17:15 〜 18:45
[BCG06-P04] Microbial carbon and sulfur cycles linked to submarine hydrothermal activities at Matachewan area, Abitibi Greenstone Belt, Canada (2.7Ga)
キーワード：太古代、熱分解、炭化水素、海底熱水活動、有機物
Light element cycles on the Archean surface environments were predominantly influenced by microbial activities, with the intricate interplay of carbon and sulfur. Notably, a significant negative shift in carbon isotope compositions (δ13C) of organic matter occurred around 2.7 Ga. Such anomalous isotope shift is attributed to the proliferation of methanotrophy in the presence of substantial methane quantities within the surface environments. However, the contribution of contemporary biosphere to the development of methane-rich environments in specific geological settings has remained unclear. To address these issues, comprehensive geological and geochemical studies were conducted on 2.7 Ga organic-rich sedimentary rocks obtained from the Matachewan area of the Abitibi Greenstone Belt, Canada.
Geological survey revealed that sandstone and organic-rich shale sequences were widespread in this area. They are normal detritus-rich sediments without significant submarine hydrothermal influences. On the other hand, sulfide-rich shales were present in the limited localities. In pillow lavas below the sulfide-rich shale, various sulfide minerals including chalcopyrite were observed in the hydrothermal chert/carbonate veins. At the interface of pillow lava and sulfide-rich shale, a peperite-like textures were observed. These occurrences in sulfide minerals suggest that the location of pillow lava and sulfide-rich shale were at the venting site for submarine hydrothermal fluids. It is found that the pillow lava contains a large amount of organic matter (OM), suggesting such OM were introduced into this site through the circulation of submarine hydrothermal fluids.
The δ13C values of OM (δ13COM) in the sandstone/shale sequence, ranged in narrow area around -29‰ (PDB). These are typical values produced by photosynthetic bacteria and do not contradict with geological constraints in this area. The δ13COM values of sulfide-rich shales showed wide variation, ranging from -32 to -17‰. The lightest value of -32‰ is similar to those observed in the sandstone/shale sequence. However, δ13COM values in pillow lavas showed heavier values, ranging from -26 to -22‰. This indicates they have suffered preferential release in 12C from OM during thermogenic methane production occurred by hydrothermal systems.
The sulfur isotope compositions (δ34S) of sulfide-rich shales and cherts ranged from 0‰ to +4 ‰ (CDT), showing bimodal distribution. Sulfides with δ34S values of around 0 ‰ represent inorganically formed sulfides that originated from isotopically homogenized hydrothermal H2S source. In contrast, sulfides having heavier δ34S values than hydrothermal source, would have formed through isotope fractionation by microbial sulfate reduction (MSR) under low sulfate concentration such as deep sediments. Similar heavy δ34S values were observed in the sandstone/shale sequence, suggesting MSR was active in these geological sequences.
Our research suggests that (1) primary carbon was fixed by shallow photosynthetic bacteria, (2) OM produced by (1) were recycled in deep sediments and submarine hydrothermal influenced area, (3) sulfur was incorporated into the biosphere via both microbial sulfate reduction and hydrothermally produced reduced sulfur, and (4) hydrocarbon, especially methane, were produced by thermal decomposition of OM. We thus propose a unique carbon and sulfur recycling model in 2.7 Ga, when submarine hydrothermal activities were intense globally.
Geological survey revealed that sandstone and organic-rich shale sequences were widespread in this area. They are normal detritus-rich sediments without significant submarine hydrothermal influences. On the other hand, sulfide-rich shales were present in the limited localities. In pillow lavas below the sulfide-rich shale, various sulfide minerals including chalcopyrite were observed in the hydrothermal chert/carbonate veins. At the interface of pillow lava and sulfide-rich shale, a peperite-like textures were observed. These occurrences in sulfide minerals suggest that the location of pillow lava and sulfide-rich shale were at the venting site for submarine hydrothermal fluids. It is found that the pillow lava contains a large amount of organic matter (OM), suggesting such OM were introduced into this site through the circulation of submarine hydrothermal fluids.
The δ13C values of OM (δ13COM) in the sandstone/shale sequence, ranged in narrow area around -29‰ (PDB). These are typical values produced by photosynthetic bacteria and do not contradict with geological constraints in this area. The δ13COM values of sulfide-rich shales showed wide variation, ranging from -32 to -17‰. The lightest value of -32‰ is similar to those observed in the sandstone/shale sequence. However, δ13COM values in pillow lavas showed heavier values, ranging from -26 to -22‰. This indicates they have suffered preferential release in 12C from OM during thermogenic methane production occurred by hydrothermal systems.
The sulfur isotope compositions (δ34S) of sulfide-rich shales and cherts ranged from 0‰ to +4 ‰ (CDT), showing bimodal distribution. Sulfides with δ34S values of around 0 ‰ represent inorganically formed sulfides that originated from isotopically homogenized hydrothermal H2S source. In contrast, sulfides having heavier δ34S values than hydrothermal source, would have formed through isotope fractionation by microbial sulfate reduction (MSR) under low sulfate concentration such as deep sediments. Similar heavy δ34S values were observed in the sandstone/shale sequence, suggesting MSR was active in these geological sequences.
Our research suggests that (1) primary carbon was fixed by shallow photosynthetic bacteria, (2) OM produced by (1) were recycled in deep sediments and submarine hydrothermal influenced area, (3) sulfur was incorporated into the biosphere via both microbial sulfate reduction and hydrothermally produced reduced sulfur, and (4) hydrocarbon, especially methane, were produced by thermal decomposition of OM. We thus propose a unique carbon and sulfur recycling model in 2.7 Ga, when submarine hydrothermal activities were intense globally.