Geochemical investigation of the Noboribetsu Oyunuma explosive crater lake

 Masayo MUROZUMI, Tsutomu ABIKO and Seiji NAKAMURA

(The Muroran Institute of Techonlogy, Japan)

Bull. Volcanol. Soc. Japan 11, 1-16 (1966) in Japanese with English abstract

 Two explosion craters, Oyunuma and Jigokudani, were formed by the last activity of the Kuttara volcanism.  The present volcanic activity makes Oyunuma a typical craterlake filled with 9x104 tons of thermal water enriched in chemical constituents.  This crater lake were found to have two layers of thermal water distinctly divided by a boundary surface.  The upper layer which is well developed all over the crater lake is 50C in temperature and more diluted in chemical concentrations, which the lower layer, falling up to 120C and having a value of 1.3 in the specific density, contains much more abundant chemical components such as Na+ and Cl- than the former does.  The thermal water making the upper layer flows out of the crater at a daily rate of 4000 tons after being stored in the lake for 20 days.  Concerning the origin of chemical constituents, it was made clear by two test borings that the cracks in the base rock, Kuttara welded-tuff, are filled with the high temperature water, the chemical property of which is neutral and saline.  This thermal water from underground is transported to the bottom of the crater lake, namely, to the lower layer after being diluted with the fresh ground water.  Much more extended dilution takes place on the surface and makes the upper layer more diluted.  Volatile sulfur compounds such as H2S and SO2 escape from the neutral saline water and are oxidized in the upper layer to make sulfuric acid. It has been observed that the change in the intensity of the volcanic activity disturbs the above mentioned structure of the crater lake and the increasing activity concentrates the chemical constituents in the neutral saline water from underground.

 (The following text was originally written in Japanese by authors.  The Japanese text was translated by T. Ohba.  A possible miss-translation is responsible to T. Ohba)

 INTRODUCTION

   The Kuttara volcanism has produced Nishi-yama peak, Central peak, Kita peak, Kuttara caldera, Hiyori-yama peak and Funami peaks in this sequence.  After the volcanism, Oyunuma explosive crater has been generated between Hiyori-yama peak and Funami peaks.  Jigokuadani explosive crater has been also generated between Nishi-yama peak and Funami peaks, at the same time of Oyunuma crater formation.  Now, huge amount of heat and material is discharged due to the hydrothermal activity represented by hot spring water 1), 2), 3, 4).  In the present hydrothermal system, the original deep fluid is neutral and dominated by Na+, Ca2+ and Cl-.  The original fluid is separated into liquid and vapor phases in shallow depth.  At the separation, Na+, Ca2+ and Cl- are distributed to liquid phase.  On the other hand, H2S, SO2 and CO2 are distributed to vapor phase.  The liquid phase is discharged at places with low altitude as neutral NaCl hot spring water.  The vapor phase is discharged at geothermal area as fumarolic gas or absorbed by near surface water to produce a steam heated hot spring water containing sulfuric acid.  At Jigokudani crater, a shallow ground water was flowing into the crater through Noboribetsu formation.  The flux was estimated to be 23 Liter/sec.  The shallow ground water was heated by the original fluid to produce hot spring discharge, the flux of which was estimated to be 31 Liter/sec 5).  The bottom part of Oyunuma explosive crater was filled with Oyunuma lake, the size of which is 200m in N-S and 80m in E-W, and 25m deep at maximum.  The temperature of water at surface and the bottom of Oyunuma is 50C and 120C, respectively.  There is a small crater lake (Okuno-yu) beside the Oyunuma lake.  The water temperature of Okuno-yu lake is 80C at the surface.  The flux of hot spring water was estimated to be 47 Liter/sec which is flowing out of Oyunuma explosive crater. In this paper, we intend to reveal the behavior of chemical composition which was transported by liquid phase within Oyunuma explosive crater.


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.

 VOLCANIC ACTIVITY WITHIN OYUNUMA EXPLOSIVE CRATER

 A crater has been generated at the point where the eastern flank of Hiyoriyama peak and the western flank of Funami peaks are joining (Fig. 1).  The crater scooped out Noboribetsu quaternary formation and underlying Kuttara tuf formation.  The crater is called Oyunuma explosive crater, in which steam with temperature of 120C is discharged at fumaroles.  Within Oyunuma explosive crater, there are small craters, which is filled with hot water.  The two hot lakes are called as Oyunuma lake and Okunoyu lake.


Fig. 1 Topographic map of Noboribetsu spa

As shown in Table 1, the temperature of lake water of Okunoyu is 80C.  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 50C.  The temperature of bottom layer of lake reaches 120C.  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

 We employed a boat which has been used for commercial sulfur production.  With a string with spindle, the depth of lake was measured.  The lake temperature was measured with a thermistor thermometer directly.  The lake water was sampled by a reversing water bottle at various depth, and subjected to the analysis of chemical components (pH, Cl-, SO42-, Na+, K+, Mg2+, Ca2+, Fe2+).  Oyuma lake is 200m long in N-S and 80m long in E-W.  The shape is rectangle (Fig. 2).  The average depth is about 6 m.  On the bottom of lake, solid sulfur is deposited ubiquitously.  In the lake, the north shore is shallow due to the deposit of sulfur and solid material coming from Okunoyu.  In the center of Oyunuma there is a small bank.  In this study, the north depression is called as gsmall basinh and the south depression is called as glarge basinh.  Both of small and large basins have a spot where gas bubbles are discharged.  The maximum depth beneath the spot reaches 25m in the large basin.  On the spot, hollow spherules of sulfur are floating.  The spots are the essential center of volcanic activity of Oyunuma explosive crater.  The size of small and large basins are 3.5x103 and 11x103 m2, respectively.  The total amount of lake water in Oyunuma lake is estimated to be 9x104 m3.  The daily flux of lake water flowing out at Oyunuma lake is 4x103 m3/day, therefore, the residence time of lake water is about 20 days.

 
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 1C (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 120C.  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 70C.  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

 Fig. 3-1 shows the distribution of lake temperature at 1 m of depth.  The distribution of temperature is in the rage of 41.5 to 45C.  The observation was carried out in winter season.  The temperature is high in the small basin and low in the large basin.  The distribution would be established by the following factors.
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.

In 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 0.134}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 gMixed layerh.  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 gStagnant layerh.  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.

 The water flowing out Oyunuma explosive crater is not the water of stagnant layer of Oyunuma lake, but the mixed layer.  The water at Oyunuma crater flows along Kusurisanbetsu river, the bed of which is composed of Kuttara tuf formation (Fig. 1).  The concentration of chemical component in the river water change along flowing (Table 5).  The concentration of Cl-, Na+, K+, Mg2+, Ca2+ increase, but the concentration of SO42- show a little change.  The change is attributed to the addition of thermal water which is discharged at the bed of river.  As shown in Table 5, the hot spring water (A, B) found at the river bed is enriched in Cl-.  There is bore holes which were dig by a private company along Kurisanbetsu river.  The chemical composition of the water in bore hole was measured along the depth (Table 6-1 and 6-2).  The near neural water found in the bore holes is enriched in Cl-.  The near neutral water could be originated in a source common to the water in stagnant layer in Oyunuma lake.  Through the cracks developed in Kurrara tuf formation, a neutral Cl- enriched thermal fluid is moving.  The fluid is supplied to the Okunoyu lake and the stagnant layer of Oyunuma lake.


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.

Seasonal change

 Oyunuma explosive crater is exposed to open air, then, the lakes are affected by seasonal climate change.  Actually, the mixed layer of Oyunma lake is variable in terms of temperature and chemical composition as shown in Fig. 8 and 9.  The temperature of the mixed layer was about 50C in summer and 43C in winter.  In summer season (August), the surface water showed slightly high temperature relative to the bulk mixed layer (Table 9).  The high temperature at surface was not observed in the seasons other than summer due to the cooling at surface.  The concentration of chemical component was high in winter and low in spring, which is due to the variable contribution of fresh dilute surface water.  The high concentration of chemical composition in winter season suggests the low contamination of cold fresh water.  However, the temperature of mixed layer is low in winter.  The effect of cooling in winter season is significant.  Although the significant cooling effect in winter, the stratified structure of Oyunuma lake has been sustained.  The parameters on the structure depend on the volcanic activity.  The monitoring of Oyunuma lake could be the measure of volcanic activity.

 
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.

Conclusion

 The volcanic activity in Oyunuma explosive crater produces the stratified structure of Oyunuma lake.  The stratified structure was stable over seasons, being in a kind of steady state.  The steady state of lake depends on the contribution of cold fresh surface water.  The shallow mixed layer in Oyunuma lake is provided by the stagnant water in terms of enthalpy and material.  The stagnant lake water would be a mixture of essential thermal fluid and fresh groundwater.  In summary, the activity in the explosive crater is affected significantly by fresh surface and groundwater.
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 85C.  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).


References
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)


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