Loydell (2007)
provides a welcomed review of some of the problems the Palaeozoic community is currently facing when attempting to interpret evidence of ancient climate change. We have, however, some points that we would like to discuss, especially with respect to his interpretation of stable isotope geochemistry/stratigraphy:
- The carbonate weathering hypothesis for positive d 13 C carb excursions presented by Loydell ( 2007) is not the carbonate weathering hypothesis presented by Kump et al. (1999). In section 7, the author states: 'The model is attractively simple and very significantly is consistent with the many observations of the magnitude of positive d 13 C carb excursions diminishing offshore, away from the source of input.' This is not the mechanism presented by Kump et al. (1999), because as envisioned by Loydell, this would be a local cause suggesting that every location on earth that recorded an excursion was close to an actively weathering carbonate platform. The author should also note that a basinward decrease in d 13 C carb values is always seen even during non-excursion intervals as well. This is likely due to the biological pump or 'ageing' a watermass (Patterson and Walter 1994). 2. Apart from the prohibitively large quantities of carbonate that would be required for weathering to produce excursions of þ10% (Wigforss-Lange 1999) such as those are seen in the Silurian, the positive d 13 C carb excursions occurred during the most prolific episodes of reef growth during the Silurian (Brunton et al. 1998). This quantitative detail makes the carbonate weathering hypothesis highly unlikely. Although, as suggested in section 6, the importance of graptolite mortality as an individual factor in the global carbon budget has heretofore been overlooked, perhaps a few calculations would be enlightening. 3. In section 8.3 Loydell states in his reference to Bickert et al. (1997): 'By contrast with Jeppsson's original model, glaciations were seen as periods of aridity at low latitudes encouraging the growth of carbonate platforms'. This citation is incorrect. In Bickert et al. (1997) times of high isotope values are not interpreted as glacial times. 4. Although the author proposes in section 5.3 that there is a suggestion (Loydell et al. 2003) of an unconformity at Datum 2 on Gotland, we are currently unaware of any published lithological, sedimentological or biostratigraphical evidence to support this claim. 5. Loydell wrote in the introduction: 'Any model that attempts to explain the early Silurian d 13 C record needs to consider (. . .) the coincidence of positive d 13 C excursions with intervals of low eustatic sea level'. We do not agree with such a statement because in our opinion one should carefully distinguish between 'hard facts', for