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.