ABSTRACT

The distribution record of benthic foraminifera in late Pleistocene and Recent sediments of the northwestern Gulf of Mexico indicates that stratigraphic occurrences of species are affected by paleoceanography. Two species that are very common in the modern gulf, Bulimina alazanensis and Osangularia culter, were not present in the area during the last glacial stage, due to the absence of Subantarctic Intermediate Water. They reappeared at about 12.5 ka, with the reintroduction of this water mass into the gulf. At the same time, Valvulineria sp. A, common during the last glacial stage, disappeared. This was caused by the cessation of production of the Glacial North Atlantic Intermediate Water during deglaciation. These events, probably isochronous throughout the gulf, do not represent the true first and last occurrences of the species, but may prove useful for the recognition of significant stratigraphic datums.

Overall water depth changes or bathymetric shifts of water mass boundaries may lead to the disappearance of certain species in one area, but not in others. This results in a diachronous stratigraphic datum, or "stratigraphic climbing." Even planktonic foraminifera can be affected by such processes, because of narrowly stenothermal and stenohaline adaptations of many species, in the context of shoreline shifts and climatic changes.