5.6 Paleoceanography
Sea, and the Mediterranean Sea, are found in nine special
volumes edited byNaim AEM, Stehli FG, Uyeda S (19731985, Plenum Press, New York). Concise sumrnaries
stressing principal processes have been provided, e.g., by
Seibold and Berger (1996) and Hay et al. (1997). Various
aspects of the relatively young evolution of the South Atlantic are discussed in Wefer et al. (1996).
5.6.2 Processes Influencing Climate, Ocean
Circulation, and Marine Sediments
Many workers have asked the question: Is the present
the key to the past? In other words: are the observations we can rnake in the modem world also valid for
times in which the Earth and its climate were quite
different from the present situation? We are stillliving in a "glacial world" and have to take into account
that the modem oceans are poor analogs for long
time spans in the Earth's history (e.g. the Mesozoie
and Paleogene). What we can assurne with confidence is that the general physical and chemical laws
also ruled in the geological past. However, the interaction between the different compartments of the
Earth, such as that of the atmosphere and the oceans,
operated in a modified way and at different rates.
The major processes influencing the interaction of
clirnate and ocean circulation and thus also marine
sediments are:
- Solar insolation, i.e. radiation received from the
Sun, modified by the orbital parameters of the Earth
(Milankovitch cyclicity, cf. Sect. 7.8).
- Atmospheric greenhouse gases, absorbing incoming or outgoing radiation.
- Changes in ocean circulation (the present-day topography of the ocean basins remaining unchanged).
- Paleogeography, i .. e. changes in the configuration
of the ocean basins and continents, the location of
oceanic gateways, high mountain ranges, etc.
- Variations in both the vegetation on land, i.e. its
nature and areal extent, and in the organic production
of the ocean basins.
Some of these points are briefly discussed below. For
more details see, e.g., the references mentioned
above including Hay et a1. (1997).
Solar Radiation and Feedback Systems
The variations in solar radiation can be amplified or
subdued by the so-called feedback system of the
Earth. If the Earth has reached astate in which polar
ice can just begin to form, then a minor reduction in
solar radiation can cause the onset of ice sheet
growth. The growing ice sheets and, in addition,
growing desert areas reflect more solar energy (i.e.
they have a high albedo) than the surfaces of water
239
and densely vegetated land. Thus, they amplify the
effect of reduced solar radiation (positive feedback).
Conversely, an increase in solar radiation may trigger
the melting of pre-existing ice and thus diminish the
albedo of the Earth's surface, leading to global warming. A rising sea level flooding land can reinforce
this effect and enable additional warming.
A negative feedback is brought about, when the
global climate warrns due to an increase in atmospheric carbon dioxide, but the area covered by vegetation simultaneously grows and stores more atmospheric carbon than before. Similarly, higher organic
production in the ocean can reduce the atmospheric
carbon dioxide content (see below) and thus contribute to climate stabilization.
Greenhouse Gases
Atmospheric greenhouse gases comprise all air molecules with more than two atoms, such as H20, 03'
CO 2 , CH 4 and man-made gaseous compounds.
Water vapor is the most effective greenhouse gas. Its
concentration in the atmosphere strongly depends on
the air temperature. Presently it reaches relatively
high concentrations in warm equatorial and tropical
regions. During the Mesozoic, the vapor pressure for
H 2 0 in the atmosphere was generally higher than
today, particularly so in high latitudes where the temperatures dropped hardly deeper than to the freezing
point. This allowed the transfer of more heat via the
atmosphere over the entire globe; the cloud cover
was denser and absorbed more radiation (positive
feedback); the hydrological cycle became intensified
and the globe wetter than today.
Carbon dioxide is thought to be the second important
greenhouse gas. In the late Pleistocene the atmospheric CO 2 content varied by about 30% from glacial to interglacial intervals (with higher values in the
interglacials; based on "fossil" air trapped in ice
cores). Of several processes proposed as an explanation of this finding, exchange of CO 2 between the
atmosphere and the ocean, holding 50 times more
carbon dioxide than the atmosphere, is important
("biological pump" and "solution pump, see below).
In the greenhouse state of the Earth, the CO 2 content, mont likely supplied by enhanced volcanic activity, is assumed to have been as much as 4 to 5 times
higher than at present. The reduced solubility of CO 2
in warmer ocean water may have played a role in this
respect.
Methane is produced by microorganisms in swamps
and tundra regions as weIl as in marine sediments
containing organic matter. It is stored in large quantities as solid crystallized gas hydrate in the sediments
Sea, and the Mediterranean Sea, are found in nine special
volumes edited byNaim AEM, Stehli FG, Uyeda S (19731985, Plenum Press, New York). Concise sumrnaries
stressing principal processes have been provided, e.g., by
Seibold and Berger (1996) and Hay et al. (1997). Various
aspects of the relatively young evolution of the South Atlantic are discussed in Wefer et al. (1996).
5.6.2 Processes Influencing Climate, Ocean
Circulation, and Marine Sediments
Many workers have asked the question: Is the present
the key to the past? In other words: are the observations we can rnake in the modem world also valid for
times in which the Earth and its climate were quite
different from the present situation? We are stillliving in a "glacial world" and have to take into account
that the modem oceans are poor analogs for long
time spans in the Earth's history (e.g. the Mesozoie
and Paleogene). What we can assurne with confidence is that the general physical and chemical laws
also ruled in the geological past. However, the interaction between the different compartments of the
Earth, such as that of the atmosphere and the oceans,
operated in a modified way and at different rates.
The major processes influencing the interaction of
clirnate and ocean circulation and thus also marine
sediments are:
- Solar insolation, i.e. radiation received from the
Sun, modified by the orbital parameters of the Earth
(Milankovitch cyclicity, cf. Sect. 7.8).
- Atmospheric greenhouse gases, absorbing incoming or outgoing radiation.
- Changes in ocean circulation (the present-day topography of the ocean basins remaining unchanged).
- Paleogeography, i .. e. changes in the configuration
of the ocean basins and continents, the location of
oceanic gateways, high mountain ranges, etc.
- Variations in both the vegetation on land, i.e. its
nature and areal extent, and in the organic production
of the ocean basins.
Some of these points are briefly discussed below. For
more details see, e.g., the references mentioned
above including Hay et a1. (1997).
Solar Radiation and Feedback Systems
The variations in solar radiation can be amplified or
subdued by the so-called feedback system of the
Earth. If the Earth has reached astate in which polar
ice can just begin to form, then a minor reduction in
solar radiation can cause the onset of ice sheet
growth. The growing ice sheets and, in addition,
growing desert areas reflect more solar energy (i.e.
they have a high albedo) than the surfaces of water
239
and densely vegetated land. Thus, they amplify the
effect of reduced solar radiation (positive feedback).
Conversely, an increase in solar radiation may trigger
the melting of pre-existing ice and thus diminish the
albedo of the Earth's surface, leading to global warming. A rising sea level flooding land can reinforce
this effect and enable additional warming.
A negative feedback is brought about, when the
global climate warrns due to an increase in atmospheric carbon dioxide, but the area covered by vegetation simultaneously grows and stores more atmospheric carbon than before. Similarly, higher organic
production in the ocean can reduce the atmospheric
carbon dioxide content (see below) and thus contribute to climate stabilization.
Greenhouse Gases
Atmospheric greenhouse gases comprise all air molecules with more than two atoms, such as H20, 03'
CO 2 , CH 4 and man-made gaseous compounds.
Water vapor is the most effective greenhouse gas. Its
concentration in the atmosphere strongly depends on
the air temperature. Presently it reaches relatively
high concentrations in warm equatorial and tropical
regions. During the Mesozoic, the vapor pressure for
H 2 0 in the atmosphere was generally higher than
today, particularly so in high latitudes where the temperatures dropped hardly deeper than to the freezing
point. This allowed the transfer of more heat via the
atmosphere over the entire globe; the cloud cover
was denser and absorbed more radiation (positive
feedback); the hydrological cycle became intensified
and the globe wetter than today.
Carbon dioxide is thought to be the second important
greenhouse gas. In the late Pleistocene the atmospheric CO 2 content varied by about 30% from glacial to interglacial intervals (with higher values in the
interglacials; based on "fossil" air trapped in ice
cores). Of several processes proposed as an explanation of this finding, exchange of CO 2 between the
atmosphere and the ocean, holding 50 times more
carbon dioxide than the atmosphere, is important
("biological pump" and "solution pump, see below).
In the greenhouse state of the Earth, the CO 2 content, mont likely supplied by enhanced volcanic activity, is assumed to have been as much as 4 to 5 times
higher than at present. The reduced solubility of CO 2
in warmer ocean water may have played a role in this
respect.
Methane is produced by microorganisms in swamps
and tundra regions as weIl as in marine sediments
containing organic matter. It is stored in large quantities as solid crystallized gas hydrate in the sediments