Hadley cells and deserts
Much of the geological literature presumes that thick sequences of bedded Phanerozoic evaporites accumulated in hot arid zones of the world that are tied to the distribution of the world’s deserts. Deserts tend to be located beneath regions of descending cool dry air, linked to Hadley Cell circulation across a latitudinal belt that is typically located 15 to 45 degrees north or south of the equator. As this sinking cool air mass approaches the landsurface beneath the descending arm of a Hadley Cell, it warms, and so its moisture-carrying capacity increases. Today, in Africa and Australia, more than 60% of the landsurface is desert, and the areas are increasing.
Global-scale atmospheric circulation is driven by varying intensities of solar irradiation, which is most intense directly above the equator and lessens toward the poles. Hence, the equatorial belt experiences higher insolation than the adjacent temperate latitudes. Equatorial air warms as it rises, creating a tropical belt of low pressure. As it rises, it cools, losing most of the water vapour as rainfall to the tropical jungles and rainforests below. This now moisture-depleted air moves up and away from the equator and further cools and compresses. Finally, it sinks back to the earth’s surface at around 30° latitude north and south of the equator. The cool descending dry air is reheated as it returns to the lower atmosphere, garnering an enhanced potential to absorb moisture. Belts of aridity sit beneath cold dry descending air masses of the Hadley Cells, which define high-pressure belts known as the subtropical Trade-Wind Belts or Horse Latitudes. The trade winds form the equatorward limb of the Hadley circulation in each hemisphere. Such areas are seldom subjected to moisture-bearing disturbances or pressure depressions (lows), either from the Intertropical Convergence Zone (ITCZ) of the tropics or from the belt of mid-latitude depressions associated with the circumpolar westerlies. The trade winds that blow across these zones are evaporating winds, and, because of the trade-wind inversion, they tend to be areas of atmospheric subsidence and stability. That is, subtropical high-pressure belts are the primary cause of aridity, though deserts are not always continuous around the earth at 30 N.
For instance, the Asian monsoon supplies large quantities of rain over northern India, and elsewhere the high-pressure cells are disrupted into a series of local cells, notably over the oceans, where air moving clockwise around the equatorial side of the cell brings moisture-laden air to the eastern margins of the continents. The earth-scale distribution of Holocene desert belts reflects broad-scale atmospheric dynamics. When ice caps expand the atmospheric circulation belts are pushed and compressed toward the equator, and they have done so numerous times in glacial maxima of the current icehouse climate mode. Equatorward compression increases the intensity of atmospheric circulation and alters the latitudinal distribution of climate belts. This has a marked effect on climate in Quaternary deserts, so that almost all modern continental playas in the Horse Latitudes have experienced numerous water-full versus dry stages in the last few hundred thousand years, what follows are examples from different continents to underline this fluctuation. Strontium-isotopic composition of preserved gypsum in Lake Frome a large discharge playa in South Australia, records periods of high rainfall at 3-6, 12-15, and >17 Ka, and drier periods at ≈10 and ≈17 Ka (Ullman and Collerson, 1994). In the last interglacial (early in oxygen isotope stage 5) a nearby and much enlarged Lake Eyre maintained a perennial freshened water body up to 25 m deep (Magee et al., 1995; Cohen et al., 2012). Subsequently, as climate deteriorated into a glacial mode, there were several dry periods separating successively less-effective wet phases, culminating in the deposition of a substantial halite salt crust around the time of the last glacial maximum. Lake Eyre only attained its present saline mudflat/ephemeral playa status some 3-4 Ka. The drying trend over the last hundred thousand years in Lake Eyre corresponds to a longterm lessening of monsoon intensity in the northern part of Australia, which is the main source area for waters flowing into the lake depression. In Death Valley, USA there were dry mudflats, characterised by abundant glauberite, gypsum and minor calcite from 0-10 Ka and 60-100 Ka (Li et al., 1997 ).
In contrast, the wet period from 10-60 Ka was typified by halite and mud layers, with relatively abundant calcite. This more humid period encompasses sediments deposited 10-35 Ka, when Death Valley contained a perennial halite lake. These millennial-scale oscillations define a predominately Na–Ca spring-fed inflow hydrology during dry climatic periods and increased volumes of bicarbonate-rich river waters in the wetter periods.