Frozen soils, or permafrost, in northeast Siberia are dominated by Yedoma deposits.  Yedoma deposits are accumulations of windblown dust and riverine sediments from the Pleistocene era, a period 2.6 million to 11,700 years ago with large temperature changes that caused warmer interglacial periods and ice ages.  These deposits settled in the interglacial periods, and froze once temperatures dropped.  Yedoma is also composed of ice wedges, huge chunks of ice trapped between the soil.  This makes Yedoma particularly vulnerable to climate warming because as the ice wedges melt, the soil collapses.  This exposes more soil to the warm air, thawing it further.  Yedoma’s vulnerability to higher temperatures is particularly important because Yedoma contains nearly a quarter of all northern permafrost organic carbon (OC), or carbon that is not in the form of carbon dioxide gas (CO₂).  It is important to understand the fate of this old carbon as the climate warms and causes more Yedoma deposits to thaw.

An excellent location to study Yedoma is Duvannyi Yar, an eroding cliff on the shore of the Kolyma River in northeast Siberia.  Here, melting ice wedge water drips down to form streams, which pick up Yedoma soil on their way to the Kolyma River below.  Past research has shown that the old OC in Yedoma streams is rapidly broken down or eaten by microorganisms, relative to waters containing more modern OC.  This ‘easy digestibility’ of Yedoma carbon means that microorganisms are more likely to eat a large portion of the carbon and respire it as CO₂.  By comparison, modern carbon, which is more difficult to break down, tends to sink before it can be used and therefore not increase CO₂.  The cause of quick degradation in Yedoma is unknown.

Our research focuses on one of the potential causes of degradation: the ice wedge meltwater that forms these Yedoma streams.  Our initial results indicate that ice wedge meltwater makes Yedoma OC quicker to break down than it would if mixed with Kolyma River water.  This suggests that water type (i.e. Kolyma River water or ice wedge water) controls how quickly old carbon is broken down.  The more rapid breakdown of Yedoma OC in ice wedge meltwater compared to Kolyma River water suggests that further ice wedge and permafrost thaw in Yedoma deposits will likely result in more carbon dioxide gas release, which will further increase climate warming.