In the Arctic, carbon dioxide goes down where methane comes up

Reports of methane bubbling up from the bottom of the East Siberian Sea may have induced some climate change anxiety. In recent years, plumes of methane bubbles rising up from what was once dry permafrost have been observed off the Siberian coast. But their context was unclear. Were they a brand-new greenhouse gas release driven by climate change or were the bubbles long-time fixtures?

Work off the coast of Svalbard provided a welcome bit of relief. Examination of similar bubble plumes off Svalbard showed that they had been present (at some rate of bubbling) for thousands of years. While estimates of the amount of methane coming out of the East Siberian Sea were surprisingly large, measurements near Svalbard showed that the methane from deeper seafloor seeps gets trapped in the water column and consumed by bacteria before it can reach the atmosphere. That helped put the Siberian activity in some global context.

A new study led by United States Geological Survey researcher John Pohlman delivers another Svalbard surprise.

Pohlman’s team looked at the other side of the ledger: how much carbon is pulled out of the atmosphere by photosynthetic critters at these sites?

Counting seeps and sheep

The team used a research ship to crisscross the waters off western Svalbard, making air and water measurements around areas of methane seeps at varying depths. Above the deeper seeps (240 meters), there was no excess methane in the surface water or air, for example, indicating the methane was consumed by microbes below.

The shallowest seeps were a little less than 100 meters below the surface. Here, there was a measurable release of methane into the air, since it isn’t as effectively contained by the shallower water column. How much methane, you ask? The researchers helpfully describe that using rather unusual units: New Zealand sheep. The cluster of seeps they were studying, covering about 100 square kilometers, emitted the equivalent of about 320 sheep. Like all ruminants, sheep produce methane as they digest grasses and other foods.

Obviously, that’s not much. You would need 90,000 seeps like this one to match the methane emissions of New Zealand’s entire sheep population. And, the researchers say, it’s unlikely there are that many of these shallow water seeps in the world.

The odd thing is that the estimated emissions rates in the East Siberian Sea are about a hundred times higher, despite being similar in most ways. The researchers’ best guess for the apparent difference is that horizontal movement of water could be carrying some of the Svalbard methane from the shallow seeps down to deeper waters. The East Siberian Sea has no deeper water nearby for methane to travel to.

Balancing the books

The CO2 measurements actually showed that the Svalbard area isn’t even creating a net addition to the greenhouse effect. While there was extra methane in the air and water above the shallow seeps, the amount of CO­2 dissolved in the water was low. This means that organisms were actively sucking it up. The amount of CO2 being taken out of the atmosphere was about 1,900 times the amount of methane being released.

Even given the fact that methane is a more potent greenhouse gas than CO2 is, the decrease of CO2 warming power is 70 to 230 times greater than the increase of methane warming power. The amount of carbon dioxide being removed here even exceeds the warming power of the stronger methane emissions in the East Siberian Sea.

Why would this be happening? Methane doesn’t seem to be the only thing coming up to the surface—nutrient-rich water from the bottom is also rising up and fertilizing the growth of photosynthetic plankton.

The question is whether this is a common situation at all (or most) shallow methane seeps. Methane is more likely to reach the surface where water is naturally upwelling, of course, which means that this kind of fertilization would be going on there, too. And where methane bubbling is especially vigorous, the plume of bubbles could drag bottom water up with it. If that’s the case, an increase in methane emissions could be offset by an accompanying increase in fertilization and CO2 uptake.

Carbon dioxide uptake will now have to be investigated at other methane seeps around the world to find out how important it is. Finding out what this process looks like in the East Siberian Sea, where it could put an encouraging asterisk on the estimates of methane emissions, will be particularly interesting.

Proceedings of the National Academy of Sciences, 2017. DOI: 10.1073/pnas.1618926114  (About DOIs).

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