Methodological perspectives on the application of compound-specific stable isotope fingerprinting for sediment source apportionment (original) (raw)
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Geoderma, 2018
In recent years, Compound Specific Stable Isotope (CSSI) techniques have enabled promising new tracers to track land-use-specific sediment sources. However, empirical data exploring the technique, particularly under controlled conditions, is still scarce. Hence, the main goal of this study is to explore the suitability of CSSI to identify sediment sources under different land use in the small agricultural site of Mistelbach (8.7 ha) located in Austria. In a previous study, the authors quantified, with a 137 Cs-based reconnaissance approach, a sedimentation magnitude of 4 mm year − 1 in the deposition zone at the outlet of that study site. To obtain detailed information on the sediment provenance, CSSI techniques based on the measurement of δ 13 C signatures of natural fatty acids (FAs), were used. A cost effective sampling approach involving composite sampling, identified potential sediment source materials from the four main agricultural fields. Two long-chain FAs (i.e. C22:0 = behenic acid; C24:0 = lignoceric acid) as well as bulk δ 13 C allowed the best statistical discrimination for apportioning the origin of the sediments. Four mixing models (i.e. IsoSource, SIAR, MixSIAR and SIMMR) applied to the data generated similar results. IsoSource performed as well as the other Bayesian models tested. The main grazed waterway of the basin, identified as one of the four sources of the sediment, was evaluated to have contributed 55.1 ± 5% (IsoSource), 53.9 ± 2.7% (SIAR), 53.9 ± 2.7% (MixSIAR) and 54.0 ± 2.7% (SIMMR) to the sediment. The estimated contributions of the sources to the sediment are consistent with the land use information and the distance of the sources to the outlet. More than 80% of the sediment deposited at the basin exit originates from the two sources which had maize cultivation, one of the more erosive crops, in particular at the beginning of the growing season. This study emphasizes that CSSI and 137 Cs techniques are complementary for establishing land sediment redistribution. Their combined use could provide key decision support knowledge for optimised decision-making of land managers to ensure the sustainability of agro-ecosystem management.
2016
Slope destabilization and associated sediment transfer are one of the major causes of aquatic ecosystems and surface waters quality impairment. Through land uses and agricultural practices, human activities modify the soils erosive risk and the catchments sedimentary connectivity, becoming a key factor of sediment dynamics. Hence, restoration and management plans of water bodies can only be efficient if the sediment sources and their respective contributions, and thus the proportion attributable to different land uses and agricultural practices are identified. Several sediment fingerprinting methods, based on the geochemical (elemental composition), color, magnetic or isotopic (137Cs) sediment properties, are currently in use. However, these tools are not suitable for a land-use based fingerprinting. New organic geochemical approaches are now developed to discriminate source-soil contributions under different land-uses: i The compound-specific stable isotopes (CSSI) technique, based on the biomarkers isotopic signature (here, fatty acids d13C) variability within the vegetal species, ii The analysis of highly specific (i.e. source-family-or even source-species-specific) biomarkers assemblages, which use is until now mainly restricted to palaeoenvironmental reconstructions, and which offer also promising prospects for tracing current sediment origin. This project aims at reconstructing the spatio-temporal variability of the main sediment sources of Baldeggersee (Lucerne Canton, Switzerland), which suffers from a substantial eutrophication, despite several restoration attempts during the last 40 years. The sediment supplying areas and the exported volumes will be identified using CSSI technique and highly specific biomarkers, coupled to a sediment connectivity model. The sediment origin variability will be defined through the analysis of plants, soils, suspended river sediments sampled at base and high flow conditions (short term), and by the analysis of a lake sediment core (long term).
Identifying subsoil sediment sources with carbon and nitrogen stable isotope ratios
Hydrological Processes, 2014
Increased sediment loads from accelerated catchment erosion significantly degrade waterways worldwide. In the South East Queensland region of Australia, sediment loads are degrading Moreton Bay, a Ramsar listed wetland of international significance. In this region, like most parts of coastal Australia, sediment is predominantly derived from gully and channel bank erosion processes. A novel approach is presented that uses carbon and nitrogen stable isotope ratios and elemental composition to discriminate between these often indistinguishable subsoil sediment sources. The conservativeness of these sediment properties is first tested by examining the effect of particle size separation (testing for consistency during transport) and the effect of sampling at different times (testing for temporal source consistency). The discrimination potential of these sediment properties is then assessed with the conservative properties, based on the particle size and temporal analyses, modelled to determine sediment provenance in three catchments. Nitrogen sediment properties were found to have significant particle size enrichment and high temporal variance indicative of non-conservative behaviour. Conversely, carbon stable isotopes had very limited particle size and temporal variability highlighting their suitability for sediment tracing. Channel erosion was modelled to be a significant source of sediment (μ 51%, σ 9%) contrasting desktop modelling research that estimated gully erosion is the predominant sediment source. To limit the supply of sediment to Moreton Bay, channel bank and gully erosion must both be targeted by sediment management programs. By distinguishing between subsoil sediment sources, this approach has the potential to enhance the management of sediment loads degrading waterways worldwide. Figure 3. Distributions for each normalised sampling occasion for all nitrogen and carbon sediment properties. Points under the distributions are the normalised samples colour coded to sampling occasion (h parameters were TOC 0.092, TN 0.040, δ 13 C 0.014, δ 15 N 0.112) J. P. LACEBY ET AL.
Hydrological Processes, 2018
The use of isotopic tracers for sediment source apportionment is gaining interest with recent introduction of compound-specific stable isotope tracers. The method relies on linear mixing of source isotopic tracers, and deconvolution of a sediment mixture initially quantifies the contribution of sources to the mixture's tracer signature. Therefore, a correction to obtain real sediment source proportions is subsequently required. As far as we are aware, all published studies to date have used total isotopic tracer content or a proxy (e.g., soil carbon content) for this post-unmixing correction. However, as the relationship between the isotopic tracer mixture and the source mixture is different for each isotopic tracer, post-unmixing corrections cannot be carried out with one single factor. This contribution presents an isotopic tracer model structure-the concentration-dependent isotope mixing model (CD-IMM)-to overcome this limitation. Herein, we aim to clarify why the "conventional" approach to converting isotopic tracer proportions to source proportions using a single factor is wrong. In an initial mathematical assessment, error incurred by not using CD-IMM (NCD-IMM) in unmixing two sources with two isotopic tracers showed a complex relation as a function of relative tracer contents. Next, three artificial mixtures with different proportions of three soil sources were prepared and deconvoluted using 13 C of fatty acids using CD-IMM and NCD-IMM. Using NCD-IMM affected both accuracy (mean average error increased up to a threefold compared with the CD-IMM output) and precision (interquartile range was up to 2.5 times larger). Finally, as an illustrative example, the proportional source contribution reported in a published study was recalculated using CD-IMM. This resulted in changes in estimated source proportions and associated uncertainties. Content of isotopic tracers is seldom reported in published work concerning use of isotopic tracers for sediment source partitioning. The magnitude of errors made by miscalculation in former studies is therefore difficult to assess. With this contribution, we hope the community will acknowledge the limitations of prior approaches and use a CD-IMM in future studies.
Biogeosciences Discussions, 2019
Application of compound-specific isotope analysis (CSIA) in sediment fingerprinting source apportionment studies is becoming more frequent, as it can potentially provide robust land-use based source attribution of suspended sediments in a freshwater system. Isotopic tracers such as δ 13 C values of vegetation-derived organic compounds are 10 considered to be suitable for CSIA based fingerprinting method. However, a rigorous evaluation of tracer conservativeness in terms of the stability of isotopic signature during detachment and transport of soil during erosion process is essential for the suitability of the method. With the aim to identify potential fractionation and shifts in tracer signature during early degradation of organic matter in surface soils, we measured concentrations and δ 13 C values of long-chain fatty acids and n-alkanes from fresh plant biomass (as vegetation is a direct source of these compounds to the 15 soils), degraded organic horizon (O horizon) as well as mineral soil (A horizon) from various forest types with different humus forms (five sites). The bulk δ 13 C values showed continuous 13 C enrichment through the degradation stages from fresh plant material to the O and A horizon, ranging between 3.5 and 5.6‰. Compound-specific δ 13 C values showed a general 13 C enrichment for both, long-chain fatty acids (up to 5‰) as well as n-alkanes (up to 3.9 ‰) from fresh plant biomass to the O horizon overlying the A horizon. However, only slight or no further changes occurred from the O to the 20 A horizon. We also compared compound-specific δ 13 C values between two soil particle-size classes (< 2 mm and < 63 µm) from four sites and found no significant differences of tracer values between them, with even less fractionation for the long-chain n-alkanes within the soil particle fractions, which points to the conclusion that sampling and analysing bulk soil material might be valid for the isotopic tracer applications. We further conclude, that our results support the suitability of studied isotopic tracers as representative source soil signature in CSIA based sediment source attribution, as 25 they demonstrated necessary stability in plant-soil system during organic matter degradation.
CHEMICAL FINGERPRINTING, A PRECISE AND EFFICIENT METHOD TO DETERMINE SEDIMENT SOURCES
An accurate method of determining source locations for detrital sediment is presented using the chemical composition of Fe-oxide minerals like a fingerprint. This method is an improvement in the use of Fe-oxide minerals for provenance determinations because it requires less time and fewer source samples. A rigorous test of the method uses a database of more than 38,000 grains from known locations. The average error of matching grains back to 45 source locations designated for this database is less than 2%. The method allows for proportional matching of a grain to multiple sources if other grains in the source database meet the compositional match criteria, which helps reduce the error of incorrect matches. Most provenance studies do not involve source basins as large as the entire Arctic Ocean, where sediment can be ice-rafted several thousand kilometers. For most studies, only a few samples (, 100 grain analyses/sample) would be required to characterize a source if strategically placed, such as near a river mouth. Deposits more than 40 million years old can be traced to specific sources using this method because Fe-oxide grains are relatively stable in most deposits.