Natural stable isotopes ratios (d13Corg and d15N) and associated elemental concentrations (i.e. total organic carbon and total nitrogen contents) preserved in marine sediments are frequently used for the determination of paleoenvironmental processes such as the organic-matter origin. Previous studies highlighted bias in the determination of such geochemical proxies due to pre-analysis acid treatment methods. This study is the first systematic comparison of the effect of acid treatment methods on bulk organic-matter using a unique sedimentary system, under glacial and interglacial conditions. We used the common method for pre-treatment analysis, which consists of acidification of bulk sediment followed by several de-ionised water rinses. We investigated the effect of acid type (i.e. Hydrochloric acid and Acetic acid), but also strength (from 0.2 to 10 mol/l) on the d13Corg, d15N, TOC, TN and C/N ratio on three samples from the Gulf of Lion. Two (i.e. S.302 and S.102) were deposited during glacial maxima whereas S.157 characterizes interglacial conditions. Samples d13Corg values range between -21.7 and -24.4 per mil with TOC varying from 0.56 and 0.84 %wt/wt. d15N values is more stable with an average value of 3.0 ± 0.1 per mil with a TN average of 0.08 ± 0.002 %wt/wt. We show that acid type did not significantly affect results. We also find that (i) glacial and interglacial samples do not react similarly to acid pre-treatment, (ii) high acid strength (>1.5 mol/l) induce significant bias on d13Corg values, TOC values and therefore on C/N ratio; (iii) 25% of an isotopically distinct pool of organic carbon was lost between the use of 0.2 mol/l and 1.5 mol/l affecting d13Corg values by more than 1.5 per mil; (iv) geochemical evidences indicate that the leachable organic-carbon pool is preferentially composed of terrestrial organic-matter. These findings call for precautions when using C/N ratios and associated d13Corgvalues for paleoenvironmental and climate reconstructions.

Supplement to: Pasquier, Virgil; Sansjofre, Pierre; Lebeau, Oanez; Liorzou, Celine; Rabineau, Marina (2018): Acid digestion on river influenced shelf sediment organic matter: Carbon and nitrogen contents and isotopic ratios. Rapid Communications in Mass Spectrometry, 32(2), 86-92