Text by: Catherine Nigrini and Annika Sanfilippo (2002)
In 1992 Catherine Nigrini and Annika Sanfilippo compiled a volume: "Cenozoic radiolarian stratigraphy for low and middle latitudes with descriptions of biomarkers and stratigraphically useful species" for the Ocean Drilling Program.
The volume was designed to accompany a set of 60 reference slides on board the Joides Resolution. Some of you might have used the slides and the volume while participating in an ODP cruise. It is a compendium of 150 stratigraphically useful species descriptions, containing a brief synonymy, morphological descriptions, dimensions, distinguishing characters, variability, geographic and stratigraphic ranges, phylogeny and an illustration. In addition, definitions of each of the commonly used Cenozoic radiolarian zones from both low and middle latitudes, a summary of radiolarian events which occur within each zone and a summary range chart have been presented.
Preparation Techniques to extract radiolarians from Cenozoic sediments
Cenozoic radiolarian tests are preserved in their original opaline silica, with only slight variations in refractive index. Because of the instability of opal through geologic time, most Mesozoic radiolarians have at least lost water and crystallized to chalcedony, and many are replaced by more stable minerals such as calcite or pyrite. These changes of the radiolarian tests occur in varying degrees in different parts of the host rock, and the matrix undergoes uneven diagenesis. Preparation techniques for Recent, Mesozoic and Paleozoic radiolarians differ somewhat from those described for Cenozoic radiolarians due to the matrix in which the radiolarians are preserved, their refractive index, total abundance and method of study etc. and are thus described in their respective sections.
Marine sediment samples containing abundant large diatoms diluting or making it difficult to observe the radiolarians can be freed from the diatoms by using a technique developed and described by Motoyama (1996). The cleaned residue is strewn on a glass petri dish. The dish is turned over on a wax paper and tapped lightly on the back. Radiolarians fall onto the wax paper while the flat diatoms adhere to the dish. The surface of the dish is brushed clean and the procedure repeated until most of the diatoms are removed. The treatment does not alter the radiolarian assemblage significantly until the diatoms become rare and flat radiolarians begin to adhere to the dish.
Oceanic soft sediments do not require a great deal of treatment before preparation of the microscope slides. The usual technique is that described by Sanfilippo et al., 1985.
- Place a 5-cm3 sample in a beaker with 150 ml of a 10% solution of hydrogen peroxide in which about 5 g of tetrasodium pyrophosphate has been dissolved.
- Boil for a few minutes to achieve oxidation of the organic matter and desegregation of the clay fraction.
- Sieve and wash through a 50 or 63 Ám mesh; dry residue and repeat from step 1 if lumps remain.
- Remove calcareous fraction with a few drops of hydrochloric acid (after eliminating the hydrogen peroxide by wet sieving).
- Desegregate and clean indurate clay coating and internal filling by treating the sample in a gentle ultrasonic bath. Further desegregation of heavily indurate sediment can be obtained by a brief treatment with kerosene, paint thinner, ammonia or a weak solution of hydroxide sodium.
- Sieve and wash thoroughly the clean residue. The size of the mesh depends on the aims of the study. Routine surveys are cleaned on 63 Ám meshes. However, samples sieved at 63 Ám are barren of many small species and juvenile stages and using a large sieve can serve to concentrate larger forms.
- Pipette a drop of the clean residue onto a labeled glass slide barely smeared with saliva, dry by evaporation.
- Add a few drops of xylene to expel air, add a few drops of the mounting medium and top with a cover glass. Dry in an oven or on a warm plate under a hood so as to avoid fumes.
Although it is more harmful and takes longer to harden than some other synthetic mounting mediums, commercially known as Norland, Pleurax, Hyrax, or Depex, Canada Balsam is usually preferred for Cenozoic material because there is some indication that radiolarians mounted in synthetic medium will not be as well preserved over time as collections mounted with Canada Balsam.
For detailed quantitative investigations involving determination of relative abundance of taxa, Moore (1973) proposed a convenient method that allows quantification of the number of radiolarian shells per unit weight of sediment.
- Dry and weigh the sediment sample.
- Clean and sieve the sample as above (1 to 6).
- Place the residue in a large (e.g., 5 l) beaker full of distilled water, on the bottom of which one or two cover glasses are installed.
- Carefully stir the water and the residue for a random distribution of the particles (avoid rotational motion which results in centrifugal fractionation), and allow the residue to settle.
- Siphon away all the water except for a 30-50 mm thick bottom layer.
- Evaporate all remaining water using an overhead infrared lamp.
- When dry, remove the cover glasses from the beaker and mount them on a glass slide as described above. The slides will contain a fraction of the radiolarian shells present in the original sample that is equivalent to the proportion of the surface of the covers relative to the surface of the bottom of the beaker.
Moore Jr., T.C., 1973. Method of randomly distributing grains for microscopic examination. Journal of Sedimentary Petrology, 43(3): 904-906.
Motoyama, I., 1996. Late Neogene radiolarian biostratigraphy in the subarctic Northwest Pacific. Micropaleontology, 42(3): 221-262.
Sanfilippo, A., Westberg-Smith, M.J., Riedel, W.R., 1985. Cenozoic Radiolaria. In: Bolli, H.M., Saunders, J.B., Perch-Nielsen, K. (editors.). Plankton stratigraphy, Cambridge University Press, 631-712.