Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 172

and references for comparison. Benthic d 18 O records should produce a better
stack than planktic records because the deep ocean is more uniform in tempera-
ture and salinity than surface water. Less local and regional climatic variability
improves the accuracy of alignment and produces a better estimate of average
d 18 O change. While a stack alone cannot [resolve] the relative contributions of ice
volume and temperature to the benthic d 18 O signal, a good stack does provide an
accurate estimate of how much total change must be explained'' (Lisiecki and
Raymo, 2005).

As Clark et al. (2006) pointed out,

''For any given species of benthic foraminifera, variations in d 18 O reflect

some combination of local to regional changes in water mass properties (largely
deep-water temperature) and global changes in seawater resulting from the
growth and decay of land ice. Determining how much each of these components
contributes to any given d 18 O benthic record, however, remains ambiguous.''

...

While it has generally been assumed that variations in d 18 O mainly record
changes in ice volume, as Clark et al. (2006) emphasized: ''Nevertheless, the
relative contributions of temperature and ice volume to this stacked record still
remain unconstrained, requiring other strategies to isolate the global ice volume
signal.'' They estimated that there was a ''60/40 ice volume/deep-water tempera-
ture contribution to the global d 18 O signal over the last several glacial cycles, with
a globally integrated deep-water cooling of 2.5 C during glaciations.''

5.2 CHRONOLOGY

The conversion of depth in sediments to age is a vital element of data processing.
One problem is that most age models are based on aligning with the astronomical
theory and, if that is done, it reduces the value of the data as a test for the astro-
nomical theory. As it turns out, most of the age models in the literature are
compounded from bits and pieces of basic information, loosely sewn together,
with orbital tuning often used as the thread to hold the whole thing together.
Interpolation between assigned time points is necessary. Tracing each element of
basic information back to its origins is often dicult.
Assigning an age to each position along the sediment core depends on
correlating the stratigraphy to a reference sequence that has been dated by inde-
pendent methods. Initially, objective dating was limited to radiocarbon dates on
sediments younger than about 30,000 years. A notable marker point in the records
was the high temperature of the last interglacial which occurred about
135,000 ybp , as suggested now by other evidence. An additional time marker point
was added by measuring the position of a polarity change (due to reversal of the
Earth's geomagnetic field) in deep-sea cores and assigning an age of 780,000 years