dilution mass spectrometry, etc. As a result, in the
late 1970s, data of some transition metals, like Cd,
Cu, and Ni were obtained by the Massachusetts Institute of Technology group and soon after confirmed
by others using different or modified methods. Their
oceanic profiles were quite consistent with known
biogeochemical cycling and scavenging processes in
the ocean. Thus, these features have often been referred as ‘oceanographically or geochemically consistent’ distribution by subsequent workers. Since
then, growing numbers of publications describing the
oceanic distributions of trace elements in sea water
based on modern technologies have appeared year by
year.
Oceanic Profiles
It is now possible to compile, with reasonable confidence, the vertical profiles in the form of Periodic
chart (Figure 1), where the data from the North
Pacific have been chosen since physical processes that
affect the distribution are relatively simple and well
documented. Figure 1 is an updated version of the
original, including new data for Nb, Ta, Hf, Os, Ag,
and rare earth elements. Now, there remains only
one element, Ru on which no real data have been
reported (see Platinum Group Elements and their
Isotopes in the Ocean). However, confirmation is
needed for many elements, including Sn, Hg, Rh, Pd,
Au, Ir, Pt, etc., since they are based on a single study
or on controversial results by different workers.
Nevertheless, it is clear that the long-standing dream
to establish the chemical composition of sea water is
about to become a reality.
Trace elements follow one or more of the categories which are described below.
Conservative type Some of the trace elements such
as U, W, and Re form stable ionic species,
UO 2 (CO 3 ) 2
2À , WO 4
2À , and ReO 4
À in sea water.
Hence, their oceanic behavior is conservative
(follow salinity) and their mean residence times in
the ocean are generally long (e.g., 410
5 years).
There is no significant variation in their
concentration between different oceanic basins.
Recycled type (nutrient-like) Many others, e.g.,
Ni, Cd, Zn, Ge, and Ba, show a gradual increase in
their concentration from the surface to deep water,
much like nutrients (nitrate, phosphate, and silicate
or alkalinity), suggesting their involvement in the
biogeochemical cycle of biological uptake in the
surface water and regeneration in deep waters. As a
result of global ocean circulation, the deep-water
concentrations of this type are higher in the Pacific
than in the Atlantic.
Scavenged type Trace metals such as Al, Co, Ce,
and Bi, show surface enrichment and depletion in
deep waters, in contrast to the opposite trend in
nutrient types. These elements are highly particlereactive and are rapidly removed from the water
column by sinking particulate matter and/or by
scavenging at the sediment–water interface. Their
mean oceanic residence times are short (o10
2
–10
3
years). Interoceanic variations in their concentration
can be large (e.g., Atlantic/Pacific concentration
ratioB40 for Al) depending on kinetic balance
between supply and removal for the specific basins.
Redox-controlled type Elements such as Cr, As,
Se, and Te exist in sea water at more than one
oxidation state. Their oceanic behavior is strongly
dependent on the chemical form. Their reduced
states are thermodynamically unstable in normal
oxygenated waters but are probably formed
through biological mediation. Reduced species can
also be formed in anoxic basins, the Black Sea,
Cariaco Trench, some fiords, and in organic-rich
sediments.
Anthropogenic and transient type Finally, Pb and
Pu are good examples of elements whose oceanic
distributions are globally influenced by human
activities (see Anthropogenic Trace Elements in the
Ocean). Their oceanic distributions are changing
with time. Although some others, such as Hg, Sn,
Cd, and Ag, are deduced to be similarly influenced,
their transient nature has not yet been proven
through direct observation.
Particle Association and Speciation
One of the important features of Figure 1 is that the
concentration, even for trace elements, varies fairly
smoothly and continuously with depth. This casts
doubt on some erratic and highly discontinuous
values unless there are obvious reasons for them,
such as hydrothermal influence or difference in the
water masses. The data shown in Figure 1 are largely
based on filtered samples and therefore, can be referred as ‘dissolved concentration.’ For conservative
elements, it does not matter whether the water
sample is filtered or not, since there is virtually no
difference in the analytical results. For most nutrienttype elements, particle association in the open ocean
is generally small (oB5%) and therefore, the gross
features of unfiltered samples remains the same as
10 ELEMENTAL DISTRIBUTION: OVERVIEW
late 1970s, data of some transition metals, like Cd,
Cu, and Ni were obtained by the Massachusetts Institute of Technology group and soon after confirmed
by others using different or modified methods. Their
oceanic profiles were quite consistent with known
biogeochemical cycling and scavenging processes in
the ocean. Thus, these features have often been referred as ‘oceanographically or geochemically consistent’ distribution by subsequent workers. Since
then, growing numbers of publications describing the
oceanic distributions of trace elements in sea water
based on modern technologies have appeared year by
year.
Oceanic Profiles
It is now possible to compile, with reasonable confidence, the vertical profiles in the form of Periodic
chart (Figure 1), where the data from the North
Pacific have been chosen since physical processes that
affect the distribution are relatively simple and well
documented. Figure 1 is an updated version of the
original, including new data for Nb, Ta, Hf, Os, Ag,
and rare earth elements. Now, there remains only
one element, Ru on which no real data have been
reported (see Platinum Group Elements and their
Isotopes in the Ocean). However, confirmation is
needed for many elements, including Sn, Hg, Rh, Pd,
Au, Ir, Pt, etc., since they are based on a single study
or on controversial results by different workers.
Nevertheless, it is clear that the long-standing dream
to establish the chemical composition of sea water is
about to become a reality.
Trace elements follow one or more of the categories which are described below.
Conservative type Some of the trace elements such
as U, W, and Re form stable ionic species,
UO 2 (CO 3 ) 2
2À , WO 4
2À , and ReO 4
À in sea water.
Hence, their oceanic behavior is conservative
(follow salinity) and their mean residence times in
the ocean are generally long (e.g., 410
5 years).
There is no significant variation in their
concentration between different oceanic basins.
Recycled type (nutrient-like) Many others, e.g.,
Ni, Cd, Zn, Ge, and Ba, show a gradual increase in
their concentration from the surface to deep water,
much like nutrients (nitrate, phosphate, and silicate
or alkalinity), suggesting their involvement in the
biogeochemical cycle of biological uptake in the
surface water and regeneration in deep waters. As a
result of global ocean circulation, the deep-water
concentrations of this type are higher in the Pacific
than in the Atlantic.
Scavenged type Trace metals such as Al, Co, Ce,
and Bi, show surface enrichment and depletion in
deep waters, in contrast to the opposite trend in
nutrient types. These elements are highly particlereactive and are rapidly removed from the water
column by sinking particulate matter and/or by
scavenging at the sediment–water interface. Their
mean oceanic residence times are short (o10
2
–10
3
years). Interoceanic variations in their concentration
can be large (e.g., Atlantic/Pacific concentration
ratioB40 for Al) depending on kinetic balance
between supply and removal for the specific basins.
Redox-controlled type Elements such as Cr, As,
Se, and Te exist in sea water at more than one
oxidation state. Their oceanic behavior is strongly
dependent on the chemical form. Their reduced
states are thermodynamically unstable in normal
oxygenated waters but are probably formed
through biological mediation. Reduced species can
also be formed in anoxic basins, the Black Sea,
Cariaco Trench, some fiords, and in organic-rich
sediments.
Anthropogenic and transient type Finally, Pb and
Pu are good examples of elements whose oceanic
distributions are globally influenced by human
activities (see Anthropogenic Trace Elements in the
Ocean). Their oceanic distributions are changing
with time. Although some others, such as Hg, Sn,
Cd, and Ag, are deduced to be similarly influenced,
their transient nature has not yet been proven
through direct observation.
Particle Association and Speciation
One of the important features of Figure 1 is that the
concentration, even for trace elements, varies fairly
smoothly and continuously with depth. This casts
doubt on some erratic and highly discontinuous
values unless there are obvious reasons for them,
such as hydrothermal influence or difference in the
water masses. The data shown in Figure 1 are largely
based on filtered samples and therefore, can be referred as ‘dissolved concentration.’ For conservative
elements, it does not matter whether the water
sample is filtered or not, since there is virtually no
difference in the analytical results. For most nutrienttype elements, particle association in the open ocean
is generally small (oB5%) and therefore, the gross
features of unfiltered samples remains the same as
10 ELEMENTAL DISTRIBUTION: OVERVIEW