5.6 Paleoceanography
5.6.5 Oceanic Gateways and Paleo-Ocean
Circulation
All observations discussed in Sect. 5.2 have pertained to modem oceans under the present climatic
conditions. Thermohaline circulation is driven by the
strong temperature gradient from the poles to the
equator causing the formation of cold bottom water
in the polar regions. This and the present-day configuration of the ocean basins prevent, for example, the
existence of large bodies of stagnant bottom water,
apart from adjacent basins and very limited coastal
sea areas (Chap. 4).
However, the shape of the ocean basins and the
location of submarine ridges and swells, island
chains, and oceanic gateways changed in the course
of the geologie history.
The development of oceanic gateways and their influence
on marine sediments over the Earth's history is a topic of
current research (see, e.g., Seibold and Berger 1996;
Heinze and Crowley 1997).
The impact of these changes on ocean cireulation and
sediments is first demonstrated here for some oeeanie
gateways, later for an entire oeean basin.
Figure 5.27b shows a paleogeographie sketch map
for the Eocene including several oeeanic gateways.
During this time, the global climate was warmer than
today, and a circum-equatorial ocean (Tethys) and
current system allowed free water exchange between
alliarge oeean basins.
Later, the eircum-equatorial gateways closed and
several higher-Iatitude gateways opened. This led to
a fundamental reorganization of the thermohaline
oceanic circulation. Since that time the three ocean
basins are fully connected only far in the south and
therefore more separated than before. Some results of
these teetonically controlled events are demonstrated
by the following examples.
The Drake Passage. The opening of a seaway, the
Drake Passage, between South America and
Antarctica around 32-30 Ma B.P. caused, in combination with the Tasmanian seaway, a drastic change
in the ocean circulation of the southern hemisphere.
Since that time, a circum-Antarctic circulation system
was established which strongly reduced the heat
transfer from low-Iatitude regions to Antaretica. As a
result, large and thick ice sheets could grow which
reinforced the albedo and promoted global eooling.
However, the outflow of Antaretic deep bottom water was weakened, except in the eastern South Atlantic. The Neogene sediments around Antarctica and in
the neighboring southern ocean basins refleet increased, but varying terrigenous sediment supply including ice-rafted material. Waters rich in nutrients
enhanced the production of biosiliceous material
245
which is well preserved in the sediments. The CCD
dropped by about 1000 m.
The Isthmus 01 Panama. Prior to the closure of this
gateway (around 5 Ma ago), a seaway existed between the North and South American continents allowing direct water exchange between the Atlantic
and the Pacific. Tradewind-driven, westward directed
warm surface currents from the Atlantic entered the
Pacific. Furthermore, it has been postulated that relatively eold Pacific waters flowed into the Caribbean
Sea for some time (Nesbitt and Y oung 1997). The
closure of the seaway led to a reorganization of the
global conveyer-belt eirculation including the Atlantic cireulation system. As a result, the intensified
Gulf Stream transported warm and relatively saline
water masses to the North Atlantic. These in turn
favored deep-water formation in the northern oceans
(e.g. in the Labrador Sea) and strengthened
southward-directed Aretie bottom water flow. Enhanced atmospheric moisture, induced by the Gulf
Stream, has probably promoted ice-sheet growth in
the northern hemisphere since 3.1-2.5 Ma.
Ocean drilling in the Caribbean Sea has revealed that the
planktonic foraminiferal assemblages changed from those
of nonnal saline water to assemblages indicating an increased surface water salinity in the time period from about
7 to 2 Ma (Haug and Tiedemann 1998). Increased oxygen
supply of the bottom water is indicated by better carbonate
preservation since the c\osure ofthe Panama Strait.
The Norwegian-Greenland Sea. The depositional
environment of this basin was not only controlled by
its asymmetrie surface current system (cf. Sect. 4.2.5
and Fig. 4.3c), but also by the Greenland-Scotland
Ridge in the south and the Fram Strait in the north
connecting it with the Arctic Ocean. These topographie features in turn strongly modified the
thermohaline circulation during low sea-Ievel stands.
The present -day situation of this basin is briefly described
in Sect. 4.2.5. The Pleistocene and Neogene history was
explored by sediment coring and deep-sea drilling (e.g.
Bohnnann et al. 1990; Jansen and Raymo 1996; Hebbien
and Wefer 1997; Henrich 1998; Thiede et al. 1998).
The overflow of water from the north was largely controlled by the sill depth of the Greenland-Scotland Ridge
which in turn was episodically affected by mantle plume
activity in the Neogene (Wright and Miller 1996). High
plume aetivity tended to separate the Norwegian-Greenland
Sea from the North Atlantic, whereas times of low activity
(in the early Miocene and early Pliocene) allowed better
water exchange over the ridge and thus caused climatic
optima in the northem basin. This example also demonstrates the great influence of minor topographic changes
within an oceanic gateway on a large basin.
In the Neogene to Quaternary, the Norwegian Sea
(i.e. the eastern basin) was characterized by both calcareous and siliceous sediment eomponents. After
5.6.5 Oceanic Gateways and Paleo-Ocean
Circulation
All observations discussed in Sect. 5.2 have pertained to modem oceans under the present climatic
conditions. Thermohaline circulation is driven by the
strong temperature gradient from the poles to the
equator causing the formation of cold bottom water
in the polar regions. This and the present-day configuration of the ocean basins prevent, for example, the
existence of large bodies of stagnant bottom water,
apart from adjacent basins and very limited coastal
sea areas (Chap. 4).
However, the shape of the ocean basins and the
location of submarine ridges and swells, island
chains, and oceanic gateways changed in the course
of the geologie history.
The development of oceanic gateways and their influence
on marine sediments over the Earth's history is a topic of
current research (see, e.g., Seibold and Berger 1996;
Heinze and Crowley 1997).
The impact of these changes on ocean cireulation and
sediments is first demonstrated here for some oeeanie
gateways, later for an entire oeean basin.
Figure 5.27b shows a paleogeographie sketch map
for the Eocene including several oeeanic gateways.
During this time, the global climate was warmer than
today, and a circum-equatorial ocean (Tethys) and
current system allowed free water exchange between
alliarge oeean basins.
Later, the eircum-equatorial gateways closed and
several higher-Iatitude gateways opened. This led to
a fundamental reorganization of the thermohaline
oceanic circulation. Since that time the three ocean
basins are fully connected only far in the south and
therefore more separated than before. Some results of
these teetonically controlled events are demonstrated
by the following examples.
The Drake Passage. The opening of a seaway, the
Drake Passage, between South America and
Antarctica around 32-30 Ma B.P. caused, in combination with the Tasmanian seaway, a drastic change
in the ocean circulation of the southern hemisphere.
Since that time, a circum-Antarctic circulation system
was established which strongly reduced the heat
transfer from low-Iatitude regions to Antaretica. As a
result, large and thick ice sheets could grow which
reinforced the albedo and promoted global eooling.
However, the outflow of Antaretic deep bottom water was weakened, except in the eastern South Atlantic. The Neogene sediments around Antarctica and in
the neighboring southern ocean basins refleet increased, but varying terrigenous sediment supply including ice-rafted material. Waters rich in nutrients
enhanced the production of biosiliceous material
245
which is well preserved in the sediments. The CCD
dropped by about 1000 m.
The Isthmus 01 Panama. Prior to the closure of this
gateway (around 5 Ma ago), a seaway existed between the North and South American continents allowing direct water exchange between the Atlantic
and the Pacific. Tradewind-driven, westward directed
warm surface currents from the Atlantic entered the
Pacific. Furthermore, it has been postulated that relatively eold Pacific waters flowed into the Caribbean
Sea for some time (Nesbitt and Y oung 1997). The
closure of the seaway led to a reorganization of the
global conveyer-belt eirculation including the Atlantic cireulation system. As a result, the intensified
Gulf Stream transported warm and relatively saline
water masses to the North Atlantic. These in turn
favored deep-water formation in the northern oceans
(e.g. in the Labrador Sea) and strengthened
southward-directed Aretie bottom water flow. Enhanced atmospheric moisture, induced by the Gulf
Stream, has probably promoted ice-sheet growth in
the northern hemisphere since 3.1-2.5 Ma.
Ocean drilling in the Caribbean Sea has revealed that the
planktonic foraminiferal assemblages changed from those
of nonnal saline water to assemblages indicating an increased surface water salinity in the time period from about
7 to 2 Ma (Haug and Tiedemann 1998). Increased oxygen
supply of the bottom water is indicated by better carbonate
preservation since the c\osure ofthe Panama Strait.
The Norwegian-Greenland Sea. The depositional
environment of this basin was not only controlled by
its asymmetrie surface current system (cf. Sect. 4.2.5
and Fig. 4.3c), but also by the Greenland-Scotland
Ridge in the south and the Fram Strait in the north
connecting it with the Arctic Ocean. These topographie features in turn strongly modified the
thermohaline circulation during low sea-Ievel stands.
The present -day situation of this basin is briefly described
in Sect. 4.2.5. The Pleistocene and Neogene history was
explored by sediment coring and deep-sea drilling (e.g.
Bohnnann et al. 1990; Jansen and Raymo 1996; Hebbien
and Wefer 1997; Henrich 1998; Thiede et al. 1998).
The overflow of water from the north was largely controlled by the sill depth of the Greenland-Scotland Ridge
which in turn was episodically affected by mantle plume
activity in the Neogene (Wright and Miller 1996). High
plume aetivity tended to separate the Norwegian-Greenland
Sea from the North Atlantic, whereas times of low activity
(in the early Miocene and early Pliocene) allowed better
water exchange over the ridge and thus caused climatic
optima in the northem basin. This example also demonstrates the great influence of minor topographic changes
within an oceanic gateway on a large basin.
In the Neogene to Quaternary, the Norwegian Sea
(i.e. the eastern basin) was characterized by both calcareous and siliceous sediment eomponents. After