Measurement of the carbon isotopic composition of organic matter in about 200 samples from the Devonian shale sequence of the eastern United States shows a definite transition from nonmarine values of around −25‰, through intermediate values in prodelta samples, to distal, basinal samples with values around −30.5‰. This trend matches very closely that found for paleocurrents in the sequence from sole markings on turbidite siltstone beds. Thus, the carbon isotopes suggest that the organic matter, and hence the enclosing shale, had the same dispersal pattern as the silt-stones. Furthermore, the steady westward change to more marine values indicates that the Cincinnati Arch was never sufficiently positive to act as a source of organic matter. The nonmarine carbon has an isotopic composition very similar to that of coal and so probably represents woody material. The basinal values, however, are much lighter than modern marine plankton. This may be a diagenetic effect or it may reflect a difference in the Devonian plankton.
- Geological Society of America