Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 177

Figure 5.2. Assignment of stages and terminations by H&W. Stages are designated by arrows
and terminations are defined by circles.
which they termed ''depth-derived ages''. However, it was claimed that none of
these were entirely satisfactory. H&W extended the depth-derived approach to 21
sediment cores and added an allowance for down-core sediment compaction. In
their model, they defined an ''event'' as a feature that can be uniquely identified in
the d 18 O vs. depth curve for each site. If an age is fixed to an event, it becomes an
age control point (ACP). Two types of events were utilized, stages and termina-
tions. Stages were defined as local minima or maxima in the d 18 O vs. depth curve
using the numbering system originally suggested by Imbrie et al. (1984).
All the stages utilized in their study corresponded to peaks in the d 18 O vs.
depth curve. Terminations were defined as an abrupt shift from glacial to inter-
glacial conditions where the assigned depth was the midpoint between the local
d 18 O minimum and maximum. Figure 5.2 shows the eight termination midpoints
and nine stages that were visually identified in each d 18 O record. H&W developed
a model for the sediment accumulation rate that included three terms: (1) a mean
sedimentation rate, (2) stochastic variability about the mean sedimentation rate,
and (3) a systematic term due to sediment compaction with age. However, the
procedure by which they obtained best values for the stochastic and systematic
terms was complex and dicult to follow.
5.3 UNIVERSALITY OF OCEAN SEDIMENT DATA
M&M emphasized that the pattern of variation of oxygen isotope content with
time over 800,000 years is remarkably similar in sea floor records from around the
world. In Figure 5.3 we show 10 measured 18 O records from forams taken from
regions that include the Pacific, Atlantic, and Indian Oceans. Since the ages of
the samples are not known, M&M assumed that the sedimentation rate for each