Geobiology of stromatolites - GWDG

Abstracts International Kalkowsky-Symposium „Geobiology of Stromatolites“, October 4-11, 2008
of the sheaths result in the formation of micrites such as peloids (Merz-Preiß
2002). A variety of micritic textures in the Girvanella–rich layers are considered to
have originated from taphonomic and diagenetic variations within identical or
similar filamentous microbial communities. In contrast, thick micritic layers include
abundant bioclasts but sporadic Girvanella, and may have resulted mainly from the
microbial (such as Girvanella) trapping and binding micrites with bioclasts. In
summary, the stromatolites considered here are thought to have been constructed
as follows: (1) microbes including Girvanella first encrusted the bioclastic micrites
or dead sponges, then (2) stabilized these substrates, and (3) produced laminated
microbialites, due to repeated changes in microbial activity (micrite precipitation or
sediment trapping and/or binding), in concert with fluctuating ambient environments.
Girvanella-dominated micritic stromatolites are widespread in the Lower Ordovician.
They are inferred to have been formed by the similar mechanisms noted
above. Further prevalent characters that are unique to Lower Ordovician stromatolites
are their co-occurrence with metazoans (e. g., lithistid sponges) within small
reefs, although the metazoans were still low in diversity. Such Lower Ordovician
reefs show a marked contrast with the subsequent shallow-marine reefs of the
Middle to Late Ordovician, when skeletal metazoans such as stromatoporoids and
corals flourished and stromatolites declined conspicuously. Lower Ordovician
stromatolites thus elucidate the interrelationship between microbes and metazoans
in the formation of age-specific reefs, as well as providing insights into the causes
of the shift from microbial- to skeletal-dominated reefs.
References
Merz-Preiß, M. 2000. Calcification in cyanobacteria. In: Riding, R. E., Awramik, S. M. (eds.): Microbial
Sediments: 50-56; Berlin (Springer).
Riding, R. 2006. Microbial carbonate abundance compared with fluctuations in metazoan diversity
over geological time. Sedimentary Geology 185 (3-4): 229-238.
Webby, B. D. 2002. Patterns of Ordovician reef development. In: Kiessling, W., Flügel, E.,
Golonka, J. (eds.): Phanerozoic reef patterns. SEPM, Special Publication 72: 129-179.
A numerical model calculating the CaCO 3 saturation state in cyanobacterial
mats [talk]
Giovanni Aloisi
Leibniz Institute for Marine Sciences (Ifm-Geomar), Wischhofstrasse, 1-3, 24148, Kiel, Germany;
E-mail: galoisi@ifm-geomar.de
A numerical model was developed which calculates the CaCO3 saturation state
(ΩCaCO3) in cyanobacterial mats as it is influenced by biogeochemical processes and
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