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Oceanography
Dirty meddies
 

The Mediterranean Sea’s high evaporative rate and low freshwater input create a unique hydrogeographic region. For years, scientists have studied how the Mediterranean’s warm, salty water outflows into the Atlantic Ocean. Now, a team of chemists at the Spanish Research Council says that the Mediterranean’s outflow may carry more than just dense water.

In the Aug. 1 online issue of Environmental Science and Technology, Soledad Martí, Josep Bayona and Joan Albaigés described how the Mediterranean outflow advectively transports organic pollutants to the northeastern Atlantic.

They collected particles throughout the water column of a northeastern Atlantic area north of the Canary Islands and south of the Azores. Their sampling showed an anomalously high concentration of hydrophobic organic compounds (HOCs), including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), at 1,000 meters or mid-depth. The unusual increase in HOC concentrations indicated that additional particles were either forming in-situ or were carried by the Mediterranean outflow.

Martí and his colleagues believe the Mediterranean-Atlantic water flux is responsible for the elevated pollutant levels, because the smaller particles they caught were distributed in patterns unfamiliar for the Atlantic but similar to those of Mediterranean deep waters. The team analyzed the lipid composition of the small-sized particulate matter, filtered through 0.1 micrometer pore size, to identify the signature of the Mediterranean water discharged at mid-depth in the northeastern Atlantic.

The Mediterranean is an enclosed sea where a high evaporation rate and low freshwater input result in water density much higher than that found in the open ocean. As this warm, salty water flows out of the Strait of Gibraltar, it sinks beneath the relatively cooler, less saline water coming in from the Atlantic Ocean.

 “The water flows over the Strait of Gibraltar like a giant river flowing down and as it goes down, it entrails water, diluting the dense water with lighter water,” explains Phil Richardson, a physical oceanographer at the Woods Hole Oceanographic Institution.

 The dense water descends along the continental slope, reaching neutral buoyancy with the surrounding ocean water at around 1,000 meters deep. The water then separates from the continental slope and drifts southwestward as a clockwise rotating eddy. This rotating flow can be mapped for some distance. Richardson is involved in several studies to identify, map and understand these so-called “meddies.” He says it “makes perfect sense” that meddies between the Azores and the Canary Islands contribute to the flow of pollutants from the Mediterranean to the Atlantic.

 Joan Albaigés, a member of the Spanish Research Council team, says that the meddies are a previously unknown mechanism for the transport of HOCs into the Atlantic. “The Mediterranean is what’s called a concentration basin where the water is more dense, so the pollutant levels are also higher than in the open seas. So, part of these pollutants goes out from the Gibraltar Strait to the Atlantic and is incorporated at mid-depth in the Atlantic,” he says.

In the 1970s and 1980s, it was believed that the main source of pollution was continental input from rivers and industrial waste. Then, in the 1990s, concerns turned to atmospheric deposition and transport of pollutants into the sea. “I am now here explaining another mechanism that can be relevant, which is the advective transport from polluted waters to the more clean waters,” Albaigés says.

Martí and his team estimate that these meddies transport eight tons per year of PAHs and 0.5 tons per year of PCBs into the northeastern Atlantic. But Albaigés adds that quantity is not the significant part of their research. “The importance here is in discovering a new process which was unknown until now, which may contribute to pollution.”

Lisa Pinsker
Geotimes contributing writer


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