Time-transgressive late cenozoic radiolarian events of the equatorial indo-pacific

A biostratigraphic study of late Cenozoic Radiolaria in the equatorial Indo-Pacific shows an asymmetrical distribution between synchronous and diachronous events. A majority of synchronous events (15 out of 19) are last occurrences; the majority of diachronous events (10 out of 13) are first occurrences. Extinctions may therefore be preferable to first occurrences in the selection of datum levels for the definition of biostratigraphic zonations and for correlation control within global time scales. Diachronous equatorial radiolarian events span 1 to 2 million years, several orders of magnitude longer than the nominal mixing time of the oceans, suggesting that the biological and physical exchange processes associated with speciation events may not follow simple advective mixing models.