Quaternary climate and aridity

Significant volumes of Quaternary evaporite salts are customarily interpreted as being allied to the distribution of the world’s hot arid deserts. In a general way, this is true, but, as the figure above shows, the correlation is an oversimplification. A hot arid desert (BWh) does not necessarily equate to occurrences of laterally extensive bedded evaporites; there must also be a significant long-term brine inflow to the evaporative sump, incoming waters may be meteoric, marine, a hybrid and perhaps the sump is fed brines coming from dissolution of earlier formed salts in the drainage basin, including diapiric salt (Warren, 2016).

Actually, there are different ways of defining a desert, and by implication, its associated evaporites. One conventional approach is to define a desert as a terrestrial area receiving less than 250 mm (10 inches) of annual precipitation. Using this definition some 26.2% of the world’s landsurface is desert. But, in terms of evaporite distribution and the economics of the associated salts, this climatic generalization related to annual rainfall conceals three important hydrological truisms. All three need to be met in order to accumulate thick sequences of bedded salts: 1) For any substantial volume of evaporite to precipitate and be preserved, there must be a sufficient volume of cations and anions in the inflow waters to allow thick sequences of salts to form; 2) The depositional setting and its climate must be located within a longer-term basin hydrology that favours preservation of the bedded salt, so the accumulating salt mass can pass into the burial realm; 3) There must be a negative water balance in the basin with the potential for more water to leave the local hydrological sump than enter. When using a rainfall (precipitation) based definition of a desert, the significance of these three simple hydrological axioms and the consequences, as to where bedded evaporites accumulate, is lost in the generalization that “evaporites form in the world’s deserts.”

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Koeppen climate zone and regions of saline areas > 250km2

Köppen climate zones and Quaternary evaporites

 In terms of evaporite occurrences, the most important of the Köppen climate groups is Köppen Group B (arid or semiarid); it defines climate areas on the continents where annual precipitation is less than potential evapotranspiration. Group B climates cover 12.2% of the world’s land surface, much less than the 26.2% desert area when defined purely by rainfall. Group B is further divided into desert (BW) zones, where annual precipitation is low, and steppe or semiarid (BS) areas where precipitation is somewhat higher relative to evapotranspiration.

A third letter is added to a Köppen classification of deserts to indicate temperature, for example, h indicates hot and k indicates cold in the BW group. Definitions vary on what is the actual thermal boundary between h and k; in the classification followed in this book Köppen’s original separation of the average annual temperature being more or less than 18°C is used. Common practice in the USA today is to assign k if there is one month or more in a year when the average monthly temperature is below zero (°C).

Group A encompasses tropical (megathermal) climates, it is assigned to regions that have year-round monthly temperature averages above 18°C. The tropics are further subdivided in Af for rainforest, Am for monsoonal, while seasonally variable tropical climates are called savannah. As denotes a tropical savannah zone with a dry summer, and Aw indicates tropical savannah with a dry winter. The savannah group encompasses most tropical setting where bedded evaporites can occur (e.g. Aw characterises many saline African rift lakes and salt pans on the leeward of some oceanic islands, such as Inagua in the Caribbean, while an As climate typifies the climate over anthropogenic salt pans near Macau in central Brazil (see Warren 2016, Chapter 11 for discussion of anthropogenic coastal salt factories). 

Group C is the temperate (mesothermal) climate group and includes the dry summer subtropical or Mediterranean climate (Csa, Csb) zones showing marked seasonal separation between hotter drier summers and cooler wetter winters. This climate setting facilitates seasonal salt harvesting in many solar salt farms and pans about the Mediterranean and southeastern Australia (Chapter 11). There is also a humid subtropical group (Cfa and Cwa) that tends to occupy temperate continental interiors. Maritime or oceanic temperate climate zones (Cfb, Cwb, Cfc) have milder summers and winters; they tend to occur on western sides of continents, polewards of the Mediterranean climate zones of the world. Then there are the temperate climate zones with dry winters and cool summers (Cwb) and the Maritime subarctic climates (Cfc), typically located poleward to the maritime temperate zones. 

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Land areas of the various Köppen climate zones (extracted and plotted from Kottek et al., 2006). The earth’s total land area is 148,940,000 km2.

Group D encompasses continental climate regimes, sometimes called the snow climates, which tend be located within continents and subject to greater temperature extremes compared to the maritime zones. They are subdivided into continental regions with hot summers (Dfa, Dwa, Dsa), continental regions with warm summers, also called hemiboreal (Dfb, Dwb, Dsb) and a third grouping that is subarctic or boreal (Taiga) and termed Dfc, Dwc and Dsc. Cryogenic evaporites are not uncommon in regions covered by this D climatic group
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Finally, there are the polar climates of Group E, characterised year round by monthly average temperatures below 10°C. Group E is further divided into icecap or frost climate (EF) and tundra (ET) which includes alpine or high-altitude climates and encompasses many natural accumulations of borate and low temperature sodium sulphate salts and along with lithium brines in the adiabatic high-altitude deserts of the Andes and the Himalayas (Warren, 2016, Chapter 11). Above 4000m most of these saline ET accumulations are relatively small deposits (>10 km2)

Evaporites, latitude and elevation

Modern continental evaporites typically accumulate within groundwater discharge regions in semi-arid to hyperarid deserts. Marine-fed coastal evaporites can occur at the oceanic edge of the same deserts, usually in coastal depressions fed by marine seeps or by rising groundwaters along coast-parallel mudflats. Evaporites also form lake precipitates and efflorescences in cold polar deserts in Antarctica, but the volumes of salt in these cryogenic regions pale to insignificance compared to arid settings closer to the equator. Brine freezing and mixing (cryogenesis), rather than direct solar concentration, plays a much more critical role in the crystallisation of most cold climate salts, not just in Antarctica but also in the interior plains of Russia and Canada and high alpine deserts of the Andes and Tibet.

Details of some of the larger Quaternary examples can be found by clicking the link below.

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Large saline areas (>250 km2) on the earth’s landsurface in terms of Köppen climate zones. A) Plot of locations in terms of climate versus latitude and elevation. B) Count of occurrences (>250 km2) as bar charts showing percentage occurrences in climate zone and elevation ranges (metres sea level).

Climatic distribution of saline deposits

The range of large (>250 km2) saline systems in the world’s arid landscape is more climatically diverse than just evaporite occurrences within a hot arid desert (BWh), although such associations do constitute some 38% of saline occurrences. Cold arid deserts (BWk) host 23% of large saline occurrences, making a combined total of 61% for large evaporite accumulation (area >250 km2) occurrences in modern arid deserts (BW group), while the arid steppes (BSh and BSk) host another 22%. In total, the world’s arid climatic zones host 83% of today’s larger evaporite occurrences. This leaves another substantial, but not widely recognized, climate zone where significant volumes of Quaternary evaporites can accumulate, this is the polar tundra (ET); an environment where some 11% of large evaporite areas occur. In terms of evaporite volumes, the polar tundra (ET) is typically an arid high altitude belt, mostly in the Horse Latitude (Trade Wind) belts, and not located in polar or near-polar higher latitudes. The lakes and saline pans of the high plateaus of the Andes (Altiplano) and the Himalayas (Tibetan Plateau) typify this style of tundra (ET) evaporite. Water may be commonplace in the ET zone, but is there mostly as ice, and cryogenic salts are commonplace (see Salty Matters, Feb 24, 2015). The remaining region where significant evaporite volumes are found, some 6% of the total of large saline occurrences is a group of deposits defined by continental interior snow climates (group D), some with hot dry summers with solar evaporites alternating with dry winters favouring the possible accumulation of cryogenic salts (e.g. Great Salt Lake, USA).In the Northern Hemisphere the occurrence of large evaporite systems within arid deserts and steppe climates (BW and BS settings) extends much further south toward the equator and much further poleward (from 5-55°N) than the narrower range of large evaporite occurrences and associated climates in the southern hemisphere.

This hemispheric asymmetry in evaporite occurrence in the northern versus the southern hemisphere is mostly a response to world-scale adiabatic effects associated with the collision of India with Eurasia and growth of the Himalayas. Today, a Cainozoic mountain range, centred on the Himalayas, diverts world-scale atmospheric air flows from the more north-south trajectory, usually associated with Hadley Cell circulation. For example, the Kunlun Mountains, first formed some 5.3 Ma, prevents moisture from the Indian Monsoon reaching much of the adjacent Tibet Plateau. Its adiabatic rain shadow creates the Taklamakan desert, the second-largest active sand desert in the world (BWk). The uplift of the Himalayas also produces a dry easterly jet stream, moving cool arid air across the Tibet Plateau, around the northern side of the Himalayas, and then equatorward across the Arabian Peninsula toward Somalia where it descends and gains heat.

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Adiabatic funnelling influenced large scale atmospheric circulation so that the northern hemisphere desert belt extends to within 5° of the equator.

That is, this stream of cool southwesterly-flowing dry air warms as it moves across the Eastern Mediterranean land areas and so heightens existing aridity. This helps create an adiabatic desert zone that today ranges across Arabia and northern Africa almost to the Equator. In the southern hemisphere, the uplift of the Andes has formed high intermontane depressions and the allied adiabatic aridity that are cooler with lower evaporation rates and higher relief in the immediate basin compared to groundwater depressions in flatter lower-elevation continental interior deserts like the Sahara. This higher stability hydrology favours salars over dry mudflats, as typified by Salar de Atacama and Salar de Uyuni.

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Salt di Atacama, Chile

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Salar di Uyuni, Bolivia

Atacama has a Quaternary saline sediment fill made up of a more than 900 m thickness of interlayered salt and clay, while Uyuni holds a more than 120 m thick interval of interbedded salt and clay infill, with areas of 3,064 km2 and 9,654 km2 and elevations of 2250 m and 3650 m, respectively. These salars are the two largest known examples of Quaternary bedded halite accumulation, worldwide. Yet neither resides in hot arid desert settings (BWh); both are located in cold arid deserts (BWk) and in actively subsiding, high altitude (>2500m) intermontane (high relief) endorheic depressions. Worldwide, distribution of most of the larger (>250 km2) Quaternary evaporite settings located in hot arid (BWh) desert settings, tie either to; 1) endorheic river terminations along desert margins, especially if adjacent to mountain belts (e.g. the various circum-Saharan chotts, playas and sabkhas adjacent to the Atlas Mountains), or 2) to ancient inherited paleodrainage depressions (as in the majority of the interior salt lakes of Australia or the southern Africa pans). Another hot arid desert (BWh) evaporite association is defined by termination outflow rims of deep artesian systems, as in Lake Eyre North, Australia (8,528 km2, with an ephemeral halite crust up to 2 m thick in its southern portion). Similar, deeply-circulating, meteoric artesian hydrologies help explain the distribution of chotts in the BWh zone of NE Africa. Unlike Atacama and Uyuni in the Andes, none of these modern BWh artesian systems preserve stacked decametre-thick salt beds, nor did they do so at any time in the Quaternary. Rather, the most extensive style of BWh salt in meteoric-fed artesian outflow zones is as dispersed crystals of gypsum and halite in a terrigenous redbed matrix (sabkha) or as visually impressive large ephemeral saline flats and pans covered by metre-scale salt crusts that dissolve and reform with the occasional decadal freshwater flood (Warren 2016; Chapter 3).

Eolian processes typically rework sediments of such continental groundwater outflow zones and, due to a lack of long term watertable stability, the longterm sediment fill is matrix-rich and evaporite-poor, with the Quaternary sediment column typified by episodes of deflation, driven by 10,000-100,000 year cycles of glacial-interglacial climate changes. It seems that to form and preserve laterally-extensive decametre-thick stacked beds of halite in a Quaternary time-framework requires an actively subsiding tectonic depression in a cooler high-altitude continental desert, where temperatures and evaporation rates are somewhat lower than in BWh settings, allowing the brine to pond and remain at or near the surface for more extended periods. But perhaps more importantly, all of the larger regions of the Quaternary world, where thick stacked bedded (not dispersed) evaporites are accumulating, are located in continental regions with drainage hinterlands where the dissolution of older halokinetic marine-fed salt masses are actively supplying substantial volumes of brine to the near-surface hydrology. This halokinetic-supplied set of deposits includes Salar de Uyuni and Salar de Atacama in the Andean Altiplano, the Kavir salt lakes of Iran, the Qaidam depression of China and the Dead Sea.