Microbes in caves: agents of calcite corrosion and precipitation - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 19

Fig. 7. Microbes in pool fingers fromCottonwood Cave, NewMexico. (a) Group of pool fingers with palaeo water level
indicated. Vertical height is c. 1 m. Photograph by Kenneth Ingram. (b, c) SEMphotomicrographs filamentous microbes
with distinctive reticulate morphology. Microbes revealed by etching of calcite from a pool finger. SEM images
courtesy of Leslie Melim.

calcite moonmilk has been attributed to abiogenic
processes (Shumenko & Olimpiev 1977; Harmon
et al. 1983; Onac & Ghergari 1993; Hill & Forti
1997; Borsato et al. 2000).
A number of issues hinder interpretation of the
origin of moonmilk. The fact that the crystallo-
graphic form of fibrous calcite crystals may vary
because of growth conditions and/or subsequent
diagenetic modifications (Jones & Kahle 1993;
Verrecchia & Verrecchia 1994; Ca˜averas et al.
1999) is commonly disregarded in discussions con-
cerning the origin of moonmilk. Thus, any infor-
mation that may be elicited from the morphology
or the crystals is ignored. Attributing the formation
of fibrous calcite crystals to the direct calcification
of filamentous microbes is difficult to support
because: (1) such crystals are typically straight
and very long whereas filamentous microbes are
typically sinuous; (2) there are no examples of
fibre crystals that have a central, longitudinal open
lumen that runs the length of the crystal; and
(3) there are no examples of fibre crystals that
encompass calcified microbes in their crystalline
structure. Nevertheless, Ca˜averas et al. (2006)
suggested that a two-stage process may be involved
in the formation of moonmilk whereby: (1) pre-
cipitation of monocrystalline rods, preserving
filament morphology, from fluids that had high

supersaturation levels due to microbial activity;
and (2) epitaxial growth of the fibers as the cessation
of microbial activity leads to decreased saturation
levels and nucleation rates.

Effective role microbes in speleothem
formation

Calcitic speleothems, which are common in caves
throughout the world, may form through abiogenic,
biogenic, or a combination of abiogenic and bio-
genic processes. Many issues, however, conspire
to make the assessment of biogenicity difficult,
especially when the focus is on old speleothemic
deposits. In lithified speleothems, for example, abio-
genic mineral precipitates can easily be mistaken for
mineralized microbes (e.g. Barton et al. 2001, table
1). Accordingly, Barton et al. (2001) suggested that
putative fossil microbes should be assessed by the
same criteria that Schopf (1999) validated fossil
microbes from ancient strata, namely: (1) prove-
nance; (2) age; (3) indigenousness; (4) syngenicity;
and (5) biogenicity. The first four criteria are
designed to identify contaminants that were intro-
duced long after the deposit had formed, whereas
the biogenicity assesses the biogenic origin.
Ideally, biogenicity should only be accepted if the