that the 13C excursion affected the terrestrial and atmospheric carbon reservoirs in addition to the marine reservoir, and this information alone would suggest that the event was highly likely to have been global in extent, rather than being of a local or regional nature.
McArthur attempts to discredit the significance of the characteristic 13C profile of the Toarcian CIE by misrepresenting the findings of a recent study that documented the effects of a localized Late Jurassic methane seep (Louis-Schmid et al. 2007). Louis-Schmid et al. (2007) showed that anaerobic oxidation of methane caused the local precipitation of 13C-depleted authigenic carbonate in a Late Jurassic hemipelagic succession. On the basis of their findings, Louis-Schmid et al. (2007) wisely cautioned against the interpretation of negative 13C excursions that ‘are local and/or only present in carbonate and not in organic matter’. But they also stated very clearly that ‘Excursions recognized in several records from different paleolatitudes and paleoenvironments and in both inorganic- and organic-carbon records represent global perturbations to the carbon cycle’, and they specifically mentioned both the PETM and the Toarcian as examples of global events.
(4) In earlier publications, McArthur and colleagues have claimed that changes in the rate of increase of seawater 87Sr/86Sr during the Toarcian OAE were caused by fluctuations in sedimentation rate, rather than by changes in the balance between the major inputs of Sr to the oceans (McArthur et al. 2000). Under this assumption, McArthur et al. (2000) attempted to estimate intervals of time within the Toarcian by ascribing variations in the seawater 87Sr/86Sr solely to changes in sedimentation rate. We and others (Cohen et al. 2004, 2007; Waltham & Gröcke 2006) have shown that the approach used by McArthur et al. (2000) is flawed because the slope of the seawater 87Sr/86Sr changed from sharply decreasing to sharply increasing close to the Pliensbachian–Toarcian boundary, perhaps only a few hundred thousand years before the start of the exaratum Subzone. Fluctuating sedimentation rates cannot have been responsible for the change in the slope of the seawater 87Sr/86Sr from decreasing to increasing; and thus, as we have stated elsewhere (Cohen et al. 2004, 2007), changes in the seawater 87Sr/86Sr must have arisen from sudden changes in the continental weathering flux. It therefore follows that it is impossible to ascribe changes in the slope of the seawater 87Sr/86Sr curve to local variations in sedimentation rate as McArthur has attempted to do.
Thus the seawater Sr-isotope data for the Toarcian cannot be used to estimate intervals of time and they do not support the assertion that the exaratum Subzone is highly condensed. With this in mind, there is a remarkable degree of coherence between the varied observations indicating the global scale and impact of the environmental perturbations that occurred during the Toarcian. Importantly, both the Sr- and Os-isotope data indicate that there were large increases in continental weathering rates (Cohen et al. 2004, 2007); these inferences are backed up by the observation of substantial changes in clay mineralogy in many Toarcian sections (Erba 2004; Woodfine et al. 2008). Collectively, these observations for enhanced continental weathering are in full accord with the large global temperature increase suggested by the data of McArthur and colleagues (Bailey et al. 2003), as has been discussed fully in our review and elsewhere (Jenkyns 2003; Cohen et al. 2004, 2007; Erba 2004; Cohen & Coe 2007).
(5) In criticizing the evidence for the c. 300 ka duration of the negative Toarcian CIE cited by Cohen et al. (2007), the publications that McArthur mentions are incorrect. The c. 300 ka duration of the negative CIE is based on data presented by Kemp (2006) that show more than one frequency and are stratigraphically far more extensive than those presented by Kemp et al. (2005). Furthermore, we pointed out on p. 1098 of our review that tuning and filtering of data from a stratigraphic interval spanning the entire excursion, as well as above and below, suggested that the 81 cm cyclicity found in intervals 1 and 2 was in fact pervasive through the entire excursion interval (Coe & Weedon 2006). It was on this basis that Cohen et al. (2007) suggested that the duration of the entire excursion was c. 300 ka. The c. 900 ka duration for the Toarcian CIE suggested by Suan et al. (2008a) is for a stratigraphic interval from Portugal that is appreciably longer than the stratigraphic interval that defines the CIE in Yorkshire (Cohen et al. 2007). Additionally, the shift towards lower 13C values was estimated at c. 150 ka by Suan et al. (2008a), which is very similar to the duration of the onset of the CIE in the Yorkshire section (c. 120 ka) as established by Kemp et al. (2005). At present, therefore, it is incorrect to state that there is a large discrepancy between the results from these two cyclostratigraphic studies. We simply draw attention here, as we did in our paper (Cohen et al. 2007), to the many other similarities between the events that were associated with the Toarcian and PETM CIEs.
Concluding remarks. In the final two paragraphs of his comment, McArthur writes in a manner suggesting that we have provided a biased and partial review of this topic, stating, for example; ‘Toarcian data presented by Cohen et al. (2007) and some others are capable of interpretations different from those accepted in their review’, and ‘Toarcian events should not be evaluated without considering the alternative views given in papers cited here’. This claim that we have omitted analysis of alternative views is demonstrably incorrect. For example, we discussed in a number of places in our review Küspert's restricted basin overturn model that McArthur favours (Küspert 1982); and recognizing its historical importance, we devoted an entire sub-section to it on pages 1099–1100 (Cohen et al. 2007). But importantly, we noted that new data from the NIOZ group in the Netherlands (van Breugel et al. 2006) demonstrated that the contribution of respired C to the Toarcian CIE (as recorded by samples from the Paris basin) was very low, at no more than c. 11%, leading those workers to conclude that ‘recycling of respired CO2 as originally proposed by Küspert (1982) is unlikely to have been the main cause for the large negative 13C shift’. The conclusions of van Breugel et al. (2006) supersede those based upon older data from the same group, which had previously supported a restricted basin overturn model as being the cause of the Toarcian CIE (Schouten et al. 2000). In continuing to champion Küspert's restricted basin overturn model, McArthur has mentioned neither the implications of the study by van Breugel et al. (2006) nor the evidence from numerous other studies that do not lend support to the Küspert model.
In summary, no new facts or interpretations were presented in McArthur's comment. We stand fully by the information that we presented in our paper (Cohen et al. 2007), by the manner in which this information was presented and discussed, and by the conclusions at which we arrived.
20 February 2008
 |
References |
|---|
 TOP References |
|---|
Bailey, T.R., Rosenthal, Y., McArthur, J.M., van de Schootbrugge, B. & Thirlwall, M.F. 2003. Paleoceanographic changes of the Late Pliensbachian–Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event. Earth and Planetary Science Letters, 212, 307–320.[CrossRef][ISI][GeoRef]
Coe, A.L. & Weedon, G.P. 2006. Jurassic cyclostratigraphy: Recent advances, implications and problems. Volumina Jurassica, 4, 156–157.
Cohen, A.S. & Coe, A.L. 2007. The impact of the Central Atlantic Magmatic Province on climate and on the Sr- and Os-isotope evolution of seawater. Palaeogeography, Palaeoclimatology, Palaeoecology, 244, 374–390.[CrossRef][GeoRef]
Cohen, A.S., Coe, A.L., Harding, S.M. & Schwark, L. 2004. Osmium isotope evidence for the regulation of atmospheric CO2 by continental weathering. Geology, 32, 157–160.
Cohen, A.S., Coe, A.L. & Kemp, D.B. 2007. The Late Palaeocene, Early Eocene and Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales, associated environmental changes, causes and consequences. Journal of the Geological Society, London, 164, 1093–1108.
Cope, J.C.W., 1998. Discussion on estimates of the amount and rate of sea-level change across the Rhaetian–Hettangian and Pliensbachian–Toarcian boundaries (latest Triassic to early Jurassic). Journal of the Geological Society, London, 155, 421–422.
Erba, E., 2004. Calcareous nannofossils and Mesozoic oceanic anoxic events. Marine Micropaleontology, 52, 85–106.[CrossRef][ISI][GeoRef]
Gradstein, F.M., Ogg, J.G. & Smith, A.G. (eds) 2005. A Geologic Time Scale 2004. Cambridge University Press, Cambridge.
Hallam, A., 1997. Estimates of the amount and rate of sea-level change across the Rhaetian–Hettangian and Pliensbachian–Toarcian boundaries (latest Triassic to early Jurassic). Journal of the Geological Society, London, 154, 773–779.
Hermoso, M. 2007. Les perturbations environnementales au cours du Toarcien inférieur. Apport de l'étude sédimentologique et géochimique de séries boréales et ouest-téthysiennes. PhD thesis, Université Pierre et Marie Curie, Paris.
Hesselbo, S.P., Gröcke, D.R., Jenkyns, H.C., Bjerrum, C.J., Farrimond, P., Bell, H.S.M. & Green, O.R. 2000. Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event. Nature, 406, 392–395.[CrossRef][Medline][GeoRef]
Hesselbo, S.P., Jenkyns, H.C., Duarte, L.V. & Oliveira, L.C.V. 2007a. Carbon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal). Earth and Planetary Science Letters, 253, 455–470.[CrossRef][ISI][GeoRef]
Hesselbo, S.P., Jenkyns, H.C., Duarte, L.V. & Oliveir, L.C.V. 2007b. Reply to comment on ‘Carbon-isotope record of the Early Jurassic (Toarcian) oceanic anoxic event from fossil wood and marine carbonate (Lusitanian Basin, Portugal)’. Earth and Planetary Science Letters, 259, 640–641.[CrossRef][ISI][GeoRef]
Higgins, J.A. & Schrag, D.P. 2006. Beyond methane: Towards a theory for the Paleocene–Eocene Thermal Maximum. Earth and Planetary Science Letters, 245, 523–537.[CrossRef][ISI][GeoRef]
Jenkyns, H.C., 1988. The Early Toarcian (Jurassic) Anoxic Event: stratigraphic, sedimentary, and geochemical evidence. American Journal of Science, 288, 101–151.
Jenkyns, H.C., 2003. Evidence for rapid climate change in the Mesozoic–Palaeogene greenhouse world. Philosophical Transactions of the Royal Society of London, Series A, 361, 1885–1916.[CrossRef][Medline]
Jenkyns, H.C. & Clayton, C.J. 1997. Lower Jurassic epicontinental carbonates and mudstones from England and Wales: chemostratigraphic signals and the early Toarcian anoxic event. Sedimentology, 44, 687–706.[CrossRef][ISI][GeoRef]
Jenkyns, H.C., Gröcke, D.R. & Hesselbo, S.P. 2001. Nitrogen isotope evidence for water mass denitrification during the early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography, 16, 593–603.[CrossRef][ISI][GeoRef]
Kemp, D.B. 2006. Astronomical forcing during the early Toarcian and Late Triassic: Implications for the cause and duration of global environmental change and stratigraphic correlation. PhD thesis, The Open University, Milton Keynes.
Kemp, D.B., Coe, A.L., Cohen, A.S. & Schwark, L. 2005. Astronomical pacing of methane release in the Early Jurassic period. Nature, 437, 396–399.[CrossRef][Medline]
Kemp, D.B., Coe, A.L., Cohen, A.S. & Schwark, L. 2006. Reply to the Comment by Wignall et al. on ’Astronomical pacing of methane release in the Early Jurassic period'. Nature, 441doi:10.1038/nature04906.
Küspert, W., 1982. Environmental changes during oil shale deposition as deduced from stable isotope ratios. In: Einsele, G. & Seilacher, A. (eds) Cyclic and Event Stratification. Springer, Berlin, 482–501.
Louis-Schmid, B., Rais, P., Logvinovich, D., Bernasconi, S.M. & Weissert, H. 2007. Impact of methane seeps on the local carbon-isotope record: a case study from a Late Jurassic hemipelagic section. Terra Nova, 19, 259–265.[CrossRef][ISI][GeoRef]
McArthur, J.M. 2007a. Comment on ‘The impact of the Central Atlantic Magmatic Province on climate and on the Sr- and Os-isotope evolution of seawater by Cohen A. S. and Coe A. L. 2007, Palaeogeography, Palaeoclimatology, Palaeoecology, 244, 374–390’. Palaeogeography, Palaeoclimatology, Palaeoecology, doi:10.1016/j.palaeo.2007.09.008.
McArthur, J.M., 2007b. 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. Earth and Planetary Science Letters, 259, 634–639.[CrossRef][ISI][GeoRef]
McArthur, J.M. & Wignall, P.B. 2007. Comment on ‘Non-uniqueness and interpretation of the seawater Sr-87/Sr-86 curve’ by Dave Waltham and Darren R. Grocke (GCA, 70, 2006, 384–394). Geochimica et Cosmochimica Acta, 71, 3382–3386.[CrossRef][ISI][GeoRef]
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, and belemnite palaeotemperatures. Earth and Planetary Science Letters, 179, 269–285.[CrossRef][ISI][GeoRef]
McArthur, J.M., Janssen, N.M.M., Reboulet, S., Leng, M.J., Thirlwall, M.F., van de Schootbrugge, B. 2007. Early Cretaceous ice-cap volume, palaeo-temperatures (Mg, 18O), and isotope stratigraphy (13C, 87Sr/86Sr) from Tethyan belemnites. Palaeogeography, Palaeoclimatology, Palaeoecology, 248, 391–430.[CrossRef][GeoRef]
McElwain, J.C., Wade-Murphy, J. & Hesselbo, S.P. 2005. Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals. Nature, 435, 479–482.[CrossRef][Medline]
Newton, R. & Bottrell, S. 2007. Stable isotopes of carbon and sulphur as indicators of environmental change: past and present. Journal of the Geological Society, London, 164, 691–708.
Osete, M.-L., Gialanella, P.-R., Gómez, J.J., Villalaín, J.J., Goy, A. & Heller, F. 2007. Magnetostratigraphy of Early–Middle Toarcian expanded sections from the Iberian Range (central Spain). Earth and Planetary Science Letters, 259, 319–332.[CrossRef][ISI][GeoRef]
Page, K.N., 2003. The Lower Jurassic of Europe: its subdivision and correlation. Geological Survey of Denmark and Greenland Bulletin, 1, 23–59.[GeoRef]
Pearce, C.R., Cohen, A.S., Coe, A.L. & Burton, K.W. 2008. Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic. Geology, 36, 231–234.
Peuker-Ehrenbrink, B. & Ravizza, G. 2000. The marine osmium isotope record. Terra Nova, 12, 205–219.[CrossRef][ISI][GeoRef]
Rohl, U., Bralower, T.J., Norris, R.D. & Wefer, G. 2000. New chronology for the late Palaeocene thermal maximum and its environmental implications. Geology, 28, 927–930.
Sælen, G., Doyle, P. & Talbot, M.R. 1996. Stable isotope analyses of belemnite rostra from the Whitby Mudstone Fm., England: surface water conditions during deposition of a marine black shale. Palaios, 11, 97–117.
Schouten, S., Van Kaam-Peters, H.M.E., Rijpstra, W.I.C., Schoell, M. & Damste, J.S.S. 2000. Effects of an oceanic anoxic event on the stable carbon isotopic composition of Early Toarcian carbon. American Journal of Science, 300, 1–22.
Suan, G., Mattioli, E., Pittet, B., Mailliot, S. and Lecuyer, C. 2008a. Evidence for major environmental perturbation prior to and during the Toarcian (Early Jurassic) oceanic anoxic event from the Lusitanian Basin, Portugal. Paleoceanography, 23, doi:10.1029/2007PA001459.
Suan, G., Pittet, B., Bour, I., Mattioli, E., Duarte, L.V. and Mailliot, S. 2008b. Duration of the Early Toarcian carbon isotope excursion deduced from spectral analysis: Consequences for its possible causes. Earth and Planetary Science Letters, doi:10.1016/j.epsl.2007.12.017.
van Breugel, Y., Baas, M., Schouten, S., Mattioli, E. and Damste, J.S.S. 2006. Isorenieratane record in black shales from the Paris Basin, France: Constraints on recycling of respired CO2 as a mechanism for negative carbon isotope shifts during the Toarcian oceanic anoxic event. Paleoceanography, 21, doi:10.1029/2006PA001305.
van de Schootbrugge, B., McArthur, J.M., Bailey, T.R., Rosenthal, Y., Wright, J.D. and Miller, K.G. 2005. Toarcian oceanic anoxic event: An assessment of global causes using belemnite C isotope records. Paleoceanography, 20, doi:10.1029/2004PA001102.
Vaughan, S., 2005. A simple test for periodic signals in red noise. Astronomy & Astrophysics, 431, 391–403.[CrossRef][ISI]
Waltham, D., Gröcke, D.R. 2006. Non-uniqueness and interpretation of the seawater 87Sr/86Sr curve. Geochimica et Cosmochimica Acta, 70, 384–394.[CrossRef][ISI][GeoRef]
Weedon, G.P., 2003. Time Series Analysis and Cyclostratigraphy: Examining Stratigraphic Records of Environmental Cycles. Cambridge University Press, Cambridge.
Wignall, P.B., Newton, R.J. & Little, C.T.S. 2005. The timing of paleoenvironmental change and cause-and-effect relationships during the Early Jurassic mass extinction in Europe. American Journal of Science, 305, 1014–1032.
Wignall, P.B., Hallam, A., Newton, R.J., Sha, G.J., Reeves, E., Mattioli, E. & Crowley, S. 2006a. An eastern Tethyan (Tibetan) record of the Early Jurassic (Toarcian) mass extinction event. Geobiology, 4, 179–190.[CrossRef]
Wignall, P.B., McArthur, J.M., Little, C.T.S. & Hallam, A. 2006b. Palaeoceanography—Methane release in the Early Jurassic period. Nature, 441, E5–E6.[Medline]
Woodfine, R.G., Jenkyns, H.C., Sarti, M., Baroncini, F. and Violante, C. 2008. The response of two Tethyan carbonate platforms to the early Toarcian (Jurassic) oceanic anoxic event: environmental change and differential subsidence. Sedimentology, doi:10.1111/j.1365-3091.2007.00934.x.
Wunsch, C., 2004. Quantitative estimate of the Milankovitch-forced contribution to Quaternary climate change. Quaternary Science Reviews, 23, 1001–1012.[CrossRef][ISI][GeoRef]
Zachos, J.C., Bohaty, S.M., John, C.M., McCarren, H., Kelly, D.C. and Nielsen, T. 2007. The Palaeocene–Eocene carbon isotope excursion: constraints from individual shell planktonic foraminifer records. Philosophical Transactions of the Royal Society of London, Series A, 365, 1829-1842, doi:10.1098/rsta.2007.2045.
| ||||||||||||||||||||||||||||||||||||||||||||||||||
