by William M. Last and Fawn M. Ginn
Department of Geological Sciences, University of Manitoba, Winnipeg, Canada R3T 2N2
Introduction
"We passed during the day many salt lakes, fringed round the edges with thick encrustations of salt, highly indicative of the rapid evaporation that takes place in these arid regions." (Palliser, 1862)
As pointed out in this quote from one of the first scientific efforts in the northern Great Plains of western Canada, in much of the region ponded saline brines are the only natural surface waters present. In fact, this region contains an estimated 3.5 million lakes and another 6-8 million 'sloughs', most of which are saline or hypersaline (Last, 1988; 1989a). These lakes show a tremendous diversity in size, basin morphology, hydrology, hydrochemistry, and sedimentary and biological characteristics. In this water stressed region of North America, any changes in lake salinity or chemistry are of major concern to the environmental manager.
Salinity has a significant impact on the emergent vegetation of the lake's littoral zone and thus influences the value of the area as a waterfowl nesting and staging ground (Conly and van der Kamp, 2001). These lakes, collectively, act as a major breeding ground for over 80% of North America's ducks (Batt et al., 1989; Scott and Scott, 1986; Sorenson et al., 1998). In addition to the importance of this surface water within the realm of wildlife conservation, future industrial and agricultural development in the Great Plains will likely lead to conflicts and potential environmental problems associated with the lakes in the region (Cameron, 1986; Harrington et al., 1997). For example, historically western Canada has been the world's leading producer of natural sodium sulfate (‘glauber's salt' or mirabilite: Na2SO4 10H2O), with sales of the product contributing over $50,000,000 annually to the Canadian prairie economy (Last, 1994; Last and Slezak, 1987). This valuable industrial mineral was extracted by solution and open pit mining of deposits in the saline lakes of the region. If the North American demand for Na2SO4 returns to levels of the late 20th century, more lakes will be leased and more mining operations initiated. Increased industrial development of this type will have a profound impact on the wetlands resource. In short, high priority must be given to the study and understanding of the salinity and chemistry of the lakes in this region and the factors that control their chemical variability.
Comments About Units And Nomenclature
Water salinity, while a very simple concept, is often confusing due to the large variety of methods used to measure this basic parameter (Williams and Sherwood, 1994) and a plethora of nomenclature applied. Biological limnologists and ecologists often use conductivity as a measure of salinity. Conductivity, or specific conductance, is a measure of the ease with which electrical current will pass through the water: in general, the greater the salinity, the greater the conductivity. This relationship, however, is not straightforward and is controlled by the specific ions present in the solution as well as the level of concentration of the ions. For example, a conductivity of 126 mS20 in a lake such as Bitter Lake in southwestern Saskatchewan, dominated by sodium and chloride ions, would be equivalent to about 100 parts per thousand (ppt or ‰) total dissolved solids (TDS), but this same conductivity value would be recorded in a brine having only about 75 ppt TDS that was dominated by magnesium and sulfate ions (Desai and Moore, 1969). Clearly, the use of conductivity to quantitatively evaluate salinity in lakes having diverse chemical compositions, such as these basins in western Canada, should be avoided.
The quantity or abundance of individual ionic components in the water is usually reported as weight concentration of the ion in solution (i.e., the weight of the dissolved ion in g or mg per kg of solution). This is preferred to the use of weight per litre because of the large increase in density of the solution at high salinities. Care must be exercised in evaluating and comparing published reports using g L-1 units; unless density is taken into account by the analyst, the difference can be substantial at elevated salinities (>7 ppt TDS). For example, the bottom water of Freefight Lake, a deep, meromictic lake in Saskatchewan, is 275 ppt (weight/weight) or 340 g L-1. It follows then that the total salinity (total dissolved solids) should be the sum of the measured ionic components.
Terminology also varies widely for the various levels of salinity, as reviewed by Williams (1967), Carpenter (1978) and Hammer (1986). Most biological limnologists use the classification scheme of: fresh water (< 1‰), subsaline (1-3‰), hyposaline (3-20‰), mesosaline (20-50‰), and hypersaline (> 50‰). Groundwater researchers often refer to fresh water as < 1‰, brackish water as 1-10‰, saline water as 10-100‰, and brine as > 100‰. Finally, most geoscientific literature uses: fresh water (< 3‰), saline (3-35‰), and hypersaline (> 35 ‰).
The Northern Great Plains: A Regional Picture
The northern Great Plains (NGP) geomorphic province of western Canada is a vast region of North America comprising over 350,000 km2. Stretching from the Precambrian Shield near Winnipeg, Manitoba, westward for over 1600 km to the foothills of the Rocky Mountains (Figure 1), the region is the agricultural heartland of Canada and also contains most of the population of western Canada. The NGP is characterized by flat to gently rolling topography and experiences a cold, semi-arid climate. Pleistocene continental glaciation has resulted in a thick sequence of unconsolidated glacial, glaciofluvial, and glaciolacustrine sediment mantling the generally flat-lying Cretaceous and Tertiary sedimentary bedrock.
From the standpoint of salt lake geochemistry, the two most important physical features of the region are the high evaporation to precipitation ratios, and the presence of large areas of endorheic drainage. Although the mean annual temperature of ~3°C would imply relatively low evaporation rates, the high winds, low humidity, and warm summer temperatures create evaporation/precipitation values of generally between 3 and 10 (Figure 2). The average annual moisture deficit over the region is about 350 mm. These climatic features combined with the poorly integrated drainage, in which nearly 45% of southern Saskatchewan and eastern Alberta is topographically closed (Figure 3), result in a large number of saline lakes of diverse morphologies and geochemical characteristics. In contrast to many other areas of the world in which there is an abundance of salt lakes (e.g., see summaries in Jones and Bowser, 1978; Jones and Deocampo, 2003), the northern Great Plains region is tectonically very stable, does not exhibit striking topographic relief, nor is there dramatic lithologic diversity of either the glacial deposits nor the bedrock on a local or regional scale.
Compiling A Regional Database: 80 Years Of Lacustrine Hydrochemistry
The saline nature of the surface waters and associated lake deposits of the northern Great Plains was well known to local aboriginal groups at the time of European expansion and settlement in the region during the mid-19th century. Although the first chemical analyses of salt lake waters from this region were published in the 1890's, it was not until nearly half a century later that the full extent and degree of diversity of the salt lakes were recognized. For over thirty years after the 1890's, the only scientific efforts and data on these lakes came from economic geologists interested in exploitable reserves of initially nitrate salts and later sodium and magnesium salts in the lakes. Indeed, the results of a regional survey of these economic deposits undertaken by the Canadian government in the early 1920's (Cole, 1926) still ranks as one of the best and most extensive summaries of the salt lake hydrogeochemistry.
Although still not as advanced as in some other parts of North America, our knowledge of the chemistry of the surface waters in the Canadian Great Plains has progressed somewhat from these early geological/geochemical reconnaissance efforts. In one of the first systematic limnological surveys in the region, Rawson and Moore (1944) reported the water chemistry of 53 lakes from southern Saskatchewan. Rutherford (1970) compiled the composition data for several hundred lakes in Saskatchewan (including both fresh and saline basins). Hammer (1978) reported the water chemistry for 60 mainly perennial saline lakes in southern Saskatchewan. The results of much of this early regional work have been summarized by Northcote and Larkin (1963), Hammer (1986) and Last (1989a).
Other important contributions covering smaller geographic areas of the Great Plains include: Govett (1958) Bierhuizen and Prepas (1985), Derry et al. (2003), and Evans and Prepas (1996) in central and eastern Alberta; Hartland-Rowe (1966) in southeastern Alberta; Rozkowski (1967) and Roskowska and Roskowski (1969) in the Moose Mountain area of southern Saskatchewan; Lieffers and Shay (1983) and Driver and Peden (1977) in central Saskatchewan; and Driver (1965) and Barica (1975; 1977) in western Manitoba. More recently, reports by Pham et al. (2008; 2009), Kelly and Holmden (2001), and Lemmen and Vance (1999) included water chemistry data of ~65 saline lakes in western and central Saskatchewan.
We now have brine chemistry data from about 800 of the salt lakes in the northern Great Plains of Canada (Figure 4). Although most of these data represent analyses of single samples, some are averages of numerous samples collected over a period of months, years, or decades. In general, the larger lakes (e.g., Lakes Manitoba, Quill, Manito, etc.) have the longest temporal records, in some cases dating back to the early twentieth century. However, no lake in the Canadian prairies has a continuous monitoring record of more than four decades in duration. Of the lakes for which there are data, 10% are located in Manitoba, 72% in Saskatchewan, and 18% in Alberta.
What Do We Know About The Chemistry Of These Salt Lakes?
Even though most of the lakes in the Great Plains of western Canada have similar overall origins, nonetheless, the waters show consider diversity in terms ionic composition and concentration. The early investigators (mainly economic geologists concentrating on the most concentrated saline brines), emphasized a strong predominance of Na and SO4 in the lakes (e.g., Cole, 1926; Tomkins, 1954a, b). Rutherford (1970) and Hammer (1978) similarly stressed the importance of sodium, magnesium and sulfate components in the perennial lakes of Saskatchewan, but also recognized a broad spectrum of water types on the basis of ionic ratios. Rutherford (1970) was able to relate spatial variation in water types to climatic gradients within the province and to shallow groundwater composition. We now realize that not only is there a complete spectrum of salinities from relatively dilute water (0.1 ppt TDS) to brines more than an order of magnitude greater than sea water (Figure 5), but also virtually every water chemistry type is represented in lakes of the region (Figure 6). Although it is obviously misleading to generalize by quoting means and averages, the 'average' lake water has about 30 ppt TDS and shows (in meq%): Na≈Mg>Ca>K and SO4>HCO3>Cl>CO3.
It is hardly surprising that the lake waters of the northern Great Plains show such a considerable range in ionic composition and concentration, considering the enormous geographic area and the varying hydrologic, geomorphic, and climatic settings. With such a vast range of salinities, it follows that the concentrations of the individual ionic components also vary greatly. The frequency distributions of Mg, Na, Cl, and SO4 concentrations in the lake waters tend to be multimodal as opposed to the Ca and HCO3 ions which show a much narrower distribution pattern (Figure 7). Sulfate and carbonate-rich lakes clearly dominate the Great Plains, comprising over 95% of the total lakes. This paucity of Cl-rich lakes makes the region unusual compared with many other areas of the world (e.g., Australia, western USA; Eugster and Hardie, 1978; Williams, 1981). The cation ratios are much more diverse, with the abundance of all three major types showing approximately subequal proportions.
As would be expected, most of the solutes in the lake waters increase in concentration with increasing total salinity (Figure 8). Sulfate, chloride, and sodium ions show the most statistically significant correlations with TDS, whereas calcium and carbonate concentrations are less directly related to salinity. The proportions of some of the solutes also show a systematic change with salinity (Figure 9). Sulfate increases in relative ionic proportion from less than 30% equivalents in dilute lakes to generally more than 70% in lakes with more than 10 ppt TDS. Calcium and bicarbonate + carbonate proportions show an inverse relationship with salinity, decreasing from over 70% equivalents in the dilute waters to nearly 5% in lakes with more than 25 ppt TDS.
The relatively uniform distribution of lakes in the northern Great Plains for which water chemistry data allows us to examine of the ionic contents on a spatial basis. Last and Schweyen (1983) and Last (1988; 1989a) discuss these regional trends and present isohaline maps for the saline lakes of Saskatchewan, Alberta, North Dakota, and Montana (see Table 1). Lakes with highest Na, Mg, and SO4 concentrations generally occur in the east central Alberta, west central and southern Saskatchewan area, whereas lakes with high alkalinity and Cl contents are found in central Alberta and western Saskatchewan. Lakes with relatively low proportions of Ca and Mg occur in the northern and central parts of the Plains.
A Statistical Analysis Approach: Insight Into Water Composition Controlling Factors
The major ion composition and concentration of these lakes is the result of: (i) a complex interaction between unconsolidated glacial sediments, bedrock, and precipitation/meltwater in the drainage basin, (ii) the composition and amount of groundwater recharge (and discharge) and streamflow in each basin, and (iii) a wide variety of other physical, chemical, and biological processes operating within the water column itself. In general, several types of geochemical approaches have been taken to help understand the major factors controlling surface water chemistry. These include mass balance calculations, thermodynamic equilibrium considerations, and statistical evaluations (see overview in Drever, 1988). In western Canada, both mass balance and thermodynamic calculations have proved valuable in deciphering many of the intrinsic (within-drainage basin) processes important in water composition on a local scale (e.g., Roskowski, 1965; Wallick and Krouse, 1977; Wallick, 1981; Last, 1984). In contrast, on a regional scale, various statistical techniques have been successfully applied to help understand the relationships between the water chemistry and extrinsic environmental factors such as climate, bedrock, geomorphology, and till composition (e.g., Dean and Gorham, 1976; Last, 1992; Winter, 1977). However, these statistical approaches lack the ability to resolve the often important local conditions and processes, however they are essential to our overall understanding of the lacustrine geochemical setting of the region as a whole.
One of the most straightforward ways to analyze the interrelationships within a data set is to examine the simple linear correlations that exist among the various parameters. The concentrations of Na, Ca and Mg in the brines of these lakes are all significantly positively correlated, as are SO4 and Cl. In addition, the ion pairs of Mg-SO4, Mg-Cl, and Na-Cl tend to strongly covary. Importantly, the concentrations of Na and SO4 do not show statistically significant linear correlation, suggesting different suites of processes affect the abundance of each of these ions. The proportions of Ca and HCO3 exhibit significant positive covariation, whereas the proportions of Mg and Na, and HCO3 and SO4 are inversely related.
Using a Q-mode cluster analysis (associations among lakes) Last (1992) subdivided the lakes into two major categories: a group of high salinity (> 20 ppt TDS) lakes and a group characterized by relatively lower TDS values. Each of these major clusters was further divided into smaller groups of lakes as related to their major ion composition (Figure 10).
By combining a variety of morphological (basin area, maximum depth), geological (bedrock type, depth to bedrock, till type), hydrological (drainage basin area, number of streams entering lake, elevation, groundwater composition), and climatic (mean annual precipitation, evaporation, temperature) variables with a lake water chemistry database, Last (1988, 1992) used R-mode factor analysis to identify a set of seven statistical factors that explained over 90% of the variance in the data. These statistical factors can be interpreted in terms of the most important intrinsic and extrinsic controls of water composition and concentration on a regional basis as follows: (i) Over a third of the total variance in the data is explained by the composition of inflowing groundwater. (ii) The evaporation to precipitation ratio of the basin accounted for about 20% of the variance, followed by (iii) the elevation or position of the lake within the drainage basin. Variables related to bedrock type, glacial drift composition, fluvial input, and lake morphology are statistically less important.
Source Of Salts
It is generally well accepted that groundwater plays a very important role not only in the overall hydrology of saline lakes, but also in dictating their hydrochemistry. However, with a few notable exceptions (e.g., Birks and Remenda, 1999; Freeze, 1969; Kelley and Holmden, 2001; Van der Kamp and Hayashi, 1998; Wallick, 1981), groundwater interaction processes with individual salt lake basins in the northern Great Plains are still poorly understood. In contrast, regional subsurface water composition, variation, and hydrodynamics are reasonably well known. As summarized elsewhere (see overviews in Betcher et al. (1995), Brown (1967), Lennox et al. (1988), Pupp et al. (1981), Remenda and Birks (1999), Rutherford (1967)), subsurface water compositions in the region are of several main types. Most of the groundwater in unconsolidated surficial deposits is of low to moderate salinity (< 3 ppt TDS) and dominated by Ca, Mg, and HCO3 ions. In the areas of lowest precipitation (southwestern Saskatchewan and southeastern Alberta), shallow drift groundwater is usually dominated by the SO4 ion rather than HCO3. The shallow bedrock aquifers (Upper Cretaceous and younger rocks) are mainly sodium-bicarbonate in southern Alberta, calcium-magnesium-sodium-sulfate in Saskatchewan, and calcium-magnesium-sodium-bicarbonate in western Manitoba. The deeper Paleozoic and Cenozoic bedrock contains much higher salinity water (up to 300 ppt TDS) that is usually dominated by Na and Cl.
While there is little disagreement that groundwater is an important factor in the hydrology of the salt lakes, the specific origin and ultimate source of the ions in the lakes of the northern Great Plains have been topics of considerable discussion. Some of the early work suggested that the deeply-buried Paleozoic evaporites which occur in the subsurface could be a possible source for the salts in the lakes. Grossman (1968) showed that there is a correlation between the occurrence of sodium sulfate deposits in lakes at the surface and the presence and trends of various salt units in the Devonian Prairie Formation in the region. In contrast, shallow Cretaceous and Tertiary bedrock, as opposed to the deep Paleozoic sequence, has been implicated as the source of at least some of the dissolved components in the lakes (Cole, 1926; Sahinen, 1948; Wallick and Krouse, 1997).
Finally, rather than invoking bedrock sources, there is considerable support for the source of the ions being the Quaternary deposits within which the lakes are immediately situated (Kelley and Holmden, 2001; Rozkowski, 1967; Rutherford, 1970). A variety of physiochemical and biochemical reactions, including cation exchange, dissolution of feldspars, and precipitation of authigenic sulfate, carbonate, and silicate phases in the tills can be documented which support this latter hypothesis. Furthermore, many researchers (e.g., Freeze, 1969; Last, 1984b; Rueffel, 1968a; Rueffel, 1968b; Witkind, 1952) have stressed the close association of the more saline lacustrine brines with buried preglacial and glacial valleys, and have concluded that these buried valleys act as conduits for groundwater supplying dissolved material to the lakes.
Other Important Considerations
Short-term Temporal Variation: A major complicating factor in characterizing the chemistry of the salt lakes of the NGP is that many of the lakes exhibit playa characteristics, filling with water during the spring and early summer and drying completely by late summer or fall. Last and Ginn (2005) estimate that 85% of the salt lakes in this region are influenced by this type of seasonal hydrologic cycle. This strong seasonality of water levels gives rise to dramatic changes in both ion concentrations and ratios, as demonstrated by numerous studies. For example, Ceylon Lake, a salt-dominated playa in southern Saskatchewan, annually undergoes changes in concentration from about 30 ppt TDS to greater than 300 ppt (Last, 1990). This lake also exhibits dramatic fluctuations in ionic ratios on a seasonal basis from a Ma-(Mg)-SO4-HCO3 type in early spring to a Mg-(Na)-Cl-SO4 composition by fall (Last, 1989b). Hammer (1978, 1986) and Last (1984a) summarize the short-term temporal changes in salinity and chemistry of several other saline lakes in the region. Unfortunately, only a few basins in the northern Great Plains have undergone periodic detailed sampling over a period of years.
Brine Evolution: The composition of any closed basin lacustrine brine is ultimately determined by two main factors: (i) the solutes are acquired by dilute inflow waters through weathering processes and by atmospheric fallout, and (ii) subsequent evaporation and concentration of ions leads to precipitation of minerals, which further affects the final brine composition. This latter change in lake water composition is referred to as brine evolution and has been the subject of considerable scientific interest (e.g., Jones, 1966; Jones and van Denburgh, 1966). Working with natural waters of the Sierra Nevada region in western United States, Garrels and McKenzie (1967) first pointed out that mineral precipitation, triggered by evaporative concentration, is the primary control of brine evolution. Hardie and Eugster (1970) subsequently generalized the evolutionary scheme and concluded that are three main brine evolutionary pathways resulting in five dominant brine types in evaporitic lacustrine basins (Figure 11).
Although there have been many modifications of the basic Hardie-Eugster evolutionary scheme, the most important contribution of the model is that of the chemical divide. A chemical divide is a point in the evolution sequence of a brine in which precipitation of a mineral depletes the water in certain cations or anions and further evaporation moves the solution along a distinct pathway. The result of this process is that small differences in ionic ratios in the dilute starting composition of the lake are amplified as the lake water evolves and produce brines of different and diverse composition.
Because of their relatively low solubility, the calcium carbonates (calcite, aragonite) are usually the first to precipitate; these comprise the first divide for most continental brines (Figure 11). The proportions of Mg, Ca, and HCO3 in the dilute parent solution then determine the subsequent evaporation pathway and the Mg/Ca ratio determines which specific carbonate mineral will precipitate. If calcium is enriched relative to carbonate alkalinity, then the brine will follow pathway I after the initial CaCO3 precipitate divide. Further evaporation of this type of brine will lead to a second divide: the precipitation of gypsum. After this, further evolution will be controlled by the relative proportions of Ca and SO4 (i.e., pathways III and IV in Figure 11).
If, however, HCO3 is enriched compared to Ca in the dilute inflow solution, then the brine evolution will follow pathway II after the initial divide. In this path, calcium will ultimately be depleted leaving an excess of HCO3, which, in turn, may combine with Mg and Na to produce a variety of complex Na-Mg-carbonate-sulfate evaporites. A second divide in this evolution pathway is that of sepiolite [Mg4Si6O15 6H2O]. Further brine evolution after this divide is controlled by the relative concentrations of Mg2+ versus HCO3-. If the magnesium concentration is greater than the remaining alkalinity, the brine will evolve to a sulfate or chloride end member (V in Figure 11). Conversely, the water will become an alkali carbonate brine (VI in Figure 11) if magnesium is less than HCO3 alkalinity after sepiolite precipitation.
While this model works well in theory and the principles of chemical divides and evolutionary paths are valid, clearly the model is an oversimplification of a complex series of sedimentary and geochemical processes. Only relatively recently have we begun to understand this complexity (e.g., Drever, 1988; Herczeg et al., 2001; Jones and Deocampo, 2003). In fact, few continental brines actually follow any of the paths outlined in this model. For example, in path II, Mg-silicate (sepiolite) is rarely found as a primary mineral and thus does not seem like a reasonable divide. Also, the model implies that only a relatively small number of brine types will evolve from a typical dilute inflow; this clearly is not the case and there are many common brine types not represented. For example, the typical Na-Mg-SO4-HCO3 brine that is very common in the northern Great Plains is not represented.
However, because of the wide spectrum of water chemistry types exhibited by the lakes of the Great Plains, these lakes provide critical information to help better understand continental brine evolution. Since salt minerals are thermodynamically and kinetically responsive to even relatively minor changes in brine composition, the basins in the Great Plains having relatively thick, continuous sequences of Holocene evaporites provide a glimpse at a complex series of evolutionary sequences. Last (1995) used Markov chain analysis to identify four generalized anion sequences and five cation sequences in the Holocene evaporates of several dozen Great Plains lakes. The most commonly occurring cyclicity among the anions (occurring in ~50% of the lakes) is: CO3 → CO3-SO4 → SO4. This anion sequence was best represented in Ceylon Lake in south-central Saskatchewan and was thus termed the Ceylon type. The three other anion sequences, which occur less frequently, are:
Alsak type (~20%): CO3 → Cl-SO4 → SO4
Metiskow type (~10%): SO4 → CO3-SO4 → CO3
Waldsea type (~10%): SO4 → CO3
The cation evolutionary sequences present in the 24 study lakes were considerably more complex than the anion sequences and about 20% of the stratigraphic sections exhibited no statistically significant temporal compositional trends. The most common cation sequences present in about 60% of the lakes, are the Lydden type (33%): Ca → Ca-Mg → Na → Na-Mg-Ca and Ingebright type (28%): Na-Mg → Ca-Na-Mg → Na-Mg → Na
followed by:
Metiskow type (~15%): Ca → Na-Mg → Ca-Mg-Na → Na
Little Manitou type (~10%): Ca-Mg → Mg-Na → Mg
Freefight type (~5%): Ca → Mg-Ca → Mg-Na
Because of the complexity of the interplay between intrinsic processes (i.e., sedimentary, geochemical, hydrologic, and biological processes operating within the lake basin itself) and extrinsic processes (i.e., 'external' factors, such as climate change, drainage basin modification), identification of the causal mechanisms for these various evolutionary sequences is not straightforward. Clearly, much more quantitative data from the evaporites of these and other saline lakes in the region need to be collected in order to explain and properly model the observed composition trends.
Biological Processes Affecting Salt Lake Chemistry: Overall, the biological processes in salt lakes of western Canada are similar to those in fresh standing waters, notwithstanding their physical and chemical extremes. The biota, however, differ significantly between fresh and saline lakes (Hammer, 1986). At low salinities the species composition of salt lakes is comparable to that of their fresh water counterparts (Evans, 1993). As salinity increases, the diversity of species declines (Haynes and Hammer, 1978), and as salinities reach extremely high values, species diversity becomes very low. At these elevated salinities, the lake is usually dominated only by halotolerant organisms.
Saline and hypersaline lakes have some of the highest measured rates of organic productivity in the world (Warren, 1986). At moderate to high salinities (30-100 ppt TDS), the main contributors to this biomass are green algae and cyanobacteria. At more elevated salinities halophilic bacteria dominate the ecosystem.
Many biological processes can affect the chemistry of saline lakes (Figure 12). For example, photosynthesis by aquatic plants, ammonification, denitrification, sulfate reduction and anaerobic sulfide oxidation cause a rise in pH as carbon dioxide is utilized by the flora, and an increase in the concentration of HCO3. Decay of organisms, in turn, can lead to a release of ions such as Mg and Ca, as well as elevated levels of HCO3 creating favorable conditions for carbonate precipitation (Castanier, 1999; Riding, 2000; Visscher et al., 1992).
The organisms that can thrive at high salinities in a saline lake are greatly restricted. Life at high salt concentrations requires considerable energy to maintain the steep ion gradient across the membrane required for osmoregulation (Orhen, 2002). The specific type of metabolism also determines the limit of salt concentration that an organism can withstand. Thus, in most saline lake environments, as salinity increases, the diversity of organisms decreases.
Ionic composition also affects species diversity. Chloride, bicarbonate and sulfate are most important in controlling species composition of salt lakes (Herbst, 2001). The striking increase in proportion of SO4 with increasing salinity in the lakes of the northern Great Plains (Figure 9b) means that sulfate reducing bacteria (SRB) are a dominant taxa. The reduction of sulfate by SRB leads to the production of bicarbonate ions and thus the generation of alkalinity according to:
SO42- + 2CH2O → HS- + 2HCO3-, where 2CH2O represents organic matter. The direct effects that SRB have on the environment (i.e., reduction of sulfate and production of H2S and alkalinity) can, in turn, directly affect the solubility and precipitation/dissolution of a wide variety of minerals, including carbonates, silicates, oxides, sulfides, and many evaporites Thus, bacterial sulfate reduction is an important mineralization process in the saline and hypersaline systems of western Canada.
When the end product of sulfate reduction, S2-, is produced, the fate of the sulfide is key in determining whether or not certain minerals will precipitate (Castanier, 1999). If the generated sulfide degasses, the sulfate reduction process will result in an increase in pH in the aqueous environment, thereby encouraging carbonate mineral precipitation. Similarly, if the sulfide is taken up by sulfide-oxidizing microbes, pH will also increase and carbonate will precipitate. In contrast, if the sulfide is oxidized back to sulfate within the aqueous environment, a strong acid, H2SO4, may form, decreasing the pH and discouraging carbonate precipitation. Finally, if the sulfide remains in the environment, the pH will decrease and no carbonate will precipitate.
The role of these organisms in the saline lake ecosystems of the northern Great Plains are critical areas of investigation, especially with respect to their potentially important role in the formation and diagenesis of carbonate minerals. A better understanding of the biomineralization processes in these lakes provides important insight into the evolution of the brine systems and will allow for the development of critical proxies for changes in the environmental conditions. Furthermore, these biomineralizaton processes will help us better understand what to look for in the search for life in extraterrestrial environments.
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Figure Captions
Figure 1. Map showing the location of the northern Great Plains of western Canada (yellow) in North America.
Figure 2. Sketch map of the northern Great Plains of western Canada showing major drainage systems and areas of internal drainage (red).
Figure 3. Sketch map of the northern Great Plains of western Canada showing major drainage systems and areas of internal drainage (red).
Figure 4. Maps showing the distribution of saline lakes in the northern Great Plains for which water chemistry (top) and modern sediment composition (bottom) data have been compiled.
Figure 5. Bar graph showing the frequency distribution of salinities (ppt TDS) of the lakes of the northern Great Plains.
Figure 6. Trilinear diagrams (meq%) showing the range of ionic compositions of saline lakes of the northern Great Plains. Each dot represents one lake.
Figure 7. Bar graphs showing the frequency distributions of the major cations (A) and anions (B) relative to salinity in the lakes of the northern Great Plains.
Figure 8. Scatter plots of cation (A) and anion (B) concentration versus salinity. Note that both axes are logarithmic. Best-fit trend lines are significant at the 0.01 confidence level.
Figure 9. Scatter plots of cation (A) and anion (B) proportions (%meq) versus salinity.
Figure 10. Q-mode cluster analysis of water chemistry data from the Northern Great Plains showing (A) the subdivision of lakes into two major groups: Cluster I (high salinity) and Cluster II (low salinity) and (B) the mean composition of each subcluster. Percentage figures on the bars of the histogram are the proportion of lakes in each subgroup.
Figure 11. Summary of the Hardie-Eugster model for evolution of non-marine waters by evaporative concentration (modified from Drever, 1988).
Figure 12. Summary diagram showing some of the major biological processes affecting water composition in a typical perennial stratified saline lake in the northern Great Plains.