Effect of removal of organic material on stable isotope ratios in skeletal carbonate from taxonomic groups with complex mineralogies

Abstract

Rationale Stable oxygen and carbon isotope ratios are one of the most accurate ways of determining environmental changes in the past, which are used to predict future environmental change. Biogenic carbonates from marine organisms are the most common source of samples for stable isotope analysis. Before they are analyzed by mass spectrometry, any organic material is traditionally removed by one of three common pretreatment methods: roasting, bleaching, or with hydrogen peroxide at various strengths and durations. Methods This study compares δ18O and δ13C values in a control with no pretreatment with those from five different pretreatment methods using conventional acid digestion mass spectrometry. The objectives are to: assess the impact of the most common pretreatment methods on δ18O and δ13C values from (1) taxonomically underrepresented groups in previous studies, and (2) those that precipitate a wide range of biomineralogies, in the debate of whether to pretreat or not to pretreat. We analyzed the following biomineralogically complex temperate marine organisms from southern New Zealand: four species of bryozoans, four species of molluscs, two species of arthropods, and one species each of annelid, red alga, brachiopod, and echinoderm (test plates and spines treated separately). These species precipitate aragonite, High‐, Intermediate‐, and/or Low‐Mg calcite (LMC) in their skeletons. We used linear mixed statistical models to compare the effects of the pretreatments and mineralogical composition on their δ18O and δ13C values. Results Roasting was the most effective pretreatment for the removal of organic matter and light H2O2 the least, but the former had corresponding impacts on isotope ratios. δ18O values were not directly affected by wt% MgCO3, but they were significantly affected by the interaction of roasting and wt% MgCO3. This same negative effect of roasting on species with higher wt% MgCO3 occurred in δ13C values, but it was much more pronounced in δ18O values. Both H2O2 pretreatments significantly and negatively affected δ18O values at higher wt% MgCO3. Neither bleaching pretreatment significantly affected δ18O values. δ13C values were most negatively affected in skeletons with high wt% MgCO3. There was also a strong negative roasting effect and more so at higher wt% MgCO3. Bleaching and H2O2 did not significantly affect δ13C values. Conclusions Based on these results, and when using skeletal carbonate of complex mineralogies, we recommend considering the abandonment of pretreatment of biogenic carbonate for stable isotope analysis due to confounded results from previous studies, difficulties with preparation, and/or the absence of significant effects of organic material on isotope ratios. If pretreatment is necessary, avoid roasting especially at higher temperatures and durations, use minimal bleaching, and in general avoid using High‐Mg calcite species in O and C stable isotope studies. If bleaching is used, clearly indicate the concentration and duration of exposure.

Characteristics of Carbonate Facies and Depositional Environment of Tapak Formation in the Ajibarang Area, Central Java, Indonesia

“…For micropaleontological analysis, foraminifera were microfauna used as an indicator in this study. Analyzes were performed using standard methods, which using a solution of hydrogen peroxide (H2O2 3%) in sample preparation [26] to remove organic matters [27]. Samples were sieved by a 63 μm mesh, the residues oven-dried at <60°C, weighed and sieved into 63-150 μm and >150 μm size fractions [28].…”

Re‐Assessing the Need for Apatite‐ and Dolomite‐Specific Calibrations of the Carbonate Clumped Isotope Thermometer

“…We note that Eagle et al (2010) found no effect of pre-treatment on clumped isotope analyses of enamel but found an effect on analyses of dentine, suggesting that organic-rich apatites could be significantly affected by sample pre-treatment. Because we measured organic-poor enamel in this study, we follow the recommendations of Eagle et al (2010), Key et al (2020), Löffler et al (2019), andWacker et al (2016) and opt not to pre-treat apatite samples. Another source of uncertainty in paleothermometry of bioapatites involves animal behavior and seasonal cycles.…”

Limited evidence for species‐specific sensitivity of temperature‐dependent fractionation of oxygen stable isotope in biominerals: A meta‐analysis

“…Organic removal, depending on the selected method (e.g. heating or peroxide digestion), has been shown to have a potential effect on stable isotope values (Key Jr et al, 2020), but no consistent effect directionality was identified. Diet is another potential factor influencing experimental results, but the isotopic composition of oxygen in diet is generally not reported in experimental studies and is not explicitly considered here.…”