Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 123

increased dust concentration. LLS was a very valuable dating tool throughout
most of the length of the core, particularly in the deeper ice at GISP2 where the
other techniques either failed or became increasingly unreliable.
Additionally, d 18 O values were used to identify seasonal cycles to depths of
300m. The effects of diffusion rapidly obliterated the seasonal signal in deeper ice.
Most other parameters were useful to at least 600m. Visual stratigraphy was a
consistent parameter throughout most of the core. ECM and LLS were more
valuable in some sections than others depending on the atmospheric chemistry
and climate at that time.
3.2.9 Tuning
The process of dating sediment cores by comparing patterns of isotope variability
with patterns of solar variability based on the astronomical theory is usually
referred to as ''orbital tuning''. This is discussed in Sections 5.2 and 9.6. Orbital
tuning has been extensively used to date ocean sediments and has also been used
to date polar ice cores (Waelbroeck et al. 1995).
According to Parrenin et al. (2007b):
''Unfortunately, layer counting is not feasible in central Antarctica where
annual cycles are barely distinguishable. Comparison of paleoclimatic records to
insolation variations (so-called orbital tuning methods) are generally applicable
to a whole ice core, as long as the stratigraphy is preserved. On the other hand:
(1) the accuracy in terms of event durations is poor, (2) the accuracy in terms of
absolute ages is limited by the hypothesis of a constant phasing between the
climatic record used for the orbital tuning procedure and the insolation varia-
tions (and, by definition, does not allow one to infer this phasing). The advantage
is that the achieved accuracy does not decrease with depth (assuming the
underlying mechanism stays constant). As a consequence, it is currently the most
precise method to date the bottom of deep ice cores.''
3.2.10 Flimsy logic
In general, the dependence of age on depth in ice cores is highly nonlinear with
the lower regions of the ice core much more highly compressed than the upper
parts. Most of the time range of an ice core is relegated to the lowermost few
hundred meters. It is dicult to develop absolute standards for dating ice core
and sediment data. Therefore, it is common practice in dating cores and sediments
to compare the morphology of the time series curve (typically, isotope ratio vs.
depth) at a site with other data for which (supposedly) firm estimates of signal vs.
time have been derived. If the two curves have similar morphologies, one can
argue that the dates corresponding to specific features in the reference curve
can be attributed to corresponding features in the curve at the site that is being
investigated. This is illustrated schematically in Figure 3.13 .