PALEOBIOGEOGRAPHIC ANALYSES OF NEOGENE PLANKTIC FORAMINIFERA REVEAL DISPERSAL STRATEGIES TO ACHIEVE BI-POLAR DISTRIBUTIONS

Genetic analyses of extant planktic foraminifera species with bi-polar distributions indicate gene exchange between Arctic and Antarctic populations, with such gene exchange hypothesized to have occur since the establishment of cool-water provinces in the Miocene. It has been proposed that species utilize equatorial upwelling zones during times of global cooling to disperse into cooler waters and maintain genetic connectivity among such populations. Using fossil occurrences of the temperate water planktic foraminiferal genus Globoconella, we reconstruct dispersal paths for six species across the global ocean through the Neogene to investigate dispersal dynamics of species with bi-polar distributions. Dispersal patterns were reconstructed for the species using biostratigraphically and magnetostratigraphically constrained ages of first occurrences at each site. Reconstructed dispersal paths indicate all six species first evolved in the Southern Hemisphere, and utilized the Antarctic Circumpolar Current to disperse into different ocean basins. From there, species were able to move into the Northern Hemisphere via western and eastern boundary currents. Two Miocene species, Globoconella miozea and Gc. praescitula, were able to disperse from the southwest Pacific across the tropics and into the Northern Hemisphere during the mid-Miocene Climate Transition. This time was characterized by intensified eastern equatorial Pacific thermocline upwelling, and cooling of the western equatorial Pacific thermocline. Globoconella miozea utilized cooling thermoclines as a ‘stepping stone’ across the tropics, whereas Gc. praescitula utilized the cool-water upwelling regions. Thus, cooling thermoclines and upwelling during the Miocene played a major role in the dispersal and establishment of bi-polar Pacific populations of Globoconella. This study sheds light on mechanisms that may enable pelagic species to maintain bi-polar distributions and accomplish long-distance genetic exchange.