244
mix with cold Antarctic water. Then they flow eastward
around Australia and finally back into the North Pacific
where they come up to the surface. Minor complications of
this huge transport system of oceanic water masses, inc1uding heat and nutrient transfer, are neglected here.
This transport system can be shut down by increased
freshwater inflow into Arctic or sub-Arctic ocean
basins (e.g. the northern Atlantic and NorwegianGreenland Sea) or by a strongly reduced equator-pole
temperature gradient. Both mechanisms prevent a
sufficient increase in surface water density to establish substantial subpolar downwelling. If this happens the enormous heat transfer by the Gulf Stream
to the northern hemisphere is weakened or completely terminated. As a result, the climate in the
countries around the North Atlantic would drastically
deteriorate and the marine sediments would become
rich in cold-water organisms (e.g. diatoms) rather
than in temperate-water biota (e.g. foraminifers).
In the Pleistocene interglacial periods, invasions of temperate Atlantic waters into the Norwegian-Greenland Sea were
only short-lived and irregular (Thiede et al. 1998). The efficiency of the conveyer belt was also reduced in the Younger Dryas (a time period of about 1 ka duration, around 11
ka B.P.) in which the temperature dropped and glaciers
again prograded (Broecker 1999). In addition, submarine
ridges and highs strongly affected the conveyer belt during
the low sea level stands of the glacial maxima.
5.6.4 Effects of Low Sea Level
Sea-level lowstands were caused by several proces ses during the Earth's history (cf. Chap. 7). The
glacio-eustatic lowstands of the Pleistocene have
been intensely studied in the large ocean basins as
well as in marginal seas (cf. Chap. 4). The impact of
a lowered sea level may differ from basin to basin;
submarine ridges and swells and other topographic
features of the sea floor can strongly modify their
circulation patterns. Nevertheless, some general mIes
inferred from times of glacial maximum can also be
applied to basins outside ofthe ice-covered regions:
- The input of terrigenous material to the outer shelf,
slope, and deep-sea fans increases. The input of dissolved matter from the continents into the sea probably did not change significantly because emerged
shelf areas compensated for ice-covered areas (e.g.
Gibbs and Rumpp 1994).
- Benthic carbonate production on emerging carbonate shelves and platforms ceases. It either continues
to some extent on the slopes of these carbonate
buildups, andJor it can be partially or completely replaced by increased planktonic carbonate production,
depending on the supply of nutrients. If the entire
oceanic carbonate production decreases and river
supply of calcium remains constant, then less carbonChapter 5 Oceanic Sediments
ate dissolution in the deep sea is required for
biogenic carbonate production than during high sea
level. This means that the calcite compensation depth
(CCD) drops.
- Whether or not thermohaline circulation becomes
intensified is not clear. It appears that the response of
ocean circulation to a lowered sea level differs regionally. Furthermore, one has to distinguish between bottom currents and the behavior of intermediate water masses. During glacial times with a greater
temperature gradient between the ice-covered polar
region or polar front and the equator, winds driving
surface currents may be stronger and strengthen
upwelling. These effects can lead to a higher organic
productivity, particularly that of diatoms, in mid- and
high latitude regions. In the central equatorial Pacific, the sediments of glacials are richer in carbonate
than those of interglacials which are markedly affected by dissolution. Other ocean regions may behave in a different way. It is possible that bottom
currents rich in carbon dioxide locally dissolve more
carbonate than during high sea level.
Some of these points have been controversially discussed
(see, e.g., Struck et al 1993; Walker and Opdyke 1995;
LaMontagne et al. 1996). The mechanism of reducing atmospheric CO2 in the glacial phases, as found in the polar
ice cores, seems to be not fully c1arified. Among the various factors mentioned above, an increase or decrease in
global carbonate deposition or carbonate dissolution plays
an important role (e.g. Howard and Prell 1994). In addItion, a certain lag time between an orbital signal (change in
solar radiation) and the response of processes operating in
the ocean and atmosphere have to be taken into ac count.
It appears that the ice-core CO2 data for the last 150 ka
can be correlated with the deep-sea Ö l8 0 record preserved
in the shells of foraminifera (Berger 1996). This correlation
may allow the reconstruction of the atmospheric CO 2 of
older periods.
- The reduced surface area of the ocean and colder
water diminish evaporation and generate a dryer climate, particularly in high latitudes.
- Polar seas covered by ice may become more stagnant and poor in oxygen. Their fertility is limited and
terrigenous sediment influx is also low during this
stage.
In periods of deglaciation (glacial terminations) the
situation in high- and partially also mid-latitude regions drastically changes. Ocean circulation tends to
become instable because substantial amounts ofmeltwater reduce or inhibit downwelling in subpolar regions (e.g. Zahn et al. 1997). Oxygenation of deeper
water masses deteriorates and may lead to the deposition of layers rich in organic matter. Red deep-sea
clays are replaced by grey material. The climate of
this transitional interval is characterized by rapid
changes.
mix with cold Antarctic water. Then they flow eastward
around Australia and finally back into the North Pacific
where they come up to the surface. Minor complications of
this huge transport system of oceanic water masses, inc1uding heat and nutrient transfer, are neglected here.
This transport system can be shut down by increased
freshwater inflow into Arctic or sub-Arctic ocean
basins (e.g. the northern Atlantic and NorwegianGreenland Sea) or by a strongly reduced equator-pole
temperature gradient. Both mechanisms prevent a
sufficient increase in surface water density to establish substantial subpolar downwelling. If this happens the enormous heat transfer by the Gulf Stream
to the northern hemisphere is weakened or completely terminated. As a result, the climate in the
countries around the North Atlantic would drastically
deteriorate and the marine sediments would become
rich in cold-water organisms (e.g. diatoms) rather
than in temperate-water biota (e.g. foraminifers).
In the Pleistocene interglacial periods, invasions of temperate Atlantic waters into the Norwegian-Greenland Sea were
only short-lived and irregular (Thiede et al. 1998). The efficiency of the conveyer belt was also reduced in the Younger Dryas (a time period of about 1 ka duration, around 11
ka B.P.) in which the temperature dropped and glaciers
again prograded (Broecker 1999). In addition, submarine
ridges and highs strongly affected the conveyer belt during
the low sea level stands of the glacial maxima.
5.6.4 Effects of Low Sea Level
Sea-level lowstands were caused by several proces ses during the Earth's history (cf. Chap. 7). The
glacio-eustatic lowstands of the Pleistocene have
been intensely studied in the large ocean basins as
well as in marginal seas (cf. Chap. 4). The impact of
a lowered sea level may differ from basin to basin;
submarine ridges and swells and other topographic
features of the sea floor can strongly modify their
circulation patterns. Nevertheless, some general mIes
inferred from times of glacial maximum can also be
applied to basins outside ofthe ice-covered regions:
- The input of terrigenous material to the outer shelf,
slope, and deep-sea fans increases. The input of dissolved matter from the continents into the sea probably did not change significantly because emerged
shelf areas compensated for ice-covered areas (e.g.
Gibbs and Rumpp 1994).
- Benthic carbonate production on emerging carbonate shelves and platforms ceases. It either continues
to some extent on the slopes of these carbonate
buildups, andJor it can be partially or completely replaced by increased planktonic carbonate production,
depending on the supply of nutrients. If the entire
oceanic carbonate production decreases and river
supply of calcium remains constant, then less carbonChapter 5 Oceanic Sediments
ate dissolution in the deep sea is required for
biogenic carbonate production than during high sea
level. This means that the calcite compensation depth
(CCD) drops.
- Whether or not thermohaline circulation becomes
intensified is not clear. It appears that the response of
ocean circulation to a lowered sea level differs regionally. Furthermore, one has to distinguish between bottom currents and the behavior of intermediate water masses. During glacial times with a greater
temperature gradient between the ice-covered polar
region or polar front and the equator, winds driving
surface currents may be stronger and strengthen
upwelling. These effects can lead to a higher organic
productivity, particularly that of diatoms, in mid- and
high latitude regions. In the central equatorial Pacific, the sediments of glacials are richer in carbonate
than those of interglacials which are markedly affected by dissolution. Other ocean regions may behave in a different way. It is possible that bottom
currents rich in carbon dioxide locally dissolve more
carbonate than during high sea level.
Some of these points have been controversially discussed
(see, e.g., Struck et al 1993; Walker and Opdyke 1995;
LaMontagne et al. 1996). The mechanism of reducing atmospheric CO2 in the glacial phases, as found in the polar
ice cores, seems to be not fully c1arified. Among the various factors mentioned above, an increase or decrease in
global carbonate deposition or carbonate dissolution plays
an important role (e.g. Howard and Prell 1994). In addItion, a certain lag time between an orbital signal (change in
solar radiation) and the response of processes operating in
the ocean and atmosphere have to be taken into ac count.
It appears that the ice-core CO2 data for the last 150 ka
can be correlated with the deep-sea Ö l8 0 record preserved
in the shells of foraminifera (Berger 1996). This correlation
may allow the reconstruction of the atmospheric CO 2 of
older periods.
- The reduced surface area of the ocean and colder
water diminish evaporation and generate a dryer climate, particularly in high latitudes.
- Polar seas covered by ice may become more stagnant and poor in oxygen. Their fertility is limited and
terrigenous sediment influx is also low during this
stage.
In periods of deglaciation (glacial terminations) the
situation in high- and partially also mid-latitude regions drastically changes. Ocean circulation tends to
become instable because substantial amounts ofmeltwater reduce or inhibit downwelling in subpolar regions (e.g. Zahn et al. 1997). Oxygenation of deeper
water masses deteriorates and may lead to the deposition of layers rich in organic matter. Red deep-sea
clays are replaced by grey material. The climate of
this transitional interval is characterized by rapid
changes.