Sediment
particle size analysis and characterisation: studies on the
atoll of Tikehau
from
Intès A., Arnaudin, H. (1987)
Particle size measurements
The
sediments in the Tikehau lagoon were studied in the framework of
a research programme focusing on how the atoll lagoon ecosystems
function. Here the characteristics of the sediments are among
the most decisive factors on which the composition, the
distribution and the production of the phyto- and zoobenthic
populations depend.
Introduction
The
Tikehau lagoon on the Tuamotu archipelago is characterised by
the presence of a pass crossing the western part of the emergent
coral ring, as well as by a large number of hoas, which are
mainly to be found in the south-eastern corner. The presence of
these breaches in the coral ring has enabled active exchanges to
occur between the lagoon and the surrounding ocean waters, and
the resulting hydrodynamic activity has affected the
distribution of the sediments in the lagoon.
The
sediments cover most of the lagoon floor, although the
underwater landscape also includes many pinnacles and coral
clusters, especially towards the west. There are only 5 emergent
islets, one of which, the motu called PUARUA located in the east
of the lagoon, has a poorly tended coconut grove growing beside
the remnants of a primaeval forest. The lagoon has a mean depth
of about 25 metres, and it is up to 40 metres deep in some parts.
Material
and methods
Samples
were collected by deep-sea divers during the months of August
and December 1986. The sediments were collected with scoops at
depths of several tens of centimetres (whenever the sediments
were deep enough for this to be possible), and the samples were
placed in plastic bags. All transfer operations were carried out
inside the bags so as not to lose any fine suspended particles
when unloading the samples from the bags. Each sampling point
was noted by taking the compass bearings. Upon landing, the
samples were lightly fixed with a neutralised formol solution to
prevent the organic matter from decaying.
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The
lagoon sediments were sampled at 54 stations spread all over the
Tikehau lagoon, from inside the hoas to the deepest parts of the
lagoon. The values given in brackets on the right of each samplng
station number are the depth in metres. |
Each
specimen was placed at the laboratory in a shaker containing
fresh water, which was agitated manually to remove the salt.
After a 24-hour period of decantation, the clear supernantant
water was pumped off and a second rinsing step was carried out.
The sediment was then left to dry in an oven at 70° for 36
hours. During this time, the whole mass of the sediment was
stirred gently but thoroughly three or four times to prevent it
from setting into a block, with the fine particles acting as a
cement. After being removed from the oven, the samples were
homogenized by stirring them manually and 500 grammes were
collected for analysis.
The
final dry samples were screened with a set of 22 sieves arranged
in 3 columns on a vibrating table. The 16 coarsest sieves (the
first 8 had screen sizes ranging from 5 to 1 mm, and the other 7
had screen sizes ranging from 0.8 to 0.16 mm) were subjected to
vibration for 30 minutes, whereas the 6 finest sieves (those
with screen sizes ranging from 0.125 to 0.04 mm) were vibrated
for a whole hour. The particles remaining on each seive were
collected and weighed on scales to within the nearest mg.
Particle
size analysis and calculation of the indices
The
weights recorded with each sample of sediment were used to
draw up a curve giving the combined percentages corresponding
to the fractions remaining on all the screens. These curves
were used to determine the values of the size parameters,
which were required to calculate the various sediment indexes
using the formulae reviewed by
Weydert (1976)
For
details, click here
Qualitative
morphological analysis
The
material examined here consisted mainly of
the residues remaining on the sieves ranging between 5 and
0.4 mm in size (i.e., the first 12 sieves in the series).
Composition
of the sediments
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The
lagoon sediments were found to be mainly sand, at all the
bathymetric depths prospected (ranging from 2 to 36 metres).
The finest sands form a patch (at depths of 15-30m) in the
central northern part of the lagoon
as well as a strip (at depths of 9-30m) on the
south-eastern edge The medium-fine sands form a large
stretch on the edge of the deep basin (at depths of
20-36m), extending in scattered patches at depths of 10m
towards the south-east, the east, the north-east, and at
depths of 10 to 15m towards the the western
edge of the lagoon. |
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The
pattern of distribution of very fine (STF) and medium-fine
(SM) sands reflects the occurrence of disturbances due to
the hydrodynamic activity: the very fine sands observed on
the eastern-to-south-eastern edge at depths of
9 to 30 m form a strip which is protected from
the prevailing winds, and borders on the cones of rubble
resulting from the hydrodynamic activity going on in the
hoas at depths of approximately 10 m; the patches of
medium-fine sand spread around the mouth of the pass
probably resulted from the hydrodynamic activity generated
by the pinnacles. |
The
sediments in the Tikehau lagoon consist mainly of chalky organogenic
debris, including the following main components:
-
the
coarser fractions (>1.25mm) are sands mainly consisting of
Halimeda components,
-
the
medium-fine fractions are sands containing foraminiferan
components (Miliolidae) with particle sizes ranging between
0.8mm and 0.2mm (sieves 9 to 15).
-
and
crustacean debris are present everywhere, but are not
particularly abundant.
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Sands
mixed with Halimeda components are mainly to be found in
the western quarter of the lagoon at depths ranging between 15 and
28m. These sands therefore occupy only part of the deep basin. Their
abundance decreases from west to east and at depths of more than
twenty metres.
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The
foraminiferan components belonging to the Miliolidae family are to be
found at depths of less than 10m near the most southerly and easterly
shores of the lagoon, as well as in the central northern part, on the
edge of the deep basin, at depths of up to 36m.
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Conclusion
The
organogenic particles composing the coarser fractions of the
lagoon sediments consist mainly of Halimeda and foraminiferan
components. However, most of the lagoon sediments are fine sands
which have been carefully sorted and classified. Beaches of
medium-fine to very fine sand are also still to be found near
the mouths of the hoas and around the pass (these are
hydrodynamically active regions), and there is also a sandy
beach of this kind in the middle of the lagoon.
This
page was based on :
Intès
A., Arnaudin H. (1987) Esquisse sédimentologique du lagon in Contribution
à l'étude de l'atoll de Tikehau : IV . ORSTOM Notes et documents N°35:
71-100
References
Weydert
P. (1976) Manuel de sédimentométrie et d'arénologie. Principe, définition,
méthodes d'interprétation et critique du protocole d'analyse des sédiments
par tamisage. Marseille-Luminy: 1-61
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