Geology Reference
In-Depth Information
• benthic sessile organisms (sponges, corals, bryozoans),
• shell-bearing organisms with valves (brachiopods,
mollusks, serpulids, ostracods) and multi-element/
multi-plate shells (trilobites, crustaceans, echino-
derms),
• rare thin-section fossils (including vertebrates, con-
odonts, graptolites, and wood),
• fossils of uncertain systematic position (microprob-
lematica).
cation has been interpreted as a light-shading function,
protection against predators and epiphytes, or as means
of receiving higher amounts of light and nutrients.
The mode and objective of calcification are topics
of ongoing research (Borowitzka 1989; Pentecost
1991). The precipitation of calcium carbonate may oc-
cur within the cell (planktonic coccolithophorids), in
association with cell walls (coralline red algae), out-
side the cell (many green algae) or as a surface deposit
on the thallus (many blue-greens). The skeleton of cal-
cified green and red algae displays distinct ultrastruc-
tures (Flajs 1977). Cyanobacterial calcification results
from complex, in places, bacterially-initiated processes
within and upon the mucilaginous sheath surrounding
the cell wall. Binding and trapping of carbonate at the
sheat surface enhance the formation of structures ca-
pable of fossilization (calcimicrobes).
In all groups, calcification is associated with organic
matter, primarily polysaccharides, which may stimu-
late nucleation and control subsequent growth of
calcium carbonate crystals (Merz 1992). Less than 10%
of the total of modern benthic algal species and cyano-
bacteria exhibit calcification, but the percentage of cal-
careous algae may increase to about 20% in warm
shallow-marine environments. In the cyanobacteria,
considerably more calcified species are known from
freshwater than from marine environments, in contrast
to green algae, with a reversed situation. Calcification
of red algae is almost completely restricted to the oceans.
Disregarding the diatoms (a group of the Chromo-
phyta) whose cell walls are impregnated with silica,
modern skeletal algae exhibit skeletons consisting of
either aragonite or calcite (see Box 4.9). Aragonite is
the exclusive mineral of marine green algae, but also
occurs in some red algae (e.g. squamariaceans). The
skeletons of corallinacean red algae consist of High-
Mg calcite containing magnesium amounts ranging up
to 30 mole percent. Low-Mg calcite is the common min-
eral of nonmarine cyanobacteria as well as of another
group of the Chromophyta, the marine planktonic coc-
colithophorids. The calcified reproductive organs of
freshwater charophyceans consist of Low-Mg or High-
Mg calcite.
The first well-calcified cyanobacteria are known
from the Late Precambrian, the first calcified algae from
the lowermost Cambrian. Since the calcification of
cyanobacterians depends strongly on environmental
conditions favoring precipitation of calcium carbonate,
the abrupt appearance of calcified cyanobacteria near
the Precambrian-Cambrian boundary has been inter-
preted as possibly reflecting a change in seawater chem-
istry (decrease of the Mg
2+
/Ca
2+
ratio: Riding 1982).
Environmental controls on algae:
Controls on the
These groups and their major systematic units are
presented by instructive plates and with accompany-
ing text. The text starts with a definition of the group,
followed by the diagnostically important morphologi-
cal criteria, takes a short glance at the classification
and discusses the environment and the biofacies, and
finally gives an overview of the temporal distribution.
The significance of the group as well as hints for avoid-
ing confusions with other fossil groups are summarized
in the last part.
10.2.1 Cyanobacteria and Calcareous Algae
Definition
: Algae are aquatic photosynthetic benthic and
planktonic plants, ranging from micron-sized unicel-
lular forms to giant kelps several meters long. These
eucaryotic organisms display extremely varied life-
cycles, characterized by different complexes of photo-
synthetic pigments, which together with other charac-
ters are used to distinguish major groups, i.e. red, green
and brown algae (Rhodophyta, Chlorophyta, Phaeo-
phyta). In contrast to these groups, the so-called 'blue-
green algae' (Cyanophyta) are procaryotes, lacking a
discrete nucleus and membrane-bound organelles
within the cell. They are physiologically more closely
related to bacteria and are included within a separate
division of the Procaryota ('Cyanobacteria'). In com-
parison with eucaryotic algae, the cyanobacteria are
adapted to a wider range of environmental conditions
(which may include the loss of photosynthetic activ-
ity) and occur both in freshwater and marine environ-
ments. Because of these differences, the paleoenviron-
mental interpretation of 'algae' has to consider the sys-
tematic membership of the fossils as well as the differ-
ences in the calcification potential.
Calcification:
Calcareous algae constitute a highly
artificial group comprising various benthic and plank-
tonic algae belonging to different systematic groups,
which are capable of removing carbon dioxide from
the water in which they live and secreting or deposit-
ing carbonate around the algal body (thallus). Calcifi-
