Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter

Carbonate-rich sandstones (samples 0708 and 0704) were collected from outcrop of the Gunflint Formation, Ontario, Canada (sample 0708: 48°29 ′24.4′′ N, 89°11′34.2′′ W; sample 0704: 48°29′24.6′′ N, 89°11′35.3′′ W). Samples contain 0.34 and 0.23wt% organic carbon. The estimated age for the Gunflint Formation is ca. 1.9 Ga (Fralick et al., 2002), and its metamorphic grade is considered to be sub-greenschist facies. Kerogen was extracted using standard method (Vandenbroucke, 2003). Organic solvent treatment was omitted because bitumen was present in the samples at only negligible levels. Elemental and isotopic analyses were performed on the Balzers QMG420 quadrupole mass spectrometer at Osaka University, Japan, following the method described by Yamamoto et al. (1998) and Pinti et al. (2007). The platinum-wrapped sample is heated for 30 minutes during each combustion step in the O2 atmosphere. Nitrogen contamination associated with the O2 gas is typically ~0.02 ng. The pressure of the O2 atmosphere during each analysis was 0.45Torr for sample 0708 and 0.85Torr for sample 0704. Stepwise combustion analyses of distinct nitrogen isotopic compositions on Paleoproterozoic organic matter