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Barite, BIFs and bugs: evidence for the evolution ofthe Earth’s
early hydrosphere§
David L. Huston �, Graham A. LoganGeoscience Australia, GPO Box
378, Canberra, ACT 2601, Australia
Received 16 June 2003; received in revised form 19 November
2003; accepted 7 January 2004
Abstract
The presence of relatively abundant bedded sulfate deposits
before 3.2 Ga and after 1.8 Ga, the peak in ironformation abundance
between 3.2 and 1.8 Ga, and the aqueous geochemistry of sulfur and
iron together suggest thatthe redox state and the abundances of
sulfur and iron in the hydrosphere varied widely during the Archean
andProterozoic. We propose a layered hydrosphere prior to 3.2 Ga in
which sulfate produced by atmospheric photolyticreactions was
enriched in an upper layer, whereas the underlying layer was
reduced and sulfur-poor. Between 3.2 and2.4 Ga, sulfate reduction
removed sulfate from the upper layer, producing broadly uniform,
reduced, sulfur-poor andiron-rich oceans. As a result of increasing
atmospheric oxygenation around 2.4 Ga, the flux of sulfate into
thehydrosphere by oxidative weathering was greatly enhanced,
producing layered oceans, with sulfate-enriched, iron-poor surface
waters and reduced, sulfur-poor and iron-rich bottom waters. The
rate at which this process proceededvaried between basins depending
on the size and local environment of the basin. By 1.8 Ga, the
hydrosphere wasrelatively sulfate-rich and iron-poor throughout.
Variations in sulfur and iron abundances suggest that the redox
stateof the oceans was buffered by iron before 2.4 Ga and by sulfur
after 1.8 Ga.Crown Copyright ; 2004 Elsevier B.V. All rights
reserved.
Keywords: hydrosphere evolution; atmosphere evolution; Archean;
Proterozoic; biogeochemistry
1. Introduction
Variations in the amount of free atmosphericoxygen through
geologic time have been the sub-
ject of considerable debate since Cloud [1] sug-gested that the
Archean atmosphere containedmuch less oxygen than at present. He
observedthat uraninite and pyrite, unstable in an oxygen-rich
atmosphere, are present as detrital grains inArchean conglomerates.
Subsequent work on Ar-chean and early Proterozoic soil pro¢les
indicatesthat iron, immobile in an oxygen-rich atmosphere,was
mobile [2,3]. Although disputed by some [4],most workers accept
that these geologic observa-tions indicate that Archean atmospheric
oxygenlevels were substantially lower than at present[1,2].
0012-821X / 04 / $ ^ see front matter Crown Copyright ; 2004
Elsevier B.V. All rights
reserved.doi:10.1016/S0012-821X(04)00034-2
* Corresponding author. Tel. : +62-2-6249-9577;Fax:
+62-2-6249-9971.E-mail addresses: [email protected] (D.L.
Huston),
[email protected] (G.A. Logan).
§ Supplementary data associated with this article can befound at
doi:10.1016/S0012-821X(04)00034-2.
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Available online at www.sciencedirect.com
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It is tempting to infer that because Archeanatmospheric oxygen
levels were low, the coexist-ing hydrosphere was reduced. However,
the pres-ence of syngenetic sulfate in the early Archeansuggests at
least local oxidizing redox conditionsin the hydrosphere1 [5].
These deposits, alongwith the abundance of banded iron
formation(BIF), an indicator of reduced bottom waters [6]in the
late Archean and early Proterozoic, suggestthe redox state of
seawater may have varied sub-stantially.A substantial body of
literature exists regarding
the evolution of the atmosphere and hydrosphereduring the early
part of the Earth’s history, atopic that still receives a large
amount of research,and perhaps more than its fair share of
contro-versy. In the pursuit of research interests outsidethis
general ¢eld, we have inferred a number ofgeological and
geochemical anomalies that are notexplained by current models for
the evolution ofthe Earth’s atmosphere and, particularly,
hydro-sphere. As discussed below, these anomalies in-clude the
abundance of bedded barite deposits,and the temporal distribution
and geochemistryof BIFs.In this paper we use temporal distributions
of
sulfate deposits and iron formations along withthe aqueous
geochemistry of iron, sulfur, and ba-rium to present a unifying
concept for the evolu-tion of the Earth’s hydrosphere before 1.0
Ga.This model builds on previous models, butpresents important,
although in some cases subtle,di¡erences in an attempt to account
for theanomalies mentioned above. The model also pla-ces
quantitative constraints on variations in theabundance and redox
state of aqueous compo-nents such as sulfur and iron, parameters
thatare not well established in many current models.Although
previous models invoking layered
oceans [7,8] partly explain our observations, thedata suggest a
more complex evolution of the hy-drosphere through time. We intend
to show thatchanges in the oxidation state of the atmosphereand the
resulting impact on weathering had aprofound and perhaps unexpected
impact on thehydrosphere. In particular, we hope to show
thatchanges occur in both availability of sulfate andiron through
time and that the dominant multi-valent ions that help bu¡er
oxidation states of thehydrosphere varied in a surprising way.
2. The temporal distributions of sulfate and ironformation
The geologic record contains various sedimentsand deposits that
re£ect the oxidation state of thehydrosphere and/or the atmosphere.
We have re-viewed the temporal and spatial occurrences ofthe key
sediment types (see Tables 1 and 22), thedetails of which are
summarized below. Fig. 1,which is based on Tables 1 and 22,
illustrates thedistributions of bedded sulfate deposits and
BIFsprior to 1.0 Ga. These tables were compiled fromexisting
literature. Table 22 is extensively updatedfrom compilations by
James [9], Walker et al. [2],Isley [10] and Isley and Abbott [11].
It providesmore constrained ages for many BIFs and in-cludes data
for BIFs not presented in the previouscompilations. In Table 22,
BIFs are classi¢ed aseither Algoma- or Superior-type.
Algoma-typeBIFs are generally small in size and associatedwith
contemporaneous volcanic suites. In con-trast, Superior-type BIFs
(Hamersley-type in Aus-tralia) are laterally extensive, generally
in a shelfenvironment without a volcanic association [12].Before
3.2 Ga, bedded barite deposits are rela-
tively common (18 in Table 12), whereas otherbedded sulfates are
not known and BIFs, al-though present, are not common and are
exclu-sively of Algoma-type. Between 3.2 and 2.4 Ga,the geologic
record is characterized by abundantBIF and a paucity of bedded
sulfate deposits(only two known). The period between 2.4 and
1 Unless stated otherwise, comments relating to redox
con-ditions refer only to the hydrosphere. No inference should
bemade about atmospheric conditions, unless speci¢cally stated.By
the term ‘oxidizing’ we imply that sulfate was the dominanthydrous
sulfur species. By the term ‘reduced’ we imply thatH2S and/or HS3
were the dominant sulfur species. Under dis-equilibrium conditions
with intermediate redox, sul¢te may beimportant. 2 See the online
version of this article.
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1.8 Ga is characterized by abundant BIF, andincreasingly
abundant sulfate deposits, with gyp-sum and anhydrite becoming more
common. Be-tween 3.0 and 1.8 Ga, both Superior- and Algo-ma-type
BIFs are present, but the Superior-typedeposits account for the
vast majority of BIF. Theperiod between 1.8 and 1.0 Ga lacks BIF
butcontains abundant sulfate deposits. AlthoughBIFs of late
Proterozoic and Phanerozoic agesare known (e.g. Rapitan), these
deposits are theproducts of either unusual geologic conditions(e.g.
global glaciation [13]) or hydrothermal vent-ing.As both the style
and the abundance of BIF
vary with time, it is unlikely that these variationsare solely
controlled by tectonic preservation. Inaddition, the true frequency
of bedded sulfate de-posits is likely to be understated in the
older rocksas these rocks are less likely to be preserved[14].
3. Sulfur isotope variations through time
Prior to 2.4 Ga, N34Ssulfate is relatively uniform(Fig. 1) with
average values for all deposits inthe range of 3.8^5.4x (all N34S
values are re-ported relative to the CDT standard), bar the
BigStubby and Geco volcanic-hosted massive sul¢de(VHMS) deposits.
Moreover, with the exceptionof the Dresser deposits [15],
coexisting sul¢deminerals have a narrow range of 0P 4x
[16].Sedimentary sul¢de elsewhere has N34S in therange of 0P 10x
[13,17]. In contrast, depositsyounger than 2.4 Ga have higher and
more vari-able N34Ssulfate, ranging from 4 to 39x (Fig. 1),similar
to values in the late Proterozoic and Phan-erozoic [18].
Sedimentary sul¢des also have alarge range in N34S, from 330 to 60x
[13,19,20].Farquhar et al. [21] showed that the fractiona-
tion of 33S relative to 32S also changes at about2.4 Ga. Syn- or
diagenetic sulfate and sul¢deminerals deposited before 2.45 Ga show
mass-independent fractionation of 33S, interpreted toresult from
gas-phase photolysis of SO2, yieldinga large range of oxidized and
reduced sulfur-bear-ing species [22]. Between 2.45 and 2.09 Ga
33Sfractionation shifted from mass independent tomass dependent.
The lack of mass-independentfractionation after 2.09 Ga suggests
that photo-chemical reactions ceased to be important, withbiogenic
and redox reactions controlling sulfurspeciation. These
observations have been indepen-dently supported by Mojzsis et al.
[23].
4. The low-temperature geochemistry of sulfur,iron and
barium
Fig. 2 illustrates the stability of Fe^S^O miner-als and barite,
and the solubility of iron as afunction of sulfur content and redox
at pH (7.8)and salinity of modern seawater (3% NaCl).These diagrams
were calculated for 25 and 75‡Cto illustrate constraints on
deposition of BIF andbarite.At 25 and 75‡C, magnetite is only
stable at
sulfur levels below 1035 and 1033 that of modernseawater. Iron
is most soluble in the magnetitestability ¢eld and least soluble in
the pyrite ¢eld.
Fig. 1. Variations in the abundances of bedded sulfate depos-its
and BIF and of N34S through time (based on Tables 1and 22).
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220 (2004) 41^55 43
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In the magnetite ¢eld, iron solubility locally ex-ceeds 100 ppm
and decreases as gSO4/gH2S in-creases. Even if slow reaction rates
inhibit pyriteformation, and the magnetite ¢eld expands tohigher
sulfur levels (dashed lines in Fig. 2), sulfurconcentrations must
still be much lower than thatof modern seawater. Under reduced
conditions atcurrent seawater sulfur levels, the maximum
ironsolubility in the pyrite ¢eld is only V1033 ppm,some ¢ve orders
of magnitude lower than that inthe magnetite ¢eld. Although iron
solubility ex-ceeds 1 ppm at the base of the hematite ¢eld,
itdecreases with increasing gSO4/gH2S. Therefore,under sulfur-rich,
highly oxidized conditions char-acteristic of modern seawater, iron
is highly in-soluble. Magnetite is only stable under
reducedconditions (gSO4/gH2S6 1032:5 at 25‡C), andhigh iron
solubility is only possible under verylow ambient sulfur
concentrations.In the presence of even low concentrations of
aqueous sulfate, barium is insoluble, forming bar-
ite. Therefore, barite deposition indicates rela-tively oxidized
conditions (gSO4/gH2Ss 1032),even for £uids with low sulfur levels
(at most1033 current seawater concentrations). In con-trast, the
solubilities of gypsum and anhydriteare much higher: a high degree
of evaporationis required to precipitate gypsum. Although bariteis
the only sulfate mineral reported prior to 3.2Ga, the initial
workers on the Dresser barite de-posit suggested that the barite
replaced originalevaporative gypsum [25,26]. However, recentwork
[27] suggests that most sulfate precipitatedoriginally as barite
from low-temperature hydro-thermal emanations analogous to white
smokers.Thus, the distributions of bedded sulfate min-
erals and iron formations plus constraints basedon the aqueous
geochemistry of iron, sulfur andbarium set limits for gSO4/gH2S,
total sulfur, andiron abundances in ancient seawater. These
datarequire that before 1.8 Ga, ancient seawater hadmuch lower
total sulfur and higher iron abundan-ces than modern seawater.
Furthermore, total sul-fate levels between 3.2 and 2.4 Ga must have
beenextremely low, although before 3.2 Ga sulfate lev-els may
locally have approached modern levels.Variations in the abundances
of the elementsmay be linked to the evolution of the atmosphereand
hydrosphere, as suggested by Walker andBrimblecombe [28].
5. The origin of bedded sulfate deposits
Interpretation of the genetic signi¢cance of thedistribution of
bedded sulfate deposits throughgeologic time requires an
understanding of howthese deposits formed. Of particular interest
arethe barite deposits : barite is the dominant or onlysulfate
mineral in bedded sulfate deposits formedprior to 2.4 Ga.Because of
the highly insoluble nature of barite,
formation of this mineral requires two discretesources, one for
barium and another for sulfate.Jewell [29] suggests that barite can
form in threeways: (1) diagenetic replacement, (2) hydrother-mal
exhalation, or (3) biological precipitation.The ¢rst two mechanisms
are most important,particularly in the Archean.
Fig. 2. gSO4/gH2S versus total sulfur diagrams calculated at25
and 75‡C at modern oceanic pH and salinity showing Fe^Ba^S^O
mineral stabilities and Fe solubilities (calculated us-ing data
generated from HCh [24]). Total sulfur concentra-tions are
normalized to the level in modern open ocean sea-water.
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Diagenetic replacement of original bedded gyp-sum has been
proposed at Dresser in the PilbaraCraton and several occurrences in
the KaapvaalCraton [25]. However, the most common originfor barite
in the Archean appears to be hydro-thermal exhalation. Barite
precipitates when a re-duced £uid carrying Ba2þ mixes with a £uid
car-rying SO234 . In Phanerozoic VHMS deposits,barium-bearing
hydrothermal £uid mixed withsulfate-bearing seawater at or just
below the sea-£oor. As a consequence, barite concentrated to-ward
the top of VHMS deposits [30].It is tempting to draw a direct
analogy between
Phanerozoic VHMS barite and Archean barite,but given the
controversy regarding the redoxstate of the Archean hydrosphere,
other alterna-tives should be considered. The most likely
alter-native is that an oxidized, sulfate-bearing hydro-thermal
£uid mixed with barium-bearing, reducedseawater. Hydrothermal
sulfate can be derived ei-ther from leaching of sulfate in
evaporative, sedi-mentary rocks or by disproportionation of
mag-matic SO2 [31]. As evaporative sulfate mineralsform by the
evaporation of sulfate-bearing sea-water, the only alternative that
is not ultimatelyderived from seawater is magmatically
derivedsulfate.However, a number of characteristics of Arche-
an barite-bearing deposits favor seawater over amagmatic source
for sulfate. Firstly, barite depos-its were more abundant before
3.2 Ga, whereasmagmatic activity was more abundant between3.2 and
2.4 Ga, a period of extensive formationof continental crust [32].
The formation of muchof the Yilgarn, Slave and Superior provinces
be-tween 3.0 and 2.6 Ga involved extensive volca-nism. If magmatic
derived sulfate was importantin forming Archean barite deposits,
more depositsshould have formed between 3.2 and 2.4 Ga.Secondly,
the isotope characteristics of the bar-
ite deposits are also inconsistent with a magmaticsulfate
source. In Phanerozoic deposits where dis-proportionation of
magmatic SO2 has been dem-onstrated, sulfate tends to be 15^30x
enriched in34S relative to the parent SO2 [31]. Hence,
unlessmagmatic N34S values during the Archean weresubstantially
lower than the general range of2P 5x [33], the N34S of Archean
barite is not
consistent with derivation from disproportionatedmagmatic SO2.
Moreover, the mass-independent33S fractionation observed in Archean
barite indi-cates that the sulfate formed in the atmosphere[21,23],
which, again, is inconsistent with a mag-matic source.Thirdly,
Huston et al. [34] interpreted that re-
gional alteration in the 3.24 Ga Panorama VHMSdistrict involving
increases in Fe2O3/FeO of vol-canic rocks indicated extensive
inorganic reduc-tion of seawater sulfate. Moreover, early
ArcheanVHMS deposits have similar mineral zonation toPhanerozoic
deposits, suggesting similar deposi-tional mechanisms, including
interaction with sul-fate-rich seawater.In summary, the temporal
distribution, sulfur
isotope characteristics and alteration/mineraliza-tion
associated with bedded Archean barite-bear-ing deposits all point
to seawater being the sourceof sulfate. Hence we take the presence
of beddedbarite to indicate the presence of sulfate in
con-temporaneous seawater.
6. The origin of BIF
Cloud [35] and Holland [36] ¢rst suggested thatiron and silica,
the main components of BIFs,were derived from seawater. This
concept is nowgenerally accepted, although some disagreementstill
exists as to the ultimate origin of these com-ponents. Holland [36]
suggested that Fe2þ couldbe provided either from terrestrial
weathering inan anoxic environment, volcanic emanations intothe
ocean, or bottom waters. Low-temperatureweathering and
high-temperature alteration [37]of the ocean £oor could also be
sources of iron.The concept that the iron and silica in BIFs
were derived from high-temperature alteration ofrocks below the
sea£oor stems from comparingthe rare earth element patterns of BIFs
with thosefrom modern hydrothermal £uids venting on thesea£oor.
Both the BIFs and the venting £uids arecharacterized by positive
europium anomalies,which are indicative of high-temperature
altera-tion of volcanic rocks [37^39]. Although thismodel is
generally accepted, systematic di¡erencesin the intensity of the
europium anomaly exist
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220 (2004) 41^55 45
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between Algoma- and Superior-type BIFs (Fig.3).Algoma-type BIFs
are characterized by much
larger europium anomalies (s 1.8) than Superi-or-type BIFs (6
1.8). This di¡erence suggeststhat there is a much larger component
of vol-canic-related hydrothermal emanations in Algo-ma-type BIFs,
consistent with their close associa-tion with greenstone belts. The
smaller europiumanomalies that characterize Superior-type
BIFssuggest a lower input from volcanic-related hy-drothermal
emissions and a higher contributionfrom other sources, including
oceanic bottomwaters. Therefore the distribution and
character-istics of Superior-type BIFs are a more accuraterecord of
background processes that controlledthe overall chemistry of
Archean and Proterozoicoceans.Isley [10] and Isley and Abbott [11]
demon-
strated that major BIF depositional events corre-late strongly
with global plume events. This anal-ysis was extended by Abbott and
Isley [40] toconsider Algoma- and Superior-type BIFs sepa-rately.
Inspection of their diagrams con¢rmsthat peaks in Algoma-type BIF
deposition corre-spond to major global plume events, but that
notall plume events correspond to Algoma-type BIFdeposition. The
largest peak in Algoma-type BIFscorresponds to a major plume event
at 2.75^2.70Ga, which can be related to extensive crustal
growth in many Archean greenstone belts. How-ever, although
Abbot and Isley [40] also suggesteda correlation between mantle
plumes for Superior-type BIFs, the updated age constraints
providedin Table 2 indicate more complicated relation-ships. These
constraints suggest that the V2.463Ga Superior-type BIF peak
identi¢ed by Abbotand Isley [40] includes at least three
discreteevents over a period of at least 163 million years,and
these BIF events do not all correlate directlyto identi¢ed plume
events. Moreover, theV1.890Ga Superior-type BIF event occurs toward
theend of a V700 million year period of pulsedplume activity.
Therefore, although some link be-tween mantle plume events and
Superior-typeBIFs is probable, that link is not direct.
Rather,hydrothermal activity associated with superplumeevents,
along with other processes, contributediron to an oceanic
reservoir, which depositediron during upwelling [2] or oxidation
events notdirectly related to the plume events.
7. A model for the geochemical evolution of thehydrosphere prior
to 1.0 Ga
Temporal variations in the abundances of ironformations and
bedded sulfate, together with var-iations in N34Ssulfate, suggest
that the evolution ofthe hydrosphere before 1.0 Ga can be divided
intofour periods: (1) s 3.2 Ga, (2) 3.2^2.4 Ga, (3)2.4^1.8 Ga, and
(4) 1.8^1.0 Ga. The geologicand isotopic characteristics, along
with the low-temperature geochemistry of sulfur, iron and ba-rium,
suggest that the evolution of the hydro-sphere was more complex
than generally thought[7,13]. Figs. 4 and 5 present our model for
thisevolution, which accounts for most of the con-straints outlined
above.
7.1. s 3.2 Ga: sulfur-poor, strati¢ed oceans
The presence of barite and iron formation priorto 3.2 Ga
indicate variable seawater sulfur con-tents and redox conditions.
Magnetite, the mainmineral in iron formations, only forms at
lowtemperatures under reduced redox conditionsand low sulfur
levels. Barite forms at higher sulfur
Fig. 3. Variations with time of the average NASC-normal-ized
europium anomaly for Algoma- (open circles) and Supe-rior-type
(solid diamonds) BIFs. Table 2 presents the data,which were culled
to minimize clastic input, used to makethis diagram.
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220 (2004) 41^5546
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levels, but only under oxidized conditions. Thewidespread
occurrence of barite before 3.2 Gasuggests that sulfate-bearing
seawater was a glob-al, not local, phenomenon.
The variable redox condition could be ex-plained by a strati¢ed
ocean, with barite depositsforming in an oxidized upper layer, and
Algoma-type BIF forming from Fe2þ-rich hydrothermalemanations in
the reduced lower layer (Fig. 4).The presence of an upper oxidized
layer coveringshelf areas may have prevented iron depositionand
suppressed Superior-type BIF. This mayhave been further in£uenced
by lack of shelf en-vironments early in Earth’s history. Di¡erences
inthe operation of plate tectonics may have limitedthe extent of
some of the environments displayedin Fig. 4 compared to later
periods.As argued by a number of authors [1], the
Earth’s early atmosphere was oxygen-poor andreduced. Such
conditions would stabilize gasessuch as H2S and SO2, both of which
can undergophotolytic reactions, producing mass-independentsulfur
isotope fractionation and oxidized sulfur-bearing gases such as SO3
[21]. Modelling by Pav-lov and Kasting [22] suggests that the
products ofthese photolytic reactions will be incorporatedinto the
hydrosphere through a combination of‘rainout’ and dissolution at
the surface of theocean, with SO3 and SO2 being the two
mostimportant oxidized sulfur-bearing gases. Thesegases would be
dissolved into the hydrosphereby hydrolysis and disproportionation
reactions,as follows:
SO3ðgÞ þH2OðlÞ ¼ SO234 þ 2Hþ; ð1Þ
Fig. 4. Model for hydrosphere evolution prior to 1.0
Ga.Photochemical sulfate oxidation e¡ectively shuts down be-tween
2.4 and 1.8 Ga and is not displayed in the diagramafter 2.4 Ga. The
term oxidized refers to the presence of oxi-dized species such as
sulfate and does not imply the presenceof free oxygen. Low levels
of oxygen would have beenpresent in the photic zone and a
signi¢cant oxycline mayhave developed by 1.8 Ga.
Fig. 5. Schematic diagram illustrating inferred changes in
at-mospheric oxygen levels and oceanic gSO4/gH2S, total sulfurand
iron abundances.
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SO2ðgÞ þH2OðlÞ ¼ SO233 þ 2Hþ; ð2Þ
and
4SO2ðgÞ þ 4H2OðlÞ ¼ 3SO234 þH2SðaqÞ þ 6Hþ:
These processes would produce an upper oceaniclayer enriched in
sulfate (cf. [41]), and possiblysul¢te [42]. The presence of
sulfate in this layerwould then have allowed deposition of
beddedsulfate minerals in hydrothermal and evaporativeenvironments,
or possibly at the interface betweenthe oceanic redox layers. Many
Paleoarchean bar-ite deposits are interpreted to have formed in
shal-low water [25,26,43], and analogies with modernarc-related
black smoker deposits suggest depthsof less than 2000 m for the
VHMS deposits.Moreover, reduced atmospheric conditions wouldallow
the 33S anomalies noted by Farquhar et al.[21].However, below this
sulfate-enriched oxidized
upper layer, the ocean was reduced. Inorganic sul-fate reduction
processes, where sulfate (and pos-sibly sul¢te) from this upper
layer was reducedhydrothermally via interactions with Fe2þ inrock
below [34], and biological sulfate (or sul¢te[42]) reduction, which
may have evolved by V3.4Ga [15], combined to reduce sulfate
produced byphotochemical reactions, eventually forming asteady
state. These interactions bu¡ered seawaterredox, sulfur abundances
and N34S. During thisperiod, N34Ssulfate and N34Ssulfide were both
closeto zero, consistent with crustal values. These
in-terpretations are consistent with those of Veizer etal. [44],
who suggested that many aspects of Ar-chean oceanic chemistry were
bu¡ered by equilib-rium with basaltic oceanic crust.
7.2. 3.2^2.4 Ga: sulfur-poor, reduced oceans
The period from 3.2 to 2.4 Ga di¡ers from theearlier period in
lacking bedded sulfate depositsand having abundant BIFs, including
abundantSuperior-type BIFs. This suggests that the oceansat this
time lacked a signi¢cant sulfate-bearinglayer (Figs. 3 and 4).
Moreover, the abundanceof BIF suggests that the ocean was
generallymore reduced than the previous period, allowinghigh
concentrations of Fe2þ in the bottom waters,
which may have been supplemented by hydrother-mal emanations as
recorded in Algoma-type ironformations.As mass-independent 33S
fractionation is re-
corded during this period [21,23], photochemicalreactions with
atmospheric sulfur-bearing gaseswere still important. As the
atmosphere was stillreduced during this period, the oxidative
weath-ering of terrestrial pyrite to sulfate would nothave supplied
signi¢cant levels of sulfur to theoceans. Importantly, the advent
of sulfate-reduc-ing bacteria, in combination with
hydrothermalinorganic sulfate reduction, would help to keepthe
oceans virtually sulfate-free because in waterswith low sulfate
concentrations biological sulfatereduction rapidly removes sulfate,
precipitatingiron sul¢des from iron-rich ocean waters. Becauseof
low atmospheric oxygen levels, the terrestrialweathering of sul¢des
would not have suppliedsulfate at the rate of removal, and the
oceanswould e¡ectively be scrubbed of sulfur throughiron sul¢de
precipitation. By 3.2 Ga sulfate depos-its were virtually absent
and photolytic reactionswere the main process of sulfate
production[21,23]. We infer that biological sulfate reductionmay
have been an important process by 3.2 Gaand that this led to the
removal of sulfate as asigni¢cant component within ocean waters
be-tween 3.2 and 2.4 Ga. This process may havebeen assisted or even
initiated by a major meteor-ite bombardment at V3.2 Ga [46], which
wouldhave perturbed the steady-state layered hydro-sphere inferred
prior to 3.2 Ga. Like the periodbefore 3.2 Ga, the N34S composition
of preservedsulfates and sul¢des from 3.2 to 2.4 Ga was lim-ited to
that of crustal average [17,45].Biogeochemical evidence for
oxygenic photosy-
thesis exists in sediments as old as 2.7 Ga [47].Eukaryotic
steroids have a biosynthetic require-ment for free oxygen, these
steroids being pre-served as steranes, which have been found in
2.7Ga sediments [48]. This helps set a lower limit foroxygen levels
in the photic zone to at least 1%PAL at this time [49]. Upwelling
of reduced, Fe-bearing waters into the photic zone would resultin
extensive iron precipitation to form Superior-type BIF [2], as
shown in Figs. 2 and 4.Further supporting evidence for
sulfate-poor
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220 (2004) 41^5548
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seawater may also come from a global excursionin the N13C of
kerogen between 2.8 and 2.5 Ga[50]. A model involving signi¢cant
rates of meth-anogenesis coupled with methanotrophy has
beenproposed by Hayes [54,55] to explain this excur-sion. For a
methane cycle to be of global signi¢-cance it would require that
other modes of organ-ic matter recycling, such as microbial
aerobicrespiration, and bacterial sulfate reduction maynot have
been as important within the water col-umn and upper sediments.
Such a situation wouldcertainly be the case in anoxic, sulfate-poor
andFe2þ-rich waters.
7.3. 2.4^1.8 Ga: transition to sulfur-rich, oxidizedoceans
The rise of atmospheric oxygen levels, whichstarted at 2.4^2.2
Ga [3], initiated one of themost sweeping changes in geologic
processes af-fecting Earth’s history. This rise initiated
condi-tions that in£uenced the evolution of the hydro-sphere over
the next 600 million years (Figs. 3 and4). The rise in atmospheric
oxygen levels has beenlinked to the increased sequestration of
reducedcarbon by burial [50], to the loss of hydrogen tospace in a
methane-rich atmosphere [51], tochanges in the redox of volcanic
gases and/or hy-drothermal £uid that interacted with the
crust[52,53], or to a major continental growth periodat the
Archean^Proterozoic boundary [45]. Muchhigher levels of
methanogenesis suggested for theprevious period of Earth’s history
may have driv-en atmospheric methane levels and thus initiatedthe
loss of hydrogen [51].The most signi¢cant consequence of the rise
of
atmospheric oxygen levels was the initiation ofoxidative
weathering [3]. An increasingly oxy-gen-rich atmosphere has
important rami¢cationsfor the geochemistry of sulfur and iron [28].
Themobility of these elements reverses : sulfur be-comes
increasingly mobile as oxidative weatheringof terrestrial sul¢des
produces soluble sulfate, in-creasing sulfate levels in seawater.
The earliestknown examples of bedded calcium sulfate depos-its are
the 2.26 Ga anhydrite lenses in the GordonLake Formation in Ontario
[56]. From this timethe abundance of bedded sulfate deposits
in-
creased, so that by 1.8 Ga, these deposits wererelatively common
(Table 1).The rise of atmospheric oxygen also would
have ended photochemical oxidation of reducedsulfur-bearing
gases. Farquhar et al. [21] inter-preted the decrease in
mass-independent fraction-ation of 33S between 2.45 and 2.09 Ga as
havingresulted from ‘either a change in atmosphericcomposition or
actinic £ux’. The ¢rst alternativeis consistent with our data, and
suggests thatatmospheric oxygenation destabilized reducedsulfur
gases, shutting down the photochemicalreactions responsible. During
this period, the in-creasing £ux of sulfate derived from
oxidativeweathering swamped the bu¡ering capacity of at-mospheric
and volcanic sulfur reservoirs. Biolog-ical sulfate reduction,
coupled with higher sea-water sulfate levels led to a larger range
inv34Ssulfate�sulfide.The consequences of the oxygenation of
the
Earth’s atmosphere did not proceed uniformly intime or space.
The processes alluded to aboveprobably occurred at di¡erent rates,
and the rateof change di¡ered in di¡erent geologic environ-ments.
For instance, sulfate concentrations wouldhave risen more quickly
in a small, closed basinrelative to an open oceanic basin (Fig. 4).
As aconsequence, the e¡ects of the oxygenation of theatmosphere
would have lead to both temporaland spatial variations in the
distribution of sulfateand sul¢des.The cessation of signi¢cant BIF
at about 1.8
Ga is one of the last consequences in the hydro-sphere of the
oxygenation of the atmosphere. The¢nal increase in the oxidation
state of bottomwaters would result in extensive deposition ofBIF,
which may account for the spike in thequantity of iron formation
between 1.9 and 1.8Ga (Fig. 1).During the period between 1.9 and
1.8 Ga, at
least 1.2U108 Mt of BIF was deposited (Table 2:[9]), which
equates to 3.6U107 Mt of iron assum-ing an average BIF iron content
of 30%. Giventhat the mass of seawater in the Proterozoic
wassimilar to current day, the average iron concen-tration in
Proterozoic seawater required to precip-itate this iron is 25 ppm.
This value, or a value oftwo or three times higher, is consistent
with con-
EPSL 6994 5-3-04
D.L. Huston, G.A. Logan / Earth and Planetary Science Letters
220 (2004) 41^55 49
-
straints on iron levels for a reduced ocean indi-cated in Fig.
2. Even if the amount of BIF pro-duced was a factor of two or three
more than thatpreserved, the concentration of iron required
isconsistent with the values indicated in Fig. 2.The presence of
sulfate deposits between 2.4
and 1.8 Ga indicates that the shallower parts ofthe oceans
became progressively sulfate-rich, yetthe production of iron
formations suggests thatthe bottom waters were reduced and
sulfur-poor.Like the time prior to 3.2 Ga, this suggests aredox
strati¢ed ocean. Several processes couldcause this inferred
strati¢cation. Photosyntheticproduction of organic matter and its
oxidativedestruction by microbial aerobic respiration, bac-terial
sulfate reduction or even bacterial Fe3þ re-duction would also play
important roles in de¢n-ing the redox gradients within ocean
waters.Moreover, Fe2þ introduced via hydrothermal em-anations would
also have maintained high ironlevels in bottom waters. Biological
sulfate reduc-tion would have been an active process in oceanwaters
and the sul¢de produced would have beentitrated by the Fe2þ-rich
bottom waters, keepingthe lower ocean virtually sulfate-free.
7.4. 1.8^1.0 Ga: sulfur-rich, oxidized hydrosphere
The abundance of bedded sulfate deposits com-bined with the lack
of iron formation between 1.8and 1.0 Ga suggests that the
hydrosphere wassu⁄ciently oxidized and sulfur-rich to suppressiron
solubility. Moreover, N34S variations re£ectbiogenic
sulfate-reducing reactions [13]. Once the£ux of sulfate from
terrestrial weathering of sul-¢des was high enough to swamp
hydrothermalFe2þ £ux, biogenic sulfate reduction could thenbegin to
titrate iron from ocean waters. Thegreatly increased £ux of sulfate
from weatheringcompared to the supply of Fe2þ in the hydro-sphere
ultimately led to the removal of Fe2þ
from the oceans [28]. This reverses the way thatiron titrated
sulfur between 3.2 and 2.4 Ga. Thechemistry of the hydrosphere
between 1.0 and 1.8Ga was more like modern day seawater, in
com-parison to the period before 2.4 Ga. However,sul¢dic bottom
water and redox gradients mayhave had a profound e¡ect on the
distribution
of redox-sensitive bio-essential elements [7,8,57,58].
7.5. Redox bu¡ering of the hydrosphere
Data presented in this paper suggest not onlythat there were
large variations in the redox stateand sulfur concentrations in the
hydrosphere, butthat the element that bu¡ers the redox statechanged
with time. Sulfur is presently the mostabundant multivalent element
in seawater, its ox-idation state bu¡ering the redox state of
seawater.As sulfate is virtually the only sulfur species inseawater
in the open ocean, this seawater is geo-chemically oxidizing. To
appreciably change theredox state of seawater requires the
conversionof sulfate to sul¢de, as happens biologically inanoxic
basins or thermochemically in sea£oor hy-drothermal systems.
However, if the total sulfurcontent of seawater was low, and the
iron contentwas high, the speciation of iron would govern theredox
state of seawater. Moreover, the presence ofhigh Fe2þ would also
prevent a rise in total sulfurconcentrations as sulfur could be
titrated by bac-terial sulfate reduction. Therefore, the sulfur
cycleprior to 2.4 Ga would be governed by high ironconcentrations
along with the low rate of sulfateproduction coupled to rapid
bacterial consump-tion. Prior to 3.2 Ga, bacterial sulfate
reductionmay not have been a global phenomenon and sea-water did
contain sulfate in the upper ocean.The rise in atmospheric O2 led
to oxidative
weathering and an increased £ux of sulfate tothe ocean.
Bacterial sulfate reduction wouldhave led to the titration of Fe2þ
from the globaloceans. The O2 rise may have had its roots in
low-sulfate, high-Fe2þ oceans, where methanogenesiswould have been
a key component of the carboncycle. This may have provided the
methane-richatmosphere where hydrogen loss gradually led toO2
increase [51].
8. Implications of hydrosphere evolution tomineralization
In addition to BIFs and barite deposits, a num-ber of other
mineral deposit types have restricted
EPSL 6994 5-3-04
D.L. Huston, G.A. Logan / Earth and Planetary Science Letters
220 (2004) 41^5550
-
distributions in time [59,60]. For instance, paleo-placer
uranium deposits, which require a reducedatmosphere and hydrosphere
to form, are foundonly in rocks older than V2.0 Ga, whereas
un-conformity-related uranium deposits formed afterthis time. In
addition, sediment-hosted copperand shale-hosted zinc^lead (e.g. Mt
Isa-type) de-posits entered the geologic record at 2.0 and 1.7Ga,
respectively [60]. The unifying theme of thelatter three deposit
types is the inference that theyformed from oxidized, sulfate-rich
ore £uids. Aspointed out by Lambert et al. [60], Veizer [59]
andothers, the oxygenation of the atmosphere, whichbegan at 2.4^2.2
Ga, allowed the generation ofsuch oxidized £uids. Another type of
depositwhich appears to require oxidized £uids for eithermetal
transport or deposition is iron oxide-hostedCu^Au deposits,
including the giant OlympicDam deposit in South Australia. The
oldest, andone of the better studied districts ^ Tennant Creekin
the Northern Territory, Australia ^ has beendated at about 1.83 Ga
[61]. Again the emergenceof this class of deposit appears to be
linked to thedevelopment of an oxygenated atmosphere
andhydrosphere.
9. Implications of hydrosphere evolution to leadisotope
growth
The oxidation of the hydrosphere also in£u-enced temporal
changes in the crust and mantle,including Th/U ratios and the
growth of lead iso-topes. Modelling by Kramer and Tolstikhin
[62]suggested that the ‘future’ paradox of lead iso-topes can be
resolved by oxidation of the atmo-sphere (and hydrosphere) at about
2.0 Ga, a pro-cess which mobilized uranium into the oceans
andoceanic crust. Collerson and Kamber [32] sug-gested that
subduction of oceanic crust resultedin a decrease in depleted
mantle Th/U ratios, be-ginning at between 2.2 and 1.8 Ga.Our
analysis suggests that many of the conse-
quences of atmospheric oxidation proceeded un-evenly across the
Earth’s surface. This may alsoapply to the introduction of uranium
into thecrust. This hypothesis is illustrated by consideringlead
from mineral prospects in the Pilbara Craton
(cf. [63]). In this region, crust was formed fromthe early
Archean (V3.5 Ga) to the very earliestProterozoic (V2.4 Ga), with
mineralizing eventsspanning virtually this entire history and
continu-ing to even younger periods. Fig. 6 shows thechanges in the
isotopic ratios of ore leads fromthis region relative to a
theoretical lead isotopeevolution curve calculated using the
Wabigoon^Uchi model of Thorpe et al. [64]. The ore leadsdeviate
from the model evolution curve towardlower 208Pb/206Pb values at a
model age of around2.4 Ga, which indicates U (232Th/238U
integratedto present day) decreased and that uranium wasintroduced
into the source rocks of the ores atthis time.As all of the
deposits sampled are small in size,
it is likely that the source of Pb was local. There-fore, we
hypothesize that uranium was introducedinto the Pilbara upper crust
by circulation of oxi-dized sur¢cial £uids during incipient
oxidation ofthe atmosphere and hydrosphere. This model isconsistent
with the suggestion by Powell et al.[65] that BIFs in the Hamersley
Province wereupgraded by oxidized £uids at 2.2 Ga. However,we are
not implying that the uranium introduc-tion recorded by Pilbara
lead isotopes is a globalevent. Rather, the inferred uranium
metasoma-tism of the upper crust is probably a local phe-
Fig. 6. 208Pb/204Pb versus 206Pb/204Pb diagram showing
theisotopic composition of ore leads from the Pilbara Craton.At
V2.4 Ga the data progressively deviate from the Wabi-goon^Uchi
growth curve of Thorpe et al. [64] to lower U val-ues.
EPSL 6994 5-3-04
D.L. Huston, G.A. Logan / Earth and Planetary Science Letters
220 (2004) 41^55 51
-
nomenon, with pandemic introduction of uraniuminto the crust and
upper mantle at V2.0 Ga[32,62].
10. Conclusions
1. Bedded barite deposits are common in the Pa-leoarchean (s 3.2
Ga), but virtually lacking inthe Meso- and Neoarchean (3.2^2.4
Ga).
2. Variations in europium anomalies betweenSuperior- and
Algoma-type BIFs indicate hy-drothermal emanations are not as
importantan iron source for Superior-type deposits.
3. The evolution of the hydrosphere prior to 1.0Ga is complex,
as follows:3.1. s 3.2 Ga: The oceans were mostly re-
duced and sulfur-poor, although a sul-fate-enriched upper layer
was present.Sulfate in this upper layer was sourcedfrom atmospheric
photolytic reactions.
3.2. 3.2^2.4 Ga: The oceans were relativelyuniform, reduced,
sulfur-poor and iron-rich, although oxyclines may have ex-isted
near the surface. These conditionsallowed the extensive development
ofSuperior-type BIFs. Although atmo-spheric photolytic sulfate
productionstill occurred, this sulfate was rapidlyreduced by
biogenic and hydrothermalactivity, preventing the maintenance ofa
sulfate-rich upper layer.
3.3. 2.4^1.8 Ga: Increasing atmosphericoxygen levels allowed
oxidative weath-ering, which added sulfate to theoceans. This
process proceeded at vary-ing rates across the globe,
producinglayered oceans and an increasing num-ber of sulfate
deposits in the geologicrecord, and culminating with the
depo-sition of extensive BIF at 1.9^1.8 Ga asthe ¢nal oxidation of
the hydrospherescrubbed the oceans of iron. Becausethe oxidation of
the atmosphere re-moved reduced sulfur gases, photolyticsulfate
reduction progressively shutdown.
3.4. 1.8^1.0 Ga: The oceans were uniformlysulfate-enriched and
iron-poor. Beddedsulfate deposits became common andBIFs rare.
4. Prior to 2.4 Ga, Fe2þ was the main oceanicredox bu¡er. After
1.8 Ga, sulfate was themain redox bu¡er. From 2.4 to 1.8 Ga,
theredox bu¡er shifted from Fe2þ to sulfate atvarious rates across
the globe depending onbasin size and environmental factors.
5. The evolution of the hydrosphere had majorimpacts on the
types of mineral depositsformed. Oxidized £uids became important
be-tween 2.4 and 1.8 Ga, which allowed the for-mation of
sediment-hosted copper, Mt Isa-typezinc^lead and iron oxide-hosted
copper^golddeposits. Moreover, the presence of these oxi-dized
£uids allowed upgrading of BIFs to formiron oxide deposits.
6. The introduction of uranium into the mantleby subduction of
oxidized oceanic crust, there-by decreasing mantle Th/U ratios, was
a globalprocess between 2.2 and 1.8 Ga. Lead isotopedata from the
Pilbara Craton suggest that ura-nium addition to the upper crust
may haveoccurred locally at V2.4 Ga in response tothe initial
oxidation of the hydrosphere andatmosphere.
Acknowledgements
This contribution is the result of encourage-ment by Martin Van
Kranendonk to expandupon ideas generated as part of the North
PilbaraProject, a joint Geoscience Australia-GeologicalSurvey of
Western Australia National GeoscienceMapping Accord Project.
Alexander Larionovand Hannu Huhma are thanked for their help
intracking down information on the age of someBIFs. Karin Orth
provided unpublished geochem-ical analyses of BIF in the Kimberley
Block ofWestern Australia. James Kasting, Ian Lambert,Karen
MacKenzie, Peter Southgate, Shen-Su Sun,Jan Veizer and Malcolm
Walter are thanked forreviewing early drafts of this contribution,
andDean Hoatson and Janet Hope provided ¢nalpolishing. These
reviews and ongoing discussion
EPSL 6994 5-3-04
D.L. Huston, G.A. Logan / Earth and Planetary Science Letters
220 (2004) 41^5552
-
with some of the reviewers, particularly MalcolmWalter and
Shen-Su Sun, greatly improved the¢nal version of this contribution,
which is pub-lished with permission of the CEO of
GeoscienceAustralia. Terry Mernagh generated the data usedto
construct Fig. 2.[BOYLE]
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Barite, BIFs and bugs: evidence for the evolution of the Earth’s
early hydrosphereIntroductionThe temporal distributions of sulfate
and iron formationSulfur isotope variations through timeThe
low-temperature geochemistry of sulfur, iron and bariumThe origin
of bedded sulfate depositsThe origin of BIFA model for the
geochemical evolution of the hydrosphere prior to 1.0 Ga3.2-2.4 Ga:
sulfur-poor, reduced oceans2.4-1.8 Ga: transition to sulfur-rich,
oxidized oceans1.8-1.0 Ga: sulfur-rich, oxidized hydrosphereRedox
buffering of the hydrosphere
Implications of hydrosphere evolution to
mineralizationImplications of hydrosphere evolution to lead isotope
growthConclusionsAcknowledgementsReferences