Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 175

dating studies. This makes the task of tracing dating procedures back to origins
more dicult. One example is the study by Lisiecki and Raymo (2005) who relied
on previous chronologies over the past 135 kybp

by graphically comparing

features with previously dated sediment studies:

.

The top 22 kybp of the stack were dated by correlating key features to a previous
14 C-dated benthic d 18 O record and assuming that all ocean sediment sites display
the same features at the same times.

From 22-120 kybp , the stack was aligned with a high-resolution benthic d 18 O
record of a site that was dated by correlating millennial-scale features in the
planktic d 18 O curve to the features in the ice d 18 O from the GRIP ice core that
were dated by layer counting. Here, the assumptions are (1) that the Greenland
ice core chronology is the same as the ocean benthic chronology and (2) that
chronologies are the same at all benthic sites.

.

. The age of the termination of the previous ice age (the one preceding the last ice

age) was taken from U-Th dating of coral terraces. This is now accepted to be
135 kybp .

Lisiecki and Raymo (2005) derived chronologies for earlier times with an
orbital-tuning model based on previous work by Imbrie et al. (1984), in which the
rate of change of ice volume in ice sheets (as inferred from the derivative of the
isotope record) was expressed in terms of a forcing function: the insolation curve
calculated for 65 N, with two parameters that were age-adjusted to allow a long-
term increase in ice volume over the past few million years. This procedure is
discussed in some detail in Sections 9.6.1 and 9.6.2. However, Lisiecki and Raymo
(2005) adjusted the two constants (B and T) in the Imbrie model to increase with
time toward the present because the data suggest a long-term increase in global ice
over the past few million years. Whereas Imbrie et al. found a best fit with B ¼ 0.6
and T ¼ 17,000 years for the time period over the past 150,000 years, Lisiecki and
Raymo (2005) used B ¼ 0.3 and T ¼ 5,000 years for the time period from 5.3 to
3.0 mybp , a linear increase to B ¼ 0.6 and T ¼ 15,000 years from 3.0 to 1.5 mybp ,
and constant values of B ¼ 0.6 and T ¼ 15,000 years from 1.5 mybp to the
present. This had the effect of compressing the time scale at early times and
stretching it out during more recent times. Then, by overlaying the d 18 O curve on
the ice model curve, they established a time scale for the d 18 O curve. This, of
course, required an elastic horizontal axis for the d 18 O curve so that the features
of each glaciation-deglaciation transition could be matched to the features in the
ice model curve. One portion of their fitting procedure is illustrated in Figure 5.1 .
The agreement between the model and the data is notable. But the significance
of this result is not obvious. On the one hand, the fact that a model as simplistic
as the Imbrie model (Sections 9.6.1 and 9.6.2) could lead to this degree of correla-
tion is impressive. On the other hand, the parameters of the Imbrie model were
adjusted to get a best fit. Furthermore, the absolute time scale is tied to the
astronomical theory via the Imbrie ice model with arbitrarily chosen parameters.
This approach seems to lie somewhere between mathematical curve fitting and a