Carbon flux and climate change effects on Capo Caccia karst ecosystem (Sardinia, Italy)

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: [email protected]

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

SISC, Second Annual Conference Climate change: scenarios, impacts and policy

Advances in Climate Science

509

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].

Advances in Climate Change


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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.

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Carbon flux and climate change effects on Capo Caccia karst ecosystem (Sardinia, Italy)

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