METHODS OB SAMPLING TlIE BENTHOS
239
marine sediments, will produce erroneous results.
Many
sediments contain high percentages, 50% or more, of shell
fragments, and to dissolve these makes nonsense of the results.
Even a separate analysis of calcium carbonate is insufficient,
as this takes no account of particle size.
The extent to which the original structure of the sediment
should be broken down before analysis varies according to the
result desired. Clearly some breaking up of cohesive lumps is
required, but when this is continued to the point of breaking
down the " crumb "-structure of the soil the results, for biological purposes, may be misleading. This would seem to
preclude any preliminary drying of the sediment, which should
be sieved in " natural " water during analysis (Morgans, 1956).
Analysis of coarse and unconsolidated sediments presents no great
difficulty. The large particles (over about & mm) are separated by
sieving, the fine fraction being determined by decantation or by the
pipette method. Techniques may be found in Krumbein and Pettijohn
(1938), and in Holme (1954), Morgans (1956), Sanders (1956), and Wigley (1961) for marine sediments. The Wentworth scale, in which each
interval is one half the last (e.g. 2, 1, 0.5 mm, etc.) is now widely used,
and intermediate sieves may be introduced if further subdivision is
necessary.
Separation by means of sieves of standard measuring gauze is
convenient where a small number of samples are to be worked, but
it is a time-consuming task. If a large number of samples are to be
snalysed without too much attention to the finer fractions, a sedimentation tube may be used. This is a tall vertical tube in which separation
is based on the sinking rate of the particles through water. Emery
(1938) developed such a tube in which an analysis could be carried out
in about 5 min, and more recently Zeigler et al. (1960) have described
a settling tube in which pressure changes related to the settling of
particles in the tube are recorded automatically. It is claimed that
up to 150 samples can be worked in a day with this apparatus.
Rising-current elutriation for separating particles of diffcrcnt size
was used by Borley (1923) and by some later workers (see Morgans,
1956, p. 370). However, this mcthod is little used today, as it is rather
time-consuming and there is a source of error due to eddy currents
near the wall of the tube (Krumbeiii and Pettijohn, 1938, p. 122).
While it is relatively easy to separate the coarse fractions by such
methods, and to lump the finer constituents as " silt plus clay '' a
more detailed analysis of the lattor is a lengthy process. A number
239
marine sediments, will produce erroneous results.
Many
sediments contain high percentages, 50% or more, of shell
fragments, and to dissolve these makes nonsense of the results.
Even a separate analysis of calcium carbonate is insufficient,
as this takes no account of particle size.
The extent to which the original structure of the sediment
should be broken down before analysis varies according to the
result desired. Clearly some breaking up of cohesive lumps is
required, but when this is continued to the point of breaking
down the " crumb "-structure of the soil the results, for biological purposes, may be misleading. This would seem to
preclude any preliminary drying of the sediment, which should
be sieved in " natural " water during analysis (Morgans, 1956).
Analysis of coarse and unconsolidated sediments presents no great
difficulty. The large particles (over about & mm) are separated by
sieving, the fine fraction being determined by decantation or by the
pipette method. Techniques may be found in Krumbein and Pettijohn
(1938), and in Holme (1954), Morgans (1956), Sanders (1956), and Wigley (1961) for marine sediments. The Wentworth scale, in which each
interval is one half the last (e.g. 2, 1, 0.5 mm, etc.) is now widely used,
and intermediate sieves may be introduced if further subdivision is
necessary.
Separation by means of sieves of standard measuring gauze is
convenient where a small number of samples are to be worked, but
it is a time-consuming task. If a large number of samples are to be
snalysed without too much attention to the finer fractions, a sedimentation tube may be used. This is a tall vertical tube in which separation
is based on the sinking rate of the particles through water. Emery
(1938) developed such a tube in which an analysis could be carried out
in about 5 min, and more recently Zeigler et al. (1960) have described
a settling tube in which pressure changes related to the settling of
particles in the tube are recorded automatically. It is claimed that
up to 150 samples can be worked in a day with this apparatus.
Rising-current elutriation for separating particles of diffcrcnt size
was used by Borley (1923) and by some later workers (see Morgans,
1956, p. 370). However, this mcthod is little used today, as it is rather
time-consuming and there is a source of error due to eddy currents
near the wall of the tube (Krumbeiii and Pettijohn, 1938, p. 122).
While it is relatively easy to separate the coarse fractions by such
methods, and to lump the finer constituents as " silt plus clay '' a
more detailed analysis of the lattor is a lengthy process. A number