The distribution of planktonic foraminifera has been studied in 28 piston cores of Late Pleistocene age from the western Gulf of Mexico. Detailed correlation between the cores has been made possible by a high degree of similarity of frequency changes within several species; by coiling direction changes within Globorotalia truncatulinoides; by a datum level representing the near extinction of Globorotalia menardii flexuosa, and Globorotaloides hexagona at the end of the last interglacial; by three distinct volcanic ash horizons, and by calcium carbonate dissolution effects at distinct intervals. Almost all species demonstrate distinct frequency oscillations that are correlatable between cores. A high proportion of these are clearly related to paleoclimatic oscillations and reflect rapidly changing water-mass conditions within the Gulf of Mexico during the latest Pleistocene. No interval appears to have been represented by stable environmental conditions. Causes of frequency changes within several other species are not clearly related to inferred paleoenvironmental changes. High similarity of faunas exists at all times between the northwest and southwest Gulf of Mexico, reflecting similarity of water-mass conditions over a wide latitudinal range. High sedimentation rates, which average between 10 cm/1000 years and 30 cm/1000 years, have enabled a detailed paleoclimatic curve to be established for the last 200,000 years. Three interglacials and two glacials are recognized. Distinct foraminiferal assemblages have enabled definition of 18 zones most of which are related to paleoclimatic changes. Most intense coolings occurred at the end of the penultimate glaciation (zone W) and during the middle of the Wisconsin glaciation (zone Y).The Gulf of Mexico curve is somewhat similar to those of other regions based on O18/O16 ratios, except that, despite close control, no intense cooling is apparent near the end of the last glaciation. The most sensitive warm water indicators are the G. menardii complex and Pulleniatina obliquiloculata; the most sensitive cooler water forms are Globorotalia inflata, and Globigerina falconensis. Several species have intermediate temperature tolerances. Much climatic information is lost when only the G. menardii complex is utilized in climatic studies, because these forms are essentially absent during glacial episodes. The most distinct faunal change associated with the Holocene warming does not coincide with the Z-Y boundary defined by the first consistent occurrence of G. menardii but occurs slightly earlier during the latest Wisconsin. The X zone, based on the consistent occurrence of the G. menardii group is of shorter duration in Gulf of Mexico cores than in cores from the central Caribbean Sea and the equatorial Atlantic Ocean.Three major volcanic ash horizons coincide with climatic coolings; almost immediately preceding the last interglacial (135,000 years B.P.); towards the end of the last interglacial (end of zone X; 90–95,000 years B.P.), and in the earliest part of the last glaciation (75,000 years B.P.). A drastic reduction of G. menardii flexuosa and G. hexagona coincides with the middle ash horizon.
