Second author

No one knows much about the Arctic Ocean during the Late Cretaceous, a period between 65 million and 99 million years ago when carbon dioxide levels in Earth's atmosphere were higher than they are today. Yet an understanding of conditions during that time might provide scientists with insight into what lies ahead, as CO2 levels rise as a result of increasing greenhouse-gas emissions.

From analysis of today's oceans, scientists had thought that phytoplankton growth during that era was fuelled by upwelling — the wind-driven movement of nutrient-rich water towards the surface. But Alan Kemp, a palaeo-oceanographer from the UK National Oceanography Centre at the University of Southampton, and his colleagues have found that one of the main types of phytoplankton, diatom algae, became adapted to grow in different layers of ocean temperatures. This stratification let them bloom in spring and through summer when sunlight warmed the sea's upper layers (see page 254). Kemp tells Nature more.

Why is this finding important?

We know that the ocean is again becoming more stratified, as it was during the Late Cretaceous. Climatically, that period is an analogue for a warmer time with higher levels of CO2, a likely scenario in the near future. This evidence of adaptability from the Cretaceous-era diatom fossils could be a clue to how an ocean may behave in the future, with more diatom algae species that can adapt to stratification. Upwelling-adapted diatoms tend to be small and reproduce rapidly, whereas stratified-adapted diatoms are usually larger and grow more slowly.

What does this mean for ocean ecosystems?

There could be a shift in the dominant or more successful species of plankton in the ocean, which is significant because the diatom algae are at the base of the whole food chain. If there are changes in the location of the dominant diatoms, including where they are most abundant, it could affect the fisheries and the entire ecosystem.

Did you go to the Arctic Ocean to get sediment samples?

No. In 1983, a team of Canadian scientists recovered shallow cores from the area's sea bed; incredibly, one contained diatom-rich sediments about 70 million years old. The cores were curated at the Bedford Institute of Oceanography in Nova Scotia, Canada, whose staff let us take samples. We used scanning electron microscopy to identify the types of diatom in these cores. Several genera and even some species from the Cretaceous are still present in the modern ocean, enabling us to make links with that period.