Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Poster

B (Biogeosciences) » B-CG Complex & General

[B-CG09] Decoding the history of Earth: From Hadean to the present

Tue. May 22, 2018 10:45 AM - 12:15 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Tsuyoshi Komiya(Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo), Yasuhiro Kato(Department of Systems Innovation, Graduate School of Engineering, University of Tokyo), Katsuhiko Suzuki(国立研究開発法人海洋研究開発機構・海底資源研究開発センター)

[BCG09-P07] Origin of negative carbon isotope anomaly in a 2.7 Ga shallow water deposits

*Beomsik Kim1, Yuichiro Ueno1,2,3, Alexis Gilbert1,2 (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551, Japan (Dept. EPS, Tokyo Tech.), 2.Earth-Life Science Institute (WPI-ELSI), Tokyo Institute of Technology, Meguro, Tokyo, 152- 8550, Japan (ELSI, Tokyo Tech.), 3.Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima-cho, Yokosuka 237-0061, Japan)

Keywords:Archean, Carbon isotopes, Stromatolite, Methanotrophs, Organic haze

Sedimentary organic carbon in some Late Archean rocks are anomalously depleted in 13C (δ13Corg = -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 δ13Corg seems not correlated with δ13Ccarb 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 δ13Corg value and TOC contents is consistent with a mixing of two organic end-members with different isotopic ratios. The observed δ13Corg-TOC trend appears to occur in each lithology of the sedimentary rocks, suggesting that the source of the low δ13Corg 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.