TO PRETREAT OR NOT TO PRETREAT: REMOVAL OF ORGANIC MATERIAL FOR STABLE ISOTOPE ANALYSIS IN TEMPERATE SKELETAL CARBONATE

Stable C and O isotope ratios are one of the most accurate ways of determining past environmental change needed 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 hydrogen peroxide at various strengths and durations. This study compares δ¹³C and δ¹⁸O values in a control with no pretreatment with 5 different pretreatment methods. The objectives are to: (1) assess the impact of the most common pretreatment methods on δ¹³C and δ¹⁸O values in biogenic carbonates, and (2) expand the taxonomic range and mineralogical diversity of carbonate biomineralizers in the debate of whether to pretreat or not to pretreat. We analyzed the following temperate, biomineralogically diverse marine organisms from southern New Zealand that include 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- (HMC), Intermediate- (IMC), and/or Low-Mg calcite (LMC) skeletons. We used a linear statistical model to compare the effects of the treatments and mineralogical composition on the δ¹³C and δ¹⁸O values. To accommodate the nested nature of the study design, a random-effect intercept term was modeled for each of the subsamples. All pretreatments removed organic material as compared to the control, with roasting being most effective. The δ¹³C and δ¹⁸O values after bleaching and hydrogen peroxide treatments did not significantly differ from the control. The exception was roasting which significantly decreased δ¹³C and δ¹⁸O values in all mineral groups except IMC for δ¹³C. As expected, metastable HMC species were problematic. Their δ13C values were also positively affected by bleaching, and their δ¹⁸O values were also negatively affected by hydrogen peroxide treatment, both of which were significant. Our recommendation is to consider not pretreating at all, but definitely avoid roasting and using HMC species.