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Geochemical Journal, Vol. 34, pp. 121 to 133, 2000 Sulfur and carbon isotope analyses of the 2.7 Ga Jeerinah Formation, Fortescue Group, Australia TAKESHI KAKEGAWA, 1 YUKIO K ASAHARA,1 K EN-ICHIRO H AYASHI1 and HIROSHI OHMOTO2 1 2 Tohoku University, Graduate School of Science, Sendai 980-8578, Japan The Pennsylvania State University, Astrobiology Research Center, University Park, PA 16802, U.S.A. (Received January 14, 1998; Accepted October 6, 1999) Sulfur and carbon isotope ratios have been determined on carbonaceous shales of the 2.7 Ga Jeerinah Formation, Hamersley Group, Australia. The analyses were performed using the Nd-YAG laser microprobe method. The δ 13C(PDB) values of organic matter range from –38.3‰ to –35.1‰ with an average of –37.0‰ (44 analyses). These carbon isotope compositions support the previously proposed hypothesis that methanogens and methanotrophs were involved in the carbon cycles in the 2.7 Ga Hamersley ocean. Sulfur isotope compositions are ranging from +0.4‰ to +10.2‰ with an average of +4.6‰ (90 analyses). A notable feature is variable δ 34S values within a micro-scale area; approximately 6 to 7‰ variations of δ34S values were detected within 5 × 5 mm areas. Such δ 34S variations clearly indicate that pyrites in the Jeerinah shales were formed as a result of the sulfate reduction. This suggests that the 2.7 Ga Hamersley ocean contained appreciable amount of dissolved sulfate, opposing to the previously popular H2S-rich ocean model. Because of no evidence for the hydrothermal alterration on the examined samples, pyrites in the 2.7 Ga Jeerinah shales were most likely formed by the biological sulfate reduction in sediments. ganisms were active in the 2.7 Ga ocean and that these organisms played an important role in the global carbon cycle. These suggestions were based on the discovery of organic matter with extremely light δ 13C values, –60 to –40‰, in some sedimentary rocks in the Hamersley Basin in Australia, in the Abitibi greenstone belt in Canada and the Transvaal basin in South Africa (Hayes et al., 1983, 1992; Strauss, 1986; Schidlowski, 1988; Strauss and Moore, 1992; Strauss and Beukes, 1996). Controversy has also persisted as to whether sulfate-reducing bacteria were already active in the 2.7 Ga ocean, because the previously reported sulfur isotope data on 2.7 Ga sedimentary rocks were ambiguous (Goodwin, 1976; Lambert and Donnelly, 1992). The shale samples of the Jeerinah Formation are characterized by high organic carbon contents (up to 5 wt %) and no hydrothermal alteration (Trendall, 1979). Therefore, they are suitable for an investigation of the ecological system and the I NTRODUCTION The carbon isotopic composition of organic matter in sedimentary rocks provides information about the nature of ancient organisms and ecological conditions (Schidlowski et al., 1983). For Archean sedimentary rocks without any fossils, carbon isotope records of organic matter are the only clue for the evolution of life during the early stage of Earth’s history. Sulfur isotopic compositions of pyrite in Archean sedimentary rocks may provide information if sulfate-reducing bacteria were active in sedimentary environments (Raiswell and Berner, 1986). In order to investigate the sedimentary environments and ecological conditions in oceans 2.7 Ga ago, carbonaceous shales were collected from the Jeerinah Formation of the Fortescue Group in the Hamersley district, western Australia. Some researchers (e.g., Hayes et al., 1983) have suggested that methanogenic and methanotrophic or121 122 T. Kakegawa et al. Fig. 1. Geological map of the Hamersley-Pilbara district. activity of sulfate-reducing bacteria in the 2.7 Ga ocean. In this study, in situ analyses of sulfur and carbon isotopes were performed on 4 rock specimens of the Jeerinah shales using a Nd-YAG laser microprobe system. The relationship between sulfate-reducing and methanogenic bacteria in the 2.7 Ga Hamersley ocean will be discussed in this paper. G EOLOGY The Pilbara Craton is mainly comprised of 3.58–2.77 Ga old granite and greenstone. The late Archean to early Proterozoic sequence, called the Mt. Bruce Supergroup, overlies the Pilbara Craton (Fig. 1). The Mt. Bruce Supergroup is subdivided into three groups based on lithostratigraphy: the mostly volcanic Fortescue Group, the Hamersley Group, and the dominantly clastic sedimentary Turee Creek Group (Trendall, 1979; Arndt et al., 1991). The Fortescue Group, with a thickness of 150 to 500 m, is comprised of a variety of igneous and sedimentary rocks. The Jeerinah Forma- Fig. 2. Stratigraphy of the Fortescue Group. S and C isotope analyses of the 2.7 Ga Jeerinah Formation Fig. 3. Petrographic feature of pyrite in a Jeerinah sample. tion is the uppermost unit of the Fortescue Group that crops out mostly along the boundary between the Pilbara block and the Hamersley Basin (Figs. 1 and 2). This formation is predominantly comprised of clastic sedimentary rocks (shale and sandstone). A U-Pb zircon age of 2687 Ma is reported for a tuffaceous unit of the Jeerinah Formation (Arndt et al., 1991). Many previous investigators (Blake and Barley, 1992; Simonson et al., 1993) proposed the shallow marine conditions as a sedimentary environment of the Jeerinah Formation. SAMPLES Five shale samples were collected from outcrops ~1 km southeast of the Millstream National Park (Fig. 1). Two representative samples were analyzed in detail for the sulfur and carbon isotopic compositions using the laser microprobe method. The Jeerinah samples are finely laminated, and cross laminae were also recognized in some samples. Fine-grained quartz and chlorite (<10 µm) are the most abundant silicate minerals. Quartz occurs as rounded-grains, showing equigranular texture, and chlorite occurs as aggregates of green fibrous crystals. Epidote and actinolite are also present as minor silicate minerals. Organic matter is laminated concordantly to the sedimentary 123 bedding. Most pyrite crystals occur in anhedral shapes with sizes of ~10 to 100 µm (see Fig. 3). Coarse-grained pyrite crystals (>200 µm in diameter) are only found in later carbonate veins which cut the sedimentary bedding. Goethite, pseudomorphic after pyrite, occurs as a minor mineral in some samples. Samples containing goethite were avoided for the sulfur isotope analyses. The metamorphic grade of the examined samples is interpreted as lower than greenschist facies, because the amounts of biotite and muscovite are small, the sedimentary bedding is well preserved, and schistosity is absent (Trendall, 1979; Trendall et al., 1990). ANALYSES Sulfur and carbon concentrations of shales Bulk rock samples were prepared by crushing rock chips of ~1 cm3 in volume. Approximately 10 mg of powdered samples were used for each analysis. Sulfur and carbon were analyzed using a Carlo-Erba Elemental Analyzer. The detection limit by this analytical method is approximately 0.1 wt % for both sulfur and carbon. The reproducibility of the carbon and sulfur concentrations is better than 0.2 wt %. Carbonate and sulfate minerals were not detected through the petrographic and X-ray diffraction studies. This suggests that carbon and sulfur concentrations determined on the Jeerinah samples represent organic carbon and pyrite sulfur contents. Sulfur isotopes Samples used in this study contain micro-sized pyrite crystals. The primary interest of this study is to determine the micro-scale variations of δ 34S values of pyrite and to discuss origins of pyrite. It is difficult, however, to physically separate each pyrite. In addition, the conventional Cu 2O combustion method to prepare SO 2 gas for sulfur isotope analyses (Robinson and Kusakabe, 1975) requires a concentrate of approximately 3 mg of pyrite; this is difficult to achieve if pyrite is disseminated in very fine grains. Because the laser microprobe method (Kelly and Fallick, 1990; 124 T. Kakegawa et al. Fig. 4. Schematic diagram of the Nd-YAG laser microprobe system at Tohoku University. Kakegawa, 1993; Kakegawa et al., 1998) can deal with much smaller amounts of pyrite, we can expect to determine the δ34S values in a micro-scale area. Therefore, δ 34S values were determined using a laser microprobe system. The Nd-YAG laser system used at Tohoku University (Fig. 4) consists of: (a) a Nd-YAG laser microprobe unit, (b) a sample chamber, (c) an optical system (a binocular microscope and a TV monitor), and (d) an oxygen flask. The selected samples were cut into rock chips with a volume of ~2 cm3 and then placed into the sample chamber. The laser ablation was carried out using a continuous-wave (C.W.) Nd-YAG laser, under an O2 atmosphere (8 torr of PO 2 ). The diameter of the laser beam can be changed by an adjustable aperture from the multiple mode (~150 µm in laser diameter) and to the TEM00 mode (~30 µm in laser diameter). Only the TEM 00 mode was adopted for all laser microprobe analyses at Tohoku University, because pyrite in the examined samples is smaller than the laser diameter in the multiple mode. The Nd-YAG laser was fired on a sample for 5 seconds; the typical output power used in this study was ~5 watts. The laser ablation typically created a pit, ~100 µm in diameter, on the surface of the pyrite sample. The laser beam also combusted the matrix of rocks as well as the pyrite crystals, when the pyrite grain sizes were smaller than 100 µm. In addition to SO2 , variable amounts of H2O, CO2 , and various hydrocarbon gases are usually generated during laser ablation of sedimentary rocks. The SO2 was purified before introduction to the mass spectrometer. After laser ablation, the sample chamber was opened to the cryogenic traps 1 and 2 (Fig. 4). The H 2O was condensed in traps 1 and 2, and cooled by an acetone slush (T ≈ –95.4°C). A platinum furnace, located between traps 1 and 2, was heated to 850°C to convert hydrocarbons to CO2 and H 2O. Another cryogenic trap (trap 3), located next to the platinum furnace, condensed SO2 and CO2 gases when cooled by liquid nitrogen (T = –195°C). After freezing, the non-condensable gases (mostly O2) were pumped away. Next, trap 3 was isolated from trap 2 and the vacuum pump system, and CO2 was pumped out when trap 3 was cooled by liquid nitrogen-pentane slush (T ≈ –129.7°C). The purified SO2 gas was transferred from the cryogenic pentane trap of the purification line to the cold finger of the micro-inlet line of the mass spectrometer (Fig. 4) by cooling the cold finger with liquid nitrogen; the cold finger was then isolated and heated to 40°C. A micro-inlet line was heated constantly at 40°C to avoid absorption of SO2 on the metal surface. A Finnigan MAT252 mass spectrometer analyzed the abundance ratio of masses 66 and 64; these ratios were converted to the δ34S value. The δ34S values are reported with respect to the CDT (Canyon Diablo troilite) standard. S and C isotope analyses of the 2.7 Ga Jeerinah Formation 125 Table 1. S and C concentrations of the Jeerinah shales Sample # Fig. 5. Schematic diagram showing the method of sample preparation and laser ablation for the Jeerinah shales. Sulfur isotope compositions of each area were determined on SO2 gas generated from several spots. Approximately 25 analyses were performed on each plate. The reproducibility of the above analyses is ±0.2‰. In general, δ34S values by the laser microprobe tend to be lighter than those by the conventional method by 1 ± 0.2‰ (Kakegawa, 1993). The δ 34S values shown in the succeeding sections are those corrected by adding 1‰ to the measured values. For accurate sulfur isotope analyses using the laser microprobe, it is necessary to generate more than 200 nmol of SO2 gas. This corresponds to the amount of SO 2 generated from a pyrite crystal of 150 µm in size. However, because the sizes of most pyrites in the Jeerinah samples are smaller than the diameter of a laser beam (20 µm), and the pyrites are sparsely distributed in the shaly matrix, it was not possible to analyze individual grains of pyrite. In order to investigate micro-scale δ 34S variations in both horizontal and vertical directions (i.e., on and across the bedding planes) in the shale samples, each sample was cut into a chip with a volume of ~1 cm3, and sliced into two plates along the bedding plane (Fig. 5). Different micro-areas on the surface of each plane were analyzed for δ 34S. A sufficient amount of SO2 gas for isotopic analysis was generated from a micro-area of ~0.5 S (wt %) C (wt %) S/C 17A 18 19 21 16 0.38 1.11 0.08 0.53 0.20 2.70 2.16 2.26 4.13 2.59 0.14 0.51 0.04 0.13 0.08 average 0.46 2.78 0.18 mm2 by hitting approximately 5 to 20 spots (each spot is ~20 µm in diameter) by a Nd-YAG laser under an O2 atmosphere. In this analysis mode, matrix of sample (i.e., silicate minerals and organic matter) are combusted with pyrite. It is found that the oxygen isotopes of sample matrix did not affect on the accuracy or reproducibility of sulfur isotope data, probably because SO2 gas is generated simply using O2 gas in the sample chamber (Kakegawa, 1993). Approximately 25 area analyses were performed on each bedding plane (laser microprobe area analysis). Carbon isotopes During the laser ablation of fine-grained pyrite in carbonaceous shales, a large amount of CO 2 gas is produced from organic matter in addition to SO 2 gas from pyrite. CO 2 gas is cryogenically separated from SO2 gas in the standard procedures of sulfur isotope analyses, and then carbon isotope compositions were determined on separated CO2 gases. The reproducibility and accuracy of δ 13C analyses by this method is determined to be better than ±0.2‰ (1σ) (Liu, 1996). Carbon isotopic ratios are expressed using the δ notation relative to Peedee belemnite standard (PDB). Carbon (or sulfur) isotope analyses were performed only when the enough amounts of sample gases (>200 nmol in the cold finger part; Fig. 4) were obtained. RESULTS S and C concentrations Bulk analyses of pyrite sulfur (Spy) and organic carbon (Corg) concentrations were performed on 5 Table 2. δ 34S values of pyrite in the Jeerinah shales determined by the laser microprobe method 126 T. Kakegawa et al. S and C isotope analyses of the 2.7 Ga Jeerinah Formation rock specimens. The results show that the Corg concentration ranges from 2.2 wt % to 4.1 wt % with an average of 2.8 wt %, and Spy concentration ranges from 0.1 wt % to 1.1 wt % with an average of 0.5 wt % (Table 1). The S/C ratios range from 0.04 to 0.51 with an average of 0.18. 127 Sulfur isotopic compositions Laser microprobe analyses were performed on a total of 90 micro-areas in four rock specimens cut from two samples (Aus 18 and Aus 21; Table 2). Approximately 20 analyses were completed on two different sedimentary bedding planes in each sample, separated by 0.5 cm in stratigraphy. The results of the laser microprobe analyses are summarized in Fig. 6. δ 34S values on each sedimentary bedding vary by 6 to 7‰. The two different planes of sample Aus 18 (18-2 and 18-3 in Fig. 6) show similar variations in δ 34S values, ranging from 0.4‰ to 7.4‰ with an average of 3.8‰. The δ 34 S frequency patterns of each plane are also similar (Figs. 6(a) and (b)). The two planes of sample Aus 21 (21-1 and 21-2) show slightly different fre- Table 3. δ 13C of organic matter in the Jeerinah shales determined by laser microprobe analysis Fig. 6. δ34S variations for disseminated, fine-grained pyrite in the Jeerinah shales determined by the laser microprobe analysis. (a) First plane of sample Aus 18; (b) second plane of sample Aus 18; (c) first plane of sample Aus 21; (d) second plane of sample Aus 21; (e) total range. Sample I.D. δ1 3 C (‰) Aus18 Aus18-2-2-a Aus18-2-2-b Aus18-2-2-c Aus18-2-2-d Aus18-2-2-e Aus18-2-2-f Aus18-4-2-a Aus18-4-2-b Aus18-4-2-c Aus18-4-3-a Aus18-4-3-b Aus18-4-3-c Aus18-4-3-d Aus18-4-3-e Aus18-4-3-f Aus18-4-3-g Aus18-4-3-h –35.1 –35.7 –35.7 –36.0 –36.0 –35.7 –36.3 –36.5 –36.4 –36.3 –36.2 –36.2 –35.9 –35.4 –36.0 –36.3 –36.3 average –36.0 Sample I.D. δ1 3 C (‰) Aus21 Aus21-0-a Aus21-0-b Aus21-0-c Aus21-0-d Aus21-0-e Aus21-0-f Aus21-0-g Aus21-0-h Aus21-0-i Aus21-0-j Aus21-0-k Aus21-1-1-a Aus21-1-1-b Aus21-1-1-c Aus21-1-1-d Aus21-1-1-e Aus21-1-1-f Aus21-1-1-g Aus21-1-2-a Aus21-1-2-b Aus21-1-2-c Aus21-1-2-d Aus21-2-1 Aus21-3-2-a Aus21-3-2-b Aus21-3-2-c Aus21-3-2-d –37.7 –37.6 –38.1 –37.7 –38.1 –37.7 –37.3 –37.5 –37.2 –37.5 –37.3 –37.2 –37.9 –38.0 –37.6 –38.1 –38.1 –36.7 –37.3 –37.8 –38.2 –37.2 –37.4 –37.9 –38.4 –38.0 –38.3 average –37.7 128 T. Kakegawa et al. Table 4. δ34S values of pyrite and δ13C values of organic matter in the Jeerinah shales determined by the conventional method Sample I.D. Aus18 (1) (2) (3) (4) (5) Aus21 (1) (2) (3) (4) δ3 4 C (‰) +3.8 +4.8 +5.2 +5.2 +5.3 +5.1 +5.3 +6.0 +4.1 Sample I.D. Aus18 (1) (2) (3) (4) (5) Aus21 (1) (2) (3) δ1 3 C (‰) –36.9 –36.8 –37.0 –37.1 –37.2 –38.9 –39.0 –38.9 –37.7‰ (Fig. 7). These results show that δ13 C values are relatively constant in micro-areas, compared to the δ 34S variations (Fig. 6). Previous investigators reported δ 13Corg values, ranging from –48.6‰ to –27.9‰ with an average of –39.6‰ for 12 analyses of the Jeerinah Formation (Fig. 7(c); Schidlowski et al., 1983; Strauss and Moore, 1992). The new set of δ 13C values determined in this study fall in the range of previously reported values. Fig. 7. Carbon isotope variations of organic matter in the Jeerinah shales. δ 13C values were determined by the laser microprobe analysis. (a) Sample Aus 18; (b) sample Aus 21; (c) previously published data (Schidlowski et al., 1983; Strauss and Moore, 1992). quency patterns and ranges of δ 34S values: from 3.5‰ to 10.2‰ with an average of 6.0‰ for the first plane, and from 2.2‰ to 8.8‰ with an average of 5.4‰ for the second plane (Figs. 6(c) and (d)). Bulk rock analyses of sulfur and carbon isotope compositions Results of bulk rock analyses of sulfur and carbon isotope compositions by the conventional method are listed in Table 4. Sulfur isotope compositions are ranging from +3.8‰ to +5.2‰ for sample Aus 18 and +4.1‰ to +5.3‰ for sample Aus 21. Carbon isotope compositions are ranging from –37.2‰ to –36.8‰ for sample Aus 18 and –39.0‰ to –38.9‰ for sample Aus 21. DISCUSSION Carbon isotopic compositions Laser microprobe analyses were performed on a total of 44 micro-areas in two samples (Aus 18 and Aus 21; Table 3). The δ13C values of organic matter range from –36.5‰ to –35.1‰ in sample Aus 18 with an average of –36.0‰, and –38.4‰ to –36.7‰ in sample Aus 21 with an average of Significance of carbon isotope compositions Extremely light carbon isotopic compositions, such as a δ 13C value of –48.6‰, have been reported by Schidlowski et al. (1983) and Strauss and Moore (1992) for the Jeerinah Formation (Fig. 7(c)). These light δ 13C values have previously S and C isotope analyses of the 2.7 Ga Jeerinah Formation 129 Fig. 8. Cartoons comparing three models in terms of chemistry and ecological conditions of the 2.7 Ga Hamersley ocean. Symbols A, T, F and S represent oxygenic photoautotrophs, methanotrophs, methanogens and sulfate reducers. (A) is the model proposed by Hayes (1994) for the 2.7 Ga Hamersley ocean. (B) and (C) are the proposed models in this study. been interpreted to suggest that methanogens and methanotrophs were involved in carbon cycles in the surface environments of the early Earth (Hayes et al., 1983; Hayes, 1994). In this model, methanogens produce CH 4 in deep oceans, and the methanotrophs oxidize CH4 to 13C-depleted CO2 using O2 produced by photosynthetic bacteria in the upper part of the oceans (Fig. 8(A); Hayes, 1994). Incorporation of such 13 C-depleted CO2 (or CH 4 ) into biological activity produces organic matter with extremely light δ 13C values (<–35‰). Many previous investigators suggested that diagenetic or metamorphic degradation of organic matter causes the shift of δ 13C org compositions towards heavier values, and the shift has been related to the H/C ratio of the kerogen (Hayes et al., 1983; Schidlowski, 1988; Watanabe et al., 1997). The disadvantage of carbon isotope analysis using the laser microprobe is that the H/C ratios of kerogen cannot be determined on the same point where the δ 13Corg values are determined. Although the absence of H/C ratios of the examined samples makes the interpretation of the measured δ13C values difficult, the original δ13C values could be lighter than the measured δ13C values (–38 to –35‰). If this is the case, the δ 13C values of this study give more supportive data for the hypothesis that methane was incorporated in biological activity in the Hamersley ocean at 2.7 Ga. Origin of pyrite in the Jeerinah Formation and activity of sulfate-reducing bacteria The primary object of this study is to examine if sulfate was abundant and sulfate-reducing bacteria were active in the 2.7 Ga Hamersley Basin. Sulfate-reducing bacteria are able to form Fesulfides by secreting H 2S as a product of the reduction of dissolved sulfate. In general, the Fesulfides form by reaction between H2S and Fehydroxides (Berner, 1984). The reduction of sulfate is accompanied by a kinetic isotope effect (∆ sulfate-sulfide) in which H 2S produced by sulfate reduction is enriched in 32S with respect to the substrate SO 42–. The environmental systems for pyrite formation by sulfate-reducing bacteria are mainly divided into: (1) a closed-bottom system with respect to sulfate supply, where the rate of reduction is faster than the rate of sulfate supply; and (2) an open-bottom system with respect to sulfate supply, where the rate of sulfate supply is greater than the rate of sulfate reduction (Schwarcz and Burnie, 1973; Ohmoto et al., 1991; Ohmoto, 1992). The closed-bottom system is represented by interstitial water in a wet sediment column where the diffusive supply of seawater sulfate is limited. Sulfate reduction in a system closed with respect to sulfate supply is analogous to the Rayleigh distillation process. At the top of the sediment- 130 T. Kakegawa et al. seawater interface, δ 34S value of pyrite becomes lighter than the δ 34S value of seawater sulfate by the ∆ value. The δ 34S value of sulfate becomes continuously heavier with depth, because the lighter isotope (32S) is preferentially removed to form pyrite. Correspondingly, δ34S value of pyrite in the deeper part of a closed-bottom system becomes isotopically heavier and the total range of δ34S values of pyrite grains formed in the closed system becomes variable. δ34S values of pyrite formed in an open-bottom system (syngenetic pyrite) may have different compositions compared to pyrite formed in a closed-bottom system (diagenetic pyrite). In an open-bottom system, the kinetic isotope effect, ∆sulfate-sulfide, would essentially remain constant at different parts of an open-bottom system and also among different systems if the nature of organic matter and temperature are essentially the same (Ohmoto, 1992). The sulfides formed in such a system should show uniform δ 34S values that deviate from the seawater sulfate values by the kinetic isotope effect. An anoxic water-column in an euxinic basin, such as the Black Sea and the Baltic Sea, is a typical example of the open-bottom system (e.g., Sweeney and Kaplan, 1980; Leventhal, 1983; Boesen and Postma, 1988). Homogeneous sulfur isotope compositions of pyrite formed in water-column of the Black Sea indicate that bacterial sulfate reduction in the open-bottom system results in homogeneous sulfur isotope compositions of pyrite (Lyons, 1997). Therefore, it is possible to constrain if pyrite was formed in anoxic water column or within sediments by examining the heterogeneity of sulfur isotope compositions. Highly variable δ 34S values are found in micro-scale areas of the four Jeerinah samples (Fig. 6); approximately 6 to 7‰ variations are found on each small area (5 × 5 mm). These variable δ34S values within a micro-scale area are considered to be the evidence of sulfate reduction in sediments during early diagenesis. Dissolved sulfate may have been reduced to sulfide either by an abiological or biological process (Trudinger and Chamber, 1985; Goldhaber and Orr, 1995; Ohmoto and Goldhaber, 1997). Because the Jeerinah samples preserved original sedimentary textures and do not show any features of hydrothermal alteration, abiological (thermochemical) sulfate reduction is unlikely to explain the sulfur isotope heterogeneity shown in Fig. 6. Therefore, the sulfate-reducing bacteria were responsible for pyrite formation in the Jeerinah shales. This postulates that the 2.7 Ga Hamersley ocean contained an appreciable amount of dissolved sulfate (>1/3 of the present ocean value) to be reduced (Ohmoto and Felder, 1987). It is known that sulfur concentrations in modern fresh water sediments are extremely low (<0.1 wt %; Berner, 1984), because of low concentrations of dissolved sulfate in fresh water. Such a fresh water environment is a favored model for some researchers as a depositional environment of the Jeerinah Formation (Fig. 8(A); Hayes, 1994). However, the fresh water or lacustrine environment model cannot explain the moderate amount of sulfur concentrations of Jeerinah samples (Fig. 9). Therefore, a marine environment is more likely for the depositional environment as proposed by previous investigators (e.g., Blake and Barley, 1992). The results obtained through this study oppose to the previously popular theory that the Archean oceans were sulfate-poor and most pyrites in Archean sediments are products of inorganic reaction between dissolved Fe2+ and H2S in oceans (Canfield and Teske, 1996). Fig. 9. Correlation between pyrite sulfur and organic carbon of the Jeerinah shales. A regression line of modern sediments are also shown (solid line) for comparison S and C isotope analyses of the 2.7 Ga Jeerinah Formation 131 Strauss (1986) porposed that the δ 34S value of the 2.7 Ga ocean was approximately +5‰. Because the lightest δ 34S value among the examined samples is approximately 0‰, the minimum kinetic isotope effect (∆ sulfate-sulfide) associated with microbial sulfate reduction is, therefore, estimated to be ~5‰ using the seawater value by Strauss (1986; Fig. 6). The kinetic isotope effects of modern sediments often exceed 45‰ (Canfield and Teske, 1996). On the other hand, the kinetic isotope effects found in the Jeerinah samples and other Archean samples are much smaller than these of modern or Phanerozoic sediments (Ohmoto et al., 1993). The magnitude of the kinetic isotope effect associated with microbial sulfate reduction will depend on environmental factors such as temperature, types of hydrogen donor (sugar, organic acid, molecular hydrogen), and etc. (Kemp and Thode, 1968; Ohmoto and Felder, 1987). In general, the surface temperature of the Archean Earth is considered to have been higher than that of the present Earth due to the high PCO 2 level of the atmosphere (Kasting, 1993). Nature of organic matter in Archean sediments could have been more digestible for sulfate-reducing bacteria, compared to the modern sediments (Hayes et al., 1983). These factors may cause the high activity of sulfate-reducing bacteria, resulting in the small kinetic isotope effect in the 2.7 Ga Jeerinah shales. sediments, and they are not coexisting as a mixed culture. This is because sulfate reducers outcompete methanogens in sulfate-rich environments (Lovely and Klug, 1983). Heterogeneous sulfur isotope compositions of Jeerinah shales suggest that sulfate-reducing bacteria were probably restricted in sediments rather than in watercolumn. If this is the case, two models can be proposed for the relationship between methanogenic and sulfate-reducing bacteria. One is the case (case (B) in Fig. 8) that only methanogenic bacteria were active in the anoxic parts of oceans, although sulfate-reducing bacteria were active in sediments. Such ecological relationship is the reversed relationship compared to the modern environments. The other is the case that both anaerobic bacteria were restricted in sediments ((C) in Fig. 8). This case is very similar to the Phanerozoic conditions, although a large uncertainty exists as to weather such a modern style of ecological relationship was already established by 2.7 Ga. If (C) is the case, the activity of methanogens in sediments had to be rather high in 2.7 Ga sediments, in order to produce large amount of CH4 to be oxidized by methanotrophs. It is still uncertain which model is correct or if the other possibility exists. Relationship between sulfate-reducing and methnogenic bacteria in the 2.7 Ga Hamersley ocean It is discussed in the previous section that extremely light carbon isotope compositions were caused by involvement of methanogenic and methanotrophic bacteria in the carbon cycle in 2.7 Ga oceans (Fig. 7). Heterogeneous sulfur isotope compositions of pyrite are most likely caused by an action of sulfate-reducing bacteria (Fig. 6). Aerobic methanotrophic bacteria were probably living in oxygenated parts of 2.7 Ga oceans, separated from anaerobic methanogenic and sulfatereducing bacteria. In modern marine sediments, both methanogenic and sulfate-reducing bacteria are restricted to live in anoxic parts of oceans or (1) δ13C values of disseminated organic matter in sedimentary rocks of the Jeerinah samples range from –38.4‰ to –35.1‰ (PDB). The 13Cdepleted δ 13C values were probably caused by involvement of methane in the biological carbon cycle in the 2.7 Ga Hamersley ocean. (2) Highly variable δ 34 S values (+0.4‰ to +10.2‰) were found in micro-scale areas of the Jeerinah shales. This suggests that (a) the 2.7 Ga Hamersley ocean was sulfate-rich and (b) pyrite in the Jeerinah samples was formed by reduction of seawater sulfate in sediments rather than in the water-column. Sulfate-reducing bacteria were responsible for sulfide formation in Jeerinah shales. (3) The kinetic isotope effect (∆ sulfate-sulfide) associated with the bacterial sulfate-reduction was CONCLUSIONS 132 T. Kakegawa et al. 5‰, much smaller than these of Phanerozoic sediments. 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