Geoscience Reference
In-Depth Information
1.2 Data Sources for High-Resolution Paleoclimatology
The critical requirements for high-resolution paleoclimatology are that:
An accurate chronology can be established; this generally requires replication of
the archive being sampled.
The archive can be sampled in detail, ideally at seasonal to annual resolutions,
butatleastattheresolutionofafewyears.
The parameter being measured is reasonably well understood in terms of its rela-
tionship to climate (i.e., its mechanistic and seasonal response) so that it can be
calibrated in terms of climate, by using the instrumental record as a yardstick for
interpreting the paleorecord.
The relationship between the proxy and climate observed today has been similar
in the past (the principle of uniformitarianism).
The record captures variance of climate over a wide range of frequencies, or at
least the window of variance that the proxy does capture is known.
In the next section, these issues are examined with reference to the main
archives that are available for high-resolution paleoclimatology: tree rings, corals,
speleothems, ice cores, and varved sediments. This examination is followed by a
discussion of the opportunities and challenges in high-resolution paleoclimatology,
with particular reference to dendroclimatology.
1.3 Chronology and Replication
An accurate timescale is essential in high-resolution paleoclimatology. A chronol-
ogy is commonly obtained by counting annual increments, by using variations in
some parameter to mark the passage of time. This might be the cyclical 18 O maxi-
mum in a coral record, registering the sea surface temperature (SST) minimum over
each annual cycle; or the presence of a 'clay cap' in varved lacustrine sediments,
marking each winter's sediment layer; or the width of a tree ring between the large,
open-walled spring cells that form each year. However, simply counting these recur-
rent features in a sample (even if they are counted several times by different analysts)
does not guarantee an accurate chronology. The best procedure is to replicate the
record by using more than one sample (core), to eliminate potential uncertainties
due to 'missing' layers and to avoid misinterpretation of dubious sections. On this
matter, dendroclimatic studies have a clear and unambiguous advantage over most
other paleoclimate proxies. Duplicate cores are easily recovered, and crossdating
using one or more samples is routinely done. Tree-ring chronologies are thus as
good as a natural chronometer can be, at least for those regions where there is
an annual cycle of temperature or rainfall and trees are selected to record such
changes in their growth. However, for those vast areas of equatorial and tropical
forests, where trees are not under climatic stress and so do not produce annual rings,
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