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and Von der Haar 1975; Shark Bay, Australia: Monty
1976; Gulf of Aqaba: Friedman et al. 1973, Gerdes and
Krumbein 1987).
cal precipitation of the aragonitic muds, (b) SEM stud-
ies show that the aragonite needles in the Great Ba-
hama Bank muds are bladed and have poorly devel-
oped pointed ends, in contrast to algal needles with
blunt-ended prisms (Loreau 1982; Macintyre and Reid
1992). Nonskeletal aragonite needles are apparently
precipitated ubiquitously over the Bahama Bank; they
accumulate within the low-energy bank leeward of
Andros Island and are exported to the deeper leeward
slope (down to -90 m), and even to deep settings, as
indicated by the position of the aragonite lysocline in a
depth of 1500 m (Droxler et al. 1988), (c) Present-day
standing stocks of green algae and carbonate produc-
tion appear too low to account for the volume of Ho-
locene aragonitic mud on top of the Bahama Bank. Algal
mud production is estimated to be greater than an or-
der of magnitude less than the precipitation rate calcu-
lated for the mud on the bank.
Of course, a significant algal contribution to the sub-
microscopic needles of the muds can not be ruled out
and has to be re-evaluated. Measurements of growth
rates of
Halimeda
and other calcified udoteaceans sug-
gest that these algae renew their standing stocks within
a few weeks (Wefer 1980; Multer 1988; Hillis 1991,
1999) and could, therefore, produce tremendous amounts
of fine-grained carbonate sediment in shallow lagoonal
and reef environments.
Halimeda
mounds and banks,
formed in deeper and shallow slope settings in eastern
Australia, Indonesia and the Caribbean, document the
enormous sediment-producing potential of these algae
(see publications in Coral Reefs, vol. 6, 1988). The dis-
integration of algae, not only udoteaceans, is an addi-
tional important source of fine-grained sediments in
modern reefs. Breakage and subsequent diminution of
exposed calcified filaments of endolithic (boring) green
algae (e.g.
Ostreobium
) and of filamentous cyanobac-
teria result in the production of silt- and micrite-size
material (Kobluk and Risk 1977).
Interpretation of limestones: The disintegration of
externally calcified cyanobacteria is regarded as an im-
portant source of micritic reef limestones, especially
of Paleozoic age (e.g. Pratt 2001).
Allomicrites
Allomicrites (allochthonous micrites) originate from
the destruction of calcareous algae and invertebrate
skeletons, bioerosion, accumulation of calcareous
plankton as well as from the erosion and abrasion of
lithified carbonates (Fig. 4.1). In shallow waters, tropi-
cal storms and ebb-tidal currents causing a mixing of
bank and ocean waters can lead to a redistribution of
genetically different carbonate muds. Carbonate muds
produced on shallow-marine platforms are also a ma-
jor source for periplatform sediments deposited on
slopes or in basins (periplatform carbonates, see Sect.
2.4.5.7)
(5) Disintegration of benthic calcareous algae: The
Halimeda
model.
The disintegration of the skeletons
of modern calcareous green algae causes the formation
of sand- to clay-sized particles. Processes involved in
the disintegration, deposition and redeposition of the
algal particles are exemplified by the udoteacean alga
Halimeda
(Fig. 4.3; Pl. 7/1-2, Pl. 57/1-2). The ultrastruc-
tural elements of the calcareous skeleton of this and
other green algae are aragonite needles. These needles
are set free after decomposition of the organic matter.
Morphologically similar needles are abundant in car-
bonate muds of the Great Bahama Bank and Florida
Bay as well as in Pacific lagoons (Lowenstam and
Epstein 1957; Stockman et al. 1967; Neumann and Land
1975; Nelsen and Ginsburg 1986).
The algal origin of the needles was supported by
(a) comparable stable isotope data of muds and algal
needles, (b) SEM photomicrographs (Pl. 7/2) indicat-
ing morphological similarities between the aragonite
produced by udoteacean algae and the aragonite needles
occurring within the aragonite muds, and (c) excess pro-
ductivity of udoteacean algae in the Bight of Abaco
(Little Bahama Bank) where the algae produce more
than twice the amount of fine-grained aragonite that
has accumulated in the Bight (Neumann and Land
1975). Aragonite muds occurring in the deeper North-
west Providence Channel are, therefore, explained as
the result of the extended offbank export of algal muds.
These points, however, have to be discussed in the light
of new data: (a) Isotope data could also indicate chemi-
Interpretation of limestones: Green algae, closely
comparable with modern
Halimeda
, have been known
since the Late Permian (Poncet 1989), were more com-
mon in the Late Triassic (Flügel 1988; Pl. 57/3), and
became rock-building constituents of warm-water plat-
form carbonates during the Cretaceous and in the Ter-
tiary (Hillis 2001). Halimediform algae exhibiting a
highly identical internal structure, however, occur as
early as in the Ordovician and are more common in the
Devonian (Pl. 57/4, 5).
(6) Disintegration of epibionts living on seagrass and
macroalgae:
Seagrass (
Thalassia
,
Posidonia
,
Zostera
and others) is abundant in shallow subtidal and inter-
