Carbon isotopic values reported in the literature for low-magnesium calcite Pleistocene limestones are generally more negative than −4‰ PDB, while values reported for most ancient limestones are more positive than −2‰ PDB. This relationship has recently led to suggestions that subaerial diagenesis was of minor importance in forming ancient limestones. Isotopic analyses of Barbados subsurface Pleistocene limestone samples show that samples from shallow subsurface diagenetic regimes have δC13 compositions much closer to those reported for ancient rocks.
In the vadose environment, Barbados limestones become systematically enriched in C13 by 3 to 4‰ with increasing depth. This δC13 versus depth relationship may prove useful for identifying subaerial exposure surfaces in ancient sequences.
Barbados subsurface limestone samples, as a group, are characterized by a narrow range of δO18 compositions and a much wider range of δC13 compositions. This relationship may provide a means for identifying ancient limestones that have undergone subaerial diagenesis.
Finally, our data indicate that positive δC13 PDB values are not inconsistent with diagenesis within the near-surface freshwater phreatic environment.