Underwater 'Tree Rings': Seafloor Algae Holds Clues To The Better Understanding of Climate Change

By Ajit Jha on November 19, 2013 11:39 AM EST

Coralline algae on the undersides of coral.
Coralline algae on the undersides of coral. (Photo: Derek Keats, CC BY 2.0)

Coralline algae or seafloor algae, of which Clathromorphum compactum is an example, will hardly attract any one's attention unless also informed, according to a new study published in the Proceedings of the National Academy of Sciences, that they hold the clue to deciphering 650 years of annual change in sea ice cover.

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The study, supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Ecological Systems Technology, claims to have produced for the first time ever a near-perfect picture understanding of sea ice formation on an annual scale, according to Jochen Halfar, lead study author and associate professor in Department of Chemical and Physical Sciences at the University of Toronto in Mississauga, Canada.

Collaborating with Halfar in the study were colleagues from Smithsonian Institution, Germany and Newfoundland. They collected and analyzed some samples of the alga Clathromorphum compactum. Widely distributed in the Arctic and sub-Arctic Oceans, this plant species in shallow waters between 15 to 17 feet deep, forms thick rock-like calcite crusts on the sea floor.

Walter Adey from the Smithsonian led several research cruises to retrieve specimens from almost freezing seawater with the help of several divers.

The algae go through continuous cycle of growth and dormancy depending on the temperature of the water and the light they are exposed to. The plant's growth is stopped when snow-covered ice accumulates over it turning the sea floor dark and cold. In the warm months, the sea ice melts, so the algae grow their calcified crusts resulting in visible layers that help determine the growth of algae each year when they actually grew during the ice-free season.

The coralline algae deposit calcite crusts that coral-like, so they are called coralline algae. The algae go dormant in the winter, a lack of sunlight leading to the development of visible bands that look like tree rings, according to Halfar.

The coral's underwater "tree rings" narrowed under the impact of the Little Ice Age from 1300s to 1800s indicating extensive sea ice cover followed by brief summers. However, the algae's growth rings dramatically doubled in thickness, indicating climate shift when the Arctic sea ice declined in an unprecedented manner, according to Halfar. 

Halfar admitted that the same principle of using rings to determine a tree's age and levels of precipitation is also applicable in this case. However, radiocarbon dating, in addition to ring counting, was also used in determining the age of algal layers.

The year to year variations in sea ice over the past decade, shown in the algae records match well with what has been shown by satellites. The current gap between climate records from sediments and ice cores could likely be filled with the collection of more algae crusts, according to Halfar.

The current models, according to Halfar, are weak in predicting when we will have the next ice-free summer Arctic, because of lack of long term data from the past to be used as an input. The current study, in the opinion of Halfar will enable us to prepare better models.

An interesting outcome of the study is the finding that the oldest algae crusts, confirmed by carbon dating, contained 646 years of layers. This is of course nowhere near some of the oldest living plants like bristlecone pines estimated to be nearly 5000 years old. Yet, the discovery does add to the list of long living plants, while algae, according to Halfar can live eternally.   

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