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Stable Carbon Isotopes As Indicators for Environmental Change in Palsa Peats

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Stable Carbon Isotopes As Indicators for Environmental Change in Palsa Peats Biogeosciences, 8, 1769–1778, 2011 www.biogeosciences.net/8/1769/2011/ Biogeosciences doi:10.5194/bg-8-1769-2011 © Author(s) 2011. CC Attribution 3.0 License. Stable carbon isotopes as indicators for environmental change in palsa peats C. Alewell1, R. Giesler2, J. Klaminder2, J. Leifeld3, and M. Rollog1 1Institute of Environmental Geosciences, University of Basel, Switzerland 2Climate Impacts Research Centre, Dept. of Ecology and Environmental Science, Umea˚ University, Abisko, Sweden 3Agroscope Reckenholz-Tanikon¨ Research Station ART, Switzerland Received: 27 December 2010 – Published in Biogeosciences Discuss.: 19 January 2011 Revised: 20 June 2011 – Accepted: 20 June 2011 – Published: 8 July 2011 Abstract. Palsa peats are unique northern ecosystems degradation at a peat depth between 4 and 25 cm. The age of formed under an arctic climate and characterized by a high these turning points was 14C dated between 150 and 670 yr biodiversity and sensitive ecology. The stability of the pal- and could thus not be caused by anthropogenically induced sas are seriously threatened by climate warming which will climate change. We found the uplifting of the hummocks change the permafrost dynamic and induce a degradation of due to permafrost heave the most likely explanation for our the mires. findings. We thus concluded that differences in carbon iso- We used stable carbon isotope depth profiles in two palsa tope profiles of the hollows might point to the disturbance mires of Northern Sweden to track environmental change of the mires due to climate warming or due to differences in during the formation of the mires. Soils dominated by aer- hydrology. The characteristic profiles of the hummocks are obic degradation can be expected to have a clear increase indicators for micro-geomorphic change during permafrost of carbon isotopes (δ13C) with depth, due to preferential re- up heaving. lease of 12C during aerobic mineralization. In soils with sup- pressed degradation due to anoxic conditions, stable carbon isotope depth profiles are either more or less uniform indi- 1 Introduction cating no or very low degradation or depth profiles turn to lighter values due to an enrichment of recalcitrant organic Global climate change is significantly threatening stability substances during anaerobic mineralisation which are de- and functioning of permafrost soils in extended areas of the pleted in 13C. northern latitudes and/or at high altitudes (Luoto et al., 2004; The isotope depth profile of the peat in the water saturated Brown and Romanowsky, 2008). A thawing of permafrost depressions (hollows) at the yet undisturbed mire Storflaket soils will most likely result in a positive feedback mechanism indicated very low to no degradation but increased rates of due to accelerated degradation of soil organic matter (Schuur anaerobic degradation at the Stordalen site. The latter might et al., 2009; Dorrepaal et al., 2009). Furthermore, biodiver- be induced by degradation of the permafrost cores in the up- sity and functioning of these unique ecosystems are under lifted areas (hummocks) and subsequent breaking and sub- immediate threat (Luoto et al., 2004). One very unique north- merging of the hummock peat into the hollows due to cli- ern ecosystem type are palsa peats, also called palsa mires. mate warming. Carbon isotope depth profiles of hummocks Palsa mires are a type of peat land typified by characteristic indicated a turn from aerobic mineralisation to anaerobic high mounds (called palsa or palsa hummocks), each with a permanently frozen core. Cryoturbation induces formation of the hummocks, where the volumetric expansion follow- Correspondence to: C. Alewell ing freezing of the underlying horizons uplift the peat out ([email protected]) of the groundwater saturated zone (Vasil’chuk et al., 2002; Published by Copernicus Publications on behalf of the European Geosciences Union. 1770 C. Alewell et al.: Stable carbon isotopes as indicators for environmental change 2003). Between the hummocks are wet depressions (called showed that δ13C of a 7 m deep Scottish bog was rather uni- hollows), where permafrost is less extensive or absent. Palsa form because opposite fractionation effects of CO2 and CH4 mires are common in high-latitude areas across the north- formation resulted in similar δ13C signatures of degradation ern hemisphere including the northern parts of Scandinavia product and sources (relative enrichment and depletion rela- and characterized by a unique geochemistry and biodiversity tive to source material, respectively). Thus, anaerobic decay (Railton and Sparling, 1973; Masing et al., 2010). Hydrol- with methane production, which requires low redox potential ogy and vegetation composition as well as degradation and under anaerobic conditions (e.g., acetate fermentation) might mineralisation patterns will change in these sensitive ecosys- also result in uniform δ13C depth profiles. tems because they are currently exposed to climate change (Akerman˚ and Johansson, 2008; Lemke et al., 2007). The 2.2 A δ13C depth trend towards slightly lower values latter should be reflected in stable carbon isotope depth pat- terns of the peat horizons. Trends in carbon isotope depth profiles towards slightly The depth distribution of stable carbon isotopes (12C and lower values are common for soils that are constantly wa- 13C) reflects the combined effects of plant fractionation pro- terlogged such as peat-producing histosols (Fig. 1b; Krull cesses and microbial decomposition (Krull and Retallack, and Retallack, 2000) but have significant anaerobic degra- dation. The slight decrease in δ13C is due to preservation 2000). The fractionation within the plant towards slowly 13 decomposing substances depleted in 13C and more easily of slowly decomposing C depleted substances like lignin degradable material relatively enriched in 13C leads to a de- (Benner et al., 1987). Since Sphagnum species have pheno- pletion of 13C in the remaining, recalcitrant organic matter lic compounds very similar to lignin (Nimz and Tutschek, in the soil litter (Agren˚ et al., 1996). In contrast, the pref- 1977; Rasmussen et al., 1995; Farmer and Morrison, 1964), 12 a similar fractionation pattern can be expected in sphagnum erential respiration of C from decomposers may lead to 13 an enrichment of 13C in the remaining soil organic matter peats. Thus, if we see a depth trend towards lower δ C val- (Agren˚ et al., 1996; Nadelhoffer and Fry, 1988). The balance ues this indicates an environment, where the enrichment of between the latter two mechanisms will shape the carbon iso- recalcitrant material dominates the isotopic profile. tope depth profiles in soils, which then reflects the dominat- 2.3 Pronounced δ13C increases with depth of up to 5 ‰ ing fractionation mechanism. The aim of this study was to use stable carbon isotope Pronounced δ13C increases with depth of up to 5 ‰ are typi- depth profiles in northern palsa peat complexes as indica- cal for mature, well drained soils, because aerobic decom- tors of environmental change and/ or soil forming processes position favours selective loss of 12C (Fig. 1c; Nadelhof- in space and time. Our hypothesis was that (i) hummocks fer and Fry, 1988; Beckerheidmann and Scharpenseel, 1989; and hollows should differ significantly in their stable isotope Agren˚ et al., 1996). Clay minerals in deeper soil horizons depth profiles and (ii) that vertical trends reflect hydrological also favour this pattern in preferentially adsorbing the heav- and botanical conditions mainly controlling decomposition ier 13C (Beckerheidmann and Scharpenseel, 1986), but the processes at the time when the peat was deposited. latter affect should be negligible in the peats we investigate in this study. 2 Theoretical concepts to interpret δ13 C depth profiles in soils 3 Site description ◦ 0 00 ◦ 0 00 Isotopic depth profiles in soils, which are not influences The two studied sites, Storflaket (68 20 51 N, 18 15 55 E) ◦ 0 00 ◦ 0 00 by a change from C3 to C4 plants or by major changes in and the eastern Stordalen mire (68 20 90 N, 18 58 57 E) species composition have been reported as three different are situated about 3 km apart within a large palsa peat com- trends (Fig. 1): plex in the Abisko valley, northern Sweden. Storflaket is characterized by a stable palsa plateau with a fairly homoge- 2.1 Uniform or slightly increasing depth trend in nous thickness of about 0.5 m. The eastern Stordalen site is the δ13C a partly degraded palsa system having an average peat depth around 0.5 m, but with large local variations. The thickness A uniform or only slightly increasing depth trend in the car- of the active layer, i.e. the seasonally thawing zone is in late bon isotopic signature of bulk soils can be found in rela- September typically about 0.5 m in the hummocks and be- tively young and/or poorly drained soils with little time for tween 1 and 3 m in the hollows in both mires. soil formation, and/or limited decomposition and thus lim- Permafrost heave drives the topography of the mires; ited fractionation (Fig. 1a). Several studies found uniform higher uplifted palsas hummocks are typically situated be- depth trends in water saturated peats with little or no fraction- tween 1 to 3 m above the surrounding, less uplifted wetter ar- ation of δ13C (Kracht and Gleixner, 2000; Clymo and Bryant, eas. Hummocks are mainly dominated by nutrient poor vege- 2008; Skrzypek et al., 2008). Clymo and Bryant (2008) tation such as dwarf shrubs (Empetrum hemafroditum, Betula Biogeosciences, 8, 1769–1778, 2011 www.biogeosciences.net/8/1769/2011/ C. Alewell et al.: Stable carbon isotopes as indicators for environmental change 1771 -30 δ13C -20 -30 δ13C -20 -30 δ13C -20 0 0 0 a) b) c) depth (cm) depth (cm) depth (cm) type 1: type 2: type 3: young and/or poorly constantly water logged: mature, well drained soils anaerobic decomposition drained soils Fig. 1. Theoretical concept of isotope depth profiles in soils under regimes of differing metabolisms due to differences in water saturation and/or age.
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