Deserts: modern studies influence interpretation of origins and demise of ancient deserts

Deserts are areas of barren land over which rainfall is too limited or spasmodic to support vegetation adequately, where the potential rate of evaporation can far exceed precipitation. They often exhibit both siliciclastic and, in coastal areas, carbonate eolian sands and evaporites, the latter from marine shorelines, ephemeral lakes and near-surface water tables (sabkhas). Prior to the widespread occurrence of land plants (effectively mid Devonian) eolian deposition extending back to the Phanerozoic was sporadic and difficult to substantiate as arid; deserts were probably controlled in part by their plate-tectonic location and orientation relative to prevailing subtropical winds. From the Late Permian onward, tropical deserts became more distinctive in various parts of the world but their existence was neither locally nor globally continuous. The best known modern deserts are found in the sub-tropical parts of major continents (e.g. Sahara, Arabia, Australia). Their growth and decline seems to have been controlled by the global effects of high-latitude glaciations, which squeeze alternating belts of high and low air pressure toward the equator, winds strengthen and sea levels fall. During interglacials, ice melts, wind systems become more variable with unpredictable higher rainfall, and sea level rises causing mass wasting at desert margins. The few modern temperate deserts form because of down-wind distance from a source of moisture.