Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 171

ratio in the water that the organisms lived in. Note that the length of sediment
cores is typically about 10 to 15m, in contrast to the depth of up to 3,000 m in
ice cores. The standard used in this case was not Standard Mean Ocean Water (or
SMOW), but was so-called PDB, a crushed belemnite (Belemnitella americana)
from the Peedee formation (Cretaceous) in South Carolina. The original PDB
material has long since been exhausted, but other standards have been calibrated
against PDB and are used as an intermediate reference standard through which
a PDB value can be calculated. Models have been developed to relate
paleotemperature to this isotope ratio (Wright, 1999).
M&M provided the following estimate of the depletion of 16 O during a major
ice age. 1 There is evidence that the level of the oceans was more than 100m lower
during the last major ice age ( 20,000 years ago) due to the agglomeration of
ice in glaciers. The average depth of the oceans is 3,800m. Thus, about 100/
3,800 2.6% of the original oceans became stored as ice during the height of the
last ice age. Measurements indicate that the water vapor that evaporates from
oceans is roughly 40

% ¼ 4% enhanced in 16 O. Therefore, the remaining ocean
water (after removal of 2.6% via evaporation and deposition onto glaciers) is
enriched in

18 O by about 4% 2.6% 0.1% ¼ 1

. According to M&M, this
agrees with typical depletion levels observed in sediments from glacial maximum
periods. 2 To the extent to which this effect dominates, 18 O is a valid proxy for
global ice volume. M&M further pointed out that, since the mixing time for sea-
water around the world is about a thousand years, the 18 O records in the northern
Atlantic are similar to those in the equatorial Pacific. However, M&M also
cautioned that isotopic separation could also reflect local temperatures and other
climate conditions, even if global glaciation has not changed appreciably. Thus,
the derivation of ice volume from isotope ratios may not be as straightforward as
this simple discussion suggests:

%

''Time series of the d 18 O of foraminiferal calcite tests provide an important
record of climate change. Foraminiferal d 18 O is a function of the temperature
and d 18 O of the water in which it forms, and the d 18 O of seawater is a function of
global ice volume and water salinity. (The scaling between d 18 O and these two
factors can vary with patterns of sea ice formation, evaporation, and precipita-
tion.) Owing to the observed similarity of most marine d 18 O records and the
global nature of the ice volume signal, d 18 O measurements also serve as the
primary means for placing marine climate records on a common timescale.
Stacks, which are averages of d 18 O records from multiple sites, improve the
signal-to-noise ratio of the climate signal and make useful alignment targets

1 J. D. Wright provided a similar estimate at http://geology.rutgers.edu/ jdwright/JDWWeb/
2000/Wright2000.pdf
2 However, note that Wilson et al. (2000) pointed out ''the last glaciation was equivalent to
removing a 165m layer from the oceans, and the net drop in sea level around the world was
only 115m. This was because the water loading [during the LGM] caused the crust beneath the
oceans to drop by 50m.