THE RECORD OF ELONGATE, CYLINDRICAL, DEEP-SEA FORAMINIFERA IN THE MIDDLE MIOCENE (15-13 MA) SUBTROPICAL PACIFIC OCEAN

Fifty-eight species of elongate, cylindrical benthic foraminifera (here referred to as the Extinction Group) belonging to genera that became extinct during the mid-Pleistocene Climate Transition (MPT), were documented (~50 kyr resolution) through the early middle Miocene (15-13 Ma) at two sites across the subtropical Pacific Ocean (ODP Sites 1146, South China Sea; ODP Site 1237, southeast Pacific). The study was undertaken to investigate the response of the Extinction Group to the major cooling during the middle Miocene Climate Transition (MCT) to look for explanations for the causes of the extinction during the glacials of the MPT. Extinction Group faunal differences between the two sites (attributed to regional and bathymetric differences in food supply to the seafloor) are greater than those that occurred through the 2 myr time span at either site.

The MCT was not an interval of enhanced species turnover nor decline in Extinction Group abundance, in contrast to the major extinctions that occurred during the MPT. Distinct changes in the composition of the Extinction Group faunas did occur through this time (more pronounced in Site 1237). At both sites the pre-MCT faunas were transformed into their post-MCT composition during the period of major cooling (14.0-13.7 Ma). During this transition interval the faunal composition swung back and forth between the two end member faunas. These faunal changes are attributed to changes in productivity (decrease in South China Sea; increase in southeast Pacific), brought about by major changes in global climate and continental aridity.