11:30 〜 11:45
[U19-04] Subseafloor biosphere in the ancient Earth: constraint from multiple S isotopes records
★Invited Papers
キーワード：Subseafloor biosphere, Archean, microbial sulfate reduction, multiple sulfur isotopes
Subseafloor environment linked to hydrothermal systems has been proposed as the cradle of life, though the ancient activity of the subseafloor biosphere is poorly understood. Furnes et al., (2004) reported micrometer-scale mineralized tubes in ca. 3.5 Ga pillow lava rims from the Barberton Greenstone Belt, South Africa, which putatively trace microbial activity below Archean seafloor. However, these textures may also originate from metamorphic processes; thus, their microbial origin is still being debated (e.g., Lepot et al., 2011). Moreover, the putative microbial textures may have been produced on the seafloor. Therefore, such structure may not be useful for the subseafloor biosphere.
Microbial sulfate reduction (MSR) is a metabolism in which sulfate is reduced into sulfide and among the most ubiquitous metabolic processes on earth. Traditionally the activity of MSR was detected by sulfur isotopes (delta-S-34) because MSR is characterized by large sulfur isotopic fractionation up to −65‰ (Sim et al., 2011). Microbial cultivation studies reported not only the traditional delta-S-34 but also minor sulfur isotopes (capital-delta-S-33&36) are useful for MSR (e.g., Johnston et al., 2007). The oldest activity of MSR was evidenced by multiple sulfur isotopes of sulfate and sulfide minerals deposited on the ancient seafloor from ca. 3.5 Ga Dresser Formation in the North Pole area of Pilbara Craton in Western Australia (Ueno et al., 2008).
To verify the activity of MSR below the ancient seafloor, multiple sulfur isotopic compositions of sulfide minerals within hydrothermally altered seafloor basalt and basaltic komatiite collected from the ca. 3.5 Ga North Pole area were analyzed. The results suggested (1) capital-delta-S-33 values of the sulfide minerals were non-zero negative, demonstrating that sulfate reduction occurred below the Archean seafloor, (2) sulfate below seafloor before reduction was different from seafloor sulfate (i.e., barite deposit), and more rich in a mass-dependent component which was probably produced by local hydrothermal processes and (3) The maximum delta-S-34 fractionation between the putative substrate sulfate and the observed sulfide minerals was −35‰, suggesting MSR was involved below ca. 3.5 Ga seafloor. In this case, MSR may have occurred ~1000 meters below the Archean seafloor based on the geologically reconstructed depth below seafloor.
Microbial sulfate reduction (MSR) is a metabolism in which sulfate is reduced into sulfide and among the most ubiquitous metabolic processes on earth. Traditionally the activity of MSR was detected by sulfur isotopes (delta-S-34) because MSR is characterized by large sulfur isotopic fractionation up to −65‰ (Sim et al., 2011). Microbial cultivation studies reported not only the traditional delta-S-34 but also minor sulfur isotopes (capital-delta-S-33&36) are useful for MSR (e.g., Johnston et al., 2007). The oldest activity of MSR was evidenced by multiple sulfur isotopes of sulfate and sulfide minerals deposited on the ancient seafloor from ca. 3.5 Ga Dresser Formation in the North Pole area of Pilbara Craton in Western Australia (Ueno et al., 2008).
To verify the activity of MSR below the ancient seafloor, multiple sulfur isotopic compositions of sulfide minerals within hydrothermally altered seafloor basalt and basaltic komatiite collected from the ca. 3.5 Ga North Pole area were analyzed. The results suggested (1) capital-delta-S-33 values of the sulfide minerals were non-zero negative, demonstrating that sulfate reduction occurred below the Archean seafloor, (2) sulfate below seafloor before reduction was different from seafloor sulfate (i.e., barite deposit), and more rich in a mass-dependent component which was probably produced by local hydrothermal processes and (3) The maximum delta-S-34 fractionation between the putative substrate sulfate and the observed sulfide minerals was −35‰, suggesting MSR was involved below ca. 3.5 Ga seafloor. In this case, MSR may have occurred ~1000 meters below the Archean seafloor based on the geologically reconstructed depth below seafloor.