Microplankton provide 9 Ma age for sediment in the Macquarie Island ophiolite complex

Macquarie Island in the southwest Pacific Ocean (558S) is unique as an exposed location for studying oceanic crust generated by slow seafloor spreading—regions where rocks are difficult to date using radiometric methods. Bolboforms, an extinct group of poorly known microplankton, in sediment intercalated with pillow lavas yield tight constraints (9.01–8.78 Ma) on the age of formation of the dominantly seafloor volcanic sequence constituting the south of the island. The occurrence of Bolboforma metzmacheri extends the known geographic range of this Late Miocene zonal marker species in the southwest Pacific. A monospecific calcareous nannoplankton flora (Reticulofenestra perplexa) accompanied by the foraminifer Neogloboquadrina pachyderma in sediment from the north part of the island indicates a slightly older age (9.5–9.3 Ma), consistent with a radiometric date (9.2+0.4 Ma) from nearby volcanics. The new age data indicate that the ocean floor volcanic sequence formed early in the Late Miocene, possibly along short segments of a slow-spreading midocean ridge. Bolboforms have potential to provide fine-scale dating in other similarly complex ridge systems that are difficult to date by other means.