Reply to ‘Early Silurian positive δ13C excursions and their relationship to glaciations, sea-level changes and extinction events: discussion’ by Bradley D. Cramer and Axel Munnecke

I am pleased that Cramer and Munnecke consider my paper to be ‘welcome’. When my interest in carbon isotopes was sparked (by the excellent presentations at ‘The Dynamic Silurian Earth’ meeting on Gotland in 2005 and the IGCP 503 meeting in Glasgow in 2006), I was struck by the coincidence of the commencement of positive d13C excursions with intervals of falling eustatic sea-level, as shown by my sea-level curve (Loydell 1998). A reasonable explanation I initially surmized (before reading any of the literature), would be that increased organic carbon burial would result from more rapid sedimentation associated with falling/low sea-levels. I was surprised that TOC values did not offer full support for this simple model. In particular, the Hirnantian excursion, which is so clearly linked temporally to the glaciation at this time (and its associated low sea levels), is often recorded in rocks with reduced TOC levels, even in basinal settings, such as represented by Dob’s Linn. For the Silurian, however, data from some sections do support carbon burial as being at least partially responsible for the positive d13C excursions (in addition to those papers listed in Loydell 2007, see also Loydell and Fry´da 2007). Nevertheless, other sections show no increase in TOC coincident with the excursions and thus some other mechanism associated with low sea-level was required. I concluded, based on the available evidence, that Melchin and Holmden’s (2006a, b) carbonate weathering hypothesis was the best available explanation for the observed coincidence of the onset of positive d13C excursions with intervals of falling eustatic sea-level.