Sensitivity of karst process to environmental change along the PEP II transect

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QuaternaryInternational,Vol. 37, pp. 105-113, 1997.

tPergamon PII: S1040-6182(96)00012-2

Copyright© 1996INQUA/ElsevierScienceLtd Printed in Great Britain. All rights reserved. 1040--6182/97$32.00

SENSITIVITY OF KARST PROCESS TO ENVIRONMENTAL CHANGE ALONG THE PEP II TRANSECT Yuan Daoxian

The Institute of Karst Geology, Guilin, Guangxi 541004, China It has been known since as early as the last century that karst formation is a geologicprocess related to chemical reaction, but not until the last couple of decades were karst processes viewed as being sensitive to environmental change. The direction and intensity of karst processes are controlled by environmental factors such as temperature, climate, hydrology, vegetation, geology, and the openness of the system to the atmosphere. Accordingly,karst features, as a productof the carbon cycle, differ in space and time. This is clearlyevident from the world karst correlation project, IGCP 299. There is a sharp contrast between karst types on both sides of the QinglingMountainrange of central China. Semi-add karst is located to the north, and humid subtropical karst to the south. Karst features are capable of recordinghigh resolution paleoclimatic change. AMS ~4C, isotope and geochemicalstudies of thin laminae from a giant stalagmite locatednear Guilin, in southernChina, have clearly identified rapid climate changes during the past 40 ka. In karst areas with active neotectonism, huge deposits of calcareous travertine record the amount of deeply sourced CO2 emitted into the atmosphere and can aid studies on modern tectonism because of the association of calcareous travertine with active faults. Copyright © 1996 INQUA/Elsevier Science Ltd

INTRODUCTION

winds to the modem monsoon system. Comprehensive research on thin laminae from a giant stalagmite located in south China has revealed the occurrence of rapid climate changes during the past 40 ka. The amount of CO2 emitted from modem volcanoes, geothermal fields and active faults is generally known. However, the evaluation of calcareous travertine could provide an estimation of the total amount of CO2 emitted, and aid research on modem tectonism. It is clearly evident that the evaluation of karst landforms can provide valuable information regarding greenhouse g a s emissions and paleoenvironmentai change. The purpose of this report is to summarize recent research on karst processes and landforms in China, to elucidate the correlation of Chinese karst to other areas of the world (IGCP Project 299), and to outline the relationship between active faults, karst processes and paleoenvironmental change.

The development of modem karstology has been characterized by two main points since the 1970s: the introduction of karstology to the earth system sciences, and the study of karst processes and landforms on a global scale. The implementation of the UNESCO/IUGS supported IGCP Project 299 'Geology, Climate, Hydrology and Karst Formation' (1990-1994) is regarded as a milestone toward the development of karstology. By carefully measuring atmospheric CO2, HCO3_ content and the pH of groundwater from karst regions around the world, the relationship between the carbon cycle and karst formation is illuminated. The largest carbon reservoir on earth is the carbonate rock body with a total capacity of 1016 tons of carbon (Berner and Berner, 1987). Karst processes are active in various climatic and ecologic zones around the world. For example, there is a significant contrast between karst features to the north and south of the Qinling Mountains of central China. This east-west trending mountain range is the major geologic, climatic, biologic, and hydrologic divide in China (Yuan, 1993). Karst research contributes to global climate studies in two ways: first, studying the sink-source effect of karst processes as a contribution to greenhouse gases. Karst processes can be both a sink (dissolution of carbonate) or source (CO2 degassing along with the precipitation of carbonate); secondly, to provide high resolution information on paleoenvironmental change associated with karst processes. In south China, a Late Cretaceous, redcarbonate breccia, reflecting a dry climate, is widespread in Guangxi, Hunan, Hubei, Guizhou, and Yunnan Provinces. Detailed work on the breccia will provide information for modeling the change from planetary

SENSITIVITY OF KARST PROCESSES TO E N V I R O N M E N T A L C H A N G E AND P R O G R E S S IN RESEARCH M E T H O D S The karst process occurs in an open, triphase, disequilibrium CO2-H20-carbonate system (Fig. 2). In nature, this system couples with the carbon cycle; e.g. the CO2-organic carbon-carbonate system (Fig. 1). It has long been known that karst processes are part of the global carbon cycle, but not until the application of field equipment such as the portable temperature autocompensated pH meter, CO2 gas detector and the Aquamerck kit did scientists get an idea on how sensitive the system is to environmental change. Figure 2 shows a close relationship between the atmosphere, hydrosphere, and litho105

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sphere by shifting carbon through the karst dynamic system. The behavior of the system can be described by three parameters: the concentration of atmospheric CO2 (pCO2), the pH and HCO3_ content of groundwater; all of which can be easily defined in the field. The addition of gaseous CO2 into the system will result in a lower pH and a deficiency in HCO3_, thus carbonate dissolves. On the contrary, CO2 degassing results in a higher pH and an excess of HCO3_, consequently precipitation occurs. Many other environmental factors can effect the direction of karst processes. Biogenic processes and the depth, temperature and turbulence of water bodies can all effect

the direction of CO2 shifting, and thus alter the intensity and direction of karstification. In natural water bodies the direction and intensity of karstification can also differ from deep to shallow areas, under sunshine or shadow, rapid or quiet flow or with many or few aquatic biota. The karst system is so sensitive to the environmental change that laboratory analysis cannot mimic all the factors effecting carbonate solution or dissolution. Field measurements show that carbonate rocks are active in the global carbon cycle and play a remarkable role in the sink-source relationship to greenhouse gases. A field experiment near Guilin indicates that HCO3_

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Sensitivity of Karst Process to Environmental Change

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FIG. 3. Correlationbetween the CO 2 content in soil atmosphere and H C O 3. in water from the underlyinglimestone at the Guilin Monitoring Site, China. hardness rises from 10 ppm in meteoric water to 80 ppm after flowing about 10 m along a limestone fissure. Monitoring in Guilin also revealed that when the CO2 content of a thin soil rises from 1000 ppm in the winter to 20,000-30,000 ppm in the summer, the HCO3- content in water from an underlying limestone will rise simultaneously from 140 ppm to 315 ppm (Fig. 3). In the karst forest of southwest China, creek water usually has a low pH and higher HCO3_ content, but pH rises gradually downstream along with calcarous tufa deposition. This process is more rapid along waterfalls. In situ observations along the creek show that the tufa deposition rate is much higher in areas with a high flow velocity, likely resulting from more favorable conditions for CO2 degassing. These examples are arguments for the sensitive response of the triphase CO2-H20-CO3 system to climate, temperature, biogenic process, as well as water-gas movement. Karst landforms are visible records suggesting that the carbon cycle is dynamic. Karst features imply variations in greenhouse gas composition, and thus indicate spatial and temporal changes in relevant climatic, hydrologic, geologic, and biologic environments.

WORLD KARST CORRELATION RECORDS Six major international field excursions took place for IGCP Project 299 namely: Turkey (1990), Perm

region of Russia (1992), a north-south trip across continental China (1991), an east-west trip in Southern Australia (1992), U.S.A. (1992), and Canada (1993). Moreover, there are data from affiliated countries as contributions from relevant National Working Groups (Fig. 4). Project members developed the idea of a karst feature complex for international correlation. A karst complex is defined as a set of well-matched karst features that developed under a similar environment. Karst features include micro and macroforms, surface and subsurface karst features, as well as dissolutional and depositional karst forms. The Perm karst at 58 ° N latitude in Russia, with a mean annual precipitation of 600-700 mm and a mean annual temperature of less than 10°C, is a cold, temperate humid karst. Taiga forest with marshes are widespread. The acidic water of bogs with a pH of 6-7, and a HCO3_ content of about 120 ppm are favourable for carbonate dissolution. However, the carbon cycle here is not exceedingly active because of a low ambient temperature. Consequently, the karst feature complex is characterized by remarkable dissolution features such as dolinen, karren on the surface, and big caves and scallops underground. However, speleothems are generally small, because calcite is saturated slowly under such a sluggish carbon cycle. Bare carbonate rock, widespread in tropical or Mediterranean-type karst, is rare. This is because the area has a low geomorphic entropy, and is near an ice sheet of the last glaciation, providing abundant source material for

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surficial deposits. This area is also unique for exhibiting evaporite karst and permanent ice in some caves. There are three major climate-associated karst types in continental China: arid and semi-add karst in north China; subtropical humid karst in south China; and, high mountain and plateau karst in southwest China (Yuan et al., 1991). The Qinling Mountains of central China sharply divide semi-arid karst to the north from subtropical humid karst to the south. Arid and semi-arid karst lays north of 35 ° N latitude. This area has a mean annual precipitation of 200-500 mm, and a mean annual temperature 10-15°C. Although biogenic processes are still active, water circulation is slow, hence groundwater is easily carbonate-saturated. Accordingly, the modem karst feature complex of this area is characterized by normal shaped mountains, tiny karren, calareous concretions, and limestone breccia resulting from mechanical weathering. Caves and speleothems are rare. However, some of the most extensive caves in the area with giant speleothems, scallops, and gravel are largely the products of a previous warm and humid period, hence these caves are useful for paleoenvironmental research. South China subtropical humid karst is pan of a more extensive karst region covering all of southeast Asia including Vietnam, Malaysia, Thailand, the Phillipines, and Indonesia. Subtropical humid karst is characterized by tower karst, dolinen, deep sharp karren, red clay, many surface tufa, and many extensive cave systems with underground streams and giant speleothems. Because of a high geomorphic entropy and being far from any ice sheets, bare carbonate rocks are abundantly exposed. Underlain by porous Tertiary limestone, tower karst on

Bohol Island of the Phillipines and Gunung Sewu of Indonesia are lower and more rounded than the tower karst of south China, north Vietnam or Thailand that developed on pre-Triassic limestone. Modem karst landforms on the Tibet Plateau are recognized as high mountain or plateau karst and are characterized by more mechanical weathering than chemical dissolution. The Tibetan karst complex is represented by many limestone peaks, walls and arches scattered on slopes together with a large amount of limestone scree. These features are the result of a long duration of frost denudation processes. Small scale dissolution features such as karren, and dolinen are also present. Caves and speleothems are very rare. Field observations of red clay in limestone fissures, speleothem in caves, and Hipparion fossils in the Bulong Basin of northern Tibet provide evidence of a previous warm, humid environment. This information and other karst records from eastern South China karst may provide a regional picture of environmental change during the Cenozoic Era. The Akiyoshidai karst of Honshu, Japan is located at 34 ° N latitude. This area is near the same latitude as the North China semi-add karst and with a similar mean annual temperature of 15°C, but the area has a higher mean annual precipitation of about 1800 mm. Thus the area displays typical temperate humid karst features such as dolinen, deep sharp karren, extensive caves with underground streams and speleothems. However, there is no tropical-humid type tower karst. The karst features of northeastern China, at a latitude greater than 40 ° N, are similar to the Akiyoshidai karst. A comparison of karst

Sensitivity of Karst Process to Environmental Change feature complexes from four areas (Akiyoshidai, the North China semi-add zone, the northeastern China semihumid zone, and the South China subtropical zone) indicates that precipitation plays a more important role than temperature or latitude for intense karstification. However, a warm climate is essental for the developing tower karst. Many reef lagoons of the Pacific and Indian Oceans are actually drowned karst depressions (dolinen) developed during the low sea stand of the last glaciation. Statistics on the depth of these lagoons show that drowned karst depressions of low latitudes are deeper than those of high latitudes. This may be due to higher precipitation in the lower latitude lagoons (Hori, 1992). There are three types of karst in Australia: subtropical karst of north Queensland; temperate humid karst in eastern Victoria and New South Wales, and semi-add karst in Nullarbor. The Chillagoe karst complex in north Queensland is similar to the south China karst. Both areas display tower karst with deep sharp karren, red clay and other subtropical karst features. The Buchan karst in eastern Victoria is a temperate-humid type. The geologic setting favors an abundance of allogenic, aggressive water flowing into the carbonate bedrock (impounded karst); thus, there are dolinen and extensive cave systems. Moreover, unlike other temperate humid karst forms, there are welldeveloped speleothems in the Buchan caves. Apparently, the dense Eucalyptus Forest present in the area has intensified the carbon cycle. The cave pCO2 is as high as 2000-100,000 ppm. The HCO3_ content of allogenic water is only 20 ppm, but it is more than 260 ppm in dripping water in the caves. Rich organic material in some of the stalagmite laminae is believed to be a relics of bush fires, and could be as old as 200,000 years (Gillieson, 1992). The Nullarbor plain (100 m a.s.l.) is in an arid region with a mean annual precipitation of 200 mm, and a xerophilic grass cover. The plain is underlain by Tertiary porous carbonates. Dish-shaped dolinen with blowholes are present..The blowhole shafts can swallow a great amount of water during storms, and thus give rise to extensive cave systems about 1-4 km long. These caves, with great halls and underground lakes and streams, are abundantly decorated with carbonate and halite speleothems. The early-growing carbonate speleothems are found seriously destroyed later by the exsudation process during the late growth of halite speleothems. Using 230Th/234U dating, the age of calcite speleothems ranges between 300,000 and more than 530,000 BP, whereas the halite speleothems range from 20,000 to 37,000 BP or are Holocene-aged (Gillieson, 1992). Calcite speleothems form in a warm, humid climate, whereas evaporites require an add climate; thus, Pleistocene climate can be reconstructed by studying speleothems from the Nullarbor caves.

L I M E S T O N E BRECCIA: KARST RECORDS OF LATE CRETACEOUS C L I M A T E IN SOUTH CHINA Global correlation of modem karst landforms from

109

different climates indicates that limestone screes can form in dry or cold conditions, however mechanic weathering must predominant. The exception is collapsed carbonate blocks, along cliff faces, in warm humid climates because of the difficulty in surviving intensive chemical dissolution in a tropical environment. Mesozoic to Paleogene Red Beds (breccia and etc.) with abundant evaporites are distributed in many parts of south China, such as the Sichuan, Hunnan, Jiangxi and central Yunnan Basins, where tectonic subsidence is evident. Red Beds record long periods of dry climate, however, karst features in south China are the result of a warm, humid climate that has been traced back to the late Tertiary. Accordingly, the age and distribution of the red limestone breccia may cast insight into the problems of when and how the climate of south China changed from dry to the warm humid climate. Modeling the change from a planetary wind system to the present monsoon system will benefit from this research. Long periods of denudation in the karst plateau of south China has made locating sediments deposited during the transition, of the two wind systems very difficult. Research will occur from early to late using the dry record of the Red Beds, and from late to early using the warm, humid record from karst features. In the 2000 km 2 karst area near Guilin, most of the Cretaceous red beds have been eroded. However, there are more than 100 small exposures, a few m e to 100 m 2, of red breccia scattered at different altitudes (Fig. 5), even at the top of some isolated limestone peaks (Fig. 6). The age of the breccia has been identified as late Cretacious by Atopochara flora found abundant in the cements. Accordingly, the dry Mesozoic climate of the area is considered to continue to as late as the Late Cretaceous. This kind of red limestone breccia is widespread in south China karst complexes (Fig. 7). Detailed work at carefully selected sites will aid in reconstructing the history of major climatic changes in the region, will illuminate the background to the change in global wind systems, and perhaps suggest an interaction between climate and uplift of Tibet Plateau.

S T A L A G M I T E FROM T H E P E N G L O N G D O N G CAVE: RECORDS OF LATE P L E I S T O C E N E TO H O L O C E N E CLIMATE IN SOUTH CHINA A 1.2 m high stalagmite was taken from the Penglongdong Cave, 37 km to the south of Guilin City. On the bases of a careful sedimentological investigation of thin laminae, high resolution dating using the AMS C14 method was carried out along with geochemical, and stable isotope studies on the stalagmite. Our analysis indicated that the growth rate of the stalagmite increased remarkably after 7700 BP. The growth rate averaged 0.1 mm/10 years before 7700 BP and 3 mm/10 years afterward. This clearly indicates a trend to a humid and warm Holocene. This event is also recognizable on the stable isotope curve (Fig. 8). From 36 ka (bottom of the stalagmite) to 11 ka

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a cold trend is evident, but a warm trend is evident after 11 ka. Moreover, the curve also shows several abrupt changes. For instance, around 10,830 BP there is a sharp increase in 8180 before trending to lower values. Additional samples are being dated using the same approach, however it is likely that the Younger Dryas event is recorded in the Guilin stalagmites (Williams et al., 1993). A geochemical study of different coloured laminae shows that there is a clear Mn, Zn, Co, Fe203, P205, and organic carbon peak in the dark laminae of the 26,250 BP to 7700 BP cold period (K41; Fig. 8). This indicates that paleoclimate change is geochemically recorded in the Penglongdong stalagmite. Preliminary dating using paleomagnetics and the uraniam disequilibrium method shows that there are many stalagmites in Guilin caves older than 300 ka. Thus. a high resolution reconstruction environmental change for the past 200 ka using karst records in south China can be accomplished where other paleoenvironmental information proxies such as ice core, loess, or lake deposits are not available.

TRAVERTINE: A RECORD OF MANTLE CO2 EMISSION AND NEOTECTONISM Many modern active faults emit an abundance CO2 in concentration of 30-90%. In karst regions, CO2 degassing is associated with the deposition of a large amount of calcareous travertine. Calculations indicate that: 120 kg of carbon are released to the atmosphere for every ton of travertine deposition. For example, the Huanglong Resort, Sichuan Province is characterized by many travertine cascades. Field measurement show that the CO2 concentration on the surface of two geothermal springs is 23% and 78% respectively, while the 813C ratios (-6.79 and -4.82) indicate that the springs have been in contact with the mantle. Obviously, the travertine depositional rate is related to fault activity. This phenomena is very common in all carbonate rock areas near plate contact zones, such as in central Italy, Turkey, Alborz Mtn. in Northern Iran, and Yukon, Canada. The amount of travertine present suggests that fault-related travertine research could aid source-sink estimations of atmospheric greenhouse gases.

Sensitivity of Karst Process to Environmental Change

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CONCLUSION With the aid of IGBP Project 299 global karst research has made great strides in elucidating information useful paleoenvironmental studies, modem tectonism research and studies on the climatic affects of atmospheric greenhouse gas concentrations. The extreme sensitivity of karst processes to environmental change allows for high resolution climate studies such as are occurring at

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Guilin, China. As chronostratigraphy of karst complexes around the world improves, a coherent picture of global climate change can be constructed. Detailed studies of red beds and karst features in south China suggests periods of dry to humid climate changes during the Cenozoic and suggests that the Early Tertiary dry climate evolved from a similar Late Cretaceous dry climate. Geochemical studies on stalagmites from Penglongdong indicate that paleoclimate change is geochemically recorded in

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FIG. 7. Distribution of limestone breccia in South China. 1. Bare karst; 2. Non-solublerocks; 3. Buried karst; 4. Coveredkarst; 5. Sites with limestone breccia.

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stalagmites. Modeling the change from planetary wind systems to the present monsoon system will benefit from karst research in south China. Finally, the abundance of calcareous travertine associated with active faults and plate contact zones indicates that a m a n t l e - C O 2 - t r a v e r tine degassing relationship should be considered in s i n k source estimations o f atmospheric CO2 content. It is apparent that all karst features, whether dissolutional or depositional, imply that the carbon cycle is

d y n a m i c and that karst processes and landforms intimately linked with p a l e o e n v i r o n m e n t a l change.

are

ACKNOWLEDGEMENTS This work is supported by UNESCO/IUGS IGCP, the Academy of Geological Science and the National Science Foundation of China. Colleagues from the Institute of Karst Geology Lin Yushi, Qin Jiaming, Li Bin, Xu Shenyou and He Siyi, did most of the work on the Guilin stalagmite, while Liu Zaihua, Xu Shenyou, and He Siyi worked on

Sensitivity of Karst Process to Environmental Change Huanglong CO2 emissions. The AMS C14 dating was done by the

Department of Technical Physics, Beijing University.

REFERENCES Berner, E.K. and Berner, R.A. (1987). The Global Water Cycle. Prentice-Hall Inc., Englwood Cliffs, New Jersey, 38 pp.. Deng Z., Lin Y., Zhang, M., Liu, G. and Wei, Z. (1988). Karst and Geological Structure in Guilin. Chongqing Publishing House, Chongqing, China, 129 pp. Gillieson, D.S. (ed.) (1992). Geology, climate, hydrology and karst formation. Guidebook of Field Symposium in Australia, Department of Geography and Oceanography, University College, Australian Defence Force, Special Publication 4, 115 pp.

113

Hori, N. (1992). Origin of Reef Lagoon: The Drowned Karst Topography and Paleoclimate. Contribution to IGCP 299, IGCP National Committee of Japan, pp. 63-66. Qin, J. (1994). Joint report of research project (led by Yuan, D.). NSFCC Project 49070155 and Ministery of Geology (PRC) Project 8502218. Williams, M.A.J., Dunkerly, D.L., de Decker, P., Kershaw, A.P. and Stokes, T. (1993). Quaternary Environments. Edward Arnold, 329 pP. Yuan, D., Zhu, D., Weng, J., Zhu, X., Han, X., Wang, X., Cai, G., Zhu, Y., Cui, G. and Deng, Z. (1991). Karst of China. Geological Publishing House, Beijing, China, 224 pp. Yuan, D. (ed.) (1993). World Karst Correlation. Carsologica Sinica, supplement 16, Institute of Karst Geology, Guilin, China, 203 pp.