Arctic current flowed under deep freeze of last ice age, study says

Arctic sea ice formation feeds global ocean circulation. New evidence suggests that this dynamic process persisted through the last ice age. -  National Snow & Ice Data Center
Arctic sea ice formation feeds global ocean circulation. New evidence suggests that this dynamic process persisted through the last ice age. – National Snow & Ice Data Center

During the last ice age, when thick ice covered the Arctic, many scientists assumed that the deep currents below that feed the North Atlantic Ocean and help drive global ocean currents slowed or even stopped. But in a new study in Nature, researchers show that the deep Arctic Ocean has been churning briskly for the last 35,000 years, through the chill of the last ice age and warmth of modern times, suggesting that at least one arm of the system of global ocean currents that move heat around the planet has behaved similarly under vastly different climates.

“The Arctic Ocean must have been flushed at approximately the same rate it is today regardless of how different things were at the surface,” said study co-author Jerry McManus, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory.

Researchers reconstructed Arctic circulation through deep time by measuring radioactive trace elements buried in sediments on the Arctic seafloor. Uranium eroded from the continents and delivered to the ocean by rivers, decays into sister elements thorium and protactinium. Thorium and protactinium eventually attach to particles falling through the water and wind up in mud at the bottom. By comparing expected ratios of thorium and protactinium in those ocean sediments to observed amounts, the authors showed that protactinium was being swept out of the Arctic before it could settle to the ocean bottom.From the amount of missing protactinium, scientists can infer how quickly the overlying water must have been flushed at the time the sediments were accumulating.

“The water couldn’t have been stagnant, because we see the export of protactinium,” said the study’s lead author, Sharon Hoffmann, a geochemist at Lamont-Doherty.

The upper part of the modern Arctic Ocean is flushed by North Atlantic currents while the Arctic’s deep basins are flushed by salty currents formed during sea ice formation at the surface. “The study shows that both mechanisms must have been active from the height of glaciation until now,” said Robert Newton, an oceanographer at Lamont-Doherty who was not involved in the research. “There must have been significant melt-back of sea ice each summer even at the height of the last ice age to have sea ice formation on the shelves each year. This will be a surprise to many Arctic researchers who believe deep water formation shuts down during glaciations.”

The researchers analyzed sediment cores collected during the U.S.-Canada Arctic Ocean Section cruise in 1994, a major Arctic research expedition that involved several Lamont-Doherty scientists. In each location, the cores showed that protactinium has been lower than expected for at least the past 35,000 years. By sampling cores from a range of depths, including the bottom of the Arctic deep basins, the researchers show that even the deepest waters were being flushed out at about the same rate as in the modern Arctic.

The only deep exit from the Arctic is through Fram Strait, which divides Greenland and Norway’s Svalbard islands. The deep waters of the modern Arctic flow into the North Atlantic via the Nordic seas, contributing up to 40 percent of the water that becomes North Atlantic Deep Water-known as the “ocean’s lungs” for delivering oxygen and salt to the rest of world’s oceans.

One direction for future research is to find out where the missing Arctic protactinium of the past ended up. “It’s somewhere,” said McManus. “All the protactinium in the ocean is buried in ocean sediments. If it’s not buried in one place, it’s buried in another. Our evidence suggests it’s leaving the Arctic but we think it’s unlikely to get very far before being removed.”

Scientist finds topography of Eastern Seaboard muddles ancient sea level changes

The distortion of the ancient shoreline and flooding surface of the U.S. Atlantic Coastal Plain are the direct result of fluctuations in topography in the region and could have implications on understanding long-term climate change, according to a new study.

Sedimentary rocks from Virginia through Florida show marine flooding during the mid-Pliocene Epoch, which correlates to approximately 4 million years ago. Several wave-cut scarps, (rock exposures) which originally would have been horizontal, are now draped over a warped surface with up to 60 meters variation.

Nathan Simmons of Lawrence Livermore National Laboratory and colleagues from the University of Chicago, Université du Québec à Montréal, Syracuse University, Harvard University and the University of Texas at Austin modeled the active topography using mantle convection simulations that predict the amplitude and broad spatial distribution of this distortion. The results imply that dynamic topography and, to a lesser extent, glacial adjustment, account for the current architecture of the coastal plain and nearby shelf.

The results appear in the May 16 edition of Science Express, and will appear at a later date in Science Magazine,

“Our simulations of dynamic topography of the Eastern Seaboard have implications for inferences of global long-term sea-level change,” Simmons said.

The eastern coast of the United States is considered an archetypal Atlantic-type or passive-type continental margin.

“The highlight is that mantle flow is a major component in distorting the Earth’s surface over geologic time, even in so-called ‘passive’ continental margins,” Simmons said. “Reconstructing long-term global sea-level change based on stratigraphic relations must account for this effect. In other words, did the water level change or did the ground move? This could have implications on understanding very long-term climate change.”

The mantle is not a passive player in determining long-term sea level changes. Mantle flow influences surface topography, through perturbations of the dynamic topography, in a manner that varies both spatially and temporally. As a result, it is it difficult to invert for the global long-term sea level signal and, in turn, the size of the Antarctic Ice Sheet, using east coast shoreline data.

Simmons said the new results provide another powerful piece of evidence that mantle flow is intimately involved in shaping the Earth’s surface and must be considered when attempting to unravel numerous long-term Earth processes such as sea-level variations over millions of years.

The water temperature in the subtropical Atlantic falls due to wind action

This is the argo profiler. -  Argo-Spain
This is the argo profiler. – Argo-Spain

The temperature of water situated in the subtropical Atlantic experienced a drop of 0.15ºC between 1998 and 2006. This has been revealed by a study led by the IEO (Spanish Oceanography Institute) which suggests that circulation caused by wind could be responsible for this “unusual” behavior.

Whilst the water temperature in this area, situated along the 24.5º north parallel, from the African coast to the Caribbean, rose by 0.27ºC between 1957 and 1998, researchers have recorded a drop of 0.15ºC in the same area between 1998 and 2006.

“In the ocean there are very pronounced cycles of change, and therefore, changes like those which took place in the coordinates analysed can reoccur in any location and at any time”, Pedro Joaquín Vélez Belchí, main author and researcher for the IEO’s Canarian Oceanography Centre, stresses to SINC.

According to the study, which was published recently in the Journal of Physical Oceanography, this phenomenon should not be linked to climate change. “The ocean’s natural variability mechanisms are more significant than we thought”, declares Vélez Belchí. The team is considering various hypotheses to explain the change in temperatures.

For the scientists, this cooling could be due to “circulation forced by the wind”. “Changes in the global structure of winds in the north Atlantic cause oscillations on the ocean’s surface layer which can be felt up to 2000 metres deep”, the expert points out.

However, the scientists discard the hypothesis of thawing despite the fact that some water masses, originating in the Antarctic and the Mediterranean, have an influence in the area analysed. The temperature drop “should have been observed clearly in the areas close to the North Pole”, maintains Vélez Belchí. And this was not the case.

The scientists measured the temperature and salinity of three oceanic layers: waters from the thermocline (300-800 metres), surface ocean (600-1800 metres) and intermediate waters (800-1.800 metros). The salinity recorded “similar” behaviour, as it is always linked to the temperature.

A new image of the ocean

The research team combined two methodologies: measurements using stations carried out from oceanographic research vessels, and the Argo network. With this network, consisting of 3000 indicators in all the oceans, “a new image of the surface ocean” is obtained. Spain is taking part in the Argo-Spain programme.

Through the new system, the scientists developed synthetic sections for each year (carried out from the laboratory with data from the Argo network’s buoys), and analysed the annual variability for 2005, 2006, 2007 and 2008. “Between 2006 and 2008 there were no significant changes”, the scientist declares.

As part of the Expedición Malaspina 2010, the team will go on a new expedition to the same area in the coming weeks. “The work is pioneering in verifying how the Argo network can be of use for large-scale studies into oceanic variability”, concludes Vélez Belchí.

Study probes mystery of loop current in eastern Gulf of Mexico

A study released by the Minerals Management Service today examines the circulation in the Eastern Gulf of Mexico (GOM) and sheds new light on the behavior of the Loop Current (LC) and Loop Current Eddies (LCEs), the relation between the upper- and lower-layer currents, and the variability of water mass characteristics in deepwater.

When the LC and the LCE are present in the Gulf near oil and gas activities, operators may have to curtail or amend their operations due to the strength of the current or eddy.

“The observations from this study will help MMS and other scientists better understand the Loop Current and improve our forecasting of its behavior in the Gulf of Mexico,” said Dr. Alexis Lugo-Fernandez, the MMS physical oceanographer responsible for the study. “This is important because oil and gas activities in the deepwater Gulf are affected by the presence of the Loop Current and the Loop Current Eddies.”

Prepared under a cooperative agreement by Louisiana State University’s Coastal Marine Institute, Observation of the Deepwater Manifestation of the Loop Current and Loop Current Rings in the Eastern Gulf of Mexico chronicled the deployment in the Eastern Gulf of a deepwater mooring cable measuring more than 11,800 feet for two years. The study supplements information gathered from a previous three year deployment.

The mooring data suggest the LC and LCEs that dominate upper-layer circulation in the Eastern GOM also influence the deeper currents in the Eastern GOM.

Dr. Lugo-Fernandez noted that a method to transmit significant energy in the form of heat to deep water in the GOM during the 2005 hurricane season was observed during this study. As sea levels rise near the center of tropical storms, the resulting higher pressure causes a small but measurable increase in temperature at all water depths. He explained that “Simply due to the large number of storm occurrences within the GOM, these findings represent an important process for transmitting energy to the deepwater.”