LAND–SEA GLOBAL TRANSFERS
F. T. Mackenzie and L. M. Ver, University of Hawaii,
Honolulu, HI, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
The interface between the land and the sea is an
important boundary connecting processes operating
on land with those in the ocean. It is a site of rapid
population growth, industrial and agricultural practices, and urban development. Large river drainage
basins connect the vast interiors of continents with
the coastal zone through river and groundwater
discharges. The atmosphere is a medium of transport
of substances from the land to the sea surface and
from that surface back to the land. During the past
several centuries, the activities of humankind have
significantly modified the exchange of materials between the land and sea on a global scale – humans
have become, along with natural processes, agents of
environmental change. For example, because of the
combustion of the fossil fuels coal, oil, and gas and
changes in land-use practices including deforestation,
shifting cultivation, and urbanization, the direction
of net atmospheric transport of carbon (C) and sulfur
(S) gases between the land and sea has been reversed.
The global ocean prior to these human activities was
a source of the gas carbon dioxide (CO 2 ) to the atmosphere and hence to the continents. The ocean
now absorbs more CO 2 than it releases. In preindustrial time, the flux of reduced sulfur gases to the
atmosphere and their subsequent oxidation and deposition on the continental surface exceeded the
transport of oxidized sulfur to the ocean via the atmosphere. The situation is now reversed because of
the emissions of sulfur to the atmosphere from the
burning of fossil fuels and biomass on land. In addition, river and groundwater fluxes of the bioessential elements carbon, nitrogen (N), and phosphorus
(P), and certain trace elements have increased because of human activities on land. For example, the
increased global riverine (and atmospheric) transport
of lead (Pb) corresponds to its increased industrial
use. Also, recent changes in the concentration of lead
in coastal sediments appear to be directly related to
changes in the use of leaded gasoline in internal
combustion engines. Synthetic substances manufactured by modern society, such as pesticides and
pharmaceutical products, are now appearing in river
and groundwater flows, thus moving toward the sea
in a greater degree than before.
The Changing Picture of Land–Sea
Global Transfers
Although the exchange through the atmosphere of
certain trace metals and gases between the land and
the sea surface is important, rivers are the main
purveyors of materials to the ocean. The total water
discharge of the major rivers of the world to the
ocean is 36 000 km
3 yr
À 1 . At any time, the world’s
rivers contain about 0.000 1% of the total water
volume of 1459 Â 10
6 km
3 near the surface of the
Earth and have a total dissolved and suspended solid
concentration of c. 110 and 540 ppm l
À 1
, respectively. The residence time of water in the world’s
rivers calculated with respect to total net precipitation on land is only 18 days. Thus the water in the
world’s rivers is replaced every 18 days by precipitation. The global annual direct discharge to the
ocean of groundwater is about 10% of the surface
flow, with a recent estimate of 2400 km
3 yr
À 1
. The
dissolved constituent content of groundwater is
poorly known, but one recent estimate of the dissolved salt groundwater flux is 1300 Â 10
6 t yr
À 1
.
The chemical composition of average river water
is shown in Table 1. Note that the major anion
in river water is bicarbonate, HCO
À
3 ; the major
cation is calcium, Ca
2þ , and that even the major
constituent concentrations of river water on a global
scale are influenced by human activities. The dissolved load of the major constituents of the world’s
rivers is derived from the following sources: about
7% from beds of salt and disseminated salt in sedimentary rocks, 10% from gypsum beds and sulfate
salts disseminated in rocks, 38% from carbonates,
and 45% from the weathering of silicate minerals.
Two-thirds of the HCO 3
À in river waters are derived
from atmospheric CO 2 via the respiration and decomposition of organic matter and subsequent conversion to HCO
À
3 through the chemical weathering
of silicate (B30% of total) or carbonate (B70% of
total) minerals. The other third of the river HCO 3
À
comes directly from carbonate minerals undergoing
weathering.
It is estimated that only about 20% of the world’s
drainage basins have pristine water quality. The organic productivity of coastal aquatic environments has been heavily impacted by changes in the
485
F. T. Mackenzie and L. M. Ver, University of Hawaii,
Honolulu, HI, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
The interface between the land and the sea is an
important boundary connecting processes operating
on land with those in the ocean. It is a site of rapid
population growth, industrial and agricultural practices, and urban development. Large river drainage
basins connect the vast interiors of continents with
the coastal zone through river and groundwater
discharges. The atmosphere is a medium of transport
of substances from the land to the sea surface and
from that surface back to the land. During the past
several centuries, the activities of humankind have
significantly modified the exchange of materials between the land and sea on a global scale – humans
have become, along with natural processes, agents of
environmental change. For example, because of the
combustion of the fossil fuels coal, oil, and gas and
changes in land-use practices including deforestation,
shifting cultivation, and urbanization, the direction
of net atmospheric transport of carbon (C) and sulfur
(S) gases between the land and sea has been reversed.
The global ocean prior to these human activities was
a source of the gas carbon dioxide (CO 2 ) to the atmosphere and hence to the continents. The ocean
now absorbs more CO 2 than it releases. In preindustrial time, the flux of reduced sulfur gases to the
atmosphere and their subsequent oxidation and deposition on the continental surface exceeded the
transport of oxidized sulfur to the ocean via the atmosphere. The situation is now reversed because of
the emissions of sulfur to the atmosphere from the
burning of fossil fuels and biomass on land. In addition, river and groundwater fluxes of the bioessential elements carbon, nitrogen (N), and phosphorus
(P), and certain trace elements have increased because of human activities on land. For example, the
increased global riverine (and atmospheric) transport
of lead (Pb) corresponds to its increased industrial
use. Also, recent changes in the concentration of lead
in coastal sediments appear to be directly related to
changes in the use of leaded gasoline in internal
combustion engines. Synthetic substances manufactured by modern society, such as pesticides and
pharmaceutical products, are now appearing in river
and groundwater flows, thus moving toward the sea
in a greater degree than before.
The Changing Picture of Land–Sea
Global Transfers
Although the exchange through the atmosphere of
certain trace metals and gases between the land and
the sea surface is important, rivers are the main
purveyors of materials to the ocean. The total water
discharge of the major rivers of the world to the
ocean is 36 000 km
3 yr
À 1 . At any time, the world’s
rivers contain about 0.000 1% of the total water
volume of 1459 Â 10
6 km
3 near the surface of the
Earth and have a total dissolved and suspended solid
concentration of c. 110 and 540 ppm l
À 1
, respectively. The residence time of water in the world’s
rivers calculated with respect to total net precipitation on land is only 18 days. Thus the water in the
world’s rivers is replaced every 18 days by precipitation. The global annual direct discharge to the
ocean of groundwater is about 10% of the surface
flow, with a recent estimate of 2400 km
3 yr
À 1
. The
dissolved constituent content of groundwater is
poorly known, but one recent estimate of the dissolved salt groundwater flux is 1300 Â 10
6 t yr
À 1
.
The chemical composition of average river water
is shown in Table 1. Note that the major anion
in river water is bicarbonate, HCO
À
3 ; the major
cation is calcium, Ca
2þ , and that even the major
constituent concentrations of river water on a global
scale are influenced by human activities. The dissolved load of the major constituents of the world’s
rivers is derived from the following sources: about
7% from beds of salt and disseminated salt in sedimentary rocks, 10% from gypsum beds and sulfate
salts disseminated in rocks, 38% from carbonates,
and 45% from the weathering of silicate minerals.
Two-thirds of the HCO 3
À in river waters are derived
from atmospheric CO 2 via the respiration and decomposition of organic matter and subsequent conversion to HCO
À
3 through the chemical weathering
of silicate (B30% of total) or carbonate (B70% of
total) minerals. The other third of the river HCO 3
À
comes directly from carbonate minerals undergoing
weathering.
It is estimated that only about 20% of the world’s
drainage basins have pristine water quality. The organic productivity of coastal aquatic environments has been heavily impacted by changes in the
485