194
N. A . HOLME
0.2 m2. The grab type of sampler has been modified in various ways to
improve its efficiency, but remains the basis of the majority of bottomsamplers. (Certain instrumcnts such as the orange-peel grab, have
been variously described as either dredges or grabs. I n this review
the term dredge is confined to instruments towed by the ship.) Coresamplers, which have the advantage of sampling a uniform area a t all
depths in the sediment, are also sometimes employed, but the majority
do not cover a sufficiciitly large area for sampling the macrofauna.
The main problems in sampling are first to ensure adequate penetration of the sedimeht, secondly to retain the sample while it is
brought to the surface, and thirdly the means of actuating the release
so that the mechanism operates at the correct time when the sampler
is on the sea bottom. When sampling soft sediments in enclosed
lochs or fiords none of these problems presents any great difficulty,
but in the open sea on the coarser and harder sediments of the continental shelf there is considerable difficulty in obtaining an adequate
sample, and we shall now consider how sampling can best be carried
out under such conditions.
1. Penetration of the sampler
While the effective force available when dredging (i.e. the horizontal component of the force exerted by the warp on the dredge)
may be very considerable, of the order of several tons, that available
to grabs and similar instruments lowered vertically from the ship is
clearly very much less, the dead weight of the instrument playing a
large part in achieving penetration.
If weight alone were used to push a grab or wide coring tube into
the sediment,, this might have to be very considerable. Soft muds
are easily penetrated, and in fact the whole instrument may Rink
below the surface, but on hard-packed sands a force of several hundred
kilograms may be necessary to ensure full penetration. Any stone in
the path of the instrument will, moreover, prevent further penetration.
Gravels are often more loosely packed, and if larger stones are not
included they may be sampled to some extent with the grab (but much
more easily with a dredge, cp. Smith, 1932). Grounds of coarser
materials can only be sampled by the dredge.
Most samplers depend to a large extent on weight to ensure penetration, and instruments weighing up to 150 kg are commonly used
(e.g. the clam-shell grab used by Lee, 1944, and the Okean-50 grab
which may be weighted up to 150 kg, see Lisitsin and Udintsev, 1955).
The Campbell grab (Fig. 14) used by Hartman (Hartman, 1965;
Hartman and Rarnard, 1958) weighs over 400 kg.
N. A . HOLME
0.2 m2. The grab type of sampler has been modified in various ways to
improve its efficiency, but remains the basis of the majority of bottomsamplers. (Certain instrumcnts such as the orange-peel grab, have
been variously described as either dredges or grabs. I n this review
the term dredge is confined to instruments towed by the ship.) Coresamplers, which have the advantage of sampling a uniform area a t all
depths in the sediment, are also sometimes employed, but the majority
do not cover a sufficiciitly large area for sampling the macrofauna.
The main problems in sampling are first to ensure adequate penetration of the sedimeht, secondly to retain the sample while it is
brought to the surface, and thirdly the means of actuating the release
so that the mechanism operates at the correct time when the sampler
is on the sea bottom. When sampling soft sediments in enclosed
lochs or fiords none of these problems presents any great difficulty,
but in the open sea on the coarser and harder sediments of the continental shelf there is considerable difficulty in obtaining an adequate
sample, and we shall now consider how sampling can best be carried
out under such conditions.
1. Penetration of the sampler
While the effective force available when dredging (i.e. the horizontal component of the force exerted by the warp on the dredge)
may be very considerable, of the order of several tons, that available
to grabs and similar instruments lowered vertically from the ship is
clearly very much less, the dead weight of the instrument playing a
large part in achieving penetration.
If weight alone were used to push a grab or wide coring tube into
the sediment,, this might have to be very considerable. Soft muds
are easily penetrated, and in fact the whole instrument may Rink
below the surface, but on hard-packed sands a force of several hundred
kilograms may be necessary to ensure full penetration. Any stone in
the path of the instrument will, moreover, prevent further penetration.
Gravels are often more loosely packed, and if larger stones are not
included they may be sampled to some extent with the grab (but much
more easily with a dredge, cp. Smith, 1932). Grounds of coarser
materials can only be sampled by the dredge.
Most samplers depend to a large extent on weight to ensure penetration, and instruments weighing up to 150 kg are commonly used
(e.g. the clam-shell grab used by Lee, 1944, and the Okean-50 grab
which may be weighted up to 150 kg, see Lisitsin and Udintsev, 1955).
The Campbell grab (Fig. 14) used by Hartman (Hartman, 1965;
Hartman and Rarnard, 1958) weighs over 400 kg.