UNDERSTANDING DIACHRONEITY: PALAEOENVIRONMENTAL CONTROLS ON DISPERSAL OF PLANKTONIC FORAMINIFERA IN THE HIGH-LATITUDE PLIO-PLEISTOCENE OCEANS

The lack of physical barriers in the ocean should allow for rapid dispersal and colonization of new habitats following speciation. However, evidence suggests gene flow is more limited than expected: diachronous first occurrence dates among and within ocean basins of many marine plankton species suggest that populations are slow to become widely established geographically. These observations indicate that the working mechanisms of physical barriers to dispersion in the open ocean are still poorly understood.

The high-resolution fossil record of planktonic foraminifera is ideally suited for in-depth studies of dispersal dynamics across space and through time. Using sediments recovered during IODP Expedition 383, we studied the environmental drivers of species dispersal and migration in the sub-polar Southern Pacific Ocean, a region that, via the Drake Passage, connects the Pacific and Atlantic Oceans but lies at the edge of many species' geographical range.

The Neogene planktonic foraminifera species Truncorotalia truncatulinoides, Globoconella puncticulata puncticuloides and Globoconella inflata all originate in the Southwest Pacific Ocean in the early Pliocene. However, G. puncticulata puncticuloides has not been recorded in any other ocean basin, and T. truncatulinoides and G. inflata do not appear in the Atlantic Ocean until 0.5-1.0 Myr after their first appearance in the mid-latitude Pacific. Interglacial assemblages at Site U1541 contain many temperate species including the Truncorotalia and Globoconella taxa, but glacial assemblages are dominated by polar species. These findings suggest that the high latitudes initially presented an environmental barrier to the dispersal of temperate-water species early in their evolutionary history, whereas interglacials weakened barriers to dispersal and facilitated adaptation.