10:15 AM - 10:30 AM
[BCG06-06] In-situ quadruple sulfur isotope analysis of sulfides from the Nulliak supracrustal rocks, Labrador, Canada: Microbial sulfate reduction in the Eoarchean
★Invited Papers
Keywords:Archean, early life, Microbial sulfate reduction (MSR), The oldest evidence for MSR, Mass-independent fractionation (MIF)
Microbial sulfate reduction (MSR) is considered to be one of the oldest metabolisms1, whose evidence of quadruple sulfur isotope compositions of sulfides and large isotopic fractionation of sulfur between sulfides and sulfates, has been found from ca. 3.5 Ga rock samples2,3,4.
Sulfur isotope compositions basically follow a mass-dependent law, but some materials deviate from the estimated value from this law, whose deviation is called mass-independent fractionation (MIF). The deviations in δ33S and δ36S relative to δ34S are expressed as Δ33S and Δ36S, respectively. An experimental study showed that MSR caused negative Δ36S values down to -2‰5. Many samples from the Archean geologic terrains have remarkable Δ33S and Δ36S, with a strong negative correlation between Δ33S and Δ36S (Δ36S/Δ33S ~ -1)6; thus, it is considered that deviation in the negative Δ36S direction from this array is strong isotopic evidence for MSR4 in the Archean. However, the isotope anomaly has not been found in the Eoarchean samples, possibly because in-situ quadruple sulfur isotope analysis was employed to only one sample in the Eoarchean7, and the depositional environment of the sample did not correspond to a habitat of MSR. Thus, a more comprehensive study of the in-situ quadruple sulfur isotope analysis of sulfide minerals from sedimentary rocks deposited in various tectonic setting should be required. We conducted in-situ quadruple sulfur isotopic analysis of sulfides from various sedimentary rocks in the Nulliak supracrustal rock, Labrador, Canada, to elucidate sulfate reduction, especially MSR, in the Eoarchean.
We analyzed 78 points of sulfide grains from 16 rock samples using a multi-collector secondary ion mass spectrometer CAMECA IMS 1280HR at the Kochi Institute, JAMSTEC. δ34S, Δ33S and Δ36S values range from -3.47 to +31.60‰, from -1.45 to +1.14‰, from -1.71 to +2.07‰, respectively. Sulfides from carbonate rocks have positive Δ33S values, whereas those from cherts have negative Δ33S values, indicating that sulfate from the atmospheric reaction was incorporated into sulfide in cherts via sulfate reduction. Some sulfides show an increase in δ34S values towards the rim across grains, suggesting that they originated from diagenetic MSR-sulfide, in contrast to thermochemical sulfate reduction (TSR)-sulfide with uniform δ34S values across the crystal9. In addition, a positive Δ33S compared to the reactant, which is the isotopic characteristic of TSR10,11, was not confirmed. This line of evidence suggests that the sulfides in the chert were formed via MSR rather than via TSR. In addition, sulfides in a chert sample (LAF0362) have large negative Δ36S values down to -1.53‰ and near-zero Δ33S values. Linear regression using sulfur isotope data of sulfides in carbonate rocks and cherts, except for LAF0362, yields a Δ36S/Δ33S ratio of -1.35 (R2 = 0.97, 2σ = 0.08). In contrast, LAF0362 plots off the regression line in the lower Δ36S direction, which is evident for MSR. Sulfides in LAF0362 have positive δ34S values of up to 31.60‰, which can be explained by Rayleigh fractionation with a quite large fractionation factor in the closed system. Assuming Rayleigh fractionation in the closed system, we calculated the shift of quadruple sulfur isotope compositions for LAF0362 and obtained mass-dependent fractionation factors of 1.90 and <-31‰ in 36θ and 34αsulfide-sulfate, respectively, for Δ36S = -1.53‰. Such large fractionation could be caused by only MSR, providing the first evidence for appearance of MSR in the Eoarchean.
1. Wagner et al. J Bacteriol. 180, 2975-2982 (1988) 2. Shen et al. Nature 410, 77-81 (2001) 3. Ueno et al. Geochim. Cosmochim. Acta 72, 5675-5691 (2008) 4. Shen et al. Earth Planet. Sci. Lett. 279, 383-391 (2009) 5. Johnston et al. Geochim. Cosmochim. Acta 71, 3929-3947 (2007) 6. Ono et al. Annu. Rev. Earth Planet. Sci. 45, 301-329 (2017) 7. Whitehouse et al. Geostand. Geoanal. Res. 37, 19-33 (2013) 8. Papineau and Mojzsis Geobiology 4, 227-238 (2006) 9. Cai et al. Chem. Geol. 607, 121018 (2020) 10 Oduro et al. Proc. Natl. Acad. Sci. 108, 17635-17638 (2012) 11. Young et al. Earth Planet. Sci. Lett. 367, 1-14 (2013)
Sulfur isotope compositions basically follow a mass-dependent law, but some materials deviate from the estimated value from this law, whose deviation is called mass-independent fractionation (MIF). The deviations in δ33S and δ36S relative to δ34S are expressed as Δ33S and Δ36S, respectively. An experimental study showed that MSR caused negative Δ36S values down to -2‰5. Many samples from the Archean geologic terrains have remarkable Δ33S and Δ36S, with a strong negative correlation between Δ33S and Δ36S (Δ36S/Δ33S ~ -1)6; thus, it is considered that deviation in the negative Δ36S direction from this array is strong isotopic evidence for MSR4 in the Archean. However, the isotope anomaly has not been found in the Eoarchean samples, possibly because in-situ quadruple sulfur isotope analysis was employed to only one sample in the Eoarchean7, and the depositional environment of the sample did not correspond to a habitat of MSR. Thus, a more comprehensive study of the in-situ quadruple sulfur isotope analysis of sulfide minerals from sedimentary rocks deposited in various tectonic setting should be required. We conducted in-situ quadruple sulfur isotopic analysis of sulfides from various sedimentary rocks in the Nulliak supracrustal rock, Labrador, Canada, to elucidate sulfate reduction, especially MSR, in the Eoarchean.
We analyzed 78 points of sulfide grains from 16 rock samples using a multi-collector secondary ion mass spectrometer CAMECA IMS 1280HR at the Kochi Institute, JAMSTEC. δ34S, Δ33S and Δ36S values range from -3.47 to +31.60‰, from -1.45 to +1.14‰, from -1.71 to +2.07‰, respectively. Sulfides from carbonate rocks have positive Δ33S values, whereas those from cherts have negative Δ33S values, indicating that sulfate from the atmospheric reaction was incorporated into sulfide in cherts via sulfate reduction. Some sulfides show an increase in δ34S values towards the rim across grains, suggesting that they originated from diagenetic MSR-sulfide, in contrast to thermochemical sulfate reduction (TSR)-sulfide with uniform δ34S values across the crystal9. In addition, a positive Δ33S compared to the reactant, which is the isotopic characteristic of TSR10,11, was not confirmed. This line of evidence suggests that the sulfides in the chert were formed via MSR rather than via TSR. In addition, sulfides in a chert sample (LAF0362) have large negative Δ36S values down to -1.53‰ and near-zero Δ33S values. Linear regression using sulfur isotope data of sulfides in carbonate rocks and cherts, except for LAF0362, yields a Δ36S/Δ33S ratio of -1.35 (R2 = 0.97, 2σ = 0.08). In contrast, LAF0362 plots off the regression line in the lower Δ36S direction, which is evident for MSR. Sulfides in LAF0362 have positive δ34S values of up to 31.60‰, which can be explained by Rayleigh fractionation with a quite large fractionation factor in the closed system. Assuming Rayleigh fractionation in the closed system, we calculated the shift of quadruple sulfur isotope compositions for LAF0362 and obtained mass-dependent fractionation factors of 1.90 and <-31‰ in 36θ and 34αsulfide-sulfate, respectively, for Δ36S = -1.53‰. Such large fractionation could be caused by only MSR, providing the first evidence for appearance of MSR in the Eoarchean.
1. Wagner et al. J Bacteriol. 180, 2975-2982 (1988) 2. Shen et al. Nature 410, 77-81 (2001) 3. Ueno et al. Geochim. Cosmochim. Acta 72, 5675-5691 (2008) 4. Shen et al. Earth Planet. Sci. Lett. 279, 383-391 (2009) 5. Johnston et al. Geochim. Cosmochim. Acta 71, 3929-3947 (2007) 6. Ono et al. Annu. Rev. Earth Planet. Sci. 45, 301-329 (2017) 7. Whitehouse et al. Geostand. Geoanal. Res. 37, 19-33 (2013) 8. Papineau and Mojzsis Geobiology 4, 227-238 (2006) 9. Cai et al. Chem. Geol. 607, 121018 (2020) 10 Oduro et al. Proc. Natl. Acad. Sci. 108, 17635-17638 (2012) 11. Young et al. Earth Planet. Sci. Lett. 367, 1-14 (2013)