Contribution of Carbonate Rock Weathering to the Atmospheric
CO₂ Sink

Zaihua Liu, Daoxian Yuan, Shiyi He
(Institute of Karst Geology, Guilin, 541004, P.R. China)
Jinbo Zhao
(Xi’an College of Technology, Xi’an, 710054, P.R. China)

ABSTRACT

To accurately predict future CO₂ levels in atmosphere, which is crucial in predicting global climate change, we must determine the sources and sinks of the atmospheric CO₂. In this paper, case studies are reviewed using published and unpublished data. Firstly the sensitivity of carbonate rock dissolution to the change of soil CO₂ and runoff will be discussed, and then the amount of CO₂ removed from the atmosphere in the carbonate rock areas of mainland China and the world will be determined by hydrochem-discharge method, carbonate-rock-tablet-method and the DBL (Diffusion Boundary Layer)-model calculation, to get an estimate of the contribution of carbonate rock weathering to the atmospheric CO₂ sink. This contribution (in the range of 0.11~0.41 billion metric tons of carbon/a) can not be neglected and should be taken into consideration in the global carbon cycle model.

It has been known (Quay, 1992; Watson et al., 1990) that the combustion of fossil fuels releases about 5.4 billion tons of carbon a year as CO₂ into atmosphere. In addition, deforestation practices contribute about 1.6 billion tons of carbon a year to atmospheric CO₂. So, the total input of CO₂ from human activities is about 7.0 billion tons of carbon a year. However, only about 3.4 billion tons of carbon a year accumulates in the atmosphere. That means there is an atmospheric CO₂ sink of about 3.6 billion tons of carbon a year.

To accurately predict future CO₂ levels in atmosphere, which is crucial in predicting climate change, we must determine the CO₂ sinks. Although extensive efforts have been made to trace the missing carbon (Berner, 1997; Degens et al., 1991; Hesshaimer et al., 1994; Quay et al.,, 1992; Ritschard, 1992; Sarmiento & Sundquist, 1992; Siegenthaler & Sarmiento, 1993; Tans et al., 1990; Yager et al., 1995; Yoshimura, 1997; Yuan, 1997), the explanation is still unclear.

As the world? biggest carbon reservoir, carbonate rocks contain about 6.1x10⁷ billion tons of carbon, which is 1694 times and 1.1x10⁵ times larger than those of oceans and world vegetation respectively (Houghton & Woodwell, 1989). Carbonate rocks occupy an area of about 22 million square kilometers in the world (Yuan, 1997).

The basic reactions for carbonate rocks weathering can be expressed by:

  CaCO₃+CO₂+H₂O <----> Ca²⁺+2HCO₃^- (1) for limestone

  CaMg(CO₃)₂ +2CO₂+2H₂O <---> Ca²⁺+Mg²⁺+4HCO₃^- (2) for dolomite.

where CO₂ may come from the atmosphere directly in bare carbonate rock areas, or from soil in overlying and/or buried carbonate rock regions.

It can be easily visulized from the above reactions that the carbonate rock weathering contributes to the atmospheric CO₂ sink (Note that the consumption of CO₂ in soil by the weathering decreases the release of soil CO₂ into atmosphere, and thus also contributes to the atmospheric CO₂ sink). For limestone weathering, the removal of 1 mol CaCO₃ needs 1 mol of CO₂ from atmosphere; and for dolomite weathering, the removal of 1 mol CaMg(CO₃)₂ needs 2 mol of CO₂ from atmosphere. It is also clear that only half of the carbon in solution is from atmospheric CO₂ (eqns. (1) and (2)). On the other hand, the backward reactions of (1) and (2), e.g., the formation of tufas, are related to the release of CO₂ into the atmosphere. It is very difficult to estimate the net CO₂ flux at individual cases. For example, the corrosion rate of limestone tablets in soil indicates only the CO₂ exhausted in the soil, where the deposition of calcite may occur. In addition, the net amount of CO₂ removed from the atmosphere in a given catchment area is equivalent to the total amount of limestone dissolved and transported outside the area via groundwater flow and/or rivers. Therefore, the net amount can be estimated by the limestone corrosion and the discharge of groundwater and/or rivers.

In this paper, case studies are reviewed using published (Liu, 1992; Liu et al., 1997; Ogden, 1982; White, 1984; Yoshimura, 1997; Yuan, 1997) and unpublished data. Firstly the sensitivity of carbonate rock dissolution to the change of soil CO₂ and runoff will be discussed, and then the amount of CO₂ removed from the atmosphere in the carbonate rock areas of mainland China and the world will be determined by hydrochem-discharge method, carbonate-rock-tablet-method and the DBL-model calculation, to get an estimate of the contribution of carbonate rock weathering to the atmospheric CO₂ sink. It will be shown that this contribution can not be neglected and should be taken into consideration in the global carbon cycle model.

a. CO₂ partial pressure (Pco₂ ) at depth of 50 cm in soil was measured in situ with CO₂-GASTEC meter monthly to monitor the variation in soil CO₂ with time (Liu, 1992; Yoshimura, 1997).

b. Temperature, pH, [Ca²⁺] and [HCO₃^-] of water were measured in situ with portable pH-meter and alkalinity meter monthly (Liu, 1992). CO₂ partial pressure in water was calculated with WATSPEC computer program (Wigley, 1972), by using the field observation data. These data were used to examine the sensitivity of carbonate rock weathering to the change in soil CO₂ (Liu et al., 1997), and to estimate its contribution to the atmospheric CO₂ sink with data of discharge (Liu, 1992; Yoshimura, 1997; Yuan, 1997) (hydrochem-discharge method, see below).

c. To compare the results by the hydrochem-discharge method, corrosion tests with limestone tablets in atmosphere and soil (carbonate-rock-tablet-test method, see below) were used (Yuan, 1997), and finally the maximum potential contribution of carbonate rock weathering to the atmospheric CO₂ sink was given by using the DBL Model (Dreybrodt & Buhmann, 1991; Liu & Dreybrodt, 1997).

As examples, two cases will be shown in the following.

Fig.1 (a) shows seasonal change in [Ca²⁺], [HCO₃^-] and CO₂ partial pressure (Pco₂) in water, and soil CO₂ partial pressure at the observation site of Yudong (Fish-cave) Underground Stream, which is located at Zhen?n County of Shanxi Province, in a climatically transitional zone between North and South China. The mean annual air temperature here is about 11^oC, and 850 mm for mean annual precipitation. The karstified rock is predominately Carboniferous-Permian limestone. Due to the sinkholes in recharge area, the Yudong Underground Stream is connected to the peak-cluster depressions, where terra rossa and loess formed. The length of the stream is about 30 km, with a catchment area of 85 km² and flood peak discharge of about 10 m³/s.

Figure 1(a)&1(b)  Seasonal and multi-year change of hydrochemistry and its sensitivity to the change in soil CO₂ partial pressure

It can be seen that soil CO₂ partial pressure changes remarkably during a year, with the maximum in summer growing season, and minimum in cold winter. Related to this, the [Ca²⁺], [HCO₃^-] and Pco₂ in water also show remarkable coincident change. That means that carbonate rock weathering is very sensitive to the soil CO₂ change (refer to eqn.(1)).

The sensitivity of carbonate rock weathering to soil CO₂ change was also found at the Guilin Karst Experimental Site, which is situated in the southeast of Guilin, about 8 km away from Guilin City, and near Yaji village (Fig.1 (b)). The site is at the boundary of peak-cluster depression and peak-forest plain. The catchment area of the site is about 1.1 km². The strata of the experimental site is mainly pure limestone of Upper Devonian, with thin soil cover in the depressions. The major vegetation are bushes and grasses. The annual mean air temperature and the annual mean precipitation are 19^oC and 1900 mm, respectively. Precipitation is the sole recharge to groundwater in the site (Liu, 1992).

In addition to the seasonal change, Fig.1(b) also shows the increase in soil Pco₂ in a multi-year scale. The latter is related to the reforestation at the site since 1993, and/or the increase in the atmospheric CO₂ content (Harrison et al., 1993). The increase in soil Pco₂ drives the weathering of carbonate rock, resulting in the increase in [Ca²⁺], [HCO₃^-] of karst water (Fig.1 (b)). This is also proven by the fact that the corrosion flux of limestone tablets in the Guilin Exp. Site increased from 1993 to 1995 (Tab.1).

Table 1 Change in corrosion flux of limestone tablet in the Guilin Exp.  Site from
1993 to 1995(unit:mg×cm^-2a^-1)

Sensitivity of carbonate rock weathering to the precipitation-evapotranspiration (P-E) or runoff change is reflected in the relationship between denudation rate of carbonate rocks and runoff. Karst denudation rate is defined as the annual removal of carbonate rock from a carbonate area and is measured in m³km^-2a^-1. This unit corresponds to an average lowering of the area by 1mm in thousand years (1mm/ka).

Fig.2 gives some reported denudation rates as a function of (P-E) (Yoshimura, 1997; White, 1984). The data points can be fitted by a linear relation DR=0.0544(P-E)-0.0215 with a correlation coefficient r=0.98. That means the carbonate rock weathering is very sensitive to the runoff, i.e., the larger the runoff, the more intensive the carbonate rock weathering. This may be the main reason for the great difference in karstification between south and north China. It also explains why the contribution of carbonate rock weathering to the atmospheric CO₂ is much larger in south China, where there is a strong runoff, than that in north China (see below).             

Figure 2 Relationship between denudation rate of carbonate rocks and runoff

By using hydrochemical and discharge data, the flux of atmospheric CO₂ consumed in carbonate rock weathering can be estimated by:

      F=1/2× [HCO₃^-]× Q× M_CO2/M_HCO3 (3)

where [HCO₃^-] is HCO₃^- concentration in water (g/l); 1/2 means that only half of the carbon in solution is from atmospheric CO₂ (eqns. (1) and (2)); Q is discharge of water in a studied area(l/s), equal to the product of the area and the runoff module; M_CO2 and M_HCO3 are molecular weight of CO₂ and HCO₃^- respectively.

Tab.2 gives the area of the bare carbonate rocks, HCO₃^- concentration in karst water, runoff module of karst water in China (after 1:4,000,000 karst hydrogeological map of China by Li Guofen, 1992), and then the amount of atmospheric CO₂ consumed during carbonate rock weathering in both south and north China could be calculated by the equ.(3) (Tab.2). It is seen that the contribution of carbonate rock weathering to the atmospheric CO₂ sink is 1.397× 10¹³ g/a in the bare karst areas of south China, and 0.337× 10¹³ g/a in the bare karst areas of north China. The total is 1.734 × 10 ¹³ g/a. It would be 6.577× 10¹³ g/a and 4.206× 10¹⁴ g/a if the latter is applied to the whole China karst areas (3.44 million km² ) and the whole world karst areas (22 million km² ) respectively.

Table 2 The relevant parameters to calculate CO₂ sink during carbonate rock weathering in south and north China

Seven monitoring stations which represent the typical karst areas in China were built (Fig.3), where corrosion tests of the standard limestone tablet (with a surface area of 28.91cm² and insoluble matter content of 0.97% ) was carried out in the implementation of IGCP 299 "Geology, Climate, Hydrology and Karst Formation" and 379 "Karst Processes and the Carbon Cycle" (Yuan, 1997).

Figure 3 Distribution of bare karst areas and monitoring stations of karst  system in China   1 bare carbonate rocks; 2 reef ; 3 monitoring station

Tab.3 gives the corrosion test results from six of these seven monitoring stations (one failed due to the loss of the samples), then the amount (right) of atmospheric CO₂ consumed in carbonate rock weathering can be estimated with the following formula:

A= F× S× C× Mco₂/M_CaCO3 (4)

where F is corrosion flux of limestone tablet (g/cm²× a ) , S- the area of studied carbonate rock (cm²), C-the CaCO₃ content of the limestone tablet, Mco₂ and M_CaCO3 - the molar weight of CO₂ and CaCO₃ respectively.

Table 3 Corrosion of the standard limestone tablets in some monitoring stations in 1994
(unit: g× a^-1)

^*-the value in 1995.

The calculation results of the amount of atmospheric CO₂ consumed in carbonate rock weathering in various karst types in China are shown in Tab.4. The total amonut of atmospheric CO₂ consumed in carbonate rock weathering is about 1.696 × 10¹³ g/a in China bare karst areas. It would be 6.432× 10¹³ g/a and 4.114× 10¹⁴ g/a if this value is applied to the whole China karst areas and the whole world karst areas respectively.

Table 4 Distribution of the main karst types and the atmospheric CO₂ sink  during
carbonate rock weathering in China

According to Liu & Dreybrodt (1997), the dissolution rate of calcite in CO₂-H₂O solutions with turbulent motion can be approximated by a linear rate law R=a (C_eq-C), where c_eq is the equilibrium concentration with respect to calcite and a a rate constant, dependent on temperature T, CO₂ partial pressure Pco₂, DBL (diffusion boundary layer) thickness e and the thickness of the water sheet flowing on the mineral d . If we take T=10^oC, Pco₂=5× 10^-3atm, d =1cm and e =5× 10^-3cm, which are the reasonable values in the nature (Dreybrodt, 1988), then a =2.03× 10^-5cm/s and C_eq=1.62× 10^-3 mmol/cm³. Taking the average value 2× 10^-4mmol/cm³ of [Ca²⁺] in rainfall, we obtain R=2.88× 10^-8 mmol× cm^-2s^-1. This corresponds to 288 mm/ka if water runs down continuously. Assuming rainfall to occur only during 20% of the time (Dreybrodt, 1988), an annual retreat of bedrock of about 57.6mm/ka will result. That means potential atmospheric CO₂ sink by this carbonate rock weathering is estimated to be 2.354× 10¹⁴ g/a (or 0.0642 billion metric tons of carbon/a) and 1.505× 10¹⁵ g/a (or 0.41 billion metric tons of carbon/a) in whole China carbonate rock area and the whole world carbonate rock area respectively. These values are about 3.66 times those obtained by hydrochem-dicharge method or by carbonate-rock-tablet-test method (average 0.11 billion metric tons of carbon/a). The latter may represent the net effect of carbonate rock dissolution and deposition. If these values were accurate enough, the effect of carbonate deposition would be 0.3 (i.e., 0.41-0.11=0.3) billion metric tons of carbon/a (returning to the atmosphere).

The contribution of carbonate rock weathering to the atmospheric CO₂ sink is about 0.11 billion metric tons of carbon/a by both hydrochem-discharge method and carbonate-rock-tablet-test method. The potential value, by the DBL-model calculation, is about 0.41 billion metric tons of carbon/a. These values correspond to about 5.5% to 20.5% of the "missing CO₂ sink", which is 2.0 billion metric tons of carbon/a (Tans et al., 1990).

Moreover, according to the data from the Guilin monitoring station in 1993-1996, the consumption of atmospheric CO₂ during carbonate rock weathering increased from 6.129× 10⁹ g c/a in 1993 to 11.582× 10⁹ g c/a in 1995 (Tab.1) due to the increase of soil CO₂ (Fig.1), which was related to reforestation and/or global increase in atmospheric CO₂ (Harrison et al., 1993). This means that the contribution of carbonate rock weathering to the atmospheric CO₂ sink increases with the lifting of the atmospheric CO₂ content. So, the carbonate rock functions as an adjustor to the atmospheric CO₂ .

Therefore, as an important and potential sink for the atmospheric CO₂, the carbonate rock weathering should be considered in the global carbon cycle model.

This work is based on the UNESCO/IUGS IGCP 299 and 379 Projects, and supported jointly by the Ministry of Geology and Mineral Resources, and the National Natural Science Foundation of China.

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