THE CO₂ REGIME OF SOIL PROFILE AND ITS DRIVE TO
DISSOLUTION OF CARBONATE ROCK

Xu Shengyou, He Shiyi

 
1 INTRODUCTION

Carbonate rocks have a close relation with CO₂ of the atmosphere^[1-3]. But few attention were paid to the behavior and regime of CO₂ profile on the interface between carbonate rock and the atmosphere^[4-5]. The carbonate

rock-atmosphere system is a completely open one controlled by various environmental factors, specifically climate and men's activities. It is difficult to directly observe the regime of CO₂ profile on the interface between carbonate rock and atmosphere. Therefore, the authors took the soil profile as a medium for CO₂ production, storage and transport. One natural soil profile overlying carbonate rock was selected to study its CO₂ regime, the dissolution intensity of carbonate rock by means of the standard limestone tablets buried at different depths, and the regime of some hydrochemical parameters of karst underground water. Combining with the data of other regions, the relationship between the CO₂ regime on the soil profile and the dissolution of the underlying carbonate rock is discussed.

2. THE REGIME OF CO₂ ON THE SOIL PROFILE

2.1 The Position, Main Property of the Profile and Methodology

The soil profile is on an unsaturated zone, near the boundary between the peak cluster-depression and peak forest-plain within Yaji Karst Hydrogeological Experimental Field 9km southeast to Guilin City(Fig. 1). It is about 3m thick, underlaid by the pure limestone of Upper Devonian and covered by sparse herb vegetation. The soil is classified to brown limestone soil, of which some chemical parameters are showed on Tab. 1.

In order to understand the distribution and regime of CO₂ on the soil profile, 6 plastic tubes were installed at different depths such as -20 cm, -50cm, -120cm, -180cm, -250cm, -290cm to measure monthly the CO₂ concentration with CO₂ detector from June of 1994 to June of 1995 and get its distribution curve on the profile every month.

2.2 The Regime of CO₂ on the Profile

Based on the observation, the properties of the CO₂ regime on the profile can be summarized as follows:

(1) The concentration and distribution of CO₂ on the profile is close tied up with climate. The larger difference in CO₂ concentration on the profile occurs in dry season without leaching by rain. For instance, the result observed from June to September of 1994 shows the curves of distribution of CO₂ have bigger curvature
(Fig. 2). On the contrary, there is a less difference in the CO₂ concentration from top to bottom in rain season (e.g. in May, 1995). It indicates that the leaching process can destroy the law of distribution and diffusion of CO₂ on the soil profile, which makes the gas tend to be distributed more evenly from top to bottom. Moreover, the concentration of CO₂ on the profile changes with seasons. The data illustrates that the higher content of CO₂ appeared from July to October of 1994 and the top value(40166 ppm) occurred in August of the year, and CO₂ had lower value in concentration from November, 1994 to February, 1995 with the minimum of 4583 ppm.

Fig. 1 Sketch Map of the position of the profile

Tab.1 Some chemical parameters of the soil profile

Depth ( cm )	pH ( H2O )	Org.C ( % )	Ca2+ ( % )	Mg2+ ( % )	K+ ( % )
-10	5.62	5.10	0.073	0.435	0.86
-50	5.45	2.42	0.062	0.432	1.01
-120	5.80	0.70	0.078	0.433	1.09
-180	5.91	0.65	0.094	0.444	1.08
-250	5.45	0.70	0.052	0.462	1.06

(2) Generally, the concentration of CO₂ increases with depth and up to the top value at the depth of - 100 ~ - 120 cm; and then, decreases with depth. Accordingly, two concentration gradients, soil to atmosphere and soil to carbonate rock, are formed. That the soil from 0 cm to -20 cm is drier being not favorable to the activity of microbes, the CO₂ in the soil diffuses continuously into the atmosphere and the underlying carbonate rock extracts intensely the CO₂ from the soil in the process of dissolution, probably, is the main reason for the two gradients of CO₂ concentration formation.

Fig.2 The curves of CO₂ concentration on the profile from June. 1994 to May. 1995

During the lower temperature period from December of 1994 to February of 1995, the concentration of CO₂ on the profile increased from top to bottom forming only one soil-atmosphere gradient.

3.THE DISSOLUTION OF THE STANDARD LIMESTONE TABLETS ON THE SOIL PROFILE

In order to understand the dissolution intensity of carbonate rock at different depths, the standard limestone tablets were fixed at different places such as 100 cm (above the ground), 0 cm(on the ground), -30 cm, -60 cm, -150 cm, -220 cm,-270 cm (on the profile). They were weighed after one year to get their loss. Comparing the dissolution rates with the average concentration of CO₂ and the concentration gradients of CO₂ (Fig.3, a, b and Tab.2), some new knowledge were got as follows:

1. The tablets in the air (100 cm above the ground) have the least dissolution rate (1.5%), the ones on the ground (0 cm) have the largest dissolution rate (2.78%). The dissolution rates of the tablets in the soil range from 1.71% to 2.11% and decrease with depth coming to the rate at -220 cm just being the next to the one in the air, and then increase with depth.
2. The dissolution rate of the tablet at certain depth has no relationship with the absolute concentration of CO₂. For example, the tablets on the ground have the largest dissolution rate, but the absolute CO₂ concentration at this place is only 500 ppm. On the contrary, the dissolution rate at - 220 cm is only 1.71%, but the CO₂ concentration there is about 50.6 times as much as the one on the ground, and 2.7 times as much as the one at - 20 cm.
3. The dissolution intensity of the limestone tablets depends upon the activity of CO₂ nearby. In other words, the larger the gradient of CO₂ concentration is, the larger the dissolution rate of the tablet is. It is illustrated on Fig.3 that the dissolution rate nearly coincides with the gradient of CO₂ concentration.

Tab. 2 The relation between the dissolution intensity of limestone
tablets and the regime of Co₂ on the soil profile

Fig.3 The curves of the dissolution rate and the average content of CO₂ on the profile (a)
and the curve of the gradient of CO₂ concentration on the soil profile (b)

4 THE RELATION BETWEEN CO₂ IN THE SOIL AND THE REGIME OF THE HYDROCHEMICAL PARAMETERS OF KARST UNDERGROUND WATER

The pH value and HCO₃^- content of a karst spring (S₃₁) near the soil profile were measured during the same period. Though only 7 months' data was obtained in consequence of damaged equipment, some important informations can be got from the Tab.3 and Fig.4. The absolute concentration of CO₂ in the soil is up to the top value in August, but the content of HCO₃^- of the spring has the maximum in October. There is a difference of two months between both sides, which can not be explained by the sensitivity of epikarst zone to the environmental changes. Replacing the absolute concentration of CO₂ with the gradient of CO₂ concentration, both of them nearly synchronously change and come to the maximum in October. The fact is supported by another case in Zhenan County of Shaanxi Province. In the period from 1993 to 1994, the pH value, HCO₃^- content of Yudong underground stream and the CO₂ regime in soil profile nearby were monthly measured. The data shows that the pH and HCO₃- do not change completely along with the absolute concentration of CO₂ there is a difference of two months (Fig.5, Fig.6). However, contrasting them with the gradient of CO₂ concentration, it can be seen that the pH value and HCO₃^- content fluctuate synchronously with the gradient of CO₂ concentration. The result indicates only absolute concentration of CO₂ can not state clearly the relation between the regime of CO₂ and dissolution, but gradient more available.

Fig. 4 Curves of the absolute concentration of CO₂ the gradient of CO₂ and HCO₃^- of  spring S₃₁

Tab.3 Regime of CO₂ and HCO₃^- content changes of S₃₁

Fig.5 The absolute concentration of CO₂ in the soil near Yudong underground stream, Zhenan, Shaanxi(1993-1994)

Fig.6 Diagram showing the relation between pH, HCO₃^- of Yudong underground stream and the gradient of CO₂ concentration in the soil nearby, Zhenan, Shaanxi(1993-1994)

5 CONCLUSIONS

1. The distribution and regime of CO₂ on the soil profile in carbonate rock area is close related to climate and its seasonal changes.
2. In general, there are two gradients of CO₂ concentration, i.e. one from soil into the atmosphere, another from soil to carbonate rock, but in winter, only one gradient from soil into the atmosphere.
3. The dissolution intensity of limestone tablets in soil depends on the gradient of CO₂ rather than the absolute concentration of CO₂. Furthermore, the pH value and HCO₃^- content of karst underground water change synchronously with the gradient of CO₂ concentration. The fact indicates that karst process in karst dynamic system is not only related to the concentration of CO₂, but more related to the activity of CO₂.

Acknowledgments

This is funded by the project of the Ministry of Geology and Mineral Resources (No. 8502218) and the project of National Natural Science Foundation (No. 49070155), and supported by Prof. Yuan Daoxian, an Academician of the Chinese Academy of Science.Also Cao Jianhua, Jiang Zhongcheng did some work in the field.

References

1. Yuan Daoxian. Sensitivity of karst process to environmental change and its records along PEP-II Transect. Paper for a PAGES/IGBP PEP II Working Group Meeting, Apr. 9-12,1994, Beijng, China
2. Yuan Daoxian. Carbon cycle and global karst. Quaternary Sciences.1993, (l): 1-6 (in Chinese)
3. Weng Jintao. The effect of carbonate rocks on global carbon cycle. Advance in Earth Sciences, 1995,10(2): 154-158 (in Chinese with English abstract)
4. P. A. Suchet et al.. Modelling of atmospheric CO₂ consumption by chemical weathering of rocks: Application to the Geronne, Congo and Amazon Basins. Chemical Geology, 1993,107:205-210
5. Striegl, R.G., Armstrong, D.E.. Carbon dioxide retention and carbon exchange on unsaturated Quaternary sediments. Geochem. Cosmochem. Acta, 1990,(54): 2277-2283

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