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seasonally (Genty & Quinif 1996; Baker et al.
2008). However, we have demonstrated from d
13
C
and sulphate evidence that the effects of low PCO
2
in winter do have a distinct chemical expression,
which accords with theory and previous experimen-
tal evidence. Since seasonal changes in PCO
2
are
normally found in cave monitoring studies, these
effects could be recognized widely in speleothems.
Unlike the Ernesto cave, the rate of speleothem
growth is not proportional to temperature, but
instead has decreased since the mid 19th century.
It is plausible that this may relate to a reduction in
the severity of winters, leading to less influence of
winter ventilation, associated with high supersatura-
tions. We do not present here our work on statistical
comparison with climatic indices, but it is clear that
any correlations are weak. Obir speleothems, like
the physiological variation of the cave system
through the year, are remarkable, and should yet
yield more specific information that can be inter-
preted in terms of seasonality under past conditions.
We are currently working on the premise that it is
the length of the winter season during which
strong ventilation applies (currently around 4
months) which is the key factor and have recently
obtained more high-resolution S profiles to try to
test S variation as a proxy for the duration of the
winter season.
P. Micrometre-scale zones of trace element
enrichment within broader event laminae and
the thin zones must represent pulses of
trace-element incorporation during not more
than a few days growth. Such short events
have only rarely been imaged before, and
only by UV-fluorescence imaging, not by
chemical analysis. They may be present
widely in speleothems from humid temperate
environments and are particularly clearly
shown here because of fortuitous analytical
conditions.
(d)
The enrichments in Zn and Pb result in X-ray
interferences that prevent quantitative determi-
nation of some other elements, but there is
sufficient evidence to draw a close parallel in
element behaviour with event laminae at
Ernesto cave where a spatial and interpreted
genetic association with colloidal humic sub-
stances is found. This comparison is reinforced
by enrichments also in the halides F, Br and
I since these elements also display organic
associations in near-surface environments.
This is the first demonstration of I enrichments
in speleothems. There is a clear contrast with
the uniform stalagmite concentrations of Cl,
which exists as a free ion in solution. The evi-
dence from Obir strengthens the idea of a
characteristic suite of elements, transported in
colloidal form, which tend to covary in spe-
leothems, and be associated with a particular
season in mid-latitudes.
Conclusions
The results of a comprehensive range of geochem-
ical and petrological studies of stalagmites from a
chamber in the interior of the humid alpine Obir
cave site, 70 m below the surface, show that:
(a)
(e)
A quantitative comparison between chem-
istries of archived dripwaters and speleothems
draws attention to very different selectivities
of element partitioning into calcite, but an
origin of seasonally high Pb and Zn purely
from local dripwater is only possible if vir-
tually all of these metals is sequestered into
calcite. Because of this observation, and a
lack of evidence for significantly higher drip
discharge during the autumn, it is proposed
that chemical enrichments and the formation
of event laminae might arise by redistribution
of colloids as aerosols during particularly
intense air flows at the start of the winter
season. This draws attention to aerosol analysis
as an under-researched area in cave studies.
The speleothems have a distinct Zn- and Pb-
enriched chemistry, related to local host-rock
mineralization. This has resulted in changed
unit-cell size, and a distinctive microstructure
is present of sub-parallel crystallites with alter-
nating rough and smooth surfaces, the latter
representing the preferential development of
one specific face in the f10 - 11g form. The dis-
tinctive chemistry has not significantly affected
fractionation of stable carbon and oxygen
isotopes or the bulk partitioning of Mg.
(b)
A pronounced visible lamination, forming
during the autumn, takes the form of annual
packages or individual 'event laminae' separ-
ated by dominant inclusion-free calcite. In
structure and spacing, they are similar to
laminae associated with fluorescent organic
matter in other humid temperate sites; fluor-
escent organic matter is present, but less abun-
dant at the deep Obir site.
(f)
The strongly developed seasonal variation in
airflow at Obir results in low PCO
2
in cave air
in winter, which is associated with higher
d
13
C and pH of dripwaters. Enrichments in
13
C were indeed found in winter growths of
most laminae traversed during ion microprobe
studies, but the observations were limited by
the extreme technical difficulties in analysis.
Sulphate also displays a seasonal variation,
offset from the distribution of P, and which is
(c)
The event laminae are associated with distinct
trace
element
enrichments
of
Zn,
Pb
and
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