Mercury is a widespread pollutant in our oceans, most of which found its way there as a consequence of human activity. Everything from coal-fired power plants to mining operations, incinerators and cement factories pumped mercury into the atmosphere which then entered the ocean either by being deposited directly or as runoff from rivers and streams. The result is a cumulative buildup of mercury in the food webs of marine ecosystems, including the seafood we eat, and new research has discovered the unusual way mercury can even travel to the deepest parts of the ocean.
The research, published in the journal Proceedings of the National Academy of Sciences (PNAS), describes how the sinking carcasses of fish from near-surface waters carry mercury pollution in their tissues to even the most remote and inaccessible parts of the world's oceans. Mercury deposits from fish-fall were even found in deepest recesses of the sea floor including the Marian Trench in the northwest Pacific, which sits a modest 10,972 meters (36,000 feet) below the water.
The research first analyzed the isotopic composition of mercury in marine life on the seabed located at two deep-sea trenches in the Pacific. Their results revealed evidence of human-derived mercury in deep-sea-trench organisms including fish and crustaceans. A research paper published in the journal Nature in July pointed the finger at sinking organic particles as the vessel on which this mercury hitched a ride to the bottom of the sea, but the team on the latest paper thought carcasses were a more likely culprit.
Mercury is made up of different isotopes and the differing combinations of these act like a unique fingerprint that can be used to identify a potential source of origin for mercury deposits. Using this fingerprinting technique, the team were able to recognize that the mercury found in the deep-sea-trench organisms they tested, which included snailfish and amphipods, matched that of fish species nearer the surface in the Pacific but not that of sinking detritus. Their results therefore supported the hypothesis that sinking carcasses were a more likely source of mercury on the seabed than the sinking particles which were identified as the cause in the Nature study.
"We studied the trench biota because they live in the deepest and most remote place on Earth, and we expected the mercury there to be almost exclusively of geologic origin--that is, from deep-sea volcanic sources," said University of Michigan environmental geochemist and lead author on the PNAS paper Joel Blum in a statement. "Our most surprising finding was that we found mercury in organisms from deep-sea trenches that shows evidence for originating in the sunlit surface zone of the ocean.
“It was widely thought that anthropogenic mercury was mainly restricted to the upper 1,000 meters of the oceans, but we found that while some of the mercury in these deep-sea trenches has a natural origin, it is likely that most of it comes from human activity."