In my paper (Calner, 2005a), I presented stratigraphic and sedimentary data from the Late Silurian of Gotland, Sweden, that supported a substantial increase of microbially mediated carbonate facies and sedimentary structures during the time interval corresponding to the Lau Event. I noted that the sudden appearance of normal-marine stromatolites, wrinkle structures, and a mass occurrence of oncoids during the event suggested that minor extinction events also resulted in anachronistic periods that were less devastating but fully comparable to those reported from the aftermath of the end-Ordovician and end-Permian mass extinctions.
In his comment, Riding also addresses the seawater saturation state of CaCO3 minerals as an important control on the secular variation and preservation of microbial carbonate deposits in the Phanerozoic. It indeed has been adequately shown that the combined efforts of metazoan evolution and changing seawater saturation state likely have determined the general long-term pattern of microbial carbonate abundance (Riding and Liang, 2005a). As Riding notes, attempts to go further and gauge the relative importance of biotic competition and seawater carbonate saturation state are important to understanding the significance of disaster biotas, and this can certainly be discussed in more detail. There seems to be little doubt that the Lau Event occurred during an extended period of elevated seawater saturation state, and I agree that this, at least partly, has contributed to the relative increase in microbial carbonate facies noted in the middle and Late Silurian (Arp et al., 2001). I do not agree, however, that seawater saturation state would be the overriding control on the increase of microbial facies during the Lau Event, as indicated by Riding. A point that should be addressed in better detail is that of temporal stratigraphic resolution. The seawater saturation curve presented by Riding and Liang (2005b), and herein, shows a period of increasing seawater saturation state through the Silurian, peaking at ~420 Ma. Thereafter, the curve shows a distinct decline that does not end until approximately the Frasnian-Famennian boundary (at ~375 Ma), which is close to the start of an extended period of elevated seawater saturation state. As is always and inevitably the case with long-term curves, this curve does not provide the necessary resolution for separating short-term anomalies from the long-term trend; in this case, short-term biotic anomalies superimposed on the general seawater saturation curve. The most striking and intriguing points with the microbial resurgence during the Lau Event are its well-defined stratigraphic range and its contemporaneous occurrence in various environments over that same period. A similar—but on Gotland, less well-developed—increase of microbial facies is associated with the middle Silurian Mulde Event only a few m.y. earlier (at ~425 Ma; Calner 2005b; Fig. 1). Microbial carbonate facies such as micro- and macro-oncoids notably increase in abundance also at this stratigraphic level, and the reef composition also changes (Calner, 2005b). In fact, this stratigraphic level conforms very well to the maximum abundance of marine calcified cyanobacteria reported by Arp et al. (2001, supplementary Table 4). Importantly, the several-million-year time interval between the Mulde and Lau Events does not show any significant increase in microbial facies or oolites, although the succession on Gotland was formed in very shallow tropical waters and under a successively increasing seawater saturation state, according to the curve of Riding and Liang (2005b). Instead, this interval is associated with carbonate platforms with major, often stromatoporoid-dominated reef complexes (Fig. 1). Hence, although the biodiversity and paleoecological trends of Silurian extinctions still need to be monitored quantitatively, it can be argued that these short-term biotic events repeatedly are associated with increases in microbial carbonates and nonskeletal carbonates such as oolites. As noted by Riding, the long-term changes in seawater saturation state, on the other hand, occur over millions of years and the elevated saturation state of the Late Silurian oceans persisted for at least 10 m.y. Thus, it appears as the biotic perturbations of the Silurian events overprint the long-term seawater saturation changes. This agrees well with the observations of Riding (2005) that Phanerozoic microbial carbonate abundance peaks during periods when an elevated saturation state coincided with low metazoan diversity.
The increase of oolitic strata in association with the Mulde and Lau Events (Groves and Calner, 2004; Calner, 2005b) may also have been partly promoted by the Silurian elevated seawater saturation state. But again, oolites of any significant thickness (several meters) on Gotland occur only in association with the Mulde and Lau Events, although there is a long-term increase in seawater saturation state through the middle and Late Silurian. Hence, the overriding control on oolite formation, is better explained by short-term anomalies related to the biotic events (Groves and Calner, 2004).