Geology Reference
In-Depth Information
found. SH2, which feeds a stalagmite that has not
been collected, was an exception in that at the
slowest drip rates (to 0.36 ml hr
21
), EC was low
and correlated with low Ca and hence high
Mg/Ca; d
13
C was also high. This is indicative of
prior calcite precipitation stimulated by degassing
(Fairchild et al. 2000; Fairchild & McMillan 2007).
Data on drip rates (from a tipping bucket drip
logger), water and air temperature is illustrated in
Figure 1. None display a simple seasonal pattern
of variation; in particular there is no sign of an
increased drip rate during peak infiltration during
the autumn season. Drip SH4, feeding Obi84, was
not continuously logged, but over a period of two
years maintained a relatively high drip rate that
would have ensured steady growth. Drips SH1 to
SH3 all displayed inter-annual growth variations;
the most pronounced relative change was SH1
(Obi12) which displayed a pronounced decline
from 2000 - 2003 (confirming instantaneous drip
rates presented in Sp¨tl et al. 2005) when monthly
volume collections indicated drip rates as low as
0.36 ml hr
21
.
Figure 1 illustrates a more regular pattern of
variation of temperature of drip water (using
SH3 - Obi55 as a typical example), displaying an
upward deviation compared with summer values
of around 0.1 8C in the first part of winter followed
by a larger drop and gradual recovery later in the
winter. Air temperature in S¨ulenhalle displays a
similar pattern, although the upward displacement
is more typically 0.2 - 0.3 8C. Since the air circula-
tion is so extensive (Sp¨tl et al. 2005), we argue
that the deviations in the water temperature are con-
trolled by the seasonal air circulation. Some spot
measurements of air velocity have been made at
the narrow squeeze at the entrance to S¨ulenhalle
as well at the next one which leads to subsequent
chambers:
are commonly referred to as 'crystallites', following
Kendall & Broughton (1978), to distinguish them
from the composite columnar individuals which
are
typically
recognized
by
their
systematic
extinction.
The fabrics in the most intensively studied
example (Obi84) are transitional from columnar to
microcrystalline types as defined by Frisia et al.
(2000). Columnar fabric is commonly characterized
by the parallel arrangement of prismatic crystals
(Onac 1997), where the equal orientation of the crys-
tals is related to a common direction of most rapid
growth. In microcrystalline fabric, some individuals
within the aggregate are not parallel or sub-parallel
to the adjacent individuals and their direction of
most rapid growth is not the same as that of the
majority of the individuals within the aggregate. In
the centre of Obi84 (Fig. 2b), extinction sweeps reg-
ularly as a wave through the crystals (with a total
variation of up to 208), similar to the variation in
orientation of growth layers but crystal boundaries
are typical of columnar calcite. However, the extinc-
tion sweeps counter to the crystal elongation (see
crystal S in Fig. 2b) in the manner of radiaxial
calcite, which has recently been recognized in a
Mg-bearing calcitic speleothem sample by Neuser
& Richter (2007). In speleothems, radiaxial fibrous
calcite aggregates have been interpreted as being
the product of crystal splitting, that is, where indivi-
dual crystals split through various mechanisms,
among which because ions with a 'poisoning' effect
on growth sites are present in the parent solution or
because ions with a larger ionic radius substitute
for ions in the mineral structure. The most extreme
form of crystal splitting found elsewhere is a spher-
ulite (Onac 1997). Further from the growth axis, the
composite crystal aggregates are closer to the micro-
crystalline fabrics described by Frisia et al. (2000),
the latter representing a type of columnar fabric
forming when growth inhibitors or drip rate variabil-
ity characterize the system (Frisia et al. 2000). High-
resolution analysis of this microcrystalline calcite
has been achieved using electron backscatter diffrac-
tion in an area c. 8 mm below the top of sample
Obi84 (Fig. 2b) and is illustrated in Figure 2c.
Figure 2c shows a mosaic of small crystallites, with
grain boundaries which range from well defined to
more 'diffuse', typically around 20 10 mm. There
is a clear preferred orientation of the indivi-
duals roughly from the left bottom corner to the
right upper corner of Figure 2c. This direction
coincides with the overall orientation of the aggre-
gate crystal which has been analysed (see the area
of the map in Fig. 2b). The relative crystallographic
orientation of each small crystal (or crystallite)
within the aggregate is indicated by the false
colours of Figure 2c and correspond to the orien-
tations shown in the stereographic projections of
maximum
air
velocities
of
c.
0.5 m
sec
21
are reached during winter.
Petrology
Stalagmite fabrics
The successive growth layers of the stalagmite are
made visible by the presence of internal laminae
which are parallel to the external surface of each sta-
lagmite (Fig. 2a). A slight depression toward the
centre of the stalagmites likely reflects the impact
point of the feeding droplets. Figure 2b highlights
the characteristics of the columnar fabrics which
compose Obir stalagmites. These, as for other spe-
leothems, are aggregates of crystals that precipitated
synchronously from the same medium to form
single growth layers. The 'synchronous' crystals
precipitated in each growth layer (where the unit
of 'synchronous' time could be a year or a season)
Search WWH ::
Custom Search