Salty Matters

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Gases in Evaporites, Part 2 of 3: Nature, distribution and sources

John Warren - Wednesday, November 30, 2016

This, the second of three articles on gases held within salt deposits, focuses on the types of gases found in salt and their origins. The first article (Salty Matters October 31, 2016) dealt with the impacts of intersecting gassy salt pockets during mining or drilling operations. The third will discuss the distribution of the various gases with respect to broad patterns of salt mass shape and structure (bedded, halokinetic and fractured)

What’s the gas?

Gases held in evaporites are typically mixtures of varying proportions of nitrogen, methane, carbon dioxide, hydrogen, hydrogen sulphide, as well as brines and minor amounts of other gases such as argon and various short chain hydrocarbons (Table 2). There is no single dominant gas stored in salt across all evaporite deposits, although a particular gas type may dominate or be more common in a particular region. For example, CO2 is commonplace in the Zechstein salts of the Wessen region of Germany (Knipping, 1989), methane is common in a number of salt dome mines in central Germany and the Five-Islands region in Louisiana, USA (Kupfer, 1990), nitrogen is dominant in other salt mines in Germany and New Mexico, while hydrogen can occur in elevated proportions in the Verkhnekamskoe salt deposits of the Ural foredeep (Savchenko, 1958).

Before considering the distribution of the various gases, we should note that older and younger sets of gas analyses conducted over the years in various salt deposits are not necessarily directly comparable. Raman micro-spectroscopy is a modern, non-destructive method for investigating the unique content of a single inclusion in a salt crystal. There is a significant difference in terms of what is measured in analysing gas content seeping from a fissure in a salt mass or if comparisons are made with conventional wet-chemical methods which were the pre Raman-microscopy method that is sometimes still used. Wet chemical methods require sample destruction, via crushing and subsequent dissolution, prior to analysis. This can lead to the escape of a variable proportion of the volatile compounds during the crush stage, such as methane, hydrogen, ethane and aromatic hydrocarbons, especially of those components held in fissures and more open intercrystalline positions. Any wet chemical technique gives values that represent the average of all the inclusion residues and intercrystalline gases left in the studied sample, post preparation. In contrast, Raman Microspectroscopy indicates content and proportion within a single inclusion in a salt crystal. So, free gas results and wet chemical compositions, when compared to Raman microscopy determinations from inclusions, are not necessarily directly comparable. With this limitation in mind, let us now look at major gas phases occluded in salt.


Nitrogen

Gassy accumulations in salt with elevated levels of N2 occur in many salt basins in regions not influenced by magmatic intrusions (Table 1). In an interesting study of spectroscopic gases held in inclusions in the Zechstein salt of Germany, Siemann and Elendorff (2001) document a bipartite distribution of inclusion gases. With rare exceptions, the first group, made up of N2 and N2-O2 inclusions reveals N2/O2 ratios close to that of modern atmosphere, which they interpret as indicating trapped paleoatmosphere (Figure 1). Similar conclusions are reached in earlier studies of nitrogen gas held in Zechstein salts, using wet chemical techniques (Freyer and Wagener, 1975). The second group documented by Sieman and Elendoorff (2001) is represented by inclusions that contain mixtures of N2, CH4 occasionally H2 or H2S. The most abundant subgroups in this second group are N2-CH4 and N2-CH4-H2 mixtures, that is, the methane association (Figure 1). Siemann and Elendorff (2001) argue that these methanogenic and hydrogenic gas mixtures of the second group are the product of decomposition of organic material under anoxic subsurface conditions. They note that the methane and hydrogenic compounds, as well as some portion of the nitrogen, are not necessarily derived from decomposing organics held within the salt. They could have been generated by degassing of underlying Early Permian (Rotliegendes) or Carboniferous organic-rich sedimentary rocks with subsequent entrapment during early stages of fluid migration, possibly driven by Zechstein halokinesis.


Different origins and timings of both main nitrogen gas groupings in inclusions in the salt host is supported by stratigraphic correlations (Siemann and Elendorff, 2001). In the stratigraphic layers which contain mainly mixtures of N2 and O2 or pure N2, inclusions of the N2-CH4-H2-H2S-group are rare (A in Figure 1) and vice versa: layers which are rich in N2-CH4-H2-H2S do not contain many pure N2-O2inclusions (B in Figure 1). The majority of layers investigated in the salt mostly contain inclusions of the N2-O2 group, sans methane. Only two anhydrite-rich layers of Zechstein 3 (Main Anhydrite and Anhydrite-intercalated Salt) contain mainly inclusions of the second group (i.e. with abundant methane) as seen in B in Figure 1. The Zechstein 3 potash seams, as well as secondary halites, contain more or less the same population of inclusions from every main group (C in Figure 1). A comparison of the gas-rich inclusions and the gases in the brine-rich inclusions of the Zechstein 2 layer, Main Rock Salt 3, also shows distinct differences. Whereas, the gas-rich inclusions are mostly of the N2-O2 grouping, the gases from the brine-rich inclusions are mostly of the N2-CH4 group, emphasizing different origins for the gas-rich and brine-rich inclusions Siemann and Elendorff (2001) conclude that the latter gas group is a product of thermally evolved anhydrite-rich parts or potash seams that have generated hydrocarbons catagenically, with these products migrating into the overlying and deforming Main Rock Salt 3.

      

Work on the free gas released during mining of the Permian Starobinsky potash salt deposit in the Krasnoslobodsky Mine, Soligorsk mining region, Russia shows that the dominant free gas is nitrogen, along with a range of hydrocarbons, including methane (Figure 2; Andreyko et al., 2013). The compositional plot is based on free gases released from the main pay horizon of the Krasnoslobodsky Mine, which it the Potash Salt Horizon 3. The exploited stratigraphy is 16 to 18 m thick in the centre of the minefield and thins to 1 m thick at the edges of the ore deposit. Depth to the potash horizon varies from 477 to 848 m below the landsurface. It consists of three units: 1) top sylvinite unit, which is classified as non-commercial due to high insoluble residue content; 2) mid clay–carnallite unit, which is composed of alternating rock salt, clay and carnallite; and 3) bottom sylvinite unit, which is the main ore target and is composed of six sylvinite layers (I-VI), alternating with rock salt bands (Figure 3). The distribution of gas across the stratigraphy of units I-VI shows that the free gas yields are consistently higher in the sylvinite bands (Figure 3).

      

Oxygen levels in salt are not studied in as much detail as the other gas phases due to their more benign nature when released in the subsurface. Work by Freyer and Wagener (1975) focusing on the relative proportions of oxygen to nitrogen held in Zechstein salts was consistent with the inclusions retaining the same relative proportions of the two gases as were present in the Permian atmosphere when the salts first precipitated.

As well as being held within the salt mass, substantial nitrogen accumulations can be hosted in inter-salt and sub-salt lithologies. For example, the resources of nitrogen in the Nesson anticline in the Williston Basin are ≈53 billion m3 and held in sandstones intercalated with anhydrite in the Permian Minnelusa Fm (Marchant, 1966; Anderson and Eastwood, 1968) and those in Udmurtia in the Volga–Ural Basin are ≈33 billion m3 (Tikhomirov, 2014). In both these non-salt enclosed cases the evolution of the nitrogen gas is related to the catagenic and diagenetic evolution of organic matter. Tikhomirov (2014) concludes that nitrogen in the various subsalt fluids in the Volga–Ural Basin originates from two major sources. Most of the nitrogen in the subsalt has δ15N > 0‰ and is genetically related to concentrated calcium chloride brines, heavy oils, and bitumen in the platform portion of the basin and so ties to a catagenic origin. The other N2 source is seen in subordinate amounts of nitrogen across the basin with δ15N values < 0‰. According to Tikhomirov (2014), this second group seems to be genetically related to methane derived at significant depths in the basement lithologies of Ural Foredeep and Caspian depression (possibly a form of mantle gas?).

Methane

Unexpected intersections with gas pockets containing significant proportions of methane can be dangerous, as evidence by the Belle Isle Salt Mine disaster in 1979 as well as others (see article 1). Many methane (earth-damp) intersections and rockbursts in US Gulf Coast salt mines can be tied to proximity to a shaley salt anomaly (Molinda 1988; Kupfer 1990).

Methane contents of normal salt (non-anomaly salt) in salt domes of the Five-Islands region of the US Gulf Coast were typically low (Kupfer, 1990). For example, the majority of the samples of normal salt, as tested by Schatzel and Hyman, (1984), contained less than 0.01 cm3 methane per 100 g NaCI. Although there can be wide ranges of methane enrichment in normal versus outburst salts, outburst salts are typified by increases in halite crystal size, the number of included methane gas bubbles, contorted cleavage surfaces related to increased overpressured gas contents, and an increase in clay impurities in some of the more methane-rich salt samples.

 

Probably the most detailed study of controls on methane distribution in domal salt was conducted at the Cote Blanche salt mine in southern Louisiana (Molinda, 1988). Because outbursts were the primary mode of methane emission into the mine, he mapped more than 80 outbursts, ranging in size from 1 to 50 ft in diameter. The outbursts were aligned and elongate parallel to the direction of salt layering and such zones correlate well with high methane content (Figure 4). Halite crystal size abruptly increased upon entry into gassy zones subject to rockburst. The intensity of folding and kinking of the salt layering within the outburst zone also increased. The interlayered sand, shown in Figure 4, also occurred throughout the mine and not just in the mapped area shown, but was not a significant source of methane. Molinda (1988) and Schatzel and Hyman (188) all concluded that not all rockbursts were hosted by coarsely crystalline fine-grained salt, so the absence of coarsely crystalline salt may not be an indication that a rockburst cannot occur, although it is less likely. Sampling the salt for methane levels may be a better approach for rockburst prediction.

In some methane occurrences in Europe (in addition to generation from clayey intrasalt organic entraining bands) there is a further association with igneous-driven volatilization from nearby, typically underlying, coaly deposits. This igneous association with coals and carbonates likely creates an additional association with CO2 and possibly H2S.

CO2

Many CO2 rich gas intersections tie to regions that have been heated or cross-cut with igneous intrusives. For example, many of the CO2-bearing gas mixtures that were encountered in the Werra region during the initial exploratory drillings for potash salts(Table 1 in article 1; Frantzen, 1894). In 1901, shortly after mining at Hämbach had begun, coincident intersections of basalt dykes and releases of gas were observed (Gropp, 1919). Dietz (1928a,b) noted that a fluid phase was always involved in the fixation of the CO2gas mixtures in the Zechstein evaporites, while Bessert (1933) reported on the enrichment of anhydrite, kainite, and polyhalite at the contact with the basaltic intrusive. Accumulations of CO2-rich free gas in many Wessen mines became a safety issue and many subsequent studies underlined the association of CO2 enriched gases with basalt occurrences (Knipping, 1989). In almost all instances in the Zechstein where native sulphur forms the at the contact of a basaltic dyke, knistersalz dominates the evaporite portion of the samples. According to Ackermann et al. (1964) gas-bearing drill core samples collected in the Zechstein K1Th unit (carnallitite, sylvinite) in the Marx-Engels mine (formerly Menzengraben, East Germany) contained up to 0.6 - 14.0 ml gas/100 gm rock, with an average of 3.6 ml of gas fixed in 100 g of salt rock (Table 1)of. On average, the gas inclusions were composed of 84 vol% CO2. Knipping (1989) concludes that quantities of volatile phases (mainly H20 and CO2) penetrated the evaporites during intrusion of basaltic melts. These gases influenced mineral reactions, particularly when intersecting with reactive K-Mg rock layers of the Hessen (K1H) and Thuringen (K1Th) potash seams in the former East Germany. The intensity of this reaction was likely greater when the evaporite layers contain hydrated salts such as carnallite and kainite. Such salts tend to release large volumes of water at relatively low temperatures when heat by a nearby intrusive (Warren, 2016; Chapter 16; Schofield et al., 2014). In doing so, significant volumes of CO2 enriched gases were trapped in the altered and recrystallising evaporites, so forming knistersalz.


While discussing CO2 elevated levels, it is probably taking a little time to illustrate what makes this area of CO2 occurrence so interesting in terms of the differential levels of reactivity when hydrated versus non-hydrated salt units are intruded and how this process facilitates penetration of volcanic volatiles (including CO2) into such zones. The Herfa-Neurode potash mine is located in the Werra-Fulda Basin in the Hessian district of central Germany (Figure 5a). The targeted ore levels consist of the carnallite-rich Kaliflöz Hessen (K1H) and Kaliflöz Thüringen (K1Th) intervals, which form part of the Zechstein 1 (Z1) bedded Werra salt succession(Warren, 2016). In the mine the K1H and K1Th units range in thickness from 2 m to 10 m, are generally subhorizontal and occur at a depth of 650–710 m below the present-day surface. In the later Tertiary, basaltic melts intruded these Zechstein evaporites as numerous sub-vertical dykes, but only a few dykes attained the Miocene landsurface. Basaltic melt production was related to regional volcanic activity some 10 to 25 Ma. Basalts exposed in the mine walls, where it cuts non-hydrous units of halite or anhydrite, are typically subvertical dykes, rather than subhorizontal sills. The basalts are phonolitic tephrites, limburgites, basanites and olivine nephelinites. Dyke margins are usually vitrified, forming a microlitic limburgite glass along dyke edges in contact with salt (Figure 5b; Knipping, 1989). At the contact on the evaporite side of the glassy rim there is a cm-wide carapace of high-temperature salts (mostly anhydrite and ferroan carbonates). Further out, the effect of the high-temperature envelope is denoted by transitions to clear halite, with higher temperature fluid inclusions (Knipping 1989). All of this metre-scale alteration is an anhydrous alteration halo, the salt did not melt (melting temperature of 804°C), rather than migrating, the fluid driving recrystallisation was largely from entrained brine/gas inclusions. The dolerite/basalt interior of the basaltic dyke is likewise altered and salt soaked, with clear, largely inclusion-free halite typically filling vesicles in the basalt.

Heating of hydrated salt layers, adjacent to a dyke or sill, tends to drive off the water of crystallisation (chemical or hydration thixotropy) at much lower temperatures than that at which anhydrous salts, such as halite or anhydrite, thermally melt (Figure 5c; Schofield et al., 2014). For example, in the Fulda region, the thermally-driven release of water of crystallisation within particular salt beds creates thixotropic or subsurface “peperite” textures in carnallitite ore layers. These are layers where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular subhorizontal horizons in the salt mass, wherever hydrated salt layers were present. In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals and the intrusive is sub-vertical to steeply dipping (Figure 5b versus 5c).

Accordingly, away from the immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter a former carnallite halite bed, and drive the creation of extensive soft sediment deformation and peperite textures in the former hydrated layer (Figure 5c). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals of the Herfa-Neurode mine. Sylvite in these altered zone is a form of dehydrated carnallite, not a primary-textured salt. Across the Fulda region, such altered zones and deformed units can extend along former carnallite layer to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes back out into the unaltered bed, which retains abundant inclusion-rich halite and carnallite (Schofield et al., 2014). That is, nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids and gases originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine/gas mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement. The contrast in alteration extent between anhydrous and hydrous salt layers shows alteration effects are minimal wherever the emplacement temperature of the magma is below that of the anhydrous salt body as it is next to a basalt dyke. If this is the mechanism driving entry of igneous-related volatiles (gases and liquids) into a salt body then the distribution of products (including CO2) will be highly inhomogeneous and related to the minerally of the salt unit adjacent to the intrusive.


Hydrogen

Many hydrogen occurrences are co-associated with occurrences of potash minerals, especially the minerals carnallite and sylvite. For example, mine gases (free gas) at Leopoldshall Salt Mine (Zechstein, Permian of Stassfurt, Germany) flowed for at least 4.5 years, producing hydrogen at a rate of 128 cubic feet per day (Rogers 1921). Bohdanowicz (1934) lists hydrogen gas as being present in evaporite intersection in the Chusovskie Gorodki well, drilled in 1928 near the city of Perm to help define the southern extent of the Soligamsk potash. Gases in the carnallitite interval in that well contained 33.6% methane and 17.4% hydrogen. More recent work in the same region clearly shows hydrogen is a commonplace gas in the mined Irenskii unit in the Verkhnekamskoe potash deposit within the central part of the Solikamsk depression in the Ural foredeep. Based on a study of free gas and inclusion-held gas in the Bereznikovshii Mine, Smetannikov (2011) found that the elevated H2 levels are consistently correlated with the carnallite and carnallite-bearing layers (Table 2). Other gases present in significant amounts, along with the hydrogen, in the potash zones include nitrogen and methane. Interestingly, methane is present in much higher proportions in the free gas fraction in the ore zones compared to gases held in inclusions in the potash crystals (Table 2).  

Smetannikov (2011) goes on to suggest that likely H2 source is via radiogenic evolution of released crystallisation water hence the higher volumes of hydrogen in the carnallitite units in the mine (Table 2). He argues the most probable mechanism generating H2 is the radiolysis of the crystallisation water of carnallite (CaMgCl3.6H2O) driven by the effects of radioactive radiation. The most likely radiogenic candidates are 40K and 87Rb, rather than such heavy radiogenic isotopes as 238U, 235U, 234U, 232Th, and 226Ra. His reasons for this are as follows: 1) U, Th, and partly Ra are sources of α radiation. U, Th, and Ra are concentrated in the insoluble residues of the salts, and the chloride masses contain only minor amounts of Th. Hence these components have no radioactive effect on carnallite because of the short distances of travel of α particles. Because of this, Smetannikov concludes these elements and not likely sources of radioactive radiation. He argues it is more likely that crystallisation water is more intensely affected by β and γ radiation generated by 40K and 87Rb. Hence, bombardment by β and γ radiation drives the radiolysis (splitting) of this water of crystallisation, so driving the release of hydrogen and hydroxyls. Free hydroxyls can then interact with Fe oxides to form hydro-goethite and lepidocrocite, i.e., both these minerals occur in the carnallite but are absent in the sylvinite.

The notion of hydrogen being created by radiolysis of potash salt layers is not new; it was used as the explanation of the hydrogen association with various potash units by Nesmelova & Travnikova (1973), Vovk (1978) and Knape (1989). Headlee (1962) attributed the occurrence of hydrogen in salt mines to the absence of substances with which hydrogen could react within the salt beds once it was generated. It is likely that there are several different origins for hydrogen gas in evaporites: 1) Production during early biodegradation of organic matter, co-deposited with the halite or potash salts and trapped in inclusions as the crystal grew. This can explain some of the associated nitrogen and oxygen; 2) A significant proportion can be produced by radiolysis associated with potassium salts (when present) and 3) the hydrogen may be exogenic and have migrated into the halite formations, along with nitrogen. 

Temperature and mineralogical effects on gas generation and distribution in salt (in part after Winterle et al., 2012)

Temperature can affect brine chemistry of volatiles released as natural rock salt is heated (is this an analogue to the generation of some types of free gas and other volatile released as salt enters the metamorphic realm? –see Warren 2013; Chapter 14). Uerpmann and Jockwer (1982) and Jockwer (1984) showed that, upon heating to 350°C [662°F], the gases H2S, HCl, CO2, and SO2 were released from blocks of natural salt collected from the Asse mine in Germany. Pederson (1984) reported the evolution of HCl, SO2, CO2, and H2S upon heating of Palo Duro and Paradox Basin rock salt to 250°C [482°F]. Impurities within the salt apparently contain one or more thermally unstable, acidic components. These components can volatilize during heating and increase the alkalinity of residual brines. For example, pH of brines increased from near neutral to approximately 10 in solutions prepared by dissolving Permian Basin salt samples that were annealed at progressively higher temperature [up to 167°C [333°F]  (Panno and Soo, 1983).

Zones of igneous emplacement and intrusion of interlayered halite and potash units create a natural laboratory for the study of the generation and migration of free and inclusion gases during the heating of various salts (Figures 5, 6 and Table 1). In the Cambrian succession of the Siberian platform evaporite intervals are dominated by thick alternating carbonate- sulphate and halite beds. Numerous basaltic dykes and sills intrude these beds. In a benchmark paper dealing with the zone of alteration of intrusives in evaporites, Grishina et al. 1992 found that, in potash-free halite zones intersected by basaltic intrusions, the evolution of the inclusion fluid chemistry is described as a function of the thickness of the intrusion (h) and the distance of the sample from the contact with the intrusion (d) and expressed as a response to the measure d/h. The associated gas in the halite is dominated by CO2 (Table 1). Primary chevron structures with aqueous inclusions progressively disappear as d/h decreases; at d/h < 5 a low-density CO2 vapour phase appears in the brine inclusions; at d/h < 2, a H2S-bearing liquid-CO2 inclusions occur, sometimes associated with carbonaceous material and orthorhombic sulphur, and for d/h < 0.9, CaCl2, CaCl2.KCl and n CaCl2.n MgCl2 solids occur in association with free water and liquid CO2 inclusions, with H2S, SCO, and Sg. The d/h values marking the transitions outlined above occur both above and below sills, but ratios are lower below the sills than above, indicating mainly conductive heating below and upward vertical fluid circulation above the sill. The water content of the inclusions progressively decreases on approaching the sills, whereas their CO2 content and density increase.


Carnallite, sylvite and calcium chloride salts occur as solid inclusions in the two associations nearest to the sill for d/h<2. Carnallite and sylvite occur as daughter minerals in brine inclusions. The presence of carbon dioxide is interpreted to indicate fluid circulation and dissolution/recrystallization phenomena induced by the basalt intrusions. The origin of carbon dioxide is related to carbonate dissolution during magmatism. Similar conclusions as to the origin of the CO2 in heated halite-dominant units were reached by many authors studying gases in the Zechstein salts in the Werra Fulda region of Germany (Figure 6; Table 1; see Knipping et al., 1989, Hermann and Knipping 1993 for a summary).

When the gas distributions measured in inclusions in potash units, other than the Cambrian salts of Siberia, are compared to those salts that have not experienced the effects of igneous heating, there is a clear separation in terms of the dominant inclusions gases (Table 1; Grishina et al., 1998). For example, inclusions in the Verhnekamsk deposit (Russian platform) are N2-rich, in regions not influenced by magmatic intrusives (Figure 2, 3). It is an area marked by the presence of ammonium in sylvite (0.01-0.15% in sylvinite and 0.5% in carnallite, Apollonov, 1976). Likewise, nitrogen (via crush release of the samples) is the dominant gas according to the bulk analyses of the same salts by Fiveg (1973). 

Later Raman studies of individual inclusions in these Cambrian salts reveals a more complicated inclusion story. There are three types of inclusion fill; a) gas, b) oil and c) brine + carnallite-bearing inclusions. Fe-oxides are sometimes associated with inclusions containing the carnallite daughter minerals. Detailed work by Grishina et al. (1998) shows there two kinds of gassy inclusion: 3) N2-rich and 2) CH4-rich 3) CO2-rich in the same age salt (Table 1; Figure 6). That is, not all gassy brine inclusion in the Cambrian salts are nitrogenous. N2 gas inclusions that also contain CO2 and are associated with sylvite with a low ammonium content (0.04 mol% NH4C1). In contrast, CH4 inclusions are associated with ammonium-rich sylvite (0.4 mol% NH4Cl) (Table 2). Older bulk analysis studies(Apollonov, 1976) showed that red sylvinite  has a lower molar NH4Cl content (0.01%) than pink and white sylvinites (0.05 to 0.19%)

Raman studies of inclusions in the potash-entraining Eocene basin of Navarra, (Spain) outside of any region with magmatic influence show that the gaseous inclusions are mostly N2-rich with 10% to 20% methane (Table 1; Figure 6; Grishina et al., 1998). Traces of CO2 are also detected in some of the Spanish inclusions. Sylvite inclusions in CO2-free inclusions in Spain contain up to 0.3 mol% NH4C1 (Table 2). Grishina et al. (1998) notes that salt formations in the Bresse basin (France) and Ogooue delta (Gabon) have no basalt intrusions and both occur in N2-free, oil-rich environments. The inference is that nitrogen in some salt units is not an atmospheric residual.


To test if there may be a mineralogical association with a gas composition in inclusions in various salt and evaporitic carbonate layers we shall return to the Zechstein of Germany and the excellent detailed analytical work of Knipping (1989) and Hermann and Knipping (1993). This work is perhaps the most detailed listing in the public realm of gas compositions inclusions sampled down to the scale of salt layers and their mineralogies. Figure 7 is a plot I made based on the analyses listed in Table 9 in Hermann and Knipping (1993).  It clearly shows that for  Zechstein salts collected across the mining districts of central Germany this is an obvious tie of salt mineralogy to the dominant gas composition in the inclusions. In this context, it should be noted that all Zechstein salt mines are located in halokinetic structures with mining activities focused into areas where the targeted potash intervals are relatively flat-lying. There is little preservation of primary chevrons in these sediments. Nitrogen is the dominant, often sole gas in the halite-dominant units, CO2 is dominant in carbonate and anhydrite dominant layers, this is especially obvious in units originally deposited near the base of the Zechstein succession. Hydrogen in small amounts has an association with inclusions the same carbonates and anhydrites, but elevated hydrogen levels are much more typical of potash units, clays and in juxtaposed layers.  

In my opinion, the gas compositions in inclusions that we see today in any salt mass that has flowed at some time during its diagenetic history will likely have emigrated and been modified to varying degrees within the salt mass. This is true for all the gases in salt, independent of whether the gas is now held in isolated pockets, fractures or fluid inclusions, Non of the gas in halokinetic salt is not in the primary position. Movement and modification of various gas accumulations in halokinetic salt is inherent to the nature of salt flow processes. Salt and its textures in any salt structure have migrated and been mixed and modified, at least at the scale of millimetres to centimetres, driven by vagaries of recrystallisation as a flowing salt mass flows (Urai et al., 2008). All constituent crystal sizes and hence gas distributions across various inclusions in the salts are modified via flow-induced pressure fields, driving pressure solution and reannealing (See Warren 2016 Chapter 6 for detail).

With this in mind we can conclude that for the Zechstein of central Germany, nitrogen was likely the earliest gas phase as it occurs in all units. On the other hand, CO2, with its prevalence in units near the base of the succession or in potash units that  have once contained hydrated salts at the time of igneous intrusion, entered along permeability pathways. This may also be true of carbonates and anhydrites which would have responded in more brittle fashion. Hydrogen is clearly associated with potash occurrence or clays and an origin via radiolysis is reasonable.


This leaves methane, which as we saw earlier is variable present in the Zechstein, but not studied in detail by Knipping (1989) or Hermann and Knipping (1993). There is another excellent paper by Potter et al. (2004) that focuses on the nature of methane in the Zechstein 2 in a core taken in the Zielitz mine, Northeastern Germany Bromine values show a salting-upward profile with values exceeding 200 ppm in the region of potash bitterns (Figure 8a). This is a typical depositional association, preserved even though textures show a degree of recrystallisation and implying there have not been massive fluid transfers since the time the salt was first deposited. Methane is present in sufficient volumes to be sampled in the lower 10 metres of the halite (Z2NAa) and in the upper halite (Z2Nac) and the overlying potash (Z2Kst). If was variably present in the intervening middle halite. When carbon and deuterium isotope values from the methane in the lower and upper parts of the stratigraphy are cross plotted. Values from the lower few meters of the halite plot in the thermogenic range and imply a typical methane derived via catagenesis and possible entry into the lowermost portion of a salt seal. The values from the upper halite and the potash interval have very positive carbon values so that the resulting plot field lies outside that  typical of a variety of methane sources (Figure 8b). Potter et al. (2004) propose that these positive values show preserve primary values and that this methane was sealed in salt since the rock was first deposited. That is positive values preserve evidence of the dominant isotopic fractionation process, which was evaporation of the mother brines. This generated a progressive 13C enrichment in the carbon in the residual brines due to preferential loss of 12CO2 to the atmosphere. The resulting CH4 generated in the sediments, as evaporation and precipitation advanced, so recording this 13C enrichment in the carbon reservoir. Therefore, the isotopic profile observed in this sequence today represents a relict primary feature with little evidence for postdepositional migration. This is a very different association to the methane interpretation based on gases held the US Gulf coast or the Siberian salts. 

The most obvious conclusion across everything we have considered in this article is that, at the level of gas in an individual brine inclusion measure, there is not a single process set that explains gas compositions in salt. Any gas association can only be tied back to its origins if one studies gas compositions in the framework of the geological history of each salt basin. We shall return to this notion in the third article in this series when we will lock at emplacement mechanisms that can be tied to depositional and diagenetic features and compositions at the macro scale.

References

Anderson, S. B., and W. P. Eastwood, 1968, Natural Gas in North Dakota, Natural Gases of North America, Volume Two, American Association of Petroleum Geologists Memoir 9, p. 1304-1326.

Andreyko, S., O. V. Ivanov, E. A. Nesterov, I. I. Golovaty, and S. P. Beresenev, 2015, Research of Salt Rocks Gas Content of III Potash Layer in the Krasnoslobodsky Mine Field: Eurazian Mining - Gornyi Zhurnal, v. 2, p. 38-41.

Apollonov, V. N., 1976, Ammonium ions in sylvine of the Upper Kama deposit. Doklady Akademii Nauk SSSR: Earth Science Section 231, 101. English Translation American Geological Institution.

Bessert, F., 1933, Geologisch-petrographische Untersuchungen der Kalilager des Werragebietes: Archiv flit Lagerstättenforschung, H. 57, 45 S., Berlin.

Dietz, C., 1928a, Überblick über die Salzlagerstätte des Werra-Kalireviers und Beschreibung der Schāchte "Sachsen-Weimar" und "Hattorf": Z. dt. Geol. Ges., Mb., v. 1/2, p. 68-93.

Dietz, C., 1928b, Die Salzlagerstätte des Werra-Kaligebietes. - Archiv für Lagersttättenforschung, H. 40, 129 S., Berlin. .

Fiveg, M. P., 1973, Gases in salts of Solikamsk deposit (in Russian): Trudi VNIIG 64, 62-63.

Frantzen, W., 1894, Bericht über neue Erfarungen beim Kalibergbau in der Umgebung des Thüringer Waldes: Jb. kgl. preuB, geol. L.-A. u. Bergakad., v. 15, p. 60-61.

Freyer, H. D., and K. Wagener, 1975, Review on present results on fossil atmospheric gases trapped in evaporites: pure and applied geophysics, v. 113, p. 403-418.

Grishina, S., J. Dubessy, A. Kontorovich, and J. Pironon, 1992, Inclusions in salt beds resulting from thermal metamorphism by dolerite sills (eastern Siberia, Russia): European Journal of Mineralogy, v. 4, p. 1187-1202.

Grishina, S., J. Pironon, M. Mazurov, S. Goryainov, A. Pustilnikov, G. Fonderflaas, and A. Guerci, 1998, Organic inclusions in salt - Part 3 - Oil and gas inclusions in Cambrian evaporite deposits from east Siberia - A contribution to the understanding of nitrogen generation in evaporite: Organic Geochemistry, v. 28, p. 297-310.

Gropp, 1919, Gas deposits in potash mines in the years 1907-1917 (in German): Kali and Steinsalz, v. 13, p. 33-42, 70-76.

Headlee, A. J. W., 1962, Hydrogen sulfide, free hydrogen are vital exploration clues: World Oil, Nov, 78-83.

Herrmann, A. G., and B. J. Knipping, 1993, Waste Disposal and Evaporites: Contributions to Long-Term Safety: Berlin, Heidelberg, Springer, 190 p.

Jockwer, N., 1984, Laboratory investigations on radiolysis effects on rock salt with regard to the disposal of high-level radioactive wastes: McVay, G. L. Scientific basis for nuclear waste management Vii. Battelle, Pac. Northwest Lab., Richland, Wa, United States. Materials Research Society Symposia Proceedings, v. 26, p. 17-23.

Knabe, H.-J., 1989, Zur analytischen Bestimmung und geochemischen Verteilung der gesteinsgebundenen Gase im Salinar (Concerning the analytical determination and geochemical distribution of rock-bound gases in salt): Zeitschrift für Geologische Wissenschaft, v. 17, p. 353-368.

Knipping, B., 1989, Basalt intrusions in evaporites: Lecture Notes in Earth Sciences (Springer-Verlag), v. 24, p. 132 pp.

Kupfer, D. H., 1990, Anomalous features in the Five Islands salt stocks, Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 40, p. 425-437.

Marchant, L. C., 1966, Nitrogen gas in five oilfields on the Nesson anticline: US Bureau Mines, Report Invest., no. 6848.

Molinda, G. M., 1988, Investigation of Methane Occurrence and Outbursts in the Cote Blanche Domal Salt Mine, Louisiana US Bureau of Mines Report of Investigation No. 9186, 31 p.

Nesmelova, Z. N., and L. G. Travnikova, 1973, Radiogenic gases in ancient salt deposits: Geochemistry International, v. 10, p. 554-555.

Panno, S. V., and P. Soo, 1983, An evaluation of chemical conditions caused by gamma irradiation of natural rock salt.: Brookhaven National Laboratory Report NUREG-33658.

Potter, J., M. G. Siemann, and M. Tsypukov, 2004, Large-scale carbon isotope fractionation in evaporites and the generation of extremely 13C-enriched methane: Geology, v. 32, p. 533-536.

Savchenko, V. P., 1958, The formation of free hydrogen in the earth's crust, as determined by the reducing action of the products of radioactive transformations of isotopes: Geochemistry (Geokhimiya)

Schatzel, S. J., and D. M. Hyman, 1984, Methane content of Gulf Coast domal rock salt, United States Dept. of the Interior, Bureau of Mines Report of Investigation, No 8889, 18 p.

Schoell, M., 1988, Multiple origins of CH4 in the Earth: Chemical Geology, v. 71, p. 1-10.

Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interactions: Geology, v. 42, p. 599-602.

Siemann, M. G., and B. Ellendorff, 2001, The composition of gases in fluid inclusions of late Permian (Zechstein) marine evaporites in Northern Germany: Chemical Geology, v. 173, p. 31-44.

Smetannikov, A. F., 2011, Hydrogen generation during the radiolysis of crystallization water in carnallite and possible consequences of this process: Geochemistry International, v. 49, p. 916-924.

Tikhomirov, V. V., 2014, Molecular nitrogen in salts and subsalt fluids in the Volga-Ural Basin: Geochemistry International, v. 52, p. 628-642.

Uerpmann, E. P., and N. Jockwer, 1982, Salt as a Host Rock for Radioactive Waste Disposal: In: Geological Disposal of Radioactive Waste: Geochemical Progress. Paris, France: Organization for Economic Cooperation and Development, Nuclear Energy Agency.

Urai, J. L., Z. Schléder, C. J. Spiers, and P. A. Kukla, 2008, Flow and Transport Properties of Salt Rocks, in R. Littke, ed., Dynamics of complex intracontinental basins: The Central European Basin System, Elsevier, p. 277-290.

Vovk, I. F., 1978, On the source of hydrogen in potassium deposits: Geochem. Int., v. 15, p. 86-90.

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


 

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.

References

Bildstein, K. L., C. B. Golden, B. J. McCraith, and B. W. Bohmke, 1993, Feeding Behavior, Aggression, and the Conservation Biology of e: Integrating Studies of Captive and Free-ranging Birds: American Zoologist, v. 33, p. 117-125.

Childress, B., S. Nagy, and B. Hughes, 2008, International Single Species Action Plan for Conservation of the Lesser Flamingo (Phoenicopterus minor): CMS Technical Series No. 18, AEWA Technical Series No. 34, Bonn, Germany, 59 pp.

Gould, S. J., 1987, The Flamingo's Smile: Reflections in NAtural History: New York, W. W. Norton and Company.

Grant, S., W. D. Grant, B. E. Jones, C. Kato, and L. Li, 1999, Novel archaeal phylotypes from an East African alkaline saltern: Extremophiles, v. 3, p. 139-145.

Jenkin, P. M., 1957, The Filter-Feeding and Food of Flamingos (Phoenicopteri): Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, v. 240, p. 401-493.

Jones, B. E., W. D. Grant, A. W. Duckworth, and G. G. Owenson, 1998, Microbial diversity of soda lakes: Extremophiles, v. 2, p. 191-200.

Kirkland, D. W., and R. Evans, 1981, Source-rock potential of evaporitic environment: Bulletin American Association of Petroleum Geologists, v. 65, p. 181-190.

Kotut, K., and L. Krienitz, 2011, Does the potentially toxic cyanobacterium Microcystis exist in the soda lakes of East Africa?: Hydrobiologia, v. 664, p. 219-225.

Krienitz, L., A. Ballot, K. Kotut, C. Wiegand, S. Pütz, J. S. Metcalf, G. A. Codd, and S. Pflugmache, 2003, Contribution of hot spring cyanobacteria to the mysterious deaths of Lesser Flamingos at Lake Bogoria, Kenya: FEMS Microbiology Ecology, v. 43, p. 141-148.

Krienitz, L., and K. Kotut, 2010, Fluctuating algal food populations and the occurrence of Lesser Flamingos (Phoeniconaias minor) in three Kenyan rift valley lakes: Journal of Phycology, v. 46, p. 1088-1096.

Melack, J. M., and P. Kilham, 1974, Photosynthetic rates of phytoplankton in East-African alkaline saline lakes: Limnology and Oceanography, v. 19, p. 743-755.

Melchor, R. N., M. C. Cardonatto, and G. Visconti, 2012, Palaeonvironmental and palaeoecological significance of flamingo-like footprints in shallow-lacustrine rocks: An example from the Oligocene-Miocene Vinchina Formation, Argentina: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 315-316, p. 181-198.

Schagerl, M., A. Burian, M. Gruber-Dorninger, S. O. Oduor, and M. N. Kaggwa, 2015, Algal communities of Kenyan soda lakes with a special focus on Arthrospira fusiformis: Fottea, v. 15, p. 245-257.

Scott, J. J., R. W. Renaut, L. A. Buatois, and R. B. Owen, 2009, Biogenic structures in exhumed surfaces around saline lakes: An example from Lake Bogoria, Kenya Rift Valley: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 272, p. 176-198.

Scott, J. J., R. W. Renaut, and R. B. Owen, 2012, Impacts of flamingos on saline lake margin and shallow lacustrine sediments in the Kenya Rift Valley: Sedimentary Geology, v. 277-278, p. 32-51.

Shiba, H., and K. Horikoshi, 1988, Isolation and characterization of novel anaerobic, halophilic eubacteria from hypersaline environments of western America and Kenya: In: Proceedings of the FEMS symposium—The microbiology of extreme environments and its biotechnological potential, Portugal, p. 371-373.

Simmons, R. E., 1995, Population declines, viable breeding areas and management options for flamingos in southern Africa: Conservation Biology, v. 10, p. 504-514.

Svengren, H., 2002, A study of the environmental conditions in Lake Nakuru, Kenya, using isotope dating and heavy metal analysis of sediments: Masters thesis, Dept. Structural Chemistry, University of Stockholm, Swededn.

Tuite, C. H., 2000, The Distribution and Density of Lesser Flamingos in East Africa in Relation to Food Availability and Productivity: Waterbirds: The International Journal of Waterbird Biology, v. 23, Special Publication 1: Conservation Biology of Flamingos, p. 52-63.

Vareschi, E., 1978, The ecology of Lake Nakuru (Kenya). I. Abundance and feeding of the lesser flamingo: Oecologia, v. 32, p. 11-35.

Vareschi, E., 1982, The ecology of Lake Nakuru (Kenya). III. Abiotic factors and primary production: Oecologia, v. 55, p. 81-101.

Vonshak, A., 1997, Spirulina: Growth, physiology and biochemistry, in A. Vonshak, ed., Spirulina platensis (Arthrospina): Physiology, cell-biology and biotechnology: London, Taylor and Francis, p. 43-65.

Warren, J. K., 1986, Shallow water evaporitic environments and their source rock potential: Journal Sedimentary Petrology, v. 56, p. 442-454.

Warren, J. K., 2011, Evaporitic source rocks: mesohaline responses to cycles of “famine or feast” in layered brines, Doug Shearman Memorial Volume, (Wiley-Blackwell) IAS Special Publication Number 43, p. 315-392.

Zhilina, T. N., and G. A. Zavarzin, 1994, Alkaliphilic anaerobic community at pH 10: Curr. Microbiol., v. 29, p. 109-112.

Zhilina, T. N., G. A. Zavarzin, F. Rainey, V. V. Kevbrin, N. A. Kostrikina, and A. M. Lysenko, 1996, Spirochaeta alkalica sp nov, Spirochaeta africana sp nov, and Spirochaeta asiatica sp nov, alkaliphilic anaerobes from the Continental soda lakes in Central Asia and the East African Rift: International Journal of Systematic Bacteriology, v. 46, p. 305-312.

 

 


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