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| Titel |
Carbon dioxide budgets in cave air and carbon in speleothems; insights from
a shallow cave in Ireland |
| VerfasserIn |
Frank McDermott, Dominika Phillips |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141876
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| Publikation (Nr.) |
 EGU/EGU2017-5429.pdf |
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| Zusammenfassung |
| The conventional view that hydrological inputs (e.g. drip-water degassing) comprise the
dominant source of cave air CO2 has been challenged by recent studies that emphasise
the importance of direct advection of gaseous CO2from above and beneath cave
voids (e.g. ‘soil air’ and ‘ground air’). A better understanding of CO2 gas budgets
in caves is important, not only for the correct interpretation of δ13C values and
14C activity data in speleothems, but also for an understanding of the wider role
of karst in the global carbon cycle as a source or sink of atmospheric CO2. This
study presents new results from a combined air-temperature and CO2 monitoring
programme at a small multi-chamber cave in SE Ireland (Ballynamintra cave, Co.
Waterford), building on an earlier study at this cave (Baldini et al., 2006). Episodic,
low-amplitude but temporally coherent diurnal-scale cave air temperature fluctuations
detected almost simultaneously by a series of temperature loggers within the cave
were used to detect air mass advection. The sequence and pattern of temperature
fluctuations at different locations within the cave enabled the identification of discrete
air-inflow and air-outflow events. These diurnal-scale events occur episodically
throughout the year in the winter/ spring and summer/autumn temperature ventilation
regimes of the cave. Importantly, changes in cave air pCO2 values recorded by
an infra-red logger located in the inner chamber a few metres from the back of
the cave occur contemporaneously with the air-mass displacement events, and are
consistent with direct advection of CO2-rich soil air via fractures in the subjacent
cave roof and walls. In the winter regime, episodic diurnal-scale air outflow events
draw CO2-rich air over the logger, resulting in short-lived pulses of air, typically
containing c. 0.7% CO2 (by volume), several times the ambient cave air CO2 values
at this site. Similar events occur during the summer/autumn thermal regime, but
these reach higher CO2values (1-1.2%), similar to those measured previously in the
overlying soil. Overall, the data confirm an important role for soil and/or ground air
sources at this cave and indicate that the episodic CO2 inputs are not controlled
by drip-water inputs,. Some recent studies have additionally argued that advected
‘ground-air’ is not only an important constituent of cave air, but also an important
source of carbon in speleothems. This claim is critically evaluated here using 14C
activity measurements from actively growing zero-age soda-straw stalactites from
the small inner chamber of the cave where the CO2 monitoring was carried out.
Surprisingly, soda-straws collected from within a few metres of each other in this inner
chamber exhibit quite different 14C activities (93-101 pMC), and are not identical as
might be expected if complete carbon isotope exchange had occurred between the
dissolved inorganic carbon and the cave atmosphere. The reasons for this will be
discussed, drawing on the results of published kinetic models for degassing and
isotope exchange. Overall, it is concluded that while the CO2 budget of the air in
Ballynamintra cave is dominated by directly advected soil air, water transported
dissolved inorganic carbon (DIC) likely remains an important carbon source for its
speleothems.
Baldini, J.U.L., Baldini, L.M., McDermott, F. and Clipson, N. (2006) Carbon dioxide
sources, sinks, and spatial variability in shallow temperate zone caves: evidence
from Ballynamintra Cave, Ireland. Journal of Cave and Karst Studies, 68, 4-11. |
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