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 (

Salt, Oil, Gas & Metals: What Drives the Link?

John Warren - Friday, March 31, 2017


This article is based on a review written for CSPG Reservoirs and presented orally at the AAPG 2016 meeting in Calgary. It endeavours to give an up-to-date synopsis of how and why ancient salty basins tend to contain elevated levels of oil, gas and metals. It begins with an overview of hydrologies, then places ancient salt bodies in their climatic and tectonic context, and lastly looks at industrial associations and predictors. For brevity, details of many regions, deposits and references are summarised as tables, more complete discussion and referencing can be found in appropriate sections in Warren (2016), or feel free to contact me ( or via with requests for more comprehensive documentation of particular examples.

Evaporite styles reflect intrabasin brine hydrology

If we accept the definition of an evaporite as “A salt rock originally precipitated from a saturated surface or near-surface brine in hydrologies driven by solar evaporation,” then the greater volume of saline mineral salts in the earth’s sedimentary realm are the product of solar heating of brine. There are other sets of mineral salts in the depositional and diagenetic realm with the same mineral composition as evaporite salts, but these salts result from cryogenic, hydrothermal or burial re-equilibration processes (Warren, 2016). For a water molecule to escape into the vapour phase in an evaporitic setting, and so increase the salinity of an enduring brine body or pore water in the capillary zone of a sabkha, the water molecule must; 1) absorb heat energy, 2) be located near the liquid surface, 3) be moving in the proper direction, 4) have sufficient energy to overcome liquid-phase intermolecular forces and, 5) pass through the surface tension inter-face (Figure 1).

Simple physics of molecular escape (rate and intensity) during solar evaporation essentially controls the potency of re-maining brine. Bed textures and mineralogies entomb evidence of concentration levels in the brine and its hydrological setting/stability (Figures 2, 3). This makes evaporites excellent gauges of climate present and past. In combination, relative brine density and specific heat capacity of adjacent brine masses control density and thermal stratification in saline brine bodies located at or near the earth’s surface (Figure 2). Subsequent burial alteration controls later textural evolution by interactions with regional shallow and deep phreatic crossflows (Warren, 2016).

Specific heat is the amount of heat needed to raise one gramme of a substance by 1 °C. For a given amount of heat input, a unit volume of hypersaline water will show a greater increase in temperature than a less salty water (Figure 2c). Given the same degree of insolation, this means density-stratified water bodies tend to be heliothermic, with the lower, denser bottom brine layer being warmer than the somewhat less saline, less dense, upper water layer (Figure 2a). The level in any water column where a marked change in temperature occurs is the thermocline, and in a density-stratified brine mass corresponds to the halocline, also termed a chemocline (Figure 3a). The combination of high temperature, high salinity and lower oxygen levels in the lower brine mass in a heliothermal brine system means only a specialised biota can survive there, often with specialised bacterial populations living in waters just above a saline thermocline. A modern example is the purple sulphur-oxidising community flourishing immediately above the halocline in Lake Mahoney, British Columbia. Fluctuating salinity and nutrient levels endemic to many evaporite depositing regions encourage preservation of elevated levels of organic matter in a variety of hypersaline settings past and present (“feast and famine” associations; Warren, 2011).

As any brine concentrates, its density increases (Figure 2b). The overlying water body must be holomictic for bottom-nucleated salts to accumulate across the subaqueous floor of a brine mass and for dense, saturated brine to sink (reflux) into underlying sediments (Figure 3a). Holomixis means a near homogenous distribution of brine density, temperature, and salinity throughout the brine mass, with internal mixing being ongoing and mostly maintained by wind movement. In contrast, a meromictic brine body is internally stratified, with a lower more-saline, denser, warmer water mass separated across a halocline from an upper, less saline, less-dense, cooler water mass. A longterm halocline hinders chemical or physical changes in the underlying denser waters, so shutting down bottom nucleation, as well as slowing and ultimately stopping brine reflux. A permanently stratified system is ectogenic, while a brine column that is temporarily stratified is endogenic.

Over decades, saline water masses can change from ectogenic to endogenic. In February 1979, salinity equalisation drove the mixing of upper and lower water masses in the Dead Sea, resulting in a holomictic water body. Since then, aside from short episodes of storm-flood-driven freshening of the upper water mass in 1980 and 1994 (meromixis), the Dead Sea has been holomictic, and halite has been accumulating on the deep lake floor (Gertman and Hecht, 2002). Before 1979, the Dead Sea had been a stratified system for at least 400 years and only pelagic carbonate laminites with minor gypsum, not halite, accumulated on the deep lake floor, beneath a 370-380 m deep brine column.

Holomixis permits deposition of a coherent salt layer across the whole basin floor, beneath both shallow and deep brine columns. Density stratification allows evaporitic salts to crystallise only in the upper water mass or at the upper brine - lower brine interface, so bottom nucleation tends to occur on the shallower lake floor, where it lies above the halocline (Figure 3a). That is, long-term (ectogenic) column stratification mean bottom nucleation of salts can only occur where the upper salt-saturated brine mass intersects the sediment bottom, with a pelagic settling of salts occurring deeper out in the depositional basin, as in the Dead Sea prior to February 1979 (Figure 3a). The bottom growth of crystals cannot occur on a deep bottom located beneath a density-stratified system, as there is no mechanism to drive ongoing supersaturation in the lower water mass. For the same reason, constant brine reflux driving sinking of a dense brine into sediments beneath the floor of the evaporite basin can only proceed if significant regions of the overlying brine mass are holomictic. Deposition of capillary salts (sabkha deposits) occurs in subaerial settings, wherever the saline capillary zone inter-sects the land surface (Figure 3b).

When salts are accumulating beneath a holomictic brine mass, textures in bottom nucleates is controlled by the stability of the overlying brine column (Figure 3b). When the overlying column is deep (>30-100m) then, other than areas on the deep bottom of local phreatic spring-fed outflows, there is no general hydrochemical mechanism to drive fluctuations in bottom-brine chemistry. The resulting deep bottom precipitates tend to be monomineralogic crystal clusters, possibly encased by re-transported material washed in from the shallower surrounds (Figure 3a, 3b). In contrast, when the overlying brine column is shallow (<30m and typically <5-10m) then the chemistry and stability of the brine varies on a shorter term (daily-weekly) basis, so more layered bi-mineralogic bottom-nucleates can accumulate as layered to laminated salt beds. In addition, all evaporite sediments can be reworked by bottom currents, with similar textures to those that characterise siliciclastic and mechanically-modified carbonate sediments (Figure 3b).

"Now" versus "then" in evaporite deposition

Uniformitarianism is an essential tenet of geological understanding. Yet, when we look at evaporite volumes and depositional settings across deep time, we see that the diversity of modern evaporite analogues is constrained by a deficit in two conditions, specifically; 1) the current lack of greenhouse eustasy; contemporaneous atmospheric conditions and sea levels are controlled by the earth’s current icehouse climate mode and have been for the last 10-12 million years, and 2) the current lack at the plate-edge scale of marine seepage into large hydrographically-isolated oceanic sump basins (Warren, 2010). Both situations circumscribe different hydrologies and eustasies compared to continental-fed in-flows that typify the world's current larger evaporite basins. Today, and across the Quaternary, the largest and thickest salt stacks, with areal extents up to 10,000 km2 and thickness up to 900 m, tend to precipitate in the lower parts of suprasealevel intermontane lacustrine sumps located in tectonically active parts of continental interiors, such as Salar Atacama and Salar di Uyuni in the Andean Altiplano (Figure 4a). Most ancient evaporites are marine-fed and were deposited in huge hydrographically-isolated subsealevel marine-seepage sumps located in intracratonic basins or within rifts or compressional sutures. Often, the areal extents of these ancient systems were more than 250,000 km2, this is more than two orders of magnitude larger than any Quaternary evaporite deposit. Bedded (pre-halokinetic) thicknesses could be more than a kilometre.

Ancient marine saline giants (megahalites and megasulphates) accrued in either of two plate-scale settings, which at times merged into one another, namely; 1) Platform evaporites (Figure 5) and 2) Basinwide evaporites (Figure 6). The first major contrast with nonmarine continental dominance in Quaternary evaporite settings is the fact that platform evaporites require greenhouse eustasy, the second is that basinwide evaporites require tectonically- and hydrographically-isolated widespread subsealevel depressions, typically found along plate edges with continent-continent proximity (Figure 5).

Neither condition is present on the current earth surface. For basinwides, suitable hydrologic conditions were last present during the Messinian Salinity Crisis in the Mediterranean region, and platform evaporite settings were last present on earth across large parts of the Middle East carbonate platform during the Eocene (Tables 1, 2). There is a third group of ancient evaporite deposits; it encompasses all nonmarine lacustrine beds past and present (Table 3). This group has same-scale modern-ancient counterparts, unlike ancient marine platform and basinwide evaporites (Figure 4a; Warren, 2010, 2016).

Platform evaporites

Are made up of stratiform beds, usually <50 m thick and composed of stacked <1 to 5 m thick parasequences or evaporite cycles, with a variably-present restricted-marine carbonate unit at a cycle base (Table 1). Salts were deposited as mixed evaporitic mudflat and saltern evaporites, sometimes with local accumulations of bittern salts. Typically, platform salts were deposited in laterally extensive (>50-100 km wide), hydrographically-isolated, subsealevel marine-seepage lagoons (salterns) or evaporitic mudflats (sabkhas and salinas). These regions had no same-scale modern coun-terparts and extended as widespread depositional sheets across large portions of hydrographically isolated marine platform areas, which passed seaward across a subaerial seepage barrier into open marine sediments (Figure 5). In marine margin epeiric settings, such as the Jurassic Arab/Hith and Permian Khuff cycles of the Middle East or the Cretaceous Ferry Lake Anhydrite in the Gulf of Mexico, these platform evaporites are intercalated with shoalwater marine-influenced carbonate shelf/ramp sediments, which in turn pass basinward across a subaerial sill into open marine carbonates. Landward they pass into arid zone continental siliciclastics or carbonate mudflats.

Platform evaporite deposition occurred in both pericontinental and epicontinental settings, at times of low-amplitude 4th and 5th order sealevel changes, which typify greenhouse eustasy (Figure 5; Warren, 2010). Platform evaporites also typify the saline stages of some intracratonic basins. Platform evaporites cannot form in the high-amplitude, high-frequency sealevel changes of icehouse eustasy. The 100m+ amplitude oscillations of icehouse times mean sealevel falls off the shelf edge every 100,000 years, so any evaporite that had formed on the platform is subaerially exposed and leached. Fourth order high-amplitude icehouse eustatic cycles also tend to prevent laterally-continuous carbonate sediment barriers forming at the top of the shelf to slope break and so icehouse evaporite systems tend not to be hydrographically isolated (drawdown) at the platform scale. Rather icehouse eustasy favours nonmarine evaporites as the dominant style, along with small ephemeral marine-margin salt bodies, as seen today in the bedded Holocene halites and gypsums of Lake Macleod in coastal West Australia.

Ancient platform evaporite successions may contain halite beds, especially in intracratonic basinwide settings, but periplatform settings, outside of intracratonic basins, are typically dominated by 5–40 m thick Ca-sulphate beds intercalated with normal-marine platform carbonates (Table 1). The lateral extent of these epeiric platform sulphate bodies, like the Middle Anhydrite Member of the Permian Khuff Fm. of Saudi Arabia and the UAE, with a current area of more than 1,206,700 km2, constitute some of the most aerially-extensive evaporite beds ever deposited.

Basinwide evaporites

Are made up of thick evaporite units >50–100 m thick made up of varying combinations of deepwater and shallow wa-]ter evaporites (Table 2). They retain textural evidence of different but synchronous local depositional settings, including mudflat, saltern, slope and basin (Figures 6). When basinwide evaporite deposition occurs, the whole basin hydrology is evaporitic, holomictic, and typically saturated with the same mineral phase across vast areas of the basin floor, as in the Dead Sea basin today. The Dead Sea has a more limited lateral scale than ancient basinwides but currently has halite forming simultaneously as; 1) decimeter-thick chevron-dominated beds on the saline-pan floor of the shallow parts around the basin edge in waters typically less than 1-10 metres deep, and 2) as coarse inclusion-poor crystal meshworks of halite on the deep basin floor that sits below a halite-saturated brine column up to hundreds of metres deep (Figure 3a). Ancient basinwide successions are usually dominated by thick massive salt beds, generally more than 100-500 m thick. Deposits are made up of stacked thick halite beds, but can also contain substantial volumes of thick-bedded Ca-sulphate and evaporitic carbonate, as in the intracratonic basinwide accumulations of the Delaware and Otto Fiord Basins (Table 2).

Owing to inherent purity and thickness of the deposited halite, many halite-dominant basinwide beds are also remobilized, via loading or tectonics, into various halokinetic geometries (Hudec and Jackson, 2007). Some basinwide systems (mostly marine-fed intracratonic settings) entrain significant accumulations of marine-fed potash salts, as in the Devoni-an Prairie Evaporite of western Canada. In contrast, all Quaternary examples of commercial potash deposits are accumulating in continental lacustrine systems (Warren 2016; Chapter 11).

Basinwide evaporite deposits are the result of a combination of tectonic and hydrological circumstances that are not currently active on the world’s surface (Figure 4b). They were last active in the Late Miocene (Messinian), in association with soft-suture collision basins tied to the Alpine-Himalaya orogenic belt, and in Middle Miocene (Badenian) basins developed in the early rift stages of the Red Sea. Basinwide systems will be active again in the future at sites and times of appropriate plate-plate interaction, when two continental plate edges are nearby, and the intervening seafloor is in or near a plate-edge rift or suture and is both subsealevel and hydrographically isolated (Figure 6).

Lacustrine (nonmarine) evaporites

Quaternary continental playa/lacustrine are constructed of stratiform salt units, with the greater volume of saline sediment accumulating in lower, more-saline portions of the lacustrine landscape. Beds are usually dominated by nodular gypsum and displacive halite, deposited in extensive evaporitic mudflats and saltpans with textures heavily overprinted by capillary wicking, rather than as bedded bottom-nucleated layers on the subaqueous floors of perennial brine lakes (Figure 3b; Ruch et al., 2012). In ancient counterparts, the total saline lacustrine thickness ranged from meters to hundreds of meters, with lateral extents measured in tens to hundreds of kilometres (Table 3; Figure 4a). Lacustrine salt beds are separated vertically, and usually surrounded by, deposits of lacustrine muds, alluvial fans, ephemeral streams, sheet floods, eolian sands, and redbeds. As today, ancient lacustrine salts accumulated in endorheic or highly restricted discharge basins, with perennial saline water masses tending to occur in the drainage sumps of steep-sided drainage basins (Warren, 2010, 2016). Saline lake basins accumulating gypsum, or more saline salts like halite or glauberite, typically have a shallow water table in peripheral saline mudflat areas and so are dominated by continental sabkha textures. Nearby is the lowermost part of the lacustrine depression or sump where deposition is typified by ephemeral ponded brine pan deposits, rather than permanent saline waters.

Saline lacustrine mineralogies depend on compositions of inflowing waters, so depositional sumps in regions with non-marine ionic proportions in the feeder inflow, accumulate thick sequences of nonmarine bedded salts dominated by trona, glauberite, and thenardite. In contrast, nonmarine areas with thalassic (seawater-like) inflows tend to accumulate more typical sequences of halite, gypsum, and anhydrite.

Across the Quaternary, less-saline perennial saline-lake beds tend to occur during more humid climate periods in the same continental-lacustrine depressions where saline-pan beds form (e.g., Lake Magadi, Great Salt Lake, Lake Urmia). On a smaller scale, in some modern saline lake basins, parts of the lake floor can be permanently located below the water surface (Northern Basin in the Dead Sea or Lake Asal). In some modern saline sumps dominated by mudflats, a perennial saline lake water mass is located toward the edge of a more central salt-flat zone, forming a perennial water filled “moat” facies surrounding a seasonally desiccated saline pan (as in Salar, de Atacama, Salar de Uyuni, Lake Magadi, Lake Natron). These permanent to near-permanent saline water “moat” regions are typically created where fresher inflows encounter saltier beds of the lake centre, dissolve them, and so form water-filled peripheral depressions. Bottom sediment in the moats tend to be mesohaline carbonate laminites, which can contain TOC levels as high as 12%.

High-water stage perennial saline lacustrine sediments tend to be carbonate-rich or silica-rich (diatomaceous) laminites. Ancient examples of large saline lacustrine deposits made up of alternating humid and desiccated lacustrine units include the Eocene Green River Formation of Wyoming and the Permian Pingdiquan Formation of the Junggar Basin, Chi-na (Table 4). Evaporites deposited in a suprasealevel lacustrine basin (especially Neogene deposits) have numerous same-scale Quaternary analogues, unlike the more voluminous ancient marine platform and basinwide evaporites (Figure 4a).

Salt punches above its weight, but why? (facilitator for economic accumulations of oil, gas and metals)

In terms of total mass in sedimentary basins, the proportion of evaporite across the world’s Phanerozoic basins is rarely more than 2% (Figure 7; Ronov, 1980). Today we have comprehensive documentation that salt horizons, their brines, associated dissolution and alteration conduits control significant economic associations of oil, gas and metals (Warren, 2016):

  • 50% of world’s carbonate reservoirs (seals, traps and source rocks)
  • All the world’s supergiant oil and gas fields in thrusts (seals and structural traps).
  • All supergiant sedimentary copper deposits (halokinetic brine focus)
  • 50% of world’s giant SedEx deposits (halokinetic brine focus).
  • 80% of giant MVT deposits (sulphate-fixer & brine interface)
  • World’s largest Phanerozoic Ni deposit (meta-igneous – Noril’sk).
  • Many larger IOCG deposits (meta-evaporite, brine and hydrothermal).
  • This enrichment runs counter to a proportion of 2% of the world's Phanerozoic sediments. The exact why or how of these associations is still not well understood. Most geologists working with oil, gas or metal buildups in a salt-rich basin will come to have a suspicion, and for some, the conviction, that salt or its subsurface alteration plays a role in defining the position or enrichment level of the commodity of interest. Evaporite masses in the subsurface, especially if halite-dominant, enable both physical and chemical alterations, which tend to improve economic prospectivity. The unique properties of salt in the diagenetic realm tend to facilitate, focus and stabilise processes that lead to elevated levels of accumulations of various commodities (Figure 8). That is, evaporites in a basin tend to enhance the volume of oil, gas or metals in an accumulation, but are not necessarily the direct cause of the accumulation/precipitation.

    Evaporite-hydrocarbon association

    When the reservoired hydrocarbon below a salt seal is oil, little or no leakage can take place through a laterally continuous evaporite. Even when the reservoired hydrocarbon is methane, little or no loss occurs, even by diffusion (Ehgartner et al., 1998). The much greater efficiency of evaporite seals, compared to shales, is clearly seen in the total hydrocarbon volumes held back by the two lithologies. Total worldwide shale sediment volume in the Earth's sedimentary crust is more that an order of magnitude greater than that of evaporites, yet the split between reservoired hydrocarbons below a shale or an evaporite seal is roughly 50:50 (Grunau, 1987). Likewise, typical volumes of salt-sealed giant fields dis-covered in the last decade are much grander than that held in mudrock-sealed systems (Bai and Yu, 2014).

    Of the 120 giant oil and gas fields discovered in the period 2000 - 2012 some 54.6 % are hosted in marine carbonates and 12% in lacustrine carbonates, meaning less than a third of new giant discoveries are in siliciclastic reservoirs (Figure 9a). Some 56% of these oil and gas giants have an evaporite seal, with 82% of the marine carbonates having an evaporite seal and 91% of the lacustrine carbonates having an evaporite seal (Figure 9b, c). Clearly, carbonate reservoirs with evaporite seals constitute most of the giant oil and gas discoveries across the period 2000-2012, and the proportions of this association are likely to increase in conventional discoveries across the next decades (Bai and Yu, 2014).

    Of these 120 oil and gas giants, one is a megagiant field with a recoverable reserve of 50 Bboe (Billion barrels oil equivalent) or more, and 6 are supergiant fields with recoverable reserves of 5 Bboe or more. The seven largest fields are: Galkynysh (aka Yolotan or Osman) gas field in the Ama-Darya Basin of Turkmenistan with an original 2P reserve of 67.1 Bboe, making it likely the second largest known gas field in the world; Kashagan oil field (18.1 Bboe) in the North Caspian basin; Kish 2 gas field in the Arabian Basin (Iran): Lula (previously known as Tupi), Franco and Libra oil fields in the Santos Basin of offshore Brazil; and Sulige gas field (5.7 Bboe) in the Ordos Basin, China. In this listing, the geology of Galkynysh is not yet reliably published, but of the remaining 6, only Sulige does not have an evaporite association.

    Typical bedded evaporite seals, especially beds composed of monomineralogic assemblages such as massive nodular anhydrite or massive halite and have measured entry pressures more than 3000 psi. Most impure evaporite beds have entry pressures greater than 1000 psi, as do many evaporite-plugged reflux dolomites. Contrast this with most shales, which tend to be water-bearing (mostly bound and structural water), with typical entry pressures between 900 and 1500 psi (Sneider et al., 1997). Although such shales are respectable seals, over time shale allows substantial diffu-sive leakage of methane and even liquid hydrocarbons via inherent microporosity, less so if the shales are organic-rich. Even ignoring halite's ability to reanneal and flow under stress, any evaporite seal has much lower intrinsic permeability than shale, and this helps maintain its seal integrity. With permeabilities of 10-7 md, a hydraulic gradient of 0.01, and a porosity less than  0.01, a brine would take somewhere between 3 and 30 million years to flow 1 metre into an unfractured halite seal. For anhydrite, which has permeability some 100 times higher than halite, a brine would take between 30,000 and 300,000 years to flow 1 metre into the seal (Beauheim and Roberts, 2002).

    Evaporites are excellent longterm seals to substantial hydrocarbon columns. This applies to any siliciclastic or carbonate reservoir adjacent to bedded or halokinetic salty seals (Figure 10). The exemplary ability of bedded evaporites to act as seals holding back massive hydrocarbon columns is clearly seen in the Middle East, where Ghawar, the world’s largest oil field, is sealed by bedded anhydrites of the Jurassic Arab D Formation and the overlying Hith Anhydrite seal. This evaporite seal association in Ghawar holds back an estimated remaining reserve of more than 100-200 billion barrels. Bedded platform anhydrites also seal Safaniya, the world’s largest offshore field, also in Saudi Arabia, with estimated reserves of more than 25-30 billion barrels of oil and 5 billion cubic feet of natural gas. Likewise, Permian platform anhydrites are the regional seal to North Field in offshore Qatar, the world’s largest single gas field (non-associated gas) with more than 500 tcf of reserves (Alsharhan and Nairn, 1997). This gas is reservoired and sealed in the evaporitic dolomites of the Permian Khuff Formation and recent announcements by the Qatari Government have postulated more than 900 tcf of certified non-associated gas sealed in the North Field structure.

    In terms of physical processes, buried near-pure halite beds and masses (rock salt) in the sedimentary realm are rheologically unusual compared to other nearby sediments, in that at geological time scales rock salt can set up deformation responses that mimic Newtonian fluid responses. At the same time, intercrystalline textures in the flowing salt mass maintain seal integrity, even as local crystals dissolve and reprecipitate. That is, down to depths of 6-8 km rock salt flows and maintains seal integrity, while adjacent non-salt sediments tend to fault and fracture. The ability of bedded salt to seal (Figure 10a) and of flowing salt to create and seal supra-, intra- and sub-salt reservoirs is well documented (Figure 10b).

    Dense evaporitic brines can pass through adjacent or underlying sediments both at the time the bedded salts are accumulating, or later as a subsurface salt mass dissolves. For example, chemical responses, created within the set-up hydrology of widespread salt deposition and early burial, drive brine reflux, moving magnesium-rich modified seawater brines into and through the underlying carbonate sediments. This hydrology crafts broad reflux dolomite haloes, with local burial anhydrite patches and overall improves intercrystalline connectivity, at least until the dolomite reservoir becomes overdolomitised (Figure 11). Once this happens, all effective polyhedral porosity is lost in the dolomite and the reservoir potential is destroyed. Later dissolution brines can create hydrothermal waters capable of leaching and burial dolomitization, as in the burial-dolomite reservoirs of the Western Canada Sedimentary Basin (Davies and Smith, 2006).

    Organic-hydrocarbon association

    As long ago as the middle of last century, Weeks (1961) emphasised the importance of evaporites as seals to many of the world's major hydrocarbon accumulations. He also pointed out that many of the cycles of deposition that involve organic-rich carbonate marls or muds also end with evaporites. Clearly, in evaporitic settings, there is an association with Type I-II hydrogen-prone kerogens in mesohaline source rocks, and this is related to the ability of halotolerant photosynthetic algae and cyanobacteria to flourish in periodically mesohaline waters. Such kerogens tend to be oil-prone rather than gas prone and typified by long-chain hydrocarbons (Warren, 2011).

    Much of the mesohaline organic matter preserved in evaporitic carbonates, and the resulting source rocks, originated as planktonic blooms (pelagic “rain from heaven”) or from the benthic biomass (“in situ” accumulations). Such organics typically settled out as seriate pulses of organic matter (often pelleted and laminated) that sank to the bottom of a layered brine column. Each pulse was tied to a short period when surface brines were diluted and halotolerant producers (mostly cyanobacteria and algae) flourished in the freshened lit zone. That is, laminated mesohaline mudstones that constitute most evaporitic source rocks reflect biological responses to conditions of “feast or famine” in vacillatingly-layered brine bodies (Figure 12; Warren, 1986, 2011).

    Worldwide, studies of schizohaline evaporitic basins have shown that organic-rich mesohaline sediments can accumulate beneath ephemeral surface brines in salterns, or in basin and slope settings in both marine and continental settings (Figure 12; Kirkland and Evans, 1981; Oehler, 1984; Warren, 1986, 2011; Rouchy, 1988; Busson, 1992). The most prolific accumulations of organics in ancient evaporitic settings tend to be laminated micritic carbonates deposited be-neath intermittently stratified moderately-saline (mesohaline) anoxic water columns of varying brine depth.

    There are three, possibly four, major mesohaline density-stratified settings where organic-rich laminites (source rocks) accumulate in saline environments that are also associated with, or evolve into, evaporite deposits (Warren, 2011):

    1) Basin-centre lows in marine-fed evaporitic drawdown basins (basinwide salts).

    2) Mesohaline intrashelf lows atop epeiric evaporitic platforms.

    3) Saline-bottomed lows in perennial underfilled saline lacustrine basins.

    4) Closed seafloor depressions in halokinetic deepwater marine slope and rise terrains.

    The Metal-evaporite association

    In addition to the evaporite hydrocarbon association, there is an association of evaporite settings with the larger of the known MVT, Sedex, Stratiform Sedimentary Copper and some IOCG associations (Figure 13; see Warren 2016, Chapter 15 and 16 for detailed case histories and models). The role of evaporites in focusing metalliferous ore accumulations is two-fold; 1) In solution (halite-dominant precursor) they can act as chloride-rich metal carriers and 2) Locally, as beds or masses (especially of CaSO4), their dissolution products, especially if trapped, can supply sulphur (mostly as bacteriogenic or thermogenic H2S) and also set up chemical interfaces that act as foci in the setup of brine mixing conditions suitable for precipitation of metal sulphides or native elements. Hence, most evaporite-associated ore systems tend to be epigenetic, rather than syngenetic. Subsurface salt beds and masses are merely the solid part of a large ionic recycling system; dissolved metals are another part, and zones of mixing between the two are typically sites where ores tend to accumulate. Halokinesis steadies the position of a redox interface, tied to a salt dissolution brine halo, and enables an extended phase of focus to a metal precipitative (redox) interface at a stable location in subsurface earth-space (Warren, 2016).

    At the world-scale, evaporite-associated metalliferous systems are driven by plate tectonics. Halite-dominated sequences, deposited in the drawdown basin centres, tend to dissolve in burial, and so supply chloride ions to the brine system. Salt beds that are thick enough tend to flow and so focus the upward, and centripetal passage of basinal and hydrothermal fluid flows. Dissolving gypsum or anhydrite beds, typically deposited higher on the basin platform or diagenetically accumulated along salt dissolution edges and touchdowns can supply sulphur, via bacterial or thermochemical sulphate reduction, while simultaneously focusing metalliferous brine flows into the precipitation interface.

    When the chemistries of the dissolving salt beds and the metal carriers interact so that redox fronts, salinity contrasts, and other precipitative interfaces are set up, an ore deposit can form. Thus, in base and precious metal exploration within evaporitic terranes, we are ultimately searching for those parts of a subsurface ionic cycling system where the salt dissolution, salt beds and metal systems have interacted to create economic levels of metalliferous precipitates.

    If salt is more than a seal, then......

    From a time in the 1950's and 1960's when evaporites were mostly seen as seals, knowledge systems developed over the next five decades now allow us to do much more with respect to predictive industrial geology, centred on the following evaporite facts:

    1) The distribution of evaporite depositional textures maps paleotopography across the underlying carbonates and siliciclastics. So, depositional signatures preserved in the seal can be used to map reservoir quality trends in terms of reflux dolomite intensity, anhydrite cement-patch distributions and zones of subaerial diagenesis

    2) Thick beds of halite tend to accumulate in particular plate tectonic settings. Beds deposited in plate-edge saline sumps tend to flow via sediment loading and extension, without later superimposed tectonic stresses. In contrast, bed deposited in intracratonic settings tend to require later externally-imposed tectonic stress in order to flow. Hydrocarbon trapping geometries are tied to particular styles of salt tectonics

    3) Mesohaline source rock distribution is related to evaporite basin architecture and likely fluid escape pathways, which in turn are related to seal type and timing of salt dissolution or halokinetic withdrawal.

    4) Potash ore quality controls are related to the timing of various brine crossflows generated during deposition, mesogenesis and telogenesis.

    5) Positions of likely base metal and copper accumulations relate to set-ups of dissolution-related of redox interfaces, mesogenetic cross-flows, and in some cases, halokinetic geometries. At the plate tectonic scale, these accumulations occur at particular hydrological interfaces within the basin architecture.


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    Lake Nakuru flamingos– Life's response to feast and famine in schizohaline lacustrine hydrologies

    John Warren - Monday, May 23, 2016

    The Flamingo Connection

    An aviator once described Lake Nakuru as “a crucible of pink and crimson fire,” with a million flamingos painting an astonishing band of colour that burst into pieces as the birds took flight (Figure 1).

    Flamingo population levels in Lake Nakuru and any mass “die-offs” are popularly considered as indicators of the environmental health of Nakuru and other lakes in the African rift valley with significant flamingo populations. In 2006, more than 30,000 of the birds were found dead at Nakuru, leaving enough pink carcases to spur an international newspaper to describe the lake as a “flamingo death camp.” Two years prior, 43,800 of the birds had perished at Tanzania’s Lake Manyara, the first major die-off documented at that alkaline, soda-rich lake. Previous mass die-offs occurred at Lake Nakuru and two other Kenyan lakes in 1993, 1995, and 1997, as well as at two lakes in Tanzania in 2002. At the same time, birds were gathering in places they have never been documented before. Since 2006 there have been additional population crashes at Nakuru and Elmentia (Table 1).


    Flamingos numbers in Lake Nakuru are perhaps one of the most visually impressive responses to episodic but very high levels of organic productivity, driven by a well-adapted species feeding in a layered saline water body subject to periodic salinity stress (Warren, 2011). The fluctuating richness of the lake’s flamingo population was dubbed the “flamingo connection” in a benchmark paper by Kirkland and Evans (1981) that considered mesohaline evaporitic carbonates as hydrocarbon source rocks.

    Flamingo biology

    Flamingos (Aves, Phoenicopteridae) are an ancient lineage of long-legged, microphagous, colonial wading birds. Although popularly misperceived as tropical species, flamingo distribution is more closely tied to the great deserts of the world and to hypersaline lake sites, than it is to equatorial regions (Bildstein, 1993). Flamingos are filter feeders that thrive on halotolerant cyanobacterial blooms in mesohaline shallows of saline lakes around the world. This creates the context between flamingos, mesohaline planktonic blooms, and saline lakes, well documented in Lake Nakuru by Vareschi (1982) and first noted in the geological literature in that benchmark Kirkland and Evans paper. Sedimentary textures and structures associated with flamingo lifestyles, where these birds dominate the macrofauna in some modern saline lakes, are described by Scott et al. (2009, 2012). Ancient avian counterparts can leave a characteristic set of trackways and trace fossils (including nest mounds) that can be used to refine mid-late Tertiary lacustrine depositional models (Melchor et al., 2012). Flamingo-like ancestors, which would give rise to modern ducks, even left traces in the shallow saline mudflats of Eocene soda lake sediments that define the saline portions of the trona/nahcolite-bearing sediments of the Green River formation in Utah (Figure 2).

    Two species of flamingo gather in huge numbers in Lake Nakuru and a few other East African rift lakes, namely, the greater and the lesser flamingo (Phoenicoptus ruber roseus and P. minor respectively), with the lesser flamingo having characteristic and spectacular pink-red colouration in their feathers. These bright pink waders feed and breed in mesohaline rift lake waters where cyanobacterial blooms can be so dense that a Secchi disc disappears within a few centimetres of the lake’s water surface (Warren, 1986, 2011). Lake Natron (Koeppen climate Aw), where trona is the dominant evaporite, is a major breeding ground for flamingos in East Africa and is the only regular breeding site for the lesser flamingo in Africa (Simmons 1995). Lesser flamingos build nesting platforms on the trona pavement in the more central parts of the lake. These spectacular birds not only feed in saline waters, they can choose to build nest mounds on evaporite pavements!

    Worldwide, only six sites are used for breeding by the lesser flamingos: Lake Natron (Tanzania), Etosha Pan (Namibia), Makgadikgadi-Pan (Botswana), Kamfers Dam (South Africa), as well as two pans in the “Little Rann of Kachchh” (India). Recent estimates of lesser flamingos at the main distribution areas are as follows: 1.5–2.5 million in eastern Africa; 390,000 in northwestern India; 55,000-65,000 in southwestern Africa; and 15,000–25,000 in western Africa. The highest population densities occur in Kenya (1.5 million) and Tanzania (600,000) (Childress et al. 2008). Lesser flamingos are well adapted to the harsh conditions associated with living and breeding in hypersaline alkaline conditions. Worldwide, they follow an itinerant lifestyle, ranging across their distribution areas in search of saline water bodies with appropriate cyanobacterial blooms. In the east African rift the flocks can travel up to 200 km a day between feeding and breeding sites, which are generally at geographically separate locations (Figure 3).

    Lake Natron has the highest concentration of breeding flamingos of any lake in East Africa. Both the greater and the lesser flamingo are found there, with the lesser flamingo outnumbering the greater by a hundred to one. Lesser flamingos bred at Lake Natron in 9 out of 14 years from 1954 to 1967. But while the trona-rich nearby Lake Natron is an essential breeding site, it is not a focal feeding site for flamingos. Major feeding sites in the Africa rift valley are Lakes Nakuru and Bogoria (formerly Lake Hannington) in Kenya and entrain waters that are mesohaline with an abundance of halotolerant cyanobacteria, dominated by Arthrospira (Schagerl et al., 2015). Lake Nakuru is a mesohaline soda lake with a pH ≈ 10.5 and a typical annual salinity range of 15-45‰, nearby lake Bogoria is somewhat larger, also an alkaline soda lake, with somewhat higher salinities. The Lake Nakuru depression measures some 6.5 km by 10 km, with a water covered area of some 5-45 km,2 experiencing an annual pan evaporation rate of 1500 mm beneath a Cfb Köppen climate (Figure 3b; Vareschi,1982; Krienitz and Kotut, 2010). It contains a eutrophic bottom water mass in the lake centre, with thermally stratified water column that can be up to 4.5 metres deep. The Lake Bogoria’ water mass is up to 4 km wide, some 17 km long with thermally stratified eutrophic waters up to 10 m deep. It lies beneath a Cfb climate with a pan evaporation of 2600 mm. It is fed by a combination of rainfall (≈760mm/year) and numerous (>200) hydrothermal springs about the lake edge (Figure 3c).

    Lake Bogoria hydrochemistry, it seems, is more stable compared to other endorheic lakes in Kenya, because of its greater depth (10 m), steep shores and larger water volume preventing it from drying up. In contrast, Lake Nakuru resembles more a flat pan; it is much shallower (1 m) and is more subject to changes in size related to changes in water levels, at times Lake Nakuru can dry up completely. Water depths in both lakes vary from year to year, in large part depending on the vagaries of annual runoff/ inflow. The higher level of hydrothermal inflow in Bogoria’s hydrology, along with its somewhat steeper hydrographic profile, means the lake area salinity and water depth vary less from year to year compared to Nakuru (Figure 4).

    Lakes Nakuru and Bogoria, which at peak breeding times can support lesser flamingo populations in excess of 1.5 million birds, have surface areas that are less than half those of lakes Natron and Magadi, which in turn have small areas compared to most ancient lacustrine evaporites. Yet Nakuru and Bogoria are two of the most organically productive ecosystems in the world (Warren, 2011). What makes both lakes so productive are dense populations of halotolerant cyanobacteria, especially Arthrospira sp, which flourish and periodically reach peak growth in their waters yet at other times organic productivity can crash. The morphological and hydrochemical contrasts likely accounts for the more frequent Arthrospira crashes and biotal community changes in Nakuru when compared to Bogoria and other lakes in the region (Table 1).

    Flamingos as filter feeders

    Flamingos pass water through their bill filters in two ways (as documented by Penelope M. Jenkin in her classic article of 1957): either by swinging their heads back and forth just below the water surface, so permitting the water to flow passively through the filters of their beak, or by more efficient and more usual system of an active beak pumping. The latter is maintained by a large and powerful tongue that fills a large channel in the lower beak. As it moves rapidly back and forth, up to four times a second, it drawing water and plankton through the beak filters on the backwards pull and expelling it on the forward drive. The tongue’s surface also sports numerous denticles that scrape the collected food from the filters.

    Flamingo beaks have evolved into highly efficient plankton-extraction apparati that exploit the dense cyanobacterial populations periodically found in mesohaline lakes worldwide (Figure 5; Gould, 1987; Bildstein, 1993). The beak is unlike that possessed by any other bird group on earth; the affinity is more to the baleen of whales used to filter planktonic krill from the lit upper waters of world's oceans. A flamingo beak houses a high volume water-filtering system made up of a piston-like tongue and hair-like structures called lamellae made up of rows of fringed platelets that line the inside of the mandible. In the lesser flamingo, the lamellae fibres have the appropriated spacing for capturing coiled filaments of Arthrospira. Lamellar spacings are wider in the beaks of the greater flamingo than those in the beaks of the lesser flamingo so these larger-sized birds are more generalist feeders of lake zooplankton. Thus, in any mesohaline lake where the two bird species feed and co-exist, they do not compete for the same food source. By swinging their upside-down heads from side to side just below the water surface and using the piston-like tongue to swish water through their lamella-lined beaks, flamingos can syphon the lake plankton into their gullets at phenomenal rates. Lesser flamingos can pump and filter as many as 4 beakfuls of plankton-rich water a second. This means some individuals filter upwards of 20,000 litres of water per day.

    How the birds manage to cycle so much brackish to mesohaline waters, while maintaining their osmotic integrity, remains a mystery. When the rift lakes are typified by dense cyanobacterial blooms, each adult bird ingests around 72 g dry weight of Arthrospira per day (Vareschi 1978). This means the Nakuru lesser flamingo flock is able to ingest 50–94% of the daily primary production in the lake (about 60-80 tons). Rates of planktonic renewal in these rift lakes is obviously extremely high and the required rates of biomass production by Arthrospira can be spectacular. Vonshak (1997) reported doubling times of 11–20 hours of Arthrospira in a culture growing under mesohaline conditions at 35°C.

    Flamingos (flamingoes) are mostly nocturnal feeders and will feed for up to 12-13 hours in a 24 hour period. The preferred planktonic food of the lesser flamingo is the cyanobacterium Arthrospira platensis (formerly known as Spirulina platensis), which for much of an average year is the widespread phytoplankton component in Lake Nakuru and Lake Bogoria waters (Figure 6). Unfortunately, dense populations of Arthrospira function best at a preferred range of temperature and salinity, meaning acme populations tend to collapse irregularly and unpredictably in both Lake Nakuru and Bogoria, leading to highly-stressed malnourished flocks of flamingos subject to mass dieback (Figure 6; Vareschi 1978; Kreinitz and Kotut, 2010).

    Arthrospira has high levels of the red pigment phycoerythrin and so when ingested in large volumes it accumulates in flamingo feathers to give the birds their world famous colouration, hence the “flamingo connection.” Once digested, the carotenoid pigment dissolves in fats, which are then deposited in the growing feathers. The same effect is seen when shrimp change colour during cooking due to carotenoid alteration. The amount of pigment laid down in the feathers depends on the quantity of pigment in the flamingo’s diet. Lesser flamingos, with beak design maximised to feed on Arthrospira have a more intense pink colour in their feathers than greater flamingos. The latter species sits higher in the Lake Nakuru food chain and so gets the slight pink tinge in its feather colour, mostly second-hand from the lake zooplankton, which also feed on Arthrospira.

    As well as possessing very high levels of phycoerythrin in its cytoplasm, Arthrospira is also unusual among the cyanobacteria in its unusually high protein content (some ten times that of soya). This, and the high growth rate of this species, explains why Nakuru and Bogoria acme populations can support such spectacular population levels of flamingos. A lack of cellulose in the Arthrospira cell wall means it is a source of plant protein readily absorbed by the gut, making it a potentially harvestable human food source in saline water bodies in regions of desertification. In Lake Chad, and in some saline lakes in Mexico, Arthrospira accumulates as a lake edge scum that has been harvested for millennia by the local people (including the ancient Aztecs) and used to make nutritious biscuits.

    When Arthrospira stocks are low in the rift lakes, the lesser flamingo will consume benthic diatoms. However, net primary productivity of benthic diatoms in East African soda lakes is one to two orders of magnitude less than that of Arthrospira, and the carrying capacity of the habitat with diatoms is lower by the same order (Tuite 2000). This lower productivity is seen in the peak 25,000 bird population, which are diatom feeders, in Laguna de Pozuelos in Argentina (area ≈100-130 km2, more than 3 times that of Nakuru, yet the peak flamingo numbers are two order of magnitude lower). In Lake Nakuru, Arthrospira and other lake plankton are also consumed by one species of introduced tilapid fish and one species of copepod and a crustacean. Rotifers, waterboatmen, and midge larvae also flourish in the mesohaline waters of Lake Nakuru. The mouth-breeding tilapid Sarotherodon alcalicum grahami was introduced to the lake in the 1950s to control the mosquito problem and fish-feeding birds (such as pelicans) have flourished ever since (Vareschi, 1978). During times of non-optimum water conditions, when either freshening or somewhat elevated hypersalinity lessens the number of Arthrospira, the tilapids can displace the flamingos as the primary consumers of planktonic algae.

    In 1972 Lake Nakuru waters held a surface biomass of 270 g/m3 and an average biomass of 194g/m3 but, as in most hypersaline ecosystems, Nakuru’s organic production rate varies drastically from year to year as water conditions fluctuate (Figure 7; Vareschi, 1978). Arthrospira was in a long-lasting bloom in 1971-1973, and accounted for 80-100% of the copious phytoplankton biomass in those years. In 1974, however, Arthrospira almost disappeared from the lake and was replaced by much smaller-diameter planktonic species, such as coccoid cyanobacteria that dealt better with elevated salinities. This transfer in primary producer make-up in the lake waters also made the lesser flamingos less efficient feeders. When the relatively large filaments of Arthrospira dominate the lake plankton, the flamingo’s beak filters between 64 and 86% of the plankton held in each mouthful of lake water (Vareschi, 1978). When the much smaller coccoids come to dominate, the filters are a much less efficient feeding mechanism. The change in plankton species was also tied to a severe reduction in algal biomass (and protein availability), which in 1974 was down to 71 g/m3 in surface waters and averaged 137 g/m3 in the total water mass. As a result, the flamingo numbers feeding in the lake declined from 1 million to several thousand, driving a significant die-back as the salinity-stressed flamingo population moved to other lakes, like Bogoria, where Arthrospira were flourishing (Vareschi, 1978).

    The lower salinity limit for an Arthrospira bloom is ≈ 5‰, but it does better when salinity is more than 20‰. The species dominance of Arthrospira and its higher biomass in somewhat more saline lake waters in the African Rift lakes is clearly seen in the near unispecific year-round biomass of nearby Lake Bogoria (salinity 40-50‰). Its surface salinity is higher year round than Lake Nakuru (salinity ≈ 30‰) and the somewhat fresher waters of Lake Elmenteita (salinity ≈ 20‰; Figure 3a). Because of this, Lake Bogoria is a more reliable food source for feeding flamingos compared to either Nakuru or Elmenteita. Birds tend to migrate there to feed when conditions for Arthrospira growth are not ideal in other nearby alkaline lakes (too fresh or too saline). This was the case in 1999 when high rainfall and dilution of lake waters caused the Arthrospira levels to fall in both Nakuru and Elmenteita. It was also true in late 2012 when Nakuru water levels were at near-historic highs and the waters too fresh to support a healthy Arthrospira population.

    Flamingo biomass controls

    A driving mechanism for the abrupt change in biomass in Lake Nakuru in the period 1972 -1974 was not clearly defined. It was thought to be related to increased salinity and lowering of lake levels, driving the growth of coccoid species other than Arthrospira sp. that are better adapted to higher salinity, but offering less protein to the feeding birds (Figure 7a; Vareschi, 1978). There is also the simple fact that in a lake with no surface outflow, ever more saline waters cover ever-smaller areas on the lake floor. There have been times in the last 70 years when most of Lake Nakuru has dried up and pools of saline water only a few tens of centimetres deep remained. This was the case in 1962 and most recently the case in 2008 (Figure 4).

    After the lake level lows of the 1960s, during the mid to late 1970s and in the 1980s the Nakuru hydrology returned to more typical inherent oscillations in water level and salinity (schizohalinity). The flamingo populations in Nakuru returned to impressive numbers but followed the vagaries of Arthrospira blooms. Since the early 1990s more reliable long-term datasets on physical lake condition and flamingo numbers have been compiled (Figures 5, 7b). In that time frame, in 1993, 1995 1998, 2008 and 2012, the flamingo populations feeding in Lake Nakuru were once again at very low levels and the remaining bird populations were stressed and subject to mass dieback.

    In 1998, unlike 1974, the stress on the flamingo population was related to lake freshening and rising water levels driving the decrease in Arthrospira biomass, not increased salinity and desiccation. In the preceding bountiful years, the Arthrospira-dominated biomass had bloomed at times when salinities were favourable and died back at times of elevated salinities and lake desiccation, as in 1974. By 2000 formerly low salinities had once again increased making surface waters suitable for another widespread Arthrospira bloom and the associated return of high numbers of feeding flamingos, which continued until 2007 (Figure 7c). In 2013 there was another freshening event, with associated rising water levels and the lakes flamingo population moved to Lake Bogonia to feed.

    Freshening favours a cyanobacterial assemblage dominated by picoplanktic chlorophytes (Picocyctis salinarum) and the nostocalean Anabaenopsis; the latter creates slimy masses that clog the flamingo’s feeding apparatus. The combination can drive much of the flamingo population to starvation or migration to other lakes with suitable salinities (Krienitz and Kotut, 2010). With freshening, comes also the possibility of the growth of strains that produce toxins (such as Anabaenopsis or Microcystis), possibly not in the feeding areas, which tend to remain too saline for Microcyctis, but in the spring waters where the flamingos fly in order to bathe and cleanse their feathers after a night spent feeding (Kotut and Krienitz, 2011).

    It seems that breeding flamingos come to Lake Nakuru to feed in large numbers when there is water in the lake with appropriate salinity and nutrient levels to facilitate an Arthrospira bloom. In some years when heavy rains occur, lake levels rise significantly and the lake waters, although perennial, stay in the lower salinity tolerance range for Arthrospira platensis, keeping cyanobacterial numbers and protein levels at the lower end of the spectrum, as in the El Niño period between October 1997 to April 1998 and again in 2013. Once lake levels start to fall, salinities and rates of salinity change return to higher levels, then water conditions once again become appropriate for an Arthrospira bloom. But the environmental stress on the flamingos also comes with further-elevated salinities and desiccation moving lake hydrochemistry into salinities at the upper end of Arthrospira tolerance.

    One of the reasons Lake Nakuru is suitable for phenomenal cyanobacterial and algal growth at times of Arthrospira bloom is the maintenance of suitable temperatures and oxygenation in the upper water mass, where the photosynthetic Arthrospira thrive. Nakuru develops a daily thermocline in the top 1.5 metres of the water column that dissipates each day in the late afternoon via wind mixing. Overturn recycles nutrients (derived from the decomposition of bird and other droppings, including those of resident hippopotami) back to the oxygenated lit surface waters to facilitate an ongoing bloom the next day (Figure 7c).

    Numbers of flamingos feeding in Lake Nakuru and Lake Bogoria are used in the popular press as indicators of the environmental health of the lakes. Thousands of birds died in Lake Nakuru in 1995 and more than 30,000 birds may have died in Lake Bogoria in the first half of 1999. The most dramatic die-offs in the last two decades were at Lake Nakuru in August 2006, when some 30,000 died, and Lake Bogoria in July 2008, when 30,000 birds died. Some environmentalists have argued in the popular press that mass die-offs and their perception of lowered numbers of flamingos in Lake Nakuru and Lake Bogoria across the 1990s and 2000s were indicators of uncontrolled forest clearance, an uncontrolled increase in sewerage encouraging eutrophication, and increase in heavy metals from increasing industrial pollutants in the lake, along with general stress on the bird population from tourists and the drastic increase in local human population centred on the town of Nakuru (third largest in Kenya). Numbers of people in the town, which is the main city in the rift valley, have grown by an average of 10% every decade for the past 30 years.

    But like much environmental doomsday argument, it is more based on opinionated prediction than on scientific fact. When numbers of feeding flamingos in Lake Nakuru are plotted across last few decades, it is evident that flamingo numbers oscillate widely, but it is also apparent that the peak numbers in 2000s are equivalent to the peak numbers in 1990s. A notion of longterm fall in numbers rather than wide natural fluctuations in numbers of feeding flamingos in the lake is not based on scientific reality.

    Likewise, when studies were done on the cause of the mass die-off in Lake Bogoria in 1999, it was found to have a natural, not an anthropogenic cause (Krienitz at al., 2003). The flamingos had ingested the remains of toxic cyanobacteria that constitute part of the population of the microbial mats that had bloomed to form a floor to the fresh water thermal spring areas about the lake edge. There the mats are dominated by thermally tolerant species; Phormidium terebriformis, Oscillatoria willei, Arthrospira subsalsa and Synechococcus bigranulatus. The influence of cyanotoxins in the deaths of the birds is reflected in autopsies which revealed: (a), the presence of hot spring cyanobacterial cells and cell fragments (especially Oscillatoria willei), and high concentrations of the cyanobacterial hepato- and neurotoxins in flamingo stomach contents and faecal pellets; (b), observations of neurological signs of bird poisoning - birds died with classic indications of neurotoxin poisoning - the ophistotonus behaviour (neck snapped back like a snake) of the flamingos in the dying phase, and the convulsed position of the extremities and neck at the time of death. Cyanobacterial toxins in stomach contents, intestine and faecal pellets were 0.196 g g-1 fresh weight (FW) for the microcystins and 4.34 g g-1 FW for anatoxin-a. Intoxication with cyanobacterial toxins probably occurred via uptake of detached cyanobacterial cells when birds come daily to the springs to drink and wash their feathers after an overnight feeding session in the saline waters of the lake proper.

    When heavy metal studies were undertaken in Nakuru lake sediments, the amount of heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) were found to be in the typical range of metals in sediments in lakes worldwide. The exception is Cd, which is elevated and can perhaps be ascribed to anthropogenic activity (Svengren, 2002). All other metals are present at low levels, especially if one considers that Lake Nakuru lies within a labile catchment where the bedrock is an active volcanogenic-magmatic terrane.

    Nearby Lake Magadi is also characterised by seasonal freshening, high productivity levels and bright red waters. In this case, the colour comes from haloalkaliphilic archaea, not cyanobacteria. Archaeal species belonging to the genera Natronococcus, Natronobacterium, Natrialba, Halorubrum, Natronorubrum and Natronomonas, all occur in soda brines of Lake Magadi. Lake centre brines where this biota flourishes is at trona/halite saturation with a pH up to 12. Stratified moat waters around the trona platform edge are less chemically extreme and moat bottom sediments preserve elevated levels of organics (≈6-8%). Lake Magadi also harbours a varied anaerobic bacterial community in the moat waters, including cellulolytic, proteolytic, saccharolytic, and homoacetogenic bacteria (Shiba and Horikoshi, 1988; Zhilina and Zavarzin, 1994; Zhilina et al., 1996). When the homoacetogen Natroniella acetigena was isolated from this environment, its pH growth optimum was found to be 9.8–10.0, and it continued to grow in waters with pH up to 10.7 (Zhilina et al., 1996).


    Flamingo numbers track feast and famine

    Rise and fall of lake levels, drastic changes in salinity, a periodically stressed biota, and a lack of predictability in water character are endemic to life in saline ecosystems (cycles of “feast or famine”). Natural variations in hydrochemistry control the number of feeding flamingos in Nakuru and Bogoria. In general, sufficient base-line scientific data in these schizohaline ecosystems is not yet available and so accurate determinations of the relative import of increased human activity versus natural environmental stresses on longterm bird numbers are not possible.


    Regionally, salinities in the east African rift valley lakes range from around 30-50‰ total salts (w/v) in the more northerly lakes in the rift (Figure 3a; Bogoria, Nakuru, Elmenteita, and Sonachi) to trona and halite saturation (>200‰) in lakes to the south (Lakes Magadi and Natron). Yet across this salinity range a combination of high ambient temperature, high light intensity and a continuous resupply of CO2, makes some of these soda lakes amongst the highest in the world in terms of their seasonal planktonic biomass (Grant et al., 1999) and also places them among the world’s most productive ecosystems (Figure 8; Melack and Kilham, 1974). Organic production is periodic, and pulses of organic product periodically swamp the ability of the decomposers and so accumulate as laminites in the perennial water-covered areas of some lake centres.

    Less-alkaline lakes in the rift valley are dominated by periodic blooms of cyanobacteria, while the hypersaline lakes, such as Magadi, can on occasion support blooms of cyanobacteria, archaea and alkaliphilic phototrophic bacteria (Jones et al., 1998). Halotolerant and halophilic biota living in the variably saline and layered water columns constitute small-scale “feast or famine” ecosystems, which at times of “feast” are far more productive than either tropical seagrass meadows or zones of marine upwelling (Figure 8).

    The “flamingo connection” across the African Rift Lakes supports a general observation that short periods of enhanced organic productivity are followed by episodes of lessened productivity in various schizohaline saline lake and seaway waters worldwide. both past and present (Warren, 2011). It reflects the general principle that increased environmental stress favours the survival of a few well-adapted and specialised halotolerant species. This biota is well adapted to the feast and famine life-cycle that exists in most saline depressions and means their numbers are subject to wide fluctuations tied to wide fluctuations in a saline lakes hydrochemistry (schizohaline waters). This same general principle of schizohaline ecosystem adaption is clearly seen in the periodic decrease in invertebrate species (grazers and predators) numbers with increasing salinity in the carbonate lakes of the Coorong of Southern Australia, where the only metazoan to remain alive in waters with salinities more than 200‰ are the southern hemisphere brine shrimp (Paratemia zietziania). It is seen in population fluctuations of the motile alga Dunaliella sp. in the Dead Sea, and in the fluctuating purple bacterial communities of Lake Mahoney in Canada (Warren, 2011). All these examples underline a general principle of “life will expand into the available niche” a paradigm that in the cases we have discussed is driven by fluctuating salinities inherent to saline-tolerant and saline-adapted ecosystems.


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