Influence of a chromium reduction treatment in the analysis of organic carbon isotopes in Quaternary coastal plain incised valley sediments

Abstract

Abstract This paper reports bulk organic carbon isotope data from a laboratory study designed to evaluate whether acid chromium reduction (ACR), commonly used to prepare rock/sediment samples for sulfur isotope analysis, significantly fractionates bulk organic carbon isotopes compared with the standard decarbonation (SD) method in Quaternary coastal plain incised valley sediments. In this study, 36 sediment samples obtained from a Quaternary sediment core from the Qiantang River incised valley on the East China coastal plain were tested. The results show differences in the δ13Corg values between the ACR and SD methods for Quaternary incised valley sediments ranging from – 0.97‰ to + 0.34‰, with most values (86%) showing a negative bias. Approximately half of the samples yield δ13Corg values that differ by ≥ ± 0.3‰. The carbonate and total organic carbon (TOC) contents of the sediment samples have no significant effects on the bulk organic carbon isotopes derived by ACR. There is also no obvious correlation between the organic carbon loss by ACR and the difference of the δ13Corg values. We believe that ACR loses more 13C-enriched labile organic carbon and/or volatile organic compounds (VOCs) than SD, which leads to a negative bias in the δ13Corg values. The mineral composition was speculated to be related to a significant positive bias in the δ13Corg values by ACR observed in two particular samples and requires further study. Consequently, ACR should be used with caution for sediments containing primarily young labile organic carbon with medium to high TOC contents (TOC ≥ 0.5 wt. %), as it may cause a large offset of the δ13Corg values compared with SD. However, ACR may be used for sediments containing organic matter primarily from terrestrial plants with relatively low TOC contents (TOC