7
The Biogeochemistry of Iron
262
ficant if reduction is predominantly coupled to
dissimilation and chemical reduction is unimportant. This is the case for the manganese
dominated sites Skagerrak S9 and Panama Basin
where neither the production of HS
-
nor of Fe
2+
was
found. O 2 consumption was (almost) completely
attributed to the reoxidation of Mn
2+
in these cases.
In contrast, Skagerrak sites S4 and S6 as well as
sediments off Chile resemble situations where iron
is reduced chemically (HS
-
) by up to ~ 50 % and
only smaller amounts of ferric iron are available for
the dissimilation. A third situation can be inferred
for typical open ocean sediments where low Fe/Mn
and organic matter deposition along with low
bioturbation intensity occurs. Here organic matter
decomposition is restricted to aerobic respiration
and denitrification. Iron and manganese reduction
rates are presumably negligible, yet over a long
period of time a significant proportion of
manganese is redistributed and the composition of
iron oxide phases changes (Haese et al. 1998).
An extensive study in a shallow water, estuarine mixing zone elucidated ideal conditions for
efficient manganese cycling (Aller 1994).
Manganese turn-over was found to be most
intense during warm periods with intensive
bioturbation, well-oxygenated bottom water, and
moderate organic matter input. Under such
conditions manganese reoxidation consumed 3050 % of the benthic oxygen flux and manganese
reduction was mainly induced by HS
-
, FeS and
FeS 2 . As soon as bottom water became O 2 -
depleted the sedimentary Mn-cycle was reduced
as dissolved Mn escaped out of the sediment. The
influence of bioirrigation has not yet been
explicitly investigated but modeling results by
Wang and Van Capellen (1996) imply enhanced
metal cycling efficiency with increasing irrigation
due to more rapid Fe
2+
- / Mn
2+
-oxidation.
A conceptual model describing the importance
of iron and manganese reactivity in different
environments is only sketchy as results are scarce
and manifold aspects deserve further investigations. A general complication concerns the
differentiation of dissimilatory and chemical
reduction of Mn-oxide when concurrent iron
reduction is apparent and further investigations
need to discriminate each reaction pathway. The
reactivity of smectite-bound iron has only been
shown qualitatively (Kostka et al. 1996; König et
al. 1997), yet quantifications with respect to
dissimilatory and/or chemical reactions are
missing. An important role of adsorbed Fe
2+
/
Mn
2+
is indicated (Sørensen and Thorling 1991;
Roden and Zachara 1996) but not proved by
sediment studies. Investigations focusing on the
importance of metal cycling and its influence on
pathways of organic matter decomposition are
scarce (Canfield et al. 1993a, Wang and Van
Capellen 1996), yet necessary to understand an
important link of the carbon cycle.
7.5
The Assay for Ferric and
Ferrous Iron
In order to study iron reactivity qualitatively and
quantitatively it is essential to quantify the
ferrous and ferric iron fractions of the present
minerals or mineral groups. With respect to the
determination of iron speciation the principal
problem is the rapid oxidation of ferrous iron.
Atmospheric oxygen diffuses into pore water
where it oxidizes dissolved ferrous iron ‘immediately’ and starts oxidizing FeS and FeS 2 . Reduced smectites may also become oxidized under air
atmosphere within hours. Therefore, if dissolved
iron and iron speciation of solid phase are to be
determined samples need to be conserved under
inert gas atmosphere. No extra care is needed for
the determination of total iron of solid phase.
Above a pH of 3 and in the absence of chelators dissolved iron is only present as ferrous iron
under natural conditions. Therefore, the colored
complex that results from the reaction between
ferrous iron and a reagent can be analysed
colorimetrically and correlates with the concentration of total dissolved iron. Most conveniently,
one can mix a drop of Ferrozine® solution
(Stookey 1970), one drop of H 2 SO 4 (diluted 1:4)
and 1 ml of pore water in the glove box, wait until
complex formation is completed (20-30 minutes)
and quantify the iron concentration by the
intensity of the color at a wavelength of 562 nm.
To avoid matrix effects standards should be
prepared with artificial seawater.
The assay of solid phase ferrous and ferric
iron has a long tradition due to the early interest
in soil chemistry. Publications on extraction /
leaching conditions and results from varying
soils and sediments are extensive and thus,
within the scope of a textbook, only important
principals and a description of the (subjectively)
most important extractions can be given. Since a
The Biogeochemistry of Iron
262
ficant if reduction is predominantly coupled to
dissimilation and chemical reduction is unimportant. This is the case for the manganese
dominated sites Skagerrak S9 and Panama Basin
where neither the production of HS
-
nor of Fe
2+
was
found. O 2 consumption was (almost) completely
attributed to the reoxidation of Mn
2+
in these cases.
In contrast, Skagerrak sites S4 and S6 as well as
sediments off Chile resemble situations where iron
is reduced chemically (HS
-
) by up to ~ 50 % and
only smaller amounts of ferric iron are available for
the dissimilation. A third situation can be inferred
for typical open ocean sediments where low Fe/Mn
and organic matter deposition along with low
bioturbation intensity occurs. Here organic matter
decomposition is restricted to aerobic respiration
and denitrification. Iron and manganese reduction
rates are presumably negligible, yet over a long
period of time a significant proportion of
manganese is redistributed and the composition of
iron oxide phases changes (Haese et al. 1998).
An extensive study in a shallow water, estuarine mixing zone elucidated ideal conditions for
efficient manganese cycling (Aller 1994).
Manganese turn-over was found to be most
intense during warm periods with intensive
bioturbation, well-oxygenated bottom water, and
moderate organic matter input. Under such
conditions manganese reoxidation consumed 3050 % of the benthic oxygen flux and manganese
reduction was mainly induced by HS
-
, FeS and
FeS 2 . As soon as bottom water became O 2 -
depleted the sedimentary Mn-cycle was reduced
as dissolved Mn escaped out of the sediment. The
influence of bioirrigation has not yet been
explicitly investigated but modeling results by
Wang and Van Capellen (1996) imply enhanced
metal cycling efficiency with increasing irrigation
due to more rapid Fe
2+
- / Mn
2+
-oxidation.
A conceptual model describing the importance
of iron and manganese reactivity in different
environments is only sketchy as results are scarce
and manifold aspects deserve further investigations. A general complication concerns the
differentiation of dissimilatory and chemical
reduction of Mn-oxide when concurrent iron
reduction is apparent and further investigations
need to discriminate each reaction pathway. The
reactivity of smectite-bound iron has only been
shown qualitatively (Kostka et al. 1996; König et
al. 1997), yet quantifications with respect to
dissimilatory and/or chemical reactions are
missing. An important role of adsorbed Fe
2+
/
Mn
2+
is indicated (Sørensen and Thorling 1991;
Roden and Zachara 1996) but not proved by
sediment studies. Investigations focusing on the
importance of metal cycling and its influence on
pathways of organic matter decomposition are
scarce (Canfield et al. 1993a, Wang and Van
Capellen 1996), yet necessary to understand an
important link of the carbon cycle.
7.5
The Assay for Ferric and
Ferrous Iron
In order to study iron reactivity qualitatively and
quantitatively it is essential to quantify the
ferrous and ferric iron fractions of the present
minerals or mineral groups. With respect to the
determination of iron speciation the principal
problem is the rapid oxidation of ferrous iron.
Atmospheric oxygen diffuses into pore water
where it oxidizes dissolved ferrous iron ‘immediately’ and starts oxidizing FeS and FeS 2 . Reduced smectites may also become oxidized under air
atmosphere within hours. Therefore, if dissolved
iron and iron speciation of solid phase are to be
determined samples need to be conserved under
inert gas atmosphere. No extra care is needed for
the determination of total iron of solid phase.
Above a pH of 3 and in the absence of chelators dissolved iron is only present as ferrous iron
under natural conditions. Therefore, the colored
complex that results from the reaction between
ferrous iron and a reagent can be analysed
colorimetrically and correlates with the concentration of total dissolved iron. Most conveniently,
one can mix a drop of Ferrozine® solution
(Stookey 1970), one drop of H 2 SO 4 (diluted 1:4)
and 1 ml of pore water in the glove box, wait until
complex formation is completed (20-30 minutes)
and quantify the iron concentration by the
intensity of the color at a wavelength of 562 nm.
To avoid matrix effects standards should be
prepared with artificial seawater.
The assay of solid phase ferrous and ferric
iron has a long tradition due to the early interest
in soil chemistry. Publications on extraction /
leaching conditions and results from varying
soils and sediments are extensive and thus,
within the scope of a textbook, only important
principals and a description of the (subjectively)
most important extractions can be given. Since a