Human activity since the beginning of the Industrial Revolution has more than tripled mercury levels in surface waters of the ocean, according to a new study in this week’s Nature.
Emissions from the toxic trace metal have increased considerably as a result of anthropogenic activities, like gold mining and fossil fuel combustion. In the ocean, inorganic mercury is converted into toxic methyl mercury, which can accumulate in fish... and people, ultimately. "It would seem that, if we want to regulate the mercury emissions into the environment and in the food we eat, then we should first know how much is there and how much human activity is adding every year," says Carl Lamborg from Woods Hole Oceanographic Institution in a news release.
Current estimates of mercury present in the ocean are mostly based on computer modeling studies -- and scientists remain uncertain about these values. So, for an observation-based estimate of anthropogenic mercury, Lamborg and colleagues measured mercury levels in data gathered from 12 sampling expeditions to the Atlantic, Pacific, Southern, and Arctic oceans over the past eight years.
First, they needed to find a way to separate the bulk contributions of natural and human sources over time. “At the moment,” Lamborg explains, “there is no way to look at a water sample and tell the difference between mercury that came from pollution and mercury that came from natural sources.” So his team looked at data on oceanic levels of phosphate, which behaves like mercury but is better studied. By determining the ratio of phosphate to mercury in water deeper than 1,000 meters -- which hasn’t been in contact with Earth's atmosphere since the 19th century -- the group was able to estimate mercury in the ocean that originated from natural sources, such as the weathering of rocks on land.
Next, to determine the contribution of anthropogenic mercury in shallower waters, the team used carbon dioxide as a tracer. The well-documented greenhouse gas can be linked back to major activities that released mercury into the environment in the first place.
Their findings show that deep North Atlantic waters and most intermediate waters (between 100 and 1,000 meters) are “anomalously enriched” in mercury, compared to the deep waters of the South Atlantic, Southern, and Pacific oceans. Since industrialization, human disturbances have led to a 150 percent increase in the amount of mercury in intermediate waters -- and we’ve more than tripled the mercury content of surface waters in the top 100 meters.
According to their estimates, the total amount of pollution mercury present in the global ocean is 60,000 to 80,000 tons -- with nearly two-thirds residing in water shallower than 1,000 meters.
"The next 50 years could very well add the same amount we've seen in the past 150," Lamborg says. “The key is now we have some solid numbers on which to base continued work."
Images: Brett Longworth, Woods Hole Oceanographic Institution