www.saltworkconsultants.com Salty MattersJohn Warren - Tuesday January 31, 2017 Evaporites and climate: Part 1 of 2 - Are modern deserts the key? desert (Figure 1b). But, in terms of evaporite distribution and the Salt deposits and deserts economics of the associated salts, this climatic generalization Much of the geological literature presumes that thick sequences related to annual rainfall conceals three significant hydrological of bedded Phanerozoic evaporites accumulated in hot arid zones truisms. All three need to be met in order to accumulate thick tied to the distribution of the world’s deserts1 beneath regions of sequences of bedded salts (Warren, 2010): 1) For any substantial descending air within Hadley Cells in a latitudinal belt that is typ- volume of evaporite to precipitate and be preserved, there must ically located 15 to 45 degrees north or south of the equator (Fig- be a sufficient volume of cations and anions in the inflow waters ure 1a: Gordon, 1975). As this sinking cool air mass approaches to allow thick sequences of salts to form; 2) The depositional the landsurface beneath the descending arm of a Hadley Cell it setting and its climate must be located within a longer term basin warms, and so its moisture-carrying capacity increases. The next hydrology that favours preservation of the bedded salt, so the two articles will discuss the validity of this assumption of evap- orites tying to hot arid desert belts in the trade wind belts, first, by a consid- eration of actual Quaternary evaporite Stratosphere distributions as plotted in a GIS data- 15 km Tropopause base with modern climate overlays, Subtropical Jet then in the second article via a look at ancient salt/climate distributions. Polar Jet Significant volumes of Quaternary Hadley evaporite salts are normally interpret- Ferrel ed as being allied to the distribution Cell Polar Cell of the world’s hot arid deserts (Figure Cell 1b). In a general way this is true, but, as Warren (2010) shows, the correla- tion is an oversimplification. A hot Pole 60° 30° (Trade winds) Equator arid desert does not necessarily equate A. Desert belt (ITCZ) to occurrences of laterally extensive bedded evaporites; there must also be a significant long-term brine inflow Australia 83% to the evaporative sump, incoming 20 Africa 64% Asia 39% waters may be meteoric, marine, a South America 17% hybrid and perhaps the sump is fed North America 16% brines coming from dissolution of earlier formed salts in the drainage 10 basin, including diapiric salt (Table 1; Warren, 2016). Actually, there are different ways of (%) covered Ground 0 defining a desert and by implication 90 60 30 0 30 60 90 N S its associated evaporites. One ac- Latitude cepted approach is to define desert as B. a terrestrial area receiving less that Figure 1. Hadley celles and deserts A) Longitudinal cross section through the earth’s atmosphere 250 mm (10 inches) of annual pre- showing major circulation cells, including jet streams. ITCZ = intertropicl convergence zone. Belts cipitation. Using this definition some of cool dry descending air at 30° N and S of the equator create the main arid zones of the world. 26.2% of the world’s landsurface is B) Latitudinal distribution of the world’s modern deserts. Inset gives proportion of each conti- nent that is arid or semiarid. (after Warren, 2016). Page 1 www.saltworkconsultants.com Af Am As Aw BSh BSk BWh BWk Cfa Cfb Cfc Csa Csb Csc Cwa Cwb Cwc Dfa Dfb Dfc Dfd Dsa Dsb Dsc Dwa Dwb Dwc Dwd EF ET Figure 2. Quaternary evaporites with areas >250 km2 and Köppen climate setting. Centroids are plotted on the earth surface where an evaporite depositional setting (sabkha, salina, playa, saline lake, saline soil) extends across an area of more than 250 km2 This plot set is overlain on a Köppen climate base (after Kottek et al., 2006). Main Climates: A; tropical, B; arid, C; warm temperate, D; snow, E; polar. Precipitation: W; desert, S; steppe, f: fully humid, s: summer dry, w; winter dry, m; monsoonal. Temperature: h; hot arid, k; cold arid, a; hot summer, b; warm summer, c; cool summer, d; extremely continental, F: polar frost, T; polar tundra. accumulating salt mass can pass into the burial realm; 3) There the same hydrological consideration that is required for evap- must be a negative water balance in the basin with the potential orite salts to form. That is, a desert is an area of land where for more water to leave the local hydrological sump than enter. annual precipitation (inflow) is less than potential evapotrans- When using a rainfall (precipitation) based definition of desert, piration (outflow). This definition of a desert is the one used the significance of these three simple hydrological axioms and by Köppen (1900). With slight modification, his climatological the consequences, as to where bedded evaporites accumulate, scheme is still in widespread use to breakout the various climatic is lost in the generalization that “evaporites form in the world’s zones across the world’s landsurface (Kottek et al., 2006). Using deserts.” a Köppen climate base, figure 2 plots worldwide occurrences of modern saline depositional systems with areas greater than 2 Continental-interior evaporites 250 km . Table 1 compares characteristics of some of the larger In an evaporite context it is better to define and plot saline hy- Quaternary bedded evaporite settings in marine edge and conti- drologies within a climatic framework where deserts are given nental interiors. These regions contain evaporite salts accumu- lating in saline soils, sabkhas, salinas, saline lakes, playas and salt flats, with more than 5000 metres 1% textural forms ranging from; isolated 4500-5000 6% B. crystals and nodules in a terrigenous 4000-4500 3% matrix, to salt crusts, to stacked beds 3500-4000 4% of salts that can be more than 10 me- 3000-3500 4% tres thick. The majority of the plotted Cfa 1% 2500-3000 3% saline areas are in arid regions, as de- Csa 1% 2000-2500 1% fined by Köppen (Zone B), but not all Dfb 1% 1500-2000 2% such areas of widespread salts are in Dsb 1% 1000-1500 9% deserts (as defined by Köppen) and Aw 1% 500-1000 13% Cwb 1% not all are in hot climates. 0-500 43% Dfa 1% The range of large (>250 km2) saline Dwc 2% below sealevel 12% systems in the world’s arid landscape BSh 9% 0 10 20 30 40 50 60 70 80 ET 11% OCCURRENCES is more climatically diverse than just BSk 13% evaporite occurrences within a hot BWk 23% arid desert (BWh), although such as- BWh 38% sociations do constitute some 38% of saline occurrences (Figure 3a; War- A. 0 10 20 30 40 50 60 70 80 OCCURRENCES ren, 2010; 2016). Cold arid deserts Figure 3. Occurrences of saline areas (>250 km2) as bar charts showing percentage occurrences in A) climate zone (BWk) host 23% of large saline oc- and B) elevation ranges (metres sea level). currences, making a combined total Page 2 www.saltworkconsultants.com Name Country Köppen Area (km2) Elev. (m) Diapirs and not located in polar or near-polar high- er latitudes. The lakes and saline pans of the Continental interior high plateaus of the Andes (Altiplano) and the Lake Eyre Australia BWh 8528 -15 No Himalayas (Tibetan Plateau) typify this style Salar de Uyuni Bolivia BWk 9,654 3650 Yes of tundra (ET) evaporite. Water may be com- monplace in the ET zone, but is there mostly Sua Pan (Makgadikgadi Pans) Botswana BSh 3,307 888 No as ice, and cryogenic salts are commonplace Salar de Atacama Chile BWk 3,065 2250 Yes (see Salty Matters, Feb 24, 2015). The re- Chagannur Salt Lake China BSk 15 968 No maining region where significant evaporite volumes are found, some 6% of the total of Dabuxum Lake China BWk 9 2681 Yes large saline occurrences is a group of deposits Zabuye Salt Lake China ET 251 4426 No defined by continental interior snow climates Wadi Natrun Egypt BWh 20 -23 No (group D), some with hot dry summers with solar evaporites alternating with dry winters Gavkhoni Playa Iran BWk 477 1450 Yes favouring the possible accumulation of cryo- Lake Magadi Kenya Aw 111 643 No genic salts (e.g. Great Salt Lake, USA). Issyk Kul Kyrgystan BSk 6,235 1619 No In the Northern Hemisphere the occurrence Dead Sea Middle East BSh 1,023 -415 Yes of large evaporite systems within arid deserts Sabkha Matti Middle East BWh 2,954 30 No and steppe climates (BW and BS settings) ex- tends much further south toward the equator Umm Al Sammim mudflat Oman BWh 2,969 81 Yes and much further poleward (from 5-55°N) Chott el Djerid (Jerid) Tunisia BWh 5,728 14 Yes than the narrower range of large evaporite Death Valley Badwater Pan USA BWh 54 -75 No occurrences and associated climates in the southern hemisphere (Figure 4). This hemi- Salton Sea USA BWh 929 -74 No spheric asymmetry in evaporite occurrence Great Salt Lake USA Cfa 4,998 1280 No is mostly a response to world-scale adiabatic Marine margin effects associated with the collision of India with Eurasia and growth of the Himalayas. Lake Macleod Australia BWh 2,067 -0.6 No Today, a Cainozoic mountain range, centred Lake Macdonnell Australia BSk 108 -0.5 No on the Himalyas, diverts world-scale atmo- Lake Asal Djibouti BWh 54 -155 No spheric air flows from the more north-south trajectory, usually associated with Hadley Dallol saline pan Ethiopia BWh 344 -115 Yes Cell circulation. For example, the Kunlun Sabkhat Maradah Libya BWh 473 -0.5 No Mountains, first formed some 5.3 Ma, pre- Oje de Liebr Mexico BWh 373 -0.1 No vents moisture from the Indian Monsoon reaching much of the adjacent Tibet Plateau.