Appraisal of biomass combustion biomarkers to track the paleo-occurrence of forest fires

Abstract:

Wildfires influence many aspects of the Earth system, including ecosystem distribution, biodiversity, the carbon cycle, atmospheric chemistry and climate. The challenge is disentangling the various controls of fire, partly because of their diversity, and also because fire was impossible to observe and analyse as a global phenomenon until the satellite era. The study of ancient climates can be helpful to understand the natural drivers of wildfires. However, the reconstruction of wildfires is limited by the nature of the proxies available, chiefly charcoal, which only represents a portion of the carbon combustion continuum.

In here we evaluate the application molecular combustion biomarkers. For this purpose we have compiled an extensive collection of soils and lacustrine sediments representative of the humid to arid environments, which encompass the wide range of climates and ecosystems within the Iberian peninsula. We have measured the abundance of a monosaccharide anhydride (MA) biomarker called levoglucosan (1,6-anhydro-β-D-glucopyranose) and polyaromatic hydrocarbons (PAHs), as well as general plant biomarkers such as n-alkanes. To discern between biogenic and/or anthropogenic combustion sources and the nature of fires we have investigated the use of levoglucosan, retene (PAH generated during combustion of conifer trees) and PAHs ratios such as phenantrane/anthracene and fluoranthene/pyrene. Charcoal (>150 microns) has also been measured in the lake samples to contribute in the assessment of local vs. regions fire signals. The final objective is to constrain the use of the molecular proxies as quantitative biomass combustion paleoproxies.

The data obtained has been mapped and compared to the documented occurrence of wildfires in Spain over the last two decades, and mesoescale patterns of atmospheric circulation and particle transport. Results show that the occurrence of levoglucosan and PAHs is widespread in modern soils and sediments in Iberia. Chemical markers are indicative of regional patterns of biomass burning rather than just local fires. Moreover, the interpretation of the molecular data requires careful consideration of the preservation processes of biomarkers as well as their input fluxes to sediments and soils.