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Arctic Sea Ice Decline Fastest For 650 Years

19.11.2013
19.11.2013 07:47 Age: 4 yrs

New research shows that Arctic sea ice cover has been variable but that the current decline is the fastest for 646 years. The scientists used so called underwater 'tree rings': calcite crusts of Arctic algae that record sea ice change to determine changes in historic sea ice cover.

Thick Crusts of the Sea-Floor Red Alga Clathromorphum. This alga can be found in coastal regions of the North Atlantic, North Pacific and Arctic Ocean, where it can live for hundreds of years. Courtesy: Nick Caloyianus. Click to enlarge.

Divers used hammer and chisel while enduring the near-freezing water temperatures of the Labrador Sea. This frame from You Tube video. Link at foot of page under sources. Click to enlarge.

 

Almost 650 years of annual change in sea-ice cover can been seen in the calcite crust growth layers of seafloor algae, says a new study from the University of Toronto Mississauga (UTM).

"This is the first time coralline algae have been used to track changes in Arctic sea ice," says 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."

With colleagues from the Smithsonian Institution, Germany and Newfoundland, Halfar collected and analyzed samples of the alga Clathromorphum compactum. This long-lived plant species forms thick rock-like calcite crusts on the seafloor in shallow waters 15 to 17 metres deep. It is widely distributed in the Arctic and sub-Arctic Oceans.

Divers retrieved the specimens from near-freezing seawater during several research cruises led by Walter Adey from the Smithsonian.

The algae's growth rates depend on the temperature of the water and the light they receive. As snow-covered sea ice accumulates on the water over the algae, it turns the sea floor dark and cold, stopping the plants' growth. When the sea ice melts in the warm months, the algae resume growing their calcified crusts.

 

Like tree rings

This continuous cycle of dormancy and growth results in visible layers 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," says Halfar. "In addition to ring counting, we used radiocarbon dating to confirm the age of the algal layers."

After cutting and polishing the algae, Halfar used a specialized microscope to take thousands of images of each sample. The images were combined to give a complete overview of the fist-sized specimens.

Halfar corroborated the length of the algal growth periods through the magnesium levels preserved in each growth layer. The amount of magnesium is dependent on both the light reaching the algae and the temperature of the sea water. Longer periods of open and warm water result in a higher amount of algal magnesium.

During the Little Ice Age, a period of global cooling that lasted from the mid-1500s to the mid-1800s, the algae's annual growth increments were as narrow as 30 microns due to the extensive sea-ice cover, Halfar says. However, since 1850, the thickness of the algae's growth increments have more than doubled, bearing witness to an unprecedented decline in sea ice coverage that has accelerated in recent decades.

 

New method

Halfar says the coralline algae represent not only a new method for climate reconstruction, but are vital to extending knowledge of the climate record back in time to permit more accurate modeling of future climate change.

Currently, observational information about annual changes in the Earth's temperature and climate go back 150 years. Reliable information about sea-ice coverage comes from satellites and dates back only to the late 1970s.

"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," says Halfar. "These algae, which live over a thousand years, can now provide us with that information."

The research, which was published in the Proceedings of the National Academy of Sciences, was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Ecological Systems Technology.

The paper

 

Abstract

Northern Hemisphere sea ice has been declining sharply over the past decades and 2012 exhibited the lowest Arctic summer sea-ice cover in historic times. Whereas ongoing changes are closely monitored through satellite observations, we have only limited data of past Arctic sea-ice cover derived from short historical records, indirect terrestrial proxies, and low-resolution marine sediment cores. A multicentury time series from extremely long-lived annual increment-forming crustose coralline algal buildups now provides the first high-resolution in situ marine proxy for sea-ice cover. Growth and Mg/Ca ratios of these Arctic-wide occurring calcified algae are sensitive to changes in both temperature and solar radiation. Growth sharply declines with increasing sea-ice blockage of light from the benthic algal habitat. The 646-y multisite record from the Canadian Arctic indicates that during the Little Ice Age, sea ice was extensive but highly variable on subdecadal time scales and coincided with an expansion of ice-dependent Thule/Labrador Inuit sea mammal hunters in the region. The past 150 y instead have been characterized by sea ice exhibiting multidecadal variability with a long-term decline distinctly steeper than at any time since the 14th century.

 

Significance as reported by PNAS

 

 

Sources:

This article based on a report Nicolle Wahl of the University of Toronto here.

You Tube video here

 

Citation:

Arctic sea-ice decline archived by multicentury annual-resolution record from crustose coralline algal proxy by Jochen Halfar, Walter H. Adey, Andreas Kronz, Steffen Hetzinger, Evan Edinger and William W. Fitzhugh

Published online before print November 18, 2013,

doi: 10.1073/pnas.1313775110
PNAS November 18, 2013

Read abstract and get research here.