THE ACID-HOT WATER METHOD

A faster and more efficient method for disaggregating coccolithic White Chalk is presented below. The method involves the use of concentrated acetic acid (99-100% pure acid) and must be employed with great care in a well-ventilated fume hood. Because the acetic acid does not completely dissociate into its component ions when dissolved in an aqueous solution, it is classified as a weak acid. A solution of acetic acid is a dynamic equilibrium between acetate ions (CH3COO-), hydronium ions (H3O+) and neutral molecules. The dissociation constant for acetic acid is 1.8 x 10-5 at 25ÉC. The essence of the method is that owing to the low dissociation, concentrated acetic acid, when applied to calcareous sediment, does not affect the carbonate until mixed with water. The key step is to cover acid-saturated samples with plenty of boiling water. This may subsequently lead to complete disaggregation of the samples. Accordingly, the method described here involves the use of concentrated acetic acid prior to disaggregation in boiling water.

The White Chalk samples used in our experiment are 60 x 60 x 60 mm in size and weigh about 190 g. The samples must be thoroughly dried overnight in a ventilated oven prior to immersion in acid. To avoid hardening of the sediment, the temperature should be less than about 50ÉC. After soaking for 40 minutes (normally sufficient for a complete saturation of the sample), the acid is decanted, and the sample is covered with boiling water containing a buffer solution of sodium carbonate (i.e., soda ash). When the acid is mixed with the water, an immediate chemical reaction takes place. The formation of carbon dioxide causes breakdown of the sample into finer particles. Although some of the fossils may be damaged during this process, sedimentary rocks generally disaggregate at their weakest points, which are the contacts between the fossils and the matrix. After 30 minutes, the sample is almost entirely broken down and can be sieved through a set of mesh screens that retain the larger fossils and other particles. The residue is now thoroughly washed in cold running water. The fossil rich residue consisting mostly of foraminifers, ostracods, brachiopods and remains of larger biogenic fragments, is oven dried at 90ÉC.

The dried residue is covered with concentrated acetic acid until saturated. Treatment is renewed with the addition of boiling water containing a buffered solution of sodium carbonate (Na2CO3) in order to keep the solution neutral. A few minutes later, the residue is entirely disaggregated by the action of carbon dioxide formation. After this second treatment, the residue is washed in cold water and separated into different size fractions. Although the formation of carbon dioxide implies etching of carbonate, the fossils are free of dissolution features at 40x magnification. The extent of etching on microfossils and nannofossils remains to be examined.

Safety notice: as the fluid and vapour of concentrated acetic acid are toxic, the method may be hazardous. The procedure should be carried out within a fume hood. Because concentrated acetic acid is very corrosive and can cause painful burns, safety gloves and glasses must be worn.