CHARACTERIZING THE LATE NEOGENE PLANKTIC FORAMINIFERAL LATITUDINAL DIVERSITY GRADIENT ACROSS THE SOUTHWEST PACIFIC OCEAN

The latitudinal diversity gradient (LDG) is a phenomenon which indicates, for some animal groups, peak diversity in equatorial regions. However, the LDG for marine invertebrates and plankton is quite different, with peaks in the mid-latitude regions today and in the more recent geologic past. Modern studies characterizing the marine plankton LDG employ the distributional records of planktic foraminifera, a protist group commonly used in paleontological studies with a well-documented and robust fossil record. Analyses using global datasets to assess modern diversity gradients indicate the planktic foraminiferal LDG peaks around 20° in the North and South Hemispheres. Local data on plankton diversity, however, indicate this pattern may not hold across western boundary currents (WBCs) and their extensions, major oceanic surface currents as part of subpolar and subtropical gyre systems. Here, we use previously published planktic foraminiferal biostratigraphies from Ocean Drilling Program Leg 90 sites which transect the Tasman Front, a southwest Pacific WBC extension, to investigate changes in the plankton LDG through the late Neogene (~7.3–0 Ma) across warming and cooling events as well as tectonic gateway closures. From 21.20°S to 31.59°S, the LDG peak shifts between highest diversity at lower latitudes to highest diversity peaks in the mid-latitude sites throughout the interval. Highest diversity in the mid-latitudes occurs from ~6–4.6 Ma, with a switch to highest diversity in the lower latitudes after this time. The shift to an LDG low-latitude peak corresponds to increased constriction of the Central American Seaway and Indonesian Throughflow, and occurs at a time of sea surface temperature cooling as inferred from stable isotopic data. From 36.57°S to 40.63°S, diversity is much lower throughout the interval compared to the tropics, indicating diversity drops significantly between 31.59°S and 36.57°S latitude across the Tasman Front. The planktic foraminiferal LDG in the southwest Pacific is in stark contrast to the northwest Pacific LDG. In the northwest across the Kuroshio Current Extension, the LDG peak is prominent throughout the late Neogene at 37°N. This study indicates that the planktic foraminiferal LDG across WBCs may be unique to the ocean basin and current configuration.