Mires and Peat, Volume 10 (2012), Article 01, 1–25, http://www.mires-and-peat.net/, ISSN 1819-754X © 2012 International Mire Conservation Group and International Peat Society 1 Microscopic charcoal and tar (CHAT) particles in peat: a 6500-year record of palaeo-fires in southern Sweden L.G. Franzén 1 and B.A. Malmgren 2 1 Physical Geography, Department of Earth Sciences, University of Gothenburg, Sweden 2 Department of Palaeobiology, Uppsala University, Sweden _______________________________________________________________________________________ SUMMARY 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 14 C 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 14 C BP signify a transition from local to regional fires. With a few exceptions, the uppermost weakly decomposed ombrotrophic peats formed after ca. 2500 cal 14 C 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. KEY WORDS: charcoal and tar particle classification; fire frequency; late and middle Holocene _______________________________________________________________________________________ INTRODUCTION Information on fire history is provided by two types of proxy data, namely fire scar records dated by dendrochronology and charcoal records from lake sediments and peat deposits. Whereas tree-ring information provides temporally precise short-term reconstructions of fire events, usually spanning the last 400 years or less (NOAA 2011), charcoal and tar (CHAT) particle records from sediments and peat can be used to reconstruct much longer fire histories, although with lower temporal and spatial precision (e.g. Olsson et al. 2010). The occurrence of CHAT in peat may have different causes. In the first place, the production of such particles is related to the characteristics of vegetation and type of fire, including the supply and quality of fuel and the efficiency of combustion. The fires could have natural causes, such as volcanism (e.g. Scott & Jones 1996, Wilmshurst & McGlone 1996, Samaniego et al. 1998) or lightning (Scott & Jones 1996, McCafferty & Owen 1996); or human causes, i.e. slash-and-burn clearances and regular large scale burn-beatings. The destiny of small particles produced by these fires is largely dependent on weather conditions. In the case of a strong and rapid fire lapse, the smallest particles carried up into the atmosphere at high vertical velocities by thermal uplift will simply fall to the ground locally if side winds are weak or absent. With stronger winds, however, these particles can be transported over vast distances before they are deposited under the influence of gravity or turbulent air flows (Whitlock & Larsen 2001). Deposition can be dry or wet, i.e. the microscopic carbonaceous particles can either drizzle down as dust or act as condensation nuclei in aggregates with sulphate (Posfai et al. 1997, Buseck & Posfai 1999) and fall with rain and snow. The CHAT particles found in peat are either autochthonous particles formed by fires in the peatland, e.g. macroscopic or microscopic pieces of charred wood, herbs and mosses; or allochthonous microscopic particles brought in by wind and/or