Geology Reference
In-Depth Information
distribution of recent benthic algae comprise (a) physi-
cal factors: intensity and quality of light (necessary for
photosynthesis), water temperature, water energy and
substrate; (b) chemical factors: water chemistry, salin-
ity, dissolved gases, pH values; and (c) biological fac-
tors: nutrients, competing and herbivorous organisms,
and ecological successions. Generally, the intensity of
light decreases with increasing water depth. About 50 %
of the light penetrating clear water is absorbed in the
uppermost ten meters. Despite this, photosynthetic al-
gae can occur at greater water depths if they have spe-
cial pigments that can utilize minimal light intensities
and specific wave lengths. Modern red algae, adapted
to blue and green light sectors, also occur at greater
water depths, whereas calcified green algae assimilate
most strongly in the red sector and are, therefore, pref-
erentially limited to shallow seas.
pending on varying magnesium contents. Low-Mg cal-
cite algae and calcimicrobes are commonly well-pre-
served.
Classification: Calcified
cyanobacteria
have been
classified into formal taxonomic units (Chuvashov
1987; Dragastan et al. 1996) or into informal morpho-
logical groups (Riding 1991). The latter classification
is more practicable (Fig. 10.4).
Living
algae
are classified according to pigments,
nature of the food reserve, kind of flagellation, and chlo-
roplast ultrastructure. The presence of skeletal carbon-
ate is not a classification criterium.
Fossil calcareous
algae
are classified according to morphological char-
acteristics used in classification schemes of analogous
living forms. These characters include external mor-
phology, preserved tissue, and reproductive bodies.
Many fossil algae, however, can not be related to ma-
jor groups of living algae. These algae are included in
non-systematic collective groups (e.g. ancestral red al-
gae, phylloid algae) or in taxonomic units which can
only be tentatively assigned to major groups of green
or red algae (e.g. gymnocodiacean algae). Still other
ancient extant algae exhibit morphological characters
that are similar to green or red algae, but do not allow
an attribution to known systematic subgroups (e.g. ung-
Preservation of calcareous algae:
Because of the
widely differing solubilities of calcium carbonate
phases, the primary mineralogy of calcareous algae is
a critical factor in their diagenesis and in the preserva-
tion of algal fossils. Aragonitic skeletons of udoteacean
and dasyclad green algae are susceptible to solution
and replacement by sparry calcite. Coralline red algae
display differing degrees of diagenetic alterations, de-
C
O S D
C P
T
J K TT Q
Cyanobacteria
Corallinacean
red algae
Squamariacean
red algae
Ancestral
red algae
Solenoporacean
red algae
Udoteacean
green algae
Gymnocodiacean
green algae
Phylloid algae
Dasyclad
green algae
Fig. 10.4.
Distribution of Phanerozoic calcimicrobes and major calcareous algal groups.
Note that the thickness of the bars
does not reflect taxonomic diversity, but instead indicates the relative frequency in thin sections.
