Search

Over the past decades, the abundance and area of macrocharcoal (i.e. ≥ 150 μm in diameter) fragments from sedimentary sequences have been quantified using visual or semi-automated methods to reconstruct fire histories. However, the lack of uniformity between counting methods used in each study could introduce methodological biases influencing fire frequency reconstructions, and therefore impact their interpretation and limit their comparisons. To overcome this issue, we propose here a new automated method to quantify the number of macrocharcoal fragments and measure their areas from high-definition image capture, based on the analysis of colorimetric parameters. We tested the efficiency of our method and reconstructed charcoal influx over the last 8000 years by comparing visual and automatic counting methods along a north-south transect from eastern Canada, estimating number and size, and the associated local and regional fire frequencies. Results show that our automated method is efficient in detecting charcoal particles, except for highly minerogenic samples, and suggest that the traditional visual inspection tends to overestimate the size of macrocharcoal fragments. Local fire frequencies varied greatly depending on the macrocharcoal detection method used. At the regional scale they seem closer, and our automated method reproduces similar trends to published studies in our study area. However, it does represent a methodological advancement, particularly for recent centuries, that should be considered for future paleoecological studies. • An open-source program was developed on ImageJ software to detect macrocharcoals. • ImageJ software allows an efficient detection of macrocharcoals. • Charcoal number and area are highly correlated, but local fire history differs. • Manual counting method strongly overestimates total charcoal area. • Methodological choice impacts fire history more at local than regional scale. [ABSTRACT FROM AUTHOR]

The atmospheric deposition of polycyclic aromatic compounds (PACs) is considered a major pathway to isolated lakes and bogs in the Athabasca oil sands region (AOSR), Canada. However, the suite of PACs measured has been limited. We report the detailed depositional history of nitrogen and sulphur heterocyclic PACs using a 210Pb dated sediment core (1914–2015) near major developments in the AOSR. We observed (1) an exponential growth in the deposition of heterocyclic PACs to recent times with an average doubling time of 12 years, (2) significant breakpoints in PAC fluxes in the mid to late 1980s, and (3) a synchronous increase of PACs with crude oil production (r2 = 0.82, p = 0.001). NPACs were not detected prior to the 1960s in the sediment core studied, suggesting they may hold promise in serving as indicators for atmospheric PAC deposition of industrial origin. Furthermore, a change in heterocyclic PAC distribution profiles beginning in the 1970–1980s, after the onset of mining, resembling a petcoke signature, was also observed. Significant positive correlations (p < 0.05) were observed between heterocyclic PACs, and several metal(loid)s, including priority pollutant elements, chromium and beryllium, and rare earth elements, cerium, lanthanum and yttrium (r2 > 0.75), suggesting the potential of a common source or similar transport and fate mechanisms. Significant negative or no correlations were observed between heterocyclic PACs and other metal(loid)s, including vanadium, total mercury and lead, possibly reflecting the impact of broader regulatory controls introduced in the mid-1970s on some metal(loids) but not on PACs, including the installation of electrostatic precipitators in major upgrader stacks. Image 1 • Sulphur polycyclic aromatic compounds (PACs) were more abundant than nitrogen PACs. • Sulphur PACs detected since 1930s, while nitrogen PACs not detected before 1960s. • Heterocyclic PACs doubling time of 12 yrs, while routine PACs was between 18 and 25 yrs. • Heterocyclic PAC distribution profiles change began in 1970s post oil sands onset. [ABSTRACT FROM AUTHOR]