As we’ve been telling you this week, the big mercury conference going on in Guiyang, China is the place to be if you’re looking for answers to those burning questions we all have about that slippery, silvery, heavy metal. We answered some of those questions for the fish-industry newsletter Seafood Source this morning. At the conference yesterday, the question du jour was this: Where does the mercury in the open ocean come from, anyway? The bottom line is that nobody is really sure how swordfish, halibut, sea bass, and other ocean fish wind up with mercury in their flesh, but Mother Nature remains the lead suspect. Wasn’t this issue already decided in a California courtroom? Yep. (Twice. Click here for all the details.) But leave it to scientists to keep digging anyway. Here’s what they’re saying.
One group  of researchers from Harvard and the U.S. Geological Survey aren’t satisfied with the idea that mercury in seawater originates in deep ocean vents, even though they produce four times as much mercury as the total amount that accumulates in all ocean fish. This team says mercury in Pacific Ocean fish comes from Asian pollution. It would be nice if they had actually tested a few fish to back up their hypothesis. (They didn’t.) So for now, mercury that we know is passing up through the ocean floor trumps mercury we think might be making the trek eastward from China.
Another study (from a group headed by a University of Michigan geologist) is playing a process-of-elimination game to determine just how migratory ocean fish absorb their mercury. Do they get it from eating other fish in coastal waters? Does their mercury enter the ocean on the coasts (as the result of human activity) and then somehow find its way to the open ocean where fish magically absorb it through the water? Or does it start out in those undersea vents and begin a slow trek up the food chain, from smaller fish to bigger and bigger fish? This research team has ruled out choice #1; choice #3 (Mother Nature’s pesky ocean vents) seems to be the simplest, most elegant answer.
Why? Because thermal vents in the ocean basically produce underwater volcanic activity, releasing gases from molten magma into the sea. And we heard about two different studies today confirming just how much mercury exits volcanoes as their gases—all natural, mind you—spew into the environment.
The first study reported on volcanoes in Nicaragua, Italy, Taiwan, the Philippines, and Hawaii, suggesting that “degassing of basaltic magma plays an important part” in the global mercury picture. The second determined that “air Hg [mercury] concentrations are elevated at active faults and volcano areas.” Exhibit A is Mt. Puyehue in Chile, which the researchers found “has naturally elevated mercury levels with [an] absence of anthropogenic activity [i.e., man-made pollution].” Their conclusion? “A possible source of DGM [dissolved gaseous mercury] in deeper and bottom waters could be intensive tectonic activity of the seafloor,” which feeds those undersea volcanic vents.
If this sounds a bit complicated, fear not: The bottom line is that man-made mercury emissions don’t seem to be finding their way into your tuna. And as we detailed yesterday, a growing scientific consensus has fish back in the “healthy” column anyway, since omega-3 fatty acids, selenium, and other nutrients in fish appear to be nature’s trump card against traces of mercury.
We attempted to measure that consensus today with a brief survey which we asked all 540 conference attendees to complete. More than half of them participated, and we’ll be reporting their answers to our questions tomorrow.
Meanwhile, we’re following the rest of the Guiyang mercury conference until it closes on Friday afternoon; you can read our observations in real-time on Twitter.