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the sediments. It is likely that such productivity increases may be amplified by
changes in external nutrient loading driven by the warming of catchment soils.
Battarbee
et al
. (2001) showed similar cyclical variations in the content of lake
sediment organic matter from a study of a remote mountain lake in the Scottish
Cairngorm Mountains. As the cycles had an approximately 200-year periodicity,
corresponding quite closely to the periodicity of the Suess solar cycle (see above),
the authors concluded, following Willemse and Tornqvist (1999), that the
organic matter variability was driven by temperature-related variability in lake
productivity. This conclusion was reinforced by the observation that the organic
matter cyclicity was matched almost perfectly by similar cyclicity in the
concentration of chironomid head capsules in the core (Battarbee
et al
. 2001). If
this interpretation is correct, it is apparent, however, that the amplitude of
variability in the LOI records is much higher than the amplitude of variability in
air temperature, suggesting that feedbacks affecting the production and
preservation of organic matter within the lake system were operating. Battarbee
et al
. (2001) speculated that small changes in air temperature might lead to more
substantial changes in sediment organic matter concentration if increased algal
productivity and stronger stratification caused a decrease in hypolimnetic oxygen
concentrations, enhanced nutrient recycling from sediments and improved
conditions for organic matter preservation.
Seasonal, inter-annual and decadal change
Most lake sediments accumulate too slowly or are too disturbed to reveal change
in lake behaviour on seasonal to inter-annual timescales, although where sediment
cores are very finely sampled, it is usually possible to identify decadal-scale
change. The exception is where sediments are annually laminated. Such lakes are
rare in most regions of the world, but where they exist, they can offer remarkably
precise insights into the past.
Some of the best annually laminated sediments are found in relatively deep
lakes in boreal climates that are ice covered in winter and are strongly stratified
in summer. Productive lakes with low hypolimnetic oxygen concentrations are
especially likely to have laminated sediments as mixing by benthic invertebrates
is usually greatly reduced in anoxic environments. Extreme care is essential in
coring such sites to avoid disturbance of the laminae, and specialized analytical
techniques are usually needed to take advantage of the particular nature of the
record. Freeze coring is often favoured followed by counting of laminae using,
for example, photography and thin-section analysis (Zolitschka 2003) or image
analysis (Petterson
et al
. 1999). In diatom-rich sediments, it is possible to use
light microscopic examinations of thin sections (e.g. Card 2008) or tape peels
(e.g. Simola 1977) to identify changes in the seasonal succession and inter-annual
variability of diatom plankton that could be associated with variations in weather
patterns, especially where those are known precisely from long-term instrumental
records.
Annually laminated sediments favour good microfossil preservation, but it
is the accuracy and precision of the chronology of such sediments that are
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