FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
Evolution of the geochemical cycles of redox-sensitive elements
Kosei E. Yamaguchi 1, 2, 3
Research Program for Paleoenvironment, Institute for Frontier Research on Earth Evolution (IFREE)
Department of Geology and Geophysics, University of Wisconsin–Madison
3
NASA Astrobiology Institute
1
2
Fiin, ocean = Fidis + Fihyd + Fireg ..........................................(2)
Introduction
Removal of an element from oceans (flux: Fout, ocean) occurs
in two ways: burial in sediments (flux: Fppt; ppt: precipitation)
and fixation in MOR system through its seawater circulation
(flux: FMOR).
The concentrations of elements in the atmosphere and
oceans have been regulated by a variety of geochemical interactions with the continental and oceanic lithospheres. The
cycling of an element between the Earth’s major reservoirs is
referred to as the geochemical cycle of that element. The geochemical cycles of redox-sensitive elements in the Earth’s surface environments have been mediated by complex redox
reactions involving both biological and non-biological controls. The evolution of the geochemical cycles of redox-sensitive elements is therefore intimately linked to the evolution of
the atmosphere, oceans, and biosphere. This study assesses the
evolution of the geochemical cycles of some redox-sensitive
elements (C, S, N, Fe, P, Mo, and U) through geologic time,
and briefly summarizes the geochemical cycling of these
redox-sensitive elements through the Earth's major reservoirs
and the behaviors of these elements in typical modern marine
sediments. The geochemical cycles of the above-mentioned
individual elements and their evolution are treated in more
detail by Yamaguchi (2002).
Fiout, ocean = Fippt + FiMOR ................................................(3)
At a steady state (Fiin, ocean = Fiout, ocean),
Fidis + Fihyd + Fireg = Fippt + FiMOR ......................................(4)
All of the fluxes Fiweath, Fidis (Fichemw), Fidet (Fiphysw), Fihyd, Fireg,
F , and FiMOR are complex functions with numerous parameters, such as pO2, pCO2, topography, climate, ocean chemistry,
heat flux, etc. We extract important differences in these fluxes
mainly between globally oxic and globally anoxic environments. In a globally anoxic world, where the atmosphere and
entire oceans are anoxic, continental weathering (Fweath) may
be dominated by physical weathering (Fphysw) and by reductive
chemical weathering. Organic carbon, S, N, Fe, Mo, and U
may be transported through rivers to the oceans mainly as
detrital forms with their reduced valency (C0, S2-, N3-, Fe2+,
Mo4+, and U4+; Fig. 1). Iron may also be transported as dissolved Fe2+ by the reductive dissolution of Fe3+-bearing minerals (e.g., magnetite) as well in congruent dissolution of the
Fe2+-bearing minerals. Accordingly, the following relationships would be expected for the redox-sensitive elements.
i
ppt
Geochemical cycles of redox-sensitive elements
We consider a simple mass-balance model for the geochemical cycles of C, S, N, Fe, P, Mo, and U using a system
composed of four major reservoirs: continents, oceans, sediments, and mid-oceanic ridges (MOR) (Fig. 1). The atmosphere is implicitly included in the continents and oceans reservoirs. The mantle is another important reservoir; however, it is
not explicitly considered here because we are primarily concerned with the geochemical cycles of those elements in the
Earth's surface environments.
Through continental weathering (flux: Fweath), the physically /
chemically weathered materials (fluxes: Fphysw and Fchemw) are
transported through rivers to the oceans via two mechanisms:
the detrital (particulate) transport (flux: Fdet) and the dissolved
transport (flux: Fdis). For an element i,
For C, S, N, Mo, and U:
Fphysw/Fchemw (anoxic atmosph.)
> Fphysw/Fchemw (oxic atmosph.) ......................................(5)
For Fe:
Fphysw/Fchemw (anoxic atmosph.)
< Fphysw/Fchemw (oxic atmosph.) ......................................(6)
In contrast, in a globally oxic world where the entire atmosphere and oceans are oxic with local anoxic environments
(e.g., mid-depth O 2 minimum zone and anoxic basins) as
today, continental weathering (Fweath) would be dominated by
the oxidative chemical weathering and the physical weathering
(Fphysw). Organic carbon, S, N, Mo, and U would be transported through rivers to the oceans mainly as dissolved (oxidized)
forms with their increased valency (C4+, S6+, N5+, Mo6+, and
U6+; Fig. 1). Iron would be transported in oxidized (Fe3+),
detrital forms. Global redox conditions would control the
valence of elements and their total weathering flux.
Fiweath = Fiphysw + Fichemw = Fidis + Fidet ................................(1)
Continental chemical weathering could be oxidative or reductive weathering depending on the global / local redox conditions. The hydrothermal flux (Fhyd) from the submarine MOR
system also enters into the oceans, although Fhyd is variable in
space and time depending on the activity of the MOR system.
Some elements are regenerated from the sediments into oceans
(flux: F reg), depending on the redox conditions of bottom
water. Therefore the input flux of an element to the oceans
(dissolved component; Fin, ocean) is the sum of the Fdis, Fhyd,
and Freg.
For C, S, P, N, Mo, and U:
Fweath (anoxic atmosph.) < Fweath (oxic atmosph.) ............(7)
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For Fe:
Examples from the Precambrian and the Cretaceous OAE
Fweath (anoxic atmosph.) > Fweath (oxic atmosph.) ............(8)
Phosphorus does not change its valency during chemical
weathering; however, its behavior in the oceans does depend
on the redox state of the water body. This is because P
adsorbed on Fe (Mn)-oxyhydroxides formed in oxic oceans is
released into seawater upon reductive dissolution of Fe (Mn)oxyhydroxides during settling in local anoxic water columns
and/or early diagenesis in sediments.
In order to constrain the redox evolution of the atmosphere
and oceans, distribution of the above-mentioned redox-sensitive elements has been investigated by Yamaguchi (2002)
using modern weathering-free Archean–Paleoproterozoic sedimentary rocks. Yamaguchi (2002) used samples of mostly carbonaceous black shales, with minor graywackes and red
shales, their depositional ages ranging from 3.25 to 2.2 billion
years old. It has been demonstrated that the abundance of
redox-sensitive elements in clastic sedimentary rocks can be
used as a powerful guide to examine the redox conditions of
their sedimentary environments. The results will be published
elsewhere.
Cretaceous has been characterized by globally warm climatic conditions, probably due to enhanced greenhouse effects
of greenhouse gases such as CO2, and the intermittent developments of the “Oceanic Anoxic Events” (OAE: e.g.,
Schlanger et al., 1987; Arthur et al., 1990). Causes and consequences of the Cretaceous OAE have not been well understood; however, they have important implications for understanding the surface environmental evolution, both in the past
and in future. For example, the Cretaceous OAE can be used
as an analogue to the Archean ocean, which could have been
(1) globally anoxic because of a lack of oxygenated atmosphere (e.g., Holland, 1984) and (2) substantially warmer compared to modern ocean because of (i) significant greenhouse
effect by much higher pCO2 (x 100 ~ x 1000) (e.g., Kasting,
1993) and (ii) heat flow (Sclater et al., 1980). For another
example, the Cretaceous OAE may provide us with some
information regarding the future of marine ecosystem. Development of the OAE in the Cretaceous oceans had a significant
impact on the diversity of marine organisms living in the
coeval oceans (e.g., extinction). The modern-day marine community may be threatened by environmental changes such as
the inferred global warming and the subsequent possible
development of anoxic water bodies; the density stratification
of an oceanic basin may lead to stagnation of its bottom water
and its anoxia).
Distribution of redox-sensitive elements in the sedimentary
rocks of the Cretaceous OAE (and of stratigraphically near-by
units) will be investigated to obtain geochemical insights into
the nature of Cretaceous OAE as a planned research project at
IFREE. Another planned target for this type of research at
IFREE is modern-day anoxic sediments and the overlying
(anoxic/oxic) water column (e.g., Lake Kai-ike in Kyushu,
Japan). Such information should shed light on the chemical and
redox evolution of a water body, especially the mechanisms of
appearance and disappearance of water column anoxia.
FPreg (anoxic bottom water) > FPreg (oxic bottom water) ....(9)
A higher surface temperature and higher pCO 2 in the
Archean compared to today (e.g., Sclater et al., 1980; Kasting,
1993) would have enhanced continental weathering rates;
however, this could have been compensated to some degree by
the suppressed continental (oxidative) chemical weathering
rate if the Archean atmosphere had a low pCO2 level. Additionally, the inferred higher heat flux in the Archean would
have enhanced both the F hyd and F MOR values because of
enhanced seawater hydrothermal circulation through MOR.
Fihyd (Archean) > Fihyd (today) ......................................(10)
FiMOR (Archean) > FiMOR (today) ..................................(11)
Influence of pO2 on the geochemical cycles of
redox-sensitive elements
A sequence of redox reactions mediated by microorganisms
in a typical profile of modern marine sediments is calculated
using the thermodynamic data in literature and shown in Fig. 2.
Organic matter is the principal source of energy to promote these
reactions. The general sequence of oxidants used in the decomposition of OM is O2 (aerobic respiration) →N(5+)O3- (denitrification) →Mn(4+)O2 →Fe(3+)(OH)3 →U(6+)O2(CO3)22- →
Mo(6+)O42- →S(6+)O42- (sulfate reduction) →C(4+)O2 (methanogenesis). This sequence corresponds to decreases in the redox
potential of the oxidants (electron donors) and thus decreases in
the free energy available by respiration with the different reductants (electron acceptors) (Fig. 2). The diagram in Fig. 2 is also
important in the evolution of the Fdis/Fdet ratios. At pCO2 levels
higher than 10-10 atm, all the redox-sensitive elements of interest
(C, S, N, Fe, Mo, and U) are in their oxidized valence states and
Fdis is expected to be more important than Fdet, although the
kinetics of chemical weathering reactions should be taken into
account.
The development of these redox sequences in marine sediments is controlled by a variety of microbiological organisms
that depend on the development of anoxic conditions within
sediments and/or in the overlying water column. Various types
of authigenic minerals are formed in marine sediments during
early diagenesis. Iron sulfide is quantitatively the most important phase. Useful information regarding the geochemical
cycling of elements, redox conditions and biological activity
can be obtained by examining the enrichment patterns of
redox-sensitive elements in sediments and sedimentary rocks.
In particular, a coupled approach using the inorganic geochemistry of redox-sensitive elements and organic and stable
isotope geochemistry has proven to be a powerful tool to
extract (paleo)environmental information.
Acknowledgements. Profs. Hiroshi Ohmoto, Mike Arthur, Lee
Kump, and Ray Najjar of the Pennsylvania State University are appreciated for their continued encouragement and discussion about the
material presented in this paper. Generous financial supports from
NSF, NASA Exobiology Program, and NASA Astrobiology Institute
are appreciated.
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References
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Holland, H. D., Chemical evolution of the atmosphere and ocean,
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Figure 2. Schematic diagram showing the characteristic pE values for
redox reactions of trace metals and major electron acceptors during
decomposition of organic matter in sediments. The corresponding pO2
levels are also shown. The pE values are for [NO 3 - ]=50µM,
[Mn2+]=1µM, [Fe2+]=5µM, [SO42-]=28mM, [H2S]=0.01µM, [MoO42]=106nM, [UO2(CO3)34-]=13nM, and [H2VO4-]=40nM. The data used
for calculation are from Crusius et al. (1996) and Piper (1994). Adapted from Yamaguchi (2002).
Figure 1. Geochemical cycles of C, S, N, Fe, Mo, and U in the continents-oceans-sediments system. The fluxes (F) between the resevoirs
are also indicated. Fdet=detrital weathering flux, Fdis=dissolved weathering flux, F ppt =precipitation flux (biogenic, scavenging, etc.),
Freg=regeneration flux, Fhyd=hydrothermal flux, and Fburial=burial flux.
At a steady state, the input flux (Fin) to the oceans and the output flux
(Fout) from the oceans are in balance: Fin=Fdis+Fhyd+Freg=Fout=Fppt+FMOR.
Adapted from Yamaguchi (2002).
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