Methodological perspectives on the application of compound-specific stable isotope fingerprinting for sediment source apportionment

Abstract

Compound-specific stable isotope (CSSI) fingerprinting of sediment sources is a recently introduced tool to overcome some limitations of conventional approaches for sediment source apportionment. The technique uses the C-13 CSSI signature of plant-derived fatty acids (delta C-13-fatty acids) associated with soil minerals as a tracer. This paper provides methodological perspectives to advance the use of CSSI fingerprinting in combination with stable isotope mixing models (SIMMs) to apportion the relative contributions of different sediment sources (i.e. land uses) to sediments. CSSI fingerprinting allows quantitative estimation of the relative contribution of sediment sources within a catchment at a spatio-temporal resolution, taking into account the following approaches. First, application of CSSI fingerprinting techniques to complex catchments presents particular challenges and calls for well-designed sampling strategies and data handling. Hereby, it is essential to balance the effort required for representative sample collection and analyses against the need to accurately quantify the variability within the system. Second, robustness of the CSSI approach depends on the specificity and conservativeness of the delta C-13-FA fingerprint. Therefore, saturated long-chain (> 20 carbon atoms) FAs, which are biosynthesised exclusively by higher plants and are more stable than the more commonly used short-chain FAs, should be used. Third, given that FA concentrations can vary largely between sources, concentration-dependent SIMMs that are also able to incorporate delta C-13-FA variability should be standard operation procedures to correctly assess the contribution of sediment sources via SIMMs. This paper reflects on the use of delta C-13-FAs in erosion studies and provides recommendations for its application. We strongly advise the use of saturated long-chain (> 20 carbon atoms) FAs as tracers and concentration-dependent Bayesian SIMMs. We anticipate progress in CSSI sediment fingerprinting from two current developments: (i) development of hierarchical Bayesian SIMMs to better address catchment complexity and (ii) incorporation of dual isotope approaches (delta C-13- and delta H-2-FA) to improve estimates of sediment sources

Estimating soil redistribution patterns with 137Cs measurements in a Mediterranean mountain catchment affected by land abandonment

“…Other tracers as compound-specific stable isotopes may offer potential to further discriminate among vegetation covers. This technique has been successfully tested in recent research to discriminate land uses as sediment sources (Alewell, Birkholz, Meusburger, Schindler Wildhaber, & Mabit, 2016;Blake, Ficken, Taylor, Russell, & Walling, 2012;Upadhayay et al, 2017) in different parts of the world.…”

Understanding the effects of early degradation on isotopic tracers: implications for sediment source attribution using compound-specific isotope analysis (CSIA)

“…These plant lipids are continuously transferred into the soil after their production, through litter deposition and abrasion of cuticular waxes (Glaser, 2005;Jansen and Wiesenberg, 2017). Compound-specific isotope analysis (CSIA) of lipid constituents such as long-chain fatty acids and n-alkanes has been routinely used as a valuable tool to infer soil OM source and turnover (Wiesenberg et al, 2004;Zocatelli et al, 2012), reconstruct paleoenvironments and paleovegetation (Glaser and Zech, 2005;Eglinton and Eglinton, 2008), and fingerprint sediment-bound OM sources in freshwater and marine environments (Canuel et al, 1997;Galy et al, 2011). As freshwater ecosystems are currently under the threat of high sediment input due to the intensification of anthropogenic activities and associated land-use changes, the CSIA technique has recently been adopted for improved quantification of the sediment sources from different land uses (Gibbs, 2008).…”

Isotope mixing models require individual isotopic tracer content for correct quantification of sediment source contributions

“…Importantly, Alewell et al (2016) proposed and used long-chain FAs for source apportionment. Data from other published studies (e.g., Blake et al, 2012;Hancock & Revill, 2013) applied short-chain FAs for which isotopic composition is considered to be non-conservative due to degradation and in situ production by microorganisms in soil and aquatic systems (Reiffarth et al, 2016;Upadhayay et al, 2017). Indeed, unpublished FA content data of Blake et al (2012) showed that short-chain (C 16 and C 18 ) sediment FA contents were an order of magnitude greater than in the soil source, questioning the selected tracer reliability in this study (Reiffarth et al, 2016).…”

“…Using an IMM to unravel a mixture using isotopic tracers thus provides a proportional contribution of the sources to the isotopic tracer in the mixture, while it is the proportional contribution of the source to the sediment mix itself, which is of interest for apportionment studies. The relationship between π n,s and the source proportion P s is given by with C n,s being the content of the isotopic tracer n in source s (Upadhayay et al, ).…”