Digital images analysis of macroscopic charcoal particles from lake and peat sediments for palaeogeographic reconstruction

The analysis of macroscopic charcoal particles in sediments of different genesis is one of the most common approaches to reconstruct the past fire regimes. The method requires a great deal of time and effort on the part of the researcher. It implies continuous sampling of the sediment core and counting of all charcoal particles with linear dimensions greater than 125 µm in a sample of fixed volume. The purpose of this paper is to present an automatic method that we have developed for the calculation of macroscopic charcoal particles using image analysis. This method is easily reproducible, not technologically demanding, and fast. It allows us to obtain additional palaeoecological information based on the study of geometric characteristics and particle area. A comparison of the results obtained by a standard manual count of the charcoal particles in the test samples and the number of particles determined from the image showed that the method was accurate enough for palaeogeographic reconstructions: Spearman correlation coefficient R = 0.85, R2 = 0.71, MAPE = 31.58% (the mean absolute percentage error), determined particle area comparison revealed R = 0.99, R2 = 0.98, MAPE = 21.45%. The results of macroscopic charcoal analysis of the peat core from Pobochnoye peatland (Buzuluksky Bor National Park, Orenburg region) are presented to demonstrate the capabilities of the developed method. One thousand samples collected from 10 m of peat sediments accumulated over 11.4 ka years were analyzed, and 6,000 images were processed. The results of the analysis include determined charcoal accumulation rates, fire episodes and inter-fire intervals, as well as classification of charcoal particles into grass and wood morphotypes. The variation in charcoal particle size was also estimated for each fire episode, providing additional palaeoecological information about Holocene fires.

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