Geochemical investigation of the Noboribetsu Oyunuma explosive crater lake
Muroran Institute of Techonlogy, Japan)
Bull. Volcanol. Soc. Japan 11, 1-16 (1966) in Japanese with English abstract
Photo-1. Oyunuma lake taken by T.Abiko. Left and right is the small basin and large basin, respectively.
Photo-2. Okunoyu lake (short) and Oyunuma lake (back) taken by T.Abiko.
Fig. 1 Topographic map of Noboribetsu spa
As shown in Table 1, the temperature of lake water of Okunoyu is 80ĀčC. It is discharged with flow rate at 1600 ton/day. The lake water is diluted by cold fresh stream water originating in Noboribetsu quaternary formation. The mixture of lake water and spring water flows into Oyunuma through the east-north shore. The temperature of Oyunma lake is about 50ĀčC. The temperature of bottom layer of lake reaches 120ĀčC. On the bottom of lake, a pool of molten sulfur is sustained. A part of the molten sulfur float on the lake surface with the shape of hollow spherule. The black part of surface on lake is due to the floating sulfur spherules.
Table 1. Size and prperties of Oyunuma lake (first line) and Okunoyu lake (second line).
In Table 2, the temperature and chemical composition of lake water and cold spring water are listed. At Okunoyu lake, a slightly acidic water is discharged. The water is enriched in Cl- and Na+ and depleted in SO42-. The water is 2.5 times diluted with fresh water, then, transported to Oyunuma lake. In Oyunuma lake, Cl- and Na+ concentration come back to 70% of that of Okunoyu lake, and the SO42- concentration increased up to 10 times relative to Okunoyu lake with the increase in acidity. The lake water of Oyunuma flows out Kusurisanbetsu river. In Table 3, the flux of material and enthalpy flowing into the river is summarized.
Table 2. Chemical composition of Okunoyu lake (first line), surface water (second line), the water flowing into Oyunuma (third line), the stagnant layer of Oyunuma (4th line) and the water flowing out of Oyunuma (5th line).
Table 3. Heat flux in the Oyunuma crater in Kcal /day (left column). Material flux in ton/day.
Structure of Oyunuma lake
Fig.2 The depth of Oyunuma lake. The number in lake is the depth in meter. The upper small lake is Okunoyu, the lower large lake is Oyunuma.
Distribution of lake water temperature (Fig.3-1, 3-2)
The water temperature of Oyunuma lake was measured at various points at 1, 3, 6, 8 and 10 m of depth. At the gas bubbling spots, water temperature was measured with 1 m step to the bottom. The horizontal distribution of water temperature is almost homogeneous with slight local variations. The variation along the depth is small in the region shallower than 13 m. Especially in the large basin, the variation is less than 1ĀčC (Fig. 3-3). In the region deeper than 13m and 5m, respectively in the large and small basins, a rapid increase was observed in temperature. In the large basin, the maximum temperature reaches 120ĀčC. The thickness of the boundary layer between the shallow and deep layer is about 0.5 m. The lake water in Oyunuma is divided into two layers. The temperature difference between the two layer extends to 40 to 70ĀčC. Although the temperature of deep layer at the large basin is high, the temperature was less than the in-situ boiling temperature.
Fig. 3-1 The temperature of lake water at 1m of depth in degree C.
Fig. 3-2 The distribution of temperature at various depth along the line on Oyunuma lake.
Fig. 3-3. The temperature distribution along the depth on the spot with gas bubbling. The dotted and broken lines are for the small basin and solid line is for the large basin in Oyunuma.
Formation of the mixing layer, stagnant layer and flow system
1) A discharge of high temperature of spring water in small basin.
2) A cooling of water in the large basin.
The relatively low temperature at large basin is not attributed to an addition of cold fresh water because the chemical composition is homogeneous at shallow layer (Fig. 4).
Fig. 4. The distribution of Cl- concentration at the depth of 1m.
the small basin, the water coming from Okunoyu is heated with a high temperature
thermal water discharged at the bottom of small basin. The homogenized shallow water moves to the
large basin. The deep region of large
basin is filled with a thermal water, the apparent density of which is 1.33 (g/cm3). A density difference between the deep and
shallow layers prevents an effective mixing between them. The temperature variation along the depth in
the shallow layer is very small, suggesting a vigorous vertical convection
within the shallow layer.
Figure 4 shows the horizontal distribution of Cl- concentration at 1 m of depth. The concentration is 0.134Ā}0.002 (g/Liter). The variation is limited within a small range suggesting the shallow layer is mixed well horizontally. Figure 5 shows also the horizontal distribution of Na+ concentration at 1 m of depth. The homogenized distribution is similar to that of Cl-.
Fig. 5 The distribution of Na+ concentration
Fig.6 show the vertical distribution of chemical components in the large basin. The shallow layer is homogenized. The homogenization is the result of vertical convention. We call the shallow layer as ĀgMixed layerĀh. Below the end of mixed layer, the concentration of Cl- and Na+ increase significantly as same as Mg2+ and Ca2+ to the concentration two times of that in the mixed layer. We call the deep layer as ĀgStagnant layerĀh. It should be noticed that the pH of stagnant layer is low relative to the pH of mixed layer, and the SO42- concentration in the stagnant layer is not higher than that in the mixed layer. In the water of stagnant layer, sulfur particles are suspended. The sampled water of stagnant layer was separated into two phases when we put the water sample statically. The water at 25 m was separated into upper transparent liquid, the volume of which is 1/5 of original volume and the separated lower layer contains suspended sulfur particles. The volume of the separated transparent water increased to 4/5 of the original volume, for the stagnant water at 13m of depth. As shown in Fig.6, in the stagnant layer, nonhomogeneity is noticed in terms of chemical composition, suggesting the vertical convection within the stagnant layer is not vigorous, and original thermal water may be discharged at the lateral inner wall of lake. The boundary between the mixed layer and the stagnant layer is definite. The material flux from the stagnant layer to the mixed layer is limited.
Fig. 6. The distribution of temperature and conponents in the water of Oyunuma lake along the depth.
In summer season, the temperature of surface water at the spot of gas bubbling is higher than the temperature without bubbling. The high temperature disappeared in winter season. The presence of gas bubbling means a transport of material coming from the stagnant layer to the mixed layer. It is noticed that the pH of lake water at surface is slightly lower at the spot of gas bubbling relative to the surface with no bubble (Fig. 7). The low pH at the spot suggest that H2S and SO2 gases were transported by the gas bubbles, then oxidized at near surface resulting in the generation of H2SO4.
Fig. 7. The distribution of pH of water at the depth of 1m in Oyunuma lake.
The origin of chemical component.
Table 5. The chemical composition of Kusurisanbetsu river water and the hot spring water (A and B) located along the river.
Table 6-1 and 6-2. The chemical composition of water in bore hole deg near Kusurisanbetsu river.
Sulfate is enriched in the mixed layer of Oyunuma lake. As shown in Table 7, the SO42-/Cl- ratio in the mixed layer is exceptionally high relative to other fluid. It is difficult to suppose that SO42- in the mixed layer behaved with Cl- commonly. Sulfate in the mixed layer would be generated from the gaseous H2S and SO2. Those gases were oxidized to produce SO42- ion.
Table 7. The concentration of SO42- and Cl- ion in various fluid in Oyunuma explosive crater. From the first to 8th line, it is surface water, Okunoyu lake, the water flowin into Oyunuma, the mixed layer in Oyunuma, Taisho hot spring, the stagnat layer in Oyunuma, neutral bore hole water, neutral bore hole water, respectively.
Fig. 8. The distribution of water temperature at Oyunuma lake along the depth in various season
Table 8. The seasonal change in the Cl- concentration of Oyunuma lake water.
Fig. 9. The seasonl change in the water temperature in the stagnat layer in Oyunuma lake. The symbols A to I corresponds to those in Fig.8.
Table 9. The tempoaral variation of lake watrer in Oyunuma lake along the depth.
In 1951, the following changes were observed suggesting the increase in volcanic activity.
1) At the spot of gas bubbling on the lake surface, the water temperature reaches 85ĀčC. A fountain of thermal water stood on the Oyunuma lake surface, the height of which was 2m.
2) The surface of Oyunuma was filled with hollow spherules of sulfur. The commercial production rate of sulfur was 3 ton/day. On the bottom of lake, a molten sulfur was sustained. The molten sulfur could be sampled easily with a bucket ( It is impossible now).
The comparison is made in Table 10 between the present lake water and that in 1951. During the increase activity, the Cl- concentration increased. The increase in Cl- is due to the increased discharge flux of neutral thermal water. Beneath the spot of gas bubbling, the stratified structure would be disturbed, and the boundary layer between the mixed and stagnant layers would be raised.
Table 10. The temperature and chemical composition of Okuoyu and Oyunuma lake in 1951 (upper part) and in 1965 (lower part).
1) M. Murozumi: On the geochemical behavior of iron in the hot spring activity in Noboribetsu spa. Chisitugakuzassi, 69, (1) 19 (1960) ( in Japanese)
2) M. Murozumi: Exo-and end magmatic hydrothermal differentiations observed among the chemical components exhaled by Noboribetsu volcanic activity. Report of Geological Survey Japan, 12, 63 (1961)
3) M. Murozumi: Hot spring water activity in active volcano and the shift of hydrogen isotope ratio, Hot spring water activity in Noboribetsu, Hokkaido, Bull. Volcanol. Soc. Japan, 6, (1) 42 (1961) (in Japanese)
4) Y. Uzumasa, M. Murozumi,: Volcanic sublimates and volcanic activity. Bull. Volcanol., p153 (1963)
5) J.Suzuki, T.Ishikawa, M.Ishibashi: Noboribetsu spa and Lake Kuttara, Field guide for geology in Hokkaido island (1943)