University of Maine marine scientists are part of a team that discovered chloride — the most common dissolved substance in seawater — can leave the ocean by sticking to organic particles that settle out of surface water and become buried in marine sediment.
The discovery helps explain the fate of chloride in the ocean over long time periods, including ocean salt levels throughout geological history, says Lawrence Mayer and Kathleen Thornton, researchers based at the UMaine Darling Marine Center in Walpole.
Chloride is half of the power couple called sodium chloride, or table salt, says Mayer. Chloride affects ocean salinity, and thereby seawater density and ocean circulation.
Until now, scientists thought chloride only left the ocean when seawater evaporated, leaving behind salt deposits. Such ancient deposits provide salt used to flavor food and melt ice on roads.
But using high-energy X-rays produced by a particle accelerator at Brookhaven National Laboratory, the research team demonstrated that chloride bonds to carbon in marine organic matter.
Researchers found high organochlorine concentrations in natural organic matter settled into sediment traps between 800 meters (2,624 feet) and 3,200 meters (10,498 feet) deep in the Arabian Sea.
Alessandra Leri from Marymount Manhattan College led the team, which included other scientists from Marymount Manhattan College and Stony Brook University. The team showed that single-celled algae can make organic matter containing organochlorines.
This chemical reaction can occur without phytoplankton, as well, Mayer says, under conditions similar to bleaching. Sunlight promotes the reaction so organochlorines likely form at the sunlit top of the ocean.
The team concluded that transformations of marine chloride to nonvolatile organochlorine through biological and abiotic pathways represent a new oceanic sink for this element.
The study titled, “A marine sink for chlorine in natural organic matter,” has been published in “Nature Geoscience.”
Mayer and Thornton examine the ocean using biogeochemistry — or how organisms and materials chemically interact in Earth surface environments.
The findings, says Mayer, pave the way to look for yet-to-be-discovered compounds and enzyme systems. Organic molecules that contain chlorine are often potent chemicals — including antibiotics, insecticides and poisons including dioxin.
The discoveries also raise questions, he says, including: Are such compounds made on purpose or by accident in the ocean and what consequences might they have for the fate of marine organic carbon?