The study, the first to be based on widespread and long-term sampling of ocean waters, found that the increase in mercury content in Earth's waters follows the same pattern as the post-industrial increase in carbon dioxide in the planet's atmosphere – an exponential rise often described as a "hockey stick" graph.
The strongest increase was evident in waters less than 328 feet deep, where dissolved mercury was 2.6 times higher than it was before humans began burning fossil fuels on a large scale, the study found.
The data also jibed with previous estimates based on computer modeling; its measurements landed roughly in the middle of those prognostications.
Researchers analyzed samples taken at varying depths over an eight-year period from different areas of the Atlantic, Pacific, Southern and Arctic oceans. Because of the differences in the rate that deep and shallow waters circulate, those samples offer a record of water chemistry over time, somewhat like rock strata.
"There's sort of an age to water, in terms of the last time it was in contact with the atmosphere," said the study's lead author, Carl H. Lamborg, a geochemist at Woods Hole Oceanographic Institution.
The densest water, formed during winter in the North Atlantic, sinks and moves like an excruciatingly slow conveyor belt, eventually pushing into the farthest reaches of the northern Pacific Ocean, Lamborg said.
"What we found is the young, deep water – the water in the North Atlantic – has a lot more mercury in it than the older deep water," Lamborg said. That pattern suggests the difference can be explained by the steady increase in airborne mercury pollution, which precipitates from the atmosphere to Earth's surface.
Of course, the mercury in the samples did not come with a stamp that said "Belched out in Birmingham, England, 1880" or "Made in China, 2010." But, by measuring at the ratio of mercury to phosphate (which behaves in a similar manner to mercury), and indexing mercury levels to a broad database of carbon dioxide, researchers could determine what percentage of the mercury could not be attributed to natural sources and processes.
The study's findings contradict some high estimates of mercury levels, based on computer modeling. Those models assumed that past gold mining had contributed a large amount of mercury to oceans. Mercury from the Gold Rush and colonial times, the study suggests, might instead be locked up in sediments.
Although the study used a wide array of samples, there are a lot of untested waters on Earth, so more sampling and analysis will be needed. The mercury measured by the study was elemental mercury, not methylated mercury, the kind that winds up in the ocean food chain and can cause neurological disorders in humans. While it is known that some elemental mercury is converted to its more dangerous form in sea water, mystery remains about how the process works.