peatlands 1 taitto.indd - International Peat Society

Recording the Past: the Power of Peat
Peat bogs are powerful archives
containing records on
the environment and culture
of the past, such as climate
change - as indicated by
botanical remains - and bog
finds such as archaeological
materials, including human
remains (bog bodies). Some
recent research highlights
from the Netherlands are
presented.
Peat bogs are growing continuously
under certain climatic conditions such
as those that prevailed during the
Holocene - the present geological period
that, following the last ice age, has
lasted for approximately 11,500 years.
Peat bogs and palaeoclimate
By digging into terrestrial deposits, one
travels, in general, back in time from
the present to the past - the deeper,
the older. The relative occurrence of
plant species at a certain depth (and
thus at a certain time in the past)
indicates climatic conditions, like tem-
Figure 2: The variations in the natural 14 C content, as measured in tree
rings dated by dendro-chronology. The 14 C content is plotted as the
deviation from the normal level, and is corrected for radioactive decay.
46 PEATLANDS International 1/2006
perature and humidity, under which the
plants at this depth were growing. In
general terms, dominance of tree pollen
is indicative of warmth; the relative
abundance of different species gives
fi ner detail. For example, there usually
is a dominance of birch and pine during
the initial phase of an interglacial
like the Holocene, followed later by
mixed forest (oak, elm, hazel) in the
temperate regions. Thus, records of
preserved botanical remains like pollen
and macrofossils (branches, leaves)
form a proxy for the palaeoclimate.
Peat bogs consist of bog mosses
(species of Sphagnum). They are
rain-fed and grow continuously. After
death, the mosses are well preserved.
Continual growth of the peat bog surface
and burial of dead organic matter
leads to peat sequences that can exceed
10 m in depth.
Radiocarbon Dating
The chronology of such deposits can
be obtained by 14 C (or Radiocarbon)
dating of organic remains like peat,
macrofossils, or pollen. Radiocarbon is
a natural radioactive
isotope
(with a half-life
of 5730 years)
of the element
Carbon.
This isotope is
continuously
produced by
cosmic radiation
in the
upper atmosphere,
and is
taken up by
the plants by
photosynthesis.
From there
it fi nds its way
into all living
organisms via
the food chain.
There is an
equilibrium
Text and photos: J. van der Plicht
J. van der Plicht.
between uptake and decay of 14 C . After
death of the organism, there is no
longer uptake of 14 C; radioactive decay
causes the content of 14 C to decrease.
Hence, by measuring the remaining
14 C content of for example the botanical
remains from within a peat bog, the
age of these remains (more precisely:
the moment of death) can be established.
The isotope 14 C provides in fact
a clock, built into the organic residues.
Although this sounds like a simple
and straightforward principle, in practice
there are many complications of
which only one is mentioned here. For
the simple dating model to work, one
has to assume that the natural atmospheric
14 C concentration is constant
throughout time. It appears that this
is not the case: the cosmic ray fl ux impinging
on earth, and thus the amount
of 14 C produced in the atmosphere,
depends on factors like the strength
of the earth’s magnetic fi eld, and solar
activity.
In addition, the 14 C concentrations
are extremely small (1:1012-1015) so
that measuring them requires highlyspecialized
equipment, such as shown
in Figure 1.