Michel Bakalowicz (C.N.R.S. ,FRANCE )

Karst processes require a solvent for dissolving the rock, and an "engine" for water flow and transport. Classical karst may be defined by soil CO₂ as thesolvent, and gravity as the engine. Other solvents and engines (Bakalowicz , 1986 ) originate special karst features. Recent studies in karst areas have shown that CO₂ of deep origin transported by thermal waters may be locally a major karst actor. The resulting hydrothermal karst (see Bakalowicz etal. , 1989 ) is identified by its typical conduits pattern. Nevertheless, some field evidences in classical karst areas show characters inciting to look for the contribution of deep CO₂ in classical karst processes. Deep CO₂ in karst processes may be identified by the following criteria : geochemical , geological and isotopic.

The main and direct geochemical criterion is the karst water CO₂ content. Generally, the groundwater CO₂ content , expressed as a partial pressure in CO₂ ( P_CO2 ), is close to that of air in soils, about 1%. Sometimes, karst waters are much richer in CO₂ such as the Lez spring , the water supply of Montpellier ( France ), which P_CO2 is up to 10% . Such high P_CO2 are responsible for high HCO₃ and Ca contents, resulting in a very active solution of carbonate rocks. Some observation about deep CO₂ intrusion were made in the aquifer of the high Guadalatin valley , southern Spain ( Ceron Garcia ,1995); the decreasing in water heads in the carbonate aquifer, because of overexploitation , produced a sudden emergence of CO₂ ( up to 85% in P_CO2 ) , and HCO₃ rich water.

The main geological criterion is the presence of travertine near the spring . The strong CO₂ degassing produces an important calcium carbonate precipitation . In Languedoc and Roussillon ( south of France ) , many karst springs are associated with travertines, which were interpreted either as the consequence of a favorable climatic situation or as the consequence of bacterial activity , precipitating calcite . Detailed field works ( Bakalowicz , 1990 ) showed regional characters , such as a P_CO2 anomaly, from 2 to 8% , associated to high Ca and HCO₃ contents.

In addition , the carbon isotope content is the best isotopic criterion for the origin of CO₂ . The different sources of carbon are identified from their carbon-- 13 content

( d ¹³ C vs, the PDB standard ): marine carbonate rocks (0‰ );atmospheric CO₂ (-8‰); soil CO₂ ( -20 to -27‰ ) . d ¹³ C of deep CO₂ depend on its origin : about 0 ‰ ( CO₂ from de-carbonatation ); about-6 ‰ ( CO₂ from the mantle ) . In the cited examples, dissolved HCO₃ shows d ¹³C about-8 ‰ , significantly enriched in 13C compared to HCO₃ resulting in carbonate solution with soil CO₂ ( d ¹³C about-12 to-14‰ ) . The travertines give d ¹³C about-3.5 to-8.5‰ , also enriched by comparison to calcite precipitate from caves (d ¹³C about-10 to-12‰) . This isotope enrichment, related to a CO₂ enrichment , is due to a deep CO₂ contribution .

Deep CO₂ takes its origin very deep , either in the mantle or in carbonate rocks in a metamorphic context , with temperatures of a few hundred Celsius degrees . In the examples, some evidences of a mantle rise since Miocene are commonly assumed by geophysicists. Contrarily to soil CO₂ , deep CO₂ directly inputs the saturated zone and is highly efficient for carbonate solution . Consequently solution only affects the saturated zone in its deep . Deep CO₂ produces a fast and extensive enlargement of joints only in the saturated zone , increasing its storage capacity below the spring level and favoring connections of karst voids with the drain structure . Deep CO₂ is the most efficient or stimulating agent of development of the saturated karst . But it may create a specific organization of drain pattern in the saturated zone , different from that of classical karst.

References.

Bakalowicz , M . (1986 ) . La karstification : processus , modeles et exemples . 9th Internat . Speleol . Cong . , Barcelona , 3 : 59-63 .

Bakalowicz , M . (1990 ) . Geoechimie des eaux incrustantes , formation des travertins et neotectonique: I’exemple des Corbires . Bulletin Centre de Geomorphologie Caen , 38 : 67-78 .

Bakalowicz , M . et al . (1989 ) . Thermal genesis of dissolution caves in the Black Hills , South Dakota . Geol. Soc. America Bull . , 99 : 729-738.

Ceron Garcia , J.C. (1995 ) . Estudio hidrogeoquimico del acuifero del alto Guadalentin (Murcia ). Tesis

Doctoral , Departamento de geodinamica , Universidad de Granada ( Spain ), 268 p.

GEOLOGICAL ASPECTS OF MODERN CARBON CYCLE IN MAINLAND CHINA

Yuan Daoxian
The Institute of Karst Geology
Guilin,GuangXi,China 541004

Recent researches supported by NNSFC,Ministry of Geology, PRC and the UNESCO/IUGS IGCP 299 Project have revealed a series of geological processes in mainland China that may make remarkable contribution to the source and sink of atmospheric CO₂. The processes include CO₂ emission from active faults, and weathering of rocks, especially the dissolution of carbonate rocks. The changing course of this part of carbon cycle have been sensitively recorded on the terrestrial carbonates.

There are numerous point of CO₂ emission in mainland China. Most of them are along or near the 28 major active fault zones. The CO₂ concentration is usually higher than 90%. According to the Carbon and Helium isotopic data, they are either originated from the mantle or the metamorphism of carbonate rocks in the crust, thus create unnegligible source in modern global carbon cycle. The emission of deep source CO₂ takes three forms (fig.1). In eastern part of China, where silicate minerals are predominant in rocks, very few precipitation take place along with CO₂ emission. In the areas where active faults are buried under thick Cenozoic or Mesozoic beds, the uprising gases are accumulated in the over-burden. 20 CO₂ reservoirs have been found in mainland China, and more on the continental shelf. In the areas where

carbonate rocks are widespread, such as SW China, calcareous sinter precipitation are usually accompanying CO₂ emission. Tens of splendid calcareous tufa are reported from Tibet Plateau, which is a part of Tethys realm, extending westward to Iran, Turkey, Italy and SE France, where calcareous tufa are very common, and being considered as one of the most important part of the world for deep source CO₂ emission.

Seven sites distributed under different ecological system in mainland China are selected to monitor tne carbon movement between carbonate rocks, wa ters, atmosphere and biosphere. The results show that the biggest carbon reservoir on the Earth, i.e, the carbonate rock body, is still active in modern carbon cycle, and not negligible because the dissolution of every ton of limestone will remove 120 kg of Carbon from the atmosphere, e.g, the annual removal of Carbon by such dissolution on Guizhou Plateau alone is estimated to be 4.15ˇÁl0⁵ tons.

The change in the direction or intensity of carbon cycle are sensitively recorded on the carbonate rocks, which could be both sink (dissolution features)or source(precipitation features, such as calcareous sinter, stalagmite in cave) for atmosphere carbon, e.g, a 1.22 long stalagmite from Panlongdong cave in Guilin has clearly recorded the changing of last Glaciation to warming Holocene, and several rapid minor changes.

Fig.1, Three different forms of deep source CO₂ emission in mainland China

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