A novel application of wet oxidation to retrieve carbonates from large organic‐rich samples for ocean‐climate research

Abstract

For ocean‐climate research, carbonates are extracted from large organic‐rich bulk sediments for specific geochemical analyses. This is conventionally achieved by time‐consuming dry oxidation or nonoxidative preprocessing. To significantly shorten sample preparation time we designed and evaluated a rapid technique that uses sequential wet oxidization of bulk samples in a hot alkaline 18% H2O2 solution. We successfully tested this wet oxidation technique on multiple bulk aliquots from two sediment trap samples and a core top sediment that were also processed by dry oxidation and without oxidation. From all aliquots four calcitic foraminifera species and an aragonitic juvenile bivalve from the 250–315 μm fraction were analyzed for size‐normalized weight (SNW), δ18O, δ13C, Mg/Ca, and Sr/Ca. In addition, these proxies were determined on powdered bulk aliquots from the 150–250 μm fraction processed by wet oxidation, dry oxidation, and without oxidation. At an initial pH of 8 and temperature of 70°C the alkaline H2O2 solution appeared to be most stable and reactive. Carbonate dissolution did not occur as no reduction was observed in SNW of the four foraminifera species G. ruber, G. trilobus, N. dutertrei, G. bulloides, and the aragonitic bivalve shells. For sediment traps the δ18O and δ13C between the three cleaning methods only deviated between 0.05 and 0.3‰ compared to 0.1 and 0.6‰ in sediments, with equally small variation in Mg/Ca and Sr/Ca ratios (0.1–0.4 mmol/mol). Lowest Ba/Ca ratios show that wet oxidation successfully removed organic matter along with residual salts. No significant systematic differences between samples or methods were found, but a residual scatter remained, particularly in the nonoxidized sediment because of intraspecies and intrashell inhomogeneity. Given proper preprocessing all three techniques performed well, with the proposed wet oxidation method emerging as a fast technique for extracting carbonate shells from wet, gram‐sized bulk samples. Within 3 h wet oxidation produced clean, dry residues of unaltered calcareous shells from batches of four wet bulk samples without time‐consuming intermediate steps. For Mg/Ca thermometry on time series sediment trap samples, wet oxidation results approached those obtained using the Barker et al. (2003) protocol with improved sample recovery. Consequently, the proposed wet oxidation offers a rapid alternative to conventional extraction techniques for carbonate geochemistry.