The group started by looking at data sets that offer detail about oceanic levels of phosphate, a substance that is both better studied than mercury and that behaves in much the same way in the ocean. Phosphate is a nutrient that, like mercury, is taken up into the marine food web by binding with organic material. By determining the ratio of phosphate to mercury in water deeper than 1,000 meters (3,300 feet) that has not been in contact with Earth's atmosphere since the Industrial Revolution, the group was able to estimate mercury in the ocean that originated from natural sources such as the breakdown, or "weathering," of rocks on land.
Their findings agreed with what they would expect to see given the known pattern of global ocean circulation. North Atlantic waters, for example, showed the most obvious signs of mercury from pollution because that is where surface waters sink under the influence of temperature and salinity changes to form deep and intermediate water flows. The Tropical and Northeast Pacific, on the other hand, were seen to be relatively unaffected because it takes centuries for deep ocean water to circulate to those regions.
But determining the contribution of mercury from human activity required another step. To obtain estimates for shallower waters and to provide basin-wide numbers for the amount of mercury in the global ocean, the team needed a tracera substance that could be linked back to the major activities that release mercury into the environment in the first place. They found it in one of the most well studied gases of the past 40 yearscarbon dioxide. Databases of CO2 in ocean waters are extensive and readily available for every ocean basin at virtually all depths. Because much of the mercury and CO2 from human sources derive from the same activities, the team was able to derive an index relating the two and use it to calculate the amount and distribution of mercury in
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Woods Hole Oceanographic Institution