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Discussion |
1 School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK (e-mail: David.Loydell{at}port.ac.uk)
2 Department of Geology, Historical Geology & Palaeontology, Sölvegatan 13, S-223 62 Lund, Sweden (e-mail: lennart.jeppsson{at}geol.lu.se)
3 Department of Geology, University of Leicester, Leicester LE1 7RH, UK (e-mail: ra12{at}le.ac.uk)
Scientific editing by Alistair Crame.
D. K. Loydell writes: In their recent paper 
Jeppsson & Aldridge (2000, p. 1137) expressed reservations about the method behind the generation of my (Loydell 1998) sea-level curve for the early Silurian (Llandovery to earliest Ludlow). As the method used was based upon decades of consistent observations of graptolite distribution, I am rather surprised that Jeppsson & Aldridge could not accept the scientific basis of the sea-level curve’s construction.
Simply stated, graptolite diversity increases from low (usually 1–3 species) in nearshore environments to a maximum in outer shelf environments. Thus a significant rise in relative sea-level can be inferred in a section where a diverse graptolite assemblage is recovered from above an unconformity or above strata with features indicative of deposition in very shallow water. Where such a deepening pattern can be demonstrated to occur at the same stratigraphical level on a number of different palaeocontinents, a eustatic sea-level rise is the logical explanation.
Clearly, it is necessary for me to cite here some of the ‘decades of consistent observations’ that I refer to above which contrast low diversity ‘inshore’ graptolite assemblages with diverse outer shelf assemblages. I should also state here that there are no known exceptions to this pattern. The examples selected below have been chosen for their wide palaeogeographical as well as stratigraphical coverage. The list is by no means exhaustive: Berry & Boucot (1972), Watkins & Berry (1977), Kaljo (1978), Pa
kevi
ius (1979), Chen (1990), Cooper et al. (1991), Lenz et al. (1993), Finney & Berry (1997), 
torch (1998), Goldman et al. (1999).
Jeppsson & Aldridge also note that my sea-level curve (Loydell 1998) differs significantly from those published previously based on other criteria. This is simply a result of the fact that my curve was based on sections that could be precisely dated using graptolites. This allowed considerable confidence to be placed on the timing of individual sea-level changes. In the previously published sea-level curves the biostratigraphical data used were either less precise (the sections lacked graptolites), or the graptolite data used had not been critically reassessed in the light of the considerable recent advances in graptolite taxonomy at the species level. Much of this critical re-assessment was carried out by me (e.g. Loydell 1992; Loydell et al. 1998) prior to the construction of the sea-level curve.
Finally, Jeppsson & Aldridge state that the ‘current situation [with regard to the relationship between planktic diversity and their oceanic episodes] is equivocal’. To quote Melchin et al. (1998, p. 176), in their analysis of global diversity and survivorship patterns of Silurian graptoloids: ‘As noted by Loydell (1994), the relationship between graptolite diversity and other environmental changes does not match well with the predictions of the model of changing oceanic states put forward by Jeppsson (1990), Aldridge et al. (1993), and Jeppsson et al. (1995).’ Even in the single paper on graptolite diversity that appears in part to support the P and S model (Berry 1998), it is noted (p. 261) that ‘No evidence for the Hellvi Secundo Episode has been found’ and (p. 262) that there is ‘relatively little evidence for [the majority of] the Llandovery oceanic events’. The situation thus appears not to be equivocal, but to be as clearly stated by Melchin et al. above.
9 November 2000
L. Jeppsson & R. J. Aldridge reply: We would wish to assure David Loydell that we do not intend to cast doubt on the observations of graptolite specialists or on his own critical re-assessments of graptolite taxonomy. We are also convinced that sea-level changes did occur in the Silurian. We merely urge caution regarding the derivation of particular sea-level curves. We do not disagree, of course, that a local rise in relative sea-level is indicated where graptolitic shales lie on an unconformity or on strata with sedimentary structures unequivocally indicative of deposition in very shallow water, but we do have concerns about the procedure of inductively assembling a curve based on graptolite diversity and then interpreting it directly as a sea-level curve. In this regard, it is important to know how ‘inshore’ and ‘outer shelf’ deposits have been identified by the various authors cited above by Loydell; if there is any input from graptolite diversity into these interpretations, then the reasoning becomes circular. We also have to trust that all the diversity changes reported by the cited authors are statistically reliable and that differing sample sizes have been taken into account by using diversity indices or rarefaction curves; unfortunately, the primary numerical data are not always available to verify this.
Loydell (1998) has rightly endeavoured to avoid circularity in his own work on graptolite diversity by using increased carbon content of sediments as an independent indicator of transgressive phases. Changes in the amount of organic carbon incorporated into sediments, however, reflect a changing balance between the transport of oxygen and the transport of organic material to the sea bottom. Oxygenation of the sea floor is largely regulated by the renewal rate of the bottom water and the concentration of dissolved oxygen in that water, while organic production in surface waters is largely nutrient dependent. The way in which these factors contribute to the formation of black shales has been widely debated, and there is a voluminous literature (e.g. see Hay 1988 for a review), but a simple direct relationship to sea level is not frequently adduced. For example, Moore et al. (1993) found a ‘quite remarkable’ relationship between areas with graptolitic black shale during the Wenlock and regions of upwelling predicted by a general circulation model.
In summary, it is difficult to find an independent test for the sea-level curve produced by Loydell (1998). As we noted in our paper (Jeppsson & Aldridge 2000, p. 1146), determination of sea-level changes by looking at the burial histories of Silurian coastal topographies offers a potential approach (Johnson et al. 1998), but deposits on rocky shorelines are notoriously difficult to correlate biostratigraphically. The graptolite diversity curve may not be wrong, but we believe that it is appropriate to consider that ecological factors other than sea level alone might have affected graptolite diversity. As well as the physical environmental factors we have already noted (Jeppsson & Aldridge 2000, p. 1137), we might surmise that biological variables such as changes in the abundance and/or diversity of graptolite food organisms, predators or competitors may have had an effect, as may factors such as the rebound time after extinction events. Rates of sediment accumulation, differential bioturbation and preservational processes may also have influenced the final diversity of fossils in the rocks.
The major mismatches between Loydell’s (1998) curve and those based on other criteria were noted by Loydell (1998) as well as by us. It is asserted above by Loydell that these differences arise from the imprecision of correlations used in constructing the previously published curves, implying that with better biostratigraphical or other time correlations the curves would match. This may turn out to be the case; but, then again, it may not. At the moment, we cannot know, but the mismatch remains.
We accept, as we stated in our paper (Jeppsson & Aldridge 2000, p. 1138), that the relationship of the abundance and diversity of macroplankton to the oceanic states we have proposed may be in need of further consideration. From our reading of the recent graptolite literature we found different opinions. In our view, if we find contradictory statements in papers by different eminent graptolite workers (Berry 1998; Melchin et al. 1998), then (as non-graptolite specialists) we can only regard the current situation as equivocal. However, we are quite prepared to reassess this aspect of the oceanic state model if the data demand.
We have striven to present our considerations of the patterns and processes of Silurian cyclicity as hypotheses open to testing. We agree with Loydell (1998) that much of this testing will depend on the development of a more refined biostratigraphy and better correlations between sections worldwide, and we have common ground in working towards those ends. The Jeppsson (1990, 1997, 1998) oceanic model is young, and it is highly probable that further refinements and improvements will turn out to be necessary. However, we have found it to have considerable
analytical and predictive power in our own fieldwork and syntheses, leading us to anticipate correctly particular types of faunal change associated with local lithological successions and with time-correlated sedimentary changes in different facies. We do not believe that the evidence from graptolite diversity as presented to date is an adequate test on which to refute our hypotheses, nor do we accept that a sediment colour/graptolite diversity curve provides fundamental support for the notion that all Silurian cyclicity was driven by sea-level changes.
22 December 2000
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References |
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