Algae Reveal History Of Arctic Sea Ice
November 19, 2013

Algae Provide Glimpse Into The History Of Sea Ice

Brett Smith for - Your Universe Online

An international team of researchers has developed a novel method for tracking changes in Arctic sea ice through the analysis of growth layers of seafloor algae, according to their report in the Proceedings of the National Academy of Sciences.

The study found that the algae Clathromorphum compactum accumulates a calcite crust as it grows and changes to this crust over time can be analyzed to reveal historical conditions of the surrounding seas.

"This is the first time coralline algae have been used to track changes in Arctic sea ice," said Jochen Halfar, an associate physical sciences professor at University of Toronto Mississauga. "We found the algal record shows a dramatic decrease in ice cover over the last 150 years."

The algae's growth is dependent on the temperature of the surrounding water and the amount of sunlight that reaches the seafloor. As sea ice accumulates on the surface of the water during the Arctic winter, it reduces the amount of light reaching the algae and lowers the water temperature, halting the algae’s growth. When the sea ice melts in the Arctic summer, the algae resume their growth and the expansion of their calcified crusts.

"It's the same principle as using rings to determine a tree's age and the levels of precipitation," Halfar said. "In addition to ring counting, we used radiocarbon dating to confirm the age of the algal layers."

To establish their method for tracking the historical changes to sea ice, the scientists gathered and analyzed algae samples in Arctic and sub-Arctic waters.

After cutting and polishing the various sections of crust, the team used a custom microscope to record thousands of images of each sample. The images allowed the researchers to render a complete impression of the fist-sized specimens.

The team says they were able to establish algal growth periods through the layers’ varying levels of magnesium, which are dependent on both light and temperature. Longer periods of bright and warm water correlate to a higher amount of algal magnesium, the scientists said.

The study researchers saw that during the Little Ice Age, a global cooling period that stretched from about the 1550s to the 1850s, the algal growth increments were extremely thin due to the broad sea-ice cover. Since that time period, the thicknesses of these growth increments have more than doubled, corroborating a decline in sea ice coverage that has only accelerated in recent decades.

Halfar said the new method isn’t only important for climate reconstruction, but also to allow for more accurate modeling of future climate change. Most of the observational information about annual changes in global climate only goes back 150 years, while information about sea-ice coverage comes mainly from satellite imagery and dates back only about 40 years.

"In the north, there is nothing in the shallow oceans that tells us about climate, water temperature or sea ice coverage on an annual basis," Halfar said. "These algae, which live over a thousand years, can now provide us with that information."