Microscopic charcoal and tar (CHAT) particles in peat: a 6500-year record of palaeo-fires in southern Sweden

Peat stratigraphies of eleven raised bogs in southern Sweden were investigated. Measurements included the occurrence of charcoal and various tar particles. Most of the particles found were microscopic, i.e. 5–100 μm in diameter. Two distinctly different groups of particles were distinguished: (A) charred fragments of plant tissue and (B) objects formed from tar, which were classified into five sub-groups on the basis of morphology. Both charcoal and tar are indicative of mire and forest fires. We suggest that it is possible to use the different groups of particles as fire regime indicators. Hence, the high frequency of charcoal and tar (CHAT) in the lower parts of the stratigraphies, i.e. in the lower strongly decomposed fen and carr peats that were formed before ca. 4000 cal 14C BP, could be indicative of intense and frequent local fires. The decreasing abundance of CHAT and the lower relative share of Type A particles within the lower strongly decomposed Sphagnum peat ca. 4000–2500 cal 14C BP signify a transition from local to regional fires. With a few exceptions, the uppermost weakly decomposed ombrotrophic peats formed after ca. 2500 cal 14C BP, in which both charcoal and tar are rare, indicate a period of low fire frequency at both local and regional scales. There is no regional variation in the lower material, and it seems that wildfires were common phenomena throughout southern Sweden during the first few thousand years after peat formation began 6–8000 years ago. From a climatological point of view, the mass occurrence of CHAT in the lower parts of the profiles indicates a warm and dry Mid Holocene with frequent and widespread wildfires, and a moist and cool Late Holocene with more sporadic fires. Spectral analysis of the entire dataset shows significant periodicities of 610, 70, 30, 21, 17 and 14 years, the two most significant being 14 and 70 years.