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Stratigraphy of peatlands in central and northern Sweden: evidence of Holocene climatic change and peat accumulation

Abstract Early peat surveys demonstrated the potential to detect Holocene climatic changes in southern Sweden through analysis of the degree of decomposition of Sphagnum peat and motivated systematic collection of peat-stratigraphic information during subsequent geological mappings in central and northern Sweden. The resulting (published and unpublished) data was compiled and reanalysed, taking into account unpublished radiocarbon data and developments in radiocarbon calibration and pollen-stratigraphic dating. Data from 61 carefully selected sites indicate a peat initiation maximum centred in the north 9500–8000 cal. BP and interpreted to primarily reflect climatic change. The cause for a secondary maximum 6000–5500 cal. BP centred further south is less clear. Peat deposition data show a continuous increase in the period 10 500–2000 cal. BP, with almost constant values during recent millennia, indicating that peatlands in the region acted as a long-term Holocene carbon sink and increasingly so until c. 2000 years ago. Sphagnum peat decomposition data display a long-term trend towards wetter conditions after 5000 cal. BP interpreted to primarily reflect progressively stronger westerly airflow and an accompanying increase in precipitation. These data also show that wet shifts occurred more frequently across the region around 3300, 2500–2100 and 1300 cal. BP. The timing of peak frequencies of wet shifts is similar to that of supposedly climate-related wet shifts identified in southern Sweden. Variability in frequency of wet shifts is most strongly pronounced in the south-west, with an apparent periodicity of 800–1000 years believed to reflect regular changes in westerly airflow during the mid and late Holocene.