Abstract By the late nineteenth century, reports of foraminiferal distributions by pioneers such as Alcide d’Orbigny, Jacob Bailey and Henry Brady led to the realization that benthic foraminiferal populations have a biogeography, and are strongly influenced by water temperatures. Later studies, especially those by Richard Norton and Manley Natland in the 1930s, confirmed the temperature effect. The salinity effect became well known by late 1950s. In the 1950s and 1960s, environmental influences, including that of dissolved oxygen, became clearer with the extensive work of Fred Phleger and associates in the Gulf of Mexico, and in the western North Atlantic and eastern North Pacific Oceans. During this time, Orville Bandy and his students demonstrated severe changes in foraminiferal assemblages at California sewage outfalls. Furthermore, in Argentina, Esteban Boltovskoy confronted questions of species abundances and morphology, as related to natural and anthropogenic environmental constraints. A decade later, George Seiglie recognized anomalous populations and morphological abnormalities in polluted Puerto Rican bays. Thus, today’s approach to environmental micropalaeontology, using benthic foraminifera as a tool, was well established by the early 1970s. The emphasis, however, has shifted since then from environmental effects on assemblage characteristics (diversity, dominance, morphological aberrations) to the use of assemblages or species as tracers of environmental changes in historical time.

Abstract Examination of the benthic foraminifera from the northwestern Gulf of Mexico slope (90–95 o W; 283–1341 m) reveals widespread changes in their assemblages during the transition from the most recent sea-level lowstand (last glacial maximum) to the current highstand (interglacial). These glacial assemblages are defined by Q-mode cluster and R-mode principal component analyses of both relative abundance and presence/absence data from 61 samples dated at 15 ka. Distinct bathymetric positions can be assigned to the assemblages, except in the case of a deltaic outflow assemblage. They are interpreted to be associated with water masses, and basin-wide changes in water-mass position and chemistry at the glacial–postglacial transition are reflected in assemblage changes. Most important is the switch from North Atlantic Intermediate Water (NAIW), which was found in the Gulf during the last glacial maximum, to Subantarctic Intermediate Water (SAIW), which is found in the modern Gulf. This water-mass shift allowed several species associated with SAIW (e.g., Bulimina alazanensis and Osangularia culter ) to reenter the Gulf after the last glacial interval. The position of several water-mass boundaries were found to differ between the two time periods, causing the upper depth limits of some species to change with time. The benthic foraminiferal record in a core (water depth 726 m) shows three faunal events during and after the deglaciation; apparently, these events are related to water-mass shifts at 13 and 11 ka and a change in the substrate organic matter at 4.5 ka.