The Carbon Cycle in Karst
Yuan Daoxian, Guilin
with 7 figures
Summary. The traditional conceptualisation of the CO2-H2O-CaCO3 system has played a role in introducing earth system science into the study of karst. It is a model for the karst dynamic system (KDS) and is useful in explaining the carbon cycle in karst. It is a triphase open disequilibrium system characterized by a high sensitivity to environmental change, and so needs special methodologies for its research. The basic functions of the system are: to drive the formation of karst features and related environmental problems; to contribute to the regulation of greenhouse gases in the atmosphere and to the mitigation of environmental acidification; to drive the migration, enrichment and precipitation of certain elements, and thus the formation of mineral deposits, and thereby influence the biodiversity of life in karst areas; and to record the course of environmental change. The global annual removal of carbon from the atmosphere by karst processes is estimated as 6.08x108 t/a, of which Chinese karsts contribute 3.83x106 t/a .
1. Introduction
Karst processes are part of the world carbon, water and calcium cycles that occur at the interfaces between lithosphere, hydrosphere, atmosphere and biosphere. The programmes of UNESCO/IUGS IGCP 299 "Geology, Climate, Hydrology and Karst Formation" (1990-1994) and IGCP 379 "Karst Processes and the Carbon Cycle" (1995-1999) promoted investigation of these topics. The late Dr. M. M. SWEETING made an important contribution to the initiation and implementation of IGCP299 and to the application for its successor IGCP 379. She was involved in the 6700 km long second field correlation investigation of IGCP 299 in her seventieth year, which examined three major types of karst in mainland China (subtropical karst in the south, high mountain karst in the west, and semiarid karst in the north). Her participation was very helpful in reaching a common understanding on the principles of the project. Accordingly, it is pertinent to draw from the results of the two projects here as a dedication to this outstanding karstologist of the latter part of this century.
The guideline for IGCP299 was to investigate karst morphology and its associated environmental background (water, heat, and chemical and biogenic energy) and then determine the physical-chemical mechanisms of karst formation. Its objectives were:
(1) to identify the global differences in karst feature complexes and to clarify the regularities of their distribution. The ‘karst feature complex’ was adopted as the tool to avoid confusion of isomorphism that might arise in world karst correlation. It is defined as a set of karst features, including macro and micro forms, surface and subsurface forms, as well as dissolutional and depositional forms that require similar environmental backgrounds for their formation;
(2) to study the relationships between karst feature complex, karst processes and the geological, climatic and hydrological background;
(3) to reconstruct the course of paleoenvironmental change from karst records; and
(4) to apply the experiences of evaluation, prediction, and exploitation of natural resources and environmental protection to various karst areas. Investigations took place in Turkey, China, Russia, USA, Australia, Iran, Canada and the UK. Thirty-two correlation sites were provided by national working groups and a number of karst areas under different environments were selected to reveal the origin of karst formation by monitoring associated carbon cycle and water cycle conditions.
The results of IGCP299 indicate that karst studies can do much to help understand the global carbon cycle, and thereby can contribute more to an understanding of global change. This is so especially in regard to the greenhouse gas problem, but also in finding more paleoenvironmental proxies to test the output of general circulation models. Accordingly the objectives of the successor project IGCP379 are:
(1) to assess the contribution of carbonate rocks and karst processes to the content of CO2 in the atmosphere;
(2) to compare the annual balance of CO2 between the atmosphere and karst systems within different geological, climatic and ecological environments;
(3) to determine the origin and amount of annual emission of CO2 into the atmosphere from karst areas with geothermal or volcanic activities or active faults, especially in areas near plate margins; and
(4) to provide information about the processes of environmental change after the late Pleistocene in major karst regions of the world, especially in those regions without other paleoenvironmental proxies.
2. The karst dynamic system
The global carbon cycle has been discussed by many authors, including SUNDQUIST (1985), BERNER & BERNER (1987,1994), MACKENZIE & MACKENZIE (1995) and SHARP (1995). Different forms of global carbon cycle are conceptualized in fig. 1, which include not only the short-term,medium-term and long-term carbon cycling suggested by MACKENZIE & MACKENZIE (1995, 123-130), but also the karst processes. Short-term cycling includes modern photosynthesis and respiration, whereas medium-term cycling includes the formation and burning of coal, oil and gas, as well as kerogene dispersed in sedimentary rocks. Long-term cycling refers to rock weathering, sedimentation and plate subduction processes. Karst processes are mainly implicated in the latter. It is accepted to be important over geological time (responsible for warming in Cretaceous and cooling in the Permo-Carboniferous), but not on a human time scale (BERNER & BERNER 1987). Recent work, including that in IGCP 299 and 379 projects, shows that carbonate rocks, which constitute the biggest carbon reservoir on Earth, are still actively involved in the modern carbon cycle as part of the karst dynamic system (KDS).
A karst dynamic system involves the transfer of energy and matter within the carbon, water and calcium (and magnesium) cycle. It occurs at the interfaces of the lithosphere, hydrosphere, atmosphere and biosphere and controls the formation and evolution of karst, but is moderated by already formed karst features.
The traditional CO2-H2O-CaCO3 system model is conceptually presented in fig.2. It can be found in similar form in various works, such as by BOGLI (1960, 1980), SWEETING (1972), TRUDGILL (1985), and FORD & WILLIAMS (1989). Its clear expression of the involvement of CO2 in karst processes requires karst study to go beyond a consideration simply of water-rock interactions and necessitates incorporation of an earth system science approach.
2.1 Structure and configuration
The KDS is composed of solid, liquid and gaseous phases. Part of the solid phase is dominated by different types of carbonate rocks with complex networks of fissures and joints. The liquid phase involves water flow with Ca+ ( and Mg+), HCO3-, CO32- , H+ and dissolved CO2 as its major chemical constituents. There is a boundary layer in the liquid phase bordering the solid surface, with its thickness controlled by the hydrodynamic condition of the flow, and playing an important role on karst processes for both dissolution and precipitation. The gaseous phase involves CO2 as a gas and in a dissolved state.
Because the KDS is an open system, its boundaries not only are configured by the already formed karst features, but also are closely interlinked with the lithosphere, hydrosphere, atmosphere and biosphere For instance, the solid phase at its lower part, not only is connected with the whole lithosphere through the carbonate rocks and the network of fisssures within it, but also is linked with the mantle through modern active deep faults. Thus CO2 originating from the mantle takes an active part in the operation of the KDS. The aqueous phase plays the key role in the system by introducing biospheric, atmospheric and human activities into karst processes through actions such as absorbing water and carbon from the system by photosynthesis, and changing hydrodynamic conditions by hydroengineering construction.
2.2 The behaviour of the KDS
A basic characteristic of the karst system is the sensitivity of its responsiveness to environmental change, as a part of the world carbon and water cycles. The behavior of the KDS can be identified by the mutual relationship between four parameters, i.e., temperature, pH, HCO3 - and CO2 (fig.2). Data from two years’ (1993 and 1994) monitoring of 7 karst systems with different environmental backgrounds (fig.3) has provided repeated evidence that shows the sensitivity of the systems to environmental conditions of rainfall, temperature, vegetation, Pco2 and water flow .All can change the intensity and direction of CO2 flux in the system, and thus can alter the intensity, manner, and even the direction of karst processes (dissolution versus precipitation). The time scale of such sensitivity is not only seasonal, but also could be daily, hourly or less.
Because of the complicated movement of matter and energy between the lithosphere, hydrosphere, biosphere and atmosphere, isotopic approaches and physical and mathematical modelling are important for a real understanding of the operation of the KDS.
Fig. 2 Conceptual model of a karst dynamic system (modified after BOGLI 1960).
Fig. 3 Sites for monitoring the dynamics of karst systems in China
3. The functions of the KDS
The karst dynamic system drives the formation of karst features (and a series of related environmental activities); contributes to the regulation of the greenhouse gases in the atmosphere (and helps mitigate environmental acidification); drives the migration, enrichment and precipitation of a number of elements (and thus influences both the formation of mineral deposits and the biodiversity of life in karst areas); and records the course of environmental change.
3.1 Karst formation and environment
The principal function of the KDS is to drive karstification. This activity and its close relation with environmntal change is shown clearly and repeatedly from many KDS monitoring sites. Two sites from China illustrate this. In Yudong underground stream, Zhen’an county (fig.3), Shaanxi Province, monitoring was carried out in 1993 and 1994. It is on the southern part of Qinling Mountain in a subtropical area with tower karst and dolines.
The HCO3 data in water shows intensive dissolution in the system, but also great seasonal variation (fig.4) from about 150-160 mg/l in winter to 210-220 mg/l in summer. It also appears to show positive correlation with soil CO2 and negative correlation with pH value of water. The time of greatest HCO3 concentration in August is not coincident with the period of highest soil CO2 content in June. Both years seem to show a 2-months lag. However, comparing CO2 at depths of 20cm and 50cm indicates the time of the highest dissolution to coincide with the greatest CO2 difference (or CO2 gradient), just after the month of highest rainfall (July). The findings show that movement of CO2 rather than just its absolute concentration plays a most important role in karstification.
The Sunjiawo karst spring in Xunyang County is about 20 km from the previous example. It has a similar geological and geographical background, but flowing out above a steep slope. It provides a good example of negative karstification (precipitation) because of its special hydrochemical and hydrodynamic conditions. Data from monitoring (fig.5) shows a similar pattern to the previous example. However, the HCO3 drops and pH rises remarkably and quickly downstream from the spring. Hugh amount of calcareous tufa are deposited on the slope following CO2 outgassing. The example shows how a KDS reacts when water emerges from a relatively confined hydrological system into the open atmosphere and then under the turbulent hydrodynamic conditions of a waterfall.
3.2 Karst processes in relation to greenhouse gas flux and environmental acidification
The operation of karst processes indicates that karst systems can act both as a sink (by carbonate dissolution) or as a source (by carbonate precipitation) of CO2 in the atmosphere. Moreover, these processes can mitigate environmental acidification by removing excess acid to form carbonates, sulfates or other salts in the hydrosphere (TRUDGILL & INKPEN 1993).
Assuming the carbonate rock is an active component of the karst system and so is linked closely to CO2 flux in the modern carbon cycle, then the annual removal of carbon from the atmosphere in karst areas can be estimated by the following equation:
where in a karst system
C is the annual removal of carbon;
Q is the annual discharge; and
HCO3 is the annual mean concentration of HCO3.
Here it is assumed that of the carbon in the HCO3 half is derived from the rock and half from CO2 involved in the solution process and, further, that the amount of carbon present is in proportion to the atomic weights of the elements involved (12/61).
On the basis of statistical data from 123 underground streams draining subtropical karst in Guizhou Province,
average annual Q = 572 m3/s (1.8038 x 1010 m3/a)
average annual HCO3 = 233.89 g/m3.
Thus the annual removal of carbon from the atmosphere in that Province is approximately 4.1x105 t/a. Using the same method, that from karst areas in China is 3.83x106 t/a.
Moreover, on the basis of published world carbonate rock denudation data (PULINA 1974, DROGUE & YUAN 1987), the contribution of karst denudation to global CO2 uptake from the atmosphere can be estimated by simply correlating the amount of CO2 to the world denudation of carbonate rocks, because 120 kg of carbon is removed from the atmosphere for every tonne of limestone rock dissolved.
Using the 21 dissolution rate and related precipitation data summarized by DROGUE & YUAN (1987), a global mean dissolution rate for karst of 87.41 m3/km2/a can be calculated by weighted average in respect to precipitation data. With a specific gravity of 2.7 this amounts to 236 t/a per km2 of karst. Assuming that the average mean content of CaCO3 in these rocks is 97.59% and that globally they cover an area of 2.2x107 km2, then global CaCO3 denudation is approximately 5.067x 109 t/a. Hence the annual removal of carbon from the atmosphere can be estimated as
Cglobal = 5.067x109 x 120/1000 = 6.08x108 t/a
which accounts for about 15.9% of the "missing carbon sink" (38.12x108 t/a) in the global carbon cycle (MACKENZIE & MACKENZIE 1995, 292). Meanwhile, on the basis of monitoring data from Akiyoshidai, Japan, KAZUHISA (1996) has estimated the global removal of carbon from the atmosphere as 2.2x108 t/a. These estimates show that karst processes are not negligible in the global budget.
So far as IGCP 299 is concerned, epigenic karst processes are considered to act more often as a sink of atmospheric CO2 than a source. In contrast, in karst areas with active faults and geothermal or volcanic activity, CO2 is commonly measured emitting into the atmosphere at concentrations of 23-90% (measured by the author during the 8 excursions of IGCP299) and large amounts of calcareous tufa are often deposited. In Huanglong Ravine, western Sichuan, (fig.3(4)), the high content of C13 of the emitted CO2 (-0.679%,and -0.482%) and some noble gas isotopic data indicate that they originate in the mantle (LIU 1995). The CO2 outgassing can be calculated directly from measurements of hydrochemical parameters at the spring (fig.6), or by calculating the volume of the calcareous tufa deposited between known dates. International review shows that hydrothermal outgassing through karst is widespread in the world, especially in the Pacific Realm and the Tethys Belt, extending from Tibet westward through Iran, Turkey, Italy, and southeast France. This is an important source of atmospheric CO2.
When the karst system acts as a sink of atmosphere CO2, it is simultaneously favourable for the mitigation of environmental acidification. For instance, in Fangtang village, south of Liuzhou, Guangxi (fig.7), acidic precipitation (pH4.05-5.59) has made spring water from a siliceous bed aquifer quite acidic (pH4.42) and harmful for domestic use, but water from a nearby well sunk into the same aquifer, but lined with limestone blocks, becomes neutralized quickly (pH6.8), although it brings about greater carbonate hardness. Carbonate values in wells in the karst aquifer of Guilin are susceptible to changes in rainfall patterns.
3.3 Records of environmental change
The sensitivity of the karst system to environment change also makes it sensitive in recording change, as demonstrated by the studies of WINOGRAD (1992), DORALE (1992), BAKER (1993), RAILSBACK (1994), SHOPOV (1994), and RICHARDS (1994) amongst others .
In south China, where other paleoenvironmental proxies are rare, a 1.22m long stalagmite from Penglongdong Cave (37 km south of Guilin) permitted reconstruction of paleoenvironmental change over the past 36 ka using stable isotope and geochemical approaches. Dating was by AMS and scintillation 14C and by alpha counting U-series. Several events of rapid climatic change are identifiable. The much faster growth rate of the stalagmite in the Holocene warm period than in the last glaciation provides another example showing the sensitivity of karst processes to environmental conditions.
4. Perspectives
This brief discussion gives an insight into the contribution that karst science can bring to the subject of the carbon cycle, a process of global dimension and of considerable applied significance.
The mechanisms of karstification will be revealed most effectively by multidisciplinary study, which will permit more accurate estimates to be made of the relative importance of karst systems as sinks and sources of CO2.
Acknowledgements
This work is based on the UNESCO/IUGS IGCP 299 and 379 Projects, and supported jointly by the Ministry of Geology and Mineral Resources (Project8502218) and the National Natural Science Foundation of China (Project 49070155). We are indebted to Prof. P.Williams for his encouragement to prepare this paper and for editing the manuscript.
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Address of author:
Yuan Daoxian
The Institute of Karst Geology
40 Qixing Road,
Guilin, Guangxi,
CHINA 541004