Ocean sediment data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 180

Bintanja et al. (2005) provided an analysis of marine sediment data. The d 18 O
data from deep benthic sediments that vary from glacial to interglacial conditions
are mainly affected by two mechanisms

(aside from local hydrographical

influences):

(1) Changes in the oxygen isotope composition of oceans because the ice sheet
contains an excess of 18 O, leaving the oceans depleted in 18 O in proportion to
the ice sheet volume (ice sheet part).
(2) Changes in the uptake of 18 O by benthic foraminifera that depend on local deep-
water temperature at the time of crystallization of their shells (deep-water part).

Previous attempts to resolve these two effects involved the use of independent
temperature and sea level records. Evidence and models suggest that the glacial
deep ocean was 2-3 C colder than today. Bintanja et al. (2005) developed an
alternative model for estimating the contributions of each of the two major
effects. They argued that, on glacial-interglacial time scales, the main contributors
to the mean benthic oxygen isotope record—Northern Hemisphere ice sheet
isotope content and the local deep-sea temperature—are both strongly related to
Northern Hemisphere mid-latitude to sub-polar surface air temperatures. This
constrains the magnitude of surface air temperatures, which enabled them to
separate the ice sheet and deep-water parts. Based only on d 18 O data, they pro-
vided reconstructions of actual climate variables such as surface air temperature,
global sea level, ice volume, and ice isotope content. They utilized data from
Lisiecki and Raymo (2005), which we plotted previously in Figure 5.6 . They
found that the ice sheet part was typically about 60% of the total during the past
million years or so, although this dropped sharply (but briefly) during interglacial
periods (see Figure 5.7 ).

5.5 OCEAN SEDIMENT DATA AND POLAR ICE CORE DATA
COMPARED

In previous sections, we presented isotope data from Greenland ice cores,
Antarctic ice cores, and ocean sediments. Greenland ice cores date back to about
140 kybp . Antarctic ice cores date back to 400 to 800 kybp , and ocean sediment
data date back beyond 2.7 mybp .
Ocean sediment data based on benthic forams are time series of d 18 O that are
believed to primarily represent changes in deep-ocean oxygen isotope content;
hence, they are assumed to measure total ice volume on the Earth.
The raw data from Greenland and Antarctica are predominantly time series of
d 18 O and d D, respectively. Various investigators have interpreted these data as
representing regional temperatures. Therefore, in comparing ice core data with
ocean sediment data, we are comparing quantities that are not equivalent. Accord-
ing to the Imbrie model (Section 9.6.2) there are time lags between variations in
solar intensity and ice sheet formation (42,500 years) or destruction (10,600 years).