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The source of sedimentary organic matter in lakes can help to elucidate climate and catchment variation and processes that reflect lake development. Common techniques for tracing sediment organic matter sources, such as the stable isotopes and elemental concentrations of C and N, can be too imprecise to identify the specific provenance of organic matter. By contrast, organic geochemical techniques such as gas or liquid chromatography and nuclear magnetic resonance provide detailed organic molecular characterisation but are both expensive and time consuming. Fourier Transform Infrared (FTIR) spectroscopy is a rapid, non-destructive, and well-established method for determining the constituents of lake sediments. However, the potential for identifying the sources of organic matter in lake sediments has not been fully explored. In this study, we assess the extent to which FTIR can be used to identify varying organic matter sources through analysis of modern autotrophs from Blue Lake, North Stradbroke Island, Australia. We investigated spectral processing techniques to identify the approach that could most accurately classify autotroph samples. Three autotroph groups (i.e algae, aquatic macrophytes, and terrestrial) were correctly classified 90% of the time. Processed spectra then became the basis of a model that used multivariate random forests to estimate sediment organic matter composition source from a sediment record from Blue Lake that spans the last 7500 years. FTIR-based estimates suggested that throughout the history of the lake, algae contributed the highest amount of organic matter to the sediment samples. These results allow a refinement of a previous study of C:N and δ13C from the same core and suggests that alterations in C:N and, particularly, δ13C reflect chemical changes in algae through time. This study demonstrates that FTIR spectroscopy is a promising tool to elucidate sources of sediment organic matter in lake sediments.

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