No abstract

Permafrost peatlands contain globally important amounts of soil organic carbon, owing to cold conditions which suppress anaerobic decomposition. However, climate warming and permafrost thaw threaten the stability of this carbon store. The ultimate fate of permafrost peatlands and their carbon stores is unclear because of complex feedbacks between peat accumulation, hydrology and vegetation. Field monitoring campaigns only span the last few decades and therefore provide an incomplete picture of permafrost peatland response to recent rapid warming. Here we use a high-resolution palaeoecological approach to understand the longer-term response of peatlands in contrasting states of permafrost degradation to recent rapid warming. At all sites we identify a drying trend until the late-twentieth century; however, two sites subsequently experienced a rapid shift to wetter conditions as permafrost thawed in response to climatic warming, culminating in collapse of the peat domes. Commonalities between study sites lead us to propose a five-phase model for permafrost peatland response to climatic warming. This model suggests a shared ecohydrological trajectory towards a common end point: inundated Arctic fen. Although carbon accumulation is rapid in such sites, saturated soil conditions are likely to cause elevated methane emissions that have implications for climate-feedback mechanisms.

Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has been attributed to increased temperature and snow depth, and has caused expansion of wet minerotrophic areas leading to significant changes in carbon cycling in the mire. In order to track these changes through time and evaluate potential forcing mechanisms, this paper analyses a peat succession and a lake sediment sequence from within the mire, providing a record for the last 100 years, and compares these with monitored climate and active layer thickness data. The peat core was analysed for testate amoebae to reconstruct changes in peatland surface moisture conditions and water table fluctuations. The lake sediment core was analysed by near infrared spectroscopy to infer changes in the total organic carbon (TOC) concentration of the lake-water, and changes in d 13 C and C, N and d 15 N to track changes in the dissolved inorganic carbon (DIC) pool and the influence of diagenetic effects on sediment organic matter, respectively. Results showed that major shifts towards increased peat surface moisture and TOC concentration of the lake-water occurred around 1980, one to two decades earlier than a temperature driven increase in active layer thickness. Comparison with monitored temperature and precipitation from a nearby climate station indicates that this change in peat surface moisture is related to June-September (JJAS) precipitation and that the increase in lakewater TOC concentration reflects an increase in total annual precipitation. A significant depletion in 13 C of sediment organic matter in the early 1980s probably reflects the effect of a single or a few consecutive years with anomalously high summer precipitation, resulting in elevated DIC content of the lake water, predominantly originating from increased export and subsequent respiration of organic carbon from the mire. Based on these results, it was not possible to link proxy data obtained on peat and lake-sediment records directly to permafrost decay. Instead our data indicate that increased precipitation and anomalously high rainfall during summers had a significant impact on the mire and the adjacent lake ecosystem. We therefore propose that effects of increased precipitation should be considered when evaluating potential forcing mechanisms of recent changes in carbon cycling in the subarctic.

Water-level changes for the last 125 years were reconstructed from two ombrotrophic mires in eastern central Sweden using testate amoebae assemblages. The reconstructed water tables show the same overall pattem with high water tables during the 1950s and 1960s and low water levels from the 1970s until present. The similarity in the two records supports the hypothesis that water-table changes in the ombrotrophic mires of this area are driven by climate change. Correlation of decadal means of reconstructed iV water levels and instrumental meteorological data was performed to examine the relationship between \ water table and climatic variability. The results show that the reconstructed water tables are correlated with changes in mean annual temperature (p < 0.05). This contrasts with similar data for other parts of Europe where correlations have been found with summer or annual precipitation and temperature. We A suggest that low rainfall in this area of Sweden makes the peatlands more susceptible to changing tempera-HOLOCENE ture and that the lack of a response to precipitation is a function of low rainfall variability over the com-RESEARCH parison period. The results show that mire surface wetness responses to climate change are spatially PAPER variable and greater attention should be given to understanding this variability if more accurate palaeoclimatic inferences are to be drawn from longer Holocene records.

Borgmark, A. and Schoning, K. 2005. A comparative study of peat proxies from two eastern central Swedish bogs and their relation to meteorological data.ABSTRACT: Analyses of peat humification, testate amoebae, carbon and nitrogen content have been carried out on a peat sequence from Ä ltabergsmossen. At Gullbergbymossen testate amoebae and peat humification were analysed. Both bogs are located in eastern central Sweden. The longest sequence, Ä ltabergsmossen, covers the last 150 yr and the data from Gullbergbymossen covers the last 60 yr, both with a time resolution of ca. 2-4 yr cm À1 . The different proxies were compared to each other and were also compared to instrumental meteorological data. Correlation between peat humification and C/N ratios is high whereas the correlation is low between these physical/chemical parameters and reconstructed water tables inferred from the testate amoebae assemblages. High peat humification values and C/N ratios greater than 50 indicate aerobic decay and are thought to reflect the thickness of the acrotelm. High humification values and low C/N ratios are recorded in peat deposited between 1965 and 1980, whereas the dry period starts in the early 1970s according to instrumental meteorological data and inferred water-table depth. The difference in the timing of the onset of a dry-shift between the physical/chemical proxies and meteorological data and testate amoebae derived water table changes is interpreted as renewed decay of already deposited peat. The term secondary decomposition is used for this process. The secondary decomposition process has implications for interpreting physical/chemical and biological parameters in peat as they may be out of phase during the beginning of a dry-shift.