Towards reconstruction of past fire regimes from geochemical analysis of charcoal

Abstract

Production of charcoal has accompanied human life from the beginning. We aimed at evaluating the degree to which the chemical signatures of charcoal may serve as a fingerprint for burning conditions. After a compilation of fire literature we differentiated three typical fire regimes [grass and forest ground (285 ± 143°C), shrub (503 ± 211°C) and domestic fires (797 ± 165°C)] and three main factors impacting on charcoal formation: charring duration, temperature and fuel. For fingerprint calibration and validation, typical fuels of prehistoric burning events (wood and grass) were charred under laboratory conditions (300-700°C; varying duration) and compared with residues from natural fires in SE Europe. Analysis comprised assessment of benzene polycarboxylic acids (BPCAs), organic carbon (Corg) content, nitrogen content, oxygen index (OI: CO2/Corg) and hydrogen index (HI: HC/Corg), temperature of maximum heating (Tmax) and mid-infrared spectroscopy (MIRS). All parameters including mass loss increased with increasing combustion temperature, but were unaffected by charring duration. Grass charcoal had consistently lower Corg content and HI than wood, but values showed a bias towards the natural charcoals, probably because the latter contained higher amounts of mineral matter or were combusted under greater O2 supply. Nevertheless, natural charcoals could be differentiated into forest ground fires (B5CA/B6CA 1.3-1.9; OI >20; intense CH2 stretching, Tmax <488°C) and grass fires (B5CA/B6CA 0.8-1.4; OI >20; weak CH2 stretching, Tmax <425°C), whereas domestic fires revealed B5CA/B6CA values <0.8, OI values <20 and little MIRS absorbance. In summary, it appears possible to reconstruct fire regimes from the temperature sensitivity of BPCA patterns, Tmax, OI and aromatic and aliphatic MIRS signals, whereas assignment of fuel source was less reliable. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Charcoal in Organic Horizon and Surface Mineral Soil in a Boreal Forest Fire Chronosequence of Western Quebec: Stocks, Depth Distribution, Chemical Properties and a Synthesis of Related Studies

“…Two of the series (Baldock Overlaying our total C concentrations onto Figure 5A shows that most of the values fall between 490 and 660 g kg −1 , corresponding to laboratory chars produced at <300 • C. Most of the laboratory chars were lower in ash, and the bulk of our ash-free C values fall in with those produced around 350-450 • C ( Figure 5A). These are within the temperature ranges suggested from attempts to develop a "molecular thermometer" based on the ratio of BPCA products with 5 vs. 6 carboxyl groups (B5CA/B6CA; Wolf et al, 2013). Wolf et al (2013) classified three typical fire temperature regimes: grass and forest groundfires at 285 ± 143 • C, shrubland fires at 503 ± 211 • C, and domestic fires at 797 ± 165 • C. However, other field observations, especially for crown fires, show that substrates may experience much higher temperatures, albeit briefly.…”

“…These are within the temperature ranges suggested from attempts to develop a "molecular thermometer" based on the ratio of BPCA products with 5 vs. 6 carboxyl groups (B5CA/B6CA; Wolf et al, 2013). Wolf et al (2013) classified three typical fire temperature regimes: grass and forest groundfires at 285 ± 143 • C, shrubland fires at 503 ± 211 • C, and domestic fires at 797 ± 165 • C. However, other field observations, especially for crown fires, show that substrates may experience much higher temperatures, albeit briefly. In an experimental crown fire near Fort Providence NWT, Canada, dataloggers showed a pattern of a very steep initial temperature rise followed by a slower rate of cooling, which was very difficult to reproduce in a subsequent laboratory charring experiment (Santín et al, 2013).…”

Production and Composition of Pyrogenic Dissolved Organic Matter From a Logical Series of Laboratory-Generated Chars

“…Because the condensed aromatic property of pyOM has often been used to quantify pyC, we coin the term "ConAC" (condensed aromatic carbon) to refer only to this portion rather than the chemically ambiguous term "black carbon." In biochars and charcoals, the proportion of ConAC varies with biomass type and charring conditions, but it generally increases with the temperature and duration of pyrolysis (Schneider et al, 2010(Schneider et al, , 2013McBeath et al, 2011;Wolf et al, 2013).…”

The properties and functions of biochars in forest ecosystems

“…For example, Wolf et al (2013) measured the degree to which the chemical signatures provided a fingerprint to predict fire temperature using biochar properties including benzene polycarboxylic acids (BPCAs), organic C content, and nitrogen content. Thus, burning and raw materials during fire are the two major factors that determine biochar properties.…”