Hellisheidi, Iceland: Sometime next month, on the steaming fringes of an Icelandic volcano, an international team of scientists will begin pumping seltzer water into a deep hole, producing a brew that will lock away carbon dioxide forever. Chemically disposing of CO2, the chief greenhouse gas blamed for global 7warming is a kind of 21st-century alchemy that researchers and governments have hoped for to slow or halt climate change. The American and Icelandic designers of the CarbFix experiment will be capitalizing on a feature of the basalt rock underpinning 90 percent of Iceland: It is a highly reactive material that will combine its calcium with a carbon dioxide solution to form limestone, permanent, harmless limestone. The experimental transformation will take place below the dramatic landscape of this place 29 kilometers (18 miles southeast of Reykjavik, Icelands capital. On an undulating, mossy moor and surrounding volcanic hills, where the last eruption occurred 2,000 years ago, Reykjavik Energy operates a huge, 5-year-old geothermal power plant, drawing on 30 wells tapping into the superheated steam below, steam laden with carbon dioxide and hydrogen sulfide. CarbFix will first separate out those two gases, and the CO2 will be piped 3 kilometers (2 miles to the injection well, to combine with water pumped from elsewhere. That carbonated water, seltzer, will be injected down the well, where the pressure of the pumped water, by a depth of 500 meters (1,600 feet, will completely dissolve the CO2 bubbles, forming carbonic acid. The acids very corrosive, so it starts to attack the rocks, explained University of Iceland geologist Sigurdur Reynir Gislason, CarbFixs chief scientist. The basalt rock, ancient lava flows, is porous, up to 30 percent open space filled with water. The carbonic acid will be pushed out into those pores, and over time will react with the basalts calcium to form calcium carbonate, or limestone.