Chemostratigraphy and vegetation in two boreal mires during the Holocene

Abstract

Two peatlands, a fen and a bog in southern Finland, were studied for peat and carbon accumulation rates, chemical and physical properties of the peat profile, and past vegetation, to assess what stage of succession these peatlands were at the time of human impacts, drainage and restoration. Paludification started at both sites soon after the glacial retreat around 9000 radiocarbon years BP. Peat accumulated at the fen site at an average rate of 0.28mm yr-1 (range 0.16-0.58mm yr-) during development. The peat was composed mostly of Carex with remains of Pin,us sylvestris and Betula spp. The bog site had an average accumulation rate of 0.47mm yr(range 0.25-1.19mm yr-1), and a minerotrophic sedge stage of about 2000 years in its early development, after which the peat was composed mostly of Sphagna. The long-term rates of carbon accumulation (LORCA) varied from 6.6 to 15.3 g m-2 yr '(average 1l.1 g m-2 yr-') for the fen and from 6.1 to 21.1 g m-2 yr-(average 13.2g m-2 yr-') for the bog. The main trend in peat chemistry of both mires was the change from higher to lower nutrient levels with an increased acidity towards the surface peat. The main changes in peat chemistry occurred between 4600 and 4200 BP, simultaneously with the spread of Picea abies in the area. This period was preceded by a large-scale climatic change at the boundary between Atlantic and Subboreal chronozones. Drainage during the twentieth century caused compaction in the uppermost peat of the fen, whereas evidence of drainage was hardly noticeable in the bog stratigraphy.