Coccolithophore sample processing without carbonate dissolution

Dissolution of coccolith carbonate during sample processing of sediments is a long known problem and occurs even in solutions of demineralised water buffered with ammonia.

We performed some tests to find an alternative solution which had to meet three conditions: 1) dissolution of coccolith carbonate should be avoided, 2) the solution should be easily and cheaply accessible, 2) the sample processing should not be hindered.

We tested 6 different solutions (see below) on four selected samples from different environments, different age, and with a large variety of species. The samples were: 1) a surface sediment sample from the Arabian Sea of recent age, 2) a piston core sample from the continental margin off Chile of Pleistocene age, 3) a sample from northeast Germany of late Oligocene age, 4) a sample from northern Germany of early Eocene age.

Sample processing: One gram of sediment was weighed and brought into dilute suspension. The suspension was treated ultrasonically for one minute and was filtered through fleece-supported regenerated cellulose filters (Sartorius, 0.45 µm pore width). In this way a monolayer of all sediment particles was created, which was then investigated in the SEM.

The main results can be summarised as follows:

1) demineralised water pH 6.9: Observation: All coccoliths were etched. Only large species were present whereas small and weakly calcified species were totally dissolved. Interpretation: solution is corrosive to coccolith carbonate.

2) demineralised water with ammonia pH 10.9: Observation: common but not always etching of coccoliths. Interpretation: Good preservation in carbonate rich samples. Etching and loss of small and weakly calcified species in samples with low carbonate content.

3) tap water (from Hannover) pH 7.7: Observation: in general good preservation of coccoliths. Sometimes slight overgrowth, rarely etching. Interpretation: local tap water has a naturally high carbonate content which makes it appropriate for sample processing.

4) demineralised water saturated with calcium carbonate pH 9.6: Observation: some samples exhibited predominantly etched coccoliths in other samples coccoliths remained unaffected. Sometimes also overgrowth was found. Interpretation: ambiguous, possibly the carbonate content of the sample might be responsible for the differences in the carbonate preservation.

5) demineralised water saturated with calcium carbonate buffered with ammonia pH 10.6: Observation: Good preservation of coccoliths. Sometimes slight overgrowth. Interpretation: Solution is not corrosive to coccolith carbonate.

6) Ethanol: Observation: No etching of coccoliths was found. However, the alcohol makes it very difficult to produce an evenly distributed monolayer of the sediment particles. Interpretation: solution is not corrosive to coccolith carbonate but handling is difficult.

Based on the results of our small study we now use demineralised water saturated with calcium carbonate and buffered with ammonia pH 10.6 as standard solution for sample processing.

Literature

Andruleit, H., Köthe A., Stäger, S. & Bruns, A. (2000): Some comments concerning sample processing and dissolution. - Journal of Nannoplankton Research, 22, 3: 201.