Saline-alkaline Lake Research Articles

Temporal trends of ion contents and nutrients in three Kenyan Rift Valley saline–alkaline lakes and their influence on phytoplankton biomass

Temporal trends of ion contents and nutrients in three Kenyan Rift Valley saline–alkaline lakes and their influence on phytoplankton biomass

Clay mineral assemblages and analcime formation in a Palaeogene fluvial–lacustrine sequence (Maíz Gordo Formation Palaeogen) from northwestern Argentina

The Palaeogene Maíz Gordo Formation is one of the main lacustrine events recorded in northwestern Argentina. It consists of sandstone, mudstone, and limestone beds 200 m thick, deposited in a brackish–alkaline lake and braided alluvial systems. The Maíz Gordo Lake evolved mainly as a closed system, with brief periods as an open one. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study samples from seven sites, corresponding respectively to proximal, intermediate, and transitional positions of the fluvial environment and marginal and inner-lake environment, focusing on the clay mineralogy and analcime formation. The basinward zonation of diagenetic minerals identified in the Maíz Gordo Lake was: mordenite → analcime → K-feldspar. Although not a typical zonation of saline–alkaline lakes, it does indicate an increase in salinity and alkalinity towards the centre. In proximal fluvial settings, smectite predominates at the base of the sequence, with scarce kaolinite. Towards the top, a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons, towards a warm and humid climate. Kaolinite content clearly decreases in a basinward direction. Such a variation is attributable to changes in hydro-geochemistry, denoting the progressive influence of the brackish and alkaline lake water on interstitial pores. SEM images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets. In littoral and inner-lake settings the clay fraction is composed of muscovite, sometimes with subordinate smectite. Analcime occurs in variable amounts in all sedimentary facies, in rock pores or filling veins. It forms subhedral square to hexagonal, or anhedral rounded crystals, denoting that they coarsened at low to moderate degrees of supersaturation. Although the mordenite identified in a fluvial level would have been the precursor of analcime in the Maíz Gordo Basin, no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found. Hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase.

Overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the Madrid Miocene Basin, Spain

The Miocene distal alluvial fan and palustrine deposits of the Madrid Basin (Paracuellos de Jarama area) were examined to establish the sequence of its pedogenic–diagenetic processes and the main factors controlling them. In this area the diagenetic processes operated not only on carbonates, as commonly studied, but also in high magnesium clays and opaline cherts. This paper provides a dynamic model for saline–alkaline lake margins that complements the existing freshwater palustrine model. Three sections (BER, PEL and UPC) containing limestones, sepiolite, dolostones and opaline cherts were examined; these represent the entire transition from alluvial fan deposit to lake environment. The alluvial fan deposits (PEL section) show the most complex sequence of processes. After a weak edaphic imprint, the primary mudstone and siltstones are affected by silicification (opaline levels formed mainly by selective replacement of sepiolites) and later dolomitization. Several types of dolomite are present, rounded crystals, spherules and globular bodies being the most characteristic. In the ponds situated at the foot of the alluvial fans or in interfan areas, sepiolite precipitated within free water bodies (BER section) and an intense pedogenesis led to the formation of palustrine sepiolite deposits. Later, the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging. In the shallow lakes (UPC section), palustrine limestones and sepiolite were deposited. The carbonates are partially affected by silicification, the sepiolite becoming completely silicified. The opaline levels clearly preserve the structure of the limestones and sepiolite host rocks. All the opaline levels show local aging processes. The silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies. In the distal alluvial fan sediments of the PEL section, the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures. The silicification environment was therefore not easy to determine. In the BER and UPC section, the main silicification took place in groundwater-influenced environments but evidence was seen of cements and silicifications in vadose environments. In all study settings, the overlapping of pedogenic, vadose and groundwater processes was controlled by changes in the position of the water table. Four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements. The majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin. In all the studied environments, the sepiolite deposits were preferentially silicified; their fibrous structure and high absorption capacity helping to retain interstitial fluids. Moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments. The dissolution of the sepiolite caused an increase in silica in the interstitial fluids, favouring silicification and ruling out the need of a biogenic source. The replacement of limestones by silica is usually driven by variations in pH and salinity, which are very common in surface environments such as shallow lakes and soils.

Toxic cyanobacteria and their toxins in standing waters of Kenya: implications for water resource use

Phytoplankton biodiversity studies in Kenya's standing waters were carried out between 2001 and 2003. Toxin producing cyanobacteria were recorded in twelve water bodies. Microcystis and Anabaena were the most common species in freshwaters while Anabaena and Anabaenopsis were common in alkaline saline lakes. Seven lakes with cyanobacteria blooms and a hot spring had detectable levels of microcystins and anatoxin-a. Cell bound microcystins (LR equivalents) concentration ranged from 1.6-19800 microgg(-1) Dry Weight (DW) while anatoxin-a varied from below the limit of detection to 1260 microgg(-1) DW. In alkaline-saline lakes, microcystins and anatoxin-a were also present in stomach contents and liver samples of dead flamingos. Monoculture strains of A. fusiformis from Lakes Sonachi and Bogoria had detectable levels of microcystins while anatoxin-a was present in strains isolated from Lakes Sonachi, Bogoria and Nakuru. Two freshwater sites, Nyanza Gulf (L. Victoria) and Lake Baringo recorded cyanotoxin concentration exceeding WHO'S upper limit of 1.0 microgl(-1) for drinking water. The results confirm that cyanotoxins could have played a role in the mortality of flamingos in Lakes Bogoria and Nakuru. The implications of these findings on water resource use, measures to be taken to reduce the risk of exposure and eutrophication control steps to reduce cyanobacteria bloom formation are considered in this paper.

The Nature of Zeolite Occurrences in Deposits of the Olduvai Basin, Northern Tanzania

AbstractThe Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

Contribution of toxic cyanobacteria to massive deaths of Lesser Flamingos at saline-alkaline lakes of Kenya

(2005). Contribution of toxic cyanobacteria to massive deaths of Lesser Flamingos at saline-alkaline lakes of Kenya. SIL Proceedings, 1922-2010: Vol. 29, No. 2, pp. 783-786.

The Phototrophic Community Found in Lake Khilganta (an Alkaline Saline Lake Located in the Southeastern Transbaikal Region)

The structure of the phototrophic community found in Lake Khilganta (the Agin-Buryat Autonomous Area), a shallow saline soda lake (depth, 35-45 cm; water mineralization, 45 g/l; alkalinity, 30 mg-equiv/l; pH 9.5) has been studied. The bottom of the lake is covered with a 10- to 15-mm microbial mat, whose basis is formed by the filamentous cyanobacterium Microcoleus chthonoplastes. The mat exhibits pronounced layering and contains a significant amount of minerals. Six zones, which have characteristic colors and consistencies and are composed of intermittent layers, have been identified along the vertical profile. Live phototrophic bacteria have been found in the three upper zones. The bulk of the cyanobacteria is concentrated in the upper zone. In the lower zones, the development of purple bacteria has been observed. The diurnal dynamics of the vertical distribution of phototrophic microorganisms, which results from variations in the physicochemical environmental parameters, is described. Ectothiorhodospira sp. are dominant among the anoxyphotobacteria present. Their number, determined according to the inoculation method, is 10(6)-10(7) cells/ml. The purple bacteria of the genera Allochromatium, Thiocapsa, and Rhodovulum are also present. Experiments with isolated pure cultures have shown that the anoxygenic photosynthetic bacteria of Lake Khilganta are halotolerant and alkalitolerant or alkaliphilic. In liquid enrichment cultures, at pH 9.5, the ratio of anoxyphotobacteria species is close to that observed in the lake. When the pH is increased to 10.4, it is Ectothiorhodospira, which is the most adapted to life under increased mineralization and alkalinity, that predominantly develops. Photosynthetic activity has been observed in the three upper mat zones and constitutes, on average, 1.5 g C/(m2 h); the share of anoxygenic photosynthesis accounts for 75-95% of the total productivity. The main role in sulfide oxidation belongs to the phototrophic anoxyphotobacteria and cyanobacteria. In terms of the physicochemical conditions and structure of the phototrophic community, Lake Khilganta is similar to shallow saline water bodies of marine origin. The main differences consist in the increased alkalinity and in the consequent prevalence of alkaliphilic and alkalitolerant microorganisms and in the absence of representatives of the neutrophilic group of green sulfur bacteria.

Phenocryst composition as a tool for correlating fresh and altered tephra, Bed I, Olduvai Gorge, Tanzania

Olduvai Gorge, Tanzania, often cited as a textbook example of continental stratigraphic study (as depicted by Hay 1976 and others) is well known for its abundant Plio-Pleistocene fauna, hominin fossils, stone artifacts, and well-dated tephra layers used widely in the calibration of human evolution. However, precise stratigraphic correlation between various sites within Olduvai has been hampered by the rarity of unaltered volcanic glass suitable for geochemical characterization. Geochemical characterization of tephra for stratigraphic correlation purposes is difficult when fresh glass is absent or secondarily altered. A multi-component approach using the major and minor element compositions of phenocrysts and glass (where present) provides successful results at Olduvai and is proposed here as a viable methodology for use elsewhere. Six widespread Bed I (~2.1-1.79 myr) tephra layers of the Olduvai area are geochemically fingerprinted and used here to show the viability of themulti-component method. This method for geochemically fingerprinting tephra successfully provides a means for distinguishing similar looking tephra layers and for correlating fresh and altered tephra layers amongst a variety of depositional and diagenetic environments (freshwater wetlands, saline-alkaline lake, and fluvial plain). Application of this technique at Olduvai is providing high-resolution stratigraphic correlations and should prove applicable to other volcaniclastic sequences where tephra is poorly preserved.

Methylhopanoids: Molecular indicators of ancient bacteria and a petroleum correlation tool

Methylhopanoids are organic compounds synthesized by certain bacteria, that when preserved in sediments act as molecular fossils or biomarkers for organic matter inputs from specific bacterial sources. Two series of methylhopanoids occur, each mainly deriving from a distinct bacterial source: cyanobacteria (2-methyl) and methanotrophic bacteria (3-methyl). The abundance and composition of methylhopanoids within sediments of modern depositional environments varies widely, apparently due to different bacterial communities contributing to the sedimentary organic matter. Comparable molecular characteristics are found in oils and their source rocks. Consequently, methylhopanoids are valuable in oil-oil and oil-source rock correlations, distinguishing between samples related to different depositional environments. In particular, abundant 3β-methylhopanoids (from methanotrophic bacteria or an additional unknown bacterial source) are characteristic of some modern alkaline saline lake environments. Comparable compositional features in the methylhopanes of oils allow the assignment of lacustrine oils offshore West Africa to two distinct lacustrine source rock facies, and to distinguish between different marine source facies, thus refining oil-source rock correlation.

Cyanobacteria and cyanobacterial toxins in three alkaline Rift Valley lakes of Kenya--Lakes Bogoria, Nakuru and Elmenteita

For decades frequent mass mortalities of Lesser Flamingos (Phoeniconaias minor Geoffroy) have been observed at alkaline-saline Kenyan Rift Valley lakes. To estimate the potential influence of toxic cyanobacteria on these mass deaths, the phytoplankton communities were investigated in Lakes Bogoria, Nakuru and Elmenteita. Cyanobacterial toxins were analyzed both in the phytoplankton from the three lakes and in isolated monocyanobacterial strains of Arthrospira fussiformis, Anabaenopsis abjjatae, Spirulina subsalsa and Phormidium terebriformis. Lake Bogoria was dominated by the cyanobacterium A. fusiformis. In L. Nakuru and L. Elmenteita the phytoplankton mainly consisted of A. fusiformis, A. abjatae and Anabaenopsis arnoldii, and in L. Nakuru an unknown Anabaena sp. was also found. Furthermore, this is the first time A. abijatae and the unknown Anabaena sp. have been found in Kenyan lakes. Phytoplankton wet weight biomass was found to be high, reaching 777 mg L -1 in L. Bogoria, 104 mg L -1 in L. Nakuru and 202 mg L -1 in L. Elmenteita. Using HPLC, the cyanobacterial hepatotoxins microgstin-LR, -RR -γR, -LF and -LA and the neurotoxin anatoxin-a were detected in phytoplankton samples from L. Bogoria and L. Nakuru. Total microcystin concentrations amounted to 155 μg microcystin-LR equivalents g -1 DW in L. Bogoria, and 4593 μg microcystin-LR equivalents g -1 DW in L. Nakuru, with anatoxin-a concentrations at 9 μg g -1 DW in L. Bogoria and 223 μg g -1 DW in L. Nakuru. In L. Elmenteita phytoplankton, no cyanobacterial toxins were found. A. fusiformis was identified as one source of the toxins. The isolated strain of A. fusiformis from L. Bogoria was found to produce both microcystin-YR (15.0 μg g -1 DW) and anatoxin-a (10.4 μg g -1 DW), whilst the A. fusiformis strain from L. Nakuru was found to produce anatoxin-a (0.14 μg g -1 DW). Since A. fusiformis mass developments are characteristic of alkaline-saline lakes, health risks to wildlife, especial the Arthrospira-consuming Lesser Flamingo, may be expected.

Geochemical Signatures of Paleodepositional and Diagenetic Environments: A STEM/AEM Study of Authigenic Clay Minerals from an Arid Rift Basin, Olduvai Gorge, Tanzania

AbstractOlduvai Gorge, Tanzania (East African Rift) exposes a 100 m thick Plio-Pleistocene sequence of dominantly volcaniclastic sediments deposited in a 50 km wide closed basin containing a playa lake. A scanning transmission electron and analytical electron microscopy (STEM/AEM) study of authigenic clay minerals in sediments from representative depositional environments in the basin (pyroclastic fan, fluvial plain, wetland, lake margin and lake basin) was undertaken to determine whether clay compositions and textures could provide unique geochemical fingerprints characteristic of source area (Plio-Pleistocene trachytic volcanics vs. Precambrian quartzose-feldspathic basement) or paleoenvironmental conditions.Our study shows that compositional signatures obtained by clay minerals during early pedogenesis are inherited from their parent source rocks. Sediments sourced from volcanics contain highly disordered, dioctahedral smectite. Those sourced from Precambrian basement are similar, but are more Al-rich. Subsequent neoformation in the pedogenic (soil) or diagenetic (lake-margin, lake) environments results in the modification of original clay mineralogy, compositions, and textures, and unique paleoenvironmental fingerprints are acquired. Soils developed on the distal pyroclastic fan contain smectite with more Fe(III) and Mg than smectite from the proximal pyroclastic fan sediments. A trend of decreasing Al and increasing Mg content occurs in smectite compositions in samples from the fluvial to lake-margin and lake environments as a result of partial replacement of original dioctahedral Al-rich smectite by neoformed trioctahedral Mg-rich smectite (stevensite). Neoformed celadonite replaces smectite in the most saline lake sediments.The STEM/AEM data collectively indicate that diagenesis in the saline-alkaline lake results in the replacement of Al-rich dioctahedral smectite by Mg-rich trioctahedral smectite (stevensite) and Mg- and Al-rich celadonite. Thus, determination of clay mineral compositions at a basin-wide scale provides a useful tool for interpreting the spatial distribution of depositional and diagenetic environments.

Environmental changes in a tropical lake (Lake Abiyata, Ethiopia) during recent centuries

Lake Abiyata is a small, closed, saline–alkaline lake located in the central part of the Ethiopian Rift Valley, East Africa. A multi-proxy study of a sediment core, 116 cm long and with undisturbed mud–water interface, was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries. The 210Pb analyses suggest that the upper 80 cm of the core represent the past 200 years. This study complements millennial-scale environmental records available in the region. The main freshwater-climatic and biological features of the modern lake system and their variations over the past decades, as known from observations, are first summarised. Results derived from individual proxies analysed along the core are then presented (successively, major physical and chemical properties of bulk sediments, diatoms and pollen). Uncertainties on the chronological framework are discussed. Major limnological stages are finally identified based on the multi-proxy interpretation of our record. Our record shows large variations in the lake water and salt balances, in the sediment sources, and in the vegetation distribution in the basin. Using our 210Pb chronology, major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records. The upper 41 cm of the core ( 210Pb age: 1940–1998 AD) reveal several fluctuations in diatom-inferred water depth and salinity which seem to be consistent with known changes in water level. Human impact on vegetation clearly appears since about 30 years. The interval 85–41 cm suggests a period of overall water deficit. Lake Abiyata experienced episodes shallower and more saline than over the past decades, especially around 68–66 cm, 210Pb dated at ca. 1890 AD. This level may coincide with one of the worst droughts known in the Ethiopian history during 1888–1892. The lower part of the core includes a stage (108–85 cm) of lake level much higher than today and which ended before 1800 AD. Although its base is undated so far, this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century. Lake Abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past, although further work is needed to reduce the uncertainties on our record, as discussed in the conclusions.

Ciliated protozoa in Lake Nakuru, a shallow alkaline-saline lake in Kenya: Seasonal variation, potential production and role in the food web

Six sampling sites in Lake Nakuru were sampled monthly for 8 months spanning rainy and dry seasons, and including an interval when Spirulina-dominated the phytoplankton community. A total of 33 species in 29 genera were observed. Small bacterivorous ciliates, especially Cyclidium, were numerically dominant, but the greatest biomass was due to the large ciliate Condylostoma. Abundance ranged from 10.4 to 14,200 ciliates mL -1 (mean ± SE = 1,150 ± 309 ciliates mL -1 ) while biomass ranged from 0.017 to 500 mg C.L -1 (mean 18.2 ± 10.8 mg C.L -1 ). Ciliates were generally large (mean equivalent spherical diameter 43 μm) compared to previous studies of marine or freshwater plankton. Using published maximum growth rates for the species, we calculated that ciliate production could be as much as 6.4 ± 2.3 mg C L -1 , which is higher than previous estimates of primary production, and indicates that ciliates probably play a major role in energy flow in this system. Most of the estimated production was by large ciliates, the algivorous Frontonia (58 %) and the predacious Condylostoma (33 %), while the more numerous bacterivores contributed only 4%.

Cyclic variations of uranium concentrations and oxygen isotopes in tufa from the middle Miocene Barstow Formation, Mojave Desert, California

Uranium concentrations of 42 to 169 ppm in fibrous calcite from spring-deposited tufa in the middle Miocene Barstow Formation, California, are among the highest reported for calcite. Fission-track maps of multiple bands of uniformly dull-luminescent fibrous calcite show that the concentration of U increases in the outward growth direction of the calcite of each individual band. Homogeneous dull luminescence in the fibrous calcite indicates no change in redox conditions of the fluid from which the calcite was precipitated. It is proposed that the cyclic pattern of increase in U concentration reflects a cyclical change in the U/Ca ratio in the fluid. Episodic mixing between Ca-rich spring water with a low U/Ca ratio and Ca-poor saline alkaline lake water with a high U/Ca ratio could produce the episodes of formation of fibrous calcite with increasing U concentrations. The spring water supplies the Ca for calcite precipitation, and the U concentrations increase with the decreasing fraction of spring water and increasing fraction of lake water. These cycles reflect a variable recharge of groundwater into the lake by springs. The large variation in U concentrations suggests mixing of 0% to 45% spring water with the lake, while the narrow range in δ 18 O of −6.37‰ to −6.87‰ limits the variation of the proportions of spring and lake water to 5%.

Oxidation of ammonia and methane in an alkaline, saline lake

The oxidation of ammonia (NH3) and methane (CH4) was investigated in an alkaline saline lake, Mono Lake, California (U.S.A.). Ammonia oxidation was examined in April and July 1995 by comparing dark 14CO2 fixation rates in the presence or absence of methyl fluoride (MeF), an inhibitor of NH3 oxidation. Ammonia oxidizer‐mediated dark 14CO2 fixation rates were similar in surface (5–7 m) and oxycline (11–15 m) waters, ranging between 70–340 and 89–186 nM d−1, respectively, or 1–7% of primary production by phytoplankton. Ammonia oxidation rates ranged between 580–2,830 nM d−1 in surface waters and 732–1,548 nM d−1 in oxycline waters. Methane oxidation was examined using a 14 CH4 tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and rates in oxycline and anaerobic bottom waters (0.5–37 and 7–48 nM d−1, respectively) were 10‐fold higher than those in aerobic surface waters (0.04–3.8 nM d−1). The majority of CH4 oxidation, in terms of integrated activity, occurred within anoxic bottom waters. Water column oxidation reduced the potential lake‐atmosphere CH4 flux by a factor of two to three. Measured oxidation rates and water column concentrations were used to estimate the biological turnover times of NH3 and CH4. The NH3 pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH4 is cycled on 103‐d time scales in surface waters and 10−2 time scales within oxycline and bottom waters. Our data suggest an important role for NH3 oxidation in alkaline, saline lakes since the process converts volatile NH3 to soluble NO2−, thereby reducing loss via lake‐atmosphere exchange and maintaining nitrogen in a form that is readily available to phyto‐plankton.

The last glacial maximum in the Basin of Mexico: The diatom record between 34,000 and 15,000 years BP from Lake Chalco

Lacustrine sediments from a section of a core extracted from the central part of the former Lake Chalco were analysed for diatom content. Radiocarbon dates and tephra correlations bracket the period under study between 34 and 15 ka BP. According to the diatom stratigraphy, Chalco was an alkaline-saline lake during the interval 34–31.5 ka BP. After 31.5 ka BP, it went through a shallower and more concentrated phase until ca . 23.5 ka BP. At ca . 23.5 ka BP, a major environmental change took place during which Lake Chalco was transformed from an saline-alkaline pond into a freshwater lake. Previous archaeological work at Tlapaocya indicates that humans entered the Basin slightly after this environmental change, by ca . 22 ka BP. At 19 ka BP, another reduction in water level occurred with the onset of a period of quick fluctuations between slightly acidic and neutral water. These variations were probably associated with intensified volcanic activity that affected the area between 19 and 15 ka BP.

Ultrastructure and seasonal growth patterns of microbial mats in a temperate climate saline-alkaline lake: Goodenough Lake, British Columbia, Canada

Goodenough Lake is a small (ca. 1 by 0.6 km) saline-alkaline lake on the Caribou Plateau in British Columbia, Canada, which has a mean depth of < 1 m, allowing the growth of substantial prokaryote-dominated benthic microbial mats over the entire lake bottom. Microbial mats were investigated on both a detailed ultrastructural level and at a macroscopic level to determine, on a lakewide scale, the seasonally influenced changes that occurred over the course of a spring to fall growth season. The acceleration of phototrophic activity with warming of the lake water in spring could be followed by increases in dissolved oxygen levels, which quickly reached supersaturation early in the summer. The grazing activity of brine fly larvae (Ephydra hians) was found to have a significant influence on mat growth and development patterns. On an ultrastructural level, several types of filamentous cyanobacteria were the main phototrophic organisms present, even in anaerobic parts of the mats, where they were accompanied by purple bacteria. The dominant cyanobacterium present was a large oscillatoriacean form which, in certain parts of the lake, formed unusual columnar structures that had a very low diversity of microbial species in comparison to mats from other regions of the lake. The microbial community also differed in areas of the lake where less saline groundwater springs emerged from the lake bed.Key words: microbial mats, saline-alkaline lakes, cyanobacteria, ultrastructure.

The zeolite deposits of Greece

Zeolites are present in altered pyroclastic rocks at many localities in Greece, and large deposits of potential economic interest are present in three areas: (1) the Evros region of the province of Thrace in the north-eastern part of the Greek mainland; (2) the islands of Kimolos and Poliegos in the western Aegean; and (3) the island of Samos in the eastern Aegean Sea. The deposits in Thrace are of Eocene-Oligocene age and are rich in heulandite and/or clinoptilolite. Those of Kimolos and Poliegos are mainly Quaternary and are rich in mordenite. Those of Samos are Miocene, and are rich in clinoptilolite and/or analcime. The deposits in Thrace are believed to have formed in an open hydrological system by the action of meteoric water, and those of the western Aegean islands in a similar way but under conditions of high heat flow, whereas the deposits in Samos were formed in a saline-alkaline lake.

Chemical evolution of shallow groundwater along the northeast shore of Mono Lake, California

We attempted to quantitatively discriminate between hypothesized sources and geochemical processes responsible for the chemical evolution of shallow groundwater along the northeast shore of Mono Lake, an alkaline saline lake located in a hydrologically closed basin in east central California. Shallow groundwater samples from 17 sites perpendicular and 11 sites parallel to the lakeshore were analyzed for major ions. The shallow groundwater contains remnant solutes from higher lake stands, which are transported toward the lake by lateral flow and surface runoff. The flow system appears to be segregated into two different regions: a concentrated, highly saline groundwater underlying much of the northeast shore and pockets of more localized lower‐salinity groundwater. The saturation state of the groundwater with respect to certain minerals was determined, and simulations for both evaporative concentration of inflow and mixing of lake water with inflow coupled with mineral precipitation were performed. Solute trends in the shallow groundwater result primarily from the degree of mixing with historical lake water; however, evaporative concentration and redissolution cycles along with various chemical fractionation mechanisms including Ca, Mg, and Na carbonate precipitation, sulfate reduction, ion exchange, and potentially Mg silicate formation are also important controls.

Geochemical and Paleoenvironmental Analysis of Lacustrine Arthropod-Bearing Concretions of the Barstow Formation, Southern California

Calcareous concretions containing well-preserved aquatic and terrestrial arthropods are found within lacustrine sediments of the middle Miocene Barstow Formation of the Mojave Desert. The concretions occur in situ within three distinct zones (each <1 m thick) of microlaminated, non-bioturbated muds, interlaminated with thin, clay-rich carbonates. Several lithologies were recognized in the three concretion-bearing zones on the basis of color, grain size, lamination thickness and composition. Minerals identified in the laminae include: zeolites, clays, sulfates, borates, carbonates, and Fe-rich sulfides and oxides. Mineral composition varies between individual lamina. Carbon and oxygen isotopes measured within individual concretions and their surrounding sediments show an enrichment of 6180 for the uppermost (third) zone with respect to the other two zones, demonstrating an evaporative trend through time. Similarly, a depletion of 613C over the same interval, indicates progressively increased precipitation of carbonate, increase in HCO3production, and probable oxidation of organic matter during the deposition of the lake sediments. Thus, the concretions formed during early diagenesis in the distal, dysaerobic, low depositional energy, low bioturbated portions of a stratified, saline-alkaline lake. Concretion formation process was initiated after a release of CO2 and NH3, most likely from decomposition of organic matter. This release would have increased the local alkalinity in the pore waters, promoting an increase in carbonate ion concentration, and thereby facilitating carbonate precipitation. INTRODUCTION