Like Tree Rings of the Ocean, Arctic Algae Record Environmental History [VIDEO]

Like rings in a tree that identify age, the layers of calcite crust growing on sea floor algae can be analyzed, revealing the history of annual change in sea ice-cover in the Arctic.

By examining the calcite layers, researchers from University of Toronto Mississauga (UTM) were able to trace back 650 years of history in the Arctic sea ice.

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

Halfar and his colleagues have published their work in the journal Proceedings of the National Academy of Sciences.

The researchers found that samples of an algae known as Clathromorphum compactum contained a history of the world around them, much like trees record climate data in their layers.

Clathromorphum compactum is a long-lived alga that forms thick, rock-like crusts of calcite on the seafloor of shallow waters. The alga is widely distributed in the Arctic ocean.

Researchers can analyze the chemical composition of the algae samples to determine the temperature and salinity of the water.

Taking the plunge in near-freezing waters, a team of divers collected samples of the calcite crusts over the course of several research cruises.

At depths of 15 to 17 meters, the algae grows on the sea floor at a rate dependent on the temperature of the water and the amount of light they receive. As snow-covered ice accumulates on the water, it blocks the algae's light supply, turning the sea floor dark and cold and halting the plants' growth of calcite crust until the the cover melts.

This consistent cycle of growth and dormancy results in visible layers of calcite that can be used to determine the length of time the algae were able to grow each year during the ice-free season.

"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."

By analyzing magnesium levels, the researchers were able to calculate the length of the growth period per cycle. Magnesium levels in the calcite are dependent on the amount of light received and the temperature of the water.

"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."