CHAPTER 4
Marine Sediments
4.1
Separation and Fractionation of Sediments from Seawater
Sedimentary solid matter is of two main classes, that which is settled on the sea bed or
river bed and that which is present in suspension in the water column. The former can
be collected by hand in shallow waters or by using a suitable solid sampler in deeper
waters as discussed by Butman [692]. Particulates present in samples of the water
column can be isolated by a variety of means including filtration and centrifugation.
In the first place, it is generally agreed that the distribution of particle sizes in the
oceans is continuous, from the whale to the simple single molecule (Sharp [693,694]).
The size at which one calls an aggregate of molecules a particle is therefore arbitrary.
In the case of sea water, the dividing line between dissolved and particulate has been
chosen as 0.45 JlIIl, largely because the first commercially available membrane filters
had that as their pore size.
4.1.1
Separation by Filtration
It should be mentioned at this point that the acceptance of 0.45 JlIIl as the dividing line
is purely nominal, since few workers in the field actually use filters with this pore size.
The glass fibre filters used by many workers have pore sizes which are considerably
larger, ranging from 0.7 JlIIl for Whatman GF IF to 1.32 JlIIl for GF IC. With these
filters, all particles larger than the nominal pore size are retained, but many smaller
particles are also trapped. The silver filters, and most particularly the 0.4 JlIIl size,
contain relatively large and variable amounts of carbon, which must be removed by
combustion. After this combustion, the pore sizes are considerably enlarged, with the
0.45 JlIIl filter approaching 0.8 JlIIl in pore size. The nominal 0.8 JlIIl pore size filter is
used by many investigators because the pore size changes very little under heat
treatment. Thus, although 0.45 JlIIl has been accepted as the minimum size particulate
matter by definition, the filters actually used have a somewhat larger pore size, and
retain particles which are considerably smaller than the nominal cut-off size (Sheldon
and Sutcliffe [695]).
The choice of filter can determine the amount of material considered as particulate,
sometimes with unexpected results. Thus, the Whatman GF I C filter with its larger
pore size actually retains about three times as much particulate organic carbon as
Marine Sediments
4.1
Separation and Fractionation of Sediments from Seawater
Sedimentary solid matter is of two main classes, that which is settled on the sea bed or
river bed and that which is present in suspension in the water column. The former can
be collected by hand in shallow waters or by using a suitable solid sampler in deeper
waters as discussed by Butman [692]. Particulates present in samples of the water
column can be isolated by a variety of means including filtration and centrifugation.
In the first place, it is generally agreed that the distribution of particle sizes in the
oceans is continuous, from the whale to the simple single molecule (Sharp [693,694]).
The size at which one calls an aggregate of molecules a particle is therefore arbitrary.
In the case of sea water, the dividing line between dissolved and particulate has been
chosen as 0.45 JlIIl, largely because the first commercially available membrane filters
had that as their pore size.
4.1.1
Separation by Filtration
It should be mentioned at this point that the acceptance of 0.45 JlIIl as the dividing line
is purely nominal, since few workers in the field actually use filters with this pore size.
The glass fibre filters used by many workers have pore sizes which are considerably
larger, ranging from 0.7 JlIIl for Whatman GF IF to 1.32 JlIIl for GF IC. With these
filters, all particles larger than the nominal pore size are retained, but many smaller
particles are also trapped. The silver filters, and most particularly the 0.4 JlIIl size,
contain relatively large and variable amounts of carbon, which must be removed by
combustion. After this combustion, the pore sizes are considerably enlarged, with the
0.45 JlIIl filter approaching 0.8 JlIIl in pore size. The nominal 0.8 JlIIl pore size filter is
used by many investigators because the pore size changes very little under heat
treatment. Thus, although 0.45 JlIIl has been accepted as the minimum size particulate
matter by definition, the filters actually used have a somewhat larger pore size, and
retain particles which are considerably smaller than the nominal cut-off size (Sheldon
and Sutcliffe [695]).
The choice of filter can determine the amount of material considered as particulate,
sometimes with unexpected results. Thus, the Whatman GF I C filter with its larger
pore size actually retains about three times as much particulate organic carbon as