Geology Reference
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putative fossil: (1) is formed of organic matter or
is clearly mineral-replaced; (2) is complex enough
in cellular structure to rule out abiogenic origins;
(3) is represented by numerous specimens; (4) is a
member of a multicomponent assemblage; (5) exhi-
bits a range of taphonomic degradation that is
consistent with their mode of preservation; (6) exhi-
bits morphological variability; (7) is found in a
plausible living environment; (8) grew and repro-
duced by biological means of cell division; and
(9) exhibits a biogenic isotopic signature (Schopf
1999; Barton et al. 2001). Although biogenicity
should be supported by as many of these criteria
as possible, preservation commonly dictates that
many of these criteria cannot be verified.
Establishing biogenicity depends on the preser-
vation of the microbes and, in particular, how well
the original features of the microbes are preserved.
Clearly, the presence of mineralized microbes
indicates that microbes were associated with the
speleothems. The converse, however, does not
necessarily indicate that microbes were absent for
it could equally reflect a lack of preservation or
failure to identify the microbes amid the calcite of
the speleothem. Experimental work by Bartley
(1996) demonstrated that the soft tissues of
microbes will be rapidly lost to decay in a matter
of days. Similarly, L´veill´ et al. (2000) noted the
paucity of preserved cells in mineralized microbia-
lites found on cave walls in the twilight zone of
caves on the north coast of Kauai, Hawaii. Obser-
vations such as these imply that the three-
dimensional preservation of some microbes
through calcification must take place very quickly
(Jones & Kahle 1986; Gradzinski 1999) as, for
example, in the case of Geitleria (Fig. 4). Paradoxi-
cally, if microbes directly or indirectly encourage
calcite precipitation they may be authoring their
own demise as the precipitated calcite encases
them (Barton et al. 2001; Barton & Northup
2007). These issues are not unique to caves, for
the same debate arises with respect to the silicifica-
tion of microbes in hot-spring systems (e.g. Jones
et al. 1997), where some microbes remain viable
even though encased with opal-A precipitates
(Phoenix et al. 2000; Jones et al. 2001). In this
case, it was suggested that the precipitated opal-A
protected the microbe by shielding them from UV
light (Phoenix et al. 2001). By analogy, this raises
the possibility that early calcification of spelean
microbes may be advantageous to them in some
manner. The preservation potential of microbes
also appears to be a function of the type of
mineral being precipitated and the form of that pre-
cipitate. For microbes in hot spring systems, for
example, it appears that the preservation potential
of microbes is high if non-crystalline (¼ amor-
phous) precipitates are forming but low if crystalline
precipitates are forming (Jones & Renaut 2007).
Similar processes may be operative in caves as
microbes are usually more commonly detected in
micrite lamina and rarely in spar calcite lamina.
Fabrics and structures consistent with microbial
activity such as stromatolite-like structures are com-
monly apparent in hand samples or thin sections
(Figs 6a-c, f-h, 7a & 8a-b). In some cases, locat-
ing microbes is easy for they contrast readily with
the surrounding substrates (Figs 2-5). In other
cases, however, the search for mineralized microbes
can become an exercise in frustration. In the cave
pisoliths from Old Man Village cave, for example,
the microbes are hidden and camouflaged in the
micrite lamina. Thus, cursory inspection of the
micrite lamina at low magnification (e.g. Fig. 8c,
k) reveals the constituent grains but no indication
of microbes. Careful inspection at high magnifi-
cation, however, reveals some areas with numerous
spores, c.1mm in diameter (Fig. 8d) and other areas
with hints of mucus (Fig. 8i) and/or small, poorly
preserved filaments (Fig. 8l). Elsewhere in these
laminae there are spherical calcite bodies, c. 1 mm
in diameter, that contrast sharply with the small
calcite grains in the surrounding micrite (Fig. 8h).
Although lacking obvious biogenic features, their
overall similarity in size and shape to the spores
found elsewhere in the lamina (Fig. 8e) suggests
that they are calcified spores. The problem of
locating and identifying microbes in micrite is not
unique to cave deposits. Benzerara et al. (2006),
for example, noted the difficulty of locating and
identifying microbes amid finely crystalline aragon-
itic microbialites that are actively growing in Lake
Van (Turkey). Although difficult to locate using
conventional optical and scanning microscopy,
Benzerara et al. (2006) showed that nanometer-
scale analyses using scanning transmission X-ray
microscopy (STXM) combined with transmission
electron microscopy (TEM) allowed the microbes
to be located and thereby demonstrated the key
role that they played in the development of the
Lake Van microbialites. These examples, from the
Cayman cave pisoliths and the Lake Van microbia-
lites, clearly illustrate the problems that can arise in
the search for evidence of microbes.
Well-preserved calcified microbes, which
should probably be considered the exception rather
than the rule, invariably bring the desire to identify
and name them in terms of extant taxa. As with
mineralized microbes in hot-spring systems, this is
fraught with problems (Jones et al. 2001, 2004).
Although calcification leads to microbe preser-
vation, the mineralizing processes commonly
destroy or fail to preserve most of the features that
are of taxonomic importance, in the same way that
silicification does in hot spring settings (Jones
et al. 2001, 2004). Mineralization can preserve
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