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Bryostromatolite Reef. The front is showing the initial stage of the bryostromatolite, indicated by rugose corals, tabulates, sponges and stromatoporoids. Towards the background it is developing more and more into a bryostromatolite, in Claussen, Munnecke & Ernst. 2022. DOI: 10.1111/sed.12863 drawing by Anna Merkel. |
ABSTRACT
Bryozoan–stromatolite associations (bryostromatolites) formed conspicuous reef structures throughout the Sheinwoodian (Wenlock) to Ludfordian (Ludlow) stratigraphy on Gotland but have not been described so far. They are mainly composed of encrusting bryozoans forming a complex intergrowth with porostromate and spongiostromate microbes and are different from the abundant stromatoporoid–coral–algal reefs with respect to their composition. In the bryostromatolite different growth stages can be identified. The observed succession can be taken as evidence for cyclic environmental changes during reef formation. Stenohaline reef-dwelling organisms, such as echinoderms, sponges, corals and trilobites, indicate fully marine salinities. Ten localities exposing bryostromatolites were discovered. Individual bryostromatolites are small with few decimetres up to one metre in size, and occur solely in shallow marine areas. Common features of these reefs on Gotland are cauliflower-like growth, a high bryozoan diversity, a high abundance of phosphatic fossils and components such as bryozoan pearls and inarticulate phosphatic brachiopods, enhanced bioerosion, Palaeomicrocodium crusts, vadose silt and gypsum pseudomorphs. The high abundance of Palaeomicrocodium, as well as the alternation with other crust-forming contributors, suggest that it could have been formed directly at the palaeo-sea surface, probably in times of minor but high-frequency sea-level fluctuations. Vadose silt and pseudomorphs after gypsum in reef cavities indicate subaerial exposure shortly after reef growth. The high amount of phosphatic components indicates a high nutrient input, probably by dust. All bryostromatolites were formed in times of strongly elevated δ13C values. The unusual combination of sedimentological and palaeoecological features, as well as their occurrence exclusively during strong positive δ13C excursions, are evidence that the bryostromatolite development responded to climatic/oceanographic changes, which may have played an important role in reef control.
Keywords: Palaeomicrocodium, phosphate, reef, stable carbon isotopes, subaerial exposure
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| Artistic reconstruction of a bryostromatolite reef. The front is showing the initial stage of the bryostromatolite, indicated by rugose corals, tabulates, sponges and stromatoporoids. Towards the background it is developing more and more into a bryostromatolite, drawing by Anna Merkel. |
CONCLUSIONS:
The conclusions of this study can be summarized as follows:
• For the first time bryostromatolite reefs are described from the Silurian of Gotland. They have a cauliflower-like appearance in the field and consist mostly of porostromate and spongiostromate microbial carbonates and bryozoans.
• On Gotland, bryostromatolites only occur in the Högklint and Tofta formations (Sheinwoodian) and in the Eke and Hamra formations (Ludfordian). They grew in fully marine conditions, and formed a rigid framework and a topographic relief of several decimetres up to about 1 m.
• The bryostromatolites show a relatively high diversity of bryozoans (with up to twenty species in a single reef), a high abundance of reef-building and reef-dwelling organisms, and a surprisingly high abundance of phosphatic constituents.
• Repeated alternations of distinct layers dominated by either porostromate microbes, spongiostromate microbial layers, or encrusting bryozoans indicate cyclic environmental changes.
• Reef growth took place in extremely shallow water. The occurrence of vadose silt and gypsum pseudomorphs in reef cavities indicate subaerial exposure shortly after reef growth.
• The problematicum Palaeomicrocodium indicates an extreme environment and, according to Antoskhina (2006), even subaerial exposure. The high abundance and alternation with crust-forming reef-building organisms suggest short phases of subaerial exposure even during reef growth and, therefore, reef growth in extremely shallow water.
• The high abundance of phosphatic components (for example, inarticulate brachiopods, bryozoan pearls and linings) indicates a high input of phosphorous by either rivers or wind. For bryozoan pearls and linings, this hypothesis is supported by several literature studies (e.g. Oakley, 1934; Conti & Serpagli, 1988; Ma et al., 2014b). According to the studies of Kozłowski (2015) in the Baltic area, an aeolian input seems more plausible. This fits the fact that so far, no bryostromatolites have been discovered from the humid equatorial belt.
• Both the bryostromatolites on Gotland and bryostromatolites in the Silurian and Ordovician of other areas occur only in times of elevated δ13C values, indicating a close link of local bryostromatolite growth and global perturbations of the carbon cycle.
Anna Lene Claussen, Axel Munnecke and Andrej Ernst. 2022. Bryozoan-rich Stromatolites (bryostromatolites) from the Silurian of Gotland and their Relation to Climate-related Perturbations of the Global Carbon Cycle. Sedimentology. 69(1) Special Issue: Understanding carbonate factories through palaeoecological and sedimentological signals – Tribute to Luis Pomar; 162-198. DOI: 10.1111/sed.12863
