- Email: [email protected]
The origin of erratic calcite speleothems in the Dangcheomul Cave (lava tube cave), Jeju Island, Korea
ARTICLE IN PRESS
Quaternary International 176–177 (2008) 70–81
The origin of erratic calcite speleothems in the Dangcheomul Cave (lava tube cave), Jeju Island, Korea Kyung Sik Wooa,, Jeong Chan Kimb, Don Won Choia,1, Jin Kyung Kima, Ryeon Kima, Odette Nehzaa a Cave Research Institute of Korea, Kangwon National University, Chuncheon, Kangwondo 200-701, South Korea Geological and Environmental Hazards Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350, South Korea
b
Available online 25 May 2007
Abstract Dangcheomul Cave in Jeju Island, Korea, is a lava tube about 110 m long. The cave is located only a few meters below the surface under alkali basalt, and contains numerous and various calcite speleothems such as soda straws, stalactites, stalagmites, columns, cave corals, curtains, flowstones, rimstones, carbonate powders, and shelfstones. Carbonate sand dunes overlying the lava tube are responsible for the formation of calcite speleothems. The sand dunes were formed from the carbonate sediments transported from adjacent shallow seas and beaches, and are composed of mollusks, echinoderms, coralline algae, benthic foraminifers, bryozoans, etc. Oxygen isotopic compositions of some speleothems and cave water indicate that the spelothems have grown mostly by evaporation of cave water. Also, carbon isotopic compositions suggest that the majority of carbon was derived from overlying carbonates with a minor contribution of organic carbon from the overlying soil. Most speleothems in Dangcheomul Cave do not show typical morphology as can be commonly seen in limestone caves. These erratic forms imply a different mode of speleothem formation. High density of soda straws, stalactites, and columns as well as erratic morphology may also provide the evidence that the plant roots are responsible for their growth. r 2007 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Dangcheomul Cave is a lava tube in the volcanic terrane of Jeju Island, Korea (Fig. 1). This cave shows typical lava tube morphology and internal microtopographic features as can be seen in many other lava tubes elsewhere; however, it is a lime-decorated lava tube characterized by well-developed carbonate speleothems. Even though Dangcheomul Cave shows many features of lava tubes, lava tube speleothems such as lava stalactites are relatively rare, except for abundant lava helictites on the ceiling which have the same chemical composition as the surrounding basalt (Woo et al., 2000). The cave was accidentally discovered in 1995 by a local farmer when Corresponding author. Fax: +82 33 242 8556.
E-mail address: [email protected] (K.S. Woo). Present address: Property Division, Jeju Special Self-governing Province 690-700, South Korea. 1
heavy earth-moving equipment broke through the roof of the cave during plowing. Since then, the cave was restored to original natural condition, and is now again overlain by the sand dunes, which are composed of carbonate sediments. Natural forests and shrub thickets, however, are formed on the lava plains surrounding the area outside the Dangcheomul Cave. The carbonate sand dunes should have been blown by wind from beaches and shallow seas nearby even though the accurate age of the dune formation is not known at present (Woo et al., 2000). The preliminary result based on the U–Th dating of the column in Dangcheolmul Cave suggests that the formation of carbonate sand dunes is likely to be during the Holocene Epoch (Woo et al., 2004). The overlying carbonate sediments show a composition similar to the beach sediments reported around Jeju Island (Ji and Woo, 1995). Because the lava tube was developed only a few meters below the surface, it is evident that calcium and carbonate ions for the formation of carbonate speleothems
1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2007.05.009
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
71
basaltic lava flows intermittently produced more than 120 lava tubes in the island (Son, 2003). The lava flows, which are about 0.1–0.4 million years old, are widely distributed along the Jeju coast. Among them, the lava flow field that extends toward the northeast and/or north produced the Geomunoreum Lava Tube System that was formed by several lava flows from the Geomunoreum Crater (Hwang et al., 2005). Several lava tube caves were developed along the paths: Bukoreum, Daerim, Mangjang, Gimnyeong, Yongchon and Dangcheomul. Recent radiometric dating of the basaltic lava in Dangcheomul Cave shows that the cave was formed about 0.15 million years BP (Hwang et al., 2005). Dangcheomul Cave is ca. 110 m long (Fig. 2), and the tube has dimensions of 5.5–18.4 m in width and 0.3–2.7 m in height. The thickness of basalt between the top of the cave and the surface is about 1–4.8 m. The cave is more or less horizontal and shows polygonal columnar jointing in the ceiling. Lava flow structures such as ropy lava, flowlines, gutter and levee are preserved on the floor and wall of the cave. 3. Methods
Fig. 1. A map showing the location of the Dangcheomul Cave.
were supplied from overlying carbonate sediments and soils. Numerous calcareous speleothems are growing in the Dangcheomul Cave, and these are soda straws, stalactites, stalagmites, columns, cave corals, curtains, flowstones, rimstones and carbonate powder (moonmilk?). Interestingly enough, many of the stalactites, stalagmites and columns display unusual morphologies that cannot be seen in limestone caves elsewhere. Therefore, the objectives of this paper are to describe the types and distribution of calcareous speleothems in Dangcheomul Cave and to delineate the origin of speleothems based on morphologic, textural, and geochemical data. 2. Geologic setting Jeju Island, the largest island of Korea (1846 km2 in area), is located in the Korean Strait about 90 km south of Korean peninsula (Fig. 1). The island is a long ellipse, 73 km east–west and 41 km north–south. It was formed by several stages of volcanic activities during the late Quarternary (about 1 million to a few thousand years BP) by intermittent plume movements, and, as a result, is almost wholly composed of volcanic rocks, mainly trachyte, trachy-andesite, andesite, alkali basalt, and volcanoclastic rocks (Won, 1976). Along with Mt. Halla (a shield volcano), the island includes more than 380 volcanic cones, concentrated in the northeast and southwest part (Yang et al., 1997). Relatively less viscous
Cave air temperature, humidity and the partial pressure of carbon dioxide as well as the pH and temperature of cave water were measured from six sites (one site from the outside and five sites within the cave) three times from November 13, 1999 to August 14, 2002 (Table 1; Fig. 2). Three samples of carbonate sediments from carbonate dune sands above the lava tube cave and one sample from an adjacent carbonate beach were collected to examine the constituents of carbonate grains. The sampled sediments were impregnated with epoxy resin and thin sectioned. Three hundred points were counted from each thin section using a polarizing microscope. Several speleothems were carefully sampled (mostly from the broken specimens) and stained with Feigl’s solution to identify aragonite mineralogy (Friedman, 1959). The mineralogy of carbonate powders was determined with an X-ray diffractometer (Bruker D5005). Cave water and adjacent stream water were collected for stable isotopic and elemental analyses. Surface dwelling plants living on the surface were collected for carbon isotopic analysis. Stable isotopic compositions of oxygen and carbon of carbonate speleothems were measured. All the stable isotope analyses were conducted using stable isotope mass spectrometer (VG PRISM II) from the Korea Basic Science Research Institute. Analytical error range is 70.2%, and all the data in this paper are reported relative to PDB standard except for the water data (vs. SMOW). Trace elements were measured using ICP Emission Spectrophotometer (PQ3 ICPS-1000111) from the Korea Basic Science Research Institute. 4. Cave environment The environmental conditions of the Dangcheomul Cave have been monitored three times from November 1999 to
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
72
Fig. 2. (A) An outside view over the Dangcheomul Cave showing the entrance. (B) Plan and longitudinal maps of the Dangcheomul Cave. Numbers denote the measurement sites of air and water temperatures, humidity, the partial pressure of CO2, and pH.
Table 1 Air temperature, humidity, partial pressure of carbon dioxide, water temperature, and pH values measured in Dangcheomul Cave Date
Station
CO2 (ppmv)
Air temperature (1C)
Humidity
pH
Water temperature (1C)
mV
11/13/1999
Outside 2 (ENT) 3 4 5 6
400 750 988 910 – 1480
21 19 21 21 – 24
62 88 86 94 – 91
– – – 7.7 7.9 7.8
– – – 19.9 20.0 19.8
– – –
Outside 2 (ENT) 3 4 5 6
430 440 760 720 – 840
17 14 20 18 – 18
80 78 79 91 – 95
– – – – 8.2 8.2
– – – – 16.5 17.7
– – – –
Outside 2 (ENT) 3 4 5 6
460 2560 2600 2520 2480 2420
24 20 19 18 18 18
88 85 90 93 94 95
– – – – – –
– – – – – –
– – – – – –
1/11/2000
8/14/2002
August 2002, and the results are summarized in Table 1. Air tempertuare in the cave can vary from in the range 17–24 1C, and does not show significant changes with seasons. Humidity and pCO2 contents in the cave tend to be higher than the outside; especially, the pCO2 contents are about two to four times higher inside (720–1480 ppmv) compared to outside (about 400 ppmv) of the cave. Such higher pCO2 values are probably due to the decay of a large
51 62 60
70 71
amount of organic matter transported from the overlying soils. The pH values of cave water range from 7.7 to 8.2 (Table 1), which is are similar to those of limestone caves in Korea (Woo, 2000). Such weak alkaline water may be due to buffering of acidic meteoric water with the overlying carbonate sediments. Water temperature varies from 16 to 20 1C, and varies roughly parallel to the outside air temperature.
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
5. Result 5.1. Carbonate sediments over the Dangcheomul Cave Three samples of overlying sediments above the Dangcheomul Cave were collected in stratigraphic order (JJ-SS1, JJ-SS-2, and JJ-SS-3 from the bottom). The sediments are mostly fine to medium sand in size, and they are dominated by calcareous components, constituting about 95% of the sediments. Main calcareous constituents are various skeletons of mollusks (63–69%), red algae (17–23%), echinoderm fragments (2–5%), benthic foraminifera (3–5%) and bryozoans (o3%) (Table 2). Mollusks are the most abundant component, constituting about twothirds (average 67%) of these carbonate sediments. The second-most component is red algae, making up about 20% of these carbonate sediments. In addition, the sediments also include minor amounts of volcanogenic detrital grains, including volcanic rock fragment, quartz, feldspar, olivine, pyroxene, etc. (Table 2). One sample, collected from the beach sand nearby, consists of a similar composition; however, the relative proportion of each constituent is somewhat different. The beach sand is characterized by lower proportion of mollusk (50%) and higher proportion of echinoderm (6%), benthic foraminifera (8.5%) and bryozoan (6%) relative to the carbonate sediments above the cave. There are two possible explanations for this discrepancy: (1) shallow-marine
Table 2 Composition of carbonate sediments overlying the Dangcheomul Cave (%) Sediment
JJ-SS-1
JJ-SS-2
JJ-SS-3
JJ-SS-4
Mollusk Red algae Echinoderm Benthic foraminifera Bryozoan Rock fragment Detrital mineral Unknown
68.3 17.9 2.9 4.3 0.6 0.3 4.3 1.4
68.4 18.5 2.5 3.3 2.2 0.4 2.2 2.5
63.0 22.3 4.3 4.0 1.7 0.7 2.0 2.0
50.0 21.4 6.1 8.5 6.1 1.0 2.1 4.8
73
carbonates had different relative proportions of carbonate constituents in the past; or (2) the well-sorted, fine to medium carbonate sands over the cave, some of which were selectively transported by wind, do not reflect the average composition of the whole beach sediments in the past. More data are needed to clarify this relationship. 5.2. Morphological and textural features of speleothems A variety of speleothems are found in Dangcheomul Cave, including soda straws, stalactites, stalagmites, columns, cave corals, cave flowers, cave pearls, curtains, flowstones, carbonate powder, rimstones, and lava helictites (Figs. 3 and 4). All the speleothems, except for lava helictites, are composed of calcite. Lava helictites consist of plagioclase and other silicate minerals and their composition and mineralogy are similar to surrounding volcanic rocks (Won, 2000; Woo et al., 2000). Textural characteristics of calcite speleothems are as follows. 5.2.1. Soda straws and stalactites Soda straws and stalactites are the most dominant speleothem types in Dangcheomul Cave, but they are quite different in morphology from those of limestone caves. The soda straws in limestone caves tend to maintain uniform diameter (mostly 5.1 mm) as they grow (Curl, 1972), but the diameters of the Dangcheomul soda straws show lengthwise variations (Fig. 5A). Stalactites in limestone caves commonly show a carrot shape with an overall tapering down to a sharp tip (Woo, 2005), but carrotshaped stalactites are rarely found in the Dangcheomul Cave. Rather, the stalactites of the Dangcheomul Cave resemble the overall shape of plant roots (Fig. 5B). Such morphological resemblance suggests that most soda straws and stalactites in the Dangcheomul Cave have been formed by downward-flowing water over plant roots, which hung down from cave ceiling, not by dripping water. Presence of the small canals of variable size in stalactites strongly supports the proposition that several stalactites grew by calcite precipitation on plant roots and then merged into one (Fig. 5C). Such canals are mostly a few millimeters in
Fig. 3. Distribution map of speleothems of the Dangcheumul Cave.
ARTICLE IN PRESS 74
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
Fig. 4. General view of (A) Dangcheomul Cave; (B) soda straws, stalactites, and columns; (C) Lumpy Cave corals; (D) Bacon sheet; (E) cave popcorn in rimstone; (F) rimstone and shelfstone; (G) carbonate powder on the wall, and (H) lava helictites on the ceiling.
diameter and generally vacant, but in some cases they are filled with equant calcite cements. The circumference of the vacant tubes mostly consists of micrite implying fast precipitation of calcite crystals around roots, but of microspar in some cases (Fig. 5D). Most stalactites are composed of columnar calcite with either planar (Fig. 6A) or serrated (Fig. 6B) crystal boundaries. Each columnar crystal shows either unit or weakly undulatory extinction. Columnar crystals with undulatory extinction are composed of smaller subcrystals showing a splitted growth pattern (Self, 2003). Numerous growth laminae can be found within columnar crystals (Figs. 6A and B), which
were formed by intermittent input of soil-derived organic matter. Commonly observed in stalactites of Dangcheomul Cave are corroded surfaces (Fig. 6C), indicating temporary cessation of stalactite growth caused by input of slightly acidic water. It has been reported that the pH of cave water in the lava tubes of Jeju Island can be slightly acidic during high rainfall, because the residence time for buffering with overlying carbonates becomes very short (Choi et al., 2005). There are several corroded horizons within the columnar calcites of the stalactite examined (Fig. 6D), indicating fluctuations in the amount of rainfall in the past.
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
75
Fig. 5. (A) Erratic soda straws and stalactites. (B) Erratic stalactites showing the morphology of plant roots. (C) Thin-section photomicrograph of a stalactite showing several molds where plant roots were present. Plane-polarized light. (D) Thin-section photomicrograph of micritic and microsparitic calcites around the mold in the stalactite. Cross-polarized light.
Continuous successive re-growth of columnar calcite suggests that cave water regained a state of calcite saturation as the amount of rainfall decreased. Some stalactites tend to grow over the one side preferentially, indicating uneven supply rate of water along the side of stalactites. 5.2.2. Stalagmites and columns Like stalactites, most stalagmites in Dangcheomul Cave also display odd shapes of various morphologies. However, some stalagmites, growing directly below stalactites by dripping water, are similar in shape to those commonly seen in limestone caves. These stalagmites are usually flat topped (Fig. 7A), although some variations do occur depending upon the rate of water drips and the height of ceiling (Woo, 2005). Small scales of rimstone may be developed on the gently sloping sides of stalagmites when the amount of dripping water significantly increases. Some stalagmites are characterized by dented tops, resulting from the splash of water drops hitting down the surfaces. Also observed are cave corals growing on the surface of stalagmites, indicating that there is no direct supply of dripping water from the ceiling.
More commonly found in Dangcheomul Cave are stalagmites of erratic shapes, which are thought to have grown over plant roots by down-flowing water along the roots rather than by dripping water. These stalagmites show more erratic and complicated morphologies than those formed by dripping water. One of the common shapes is the stalagmite of a pinnacle shape with an acute top (Fig. 7B). The pinnacle-shaped stalagmite might be formed by the growth of long, upward-tapering coating of calcite along plant roots and the removal of upper organic part due to the decomposition of roots. Some stalagmites, like stalactites, show branched and/or rootlet-like morphologies. When the complete coating of calcite is successful over the roots, which grow down to cave floor, columns may develop. All the columns in the Dangcheomul Cave show uniform morphology. In limestone caves, most columns tend to show the combined morphology of stalactite and stalagmite and maintain uniform diameter from bottom to top, whereas columns in the Dangcheomul Cave always thicken downward, thus resulting in a vase shape (Fig. 7C). This may well result from the merging of calcite-coated plant roots, and molds of plant roots in columns also indicate the
ARTICLE IN PRESS 76
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
Fig. 6. (A) Thin-section photomicrograph of a stalactite showing columnar calcites with planar crystal boundaries. Cross-polarized light. (B) Thin-section photomicrograph of a stalactite showing fibrous to columnar calcites with serrated crystal boundaries. Cross-polarized light. (C) Thin-section photomicrograph of a stalactite showing the corroded surface. The corroded surface is also present within the stalactite (arrow). (D) A close-up view of (C). Note that laterally continuous growth laminae became disrupted due to corrosion.
similar origin of columns to stalactites and stalagmites in the Dangcheomul Cave (Fig. 7D). After coating of calcite over plant roots that penetrated through the ceiling down to the floor, precipitation took place more rapidly in the lower part of the thin column, probably because the downflowing water can reach higher state of supersaturation as evaporation increases. When several roots coated with calcite are present next to each other and become larger, they will merge eventually, making a thick, vase-shaped column.
5.2.3. Cave corals Cave corals are mostly knobby shaped and milky white to light brown in color. The basal parts of some cave corals consist of thin canals (Fig. 8A). Cave corals are found either on the basalt surface or on the surface of other speleothems. They always grow where there is no direct contact with water drips, implying that cave corals grow by evaporation of seepage water. Cave corals consist of isopachous calcites showing spherulitic fibrous texture (Fig. 8B).
5.2.4. Cave popcorns in ephemeral pools The occurrence of cave popcorns in Dangcheomul Cave is limited at the cave floor near the downstream end of the passage. Although the Dangcheomul cave popcorns resemble the thick frostworks of limestone caves in overall morphology (Hill and Forti, 1997), they are composed entirely of calcite and the thickness of calcite crystals tends to be larger (Fig. 8C), in contrast to aragonitic frostworks in many Korean limestone caves (Woo, 2000). Hence, in spite of frostwork-like morphology, their limited occurrence only in lower relief area, together with calcite mineralogy, suggests that they may well be cave popcorns formed in ephemeral rimstone pool only during rainy periods. Under the microscope, cave popcorns show randomly oriented, fibrous calcite crystals containing abundant clay-sized particles. 5.2.5. Cave pearls Cave pearls in the Dangcheomul Cave are distributed in rimstone pools. They are ovoidal in shape, and their surfaces are commonly polished. They range from 1 to 2 cm in diameter. The nucleus of each cave pearl commonly
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
77
Fig. 7. (A) Flat-topped stalagmites. (B) An erratic stalagmite with an acute top. (C) A typical column showing a vase-shaped morphology with width thickening downward. (D) A transverse section of the broken column. Note the several holes where plant roots were present.
Fig. 8. (A) A side-view of knobby-shaped cave corals. (B) Thin-section photomicrograph of a cave coral showing isopachous, spherulitic fibrous calcite texture. (C) Cave flowers on the floor. (D) Thin-section photomicrograph of a cave pearl. Note that the bottom side stopped to grow while the pearl continued to grow in other three directions.
ARTICLE IN PRESS
Fe (ppm)
– – – – – – – – – – 5.3 6.0 5.8 5.6 5.1 5.5 4.8 4.7 3.9 0.1 0.4 8.3 8.6 8.7 6.1 6.1 – – – – –
8.6 7.5 4.9 7.0 7.3 4.2 7.6 4.3 0.8 0.7 0.6 354 447 386 327 248 488 672 316 361 – – 0.6 0.7 0.4 0.3 0.3 – – – – – – – – – – – 3.5 4.8 3.8 4.7 4.7 7 1 5 3 2 5 3 4 2 – – – – – – –
Ba (ppm)
Si (ppm)
Sr (ppm)
5.2.6. Draperies The occurrence of draperies is limited to the ceiling of the Dangcheomul Cave, contrary to those generally found along the joints of walls or ceilings in limestone caves. Vacant canals, which are the molds of original plant roots, are commonly found within the draperies, suggesting that some of them are also the result of coalescence of several stalactites as the supply of cave water increased. Finegrained micritic calcites surround the wall of vacant canals as in stalactites, however columnar calcites grew only toward one direction in curtains. Columnar calcites during the early stage of growth characteristically show straight crystal boundaries on one side and serrated boundaries on another side. Many growth laminae are observed within columnar calcites. Some growth lines show poor lateral continuity, suggesting that the inflow of cave water was not uniform. Fibrous to bladed calcites are occasionally observed around the molds of plant roots, which evolved into columnar calcites as they grew.
208 62 27 5 3 244 39 36 25 – – – – – – –
d13C (%)
d18O (%)
consists of zoned, equant calcite crystals or randomly oriented bladed calcite crystals, both of which formed after the dissolution of a pre-existing nucleus. The nucleus is surrounded by fibrous calcite with well developed, distinctive growth laminae (Fig. 8D). Fibrous calcites show the preferential direction of growth only toward one direction, indicating that the cave pearl stopped rolling after the certain period of growth.
57 59 60 56 59
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81 dD (%)
78
8 3 2 1 94 32 1 7 2 – – – – – – – 1120 1097 788 806 1028 688 4921 932 1236 – – 7.7 10.6 8.5 12.9 12.9 Moonmilk Moonmilk Cave flower Soda straw Stalactite (yellowish white) Stalactite (corroded) Cave coral Stalactite Stalactite Carbonate sediment Carbonate sediment Cave water Cave water Cave water Stream Stream DCHM-1 DCHM-2 DCHM-3 DCHM-4 DCHM-5 DCHM-6 DCHM-7 DCHM-10 DCHM-11 DCHM-8 DCHM-9 DCHM-S1 DCHM-S2 DCHM-S3 JJ-R-1 (1) JJ-R-1 (2)
145 96 66 59 33 90 63 44 95 – – 8.4 12.7 9.7 23.6 23.8
Mn (ppm) Mg (ppm) Na (ppm) Description Sample no.
The stable isotopic data of speleothems, carbonate sediments, cave water, and stream water are presented in Table 3. d18O and dD values of two cave water samples are 8.7% and 60%, and 8.3% and 57%, respectively, and these data generally agree with the GMWL (global meteoric water line; Craig, 1961) suggesting that rainwater quickly becomes cave water without any isotopic changes (thus the residence time in overlying soil and rock layers is very short). Also, d18O and dD values of stream waters are 6.1% and 59%, and 6.1% and 56%, respectively. One sample follows the trend of GMWL, but the other sample shows a slight enrichment of d18O relative to dD (that is, the deviation to the right of the GMWL), possibly due to evaporation effect of surface runoff. d18O and d13C values of the overlying carbonate sediments are +0.1–0.4% and +0.6–0.7%, respectively, and these values are well within the range of shallow water carbonates around Jeju Island (Ji and Woo, 1998a, b). The d13C value of the plant living on the surface overlying the cave is 32%. d13C values of the speleothems in Dangcheomul Cave show a wide range depending upon the type of speleothems (Table 3): stalactites ( 7.3% to 0.8%); cave coral ( 7.6%); cave flower ( 4.9%); carbonate powder ( 8.6% and 7.5%) (Fig. 9). The large variation of d13C values between 8.6% and 0.8% indicates two sources of
Table 3 Stable isotope and trace element compositions of speleothems, cave water and stream
5.3. Stable isotopes
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
79
Fig. 9. Scatter diagram of d13C vs. d18O for calcareous speleothems in Dangcheomul Cave.
carbon, organic carbon from overlying soil horizon and inorganic carbon from carbonate sediments. Tentatively, more positive values of d13C imply more contribution from carbonate sediments, whereas more depleted d13C values are derived from more influence of organic carbons supplied from soil layer above. Assuming that d13C values of speleothems reflect two sources of carbon reservoirs and kinetic d13C enrichment is negligible during degassing of carbon dioxide for the formation of speleothems (evaporation-dominated process), the relative contribution of organic carbon from carbonate sediment in stalactites, cave coral, cave flower and carbonate powders can be estimated to be about 0–24%, 25%, 17% and 25–28%, respectively. Hence, it can be inferred that the carbonate sediments above the cave are a major source of carbon for the growth of speleothems in Dangcheomul Cave. Somewhat wider variations of the d13C values in stalactites relative to other speleothems may indicate that the stalactites are more sensitive to the fluctuations of the amount of soil-derived organic matter. In other words, relatively less variable d13C values in other speleothems (cave coral, cave flower, and carbonate powder) suggests that the supply rate of organic carbon was more or less constant during their growth. d18O values of speleothems range from 3.9% to 6.0%, showing small variation relative to d13C values. The temperature in the Dangcheomul Cave ranges from 18 to 24 1C, and the d18O values of cave water about 8.7% to 8.3% (vs. SMOW). The calculated equilibrium d18O values of speleothems, using the equation of Epstein et al. (1953), range from 10.5% to 8.7%. Accordingly, the measured d18O values are more enriched than calculated ones by about 3–5%. Such enrichment in d18O suggests that the formation of speleothems was mostly driven by evaporation rather than degassing of CO2. Choi et al. (2005) suggested that cave corals of the lava tubes in Jeju Island grow from thin film of fluids supplied by seepage
water from surrounding rocks. It is expected that the cave flowers in Dangcheomul Cave grew in the pool as water was evaporated. In addition, carbonate powders are always found on the surface of the wall or other speleothems, implying that they formed by evaporation of seepage water as the supply rate decreased. Similar range of d18O values of stalactites to that of cave coral may also imply that stalactites may have been dominated by evaporation for their formation. One stalactite, showing the most enriched d18O value ( 3.9%) also support this assumption. Hence, these speleothems tend to form where the effect of evaporation is larger than the effect of CO2 degassing, and it can be therefore accepted that their enriched d18O values are the result of their formation process, i.e., an evaporation-dominated process. 5.4. Major, minor and trace elements Results of elemental data are presented in Table 3. Cave water and stream water show similar Si compositions; however, other elemental contents are quite different. The higher concentration of Na and Mg in the stream water is presumed to be influenced by the weathering of silicate minerals. However, higher Ca, Sr, and K concentrations in cave water appear to be influenced by overlying carbonate sediments. Similar Si and Ca results of cave water were reported from the cave corals in lava tubes of Jeju Island (Woo et al., submitted). Na contents of speleothems are lower than those of shallow-marine carbonate sediments reported elsewhere (Milliman, 1973), indicating that Na compositions are similar to those of meteoric calcite formed in watercontrolled open system (James and Choquette, 1990). Lower contents of Mn and Fe correspond with the overall oxidizing condition in the cave. Mg contents range from 780 to 1250 ppm, which is within the range of diagenetic calcite of meteoric environment (Veizer, 1983). However, a
ARTICLE IN PRESS 80
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
cave coral with a higher content of Mg (ca. 4900 ppm) and Sr (ca. 670 ppm) than other speleothems may reflect its mode of the formation. As mentioned earlier, cave corals usually form from thin film of water by evaporation (Woo, 2005; Woo et al., submitted). Hence Mg and Sr compositions (like Mg/Ca and Sr/Ca ratios) of the thin film of fluids may have increased with calcite precipitation because partition coefficient(s) are significantly less than unity (Kinsman and Holland, 1969; Katz, 1973; Fairchild et al., 2000). 6. Discussion: origin of calcite speleothems in Dangcheomul Cave Even though Dangcheomul Cave is a lava tube, various calcite speleothems have been growing. Secondary mineralization including calcite precipitation in lava tubes may not be uncommon elsewhere in the world (for example Duck Creek Cave of Utah, USA; Halliday, personal communication), however their density of distribution and erratic morphology may be truly unique (Forti, 2004; Cultural Heritage Administration, 2006). Hence, this cave was nominated as a World Heritage site by the Korean government in 2006. Unlike other calcareous speleothems in limestone caves, most speleothems with erratic morphologies in this cave could be formed in a very straightforward way because they are simply calcite coatings around plant roots. Results of geochemical analyses suggest that calcium and carbonate ions for their growth were entirely derived from carbonate sediments overlying the cave. The precipitation of calcite should take place from cave water, which originated from meteoric water that became buffered with overlying carbonate dune sands and which easily infiltrated into the cave through plant roots. This process can be considered quite different from speleothem formation in limestone caves, because the groundwater usually percolates into limestone caves through joints or bedding planes. Thus, compared to the long residence time of the groundwater that enters into limestone caves, the residence time of groundwater in Dangcheomul Cave is relatively very short (within a few hours and sometimes a few days at most), because Dangcheomul Cave is located just a few meters below the surface and groundwater is usually transported through cracks between columnar joints and plant roots. However, some speleothems such as flowstones were formed by groundwater that was transported through fractures or cracks in surrounding basalts. Also, cave corals, shelfstone, and rimstone appear to grow in ways similar to those in limestone caves when the rate of water supply into the cave is high enough to produce flowing or standing water on cave floor. Many plant roots are hanging down from the ceiling are present (Fig. 5A), and these roots have penetrated through overlying soils, carbonate dune sands, and basalts. The length and thickness of plant roots varies according to the type and age of vegetation, and some of them may grow down to cave floor. Calcite precipitation usually begins to
take place from a water drop at the tip of root (Fig. 5A), however water droplets can be also found anywhere along root surface, meaning that calcite precipitation may start at any point. After rain, meteoric water rapidly enters into the cave along the roots of plants, and calcite precipitation takes place either by the degassing of carbon dioxide or evaporation, resulting in the formation of erratic soda straws and stalactites (Fig. 5B). Stalactites begin to grow as micritic calcite at the incipient stage (Fig. 5D), probably because the saturation state with respect to calcite is high or the surface of the roots is very irregular, thus providing more nucleation sites for calcite precipitation. If roots are decayed away during stalactite growth, the hanging stalactite may fall to the cave floor. The shape of stalactites is strongly influenced by the overall morphology of the precursor root, such as its length and/or the presence of rootlets (Fig. 5B). When the root has reached the cave floor and calcite precipitation begins to take place from the lower part, a stalagmite with an acute form may be developed after the decay of plant root in the upper part (Fig. 5C). When the roots extend from ceiling to floor and several stalagmites merge into one, a column can be developed. Numerous molds of plant roots in the transverse section of the broken column support this processes (Fig. 7D). Although the shape of speleothems takes after that of precursor plant roots to a great extent, it can be also modified by another factor such as the supply rate of cave water. For example, a vase-shaped column showing downward increase of the diameter might have been formed when the supply rate of water was high enough to cause continued lateral growth mainly in the lower part. On the other hand, some stalactites with very thin and uniform diameters might grow along the very thin root extending from the cave ceiling nearly to the cave floor when the supply rate of cave water was steady and low. Depending upon the nature of branching roots and their morphologies as well as the rate of water supply, a variety of erratic speleothems may form. Conclusively, a variety of erratically shaped calcite speleothems in the lava tube (Danhcheomul Cave) are thought to result from a combination of following conditions: (1) the formation of volcanic lava tube, providing the accommodation space for the speleothem growth; (2) the deposition of carbonate sediments above the cave, providing calcium and carbonate ions for the growth of calcite speleothems; (3) the development of vegetation with deep roots, facilitating the transport of carbonate-charged meteoric water into the cave and finally (4) the suitable surrounding climatic condition of Jeju Island, controlling vegetation type and the amount of rainfall. 7. Conclusions The Dangcheomul Cave, a typical lava tube formed in the late Quaternary volcanic rocks, is characterized by numerous calcite speleothems, which may well be truly
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
unique in the world. Dissolution of carbonate sediments above the cave provides calcium and carbonate ions for their formation. Most speleothems in the cave commonly show erratic shapes which cannot be seen in limestone caves, and such erratic shapes result from a combination of (1) the penetration of plant roots from the overlying soil into the cave, (2) the supply of meteoric water along the roots, and (3) precipitation of carbonate minerals around the roots. Geochemical analyses indicate that evaporation of cave water rather than degassing of carbon dioxide played an important role in the formation of calcite speleothems. Acknowledgments This work was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and Technology of Korea. Special thanks to R. Kim, and K.N. Jo for laboratory assistance and fieldwork. References Choi, D.W., Woo, K.S., Lee, K.C., 2005. Opal cave corals in the lava tubes in Jeju Island, Korea: implications for local paleoenvironmental change. Journal of the Geological Society of Korea 41, 465–480 (in Korean, English abstract). Craig, H., 1961. Isotopic variations in meteoric waters. Science 133, 1702–1703. Cultural Heritage Administration, 2006. Jeju Volcanic Island and Lava Tubes—A Candidate For World Heritage Inscription, 176pp. Curl, R.L., 1972. Minimum diameter stalactite. The National Speleological Society Bulletin 34, 129–136. Epstein, S., Buchbaum, R., Lowenstam, H.A., Urey, H.C., 1953. Revised carbonate water isotopic temperature scale. Geological Society of America Bulletin 64, 1315–1326. Fairchild, I., Borsato, A., Tooth, A.F., Frisia, S., Hawkesworth, C.J., Huang, Y., McDermott, F., Spiro, B., 2000. Controls on trace element (Sr–Mg) compositions of carbonate cave water: implications for speleothem climatic records. Chemical Geology 166, 255–269. Forti, P., 2004. Genetic processes of cave mineral in volcanic environments: an overview. In: Proceedings of the 11th International Symposium on Volcanospeleology, Pico Island, Azores, Portugal, pp. 12–13. Friedman, G.M., 1959. Identification of carbonate minerals by staining method. Journal of Sedimentary Petrology 29, 87–97. Hill, C., Forti, P., 1997. Cave Minerals of the World. National Speleological Society, Huntsville, 463pp. Hwang, S.G., Ahn, U.S., Lee, M.W., Yun, S.H., 2005. Formation and internal structures of the Geomunorm Lava Tube System in the
81
northeastern Jeju Island. Journal of the Geological Society of Korea 41, 385–400 (in Korean, English abstract). James, N.P., Choquette, P.W., 1990. Limestones—the meteoric diagenetic environment. In: McIlearth, Morrow, D.W. (Eds.), Diagenesis. Geoscience Canada Reprint Series, vol. 4, pp. 35–74. Ji, O.M., Woo, K.S., 1995. Compositional variations of the beach sediments in Cheju Island. The Journal of the Korean Society of Oceanography 30, 480–492 (in Korean, English abstract). Ji, O.M., Woo, K.S., 1998a. Trace (minor) elemental and isotopic compositions of calcitic skeletons in the eastern coast and Cheju Island, Korea. The Sea—Journal of the Korean Society of Oceanography 3, 124–141 (in Korean, English abstract). Ji, O.M., Woo, K.S., 1998b. Trace (minor) elemental and isotopic compositions of aragonitic skeletons in the eastern coast and Cheju Island, Korea. The Sea—Journal of the Korean Society of Oceanography 3, 112–123 (in Korean, English abstract). Katz, A., 1973. The interaction of magnesium with calcite during crystal growth at 25–90 1C and one atmostphere. Geochimica et Cosmochimica Acta 37, 1563–1586. Kinsman, D.J.J., Holland, H.D., 1969. The coprecipitation of cations with CaCO3-IV. The coprecipitation of Sr2+ with aragonite between 16 1C and 96 1C. Geochimica et Cosmochimica Acta 33, 1–17. Milliman, J.D., 1973. Marine Carbonates. Springer, New York, 374pp. Self, C.A., 2003. The internal organization of speleothems. Acta Carsologica 33, 245–255. Son, I.S., 2003. Natural caves in Jeju, Cultural Properties Administration, 263pp. Veizer, J., 1983. Chemical diagenesis of carbonates: Theory and application of trace element technique. In: Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S. (Eds.), Stable Isotopes in Sedimentary Geology. Society of Economic Paleontologists and Mineralogists, Short Course No. 10, pp. 3-1–3-100. Won, C.K., 1976. Study of petro-chemistry of volcanic rocks in Jeju Island. Journal of the Geological Society of Korea 12, 207–226. Woo, K.S., 2000. Scientific Reports of Okgye, Namdae, and Dongdae Caves in Gangneung, Gangneung City, 269pp (in Korean). Woo, K.S., 2005. Caves: A Wonderful Underground. Hallym Publisher, Seoul, 230pp. Woo, K.S., Choi, D.W., Kim, R., Kim, J.K., 2000. The origin of the speleothems in Dangcheomul Cave, Jeju Island, Korea. Journal of the Geological Society of Korea 36, 411–434 (in Korean, English abstract). Woo, K.S., Kim, R., Kim, J.K., Choi, D.W., Jo, K.N., 2004. Textural and stable isotopic results of the column in Dangcheomul Cave, Jeju Island, Korea: paleoclimatic implications. Journal of the Geological Society of Korea 40, 133–144 (in Korean, English abstract). Woo, K.S., Choi, D.W., Lee, K.C., Silicification of the cave corals in some lava tubes of Jeju Island, Korea: implications for the paleoenvironmental change during the late Quaternary. Quaternary International, submitted for publication. Yang, M.S., Moon, J.H., Kang, S.S., Hyeon, K.S., Jang, Y.J., 1997. Oreum (adventive volcano) in the Jeju Island. Jeju Provincial Government, 51–52 (in Korean).
Quaternary International 176–177 (2008) 70–81
The origin of erratic calcite speleothems in the Dangcheomul Cave (lava tube cave), Jeju Island, Korea Kyung Sik Wooa,, Jeong Chan Kimb, Don Won Choia,1, Jin Kyung Kima, Ryeon Kima, Odette Nehzaa a Cave Research Institute of Korea, Kangwon National University, Chuncheon, Kangwondo 200-701, South Korea Geological and Environmental Hazards Division, Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejeon 305-350, South Korea
b
Available online 25 May 2007
Abstract Dangcheomul Cave in Jeju Island, Korea, is a lava tube about 110 m long. The cave is located only a few meters below the surface under alkali basalt, and contains numerous and various calcite speleothems such as soda straws, stalactites, stalagmites, columns, cave corals, curtains, flowstones, rimstones, carbonate powders, and shelfstones. Carbonate sand dunes overlying the lava tube are responsible for the formation of calcite speleothems. The sand dunes were formed from the carbonate sediments transported from adjacent shallow seas and beaches, and are composed of mollusks, echinoderms, coralline algae, benthic foraminifers, bryozoans, etc. Oxygen isotopic compositions of some speleothems and cave water indicate that the spelothems have grown mostly by evaporation of cave water. Also, carbon isotopic compositions suggest that the majority of carbon was derived from overlying carbonates with a minor contribution of organic carbon from the overlying soil. Most speleothems in Dangcheomul Cave do not show typical morphology as can be commonly seen in limestone caves. These erratic forms imply a different mode of speleothem formation. High density of soda straws, stalactites, and columns as well as erratic morphology may also provide the evidence that the plant roots are responsible for their growth. r 2007 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Dangcheomul Cave is a lava tube in the volcanic terrane of Jeju Island, Korea (Fig. 1). This cave shows typical lava tube morphology and internal microtopographic features as can be seen in many other lava tubes elsewhere; however, it is a lime-decorated lava tube characterized by well-developed carbonate speleothems. Even though Dangcheomul Cave shows many features of lava tubes, lava tube speleothems such as lava stalactites are relatively rare, except for abundant lava helictites on the ceiling which have the same chemical composition as the surrounding basalt (Woo et al., 2000). The cave was accidentally discovered in 1995 by a local farmer when Corresponding author. Fax: +82 33 242 8556.
E-mail address: [email protected] (K.S. Woo). Present address: Property Division, Jeju Special Self-governing Province 690-700, South Korea. 1
heavy earth-moving equipment broke through the roof of the cave during plowing. Since then, the cave was restored to original natural condition, and is now again overlain by the sand dunes, which are composed of carbonate sediments. Natural forests and shrub thickets, however, are formed on the lava plains surrounding the area outside the Dangcheomul Cave. The carbonate sand dunes should have been blown by wind from beaches and shallow seas nearby even though the accurate age of the dune formation is not known at present (Woo et al., 2000). The preliminary result based on the U–Th dating of the column in Dangcheolmul Cave suggests that the formation of carbonate sand dunes is likely to be during the Holocene Epoch (Woo et al., 2004). The overlying carbonate sediments show a composition similar to the beach sediments reported around Jeju Island (Ji and Woo, 1995). Because the lava tube was developed only a few meters below the surface, it is evident that calcium and carbonate ions for the formation of carbonate speleothems
1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2007.05.009
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
71
basaltic lava flows intermittently produced more than 120 lava tubes in the island (Son, 2003). The lava flows, which are about 0.1–0.4 million years old, are widely distributed along the Jeju coast. Among them, the lava flow field that extends toward the northeast and/or north produced the Geomunoreum Lava Tube System that was formed by several lava flows from the Geomunoreum Crater (Hwang et al., 2005). Several lava tube caves were developed along the paths: Bukoreum, Daerim, Mangjang, Gimnyeong, Yongchon and Dangcheomul. Recent radiometric dating of the basaltic lava in Dangcheomul Cave shows that the cave was formed about 0.15 million years BP (Hwang et al., 2005). Dangcheomul Cave is ca. 110 m long (Fig. 2), and the tube has dimensions of 5.5–18.4 m in width and 0.3–2.7 m in height. The thickness of basalt between the top of the cave and the surface is about 1–4.8 m. The cave is more or less horizontal and shows polygonal columnar jointing in the ceiling. Lava flow structures such as ropy lava, flowlines, gutter and levee are preserved on the floor and wall of the cave. 3. Methods
Fig. 1. A map showing the location of the Dangcheomul Cave.
were supplied from overlying carbonate sediments and soils. Numerous calcareous speleothems are growing in the Dangcheomul Cave, and these are soda straws, stalactites, stalagmites, columns, cave corals, curtains, flowstones, rimstones and carbonate powder (moonmilk?). Interestingly enough, many of the stalactites, stalagmites and columns display unusual morphologies that cannot be seen in limestone caves elsewhere. Therefore, the objectives of this paper are to describe the types and distribution of calcareous speleothems in Dangcheomul Cave and to delineate the origin of speleothems based on morphologic, textural, and geochemical data. 2. Geologic setting Jeju Island, the largest island of Korea (1846 km2 in area), is located in the Korean Strait about 90 km south of Korean peninsula (Fig. 1). The island is a long ellipse, 73 km east–west and 41 km north–south. It was formed by several stages of volcanic activities during the late Quarternary (about 1 million to a few thousand years BP) by intermittent plume movements, and, as a result, is almost wholly composed of volcanic rocks, mainly trachyte, trachy-andesite, andesite, alkali basalt, and volcanoclastic rocks (Won, 1976). Along with Mt. Halla (a shield volcano), the island includes more than 380 volcanic cones, concentrated in the northeast and southwest part (Yang et al., 1997). Relatively less viscous
Cave air temperature, humidity and the partial pressure of carbon dioxide as well as the pH and temperature of cave water were measured from six sites (one site from the outside and five sites within the cave) three times from November 13, 1999 to August 14, 2002 (Table 1; Fig. 2). Three samples of carbonate sediments from carbonate dune sands above the lava tube cave and one sample from an adjacent carbonate beach were collected to examine the constituents of carbonate grains. The sampled sediments were impregnated with epoxy resin and thin sectioned. Three hundred points were counted from each thin section using a polarizing microscope. Several speleothems were carefully sampled (mostly from the broken specimens) and stained with Feigl’s solution to identify aragonite mineralogy (Friedman, 1959). The mineralogy of carbonate powders was determined with an X-ray diffractometer (Bruker D5005). Cave water and adjacent stream water were collected for stable isotopic and elemental analyses. Surface dwelling plants living on the surface were collected for carbon isotopic analysis. Stable isotopic compositions of oxygen and carbon of carbonate speleothems were measured. All the stable isotope analyses were conducted using stable isotope mass spectrometer (VG PRISM II) from the Korea Basic Science Research Institute. Analytical error range is 70.2%, and all the data in this paper are reported relative to PDB standard except for the water data (vs. SMOW). Trace elements were measured using ICP Emission Spectrophotometer (PQ3 ICPS-1000111) from the Korea Basic Science Research Institute. 4. Cave environment The environmental conditions of the Dangcheomul Cave have been monitored three times from November 1999 to
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
72
Fig. 2. (A) An outside view over the Dangcheomul Cave showing the entrance. (B) Plan and longitudinal maps of the Dangcheomul Cave. Numbers denote the measurement sites of air and water temperatures, humidity, the partial pressure of CO2, and pH.
Table 1 Air temperature, humidity, partial pressure of carbon dioxide, water temperature, and pH values measured in Dangcheomul Cave Date
Station
CO2 (ppmv)
Air temperature (1C)
Humidity
pH
Water temperature (1C)
mV
11/13/1999
Outside 2 (ENT) 3 4 5 6
400 750 988 910 – 1480
21 19 21 21 – 24
62 88 86 94 – 91
– – – 7.7 7.9 7.8
– – – 19.9 20.0 19.8
– – –
Outside 2 (ENT) 3 4 5 6
430 440 760 720 – 840
17 14 20 18 – 18
80 78 79 91 – 95
– – – – 8.2 8.2
– – – – 16.5 17.7
– – – –
Outside 2 (ENT) 3 4 5 6
460 2560 2600 2520 2480 2420
24 20 19 18 18 18
88 85 90 93 94 95
– – – – – –
– – – – – –
– – – – – –
1/11/2000
8/14/2002
August 2002, and the results are summarized in Table 1. Air tempertuare in the cave can vary from in the range 17–24 1C, and does not show significant changes with seasons. Humidity and pCO2 contents in the cave tend to be higher than the outside; especially, the pCO2 contents are about two to four times higher inside (720–1480 ppmv) compared to outside (about 400 ppmv) of the cave. Such higher pCO2 values are probably due to the decay of a large
51 62 60
70 71
amount of organic matter transported from the overlying soils. The pH values of cave water range from 7.7 to 8.2 (Table 1), which is are similar to those of limestone caves in Korea (Woo, 2000). Such weak alkaline water may be due to buffering of acidic meteoric water with the overlying carbonate sediments. Water temperature varies from 16 to 20 1C, and varies roughly parallel to the outside air temperature.
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
5. Result 5.1. Carbonate sediments over the Dangcheomul Cave Three samples of overlying sediments above the Dangcheomul Cave were collected in stratigraphic order (JJ-SS1, JJ-SS-2, and JJ-SS-3 from the bottom). The sediments are mostly fine to medium sand in size, and they are dominated by calcareous components, constituting about 95% of the sediments. Main calcareous constituents are various skeletons of mollusks (63–69%), red algae (17–23%), echinoderm fragments (2–5%), benthic foraminifera (3–5%) and bryozoans (o3%) (Table 2). Mollusks are the most abundant component, constituting about twothirds (average 67%) of these carbonate sediments. The second-most component is red algae, making up about 20% of these carbonate sediments. In addition, the sediments also include minor amounts of volcanogenic detrital grains, including volcanic rock fragment, quartz, feldspar, olivine, pyroxene, etc. (Table 2). One sample, collected from the beach sand nearby, consists of a similar composition; however, the relative proportion of each constituent is somewhat different. The beach sand is characterized by lower proportion of mollusk (50%) and higher proportion of echinoderm (6%), benthic foraminifera (8.5%) and bryozoan (6%) relative to the carbonate sediments above the cave. There are two possible explanations for this discrepancy: (1) shallow-marine
Table 2 Composition of carbonate sediments overlying the Dangcheomul Cave (%) Sediment
JJ-SS-1
JJ-SS-2
JJ-SS-3
JJ-SS-4
Mollusk Red algae Echinoderm Benthic foraminifera Bryozoan Rock fragment Detrital mineral Unknown
68.3 17.9 2.9 4.3 0.6 0.3 4.3 1.4
68.4 18.5 2.5 3.3 2.2 0.4 2.2 2.5
63.0 22.3 4.3 4.0 1.7 0.7 2.0 2.0
50.0 21.4 6.1 8.5 6.1 1.0 2.1 4.8
73
carbonates had different relative proportions of carbonate constituents in the past; or (2) the well-sorted, fine to medium carbonate sands over the cave, some of which were selectively transported by wind, do not reflect the average composition of the whole beach sediments in the past. More data are needed to clarify this relationship. 5.2. Morphological and textural features of speleothems A variety of speleothems are found in Dangcheomul Cave, including soda straws, stalactites, stalagmites, columns, cave corals, cave flowers, cave pearls, curtains, flowstones, carbonate powder, rimstones, and lava helictites (Figs. 3 and 4). All the speleothems, except for lava helictites, are composed of calcite. Lava helictites consist of plagioclase and other silicate minerals and their composition and mineralogy are similar to surrounding volcanic rocks (Won, 2000; Woo et al., 2000). Textural characteristics of calcite speleothems are as follows. 5.2.1. Soda straws and stalactites Soda straws and stalactites are the most dominant speleothem types in Dangcheomul Cave, but they are quite different in morphology from those of limestone caves. The soda straws in limestone caves tend to maintain uniform diameter (mostly 5.1 mm) as they grow (Curl, 1972), but the diameters of the Dangcheomul soda straws show lengthwise variations (Fig. 5A). Stalactites in limestone caves commonly show a carrot shape with an overall tapering down to a sharp tip (Woo, 2005), but carrotshaped stalactites are rarely found in the Dangcheomul Cave. Rather, the stalactites of the Dangcheomul Cave resemble the overall shape of plant roots (Fig. 5B). Such morphological resemblance suggests that most soda straws and stalactites in the Dangcheomul Cave have been formed by downward-flowing water over plant roots, which hung down from cave ceiling, not by dripping water. Presence of the small canals of variable size in stalactites strongly supports the proposition that several stalactites grew by calcite precipitation on plant roots and then merged into one (Fig. 5C). Such canals are mostly a few millimeters in
Fig. 3. Distribution map of speleothems of the Dangcheumul Cave.
ARTICLE IN PRESS 74
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
Fig. 4. General view of (A) Dangcheomul Cave; (B) soda straws, stalactites, and columns; (C) Lumpy Cave corals; (D) Bacon sheet; (E) cave popcorn in rimstone; (F) rimstone and shelfstone; (G) carbonate powder on the wall, and (H) lava helictites on the ceiling.
diameter and generally vacant, but in some cases they are filled with equant calcite cements. The circumference of the vacant tubes mostly consists of micrite implying fast precipitation of calcite crystals around roots, but of microspar in some cases (Fig. 5D). Most stalactites are composed of columnar calcite with either planar (Fig. 6A) or serrated (Fig. 6B) crystal boundaries. Each columnar crystal shows either unit or weakly undulatory extinction. Columnar crystals with undulatory extinction are composed of smaller subcrystals showing a splitted growth pattern (Self, 2003). Numerous growth laminae can be found within columnar crystals (Figs. 6A and B), which
were formed by intermittent input of soil-derived organic matter. Commonly observed in stalactites of Dangcheomul Cave are corroded surfaces (Fig. 6C), indicating temporary cessation of stalactite growth caused by input of slightly acidic water. It has been reported that the pH of cave water in the lava tubes of Jeju Island can be slightly acidic during high rainfall, because the residence time for buffering with overlying carbonates becomes very short (Choi et al., 2005). There are several corroded horizons within the columnar calcites of the stalactite examined (Fig. 6D), indicating fluctuations in the amount of rainfall in the past.
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
75
Fig. 5. (A) Erratic soda straws and stalactites. (B) Erratic stalactites showing the morphology of plant roots. (C) Thin-section photomicrograph of a stalactite showing several molds where plant roots were present. Plane-polarized light. (D) Thin-section photomicrograph of micritic and microsparitic calcites around the mold in the stalactite. Cross-polarized light.
Continuous successive re-growth of columnar calcite suggests that cave water regained a state of calcite saturation as the amount of rainfall decreased. Some stalactites tend to grow over the one side preferentially, indicating uneven supply rate of water along the side of stalactites. 5.2.2. Stalagmites and columns Like stalactites, most stalagmites in Dangcheomul Cave also display odd shapes of various morphologies. However, some stalagmites, growing directly below stalactites by dripping water, are similar in shape to those commonly seen in limestone caves. These stalagmites are usually flat topped (Fig. 7A), although some variations do occur depending upon the rate of water drips and the height of ceiling (Woo, 2005). Small scales of rimstone may be developed on the gently sloping sides of stalagmites when the amount of dripping water significantly increases. Some stalagmites are characterized by dented tops, resulting from the splash of water drops hitting down the surfaces. Also observed are cave corals growing on the surface of stalagmites, indicating that there is no direct supply of dripping water from the ceiling.
More commonly found in Dangcheomul Cave are stalagmites of erratic shapes, which are thought to have grown over plant roots by down-flowing water along the roots rather than by dripping water. These stalagmites show more erratic and complicated morphologies than those formed by dripping water. One of the common shapes is the stalagmite of a pinnacle shape with an acute top (Fig. 7B). The pinnacle-shaped stalagmite might be formed by the growth of long, upward-tapering coating of calcite along plant roots and the removal of upper organic part due to the decomposition of roots. Some stalagmites, like stalactites, show branched and/or rootlet-like morphologies. When the complete coating of calcite is successful over the roots, which grow down to cave floor, columns may develop. All the columns in the Dangcheomul Cave show uniform morphology. In limestone caves, most columns tend to show the combined morphology of stalactite and stalagmite and maintain uniform diameter from bottom to top, whereas columns in the Dangcheomul Cave always thicken downward, thus resulting in a vase shape (Fig. 7C). This may well result from the merging of calcite-coated plant roots, and molds of plant roots in columns also indicate the
ARTICLE IN PRESS 76
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
Fig. 6. (A) Thin-section photomicrograph of a stalactite showing columnar calcites with planar crystal boundaries. Cross-polarized light. (B) Thin-section photomicrograph of a stalactite showing fibrous to columnar calcites with serrated crystal boundaries. Cross-polarized light. (C) Thin-section photomicrograph of a stalactite showing the corroded surface. The corroded surface is also present within the stalactite (arrow). (D) A close-up view of (C). Note that laterally continuous growth laminae became disrupted due to corrosion.
similar origin of columns to stalactites and stalagmites in the Dangcheomul Cave (Fig. 7D). After coating of calcite over plant roots that penetrated through the ceiling down to the floor, precipitation took place more rapidly in the lower part of the thin column, probably because the downflowing water can reach higher state of supersaturation as evaporation increases. When several roots coated with calcite are present next to each other and become larger, they will merge eventually, making a thick, vase-shaped column.
5.2.3. Cave corals Cave corals are mostly knobby shaped and milky white to light brown in color. The basal parts of some cave corals consist of thin canals (Fig. 8A). Cave corals are found either on the basalt surface or on the surface of other speleothems. They always grow where there is no direct contact with water drips, implying that cave corals grow by evaporation of seepage water. Cave corals consist of isopachous calcites showing spherulitic fibrous texture (Fig. 8B).
5.2.4. Cave popcorns in ephemeral pools The occurrence of cave popcorns in Dangcheomul Cave is limited at the cave floor near the downstream end of the passage. Although the Dangcheomul cave popcorns resemble the thick frostworks of limestone caves in overall morphology (Hill and Forti, 1997), they are composed entirely of calcite and the thickness of calcite crystals tends to be larger (Fig. 8C), in contrast to aragonitic frostworks in many Korean limestone caves (Woo, 2000). Hence, in spite of frostwork-like morphology, their limited occurrence only in lower relief area, together with calcite mineralogy, suggests that they may well be cave popcorns formed in ephemeral rimstone pool only during rainy periods. Under the microscope, cave popcorns show randomly oriented, fibrous calcite crystals containing abundant clay-sized particles. 5.2.5. Cave pearls Cave pearls in the Dangcheomul Cave are distributed in rimstone pools. They are ovoidal in shape, and their surfaces are commonly polished. They range from 1 to 2 cm in diameter. The nucleus of each cave pearl commonly
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
77
Fig. 7. (A) Flat-topped stalagmites. (B) An erratic stalagmite with an acute top. (C) A typical column showing a vase-shaped morphology with width thickening downward. (D) A transverse section of the broken column. Note the several holes where plant roots were present.
Fig. 8. (A) A side-view of knobby-shaped cave corals. (B) Thin-section photomicrograph of a cave coral showing isopachous, spherulitic fibrous calcite texture. (C) Cave flowers on the floor. (D) Thin-section photomicrograph of a cave pearl. Note that the bottom side stopped to grow while the pearl continued to grow in other three directions.
ARTICLE IN PRESS
Fe (ppm)
– – – – – – – – – – 5.3 6.0 5.8 5.6 5.1 5.5 4.8 4.7 3.9 0.1 0.4 8.3 8.6 8.7 6.1 6.1 – – – – –
8.6 7.5 4.9 7.0 7.3 4.2 7.6 4.3 0.8 0.7 0.6 354 447 386 327 248 488 672 316 361 – – 0.6 0.7 0.4 0.3 0.3 – – – – – – – – – – – 3.5 4.8 3.8 4.7 4.7 7 1 5 3 2 5 3 4 2 – – – – – – –
Ba (ppm)
Si (ppm)
Sr (ppm)
5.2.6. Draperies The occurrence of draperies is limited to the ceiling of the Dangcheomul Cave, contrary to those generally found along the joints of walls or ceilings in limestone caves. Vacant canals, which are the molds of original plant roots, are commonly found within the draperies, suggesting that some of them are also the result of coalescence of several stalactites as the supply of cave water increased. Finegrained micritic calcites surround the wall of vacant canals as in stalactites, however columnar calcites grew only toward one direction in curtains. Columnar calcites during the early stage of growth characteristically show straight crystal boundaries on one side and serrated boundaries on another side. Many growth laminae are observed within columnar calcites. Some growth lines show poor lateral continuity, suggesting that the inflow of cave water was not uniform. Fibrous to bladed calcites are occasionally observed around the molds of plant roots, which evolved into columnar calcites as they grew.
208 62 27 5 3 244 39 36 25 – – – – – – –
d13C (%)
d18O (%)
consists of zoned, equant calcite crystals or randomly oriented bladed calcite crystals, both of which formed after the dissolution of a pre-existing nucleus. The nucleus is surrounded by fibrous calcite with well developed, distinctive growth laminae (Fig. 8D). Fibrous calcites show the preferential direction of growth only toward one direction, indicating that the cave pearl stopped rolling after the certain period of growth.
57 59 60 56 59
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81 dD (%)
78
8 3 2 1 94 32 1 7 2 – – – – – – – 1120 1097 788 806 1028 688 4921 932 1236 – – 7.7 10.6 8.5 12.9 12.9 Moonmilk Moonmilk Cave flower Soda straw Stalactite (yellowish white) Stalactite (corroded) Cave coral Stalactite Stalactite Carbonate sediment Carbonate sediment Cave water Cave water Cave water Stream Stream DCHM-1 DCHM-2 DCHM-3 DCHM-4 DCHM-5 DCHM-6 DCHM-7 DCHM-10 DCHM-11 DCHM-8 DCHM-9 DCHM-S1 DCHM-S2 DCHM-S3 JJ-R-1 (1) JJ-R-1 (2)
145 96 66 59 33 90 63 44 95 – – 8.4 12.7 9.7 23.6 23.8
Mn (ppm) Mg (ppm) Na (ppm) Description Sample no.
The stable isotopic data of speleothems, carbonate sediments, cave water, and stream water are presented in Table 3. d18O and dD values of two cave water samples are 8.7% and 60%, and 8.3% and 57%, respectively, and these data generally agree with the GMWL (global meteoric water line; Craig, 1961) suggesting that rainwater quickly becomes cave water without any isotopic changes (thus the residence time in overlying soil and rock layers is very short). Also, d18O and dD values of stream waters are 6.1% and 59%, and 6.1% and 56%, respectively. One sample follows the trend of GMWL, but the other sample shows a slight enrichment of d18O relative to dD (that is, the deviation to the right of the GMWL), possibly due to evaporation effect of surface runoff. d18O and d13C values of the overlying carbonate sediments are +0.1–0.4% and +0.6–0.7%, respectively, and these values are well within the range of shallow water carbonates around Jeju Island (Ji and Woo, 1998a, b). The d13C value of the plant living on the surface overlying the cave is 32%. d13C values of the speleothems in Dangcheomul Cave show a wide range depending upon the type of speleothems (Table 3): stalactites ( 7.3% to 0.8%); cave coral ( 7.6%); cave flower ( 4.9%); carbonate powder ( 8.6% and 7.5%) (Fig. 9). The large variation of d13C values between 8.6% and 0.8% indicates two sources of
Table 3 Stable isotope and trace element compositions of speleothems, cave water and stream
5.3. Stable isotopes
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
79
Fig. 9. Scatter diagram of d13C vs. d18O for calcareous speleothems in Dangcheomul Cave.
carbon, organic carbon from overlying soil horizon and inorganic carbon from carbonate sediments. Tentatively, more positive values of d13C imply more contribution from carbonate sediments, whereas more depleted d13C values are derived from more influence of organic carbons supplied from soil layer above. Assuming that d13C values of speleothems reflect two sources of carbon reservoirs and kinetic d13C enrichment is negligible during degassing of carbon dioxide for the formation of speleothems (evaporation-dominated process), the relative contribution of organic carbon from carbonate sediment in stalactites, cave coral, cave flower and carbonate powders can be estimated to be about 0–24%, 25%, 17% and 25–28%, respectively. Hence, it can be inferred that the carbonate sediments above the cave are a major source of carbon for the growth of speleothems in Dangcheomul Cave. Somewhat wider variations of the d13C values in stalactites relative to other speleothems may indicate that the stalactites are more sensitive to the fluctuations of the amount of soil-derived organic matter. In other words, relatively less variable d13C values in other speleothems (cave coral, cave flower, and carbonate powder) suggests that the supply rate of organic carbon was more or less constant during their growth. d18O values of speleothems range from 3.9% to 6.0%, showing small variation relative to d13C values. The temperature in the Dangcheomul Cave ranges from 18 to 24 1C, and the d18O values of cave water about 8.7% to 8.3% (vs. SMOW). The calculated equilibrium d18O values of speleothems, using the equation of Epstein et al. (1953), range from 10.5% to 8.7%. Accordingly, the measured d18O values are more enriched than calculated ones by about 3–5%. Such enrichment in d18O suggests that the formation of speleothems was mostly driven by evaporation rather than degassing of CO2. Choi et al. (2005) suggested that cave corals of the lava tubes in Jeju Island grow from thin film of fluids supplied by seepage
water from surrounding rocks. It is expected that the cave flowers in Dangcheomul Cave grew in the pool as water was evaporated. In addition, carbonate powders are always found on the surface of the wall or other speleothems, implying that they formed by evaporation of seepage water as the supply rate decreased. Similar range of d18O values of stalactites to that of cave coral may also imply that stalactites may have been dominated by evaporation for their formation. One stalactite, showing the most enriched d18O value ( 3.9%) also support this assumption. Hence, these speleothems tend to form where the effect of evaporation is larger than the effect of CO2 degassing, and it can be therefore accepted that their enriched d18O values are the result of their formation process, i.e., an evaporation-dominated process. 5.4. Major, minor and trace elements Results of elemental data are presented in Table 3. Cave water and stream water show similar Si compositions; however, other elemental contents are quite different. The higher concentration of Na and Mg in the stream water is presumed to be influenced by the weathering of silicate minerals. However, higher Ca, Sr, and K concentrations in cave water appear to be influenced by overlying carbonate sediments. Similar Si and Ca results of cave water were reported from the cave corals in lava tubes of Jeju Island (Woo et al., submitted). Na contents of speleothems are lower than those of shallow-marine carbonate sediments reported elsewhere (Milliman, 1973), indicating that Na compositions are similar to those of meteoric calcite formed in watercontrolled open system (James and Choquette, 1990). Lower contents of Mn and Fe correspond with the overall oxidizing condition in the cave. Mg contents range from 780 to 1250 ppm, which is within the range of diagenetic calcite of meteoric environment (Veizer, 1983). However, a
ARTICLE IN PRESS 80
K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
cave coral with a higher content of Mg (ca. 4900 ppm) and Sr (ca. 670 ppm) than other speleothems may reflect its mode of the formation. As mentioned earlier, cave corals usually form from thin film of water by evaporation (Woo, 2005; Woo et al., submitted). Hence Mg and Sr compositions (like Mg/Ca and Sr/Ca ratios) of the thin film of fluids may have increased with calcite precipitation because partition coefficient(s) are significantly less than unity (Kinsman and Holland, 1969; Katz, 1973; Fairchild et al., 2000). 6. Discussion: origin of calcite speleothems in Dangcheomul Cave Even though Dangcheomul Cave is a lava tube, various calcite speleothems have been growing. Secondary mineralization including calcite precipitation in lava tubes may not be uncommon elsewhere in the world (for example Duck Creek Cave of Utah, USA; Halliday, personal communication), however their density of distribution and erratic morphology may be truly unique (Forti, 2004; Cultural Heritage Administration, 2006). Hence, this cave was nominated as a World Heritage site by the Korean government in 2006. Unlike other calcareous speleothems in limestone caves, most speleothems with erratic morphologies in this cave could be formed in a very straightforward way because they are simply calcite coatings around plant roots. Results of geochemical analyses suggest that calcium and carbonate ions for their growth were entirely derived from carbonate sediments overlying the cave. The precipitation of calcite should take place from cave water, which originated from meteoric water that became buffered with overlying carbonate dune sands and which easily infiltrated into the cave through plant roots. This process can be considered quite different from speleothem formation in limestone caves, because the groundwater usually percolates into limestone caves through joints or bedding planes. Thus, compared to the long residence time of the groundwater that enters into limestone caves, the residence time of groundwater in Dangcheomul Cave is relatively very short (within a few hours and sometimes a few days at most), because Dangcheomul Cave is located just a few meters below the surface and groundwater is usually transported through cracks between columnar joints and plant roots. However, some speleothems such as flowstones were formed by groundwater that was transported through fractures or cracks in surrounding basalts. Also, cave corals, shelfstone, and rimstone appear to grow in ways similar to those in limestone caves when the rate of water supply into the cave is high enough to produce flowing or standing water on cave floor. Many plant roots are hanging down from the ceiling are present (Fig. 5A), and these roots have penetrated through overlying soils, carbonate dune sands, and basalts. The length and thickness of plant roots varies according to the type and age of vegetation, and some of them may grow down to cave floor. Calcite precipitation usually begins to
take place from a water drop at the tip of root (Fig. 5A), however water droplets can be also found anywhere along root surface, meaning that calcite precipitation may start at any point. After rain, meteoric water rapidly enters into the cave along the roots of plants, and calcite precipitation takes place either by the degassing of carbon dioxide or evaporation, resulting in the formation of erratic soda straws and stalactites (Fig. 5B). Stalactites begin to grow as micritic calcite at the incipient stage (Fig. 5D), probably because the saturation state with respect to calcite is high or the surface of the roots is very irregular, thus providing more nucleation sites for calcite precipitation. If roots are decayed away during stalactite growth, the hanging stalactite may fall to the cave floor. The shape of stalactites is strongly influenced by the overall morphology of the precursor root, such as its length and/or the presence of rootlets (Fig. 5B). When the root has reached the cave floor and calcite precipitation begins to take place from the lower part, a stalagmite with an acute form may be developed after the decay of plant root in the upper part (Fig. 5C). When the roots extend from ceiling to floor and several stalagmites merge into one, a column can be developed. Numerous molds of plant roots in the transverse section of the broken column support this processes (Fig. 7D). Although the shape of speleothems takes after that of precursor plant roots to a great extent, it can be also modified by another factor such as the supply rate of cave water. For example, a vase-shaped column showing downward increase of the diameter might have been formed when the supply rate of water was high enough to cause continued lateral growth mainly in the lower part. On the other hand, some stalactites with very thin and uniform diameters might grow along the very thin root extending from the cave ceiling nearly to the cave floor when the supply rate of cave water was steady and low. Depending upon the nature of branching roots and their morphologies as well as the rate of water supply, a variety of erratic speleothems may form. Conclusively, a variety of erratically shaped calcite speleothems in the lava tube (Danhcheomul Cave) are thought to result from a combination of following conditions: (1) the formation of volcanic lava tube, providing the accommodation space for the speleothem growth; (2) the deposition of carbonate sediments above the cave, providing calcium and carbonate ions for the growth of calcite speleothems; (3) the development of vegetation with deep roots, facilitating the transport of carbonate-charged meteoric water into the cave and finally (4) the suitable surrounding climatic condition of Jeju Island, controlling vegetation type and the amount of rainfall. 7. Conclusions The Dangcheomul Cave, a typical lava tube formed in the late Quaternary volcanic rocks, is characterized by numerous calcite speleothems, which may well be truly
ARTICLE IN PRESS K.S. Woo et al. / Quaternary International 176–177 (2008) 70–81
unique in the world. Dissolution of carbonate sediments above the cave provides calcium and carbonate ions for their formation. Most speleothems in the cave commonly show erratic shapes which cannot be seen in limestone caves, and such erratic shapes result from a combination of (1) the penetration of plant roots from the overlying soil into the cave, (2) the supply of meteoric water along the roots, and (3) precipitation of carbonate minerals around the roots. Geochemical analyses indicate that evaporation of cave water rather than degassing of carbon dioxide played an important role in the formation of calcite speleothems. Acknowledgments This work was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and Technology of Korea. Special thanks to R. Kim, and K.N. Jo for laboratory assistance and fieldwork. References Choi, D.W., Woo, K.S., Lee, K.C., 2005. Opal cave corals in the lava tubes in Jeju Island, Korea: implications for local paleoenvironmental change. Journal of the Geological Society of Korea 41, 465–480 (in Korean, English abstract). Craig, H., 1961. Isotopic variations in meteoric waters. Science 133, 1702–1703. Cultural Heritage Administration, 2006. Jeju Volcanic Island and Lava Tubes—A Candidate For World Heritage Inscription, 176pp. Curl, R.L., 1972. Minimum diameter stalactite. The National Speleological Society Bulletin 34, 129–136. Epstein, S., Buchbaum, R., Lowenstam, H.A., Urey, H.C., 1953. Revised carbonate water isotopic temperature scale. Geological Society of America Bulletin 64, 1315–1326. Fairchild, I., Borsato, A., Tooth, A.F., Frisia, S., Hawkesworth, C.J., Huang, Y., McDermott, F., Spiro, B., 2000. Controls on trace element (Sr–Mg) compositions of carbonate cave water: implications for speleothem climatic records. Chemical Geology 166, 255–269. Forti, P., 2004. Genetic processes of cave mineral in volcanic environments: an overview. In: Proceedings of the 11th International Symposium on Volcanospeleology, Pico Island, Azores, Portugal, pp. 12–13. Friedman, G.M., 1959. Identification of carbonate minerals by staining method. Journal of Sedimentary Petrology 29, 87–97. Hill, C., Forti, P., 1997. Cave Minerals of the World. National Speleological Society, Huntsville, 463pp. Hwang, S.G., Ahn, U.S., Lee, M.W., Yun, S.H., 2005. Formation and internal structures of the Geomunorm Lava Tube System in the
81
northeastern Jeju Island. Journal of the Geological Society of Korea 41, 385–400 (in Korean, English abstract). James, N.P., Choquette, P.W., 1990. Limestones—the meteoric diagenetic environment. In: McIlearth, Morrow, D.W. (Eds.), Diagenesis. Geoscience Canada Reprint Series, vol. 4, pp. 35–74. Ji, O.M., Woo, K.S., 1995. Compositional variations of the beach sediments in Cheju Island. The Journal of the Korean Society of Oceanography 30, 480–492 (in Korean, English abstract). Ji, O.M., Woo, K.S., 1998a. Trace (minor) elemental and isotopic compositions of calcitic skeletons in the eastern coast and Cheju Island, Korea. The Sea—Journal of the Korean Society of Oceanography 3, 124–141 (in Korean, English abstract). Ji, O.M., Woo, K.S., 1998b. Trace (minor) elemental and isotopic compositions of aragonitic skeletons in the eastern coast and Cheju Island, Korea. The Sea—Journal of the Korean Society of Oceanography 3, 112–123 (in Korean, English abstract). Katz, A., 1973. The interaction of magnesium with calcite during crystal growth at 25–90 1C and one atmostphere. Geochimica et Cosmochimica Acta 37, 1563–1586. Kinsman, D.J.J., Holland, H.D., 1969. The coprecipitation of cations with CaCO3-IV. The coprecipitation of Sr2+ with aragonite between 16 1C and 96 1C. Geochimica et Cosmochimica Acta 33, 1–17. Milliman, J.D., 1973. Marine Carbonates. Springer, New York, 374pp. Self, C.A., 2003. The internal organization of speleothems. Acta Carsologica 33, 245–255. Son, I.S., 2003. Natural caves in Jeju, Cultural Properties Administration, 263pp. Veizer, J., 1983. Chemical diagenesis of carbonates: Theory and application of trace element technique. In: Arthur, M.A., Anderson, T.F., Kaplan, I.R., Veizer, J., Land, L.S. (Eds.), Stable Isotopes in Sedimentary Geology. Society of Economic Paleontologists and Mineralogists, Short Course No. 10, pp. 3-1–3-100. Won, C.K., 1976. Study of petro-chemistry of volcanic rocks in Jeju Island. Journal of the Geological Society of Korea 12, 207–226. Woo, K.S., 2000. Scientific Reports of Okgye, Namdae, and Dongdae Caves in Gangneung, Gangneung City, 269pp (in Korean). Woo, K.S., 2005. Caves: A Wonderful Underground. Hallym Publisher, Seoul, 230pp. Woo, K.S., Choi, D.W., Kim, R., Kim, J.K., 2000. The origin of the speleothems in Dangcheomul Cave, Jeju Island, Korea. Journal of the Geological Society of Korea 36, 411–434 (in Korean, English abstract). Woo, K.S., Kim, R., Kim, J.K., Choi, D.W., Jo, K.N., 2004. Textural and stable isotopic results of the column in Dangcheomul Cave, Jeju Island, Korea: paleoclimatic implications. Journal of the Geological Society of Korea 40, 133–144 (in Korean, English abstract). Woo, K.S., Choi, D.W., Lee, K.C., Silicification of the cave corals in some lava tubes of Jeju Island, Korea: implications for the paleoenvironmental change during the late Quaternary. Quaternary International, submitted for publication. Yang, M.S., Moon, J.H., Kang, S.S., Hyeon, K.S., Jang, Y.J., 1997. Oreum (adventive volcano) in the Jeju Island. Jeju Provincial Government, 51–52 (in Korean).