that have influenced areas of up to 100,000 km
2
, there
have probably been a large number of smaller slides.
Climate coupling
Climate plays a prominent role in driving the processes
that shape the environment, and it also interacts closely
with ocean circulation and productivity. The climateinduced fluctuations of sea-level alone, with swings of
over 100 m during the Late Quaternary glacial cycles
causing broad shelf regions to alternately be dry and part
of the land and then flooded and covered by the sea, highlight the sensitivity of the ocean margins to climate. In
addition to the fact that these areas represent extremely
active transitional areas between the oceans and continents, the presence of high sedimentation rates due to the
input of terrigenous sediments is a very important
characteristic.
Direct climatological measurements, which are critical
for a basic understanding of such processes, only go back
to the year 1850. Records extending farther back in time
are documented primarily in the ocean margin sediments.
The ocean margin sediments clearly provide an impressive archive of past climatic, productivity, current, and
sedimentation conditions.
The past thousand years represent a particularly interesting time interval. In addition to historical records, there
are also numerous high-resolution climate archives available from this period (e.g., coral skeletons, tree rings, continental slope sediments) that make it possible to compare
the effects of relatively small-scale natural climate anomalies (e.g., the Middle Ages warm period and the “Little
Ice Age”) to recent variations caused by humans under
somewhat similar boundary conditions.
Another time scale of interest includes the past 30,000
years, which incorporate the transition from the last ice
age to the present warm period, the Holocene, which
began about 10,000 years ago. Another important time
interval encompasses the past 500,000 years (the last
glacial-interglacial cycles). During this period, processes
leading to variability of the global ocean in the Late Quaternary can be identified. These processes, in turn, are
closely coupled to the drastic climate fluctuations over
the course of the glacial-interglacial cycles. The ocean is
a central component of the global carbon and hydrological
cycles, and it also has a critical influence on the other two
major components of the climate system, the atmosphere
and continental ice.
Productivity
Organic matter deposition
The deposition of large amounts of organic material
resulting from high-productivity conditions along the
ocean margins leads to very high turnover rates at the seafloor, so these areas can also be characterized as efficient
biogeochemical reactors. These high turnover rates of
organic carbon, along with associated elements such as
nitrogen, phosphorus, sulfur, and iron, play a critical role
in the global mass cycles, but above all in the global carbon cycle.
In high-productivity regions, elevated flux rates of
organic substance often lead to the formation of an
oxygen-minimum zone. In the highly productive ocean
margins, these oxygen-minimum zones are in direct contact with the seafloor, strongly affecting the organism
communities and the biogeochemical processes. The oxygen deficit varies with the dynamic balance between the
flux of degradable organic detritus, stratification stability,
and exchange time of the water masses. Interactions
between the seafloor and the oxygen-depleted water column are much more intense than in the deep sea, leading
to a close coupling between the bottom-water processes
and those in the sediment. Oxygen concentration is one
of the most important biological-chemical steering parameters for these processes.
Sediments at the seafloor are the habitat for countless, so
far only partly known, microorganisms that drive these
cycles and contribute significantly to the conversion of both
organic and inorganic materials. Along with the normal flux
of materials from the water column into the sediments,
these microorganisms contribute to an intense return of
nutrients from the sediments back into the water column.
Gas hydrates
Gas hydrates are firm, icelike associations of gas molecules and water, present in water depths between
300 and 700 m (see “Marine Gas Hydrates”). Besides
methane, other hydrocarbons and other gases like carbon
dioxide and nitrogen can also form gas hydrates at higher
pressures and lower temperatures. Besides specific pressure and temperature conditions, an additional requisite
is a sufficient amount of gas. In ocean margin sediments,
the gas can be the product of microbial decomposition of
organic material. Gas can also be produced by
“thermocatalytic” processes in deep sediment columns.
Because abundant organic material is commonly available
on ocean margins, gas hydrates are common there. High
plankton production and high sedimentation rates are
favorable for the deposition of large amounts of organic
material on the seafloor (see “Sapropels”).
There are concerns regarding the instability of gas
hydrates concentrated at the ocean margins as it relates
to the warming of the oceans, which is in part caused by
humans. The temperature increase could, on the one hand,
lead to a wide-scale destabilization of the continental
slopes resulting in large slides (see “Submarine Slides”)
with possible tsunamis and, on the other hand, release
immense amounts of methane, which would contribute
to a significant strengthening of the greenhouse effect.
Methane hydrates are also often discussed as a potential
source of energy for the future. Although estimates are
highly variable, the resources might be comparable to
today’s known fossil fuel (see “Energy Resources”). If
they are eventually exploited, the exploration for methane
hydrates will probably start in permafrost areas.
OCEAN MARGIN SYSTEMS
555
2
, there
have probably been a large number of smaller slides.
Climate coupling
Climate plays a prominent role in driving the processes
that shape the environment, and it also interacts closely
with ocean circulation and productivity. The climateinduced fluctuations of sea-level alone, with swings of
over 100 m during the Late Quaternary glacial cycles
causing broad shelf regions to alternately be dry and part
of the land and then flooded and covered by the sea, highlight the sensitivity of the ocean margins to climate. In
addition to the fact that these areas represent extremely
active transitional areas between the oceans and continents, the presence of high sedimentation rates due to the
input of terrigenous sediments is a very important
characteristic.
Direct climatological measurements, which are critical
for a basic understanding of such processes, only go back
to the year 1850. Records extending farther back in time
are documented primarily in the ocean margin sediments.
The ocean margin sediments clearly provide an impressive archive of past climatic, productivity, current, and
sedimentation conditions.
The past thousand years represent a particularly interesting time interval. In addition to historical records, there
are also numerous high-resolution climate archives available from this period (e.g., coral skeletons, tree rings, continental slope sediments) that make it possible to compare
the effects of relatively small-scale natural climate anomalies (e.g., the Middle Ages warm period and the “Little
Ice Age”) to recent variations caused by humans under
somewhat similar boundary conditions.
Another time scale of interest includes the past 30,000
years, which incorporate the transition from the last ice
age to the present warm period, the Holocene, which
began about 10,000 years ago. Another important time
interval encompasses the past 500,000 years (the last
glacial-interglacial cycles). During this period, processes
leading to variability of the global ocean in the Late Quaternary can be identified. These processes, in turn, are
closely coupled to the drastic climate fluctuations over
the course of the glacial-interglacial cycles. The ocean is
a central component of the global carbon and hydrological
cycles, and it also has a critical influence on the other two
major components of the climate system, the atmosphere
and continental ice.
Productivity
Organic matter deposition
The deposition of large amounts of organic material
resulting from high-productivity conditions along the
ocean margins leads to very high turnover rates at the seafloor, so these areas can also be characterized as efficient
biogeochemical reactors. These high turnover rates of
organic carbon, along with associated elements such as
nitrogen, phosphorus, sulfur, and iron, play a critical role
in the global mass cycles, but above all in the global carbon cycle.
In high-productivity regions, elevated flux rates of
organic substance often lead to the formation of an
oxygen-minimum zone. In the highly productive ocean
margins, these oxygen-minimum zones are in direct contact with the seafloor, strongly affecting the organism
communities and the biogeochemical processes. The oxygen deficit varies with the dynamic balance between the
flux of degradable organic detritus, stratification stability,
and exchange time of the water masses. Interactions
between the seafloor and the oxygen-depleted water column are much more intense than in the deep sea, leading
to a close coupling between the bottom-water processes
and those in the sediment. Oxygen concentration is one
of the most important biological-chemical steering parameters for these processes.
Sediments at the seafloor are the habitat for countless, so
far only partly known, microorganisms that drive these
cycles and contribute significantly to the conversion of both
organic and inorganic materials. Along with the normal flux
of materials from the water column into the sediments,
these microorganisms contribute to an intense return of
nutrients from the sediments back into the water column.
Gas hydrates
Gas hydrates are firm, icelike associations of gas molecules and water, present in water depths between
300 and 700 m (see “Marine Gas Hydrates”). Besides
methane, other hydrocarbons and other gases like carbon
dioxide and nitrogen can also form gas hydrates at higher
pressures and lower temperatures. Besides specific pressure and temperature conditions, an additional requisite
is a sufficient amount of gas. In ocean margin sediments,
the gas can be the product of microbial decomposition of
organic material. Gas can also be produced by
“thermocatalytic” processes in deep sediment columns.
Because abundant organic material is commonly available
on ocean margins, gas hydrates are common there. High
plankton production and high sedimentation rates are
favorable for the deposition of large amounts of organic
material on the seafloor (see “Sapropels”).
There are concerns regarding the instability of gas
hydrates concentrated at the ocean margins as it relates
to the warming of the oceans, which is in part caused by
humans. The temperature increase could, on the one hand,
lead to a wide-scale destabilization of the continental
slopes resulting in large slides (see “Submarine Slides”)
with possible tsunamis and, on the other hand, release
immense amounts of methane, which would contribute
to a significant strengthening of the greenhouse effect.
Methane hydrates are also often discussed as a potential
source of energy for the future. Although estimates are
highly variable, the resources might be comparable to
today’s known fossil fuel (see “Energy Resources”). If
they are eventually exploited, the exploration for methane
hydrates will probably start in permafrost areas.
OCEAN MARGIN SYSTEMS
555