With little doubt, as has been documented in a
number of observational and experimental studies, a
reduction in the saturation state of ocean waters will
lead to a reduction in the rate of precipitation of both
inorganic and skeletal calcium carbonate. Conversely, increases in the degree of supersaturation
and temperature will increase the precipitation rates
of calcite and aragonite from seawater. During global
warming, rising sea surface temperatures and declining carbonate saturation states due to the absorption of anthropogenic CO 2 by surface ocean
waters result in opposing effects. However, experimental evidence suggests that within the range of
temperature change predicted for the next century
due to global warming, the effect of changes in saturation state will be the predominant factor affecting
precipitation rate. Thus decreases in precipitation
rates should lead to a decrease in the production and
accumulation of shallow-water carbonate sediments
and perhaps changes in the types and distribution of
calcifying biotic species found in shallow-water
environments.
Anticipated Response to Heightened
Human Perturbation: The Asian
Scenario
In the preceding sections it was shown that the fluxes
of C, N, and P from land to ocean have increased
because of human activities (refer to Table 1 for data
comparing the actual and natural concentrations of
C, N, P, and other elements in average river water).
During the industrial era, these fluxes mainly had
their origin in the present industrialized and developed countries. This is changing as the industrializing and developing countries move into the
twenty-first century. A case in point is the countries
of Asia.
Asia is a continent of potentially increasing contributions to the loading of the environment owing to
a combination of such factors as its increasing
population, increasing industrialization dependent
on fossil fuels, concentration of its population along
the major river drainage basins and in coastal urban
centers, and expansion of land-use practices. It is
anticipated that Asia will experience similar, possibly
even greater, loss of storage of C and nutrient N
and P on land and increased storage in coastal
marine sediments per unit area than was shown by
the developed countries during their period of industrialization. The relatively rapid growth of Asia’s
population along the oceanic coastal zone indicates
that higher inputs of both dissolved and particulate
organic nutrients may be expected to enter coastal
waters.
A similar trend of increasing population concentration in agricultural areas inland, within the
drainage basins of the main rivers flowing into the
ocean, is also expected to result in increased dissolved and particulate organic nutrient loads that
may eventually reach the ocean. Inputs from inland
regions to the ocean would be relatively more important if no entrapment or depositional storage
occurred en route, such as in the dammed sections of
rivers or in alluvial plains. In the case of many of
China’s rivers, the decline in sediment discharge from
large rivers such as the Yangtze and the Yellow
Rivers is expected to continue due to the increased
construction of dams. The average decadal sediment
discharge from the Yellow River, for example, has
decreased by 50% from the 1950s to the 1980s. If
the evidence proposed for the continental United
States applies to Asia, the damming of major rivers
would not effectively reduce the suspended material
flow to the ocean because of the changes in the
erosional patterns on land that accompany river
damming and more intensive land-use practices.
These flows on land and into coastal ocean waters
are contributing factors to the relative importance of
autotrophic and heterotrophic processes, competition between the two, and the consequences for
carbon exchange between the atmosphere and land,
and the atmosphere and ocean water. The change
from the practices of land fertilization by manure to
the more recent usage of chemical fertilizers in Asia
suggests a shift away from solid organic nutrients
and therefore a reduced flow of materials that might
promote heterotrophy in coastal environments.
Sulfur is an excellent example of how parts of Asia
can play an important role in changing land–sea
transfers of materials. Prior to extensive human
interference in the global cycle of sulfur, biogenically
produced sulfur was emitted from the sea surface
mainly in the form of the reduced gas dimethyl sulfide (DMS). DMS was the major global natural
source of sulfur for the atmosphere, excluding sulfur
in sea salt and soil dust. Some of this gas traveled far
from its source of origin. During transport the reduced gas was oxidized to micrometer-size sulfate
aerosol particles and rained out of the atmosphere
onto the sea and continental surface. The global
sulfur cycle has been dramatically perturbed by the
industrial and biomass burning activities of human
society. The flux of gaseous sulfur dioxide to the
atmosphere from the combustion of fossil fuels in
some regions of the world and its conversion to
sulfate aerosol greatly exceeds natural fluxes of
sulfur gases from the land surface. It is estimated that
this flux for the year 1990 was equivalent to 73 Â 10
6
t yr
À1
, nearly 4 times the natural DMS flux from the
492 LAND–SEA GLOBAL TRANSFERS
number of observational and experimental studies, a
reduction in the saturation state of ocean waters will
lead to a reduction in the rate of precipitation of both
inorganic and skeletal calcium carbonate. Conversely, increases in the degree of supersaturation
and temperature will increase the precipitation rates
of calcite and aragonite from seawater. During global
warming, rising sea surface temperatures and declining carbonate saturation states due to the absorption of anthropogenic CO 2 by surface ocean
waters result in opposing effects. However, experimental evidence suggests that within the range of
temperature change predicted for the next century
due to global warming, the effect of changes in saturation state will be the predominant factor affecting
precipitation rate. Thus decreases in precipitation
rates should lead to a decrease in the production and
accumulation of shallow-water carbonate sediments
and perhaps changes in the types and distribution of
calcifying biotic species found in shallow-water
environments.
Anticipated Response to Heightened
Human Perturbation: The Asian
Scenario
In the preceding sections it was shown that the fluxes
of C, N, and P from land to ocean have increased
because of human activities (refer to Table 1 for data
comparing the actual and natural concentrations of
C, N, P, and other elements in average river water).
During the industrial era, these fluxes mainly had
their origin in the present industrialized and developed countries. This is changing as the industrializing and developing countries move into the
twenty-first century. A case in point is the countries
of Asia.
Asia is a continent of potentially increasing contributions to the loading of the environment owing to
a combination of such factors as its increasing
population, increasing industrialization dependent
on fossil fuels, concentration of its population along
the major river drainage basins and in coastal urban
centers, and expansion of land-use practices. It is
anticipated that Asia will experience similar, possibly
even greater, loss of storage of C and nutrient N
and P on land and increased storage in coastal
marine sediments per unit area than was shown by
the developed countries during their period of industrialization. The relatively rapid growth of Asia’s
population along the oceanic coastal zone indicates
that higher inputs of both dissolved and particulate
organic nutrients may be expected to enter coastal
waters.
A similar trend of increasing population concentration in agricultural areas inland, within the
drainage basins of the main rivers flowing into the
ocean, is also expected to result in increased dissolved and particulate organic nutrient loads that
may eventually reach the ocean. Inputs from inland
regions to the ocean would be relatively more important if no entrapment or depositional storage
occurred en route, such as in the dammed sections of
rivers or in alluvial plains. In the case of many of
China’s rivers, the decline in sediment discharge from
large rivers such as the Yangtze and the Yellow
Rivers is expected to continue due to the increased
construction of dams. The average decadal sediment
discharge from the Yellow River, for example, has
decreased by 50% from the 1950s to the 1980s. If
the evidence proposed for the continental United
States applies to Asia, the damming of major rivers
would not effectively reduce the suspended material
flow to the ocean because of the changes in the
erosional patterns on land that accompany river
damming and more intensive land-use practices.
These flows on land and into coastal ocean waters
are contributing factors to the relative importance of
autotrophic and heterotrophic processes, competition between the two, and the consequences for
carbon exchange between the atmosphere and land,
and the atmosphere and ocean water. The change
from the practices of land fertilization by manure to
the more recent usage of chemical fertilizers in Asia
suggests a shift away from solid organic nutrients
and therefore a reduced flow of materials that might
promote heterotrophy in coastal environments.
Sulfur is an excellent example of how parts of Asia
can play an important role in changing land–sea
transfers of materials. Prior to extensive human
interference in the global cycle of sulfur, biogenically
produced sulfur was emitted from the sea surface
mainly in the form of the reduced gas dimethyl sulfide (DMS). DMS was the major global natural
source of sulfur for the atmosphere, excluding sulfur
in sea salt and soil dust. Some of this gas traveled far
from its source of origin. During transport the reduced gas was oxidized to micrometer-size sulfate
aerosol particles and rained out of the atmosphere
onto the sea and continental surface. The global
sulfur cycle has been dramatically perturbed by the
industrial and biomass burning activities of human
society. The flux of gaseous sulfur dioxide to the
atmosphere from the combustion of fossil fuels in
some regions of the world and its conversion to
sulfate aerosol greatly exceeds natural fluxes of
sulfur gases from the land surface. It is estimated that
this flux for the year 1990 was equivalent to 73 Â 10
6
t yr
À1
, nearly 4 times the natural DMS flux from the
492 LAND–SEA GLOBAL TRANSFERS