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shifts from covariations to antipathetic variation
and there are no strong correlations with mean
temperature, winter temperature or rainfall in the
instrumental period. Longer-term d 18 O trends in
Obi84 and other stalagmites from this cave show
amplitudes of about 1‰ over the Holocene which
correlate with isotopic changes identified in speleo-
thems from Spannagel Cave in the Central Alps of
Austria
(Vollweiler
et
al.
2006;
Vollweiler,
Mangini & Sp¨tl, unpublished data).
Seasonal variations in chemical
composition
Carbon isotopes
The monitoring data show that enhanced ventilation
during the winter season is a key process at Obir.
The winter season is associated with low PCO 2
which is found to control the pH of dripwater by
enhancing the degassing towards equilibrium
(Sp¨tl et al. 2005). The high d 13 C values found in
dripwater should be expected to be seen in winter
speleothem growth. In faster-growing stalagmites,
it has recently proved possible to micromill sub-
samples and demonstrate seasonal variations in
d 13 C (Frappier et al. 2002; Johnson et al. 2006;
Mattey et al. 2008), but the Obir laminae are too
thin for this to be done. Hence, we made the first
attempt to determine such variations in terrestrial
samples by ion microprobe.
The work was focused primarily around a group
of relatively thick annual laminae which formed
between 1958 - 1966. Interlaced forward-and-back
trains of ablation pits were created during the analy-
sis which was repeated in some parallel traverses
(Fig. 6b). The work was technically extremely
demanding because of the high beam current that
was required leading to frequent electrical dis-
charges that terminated the analysis. Enough data
was collected however to demonstrate that consider-
able intra-annual variability is present with an
amplitude of up to 6‰ and always at least 2‰,
although the magnitude could not be reproduced
between analytical tracks. The expectation from
cave monitoring would be that the winter growth
of calcite should be isotopically heavier and would
occur earlier in the 'hydrological' year that starts
with the visible lamina. Heavier mean values in
the first half of the growth in a particular year
was found (Fig. 6b) in 1959 - 1960 (4 replicates),
1960 - 1961 (3 of 4 replicates), 1961 - 1962 (2
clear, 2 unclear replicates) and 1962 - 1963 (both
replicates). The results can be regarded as providing
indicative support for the conceptual understanding.
At other cave sites, where faster growth permits
conventional isotopic analysis, clear annual d 13 C
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