Technical note: Acidification methodology impacts sediment decarbonation as revealed by bulk and serial oxidation measurements

Abstract. Acidification is frequently adopted to remove carbonates preceding particulate organic carbon (POC) measurements. In practice, acid rinsing and acid fumigation are two typical and well-established acidification methods eliminating inorganic carbon. However, detailed protocols therein are most likely adopted based on conventional laboratory practices, assuming that the measurement precision is unaffected by experiment conditions. In fact, acidification can cause mineral dissolution and leaching of organic components, and therefore impacts the quantity and composition of residual POC considerably. Nonetheless, the effect of acidification on POC properties and the underlying mechanisms are ambiguous when relying solely on bulk measurements. In this study, we investigated POC properties following acidification using ramped temperature pyrolysis/oxidation (RPO) technique, in combination with bulk carbon analyses, to assess the impact of different decarbonation pretreatments on the sedimentary organic carbon (OC) measurements. Our results reveal that both acidification method (rinsing or fumigation) and HCl concentrations under acid rinsing are main factors dictating OC properties measured. Notably, despite negligible differences in bulk measurements, RPO results show distinct variations between these two acidification methods. In combination with other evidence, our study suggests that the alteration of organic-inorganic associations, which is ubiquitous during acidification, drives the behaviour of POC properties that measured. Furthermore, we demonstrate that the characteristics of residual POC in acid-rinsed samples are more proximal to pristine, natural states of the raw materials, whereas the strikingly discrepant differences between two acidification methods can be attributed mainly to the perturbation caused by calcium chlorides after acid fumigation.