Of all the ‘geoengineering’ approaches that may offer some relief from global warming pumping CO₂ into deep sedimentary rocks, through carbon capture and storage (CCS) is one that most directly intervenes in the natural carbon cycle. In fact it adds an almost wholly  anthropogenic route to the movement of carbon. It is difficult if not impossible for natural processes to ‘pump’ gases downwards except when they are dissolved in water and most often through the conversion of CO₂ to solid carbonates or carbohydrates that are simply buried on the ocean floor. Artificially producing carbonate or organic matter on a sufficient scale to send meaningful amounts of anthropogenic carbon dioxide to long-term rock storage is pretty much beyond current technology, but gas sequestration seems feasible, if costly. The main issues concern making sure geological traps are ‘tight’ enough to prevent sufficient leakage to render the exercise of little use and to understand the geochemical effects of large amounts of buried gas that would inevitably move around to some extent.

The geochemistry is interesting as reactions of CO2 with rock and subsurface water are inevitable. The most obvious is that solution in water releases hydrogen ions to create weakly acidic fluids: on the one hand that might be a route for precipitation of carbonate and more secure carbon storage, through reaction with minerals (see http://earth-pages.co.uk/2012/04/10/possible-snags-and-boons-for-co2-disposal/), but another possibility is increasing solution of minerals that might eventually cause a trap to leak. A counterpart of pH change is the release of electrons, whose acceptance in chemical reactions creates reducing conditions. The most common minerals to be affected by reducing reactions are the iron oxides, hydroxides and sulfates that often coat sand-sized grains in sedimentary rocks, or occur as accessory minerals in igneous and metamorphic rocks. Iron in such minerals is in the Fe-3 valence state (ferric iron from which an electron has been lost through oxidation) which makes them among the least soluble common materials, provided conditions remain oxidising. Flooding sedimentary rocks with CO₂ inevitably produces a commensurate flow of electrons that readily interact with Fe-3. The oxidised product Fe-2 (ferrousiron) is soluble in water, and so reduction breaks down iron-rich grain coatings. Much the same happens with less abundant manganese oxides and hydroxides. One important concern is that iron hydroxide (FeO.OH or goethite) has a molecular structure so open that it becomes a kind of geochemical sponge. Goethite may lock up a large range of otherwise soluble ions, including those of arsenic and some toxic metals. Should goethite be dissolved by reduction that toxic load moves into solution and can migrate.

Except where deep, carbonated groundwater leaks to the surface in springs – the famous Perrier brand of mineral water is an example – it is difficult to judge what is happing to gases and fluids at depth. But their long-past activity can leave signatures in sedimentary rocks exhumed to the surface. Most continental sandstones, formed either through river or wind action, are strongly coloured by iron minerals simply because of strongly oxidising conditions at the Earth’s surface for the past two billion years or more. Should reducing fluids move through the, the iron is dissolved and leached away to leave streaks and patches of bleached sandstone in otherwise red rocks. In a few cases an altogether more pervasive bleaching of hundreds of metres of rock marks the site of massive fluid-leakage zones. Terrestrial Mesozoic sedimentary sequences in the Green River area of Utah, USA exhibit spectacular examples, easily amenable to field and lab study (Wigley, M. et al. 2012. Fluid-mineral reactions and trace metal mobilization in an exhumed natural CO₂ reservoir, Green River, Utah. Geology, v. 40, p. 555-558). There the bleaching rises up through the otherwise brown and yellow sandstones, cutting across the bedding. In the bleached zone, secondary calcite fills pore spaces. At the contact with unbleached sandstone there are layers of carbonate and metal oxides, enriched in cobalt, copper, zinc, nickel, lead, tin, molybdenum and chromium: not ores but clear signs confirming the general model of reductive dissolution of iron minerals and movement of metal-rich fluid. Carbon isotopes from the junction are richer in ¹³C than could be explained by the gas phase having been methane, and confirm naturally CO₂ – rich fluids.

So, Green River provides a natural analogue for a carbon capture and storage system, albeit one that leaked so profusely it would be a latter day disaster zone. In that sense the site will help in deciding where not to construct CCS facilities.

Related articles

• Earthquake risk for carbon capture and storage schemes (newscientist.com)
• Fessenden, J. 2012. Carbon sequestration and natural analogs. Geology, v. 40, p. 575-576