The Silurian events and carbonate platforms

Todays carbonate platforms - such as the Bahamas platform or the Great Barrier Reef - are major marine ecosystems that respond to a wide variety of changes in the contemporaneous oceans. They have a life cycle and a carbonate production rate that depends on the interaction of climate, relative sea-level, and biotic factors such as diversification rate. For this reason, environmental changes in coastal marine waters are reflected in the type of carbonate production and thus in the sediment composition of carbonate platforms. The changes are generally profound at times of major global crises and a number of anomalous carbonate facies can be directly related to mass extinction events. As opposed to the normal marine shelly faunas of pre- and post-extinction strata the anomalous carbonate facies is often microbially mediated and associated with a substantial increase in calcifying cyanobacteria, leading to mass occurrences of oncoids and sessile stromatolites in normal marine environments. Other anomalous facies common to extinction events are flat-pebble conglomerates (Sepkoski 1982) and wrinkle structures (Hagadorn and Bottjer 1997; Pruss et al. 2004). This type of signals of ecologic turnover are known from the Late Ordovician (Sheehan and Harris 2004), the Late Devonian (Whalen et al. 2002), and from the Permian-Triassic (Schubert and Bottjer 1992) mass extinctions. There are two prevailing ways of interpreting the overrepresentation of microbially mediated facies during mass extinctions. The first is biological and suggests that the increase of cyanobacteria is due to lowered grazing pressure by vagrant benthos (Hagadorn and Bottjer 1999). The other interpretation favours changing ocean chemistry and relates the same increase in abundance to increased carbonate saturation states in the oceans (Riding and Liang 2005).

Otto Tanks Morene

Fig. 12. Carbonate platforms are major marine ecosystems which inhabitants respond to environmental changes in the contemporaneous oceans. Fossil carbonate platforms, such as those on the island of Gotland, are important archives from which biotic changes and anomalies in the carbon cycle, as well as physical and chemical oceanographic changes, can be measured and gauged in the same stratigraphic section. Based on such integrated analysis of these strata, the Ireviken, Mulde, and Lau events are all associated with profound environmental changes in the ecology of coastal environments. The photograph shows three major reefs of the Hogklint Formation on north-western Gotland

Fig. 12. Carbonate platforms are major marine ecosystems which inhabitants respond to environmental changes in the contemporaneous oceans. Fossil carbonate platforms, such as those on the island of Gotland, are important archives from which biotic changes and anomalies in the carbon cycle, as well as physical and chemical oceanographic changes, can be measured and gauged in the same stratigraphic section. Based on such integrated analysis of these strata, the Ireviken, Mulde, and Lau events are all associated with profound environmental changes in the ecology of coastal environments. The photograph shows three major reefs of the Hogklint Formation on north-western Gotland

The several stacked carbonate platform generations that form the bedrock of Gotland (Fig. 12) have been highly useful for integrated studies of the Silurian global events. Recent studies have shown that the Mulde and Lau events are associated with ecosystem changes in low-latitude shallow seas that are similar to those during the mass extinctions discussed above. This is expressed as short-lived but prominent lapses in biotic carbonate production during which microbial carbonates regionally were important. A microbial resurgence, and the formation of microbial-oolitic-stromatolitic sedimentary units, has been demonstrated in strata formed during the Lau Event when stromatolites and wrinkle structures increases substantially in abundance or appear during or shortly after the extinction phase (Calner 2005a, b, c). These are the first wrinkle structures documented from strata formed during a smaller extinction event and the first reported from the Silurian. A similar but less conspicuous microbial resurgence occurred also during the extinction phase of the Mulde Event and after the Ireviken Event (Munnecke et al. 2003; Calner 2005a). The oolites that developed during the Mulde and Lau events are of particular interest since they have important analogues during the end-Permian mass extinction when these non-biological carbonates dominated along the shores of the Tethys Ocean (Groves and Calner 2004). The oolites formed during or immediately after the extinction events and superimpose terminated carbonate platforms with abundant skeletal carbonate production and large reefs. Widespread oolites are therefore lithologic analogues to low-diversity, post-extinction disaster faunas common to the majority of Phanerozoic events - a sign of ecological stress in the Silurian shallow seas. This may be true for Phanerozoic extinction events in general and for the widespread Holocene interglacial oolites.

The reefs that follow immediately after the Mulde and Lau events on Gotland are small and have a deviating composition as compared to the pre-event reefs. The main reef-builders, the stromatoporoids, decrease markedly in diversity and size and is to some extent replaced by dendroid or encrusting tabulates and bryozoans (Calner 2005a). The significance of these changes is not yet revealed.

Was this article helpful?

0 0

Post a comment