Post on 19-Feb-2023
Carbon flux and climate change effects on Capo Caccia karst ecosystem (Sardinia, Italy)
Sanna L.1*, Arca A.1, Ventura A.1, Zara P.1 and Duce P.1
1Institute of Biometeorology, National Research Council - Italy
*Corresponding Author: sanna@ibimet.cnr.it
Abstract
The assessment of carbon sequestration or release in the terrestrial ecosystems is
crucial for understanding carbon cycling processes. Since carbon dioxide plays a key
role in karst processes, it has to be taken into consideration when studying gas
exchanges among atmosphere, biosphere, and geosphere. Even though karst areas
have been recently used to determine the contributions of multiple carbon sink
(dissolution) or source (deposition), geological processes have not been sufficiently
addressed yet in global carbon cycle model. After a short summary on cave
microclimatology, its atmosphere dynamic and subsurface-atmosphere-biosphere
gas exchanges, this work illustrates Eddy Covariance tower and cave
micrometeorological data with the purpose of assessing the correlation between
meteorological variables and carbon dioxide concentration in the shrubland karst
ecosystem of Capo Caccia (North-West Sardinia, Italy). The preliminary results show
that the ecosystem physiological features depend not only on surface and soil
environmental conditions but also on environmental forcing related to underground
voids.
Keywords: abiotic carbon dioxide, cave atmosphere, karst ecosystem, net
ecosystem carbon balance, subsurface-atmosphere-biosphere gas exchange
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1. INTRODUCTION
Many branches of science used caves as a powerful research tool, and recognize the
importance of these ecosystems from a multidisciplinary perspective [1]. Karst areas
are currently strategic zones for research related to global change because caves
are subject to change, often largely, in response to external environmental conditions
but can also detect inertial fluctuations of the climate system [2].
Even better than most surface environments, karst systems are natural laboratories
for reconstructing past climate and predict the future, since erosion that other proxies
have suffered on the surface has been attenuated in the subsurface. Moreover, the
medium-long term monitoring programs of cave microclimate have clearly showed
the role of underground karst environment in the dynamics of subsurface-
atmosphere-biosphere gas exchanges. In fact, cave atmosphere is directly
connected to the surface through cave entrances and dense network of small
fractures of carbonate rocks. Therefore, cave, soil and vegetation cover should be
considered components of a unique ecosystem.
Although the cave environments appear to be stable and their modifications occur
slowly [3], the impact of climate change on karst systems in the coming decades may
help climate and environmental scientists to better predict future scenarios and large-
scale events through the study of the intrinsic characteristics of these environments,
even though their minor modifications. To this end, it is essential to control air
temperature, relative humidity, barometric pressure and CO2 content as a tracer gas
(as well as other potential, i.e. Radon 222Rn), in order to characterize the daily and/or
seasonal variations in underground CO2 compared to the outside atmosphere.
Understanding the role of the karst in the global carbon cycle is still unclear. Recent
monitoring studies revealed that the concentration of CO2 in cave atmosphere
showed a certain degree of seasonality [4], [5], [6], [7], [8], [9].
In addition to the biotic component tied to the decomposition of organic matter, recent
studies showed that only a fraction of the abnormal amount of CO2 present in the
caves of desert or semi-arid areas is linked to this phenomenon. In addition, they
suggested an abiotic mechanism related to the processes of dissolution-precipitation
of calcite and underground ventilation, and coincident with the seasonal reversal of
the underground air masses flow [10].
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As carbon dioxide is one of the most important greenhouse gases responsible for
global warming, the aim of this work is to describe the case of Capo Caccia (North-
West Sardinia, Italy) as an example of the study carbon flux on a shrubland karst
ecosystem.
2. CAVE ATMOSPHERE
Cave environments are characterized by the lack of light, high humidity and a
relatively constant air temperature similar to the annual average surface values. The
study of climatic variations inside cavities (cave microclimatology) is complex since
the range of parameters variation are very limited, especially in the case of
temperature and relative humidity. The underground atmosphere can be considered
stable and changes can only be detected using sophisticated measuring instruments.
The absence of radiation in the underground environment makes daily cycles just
does not exist, except those determined by the influence of the contact with the
edaphic cover and, in particular, circadian biological activity of soil. Instead, in caves
can be observed other seasonal and annual cycles with small-scale periodicity
(reflecting surface records), although the result is a slow response to environmental
changes in the outer atmosphere.
The basic cave micro-environmental control consists in the continuous monitoring of
a number of key parameters of the hypogean air [11]:
Temperature tends to stabilize at values equal to the local average annual external
values, due to the large heat capacity of the rocks that damps the variations of the
incoming fluids (air and water), even a few meters from your entry in the karst. Cave
microclimate system is a low-pass filter that reduces amplitude of temperature
fluctuations.
Relative humidity. It is a measure of the water content in the gaseous state
depending on the air temperature. In general, the relative humidity of the cave
atmosphere is almost always close to 100% (that means cave air is saturated),
except in specific cases of extreme aridity. The saturation of water vapor in the cave
atmosphere can be caused by the presence of underground waters (rivers and
pools). When the humidity reaches saturation, every small variation of these
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conditions can lead to condensation or evaporation of water vapor, with an important
role in the evolution and development of speleothems.
Ventilation. Air circulation inside the caves follows a seasonal pattern showing a
change in the direction of movement of air masses, which is a factor to be taken into
account in evaluating carbon flux, especially at a small spatial scale. Air enters the
caves from the outside and is renewed very slowly through the multiple fractures of
the karst system. Cave ventilation and gases exchange by diffusion of the
underground atmosphere within its surrounding environment (mainly soil and host
rocks) regulate the levels of CO2 in the air and therefore influence the processes of
degassing and saturation of the drip waters. A karst system can behave as a
reservoir or a natural source of CO2 at yearly-seasonal scale.
Carbon dioxide in cave atmosphere can derive from disaggregation of organic matter
and its further motion, but it can originate directly by the karst process. Dissolution of
calcium carbonate (CaCO3) by carbonic acid consumes two molecules of
bicarbonate, one coming from atmosphere or soil CO2 and one further from the
carbonate minerals. Nucleation of CaCO3 during speleothem deposition releases
CO2 to the atmosphere. These two processes are thought to be balanced over long
time and global scales, resulting in zero net flux of atmospheric CO2, but recent work
proposed that they can impact local carbon cycling at short term scale as sinks and
sources for atmospheric CO2 in certain areas [12].
3. KARST-SURFACE GAS EXCHANGES
The stability of the karst systems and the degree of energy exchange with the
outside atmosphere are controlled primarily by the difference in cave/surface
temperature and relative humidity that can act filling or emptying the pores system of
the soil, that operates as an insulating double membrane between the
microenvironment confined inside the cavity and the outside atmosphere [13], [14].
A simple model of the aerodynamics in a karst system establishes that the major
mechanisms and factors involved are:
The thermal convection: the temperature difference between the cave environment
and the outside atmosphere causes the activation of the vertical currents whose
direction depends on the surface temperature (inside caves temperature is relatively
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constant). For example, during the winter cave air is relatively warm compared with
the surface air so that there is an air flow outward cave [Fig. 1]. The flow is the more
intense the larger is the underground void, and increases with the temperature
gradient.
Barometric pressure does not produce too obvious effects in underground air
circulation but in large rooms of some caves can induce very intense episodic
responses to balance the internal pressure of the cavity with the external
atmosphere. Furthermore in many karst system it is the only way to replace the in-
cave air and the only source of the energy flow.
Wind swings: the surface wind can lead to a small increase in pressure, pushing the
mass of air contained in a cave and causing an oscillatory movement, very intense
and at a very low frequency (tens of seconds). When these variations occur, the air
flow creates a new depression, followed by a reversal of the movement. This
particular phenomenon, which can be treated as an infrasound, occurs at the
entrance of the great karst systems and is caused by the inertia of large volumes of
air nass.
Fig. 1 Cave air flowing upward is often visible at the karst system entrance during very cold nights (Photo by Riccardo De Luca).
It is worth to highlight that the emissions from geological sources in karst terrains are
masked at annual scale by large amounts of CO2 exchanged between the soil and
atmosphere due to biological activity, including elevated anthropogenic emissions.
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The correct net ecosystem carbon balance is hard to assess because, to date, the
source/sink of CO2 from underground karst environments has not been considered in
the calculation of the current balance of CO2 in the atmosphere. This is particularly
important in view of the fact that approximately 10% of the rocky outcrops worldwide
are limestones and dolomites and carbonate materials which represent the largest
carbon surface at planetary scale.
Another quantitative data that highlights the importance of karst cavities in the
dynamics of atmospheric CO2 is the concentration of this gas in the hypogean
atmosphere, which average values are around 10 times higher than the mean
atmospheric concentration (390 ppm, approximately).
4. CAPO CACCIA KARST ECOSYSTEM
Capo Caccia karst area is located on the North-West coast of Sardinia. The climate
is semi-arid with an annual mean temperature and precipitation of 15.9 °C and 588
mm, respectively, and summer drought conditions [15]. A Mediterranean maquis,
constituted by sclerophyll species with a maximum height of 2.5 m over a very thin
soil, covers the outcropping Mesozoic carbonate rocks. In this area, epigean and
hypogean karst features are widespread and, in conjunction with the intricate fracture
network related to Tertiary geodynamic, give rise to an interconnection between
external and underground atmosphere. High levels of CO2 have been detected in
cave air [16] and also into the aquifer [17].
In this natural ecosystem, carbon flux is measured continuously using an eddy
covariance tower placed over the vegetation at 3.5 m above the soil. It consists of a
3D sonic anemometer and an open-path IRGA instrument. Spot measurements
inside the caves have been performed by a portable NDIR sensor (Zenith AZ7755,
range 0-10000 ppm - accuracy ±50 ppm).
Net ecosystem exchanges measured at Capo Caccia karst area by Eddy Covariance
tower showed a clear seasonal pattern with CO2 uptake by vegetation prevailing
during spring and fall depending on water availability and temperature conditions.
The ecosystem is generally a sink of carbon also showing daily and seasonal
variations. The carbon flux in this ecosystem does not only respond to physiological
features of its vegetation but reflects environmental forcing related to atmospheric
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variables and gas exchanges with soils and underground voids, where static CO2
concentration reaches value of >10,000 ppm. Unusual peaks of this gas are detected
by Eddy Covariance tower and seem to be related to karst emission when the
temperature falls.
In winter, and, in general, when external air temperature is below the annual mean
value, cave air density results lower than external. Therefore, cave air can flow
upward and outside while cold external air cannot enter into underground system.
This ventilation moves cave air masses, rich in carbon dioxide, in the surrounding
atmosphere. The CO2 seasonality tends to covary with temperature, which influences
both the production of CO2 in soils and the cave ventilation [Fig. 2].
Fig. 2 Inverse correlation between atmospheric CO2 molar fraction and temperature patterns in winter (January), spring (April), summer (July) and autumn (October) during 2012. The daily CO2
release is inhibited when the external temperature standstills over its annual average (blue areas).
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The climate change and its variations can amplify the effects of this process. In fact,
the temperature rising induces an increase of CO2 production in the soils and then in
the vadose zone leading to a higher partial pressure of carbon dioxide (pCO2) at the
top of phreatic zone. The increased CO2 concentration dissolved in the water
enhances carbonate dissolution. The gas is stored in the water until supersaturation
determines speleothem formation. Each molecule of CaCO3 is produced, two of CO2
are degassed. The same ventilation, triggered by temperature gradient between
outside-cave atmospheres, induces carbonate precipitation by evaporation [Fig. 3].
Fig. 3 Flow sheet for the movements of carbon dioxide through Capo Caccia karst ecosystem.
Moreover, it is worth to note that daily CO2 cycles are not related to specific wind
directions. On the contrary, the detected CO2 molar fraction values are lower during
windy days (average 30’-values above 2.5-3.0 m s-1) due to the effect of air masses
mixing [Fig. 4].
Fig. 4 Daily CO2 molar fraction and wind speed patterns at Capo Caccia in August 2012. The orange
areas highlight the absence of CO2 peaks during the atmospheric and underground air mixing events.
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5. CONCLUSIONS
Several researches conducted in various karst areas of the world have been shown
that the detailed study of the cave micrometeorology is a high-resolution tool for
better understanding global change processes. Caves are characterized by the
absence of light and environmental conditions largely different from the outside
temperature, humidity, partial pressure of carbon dioxide, etc. However, the physical
processes occurring inside the caves are closely related to the external environment
and this influences the microclimate of the karst systems. It has been shown that the
thermal inertia of the host rock results in a mismatch between the climatic variations
in the underground voids and the external environment [18]. In a first approximation,
a cave can be considered as a insulated environment in terms of air temperature and
water flow system. The study of the gradient of microclimate within a karst system
reflects the responses to fluctuations in the external state [19]. Experimental data of
Tarhule-Lips and Ford [20] present both short- and long-term responses to climate
change. In this context, the cave micrometeorological models can be interpreted as
the fluid dynamics inside the caves that affects their development and interacts with
the surface atmosphere and vegetation cover [21].
Understanding the underground micrometeorology is a critical goal, in particular
when the investigation focuses on the impacts of climate changes, caused by the
temperature difference between inside and outside air, on the cave environment. The
exchange of air between the cave exterior and interior increases with the
temperature difference, accelerating the corrosion which leads to carbon dioxide sink
during warm periods [22], [23]. The increase in global temperature and CO2
concentration in the air can induce the increasing of condensation and corrosion on
cave walls and speleothems [24]. For example, studies made in the karstic systems
of gypsiferous area of Bologna (Italy) where speleothems precipitation is not
influence by host rock, showed that carbonate concretions are correlated with
atmospheric CO2 variations experienced in the area [25], and this can have socio-
economic impacts on tourist caves management. Micrometeorological data collected
in Capo Caccia karst ecosystem show a strong daily correlation between the release
of carbon dioxide from the karst system and outside air temperature. The carbon
source behavior is inhibited when the external temperature standstills over its annual
average. This result indicates that the study of atmosphere-biosphere-geosphere
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interactions could allow a detailed analysis of the vulnerability of different areas to
future changes in temperature and/or humidity. Indicators for assessing the impacts
of global change could be useful in formulating a hypothesis and in helping define
and predict both the effects of climate change and their socio-economic impacts and
in turn to assess the potential anthropic impacts on underground systems. This could
give rise the opportunity for a more precise spatial-temporal characterization of cave-
surface atmosphere gases exchange, which is an important factor in determining the
processes associated with climate change.
The integrative knowledge of biological/geological origin of subterranean carbon
dioxide storage together with the understanding of mechanisms associated with
degassing or ventilation processes will allow a more accurate estimation of carbon
balances at regional spatial scales for ecosystems that insist on karst areas [26].
6. ACKNOWLEDGEMENTS
This study was conducted under the research project “An integrated system for
quantifying the net exchange of CO2 and evaluating mitigation strategies at urban and
landscape level”, funded by the Regional Administration of Sardinia, Regional Law n.
7, 7 August 2007. Many thanks to Giovanni Badino for his comments and to Riccardo
De Luca for providing his photography.
7. REFERENCES
1. Wilkens H., Culver D.C. & Humphreys A. (2000), Subterranean ecosystems,
Elsiever, pp. 791.
2. Badino G. (2010), Underground meteorology – “What’s the weather
underground?”, Acta Carsologica, No. 30(3), pp. 427-448.
3. Badino G. (2004), Cave temperatures and global climatic change, International
Journal of Speleology, No. 33(1-4), pp. 103-114.
4. Baldini J.U.L., Baldini L.M., McDermott F. & Clipson N. (2006), Carbon dioxide
sources, sinks, and spatial variability in shallow temperate zone caves:
evidence from Ballynamintra Cave, Ireland, Journal of Cave Karst Studies, No.
68, pp. 4-11.
Advances in Climate Change
SISC, Second Annual Conference Climate change: scenarios, impacts and policy
518
5. Badino G. (2009), The legend of carbon dioxide heaviness, Journal of Cave
and Karst Studies, No. 71(1), pp. 100-107.
6. Fernández-Cortés A., Sanchez-Moral S., Cuezva S., Cañaveras J.C. & Abella
R. (2009), Annual and transient signatures of gas exchange and transport in
the Castañar de Ibor cave (Spain), International Journal of Speleology, No.
38(2), pp. 153-162.
7. Ginés A., Hernández J., Ginés J. & Pol A. (1987), Observaciones sobre la
concentración de dióxido de carbono en la atmósfera de la Cova de les Rodes
(Pollenga, Mallorca), Endins, No. 13, pp. 27-38.
8. Liñán C., Vadillo I. & Carrasco F. (2008), Carbon dioxide concentration in air
within the Nerja Cave (Malaga, Andalusia, Spain), International Journal of
Speleology, No 37(2), pp. 99-106.
9. Madonia P., Bellanca A., Di Pietro R. & Mirabello L. (2012), The role of near-
surface cavities in the carbon dioxide cycle of karst areas: evidence from the
Carburangeli Cave Natural Reserve (Italy), Environmental Earth Science, No.
67, pp. 2423-2439.
10. Serrano-Ortiz P., Roland M., Sanchez-Moral S., Janssens I.A., Domingo F.,
Goddéris Y. & Kowalski A.S. (2010), Hidden, abiotic CO2 flows and gaseous
reservoirs in the terrestrial carbon cycle: Review and perspectives, Agricultural
and Forest Meteorology, No. 150, pp. 321–329.
11. Badino G. (1995), Fisica del Clima Sotterraneo, Memorie Istituto Italiano di
Speleologia, No. 7, pp. 136.
12. White W.B. (2013), Carbon fluxes in karst aquifers: sources, sinks, and the
effect of storm flow, Acta Carsologica, No. 42 (2-3), pp. 177-186.
13.Cuezva S. (2008), Dinámica microambiental de un medio kárstico somero
(Cueva de Altamira, Cantabria): microclima, geomicrobiología y mecanismos
de interacción cavidad/exterior, Doctoral Thesis, Universidad Complutense de
Madrid.
14. Fernández-Cortés A., Sanchez-Moral S., Cuezva S., Benavente D. & Abella
R. (2009), Characterization of tracer gas fluctuations in a “low energy” cave
Advances in Climate Change
SISC, Second Annual Conference Climate change: scenarios, impacts and policy
519
(Castañar de Ibor, Spain) using techniques of entropy of curves, International
Journal of Climatology, DOI: 10.1002/joc.2057.
15. Spano D., Duce P., Sirca C., Zara P., Marras S. & Pisanu S. (2006), Energy
and CO2 exchanges of a Mediterranean shrubland ecosystem, Proceedings of
the 27th Conference on Agricultural and Forest Meteorology of the American
Meteorological Society, San Diego, California.
16.Mucedda M. (1998), Grotte e pozzi della Nurra di Sassari, Bollettino del
Gruppo Speleologico Sassarese, No. 7, pp. 10-26.
17.Ghiglieri G., Oggiano G., Fidelibus M.D., Alemayehu T., Barbieri G. & Vernier
A. (2009), Hydrogeology of the Nurra Region, Sardinia (Italy): basement-cover
influences on groundwater occurrence and hydrogeochemistry, Journal, No. 7,
pp. 447-466.
18. Badino G. (2004), Clouds in cave, Speleogenesis and Evolution of Karst
Aquifers, No. 2(2), pp. 1-8.
19.Taboroši D., Hirakawa K. & Sawagaki T. (2005), Carbonate precipitation along
a microclimatic gradient in a Thailand cave - Continuum of calcareous tufa and
speleothems, Journal of Cave and Karst Studies, No. 67(1), pp. 69-87.
20.Tarhule-Lips R.F.A. & Derek F.C.,(1998), Condensation Corrosion in Caves on
Cayman Brac and Isla de Mona, Journal of Cave and Karst Studies, No. 60(2),
pp. 84-95.
21. Shindo S. (2005), Micrometeorological modeling of an idealized cave and
application to Carlsbad Cavern, NM, Master of Science thesis – Socorro
University, New Mexico, pp. 244.
22. Schmekal A.A. & de Freitas C.R. (2001), Condensation in Glow-worm Cave,
Waitomo, New Zealand: management guidelines, Doc Science Internal Series,
No. 15, pp. 5-12.
23. Russell M.J. & MacLean V.L. (2008), Management issues in a Tasmanian
tourist cave: Potential microclimatic impacts of cave modifications, Journal of
Environmental Management, No. 87, pp. 474-483.
24.de Freitas C.R. & Schmekal A.A. (2005), Prediction of condensation in caves,
Speleogenesis and Evolution of Karst Aquifers, No. 3(2), pp. 9.
Advances in Climate Change
SISC, Second Annual Conference Climate change: scenarios, impacts and policy
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25.Shopov Y.Y., Stoykova D. & Forti P. (2009), Luminescence of speleothems in
Italian gypsum caves: preliminary report, In: Gypsum Karst Areas in the World:
their protection and tourist development, Memorie Istituto Italiano di
Speleologia, No. 16, pp. 87-90.
26. Pérez-Priego O., Serrano-Ortiz P., Sánchez-Canete E.P., Domingo F. &
Kowalski A.S. (2013), Isolating the effect of subterranean ventilation on CO2
emissions from drylands to the atmosphere, Agricultural and Forest
Meteorology, No. 180, pp. 194-202.
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