Planet Earth Online

Ice age ocean full of life

25 June 2009

Around 20,000 years ago, when the Earth last went through an ice age, levels of CO₂ in the atmosphere were around 100 parts per million (ppm) lower than they were at the start of the industrial revolution - 190 ppm compared with 280 ppm.

But until now, scientists weren't sure exactly where the CO₂ was going.

Some suggested that lower CO₂ levels could be down to a slower ocean circulation. If ocean circulation is strong, carbon gets moved around more and is released as CO₂ to the atmosphere, whereas a slower ocean is less able to give off CO₂.

Another idea put forward by other researchers, was that tiny marine plants called phytoplankton that live in the ocean and which need CO₂ and iron to grow could have been using up the CO₂.

When they die, these creatures sink to the ocean floor, staying there for thousands of years. If phytoplankton were using CO₂ from the atmosphere, researchers would see more carbon in sediment cores, which is indeed what they found.

Paradox

But they were faced with a paradox. Although sediment records show that the last ice age must have been a boom time for phytoplankton, because there was more iron being supplied to the ocean and more carbon sinking to the ocean floor, other researchers showed contradictory results.

Using a different way to measure how many diatoms lived in the ocean during the last ice age, they showed that this region of the ocean couldn't have been full of life.

Diatoms are a type of phytoplankton that use silicon to make their shells. When they die and sink to the ocean floor, their shells turn into opal. The second group of researchers found that opal levels were low in sediments underneath the equatorial Pacific Ocean - too low for a region of ocean supposedly rich in biological activity.

'This contradicted previous evidence based on organic matter data,' explains Dr Laetitia Pichevin of the University of Edinburgh.

Pichevin and her team decided to analyse a 35,000 year-old sediment core from the region in detail to try to resolve the problem.

'We needed clearer evidence that the ocean off the coast of Ecuador and the equatorial Pacific as a whole played a role in locking carbon away during the last ice age,' adds Pichevin.

Pichevin and her team's results, reported in Nature this week, show that lower levels of opal in the sediment weren't down to fewer phytoplankton. Instead, they show that diatoms changed the way they used silicon when iron was abundant.

They demonstate that diatoms don't use much silicon when iron is plentiful, but they continue to use carbon as CO₂. When they die, carbon gets locked away to the ocean floor, but less opal is produced. Paradox solved.

Modern iron fertilisation experiments in the equatorial Pacific Ocean support these results. When surrounded by an iron-rich ocean, diatoms don't use much silicon.

Pichevin's findings could ultimately be used by climate modellers to help predict what might happen in this region of the ocean as CO₂ levels increase over the coming years, but she stresses that 'this clearly depends on future dust and iron supply to the ocean.'

Keywords: Atmosphere, Environmental change, Geology, Oceans, Pacific Ocean,

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