The most common dissolved substance in the ocean is chloride – one half of the power couple known as sodium chloride (“table salt” to most of us). Chloride is abundant because it is not very interested in combining with other material in the oceans and therefore stays dissolved for a long time. Up to now, scientists thought that the only way chloride would leave the ocean is if seawater would evaporate and leave salt deposits behind. Such ancient deposits provide the salt we use to flavor food and melt ice on roads.
Experiments by Associate Professor of Chemistry Alessandra Leri and her students at Marymount Manhattan College have just led to the discovery that chloride can leave the ocean in another way, by sticking to organic particles that settle out of surface waters and are buried in marine sediments. Using high-energy X-rays produced by the particle accelerator at Brookhaven National Laboratory, the team showed that chloride bonds to carbon in marine organic matter. Further research in collaboration with Larry Mayer at the University of Maine revealed that single-celled algae can produce organic matter containing such organochlorines. Additional experiments showed that this chemical reaction can also occur without algal help, under conditions similar to bleaching. Sunlight facilitates the reaction, just as it is necessary for algae to grow, so these organochlorines likely form at the sunlit top of the ocean. These results have just been published in Nature Geoscience, the #1-ranked journal in geosciences. Three MMC students, Marisa Dunigan, Katherine Ness, and Austin Gellis, appear as co-authors on the paper.
Why should we care about marine chlorination? For scientists interested in the history of the oceans, it helps explain the fate of chloride over very long time periods and may affect salt levels of the ocean over geological history. The discovery also paves the way to look for yet-undiscovered compounds and enzyme systems. Organic molecules with chlorine in them are often very potent chemicals, such as antibiotics, insecticides, or poisons like dioxin. Are such compounds made deliberately or by accident in the ocean? What consequences might they have for the fate of marine organic carbon? This exciting discovery, like many, opens up more questions than it answers.