Planet Earth Online

Ice cover linked to climate for at least 30,000 years

26 October 2009, by Tamera Jones

This strongly suggests the loss of sea ice from the region over the last 30 years is directly related to recent changes in our climate.

Arctic sea ice has both thinned and retreated dramatically in recent decades, but until now, scientists weren't sure if this was just part of a natural cycle. Although they were fairly certain that sea-ice cover and climate have always been intimately linked, until now, there wasn't much concrete evidence for this.

'There's been a burning desire for more datasets on sea-ice cover,' says Professor Simon Belt from the University of Plymouth, who led the study.

Muddy sediment

Now Belt and other colleagues from the University of Plymouth and the Alfred Wegener Institute for Polar and Marine Research have worked out exactly how the amount of ice covering one region of the Arctic has varied over the last 30,000 years.

The researchers analysed muddy sediment samples from the sea floor of the Fram Strait, between Greenland and Spitsbergen.

'We chose the Fram Strait, because it's the gateway to the Arctic and it's a region that's very sensitive to change, which means it reflects major changes in climate conditions,' says Belt.

To find out how the amount of ice varied over the last 30,000 years, they used the fact that two microscopic marine plants prefer slightly different conditions, and make different chemicals when they die.

Tiny marine algae called diatoms grow well when there is some sea ice as well as sunlight. When they die, they sink to the ocean floor and decompose. They produce a chemical called IP25. The good news is as far as scientists know, no other creature makes this chemical, which stays in sea floor sediments for thousands of years.

'This method is much easier than trying to find and identify intact diatoms, which are incredibly fragile. IP25 is stable,' says Belt.

Other miniature marine plants, like Arctic phytoplankton, do better when there's lots of sunlight but no ice. They use the sun's energy to photosynthesize and make energy for themselves. When they decompose, they release a completely different chemical, called brassicasterol, which also sits around in sea floor sediments.

Belt and his colleagues reasoned that high levels of IP25 in sediments suggest that ice cover varied according to the seasons, because diatoms would be able to grow only in these conditions. No trace of this chemical in sediments means that the Arctic sea was either free of ice, or completely covered with ice (that doesn't melt in the summer) when that sediment was laid down.

Covered with ice

When the sea is covered with thick ice, no sunlight can get through, so neither diatoms nor phytoplankton can get a foothold.

On the other hand, in an ice-free Arctic, phytoplankton would have flourished, meaning that brassicasterol would have piled up on the ocean floor when the phytoplankton died.

Using this information, they found that during the last ice age, 30,000 to 17,000 years ago, there's practically no evidence for diatom or phytoplankton growth, suggesting the northern Atlantic Ocean was, not surprisingly, almost entirely covered with ice.

But 14,000 years ago, the region warmed. The researchers found evidence that at this time phytoplankton did very well, but diatoms didn't, because the region was essentially ice-free.

From 11,700 years ago until today, medium levels of IP25 and brassicasterol reflect sea-ice conditions that varied each year according to the seasons.

The team's results fit well with other researchers' findings about the Fram Strait's ancient climate.

'Our record of sea-ice cover coincides with other reconstructions of warm water flowing into the Nordic Seas,' says Belt.

The study is the first time scientists have used changing levels of IP25 to figure out how sea ice varied in the Arctic so long ago.

Before this, the oldest documented records of sea-ice cover, going back 1000 years, came from Iceland. But because these records were made by people, they were inevitably subjective.

When Belt realised the potential of IP25 as a proxy for ice cover, he tested it on samples corresponding to the 1000-year-old samples from Iceland. He was surprised to find that his results tallied extremely well with the subjective records, which gave him confidence that the proxy would work on older cores.

'Before this, we just didn't have the know-how to work out what Arctic sea-ice conditions were like,' says Belt.

Keywords: Arctic, Environmental change, Polar,

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