Permafrost – High Biolability of Carbon in Ancient Permafrost upon Thaw

Australia: The Land Where Time Began

Permafrost in the Arctic will be thawed by continuing climate changes which will remobilise substantial terrestrial organic carbon (OC) pools. About ¼ of northern permafrost organic carbon is contained in the Yedoma deposits in Siberia, which is the oldest form of permafrost carbon. However, understanding of the degradation and fate of this ancient OC in coastal and fluvial environments is still rudimentary. Vonk et al. show in this paper that ancient dissolved organic carbon (DOC, >21,000 ¹⁴C years), the oldest DOC that has ever been reported, has been found to be mobilised in the waters of streams that drain Yedoma outcrops. Also, this dissolved organic carbon is highly biolabile: during incubation under dark, oxygenated conditions at ambient river temperatures for 14 days 34 ± 0.8% was lost. Mixtures of the DOC from Yedoma stream with main river waters of the ocean mimicking the in situ mixing processes, also showed high losses of DOC (14 days; 17 ± 0.8% to 33 ± 1.0%). It is suggested by this that exceptionally old DOC is among the most biolabile DOC in any contemporary stream in the Arctic that has been previously reported.

The carbon pool in the northern soil contains about 1,672 Pg of carbon (Tarnocai et al., 2009) more than ¼ of which (>500 Pg C) is stored in frozen Yedoma (Zimov et al., 2006) deposits in the Siberian Arctic. This is approximately equal to the total that is stored in total global forest biomass (Pan et al., 2011). According to Vonk et al. Yedoma deposits formed in the Late Pleistocene (Zimov et al., 2006; Schirrmeister et al., 2011) in Siberia that was unglaciated at the time and covered about 1 million km². It has been hypothesised that as a result of a lack of processing and bacterial survival (Dutta et al., 2006: Rivkina et al., 1998) during formation, organic carbon that is held in the Yedoma deposits is highly labile when thawed. Yedoma formation was initiated by the accumulation of sediments (Zimov et al., 2006), and therefore is less exposed to repetitive freeze-thaw cycles, that process and degrade organic carbon, compared to other soil ecosystems. Permafrost soils, such as Yedoma, thaw and decompose as the climate warms which leads to the production of greenhouse gases, which in turn accelerates warming of the climate (Dutta et al., 2006; Schaefer et al., 2011), which is the “permafrost carbon feedback” (PCF). It is suggested by initial estimates that the PCF can be substantial (Zimov et al., 2006; Schuur et al., 2011), though remarkably, according to Vonk et al., this is not included in any Intergovernmental Panel on Climate Change (IPCC) (IPCC, 2007) scenario of future climate change. An understanding of how frozen pools of organic carbon are processed upon thawing is, therefore, crucial for an understanding of climate change and carbon dynamics in the Arctic and globally.

There are 4 physical mechanisms that release organic carbon from permafrost to the atmosphere (Schurr et al., 2008):

1) Active layer deepening;

2) Talik formation,

3) Thermokarst development, and

4) Erosion.

Erosion of Yedoma deposits on the coast and riverbanks can occur over vast distances (such as along >5,000 km of the East Siberian Arctic coastline) (Vonk et al., 2012) and may release organic carbon from entire soil depth profiles, up to about 40 m, direct to aquatic environments. A deepening g of the active layer – the surface soil that is thawed seasonally, may also lead to an increased supply of organic carbon that is derived from permafrost to inland waters over regional scales. Following thawing, the crucial current unknowns are how much and how fast does the organic carbon that is contained in these deposits enter the contemporary carbon cycle. It is increasingly recognised that inland and coastal waters are important processors of terrestrial carbon, which generates a substantial flux of CO₂ to the atmosphere (Aufdenkampe et al., 2011; Battin et al., 2008; Bianchi, 2011). During coastal release in the Siberian Arctic carbon processing has been shown to be substantial (Alling et al., 2010; Sánchez-García et al., 2011). In this study Vonk et al. investigated the “hydrological biolability”, i.e., the potential biodegradability within the time of residence in the aquatic system, of organic carbon from Yedoma deposits that had thawed recently were introduced to aquatic ecosystems.

On the banks of the Kolyma River, Northeast Siberia, the “Duvannyi Yar” exposure is a classical and relatively well studied Yedoma site (e.g. Vasil’chuk & Vasil’chuk, 1997; Dutta et al., 2006). Deposits that are as much as about 40 m high have been dated by radiocarbon to between 45,000 BP and 13,000 BP (Vasil’chuk & Vasil’chuk, 1997). Mean retreat rates of 3-5 m/yr have been recorded that resulted from thawing of permafrost and/or erosion of the river bank. Streams that are fed by melting ice wedges carry recently thawed Yedoma off the cliff into the Kolyma River. These first-order streams, which are laden with sediment, represent an integrated signal of thawed Yedoma.

These first-order streams form in early in summer when the ice wedges begin to melt, and their course and magnitude depend on local collapse of the river bank, rates of thaw and relief. Vonk et al. used the organic carbon dissolved in the stream for experiments of incubation and mixing with water from the Kolyma River and water from the East Siberian Sea, with the aim of estimating the biolability during fluvial and coastal processing of ancient permafrost organic carbon.

Conclusions

In this paper Vonk et al. show that the Yedoma dissolved organic carbon (DOC), with >21,000 ¹⁴C years the oldest ever reported, upon release is highly biolabile, in both fluvial (i.e., Kolyma River) and in coastal environments (i.e., the East Siberian Sea. Estimates of the exact contribution of the Yedoma organic carbon in Arctic carbon cycling are still subject to considerable uncertainties, e.g., due to a lack of adequate spatial coverage of Yedoma deposits (Romanovskii, 1993), and accurate estimates of erosion of riverbanks and deltas, though they are improving. It is expected that as a result of ongoing climate warming the thermal exposure, thawing and erosion of the Yedoma will increase, particularly along the extensive East Siberian Arctic coastline, where Yedoma deposits are ubiquitously present and are increasingly exposed to wave fetch and storms as a result of recent reduction of sea ice (IPCC, 2007; Stroeve et al., 2007). Yedoma which hosts about 25% of the below ground permafrost soil organic carbon (Zimov et al., 2006; Tarnocai et al., 2009) is very old, yet upon mobilisation it is very biologically reactive. It seems likely that the biolability 0f this material will amplify the effect of the PCF scenario in the Arctic. Also, it may not be representative to examine degradation of permafrost ¹⁴C-DOC at the mouths of rivers for the actual mobilisation and turnover of permafrost C, as it may be that processing of permafrost derived-C within the watershed is masking the signal at the river mouths. It is apparent that further studies are required to address the magnitude of the Yedoma organic carbon thawing, though also, it is important that the reactivity of this material be incorporated in regional and global carbon budgets.

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