Geoscience Reference
In-Depth Information
In Quaternary research, it is all too easy to view the glacial ages simplistically as a
succession of glacial and interglacial culminations during which the extent and volume
of glacier ice were at a maximum or minimum.
In reality, and this is shown very clearly by the marine oxygen isotope records
(fi gures 13.3 and 13.5a,b), conditions during the Quaternary Period were, for much
of the time,
intermediate
between these extremes and, after all, the peaks of glacial
and interglacial periods were relatively short-lived. Porter (1989) argued that this
was an important consideration when examining geomorphological and ecological
processes during the Quaternary Period. It is clear, however, that for much of the
Quaternary, global ice volume was much greater than present-day values. Figure
13.5b shows a relatively slow build-up of continental ice during the course of marine
isotope stages 2 and 6 with extended periods of very harsh conditions ending in a
brief period of rapid ice sheet melting known as a Termination. However, the high-
resolution data from the North Atlantic and from the Greenland ice cores have
dispelled any notions of long-term ice sheet stability and glacial monotony as is
shown in fi gure 13.5c,d. One of the most remarkable discoveries of the 1990s was
that the last cold stage was punctuated by centennial- to millennial-scale variations
in climate and these are clearly recorded in the ice core records (fi gure 13.5c) with
signifi cant fl uctuations in air temperatures in Greenland throughout this period.
These are known as Dansgaard-Oeschger cycles and they represent air temperature
shifts of the order of 15ºC (Dansgaard et al., 1993).
Another key discovery of the last two decades was the presence of ice-rafted
debris in the marine sediment record across the North Atlantic (fi gure 13.5d). These
sediments show that the North American (Laurentide) ice sheet was highly dynamic
throughout the last cold stage as large discharges of icebergs periodically fl owed
out across the North Atlantic and cooled the ocean surface. These are known as
Heinrich Events and their impact on the climate system has been recorded in a
variety of proxy records across the European continent (Bell and Walker, 2005;
Anderson et al., 2007). As the drifting ice melted, it lowered the salinity of surface
waters and this is clearly recorded in the oxygen isotope signal from foram species
that lived in the upper part of the water column (Bond et al., 1993). The recognition
of Heinrich Events showed the potential scale and rapidity of cryosphere-ocean-
atmosphere interactions during the last cold stage.
For the second time within two decades, revelations from the marine record have
forced Quaternary scientists to revise their ideas about long-term ice sheet dynamics
and the drivers of environmental change, and to ask new questions of the terrestrial
records. Indeed, the extract from Bowen (1979) cited above is just as relevant almost
30 years on as the combined impact of these fi ndings alongside the Greenland ice
core records has been profound across both the Quaternary science and archaeologi-
cal communities. A direct result of these revelations is that most research is now
done at much higher resolution than before, with more fi nely resolved sampling and
better dating control. The impact of these changes for the study of long-term human-
environment interactions will be discussed below.
A Mediterranean Perspective: High-Resolution Records
Another important development in European Quaternary research in the last two
decades has seen a signifi cant increase in the volume of work conducted south of
