Most palynologists have probably had an occasional uncomfortable
sense that the slide they were working on was not homogeneous, or discovered that
two slides of the same sample produced somewhat different results. I have
always been struck by one apparent anomaly - that sometimes no specimens of
a rare taxon would appear for many traverses and then two would appear
nearly adjacent. Other palynologists to whom I have spoken have remarked
on the same phenomenon. Knowing palynology's ability to produce
biologically and geologically reasonable results, and also knowing how much
effort would be required to substantiate suspicions of non-homogeneity, I
have always ignored the problem.
When I joined the GSC to work on Tertiary palynology I experimented
with an adaptation of the Quaternarist's exotic spike technique to
biostratigraphy. The results, which led in an unexpected direction, have
been reported in White (1988). A short summary follows.
Palynomorphs observed in pre-Quaternary samples are often brown to black
as a result of heating during burial. Initially I wondered if the specific
gravity of the palynonomorphs might change during diagenesis, so that an
exotic spike such as recent Lycopodium, might not settle homogeneously with
the fossil palynomorphs.
Measuring the specific gravity of palynomorphs is not a simple
undertaking. The experimental technique finally arrived at was to mix a
stock suspension containing three different batches of palynomorphs: alder
pollen collected from trees near Calgary; medium maturity Carboniferous
spores from the Horton Bluff Formation, Nova Scotia, and very mature
Cenozoic palynomorphs from the White Lake Formation, B.C. The suspension
was homogenized in a vortex mixer while equal aliquots were withdrawn and
put into 11 test tubes. Solutions of water and zinc bromide of specific
gravities 1.0, 1.1, 1.2, 1.3, ....to 2.0 were added to the test tubes, the
samples stirred, and let sit. The experiment was performed twice with
slightly different techniques. The ring of floated material formed at the
top of each test tube was recorded, and the float fraction and some sink
fractions removed for analysis of their composition.
The results were similar for both experiments. Almost no palynomorphs
floated at sp. gr. 1.0 (distilled water), and few floated at 1.1 or 1.2.
At sp. gr. 1.3 and above a distinct ring of palynomorphs formed around the
top of each test tube. The width of the ring increased with each increasing
step in specific gravity. There were still small fractions which sank to
the bottom of the test tube even at sp. gr. 2.0. Analysis of slides made
from the float fractions showed that high maturity palynomorphs tended to
be more abundant in the high specific gravity samples. However, low and
medium maturity palynomorphs could be found in the sink fraction at high
specific gravities. Although I found it intuitively difficult to accept
that such a broad range of specific gravities could exist amongst
palynomorphs, the observations were hard to ignore. I could not
satisfactorily determine whether the specific gravity range existed in the
original palynomorphs (a possibility which would have implications for
pollination ecology), or whether it is somehow a product of the processing,
perhaps of the heavy liquid solution itself.
Stokes' Law allows an approximation of the terminal velocity of a
palynomorph during settling assuming that the palynomorph is a solid
sphere. R2 (radius squared) is the most important term in Stokes'
equation. If one generates a matrix including a range of specific
gravities from 1.1 to 1.9 and particle radii from 6 to 50 microns, at one
gravity, about three orders of magnitude difference in theoretical terminal
velocity can be demonstrated. Although the results are surely too extreme
for many reasons, they are sufficient to emphatically dispel one's illusion
that palynomorphs are setting homogeneously in the test tubes during
settling or centrifugation.
Why are non-homogeneous results not more obvious on the slides?
Possibly because when the sample is stirred and resuspended throughout the test tube
column, the arrival order of palynomorphs at the bottom is randomized.
Possibly also because one's traverse pattern on a slide transects, rather
than follows, the smear pattern of slide preparation.
One of the fascinating questions is the putative adjacent
appearance of rare types. Could differential sorting explain such a phenomenon? My
mentor in fluid mechanics, B. Karney (now at U. of T.), advised me that if
two particles did not begin in close proximity, sorting processes would not
bring them together. Perhaps the phenological pattern of a plant results
in juxtaposition of its pollen or spores in sediment, and some of this
pattern is not eradicated by bioturbation or sample processing. However,
the question has not been resolved whether this phenomenon is real or only
a product of selective memory of a few rare events? A plausible answer to
these questions would be interesting to many palynologists.
While it is not possible to avoid differential settling, it may be
possible to assess the non-homogeneity it may induce. The possibility of
testing the effectiveness of mixing during sample processing emerges from
Stokes' Law; particle size is a more important term than the difference in
specific gravity between the liquid and the particle. As micro-spheres are
available in different sizes, two suspensions of different sized
microspheres could be made and added to a sample before processing. A
comparison of their ratios in the original suspensions and final slide
would be a measure of sample bias caused by differential settling. This
analysis could be more important to palynologists practising high
resolution Quaternary palynology than those involved in stratigraphic
palynology. Possibly some of the noise typical of influx diagrams could be
eliminated by a measure of sample bias.
White, J.M. 1988. Methodology of the exotic spike: differential settling
of palynomorphs during sample preparation. Pollen et Spores 30:131-148.
Notes added in April, 2003
On rereading Benninghoff 's (1962) paper, I noticed that he
suggested using particles of different size to test for differential
Ogden (1986) documented the high resistance of NEN microspheres to
most standard palynological preparation techniques, except for strong
oxidation. However, months after leaving a suspension of NEN microspheres
stirring in glycerine on a vortex mixer, we checked the suspension, and
found that the microspheres had disappeared! A NEN product representative
speculated that the stirring bar had eventually eroded the coating on the
microspheres, and the contents had dissolved. From his perspective, this was
potentially a useful technique for recovering valuable radioisotopes from
bad batches of microspheres! Obviously, anybody interested in using
microspheres would need to develop a different method of mixing a
homogeneous suspension, or be assured of their stability by the supplier.
We reverted to using Lycopodium tablets as a spike because of the routine
convenience of the technique.
Biostratigraphers do not normally use an exotic spike in a
geological study of continental rocks because it is unusual to have any fine
control of sedimentation rate, so that useful influx estimates are
unobtainable. However, in the analysis of the Mallik 2L-38 gas hydrate
research borehole in the Mackenzie River delta, I have found palynomorph
concentration estimates to be a useful tool for differentiating different
sedimentary units, even if the changes were due to different pollen
productivity or different sedimentation rate.
Benninghoff, W. S. Calculation of pollen and spores density in sediments by
addition of exotic pollen in known quantities. Pollen et Spores, IV, 2,
Ogden, J. G. III, 1986. An alternative to exotic spore or pollen addition in
quantitative microfossil studies. Canadian Journal of Earth Sciences 23:102-106.
Note: This item orginally appeared in CAP Newsletter 12(2):28-30, 1989.
It has been slightly updated.