Geology Reference
In-Depth Information
Riding, R., Guo, L. (1991): Permian marine calcareous al-
gae. - In: Riding, R. (ed.): Calcareous algae and stroma-
tolites. - 452-480, Berlin (Springer)
Roux, A. (1985): Introduction a l'étude des algues fossiles Paléo-
zoiques (de la bacteria a la tectonique des plaques). - Bull.
Centre Rech. Explora.-Prod. Elf-Aquitaine,
9
, 465-699
Roux, A. (1991): Ordovician algae and global tectonics. - In:
Riding, R. (ed.): Calcareous algae and stromatolites. - 335-
348, Berlin (Springer)
Roux, A. (1991): Ordovician to Devonian marine calcareous
algae. - In: Riding, R. (ed.): Calcareous algae and stro-
matolites. - 349-368, Berlin (Springer)
Further reading:
K129 to K136
etal stromatolites and oncoids, sometimes exhibiting dis-
crete tubular microstructures. Other growth forms are
microbushes, thrombolites and dendrolites.
Classification:
Cyanobacteria are extremely vari-
able with regard to their morphology. Some fossil calci-
microbes exhibit high morphological similarities with
groups of modern cyanobacteria, indicating the exist-
ence of very conservative long-range characters. The
tubular microstructure was used to define the artificial
group 'Porostromata'.
Some authors still prefer the term 'porostromate al-
gae' but one should be aware that this group includes
cyanobacterian fossils with close morphological simi-
larity to recent calcified blue-greens, but also various
green algae.
A handy as well as morphological classification of
calcimicrobes proposed by Riding (1991) is shown in
Fig. 10.5. Another approach, based on comparative
studies of porostromate taxa and recent cyanobacteria,
results in a classification that relates ancient taxa to
recent counterparts (Dragastan 1985, 1988, 1989, 1990;
Dragastan et al. 1996).
10.2.1.1 Cyanobacteria and Calcimicrobes
Definition, calcification and morphology:
Cyanobac-
teria are phototroph Procaryota. The cells are arranged
in colonies (coccoid forms; Croococcales) or arranged
within threads forming trichomes (filamentous forms;
Hormogonophyceae). The threads can be branched and
enclosed in a protecting mucilaginous sheath. The de-
velopment of organic sheaths is strongly environmen-
tally controlled. Some modern filamentous and coc-
coid cyanobacteria calcify through impregnation within
or encrustation upon the extracellular sheaths with car-
bonate. Encrusted filaments are potential fossils, where-
as impregnated filaments commonly disintegrate to car-
bonate mud. The mineralogy reflects water chemistry
(freshwater: Low-Mg calcite; marine and restricted en-
vironments: Aragonite and High-Mg calcite). Calcifi-
cation is species-specific. Calcification in recent cy-
anobacteria corresponds to a nucleation of calcium car-
bonate upon or within the mucilage sheath (Pentecost
and Riding 1986; Pentecost 1990). Considerably more
cyanobacteria have been observed in modern freshwa-
ter lakes and streams compared with the sea. However,
calcification occurred widely in marine cyanobacteria
during the Paleozoic and Mesozoic, perhaps reflecting
temporal changes in oceanic carbonate chemistry.
Merz-Preiss and Riding (1999) comparing calcifi-
cation patterns of cyanobacteria from the Everglades
in Florida and from spring tuffs stress the point that the
preservation potential of fossilized filamentous cyano-
bacteria probably depends on the degree of carbonate
oversaturation of the ocean water. Calcification of fila-
ments produces tubelike microfossils, varying in ex-
ternal diameter between ten and several tens of microns.
The tubes are bordered by microcrystalline calcitic
'walls'. Calcified cyanobacteria occur as millimeter-
to centimeter-sized discrete microfossils (calcimi-
crobes) consisting of tubes or irregular bodies, or as
amalgamated masses exhibiting laminated or massive
growth forms. Laminated growth forms include skel-
Environmental constraints:
Modern cyanobacteria
have a wide range of habitats. As pioneer settlers in
marine freshwater and on terrestrial sedimentary de-
posits, benthic cyanobacteria prepare the ground for
the successive invasion and expansion of eukaryotic
flora and fauna (Golubic et al. 2000). Benthic cyano-
bacteria are the major constituents of microbial mats,
specifically of the topmost oxic layers. The mats form
in inter- and supratidal, sometimes hypersaline coastal
environments as well as in soda and freshwater lakes
or hot springs. Mat-building cyanobacteria exhibit sig-
nificant tolerance to changes in light intensity, anoxic
conditions or sporadic drying. Effective environmen-
tal factors are light, water temperature and nutrients
(C, N, P, Fe). Planktonic cyanobacteria are common in
freshwater but also occur in open oceans.
Ancient benthic cyanobacteria are known both from
marine and nonmarine environments. Calcimicrobes are
common constituents of Mesozoic restricted, lagoonal
as well as on open-marine platform carbonates and con-
tributes to the formation of reefs and mud mounds.
Distribution
: Nonskeletal cyanobacteria were abun-
dant in the Precambrian and have been recorded from
rocks as old as 3.5 billion years. Heavily calcified cal-
cimicrobes appeared in the Vendian (1.0-0.85 billion
years) coincident with the decline in stromatolites.
Vendian reefs are composed of abundant thrombolites
and calcimicrobes.
The absence of Proterozoic calcified cyanobacteria
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