102
Chemical Oceanography, 4th Edition
3.3 Distribution of Trace Elements in the Oceans
In recent years, there has been a rapid increase in our knowledge of the distribution of
minor trace elements (mostly metals) in the oceans. This recent revolution is related to
major advances in instrumentation and the elimination or control of contamination during sampling, storage, and analysis. Bruland (1983) has reviewed these developments.
Preconcentration techniques such as chelation on resins and coprecipitation have been
used to separate an element from the major components of seawater, and measurements
of trace elements at levels of nanomoles and picomoles per kilogram have been achieved.
The distributions of the elements were found to be consistent with known biological and
physical behavior. Ultraclean room techniques pioneered by Patterson and coworkers
(Schaule and Patterson, 1983) in studies of lead and the use of specially designed hydrowires (Kevlar) and sample bottles (Teflon coated) provided reliable data for trace metals
such as Mn, Cu, Cd, Ni, Ba, and Fe. Surface values for Pb, Hg, Cu, Ni, and Zn were obtained
using rafts and by collecting the samples by hand. More recently, Biller and Bruland (2012),
as part of the GEOTRACES program, have determined profiles of Mn, Pb, Fe, Co, Zn, Cu,
Ni, and Cd in the North Atlantic and Pacific Oceans. These new results are a prelude to
the expected results for these metals throughout the world oceans. The types of profiles
found for various elements can be divided into a number of general categories. Some for
the various types of profiles are as follows:
1. Conservative profile: A constant ratio of the concentration of the element to
chlorinity or salinity is found for some elements because of their low reactivity.
The major components of seawater and trace metals such as Li + , Rb + , and Cs + and
anions such as molybdenum (MoO 4
2– ) and tungsten (WO 4
2– ) exhibit this type of
behavior (see Figure 3.5).
2. Nutrient type profile: A depletion of an element in surface waters and enrichment
at depth is a nutrient type profile. The element is removed from the surface waters
by uptake by plankton or adsorption to biologically produced particulate matter.
It is regenerated in deep waters when the biologically produced particulate matter
is oxidized by bacteria. Three types of nutrient type profiles are found:
a. The shallow water regeneration that leads to a maximum near 1 km, similar
to the nutrients PO 4
3– and NO 3
– . The metal Cd 2+ provides a good example of
this type of nutrient behavior (see Figure 3.6). This behavior indicates that the
element is associated with the soft parts of living and dead biological material.
De Baar and coworkers (De Baar et al., 1994; Löscher et al., 1997) have shown
that many metals show a near- linear correlation with nitrate and phosphate
(Figure 3.7). Boyle (1992) has shown that this relationship can be used to estimate the phosphate in ocean water from the coprecipitated Cd in shells. By
dating the shells, he has been able to estimate the past phosphate in North
Atlantic deep waters as a function of time.
b. The deep regeneration cycle leading to a deep maximum is observed for metals
of this type, similar to the distribution of silica and total alkalinity. Examples of
this type include the elements Zn 2+ , Ge 3+ , Ba 2+ , and Ni 2+ (see Figure 3.8 through
Figure 3.11).
Chemical Oceanography, 4th Edition
3.3 Distribution of Trace Elements in the Oceans
In recent years, there has been a rapid increase in our knowledge of the distribution of
minor trace elements (mostly metals) in the oceans. This recent revolution is related to
major advances in instrumentation and the elimination or control of contamination during sampling, storage, and analysis. Bruland (1983) has reviewed these developments.
Preconcentration techniques such as chelation on resins and coprecipitation have been
used to separate an element from the major components of seawater, and measurements
of trace elements at levels of nanomoles and picomoles per kilogram have been achieved.
The distributions of the elements were found to be consistent with known biological and
physical behavior. Ultraclean room techniques pioneered by Patterson and coworkers
(Schaule and Patterson, 1983) in studies of lead and the use of specially designed hydrowires (Kevlar) and sample bottles (Teflon coated) provided reliable data for trace metals
such as Mn, Cu, Cd, Ni, Ba, and Fe. Surface values for Pb, Hg, Cu, Ni, and Zn were obtained
using rafts and by collecting the samples by hand. More recently, Biller and Bruland (2012),
as part of the GEOTRACES program, have determined profiles of Mn, Pb, Fe, Co, Zn, Cu,
Ni, and Cd in the North Atlantic and Pacific Oceans. These new results are a prelude to
the expected results for these metals throughout the world oceans. The types of profiles
found for various elements can be divided into a number of general categories. Some for
the various types of profiles are as follows:
1. Conservative profile: A constant ratio of the concentration of the element to
chlorinity or salinity is found for some elements because of their low reactivity.
The major components of seawater and trace metals such as Li + , Rb + , and Cs + and
anions such as molybdenum (MoO 4
2– ) and tungsten (WO 4
2– ) exhibit this type of
behavior (see Figure 3.5).
2. Nutrient type profile: A depletion of an element in surface waters and enrichment
at depth is a nutrient type profile. The element is removed from the surface waters
by uptake by plankton or adsorption to biologically produced particulate matter.
It is regenerated in deep waters when the biologically produced particulate matter
is oxidized by bacteria. Three types of nutrient type profiles are found:
a. The shallow water regeneration that leads to a maximum near 1 km, similar
to the nutrients PO 4
3– and NO 3
– . The metal Cd 2+ provides a good example of
this type of nutrient behavior (see Figure 3.6). This behavior indicates that the
element is associated with the soft parts of living and dead biological material.
De Baar and coworkers (De Baar et al., 1994; Löscher et al., 1997) have shown
that many metals show a near- linear correlation with nitrate and phosphate
(Figure 3.7). Boyle (1992) has shown that this relationship can be used to estimate the phosphate in ocean water from the coprecipitated Cd in shells. By
dating the shells, he has been able to estimate the past phosphate in North
Atlantic deep waters as a function of time.
b. The deep regeneration cycle leading to a deep maximum is observed for metals
of this type, similar to the distribution of silica and total alkalinity. Examples of
this type include the elements Zn 2+ , Ge 3+ , Ba 2+ , and Ni 2+ (see Figure 3.8 through
Figure 3.11).