246
the closure of the Isthmus of Panama and with an
increasingly cooling climate in the upper Miocene
and Pliocene, the Norwegian Current and import of
pelagic carbonate became stronger. Ocean circulation
varied from stronger to more reduced inflow of Atlantic surface water. As a result, the sediment composition fluctuated from carbonate-rich to opal-rich sections. Periods of intensified bottom water circulation
are also evident from several hiatuses or condensed
horizons and phases of enhanced carbonate dissolution observed in drill cores. Ice-rafted material in
significant quantities and a pronounced cyclicity in
the sediments (cycles of 41 ka and later 100 ka) began not earlier than about 3 Ma B.P. along the Norwegian margin. During cold periods, the southward
prograding polar front, sea ice, and lowered sea level
led to a marked decrease in water exchange with the
Atlantic, reduced oxygenation of the deep water
masses, and sediments characterized by higher proportions of organic matter and biogenic silica.
In the Greenland Sea, i.e. in the western part of
the basin, biosiliceous sediments relatively rich in
warm-water species and marine organic matter prevailed in the lower to rniddle Miocene. High fertility
may have been caused by river-supplied nutrients
(not by upwelling). Ice-rafted debris in deep-sea sediments can be traced back to the rniddle Miocene. In
the Fram Strait it first appeared 14 Ma B.P.
Simultaneously, bottom water from the Norwegian-Greenland Sea began to flow over the Greenland-Scotland Ridge
into the North Atlantic Ocean and caused the formation of
very large, e10ngate sediment ridges (or "sediment drifts";
Fig. 4.3c) described in Sect. 5.5.
5.6.6 Tbe Cretaceous Ocean
The Mesozoic "greenhouse" world was characterized
by a warm, relatively equal climate with a reduced
temperature gradient between the poles and the equator. The CO2 content of the atmosphere nay have
been four times that of the present-day, and the average global temperature is estimated to have been
higher by about 6°C. The South Atlantic opened during the Late Jurassic and Early Cretaceous, but the
geographical position and morphology of the other
ocean basins also differed from the present situation.
The circum-equatorial Tethys Ocean was still wide
and therefore significantly influenced the global
ocean circulation. However, ocean circulation was
even more controlled by regional differences in salinity, reinforced by high freshwater inflow from "wet"
continental areas. Temperature-induced changes in
water density probably were less important. Thus,
thermohaline circulation was fundamentally different
from the present situation, which does not differ profoundly from the that of earlier icehouse states in the
Earth's history (cf. Sect. 7.8 and Fig. 7.16).
Chapter 5 Oceanic Sediments
The Cretaceous oceans tended to develop wann,
stagnant and more saline bottom waters with temperatures sirnilar to those of surface waters. These waters presumably formed in low-Iatitude regions where
evaporation was high on shelves and, due to high sea
level, flooded wide land areas. Thus, most of the oceanic circulation at this time may have been restricted
to surface and intermediate waters. Some upwelling
of intermediate waters possibly occurred in polar and
subpolar regions, and the climatic belts on the continents, e.g. the location of deserts, probably differed
significantly from the present situation. Some of the
consequences of such a pattern are mentioned in
Sects. 10.3.3 and 7.8.
Warmer oceans with slower circulation caused
poorly oxygenated waters and thus promoted the deposition of black shales (cf. Sects. 5.2 and 10.3.3),
either in basins of stagnant deep water or in enlarged
zones of weak coastal upwelling and oceanic divergence (e.g. Parrish 1987). At the same time, the vegetation. cover on the continents expanded into high
latitude regions and may have contributed to the enrichment of organic matter in marine sediments (e.g.
Stein et al. 1989).
The situation ofthe Middle and Upper Cretaceous and particularly the Cretaceous "anoxie events" have been discussed by many workers (e.g., Thierstein 1979; Berger
1981; De Graciansky et al. 1982; Hay 1987, 1997; several
papers in Brooks and Fleet 1987; Larsen 1991; Arthur and
Sageman 1994; Bralower et al. 1994; Barron et al. 1995;
Norris and Wilson 1998; Poulsen et al. 1998; and many
others).
In the mid-Cretaceous, rudists experienced a rapid evolution and expansion in the shallow parts of the warm
"Supertethys" while hermatotypic corals retreated to marginal regions of this ocean (e.g. Stanley 1995). Black shale
deposition in deep water was accompanied by a decrease in
the ö l3 C values of carbonate rocks (e.g. Arthur et al. 1988;
discussed further, e.g., by Menegatti et al. 1998) reflecting
the transfer of sedimentary organic carbon to the oxidized
inorganic carbon in limestones. At the same time, sulfur
was transferred from evaporitic sulfate rocks to the reduced
sulfur reservoir of pyrite in sediments, resulting in an increase of Ö 34 S values of evaporitic sulfate minerals (Mackenzie 1990). In addition, oolitic ironstones and
phosphorites appear to have been formed in re1ativeiy large
quantities (V an Houten and Arthur 1989).
Adjacent or marginal seas of the Cretaceous oceans
should also have recorded the greenhouse state of the
globe. The question whether the Cretaceous Western Interior seaway ofNorth America had a brackish lid and/or was
subjected to estuarine circulation is still open (e.g.
Slingeriand et al. 1998; Jewell 1998). A mid-Cretaceous
rapid lateral facies change between calcareous and
noncalcareous shale, observed in Montana, is thought to
have been generated at the boundary between two different
water masses (Fisher et al. 1994). One of these entered
from the Arctic Ocean, the other from the Tethys Ocean.
Toward the end of the Cretaceous, the climate seems
to have become unstable. According to a stable iso-
the closure of the Isthmus of Panama and with an
increasingly cooling climate in the upper Miocene
and Pliocene, the Norwegian Current and import of
pelagic carbonate became stronger. Ocean circulation
varied from stronger to more reduced inflow of Atlantic surface water. As a result, the sediment composition fluctuated from carbonate-rich to opal-rich sections. Periods of intensified bottom water circulation
are also evident from several hiatuses or condensed
horizons and phases of enhanced carbonate dissolution observed in drill cores. Ice-rafted material in
significant quantities and a pronounced cyclicity in
the sediments (cycles of 41 ka and later 100 ka) began not earlier than about 3 Ma B.P. along the Norwegian margin. During cold periods, the southward
prograding polar front, sea ice, and lowered sea level
led to a marked decrease in water exchange with the
Atlantic, reduced oxygenation of the deep water
masses, and sediments characterized by higher proportions of organic matter and biogenic silica.
In the Greenland Sea, i.e. in the western part of
the basin, biosiliceous sediments relatively rich in
warm-water species and marine organic matter prevailed in the lower to rniddle Miocene. High fertility
may have been caused by river-supplied nutrients
(not by upwelling). Ice-rafted debris in deep-sea sediments can be traced back to the rniddle Miocene. In
the Fram Strait it first appeared 14 Ma B.P.
Simultaneously, bottom water from the Norwegian-Greenland Sea began to flow over the Greenland-Scotland Ridge
into the North Atlantic Ocean and caused the formation of
very large, e10ngate sediment ridges (or "sediment drifts";
Fig. 4.3c) described in Sect. 5.5.
5.6.6 Tbe Cretaceous Ocean
The Mesozoic "greenhouse" world was characterized
by a warm, relatively equal climate with a reduced
temperature gradient between the poles and the equator. The CO2 content of the atmosphere nay have
been four times that of the present-day, and the average global temperature is estimated to have been
higher by about 6°C. The South Atlantic opened during the Late Jurassic and Early Cretaceous, but the
geographical position and morphology of the other
ocean basins also differed from the present situation.
The circum-equatorial Tethys Ocean was still wide
and therefore significantly influenced the global
ocean circulation. However, ocean circulation was
even more controlled by regional differences in salinity, reinforced by high freshwater inflow from "wet"
continental areas. Temperature-induced changes in
water density probably were less important. Thus,
thermohaline circulation was fundamentally different
from the present situation, which does not differ profoundly from the that of earlier icehouse states in the
Earth's history (cf. Sect. 7.8 and Fig. 7.16).
Chapter 5 Oceanic Sediments
The Cretaceous oceans tended to develop wann,
stagnant and more saline bottom waters with temperatures sirnilar to those of surface waters. These waters presumably formed in low-Iatitude regions where
evaporation was high on shelves and, due to high sea
level, flooded wide land areas. Thus, most of the oceanic circulation at this time may have been restricted
to surface and intermediate waters. Some upwelling
of intermediate waters possibly occurred in polar and
subpolar regions, and the climatic belts on the continents, e.g. the location of deserts, probably differed
significantly from the present situation. Some of the
consequences of such a pattern are mentioned in
Sects. 10.3.3 and 7.8.
Warmer oceans with slower circulation caused
poorly oxygenated waters and thus promoted the deposition of black shales (cf. Sects. 5.2 and 10.3.3),
either in basins of stagnant deep water or in enlarged
zones of weak coastal upwelling and oceanic divergence (e.g. Parrish 1987). At the same time, the vegetation. cover on the continents expanded into high
latitude regions and may have contributed to the enrichment of organic matter in marine sediments (e.g.
Stein et al. 1989).
The situation ofthe Middle and Upper Cretaceous and particularly the Cretaceous "anoxie events" have been discussed by many workers (e.g., Thierstein 1979; Berger
1981; De Graciansky et al. 1982; Hay 1987, 1997; several
papers in Brooks and Fleet 1987; Larsen 1991; Arthur and
Sageman 1994; Bralower et al. 1994; Barron et al. 1995;
Norris and Wilson 1998; Poulsen et al. 1998; and many
others).
In the mid-Cretaceous, rudists experienced a rapid evolution and expansion in the shallow parts of the warm
"Supertethys" while hermatotypic corals retreated to marginal regions of this ocean (e.g. Stanley 1995). Black shale
deposition in deep water was accompanied by a decrease in
the ö l3 C values of carbonate rocks (e.g. Arthur et al. 1988;
discussed further, e.g., by Menegatti et al. 1998) reflecting
the transfer of sedimentary organic carbon to the oxidized
inorganic carbon in limestones. At the same time, sulfur
was transferred from evaporitic sulfate rocks to the reduced
sulfur reservoir of pyrite in sediments, resulting in an increase of Ö 34 S values of evaporitic sulfate minerals (Mackenzie 1990). In addition, oolitic ironstones and
phosphorites appear to have been formed in re1ativeiy large
quantities (V an Houten and Arthur 1989).
Adjacent or marginal seas of the Cretaceous oceans
should also have recorded the greenhouse state of the
globe. The question whether the Cretaceous Western Interior seaway ofNorth America had a brackish lid and/or was
subjected to estuarine circulation is still open (e.g.
Slingeriand et al. 1998; Jewell 1998). A mid-Cretaceous
rapid lateral facies change between calcareous and
noncalcareous shale, observed in Montana, is thought to
have been generated at the boundary between two different
water masses (Fisher et al. 1994). One of these entered
from the Arctic Ocean, the other from the Tethys Ocean.
Toward the end of the Cretaceous, the climate seems
to have become unstable. According to a stable iso-