Geology Reference
In-Depth Information
the disintegration of encrusting coralline algae
and delicate bryozoans together with lime sand
from the breakdown of articulated (geniculate)
corallines, bryozoans and benthic foraminifera.
This leads, in part, to the typically poorly sorted
character of grassbank sediment. Lowenstam
(1955) observed that 'the disaggregation of coral-
lines may prevent recognition of perhaps quite
signifi cant contributions'. Nelsen & Ginsburg
(1986) also pointed out that encrusting corallines
(
Melobesia membranacea
and
Fosliella farinose
)
would disintegrate rapidly when seagrass blades
are broken or shed. This is true for much of the
coralline algae produced in these grass banks
that probably becomes carbonate mud. Sand
particles, on the other hand will be preserved to
help fi ngerprint grassbank environments in the
rock record. This is especially true of the ben-
thic foraminifera
Elphidium
,
Nubecularia
and
Discorbis
.
Nubecularia
in particular contributes
large amounts of sediment to the modern and
late Pleistocene of the gulfs (Cann
et al
., 2000),
to the shelf proper during sea-level lowstands
(Rivers
et al
., 2007), and to the Pliocene of the
Great Australian Bight (James
et al
., 2006; James &
Bone, 2007).
Preservation, however, is a function of biolo-
gical and physical processes. Numerous animals
feed on the grass biota. Few animals, however,
can digest large quantities of cellulose, with
the exception of swans, garfi sh, mullet and
leatherjackets, so the leaves themselves are not
grazed (Edgar, 2001). Instead animals prefer the
detritus and algal resources associated with the
plants. The major food source of foragers in grass
beds is the epiphytes on the seagrass surfaces,
especially single-celled diatoms, encrusting and
fi lamentous algae, and small fl eshy macroalgae.
Such epiphytic algae in turn provide an attractive
habitat and food source for a large suite of gastro-
pods, crustaceans and polychaetes, most of which
graze while foraging over the seagrass. These
mesograsers are in turn fed upon by fi sh, rock lob-
sters, prawns and crabs. Thus, much of the calcar-
eous material may pass through the guts of several
organisms before accumulating as sediment.
the sediment itself. This biota is dominated by
infaunal bivalves, gastropods, and locally by large,
symbiont-bearing benthic foraminifera (Burne &
Colwell, 1982).
The resulting Quaternary sediment has been
well documented beneath grass banks in the
gulfs (Hails
et al
., 1984; Barnett
et al
., 1997; Cann
et al
., 2000) and the poorly sorted character
refl ects the nature of this factory. It is dominated
by siliciclastic and carbonate muds, sands rich
in coralline algae (
Metagoniolithon, Corallina,
Jania
), benthic foraminifera (
Nubecularia
, mili-
olids (
Quinqueloculina, Triloculina, Milionella,
Spirolculina
), rotaliids (
Discorbis, Elphidium
),
large sortids (
Peneropolis
)), and mollusc frag-
ments, together with a mollusc-dominated macro-
biota (Gostin
et al
., 1988; Cann
et al
., 2002). Thus,
it would appear that a signifi cant proportion of
the sediment produced here remains in the envi-
ronment of formation.
Nevertheless, Thomas & Clarke (2001) using
sediment tracers showed that sediment moved
through a grassbed at a similar rate to that from
an unvegetated site, generally in an onshore
direction. This is even more so under storm con-
ditions where sediment, both sand and mud,
is suspended equally from within the grass and
unvegetated areas. Lack of epiphyte carbonate in
siliclastic-dominated grass-bed environments in
Mozambique has led Perry & Beavington-Penny
(2005) also to propose that most of the sediment is
transported away from the area of production.
Consideration of environments adjacent to
grassbanks in southern Australia, particularly
low-energy peritidal mudfl ats and high-energy
beach-aeolianite systems, indicates that the grass-
beds are acting as a source for sediments in these
areas as well. Grass banks in the gulfs commonly
grade shoreward into supratidal mud fl ats
(Burne & Colwell, 1982; Gostin
et al
., 1984) and
the mud on these fl ats can come only from off-
shore, and the only source is epiphyte carbonate.
The southern Australian coastline has some of
the most extensive Quaternary aeolianites on the
globe. Examination of these aeolianites across the
same regions as this study (Gostin
et al
., 1988;
Wilson, 1991) clearly shows that offshore grass
beds are the source of most of this sediment as
refl ected by their carbonate composition; princip-
ally mollusc, bryozoan, benthic foraminifera, echi-
noid and coralline algae particles. Foraminifera
are again miliolids (
Spiroloculina, Triloculina,
Quinqueloculina
), rotaliids (
Elphidium, Discorbis
)
and the large form
Peneropolis
. Thus, marine
Sediment dynamics
The sediment is, however, generated in two
places, on the grass itself (the topic of this study),
and on the seafl oor. In the temperate marine envir-
onments of southern Australia the benthic biota
is present on the sediment surface and within
