15:00 〜 15:15
[BCG06-12] 古原生代のストロマトライト形成場における, 浅部海底熱水活動に関連した炭素・鉄循環
★招待講演
Paleoproterozoic era is an important age in the evolution of life, including the rise of eukaryotes and the diversification of prokaryotes. Paleoproterozoic stromatolites sometimes host microfossils and record the diversity of microbes in that age. However, it has been unclear how environmental factors made stromatolites habitable at the specific sites. Here we report the positive roles of shallow submarine hydrothermal activities to develop Paleoproterozoic stromatolites.
Geological surveys were performed in two localities on the 1.9 Ga Gunflint Formation: Sandstone Lake, and Mink Mountain, Ontario, Canada. At Sandstone Lake locality, three stromatolite-bearing layers were highly silicified and hematized. They occurred along 200 m conformable grainstone sections. Jasper veins are predominantly confined near the layers of these stromatolites. In addition, finely curved or irregular quartz veins and hematite jasper veins were observed. These facts imply the iron- and silica-rich fluids penetration laterally in the unconsolidated sediments during stromatolite formation.
At Mink Mountain locality, stromatolite-bearing layers extend horizontally for at least 2 km. Stromatolites hosted multiple quartz veins, sometimes accompanied by pyrite, as well as calcite veins. These occurrences and microscopic observations of vein samples revealed that the chemistry of the hydrothermal fluids transited from CO2-rich to SiO2-rich, and then to H2S-rich stages.
At this locality, submillimeter-scale of calcite aggregates along the microscopic lamina were observed in one stromatolite. Ambiguous shape of organic matter, possibly pyrobitumen, was also found inside these calcite blocks. These observations suggest the hydrocarbon seeping activity during the stromatolite formation, most likely accompanied by shallow hydrothermal activities. This seep most likely produced oil-bearing carbonate mounds, which collapsed and deposited onto stromatolites. The δ13C values (VPDB) of these calcite blocks were around -12.9 ‰ and those of calcite veins ranged from -11.0 to -8.8 ‰. These values suggest that a part of carbon of carbonates was sourced from organic matter degraded by hot seep activities. The hydrothermal/seeping sites suggested by this study could have served as the cradle for the Paleoproterozoic microbial life.
Geological surveys were performed in two localities on the 1.9 Ga Gunflint Formation: Sandstone Lake, and Mink Mountain, Ontario, Canada. At Sandstone Lake locality, three stromatolite-bearing layers were highly silicified and hematized. They occurred along 200 m conformable grainstone sections. Jasper veins are predominantly confined near the layers of these stromatolites. In addition, finely curved or irregular quartz veins and hematite jasper veins were observed. These facts imply the iron- and silica-rich fluids penetration laterally in the unconsolidated sediments during stromatolite formation.
At Mink Mountain locality, stromatolite-bearing layers extend horizontally for at least 2 km. Stromatolites hosted multiple quartz veins, sometimes accompanied by pyrite, as well as calcite veins. These occurrences and microscopic observations of vein samples revealed that the chemistry of the hydrothermal fluids transited from CO2-rich to SiO2-rich, and then to H2S-rich stages.
At this locality, submillimeter-scale of calcite aggregates along the microscopic lamina were observed in one stromatolite. Ambiguous shape of organic matter, possibly pyrobitumen, was also found inside these calcite blocks. These observations suggest the hydrocarbon seeping activity during the stromatolite formation, most likely accompanied by shallow hydrothermal activities. This seep most likely produced oil-bearing carbonate mounds, which collapsed and deposited onto stromatolites. The δ13C values (VPDB) of these calcite blocks were around -12.9 ‰ and those of calcite veins ranged from -11.0 to -8.8 ‰. These values suggest that a part of carbon of carbonates was sourced from organic matter degraded by hot seep activities. The hydrothermal/seeping sites suggested by this study could have served as the cradle for the Paleoproterozoic microbial life.