Florias Mees

Holocene evolution of the crater lake at Malha, Northwest Sudan

A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 14C years BP.Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lakes Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso- to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool.The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early- to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lakes history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point.

Stable isotope chemistry of gaylussite and nahcolite from the deposits of the crater lake at Malha, northern Sudan

Abstract Gaylussite (Na 2 CO 3 ·CaCO 3 ·5H 2 O) and nahcolite (NaHCO 3 ) in deposits of the crater lake at Malha are predictably characterized by a strong 18 O enrichment that is typical of evaporitic environments. Both minerals also yield high δ 13 C values, which, in the case of gaylussite, are covariant with the δ 18 O values. The fact that the covariant trend, which is a feature that is commonly recognized for the deposits of closed basins, pertains both to the synsedimentary and pedogenic gaylussite deposits of the Malha basin demonstrates that 13 C enrichment was mainly controlled by inorganic processes. An important biological control on the carbon isotope composition is also contradicted by the occurrence of a covariance for synsedimentary gaylussite that mainly formed by bottom growth in a well-mixed water mass. Degassing of carbon dioxide is the most likely cause of 13 C enrichment, although this implies that CO 2 loss is a process that occurs concurrently with the evaporation of water, without, as could be expected, being driven by changes in the relative abundance of compounds within the CO 2 –H 2 O system. The lack of a covariance of δ 18 O and δ 13 C for nahcolite seems to be partly related to bacterial activity, which did play an important role during lake stages with nahcolite formation, and partly to the lack of a further increase of δ 18 O with continued evaporation at advanced stages of the evaporitic concentration of brines.

Distribution patterns of gypsum and kalistrontite in a dry lake basin of the southwestern Kalahari (Omongwa pan, Namibia).

The deposits of the Omongwa pan, southwestern Kalahari, Namibia, are partly gypsiferous and locally contain small amounts of kalistrontite. The distribution patterns of gypsum (CaSO4.2H2O) and kalistrontite (K2Sr(SO4)2) are mainly determined by groundwater depth and by the lithological composition of the deposits. The latter determines the hydraulic conductivity for saturated and unsaturated flow and therefore controls the depth of the interval within the deposits where evaporation and mineral precipitation can take place. Other factors that affect the distribution of the evaporite minerals are the patterns of groundwater flow within the basin and the occurrence of a redistribution of salts during short flooding stages. Gypsum occurs as crystals of four distinct morphological types, which each developed in different conditions: prismatic crystals formed both as subaqueous precipitates and as crystals that developed within the sediment matrix of a brine-saturated surface layer; sub/euhedral tabular crystals formed within brine-filled macropores; hemi-bipyramidal crystals formed in a nearly permanently brine-saturated part of a subsurface horizon (phreatic zone), as the product of recrystallization of an older gypsum occurrence; and lenticular crystals formed by intrasediment growth in the vadose zone. The development of these morphological types of gypsum crystals is marked by differences in the differential inhibition of growth of the various crystal forms. The further growth of lenticular crystals, which are most strongly affected by differential growth inhibition, is still characterized by a difference in degree of inhibition between the {111} and {103} forms, expressed as a change in orientation of the plane of flattening. Copyright

First evidence of Pleistocene rock art in North Africa: securing the age of the Qurta petroglyphs (Egypt) through OSL dating

Long doubted, the existence of Pleistocene rock art in North Africa is here proven through the dating of petroglyph panels displaying aurochs and other animals at Qurta in the Upper Egyptian Nile Valley. The method used was optically stimulated luminescence (OSL) applied to deposits of wind-blown sediment covering the images. This gave a minimum age of ~15 000 calendar years making the rock engravings at Qurta the oldest so far found in North Africa.

TEXTURAL FEATURES OF HOLOCENE PERENNIAL SALINE LAKE DEPOSITS OF THE TAOUDENNI-AGORGOTT BASIN, NORTHERN MALI

Abstract The Holocene salt lake deposits of the Taoudenni–Agorgott basin, northern Mali, mainly consist of sediments with a high glauberite (Na2Ca(SO4)2) content. The remainder of the deposits largely consists of salt beds with a bloedite (Na2Mg(SO4)2·4H2O), thenardite (Na2SO4) or halite (NaCl) composition. A petrographical study of the deposits demonstrates that they formed in a perennial lake that experienced a gradual decrease in water depth. Textural features of the glauberite-dominated deposits are found to be related to water depth, through the control that this factor exerts on the sensitivity of the lake to changes in water supply and to short-term variations in evaporation rates. In this way, layering — due to variations in glauberite content and crystal size — is inferred to be typical of deposits that formed in shallow water, whereas unstratified deposits are the product of high lake level stages. Halite textures are found to be indicative of the place within the water column where crystal growth occurred (along the lake bottom or higher), which is mainly determined by water depth and partly by evaporation rates. The oldest halite beds are largely unaltered cumulate deposits, whereas the youngest layers developed exclusively through bottom growth. The basal part of one thick halite bed at a level between these two groups of halite layers developed by an alternation of both types of growth, in response to variations in evaporation rates. Variations in mineralogical composition between and within the salt beds that formed during the earliest periods with a higher salinity, up to the first stage with halite formation, record a change in lake water chemistry with time but they are in one instance also determined by an early diagenetic mineral transformation.

THE NATURE OF ZEOLITE OCCURRENCES IN DEPOSITS OF THE OLDUVAI BASIN, NORTHERN TANZANIA

The 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.

The alteration of glauberite in lacustrine deposits of the Taoudenni-Agorgott basin, northern Mali

Abstract A profile studied within Holocene salt lake deposits of the Taoudenni-Agorgott basin, northern Mali, comprises a macroscopically unstratified surface layer containing calcium sulfate minerals. This interval has previously been interpreted as a depositional unit that formed under different conditions than the underlying glauberite-bearing layered deposits. The present micromorphological study of this layer demonstrates that it formed by an alteration of the glauberite-bearing primary deposits. The first stage in the alteration of the syndepositional salts was the incongruent dissolution of glauberite (Na 2 Ca(SO 4 ) 2 ). This resulted in the formation of gypsum (CaSO 4 ·2H 2 O), both as crystals that are part of microcrystalline aggregates with a xenotopic fabric and as larger anhedral and elongated prismatic crystals. The gypsum crystals were subsequently subjected to a dehydration, resulting in a topotactic replacement of this mineral by bassanite (2CaSO 4 ·H 2 O) and anhydrite (CaSO 4 ). This replacement was characterized by a parallelism between the fibre axes of the dehydrated mineral phases and the [001] axis of the gypsum crystals. During later periods, the alteration products repeatedly underwent hydration and dehydration reactions and interactions with both more concentrated and dilute solutions, producing aggregates with gypsum, bassanite, anhydrite and secondary glauberite as constituents. Aggregates of acicular crystals with a possible eugsterite (Na 4 Ca(SO 4 ( 3 ·2H 2 O) composition also formed during this stage. In the part of the surface layer in which the alteration is most advanced, the evaporite minerals mainly occur in the form of large anhydrite nodules. The constituents of these aggregates are not exclusively the direct product of a dehydration of the large gypsum crystals from which the nodules were derived. The products of these different stages in the alteration of the primary deposits occur in separate depth intervals of the surface layer, which therefore displays a general upward increase in degree of diagenesis.

20 – Salt Minerals in Saline Soils and Salt Crusts

Publisher Summary This chapter discusses the significance of micromorphological features of soluble salt occurrences in soil environments. The micromorphological study of salt minerals in soils can be complicated by a number of specific problems that are not encountered for most other soil materials. SEM studies are more common but provide little information on some relevant aspects, such as the nature of internal features of crystals and aggregates. Many published micromorphological studies of salt occurrences are rather descriptive, with little attention to the significance of observed features. The interpretation of variations in crystal habit and type of aggregation is hindered by a lack of experimental studies on possible controlling factors. Available results for gypsum growth, which has been intensively investigated, indicate that a great complexity should be expected.

Salt mineral distribution patterns in soils of the Otjomongwa pan, Namibia

Abstract The deposits of the Otjomongwa pan, southwestern Kalahari, Namibia, contain halite, thenardite and minor amounts of calcium-bearing sulfate minerals. The deposits show vertical variations in salt content, evaporite mineralogy and thenardite morphology, with similar patterns throughout the basin. The salt minerals were formed by evaporation of groundwater, within a groundmass of lacustrine deposits that did not contain synsedimentary evaporites. Lateral variations in total salt content are related to the direction of groundwater flow during a period with high groundwater levels following the end of lacustrine sedimentation in the pan basin. This was followed by a period without any groundwater influence that continues to the present. During this stage, leaching by rainwater occurred, resulting in changes in evaporite mineralogy, morphology and distribution.

Authigenic Silicate Minerals – Sepiolite-Palygorskite, Zeolites and Sodium Silicates

Publisher Summary This chapter deals with micromorphological features of silicate neoformation in soil environments. Micromorphological techniques have been widely applied to the study of silicate neoformation, both in the field of soil science and for the study of diagenetic processes in sedimentary geology. Their use has greatly contributed to the recognition of silicate mineral authigenesis in earth surface conditions. Micromorphology also provides information about the transformation of authigenic silicates and their inherited counterparts. Electron microscopy has been most commonly used, as a technique that is ideally suited for the study of submicroscopic particles. For palygorskite and sepiolite, this has led to an over-representation in the literature of SEM and TEM images that merely illustrate a fibrous particle morphology rather than textural features with genetic implications. Electron microscopy studies of zeolites and sodium silicates show a highly characteristic and invariable crystal morphology, which makes SEM and TEM important tools for their identification.