[B-CG09] 地球史解読：冥王代から現代まで

[BCG09-P07] 27億年前の浅海堆積物における炭素同位体負異常の起源

*Kim Beomsik¹、上野 雄一郎^1,2,3、Gilbert Alexis^1,2 (1.東京工業大学理学院地球惑星科学系、2.東京工業大学地球生命研究所、3.海洋研究開発機構)

キーワード：太古代、炭素同位体、ストロマトライト、メタン資化菌、Organic haze

Sedimentary organic carbon in some Late Archean rocks are anomalously depleted in ¹³C (δ¹³C_org = -45 to -60‰). The origin of the isotope anomaly is still unknown and is possibly resulted from biological uptake of methane (methanotrophy) or deposition of hydrocarbon haze. To test the hypotheses, small-scale isotopic analyses of both inorganic and organic carbon were conducted for various lithologies of 2.7 Ga sedimentary rocks in Fortescue Group, Western Australia. For this purpose, a new analytical method was developed for measuring small samples. As a result, low δ¹³C organic matter occurs not only in stromatolite as previously pointed out, but also in black laminated mud. Also, the δ¹³C_org seems not correlated with δ¹³C_carb value as opposed to the case expected when methanotrophs are active because methanotrophs typically produce not only very ¹³C-depleted organic matter but also CO₂. These results do not support the methanotrophy scenario. Furthermore, a relationship between δ¹³C_org value and TOC contents is consistent with a mixing of two organic end-members with different isotopic ratios. The observed δ¹³C_org-TOC trend appears to occur in each lithology of the sedimentary rocks, suggesting that the source of the low δ¹³C_org distributed uniformly irrespective to the depositional environment. This may suggest that the anomalously ¹³C-depleted organics could have been deposited from atmosphere at about 2.7 Ga.