(Table 1) Planktonic foraminiferal abundance at DSDP Site 94-606

A "total fauna" curve (Ruddiman, 1971; doi:10.1130/0016-7606(1971)82[283:PSITEA]2.0.CO;2) shows that surface water overlying DSDP Site 606 (central North Atlantic Ocean) cooled suddenly at 3.1 Ma, within the Mammoth paleomagnetic reversed interval. At that time, Globorotalia puncticulata, Neogloboquadrina pachyderma, and Globorotalia crassaformis increased and Globorotalia hirsuta, Globigerinoides sacculifer, and Sphaeroidinellopsis decreased in abundance. Oxygen isotope results for Globigerina bulloides do not indicate a "stepwise" cooling, but probably reflect small changes in continental ice volume. It is hypothesized that as annual average temperatures decreased, G. bulloides delayed its bloom from spring to summer to follow a preferred temperature habitat.