Geology Reference
In-Depth Information
but peyssoneliacean red algae with aragonitic skeletons
(Pl. 64/10) can be of importance, too (Basso 1990). Late
Paleozoic rhodoids were formed by ancestral red algae
(Pl. 56). Centimeter-sized unattached nodules, predomi-
nantly consisting of encrusting organisms other than
coralline algae (e.g. foraminifera), are called macroids.
sea-level fluctuations is an important factor for the bi-
otic composition and internal growth patterns of
rhodoids. Water temperature controls the composition
of algal communities and their biogeographic distribu-
tion. Hydrodynamic energy is regarded as another very
important ecological factor for rhodoid distribution. It
controls external and internal growth forms and the suc-
cess of taxonomic successions. Salinity control is of
minor importance, because most calcareous coralline
red algae are adapted to normal marine environments.
The co-existence of corallines and cyanobacteria, how-
ever, indicates the possibility that brackish-water con-
ditions can also be tolerated (Richter and Sedat 1983).
Vertical accretion rates are high for tropical rhodoids
(about 1-2 mm/year) and an order of magnitude lower
for temperate and deeper-water rhodoids (Matsuda
1989). Growth rates of thin and thick crusts depend
strongly on the effect of grazing herbivorous organ-
isms (Steneck 1985).
Distribution of modern rhodoids:
Rhodoids are
known from tropical and subtropical, temperate and
polar seas. Living rhodoids occur in intertidal pools
(Wehrmann et al. 1995) to subtidal shelves below nor-
mal wave base, down to depths of more than 200 m.
Deeper-water settings are often dominated by macroids
(e.g. outer Florida shelf: Prager and Ginsburg 1989).
In tropical warm-water settings rhodoids (30 °S to
20 °N) occur in shallow-water environments (lagoons,
tidal channels, reef flats, back-reef areas; seagrass beds),
at shelf margins as well as in deeper forereef environ-
ments on terraces (Minoura and Nakamori 1982;
Scoffin et al. 1985; Gischler and Pisera 1999). The com-
mon lower limit in the tropics is about 80 m, but living
rhodoids also occur in depths between 160 and 180 m,
too (pers. comm. W.-Chr. Dullo). Temperate rhodoids
are known from the warm-temperate Mediterranean Sea
in depths between about 35 and 150 m (e.g. Basso
1998), and from the transition between tropical and tem-
perate waters off southern Queensland between 50 and
110 m (Lund et al. 2000). These rhodoids contribute to
the widely distributed rhodalgal facies which inter-
fingers with the bryomol facies (Sect. 12.2). On rocky
coastal slopes of the cool-temperate Atlantic, living
rhodoids grow in depths between about 20 and 40 m,
in higher latitudes between 5 to 25 m (Tromsö/Nor-
way: pers. comm. A. Freiwald). A characteristic sedi-
ment consisting of coralline algal branches, rhodoids
and their detritus, is 'maerl' (Sect. 2.4.4.3). Wide and
thick accumulations of rhodoids with only few detritus
are called 'rhodolith pavements'. In polar regions liv-
ing corallinacean algae were reported from depths be-
tween 60 m to about 100 m (Svalbard shelf: Andruleit
et al. 1996; northern Norwegian shelf). These rhodoids,
however, exhibit only thin crusts.
Some caution is necessary in a too rigorous use of
'modern' rhodoids for paleobathymetrical interpreta-
tions, as seen in originally Pleistocene 'relic rhodoids'
which formed in shallow-marine settings and now are
found in deep-water settings (e.g. South African shelf:
Siesser 1972).
How to describe rhodoids and macroids?
The methods for analyzing of modern and Tertiary
rhodoids and macroids are firmly established (Bosellini
and Ginsburg 1971; Bosence 1983a, 1983b, 1991). The
main criteria are external shape, internal growth pat-
terns, size, and the biotic composition of the cortices.
In addition, nuclei types and associated fauna are of
importance. Box 4.12 summarizes the features which
should be checked.
External shapes and internal growth patterns:
The
external shape depends on the internal growth patterns
of the algae, and the frequency of turning (Braga and
Martin 1988). Growth forms are apparently controlled
by the frequency of overturning. Increasing overturn-
ing causes flattening of branches which join laterally.
In tropical and subtropical climates branched rhodoids
are formed in shallow and quiet waters; in sheltered
areas they are spherical; in higher energy areas flat-
tened ellipsoidal shapes dominate. Rhodoliths with con-
sistent internal growth patterns indicate stable environ-
mental conditions and/or fast growth. Ellipsoidal, sphe-
roidal and laminar growth patterns are generally con-
sidered to be indicative of higher water energy. Discoi-
dal and flat external forms, and branching and colum-
nar internal growth forms are common in low-energy
environments (Piller and Rasser 1996).
Size:
Modern rhodoids range in size from 10 to about
250 mm, more frequently from about 10 to 100 mm.
Ancient coralline rhodoids have about the same size.
The size of ancient rhodoids appears to be strongly con-
trolled by the composition of the algal communities
Controls:
The most important control on the growth
of coralline algae and the formation of rhodoids is light.
Light intensity is correlated with water depth and geo-
graphic latitude. A change in water depth combined with
