The Arctic is most sensitive to climate change and global warming. Just recently (winter 2017/2018), this region experienced its warmest winter on record. The rising temperatures have dramatic effects on the normally frozen ground – permafrost – which underlies twenty-four percent of the land area in the northern hemisphere. The permafrost thaws much deeper and rapid erosion of deep, ice-rich permafrost will increase. The Pleistocene deep permafrost (Yedoma) deposits are particularly prone to rapid degradation due to the loss of their high ice-contents upon thaw. Through this degradation, large amounts of previously stored frozen organic carbon will be exposed to microbial decomposition, resulting in the release of the greenhouse gases carbon dioxide (CO2) and methane (CH4) to the atmosphere. This emission in turn acts as a positive feedback to the climate system. So far, it is difficult to predict the rates of greenhouse gas emission because information on the decomposability of the organic matter is limited. As the organic matter is stored for millennia in the deep permafrost deposits, the radiocarbon (14C) analysis on CO2 can be used to trace the decomposition of ancient (permafrost derived) vs. recent organic matter sources.
The collection and processing of the respired CO2 for accelerator mass spectrometry (AMS) 14C analysis, however, is challenging and prone to contamination. Thus, during the progress of this thesis, we constructed a robust stainless-steel sampling device and refined a method for the collection of even small amounts (50 µg C) of CO2. This method is based on a CO2 sampling technique using a molecular sieve, which acts as an adsorbent. It has the advantage over other approaches (such as sampling in glass flasks) that CO2 can be concentrated from large air volumes.
The reliability of the 14CO2 results obtained with this molecular sieve cartridge (MSC) was evaluated in detailed tests of different procedures to clean the molecular sieve (zeolite type 13X) and for the adsorption and desorption of CO2 from the zeolite using a vacuum rig. Under laboratory conditions, the contamination of exogenous carbon was determined to be less than 2.0 µg C from fossil and around 3.0 µg C from modern sources. In addition, we evaluated the direct CO2 transfer from the MSC into the automatic graphitization equipment, AGE, with the subsequent 14C AMS analysis as graphite. This semi-automatic approach is promising as it resulted in a lower modern carbon contamination of only 1.5 µg C. In addition, this analyzing procedure can be performed autonomously. To collect CO2 released from soils or sediments, additional sampling equipment, such as respiration chambers or depth samples, connected to the MSC is needed. Including the sampling equipment, a modern contamination of 3.0–4.5 µg C was obtained. Overall, these results show that the contamination becomes insignificant for large sample sizes (>500 µg C) and should be considered for smaller amounts.
With this successfully tested MSC, it became possible to investigate the decomposition of the organic matter in thawing Pleistocene Yedoma deposits. On a Yedoma outcrop in the Lena River Delta, Northeast Siberia, we measured CO2 fluxes and their 14C signature to assess whether ancient (Yedoma derived) or younger C sources are preferentially respired. The CO2 released from the different sites is generally younger (2600–6500 yrs BP) than the bulk sediment (4000–31,000 yrs BP). Using isotopic mass balance calculations, we determined that up to 70% of the respired CO2 originates from ancient OM. These data show that thawing Yedoma organic matter can be rapidly decomposed, which introduces the ancient carbon into the active carbon cycle and thus increases the permafrost carbon feedback
