Petrology and geochemistry of annually laminated stalagmites from an Alpine cave (Obir, Austria): seasonal cave physiology - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 311

near Beijing where a series of pulses of organic
carbon are found during a wet season (Tan et al.
2006; Ban et al. 2008). However, a review of all
the evidence concerning the Obir laminae indicates
that we need also to consider an alternative origin as
aerosol laminae.
The first line of evidence concerns the patterns
of chemical change associated with the laminae.
Of the colloidally-associated species identified
from Ernesto, interferences with more abundant
species prevented microanalysis of Y in Obi84,
but all the other species are found enriched together
in event laminae together with iodine, which has
been analyzed for the first time. Our unpublished
UV-fluorescence and Fourier-Transform infrared
analyses indicated that the infiltration laminae at
Ernesto host organic compounds (previously obser-
ved by standard UV-lamp excitation) in addition to
trace metals, and literature documents that most
trace metals detected could be complexed, espe-
cially by humic substances. This fits readily with
our identification of iodine enrichments since there
is also a large body of evidence, summarized by
Steinberg et al. (2008), that a large fraction of
soluble iodine is associated with colloidal humic
substances in terrestrial environments. It is signifi-
cant also that Cl, which shows minimal complexing
effects, shows no variation in the stalagmite. Hence,
this pattern of element enrichments clearly supports
a similar mechanism of formation of trace element
laminae at both cave sites. UV-fluorescence is not
readily detectable from direct excitation of the
Obir samples, but fluorescent organic matter is
recovered by dissolution of the speleothem calcite
in weak acid (A. Hartland, pers. comm. 2009).
This is consistent with the much deeper (70 m
v. 10 - 15 m) position of S¨ulenhalle relative to the
ground surface compared with Ernesto.
Secondly, we need to consider the delivery of the
elements to the speleothem. A mass balance can be
calculated for Obi84 given the mean discharge of
dripwater at this site (which varies less than the
other drips studied, Table 1), the mean composition
of dripwaters (Tables 1 and 4), and the mean compo-
sition of the top 10 mm of the stalagmite (Table 2).
The annual discharge is 287 litres and the annual
volume of calcite precipitated on the top surface
of the stalagmite is c. 0.23 g (given a thickness of
0.12 mm and diameter of the top surface of
30 mm). The results, in terms of % removal of the
species from the solution, are shown in Table 5
along with the distribution coefficients previously
discussed. It can be seen that only a small fraction
(0.63%) of Ca is removed from the dripwater
and much smaller percentages of Mg, Cu and
Y. However the calculated figures for Zn (5.7%)
and Pb (14%) are high, but still well below 100%.
Hence,

supply sufficient trace elements. However, as has
been shown, the event laminae display concen-
trations 3 - 10 times higher than outside these zones,
but there is no indication of a seasonal variability
in concentration, even though three of the water
samples were collected in November when infiltra-
tion might be expect to be at a maximum. It
appears that much higher rates of delivery of ions
is required in the autumn season and yet the drip-
water data do not clearly indicate higher rates
of discharge at this time.
One can construct a hydrological mechanism by
which this could occur, using the plumbing model
concepts in Tooth & Fairchild (2003) and Fairchild
et al. (2006b). If a proportion of the dripwater during
the autumn season derived from a fracture-fed
source with high colloidal content, it could result
in significant changes in dripwater chemistry. The
fracture-fed flow might enter a reservoir with an
overflow such that the outflow to the drip had an
approximately constant pressure head, or the
volume of fracture-fed water might be small.
However, the weakness of the argument is the lack
of clear evidence for enhanced trace element
concentrations in the archived samples of autumn
dripwater, and so an alternative needs to be

explored.
Given the uncertainty as to whether dripwater
could supply sufficient elements for the event
laminae, we now consider as an alternative expla-
nation that the trace element enrichments represent
deposition from aerosol in the cave system. This
hypothesis only emerged after our analyses and
specifically after the period of more intense cave
monitoring (2001 - 2003) when instantaneous
water samples were available at this logistically
difficult site. As a result, we currently lack specific
observations that could be used to support this
hypothesis, so we will argue in more general terms.
One point that emerged from the 14 C study in Smith
et al. (2009) was the result that the 14 C concen-
tration of the speleothems increased in parallel
with that of the external atmosphere during the era
of atmospheric bomb testing, in contrast with the
normal case where the radiocarbon bomb peak is
assumed to be purely related to aqueous transport
from the soil and hence is delayed via soil recycling.
Smith et al. (2009) concluded that there must, in
addition to soil transport, be isotopic exchange of
carbon between air and solution, despite the net
degassing of the latter. This result was unexpected,
but draws attention to the air as a potential source
of chemical species for speleothem deposition.
Aerosols (liquid droplets and fine particles) are
generated within the cave system or brought from
the outer atmosphere within the cave by air currents
(Cigna & Hill 1997). Deposition from aerosols con-
tributes to the accretion of speleothem deposits at on an annual basis, dripwaters could