Smouldering fire signatures in peat and their implications for palaeoenvironmental reconstructions

Peatland ecosystems are valued as natural archives of past climatic and vegetation changes and as such their study is essential for palaeoenvironmental reconstructions over millennia. Fires in peatlands are dominated by smouldering combustion which is the self-sustained, slow, low temperature, flameless form of burning. Most studies on peat fires to date have focused on ignition conditions, C losses or atmospheric emissions, but there is a significant gap in the understanding of the evolution of organic matter (OM) following smouldering. A key feature of smouldering fires is that they consume most of the pyrogenic char produced. Consequently, it may be that most smouldering fires are simply not visible using standard palaeontological techniques. Here we present the possibility of identifying palaeofires by following their physical and chemical signature along a peat profile. We have undertaken laboratory experiments on Sphagnum peat columns and measured physical, chemical and spectroscopic changes of OM features induced by smouldering on samples of varying moisture content. We reveal that there is a higher production of aromatic and condensed molecules, an increase of the total N and a decrease of the C/N ratio, besides significant variations of pH, electrical conductivity and ash content. Several of these changes have, in previous studies, been taken to be indicative of alterations in atmospheric dust deposition and climate-driven changes (e.g., vegetation, water table fluctuation, decomposition and mineralization processes), but are also produced by smouldering fires. Our results imply that smouldering fires should therefore also be considered in climatic and floral reconstructions drawn from peat cores and that these additional physical and chemical changes may serve to enhance our understanding of palaeofire histories.