The effect of different acid-treatments on the age spectrum of organic matter in sediments determined by Ramped PyrOx/accelerator mass spectrometry

Studies of the radiocarbon and stable carbon isotopes of organic matter (OM) in sediments have become common and are an effective way to understand the fate of sedimentary OM burial in the aquatic environment. In practice, obtaining the radiocarbon and stable carbon isotopic composition in sediments requires first removing inorganic carbon by acid treatment. Two common treatments are acid rinsing and fumigation. The radiocarbon ages and stable carbon isotopic values obtained using the different acid-treatments can differ significantly, but the details of the change of organic components have received much less attention. A new approach was recently developed for radiocarbon dating of carbonate -poor and/or rich sediments using a so-called ramped pyrolysis/oxidation (“Ramped PyrOx”) method in combination with accelerator mass spectrometry. Radiocarbon and stable carbon isotopic analysis of the CO₂ that evolves under a linear temperature program allows separation of OC components in sediments based on their thermochemical stability (Rosenheim et al., 2008). In this preliminary study, we explore the utility of the Ramped PyrOx method for determining the effect of different acid-treatments on radiocarbon ages and carbon isotopic compositions of OM in sediments. The observations indicate that the HCl rinsing method alters OM more than fumigation in lower carbonate samples while the opposite occurs in the high carbonate samples. This result has implications for studies of the transfer of carbon from the terrestrial to the marine environment because these are samples that contain low amounts of carbonate material. Recommendations for the most appropriate way to treat these samples will be made. The loss of organic matter during the HCl rinsing in the marine carbonate poor sediments could be one of the reasons that radiocarbon ages in the labile thermal fractions were older than that processed by fumigation, whereas younger in the resistant thermal fractions. The distinct alterations of stable carbon isotopic composition in the carbonate rich samples are likely results from “selective” loss of organic carbon sources during the acid-treatments. This study is the first systematic comparison of the effect of different acid treatments on the reliability of radiocarbon age by ramped pyrolysis/oxidation analysis.