PREPARATION TECHNIQUES FOR USE OF FORAMINIFERA IN THE CLASSROOM
INTRODUCTIONINTRODUCING STUDENTS to microfossils in the K-12 curriculum can be a rewarding and exciting experience for both instructor and student. Students, even those who collect fossils as a hobby, are generally unaware of microfossils. Exposure to microfossils opens up a whole new world of inquiry, and the extent to which that inquiry is developed can be adjusted to the grade-level and interest-level of the individual student.
Among the most abundant and readily studied microfossils are the foraminifera, a group of single-celled a protozoans that construct chambered shells (tests) of almost infinite variety (see Loeblich and Tappan, 1964 and 1988). Tests are either calcareous (composed of calcium carbonate secreted by the organism) or agglutinated (composed of a variety of grains selected from the seabottom and cemented together to form the test). Typically about the size of a sand grain, foraminifera (or forams as they are often called) display such an array of shell forms and surface textures that students are instantly fascinated with them. Modern foraminifera, like their fossilized ancestors, inhabit salt water environments ranging from open marine to brackish (estuaries, salt marshes, etc.). Most species live on the sea bottom (benthic), but some are floaters (planktonic) that spend their entire lives in the water column. Hence, foraminifera occur in a wide variety of sediments and sedimentary rocks, often in great abundance, and can be collected in most geographic regions.
The actual concentration of foraminiferal tests in sediments, either modern or ancient, depends largely upon the rate at which nonbiogenic (inorganic) sediments have accumulated. For instance, near the mouth of a major river the supply of nonbiogenic sediment is so great that one might need to look through thousands of sediment grains to find a single foram. Conversely, some deep-sea sediments (far removed from the source of most nonbiogenic sediments) may be almost pure concentrations of foraminiferal tests. Generally speaking, tests are scarce in coarse sands but are often abundant in silts and fine sands (Haynes, 1981).
The techniques used to prepare and concentrate samples for examination vary according to rock type (composition and grain size), how hard or resistant the sediment or rock is, how abundant the foraminifera are, and how they are preserved within the sediment matrix. The discussions that follow focus on sediment types that can be disaggregated in order to free the foraminiferal tests. This would include sands, silts and clays, and the rock types produced when these sediments are hardened (sandstones, siltstones and shales, respectively). It would not, however, include limestones, which often contain foraminifera but cannot be readily disaggregated to free the fossilized tests.
Foraminifera can be recovered from bulk sediment samples, although their presence or absence in any given sample often cannot be established until after processing. The sampling strategy is simply to collect bags of sediments/sedimentary rocks that can later be broken down and processed for foraminifera. Such samples might come from selected rock types or at predetermined stratigraphic intervals within an exposed geologic section. Another approach is to process sediment contained within larger fossils that one might collect. For example, the fossil shells of marine snails and clams are often filled with the same sediment that surrounds them. Processing these sediment fillings may yield foraminifera.
The materials needed for collecting and processing foraminifera are generally rather modest. A geology pick or small scraping tool and bags to hold sediment samples will support field collection. Bags need not be elaborate or expensive. Simple zip-lock plastic baggies work quite nicely. Try to sample a fresh surface by scraping away sediment from the surface of the exposure before taking your samples. Be sure to utilize a field notebook to record the geographic location, stratigraphic position, and any other important information regarding each sample. Assign the sample a number correspopnding to your field notes so that you can reconstruct information about the sample at a later date. Processing samples in the laboratory will require a source of running water, a sieve, a funnel and some filter paper, and perhaps detergents or chemicals to help disaggregate the sediments.
DIRECTIONS FOR PROCESSING SAMPLES TO RECOVER FORAMINIFERAThe object of all techniques described below is to isolate microfossils, in this case foraminifera, from the sediment grains that surround them. Only then can the microfossils be adequately observed and studied.
Unconsolidated sediment and some soft rocks will break down after soaking in water for a few hours, whereas harder rocks may first require crushing and then boiling. The rule of thumb here is to utilize the simplest and easiest technique that will provide the desired results. If simple soaking is all that is required to disaggregate the sediment, then forego more involved techniques. Regardless of which technique you utilize, initially breaking the sediment or rock into fragments several mm in maximum dimension, or slightly larger, will speed the process.
Precautionary Note: Make sure that labeling is carefully and accurately transcribed at every step. A mislabeled sample has little, if any, scientific value.
Simple Soaking If your sample is composed of unconsolidated sediment or sedimentary rock that can be easily disaggregated, simple soaking may be all that is required. Soaking in distilled water is most desirable, but using a dilute Calgon solution often helps to diaggregate fine sediments (muds). Calgon can be purchased in the laundry detergent section of most grocery stores. This can be done in a large beaker or any other clean glass container that is available. Experiment to see how long any given sample needs to be soaked.
Once the muds have been dispersed, the sample can be washed through a sieve (a stainless steel U. S. Standard Sieve No. 230 with mesh openings of 63 microns is recommended). This is probably the single most expensive item needed to properly prepare samples (current cost, about $90 for a sieve 8 inches in diameter and 2 inches deep). This initial expense is offset by the fact that, if properly utilized and maintained, a sieve will last for many years. Gently agitate your water/sediment mixture, introduce it gradually onto the sieve, and wash under a gentle stream of water. Most professionals recommend distilled water, but tap water may be used at this stage. The muds will pass through the sieve and be discarded. Do not do this at a standard sink that is not equipped with a sediment trap. If you do, you will have a clogged sink line in very short order. If you do not have a sink with a sediment trap, do this outdoors or use a large bucket to catch what passes through the sieve. You can then dump contents of the bucket outside. What remains on the sieve is a concentration of sand-sized material, including any foraminifera that are in the sample. Rinse this material into filter paper placed within a funnel, allow the sample to drain, and then air dry in place safe from contamination and breezes. When dry, the grains should not adhere to one another. If they do, some mud still remains and the soaking/sieving procedure should be repeated. When satisfactorily clean, the dried sample should be stored in a properly labeled vial until ready for microscopic examination.
Don't get in a hurry during the sample processing phase. A bit of extra time invested in properly cleaning your samples will save time and frustration when you examine them under the microscope.
Hydrogen Peroxide (H2O2) Method If your sample is more resistant, additional treatments may be required to breakit down. Soaking and, if necessary, boiling in a dilute solution of hydrogen peroxide is an effective means of breaking down such samples.
Precautionary Note: Concentrated H2O2 (thirty percent) is commercially available, but it is expensive and highly caustic. Handle it with extreme caution and dilute it to make a three percent solution before using it to process samples.
The steps in the H2O2 method are:
Other Techniques Literature on the foraminifera describes other methods for disaggregating sediment samples. A product called Quaternary O, a highly active but low sudsing detergent, was widely used for many years (e.g., Snyder et al., 1983). Although it is no longer available, a product called Miramine is a suitable and inexpensive substitute. It is available from the Miranol Chemical Company, 68 Culver Road, Dayton, NJ 08810. The methodology for using surfactants such as Quaternary O or Miramine is exactly like that desribed above for the use of hydrogen peroxide. Simply use the appropriately diluted detergent solution in place of the three percent H2O2 solution.
Another technique for additional cleaning involves use of a sodium pyrophosphate or a sodium metaphosphate solution (e.g., Snyder and Waters, 1984). After an intial soaking (in distilled water or a dilute Calgon solution), the sand-sized residue trapped on the No. 230 sieve is placed in 0.1 M solution (five grams of chemical to one liter of distilled water) and gently agitated for 20 to 30 minutes. This process effectively removes persistent clay-sized particles that may partially obscure important features of the test.
Finally, some of the older literature, not cited here because the techniques may be extremely hazardous, advocates the use of much harsher chemicals, including kerosene, gasoline, Varsol (similar to white gas or mineral spirits), and concentrated H2O2. Use of these methods is not recommended because they can be dangerous, both to the preparator and the environment.
CONCENTRATING THE TESTS OF FORAMINIFERAAs indicated earlier, foraminiferal tests may be rare compared to nonbiogenic sediment particles. If foraminifera are reasonably abundant, the best procedure is simple microscopic examination of the processed sample in order to find them. However, there may be instances where the time required to examine the sample in this manner is prohibitive. Then it may be desirable to float the foraminifera in order to separate them from other sediment grains. The only reason this works is because foraminifera, with their hollow chambers, have an effective density much less than solid sediment grains of comparable size. If the foraminifera are filled with sediment or secondary mineral material, they will not float.
Soap Float One of the simplest ways to concentrate foraminiferal tests is to employ a soap float. Here the detergent is not of the low sudsing variety (such as Quaternary O), but rather a standard detergent or soap that produces a sudsy froth. The processed sample is added in small increments to a solution of soap and distilled water. With frequent agitation, the foraminifera become suspended in the surface froth while solid sediment particles such as quartz grains sink to the bottom of the container. The froth can be periodically decanted onto a No. 230 sieve and washed in a gentle stream of water to eliminate the suds. What remains will be a concentration of foraminiferal tests, perhaps with some very fine sands of nonbiogenic origin. This residue can be dried and examined under the microscope.
Other techniques can provide an even cleaner separation, but many involve the use of chemicals that are extremely hazardous. For example, bromoform and carbon tetrachloride have been widely used to concentrate foraminiferal tests by floating. However, both are carcinogenic and must be used under a fume hood. The fumes are toxic and the chemicals can be absorbed through the skin. Consequently, use of these chemicals to concentrate foram tests is not recommended. The use of another, safer chemical to accomplish the same sort of separation is described below.
Flotation Using Sodium Polytungstate Sodium polytungstate [also known as sodium metatungstate: Na(H2W12O40)] is a non-toxic, high-density agent that is ecologically safe, easy to use, and recoverable so that it may be re-used several times. It has a density of 3.1 g/ml, which can be reduced to any desired lesser density simply by adding distilled water that is heated above 20°C. Although calcite has a density of 2.7 g/ml, air is usually trapped within the foraminiferal tests making them more bouyant than quartz (density = 2.65 g/ml). For best results, the sodium polytungstate solution should be diluted until a piece of gypsum (density = 2.32 g/ml) floats and a piece of orthoclase (density = 2.57 g/ml) sinks. If the gypsum and orthoclase sink, the liquid can be boiled off to increase the density.
Sodium polytungstate is available as a salt (in powder form) from GEOLIQUIDS, Inc., 15 E. Palatine Rd., Suite 109, Prospect Heights, IL 60070 and can be ordered by calling 1- 800-827-2411. The cost is about $90.00/lb.
EXAMINING AND PICKING FORAMINIFERAProcessed samples, once dried, can be stored indefinitely in labeled vials until one desires to examine them. The sample is then sprinkled sparsely across a picking tray and examined under a binocular microscope. Brass picking trays with a grid of rectanglar subdivisions, all of equal size, are typically used by professionals. The surface of the tray is a dull black (to minimize reflection) and the grid lines may be white or gold. Sources for these trays are very difficult to find, but less sophisticated trays serve nicely. Any shallow plastic tray measuring a few inches per side will suffice. If it is clear or highly lustrous, simply cut a piece of black construction paper or cardboard to fit in the bottom of the tray. This will provide a background that will not strain your eyes, and it provides a nice contrast to the foraminifera, which are typically white.
Any binocular microscope with reasonably good optics and the power to magnify 30 to 40 times will be adequate for the study of foraminifera. Of course, scopes with better optics and magnifications up to 100 times are helpful.
Individual foraminiferal specimens encountered while examining samples strewn across the picking tray can be picked and mounted for permanent reference. A recessed area in an 18-ply cardboard slide provides a black background that can be coated with a water- soluble glue (e.g., Tragacanth). The cardboard slides will also need glass cover slides and 18 ply aluminum holders. All three of these items can be ordered from: Curtis Matheson Scientific, Inc., 8291 Patuxent Range Rd., Suite F, Jessup, MD 20794 (tel. 1-800-650-0650 or 301-498-5210). An alternative supplier of cardboard slides is: SGE President, Geology Department, Kent State University, Kent, OH 44242 (e-mail: firstname.lastname@example.org). The cost of the cardboard slides is around $40 per package of 100.
Any foram specimens encountered on the picking tray can be captured using the wetted tip of an artist's brush (buy size 000, sable hair). Simply dip the tip of the brush in water, touch it to the specimen you desire to pick, and transfer the specimen to the glued slide. The glue, being water soluble, will then dry and secure the foram to the slide. At any time, wetting the specimen will release the glue so that the specimen may be turned and viewed from different perspectives. A metal clip holds a glass cover slide over the cardboard micropaleontology slide to protect specimens during prolonged storage. Using these slides, you can build a reference collection of foraminifera to share with students.
REFERENCESHAYNES, J. R., 1981, Foraminifera. John Wiley & Sons, New York, NY, 433 p.
LOEBLICH, A. R., JR., AND TAPPAN, H., 1964, Protista 2, Sarcodina, Chiefly Thecamoebians and Foraminiferida. Treatise on Invertebrate Paleontology, Geological Society of America and University of Kansas Press, Lawrence, Kansas, pt. C, 2 vols., 900 p.
_____, AND _____, 1988, Foraminiferal genera and their classification - Plates. Van Nostrand Reinhold, New York, NY, 2 vols., 970 p., 847 pls.
SNYDER, S.W., MAUGER, L.L., AND AKERS, W.H., 1983, Planktonic foraminifera and biostratigraphy of the Yorktown Formation, Lee Creek, North Carolina. Smithsonian Contributions to Paleobiology, no. 53, p. 455-482.
_____, AND WATERS, V. J., 1984, Cenozoic planktonic foraminiferal biostratigraphy of the Goban Spur region, Deep Sea Drilling Project Leg 80. In Graciansky, P.C. de, Poag, C.W., et al., Initial Reports Deep Sea Drilling Project, Washington, DC, U. S. Goverment Printing Office, v. 80. p. 439-472. ??