Geoscience Reference
In-Depth Information
forams represent a proxy for past sea surface temperatures. One of the most
remarkable proxies is the ratio of oxygen isotopes in benthic (bottom-dwelling)
forams. This ratio in ancient sediments is believed to reflect the total amount of
ice that existed on the Earth at the time the seabeds were formed. This ratio is
therefore interpreted as a proxy for global ice. To the extent that temperature and
global ice track each other, the measurements of 18 O from planktic and benthic
forams may be similar. But, one must be careful in examining data, since some
records may reflect extreme local conditions. In general, the 18 O signal in benthic
forams primarily measures total ice volume best. In planktic forams there is a
much larger contribution from water temperature. Unfortunately, changes in the
isotopic composition of ocean reservoirs are not the only complications affecting
simple temperature interpretation of d c variations. The assumption that marine
organisms precipitate calcium carbonate from seawater in equilibrium is some-
times invalid. However, by careful selection of species either with no vital effects
or where the vital effects may be quantified,
this problem can hopefully be
minimized.
In a recent paper, Lisiecki et al. (2008) discussed the assumption that benthic
d 18 O represents the phase of ice volume change despite the fact that benthic d 18 O
is also affected by deep-water temperature change. They raised the question of
how to extract an ice volume signal from benthic d 18 O records using an accurate
age model and discussed a number of attempts to do this. They concluded:
''Generating a robust age model for benthic d 18 O or ice volume without the
assumptions of orbital tuning remains an important, unsolved problem.''
However, they showed a graph that compared radiometrically dated sea level esti-
mates with an orbitally tuned benthic d 18 O stack over the past 250,000 years and
the correlation was quite good with minor discrepancies. This was put forth as a
basis for assuming that benthic d 18 O can be interpreted as representing ice volume.
In addition to stable isotope analyses, the reconstruction of paleoclimates
can also be achieved by studying the relative abundances of species or species
assemblages and their morphological variations.
As we pointed out in Sections 3.1 and 3.2.2, measured 18 O/ 16 O ratios are
compared with a standard 18 O/ 16 O ratio in order to determine historical climate
variations from ice cores. The resulting oxygen isotopic variations are expressed in
delta notation, d 18 O, where
0
!
1
18 O
16 O
@
A
Sample
d 18 O
!
1
1,000
18 O
16 O
Reference
and its counterpart d (D) are indicators of past temperatures or ice accumulations.
For ocean sediments, one analyzes the oxygen isotope content of carbonate
samples precipitated in the distant past by calcite-secreting organisms (foramini-
fera, corals, mollusks) and the oxygen isotope ratio in these carbonates reflects the
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