Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (

Evaporites and climate: Part 1 of 2 - Are modern deserts the key?

John Warren - Tuesday, January 31, 2017

Salt deposits and deserts

Much of the geological literature presumes that thick sequences of bedded Phanerozoic evaporites accumulated in hot arid zones tied to the distribution of the world’s deserts beneath regions of descending air within Hadley Cells in a latitudinal belt that is typically located 15 to 45 degrees north or south of the equator (Figure 1a: Gordon, 1975). 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. The next two articles will discuss the validity of this assumption of evaporites tying to hot arid desert belts in the trade wind belts, first, by a consideration of actual Quaternary evaporite distributions as plotted in a GIS database with modern climate overlays, then in the second article via a look at ancient salt/climate distributions.

Significant volumes of Quaternary evaporite salts are normally interpreted as being allied to the distribution of the world’s hot arid deserts (Figure 1b). In a general way this is true, but, as Warren (2010) shows, the correlation is an oversimplification. A hot arid desert 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 (Table 1; Warren, 2016).

Actually, there are different ways of defining a desert and by implication its associated evaporites. One accepted approach is to define desert as a terrestrial area receiving less that 250 mm (10 inches) of annual precipitation. Using this definition some 26.2% of the world’s landsurface is desert (Figure 1b). But, in terms of evaporite distribution and the economics of the associated salts, this climatic generalization related to annual rainfall conceals three significant hydrological truisms. All three need to be met in order to accumulate thick sequences of bedded salts (Warren, 2010): 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 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.”

Continental-interior evaporites

In an evaporite context it is better to define and plot saline hydrologies within a climatic framework where deserts are given the same hydrological consideration that is required for evaporite salts to form. That is, a desert is an area of land where annual precipitation (inflow) is less than potential evapotranspiration (outflow). This definition of a desert is the one used by Köppen (1900). With slight modification, his climatological scheme is still in widespread use to breakout the various climatic zones across the world’s landsurface (Kottek et al., 2006). Using a Köppen climate base, figure 2 plots worldwide occurrences of modern saline depositional systems with areas greater than 250 km2. Table 1 compares characteristics of some of the larger Quaternary bedded evaporite settings in marine edge and continental interiors. These regions contain evaporite salts accumulating in saline soils, sabkhas, salinas, saline lakes, playas and salt flats, with textural forms ranging from; isolated crystals and nodules in a terrigenous matrix, to salt crusts, to stacked beds of salts that can be more than 10 metres thick. The majority of the plotted saline areas are in arid regions, as defined by Köppen (Zone B), but not all such areas of widespread salts are in deserts (as defined by Köppen) and not all are in hot climates.

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 (Figure 3a; Warren, 2010; 2016). 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 (Figure 3b).

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 (Figure 4). This hemispheric asymmetry in evaporite occurrence 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 Himalyas, 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 creates a dry easterly jet stream, moving arid cool 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. 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 (Figures 5).

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 relied 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. 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 (Figure 7a). 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 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). Sediments of such continental groundwater outflow zones are typically reworked by eolian processes 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 brine to pond and remain at or near the surface for longer periods (Figures 6, 7a). 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 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 (Table 1).

Marine-edge evaporites

Thick sequences of stacked Quaternary evaporite beds with a marine-brine feed are far less common and far smaller than meteoric/halokinetic Quaternary continental evaporite occurrences. Relatively few marine-fed evaporite regions exist today with areas in excess of 250 km2 (Table 1; Figures 6, 7b). By definition, in order to be able to draw on significant volumes of seawater, these basins must operate with a subsealevel hydrology. This allows large volumes of seawater to seep into the depression and evaporate. It also means most of these deposits are located near the continental edge where a freestanding mass of seawater is not too distant

The largest known deposit of this group is Lake Macleod on the west coast of Australia, with an area of 2,067 km2 and containing a 10m-thick Holocene gypsum/halite bed (Figure 7b). It hosts a saltworks producing some 1,500,000 tonnes/year of halite from lake brines in a BWh setting (Warren, 2016). A smaller marine seepage example, with a similar Quaternary coastal carbonate dune-hosted seepage hydrology, is Lake MacDonnell near the head of the Great Australia Bight. It has an area of 451 km2, a 10m-thick fill of Holocene bedded gypsum and is located in a milder BSk setting, compared to Lake Macleod. Even so, annually, the Lake MacDonnell operation is quarrying more than 1.4 million tonnes of Holocene coarsely-crystalline near-pure gypsum and producing more that 35,000 tonnes of salt via by pan evaporation of lake brines.

Interestingly, when the climatic settings of Holocene coastal salinas of southern and western Australia are compared, all show similar interdunal sump seepage hydrologies with unconfined calcarenite aquifers, yet it is clear that gypsum evaporite beds dominate in BSk and lower precipitation levels, with more carbonate, in Csb coastal settings typified by hot dry summers. Halite dominates the marine-fed bedded fills in BWh coastal settings, while Coorong-style meteoric-fed carbonates dominate in similar interdunal coastal seepage depressions in the more the humid and somewhat cooler Csb settings of the Coorong region.

One of the most visually impressive marine seep systems in the world is Lake Asal, immediately inland of the coast of Djibouti, with an area of only 54 km2 it is much smaller than the 250 km2 cutoff used for this discussion. It is located in a BWh climate, similar to that of the Danakil depression, contains subaqueous textured gypsum and pan halites, and lies at the bottom of a hydrographically-isolated basalt-floored depression with a brine lake surface some 115m below sealevel (Figure 8; see Warren, 2016 Chapter 4 for geological detail). This difference in elevation between the nearby Red Sea and the lake floor drives a marine seep hydrology, so that seawater-derived groundwater escapes as springs along the basaltic lake margin. Most of the cations and anions in the seawater feed, that ultimately accumulate as salts in the subsealevel Asal sump, move lakeward via gravitational seepage along fractures in a basaltic ridge aquifer separating the Red Sea from the brine lake, in what is an actively extending continental rift that will shortly become and arm of the Red Sea.

That there are very few large marine-fed bedded Quaternary evaporite systems is illustrated by summing the total area of large active continental-fed evaporite areas (>250km2) listed in Figure 2, which gives a total surface area worldwide in excess of 360,000 km2. In contrast, the total area of large modern marine-fed bedded salt systems, at slightly less 10,000 km2, is more than an order of magnitude smaller. This low value for large coastal marine-fed evaporite occurrences is in part because many classic coastal sabkhas, with characteristic dispersed evaporites in their supratidal sediments and long considered to be archetypal marine-fed groundwater evaporite system, are now seen as mostly continental brine-fed hydrologies.

For example, the modern Abu Dhabi sabkha system (1,658 km2; BWh) has been shown by Wood (2010) to be a continental groundwater outflow area, where most of ions precipitating as salts in the supratidal zone are supplied by upwelling of deeply-circulated meteoric waters, not seawater flooding (Warren, 2016; Chapter 3). Similar continental hydrologies, resurging into the coastal zone. supply much of the salt accumulating in the suprasealevel sabkhas near Khobar in Saudi Arabia (BWh), Rann of Kutch in India (28,000 km2, BWh), Sabkha Matti (2,955km2, BWh) in the Emirates, and Sabkha de Ndrhamcha (634 km2, BWh) in Mauritania. And yet today all of these large sabkha occurrences are located just inland of modern coastal zones and so, without hydrological knowledge, are easily interpreted as marine. The distinction between a water budget and a salt supply budget emphasizes a general observation in Holocene evaporite systems that, in order to define the main fluid carrier for the salt volume found in any supratidal part of an evaporitic depression, it is important to understand and quantify the nature of the groundwater feed, be it by marine or nonmarine, in both continental-interior and marine-margin settings. It also means that surface runoff and seawater washovers, while at times visually impressive, typically do not supply the greater volume of preserved salts in most modern coastal sabkha systems.

Due to the highly impervious nature of muddy sabkha sediment, an occasional seawater storm flood into a coastal margin mudflat does not mean the pooled seawater ever penetrates the underlying sabkha. Most of its solute load is deposited as an ephemeral surface crust of halite that is a few centimetres thick atop the sabkha muds (Wood et al., 2005). Based on Wood’s and other hydrological studies of modern coastal sabkhas in BWh settings, centred on the Middle East, it seems that the salt supply to most modern coastal sabkha depressions is still not at hydrological equilibrium with the present sealevel, which was reached at the beginning of the Holocene some 6,000-8,000 years ago. That is, modern zones of continental groundwater outflow along an arid coast can have watertables that are up to a metre of two above sealevel and are strongly influenced by the resurgence of seaward-flowing deeply circulating, continental groundwaters (it is axiomatic that surface runoff and local meteoric recharge tend to be limited in BWh settings). In this situation resurging waters in the sabkha tend to have a dominantly continental ionic supply for their preserved salts.

Lastly, in terms of the climatic and hydrological pre-requisites for the accumulations of thick bedded Holocene salt deposits, one must recognize that marine-margin sabkha mudflats do not have the same geohydrology as a near-coastal, hydrographically-isolated seepage-fed sub-sealevel salina depression. A BWh or BSk near-coastal salina is slowly but continually resupplied ions via springs fed by the effusion of seawater, moving under a gravity drive (evaporative drawdown), through an aquifer that is the barrier separating the salina depression from the adjacent ocean. Until the depositional surface reaches hydrological equilibrium, it draws continually on the near limitless supply of ions in the adjacent salty reservoir that is the world’s ocean. Salts growing displacively in the supratidal parts of a sabkha cannot call on a drawdown hydrology and can only be supplied marine salts from salt spray as it is blown inland or from waters washed over the sabkha by the occasional storm driven wash-over and breakout (Figure 9; Warren and Kendall, 1985).

So what?

So, if we based our ideas of where ancient bedded evaporites formed on a strictly uniformitarian approach using Quaternary analogues, then the study of where bedded salts have formed best in the world over the last two million years leads to a conclusion that large bedded salt deposits are not marine fed. Rather, in the Quaternary, thick bedded salts form best in tectonically active, subsiding hypersaline groundwater sumps located in high-altitude high-relief cold-arid deserts. These depressions are hydrographically closed, with the purer, most voluminous, examples of 100m-thick evaporite successions located in tectonically active piggy-back depressions, with lake floors and hydrologies that are more than 2000 m above sea level. Their hydrologies are endorheic and strongly influenced by deeply circulating meteoric waters. In addition, the better examples of thick stacked bedded salts are supplied ions via the dissolution of older marine halokinetic salts in the surrounding drainage basin (Table 1).

Yet, anyone working in ancient (pre-Quaternary) evaporites knows from simple arguments of salt volumetrics versus brine sources, and the nature of the enclosing sediments, that they are part of a dominantly marine basin fill. This will be the focus


Gordon, W. A., 1975, Distribution by latitude of Phanerozoic evaporite deposits: Journal of Geology, v. 83, p. 671-684.

Köppen, W., 1900, Versuch einer Klassification der Klimate, vorzsugsweise nach ihren Beziehungen zur Pflanzenwelt: Geographraphische Zeitschrift, v. 6, p. 593-611.

Kottek, M., J. Grieser, C. Beck, B. Rudolf, and F. Rubel, 2006, World Map of the Köppen-Geiger climate classification updated: Meteorologische Zeitschrift, v. 15, p. 259-263.

Stieljes, L., 1973, Evolution tectonique récente du rift d'Asal: Review Geographie Physical Geologie Dynamique, v. 15, p. 425-436.

Warren, J. K., 2010, Evaporitic source rocks: mesohaline responses to cycles of “famine or feast” in layered brines, Quaternary carbonate and evaporite sedimentary facies and their ancient analogues, John Wiley & Sons Ltd., p. 315-392.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

Warren, J. K., and C. G. S. C. Kendall, 1985, Comparison of sequences formed in marine sabkha (subaerial) and salina (subaqueous) settings; modern and ancient: Bulletin American Association of Petroleum Geologists, v. 69, p. 1013-1023.

Wood, W. W., 2010, An historical odyssey: the origin of solutes in the coastal sabkha of Abu Dhabi, United Arab Emirates, Quaternary carbonate and evaporite sedimentary facies and their ancient analogues, John Wiley & Sons Ltd., p. 243-254.

Wood, W. W., W. E. Sanford, and S. K. Frape, 2005, Chemical openness and potential for misinterpretation of the solute environment of coastal sabkhat: Chemical Geology, v. 215, p. 361-372.

[1] The word desert comes from the Latin dēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon."


Seawater chemistry (1 of 2): Potash bitterns and Phanerozoic marine brine evolution

John Warren - Tuesday, August 11, 2015

The significance of evaporites as indicators of the chemical evolution of seawater across time and in relation to potash bitterns is considered in the next two Salty Matters articles. This article focuses on Phanerozoic seawater chemistry, where actual salts are widespread and the proportions of potash bittern salts are a useful pointer to the chemical makeup of the mother brine. Throughout both articles, the term “lower salinity” refers to marine brines with salinities between one and ten times that of ambient seawater. The second article considers seawater chemistry based on Precambrian evaporites, where much of the evidence of mother brine composition comes from salt pseudomorphs, rather than remnants of actual salts. In the second article we shall see that atmospheric conditions in the Early Precambrian were reducing and hotter than today, so that seawater was more saline, warmer, anoxic, with higher levels of calcium and bicarbonate compared to Phanerozoic seawater. Gypsum (CaSO4.2H2O), which requires free sulphate, was a rare precipitate during concentration of Archean seawater. Changing atmospheric proportions of CO2, CH4 and O2 meant sodium carbonate salts were significant lower-salinity early Archean marine-brine precipitates. Yet today, sodium carbonate salts, such as trona (NaHCO3.Na2CO3), nahcolite (NaHCO3) and shortite (2CaCO3.Na2CO3) cannot precipitate from a brine with the ionic proportions of modern seawater. The presence of sodium carbonate salts in any evaporite succession across the Phanerozoic is a reliable indicator of a nonmarine mother brine (Figure 1).

A Phanerozoic dichotomy: evolving marine potash bitterns

Consistently across the last 550 million years, halite and gypsum (mostly converted to anhydrite in the subsurface) are the dominant lower-salinity marine salts. But potash-bittern evaporite associations plotted across the same time framework define two end-members (Figure 1):

1) Sulphate-enriched potash deposits, with ores typically composed of halite (NaCl) with carnallite (MgCl2.KCl.6H2O) and lesser sylvite (KCl), along with varying combinations of MgSO4 salts, such as polyhalite (2CaSO4.MgSO4.K2SO4.H2O), kieserite (MgSO4.H2O) kainite (4MgSO4.4KCl.11H2O) and langbeinite (2MgSO4. K2SO4); and

2) Sulphate-depleted potash deposits are composed of halite with sylvite and carnallite, and entirely free or very poor in the magnesium-sulphate salts. The sulphate-depleted association typifies more than 65% of the world’s exploited Phanerozoic potash deposits. Sylvite ores with this association have properties that are easier to process cheaply (Warren 2016; see also blog 4 of 4 in the Salty Matters Danakhil articles).

The sulphate-enriched group of ancient potash salts contains a bittern mineral suite predicted by the evaporation and backreaction of seawater with proportions similar to modern marine brine. In contrast, the sulphate-depleted group of bittern salts must have precipitated from Na-Ca-Mg-K-Cl brines with ionic proportions quite different from that of concentrated modern seawater. The separation between the two bittern associations is defined by brine evolution across the gypsum divide. That is, once gypsum (CaSO4.2H2O) and halite (NaCl) have precipitated in the lower salinity spectrum, are the remaining brines enriched in sulphate or calcium (Figure 1)? The greater suitability for potash utilisation of the sulphate depleted bitterns makes understanding and hence predicting occurrences of the sulphate-depleted association in time and space a useful first-order potash exploration tool.

Why the dichotomy?

In the older literature dealing with Phanerozoic salt chemistry, MgSO4-depleted potash evaporites were often explained as diagenetically-modified marine evaporite brines, thought to result from backreactions during burial diagenesis of normal marine waters (Borchert, 1977; Dean, 1978; Wilson and Long, 1993). If so, then the mother seawater source across the Phanerozoic had ionic proportions like those of today, but diagenetically altered via; a) dolomitisation, b) sulphate-reducing bacterial action, c) mixing of brines with calcium bicarbonate-rich river water, or d) rock-fluid interaction during deep burial diagenesis. As another option, Hardie (1990) suggested MgSO4-depleted potash bitterns formed by the evaporative concentration of sulphate-depleted nonmarine inflow waters seeping into an evaporite basin via springs and faults. Such springs were sourced either from CaCl2-rich hot hydrothermal brines or via cooling of deep basinal brines. Such fault-fed deeply-circulating CaCl2 brines source the various springs feeding the Dead Sea, the Qaidam Basin, the Salton Sea and the Danakil Depression. In all these cases, the elevated salinities of inflow waters are related, at least in part, to the dissolution of buried evaporites. Upwelling of brines in these regions is driven either by thermally-induced density instabilities, related to magma emplacement, or by the creation of tectonically-induced topographic gradients that force deeply-circulated basinal brines to the surface. Ayora et al. (1994) demonstrated that such a deeply-circulating continental Ca–Cl brine system operated during deposition of sylvite and carnallite in the upper Eocene basin of Navarra, southern Pyrenees, Spain.

Today, a more widely accepted explanation for SO4-enriched versus SO4-depleted Phanerozoic potash bitterns, is that seawater chemistry has evolved across deep time. Background chemistry of the marine potash dichotomy is simple and can be related to brine evolution models published by Hardie more than 30 years ago (Hardie, 1984). He found that the constituent chemical proportions in the early stages of concentration of any marine brine largely controls the chemical makeup of the subsequent bittern stages. These ionic proportions control how a brine passes through the lower salinity CaCO3 and gypsum divides (Figure 1). That is, a marine brine’s bittern make-up is determined by the ionic proportions in the ambient seawater source. It determines the carbonate mineralogy during the precipitation of relatively insoluble evaporitic carbonates (aragonite, high-magnesium calcite, or low-magnesium calcite) which in turn controls its constituent chemistry as it attains gypsum saturation. These two stages are called the CaCO3 and gypsum divides. Hence, the chemical passage of a bittern is controlled by the ionic proportions in the original ambient seawater. The CaCO3 divide kicks in when a concentrating seawater brine attains a salinity around twice that of normal seawater (60‰). The gypsum divide occurs when brine concentrations are around 4-5 times that of normal seawater (140-160‰). Normal seawater has a salinity around 35-35‰ and the various potash bittern salts precipitate when concentrations are around 40-60 times that of the original seawater (Figure 2 – lower part).

As seawater concentrates and calcium carbonate mineral(s) begin to precipitate at the CaCO3 divided then, depending on the relative proportions of Ca and HCO3 in the mother seawater, either Ca is used up, or the HCO3 is used up. If the Ca is used up first, an alkaline brine (pH>10) forms, with residual CO3, along with Na, K, SO4 and Cl, but no remaining Ca (Figure 1). With ongoing concentration this brine chemistry will then form sodium bicarbonate minerals, it cannot form gypsum as all the Ca is already used up. Such an ionic proportion chemistry likely defined oceanic waters in the early Archaean but is not relevant to seawater evolution in the Proterozoic and Phanerozoic, as evidenced by widespread gypsum (anhydrite) or pseudomorphs in numerous post-Archean marine-evaporite basins. At higher concentrations, early Archean marine brines would have produced halite and sylvite bittern suites, but with no gypsum or anhydrite (Figure 1).

If, instead, HCO3 is used up during initial evaporitic carbonate precipitation, as is the case for all Phanerozoic seawaters, the concentrating brine becomes enriched in Ca and Mg, and a neutral brine, depleted in carbonate, is formed. Then the ambient Mg/Ca ratio in a concentrating Phanerozoic seawater will control whether the first-formed carbonate at the CaCO3 divide is aragonite (Mg/Ca>5) or high-magnesium calcite (2>Mg/Ca>5), or low-Mg calcite(Mg/Ca>2). The latter Mg/Ca ratio is so low it is only relevant to concentrating Cretaceous seawaters. Elevated Mg/Ca ratios favouring the precipitation of aragonite over high Mg-calcite typify modern marine seawater brines, which have Mg/Ca ratios that are always >5 (Figure 2). At lower salinities, modern marine brines are Na-Cl waters that with further concentration and removal of Na as halite evolve in Mg-SO4-Cl bitterns (Figure 2)


The next chemical divide reached by concentrating marine Phanerozoic brines (always depleted in HCO3 at the carbonate divide) occurs when gypsum precipitates at around 4-5 times the concentration of the original seawater (Gypsum divide in Figure 1). As gypsum continues to precipitate, either the Ca in the brine is used up, or the SO4 in the brine is used up. If the Ca is depleted, a calcium-free brine rich in Na, K, Mg, Cl, and SO4 will be the final product and the diagnostic bittern minerals will include magnesium sulphate minerals. This is the pathway followed by modern seawater bitterns. If, however, the sulphate is used up via gypsum precipitation, the final brine will be rich in Na, K, Mg, Ca, and Cl. Such a sulphate-depleted brine precipitates diagnostic potassium and magnesium chloride minerals such as sylvite and carnallite. If calcium-chloride levels are very high, then diagnostic (but uncommon) minerals such as tachyhydrite (CaCl2.2MgCl2.12H2O), and antarcticite (CaCl2.6H2O) can precipitate from this brine. But both these assemblages contain no sulphate bittern minerals, making potash processing relatively straightforward (Warren, 2016). In Phanerozoic marine salt assemblages, tachyhydrite, which is highly hygroscopic, is present in moderate quantities only in Cretaceous (Aptian) marine sylvite-carnallite associations in the circum-Atlantic potash basins and the Cretaceous (Albian) Maha Sarakham salts of Thailand, along with its equivalents in Laos and western China. The CaCl2-entraining bittern mineral assemblages of these deposits imply ionic proportions of Cretaceous seawater differ from those of today.

Inclusion evidence

Based on a study of brine inclusion chemistry preserved in halite chevrons, from the Early Cretaceous (Aptian, 121.0–112.2 Ma) of the Sergipe Basin, Brazil, the Congo Basin, Republic of the Congo, and the Early to Late Cretaceous (Albian to Cenomanian, 112.2–93.5 Ma) of the Khorat Plateau, Laos and Thailand, Timofeeff et al. (2006) defined a very different chemical makeup for Cretaceous seawater, compared to that of today. Brine proportions in the fluid inclusions in these halites indicate that Cretaceous seawaters were enriched several fold in Ca, depleted in Na and Mg, and had lower Na/Cl, Mg/Ca, and Mg/K ratios compared to modern seawater (Table 1). 

Elevated Ca concentrations, with Ca>SO4 at the gypsum divide, allowed Cretaceous seawater to evolve into Mg–Ca–Na–K–Cl brines lacking measurable sulphate. Aptian seawater was extreme in its Ca enrichment, more than three times higher than present day seawater, with a Mg/Ca ratio of 1.1–1.3. Younger, Albian-Cenomanian seawater had lower Ca concentrations, and a higher Mg/Ca ratio of 1.2–1.7. Cretaceous (Aptian) seawater has the lowest Mg/Ca ratios so far documented in any Phanerozoic seawater from fluid inclusions in halite, and lies well within the range chemically favourable for precipitation of low-Mg calcite ooids and cements in the marine realm.

Likewise, a detailed analysis of the ionic make-up of Silurian seawater using micro-inclusion analysis of more than 100 samples of chevron halite from various Silurian deposits around the world was published by Brennan and Lowenstein (2002), clearly supports the notion that ionic proportions in the world’s Silurian oceans were different from those of today (Figure 3). Samples were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and the Early Silurian in the Canning Basin (Australia) in the Mallowa Salt of the Carribuddy Group. The Silurian ocean had lower concentrations of Mg, Na, and SO4, and much higher concentrations of Ca relative to the ocean’s present-day composition (Table 1). Furthermore, Silurian seawater had Ca in excess of SO4. Bittern stage evaporation of Silurian seawater produced KCl-type potash minerals that lack the MgSO4-type late stage salts formed during the evaporation of present-day seawater and allowed sylvite as a primary precipitate. In a similar fashion, work by Kovalevych et al. (1998) on inclusions in primary-bedded halite from many evaporite formations of Northern Pangaea, and subsequent work using micro-analyses of fluid inclusions in numerous chevron halites (Lowenstein et al., 2001, 2003), shows that during the Phanerozoic the chemical composition of marine brines has oscillated between Na-K-Mg-Ca-Cl and Na-K-Mg-Cl-SO4 types. The former does not precipitate MgSO4 salts when concentrated, the latter does (Figure 3). A recent paper by Holt et al. (2014), focusing on chevron halite inclusions from various Carboniferous evaporite basins, further refined the transition from the Palaeozoic CaCl2 high Mg-calcite sea into a MgSO4-enriched aragonite ocean of the Permo-Carboniferous, so showing CaCl2 oceanic chemistry (and sylvite-dominant bitterns) extend somewhat further across the Palaeozoic than previously thought (Figure 4).


More recent work has shown varying sulphate levels in the Phanerozoic ocean rather than Mg/Ca variations are perhaps more significant in controlling aragonite versus calcite at the CaCO3 divide and the associated evolution of MgSO4-enriched versus MgSO4-depleted bittern suites in ancient evaporitic seaways than previously thought. Bots et al. (2011) found experimentally that an increase in dissolved SO4 decreases the Mg/Ca ratio at which calcite is destabilized and aragonite becomes the dominant CaCO3 polymorph in an ancient seaway (Figure 5). This suggests that the Mg/Ca and SO4 thresholds for the onset of ancient calcite seas are significantly lower than previous estimates and that Mg/Ca levels and SO4 levels in ancient seas are mutually dependent. Rather than variations in Mg/Ca ratio in seawater being the prime driver of the aragonite versus calcite ocean chemistries across the Phanerozoic, they conclude sulphate levels are an equally important control.


There is now convincing inclusion-based evidence that the chemistry of seawater has varied across the Phanerozoic from sulphate-depleted to sulphate-enriched, what is not so well understood are the various worldscale processes driving the change (Figure 4). Spencer and Hardie (1990) and Hardie (1996) argued that the level of Mg in the Phanerozoic oceans has been relatively constant across time, but changes in the rate of seafloor spreading have changed the levels of Ca in seawater. This postulate is also supported in publications by Lowenstein et al. (2001, 2003). Timing of the increase of Ca in the world’s oceans was likely synchronous with a decrease in the SO4 ion concentration, which at times was as much as three times lower than the present.

Simple mixing models show that changes in the flux rate of mid-oceanic hydrothermal brines can generate significant changes in the Mg/Ca, Na/K and SO4/Cl ratios in seawater (Table 1). Changes of molal ratios in seawater have generated significant changes in the type and order of potash minerals at the bittern stage. For example, Spencer and Hardie’s (1990) model predicts that an increase of only 10% in the flux of mid-ocean ridge hydrothermal brine over today’s value would create a marine bittern that precipitates sylvite and calcium-chloride salts, as occurred in the Cretaceous instead of the Mg-sulphate minerals expected during bittern evaporation of modern seawater. Such Ca-Cl potash marine bitterns correspond to times of “calcite oceans” and contrast with the lower calcium, higher magnesium, higher sulphate “aragonite oceans” of the Permo-Triassic and the Neogene (Figure 3; Hardie, 1996; Demicco et al., 2005).

Ocean crust, through its interaction with hydrothermally circulated seawater, is a sink for Mg and a source of Ca, predominantly via the formation of smectite, chlorite, and saponite via alteration of pillow basalts, sheeted dykes, and gabbros (Müller et al., 2013). Additional removal of Mg and Ca occurs during the formation of vein and vesicle-filling carbonate and carbonate-cemented breccias in basalts via interaction with low-temperature hydrothermal fluids. Hence, changing rates of seafloor spreading and ridge length likely influenced ionic proportions in the Phanerozoic ocean and this in turn controlled marine bittern proportions.

According to Müller et al., 2013, hydrothermal ocean inputs are and the relevant ionic proportions in seawater are driven by supercontinent cycles and the associated gradual growth and destruction of mid-ocean ridges and their relatively cool flanks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge flank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fluid flux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcification from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their flanks along the Pacific rim.

Holland et al. (1996), while agreeing that there are changes in ionic proportion of Phanerozoic seawater and that halite micro-inclusions preserve evidence of these changes, recalculated the effects of changing seafloor spreading rates on global seawater chemistry used by Hardie and others. They concluded changes in ionic proportions from such changes in seafloor spreading rate were modest. Instead, they pointed out that the composition of seawater can be seriously affected by secular changes in the proportion of platform carbonate dolomitised during evaporative concentration, without the need to invoke hydrothermally driven changes in seawater composition. In a later paper, Holland and Zimmermann (2000) suggest changes in the level of Mg in seawater were such that the molar Mg/Ca ratio of the more saline Palaeozoic global seawater (based on dolomite volume) was twice the present value of 5.

Using micro-inclusion studies of halites of varying ages, Zimmermann (2000a, b) has proposed that the evolving chemistry of the Phanerozoic ocean is more indicative of changing volumes of dolomite than it is of changes in the rates of seafloor spreading . Using halite inclusions, she showed that the level of Mg in seawater has increased from ≈38 mmol/kg H2O to 55 mmol/kg H2O in the past 40 million years (Figure 6). This increase is accompanied by an equimolar increase in the level of oceanic sulphate. Over the longer time frame of the Palaeozoic to the present the decrease in Mg/Ca ratio corresponds to a shift in the locus of major marine calcium carbonate deposition from Palaeozoic shelves to the deep oceans, a change tied to the evolution of the nannoplankton. Prior to the evolution of foraminifera and coccoliths, some 150 Ma, the amount of calcium carbonate accumulating in the open ocean was minimal. Since then, a progressively larger portion of calcium carbonate has been deposited on the floor of the deep ocean. Dolomitization of these deepwater carbonates has been minor.


In a study of boron isotopes in inclusions in chevron halite, Paris et al. (2010) mapped out the changes in marine boron isotope compositions over the past 40 million years (Figure 7). They propose that the correlation between δ11BSW and Mg/Ca reflects the influence of riverine fluxes on the Cenozoic evolution of oceanic chemical composition. Himalayan uplift is a major tectonic set of events that probably led to a 2.5 times increase of sediment delivery by rivers to the ocean over the past 40 m.y. They argue that chemical weathering fluxes and mechanical erosion fluxes are coupled so that the formation of the Himalaya favoured chemical weathering and hence CO2 consumption. The increased siliciclastic flux and associated weathering products led to a concomitant increase in the influx levels of Mg and Ca into the mid to late Tertiary oceans. However the levels of Ca in the world’s ocean are largely biologically limited (mostly by calcareous nannoplankton and plankton), so leading to an increase in the Mg/Ca ratio in the Neogene ocean.


a study of CaCO3 veins in ocean basement, utilising 10 cored and documented drilled sites, Rausch et al. (2013) found for the period from 165 - 30 Ma the Mg/Ca and the Sr/Ca ratios were relatively constant (1.22-2.03 mol/mol and 4.46-6.62 mmol/mol respectively (Figure 8). From 30 Ma to 2.3 Ma there was a steady increase in the Mg/Ca ratio by a factor of 3, mimicking the brine inclusion results in chevron halite. The authors suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. They go on to note that additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate. However, world-scale factors driving the increase in Mg in the world’s oceans over the past 40 million years are still not clear and are even more nebulous the further back in time we look.


Changes in Phanerozoic ocean salinity

As well as changes in Mg/Ca and SO4, the salinity of the Phanerozoic oceans shows a fluctuating but overall general decrease from the earliest Cambrian to the Present (Figure 9; Hay et al. 2006). The greatest falls in salinity are related to major extractions of NaCl into a young ocean (extensional continent-continent proximity) or foreland (compressional continent-continent proximity) ocean basins (Chapter 5). Phanerozoic seas were at their freshest in the Late Cretaceous, some 80 Ma, not today. This is because a substantial part of the Mesozoic salt mass, deposited in the megahalites of the circum-Atlantic and circum-Tethyan basins, has since been recycled back into today’s ocean via a combination of dissolution and halokinesis. Periods characterised by marked decreases in salinity (Figure 9) define times of mega-evaporite precipitation, while periods of somewhat more gradual increases in salinity define times when portions of this salt were recycled back into the oceans (Chapter 5).

The last major extractions of salt from the ocean occurred during the late Miocene in the various Mediterranean Messinian basins created by the collision of Eurasia with North Africa. This was shortly after a large-scale extraction of ocean water from the ocean to the ice cap of Antarctica and the deposition of the Middle Miocene (Badenian) Red Sea rift evaporites. Accordingly, salinities in the early Miocene oceans were between 37‰ and 39‰ compared to the 35‰ of today (Figure 9). The preceding Mesozoic period was a time of generally declining salinity associated with the salt extractions in the opening North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous) and the earliest Cambrian oceans also had some of the highest salinities in the Phanerozoic. Recently, work by Blättler and Higgins (2014) utilising Ca isotopes studies of selected Phanerozoic evaporites has confirmed the dichotomous nature of Phanerozoic ocean chemistry that was previously defined by micro-inclusion studies of chevron halite (Figure 3).

So what?

In summary, based on a growing database of worldwide synchronous changes in brine chemistry in fluid inclusions in chevron halite, echinoid fragments, vein calcites at spreading centres and Ca isotope variations, most evaporite workers would now agree that there were secular changes in Phanerozoic seawater chemistry and salinity. Ocean chemistries ranged from MgSO4-enriched to MgSO4-depleted oceans, which in turn drove the two potash endmembers What is not yet clear is what is the dominant plate-scale driving mechanism (seafloor spreading versus dolomitisation versus uplift/weathering) that is driving these changes.

In terms of marine bitterns controlling favourable potash ore associations, it is now clear that the variation in ionic proportions in the original seawater controls whether or not potash-precipitating bitterns are sulphate enriched or sulphate depleted. A lack of MgSO4 minerals as co-precipitates in a sylvite ore makes the ore processing methodology cheaper and easier (Warren, 2016). Understanding the ionic proportion chemistry of Phanerozoic seawater is a useful first-order exploration tool in ranking potash-entraining evaporite basins across the Phanerozoic.


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