144 Sea Level Processes and Effects of Sea Level Change
fluctuations of the Pleistocene. Sea level has been constantly changing over the last
several hundred thousand years as a result of the waxing and waning of large continental ice masses. The rise of the last deglaciation, which was witnessed by our
ancestors, was one of the biggest and fastest sea level changes ever, as far as we
known.
It was the result of a maximum change of climate: from a peak cold period to a
peak warm period.
About 20000 years ago, during the last major ice age (the Wurm in Europe, the
Wisconsin in North America), enough water was tied up in the continental glaciers to
depress sea level by some 120 m. Large shelf areas fell dry as a result of the
ice-caused regression of the sea. Rivers crossed the shelves and entered the sea at the
shelf edge, cutting backward into the shelf. Their sediment load was dropped in a
narrow zone at the upper slope, became unstable there and slid, starting turbidity
flows which rushed down the undersea canyons. In front of the ice, outwash plains
(or moraine ridges next to the terminal ice tongues) developed on the exposed shelf.
Dune fields evolved on shelves where the climate was favorable.
Land animals spread over the emerged shelves and used them as land bridges in
some cases, for example, between Northeast Siberia and Alaska. Mammoths roamed
over the large area covered by the North Sea today. Their remains are found on the
sea bottom now, together with the tools of prehistoric hunters. Could these people of
the late Stone Age have left a memory of the effects of rapid sea level rise, in the
ubiquitous legends of a Great Flood?
5.4.2 Pleistocene Fluctuations. There is an indication that, throughout the late Pleistocene, major transgressions were much more rapid than any of the regressions. In
general, apparently, the buildup of ice (and hence the sea level drop) proceeded at a
slower rate than the melting of ice. The evidence for this concept rests on the oxygen
isotope record of pelagic foraminifera (Fig. 5.15).
How can we measure sea level changes in the chemistry of foraminiferal shells?
The principle of this method was introduced by C. Emiliani in 1955.
The shells of the foraminifera consist of calcium carbonate, CaC03. Hence, they
contain oxygen. The water within which the shells grow, H20, also contains oxygen.
There are three kinds of oxygen, the normal one, with atomic weight 16, and two rare
ones, with atomic weights 17 and 18. The oxygen-17 is very rare and we need pay no
further attention to it. Through mass spectrometry, one can determine the ratio between oxygen-16 and oxygen-I 8 (commonly written 16 0 and 18 0). The ratio of these
two isotopes in the shells is in equilibrium with the ratio of the isotopes in the water
in which the shells grew. In other words, if the 180/160 ratio in the water changes, it
will change similarly in the shell.
This fact relates to sea level change as follows. Every time the sea level drops, the
18 0/ 16 0 ratio of the water increases, because the glacial ice is made of water which
is impoverished in the isotope 18 0. Hence, seawater is enriched in 18 0 during glacials
(Fig. 5.15b). The 18 0/ 16 0 ratio of the carbonate shells reflects this change in the
chemistry of seawater. Temperature also influences the U10/160 ratio of the shells. In
Fig. 5.15a, however, the long sediment core on which the measurements were made
fluctuations of the Pleistocene. Sea level has been constantly changing over the last
several hundred thousand years as a result of the waxing and waning of large continental ice masses. The rise of the last deglaciation, which was witnessed by our
ancestors, was one of the biggest and fastest sea level changes ever, as far as we
known.
It was the result of a maximum change of climate: from a peak cold period to a
peak warm period.
About 20000 years ago, during the last major ice age (the Wurm in Europe, the
Wisconsin in North America), enough water was tied up in the continental glaciers to
depress sea level by some 120 m. Large shelf areas fell dry as a result of the
ice-caused regression of the sea. Rivers crossed the shelves and entered the sea at the
shelf edge, cutting backward into the shelf. Their sediment load was dropped in a
narrow zone at the upper slope, became unstable there and slid, starting turbidity
flows which rushed down the undersea canyons. In front of the ice, outwash plains
(or moraine ridges next to the terminal ice tongues) developed on the exposed shelf.
Dune fields evolved on shelves where the climate was favorable.
Land animals spread over the emerged shelves and used them as land bridges in
some cases, for example, between Northeast Siberia and Alaska. Mammoths roamed
over the large area covered by the North Sea today. Their remains are found on the
sea bottom now, together with the tools of prehistoric hunters. Could these people of
the late Stone Age have left a memory of the effects of rapid sea level rise, in the
ubiquitous legends of a Great Flood?
5.4.2 Pleistocene Fluctuations. There is an indication that, throughout the late Pleistocene, major transgressions were much more rapid than any of the regressions. In
general, apparently, the buildup of ice (and hence the sea level drop) proceeded at a
slower rate than the melting of ice. The evidence for this concept rests on the oxygen
isotope record of pelagic foraminifera (Fig. 5.15).
How can we measure sea level changes in the chemistry of foraminiferal shells?
The principle of this method was introduced by C. Emiliani in 1955.
The shells of the foraminifera consist of calcium carbonate, CaC03. Hence, they
contain oxygen. The water within which the shells grow, H20, also contains oxygen.
There are three kinds of oxygen, the normal one, with atomic weight 16, and two rare
ones, with atomic weights 17 and 18. The oxygen-17 is very rare and we need pay no
further attention to it. Through mass spectrometry, one can determine the ratio between oxygen-16 and oxygen-I 8 (commonly written 16 0 and 18 0). The ratio of these
two isotopes in the shells is in equilibrium with the ratio of the isotopes in the water
in which the shells grew. In other words, if the 180/160 ratio in the water changes, it
will change similarly in the shell.
This fact relates to sea level change as follows. Every time the sea level drops, the
18 0/ 16 0 ratio of the water increases, because the glacial ice is made of water which
is impoverished in the isotope 18 0. Hence, seawater is enriched in 18 0 during glacials
(Fig. 5.15b). The 18 0/ 16 0 ratio of the carbonate shells reflects this change in the
chemistry of seawater. Temperature also influences the U10/160 ratio of the shells. In
Fig. 5.15a, however, the long sediment core on which the measurements were made