Climate warming is 3 to 4 times faster in the Arctic than the global average. Permafrost – soil frozen for at least two consecutive years – can be very rich in ice and organic matter there. One consequence of climate warming is the thawing of this permafrost, leading to ground collapse and the formation of "thermokarst" lakes.
This thaw can also lead to the introduction of organic carbon into these lakes, which can be converted into greenhouse gases (carbon dioxide, methane) and re-emitted into the atmosphere.
Cavity caused by permafrost thaw in Siberia.
Vladimir Pushkarev A field study in Siberia
Scientists from the international project
PRISMARCTYC – bringing together teams from the CNRS (see box) – have conducted field campaigns in Central Yakutia (Eastern Siberia). The scientists analyzed the concentrations and origin of dissolved and particulate organic carbon in thermokarst lakes to visualize these transfers.
To understand the impact of permafrost thaw, the scientists selected several types of lakes: recently formed thermokarst lakes (less than 50 years old) and ancient thermokarst lakes (several thousand years old). Furthermore, the scientists studied some ancient lakes that have recently been impacted by permafrost thaw due to the formation of recent landslides on their shores.
The results indicate unprecedented concentrations of dissolved organic carbon reaching several hundred mg/L in recent lakes and in ancient lakes affected by landslides. Up to 75% of this dissolved organic carbon comes from permafrost thaw. Conversely, particulate organic carbon is mostly produced within the lakes themselves and not transferred from the frozen soils.
A major result: not all carbon is transformed into greenhouse gases
A portion of the dissolved organic carbon is converted into carbon dioxide and methane. But an innovative result from the study shows that a significant fraction of the ancient organic carbon transferred to the lakes is not converted into greenhouse gases. Thus, permafrost thaw causes a modification of the carbon cycle in lakes.
This study on a large number of lakes therefore makes it possible to show the diversity of processes occurring in the Arctic due to permafrost thaw. These results provide essential elements to better integrate thermokarst lakes into global climate models and better anticipate the feedbacks between climate warming and the Arctic carbon cycle.