REFRACTORY METALS
K. J. Orians and C. L. Merrin,
The University of British Columbia,
Vancouver, BC, Canada
Copyright & 2001 Elsevier Ltd.
Introduction
The elements classified as ‘refractory’ here are those
that are not readily dissolved in sea water. Their
supply to the oceans is low relative to their abundance in the Earth’s crust. In addition, they are
rapidly removed from solution by interaction with
the surfaces of sinking particles, a process referred to
as ‘scavenging.’ This rapid removal means that they
are in the oceans for only a short time before being
removed to the seafloor. The average time they spend
in the oceans, known as the oceanic residence time,
ranges from a few tens to a few thousands of years.
Both of these factors result in low concentrations in
sea water relative to their abundance in the Earth’s
crust, a large range of oceanic concentrations, and
distributions that typically reflect their sources.
The elements in this category exist as hydroxide
species in sea water, mostly as M(OH) n
X À n , where M
is the metal, X is the oxidation state of the metal, and
n is the number of hydroxide ligands in the complex.
There is also the possibility that they may exist as
organic complexes and/or in association with colloidal phases (particles pmm). Organic complexes
have been shown to be important for iron, but not
much is known about these other forms for many of
these elements.
The most abundant of these elements, aluminum
and iron (which comprise 8.23%, and 5.63%, respectively, of the Earth’s crust, by weight) are also
the most studied. The first reliable reports on dissolved aluminum in the oceans were made in the late
1970s. Since then there have been over 50 articles on
the distribution of aluminum in the oceans. Reliable
data on iron were not available until the late 1980s
but, owing to the importance of iron in regulating
primary production in some regions of the ocean,
there has been a wealth of studies on this element in
the past decade. Most of the other elements discussed
here were not studied until the late 1980s or even the
1990s, and for many there are only a couple of articles on their oceanic distributions. There is still
much we do not know about their distributions and
the processes that control them in the oceans.
History
Advances in our understanding of trace metal distributions in the oceans began with the development of
clean sampling methods in the late 1970s and have
continued with the ongoing development of highly
sensitive analytical methods. Clean sampling and
handling methods are critical in the analysis of the
more abundant refractory metals, aluminum and
iron. Detection of the lower-abundance refractory
metals, owing to their exceptionally low concentrations in sea water, has been limited by the sensitivity of available methods. Their analysis has greatly
benefited from the increasing sensitivity of modern
analytical instruments. The development of highly
sensitive mass spectrometers that allow for aqueous
sample introduction have revolutionized this field.
Inductively coupled plasma mass spectrometers (ICPMS) using quadrupole mass analyzers, made commercially available in the mid 1980s, and the magnetic and electric sector high-resolution ICP-MS
instruments, available since the early 1990s, have
allowed the detection of these elements without requirement for excessive sample processing and preconcentration steps.
Distributions
Rapid removal of the scavenged elements results in a
low background concentration in the oceans and the
potential for a large concentration range, depending
on the variations in the magnitude of their sources.
This is especially true for aluminum, where the concentrations vary by up to 2 orders of magnitude with
depth at a given location, by 3 orders of magnitude
from one major ocean to another, and by 4 orders of
magnitude in extreme environments. This range reflects the large variations in dust sources to the oceans
from place to place and rapid removal of aluminum
from sea water away from these sources. The range of
concentrations for all of the refractory elements,
their oceanic average, and some typical concentrations
found in surface (o100 m) and deep (o2000 m)
waters of the central North Atlantic and North Pacific
Oceans are presented in Table 1. Although the range
of concentrations is largest for aluminum, this difference is most likely exaggerated in the data shown here,
as aluminum has been studied in more diverse regions.
52
K. J. Orians and C. L. Merrin,
The University of British Columbia,
Vancouver, BC, Canada
Copyright & 2001 Elsevier Ltd.
Introduction
The elements classified as ‘refractory’ here are those
that are not readily dissolved in sea water. Their
supply to the oceans is low relative to their abundance in the Earth’s crust. In addition, they are
rapidly removed from solution by interaction with
the surfaces of sinking particles, a process referred to
as ‘scavenging.’ This rapid removal means that they
are in the oceans for only a short time before being
removed to the seafloor. The average time they spend
in the oceans, known as the oceanic residence time,
ranges from a few tens to a few thousands of years.
Both of these factors result in low concentrations in
sea water relative to their abundance in the Earth’s
crust, a large range of oceanic concentrations, and
distributions that typically reflect their sources.
The elements in this category exist as hydroxide
species in sea water, mostly as M(OH) n
X À n , where M
is the metal, X is the oxidation state of the metal, and
n is the number of hydroxide ligands in the complex.
There is also the possibility that they may exist as
organic complexes and/or in association with colloidal phases (particles pmm). Organic complexes
have been shown to be important for iron, but not
much is known about these other forms for many of
these elements.
The most abundant of these elements, aluminum
and iron (which comprise 8.23%, and 5.63%, respectively, of the Earth’s crust, by weight) are also
the most studied. The first reliable reports on dissolved aluminum in the oceans were made in the late
1970s. Since then there have been over 50 articles on
the distribution of aluminum in the oceans. Reliable
data on iron were not available until the late 1980s
but, owing to the importance of iron in regulating
primary production in some regions of the ocean,
there has been a wealth of studies on this element in
the past decade. Most of the other elements discussed
here were not studied until the late 1980s or even the
1990s, and for many there are only a couple of articles on their oceanic distributions. There is still
much we do not know about their distributions and
the processes that control them in the oceans.
History
Advances in our understanding of trace metal distributions in the oceans began with the development of
clean sampling methods in the late 1970s and have
continued with the ongoing development of highly
sensitive analytical methods. Clean sampling and
handling methods are critical in the analysis of the
more abundant refractory metals, aluminum and
iron. Detection of the lower-abundance refractory
metals, owing to their exceptionally low concentrations in sea water, has been limited by the sensitivity of available methods. Their analysis has greatly
benefited from the increasing sensitivity of modern
analytical instruments. The development of highly
sensitive mass spectrometers that allow for aqueous
sample introduction have revolutionized this field.
Inductively coupled plasma mass spectrometers (ICPMS) using quadrupole mass analyzers, made commercially available in the mid 1980s, and the magnetic and electric sector high-resolution ICP-MS
instruments, available since the early 1990s, have
allowed the detection of these elements without requirement for excessive sample processing and preconcentration steps.
Distributions
Rapid removal of the scavenged elements results in a
low background concentration in the oceans and the
potential for a large concentration range, depending
on the variations in the magnitude of their sources.
This is especially true for aluminum, where the concentrations vary by up to 2 orders of magnitude with
depth at a given location, by 3 orders of magnitude
from one major ocean to another, and by 4 orders of
magnitude in extreme environments. This range reflects the large variations in dust sources to the oceans
from place to place and rapid removal of aluminum
from sea water away from these sources. The range of
concentrations for all of the refractory elements,
their oceanic average, and some typical concentrations
found in surface (o100 m) and deep (o2000 m)
waters of the central North Atlantic and North Pacific
Oceans are presented in Table 1. Although the range
of concentrations is largest for aluminum, this difference is most likely exaggerated in the data shown here,
as aluminum has been studied in more diverse regions.
52