Calner (2005) perceptively recognizes an episode of increase in “anachronistic” facies, including flat-pebble conglomerates, normal-marine stromatolites, oncoids, and subtidal wrinkle structures, in the Late Silurian of Gotland, Sweden. He suggests that this is evidence for reduced grazing and infaunal activity, causally related to an extinction event—the late Ludlow Lau Event—420 Ma.
It has long been suggested that secular variation in the abundance of stromatolites and other microbial carbonates could reflect biotic competition in its broadest sense (Fischer, 1965). The classic example is where metazoan origination and diversification may have caused Late Proterozoic stromatolite decline (Awramik, 1971). Once metazoans became established in the Phanerozoic, a reverse effect could have applied with metazoan reduction at mass extinction events, permitting recovery of stromatolites as “disaster biotas” (Schubert and Bottjer, 1992). By this reasoning, even a relatively minor extinction, such as the Lau Event, might give rise to detectable stromatolite resurgence, as Calner (2005) suggests.
However, in addition to biotic competition, environmental factors also are likely to have influenced the secular abundance of microbial carbonates. Seawater chemistry is particularly important in this respect through its effect on the synsedimentary lithification that is essential for the accretion and preservation of microbial carbonates (Riding and Liang, 2005a). Fischer (1965) suggested that microbial calcification was limited by reduction in pCO2 from the Ordovician onward. Microbial calcification is mediated by saturation ratio (Ω) with respect to CaCO3 minerals (Riding and Liang, 2005b). Seawater Ωaragonite and Ωcalcite can be calculated for the Phanerozoic from estimates of past seawater ionic composition and atmospheric CO2 levels (Riding and Liang, 2005b) (Fig. 1). Microbial carbonate abundance shows broad positive correspondence with calculated seawater saturation state throughout much of the Phanerozoic, especially prior to the Cretaceous, indicating that seawater chemistry has indeed influenced the formation and preservation of microbial carbonates (Riding and Liang, 2005a). The calculated saturation ratio trend shows generally high values 540–340 Ma, with maxima at ~520 and ~350 Ma. In between these maxima, the highest Ω values were reached ~420 Ma. This relatively brief episode coincided with a short-lived increase in microbial carbonates (Arp et al., 2001, Fig. 3D; Riding and Liang, 2005b, Fig. 1B) (Fig. 1), of which the anachronistic deposits of the Eke Formation described by Calner (2005) are an example.
It is therefore possible that, in addition to a decrease in metazoan competition and disturbance induced by the Lau Event, the Eke-Burgsvik deposits of Gotland also were influenced by a contemporaneous increase in seawater saturation state. Reduction in biotic competition caused by the Lau Event was relatively short-lived (~0.5 m.y.) (Calner, 2005, Fig. 2), whereas an elevated saturation state during the Late Silurian may have persisted for 10 m.y. or more (Riding and Liang, 2005b) (Fig. 1). An increase in saturation state could account not only for Eke-Burgsvik facies but also for increased abundance in microbial carbonates during the mid-Late Silurian generally (Arp et al., 2001, Fig. 3d), and conceivably for their continuation into the earliest Devonian too (Kiessling, 2002, Fig. 16). Furthermore, in addition to stimulating microbial carbonate formation, elevated saturation state is arguably a more significant factor than biotic competition in determining the formation of Eke Formation flat-pebble conglomerates, which Calner (2005) includes as anachronistic facies, and could also account for the marine pisoids of the overlying Burgsvik Formation (Groves and Calner, 2004). It can thus be argued that the effect of the Lau Event on the increase in microbial carbonates and anachronistic facies was relatively minor in comparison with a contemporaneous but longer-lived rise in seawater saturation state.
Such attempts to gauge the relative importance of fluctuations in competition and in seawater saturation state are central to understanding disaster biotas and anachronistic facies following mass extinction events (Riding and Liang, 2005a). Microbial carbonates do not always show resurgence in mass extinction aftermaths (Schubert and Bottjer, 1992). Comparing Phanerozoic changes in metazoan diversity, seawater saturation state, and microbial carbonate abundance, Riding (2005) concluded that microbial carbonates were most abundant when elevated saturation state coincided with low metazoan diversity, and least abundant when reduced saturation state coincided with high metazoan diversity. It seems likely that microbial carbonate development during the Late Silurian, including the Lau Event, reflects elevated saturation state as well as low biotic diversity.