Abstract
The characterization of particles in suspension in river plumes contributes to the assessment of net particulate organic carbon (POC) fluxes and to a better understanding of the anthropogenic and climatic impact on blue carbon. Prior to POC analysis in natural waters, inorganic carbon (in the form of carbonates) must be removed. This step is generally carried out by acid leaching. However, the presence of mineral matrices (in turbid waters) may hinder total decarbonation, which may result in biased measurements. This work checks the quality of decarbonation through the analysis of carbon stable isotope ratio (δ13C), considering suspended particles discharged by three rivers into coastal waters under flooding conditions. Carbonates were removed by adding variable volumes of 2N hydrochloric acid (HCl) to filters. Carbon concentrations and stable isotopic ratios were analyzed. Values of δ13Corg (stable isotope ratio of organic carbon) allow the identification of incompletely decarbonated samples. If a small amount of detrital carbonates resists the usual decarbonation treatment, δ13Corg can be significantly shifted towards less negative values, suggesting the need of more efficient decarbonation methods in order to improve the accuracy of organic carbon measurements. Even in the case of a high Corg/Ctotal ratio, the impact of remaining carbonates on the δ13Corg value is strong because δ13Cinorg is significantly different. The sensitivity of δ13Corg measurement might therefore be used to validate POC measurements in estuarine and coastal waters.
Highlights
Combining ocean color remote sensing observations (e.g., Lorthiois et al 2012; Ody et al 2016) with in situ sampling and laboratory analyses helps to characterize, chemically andThere is growing evidence of a link between climate change and the increase in extreme meteorological events such as floods and droughts (IPCC 2012)
This study shows that carbon isotopic variability can be used to validate the results of the OC elemental analysis and to ensure the completeness of the decarbonation protocol
Analyses of particulate organic carbon (POC) in natural water samples require a compromise between efficiency of decarbonation and preservation of OC, both in nature and in concentration
Summary
Combining ocean color remote sensing observations (e.g., Lorthiois et al 2012; Ody et al 2016) with in situ sampling and laboratory analyses helps to characterize, chemically andThere is growing evidence of a link between climate change and the increase in extreme meteorological events such as floods and droughts (IPCC 2012). The northwestern Mediterranean Sea is known to be significantly impacted by climate change (Metzger et al 2005; Kim et al 2019) and is Organic carbon analyses are not simple, notably because organic phases are often intimately associated with inorganic phases (King et al 1998). The removal of IC forms is generally carried out by acid treatment (“decarbonation”), usually with hydrochloric acid (HCl), prior to POC analysis, assuming that most of this carbon is contained into calcium carbonate phases. Inefficient carbonate removal has been reported for diluted acid solutions (0.1N), and potential alteration of OC has been reported for concentrated solutions (6N). Many protocols use diluted HCl 1N or 2N (e.g., Darnaude et al 2004) in aqueous phase. Fumigation is often recommended (Liénart et al 2017), but the use of more concentrated acid may induce partial loss of OC if the sample is overexposed (Komada et al 2008). There is no consensus on the most appropriate method to use, depending of the samples studied (Brodie et al 2011)
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