Amount of Coldest Antarctic Water Near Ocean Floor Decreasing for Decades

Scientists have found a large reduction in the amount of the coldest deep ocean water, called Antarctic Bottom Water, all around the Southern Ocean using data collected from 1980 to 2011. These findings, in a study now online, will likely stimulate new research on the causes of this change.

Two oceanographers from NOAA and the University of Washington find that Antarctic Bottom Water has been disappearing at an average rate of about eight million metric tons per second over the past few decades, equivalent to about fifty times the average flow of the Mississippi River or about a quarter of the flow of the Gulf Stream in the Florida Straits.

“Because of its high density, Antarctic Bottom Water fills most of the deep ocean basins around the world, but we found that the amount of this water has been decreasing at a surprisingly fast rate over the last few decades,” said lead author Sarah Purkey, graduate student at the School of Oceanography at the University of Washington in Seattle, Wash. “In every oceanographic survey repeated around the Southern Ocean since about the 1980s, Antarctic Bottom Water has been shrinking at a similar mean rate, giving us confidence that this surprisingly large contraction is robust.”

Antarctic Bottom Water is formed in a few distinct locations around Antarctica, where seawater is cooled by the overlying air and made saltier by ice formation. The dense water then sinks to the sea floor and spreads northward, filling most of the deep ocean around the world as it slowly mixes with warmer waters above it.

The world’s deep ocean currents play a critical role in transporting heat and carbon around the planet, thus regulating our climate.

While previous studies have shown that the bottom water has been warming and freshening over the past few decades, these new results suggest that significantly less of this bottom water has been formed during that time than in previous decades.

“We are not sure if the rate of bottom water reduction we have found is part of a long-term trend or a cycle,” said co-author Gregory C. Johnson, Ph.D., an oceanographer at NOAA’s Pacific Marine Environmental Laboratory in Seattle. “We need to continue to measure the full depth of the oceans, including these deep ocean waters, to assess the role and significance that these reported changes and others like them play in the Earth’s climate.”

Changes in the temperature, salinity, dissolved oxygen, and dissolved carbon dioxide of this prominent water mass have important ramifications for Earth’s climate, including contributions to sea level rise and the rate of Earth’s heat uptake.

“People often focus on fluctuations of currents in the North Atlantic Ocean as an indicator of climate change, but the Southern Ocean has undergone some very large changes over the past few decades and also plays a large role in shaping our climate,” said Johnson.

The data used in this study are highly accurate temperature data repeated at roughly 10-year intervals by an international program of repeated ship-based oceanographic surveys. Within the U.S., the collection of these data has been a collaborative effort of governmental laboratory and university scientists, funded primarily by NOAA and the National Science Foundation. However, much of the data used in this study were measured by international colleagues.

“Collection of these data involves 12-hour days, seven days a week, of painstaking, repetitive work at sea, often for weeks on end with no sight of land. We are grateful for the hard work of all those who helped in this effort,” said Purkey.

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NOAA

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This Enormous Mass Of Floating Antarctic Algae Can Be Seen From Space

Stephanie Pappas, LiveScience | Mar. 7, 2012, 8:48 PM

An enormous algae bloom off the coast of Antarctica is so huge and colorful that it can easily be seen from space.

A stunning photo of the monster algae bloom was released March 4 by the Australian Antarctic Division.

The bloom hugs the coast of eastern Antarctica and has been present since mid-February. Marine glaciologist Jan Lieser of the Antarctic Climate & Ecosystems Cooperative Research Center (ACE) in Australia said in a statement that the event is remarkable.

“We know that algal blooms are a natural occurrence down south —it’s just a part of the Southern Ocean,” Lieser told Australian website The Conversation. “But I’ve never seen one on this scale before. It’s been going on for about 15 days now, so it’s maybe about two-thirds or three-fourths of the way through the cycle.”

The bloom stretches about 124 miles (200 kilometers) east to west and 62 miles (100 km) north to south. The image of this gigantic bloom was taken by the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard NASA’s Earth-orbiting Terra satellite; together with the Aqua satellite, Terra views Earth’s entire surface every one to two days, acquiring data in several wavelengths of light.

On Feb. 27, MODIS spotted another Antarctic phytoplankton bloom, this one off the coast of the Princess Astrid Coast.

Algae blooms like these are triggered when a combination of sunlight and nutrients create fertile conditions. In the Southern Ocean, iron is the limiting nutrient, according to ACE. When iron concentrations are high enough, algae blooms follow.

This particular bloom is thought to be made up of phaeocystis, a single-celled algae well-known in polar areas. Algae also live on land in the Antarctic, sometimes in concentrations high enough to color snow banks red, green and orange. Australian research vessel Aurora Australis is venturing near the Antarctic bloom so scientists can collect samples of the algae.

Algae is the base of the ocean food chain, and in the Southern Ocean, as is the case elsewhere, they take up the greenhouse gas carbon dioxide as they photosynthesize and grow. But massive blooms occasionally cause trouble. Some species of algae produce neurotoxins that are deadly. Humans who eat shellfish that have fed on Alexandrium catanella, the algae responsible for “red tides,” can die of paralytic shellfish poisoning.

Some researchers even suspect that algae poisoning contributed to all five of Earth’s great mass extinctions, which killed off between half and 90 percent of all animal species when they occurred. According to this controversial theory, there were increased levels of algae in at least four of the five mass extinctions in Earth’s history. A cataclysmic event such as a volcanic eruption or asteroid impact could have stressed the algae, causing them to release more toxins and further harm the ecosystem.

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.

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