Geology Reference
In-Depth Information
and stalactites had been abiogenically precipitated
for no consideration was given to the possible
effects of microbial activity. Such an approach is
curious given the overwhelming evidence for diver-
sity microbial biotas that have been implicated in
the broad range of processes that influence the cor-
rosion and/or precipitation of speleothemic calcite.
Trace elements concentrations in speleothemic
calcite has been assessed using synchrontron
radiation micro X-ray fluorescence (m-XRF), ion
microprobe analyses (Borsato et al. 2007),
excimer laser-ablation inductively coupled plasma
mass spectrometry (Treble et al. 2003), and second-
ary ion mass spectrometry (SIMS) (Huang et al.
2001). All of these methods are capable of determin-
ing the concentrations of elements such as Y, Pb,
Cu, Sr, Mg (e.g. Borsato et al. 2007), Si, Sr, Fe
(e.g. Huang et al. 2001), and P, Na and H (e.g.
Fairchild et al. 2001) in the 0-1000 ppm concen-
tration range from spot analyses that are typically
,30 mm in diameter (Huang et al. 2001). The fact
that fluctuations in the concentrations of these
elements commonly follows a systematic pattern
that seem to correlate with the laminations in the
speleothems has led to the notion that such vari-
ations are proxies for seasonal climatic changes
(e.g. Roberts et al. 1998; Fairchild et al. 2001;
Huang et al. 2001; Borsato et al. 2007).
Phosphor, for example, has been related to
increased water infiltration that coincides with the
first storm of the autumn (Fairchild et al. 2001;
Huang et al. 2001; Treble et al. 2003; Treble et al.
2005; Borsato et al. 2007). Where P resides in the
speleothems is unknown. Suggestions include:
(1) individual P ions located in calcite defects
(Fairchild et al. 2001; Huang et al. 2001; Mason
et al. 2007); (2) various types of coexisting phos-
phate inclusions, including monetite (CaHPO
4
);
and (3) various unidentified crystalline phases
(Mason et al. 2007). The possibility that P and
other trace elements may be related, in some
manner, to microbes has been ignored. Jones
(2009) recently demonstrated that much of the P
in speleothems from Old Man Village cave on
Grand Cayman was preferentially associated with
the actinomycetids that colonized corrosion sur-
faces. Indeed, the actinomycetes had been minera-
lized with P. Thus, it could be argued that the P
was there because of microbial mineralization, not
because of a climatic event.
Little attempt has been made to assess the affect
that microbes may have on the geochemical attri-
butes in speleothemic calcite that are used as
climate proxies. This may prove difficult until
methods for detecting microbes in these spe-
leothems by direct observation (e.g. SEM images)
or biomarker proxies (e.g. lipids, proteins) become
well established. Scale becomes an issue because
most cave microbes are submicron in size and the
amount of an element that can be fixed by an indi-
vidual microbe is, as yet, unknown. With geochem-
ical analyses seeking palaeoclimate signals,
however, the trend is towards smaller spot sizes
(1 mm in some cases) and ever-lower concentrations
of an element (,100 ppm). Thus, such analyses are
increasingly approaching the scale of the microbes,
both in terms of the size of the sample being ana-
lyzed and the amount of an element being deter-
mined. Accordingly, there is the possibility that
such geochemical analyses may be derived from
mineralized microbe(s) rather than from the calcite
itself is also increasing. Clearly the a priori assump-
tion of abiogenicity that is inherent to many geo-
chemical studies of speleothems must be treated
with great caution for there is the very real possi-
bility that such analyses may in fact reflect
microbial mineralization processes. This is clearly
illustrated in the case of the P in the Cayman spe-
leothems that is directly associated with microbe
mineralization (Jones 2009).
Conclusions
With respect to microbes in caves, it is important to
stress and highlight the following points.
† Although most caves are home for rich, diverse
microbial biotas, the diversity and nature of
that biota is still poorly known.
† Identification of mineralized (e.g. calcified)
microbes in terms of extant taxa is difficult and
depends largely on the criteria used to define
the extant taxa. There is little chance of matching
mineralized microbes with extant taxa defined
solely by DNA characteristics. Comparisons
based on morphological attributes are also
difficult because mineralization commonly
destroys or disguises the taxonomically impor-
tant features.
† Microbial boring and microbially mediated dis-
solution leads to substrate destruction and, in
many cases, production of residual micrite.
† Microbesmediate constructive processes through
microbe calcification, trapping and binding of
detrital grains to a substrate, and/or inducing
calcite precipitation by modifying microenviron-
mental conditions.
† Recognition of microbes in speleothems depends
on the location of mineralized forms and/or the
detection of biogenic proxies.
† The mere presence of microbes in a speleothem
does not guarantee that they played a formative
role in the growth and development of that
speleothem.
† That microbes played a role in the growth and
development of a speleothem can be inferred
Search WWH ::
Custom Search