2
AN INTRODUCTION TO POLLEN ANALYSIS
Some readers have expressed interest into the short synopsis of the pollen work at
Walthamstow marshes in the previous Newsletter. In order to explain the findings in
greater detail it may be a useful exercise to explain what "pollen analysis" is.
Historically pollens have been the dominant method of analysis for climatic
investigations since the 1920's and this is now a refined tool of research, but C14 dating
has tended to give pollen analysis a lowly status.
Pollen grains are "male" parts of a flowering plant formed in the anther, these are
intended to reach the female stigma of a flower after which fertilisation can take place.
Spores produced by the lower plants follow a different life cycle but are ultimately
dispersed in the same way as pollens. Some people are particularly sensitive to these
small grains and hay fever sufferers know this only too well.
Pollens are microscopic in size but their walls are partly made of a waxy material very
resilient to decay, hence they can remain well preserved when buried. They can be
attacked by microbial organisms, oxidised by other agents or suffer from mechanical
damage.
The walls of spores and pollen have distinct characteristics such as shape, size and
surface features which can be used to identify the parent plant, (see examples).
Pollens are to be found in a wide ranging set of environments, wherever suitable
conditions allow the pollen to become entrapped and buried. Laboratory analysis
involves taking samples from exposures or borehole cores of lake and river sediments (or
Oceanic sediments). From measured points along a sample length a small amount of
material is taken. The samples are treated with various chemicals and acids to remove
unwanted mineral and certain organic fractions, so you are left with a pollen "soup"
which is stained and mounted on a microscope slide.
Under the microscope the slide is examined and identifiable pollens and spores counted
until a predetermined number is reached. This is usually around 150/200 grains (of
course the higher the number the better the resolution but counting to high numbers is
very tedious). From each slide that has been taken from a different level it is possible
therefore to build up a diagram in the form of a percentage graph of pollen composition
for each level and from inference a picture is made of vegetational composition over
time. From the changes in the plant communities over time it is also possible to infer the
climatic conditions at a given point by reference to indicator species whose ecological
requirements are well known, for example cold growing and warm growing plants.
However a range of factors which complicate the analysis have to borne in mind.
Production rates of pollens with different plants varies and this knowledge is used to
statistically alter the pollen graphs, leaving what is called a corrected diagram. There can
be diferential preservation, some pollens being more resilient, or differential dispersion
with the shapes and weights of pollens affecting the final resting place.
Interpretation of the diagrams is usually taken with reference to stratigraphic changes and
other biological eveidence such as fossil bones and molluscs. Notwithstanding the
weaknesses of pollen diagrams they are remarkably consistent taken as a whole and
remain a useful tool in reconstructing past environments.