A guide to screening charcoal peaks in macrocharcoal-area records for fire-episode reconstructions

Abstract

Macroscopic charcoal records can be used to infer spatially explicit reconstructions of past fire history. However, a current deficiency in the charcoal-analysis toolbox has been the lack of a method to consider sampling variability and charcoal-particle area distributions for peak detection with charcoal-area records. We present a screening procedure specific for datasets comprising charcoal numbers and areas to screen the charcoal-area estimates with respect to the count sums. The rationale for screening charcoal-area peaks stems from the observation that although charcoal-area records can be more suitable in a statistical sense for peak detection (e.g. as established by the signal-to-noise index), charcoal-area peaks can be questionable if they are determined by just one or a few larger charcoal particles. Our method begins with a charcoal-area time series analysed by existing methods to identify peaks representing fire episodes. To screen these peaks, the method uses bootstrap resampling of charcoal-particle areas observed in a user-defined subsection of the record around each peak to obtain the range of likely charcoal areas for different counts. Peaks with total area within the likely range of bootstrapped samples (e.g. p > 0.05) are flagged as potentially unreliable, whereas samples with total area significantly greater than expected by chance are deemed robust indicators of past fire events. In an example application of the method to a charcoal record from Lake Brazi, Romania, several peaks failed to pass the screening suggesting that, as for count-based records, unscreened charcoal-area records may include spurious fire episodes and thus potentially underestimate past fire-return intervals.

History of fires and vegetation since the Neolithic in the Cantabrian Mountains (Spain)

“…The standard protocol (Carcaillet et al, 2001(Carcaillet et al, , 2007, as adjusted by Pérez-Obiol et al (2016), was followed. KOH was used as a deflocculant solution and 15% NaClH as a bleaching solution (Finsinger, Kelly, Fevre, & Magyari, 2014). Samples were heated to 70°C for 90 min and sieved with a 150-μm mesh.…”

Charcoal and pollen analysis: Examples of Holocene fire dynamics in Mediterranean Iberian Peninsula

“…Y también con el método propuesto por Carcaillet cuando el volumen era uniforme en el conjunto de la muestra. En todos los casos se utilizó hidróxido de potasio (KOH) como la solución defloculante y para eliminar el contenido orgánico, y una solución de blanqueo de hipoclorito de sodio (NaClO) a 15% de concentración (Finsinger et al, 2014). Todas las muestras se calentaron a 70ºC durante 90 minutos y se tamizaron a través de una malla de 150 µm después de enfriarse.…”

“…Los carbones mayores de 0,250 mm2 se han graficado, junto con el número de partículas de carbón vegetal por muestra, con el fin de indicar más claramente los incendios locales , tal y como sugerían otros autores (Whitlock y Larsen, 2001;Finsinger et al, 2014).…”

Holocene treeline and timberline changes in the South Carpathians (Romania): Climatic and anthropogenic drivers on the southern slopes of the Retezat Mountains

“…To determine long-term variations in regional fire activity and local biomass burning, microCHAR and macro-CHAR records were smoothed with a 900-year-bandwidth lowess (Cleveland and Devlin, 1988). Because the sample deposition times (years sample −1 ) for Lake Bucura were too high below 100 cm depth, the record was not analysed to infer local fire episodes using the decomposition approach (Finsinger et al, 2014;Higuera et al, 2009;Kelly et al, 2011).…”