associated with particles) in natural waters and
hence is rapidly adsorbed from solution to particles.
It is likely that even the reported dissolved
232 Th
concentrations are upper limits, as recent results,
based on smaller volume samples and high sensitivity
mass-spectrometric measurements seem to show that
dissolved
232 Th in rivers is associated with smaller
particles (o0.45 mm size). Similar to
232 Th, the bulk
of
230 Th and
210 Pb is also associated with particles in
rivers and hence is transported mainly in particulate
form from continents.
226 Ra and
228 Ra are two other members of the UTh series (Figure 1) for which dissolved concentration data are available for several rivers, these
show that they are present at levels of B0.1 d.p.m.
l
À1 . The available data show that there are significant
differences between the abundances of U, Ra isotopes and
232
Th in the host rocks and in river waters.
The various physicochemical processes occurring
during the mobilization and transport of these nuclides contribute to these differences.
Rivers also transport U/Th series nuclides in particulate phase to the sea. These nuclides exist in
two forms in the particulate phase, one as a part of
their lattice structure and the other as surface coating
resulting from their adsorption from solution.
Analysis of suspended particulate matter from rivers
shows the existence of radioactive disequilibria
among the members of the same radioactive decay
chain. In general, particulate phases are characterized by
234 U/
238 U,
226 Ra/
230 Th activity ratios
o1 and
230 Th/
234
U and
210 Pb/
226 Ra41, caused by
preferential mobilization of U and Ra over Th and
Pb isotopes.
Soluble and suspended materials from rivers enter
the open ocean through estuaries. The interactions of
sea water with the riverine materials can modify the
dissolved concentrations of many nuclides and hence
their fluxes to the open sea. Studies of U/Th series
isotopes in estuaries show that in many cases their
distribution is governed by processes in addition to
simple mixing of river and sea water. For example, in
the case of U there is evidence for both its addition
and removal during transit through estuaries. Similarly, many estuaries have
226 Ra concentration
higher than that expected from water mixing considerations resulting from its desorption from riverine particles and/or its diffusion from estuarine
sediments. Estuaries also seem to act as a filter for
riverine
232 Th.
The behavior of radionuclides in estuaries could be
influenced by their association with colloids. Recent
studies of uranium in Kalix River show that a significant part is bound to colloids which is removed in
the estuaries through flocculation. Similarly, colloids
seem to have a significant control on the
230 Th–
232
Th
distribution in estuarine waters.
In situ Production
Radioactive decay of dissolved radionuclides in the
water column is an important supply mechanism for
several U/Th series nuclides. This is the dominant
mode of supply for
234 Th,
228 Th,
230 Th,
210 Po,
210 Pb,
and
231 Pa. The supply rates of these nuclides to sea
water can be precisely determined by measuring the
concentrations of their parents. This is unlike the
case of nuclides supplied via rivers whose fluxes are
relatively more difficult to ascertain because of large
spatial and temporal variations in their riverine
concentrations and their modifications in estuaries.
Supply at Air–sea and Sediment–water Interfaces
A few of the U/Th nuclides are supplied to the sea via
atmospheric deposition and diffusion through sediment pore waters. Decay of
222 Rn in the atmosphere
to
210
Pb and its subsequent removal by wet and dry
deposition is an important source of dissolved
210 Pb
to the sea. As the bulk of the
222
Rn in the atmosphere
is of continental origin, the flux of
210 Pb via this
route depends on factors such as distance from land
and aerosol residence times.
210 Po is also deposited
on the sea surface through this source, but its flux is
o10% of that of
210 Pb. Leaching of atmospheric
dust by sea water can also contribute to nuclide
fluxes near the air–sea interface, this mechanism has
been suggested as a source for dissolved
232 Th.
Diffusion out of sediments forms a significant
input for Ra isotopes,
227 Ac and
222 Rn into overlying
water. All these nuclides are produced in sediments
through a-decay (Figure 1). The recoil associated
with their production enhances their mobility from
sediments to pore waters from where they diffuse to
overlying sea water. Their diffusive fluxes depend on
the nature of sediments, their accumulation rates,
and the parent concentrations in them.
234 U is another isotope for which supply through diffusion
from sediments may be important for its oceanic
budget.
In addition to diffusion out of sediments,
226
Ra
and
222 Rn are also introduced into bottom waters
through vent waters associated with hydrothermal
circulation along the spreading ridges. The flux of
226 Ra from this source though is comparable to that
from rivers; its contribution to the overall
226
Ra
budget of the oceans is small. This flux, however, can
overwhelm
226 Ra diffusing out of sediments along
the ridges on a local scale.
URANIUM-THORIUM SERIES ISOTOPES IN OCEAN PROFILES 215
hence is rapidly adsorbed from solution to particles.
It is likely that even the reported dissolved
232 Th
concentrations are upper limits, as recent results,
based on smaller volume samples and high sensitivity
mass-spectrometric measurements seem to show that
dissolved
232 Th in rivers is associated with smaller
particles (o0.45 mm size). Similar to
232 Th, the bulk
of
230 Th and
210 Pb is also associated with particles in
rivers and hence is transported mainly in particulate
form from continents.
226 Ra and
228 Ra are two other members of the UTh series (Figure 1) for which dissolved concentration data are available for several rivers, these
show that they are present at levels of B0.1 d.p.m.
l
À1 . The available data show that there are significant
differences between the abundances of U, Ra isotopes and
232
Th in the host rocks and in river waters.
The various physicochemical processes occurring
during the mobilization and transport of these nuclides contribute to these differences.
Rivers also transport U/Th series nuclides in particulate phase to the sea. These nuclides exist in
two forms in the particulate phase, one as a part of
their lattice structure and the other as surface coating
resulting from their adsorption from solution.
Analysis of suspended particulate matter from rivers
shows the existence of radioactive disequilibria
among the members of the same radioactive decay
chain. In general, particulate phases are characterized by
234 U/
238 U,
226 Ra/
230 Th activity ratios
o1 and
230 Th/
234
U and
210 Pb/
226 Ra41, caused by
preferential mobilization of U and Ra over Th and
Pb isotopes.
Soluble and suspended materials from rivers enter
the open ocean through estuaries. The interactions of
sea water with the riverine materials can modify the
dissolved concentrations of many nuclides and hence
their fluxes to the open sea. Studies of U/Th series
isotopes in estuaries show that in many cases their
distribution is governed by processes in addition to
simple mixing of river and sea water. For example, in
the case of U there is evidence for both its addition
and removal during transit through estuaries. Similarly, many estuaries have
226 Ra concentration
higher than that expected from water mixing considerations resulting from its desorption from riverine particles and/or its diffusion from estuarine
sediments. Estuaries also seem to act as a filter for
riverine
232 Th.
The behavior of radionuclides in estuaries could be
influenced by their association with colloids. Recent
studies of uranium in Kalix River show that a significant part is bound to colloids which is removed in
the estuaries through flocculation. Similarly, colloids
seem to have a significant control on the
230 Th–
232
Th
distribution in estuarine waters.
In situ Production
Radioactive decay of dissolved radionuclides in the
water column is an important supply mechanism for
several U/Th series nuclides. This is the dominant
mode of supply for
234 Th,
228 Th,
230 Th,
210 Po,
210 Pb,
and
231 Pa. The supply rates of these nuclides to sea
water can be precisely determined by measuring the
concentrations of their parents. This is unlike the
case of nuclides supplied via rivers whose fluxes are
relatively more difficult to ascertain because of large
spatial and temporal variations in their riverine
concentrations and their modifications in estuaries.
Supply at Air–sea and Sediment–water Interfaces
A few of the U/Th nuclides are supplied to the sea via
atmospheric deposition and diffusion through sediment pore waters. Decay of
222 Rn in the atmosphere
to
210
Pb and its subsequent removal by wet and dry
deposition is an important source of dissolved
210 Pb
to the sea. As the bulk of the
222
Rn in the atmosphere
is of continental origin, the flux of
210 Pb via this
route depends on factors such as distance from land
and aerosol residence times.
210 Po is also deposited
on the sea surface through this source, but its flux is
o10% of that of
210 Pb. Leaching of atmospheric
dust by sea water can also contribute to nuclide
fluxes near the air–sea interface, this mechanism has
been suggested as a source for dissolved
232 Th.
Diffusion out of sediments forms a significant
input for Ra isotopes,
227 Ac and
222 Rn into overlying
water. All these nuclides are produced in sediments
through a-decay (Figure 1). The recoil associated
with their production enhances their mobility from
sediments to pore waters from where they diffuse to
overlying sea water. Their diffusive fluxes depend on
the nature of sediments, their accumulation rates,
and the parent concentrations in them.
234 U is another isotope for which supply through diffusion
from sediments may be important for its oceanic
budget.
In addition to diffusion out of sediments,
226
Ra
and
222 Rn are also introduced into bottom waters
through vent waters associated with hydrothermal
circulation along the spreading ridges. The flux of
226 Ra from this source though is comparable to that
from rivers; its contribution to the overall
226
Ra
budget of the oceans is small. This flux, however, can
overwhelm
226 Ra diffusing out of sediments along
the ridges on a local scale.
URANIUM-THORIUM SERIES ISOTOPES IN OCEAN PROFILES 215