Pretreatment For Removal of Organic Material Is Not Necessary For X-Ray-Diffraction Determination of Mineralogy In Temperate Skeletal Carbonate

Abstract

Quantifying the effects of ocean acidification requires understanding the skeletal carbonate mineralogy in living marine organisms. X-ray diffractometry (XRD) is the simplest and most commonly used technique for determining this. Samples being analyzed by XRD are typically pretreated to remove organic material prior to grinding to a crystallite powder. This pretreatment was traditionally performed as organic material may obscure the mineral peaks on XRD traces. This study compared controls with no pretreatment with the most common pretreatment methods: roasting, immersion in chlorine bleach, and immersion in hydrogen peroxide. The latter two methods were performed at two strengths and two durations. We test the hypothesis that bleaching and/or roasting of skeletal carbonate to remove organic material does not affect the mineralogy of temperate skeletal carbonate at a scale detectable by XRD. This was done with biogenic skeletal carbonate from temperate marine environments around southern New Zealand. Specimens included 5 species of bivalve mollusks, 4 species of bryozoans, 2 species of barnacles, as well as 1 species each of serpulid worm, echinoid, gastropod mollusk, brachiopod, and coralline algae. Comparison to the untreated control showed that all pretreatments removed some organic matter and that the presence of organic matter in temperate skeletal carbonate does not affect the ability to qualitatively interpret mineralogy or semiquantitatively measure mineralogy using XRD. Given that pretreatment does not appear to be necessary and that some methods at least can cause unacceptable changes in mineralogy, we recommend that pretreatment for the removal of organic material be abandoned.

The forgotten variable: Impact of cleaning on the skeletal composition of a marine invertebrate

“…Previous studies (e.g. Smith et al, 2016;Barker et al, 2003) have noted that low concentrations of bleach, for short times, are useful and mostly harmless for both cleaning and pre-treatment to remove organic material, but we agree with those who suggest that the lowest concentration of bleach possible should be used and for short durations. Chedd (1970) explained that the effects of ultrasonic techniques on a material are poorly understood and remain at the "empirical suck-it-and-see level" (Chedd, 1970), although some studies have since been conducted into the effects of ultrasound on calcareous specimens, such as corals (Watanabe et al, 2001) and Foraminifera (Hodgkinson, 1991).…”

“…Even if a living organism has been freshly collected, its skeletal composition may be affected by: methods of fixation/preservation (Loxton, 2014), duration and conditions of storage (Steedman, 1976), cleaning, and pretreatment for removal of organic carbon (Smith et al, 2016). The effects of cleaning on invertebrate carbonate are poorly understood, but it is necessary to consider them.…”

“…Krause-Nehring et al (2011) found a mixture of losses and gains of Mg/Ca following pre-treatment of the bivalve Arctica islandica. Smith et al ( 2016) go so far as to advise not bleaching or removing organic material to avoid geochemical alteration. Overall, in the literature, there is little consensus as…”

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

“…These are the same species as used by Smith et al, 72 although we excluded two of their species and added an additional one. We did not use the bivalves Atrina zelandica and Cardita distorta , but we added the gastropod Haliotis iris.…”

“…We did not use the bivalves Atrina zelandica and Cardita distorta , but we added the gastropod Haliotis iris. The two bivalves were excluded from this study because Smith et al 72 found them mineralogically inconsistent and thus excluded them from statistical analysis. We added the haliotid gastropod to include a bimineralic species from this diverse class of molluscs and because this genus was used in many of the pioneering pretreatment studies 23,25,77 .…”