Rhizomorphs. Fossilised roots

Rhizomorphs. Fossilised roots preserved in a calcrete layer developed below the soil horizon. These are formed in arid climates as plant roots draw in water but exclude dissolved salts such as calcium. The calcium ions accumulate in high concentrations around the living root and combine with carbon dioxide (dissolved in rain water as a week carbonic acid HCO3-), to precipitate calcite (CaCO3). Eventually the root is suffocated by the calcite and the rhizomorph remains fossilised in the calcrete layer just below the surface. These calcrete layers are common in the arid coastal region of Port Lincoln, South Australia. These calcretes probably formed more extensively during dry, cold interglacial periods but they are notoriously difficult to date and as a result the rates of formation are not well understood. Most of arid Australia is covered by calcrete just below the surface and it represents a huge reservoir of solid carbon dioxide locked up as calcite.