5.6 Paleoceanography
tope study on sediment cores from the mid-latitude
South Atlantic, a major warm pulse was followed by
rapid cooling prior to the Cretaceous-Tertiary boundary (Li and Keller (1998). In the formerly widely
opened Caribbean Sea, W-flowing bottom currents,
measured in uplifted flysch deposits, indicate the
presence of deep-water circulation during this time
(Stanley 1988). These contour currents affected
flysch deposits which were shed from north to south
into the deep basin.
5.6.7 The post-Cretaceous Cooling
The paleoceanographic and climatic development of
the post-Cretaceous world was characterized by continued plate-tectonic motions, fundamental changes
in the oceanic thermohaline circulation as mentioned
above, a trend to falling sea level (caused by decreasing ocean spreading and mountain building), and
cooling. After the tropical oceanic gateways had been
successively closed, the circum-tropical connection
of the Pacific and Atlantic ocean basins was terminated and replaced by the present-day restricted water exchange in the south ("southern exchange").
The rates of ocean spreading and subduction, as
weH as the activity of large mantle plumes, tended to
slow since the mid-Cretaceous. As a result, the rate
of outgasing greenhouse gase"s (mainly CO 2 ) into the
atmosphere decreased with time. Due to falling temperature, evaporation and cloudiness simultaneously
declined and thus weakened the greenhouse effect of
water vapor. The greenhouse effect of the atmospheric pC0 2 could be further reduced by:
- Uptake of CO 2 by the cooling ocean (the solution
pump mentioned above).
- Increase in the rate of silicate weathering, promoted by widely exposed plutonic and crystaHine
rocks and high runoff.
- The biological pump and organic carbon burial in
the ocean and in lake basins (see above). This process appears to have been particularly important because the sedimentary organic carbon reservoir has
grown since the Neogene (Derry and France-Lanord
1996).
- Dissolution of oceanic calcium carbonate (consumption of CO 2 ) and/or reduced production of
biogenic calcium carbonate in relation to earlier periods (less release of CO 2 ).
The cooling trend was reinforced by an increasing
albedo (falling relative sea level, reduction in cloudiness).
Processes releasing CO 2 into the atmosphere and
thus counteracting this trend, have obviously been
insufficient to prevent further cooling. These proces ses include:
247
- An excess of carbonate burial (including subduction of carbonate), releasing carbon dioxide, in
relation to carbonate weathering on the continents
and carbonate dissolution in the deep sea, consuming
carbon dioxide.
- Soil deterioration leading to oxidation of organic
matter.
- Reduction of the plant mass on the continents, including forest destruction by fire.
The complex global carbon cycle and the CO 2 balance have
been discussed since about two decades by many workers.
Recent publications include, e.g., Bemer (1994, 1997),
Kerrick and Caldeira (1994); Raymo (1994), Beck et al.
(1995), Keir (1995), Compton and Mallinson (1996). Two
problems stand out as particularly difficult: (1) the great
number of factors influencing ocean circulation, organic
production, production and dissolution of biogenic carbonate, and climate, and (2) the reliable quantification of the
individual processes and their interrelationships.
The present-day knowledge of the climatic and paleoceanographic evolution also relies significantly on studies
of the oxygen and carbon isotopes of planktonic and benthic foraminifera. Further information is gained from the
nature (including biomarkers) of the organic matter preserved in sediments. An increasing number of oceanographic models try to simulate both paleo-circulation and
the distribution of certain sediment types in the ocean.
The warm climate of the Cretaceous more or less
continued up to the Paleocene in which a new peak
in temperature was reached (as, e.g., in sediments of
the Caribbean Sea, mentioned above). Carbonate deposition was restricted to the shelves and continental
slopes, and biosiliceous sediments were produced in
widespread ocean regions, including low-latitudes.
Thereafter, the cooling of the globe took place in
several distinct steps (e.g. Berger and Wefer 1996):
- At the end ofthe Eocene, 35-40 Ma B.P.: begin of
deep-water cooling; switch from low-latitude to highlatitude deep-water formation; deepening of CCD
due to regression of the sea; first evidence of ice in
East Antarctica. Somewhat later: opening of the
Drake Passage and separation of Australia from
Antarctica; restriction of the circum-equatorial ocean.
- About 14 Ma B.P.: accelerated growth ofice sheets
in Antarctica, including West Antarctica; further decrease in atmospheric pC0 2 due to the highly effective biological pump (Monterey Event, mentioned
above); increased diatom production in high latitudes. The closure of the Panama Strait delays northern glaciation.
- About 3 Ma B.P.: buildup of large ice sheets in the
northern hemisphere.
The increasing meridional temperature gradients
caused increased North Atlantic deep-water formation and hence also more nutrient supply to
upwelling regions. North Atlantic deep water reached
tope study on sediment cores from the mid-latitude
South Atlantic, a major warm pulse was followed by
rapid cooling prior to the Cretaceous-Tertiary boundary (Li and Keller (1998). In the formerly widely
opened Caribbean Sea, W-flowing bottom currents,
measured in uplifted flysch deposits, indicate the
presence of deep-water circulation during this time
(Stanley 1988). These contour currents affected
flysch deposits which were shed from north to south
into the deep basin.
5.6.7 The post-Cretaceous Cooling
The paleoceanographic and climatic development of
the post-Cretaceous world was characterized by continued plate-tectonic motions, fundamental changes
in the oceanic thermohaline circulation as mentioned
above, a trend to falling sea level (caused by decreasing ocean spreading and mountain building), and
cooling. After the tropical oceanic gateways had been
successively closed, the circum-tropical connection
of the Pacific and Atlantic ocean basins was terminated and replaced by the present-day restricted water exchange in the south ("southern exchange").
The rates of ocean spreading and subduction, as
weH as the activity of large mantle plumes, tended to
slow since the mid-Cretaceous. As a result, the rate
of outgasing greenhouse gase"s (mainly CO 2 ) into the
atmosphere decreased with time. Due to falling temperature, evaporation and cloudiness simultaneously
declined and thus weakened the greenhouse effect of
water vapor. The greenhouse effect of the atmospheric pC0 2 could be further reduced by:
- Uptake of CO 2 by the cooling ocean (the solution
pump mentioned above).
- Increase in the rate of silicate weathering, promoted by widely exposed plutonic and crystaHine
rocks and high runoff.
- The biological pump and organic carbon burial in
the ocean and in lake basins (see above). This process appears to have been particularly important because the sedimentary organic carbon reservoir has
grown since the Neogene (Derry and France-Lanord
1996).
- Dissolution of oceanic calcium carbonate (consumption of CO 2 ) and/or reduced production of
biogenic calcium carbonate in relation to earlier periods (less release of CO 2 ).
The cooling trend was reinforced by an increasing
albedo (falling relative sea level, reduction in cloudiness).
Processes releasing CO 2 into the atmosphere and
thus counteracting this trend, have obviously been
insufficient to prevent further cooling. These proces ses include:
247
- An excess of carbonate burial (including subduction of carbonate), releasing carbon dioxide, in
relation to carbonate weathering on the continents
and carbonate dissolution in the deep sea, consuming
carbon dioxide.
- Soil deterioration leading to oxidation of organic
matter.
- Reduction of the plant mass on the continents, including forest destruction by fire.
The complex global carbon cycle and the CO 2 balance have
been discussed since about two decades by many workers.
Recent publications include, e.g., Bemer (1994, 1997),
Kerrick and Caldeira (1994); Raymo (1994), Beck et al.
(1995), Keir (1995), Compton and Mallinson (1996). Two
problems stand out as particularly difficult: (1) the great
number of factors influencing ocean circulation, organic
production, production and dissolution of biogenic carbonate, and climate, and (2) the reliable quantification of the
individual processes and their interrelationships.
The present-day knowledge of the climatic and paleoceanographic evolution also relies significantly on studies
of the oxygen and carbon isotopes of planktonic and benthic foraminifera. Further information is gained from the
nature (including biomarkers) of the organic matter preserved in sediments. An increasing number of oceanographic models try to simulate both paleo-circulation and
the distribution of certain sediment types in the ocean.
The warm climate of the Cretaceous more or less
continued up to the Paleocene in which a new peak
in temperature was reached (as, e.g., in sediments of
the Caribbean Sea, mentioned above). Carbonate deposition was restricted to the shelves and continental
slopes, and biosiliceous sediments were produced in
widespread ocean regions, including low-latitudes.
Thereafter, the cooling of the globe took place in
several distinct steps (e.g. Berger and Wefer 1996):
- At the end ofthe Eocene, 35-40 Ma B.P.: begin of
deep-water cooling; switch from low-latitude to highlatitude deep-water formation; deepening of CCD
due to regression of the sea; first evidence of ice in
East Antarctica. Somewhat later: opening of the
Drake Passage and separation of Australia from
Antarctica; restriction of the circum-equatorial ocean.
- About 14 Ma B.P.: accelerated growth ofice sheets
in Antarctica, including West Antarctica; further decrease in atmospheric pC0 2 due to the highly effective biological pump (Monterey Event, mentioned
above); increased diatom production in high latitudes. The closure of the Panama Strait delays northern glaciation.
- About 3 Ma B.P.: buildup of large ice sheets in the
northern hemisphere.
The increasing meridional temperature gradients
caused increased North Atlantic deep-water formation and hence also more nutrient supply to
upwelling regions. North Atlantic deep water reached