Stable carbon isotopes as indicators for environmental change in palsa peats

Abstract

Abstract. Palsa peats are unique northern ecosystems formed under an arctic climate and characterized by a high biodiversity and sensitive ecology. The stability of the palsas are seriously threatened by climate warming which will change the permafrost dynamic and induce a degradation of the mires.We used stable carbon isotope depth profiles in two palsa mires of Northern Sweden to track environmental change during the formation of the mires. Soils dominated by aerobic degradation can be expected to have a clear increase of carbon isotopes (δ 13 C) with depth, due to preferential release of 12 C during aerobic mineralization. In soils with suppressed degradation due to anoxic conditions, stable carbon isotope depth profiles are either more or less uniform indicating no or very low degradation or depth profiles turn to lighter values due to an enrichment of recalcitrant organic substances during anaerobic mineralisation which are depleted in 13 C.The isotope depth profile of the peat in the water saturated depressions (hollows) at the yet undisturbed mire Storflaket indicated very low to no degradation but increased rates of anaerobic degradation at the Stordalen site. The latter might be induced by degradation of the permafrost cores in the uplifted areas (hummocks) and subsequent breaking and submerging of the hummock peat into the hollows due to climate warming. Carbon isotope depth profiles of hummocks indicated a turn from aerobic mineralisation to anaerobicCorrespondence to: C. Alewell (christine.alewell@unibas.ch) degradation at a peat depth between 4 and 25 cm. The age of these turning points was 14 C dated between 150 and 670 yr and could thus not be caused by anthropogenically induced climate change. We found the uplifting of the hummocks due to permafrost heave the most likely explanation for our findings. We thus concluded that differences in carbon isotope profiles of the hollows might point to the disturbance of the mires due to climate warming or due to differences in hydrology. The characteristic profiles of the hummocks are indicators for micro-geomorphic change during permafrost up heaving.

Autotrophic fixation of geogenic CO<sub>2</sub> by microorganisms contributes to soil organic matter formation and alters isotope signatures in a wetland mofette

“…Microorganisms using the CBB cycle would add 13 C-depleted carbon to SOM. Indeed, δ 13 C profiles of both reference soils do not show shifts towards more positive values with depth, as is usually observed from other Gleysols, although radiocarbon data indicate that SOM becomes older with depth (Alewell et al, 2011;Bol et al, 1999). Further, both reference soils have C / N ratios close to 10 throughout the soil profile, which normally indicates a higher contribution of microbial C to SOM (Rumpel and Kogel-Knabner, 2011).…”

Microbial dynamics in a thawing world: Microbial ecology of a permafrost active layer

“…These peats, while being lower in recalcitrant lignin (bryophytes lack vascular tissue), are highly acidic, contain phenolics and are oligotrophic; thus the degradation of this more labile C is also slowed (Kleber, 2010;Alewell et al, 2011). Degradation of peat at such sites occurs mostly during the thaw season and is mediated by bryophyte associated microbes (Lindo and Gonzalez, 2010;Belova et al, 2011).…”

“…The dry elevated sections of the palsa mire (palsa hummocks) exhibit tundra type vegetation ( Figure I-12) ranging from tussock tundra (Eriophorum vaginatum dominated), ericaceous tundra (Betula nana, Empetrum spp., Rubus chamaemorus dominated) through to bare tundra which is lichen dominated (Alewell et al, 2011;Malmer et al, 2005;Malmer and Wallen, 1999). The area of the Mire covered by palsa has decreased in the last 40 years by at least 10% (T. .…”

“…The 'palsa' auto-chamber site vegetation is mostly ericaceous tundra; the pH of the peat is ~ 4. This site consistently falls within classifications of hummock, sometimes with a qualifier such as 'dry' or 'bog', 'hummock and tall shrub' and 'ombrotrophic hummock' Svensson and Thomas Rosswall, 1984;Rydén et al, 1980;Moore, 1987;Sonesson et al, 1980;Kvillner and Sonesson, 1980;Alewell et al, 2011;Malmer et al, 2005;Malmer and Wallen, 1999;Kellogg and Bridgham, 2011;Kokfelt et al, 2009). Cracks at the edge and vegetation succession suggest this Palsa has passed the aggradation stage and is now degrading.…”

“…site', 'hollow' and 'carpet' which refers to the Sphagnum spp. carpet Malmer et al, 2005;Svensson and Thomas Rosswall, 1984;Malmer and Wallen, 1999;Kokfelt et al, 2009;Alewell et al, 2011;Kvillner and Sonesson, 1980). The bog can be classified as a high C fixing site with lowest warming potential or radiative forcing.…”