Geology Reference
In-Depth Information
Fig. 4.3.
The Halimeda model
.
Halimeda
, a common calcareous green alga in lagoonal and reef environments, is one of the
most prominent producer of modern carbonate muds and sands. The erect plant is several centimeters high and attached with
a holdfast of root-like filaments to the sediment (B). The alga is segmented and branching. The segments possess an internal
structure composed of interwoven tubular filaments which are differently arranged in the central and peripheral parts of the
segments (C and D; Pl. 57/1, 2). Calcification starts at the surface between the filaments (D to F)) and progresses inwards.
The skeleton consists of minute needles of aragonite (5
m long and 0.3
m wide; G; Pl. 7/2) which, on the death of the plant
and disintegration of the segments, disperse in the sediment and provide a source for lime mud. The mud is often redeposited
within the shallow-water area or in deeper environments on the slope. Isolated algal segments (Pl. 3/5) are a major component
of calcareous back-reef sands and occur in the talus of many Cenozoic reefs.
A
: The first species of
Halimeda
was documented from the Mediterranean Sea in 1599 by the Italian naturalist Imperato.
His figure illustrates superbly the characteristic segmentation, the branching pattern and the inconspicuous holdfast of
Halimeda
tuna.
The holdfast of the pantropical species forms a mat that fixes the plant onto the rocks or corals. The width of the thallus
is about 10 cm.
B
: The basic holdfasts of the sand-growing species act as anchors in times of storms.
C
: Internal structure of
a segment. Note the different branching pattern and orientation of the filaments in the central part (medulla) and the peripheral
part (cortex).
D
: Differentiation of cortical filaments. Calcification (gray pattern) takes place around and between the filaments.
E
: Calcification on and between medullar (left) and cortical filaments.
F
: Close-up of the cortex exhibiting remains of
organic membranes; see also Fig. E.
G
: Aragonite needles. Sources: A, B and C - modified from Hillis-Colinvaux (1980);
D - modified from Hillis (1959).
tidal settings. Seagrass can tolerate a wide range of sa-
linities, it occurs most prolific in water depths between
2 and 15 m. The mud banks of Southern Florida, some-
times regarded as an analogue of ancient mud mounds
(Bosence 1985), document four important controls of
seagrass on carbonate mud production and energy baf-
fling: (1) The long and dense root systems (rhizomes),
extending a few tens of centimeters below the sea bed,
stabilize and bind the sediment. (2) The long blades
have a baffling effect, can trap passing suspension-trans-
ported particles and reduce current speeds (Ginsburg
1957; Scoffin 1970; Almasi et al. 1987). Depending
on the density of the meadows, large amounts of muddy
sediment (packstones and wackestones) will accumu-
late. (3) Seagrass stabilizes the substrate permitting
colonization by stable-bottom sediment-producing and
sediment-reworking communities, and (4) very impor-
tantly, sea-grass blades provide a constantly renewable
substrate onto which benthic organisms can attach and
grow (Ginsburg and Lowenstam 1958).
