Acid digestion on river influenced shelf sediment organic matter: Carbon and nitrogen contents and isotopic ratios.

Rationale: Natural stable isotope ratios (δ C and δ N values) and associated elemental concentrations (i.e. total organic carbon and total nitrogen contents) preserved in marine sediments are frequently used for the determination of paleo-environmental processes such as the origin of organic matter. Previous studies highlighted biases 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 two contrasting climatic contexts (i.e. glacial vs interglacial).

Methods: We used the most common method for pre-treatment analysis, which consists of the acidification of bulk sediments followed by several rinses with deionised water. We investigated the effect of acid type (hydrochloric and acetic acid), and also acid strength (from 0.2 to 10 mol.L ) on the δ C , δ N, TOC, TN and C/N values of three samples from the Gulf of Lion. Two of them (S.304 and S.102) were deposited during glacial maxima (i.e. high sedimentation rate, low porosity and high terrestrial inputs) whereas S.157 characterizes interglacial conditions (high porosity, high foraminifera content, low sedimentation rates and low terrestrial inputs).

Results: For all three samples the δ C values are between -21.7 and -24.4‰ with the TOC varying from 0.56 to 0.84 %wt/wt. The δ N values are more stable with an average value of 3.0 ± 0.1‰ with a TN average of 0.08 ± 0.002 %wt/wt.

Conclusions: 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 ) induces significant bias on δ C and TOC values, and therefore on C/N ratios; (iii) 25% of an isotopically distinct pool of organic carbon was lost when using acid concentrations of 0.2-1.5 mol.L , affecting δ C values by more than 1.5‰; and (iv) geochemical evidence indicates 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 δ C values for paleo-environmental and climate reconstructions.