A dynamic model of wetland extent and peat accumulation: results for the Holocene

[摘要] Substantial deposits of peat have accumulated since the last glacial.Since peat accumulation rates are rather low, this process was previouslyneglected in carbon cycle models. For assessments of the global carboncycle on millennial or even longer timescales, though, the carbonstorage in peat cannot be neglected any more. We have therefore developeda dynamic model of wetland extent and peat accumulation in order toassess the influence of peat accumulation on the global carbon cycle.

The model is based on the dynamic global vegetation model LPJ andconsists of a wetland module and routines describing the accumulationand decay of peat. The wetland module, based on the TOPMODEL approach,dynamically determines inundated area and water table, which changedepending on climate. Not all temporarily inundated areas accumulatepeat, though, but peat accumulates in permanently inundated areaswith rather stable water table position. Peatland area therefore ishighly uncertain, and we perform sensitivity experiments to coverthe uncertainty range for peatland extent. The peat module describesoxic and anoxic decomposition of organic matter in the acrotelm, i.e.,the part of the peat column above the permanent water table, as wellas anoxic decomposition in the catotelm, the peat below the summerminimum water table.

We apply the model to the period of the last 8000 years, during whichthe model accumulates 330 PgC as catotelm peat in the peatland areasnorth of 40° N, with an uncertainty range from 240 PgCto 490 PgC. This falls well within the range of published estimatesfor the total peat storage in high northern latitudes, consideringthe fact that these usually cover the total carbon accumulated, notjust the last 8000 years we considered in our model experiments. Inthe model, peat primarily accumulates in Scandinavia and eastern Canada,though eastern Europe and north-western Russia also show substantialaccumulation. Modelled wetland distribution is biased towards Eurasia,where inundated area is overestimated, while it is underestimatedin North America. Latitudinal sums compare favourably to measurements,though, implying that total areas, as well as climatic conditionsin these areas, are captured reasonably, though the exact positionsof peatlands are not modelled well. Since modelling the initiationof peatland growth requires a knowledge of topography below peat deposits,the temporal development of peatlands is not modelled explicitly,therefore overestimating peatland extent during the earlier part ofour experiments.

Overall our results highlight the substantial amounts of carbon takenup by peatlands during the last 8000 years. This uptake would havesubstantial impacts on the global carbon cycle and therefore cannotbe neglected.

[学科分类] 地球化学与岩石