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

The carbon isotopic composition of dissolved organic carbon (δ¹³C_DOC) 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 δ¹³C_DOC is therefore essential to putting it into practice. Actually, we have previously verified the reliability of the method for speleothem δ¹³C_DOC measurements. However, it was only based on the final δ¹³C_DOC values of homogeneous speleothem powder and is unable to evaluate the effects of different experimental conditions and the impacts of adding acid on δ¹³C_DOC 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 δ¹³C_DOC analysis and presented the resulting data to inspect the effects of the experimental processes on the analysis of δ¹³C_DOC. 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 δ¹³C_DOC. Considering the range and standard deviation of results, we proposed that the δ¹³C_DOC record could be used in various studies when the amplitudes of the δ¹³C_DOC record are larger than 0.1‰, especially greater than 0.4‰.