Biology Reference
In-Depth Information
week, the metal objects would be digested or highly corroded, while the
pollen walls would remain virtually unaltered.
Third, techniques are available for extracting plant microfossils from the
rock. This involves placing pulverized samples in a series of acids that dis-
solve the mineral matrix—hydrofl uoric acid (HF) for silicates, hydrochloric
acid (HCL) for carbonates, and nitric acid (HNO
3
) to reduce subrin, cel-
lulose, and other organic debris. The wall chemistry is stabilized, both in
modern reference material and in plant microfossils, by acetolysis which is
a mixture of nine parts acetic anhydride ([CH
3
CO]
2
O) to one part concen-
trated sulfuric acid (H
2
SO
4
). The acetolysis mixture is explosive with water,
so it is preceded and followed by rinses in dehydrating glacial acetic acid.
The residue remaining in the centrifuge tube consists of anything originally
present in the rock that is not soluble in acid, that is, mostly spores, pol-
len, and other plant microfossils. A small amount of mounting medium is
added (glycerin jelly or silicone oil) and several slides are prepared, labeled,
cataloged, and stored from each sample. The process takes about a day for
a group of eight samples depending on how long they are left in each acid
(typically several minutes; up to four hours in HF or HNO
3
for highly sili-
ceous or organic samples).
Fourth, pollen grains and spores vary in morphology, so plants can be
identifi ed on the basis of pollen and spore characters (see, e.g., fi g. 4.5; Mo-
ran et al. 2007; and David Roubik's site Pollen and Spores of Barro Colorado
Island, http://striweb.si.edu/roubik/). Identifi cations are usually to the level
of genus, but occasionally to species or only to family (e.g., Poaceae, grasses).
One exception among the grasses is
Zea mays
(corn), which has pollen triple
the size of most other grasses (100 vs. 30 mμ), allowing fossils of this impor-
tant plant to be recognized in sediments from archeological sites.
Finally, plants differ in their ecological requirements and are restricted
to varying extents to specifi c kinds of habitats (
Larrea
, desert;
Nymphaea
,
aquatic). A corollary to this generalization is that paleoenvironments can be
estimated from paleobotanical and palynological assemblages.
The goal is ultimately to be able to “sight read” (identify) at a glance
a suffi cient number of microfossils on a slide (e.g., fi g. 2.25b, c) that one
can “see” an ecosystem, that is, a mental image emerges (e.g., fi g. 2.25a).
The details of that image become better defi ned with additional identifi ca-
tions and with due consideration to ancillary and contextual information. A
preliminary impression of ecosystem history emerges when the images are
mentally superimposed like frames in a slowly moving fi lm.
Plant macrofossils and microfossils may be preserved in the same fl ora
or in different fl oras from the same region. In these cases, suggestions for
