The potential for rapid determination of charcoal from wetland sediments using infrared spectroscopy

Wetland sediments archive information about past terrestrial ecosystem change including variations in fire occurrence and terrestrial carbon fluxes. The charcoal content of sediments is important for understanding past fire regimes, as well as the role this recalcitrant carbon plays in the global carbon cycle. Infrared (IR) spectroscopy provides a rapid, non-destructive and cost effective method for simultaneously analysing numerous organic and inorganic sediment properties. The use of IR spectroscopy is well developed for determining concentrations of total organic carbon (TOC), total nitrogen (TN), biogenic silica and carbonate in lacustrine sediments. In soil science IR spectroscopy is also routinely used to determine charcoal content, however the potential for analysing charcoal content from lacustrine sediments has yet to be investigated. Here we develop IR spectroscopy and partial least squares regressions (PLSR) to predict the charcoal and TOC content of an organic, 130,000 year old sediment sequence from North Stradbroke Island (Minjerribah), Australia. Charcoal concentrations used for model development were derived using both traditional palaeoecological area measures (cm² g⁻¹) and solid state ¹³C nuclear magnetic resonance (¹³C NMR) of poly-aryl structures. The IR PLSR models yielded significant correlations for the two charcoal methodologies (area measurements, R² = 0.57, p < .05; ¹³C NMR, R² = 0.70, p < .05). We additionally find a very strong, significant, correlation for TOC (R² = 0.92, p < .05), consistent with previous studies. Hence, IR is a promising tool for determining the charcoal content of lacustrine sediments, particularly for first-order sample screening, as part of a multi-proxy framework. IR spectroscopy can therefore provide a reliable and rapid technique for the initial investigation of fire histories and organic constituents of sedimentary sequences.