Effects of handling, storage, and chemical treatments on δ13C values of terrestrial fossil organic matter

With the need to interpret small isotopic variations, delta(13) C analyses of sedimentary organic matter are more and more widespread in the field of (paleo)climatology. Recent developments require an evaluation of the reliability and reproducibility of the whole data acquisition chain. Literature abounds in protocols for sediment pretreatment prior to physical measurements. These procedures differ at every step: from sampling, handling, and storage conditions to leaching procedure, without cross evaluation. In this study, we focus on two sediment samples: a modern temperate soil and a 70 ka typical loess. We review different protocols that characterize each step of the sediment pretreatment. Handling and storage conditions are tested, e.g., finger skin contact, mild- to high-temperature oven-dry, and freeze-drying. Likewise, different decarbonation protocols are compared: wet decarbonation under cold 0.6 N HCl, 2 N HCl and boiling 1 N HCl, and acid fuming with 36% HCl. This study identifies up to 1.5 parts per thousand isotopic shifts linked to experimental bias. This large bias might be at the origin of erroneous paleoclimatic interpretation. On the basis of these results, we propose specific treatments adapted to the sample type.