UNDERSTANDING PEATLAND CARBON DYNAMICS AND SENSITIVITY TO HOLOCENE CLIMATE VARIABILITY: A FEN PEAT RECORD FROM CONTINENTAL WESTERN CANADA

Assessing carbon sink-source relationships in peatlands must be based on the understanding of processes responsible for long-term carbon-accumulation patterns. Here we investigate the possible connection of Holocene climate variability and peatland carbon dynamics. A high-resolution fen peat record and 79 basal peat dates from paludified peatlands in continental western Canada provide evidence for cyclic change in moisture conditions and in peat carbon accumulation. The ash-free bulk density, a proxy for degree of peat decomposition and thus moisture conditions, shows periodicities at both millennial (from 1,500 to 2,190 yr, with a mean of 1,785 yr) and century scales (386 yr, and 667 yr). Wet periods of 200-600 yr in duration, especially at ~6,900, 5,500, and 4,000 cal BP, correlate with rapid peat accumulation, new peatland initiation, and declines in the rate of increase of atmospheric CO₂ concentrations. These results indicate a strong connection between climate and the global carbon cycle at the millennial scale, mediated in part by peatland dynamics. That global atmospheric CO₂ concentrations have in the past responded to these changes in peatland dynamics implies a strong potential for peatlands to be a major player in affecting future global change.

We also analyze the convex peat mass-age pattern of continental fens using an extended model incorporating variable peat-addition rates to the catotelm. Sensitivity analysis suggests that decreasing vegetation production and/or increasing acrotelm decomposition, due to peatland surface drying, could have produced the convex pattern. Prior to recent human disturbance, the fen has a time-weighted mean carbon accumulation rate of 31.1 g C m^-2 yr^-1, ranging from 7.2 to 182.5 g C m^-2 yr^-1 during the last 8000 years. This large variation results from the gradual decline of long-term accumulation and short-term climate-induced accumulation “pulses”. The results imply that in the absence of climatic change peatlands with a convex accumulation pattern will reach their growth limit sooner and that their carbon sequestration capacity will decline faster than would be expected given the concave-pattern model.