Seasonal microclimate control of calcite fabrics, stable isotopes and trace elements in modern speleothem from St Michaels Cave, Gibraltar - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 335

bulk calcite d 13 C resulting from winter - summer
ventilation cycles of 2 - 2.5‰ is very similar in mag-
nitude to the d 13 C cycles preserved in Gib04a
(Mattey et al. 2008) and there is no evidence of
kinetically enhanced d 13 C values as found at Obir
cave by Sp¨tl et al. (2005).
The isotopic composition of degassed CO 2
shows a similar pattern, the initial stages of degas-
sing show little change around 223‰, rising only
in the final stages of degassing to around 222‰.
The calculated d 13 C trend for degassed CO 2 is
only slightly lower than the d 13 C of end-member
cave air CO 2 but a close match would only be
expected if the source of cave-air CO 2 was solely
from locally degassed dripwater. This cannot be
the case as the quantity of drip water percolating
into NSM is very low and the behaviour of CO 2 in
the cave suggests that advective processes introduce
'ground air' CO 2 from elsewhere.

type and trace element uptake during calcite pre-
cipitation. The correlation between paired fabrics,
calcite Sr abundance, and stable isotope compo-
sition across four growth cycles - representing
calcite deposited between 2000 - 2004 - are shown
in Figure 10. Figure 10a, taken from Mattey et al.
(2008), shows the macroscopic appearance of the
uppermost portion of Gib04a in a polished surface
where the paired LCC and DCC can be seen.
Figures 10b and 10c show the structure of the final
four cycles in plane polarized light and as an elec-
tron backscattered diffraction grain boundary map
which clearly shows the alternations of columnar
macroporous LCC calcite with the microporous
compact DCC calcite which forms the darker
bands visible on cut surfaces. The ESBD image in
Figure 8c shows that the boundary surface
between the LCC and DCC fabrics is irregular and
is marked by a sharper transition from coarse- to
fine-grained. The DCC fabric increases in grain
size and the transition back to LCC is more

Paired laminae in Gib04a: relationships
between cave microclimate and calcite fabric,
stable isotope and trace element cycles

diffuse (Fig. 8b).
A new stable isotope profile obtained by micro-
milling at 50-mm resolution was performed adjacent
to the high resolution trace element profile by
obtained by synchrotron analysis reported in Mattey
et al. (2008) to examine the topology of the isotopic
transitions between the LCC and DCC fabrics. The
stable isotope, trace element and fabric maps can
now be correlated with a confidence of +100 mm.
Each analysis in the stable isotope profile in
Figure 10 nominally represents 2 - 3 weeks of
growth. They show that the cyclical pattern in
d 13 C rises to values that remain fairly constant
before falling more sharply to the lower 'winter'
value. The abrupt switching of pCO 2 in the cave
atmosphere implies that the annual cycle of d 13 C
in the calcite should be a square wave. The high res-
olution record in Figure 10 may represent a some-
what rounded version of this, rather than the
sinusoidal patterns seen in the lower resolution
profile reported in Mattey et al. (2008). Here the
rounding is introduced because the boundary
surface between the LCC and DCC fabrics is irregu-
lar with amplitude of up to 500 mm. Micromill
sampling in 50 mm increments along a 2 mm wide
face aligned parallel to the layering of the laminae
cannot fully resolve a sharp isotopic change corre-
sponding to the jump in cave air pCO 2 that occurs
in November and April.
A Sr profile across the four annual cycles was
measured

Modern calcite deposition in NSM results in paired
laminae composed of light columnar calcite (LCC)
and dark compact calcite (DCC) which preserved
regular cycles in trace elements, d 13 C and d 18 O
(Mattey et al. 2008). Mattey et al. (2008) assigned
each lamina pair to a calendar year by counting
back from the time of collection (June 2004) and
the age model was confirmed by locating the radio-
carbon 'bomb' peak in its correct position. The
cycles in d 13 C and d 18 O reported in Mattey et al.
(2008) were defined by samples taken at 100-mm
resolution which produced quasi-regular sinusoidal
variation in d 13 C, but variations in d 18 O and Sr
were more complex. The d 13 C minima were found
to be located in the LCC fabric and the switch to
DCC fabrics occurs when d 13 C and d 18 O are reach-
ing highest values of the annual cycle but the precise
position of the LCC-DCC fabric transition relative
to stable and trace element cycles, and their timing
within the annual cycle were more difficult to locate.
The cave air monitoring results shows that the
switch between winter high-CO 2 and summer
low-CO 2 and back again is very rapid (Fig. 7) and
provide time markers for mid-April and mid-
November in the annual cycle of d 13 C in calcite,
which can be used to obtain a better understanding
of environmental and kinetic controls on fabric by synchrotron micro-XRF (Mattey

Fig. 10. (Continued) calcite (adapted from Mattey et al. (2008). (c) Grain boundary map of (b) obtained by electron
backscatter diffraction. (d) Sr profile measured by synchrotron m-XRF using a beam 1 micron across in 10 micron steps
(data from Mattey et al. 2008). (e) d 13 C and d 18 O profile measured on samples removed from slab (a) in 50 micron
steps by micromilling. Timelines for mid-November (N, grey dashed) and mid-April (A, grey dotted) are shown for
reference. The faint dark band is radiation damage cause by synchrotron microbeam analysis. See text for discussion.