Lakes of liquid CO2 in the deep sea.

T he thought of liquid CO2 conjures up different things to different folks: perhaps the decaffeination of coffee beans, perhaps the recently popularized ‘‘green’’ method for dry cleaning, or even phase diagrams that occupied a part of one’s life in past chemistry classes. What it does not conjure up is a subsurface lake at the bottom of the ocean, a lake with abundant living microbes, as reported in this issue of PNAS by Inagaki et al. (1). These authors discovered such a place near the Yonaguni Knoll in the Okinawa Trough at a depth of 1,400 m. The description in both words and video (see supporting movie 1 in ref. 1) is quite striking. First, because liquid CO2 at this depth is less dense than water (2, 3), so that such a lake should not be present. Second, because this is a phenomenon that few of us have ever seen, movie 1 in ref. 1 reveals a flowing stream of liquid CO2 that seems almost surreal. The answer to the apparent conundrum surrounding the very existence of this phenomenon is that the lake is maintained in place by a surface pavement and a subpavement cap of CO2 hydrate (CO2 6H2O) that traps the lowdensity liquid CO2 in place. At the temperature of the seafloor at this depth, such a CO2 hydrate should be stable (4), leading to a structure similar to that shown in figure 1 of the Inagaki et al. article (1), in which a surface pavement overlies a layer of CO2 hydrate that serves as a cap for the subsurface lake. The surface pavement is quite remarkable, having a very unusual elemental sulfur content of 50%. It may well be that there are clues to the origin of the sulfur (and the role of sulfur metabolism in this system) in both the isotopic composition of the sulfur and the chemical and biological nature of the ‘‘sulfurhydrate complex,’’ things that should be resolved in future studies. As discussed in a another recent article in PNAS (3), the density of liquid CO2 increases with depth, so that at depths of 3,000–3,800 m (density reaches a maximum at 3,500 m and decreases at greater depth), it is more dense than seawater [see figure 2 in House et al. (3)], forming a natural negative buoyancy zone, where one could rightfully expect to see lakes of liquid CO2. CO2 hydrates also form at these depths, suggesting that large subsurface lakes of liquid CO2 capped by hydrates could be excellent locations for the large-scale injection and disposal of CO2 (3). The notion that similar sites might exist as natural systems was not entertained in the House et al. article, but if they do, and are stable in the long term, then the notion that communities of microbes might be capable of adapting to such an environment becomes of great interest. Such knowledge also becomes of importance with regard to the establishment of such reservoirs in the deep sea. Liquid CO2 in the deep ocean is not an unprecedented finding. In 1990, Sakai et al. (4) noted the release of CO2 droplets at a depth of 1,400 m and a temperature of 3.8°C in a region near the mid-Okinawa Trough, and more recently, similar observations were made in the northern Mariana Arc (5). What is new is the concept that large bodies of liquid CO2 may exist as subsurface lakes in such zones. For example, the northern Mariana is a volcanic arc with little or no sediment deposition. Thus, one does not expect to find sedimenthosted lakes such as are reported by Inagaki et al. (1). How many such ‘‘lakes’’ are there? How stable are they, and are they potential players in the global carbon cycle? Given our paucity of knowledge about such systems, it is fair to say that these questions remain unanswered. It may be of great interest to answer such questions for a variety of different reasons, as outlined below. First, one of the proposed methods for disposal of CO2 (and amelioration of the associated effects on global climate) is the direct injection of CO2 into the deep sea (6–8). The expense of moving large amounts of CO2 to 3,000 m and deeper and the problems with rapidly injecting it at these depths could be substantial. Neither the biological (toxicity) nor the physical (effect on porewaters from injection of massive amounts of liquid CO2) impacts are known (3). Do the findings of Inagaki et al. (1) offer another potential avenue for CO2 storage? Probably not. The robustness of these systems clearly depends on the formation and long-term stability of the CO2 hydrate cap, something that may not be routine to achieve. Whereas in the deeper ocean, the hydrate cap should be stable and the underlying liquid CO2 can migrate downward until it becomes neutrally buoyant and will then move only by diffusion (3), burial in zones where liquid CO2 is less buoyant than water would almost certainly have to be deeper into the sediments themselves, thus becoming subject to temperature changes caused by the geothermal gradient. Thus, the CO2 lakes reported (1) probably will not lead the way to a new avenue for shallow-water CO2 disposal. Such environments do, however, offer accessible sites where some of the other impacts of the liquid CO2 seawater interface on the environment (including