In summary, we have discovered a site at ~1600 m depth on NW Eifuku, a submarine volcano on the northern Mariana Arc, which is venting droplets of liquid CO at an estimated rate of 8 × 10 moles/yr. This is only the second locality where submarine venting of liquid CO has been observed, the other being the mid-Okinawa Trough [Sakai et al., 1990a, 1990b]. The Champagne site on NW Eifuku is also venting hot (~100°C) vent fluid with CO contents up to 2.7 moles/kg, far above the solubility (~1.0 mole/kg) at these P, T conditions. Observations at the site indicate the presence of a subsurface liquid CO layer under a capping layer of CO hydrate. We attribute the apparent CO super-saturation in the vent fluid to entrainment of small amounts of liquid CO and/or CO hydrate into the ascending vent fluid stream. The liquid droplets are composed of >98% CO, ~1% HS, with only trace amounts of H and CH. The dissolved gases in the vent fluid have a similar composition, with a slightly greater concentration of HS (~3%). The C (CO) and CO/He ratios fall in the range typical for volcanic arcs, and indicate that the carbon is derived ~90% from marine carbonates, the remainder being mantle carbon and sedimentary organic matter. The fact that the radiocarbon is dead (age ≥ 50,000 years) suggests that the source is subducted carbonates incorporated into the melt at depth in the subduction zone and not local carbonates on the volcano edifice.
Sakai et al. [1990a, 1990b] explained their observations in the mid-Okinawa Trough in terms of separate CO-rich and HO-rich fluids that formed as the result of magma chamber degassing. They also discussed subsurface boiling as a possible mechanism for generating these two phases. It is clear that separate cold CO-rich and hot HO-rich fluids exist at the same site in close proximity at NW Eifuku. At the vent site, we envision a mechanism in which hot water is venting through an area of liquid CO and CO hydrate and entrains these, generating hot fluids with CO contents higher than predicted by the limits of CO solubility. In contrast to the 320°C fluids found in the Okinawa Trough, the 103°C fluid temperatures at NW Eifuku are ~250°C below the boiling point at 1600-m depth, and thus shallow subsurface boiling is unlikely. The 103°C fluids do not show signs of intense water/rock interaction, and their low alkali metal content is indicative of a high water/rock ratio. Given that we do not find a zero-magnesium, high-temperature fluid at NW Eifuku, it is impossible to extract enough CO from the rock into circulating seawater to form an aqueous fluid saturated with CO. Instead, the extreme CO concentrations at NW Eifuku require direct degassing of CO from a magma chamber, cooling and migration to the seafloor, resulting in the generation of the CO-rich and HO-rich fluids that we observed. The physical/chemical model we have proposed (Figure 11) differs substantially from the mid-ocean ridge model of extraction of gases from rock by circulating hot water. If our model is correct, then elemental and isotopic fractionations that occur as a result of magma degassing, CO condensation, hydrate formation, HO-CO mixing, and phase separation add considerable complexity to the interpretation of gas ratios and isotopic ratios.
The Champagne vent field and the other sites of hydrothermal activity on NW Eifuku clearly merit further study. As mentioned above, NW Eifuku is only the second locality where natural venting of liquid CO has been reported, the other being the Okinawa Trough, a back-arc basin environment. At the time of its discovery, the Champagne site was the only arc volcano where the phenomenon of liquid CO venting had been found. However, venting of a separate CO gaseous phase was recently observed at 3 other submarine arc volcanoes: Nikko volcano in the Mariana Arc [Lupton et al., 2005], and Giggenbach volcano and Volcano 1, both in the Kermadec Arc [Lupton et al., 2005; Stoffers et al., 2006]. Furthermore, to our knowledge liquid CO venting has never been found on mid-ocean ridges, suggesting that this type of activity is more prevalent on volcanic arcs and the associated back-arc basins. Experiments are being designed to accurately measure the flux and oceanic dispersal of CO at NW Eifuku. In addition to physical and chemical measurements, the hydrothermal sites on NW Eifuku are a valuable natural laboratory for studying the effects of high CO concentrations on marine ecosystems.
We thank K. Shepard, K. Tamburri, and the other members of the Canadian ROPOS team, and the captain and crew of the R/V Thompson for their excellent support during the 2004 SROF expedition, and K. Chiba and the other members of the Hyper-Dolphin team and the Captain and crew of the R/V Natsushima for their support during the NT05-18 expedition. We thank S. Merle for help with the figures and Tom Brown and the Natural C Group at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. V. Salters, D. Hilton, C. German, and T. Fischer provided constructive reviews of the manuscript. Radiocarbon measurements were supported in part by funding from CAMS through the University Collaborative Research Program. This publication was partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA cooperative agreement NA17RJ1232, contribution 1154. This work was supported by the NOAA Ocean Exploration Program and by the NOAA VENTS Program. This is PMEL contribution 2843.
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