Deciphering the surface environment of the primitive Earth from nitrogen isotopes trapped in Archean sedimentary rocks-Investigation of its trapping forms in the rocks.

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In this study, search of the surface environments of the primitive earth was performed using the information of nitrogen isotopes obtained from Archean sedimentary rocks. Though the nitrogen isotopes trapped in the rock samples have potential of providing such information, it is not always clear how we may obtain and interprete the raw data. We studied 3.5 Ga old chert samples from western Australia (from Kitty's gap and Marble bar), and a 2.7-2.9 Ga old samples of the banded iron formation from Dhawar Craton of India. Volatile compounds (N_2, Ar, C and H_2O) were extracted from samples using a stepwise combustion method, and their elemental and/or isotopic compositions were analysed.We were able to specify or infer three trapping sites/forms of nitrogen by combining the information of different volitile compounds. In Kitty's gap chert, nitrogen concentrations were observed to be correlated well with those of radiogenic ^<40>Ar, suggesting that nitrogen is trapped in K-bearing clay minerals. The δ15N values showed a wide range between-4 to +10 permil, and were correlated with the ^<40>Ar/N ratio, suggesting existence of two different types of trapping sites among the clay minerals. In Marble bar chert, nitrogen with δ^<15>N values between +6.0 to +7.3 permil was observed, associated with carbon, suggesting that nitrogen in this sample is trapped in organic form. Furthur investigations imply that the N-bearing organic matter is protected by inorganic minerals. Among the BIF samples, N with δ^<15>N values around +10 permil was observed. There seem to exist two types of N-bearing C-associated host phases with different C/N ratios. Discrimination of the N isotopic components in different chemical forms among the Archean rocks may largly help us to decipher the isotopic evolution of nitrogen of the primitive biosphere.