Geoscience Reference
In-Depth Information
contains large gaps. The interglacial periods, for example, are represented, not by
sediments, but by long phases of incision in the river valleys (table 13.1). A key
weakness of this model was the lack of a reliable time frame for the events it repre-
sented (Bowen, 1978) although Penck and Brückner (1909) did provide estimates
for the length of the interglacial periods relative to the Holocene or post-glacial
period (table 13.1). This framework became a cornerstone of Quaternary research
and records from around the world were correlated with the Alpine model (see
Bowen, 1978 for a detailed discussion).
The Marine Realm: Oxygen Isotopes and the Glacial Record
The Penck and Brückner model persisted for so long partly because there were no
convincing alternatives and partly because it could not be challenged effectively in the
absence of reliable dating frameworks for long records of change. This situation
changed in the 1960s and 1970s as attention shifted to the study of the continuous
Quaternary records in the deep ocean basins and the use of oxygen isotope analysis.
Oxygen has three stable isotopes (
16
O,
17
O and
18
O) and because the lighter
isotope evaporates more easily, atmospheric water vapour contains more
16
O and
less
18
O than the parent sea water. This process is called fractionation and it means
that continental ice sheets and glaciers are enriched in
16
O and, as ice sheets grow,
the oceans become relatively enriched in the heavier isotope
18
O. The oxygen isotope
ratio of ocean water is recorded in the calcium carbonate shells of tiny organisms
called forams. When they die they form part of the marine sediment record. These
simple creatures and these physical principles were the key to unlocking the glacial
record of the Quaternary.
Some species of forams produce shells with a composition that is in isotopic
equilibrium with the water that they inhabit and the oxygen isotopes can be meas-
ured using a mass spectrometer. This means that a long core of foram-rich marine
sediment can provide a record of long-term shifts in the isotopic composition of the
oceans. A cold stage or glacial ocean is enriched in the heavier isotope (
18
O) because
huge amounts of
16
O are locked within the continental ice sheets. Conversely, a
warm stage or interglacial ocean contains more of the lighter isotope (
16
O) because
ice sheet melting returns
16
O to the oceans. Shackleton (1967) had already shown
that the oxygen isotope record from Quaternary marine sediments was primarily a
record of changes in global ice volume and not a record of changes in ocean tem-
perature as had been argued previously (Emiliani, 1955). Thus, oxygen isotope
measurements can provide valuable insights into long-term changes in the global
hydrological cycle.
Shackleton and Opdyke (1973) worked on a 16-m sediment core (V28-238)
recovered from the Solomon Plateau on the fl oor of the equatorial Pacifi c and
measured the oxygen isotope ratio of foram samples for the entire length of the
core. Their oxygen isotope curve is shown in fi gure 13.3. The troughs in this curve
mark those periods when global ice volume reached its maximum extent during
glacial stages and these are marked with even numbers. The odd numbers represent
interglacial periods when global ice volume was much reduced and eustatic sea level
was high. These are commonly referred to as marine isotope stages (MIS) and MIS
5, for example, is the last interglacial.
This record revolutionised the study of the Quaternary because Shackleton
and Opdyke (1973) were the fi rst to set an oxygen isotope curve (and the long-
