A new method for extracting conodonts and radiolarians from chert with NaOH solution

Abstract

Microfossils are important components of sedimentary rocks used for palaeontological, biostratigraphic, palaeoenvironmental and palaeoclimatic investigations. They are usually extracted from rocks using an acid solution, which might vary depending on the embedding rock lithology. Here we propose a new method using common NaOH (sodium hydroxide; soda) to digest cherts (micro‐ and cryptocrystalline quartz) instead of the standard technique based on HF (hydrofluoric acid). This new method allows the collection of undamaged specimens of different kinds of microfossils, such as conodonts, radiolarians, teeth and dermal scales, the minerology of which is still preserved (e.g. biogenic apatite in conodonts). The use of soda is thus recommended, as it is less dangerous, less expensive, and it better preserves the extracted microfossils both in shape and mineralogy.

A dilute sodium hydroxide technique for radiolarian extraction from cherts

“…Recently, Rigo et al . 22 found that the solubility of chert in NaOH solutions increases markedly with increasing temperature. They reported the use of 3 mol/L NaOH at 80 °C to extract well-preserved conodonts from chert.…”

“…They reported the use of 3 mol/L NaOH at 80 °C to extract well-preserved conodonts from chert. Although most dissolution reactions of chert with NaOH do not occur at room temperature 21 , it may be possible to dissolve chert and obtain radiolarians at higher temperatures 21 , 22 .…”

“…5 ). Radiolarians can also be extracted from chert using 3 mol/L NaOH 22 , which has the advantage of dissolving the chert more quickly (Fig. 1 a).…”

“…Furthermore, the NaOH method is effective in extracting conodont fossils from cherts 22 . Conodonts were extracted from cherts even at the low concentrations (1 mol/L) of NaOH proposed in this study.…”

“…Conodonts were extracted from cherts even at the low concentrations (1 mol/L) of NaOH proposed in this study. Moreover, unlike HF, the NaOH method does not affect the chemical composition of apatite 22 , which is the main component of conodonts. By obtaining well-preserved conodonts that retain their original chemical composition using the NaOH method, it will be possible in the future to analyse the O isotopes of apatite 37 , 38 and reconstruct the Rare Earth Element (REE) composition of palaeo-seawater 39 .…”