Darkening snow: is soot off the hook? The culprit may be algae

Over the last 10 years or so, the finger of blame for darkening the surface of snow has largely been pointed at soot from the combustion of fossil fuels, according to Lori Ziolkowski of the University of South Carolina, US. ButZiolkowski (or Z, for short) believesthat for glaciers this may not actually be the case; instead the culprit may be algae.

“There’s been a lot of media attention,” said Ziolkowski at the International Glaciological Society British Branch meeting in Bristol, “but no convincing data to pinpoint the source as soot. To date there have not been many measurements of soot on glaciers”.

Ziolkowski was looking for collaborators who specialize in microbial work, albedo study or chemical characterization to help determine the age of the carbon in dark material from glaciers. This should pinpoint the source of the black snow. As a result of the conference she is now planning to team up with scientists from the University of Bristol and the University of Leeds in the UK and Canada’s University of Alberta.

Carbon from fossil fuels is likely to be millions of years old. But the case of algae is less clear-cut. If algae are photosynthesizing carbon dioxide from the atmosphere, their carbon date will appear recent. But they may also be taking up ancient bicarbonate ions dissolved in water or actively consuming old aerosols in the glacier, which will make them appear old.

Radiocarbon dating examines the amount of the carbon-14 isotope, which forms by cosmic rays interacting with atmospheric nitrogen and enters the food chain via photosynthesis. Roughly 60,000 years after its death an animal or plant no longer contains any carbon-14 as it has decayed away.

“You are what you eat,” said Ziolkowski. In practice this means that bulk carbon analysis of, for example, ocean run-off from glaciers “can’t tell you so much” about the source of the carbon – it could appear old because it’s from a fossil fuel or old because it’s from a microbe that absorbed ancient bicarbonate.

Ziolkowski has been frustrated by these limitations in looking at the age of glacial carbon but that’s where her compound-specific radiocarbon dating technique could come in – it can measure the age of particular carbon-containing chemicals. “Normally the radiocarbon date is for all the carbon in there,” she said. “For example, on a patterned dress, it’s for everything. I can measure just the blue parts.”

Ziolkowski has a hunch that lots of microbes are eating old carbon, and this has made researchers mistake the compounds darkening snow as soot from burning of fossil fuels. “I don’t think soot is going to be playing an important role,” she said.

Although you might think it would be straightforward to determine whether carbon comes from soot as it has an aromatic structure, some biological substances, such as moulds, can make aromatic molecules too. And black carbon becomes less aromatic the longer it’s in the ocean. “Nobody checked the age of the aromatic carbon,” said Ziolkowski.

There would be two ways to date carbon from glaciers on a compound-specific basis, Ziolkowski says – using ocean run-off or microbes. Since it’s not a “super-carbon-rich” environment, the first technique would need around 100 litres of glacial outflow water. Looking at microbes, on the other hand, would require 109 cells. Depending on the cell density, that means an amount roughly equal in size to a golf ball or a watermelon. So it looks like the microbe approach is the easiest to try first.

Ziolkowski hopes to get results in a few years’ time.


View original article at: Darkening snow: is soot off the hook?


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