Reconstructions of carbon sequestration in peatlands are often obtained from single and long records sampled in central sections of the ecosystem. As peatland margins are often hydrologically and botanically distinct and lateral expansion rates may be related to local rather than external factors, reconstructions from a single core may poorly represent accumulation patterns at the ecosystem scale.
We used multiple cores to reconstruct spatial and temporal carbon sequestration in three pristine, ombrotrophic peatlands (1.7-2.7 km²) in the Eastmain region of boreal Quebec, Canada. Peatland depth was modelled by kriging after extensive manual probing and ground-penetrating radar analyses. Peat carbon density was quantified after bulk density and loss-on-ignition analyses.
At the ecosystem scale, Holocene carbon accumulation rates averaged 16.2 g m^-2 yr^-1, with a mean age of peat inception of 5.1 ka BP. Site-specific age-depth modelling shows a long-term decrease in vertical peat accumulation after initial peat inception between 7.5-7.0 ka BP for each peatland, while lateral expansion rates culminated in the earliest stages of bog history between 7.0-6.0 ka BP. Except for a shift at the acrotelm-catotelm boundary, carbon density did not show a consistent increasing trend towards older peat. Combining vertical peat accumulation, lateral expansion and carbon density data, the most important period of carbon sequestration was centered between 5.0-3.0 ka BP.
These results show that reconstructions of both vertical accumulation and lateral expansion patterns are essential to accurately quantify carbon sequestration at the ecosystem scale and that reconstructions based on vertical rates only may provide erroneous interpretations. Moreover, as changes in expansion rates have previously been linked to local variability in basin morphology rather than climate factors, this implies that past sequestration might have been controlled to a greater extent by local, geomorphological conditions than previously thought.