Arctic: meltwater as a CO2 sink

Glacial rivers in the Canadian Arctic swallow more carbon dioxide than the Amazon rainforest

The confluence of a glacial river with Lake Hazen in the Canadian High Arctic. These waters absorb a surprising amount of CO2 from the air. © Kyra A. St. Pierre
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Surprising discovery: glacial meltwater can act as a natural CO2 smoker - and surprisingly effective. Because the minerals dissolved in the glacier water bind a lot of CO2 through their weathering and thereby remove the greenhouse gas from the air, as researchers have determined. Some meltwater flows in the Canadian Arctic bind twice as much CO2 per day per area as the Amazon rainforest.

Most rivers, lakes and especially reservoirs release more CO2 than they absorb. The reason is a high content of organic matter and rotting plant material. It is estimated that CO2 emissions from global inland waters are equivalent to about ten percent of fossil fuel CO2 emissions.

Weathering binds CO2

But there are also waters that are extremely nutrient-poor and whose water carries a mineral rather than organic cargo: meltwater rivers and lakes. However, what role they play in the CO2 cycle and whether they are sinks or emitters has remained unclear so far: "Glacially fed rivers and lakes have so far been largely overlooked in the study of the CO2 cycle, " say Kyra St. Pierre from the University of Alberta in Edmonton and her colleagues.

Wrong, as it turns out. After all, a chemical process that binds CO2 extremely effectively takes place in meltwater: "Glacier-fed waters carry little organic carbon and large amounts of freshly eroded and reactive sediments, which are susceptible to rapid chemical weathering, " the researchers explain. During weathering, compounds such as calcium carbonates or calcium-containing silicates react with CO2 to form dissolved calcium, carbon dioxide and other compounds.

Hocharktischer See as a test area

How effective this weathering is in meltwater flows and what this means for CO2 uptake has been studied by St. Pierre and her team at the tributaries of Lake Hazen in northern Ellesmere Island, Canada. This approximately 544 square kilometer lake is fed by eleven glacial rivers. From seven of them and the lake itself, the researchers took water samples and analyzed them during a summer. display

The surprising result: the CO2 content in the water of these rivers was well below the normal CO2 equilibrium with the air. The CO2 saturation of the water declined with increasing distance from the origin glacier increasingly, as the researchers report. From measurements of carbon isotopes in water, they conclude that this CO2 wastage has to be traced back to the chemical weathering of minerals.

Higher CO2 uptake than the Amazon

The exciting thing about it: "As the weathering reactions in the rivers run down, CO2 is constantly replenished from the air, " says St. Pierre and her team. As a result, these streams of meltwater extract from the atmosphere substantial amounts of this climate gas and bind it in the form of dissolved inorganic carbon, ions and silicon compounds.

As a result, the glacial rivers in Lake Hazen's catchment area absorb a good 1, 000 tons of carbon in the form of CO2 every year, according to the researchers. In terms of area, the Lake Hazen area recorded twice as much carbon per square meter per day in 2015 than the Amazon rainforest. "The maximum daily rates of up to six grams of carbon dioxide per square meter were even up to 40 times higher than those of the Amazon, " the scientists report.

"Previously overlooked CO2 sink"

"This makes the glacial rivers an important and previously overlooked CO2 sink, " states St. Pierre and her team. At least at the regional level, the uptake of atmospheric CO2 by such rivers could measurably affect the carbon budget. "With many rivers flowing from glaciers around the world, this has potentially great importance, " said the researchers. They assume that this meltwater sink is also present in other regions of the Arctic and the high mountains.

Their measurements, however, also show that the CO2 uptake of the glacial rivers fluctuates strongly: in times with a lot of meltwater and a high proportion of coalesced minerals, it is higher than in summers with rather little meltwater. But that means: at least in the near future, this Arctic CO2 sink could become even more effective. "According to forecasts, the meltwater flow in the high latitudes will increase until at least the middle of this century, " the researchers explain.

Then, however, with the disappearance of the glaciers, the melt water will cease to flow. (Proceedings of the National Academy of Sciences, 2019; doi: 10.1073 / pnas.1904241116)

Source: Proceedings of the National Academy of Sciences

- Nadja Podbregar