Holocene carbon dynamics and climate forcing of two subarctic minerotrophic fens

Historical data on three dimensional peatland development patterns, carbon (C) dynamics and radiative forcing may aid us to understand peatland - climate-related feedback mechanisms. We investigated vertical and horizontal peat growth patterns of two subarctic fens, Lompolonjänkkä (Western Finnish Lapland) and Kiposuo (Eastern Finnish Lapland). We calculated C accumulation rates and, based on these and modern gas measurements, we estimated historical CO2 and CH4 fluxes. Past C budgets and radiative forcing were reconstructed. The C accumulation rates of Lompolojänkkä fen and Kiposuo fen varied between ranges 3 - 47 and 2 - 27 g C m-2 a-1, respectively. Plant macrofossil analyses suggest that fen plant communities have changed only a little throughout the Holocene and accordingly we assume only minor variation in CH4 emissions. However, C uptake rates varied and showed extended periods of low C uptake between 8 and 2 kcal BP in Lompolojänkkä and between 9 and 4 kcal BP in Kiposuo. These periods of low C uptake coincide with warm and dry climate conditions. Lateral expansion of Lompolojänkkä was fast after peatland initiation, which happened through infilling, and during the last two millennia, but expansion was slow between 6 and 3 kcal BP. Kiposuo started to initiate simultaneously over a large area mainly on mineral soil. Subsequent lateral expansion has been negligible. In conclusion the reconstructed C dynamics suggest that the mid Holocene dry and warm conditions slowed down peat growth and C accumulation. This was especially striking in Lompolonjänkkä. Both fens had a positive radiative forcing (RF) effect to the atmosphere after the peat initiation. The RF turned negative after c. 700 years from the initiation in Lompolonjänkkä and after c. 1500 years in Kiposuo. Afterwards the radiative forcing continued to decrease until the present, reaching current radiative forcing of -1.33 x 10-8 W m-2 (Lompolonjänkkä) and -0.77 x 10-8 W m-2 (Kiposuo). Although climate may have instantaneous effect on peatlands' C dynamics, impact on radiative forcing seems to be buffered by long-term effects of previous C uptake.