The Measurement Reliability of δ¹³C of Dissolved Organic Carbon: A Validation for Speleothem Samples

The carbon isotopic composition of dissolved organic carbon (δ13CDOC) in speleothems, based on high-precision U-Th ages, has great potential to reconstruct past ecology, microbial activity, and carbon cycling. Identifying the reliability of the pretreatment and measurement of speleothem δ13CDOC is therefore essential to putting it into practice. Actually, we have previously verified the reliability of the method for speleothem δ13CDOC measurements. However, it was only based on the final δ13CDOC values of homogeneous speleothem powder and is unable to evaluate the effects of different experimental conditions and the impacts of adding acid on δ13CDOC if using speleothem as the study samples. In this study, we used an organic reagent (potassium sorbate) as the study sample and designed a conditional experiment that simulates the protocols for speleothem δ13CDOC analysis and presented the resulting data to inspect the effects of the experimental processes on the analysis of δ13CDOC. The results show that the standard deviation of duplicate samples is 0.1‰, which is close to that of the previous work for organic reagents and water samples, and the results were not affected by different experimental conditions and operation steps (such as adding orthophosphoric acid, digestion time, and storage duration of resulting CO2 in storage vessels), suggesting that this method is robust to detect the speleothem δ13CDOC. Considering the range and standard deviation of results, we proposed that the δ13CDOC record could be used in various studies when the amplitudes of the δ13CDOC record are larger than 0.1‰, especially greater than 0.4‰.