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Precambrian Ophiolites and Related Rocks
Edited by Martin J. van Kranendonk, R. Hugh Smithies and Vickie C. Bennett
Developments in Precambrian Geology, Vol. 15 (K.C. Condie, Series Editor)© 2007 Elsevier B.V. All rights reserved.
DOI: 10.1016/S0166-2635(07)15073-8
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Chapter 7.3

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STABLE CARBON AND SULFUR ISOTOPE GEOCHEMISTRY
OF THE CA. 3490 MA DRESSER FORMATIONHYDROTHERMAL DEPOSIT, PILBARA CRATON,WESTERN AUSTRALIA

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YUICHIRO UENO
Research Center for the Evolving Earth and Planet, Department of Environmental Scienceand Technology, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503, Japan

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7.3-1. INTRODUCTION

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The North Pole area in the Pilbara Craton of Western Australia is well known for theoccurrence of the oldest (∼3.5 Ga) putative microfossils (Awramik et al., 1983; Ueno etal., 2001a) and stromatolites (Walter 1980; Hofmann et al., 1999; Van Kranendonk 2006;
Allwood et al., 2006a), particularly in the chert-barite dominated sedimentary rocks of the
Dresser Formation. However, a biological origin of these putative fossils has been debated(Buick et al., 1981; Buick 1990; Schopf et al., 2002; Garcia Ruiz et al., 2003; Brasier etal., 2002, 2006). Moreover, morphology-based study of prokaryotic fossils provides only
limited information for the physiology of possible ancient life.
On the other hand, carbon and sulfur isotope geochemistry is useful to trace specificmetabolic activities in the past, including autotrophic carbon fixation, sulfate reduction,and methanogenesis. Shen et al. (2001) reported 34 S-depletion of pyrite in bedded andvein barite (δ 34 Spyrite − δ 34 Sbarite = −5 to −21h) in the Dresser Formation, which isinterpreted as the oldest evidence for microbial sulfate reduction. The recent discovery
of 13 C-depleted methane (δ 13 CCH4 < −56h) in primary fluid inclusions of the Dresser
Formation indicates that microbial methanogenesis could have been active in the Paleoarchaean (Ueno et al., 2006).
The depositional environment of the Dresser Formation has long been considered as
an evaporitic, shallow marine basin (e.g., Buick, 1990; Lowe, 1983). Hence, most of thepalaeontological and geochemical evidence for the putative early life forms have been interpreted from the viewpoint of a photosynthesis-based shallow marine ecosystem, which
includes cyanobacteria-like microfossils (Awramik et al., 1983), stromatolites build byphotosynthesizers (Walter, 1983), and sulfate-reduction in a locally sulfate-rich evaporitic
basin (Shen and Buick, 2004). However, recent geological and geochemical investigations
suggest that hydrothermal activity played an important role in the deposition of the chertbarite rocks of the Dresser Formation (Isozaki et al., 1997; Nijman et al., 1999; Runnegar

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Chapter 7.3: Stable Carbon and Sulfur Isotope Geochemistry2001; Ueno et al., 2001a, 2004; Van Kranendonk et al., 2001b; Van Kranendonk and Pirajno, 2004; Van Kranendonk, 2006). The most striking evidence for the hydrothermal
activity comes from the occurrence of a prominent swarm of silica ± barite feeder veins,which penetrated along syn-sedimentary growth faults in the footwall to bedded chertbarite rocks (Nijman et al., 1999; Ueno et al., 2001a; Van Kranendonk et al., 2001b; Van
Kranendonk and Pirajno, 2004). Hence, it is plausible that the Dresser Formation represents a hydrothermally influenced volcano-sedimentary sequence deposited in a submarine
volcanic caldera (Van Kranendonk, 2006, this volume).
With this new depositional setting in mind, we can now re-evaluate the link betweenhydrothermal activity and the palaeontological and geochemical evidence for life in the
Dresser Formation. Furthermore, recent petrological studies confirm that black silica veins,which are abundant in the footwall to the Dresser Formation, commonly contain significant amounts of organic matter (e.g., Ueno et al., 2004), as reported from other Paleoand Mesoarchaean greenstone terranes (e.g., de Wit et al., 1982; Kiyokawa et al., 2006).
The origin of the organic matter in the veins is controversial, but particularly important,because the organic matter has been produced either by biological carbon fixation (Uenoet al., 2004, 2006; Brasier et al., 2006), or by pre-biotic organic synthesis (Brasier et al.,
2002; Lindsay et al., 2005; McCollom and Seewald, 2006). In this paper, the mode ofoccurrence of the Dresser hydrothermal deposits is first briefly summarized. Then, the carbon and sulfur isotope geochemistry of the Dresser deposits is reviewed in relation to thehydrothermal activity.

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7.3-2. HYDROTHERMAL DEPOSITS IN THE NORTH POLE AREA

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7.3-2.1. The Dresser Formation

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The Dresser Formation is exposed in the North Pole Dome of the East Pilbara Terrane,
Pilbara Craton (Fig. 7.3-1). The Dresser Formation (Van Kranendonk and Morant, 1998)
consists of ca. 2-km thick basaltic greenstones intercalated with three horizons of bedded
chert (Fig. 7.3-1). The lowermost chert unit is 1 to 70 m thick and is intercalated withseveral barite horizons of 0.1 to 5 m thickness. This barite-bearing chert unit corresponds
to the “chert-barite unit” previously described by Buick and Dunlop (1990). The other two
chert units are thinner (1–13 m) than the chert-barite unit, and contain only rare barite.
The precise age of the Dresser Formation has not been directly determined. Zircon UPb dating yields an age of 3458 ± 2 Ma for felsic volcanic rocks of the stratigraphicallyoverlying Panorama Formation in the North Pole area (Thorpe et al., 1992a), indicating an
age for the Dresser Formation of >3460 Ma. A model lead age of 3490 Ma (Thorpe et al.,

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Fig. 7.3-1. Geological map of the Dresser Formation in the North Pole area, modified from Kitajima
et al. (2001).

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Chapter 7.3: Stable Carbon and Sulfur Isotope Geochemistry

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Fig. 7.3-2. Detailed geological map of part of the Dresser Formation, showing carbon isotopic values of analysed organic matter (circles), andof CO2 and CH4 in fluid inclusions (stars). Note that the contact between the chert-barite unit and the underlying pillowed basalt represents thesea-floor surface at the time of chert deposition, and therefore that the silica veins developed in the uppermost 1000 m of the oceanic crust. The
box indicates the location of Fig. 7.3-4.
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51992b) was obtained for galena from barite in the Dresser Formation, which is considered
to represent the actual depositional age of the Dresser Formation.

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7.3-2.3. Organic Matter in the Silica Veins

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In the North Pole area, numerous (>2000 identified) silica veins characteristically intruded
along normal faults, which developed in pillowed basaltic volcanic rocks below the Dresser
Formation (Figs. 7.3-1–7.3-4). They are 0.3–20 m wide and generally >100 m long, withthe longest one over 1 km. The veins are massive and are composed mainly of fine-grained
silica (<10 µm). Some silica veins show a symmetrical zonation pattern across the dikeaxis, and sometimes have agate at the center (Fig. 7.3-3(b)), in which the silica shows fanshape structures grown from the margins toward the center of the veins. This suggests theprecipitation of silica from a hydrothermal fluid (cf. Van Kranendonk and Pirajno, 2004).
The silica veins were previously called T-chert, which means “tectonic” chert (Hickman,
1973), chert vein (Nijman et al., 1999), chert dyke (Lindsay et al., 2005), silica dyke (Uenoet al., 2004, 2006), or silica vein (Van Kranendonk, 2006). The term “silica vein” is used inthis paper, because “chert” is a term used for a chemical sedimentary rock with a distinct
grain size and texture, and is thus not suitable for an intrusive rock, which has a grain sizeof generally microquartz. Also, the silica veins are clearly distinguished from quartz veins,which consist of white, coarse-grained quartz.
The silica veins terminate into chert beds of the Dresser Formation, but do not cut
through the entire chert unit, nor into the overlying pillow basalt (Figs. 7.3-1, 7.3-2, and7.3-4: Ueno et al., 2004; Van Kranendonk, 2006). The tops of the veins locally show a
gradual transition into chert beds, forming a clear T-junction. In the field, repeated cycles ofnormal faulting, vein formation, and subsequent deposition of chert beds can be recognized(Fig. 7.3-4). These relationships suggest the silica veins were formed intermittently during
the deposition of chert beds (Isozaki et al., 1997; Nijman et al., 1999; Ueno et al., 2001a;
Van Kranendonk et al., 2001b; Van Kranendonk, 2006).
In addition, 0.1 to 2 m wide barite veins also intruded the footwall basaltic rocks, andsome vein barite occurs in silica veins, as growth zones, or as discrete veins (cf. Van Kranendonk, 2006). Similar to the relationship between silica veins and bedded chert, thevein and “bedded” barite show feeder vein-deposit relationships (Nijman et al., 1999). The
distribution of vein barite is laterally discontinuous and is generally restricted to the uppermost ∼100 m of the pillow lava unit below the Dresser Formation, whereas silica veins
occur up to about 1000 m below the formation. This distribution pattern indicates that thehydrothermal fluid circulated at least 1000 m below seafloor at the time of deposition of the
Dresser Formation, but that circulation of sulfate-rich fluids was restricted to the shallower
part of the system.

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The silica veins contain considerable amounts of organic matter (kerogen), which givethe veins their black to gray color (Fig. 7.3-3). Thorough geochemical and petrological

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Fig. 7.3-3. (a) Photograph of a 1 m wide hydrothermal silica vein leading up to bedded chert andbarite. (b) Close-up view of the central part of the silica vein in (a), showing agate-textured quartz
core. Width of the photo is about 30 cm. (c) Photomicrograph of a black part of the silica vein,
showing irregular-shaped clots of organic matter and pyrite enclosed in a fine-grained silica matrix.
(d) Photomicrograph of coarse-grained quartz developed in the central part of the vein, showing
H2 O–CO2 fluid inclusions therein.

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Fig. 7.3-4. Mode of occurrence of the chert-barite beds and silica veins intruding along
syn-depositional growth faults (modified from Isozaki et al. (1997)). At least three stages of penetration of the silica vein and subsequent deposition of the bedded chert can be recognized in thisoutcrop.

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investigation of more than 600 hand specimens of the silica veins has revealed that the concentration of organic carbon in the veins would have been originally >1 mg C/g (Ueno etal., 2004). Most veins have suffered post-depositional metasomatism (oxidation) and havelost about 90% of the original organic matter. However, some black silica veins possess an
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Chapter 7.3: Stable Carbon and Sulfur Isotope Geochemistryal., 2004). These mineral compositions, as well as its silica- and barite-dominated mineral
assemblage, suggest that the veins were deposited from relatively low temperature (100–200 ◦ C), reducing hydrothermal fluids (Ueno et al., 2004). This temperature estimate isconsistent with homogenization temperatures of fluid inclusions in vein quartz (∼150 ◦ C;
Kitajima et al., 2001) and in cavity-filling quartz of pillowed basalt (90–170 ◦ C; Foriel etal., 2004).
Consequently, this suggests that organic carbon was widely distributed in fissures thatdeveloped in the upper 1000 m of the seafloor basalt. These fissures acted as conduits forcirculating hydrothermal fluids (Van Kranendonk, 2006). The estimated temperature of thefluid is low enough to permit biological activity in the sub-seafloor environment, and issuitable for thermophilic or hyperthermophilic organisms.

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7.3-3.1. Carbon Isotopic Composition of Organic Matter in the Hydrothermal Veins
The δ 13 C values of organic matter in the silica veins and bedded chert in the Dresser
Formation are −38 to −30h and −31 to −29h, respectively (Figs. 7.3-2 and 7.3-5).
The isotopic variation of the organic matter does not correlate with depth below seafloor
surface at that time, but rather with organic carbon concentrations. This indicates that postdepositional alteration would have reduced a considerable amount of the organic carbon in
the silica veins and significantly modified its carbon isotopic composition. Hence, we haveto first estimate the original isotopic composition of the organic matter before discussing
its origin.
Among the silica veins, sulfide-bearing black silica veins contain higher organic carbon
concentrations and show more 13 C-depleted isotopic compositions than gray sulfide-freesilica veins, which formed by alteration of primary sulfide-bearing veins (Ueno et al.,2004). This variation would be produced by the Rayleigh distillation process with a carbon
isotopic fractionation factor of 0.9985 (Fig. 7.3-5; Ueno et al., 2004). According to thismodel, the original organic carbon in the silica veins would have been isotopically heterogeneous (∼5h) at around −35h, and at least some material had initial δ 13 C values of−38h.
The inferred 13 C-depletion of the initial organic carbon can be explained by biologicalcarbon fixation. The isotopic composition of carbonate carbon in the veins is −2h (Uenoet al., 2004). This indicates that the equilibrated dissolved CO2 had δ 13 C values of about−4h at 100 ◦ C (Mook et al., 1974). This estimate is roughly consistent with δ 13 C valuesof CO2 entrapped in the primary fluid inclusions (−3 to −7h; Ueno et al., 2006). Thus,
the fractionation between initial organic carbon and dissolved CO2 (i.e., δ) would havebeen over 34h, for at least some of the material.
If the organic carbon was produced by autotrophic organisms, then this fractionation is
too large to be associated with Rubisco (δ  30h; e.g., House et al., 2003), the enzyme

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Fig. 7.3-5. Carbon isotopic compositions of organic matter, CO2 , and CH4 in the silica veins. (a)Relationship between organic carbon concentration and carbon isotopic composition of the organic
matter in the silica veins and bedded chert. Dashed lines tie the values from the same rock samples.
Doted lines represent Rayleigh-fractionation trajectories with fractionation factor () of 0.9985. The
three lines labeled 1, 10, and 100 started with an initial δ 13 C value of −38h, and with initial concentrations of organic carbon of 1, 10, and 100 mg C/g, respectively. (b) Relationship between δ 13 CCH4and δ 13 CCO2 values of the extracted volatiles from the fluid inclusions as compared with that of fluids vented from present-day seafloor hydrothermal systems. 13 C-depleted CH4 occurs preferentiallyin samples rich in primary fluid inclusions (Ueno et al., 2006). (c) The ranges of carbon isotopic
compositions of organic matter and carbonate in the silica vein and bedded chert.

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utilized by aerobic photoautotrophs. This is consistent with the newly recognized depositional model, in which phototrophs are unlikely to have been involved in the sub-seafloorhydrothermal environment. On the other hand, the large organic carbon fractionation could
have been produced via the reductive acetyl-CoA pathway (δ  42h; e.g., House et al.,
2003; Londry and Des Marais, 2003), which is utilized by H2 -dependent chemoautotrophs
such as methanogen, acetogen, and some sulfate-reducers. This inferred large fractionationis consistent with the reducing conditions of the sub-seafloor hydrothermal system during
the Dresser Formation. In fact, such large fractionations (up to 36h) have been demonstrated by some anaerobic and thermophilic chemoautotrophs, such as Methanobacterium
thermoautotrophicum (Fuchs et al., 1979).
Although the geological, petrological, and geochemical characteristics of the organic
matter in Dresser Formation hydrothermal veins can be fully explained by metabolic
activity of autotrophic organisms in a hydrothermal environment, the data do not eliminate a possible abiological origin for the organic matter. Under hydrothermal conditions

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Fig. 7.3-5. (Continued.)

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of 200–300 ◦ C, it has been shown that abiological Fischer–Tropsch-type (FTT) reactions
can produce organic matter with similarly large 13 C-depletions (McCollom and Seewald,2006). However, it is questionable whether FTT synthesis took place under the conditions
prevailing during silica precipitation in the Dresser Formation, because likely catalysts for
FTT reactions (i.e., native metal and oxide) would not have existed under the reducing conditions exhibited by these veins. Fe-Ni alloy does not occur, despite thorough investigationof more than 300 petrographic thin sections of the silica veins (Ueno et al., 2004). Fe oxides occur only as a secondary mineral in the veins, whereas Fe-Ni sulfide (polydymite)exists as the primary mineral phase in the veins. Significantly, the presence of sulfide is
known to poison industrial FTT reactions (Anderson, 1956). The apparent deficiency ofan effective catalyst raises doubts as to whether FTT reactions could have produced theorganic matter, under the low temperature, sulfidic conditions recorded by Dresser Formation hydrothermal veins. Note that native metal could have existed in the deeper partsof the veins than the site of silica precipitation, such that FTT reactions, if they occurredat all, may have possibly occurred under deeper and much higher temperature conditions(200 ◦ C). If this is the case, however, isotopic fractionation should be smaller than thatobserved and organic materials should increase with depths in the veins, a feature which
does not occur. In the following section, this point is further discussed in relation to theisotopic compositions of methane and carbon dioxide.

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The hydrothermal silica veins and associated quartz veins contain fluid inclusions (Fig. 7.33), some of which were entrapped during the original silica precipitation. In-situ laser
Raman microspectroscopy confirmed that the fluid mainly consists of H2 O and CO2 , witha minor, but detectable, amount of CH4 and H2 S (Fig. 7.3-6).
Crushing extraction of the CO2 and CH4 in the fluid inclusions coupled with carbon
isotope analysis revealed that the δ 13 C values of the CO2 and CH4 is −7 to 0h and −56
to −36h, respectively (Figs. 7.3-2 and 7.3-5: Ueno et al., 2006). Similar to the isotopic
compositions of the organic matter, the δ 13 CCH4 values do not correlate with the depth
below the seafloor surface at that time, but with a mixing ratio of primary and secondary
inclusions determined by petrographic observation (Ueno et al., 2006). This relationshipindicates that the primary fluid contains 13 C-depleted CH4 and CO2 (δ 13 CCH4 < −56h;δ 13 CCO2 < −4h), and that a secondary fluid was more 13 C-enriched (δ 13 CCH4 ≈ −35h;δ 13 CCO2 ≈ 0h).
The large 13 C-depletion of the primary CH4 with respect to CO2 (δ 13 CCO2–CH4 >52h) is comparable to that exhibited by microbial methanogenesis. It is known thatmethanogenic microbes reduce CO2 to produce CH4 with a distinctively large isotope effect (δ 13 CCO2–CH4 = 21 to 69h; Conrad, 2005). Although the fractionation may possibly
be smaller under a high temperature environment, the large fractionation has also been observed for hyperthermophilic methanogens grown above 80 ◦ C (Botz et al., 1996). Somemethanogens can also produce CH4 by decomposing acetate, which results in fractionations of 7–27h between the acetate and CH4 (Conrad, 2005). If it is assumed that thepotential substrate acetate had a δ 13 C value similar to that of organic matter in the silica
veins (≈−35h), then the acetate fermentation process can also explain the observed fractionation of the primary methane and organic matter (δ 13 Corg-CH4 > 21h). Therefore,
the carbon isotopic relationship among CH4 , CO2 , and organic matter is consistent withbiological methane production by reduction of CO2 and/or acetate.
The large isotopic fractionation between CO2 and CH4 is not to be expected from amagmatic process, which should result in much smaller isotopic fractionation due to hightemperature equilibrium (Fig. 7.3-5). Thermal decomposition of organic matter under thisprocess may have possibly produced >20h fractionation between primary CH4 and theorganic matter. However, the primary fluid lacks ethane and propane, which should havebeen co-produced by thermogenesis. Hence, a thermogenic origin of the primary methane
is controversial.
Fe-Ni alloy catalyzed FTT reactions at 200–300 ◦ C may produce 13 C-depleted methaneby 30–50h, relative to CO2 (Horita and Berndt, 1999; McCollom and Seewald, 2006).
Even larger isotopic fractionation between CH4 and CO2 may be achieved at lower temperature below 200 ◦ C (Horita and Berndt, 1999; McCollom and Seewald, 2006). However,as discussed in the previous section, Fe and Ni would have existed as sulfides at temperatures prevailing during the silica precipitation (100–200 ◦ C). Hence, native metal catalysis
cannot be expected to produce the methane in the veins.

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Fig. 7.3-6. Representative Raman spectra of primary fluid inclusions in the silica vein. The fluid mainly consists of H2 O (broad band around3400 cm−1 ) and CO2 (two main peaks at 1283 and 1387 cm−1 ). Inset shows the result of longer time analysis (360 seconds), indicating thepresence of N2 (2329 cm−1 ), H2 S (2329 cm−1 ), and CH4 (2914 cm−1 ) in the same fluid.
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As already mentioned above, native metal could have existed in deeper parts of the hydrothermal system than the site of silica precipitation (100–200 ◦ C). Thus, methane may
have possibly been produced abiotically under much higher temperature conditions. If thiswas the case, isotopic fractionation between CO2 and CH4 should be smaller (less than
30h above 300 ◦ C; Horita and Berndt, 1999). Hence deep and high temperature abiotic synthesis does not explain the observed isotopic fractionation of the primary fluid(δ 13 CCO2 − δ 13 CCH4 > 52h). Even if abiotic methane in deep and high temperature
regimes were introduced into the fluid inclusions, its contribution should be small in the
primary fluid preserved in the veins.
Furthermore, the isotopic relationship between the methane and the organic matter inthe veins is not expected from FTT reactions (Fig. 7.3-7). Recent experimental studies by

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Fig. 7.3-7. Relationships between carbon number and its isotopic composition observed in: (a) abiotic hydrocarbons from Kidd Creek (Sherwood-Lollar et al., 2002) and Khibina (Potter et al., 2004)
natural gas fields; (b) hydrothermal experiment (McCollom and Seewald, 2006); and (c) the Dresserhydrothermal deposit (Ueno et al., 2004, 2006).

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Fig. 7.3-7. (Continued.)

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McCollom and Seewald (2006) demonstrated that Fe-metal-catalyzed FTT reactions under
250 ◦ C hydrothermal conditions produces 13 C-depleted methane (δ 13 CCO2 − δ 13 CCH4 =36h) as well as C2 to C28 hydrocarbons with the same degree of isotopic fractionation(δ 13 CCO2 − δ 13 Chydrocarbons = 36h). They pointed out that the organic matter in the silica
veins shows similar degree of fractionation (δ 13 CCO2 −δ 13 Corg ≈ 31h) and thus may represent higher molecular weight organic compounds polymerized during the FTT synthesis.
However, their experiments also demonstrate that significant isotopic fractionation does
not occur during the polymerization step of FTT synthesis. Therefore, the observed large
13 C-depletion of the primary methane relative to surrounding organic matter in the veins(δ 13 CCH4 − δ 13 Corg < −21h) should not result from the Fe-metal-catalyzed FTT reaction (Fig. 7.3-7(b)). In addition, slight isotopic fractionation may possibly occur during thepolymerization step of some FTT reactions. It is known that abiotic hydrocarbons in several natural gas reservoirs exhibit progressive 13 C-depretion from C1 to C4 hydrocarbons(Fig. 7.3-7(a)), which may suggest that isotopically light methane selectively polymerizes
into higher hydrocarbons during FTT synthesis (Sherwood-Lollar et al., 2002). This isotopic trend is opposite to that exhibited by the primary methane and organic matter in the
Dresser hydrothermal veins. Hence, the inferred FTT processes can not be applied to theformation of organic matter in the Dresser Formation.
In summary, the carbon isotope geochemistry of carbonate, organic matter, methaneand carbon dioxide in the Dresser hydrothermal deposits can be fully explained by biological processes including thermophilic chemoautotrophic organisms, especially methanogen.
Although the organic matter and methane production by alternative abiological process is
not completely dismissed, the observed isotopic relationships are incompatible with ourcurrent knowledge of abiotic organic synthesis.

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7.3-3.3. Sulfur Isotopic Compositions of Pyrite in Barite Deposits

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Various forms of sulfur compound occur in the Dresser Formation, and generally showδ 34 S values close to 0h (δ 34 S = −5 to +5h; Lambert et al., 1978), which seems to be derived from juvenile magmatic sulfur (Fig. 7.3-8). Exceptionally, bedded and vein barites ofthe formation contain more 34 S-depleted pyrite (δ 34 S = −17 to +5h; Shen et al., 2001).
The observed large isotopic fractionation between the pyrite and the co-existing barite (upto 21h) is similar to those resulting from the metabolic activity of sulfate-reducing microbes and thus may represent the oldest evidence for microbial sulfate reduction (Shen etal., 2001; Shen and Buick, 2004). Also, this sulfur isotopic fractionation is exceptionallylarge relative to those reported from other Paleo- and Mesoarchaen deposits, such that microbial sulfate reduction may have been locally developed in sulfate-poor Archaean ocean
water (e.g., Canfield, 2005).
The microbial origin of the barite-associated pyrite, however, has been debated, because
abiotic reactions under hydrothermal conditions may have produced pyrite with a similar
degree of isotopic fractionation (Van Kranendonk, 2006). Although initial models for the
Dresser barite suggested that they were deposited as Evaporitic gypsum replaced by barite(Buick and Dunlop, 1990; Lowe 1983), recent geological and petrological investigations
have demonstrated a clear hydrothermal origin of the barite (Nijman et al., 1999; Ueno
et al., 2001a; Van Kranendonk, 2006) as a non-evaporitic, primary mineral confirmed by
X-ray CT analyses (Runnegar, 2001). It is notable that the 34 S-depleted pyrite occurs not
only in bedded barite, but also in vein barite. Even if the pyrites were produced by microbial
sulfate reduction, the depositional environment of the 34 S-depleted pyrite should not be ashallow marine setting, but a submarine hydrothermal environment.
Van Kranendonk and Pirajno (2004) proposed that the pyrite and barite were depositedby hydration of magmatic SO2 , which produced both sulfate and sulfide. In this case, equi-

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Fig. 7.3-8. Sulfur isotopic compositions of sulfide in chert and veins (black: Lambert et al., 1978;
Y. Ueno et al., unpublished data) and disseminated pyrite (white) in bedded and vein barites (gray)(Shen et al., 2001; Y. Ueno et al., unpublished data). The narrow lines indicate the full ranges ofobserved isotopic compositions, while >70% of the data are within the broad lines.

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librium isotopic fractionation between sulfate and sulfide is 22h at 250 ◦ C (Ohmoto andGoldhaber, 1997), and is thus comparable to that observed in the barite-pyrite pair described by Shen et al. (2001). Alternatively, Runnegar (2001) and Runnegar et al. (2002)
suggested that thermochemical reduction of seawater sulfate could have produced the
barite-pyrite association. These abiotic models are roughly compatible with the δ 18 O values of the barite (δ 18 OSMOW = 5 to 10h; Lambert et al., 1978), which would suggest
about 200 ◦ C conditions of precipitation (Runnegar et al., 2002). If this estimate is correct,
the temperature is somewhat high to allow microbial activity. However, the oxygen isotope geothermometry of the barite assumes that δ 18 O value of Archaean seawater would
be the same as that of modern seawater (δ 18 OSMOW = 0h). This assumption is still untenable, because the Archaean seawater might have been more 18 O-depleted (Shields andVeizer, 2002; Knauth, 2005), giving a much lower temperature estimate for barite deposition. Consequently, both biological and abiological reactions could explain the observed
isotopic fractionation between the barite and pyrite.
The origin of the barite-associated pyrite might be further constrained by multiple sulfur
isotope analysis. Discovery of non-mass-dependent sulfur isotope fractionation in pre-2.0
Ga sedimentary rocks implies that multiple sulfur isotope ratios (32 S/33 S/34 S/36 S) could bea useful new tracer for the Archean biogeochemical sulfur cycle (e.g., Farquhar and Wing,
2003). Recent multiple sulfur isotope analysis revealed that the Dresser barite has nonmass dependent isotopic composition, with negative 33 S (Ueno et al., 2003; Mojzsis,
this volume). This clearly suggests that the barite would have been derived from nonmass-dependently fractionated Archean seawater sulfate, but not from a magmatic source,which should be mass-dependent. The pyrites in both vein and bedded barite also show
negative 33 S values, again indicating that the pyrite originated from seawater sulfate(Runnegar, 2001; Ueno et al., 2003). This means that reduction of seawater sulfate would
be responsible for the ∼20h 34 S-fractionation between the barite and pyrite. However, it
is still ambiguous whether the pyrite was produced by thermochemical or microbial sulfate
reductions.

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Fig. 7.3-9 summarizes the proposed carbon and sulfur cycles in the Dresser Formationhydrothermal system. The geological and geochemical investigations suggest that the reduced forms of carbon (CH4 and organic matter) and sulfur (e.g., pyrite) were produced in
the hydrothermal system, with significantly large isotopic fractionations. This strongly indicates that low temperature reactions took place in the hydrothermal system, regardless ofa biological or abiological interpretation. In principle, large isotopic discrimination could
be expected under low temperature conditions, in which chemical reactions are usuallyprohibited due to kinetic barriers. Hence, the observed large isotopic fractionations imply
catalysis that accelerated the low temperature reactions.
For carbon, large isotopic fractionations between CH4 –CO2 and organic matter-CO2 arebest interpreted as a result of enzymatically catalyzed biological reactions, because effec-

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Fig. 7.3-9. A conceptual model of carbon and sulfur cycles in the Dresser hydrothermal system. See text for discussion.

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tive catalysts of the FTT reactions (e.g., Fe-Ni alloy) would have been unstable in the lowtemperature hydrothermal environment. Hence, it is plausible that a microbial ecosystem
would have existed in the Dresser hydrothermal system, including a sub-seafloor environment. Methanogens would have gained energy through the conversion of CO2 and H2
into CH4 and acted as a primary producer of organic compounds, which may possibly beutilized by other heterotrophs.
For sulfur, on the other hand, both thermochemical and microbial sulfate reductions
could explain the exceptionally large isotopic fractionation between barite and microscopic
pyrite crystals embedded therein. Sulfide minerals occur also in bedded chert and in silica
veins, but these show no such large isotopic fractionations. Hence, most sulfide minerals
in the Dresser Formation seem to be derived from a volcanic source (cf. Van Kranendonk, 2006), but not from seawater sulfate. Although the origin of the barite-associated34 S-depleted pyrite is still ambiguous, sulfate reduction seems to have been restricted
to the shallower part of the hydrothermal system (Fig. 7.3-9). This may suggest that theredox gradient beneath the seafloor might have controlled zonation of different types ofmetabolisms.

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ACKNOWLEDGEMENTS

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The author would like to express great appreciation and thanks to S. Maruyama and
Y. Isozaki for continuous encouragement, support and fruitful discussions over ten years.
The author acknowledges the assistance in the field work of M. Terabayashi, Y. Kato,
K. Okamoto, T. Ota, T. Kabashima, K. Kitajima, and K. Shimizu. Field collaboration with
A. Thorne, K.J. McNamara, and A.H. Hickman was helpful and much appreciated. Constructive comments by M. Van Kranendonk and an anonymous reviewer improved themanuscript. This study was supported by the 21st Century COE Program “How to build
habitable planets”, Tokyo Institute of Technology, sponsored by the Ministry of Education,
Culture, Sports, Technology and Science, Japan.

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