Calcretes

Calcretes are pedogenic accumulations of calcium carbonate that form in soils and at their surfaces, commonly resulting in hard, cemented horizons or nodular features. The term calcrete is often used for a cemented horizon rich in calcium carbonate, while caliche is used in some regions to describe the nodular or diffuse carbonate accumulations within the soil profile. Collectively, calcretes describe carbonate-rich features produced by soil-forming processes in arid and semi-arid environments.

Formation occurs when weathering of parent material releases calcium and bicarbonate, and groundwater moves these ions through the soil. In arid to semi-arid climates, evaporation concentrates calcium carbonate near the surface or within the lower part of the profile, leading to precipitation of calcite. Carbon dioxide degassing, changes in soil moisture, and vegetation influence the rate and pattern of precipitation. Over time, the carbonate can cement the soil into a durable layer or form discrete nodules or crusts.

Morphology ranges from diffuse nodules and crusts to a continuous cemented horizon. When sufficiently cemented, calcretes create petrocalcic or duric horizons (often called calcrete horizons) that can restrict rooting and water movement. Calcretes may also occur at mid-depths or near the surface, perched on older layers or paleosols.

Distribution is global but concentrated in arid and semi-arid belts, with extensive occurrences in Australia, southern Africa, the southwestern United States, and parts of South America and Asia. They are of interest in geology, pedology, archaeology, and engineering due to their influence on soil properties, groundwater recharge, and landscape interpretation, including paleoclimatic indicators preserved in carbonate isotopes and textures.