10:45 〜 11:00
[MIS22-07] Formation Processes of Oligocene Methane-seep Carbonates in Urahoro, Hokkaido, Japan.
キーワード：メタン湧水、炭酸塩、北海道、漸進世
Along the main stream of the Atsunai River in Urahoro Town, southeastern Hokkaido, a large carbonate rocks, 6.6 m thick, is intercalated in the mudstones of the Oligocene Nuibetsu Formation. The petrographic and geochemical researches on the carbonates have been scarce. This study aims to elucidate the formation environment and formation history of the carbonate rocks by detailed lithological observations and geochemical analyses.
The carbonate rocks are divided into five units (Units 1 to 5 from lower to upper parts) based on lithology, and their δ13C values range from -50.8 to -31.3‰VPDB, with the highest value in Unit 5. This indicates that the contribution of microbial methane was relatively larger in the lower units than in the uppermost unit 5 during the formation of the rock body, and that the contribution of thermogenic methane was larger in Unit 5.The δ18O values varied between -4.07 and -2.56‰VPDB, except for the carbonate nodule in Unit 1, which had a relatively high value -0.29‰VPDB compared to the other samples, and the values increased from Unit 1 to Unit 2 and decreased from Unit 2 to Unit 5. Biomarker analysis revealed that PMI, a biomarker derived from anaerobic methane-oxidizing archaea (ANME), and squalane, derived from both ANME and sulfate-reducing bacteria, were detected in all units, while crocetane, typical in ANME-2 clade with high methane oxidation efficiency, was detected only in Unit 5. In the microscopic observation, clotted fabrics, which are considered to be high microbial activities probably by the ANME and AOM (anaerobic oxidation of methane) related microbes, were observed in at least three units (the upper part of Unit 1 and Units 4 and 5), and bladed calcite, which is crystallized during active AOM, was observed in Unit 5 The clotted fabric was fractured, and carbonate mineral cement of anaerobic methane oxidation origin was also observed around the fractured rock fragments.
The above results reveal the following two major points about the formation process of the methane-seep carbonate rock body.
I. The carbonate nodules in the lower part of Unit 1 were formed by calcium from seawater and microbial methane, and carbonate precipitates filled the spaces between the nodules to form a 6.6 m-thick rock. Such a large carbonate was formed by the intermittent supply of thermogenic and microbial methane to the AOM zone from the deeper sediments.
II. The δ13C results of carbonate indicate that the methane flux is high throughout all units, especially in Unit 5, which corresponds to a period of particularly high methane flux based on the detection of bladed calcite and crocetane.
The carbonate rocks are divided into five units (Units 1 to 5 from lower to upper parts) based on lithology, and their δ13C values range from -50.8 to -31.3‰VPDB, with the highest value in Unit 5. This indicates that the contribution of microbial methane was relatively larger in the lower units than in the uppermost unit 5 during the formation of the rock body, and that the contribution of thermogenic methane was larger in Unit 5.The δ18O values varied between -4.07 and -2.56‰VPDB, except for the carbonate nodule in Unit 1, which had a relatively high value -0.29‰VPDB compared to the other samples, and the values increased from Unit 1 to Unit 2 and decreased from Unit 2 to Unit 5. Biomarker analysis revealed that PMI, a biomarker derived from anaerobic methane-oxidizing archaea (ANME), and squalane, derived from both ANME and sulfate-reducing bacteria, were detected in all units, while crocetane, typical in ANME-2 clade with high methane oxidation efficiency, was detected only in Unit 5. In the microscopic observation, clotted fabrics, which are considered to be high microbial activities probably by the ANME and AOM (anaerobic oxidation of methane) related microbes, were observed in at least three units (the upper part of Unit 1 and Units 4 and 5), and bladed calcite, which is crystallized during active AOM, was observed in Unit 5 The clotted fabric was fractured, and carbonate mineral cement of anaerobic methane oxidation origin was also observed around the fractured rock fragments.
The above results reveal the following two major points about the formation process of the methane-seep carbonate rock body.
I. The carbonate nodules in the lower part of Unit 1 were formed by calcium from seawater and microbial methane, and carbonate precipitates filled the spaces between the nodules to form a 6.6 m-thick rock. Such a large carbonate was formed by the intermittent supply of thermogenic and microbial methane to the AOM zone from the deeper sediments.
II. The δ13C results of carbonate indicate that the methane flux is high throughout all units, especially in Unit 5, which corresponds to a period of particularly high methane flux based on the detection of bladed calcite and crocetane.