Geology Reference
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certain sites. However, the main issue in the litera-
ture has been whether speleothems could be entirely
explained from aerosol deposition (Cigna & Hill
1997). At Obir, it is accepted that the main crystal
growth originates from solution: the only question
is whether aerosols might add to the solutes supplied
from
originally bound to soil humic substances. Therefore
an overall two-stage hypothesis for the development
of the event laminae can be tentatively proposed as:
1.
Delivery of elements bound to colloidal humic
substances in solutions percolating through
fracture-fed components of the karstic aquifer
into the cave system; and
dripwater
during
the
formation
of
event
laminae in particular.
In an extensive study on the physics of cave
passages, Badino (1995) suggested that aerosols
deposit onto speleothems in zones where air cur-
rents are feeble to stagnant through the phenomenon
of 'evaporation', that is, when the dimensions of the
aerosol particles decrease below a critical size
(the pressure within the aerosol droplet drops and
the surrounding cave air equilibrates with a pressure
which is similar to that of the disappearing aerosol
droplet). If these aerosols carry trace metals as par-
ticulate they may deposit their load onto the growing
speleothem surface.
An instructive case study is provided by the work
of Jeong et al. (2003) and Chang et al. (2008) on per-
vasive black carbon deposits on speleothems from
touristic caves in Korea. Microscopic and
14
C
studies were used to identify the sources of carbon
as being both from vehicle exhausts and biomass
burning (the latter potentially being both external
and internal to the cave). Elevated levels of Mn, N,
S and Pb were attributed to airborne anthropogenic
sources. The organic carbon was of colloidal dimen-
sions (20 - 50 nm) and so clearly would be much
more mobile than pollen grains which are known to
be aerially transported only close to the entrance of
cave systems (McGarry & Caseldine 2004).
We must stress that although the circulation at
Obir is efficient, strong draughts are not felt in the
cave interior (flow of up to 0.5 m s
21
at constrictions
was mentioned earlier) and flows within chambers
could even be laminar. Hence we do not anticipate
that micron-scale particulates (e.g. rock flour) are
transported, but smaller colloidal (organic-hosted)
entities could be. A suggestive piece of evidence
is the observed and irregular increase in air tempera-
ture at the beginning of the winter circulation
regime (Fig. 1). This may imply a more vigorous cir-
culation regime at the time, which could be equated
with the event laminae that overlap with and are
partly followed by the low-S winter growth. For
example in autumn 2003, four distinct pulses of
higher temperature can be recognized (Fig. 1),
which are likely to reflect changes in the intensity
of air circulation. Such pulses would be responsible
for the distinct micron-scale enrichments in metal
content observed in Figures 10 and 11.
The chemical properties of the elements found
enriched in the event laminae are distinctive and
are precisely those expected from binding to
organic matter, supporting their interpretation as
2.
Suspension of part of the colloidal material as
aerosol
and
redistribution
within
the
cave
system.
These and other models will be tested in future
observations at the study site by attempting to
sample air currents as well as the composition of
wall deposits.
Expressions of seasonality and proxies
for climate
This study has been based on samples that show
strongly developed seasonality. The most obvious
effect observed during monitoring is the develop-
ment of a stronger winter circulation, once the exter-
nal temperature falls below that of the interior,
associated with reduced PCO
2
in cave air. The
most obvious annual phenomena in the speleothems
are the event laminae (of the order of 10
21
years),
many of which are composite of individual events
representing only a few days growth. The enrich-
ment in trace elements expected to be associated
with organic colloids could be interpreted as
reflected a seasonal hydrological flushing as in the
model set up for Grotta di Ernesto (Smith et al.
2009). Borsato et al. (2007) found the most convin-
cing relationship between abundance of such
elements and surface conditions for the Ernesto
cave stalagmite arose when a period of deforestation
around World War I led to enhanced metal contents.
However, in sample Obi84, although year-to-year
variability in element abundances is strong, it is
not related to the inter-annual variability in the
intensity of autumnal rainfall (Fig. 9). Neither has
cave monitoring detected changes in the flux of
elements in dripwater, whereas there is evidence
for unusually high and fluctuating airflows at the
start of the winter season. For this reason, an
aerosol origin for event laminae is proposed as an
alternative model for testing in future work. The
'event lamina' type of seasonality appears to be
characteristic of many cave sites in humid temperate
environments and it will be interesting to explore
the extent to which the phenomena are modified
across a wider range of climatic settings.
The reduction in CO
2
content of cave air in the
winter, as observed by monitoring, in itself leaves
no physical trace in the speleothem, unlike the
changes in saturation state in caves in seasonally dry
climates where clear and cloudy calcites alternate
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