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found. Microcrystalline calcite is typically associ-
ated with perturbations of the system with selective
adsorption of species from solution on particular
surfaces. The result is a change in the morphology
of calcite crystals, with the formation of macrosteps
which indicate the presence of crystal structural
defects (Frisia et al. 2000; Jimenez-Lopez et al.
2003; Fernandez-Diaz et al. 2006). In Obi84, XRD
data demonstrates that there is a distortion of
the lattice induced by substitution for Ca of the
smaller ions Mg and Zn and the presence of non-
divalent ions such as phosphate and halides
implies significant substitution in defect sites.
These impurities may cause the development of
sweeping extinction patterns, even in the quasi-
columnar composite crystals near the growth axis
of the stalagmite. In the off-axis area studied by
EBSD, adjacent crystallites are seen to differ in
lattice orientation.
The SEM image of the top of Obi84 shows the
predominance of unstable surfaces, that is, those
characterized by macrosteps, with only subordinate
flat, defect-free faces. Also, we did not observe
calcite rhombohedra which, so far, have been typi-
cally illustrated as characteristic at the surface of
speleothems (Frisia et al. 2000). Cave precipitation
experiments carried out in other caves, and in par-
ticular at Grotta di Ernesto, highlighted the develop-
ment of stepped crystal faces with forms deviating
from the equilibrium form (i.e. f10 - 14g rhombohe-
dron) when supersaturation or presence of impuri-
ties perturbed the system (Frisia et al. 2000). In
Obi84 each crystallite displays a single, poorly
developed flat face possibly pertaining to f10 - 11g.
The appearance of macrosteps and the deviation
from the equilibrium rhombohedra morphology
have been documented by Braybrook et al. (2002)
who grew calcite crystals doped with cobalt. By
increasing the concentration of this trace metal
in the growth medium, the morphology of calcite
crystals became progressively anisotropic, probably
because of the preferential localization of the
dopant at the crystal surface (Reeder 1996) and
the preferential development of a face in the f10 -
11g form was found. Zn has a very similar ion size
to Co, and similarly adsorbs strongly to calcite sur-
faces, forming distinct phases at higher concen-
trations (Zachara et al. 1991). Zn prefers the
opposite surface site to Co (Reeder 1996) and yet
despite this, Reeder et al. (1999) showed from
X-ray absorption fine-structure studies that Co, Pb,
Zn and Ba in calcites all adopted octahedrally-
coordinated Ca-substituted lattice positions with
varying degrees of local distortion around the ions.
Many trace species are present in Obi84, of which
Zn and Mg are the most abundant, but the peculiar
morphology is not typical of Mg-calcite. Given
that the ionic radii of zinc and cobalt are nearly
identical, a homology with the morphologies ident-
ified by Braybrook et al. (2002) seems plausible.
The presence of trace metal ions with diverse
ionic radii with respect to Ca may explain also the
observed radiaxial fibrous fabric in some aggregates
in Obi84. Finally, Figure 10b illustrates differences
in composition between alternating zones across
crystallites; arguably these could be sector zones,
reflecting differential trace metal incorporation in
different
crystallographic forms
(e.g.
Reeder &
Grams 1987).
It is of interest that despite the strong disequili-
brium effects noted above, there is a consistency
of mean Mg data with published literature (Huang
et al. 2001) implying that deposition on multi-
annual timescales was not strongly out of chemical
equilibrium, consistent also with the d
18
O data.
Also d
13
C data in the speleothem can be regarded
as equilibrium values, assuming some further degas-
sing occurs in the field beyond that typical of the
dripwaters sampled.
Phenomena responsible for trace element
patterns in event laminae
The coincidence of narrow optically visible laminae
with trace element anomalies is striking in the Obir
stalagmites. In Smith et al. (2009) we had inter-
preted these event laminae as forming in the same
way as the infiltration laminae from Grotta di
Ernesto (Borsato et al. 2007). Our understanding
of the Ernesto laminae has progressed through the
work of Huang et al. (2001), Fairchild et al.
(2001) and Treble et al. (2003) who established a
link between P enrichments in stalagmites and
patterns of seasonal infiltration. Borsato et al.
(2007) went much further in demonstrating a very
specific hierarchy of association of trace element
enrichments with infiltration laminae which they
related to the relative importance of colloidal trans-
port. The most specifically associated element with
UV-fluorescent laminae was Y (a proxy also for
behaviour of the heavy rare earths), followed by
Cu, Pb and Zn, whereas P and Br were found to
display a rather broader enrichment, centred on the
infiltration lamina. Sr was depressed, which was
interpreted as due to being out-competed for
lattice defect sites. Ion microprobe analysis con-
firmed the role of Y and also showed that F, H and
Na were focused around these laminae, although
the latter two might reflect the presence of fluid
inclusions (Borsato et al. 2007). In the infiltration
lamina hypothesis, multiple pulses of trace elements
associated with infiltration laminae (Figs 10 & 11)
could be consistent with early work on UV-
fluorescence in which multiple laminae were recog-
nized (Shopov et al. 1994) and with monitoring
work in modern caves, such as the Shihua Cave
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