Understanding Holocene peat accumulation pattern of continental fens in western Canada

Assessing carbon sink–source relationships in peatlands must be based on the understanding of processes responsible for long-term carbon accumulation patterns. In contrast with ombrogeneous bogs, however, the processes in geogeneous fens are poorly understood. Here, we present high-resolution Holocene peat accumulation and macrofossil data from a rich fen (Upper Pinto Fen (UPF)) in west-central Alberta, Canada. The ~8000-year chronology of a 397 cm peat core was controlled by 20 accelerator mass spectrometry 14 C dates. The paludified peatland initially consisted of diverse brown moss species and some Larix trees but was dominated by Scorpidium scorpioides from 6500 to 1300 calibrated years BP. The last 1300 years are characterized by the reappearance of Larix together with abundant woody materials and Cyperaceae, culminating in a sharp increase in Tomenthypnum nitens in the last several decades. During the Scorpidium-dominated period, the peat accumulation pattern derived from 15 14 C dates and 260 bulk density measurements indicates declining mass accumulation rates over time (i.e., convex age–depth curve), in contrast with the standard bog growth model (i.e., concave curve). The analysis of the UPF data using an extended model incorporating variable peat addition rates (PAR) to the catotelm suggests a unidirectional sevenfold decrease in PAR from 191.8 to 26.0 g dry mass·m –2 ·year –1 during the ~5000-year "convex period". Decreasing vegetation production and (or) increasing acrotelm decomposition could have produced the convex pattern. Decreasing PAR might be owing to autogenically induced changes in local hydrology and nutrient availability, which are pronounced in the moisture-limited climate of the region and in peatlands that have a strong groundwater influence. The convex-pattern model, explicit to the height-induced long-term drying hypothesis, has important implications for building simulation models and for projecting future carbon dynamics of peatlands. Prior to recent human disturbance, the UPF site has a time-weighted mean carbon accumulation rate of 31.1 g C·m –2 ·year –1 , ranging from 7.2 to 182.5 g C·m –2 ·year –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.Key words: carbon dynamics, moisture and nutrient availability, macrofossils, peatland model, brown moss Scorpidium scorpioides.

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