In marked contrast to mass extinctions and productivity crises in much of the world' oceans at the Cretaceous-Tertiary boundary, siliceous plankton thrived in earliest Paleocene seas of Marlborough, New Zealand. Five bathyal sections show no radiolarian mass extinction across a well-defined Cretaceous-Tertiary boundary but, instead, an abrupt increase of diatoms relative to radiolarians, a general increase in both groups, and an influx of spumellarians. A pronounced increase in modal quartz and SiO2 within a 9–25-m-thick basal Paleocene interval indicates high biosiliceous productivity, the silica being derived primarily from diatoms. Enhanced upwelling in response to climatic cooling, or more efficient nutrient cycling related to sea-level changes, may explain this plankton bloom over the first 1 m.y. of the Tertiary.