Degradation changes stable carbon isotope depth profiles in palsa peatlands

J Leifeld,J P Krüger,C Alewell

doi:10.5194/bg-11-3369-2014

Abstract

Abstract. Palsa peatlands are a significant carbon pool in the global carbon cycle and are projected to change by global warming due to accelerated permafrost thaw. Our aim was to use stable carbon isotopes as indicators of palsa degradation. Depth profiles of stable carbon isotopes generally reflect organic matter dynamics in soils with an increase of δ13C values during aerobic decomposition and stable or decreasing δ13C values with depth during anaerobic decomposition. Stable carbon isotope depth profiles of undisturbed and degraded sites of hummocks as well as hollows at three palsa peatlands in northern Sweden were used to investigate the degradation processes. The depth patterns of stable isotopes clearly differ between intact and degraded hummocks at all sites. Erosion and cryoturbation at the degraded sites significantly changes the stable carbon isotope depth profiles. At the intact hummocks the uplifting of peat material by permafrost is indicated by a turning in the δ13C depth trend, and this assessment is supported by a change in the C / N ratios. For hollows isotope patterns were less clear, but some hollows and degraded hollows in the palsa peatlands show differences in their stable carbon isotope depth profiles indicating enhanced degradation rates. We conclude that the degradation of palsa peatlands by accelerated permafrost thawing can be identified with stable carbon isotope depth profiles. At intact hummocks δ13C depth patterns display the uplifting of peat material by a change in peat decomposition processes.

Highlights

Peatlands cover only 3 % of the global land surface, but they are an important component in the global carbon (C) cycle (Joosten and Clarke, 2002; Yu et al, 2011)
In this study we focused on the elevated, dry hummock and on the wet hollow parts of the Stordalen peatland
In the studied palsa peatlands, depth profiles of stable carbon isotopes show typical patterns related to their metabolism and degree of degradation

Summary

Introduction

Peatlands cover only 3 % of the global land surface, but they are an important component in the global carbon (C) cycle (Joosten and Clarke, 2002; Yu et al, 2011). Peatlands in the northern permafrost zone, where palsa mires are widespread, have accumulated more than 270 Pg carbon in their soils (Tarnocai et al, 2009). The existence of palsa mires is linked to climate conditions in the discontinuous permafrost region with low mean annual temperature, low annual precipitation and/or strong winds (Luoto and Seppälä, 2003; Luoto et al, 2004). On wind exposed sites with a thin or even lacking snow cover, a frozen core is built up (Luoto and Seppälä, 2002). The characteristics of palsa mires are mounds and plateaus called hummocks, which have been raised by the frozen core and lost connection to the groundwater. With increasing active layer depth (annual thawing soil layer), their hummocks lose stability and start to collapse at the edges by block erosion and subsidence (de Jong et al, 2010) and could create thermokarst ponds (Luoto and Seppälä, 2003)

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