Lakes of liquid CO 2 in the deep sea

The 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 low-density 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 …