CLUES TO RECOGNIZING MICROFOSSILS IN SMEAR SLIDES

(Source: G. Rothwell's "Minerals and Mineraloids," the Riedel and Tway LITHOWISE software, Kennett's "Marine Geology," and personal observations. 

                               CALCAREOUS FOSSILS

Calcareous microfossils and microfossil fragments are highly birefringent under crossed polars. Even small calcareous fragments show pink/blue/yellow colors.

FORAMINIFERS

Calcareous foraminifers and foraminiferal fragments are recognized by their high birefringence and black cross seen on each curved chamber under crossed polars. (The optic/isotropic c-axis of individual calcite crystals that form the test wall point outward. As the stage is rotated bringing other optic axes into view, the black cross migrates across the test chambers. In fragments with less curvature, the cross may appear as a dark blur. Don't confuse this "black cross" with the black cross formed in optic figures in which interference patterns are seen in a single oriented crystal grain.)

Planktonic foraminifers are generally globular with inflated chambers and rounded test outline. (They look like they would float.) Calcacareous benthic foraminifers often have flattened, serial, less globular tests or apertures on elongated necks. 

           A field of planktonic foraminifers in plane-polarized light.

             Some typical benthic foraminifer types.

Large foram fragments are recognized by general shape, often thick walls. Pores and migrating black cross or black blur under crossed polars. Some samples (probably deposited near the calcium-carbonate compensation depth (CCD) have a high percentage of planktonic foram rims, the remainder of the tests having been dissolved.The rims are recognized by arcuate shape, high birefringence around edges and association with other foraminifer fragments.
 

    Thick-walled planktonic foraminifer fragments.

Recrystallized foram tests and fragments lose distinctive calcite properties described above, but can sometimes be recognized on the basis of "ghost foram" shapes. Foram fragments may also recrystallize into pavement calcite. This appears under crossed polars as flat bluish grain with irregular black lines (the interlocking pavement blocks).

Foraminifers are found most commonly in sediments flooring open oceans at water depths above the calcium-compensation depth.

CALCAREOUS NANNOPLANKTON (COCCOLITHS AND DISCOASTERS)

Coccoliths and discoasters are very small, less than 5 microns, and usually occur in the clay-sized fraction. In plane-polarized light they appear as colorless, bright disks or pustules. Under crossed-polars many individual coccoliths show "4 points of light" that stay in place as the stage is rotated. If very abundant, coccoliths appear as a bluish fluff with black squiggly lines under crossed polars.

Discoasters are distinguished by their 6-pointed star shape. They are best seen in plane-polarized light. Discoasters are common in Tertiary sediments and died out at the end of the Pliocene. Presence or lack of discoasters (in post-Cretaceous calcareous sediments) is a good indicator of Tertiary or Quaternary sediments, respectively.

Calcareous nannoplankton seem to occur in sediments deposited in greater water depths than do foraminifers. That is, the CCD for nannoplankton is deeper than that for foraminifers. If foraminifers are present in a sample, look for nannofossils.

       Discoasters (PPL, 6-pointed) and coccoliths (XPL, 4 points of light)

 
                    Coccolithophore before plates have broken apart.

ECHINODERM SPINES

Echinoderm spines are relatively large (several mm), elongate, latticed and striated spines with an inflated base. Echinoderm spine fragments are highly birefringent under crossed polars, but the whole larger spines are easiest to identify with a lower powered reflecting microscope. Echinoderm spine fragments appear under the petrographic scope as notched, elongate, rectangular fragments.

Echinoderm spine, magnification about X 10.

PTEROPODS

Pteropods are microscopic free-floating gastropods with aragonite shells. They are found in Quaternary sediments at relatively shallow water depths. Aragonite dissolves at shallower depths than does calcite and although pteropods may live in the surface waters of deep ocean basins, their shells are often preserved in deep-water deposits.

Pteropods appear as thin brown fragments or spiral, elongated conical or fluted forms under plane-polarized light and are highly birefringent under crossed polars. In reflected light they are shiny, pearly white. Pteropods may be distinguished from forams by their shape (they look like, what they are, gastropods), lack of chambered shells and more glossy (more highly reflecting) surfaces. Peteropods, usually 1-10mm, are often larger than most other microfossils.

OSTRACODS

Ostracods are small crustaceans that form bivalved calcareous shells. They usually appear in smear slides as a single valve. Like foraminifers, a valve may show a black cross under crossed polars, but ostracods are easily distinguished from foraminifers on the basis of general morphology. They look like tiny clam shells, but are biologically quite different animals. Some ostracods valves are quite beautifully ornamented with intricate patterns of nodules, frills and lace.

              Ostracods. Scale bars equal .1 mm. 

SHELL FRAGMENTS

Small gastropod, pelecypod and bryozoan fragments look like other calcareous fragments in transmitted and reflected light. Their larger size, mollusk shell shapes (in the cases of gastropods and pelecypods) may help to distinguish fragments from large foraminifer fragments. Some thicker shell fragments appear a scuzzy yellowish brownish under crossed polars. Shell fragments often appear in cores and angular, white fragments. But large forams can also be seen with the naked eye, so it is best to examine fragments with a hand lens or microscopically.

CORAL/CALCAREOUS ALGAE

Small fragments of coral and calcareous algae look like other cacitic fragments under crossed polars: they cannot be easily recognized in smear slides. These larger invertibrate and plant forms are best identified in hand specimen or under low power microscopes.

SILICEOUS MICROFOSSILS

Siliceous microfossils are isotropic. Opaline silica is amorphous and does not split light into beams traveling at different speeds which causes the birefringence seen in the tests of calcareous microfossils. hey usually appear as clear, colorless forms in plane-polarized light and black (no light transmitted) under crossed polars. Under partially crossed polars their outline can be discerned but no birefringence or extinction patterns are seen.

N. B. Viewing a smear slide which contains both calcareous and siliceous biogenic components under partially crossed polars allows viewing both types simultaneously making easier the determination of their relative percentages.

    Diatoms: North Pacific diatom flora, (L). Antarctic diatom flora, (R).

Diatoms, the pillbox phytoplankton, come in two basic types: centric and pennate. Centric diatoms are disk shaped forms and have rows of regular pits (look like pores in smear slides). Often they are represented in smear slides only by rims that appear as small rings or half rings, little "C" or "0" shaped structures. Pennate diatoms are the elongate forms.

Centric diatoms are distinguished from radiolarians by their generally (not always) smaller size, more delicate framework, smaller pits/pores, and more regular periphery. Pennate diatoms are distinguished from sponge spicules by structure on the diatom frustules and more pointed terminations at both ends.

Ethmodiscus rex is a relatively common unusually large (up to 2mm) diatom species. It appears as angular, poorly visible fragments in plane-polarized light with lines of parallel to subparallel pores. Under high power E. rex is often seen to form a virtual pavement of fragments.

Diatoms occur often as single individuals, but colonial forms are relatively common.

Diatoms are nearly ubiquitous; they occur in fresh water as well as in marine deposits. In marine sediments they commonly occur in high latitude and near-shore deposits. Presumably they are present in other nutrient-rich surface waters, but are so diluted in the bottom sediments by the more abundant foraminifers that they are not seen in the smear slides.

Always look for diatoms under high power as well as low power to determine percentages. What looks like clay under low power will be often seen as diatom (especially pennate) frustules and E. rex pavements.

If diatoms are present, look for the other common siliceous forms: radiolarians and silicoflagellates.

RADIOLARIANS
 

Radiolarians seen under plane-polarized light. Some typical 
radiolarian forms.

Radiolarians occur in spherical, discoid, conical or ring-like forms. They are usually larger, more robust and have a more open framwork than do diatoms. Radiolarian pores are much larger and more irregular than diatom pits. Radiolarian spines never have an internal central canal which distinguished them from sponge spicules. Some radiolarian spines have ridges which may appear to be canals. But these can usually be discerned as ridges by focussing the microscope up and down.

Orospherids are a group of unusually large radiolarians (up to a few mm in diameter). In smear slides they appear as huge open latticework fragments attached to ridged spines.

Radiolarians tend to live in oceanic regions of nutrient upwelling, as occurs when two adjacent current masses are moving opposite directions. They are often found in sediments underlying deep, cold water which dissolves calcareous tests that would otherwise swamp them in smear slides.

SPONGE SPICULES

Sponge spicules in plane-polarized light. Some other sponge-spicule types.

Siliceous sponge spicules are most often seen in smear slides as robust hollow spines. They also occur in a variety of bizarre shapes resembling pick-up-jacks, spiny clubs, fish hooks, multipointed spars, spiny hairpins, anchors and more.

Sponge spicules can usually be distinguished from pennate diatoms or diatom spines by their more robust size, parallel-sided internal canal and lack of girdle.

SILICOFLAGELLATES

Silicoflagellates are a photosynthesizing, flagellated protistid. Silicoflagellate tests have a distinctive open lattice: even more open than most radiolarians. They are often present in samples rich in diatoms, but in much smaller numbers. Silicoflagellates commonly comprise only two distinctive groups/shapes in Quaternary sediments; dictyochids and distephanids. They are fairly easy to recognize on the basis of their unique morphology.

                                         PLANT MATERIAL

The brown fibrous material that often floats when water is added during smear-slide preparation is usually plant material. It looks the same in plane-polarized light: brown and fibrous, though large pieces are opaque in smear slides.

Pollen and spore grains are relatively small, rounded or lobate, translucent brown grains in plane-polarized light. They often appear to have a wrinkled or pitted surface.
 

                Pollen and spores.
          The scale bar equals .05 mm.

ICHTHYOLITHS

Ichthyoliths are fish parts: bones, teeth, scales. Fish teeth are the most easily recognized ichthyolith. They are clear or amber in plane-polarized light. One typical tooth shape is a triangular form with parallel pulp cavity and concave base. It looks like a tooth. Bone chips cannot always be identified as such, but appear in cross-polarized light a waxy gray with low birefringence.

Fish teeth are most often seen in smear slides from deep-sea clays that are deposited very slowly. Here diatoms and radiolarians (insufficient nutrients) and foraminifers (dissolved in deep, cold water) do not swamp the sediments with their tests.
 

               Fish teeth (about .4mm long) as seen in plane-polarized light.

 
(Revised for the Internet 03/05/2002, P. Worstell)