Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 109

3.2 DATING ICE CORE DATA

3.2.1 Introduction

There are two major issues in deriving historical data from ice cores:

(1) Developing a date-depth relationship that provides a chronology of ice core
measurements at any depth.
(2) Developing algorithms for proxies contained in the cores (typically based on
isotope ratios) that reveal temperature, ice accumulation, or other historical
climatological data.

The actual procedures used in dating ice cores typically involve a number of
intricately woven factors. Dating of Greenland ice cores is greatly enhanced by the
ability in many cases to visually discern annual layers, although these tend to blur
below a moderate depth. But, Greenland cores typically only go back a bit over
100,000 years.
According to Alley and Bender (1998), visual counting of annual strata for
Greenland ice is accurate to about 1% for the most recent 11,500 years. Although
accuracy is reduced in older ice from colder times, it appears to be as good as that
of other dating techniques to perhaps 50,000 years ago. Arguably, annual layers
from Greenland remain visible past 100,000 years, but they often appear distorted
because ice sheets spread and thin under the influence of gravity, and the bottom
layers of ice become extremely chaotic.
However, visual resolution of annual layers is not feasible in Antarctica where
annual precipitation is much less and the layers are more closely packed. Uncer-
tainty in the Vostok (Antarctica) time scale as of 2001 was estimated to be as high
as 15,000 years (Parrenin et al., 2001) In fact, the procedures used for Antarctica
ice cores are so complex and arcane that it is usually dicult to clarify what has
been done, what assumptions have been made, and how reliable these assumptions
are. Many investigators have relied on a comparison of key transition points in
the isotope ratio vs. depth curve with oscillations in the curve of time dependence
of solar input to high latitudes. One thereby uses the astronomical theory to assign
ages to key points in the isotope ratio vs. depth and interpolates between these
key points. Such processes are referred to as ''orbital tuning'' (or just ''tuning'').
However, the assignment of these key dates is a subjective process. More impor-
tantly, once orbital tuning is employed, the value of the results for testing the
astronomical theory is greatly diminished because the astronomical theory was
used to define the chronology and one ends up with circular reasoning. Since one
of the main reasons to obtain and analyze ice cores is to test the astronomical
theory, such circular reasoning can be counterproductive. But there seems to be
widespread bias in the community of paleoclimatologists to defend the astro-
nomical theory at all costs, using all manner of tinkering with the data to seek
agreement. When disagreements are found between the data and the theory, it is