As the Arctic gets warmer, large quantities of greenhouse gas could be released from the sediment at the bottom of lakes, a source that has previously been overlooked.
The frozen soil of the Arctic has already started to thaw, triggering the release of more methane and carbon dioxide into the atmosphere. This climate feedback is well known, but most modelling only accounts for thawing in the top 3 metres of Arctic soil.
Deep sediment may also pose a grave threat, scientists have now discovered, with tests suggesting Arctic lakes could be triggering a thaw of permafrost at much deeper levels than expected.
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Thermokarst lakes are formed when permafrost thaws, creating hollows where meltwater collects. They are a common feature in Arctic landscapes, with some being hundreds of years old, and they are increasing in number as the world warms.
Water in thermokarst lakes transfers heat into the sediment below, accelerating the thaw of deep permafrost under the lakebed. Once thawed, microbial activity emerges in this ancient sediment, triggering the release of carbon dioxide and methane.
at the University of California, Berkeley, and her colleagues set out to better understand whether thawing permafrost beneath lakes could be producing significant levels of greenhouse gases.
They used a 20-metre sediment core taken from Goldstream Lake in Alaska to assess how microbial activity differed across sediment layers and under varying temperature conditions.
Sediment at the base of the core, 20 metres below the lake bed, was 1.45掳C (34.6掳F) when extracted, and therefore not frozen. Microbial activity was already evident at this temperature, says Freitas.
When core samples were exposed to temperatures of 4掳C, 10掳C and 20掳C (39掳F, 50掳F and 68掳F), emissions increased, suggesting more greenhouse gases will be released as the sediment warms under climate change.
Deeper sediments are the most carbon-rich, and so could constitute a large source of emissions if they are disturbed under future warming. The team also observed significant production of methane 鈥� a greenhouse gas many times more powerful than carbon dioxide 鈥� under oxygen-free laboratory conditions. This implies that methane emissions can occur deep beneath the surface, says Freitas.
鈥淔indings like these indicate to us that we do not yet fully understand what some of the climate effects and their feedbacks are within these ecosystems,鈥� she says.
The study assessed greenhouse gas production in samples taken from just a single sediment core. Assuming the same processes are occurring under similar lakes across the Arctic, this could amount to an additional 30 to 90 million tonnes of emissions per year not currently accounted for in models. This is a conservative estimate, says Freitas, but nevertheless it is 鈥渟ignificant in scientific terms鈥�. 鈥淓missions that are released in the Arctic don鈥檛 just stay in the Arctic,鈥� she adds.
at Ume氓 University in Sweden says the study sheds new light on thawing happening in deep Arctic sediment. She cautions against drawing too many conclusions from analysis of a single sediment core, but says given the high number of lakes in the Arctic this could be a potentially significant emissions source for climate models to consider. 鈥淚t鈥檚 another part of the permafrost carbon feedback,鈥� she says. 鈥淲e should put more effort into quantifying it.鈥�
Nature Geoscience
