Paleoecological evidence of major declines in total organic carbon concentrations since the nineteenth century in four nemoboreal lakes
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A decade of widespread increases in surface water concentrations of total organic carbon (TOC) in some regions has raised questions about longer term patterns in this important constituent of water chemistry. This study uses near-infrared spectroscopy (NIRS) to infer lake water TOC far beyond the decade or two of observational data generally available. An expanded calibration dataset of 140 lakes across Sweden covering a TOC gradient from 0.7 to 24.7 mg L−1 was used to establish a relationship between the NIRS signal from surface sediments (0–0.5 cm) and the TOC concentration of the water mass. Internal cross-validation of the model resulted in an R 2 of 0.72 with a root mean squared error of calibration (RMSECV) of 2.6 mg L−1. The TOC concentrations reconstructed from surface sediments in four Swedish lakes were typically within the range of concentrations observed in the monitoring data during the period represented by each sediment layer. TOC reconstructions from the full sediment cores of four lakes indicated that TOC concentrations were approximately twice as high a century ago.
KeywordsCarbon cycling Dissolved organic carbon Near infrared spectroscopy (NIRS) Paleolimnology Sediment Sweden
This research was supported by the Climate Impacts Research Centre (CIRC). We would like to thank Christian Bigler for providing lake sediments and data for the northern calibration set. The SLU Dept of Aquatic Sciences and Assessment is acknowledged for funding collection of sediment from the 40 lakes in S Sweden and for the provision of environmental monitoring data. We would also like to thank Annika Holmgren, Nina Stenbacka, Evastina Grahn and Thomas Westin for field and laboratory assistance.
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