74 Climates ofthe past
−− 3838 − 36 − 34 − 32 − 30 −28 ‰ −^35 −^30
Gerzensee
Lake sediments
DYE 3
δ^18 O δ^18 O ‰
Deep ice core
9.500 yr BP
8500 yr BP
9000 yr BP
Depth of sediment
Depth of
ice core
Allerod
Younger Dryas
Bolling
10.700 BP
− 10
1
1750
1785
Pre-Boreal
Younger Dryas
1786
7 °C
∼50yr
1787
m
1800
1850
m
1700
2
3
4
m
− 8 −6 ‰
Figure 4.7Records of the variations of the oxygen isotopeδ^18 O from lake
sediments from Lake Gerzen in Switzerland and from the Greenland ice core
‘Dye 3’ showing the ‘Younger Dryas’ event and its rapid end about 10 700 years
ago. Dating of the ice core was by counting the annual layers down from the
surface; dating of the lake sediment was by the^14 C method. A change of five
parts in a thousand inδ^18 O in the ice core corresponds to about a 7◦C change
in temperature.
Broecker for the Younger Dryas event.^11 As the great ice-sheet over
north America began to melt at the end of the last ice age, the melt water
at first drained through the Mississippi into the Gulf of Mexico. Even-
tually, however, the retreat of the ice opened up a channel for the water
in the region of the St Lawrence river. This influx of fresh water into the
North Atlantic reduced its saltiness, thus, Broecker postulates, cutting
off the formation of deep water and that part of the ocean ‘conveyor
belt’.^12 Warm water was therefore prevented from flowing northward,
resulting in a reversal to much colder conditions. The suggestion is
also that a reversal of this process with the starting up of the Atlantic
‘conveyor belt’ could lead to a sudden onset of warmer conditions.
Although debate continues regarding the details of the Younger
Dryas event, there is considerable evidence from paleodata, especially
those from ocean sediments, for the main elements of the Broecker