MIDDLE MIOCENE PALEOCEANOGRAPHY OF THE WESTERN EQUATORIAL PACIFIC (DSDP SITE 289) AND THE EVOLUTION OF GLOBOROTALIA (FOHSELLA)

Evolution of the planktic foraminiferal lineage Globorotalia (Fohsella ) occurred during the Miocene between 23.7 and 11.8 Ma and forms the b asis for stratigraphic subdivision of the early middle Miocene (Zones N10 through N12). Important morphologic changes within the G. (Fohsell a) lineage included a marked increase in test size, a transition from a rounded to an acute periphery, and the development of a keel in late r forms. We found that the most rapid changes in morphology of G. (Foh sella) occurred between 13 and 12.7 Ma and coincided with an abrupt in crease in the deltaO-18 ratios of shell calcite. Comparison of isotopi c results of G. (Fohsella) with other planktic foraminifers indicate t hat deltaO-18 values of the lineage diverge from surface-dwelling spec ies and approach deep-dwelling species after 1 3.0 Ma, indicating a ch ange in depth habitat from the surface mixed layer to intermediate dep th near the thermocline. Isotopic and faunal evidence suggests that th is change in depth stratification was associated with an expansion of the thermocline in the western equatorial Pacific. After adapting to a deeper water habitat at 13.0 Ma, the G. (Fohsella) lineage became ext inct abruptly at 11.8 Ma during a period when isotopic and faunal evid ence suggest a shoaling of the thermocline. Following the extinction o f G. (Fohsella), the ecologic niche of the lineage was filled by the G loborotalia (Menardella) group, which began as a deep-water form and l ater evolved to an intermediate-water habitat. We suggest that the evo lution of G. (Fohsella) and G. (Menardella) were tightly linked to cha nges in the structure of the thermocline in the western equatorial Pac ific.