Variations in stable carbon isotope ratio values on combustible organic matter in Gulf of Mexico sediments correlate with Pleistocene warm and cold climates. Warm stages are characterized by C¹³/C¹² isotopic ratios of -20 to -22°/oo, compared with the PDB standard, whereas colder stages are characterized by -24 to -26°/oo.

Piston core samples from the present bottom show similar ^dgr C¹³ values (-20 to -22°/oo) characterizing the warm post-glacial period regardless of bathymetric environment (shelf, slope, or abyss) except where relict Pleistocene sediments crop out (-24 to -26°/oo). Sedimentation patterns and paleontology confirm the relict nature of the latter. Samples from 1,000-ft cores on the present slope represent post-Pliocene sequences; ^dgr C¹³ values alternate from more positive to more negative. These alternations from about -21°/oo to coincide with the glacial-inter-glacial stages which are independently identified by planktonic Foraminifera.

Interpretations of the data lead to the conclusion that the principal reason for the observed correlations is variation of water temperatures in the photosynthetic zone during warm interglacial climates and during cooler glacial climates. There are other alternative processes which could affect the data, one of which, the relative contribution of terrestrially-derived organic matter versus marine-derived organic matter, would change the ^dgr C¹³ values in the same relative direction as the water-temperature variation. Thus, it is not possible to unequivocally interpret the contributions of different processes for individual samples.

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