Submarine venting of liquid carbon dioxide on a Mariana Arc volcano

J. Lupton¹, D. Butterfield², M. Lilley³, L. Evans⁴, K. Nakamura⁵, W. Chadwick Jr.⁴, J. Resing², R. Embley¹, E. Olson³, G. Proskurowski^3,6, E. Baker⁷, C. de Ronde⁸, K. Roe³, R. Greene⁴, G. Lebon², and C. Young⁹

¹NOAA/Pacific Marine Environmental Laboratory, Newport, Oregon
²JISAO/University of Washington, Seattle, Washington
³School of Oceanography, University of Washington, Seattle, Washington
⁴CIMRS/Oregon State University, Newport, Oregon
⁵National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
⁶Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
⁷NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
⁸Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand
⁹6450 Eagles Crest Road, Ramona, California

Geochem. Geophys. Geosyst., 7, Q08007, doi: 10.1029/2005GC001152, 2006 .
Copyright ©2006 by the American Geophysical Union. Further electronic distribution is not allowed.

5. Discussion

In order to put the very high CO concentration of the Champagne vent fluids in perspective, Figure 9 compares the end-member CO concentrations at NW Eifuku with those at the Okinawa Trough and at various mid-ocean ridge hydrothermal sites. Although Sakai et al. [1990a] observed liquid CO venting at the JADE site in the Okinawa Trough, the 320°C vent fluid at the JADE site contained only 200 mmol/kg of CO. As shown in Figure 9, the end-member CO concentrations at the Champagne site are an order of magnitude higher than any values reported for other hydrothermal fluids, and 100 times higher than average values at MOR systems. The structure of the hydrothermal system at NW Eifuku is clearly different from that at the JADE site, where high temperature, zero-magnesium fluids are produced in a reaction zone with low water/rock ratio [Sakai et al., 1990a, 1990b].

The CO concentration at NW Eifuku is even more remarkable when it is compared against the CO solubility at these P, T conditions. The solubility of CO in seawater at 160 bars, 100°C is ~1.0 mole/kg [Wiebe and Gaddy, 1939; Takenouchi and Kennedy, 1964], much lower than the concentrations we measured in 2004. The most plausible explanation for the apparent supersaturation of CO is that the Champagne vent fluid is entraining small amounts of liquid CO and/or CO hydrate as the hot vent fluid penetrates the layers of CO liquid and hydrate that we propose exist beneath the seafloor (Figure 4). Incorporation of only 6% by volume of liquid CO into the vent fluid would increase the CO concentration from 1 to 2.7 moles/kg, and this liquid CO would likely not be visible as a separate phase in the vent fluid stream.

The 2005 vent fluid samples had lower CO concentrations and different relative proportions of dissolved gases compared to the earlier 2004 collections. In fact, none of the 2005 water samples had CO concentrations above the 1.0 mole/kg solubility of CO at the conditions at the Champagne site. Furthermore, as discussed above, the 2005 vent fluid samples had higher end-member HS/CO ratios, and higher C/He ratios compared to the 2004 samples (Table 3). For whatever reason, the liquid droplets have lower HS/CO, lower C/He, and are heavier in C compared to the vent fluids. This difference between the vent fluid compositions in 2005 versus 2004 may be due to temporal changes in the degree of entrainment of liquid CO and/or hydrate into the rising vent fluid. Our results indicate that all of the vent fluid samples are actually a mixture of the pure "subsurface" hydrothermal fluid combined with varying amounts of entrained liquid CO or CO hydrate. The end-member compositions estimated for the 2005 samples (see Table 3) may thus represent a form of the Champagne vent fluid relatively un-contaminated with entrained liquid CO, while the gas-rich 2004 vent fluids had more entrained liquid CO. For the 2004 vent fluid samples with ~2.7 moles/kg CO, most of the CO came from the entrained liquid droplets and/ or hydrate, while most of the HS and He was already dissolved in the hot fluid before it reached the near surface liquid CO layer.

In many cases, diffuse hydrothermal fluids are located near high-temperature fluids or their chemistry indicates that they are dilutions of high-temperature fluids, with overprinting lowtemperature reactions [Edmond et al., 1979; Butterfield and Massoth, 1994; Butterfield et al., 1997, 2004; Sedwick et al., 1992]. That is not the case at NW Eifuku. If we were to extrapolate the temperature and fluid composition to a zeromagnesium value, the results would be nothing like a fluid produced in a high-temperature water/rock reaction zone. For example, a zero-Mg extrapolation at NW Eifuku would yield temperatures of 500-600°C, CO concentrations of 10-20 mol/kg, and zero Li concentration. The implication of this is that we are not dealing with a high-temperature aqueous system, but with a high-temperature CO system, entraining some water that undergoes incomplete reaction to remove some seawater magnesium and extract some elements from the rock. CO migrating upward from a gas pocket in a magma chamber must cool as it ascends through volcanic rock and may entrain small amounts of seawater (Figure 11). Water and CO are immiscible at 500 bars at temperatures below 310°C [Takenouchi and Kennedy, 1964; Bowers, 1991] and separate into a CO-rich vapor and a water-rich liquid. As pressure increases, immiscibility of HO-CO occurs at lower temperatures [Bowers, 1991]. In a system dominated by the flux of hot CO from a magma chamber, the penetration of water into the core of the system will be limited at typical hydrothermal temperatures (up to ~350°C) due to the immiscibility. If seawater is not first heated by hot rock (and the water chemistry at the Champagne site indicates minimal high-T water/rock reaction), then the outer portions of the CO column will be in the P-T region of HO-CO immiscibility. As long as the flux of hot CO from the magma chamber and cooling in the pathway to the seafloor is maintained, the penetration of water into the CO-rich zone is inhibited. The presence of a gas hydrate phase at temperatures below 10°C may also inhibit penetration of water into the sub-seafloor CO-rich zone.

Figure 11. Diagram showing a model proposed for the gas flux from NW Eifuku, in which a CO-rich gas is directly degassing from the magma chamber. As this hot gas rises through the system, it cools, and CO condenses as a separate liquid phase on the periphery of the main conduit. Seawater circulates through the system, but the penetration of water into the core of the system is limited at temperatures below 250°C due to CO-HO immiscibility. At the volcano summit the liquid CO collects beneath a hydrate "cap" layer that forms where the liquid CO comes in contact with seawater. Because the penetration of water is limited and the enthalpy is carried by the CO gas, there is little high-temperature water-rock interaction.

Some insight into the origin of the high CO concentrations at NW Eifuku can be gained from the isotopic composition of the CO and the relation of CO to He. The C of the Champagne vent fluids (-1.75‰) is much heavier than the C = -13 to -4‰ typical for carbon in MOR vent fluids [Kelley et al., 2004]. The NW Eifuku CO is also heavier than that reported for the Mid-Okinawa Trough (-5.0 to -3.7‰) [Sakai et al., 1990a, 1990b], falls at the heavy end of the range reported for arc volcanoes in general (-7 to -2‰) [Sano and Williams, 1996; van Soest et al., 1998], and at the lighter end of the range for marine carbonates (-2 to +1‰) (Figure 12) [Hoefs, 1980]. The C/He ratio for the Champagne vent fluids and liquid droplets (1.3 to 9.4 × 10) is similar to that reported for the Mid-Okinawa Trough [Sakai et al., 1990a, 1990b], but an order of magnitude higher than the average value of 2 × 10 found in MOR vent fluids [Resing et al., 2004]. These C and C/He values indicate that the majority of the carbon flux originated from marine carbonates incorporated into the melt as part of the subduction zone melting process rather than from mantle carbon. Using the method outlined by Sano and Marty [1995] based on C and C/He values, and taking sedimentary organic matter to have a C value of -30‰ as did Sano and Marty [1995], we estimate that the NW Eifuku CO was derived 88% from marine carbonates, 9% from mantle carbon, and 3% from sedimentary organic matter. If instead we assume that the C of sedimentary organic matter is -20‰, then we calculate 87% from marine carbonates, 9% mantle, and 4% from sedimentary organics. These fractions are similar to those observed at subaerial arc volcanoes [Hilton et al., 2002]. The fact that the radiocarbon is "dead" suggests that the CO flux is mainly derived from subducted carbonates incorporated into the melt at depth and that local sediments are not responsible.

Figure 12. (a) Chart comparing C (‰) for CO from various MOR sites [Kelley et al., 2004], the Okinawa Trough [Sakai et al., 1990a, 1990b], NW Eifuku (this work), marine carbonates [Hoefs, 1980], and typical volcanic arcs [Sano and Williams, 1996; van Soest et al., 1998]. (b) Similar chart comparing CO/He ratios for MOR vents [Kelley et al., 2004], the Okinawa Trough [Sakai et al., 1990a, 1990b], and typical volcanic arcs [Sano and Williams, 1996; van Soest et al., 1998].

It is a simple matter to show that the extremely high concentrations of CO at NW Eifuku cannot be easily derived from either water/rock reaction or from dissolution of putative carbonates within the volcanic edifice. On the first count, it has already been shown [Butterfield et al., 1990; Sedwick et al., 1994] that by extracting all of the CO from 1 kg of MORB, assuming the maximum reported level of 8 mmol CO/kg rock [Dixon et al., 1988], into 1 kg of water (a typical water/rock ratio), the total CO concentration in the aqueous phase would not exceed ~8 mmol/kg. However, according to Wallace [2005], undegassed arc magmas contain ~3000 ppm of CO. Using the same water/rock ratio of 1, this would produce only 68 mmol/kg of CO in the hydrothermal fluid, still far below the 900-2700 mmol/kg we observe. Furthermore, the Champagne vent fluids have lithium concentrations lower than the ambient seawater concentration, indicative of minimal high-temperature water-rock reaction (a high water/rock ratio). Thus it appears to be impossible to extract enough CO from basaltic or andesitic lava by water/rock interaction to reach the levels of CO found in the Champagne vent fluids, or even in some MOR vents (e.g., Axial Volcano on the Juan de Fuca Ridge or 9°N East Pacific Rise). On the second count, the low pH of Champagne vent fluids is inconsistent with calcium carbonate dissolution. In addition, dissolution of carbonates is self-limiting unless there is an additional source of acid to drive more dissolution. In that case, the calcium concentration would also be very high, but it is lower than seawater concentration in the Champagne vent fluids. We conclude that the CO at this site must be derived from magma degassing, as there is no other plausible source.

Although it is clear that the venting at the Okinawa Trough and NW Eifuku Champagne sites locally produces very high CO concentrations, the question remains as to how significant the overall carbon flux is on a global scale. We estimated the volume flux of liquid CO droplets at the Champagne site by examining video collected during the 2004 ROV dives. We first estimated that there were about 300 droplet streams rising from the 10 m area of the Champagne site, and that each stream contained 2 droplets/s, each with an average diameter of ~1.5 cm. This gives a total liquid CO flux of ~1 liter/s or 1 kg/s (assuming a density of 1g/cm3), equivalent to ~23 moles CO/s. Using a similar method, we estimate the CO flux from the Champagne hot vents to be ~ 0.5 mole/s or only about 2% of the liquid CO flux. The CO flux from the liquid droplets at the Champagne site (8 × 10 moles/yr) approximately equals the combined carbon flux from all of the Endeavour Ridge vent fields on the Juan de Fuca Ridge [Rosenberg et al., 1988; Lilley et al., 1993], or about 0.1% of the global MOR carbon flux which is estimated at 0.5-2.0 × 10 moles/yr [Resing et al., 2004]. The carbon flux from the Champagne site is also about 0.1% of the global CO flux from subaerial arc volcanoes, estimated at ~1.6 × 10 moles/y [Hilton et al., 2002]. Although these flux estimates for NW Eifuku are admittedly only accurate to a factor of 2 or so, this carbon flux is surprising, since NW Eifuku is a small, young arc volcano and not a major volcanic edifice. Furthermore, the fact that NW Eifuku is a submarine volcano suggests that carbon fluxes based on observations of subaerial volcanoes may have underestimated the global fluxes from arcs. If there are many such submarine sites active along volcanic arcs and back arcs, then there is potential for a significant impact on oceanic carbon cycling. For completeness it should be noted that Hilton et al. [2002] estimated the carbon flux from an average subaerial arc volcano at 2 × 10 moles/y, about 25 times higher than the carbon flux at NW Eifuku. They arrived at this estimate by normalizing the CO flux to the measured SO flux at various subaerial arc volcanoes, rather than by direct measurements of the CO outgassing rate. Thus the carbon flux at NW Eifuku may be significant for submarine carbon cycling, but not necessarily for the global subaerial carbon flux.

Additional visits and possibly long-term monitoring of the site are required to determine if this high CO flux is time dependent. For example, Miyakejima volcano (Japan) underwent a months-long extremely high-volume magmatic degassing event following a caldera collapse in 2000 [Kazahaya et al., 2004]. Recent mass-wasting at the summit of NW Eifuku could have been triggered by movements along what appears to be a NW-SE fracture underlying the volcano (Figure 2c). Deep conduits could be enlarged and new ones opened during this process. Alternatively, long-term degassing during non-eruptive periods on some volcanoes has been tied to endogenous growth by magmatic intrusion [Allard, 1997].

Recently there has been considerable interest in the possible oceanic disposal of fossil fuel CO as a means to alleviate the increase of atmospheric CO [Brewer, 2000]. One important question concerns the fate of the CO after it is introduced into the ocean. Brewer et al. [2002] measured the rate of dissolution of liquid CO injected into the ocean at a depth of ~800 m, and found that 90% of the buoyant CO droplets dissolved within 200 m above the injection point. As shown in Figure 10, our water column measurements in the vicinity of NW Eifuku are in basic agreement with the Brewer et al. [2002] results. On several hydrocasts collected over the volcano in 2003 and 2004 we found large excesses in 3He and CO that co-varied almost perfectly [Lupton et al., 2003; Resing et al., 2003]. In every case the excess 3He and CO was confined to the depth range of 1490 to 1620 m and returned to background values about 150 m above the depth of the NW Eifuku vent fields (Figure 10).

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