Par Carsten Meyer-Jacob, research associate au Department of Ecology and Environmental Science, Umeå University (Suède)
Infrared spectroscopic (IR) methods are a cost-efficient alternative to conventional methods because they offer a) a simple sample pre-treatment, b) a rapid measurement time, c) the non- or minimal consumption of sample material, and d) the potential to extract quantitative and qualitative information about organic and inorganic sediment components from a single measurement. Sediment components such as total organic and inorganic carbon (TOC ; TIC) and biogenic silica (bSi) can be quantified by IR spectroscopy due to their component-specific IR signatures. Previous approaches using IR spectroscopy for quantifying bSi – a common proxy of paleoproductivity in lakes – relied on conventional wet-chemical digestion techniques as reference for calibration, and thus inherited the measurement uncertainty of these methods. This limitation was overcome by using synthetic sediment mixtures with known bSi content, which allowed establishing an independent calibration model using Fourier transformed infrared spectroscopy (FTIRS ; 3750-450 cm-1) and PLS modeling for determining bSi in sediments (Meyer-Jacob et al. 2014). Ongoing work includes adding currently underrepresented sediment components to the model (different clay types, organic matter), and applying the model in soil biogeochemistry.
IR spectroscopy can further be used to indirectly infer more complex environmental variables from sediments. Rosén (2005) originally developed a transfer function to infer past lake-water TOC levels based on visible near-infrared spectroscopy (VNIRS ; 25000-4000 cm-1). This model is based on VNIR spectra of surface sediments and the corresponding measured lake-water TOC concentration. After re-analysis and extension, this model now includes 145 nemoral, boreal, and subarctic lakes across Sweden and covers a lake-water TOC gradient from 0.7 to 22 mg L−1. Applications of the model show a good agreement between monitored and inferred lake-water TOC levels over the past decades (Meyer-Jacob et al., 2015). Centennial to millennial time scale TOC reconstructions from lakes in central and southern Sweden revealed that the recent increase in lake-water carbon was preceded by a landscape-wide, long-term decline. This decline started as early as AD 900 and occurred in response to an extensive human landscape utilization that altered carbon cycling between terrestrial and aquatic ecosystems (Rosén et al, 2011, Bragée et al., 2015 ; Meyer-Jacob et al., 2015). Ongoing work involves merging several calibration sets (Sweden, Canada, Finland) into one universal model, and studying the molecular organic matter composition of the surface sediments and how this relates to the VNIR spectra and lake-water TOC model.
- Bragée et al. (2015) Biogeosciences 12, 307-322
- Meyer-Jacob et al. (2014) J Paleolimnol 52:245-255
- Meyer-Jacob et al. (2015) P Natl Acad Sci USA 112, 6579-6584
- Rosén (2005) Biogeochemistry 76:503-516
- Rosén et al. (2011) Biogeosciences 8, 2717-2727