Geoscience Reference
In-Depth Information
level, changes in the
16
O/
18
O ratio within calcareous marine fossils can be used as a
measure of past changes in near-surface sea salinity and temperature.
However, the overwhelming factor controlling fluctuations in this ratio is thewaxing
and waning of the great continental ice sheets, as Shackleton (
1967
;
1977
;
1987
)was
the first to recognise. As progressively more water is abstracted from the ocean
and incorporated into the ice caps, the ocean becomes more enriched in the heavier
isotope, and this is reflected in the
16
O/
18
O ratio. The effect is so pronounced across
the oceans of the Earth that fluctuations in the ratio of
16
Oto
18
O, expressed as
18
Oin
parts per thousand (
, or 'per mil'), have been used to reconstruct a detailed relative
chronology (see
Chapter 6
for details) of what are now the well-knownMarine Isotope
Stages, with the interglacial phases numbered back from the Holocene interglacial
(MIS 1) using uneven numbers and the glacial phases numbered back from MIS
2 (incorporating the Last Glacial Maximum at 21 ka) using even numbers. A pair
of marine isotope stages will, in principle, coincide with a single, complete glacial-
interglacial cycle, but complications arise from the inclusion of interstadial and stadial
episodes within the MIS chronology, so they should be discussed as separate entities.
Imbrie et al. (
1984
) provided a revised version of the marine chronology, known as
the SPECMAP
‰
18
O record, and it has been widely accepted since then.
A further issue is the need to calibrate the MIS relative chronology against a
measure of absolute time, for which the preferred templates are the various orbital
cycles (
Chapter 6
), a process termed 'orbital tuning'. Here again, difficulties arise
over possible time lags between inferred insolation changes at various latitudes linked
to the changes in orbital geometry, such as the distance of the earth from the sun or
the tilt of the earth's axis, and the response of the continental ice sheets, which may
involve time lags of thousands of years. Ideally, any use of the marine isotope stage
chronology should be supplemented by independent dating of the relevant marine
core using the methods described in
Chapter 6
. In the absence of any such age control,
the use of 'wiggle matching' between sets of MIS curves (e.g., Lisiecki and Raymo,
2005
; Lisiecki and Raymo,
2007
; Raymo and Huybers,
2008
) and other evidence of
environmental change should be carried out circumspectly.
7.4 The oxygen and carbon isotope record in desert lakes and rivers
Analysis of the stable isotope ratios of oxygen and carbon in carbonates from desert
lakes, swamps and playas has shed useful light on past changes in water depth,
temperature and chemistry (Fontes and Pouchan,
1975
; Cerling et al.,
1977
; Cerling,
1979
; Fontes et al.,
1983
; Lemeille et al.,
1983
; Gasse and Fontes,
1989
). In the case
of coastal sebkhas, or saltpans, it has also been used to determine past fluctuations in
sea level (Fontes and Perthuisot,
1971
).
Subsequent isotopic work has focussed rather more on the freshwater gastro-
pod and ostracod shells within lake and river sediments rather than the sedimentary
