Environmental Engineering Reference
In-Depth Information
conditions prevailing at the time of deposition. Interpretation of this material is based on
the
principle of uniformitarianism
. In other words, where we find evidence that plants
or animals were living in the immediate area it is assumed that the conditions under
which those plants or animals live today must have prevailed at that time and location in
the past. Hence the phrase that the present is the key to the past. The most important
aspect is that we know the present requirements of that life form and that it can be
recognized as a fossil. Species that are extinct present more of a problem. Here we have
to determine the range of species preserved on the assumption that some of them will still
exist. Much investigation of Quaternary sediments and their environmental reconstruction
is based on the integration of evidence from several different sources rather than being
dependent on a single proxy.
Although some of the organic material is visible to the naked eye in sediment, the
majority of material has to be extracted by laboratory techniques and examined by
microscope or more advanced instrumentation. Each of these has become a specialist
activity that can be read about more fully elsewhere. The most widely used evidence is
pollen. As this is small and often wind blown, it is frequently preserved in water-laid
sediments, especially on land where plants producing the pollen grow. Unfortunately not
all pollen can be identified to species level, and not all species produce the same quantity
of pollen. For example, Scots pine (
Pinus sylvestris
) pollen is produced in large quantities
and can be found in most lake sediments, whilst beech (
Fagus sylvatica
) is produced in
small quantities and is usually underrepresented in sediments. Nevertheless if these and
other problems associated with palynology (the study of pollen) are appreciated, the
information it provides is extremely useful in determining former climate.
There is a range of other plants and animals which also provide information where
their environmental requirements are known. Insects, such as beetles, are particularly
useful as they are more mobile and respond more rapidly to environmental change than
do plants. Diatoms, a member of the algae family, are often preserved in lakes and
provide environmental information, as do ostracods, foraminifera and molluscs in marine
sediments. Most of these need to be studied under a microscope but occasionally
macroscopic remains of vertebrates are preserved and give more spectactular evidence of
former climate
DATING
concepts
Even if we know all about the environmental conditions inferred by biological evidence,
it is really only of value if we have some information about
when
the material was
deposited, otherwise we cannot compare material and evidence either in space or in time.
Until recently we could determine only relative ages, i.e. that one event happened prior to
or after another event. If a sediment was in its original sequence, then the beds towards
the top of the sequence would be younger than those at the base, but we did not know
how much younger. One way of determining an absolute age is by incremental methods.
In some glacial lakes, sediments called varves are laid down annually as a result of the
seasonal glacial melt, producing different bands of sediment between the active summer
season and the quiescent winter season; the cycle from coarse to fine represents one year.
If the termination date is known, or the system is still active, the varves can be counted to
produce an absolute date Similarly tree rings are essentially annual Working from a tree
