Se(VI), Te(IV) is the stable form as Te(OH) 4 . There
are few data for this element in the ocean, and profiles of tellurium in the eastern North Pacific
(Figures 5D and E) show that its concentrations are
B1000 times less than those of selenium and both
forms of tellurium show strongly scavenged behavior. It is interesting to note that the most abundant form of tellurium is Te(VI), but it is the least
thermodynamically stable. Although there are numerous biological reasons to expect reduced species
in oxic waters (i.e., as for arsenic and selenium), this
observation is somewhat difficult to explain. The
elevated concentrations of Te(VI) at the surface,
relative to Te(IV), suggest that the atmospheric or
riverine inputs of this element are enriched in this
form, but virtually no atmospheric data are available
to confirm this speculation.
Conclusions
Overall, the classic types of behavior described for
trace elements are displayed by the oxyanions. Rhenium, molybdenum, and methylated germanium
have conservative, salinity-like distributions with
depth. This does not mean that they are truly unreactive, but rather that the reactions affecting them
are minor. Vanadium, chromium, osmium, arsenic,
and selenium show nutrient-like behavior, although
this does not necessarily mean they are biologically
required. Tungsten, antimony, and tellurium show
scavenged behavior to some degree. Hybrid/mixed
behavior was originally defined for trace elements
with a single chemical form, but most of the oxyanions can exist in multiple chemical forms, creating a
new type of hybrid distributions – oxidation–reduction behavior superimposed upon the other three
categories. Finally, the types of behavior found for
these elements are strongly affected by both chemical
and biological processes, and thus it is appropriate to
say that they are ‘biogeochemically’ cycled.
See also
Anthropogenic Trace Elements in the Ocean.
Atmospheric Input of Pollutants. Carbon Cycle.
Conservative Elements. Hydrothermal Vent Fluids,
Chemistry of. Marine Silica Cycle. Metal Pollution.
Nitrogen Cycle. Platinum Group Elements and their
Isotopes in the Ocean. Refractory Metals. River
Inputs. Transition Metals and Heavy Metal
Speciation.
Further Reading
Bruland KW (1992) Trace elements in seawater. In: Riley
JP Chester R (eds.) Chemical Oceanography, vol. 8,
pp. 157–220. London: Academic Press.
Donat JR and Bruland KW (1995) Trace elements in the
oceans. In: Salbu B and Steinnes E (eds.) Trace Elements
in Natural Waters, pp. 247--281. Boca Raton, FL: CRC
Press.
Libes SM (1992) An Introduction to Marine Biogeochemistry. New York: Wiley.
Millero FJ (1996) Chemical Oceanography, 2nd edn. Boca
Raton, FL: CRC Press.
Pilson MEQ (1998) An Introduction to the Chemistry of
the Sea. Reading, NJ: Prentice Hall.
Stumm W and Morgan JJ (1996) Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. New York:
Wiley.
METALLOIDS AND OXYANIONS 71
are few data for this element in the ocean, and profiles of tellurium in the eastern North Pacific
(Figures 5D and E) show that its concentrations are
B1000 times less than those of selenium and both
forms of tellurium show strongly scavenged behavior. It is interesting to note that the most abundant form of tellurium is Te(VI), but it is the least
thermodynamically stable. Although there are numerous biological reasons to expect reduced species
in oxic waters (i.e., as for arsenic and selenium), this
observation is somewhat difficult to explain. The
elevated concentrations of Te(VI) at the surface,
relative to Te(IV), suggest that the atmospheric or
riverine inputs of this element are enriched in this
form, but virtually no atmospheric data are available
to confirm this speculation.
Conclusions
Overall, the classic types of behavior described for
trace elements are displayed by the oxyanions. Rhenium, molybdenum, and methylated germanium
have conservative, salinity-like distributions with
depth. This does not mean that they are truly unreactive, but rather that the reactions affecting them
are minor. Vanadium, chromium, osmium, arsenic,
and selenium show nutrient-like behavior, although
this does not necessarily mean they are biologically
required. Tungsten, antimony, and tellurium show
scavenged behavior to some degree. Hybrid/mixed
behavior was originally defined for trace elements
with a single chemical form, but most of the oxyanions can exist in multiple chemical forms, creating a
new type of hybrid distributions – oxidation–reduction behavior superimposed upon the other three
categories. Finally, the types of behavior found for
these elements are strongly affected by both chemical
and biological processes, and thus it is appropriate to
say that they are ‘biogeochemically’ cycled.
See also
Anthropogenic Trace Elements in the Ocean.
Atmospheric Input of Pollutants. Carbon Cycle.
Conservative Elements. Hydrothermal Vent Fluids,
Chemistry of. Marine Silica Cycle. Metal Pollution.
Nitrogen Cycle. Platinum Group Elements and their
Isotopes in the Ocean. Refractory Metals. River
Inputs. Transition Metals and Heavy Metal
Speciation.
Further Reading
Bruland KW (1992) Trace elements in seawater. In: Riley
JP Chester R (eds.) Chemical Oceanography, vol. 8,
pp. 157–220. London: Academic Press.
Donat JR and Bruland KW (1995) Trace elements in the
oceans. In: Salbu B and Steinnes E (eds.) Trace Elements
in Natural Waters, pp. 247--281. Boca Raton, FL: CRC
Press.
Libes SM (1992) An Introduction to Marine Biogeochemistry. New York: Wiley.
Millero FJ (1996) Chemical Oceanography, 2nd edn. Boca
Raton, FL: CRC Press.
Pilson MEQ (1998) An Introduction to the Chemistry of
the Sea. Reading, NJ: Prentice Hall.
Stumm W and Morgan JJ (1996) Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. New York:
Wiley.
METALLOIDS AND OXYANIONS 71