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The experimental procedures used for the determination of 1 3 C / 1 2 C ratios are essentially those of Craig (1953) and Eckelmann et al. (1962). The sediment samples were acidified to eliminate carbonate (carbon dioxide), were dried, and were combusted over copper oxide at approximately 800°C. The produced carbon dioxide was purified by passing it over dry ice to remove water and over copper metal and manganese dioxide at 500°C to remove nitrogen oxides and sulfur dioxide. The purified carbon dioxide samples were analyzed in a 60°, sector-type mass spectrometer similar to that described by Nier (1947), and described in detail by Nier, Eckelmann, and Lupton (1962). The mass spectrometric analyses are reported as per mil deviation (°/°°) from the 1 3 C / 1 2 C ratio of the Cretaceous belemnite, Belemnitella americana, from the Peedee Formation of South Carolina. In practice, a commercial lubricating oil with an assigned value of —29.4 per mil relative to the Peedee belemnite is used as a laboratory standard. This lube oil standard was obtained from the Chevron Research Laboratories. The results are reported thus:

Organic Carbon 13C Values from Cretaceous, Tertiary and Quaternary Marine Sequences in the North Atlantic