Hyper-arid

Hyper-arid environments (P/PET < 0.05) cover 7.5% of the global land surface and have very limited and highly variable rainfall amounts both interannually (up to 100%) and on a monthly basis such that there is no seasonal rainfall regime. In virtually all cases where data are available, year-long periods without rainfall have been recorded. 

Because of the irregular rainfall distribution, mean precipitation values have little meaning in the (semi)-arid zone, if not also the range of variation is indicated. This variability refers to both temporal and spatial variability. Temporal variability affects not only the onset and duration of the rains in the year, but plays also a role in year-by-year differences. The variability is highest in the hyper-arid zone, where the mean precipitation value is composed of a few intensive rainstorms. When these fall on a heated barren surface - as is often the case in the arid zone - a part of it is immediately evaporated and lost for soil processes. High rainfall intensity results on the other hand in a rapid saturation of the surface layers and creates lateral runoff and erosion, in particular on sloping land. Many arid and semi-arid soils show therefore features of gully and sheet erosion. 

Fig. 1. Distribution of copper deposits in northern Chile including those containing atacamite in the oxide zone. DFZ is the Domeyko Fault Zone and ACL is Antofagasta-Calama Lineament. The land between the High Andes and the coast is the hyper-arid central Atacama Desert.

These marine sources of reduced sulfur gases can be important as a source of sulfur to the continents. The gypsum accumulations of the hyper-arid Central Namib Desert seem to be mainly derived from non-seasalt sulfur, in particular oxidation products of marine DMS (Eckardt and Spiro, 1999). 

Sodium nitrate deposits are less widespread than the other major types of chemical crust or sediment that are found in different parts of the world. Indeed, the only deposits of any great spatial extent and thickness are those of portions of the hyper-arid Atacama Desert in South America. These materials are, in the words of Ericksen (1983, p. 366), so extraordinary that, were it not for their existence, geologists could easily conclude that such deposits could not form in nature . Ericksen notes a series of features of the deposits that defy rational explanation. These include their restricted distribution in a very salty area, their occurrence in a wide variety of topographic settings, the abundance of nitrate minerals, and the presence of a series of other minerals, such as perchlorate, that do not occur in any other saline complexes and the origin of which is obscure. 

Mueller (1968) argued that one of the unique features of the Atacama was what he described as its climatic asymmetry . Basically, he indicated that in the Andes to the east there was relatively high precipitation, whereas at lower altitudes in the west there was hyper-aridity. Weathering in the high rainfall zone produced solutes that accumulated in the closed basins at lower altitudes and formed salt deposits. Waterlogged sumps were zones of chloride and sulphate precipitation. Higher zones around the sumps were fed by capillary concentration, and it is this that led to the formation of zones of nitrate accumulation. 

The search for surface deposits that preserve anomalous oxygen isotope fractionations led fi om improbable sites such as the hyper-arid, super-hot Atacama Desert, Chile, to super-cold sampling pits dug in snow at the South Pole. The proxy dragnet focused on these remote sites because aridity and freezing are two viable methods for preserving water-soluble, direct products of atmospheric chemistry for detailed investigations. Anomalously fractionated... 

Highly specialized microbial diversity in hyper-arid polar desert. Proceedings of the National Academy of Sciences USA 106,19964-19969. 

Sabkha s are most often found in hyper-arid to semi-arid areas of the world. Today arid areas form two belts between 15° and 45° north and south of the equator. Climatic, geochemical, hydrological and geomorphological factors play different roles in the genesis of the sabkha system. The interaction of these factors... 

Karstic landforms vary enormously in character, shape and size and are formed in wet climatic conditions at present or in the geological past. When developed in the past, surface features may be buried and hidden by more recent sediments deposited in a changed climatic environment e.g., in the Middle East, currently known for its hyper-arid conditions, widespread karstification has occurred during the wet Middle Pleistocene period (325,000-560,000 years ago) affecting the Eocene limestones (Sadiq et al, 2002). The presence of dissolution cavities and other karst-related hazards may not always be visible at the ground surface, but may have to be identified by site investigations specifically aimed at karstic features. 

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