Micropaleontology lab

In the Micropalaeontology Laboratory, samples are prepared for the study of taxonomy, stratigraphy, ecology, proxy development and application of microfossils. Investigations on recent benthic foraminifera are distribution surveys and temporal sample series studies in combination with environmental observations and hydrographic measurements. Culture and field experiments are pursued as well.

Benthic foraminifera live in the uppermost centimeters of the sediment. Their species composition and population density is in balance with the prevailing environmental conditions. Therefore, many studies focus on living foraminifera. They contain cream, light green, or yellowish-brown cytoplasm. Tests containing cell plasma are difficult to distinguish from specimens that are filled with fine sediment. The plasma can be made visible with specific dyes. We use rose Bengal, which stains the proteins of the cytoplasm. Immediately after sampling, foraminiferal samples are stained and preserved with a solution of 2 grams rose Bengal in one liter ethanol (at least 70%, technical grade, e.g. denatured alcohol).

Foraminiferal assemblages from surface samples also contain empty tests of varying ages. The number of empty tests of the so-called dead assemblage is 10 to 1000 times higher than the number of living individuals. Early diagenetic processes change the composition of the dead fauna over time because the tests of each species have a different preservation potential. When they are deposited in the sediment below the surface layer, the assemblage composition remains constant. The dead assemblage spans many generations from different years, with robust shells of well-preservable species being over-represented. There are also tests of species that have lived in other places, have been transported and deposited at the sampling site.

Samples preserved and stained with ethanol have to be stored for at least two weeks to allow a complete staining of individuals that were living at the time of sampling. During the subsequent processing, we first pass the samples through a 2000 µm sieve. This is intended to remove pebbles and other large particles that would grind down fragile foraminiferal tests while washing the sample. The fraction >2000 µm is rinsed on the sieve with a shower, dried at 60°C and stored. Then, the samples are transferred to a 63 µm sieve and the washing continues by using a shower and tap water. It is checked under a microscope from time to time whether the foraminifera are sufficiently clean. After washing, the sample residue is stored under ethanol (at least 70%) if fragile, agglutinated species are to be studied. Otherwise, the 63 – 2000 µm fraction is dried at 60°C, weighed and stored for further investigations.

Foraminifera are stored in plastic or cardboard cell slides. The cell slides have a transparent coverslip so that the foraminifera can be viewed with a stereomicroscope. Collective cell slides contain all individuals picked from a sample. They are randomly arranged and not fixed. Only specimens of a specific species are contained in a single cell slide. They are fixed with glue, arranged in rows or sectors so that different sizes or aspects can be compared. Plummer cell slides contain all individuals of a sample. They are sorted by species and fixed with glue. This documents both, the identification of the species and its differentiation from similar taxa, as well as the number of individuals. Before preparing a single cell or Plummer cell slide, the bottom of the cell slide is thinly coated with a water-soluble glue. Wallpaper paste, methylhydroxyethylcellulose or traganth is used. When the glue has dried, it forms an invisible film. Foraminifera are placed and arranged on this film with a very fine, wet paint brush (marten hair, grade 000). The moisture makes the glue film swell up a bit. The foraminifera sink into the gel layer. When the moisture has evaporated, the foraminifera are fixed. They can be removed again with a wet paint brush without leaving any residue, e.g. to be placed elsewhere in the cell, or to be removed for chemical analysis or imaging with a scanning electron microscope.

All specimens of a species are arranged in rows in one or more adjacent fields in the Plummer cell slide. They are aligned with the youngest chamber at the top, and then fixed. We start with the largest specimens of the species and add the next smaller individuals. This makes it easier to discriminate a species. Even the smallest individuals can be assigned with certainty at the end. The species are then determined and the individuals are counted.