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Estimation of carbon balance in mires

This result is concerned with a quantitative and qualitative estimate of carbon balance in Baltic mires over the latter half of the Holocene.

Peatlands are stores of carbon dioxide bound from the atmosphere by plants. They are also sites of anaerobic methanogenesis and aerobic methanotrophy. As hydrology has regulated both the rates of peat accumulation and net release of methane (CH4), peatlands have been sensitive to climatic excursions, and their climatic feedback may have favoured either cooling or warming of the atmosphere during the Holocene. The aim of this part of the ACCROTELM project was to reconstruct and evaluate the climatic feedbacks of a bog, as revealed by a high-resolution multi-proxy time series over the past four millennia.

A core for multiproxy analysis was taken from the centre of Männikjärve Bog, Central Estonia. The 448 cm long core was analysed at every 4 cm for testate amoebae and at every 5 cm for plant macrofossils. The chronology of the peat layers was obtained using wiggle matching of 40 AMS-dates of macrofossils over the depth range. Fossil testate communities were used, together with modern samples over a range of microsites, in order to reconstruct the water tables during the past c. 4300 years.

Aerial digital images of the present bog surface were classified in GIS to identify pools, hollows, lawns, and hummocks. Emission of CH4 through the bog surface was estimated using microsite-specific flux rates as controlled by the water table. The palaeo-CH4 emissions at each time step were simulated by letting the microsites be wetter or drier according to the reconstructed water tables. The microsite development history was constructed by running the GIS model backwards in time, starting from the present-time microsite snapshot. A set of rules was defined in the model to constrain how the pools, hollows, and lawns follow the water table dynamics. The initial results of these analyses were presented in a poster at the HOLIVAR 2006 meeting.

Dynamics of WT do not always result in a similar response of the atmospheric intercation in different bogs, as viewed by means of GWP. Dry shifts leaded towards net cooling with lower CH4 emissions, but the two bogs showed different sensitivity to changes in water table. A warming shift in GWP at the Finnish site, Kontolanrahka bog ca. 2700 BP, is probably associated with evidence of a fire. Thus, GWP derived from a combination of palaeo-wetness index and peat bulk density profile seems a reasonable indicator of bog-atmosphere interaction.

The average annual C accumulation rate is a result of C flows in new litter deposition minus the release of C in gaseous form as CO2 and CH4, and in dissolved organic matter DOC leaching out of the mire during a year. The ACCROTELM peat cores, dated with great resolution, show continuous net C accumulation with varying apparent annual accumulation rates throughout the cores, with an apparent increase in accumulation rate close below the peat surface. Recent gas exchange studies have indicated annual net C losses in mires over the northern hemisphere. In spite of these seemingly contrasting results, there is actually no disagreement between the two results, since they only deal with different temporal scales. While annual mass losses can be estimated by integrating the gaseous and dissolved matter fluxes showing a negative balance when losses exceed the input, geological estimates based on dated peat cores integrate peat accumulation over much longer periods of time. They also cannot show the matter that has been lost. However accurately the peat layers are dated, only positive present storage situations are visible in layers between two consecutive dates.

According to the preliminary investigation, bogs have mostly acted in favour of atmospheric cooling. GWP balance has shown to be sensitive to both changes in hydrology or other allogenic factors affecting the bog. Further research and comparison with other proxies made available by the ACCROTELM collaboration lets us evaluate the detailed responses during climatic excursions.

An animation of the peat accumulation of another ACCROTELM peat site - that of Kontolanrahka Bog, in Finland, has been produced, and part of this has been incorporated in a DVD of the ACCROTELM project (see Dissemination products from the project).

The 3-D modelling of the Baltic bogs and the estimation of carbon balance and calculation of Global Warming Potential (GWP) is to be disseminated in peer-reviewed papers based on the work carried out in ACCROTELM.

These overall results of the project may be of interest to and use by climate modellers.

Reported by

The Finnish Forestry Research Institute
University of Joensuu
80101 Joensuu
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