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CO2 supply from deep-sea hydrothermal systems
Waste Management, Vol. 17, No. 5/6, pp. 385 390, 1997 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0956-053X/98 $19.00+0.00
Pergamon PH: S0956-053X(97)100,16-0
CO2 SUPPLY FROM DEEP-SEA HYDROTHERMAL SYSTEMS
Kiminori Shitashima Environmental Science Department, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba-ken 270-11, Japan
ABSTRACT. Deep-sea hydrothermal systems are aimed as an on-site field analysis on the behavior and diffusion of CO2 in deep ocean. Through ocean ridge volcanism, a large amount of elements including carbon as a form of CO2 are supplied to deep ocean. Hydrothermal vent fluids at highly enriched in CO2 and show low pH (~ pH 3) relative to seawater. Total carbonate, total CO2 in seawater, and pH were determined in samples at hydrothermal active area in S-EPR. The concentration of total carbonate and pH in the hydrothermal fluid samples ranged from 16 to 5mM and from 3.1 to 7.6, respectively. The hydrothermal fluids discharged from the vents were rapidly diluted with ambient seawater, therefore total carbonate concentration and pH value in the plume waters become close to that of ambient seawater near the vents. The positive anomaly of total carbonate and negative anomaly of pH associated with hydrothermal plumes were observed on the seafloor along S-EPR axis. The diffusion of total carbonate plumes both westward and eastward in the bottom water along 15°S across the S-EPR were also detected, but pH anomalies were not obtained in the plume. These suggest the possibility of discharging of CO2 through hydrothermal systems to the ocean. Recent estimation of CO2 fluxes to the ocean through MOR was calculated at 0.7,-q5x 1012 mol C year -I. These values are 3-4 orders of magnitude smaller than the annual CO2 fluxes through terrestrial and marine respiration, therefore the importance of CO2 input from MOR on oceanic carbon cycle is thus minimal on shorter-term time scale. However, the CO2 input from MOR is significant at 106,'-'107 years scales, and CO2 concentration in hydrothermal fluids at hotspot and back-arc basin is 10,,d00 times higher than that of MOR. The flux of CO2 from deep-sea hydrothermal systems to the ocean may be significant. © 1998 Elsevier Science Ltd. All rights reserved
Through deep-sea hydrothermal activities, a large amount of elements including carbon as a form of CO2 are discharge to deep ocean. Deep-sea hydrothermal activities occur at the active lithosphere area, such as spreading ridges, back-arc basin and undersea volcano, and the global extent of the discovered deep-sea hydrothermal activity areas is presented in Fig. 1. Seawater penetrates downward toward the m a g m a chamber from the crack of oceanic crust, the rapid heating forces the buoyant hot water, hydrothermal fluid, to the surface as a hydrothermal vent. Fig. 2 represents the schematic major processes during the seawater circulation through the oceanic crust at spreading center, called hydrothermal circulation. The typical hydrothermal vent, called black smoker, located near 18°S at the southern East Pacific Rise (S-EPR) is shown in Fig. 3. The vents of the natural CO2-rich fluid and hydrate formation were found in 1989 at the hydrothermal active area in the mid-Okinawa Trough, back-arc basin. ~ This fact suggest, that CO2 is discharged to deep ocean by hydrothermal activities at the seafloor where hydrothermal vents exist. Hydrothermal fluids are highly
INTRODUCTION The study on the fate of carbon dioxide (CO2) in the ocean is important in coping with the global climate issues, because the future CO2 concentration in atmosphere can not be precisely predicted without full knowledge on the inventory of CO2 in the ocean. Furthermore, the direct ocean disposal of CO2 recovered from fossil fuel usage is now examined as one of possible options to mitigate global warming. For considering over the possibility of direct ocean disposal of CO2, it is necessary to know that how the direct ocean disposal affect to the ocean environment. Therefore the behavior, diffusion process and environmental impacts of disposed CO2 to the ocean should be defined.
Acknowledgements--I thank Drs T. Urabe and K. Fujioka as chief scientists of the R/V Melville and D S R V S H I N K A I 6500 cruises, respectively. T h a n k s are also extended to the captains, crew, S H I N K A I team and scientists aboard the cruises for their kind help in the sampling and onboard analysis.
385
386
K. SHITASHIMA
Map No. Site I
2 3 4 5 6 7 g 9 10 11 12 13 14 15 16 17 Ig 19 20 21 22 23 24
Lucky Sttil~ (37" 17'N) Broken Spur (29"N) TAG (26°N) Snakepit/MARK(23°N) Explorer Ridge (49"45'N) Middle Valley Endeavour (48*) Axial Volcano (46"N) Cleft~North (45*) Clefl~South (44"40'N) Eseanaba Trough Guaymas Basin (270N) EPR (21 ON) EPR (13"N) EPR (I ION) EPR (10*N) Calapagos (86*W) EPR (17"-19"S) Okinawa Trough (27"30'N) Madana Trough Manus Basin Woodlark Basin N. Fiji Basin (17"S) Lau/Valu Fa (22*S)
30*
30*
120"
150"
180" 150" 120" 90°
60*
30*
30*
0.
60*
90*
FIGURE 1. Location map of discovered deep-sea hydrothermal systems at the world ocean floor (from Baker et alJ).
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F I G U R E 2. Schematic r e p r e s e n t a t i o n o f a m a j o r processes d u r i n g the h y d r o t h e r m a l circulation o f s e a w a t e r t h r o u g h the oceanic crust at s p r e a d i n g center (from M a s s o t h et al.2).
enriched in C O 2 and show low pH (about pH 3) relative to seawater. The C O 2 in deep-sea hydrothermal vent fluids is mainly of magmatic origin. This CO2 is taken up from the basalt by the fluid during high temperature seawater-magma interaction. Furthermore at the back-arc basin, CO2 is extracted from organic matter and calcium carbonate in the sediment over the subduction zone is added to the vent fluids. Therefore, hydrothermal vent fluids are highly enriched in CO2 relative to seawater. The large excess of CO2 in the hydrothermal fluid and plume produces a pH decrease. The CO2 and pH data of hydrothermal fluid end-member, original
fluid, from various hydrothermal active sites were compiled in Table 1. Deep-sea hydrothermal systems are aimed as an on-site field analysis on the fate of CO2 in deep ocean. The study of deep-sea hydrothermal systems was, therefore, focused on the observation of the behavior and diffusion of COz in the deep ocean. In studying on the hydrothermal CO2, we can collect the data utilizable in the verification of the models developed for the behavior prediction of the injected CO2 in the ocean and imagine the direct effect of the injected CO2 on the intermediate and deep oceanic environment. This paper will report the observation
CO2 SUPPLY FROM DEEP-SEA HYDROTHERMAL SYSTEMS
387
FIGURE 3. A photograph of typical hydrothermal vent located near 18°S at the S-EPR. TABLE 1 The CO2 and pH Data of Hydrothermai Fluid End-member from Various Hydrothermal Active Sites (from Gamo4). End-member Values are those Obtained by Extrapolation to an Assumed of Zero Mg Name of site
Okinawa Izu-Bonin Mid-Mariana Trough Seamount Trough
Type
Back-arc basin 710 1390 4.7 5.3 64 209
Depth (m) pH (25°C) CO2 (mM)
Arc 1380 3.7 34~42
Back-arc basin 149(~3600 3.94.4 43.4
Manus Basin Back-arc basin ~2500 5 21
North-Fiji Lau Basin Loihi Juan de Fuca Basin Seamount Ridge
Guaymas Basin
Back-arc basin 2000 4.7 l 1.1-14.4
Mid-ocean Mid-ocean ridge ridge 2000 ~2600 5.9 3.14.0 16 24 5.(~18
Back-arc basin 1720 2.0
results of the behavior of hydrothermal CO2 in deepsea at the southern East Pacific Rise (S-EPR) and consider that the influence of the input of hydrothermal CO2 to carbon cycle in deep ocean. FIELD OBSERVATION OF CO2 DISCHARGE FROM DEEP-SEA HYDROTHERMAL
SYSTEMS Total carbonate (the sum of carbonic acid, bicarbonate ion and carbonate ion dissolved in seawater) and pH were determined in samples of the nonbuoyant plume, the buoyant plume and the hydrothermal fluid collected at S-EPR by the R/V Melville 14 (November~December 1993) and the DSRV SHINKAI 6500 (September~November 1994) under the Ridge Flux project (funded by Science & Technology Agency, Japan). The concentration of total carbonate and pH in the hydrotherrnal fluid samples from black smoker chimney ranged from 5 to 16 mM (2 to 8 times greater than ambient seawater) and from 3.1 to 4.8, respectively. Total carbonate concentration of the SEPR vent fluids were relatively high in comparison to concentration reported for other EPR hydrothermal
Hotspot 1200 4.24.4 ~300
Mid-ocean ridge 1542 2400 2.7-5.5 2.7-12
East Pacific Mid-Atlantic Rise Ridge Mid-ocean ridge 1700-3700 2.64.5 -
vent fluids and 10,-~102 times lower than those of the vent fluid collected from hotspot volcano (Loihi Seamount) 2 and back-arc basin (e.g. Okinawa Trough). 3 The hydrothermal end-member fluids (original fluid) discharged from the vents are rapidly diluted between hydrothermal end-member fluid and ambient seawater by factors of 104-105.4 Figure 4 indicates the distribution of total carbonate and pH in the buoyant plume as a function of distance from hydrothermal active vents in the S-EPR. The buoyant plume samples were collected along with the trail of the buoyant plume by means of keeping the submersible buoyancy at natural and drifting with water current. Total carbonate concentration and pH value in the buoyant plume become close to those of ambient seawater within dozens meters from the vents due to drastic mixing and dilution of vent fluid with ambient seawater. What happens to the hydrothermal CO2 discharged by deep-sea hydrothermal systems in deep ocean? Along-axis transact of total carbonate anomaly in hydrothermal plumes over S-EPR axis is shown in Fig. 5. The positive anomaly of total carbonate and negative anomaly of pH associated with
388
K. SHITASHIMA
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Distance from hydrothermal vents (m) F I G U R E 4. Distribution of total carbonate (upper) and p H (lower) as a function of distance from hydrothermal active vents in the S-EPR.I 5
hydrothermal plumes were observed on the seafloor along S-EPR axis. The highest concentratin of total carbonate anomaly detected near 16°S was 2.48 mM, but total carbonate concentration of ambient seawater in water column around S-EPR is about 2.36mM. Furthermore, the highest decrease of pH anomaly showed 0.02 pH unit. Figure 6 represents the vertical profiles of total carbonate for a short east-west section along 15°S across the S-EPR axis. The total carbonate anomalies were observed in the bottom water over the slope of S-EPR axis, indicating the dispersion of a hydrothermal plumes at this depth. The total carbonate plumes spreaded out above the seafloor both westward and eastward along 15°S across the S-EPR. At the western stations, the concentrations of total carbonate were decreased due to mixing with the Pacific water with low total carbonate concentration. However, the pH anomalies were not obtained in the same depth where the total carbonate anomalies were observed. Owing to CO2 dissociation to bicarbonate ion when CO2 dissolves in seawater, the low pH in seawater by the input of hydrothermal CO2 was recovered. According to recent research for hydrothermal plumes, the carbon dioxide anomalies associated with hydrothermal plume have not been observed various hydrothermal area. These results
-1900
i
-2000
-2100
-2200
E
~"-2300
-2400
-2500
-2600
-2700
i 18°S
17°S
16°S
15°S
lz~°S
Latitude F I G U R E 5. View of total carbonate in the water column over the S-EPR axis.
CO2SUPPLY FROM DEEP-SEA HYDROTHERMAL SYSTEMS 2.35 0
2.40
2.35
389
TCO 2 (mM/I) 2.40 2.35
2.40
500 1000
i nei
................................. , aLi ...............................
1500
K 2000 2500 3000 3500
West FIGURE 6. Vertical profiles of total carbonate for a short east west section along 15°S across the S-EPR axis ("Axis" is the spreading center, each "mile" is distance from the spreading center).
demonstrate that carbon dioxide is discharged through hydrothermal systems to the ocean in the same manner as 3He, CH4 and Mn. Schematic diagram of total carbonate plume for a short east-west section along 15°S across the S-EPR axis is presented in Fig. 7. The distribution pattern of the total carbonate plumes were different from that of the 3He plume reported by Lupton and Craig. 5 Low temperature diffuse discharge typically occurs in areas surrounding black smoker chimneys. This diffuse flow transports an order of magnitude more heat than the hydrothermal vent fluid from black smoker
chimney.6,7 It is implied that the low temperature diffuse flow is significant source of hear and chemical, such as CO2, fluxes in deep-sea hydrothermal systems. These observation results suggest the possibility of discharging of CO2 through deep-sea hydrotherrnal systems to the ocean. On the other hand, the other inputs of CO2 into the deep-sea are the bacterial activity around hydrothermal vents, the oxidation of organic matter generated by hydrothermal bacteria, and the formation of calcium carbonate and CO2 from calcium bicarbonate solution supplied by hydrothermal activity.
~---'[3He plume °ver the S-EPR~ - ~ \
\
/
ilution w i t h e Pacific water
~
~
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vent
[Oceanfloorspreading( FIGURE 7. Schematic diagram of total carbonate plume for a short east west section along 15°S across the S-EPR axis? 5
390 However, these inputs in deep-sea are indirectly concerned with h y d r o t h e r m a l activity. In other words, CO2 is supplied directly and indirectly from hydrothermal systems to deep ocean. Determination o f c a r b o n isotope ratio will be effective in order to distinguish the source o f each c a r b o n dioxide.
K. SHITASHIMA geological inference. In: Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions, Humphris, S. E., Zierenberg, R. A., Mullineaux, L. S. and Thomson, R. E. (eds). Geophysical Monograph 91, American Geophysical Union, Washington, DC, pp. 47-71. 2. Massoth, G. J., Milburn, H. B., Hammond, S. R., Butterfield, D. A., McDuff, R. E. and Lupton, J. E. The geochemistry of submarine venting fluid at Axial Volcano, Juan de Fuca Ridge: new sampling methods and a VENTS Program rationale. In: Global venting, midwater, and benthic ecological processes,
F L U X O F CO2 T O T H E O C E A N T H R O U G H M I D O C E A N RIDGE Recent estimates o f CO2 fluxes to the ocean t h r o u g h mid ocean ridge ( M O R ) f r o m the CO2/3He ratio o f vent fluids and 3He budget for the ocean were calculated at 0 . 7 + 0 . 2 5 x l 0 1 2 m o l C y e a r -1 by Gerlach 8 and 1 5 x 1 0 1 2 m o l C y e a r -1 by J a v o y and Pineau. 9 These results suggest that the input o f CO2 to the ocean f r o m M O R is 3-4 orders o f magnitude smaller than the annual CO2 fluxes t h r o u g h terrestrial and marine respiration, and an order o f magnitude smaller than the flux o f c a r b o n to shelf and deep-sea sediments. The importance o f COz input from M O R on oceanic c a r b o n cycle is thus minimal on shorterterm time scales (in several years order). However, the CO2 input f r o m M O R m a y be significant at longer time scales (106,~107 years), because ridge generation that cause the h y d r o t h e r m a l activity has been continuing from 100 million years ago, m o r e active ridge generation might occur in the past or ridge generation rates might undergo rapid change. On a time scale o f years, furthermore, the concentration o f CO2 in h y d r o t h e r m a l vent fluid at hotspot submarine volcano (Loihi Seamount) 2 and back-arc basin (e.g. O k i n a w a T r o u g h ) 3 is 10-,~102 times higher than that o f M O R (Table 1), but the hydrothermal activities at back-arc basin and h o t s p o t exist locally relative to M O R . The input o f CO2 from these h y d r o t h e r m a l systems to the ocean m a y be significant. The heat flux o f the Megaplume, even plume o f the episodic large-scale h y d r o t h e r m a l activity, is 1,-~3 orders larger than that o f a n o r m a l plume, 1° but the h y d r o t h e r m a l CO2 flux f r o m M e g a p l u m e is not m a d e clear yet at present time. F r o m the facts presented above, it can be imagined that the flux o f CO2 from deep-sea hydrothermal systems to the deep ocean m a y bge significant. It is necessary for the observation o f CO2 flux f r o m the individual h y d r o t h e r m a l activity and the understanding o f contribution or effect o f the h y d r o t h e r m a l CO2 flux to oceanic c a r b o n cycle. These will shed light on the environment impact o f CO2 discharge to deep-sea.
3.
4.
5.
6.
7. 8. 9.
10.
11.
12.
13. 14.
15. REFERENCES 1. Baker, E. T., German, C. R. and Elderfild, H. Hydrothermal plumes over spreading-center axes: Global distributions and
Deluca, M. P. and Babb, I. (eds). National Undersea Research Program Report 88-4, NOAA, Rockville, MD, pp. 29-59 (1988). Sakai, H., Gamo, T., Kim, E.-S., Yanagisawa, F., Tsutumi, M., Tanaka, T., Ishibashi, J., Wakita, H., Yamano, M. and Oomori, T. Venting of carbon dioxide-rich fluid and hydrate formation in mid-Okinawa Trough backarc basin. Science 248: 1093-1096 (1990). Gamo, T. Wide variation of chemical characteristics of submarine hydrothermal fluids to secondary modification processes after high temperature water-rock interaction: a review. In: Biogeochemical Processes and Ocean Flux in the Western Pacific, Sakai, H. and Nozaki, Y. (eds). Terra Scientific Publishing Company, Tokyo, pp. 425-451. Sedwick, P. N., McMurtry, G. M. and Macdougall, J. D. Chemistry of hydrothermal solution from Pele's Vents, Loihi Seamount, Hawaii. Geohimica Cosmochimica Acta 56: 36433667 (1992). Sakai, H., Gamo, T., Kim, E.-S., Shitashima, K., Yanagisawa, F., Tsutumi, M., Ishibashi, J., Sano, Y., Wakita, H., Tanaka, T., Matsumoto, T., Naganuma, T. and Mitsuzawa, K. Unique chemistry of the hydrothermal solution in the mid-Okinawa Trough backarc basin. Geophysical Research Letters 17: 21332136 (1990). Lupton, J. E., Delaney, J. R., Johnson, H. P. and Tivy, M. K. Entrainment and vertical transport of deep-ocean water by buoyant hydrothermal plumes. Nature 316:621-623 (1985). Lupton, J. E. and Graig, H. A major helium-3 source at 15°S on the East Pacific Rise. Science 214:13-18 (1981). Schultz, A., Delaney, J. R. and McDuff, R. E. On the partioning of heat flux between diffuse and point source seafloor venting. Journal of Geophysical Research 97:12229-12314 (1992). Ginster, U., Motti, M. J. and Von Herzen, R. P. Heat flux from black smokers on the Endeavor and Cleft segments, Juan de Fuca Ridge. Journal of Geophysical Research 99:4937-4950 (1994). Gerlach, T. M. Degassing of carbon dioxide from basaltic magma at spreading centers: II. Mid-oceanic ridge basalts. Journal of Volcanology and Geothermal Research 39:221-232 (1989). Javoy, M. and Pineau, F. The volatile record of a "popping" rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles. Earth and Planetary Science Letters 107:598-611 (1991). Baker, E. T., Massoth, G. J. and Feely, R. A. Cataclysmic hydrothermal venting on the Juan de Fuca Ridge. Nature 329: 149-151 (1987). Urabe, T., Baker, E. T., Ishibashi, J., Feely, R. A., Manumo, K., Massoth, G. J., Maruyama, A., Shitashima, K., Okamura, K., Lupton, J. E., Sonoda, A., Yamazaki, T., Aoki, M., Gendron, J., Green, R., Kaiho, Y., Kishimoto, K., Lebon, G., Matsumoto, T., Nakamura, K., Nishizawa, A., Okano, O., Paradis, G., Roe, K., Shibata, T., Tennant, D., Vance, T., Walker, S. L., Yabuki, T. and Ytow, N. The effect of recent magmatic activity on hydrothermal venting along the superfast-spreading East Pacific Rise. Science 269: 1092-1095 (1995). Shitashima, K., Sonoda, A., Ishibashi, J., Urabe, T., Fujioka, K., Baker, E. T., Feely, R. A. and Butterfield, D. A. The flux of total carbonate at the southern East Pacific Rise. JAMSTEC Journal of Deep Sea Research, in press (in Japanese with English abstract).
Pergamon PH: S0956-053X(97)100,16-0
CO2 SUPPLY FROM DEEP-SEA HYDROTHERMAL SYSTEMS
Kiminori Shitashima Environmental Science Department, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba-ken 270-11, Japan
ABSTRACT. Deep-sea hydrothermal systems are aimed as an on-site field analysis on the behavior and diffusion of CO2 in deep ocean. Through ocean ridge volcanism, a large amount of elements including carbon as a form of CO2 are supplied to deep ocean. Hydrothermal vent fluids at highly enriched in CO2 and show low pH (~ pH 3) relative to seawater. Total carbonate, total CO2 in seawater, and pH were determined in samples at hydrothermal active area in S-EPR. The concentration of total carbonate and pH in the hydrothermal fluid samples ranged from 16 to 5mM and from 3.1 to 7.6, respectively. The hydrothermal fluids discharged from the vents were rapidly diluted with ambient seawater, therefore total carbonate concentration and pH value in the plume waters become close to that of ambient seawater near the vents. The positive anomaly of total carbonate and negative anomaly of pH associated with hydrothermal plumes were observed on the seafloor along S-EPR axis. The diffusion of total carbonate plumes both westward and eastward in the bottom water along 15°S across the S-EPR were also detected, but pH anomalies were not obtained in the plume. These suggest the possibility of discharging of CO2 through hydrothermal systems to the ocean. Recent estimation of CO2 fluxes to the ocean through MOR was calculated at 0.7,-q5x 1012 mol C year -I. These values are 3-4 orders of magnitude smaller than the annual CO2 fluxes through terrestrial and marine respiration, therefore the importance of CO2 input from MOR on oceanic carbon cycle is thus minimal on shorter-term time scale. However, the CO2 input from MOR is significant at 106,'-'107 years scales, and CO2 concentration in hydrothermal fluids at hotspot and back-arc basin is 10,,d00 times higher than that of MOR. The flux of CO2 from deep-sea hydrothermal systems to the ocean may be significant. © 1998 Elsevier Science Ltd. All rights reserved
Through deep-sea hydrothermal activities, a large amount of elements including carbon as a form of CO2 are discharge to deep ocean. Deep-sea hydrothermal activities occur at the active lithosphere area, such as spreading ridges, back-arc basin and undersea volcano, and the global extent of the discovered deep-sea hydrothermal activity areas is presented in Fig. 1. Seawater penetrates downward toward the m a g m a chamber from the crack of oceanic crust, the rapid heating forces the buoyant hot water, hydrothermal fluid, to the surface as a hydrothermal vent. Fig. 2 represents the schematic major processes during the seawater circulation through the oceanic crust at spreading center, called hydrothermal circulation. The typical hydrothermal vent, called black smoker, located near 18°S at the southern East Pacific Rise (S-EPR) is shown in Fig. 3. The vents of the natural CO2-rich fluid and hydrate formation were found in 1989 at the hydrothermal active area in the mid-Okinawa Trough, back-arc basin. ~ This fact suggest, that CO2 is discharged to deep ocean by hydrothermal activities at the seafloor where hydrothermal vents exist. Hydrothermal fluids are highly
INTRODUCTION The study on the fate of carbon dioxide (CO2) in the ocean is important in coping with the global climate issues, because the future CO2 concentration in atmosphere can not be precisely predicted without full knowledge on the inventory of CO2 in the ocean. Furthermore, the direct ocean disposal of CO2 recovered from fossil fuel usage is now examined as one of possible options to mitigate global warming. For considering over the possibility of direct ocean disposal of CO2, it is necessary to know that how the direct ocean disposal affect to the ocean environment. Therefore the behavior, diffusion process and environmental impacts of disposed CO2 to the ocean should be defined.
Acknowledgements--I thank Drs T. Urabe and K. Fujioka as chief scientists of the R/V Melville and D S R V S H I N K A I 6500 cruises, respectively. T h a n k s are also extended to the captains, crew, S H I N K A I team and scientists aboard the cruises for their kind help in the sampling and onboard analysis.
385
386
K. SHITASHIMA
Map No. Site I
2 3 4 5 6 7 g 9 10 11 12 13 14 15 16 17 Ig 19 20 21 22 23 24
Lucky Sttil~ (37" 17'N) Broken Spur (29"N) TAG (26°N) Snakepit/MARK(23°N) Explorer Ridge (49"45'N) Middle Valley Endeavour (48*) Axial Volcano (46"N) Cleft~North (45*) Clefl~South (44"40'N) Eseanaba Trough Guaymas Basin (270N) EPR (21 ON) EPR (13"N) EPR (I ION) EPR (10*N) Calapagos (86*W) EPR (17"-19"S) Okinawa Trough (27"30'N) Madana Trough Manus Basin Woodlark Basin N. Fiji Basin (17"S) Lau/Valu Fa (22*S)
30*
30*
120"
150"
180" 150" 120" 90°
60*
30*
30*
0.
60*
90*
FIGURE 1. Location map of discovered deep-sea hydrothermal systems at the world ocean floor (from Baker et alJ).
qD Oxylmlona (HPO4S'pHVO4Z'pCr041-~HAn04I") , IqJll~Tlrllce Molal| /¢
f
f
jr
:/..;-.i:.!.:.:..'
_~¢"
t7
::
•
•
•
•
•
_
....
.....
i4
....... •
._f_
"
/
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,~ Predpkation
~./JlkChimney
,~ '\\'~,~
Sub"S"t~°°"SuJpd"<~i A
-"3
Fe-MnCrus~
CO=,I~,[¢I'I~],'He
/
H E A T S oU R C E
I
F I G U R E 2. Schematic r e p r e s e n t a t i o n o f a m a j o r processes d u r i n g the h y d r o t h e r m a l circulation o f s e a w a t e r t h r o u g h the oceanic crust at s p r e a d i n g center (from M a s s o t h et al.2).
enriched in C O 2 and show low pH (about pH 3) relative to seawater. The C O 2 in deep-sea hydrothermal vent fluids is mainly of magmatic origin. This CO2 is taken up from the basalt by the fluid during high temperature seawater-magma interaction. Furthermore at the back-arc basin, CO2 is extracted from organic matter and calcium carbonate in the sediment over the subduction zone is added to the vent fluids. Therefore, hydrothermal vent fluids are highly enriched in CO2 relative to seawater. The large excess of CO2 in the hydrothermal fluid and plume produces a pH decrease. The CO2 and pH data of hydrothermal fluid end-member, original
fluid, from various hydrothermal active sites were compiled in Table 1. Deep-sea hydrothermal systems are aimed as an on-site field analysis on the fate of CO2 in deep ocean. The study of deep-sea hydrothermal systems was, therefore, focused on the observation of the behavior and diffusion of COz in the deep ocean. In studying on the hydrothermal CO2, we can collect the data utilizable in the verification of the models developed for the behavior prediction of the injected CO2 in the ocean and imagine the direct effect of the injected CO2 on the intermediate and deep oceanic environment. This paper will report the observation
CO2 SUPPLY FROM DEEP-SEA HYDROTHERMAL SYSTEMS
387
FIGURE 3. A photograph of typical hydrothermal vent located near 18°S at the S-EPR. TABLE 1 The CO2 and pH Data of Hydrothermai Fluid End-member from Various Hydrothermal Active Sites (from Gamo4). End-member Values are those Obtained by Extrapolation to an Assumed of Zero Mg Name of site
Okinawa Izu-Bonin Mid-Mariana Trough Seamount Trough
Type
Back-arc basin 710 1390 4.7 5.3 64 209
Depth (m) pH (25°C) CO2 (mM)
Arc 1380 3.7 34~42
Back-arc basin 149(~3600 3.94.4 43.4
Manus Basin Back-arc basin ~2500 5 21
North-Fiji Lau Basin Loihi Juan de Fuca Basin Seamount Ridge
Guaymas Basin
Back-arc basin 2000 4.7 l 1.1-14.4
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results of the behavior of hydrothermal CO2 in deepsea at the southern East Pacific Rise (S-EPR) and consider that the influence of the input of hydrothermal CO2 to carbon cycle in deep ocean. FIELD OBSERVATION OF CO2 DISCHARGE FROM DEEP-SEA HYDROTHERMAL
SYSTEMS Total carbonate (the sum of carbonic acid, bicarbonate ion and carbonate ion dissolved in seawater) and pH were determined in samples of the nonbuoyant plume, the buoyant plume and the hydrothermal fluid collected at S-EPR by the R/V Melville 14 (November~December 1993) and the DSRV SHINKAI 6500 (September~November 1994) under the Ridge Flux project (funded by Science & Technology Agency, Japan). The concentration of total carbonate and pH in the hydrotherrnal fluid samples from black smoker chimney ranged from 5 to 16 mM (2 to 8 times greater than ambient seawater) and from 3.1 to 4.8, respectively. Total carbonate concentration of the SEPR vent fluids were relatively high in comparison to concentration reported for other EPR hydrothermal
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vent fluids and 10,-~102 times lower than those of the vent fluid collected from hotspot volcano (Loihi Seamount) 2 and back-arc basin (e.g. Okinawa Trough). 3 The hydrothermal end-member fluids (original fluid) discharged from the vents are rapidly diluted between hydrothermal end-member fluid and ambient seawater by factors of 104-105.4 Figure 4 indicates the distribution of total carbonate and pH in the buoyant plume as a function of distance from hydrothermal active vents in the S-EPR. The buoyant plume samples were collected along with the trail of the buoyant plume by means of keeping the submersible buoyancy at natural and drifting with water current. Total carbonate concentration and pH value in the buoyant plume become close to those of ambient seawater within dozens meters from the vents due to drastic mixing and dilution of vent fluid with ambient seawater. What happens to the hydrothermal CO2 discharged by deep-sea hydrothermal systems in deep ocean? Along-axis transact of total carbonate anomaly in hydrothermal plumes over S-EPR axis is shown in Fig. 5. The positive anomaly of total carbonate and negative anomaly of pH associated with
388
K. SHITASHIMA
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Distance from hydrothermal vents (m) F I G U R E 4. Distribution of total carbonate (upper) and p H (lower) as a function of distance from hydrothermal active vents in the S-EPR.I 5
hydrothermal plumes were observed on the seafloor along S-EPR axis. The highest concentratin of total carbonate anomaly detected near 16°S was 2.48 mM, but total carbonate concentration of ambient seawater in water column around S-EPR is about 2.36mM. Furthermore, the highest decrease of pH anomaly showed 0.02 pH unit. Figure 6 represents the vertical profiles of total carbonate for a short east-west section along 15°S across the S-EPR axis. The total carbonate anomalies were observed in the bottom water over the slope of S-EPR axis, indicating the dispersion of a hydrothermal plumes at this depth. The total carbonate plumes spreaded out above the seafloor both westward and eastward along 15°S across the S-EPR. At the western stations, the concentrations of total carbonate were decreased due to mixing with the Pacific water with low total carbonate concentration. However, the pH anomalies were not obtained in the same depth where the total carbonate anomalies were observed. Owing to CO2 dissociation to bicarbonate ion when CO2 dissolves in seawater, the low pH in seawater by the input of hydrothermal CO2 was recovered. According to recent research for hydrothermal plumes, the carbon dioxide anomalies associated with hydrothermal plume have not been observed various hydrothermal area. These results
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CO2SUPPLY FROM DEEP-SEA HYDROTHERMAL SYSTEMS 2.35 0
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389
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demonstrate that carbon dioxide is discharged through hydrothermal systems to the ocean in the same manner as 3He, CH4 and Mn. Schematic diagram of total carbonate plume for a short east-west section along 15°S across the S-EPR axis is presented in Fig. 7. The distribution pattern of the total carbonate plumes were different from that of the 3He plume reported by Lupton and Craig. 5 Low temperature diffuse discharge typically occurs in areas surrounding black smoker chimneys. This diffuse flow transports an order of magnitude more heat than the hydrothermal vent fluid from black smoker
chimney.6,7 It is implied that the low temperature diffuse flow is significant source of hear and chemical, such as CO2, fluxes in deep-sea hydrothermal systems. These observation results suggest the possibility of discharging of CO2 through deep-sea hydrotherrnal systems to the ocean. On the other hand, the other inputs of CO2 into the deep-sea are the bacterial activity around hydrothermal vents, the oxidation of organic matter generated by hydrothermal bacteria, and the formation of calcium carbonate and CO2 from calcium bicarbonate solution supplied by hydrothermal activity.
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[Oceanfloorspreading( FIGURE 7. Schematic diagram of total carbonate plume for a short east west section along 15°S across the S-EPR axis? 5
390 However, these inputs in deep-sea are indirectly concerned with h y d r o t h e r m a l activity. In other words, CO2 is supplied directly and indirectly from hydrothermal systems to deep ocean. Determination o f c a r b o n isotope ratio will be effective in order to distinguish the source o f each c a r b o n dioxide.
K. SHITASHIMA geological inference. In: Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions, Humphris, S. E., Zierenberg, R. A., Mullineaux, L. S. and Thomson, R. E. (eds). Geophysical Monograph 91, American Geophysical Union, Washington, DC, pp. 47-71. 2. Massoth, G. J., Milburn, H. B., Hammond, S. R., Butterfield, D. A., McDuff, R. E. and Lupton, J. E. The geochemistry of submarine venting fluid at Axial Volcano, Juan de Fuca Ridge: new sampling methods and a VENTS Program rationale. In: Global venting, midwater, and benthic ecological processes,
F L U X O F CO2 T O T H E O C E A N T H R O U G H M I D O C E A N RIDGE Recent estimates o f CO2 fluxes to the ocean t h r o u g h mid ocean ridge ( M O R ) f r o m the CO2/3He ratio o f vent fluids and 3He budget for the ocean were calculated at 0 . 7 + 0 . 2 5 x l 0 1 2 m o l C y e a r -1 by Gerlach 8 and 1 5 x 1 0 1 2 m o l C y e a r -1 by J a v o y and Pineau. 9 These results suggest that the input o f CO2 to the ocean f r o m M O R is 3-4 orders o f magnitude smaller than the annual CO2 fluxes t h r o u g h terrestrial and marine respiration, and an order o f magnitude smaller than the flux o f c a r b o n to shelf and deep-sea sediments. The importance o f COz input from M O R on oceanic c a r b o n cycle is thus minimal on shorterterm time scales (in several years order). However, the CO2 input f r o m M O R m a y be significant at longer time scales (106,~107 years), because ridge generation that cause the h y d r o t h e r m a l activity has been continuing from 100 million years ago, m o r e active ridge generation might occur in the past or ridge generation rates might undergo rapid change. On a time scale o f years, furthermore, the concentration o f CO2 in h y d r o t h e r m a l vent fluid at hotspot submarine volcano (Loihi Seamount) 2 and back-arc basin (e.g. O k i n a w a T r o u g h ) 3 is 10-,~102 times higher than that o f M O R (Table 1), but the hydrothermal activities at back-arc basin and h o t s p o t exist locally relative to M O R . The input o f CO2 from these h y d r o t h e r m a l systems to the ocean m a y be significant. The heat flux o f the Megaplume, even plume o f the episodic large-scale h y d r o t h e r m a l activity, is 1,-~3 orders larger than that o f a n o r m a l plume, 1° but the h y d r o t h e r m a l CO2 flux f r o m M e g a p l u m e is not m a d e clear yet at present time. F r o m the facts presented above, it can be imagined that the flux o f CO2 from deep-sea hydrothermal systems to the deep ocean m a y bge significant. It is necessary for the observation o f CO2 flux f r o m the individual h y d r o t h e r m a l activity and the understanding o f contribution or effect o f the h y d r o t h e r m a l CO2 flux to oceanic c a r b o n cycle. These will shed light on the environment impact o f CO2 discharge to deep-sea.
3.
4.
5.
6.
7. 8. 9.
10.
11.
12.
13. 14.
15. REFERENCES 1. Baker, E. T., German, C. R. and Elderfild, H. Hydrothermal plumes over spreading-center axes: Global distributions and
Deluca, M. P. and Babb, I. (eds). National Undersea Research Program Report 88-4, NOAA, Rockville, MD, pp. 29-59 (1988). Sakai, H., Gamo, T., Kim, E.-S., Yanagisawa, F., Tsutumi, M., Tanaka, T., Ishibashi, J., Wakita, H., Yamano, M. and Oomori, T. Venting of carbon dioxide-rich fluid and hydrate formation in mid-Okinawa Trough backarc basin. Science 248: 1093-1096 (1990). Gamo, T. Wide variation of chemical characteristics of submarine hydrothermal fluids to secondary modification processes after high temperature water-rock interaction: a review. In: Biogeochemical Processes and Ocean Flux in the Western Pacific, Sakai, H. and Nozaki, Y. (eds). Terra Scientific Publishing Company, Tokyo, pp. 425-451. Sedwick, P. N., McMurtry, G. M. and Macdougall, J. D. Chemistry of hydrothermal solution from Pele's Vents, Loihi Seamount, Hawaii. Geohimica Cosmochimica Acta 56: 36433667 (1992). Sakai, H., Gamo, T., Kim, E.-S., Shitashima, K., Yanagisawa, F., Tsutumi, M., Ishibashi, J., Sano, Y., Wakita, H., Tanaka, T., Matsumoto, T., Naganuma, T. and Mitsuzawa, K. Unique chemistry of the hydrothermal solution in the mid-Okinawa Trough backarc basin. Geophysical Research Letters 17: 21332136 (1990). Lupton, J. E., Delaney, J. R., Johnson, H. P. and Tivy, M. K. Entrainment and vertical transport of deep-ocean water by buoyant hydrothermal plumes. Nature 316:621-623 (1985). Lupton, J. E. and Graig, H. A major helium-3 source at 15°S on the East Pacific Rise. Science 214:13-18 (1981). Schultz, A., Delaney, J. R. and McDuff, R. E. On the partioning of heat flux between diffuse and point source seafloor venting. Journal of Geophysical Research 97:12229-12314 (1992). Ginster, U., Motti, M. J. and Von Herzen, R. P. Heat flux from black smokers on the Endeavor and Cleft segments, Juan de Fuca Ridge. Journal of Geophysical Research 99:4937-4950 (1994). Gerlach, T. M. Degassing of carbon dioxide from basaltic magma at spreading centers: II. Mid-oceanic ridge basalts. Journal of Volcanology and Geothermal Research 39:221-232 (1989). Javoy, M. and Pineau, F. The volatile record of a "popping" rock from the Mid-Atlantic Ridge at 14°N: chemical and isotopic composition of gas trapped in the vesicles. Earth and Planetary Science Letters 107:598-611 (1991). Baker, E. T., Massoth, G. J. and Feely, R. A. Cataclysmic hydrothermal venting on the Juan de Fuca Ridge. Nature 329: 149-151 (1987). Urabe, T., Baker, E. T., Ishibashi, J., Feely, R. A., Manumo, K., Massoth, G. J., Maruyama, A., Shitashima, K., Okamura, K., Lupton, J. E., Sonoda, A., Yamazaki, T., Aoki, M., Gendron, J., Green, R., Kaiho, Y., Kishimoto, K., Lebon, G., Matsumoto, T., Nakamura, K., Nishizawa, A., Okano, O., Paradis, G., Roe, K., Shibata, T., Tennant, D., Vance, T., Walker, S. L., Yabuki, T. and Ytow, N. The effect of recent magmatic activity on hydrothermal venting along the superfast-spreading East Pacific Rise. Science 269: 1092-1095 (1995). Shitashima, K., Sonoda, A., Ishibashi, J., Urabe, T., Fujioka, K., Baker, E. T., Feely, R. A. and Butterfield, D. A. The flux of total carbonate at the southern East Pacific Rise. JAMSTEC Journal of Deep Sea Research, in press (in Japanese with English abstract).