Kump et al EPSL 2005.pdf - Bryn Mawr College

Earth and Planetary Science Letters 235 (2005) 654–662
www.elsevier.com/locate/epsl
Hydrothermal Fe fluxes during the Precambrian: Effect of low
oceanic sulfate concentrations and low hydrostatic pressure
on the composition of black smokers
Lee R. Kump a, *, William E. Seyfried Jr. b
a Department of Geosciences and Astrobiology Research Center, The Pennsylvania State University, 535 Deike Bldg.,
University Park, PA 16827, USA
b Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455-0219, USA
Received 18 June 2004; received in revised form 15 March 2005; accepted 25 April 2005
Available online 15 June 2005
Editor: E. Boyle
Abstract
Modern mid-ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved H 2 S in excess of Fe.
These fluids are the consequence of high-temperature interactions between sulfate-rich seawater and mid-ocean ridge basalt at
conditions near the critical point for seawater. The precipitation of FeS and FeS 2 during venting titrates most of the Fe from the
fluid, significantly reducing the net flux of Fe to the open ocean. Here we suggest that hydrothermal fluids emanating from
Precambrian seafloor systems older than ~1.8 Ga had Fe/H 2 S ratiosNN1, and with fH 2 higher than today, because seawater
lacked its primary oxidant, dissolved sulfate ion. This predominance of Fe over H 2 S would have promoted the establishment of
an iron-rich deep ocean and the deposition of banded iron formations (BIF). Accordingly, the end of BIF deposition at ~1.8 Ga
was the result of the buildup of sulfate in seawater from oxidative weathering, and its return at 750 Ma the result of reductions in
seawater sulfate concentrations during Snowball Earth episodes, enhanced by elevated Fe concentrations during depressurization
of hydrothermal systems by large eustatic sea-level falls. Moreover, Precambrian chemosynthetic vent communities may
have been based on H 2 synthesis rather than on H 2 S oxidation, as they largely are today.
D 2005 Elsevier B.V. All rights reserved.
Keywords: hydrothermal; banded iron formation; Precambrian; iron; sulfate
1. Introduction
* Corresponding author.
E-mail address: lkump@psu.edu (L.R. Kump).
Mid-ocean ridge vent fluids have been considered
a likely source of Fe for Precambrian iron formations
[1]. However, modern vent compositions are unlikely
to generate BIF because the concentration of H 2 S
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.04.040

L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662 655
generally exceeds the Fe concentration such that Fe
is titrated out of the vent fluid by the precipitation of
FeS and FeS 2 [2,3]. Walker and Brimblecombe [4]
pointed out, however, that in the absence of seawater
sulfate, vent fluids would be Fe dominated simply
because basalt has an abundance of Fe-minerals
relative to S-minerals.
In a similar vein, Canfield [5] suggested that the
end of BIF deposition at ~1.8 Ga resulted from the
accumulation of sulfate in the ocean following the rise
in atmospheric oxygen abundance at ~2.3 Ga. In
Canfield’s model, microbial reduction of this sulfate
led to the development of sulfidic conditions in the
deep sea that severely reduced the concentration of
ferrous iron because of the insolubility of FeS. An
alternative explanation, consistent with Walker and
Brimblecombe’s model, is that abiogenic sulfide associated
with hydrothermal venting of Paleoproterozoic
sulfate-rich seawater-derived fluids led to the
precipitation of iron-sulfide minerals, creating conditions
unfavorable for BIF production (with Fe/
H 2 Sb1). According to either model, the return of
BIF deposition during the Neoproterozoic bSnowball
EarthQ episodes would have been more related to the
depletion of seawater sulfate during the ice-covered
interval than to the establishment of oceanic anoxia
(the original argument put forth by Kirschvink [6] and
Beukes and Klein [7]); anoxia alone does not affect
the Fe/H 2 S ratio of vent fluids nor does it allow for
the buildup of Fe because of the high concentrations
of H 2 S that develop.
In this paper we explore the Walker and Brimblecombe’s
model [4] using available thermodynamic
data for seawater equilibria at elevated temperatures
and pressures. Assuming the absence of seawater
sulfate, we find that Precambrian vent fluids were
considerably more reducing, with ratios of Fe/H 2 S
that exceeded unity. Low hydrostatic pressures during
Snowball Earth episodes of the Neoproterozoic and
for much of the Archean, if mid-ocean ridge depths
were shallower then than now [1], may have further
enhanced Fe concentrations.
2. Constraints on vent fluid composition
After nearly 20 years of experimentation and theoretical
development, we now have a clearer understanding
of the effects of temperature and pressure on
the chemistry of mid-ocean ridge hydrothermal systems
[8,9]. The composition of fluids collected from
black smoker chimneys can be explained for the most
part in terms of fluid–mineral equilibria, established at
elevated temperatures and pressures in subseafloor
reaction zones near magma chambers below the
chimneys. These solutions then rise convectively to
the sea floor, mixing with cold seawater, precipitating
insoluble minerals, and in some cases, separating into
vapor and brine [10,11].
2.1. Effect of seawater sulfate on vent fluid
composition
In sharp contrast with modern hydrothermal systems,
low dissolved sulfate in the ocean [5,12,13]
would likely have rendered Precambrian hydrothermal
systems more reducing, enhancing dissolved Fe
concentrations in coexisting vent fluids. We can illustrate
this by means of a phase diagram for a portion of
the FeO–Fe 2 O 3 –H 2 S–SiO 2 –CaO–H 2 O–HCl system at
400 8C, 400 bars (Fig. 1). Reaction of basalt/gabbro or
even more reducing protolith with a fluid lacking
sulfate would permit the inherent redox capacity of
the rock to buffer the fluid resulting in relatively high
H 2 /H 2 S ratios, consistent with constraints imposed by
ferrous iron-bearing phases, such as might be approximated
by the fayalite–magnetite–pyrrhotite-bearing
system. Vent fluids impacted by magmatic degassing
effects provide an indication of this. For example,
hydrothermal vent fluids from 98 to 108 N, East
Pacific Rise and the Endeavour segment of the Juan
de Fuca Ridge, indicate high H 2 /H 2 S ratios and high
H 2 concentrations in the immediate aftermath of subseafloor
magmatic intrusions [14]. Although uncertainties
exist in terms of temperature, pressure and
phase separation effects, the high H 2 in particular is
an indicator of distinctly reducing conditions associated
with vapor release at the magmatic–hydrothermal
interface [14,15]. In all cases, however, time series
observations reveal sharp decreases in dissolved H 2
and H 2 S, with changes in H 2 greater than H 2 S [14],
suggesting more oxidizing conditions. Indeed, even
modest rock–seawater interaction can be expected to
cause the fluid to achieve saturation with respect to
anhydrite, assuming coexistence of plagioclase feldspar
and quartz (Fig. 1), which is in good agreement

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L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662
-1.0
T = 400 °C
P = 400 bars
Cl = 0.55 molal
-1.5
Pyrite
Pyrrhotite
log H 2 S(aq) (molal)
-2.0
-2.5
Hematite
Anhydrite
-2
SO 4 - bearing
seawater
Magnetite
-2
SO 4 - free seawater
Fayalite
-3.0
-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0
log H 2(aq) (molal)
Fig. 1. Phase equilibria in the FeO–Fe 2 O 3 –H 2 S–SiO 2 –CaO–H 2 O–HCl system at 400 8C, 400 bars. Reaction of fresh basalt and/or ultramafic
lithologies with sulfate-free aqueous fluid can be expected to result in relatively high H 2 and moderate H 2 S concentrations, respectively,
analogous to constraints imposed by fayalite–magnetite–pyrrhotite–fluid equilibria. In contrast, the relative abundance of sulfate in modern
seawater provides a powerful oxidizing agent that is capable of buffering H 2 at lower values, although H 2 S concentrations are generally similar
to the more reducing system. For plagioclase-bearing systems, these redox constraints ultimately result in anhydrite formation, a condition that
is consistent with the chemistry of many modern vent fluids (diagonal boundary). The two distinctly different redox conditions—reducing/
initially sulfate-free fluid (fayalite/magnetite/pyrrhotite equilibria), and more oxidizing/initially sulfate-bearing fluid (anhydrite/magnetite
equilibria) can exert a fundamental control on dissolved Fe concentrations (see below). Modern vent fluids unaffected of recent subseafloor
magmatic activity reveal H 2 /H 2 S concentrations that are in good agreement with the plagioclase buffered, anhydrite-bearing system [8,14].
Thermodynamic data used for the construction of the diagram are from Johnson et al. [41]. Activity–concentration relations for pyrrhotite and
H 2 and H 2 S are from Barton and Skinner [42], Kishima [43] and Ding and Seyfried [44], respectively.
with the vent fluid dissolved gas data [8,14,16]. Accordingly,
modern vent fluids with dissolved chloride
concentrations close to seawater values have dissolved
H 2 and H 2 S concentrations that typically do
not exceed 1 and 10 mmol/kg, respectively [3,14].
The role of dissolved sulfate in the redox evolution of
modern vent fluids, however, is also manifest by d 34 S
data, which reveal seawater and rock-derived sources
of sulfur [17], while penetration of sufficient sulfate to
render anhydrite stable in high-temperature hydrothermal
reaction zones is consistent with d 34 S of sulfate in
hydrothermally altered rocks [18].
Redox constraints imposed by dissolved sulfate not
only affect dissolved H 2 and H 2 S concentrations in
vent fluids, but also dissolved Fe. For example, calculations
for typical seafloor hydrothermal vent fluids
(400 8C, 400 bars, and an in situ pH of 5; [19])
indicate relatively high Fe concentrations for the fayalite–pyrrhotite-bearing
system, whereas more modest
Fe concentrations are predicted for sulfate-bearing
systems (Fig. 2a and b). Although dissolved Fe concentrations
change greatly from one condition to the
other, dissolved H 2 S remains relatively constant (see
Fig. 1). Thus, the absence of sulfate in seawater would
likely yield vent fluids with high Fe/H 2 S ratios,
approaching values imposed by the reducing nature
of the unaltered rock. The high ratios depicted in (a)
are likely even with modest increases in pH.
A consequence of the high Fe /H 2 S ratio vent
fluids would be more efficient delivery of Fe to the
ocean. For example, calculations show that mixing a
fluid initially buffered at 400 8C by fayalite–magnetite–pyrrhotite
equilibria with sulfate-free seawater, as
would occur on venting, fails to prevent the flux of Fe

L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662 657
80
70
Fe
a.
Concentration (mmol/kg)
60
50
40
30
20
10
0
12
11
10
H 2 S (aq)
5.0 5.2 5.4 5.6 5.8 6.0
pH
H 2 S
b.
Concentration (mmol/kg)
70
60
50
40
30
20
10
0
a.
Fe
H 2 S
0 50 100 150 200 250 300 350 400
Temperature °C
Concentration (mmol/kg)
9
8
7
6
5
4
3
2
1
Fe
Concentration (mmol/kg)
10
8
6
4
2
b.
H 2 S
Fe
5.0 5.2 5.4 5.6 5.8 6.0
pH
0
Fig. 2. The effect of pH on dissolved Fe for fayalite–magnetite–
pyrrhotite–(quartz)–fluid equilibria (a) and anhydrite–magnetite–
(quartz)–plagioclase (An80) [8]. (b). Calculations were performed
at 400 8C, 400 bars. At a pH of 5, which is typical of modern hot
spring vent fluids at mid-ocean ridges [15], high Fe and relatively
low H 2 S are predicted (high Fe/H 2 S ratio) assuming equilibria with
the more reducing assemblage (a), while the opposite is true for the
anhydrite-bearing bmodernQ system (b). The relative absence of
sulfate in ancient oceans (Archean, Neoproterozoic) would permit
more reducing assemblages in host rocks to persist, enhancing Fe
solubility. Model calculations were performed using EQ3/6 [45]
with thermodynamic data generated using SUPCRT92 [41,46],
assuming dissolved chloride of 0.55 mol/kg. See Fig. 1 for additional
sources of thermodynamic data.
into the ancient ocean. Most of the loss in Fe is by
dilution, although minor Fe-mineralization (pyrite,
pyrrhotite and at sufficiently low temperature, hematite)
also occurs (Fig. 3a). Because of the high Fe/H 2 S
ratio, however, complete removal of H 2 S from the
0 50 100 150 200 250 300 350 400
Temperature °C
Fig. 3. Reaction path model depicting the effect of mixing (cooling)
on dissolved Fe and H 2 S initially set assuming fayalite–magnetite–
pyrrhotite–(quartz)–fluid equilibria (a) and anhydrite–pyrite–magnetite–(quartz)–plagioclase
(An80) (b) (see Fig. 2). Concentrations
of Fe and H 2 S at 400 8C, 400 bars were calculated assuming pH=5,
and 0.55 mol/kg dissolved chloride. Temperature change was calculated
assuming mixing with a NaCl fluid (0.55 mol/kg) at 25 8C
(a), while modern seawater was the low temperature mix fluid for
second simulation (b). Dilution effects and temperature dependent
changes in sulfide mineral solubility (pyrite, pyrrhotite, hematite)
and homogeneous equilibria (pH change) cause the predicted
changes in Fe and H 2 S (a). These effects can result in the delivery
of relatively high Fe and high Fe/H 2 S ratio fluids to the ancient
ocean affecting BIF deposition. This is not the case for modern
sulfate-bearing systems due to the initially low Fe/H 2 S ratio of the
predicted source fluid (b). Model calculations were performed using
EQ3/6 [45] with thermodynamic data generated using SUPCRT92
[41,46], assuming dissolved chloride of 0.55 mol/kg. See Fig. 1 for
additional sources of thermodynamic data.

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L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662
hydrothermal fluid is predicted (Fig. 3a). In contrast,
the low Fe/H 2 S ratio of the more oxidizing modern
vent fluids results in complete removal of Fe when
mixed with modern sulfate-bearing seawater, a result
largely due to pyrite precipitation (Fig. 3b). In actuality,
vent mineralogy is undoubtedly dominated by a
complex series of kinetically mediated reactions [2],
although fundamental redox constraints will still dominate
the temperature dependent abundance and sequence
of mineral precipitates and corresponding
changes in vent fluid chemistry.
2.2. Effect of pressure on vent fluid composition
concentration (mmol/kg)
80 500 bars
400 bars
70
60
50
40
30
20
10
It has long been known that in addition to redox,
temperature and pressure play critical roles in controlling
Fe solubility in hydrothermal systems. At
elevated temperatures, relatively small changes in
pressure can result in large changes in dissolved
Fe. Thermodynamic calculations for the fayalite–pyrrhotite–magnetite–(quartz)-bearing
system at pH=5
show that a change in pressure from 500 to 400
bars results in a doubling of Fe, a modest change
in H 2 S, and correspondingly a substantial increase in
Fe/H 2 S (Fig. 4). Owing to the lack of thermodynamic
data, it is not possible to perform similar
calculations at lower pressures, although it can be
inferred from experimental data [20] that at temperatures
in excess of 350 8C, a further decrease in
pressure would result in additional increases in dissolved
Fe, particularly for hydrothermal systems
lacking sulfate. This sensitivity largely results from
the reduction in pH that occurs as the two-phase
boundary of seawater is approached, but is also
due to the enhanced stability of aqueous Fe-chlorocomplexes
[19,20].
Recent analyses of fluids from modern vent systems
further indicate that phase separation in response
to pressure and/or temperature change can result in
significant corresponding changes in dissolved Fe
concentrations. For example, in spite of the low salinity
of the vapor phase fluids released from subseafloor
reaction zones undergoing phase separation in
connection with subseafloor magmatic activity at EPR
9–108 N, Fe concentrations are unusually high on an
absolute and chloride normalized basis [11]. The high
Fe concentrations of the vapors likely result from
aqueous complexing effects, as noted earlier.
Fe H 2 S Fe/H 2 S
Fig. 4. The effect of pressure on dissolved Fe, H 2 S and Fe/H 2 S for a
chemical system buffered by fayalite–quartz–magnetite–pyrrhotite
coexisting with 0.55 mol/kg chloride at a pH=5 and 400 8C (see
text). The increase in dissolved Fe and Fe/H 2 S ratio predicted for
the 100 bar pressure drop likely continues at still lower pressures
owing to the effect of pressure on the stability of aqueous ferrous
chloride complexes and mineral phase relations, as suggested by
earlier experimental data [20]. These data suggest that the decrease
in hydrostatic pressure associated with global glaciations in the
Neoproterozoic may enhance the flux of hydrothermal Fe to the
ancient ocean. Relatively shallow ridges in the Archean together
with a sulfate-free ocean would similarly enhance Fe flux for BIF
deposition at that time. Pressure sufficiently low that the two-phase
boundary of the fluid (ancient or modern) is intersected would result
in formation of Fe-bearing vapors and brines. Assuming both phases
ultimately reach the seafloor, the Fe flux would be significant.
Thermodynamic data sources are as discussed in earlier figures.
The coexisting brine phase at depth is likely
exceedingly rich in Fe, owing to its high chloride
concentration and relatively low pH [8,10,21]. Although
it is commonly believed that vapor loss
causes an increase in the pH of the residual brine
phase [22], this is not the case, however, in the
presence of silicate minerals that can buffer pH at
relatively low values in response to the increase in
chloride and charge balance constraints [23], especially
at high temperatures where such reactions
occur rapidly. The Fe-bearing brines likely reside
in the crust for some time, but ultimately mix with
hydrothermal seawater and vent as well, as indicated,
for example, by the composition of vent fluids at the
southern Cleft Segment of the Juan de Fuca Ridge
[24–26]. If phase separation/segregation takes place
very near the seafloor, as has been suggested for vent

L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662 659
fluids from the Southern East Pacific Rise and East
Pacific Rise at 9–108 N, venting of Fe-bearing brine
is even more likely, especially for conditions at or
near the critical point of the NaCl source fluid [11].
Similar arguments apply to Precambrian vent systems,
although even higher Fe concentrations are
likely owing to the lack of sulfate (more reducing
conditions) in the hydrothermal root zones from
which the fluids are derived.
3. Precambrian mid-ocean ridge systems
3.1. Ridge depressurization and sulfate depletion during
Snowball Earth
Might the pressure decrease associated with the
growth of continental ice sheets cause an enhancement
of the mid-ocean ridge Fe flux sufficient to
stimulate BIF deposition? As emphasized above,
redox constrains imposed by the lack of sulfate in
Neoproterozoic seawater during global glacial events
likely play a key role in accounting for BIF deposition.
Nevertheless, pressure changes associated with
glaciation could have significant secondary effects on
Fe flux. All of the Neoproterozoic iron formations are
in glaciomarine sequences with associated glacial diamictites
[27]. Thus the coincidence of glaciation and
iron deposition is on secure footing. Mid-ocean ridge
fluxes represent a significant fraction of the total
delivery of soluble Fe to the ocean today [1,28,29],
and on a Snowball Earth, would be the only major
source of Fe to the ocean.
Estimates of Neoproterozoic sea-level fluctuations
range from z160 m [30] to more than 1000 m [31].
At the low end of these estimates (equivalent to a drop
in hydrostatic pressure of 16 bars), the enhancement
of Fe fluxes would be minor. At the upper end (a 100
bar depressurization), and with fluids in the seafloor
reaction zone equilibrated at temperatures greater than
400 8C, the enhancement of mid-ocean ridge Fe fluxes
could be appreciable.
Interestingly, although the low-end estimated increase
in Fe flux is small for one-phase systems,
even modest changes in pressure could enhance Fe
delivery for systems sufficiently close to the critical
point of the NaCl fluid, and this argument
applies equally well to the Quaternary. Even a
relatively small increase in Fe delivery, coupled
with shallower ridge crests and perhaps more vigorous
hydrothermal circulation at near critical conditions
could presumably inject Fe-rich plumes
more efficiently into the thermocline, stimulating
glacial biological productivity and a draw-down
of atmospheric pCO 2 comparable to that estimated
to result from intensified aerosol fluxes during the
Quaternary [32].
Recognizing that there were at least two and
perhaps several Neoproterozoic glaciations [33,34],
one might expect to find a correlation between the
extent of continental glaciation (inferred magnitude
of sea-level fall) and abundance of iron formation. Of
the 8 glacially associated iron formations listed by
Klein and Beukes [35], 6 are apparently bSturtianQ in
age (~750 Ma; Kingston Peak, Death Valley, California;
Rapitan, Mackenzie Mountains, Canada;
Upper Tindir Group, Alaska; Adelaide Geosyncline,
South Australia; Numees Formation, South Africa
and Namibia; and Chuos Formation, Namibia) and
2 are bVarangian or MarinoanQ (~600 Ma; Bissokpabe
Group, West Africa; and Jacadigo Group, Brazil)
[36]. Thus, iron deposition was concentrated in
the older glacial interval. But did the Sturtian event
produce thicker and/or more extensive continental ice
sheets and thus a larger sea-level fall? Unfortunately,
the magnitude of the Neoproterozoic sea-level fluctuations
remains controversial. Thus, we are left with
an untested hypothesis that the Sturtian glaciation
involved larger continental ice sheets than the Marinoan
event.
An alternative explanation for the preponderance
of BIF in the Sturtian glacial deposits is that sulfate
concentrations were lower during the Sturtian than
during the Marinoan event. This explanation was
forwarded by Hurtgen et al. [13], based on their
analysis of the trace sulfate content and sulfur isotopic
composition of cap carbonates deposited following
these events. Of course, a more intense glaciation
(presumably the Sturtian) would more effectively
eliminate the weathering supply of sulfate. Additionally,
the isolation of the ocean covered in sea ice
would favor the consumption of what little sulfate
initially existed in the ocean by bacterial sulfate reduction
or hydrothermal uptake, leading to a large
increase in the Fe/S ratio and the absolute flux of
Fe from mid-ocean ridges.

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L.R. Kump, W.E. Seyfried Jr. / Earth and Planetary Science Letters 235 (2005) 654–662
3.2. Archean and Paleoproterozoic BIF production
The abundance of BIF in older (Archean and
Paleoproterozoic) sedimentary rocks could also be
the result of a predominance of low oceanic sulfate
concentrations and lower pressure hydrothermal systems.
The observation that Archean shales are
enriched in Fe has been interpreted to indicate a
larger hydrothermal flux of Fe to the early ocean
compared to today [37]. Isley [1] implicated higher
heat flow (implying increased hydrothermal circulation)
and shallower ridges to account for this enhanced
Fe delivery; shallower ridges would have
facilitated the delivery of mid-ocean ridge Fe in
hydrothermal plumes to surface waters leading to
BIF deposition. Upwelling of Fe-rich water to the
surface may have supported anoxygenic photosynthesis
based on Fe oxidation [38,39], or the Fe may
have been oxidized anoxically and inorganically. The
predictably high heat flow in the Archean and accelerated
rates of hydrothermal circulation at ridges,
notwithstanding, Lowell and Keller [40] have
shown that simple extrapolation of the modern hydrothermal
Fe flux to the Archean using crustal
cooling models underestimates Fe in the large superior-type
BIF by approximately an order of magnitude.
This inconsistency, however, can be resolved in
large part by taking explicit account of the absence
of sulfate in the Archean ocean on hydrothermal
alteration processes (Figs. 2 and 3).
Archean vent fluids would have been relatively
sulfide poor, but H 2 -rich, with H 2 concentrations predictably
larger than modern vent fluids. Thus, the
biota they supported likely was based on H 2 chemosynthesis
(methanogenesis) than on sulfide oxidation
as a chemosynthetic mechanism, more prevalent
today.
The end of Archean–Paleoproterozoic BIF deposition
may have resulted from decreasing heat flow,
deeper ridges [1], and perhaps most importantly, the
transition to more oxidizing sulfate-bearing systems.
Such a mechanism is not inconsistent with the suggestion
that the increase in sulfate concentrations in
the Paleoproterozoic ocean stimulated sulfate reduction
and created a sulfidic deep ocean [5]. Such an
ocean could conceivably have elevated concentrations
of both sulfate and sulfide compared to those of the
Archean ocean, but would be ineffective as a transfer
agent for BIF deposition at points distal from source
regions at ridges.
4. Conclusion
Because they operate close to the critical point and
their redox state is set largely by the availability of
dissolved sulfate, the chemical composition of midocean
ridge vent fluids is particularly sensitive to
changes in temperature, pressure, and seawater sulfate
concentration. In particular, the Fe content and Fe/
H 2 S ratio of mid-ocean ridge hydrothermal fluids
should have been significantly elevated if Precambrian
seawater was depleted in sulfate and if ridges were
shallow during the Archean and during Snowball
Earth episodes of eustatic sea-level draw-down. The
abundance of BIF under these conditions would then
have been largely the consequence of changes in
hydrothermal fluid composition rather than changes
in the solubility of Fe in seawater determined by the
O 2 [28] or H 2 S content [5] of the deep sea.
Acknowledgments
This work was supported by the NASA Astrobiology
Institute (Cooperative Agreements NCC2-0157
and NNA04CC06A to LRK) and the National Science
Foundation. P. Hoffman offered valuable
insights and specific information on the geological
evidence for glacioeustatic sea-level fall during the
Neoproterozoic. H.D. Holland and N. Sleep provided
careful and useful reviews of earlier versions of the
manuscript.
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