Mercury found in glaciers
Climate change is rarely out of the headlines at the moment, and one very visible manifestation of warmer temperatures is the accelerating retreat of glaciers, which currently cover about 10 per cent of the planet. But it turns out there’s a double sting in the tail here, because scientists have discovered that, as glaciers scour their way down a valley, they scrape off the underlying rock and release minerals and metals in the process, including toxic mercury. This flows in the meltwater into the sea. And if the glaciers speed up their retreat, the delivery of that mercury into the food chain accelerates too, as Charlotte Birkmanis heard about this work, published in Nature Geoscience, from Florida State University’s Jon Hawkings...
Jon - There's certainly very high concentrations of mercury as well, which is concerning. Something doesn't really add up here. The concentrations on the surface of the ice, where all the melt-water is coming from are very low, but the concentrations in the rivers that are coming out of the ice or from beneath the ice that we were sampling were very high. So there's a missing mass in there somewhere.
Charlotte - Could you tell us exactly how you did this study? How many sites did you go and take these water quality samples from?
Jon - So we sampled three places in Southwest Greenland and two fjords that those glacial melt-waters run into. Taking water quality samples and taking them back to the lab and analysing them in that lab.
Charlotte - And you found a bit more than you bargained for, I think?
Jon - What we actually found was that some of these meltwaters had very high concentrations of mercury, and we think these concentrations of mercury are quite concerning for the export of this toxic element from ice sheets to the ocean.
Charlotte - So how do you think the mercury got there? Did it come from the glaciers and the melt-water, or did it come from somewhere else?
Jon - We think it's very likely that this mercury is coming from the bedrock beneath the sheets. Glaciers are these natural bulldozers. They move very slowly, but as they move over the bedrock, they crush it and grind it up. And they're really efficient agents of erosion. So if you put a glacier over a landscape, you can increase the amount of physical erosion in that landscape considerably. And there's also a lot of water at the base of glacier's ice/rock interface. And it's delivered there by the surface, by pipes through the ice, and crevasses through the ice from the surface to the bed of the glacier. And it also just sits there as kind of like a storage aquifer. It can sit there for quite a long time, some of the water, without freezing. And so there's interaction between the liquid water coming in from the surface of the ice and the liquid water that's stored beneath the ice, plus all this physical erosion. We think that those two, kind of, releases this mercury into the water, which is then flushed from underneath the glacier into these big glacial melt-water rivers.
Charlotte - And with this flushing, do you think it could go off shore a long way? Could it actually impact the food chain?
Jon - We really don't know if this mercury makes it into the food chain. All we do know at the moment is that there are very high concentrations of mercury in the melt-water and that these high concentrations are maintained to a degree in the coastal waters. We sampled some of the surface waters from fjords nearby, and these fjords are mixing zones between the fresh water coming from the ice sheet and the salt water coming from the ocean. And we found high concentrations in those systems that there are environmentally high concentrations in some of these fjords that are fed by these meltwaters. But as I said, we really don't know if this mercury is getting into the food chain or not. And I think that should be a focus of future research.
Charlotte - You mentioned there are high concentrations. I saw that it was comparable to rivers in industrial China. Can you give us an idea of the levels that we're looking at
Jon - Mercury is usually present in very, very low concentrations in any natural waters. So a typical river has about from 1 to 10 nanograms per litre of mercury in dissolved form. And that's like putting a grain of salt into an Olympic swimming pool sized swimming pool. So very small amounts of mercury, and the mercury we're seeing in Greenland is more like 150 to 300 nanograms per litre. So we're talking one to two orders of magnitude higher in these glacial meltwater rivers than in an average river. That's what you would expect to see in heavily contaminated rivers. So these are very high concentrations. I should also say they are still quite low. They are at, or just above, the recommended drinking limits. But the big concern with mercury is that even though it's very low in water, it can bioaccumulate in food webs. And what we mean by that is, as it goes up the trophic levels in a food web, the mercury actually accumulates in the organism and through that process, it can accumulate up to a million times. That's the main concern really