Artículos

 

Evaporite mineralogy and major element geochemistry as tools for palaeoclimatic investigations in arid regions: A synthesis

 

Mineralogía de evaporitas y geoquímica de elementos mayores como herramientas para la investigación paleoclimática en regiones áridas: una síntesis

 

Werner Smykatz–Kloss¹, Priyadarsi D. Roy²,*

 

¹ Institute of Mineralogy and Geochemistry, University of Karlsruhe, 76131, Karlsruhe, Germany.

² Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, CP 04510, México D.F., México.*E–mail: roy@geologia.unam.mx

 

Manuscript received: July 3, 2009.
Corrected manuscript received: September 14, 2009.
Manuscript accepted: May 14, 2010.

 

Abstract

This paper presents a synthesis of the applications of evaporite mineralogy and the relationship between major elements for the palaeoclimatological research of arid regions, with examples from Playa Oum el Krialate in Tunisia, Wadi Natron in Egypt, East African Rift Valley, etc. The numerous evaporite minerals serving as indicators of palaeo–drylands (salinity and evaporation) include carbonates, sulfates, and Na, K, Ca, and Mg chlorides. The occurrence of double salts, such as glauberite, carnallite, kainite, gaylussite, pirssonite, burkeite, etc., suggests disequilibrium conditions. Apart from that, the presence of very rare Fe–sulfates, such as rozenite and szomolnokite, indicates anoxic conditions with higher salinity. The formation of Na–silicates, such as magadiite and kenyaite, implies a decrease in pH of a highly alkaline Na concentrated brine. The Mg–silicates (palygorskite, Mg–montmorillonite and talc) form quickly and then re–dissolve when conditions change. Identification of fulgurites in the Sahara has been related to palaeo–lightning. We have also discussed a simple geochemical approach of using the ratios of soluble/insoluble elements to identify palaeo–arid events with examples from loess–soil sequences from Feiran Oasis in the Sinai Desert (Egypt) and salty silt lacustrine sequences from Thar Desert (India).

Keywords: Tropical deserts, evaporite minerals, geochemistry, synthesis.

 

Resumen

En este trabajo se presenta una síntesis del uso de minerales evaporíticos y relaciones entre elementos mayores en investigaciones paleo–ambientales de zonas áridas, con ejemplos de la sabkha Oum el Krialate en Túnez, Wadi Natron en Egipto, Valle de Rift del África oriental, etc. Los minerales evaporíticos indicadores de eventos secos (salinidad y evaporación) son carbonatos, sulfatos y cloruros de Na, K, Ca y Mg. La ocurrencia de sales dobles, como glauberita, carnalita, kainita, gaylusita, pirssonita, burkeita, etc., sugiere condiciones de desequilibrio. Además, la presencia de sulfatos de Fe poco comunes, como rozenita y szomolnokita, indica condiciones de anoxia con alta salinidad. La precipitación de silicatos de Na, como magadiita y kenyaita, implica la dilución de una salmuera muy alcalina y con alta concentración de Na. Los silicatos de Mg (paligorskita, Mg–montmorillonita y talco) precipitan rápidamente para disolverse nuevamente al cambiar las condiciones. La presencia de fulguritas en el Sáhara indica la incidencia de paleo–relámpagos. Asimismo, para identificar eventos de paleo–sequía, se discute un método sencillo basado en relaciones de elementos solubles e insolubles con ejemplos de secuencias de suelo tipo loess del Oasis de Feirán (Desierto de Sinaí, Egipto) y de secuencias de sedimentos lacustres del Desierto de Thar (India).

Palabras clave: Desiertos tropicales, minerales evaporíticos, geoquímica, síntesis.

 

1. Introduction

The study of (palaeo–) climatic registers is highly effective in reconstructing the past climate and other conditions present at the time of sediment formation or deposition. The knowledge of old climates and the mechanisms of climatic changes may help to identify the regularities of palaeoclimatology and could possibly lead to improve present–day agriculture and horticulture as well as the management of water, soil, and natural resources. While marine sediments on continents demand a transgressive sea, the sediments in lagoonal or even in true sabkha environments require a topographically closed basin and fluvial freshwater input. After some time, these lakes become saline in (semi–) arid environments and finally evaporate in pans, accompanied by sedimentation caused by aeolian activity and erosion, e.g. dune and loess formation, grain sorting, and formation of fluvial and lake terraces. The criteria used to recognize these geomorphological formations and processes in old sediments and soils have been treated in hundreds of geomorphological and geological articles and textbooks (e.g., Glennie, 1970; Holser, 1979; Büdel, 1982; Goudie and Pye, 1983; Lowe and Walker, 1984; Gasse et al., 1987; Thomas, 1989; Summerfield, 1991; Thomas and Shaw, 1991; Cooke et al., 1993; Kröpelin, 1993; Goudie and Wells, 1995; Lancaster, 1995; Williams and Balling, 1996; Dittmann, 1999; Glennie and Singhvi, 2002).

Diagenetic processes change the association of primary minerals (at least partly). Evaporite minerals (salts) are dissolved and transported in solution until more stable products precipitate. Physical and chemical conditions determine the transformation of salts (Braitsch, 1971; Busson and Perthuisot, 1977; Eugster and Hardie, 1978; Holser, 1979; Usdowski and Dietzel, 1998). But quite often, the development from primary limnic formations to secondary transformed products can be traced back using remnants or relict structures (Wiedemann and Smykatz–Kloss, 1981). Sophisticated physical methods may answer some questions, e.g. the determination of isotopic ratios of C, S, O, B, Sr, or H (see Hoefs, 1987; Heine, 1998; von Grafenstein et al., 1998; Eitel, 2000), including age determinations by ¹⁴C methods (Geyh and Jäkel, 1974; Yadav, 1997; Geyh, 2005). Even in case of lack or absence of organic material, electron spin resonance (ESR) (Dhir et al., 2004), thermoluminescence (TL) or optically stimulated luminescence (OSL) methods may help (Rögner et al., 1999; Eitel et al., 2004), provided that the tolerance of the method is suitable to solve a certain problem (Zöller, 1995; Jäkel, 2004). Considering the peculiarities in formation and dissolution behaviour of salt or clay minerals, a certain reconstruction of a primary limnic environment is possible. Both the degree of interaction between sediments (or soils) and water and the development of secondary products are dependent on several environmental conditions, namely temperature, pH and Eh values, pressure, activity of water and ions, soil and sediment porosity and packing density, solubility of species and interfering (disturbing) compounds, equilibrium or disequilibrium conditions, and time.

 

2. Preliminary remarks on geomorphology, geology, biology, physics, and regional distribution

2.1. Geomorphological criteria

The formation of terraces by lake and sea level fluctuations is a widely observed geomorphological phenomenon (Büdel, 1982; Rust, 1989; Vogel, 1989; Heine, 1990). Similarly, palaeo–dunes are indicators of wind directions or variations in past wind currents (Bowler, 1973; Sarnthein, 1978; Lancaster, 1981; Wasson et al., 1983, 1984; Gaylord, 1990; Kar, 1990; Pye and Tsoar, 1990; Felix–Henningsen, 2000, 2004; Grunert et al., 2000; Grunert and Lehmkuhl, 2004; Heine, 2004). Lake and sea level fluctuations mirror past changes in precipitation as well as marine transgressions and regressions due to climatic and tectonic activities (Faure and Elouard, 1967; Nir, 1974; Perthuisot, 1976; Gasse et al., 1987; Tooley and Shennan, 1987; Koessl, 1988; Lancaster, 1989; Baumhauer, 1990; Baumhauer et al., 2004; Fang, 1991; Wenigwieser, 1992; Qin and Yu, 1998; Wünnemann et al., 1998; Eitel et al., 2004; Sinha and Raymahashay, 2004; Sinha et al., 2006; Roy et al., 2006, 2008a, 2009; Achyuthan et al., 2007). Increasing aridity leads to salt evaporation in pans and playas (Lancaster, 1981; Petit–Maire, 1986; Goudie and Wells, 1995; Eitel and Blümel, 1997; Schütt, 1998, 2004; Smykatz–Kloss et al., 1999/2000) and enables loess and silt accumulation (Sirocko and Sarnthein, 1989; Vogel, 1989; Derbyshire et al., 1995; Gallet et al., 1996; Smykatz–Kloss et al., 1998; Brunotte and Sander, 2000; Grunert and Lehmkuhl, 2004; Singhvi and Kar, 2004). Special conditions are required for the formation of fulgurites as observed by Sponholz et al. (1993) in the Sahara. The presence of fulgurites demonstrates the interaction of lightning and lake surfaces.

2.2. Geological and biological criteria

Climatic changes are also mirrored in geological, sedimentological, and biological events (Tricart and Cailleux, 1972; Carbonel et al., 1988; Sirocko and Sarnthein, 1989; Vogel, 1989; Geyh and Eitel, 1998; Smykatz–Kloss et al., 1998; Heine and Heine, 2002; Jain and Tandon, 2003; Mischke et al., 2004). The organic carbon content on the continental slope of the Arabian Sea (Indian Ocean) tells of palaeoclimatic changes and monsoon development in the adjacent continents (Berner and Lasaga, 1989; Sirocko, 1995). Sea level fluctuations show global climatic changes as well (Faure and Elouard, 1967; Einsele et al., 1974). Geophysical methods also contribute (Schumm, 1977; Prell and Kutzbach, 1987) and astronomic theories, like Milankovitch's, have led to the explanation of climatic changes (Berger and Tricot, 1986; Blum and Törnquist, 2000).

Biological aspects may play an important role in reconstructing palaeontologic developments (Maley, 1983; Gasse et al., 1987; Schulz, 1987). The development of ostracod and diatom species, in particular, has been used for palaeoenvironment reconstructions (Carbonel et al., 1988; Caballero, 1997; Mezquita et al., 1999; Palacios Fest et al., 2002; Mischke et al., 2004; Caballero et al., 2005).

2.3. Physical and chemical criteria

The influence of ice and ice transport on rocky grounds and soils may be mentioned briefly. This includes all physical processes, such as freezing and thawing, and those processes related to glaciers, wind, and thunderstorms, as well as the formation of (peri–) glacial and structured soils etc. (Fink and Kukla, 1977; Washburn, 1979; Büdel, 1982; Harris, 1986; Tyson, 1986). Roberts and Spencer (1995) and Spencer et al. (2003) used fluid inclusions in halite (evaporitic crusts in Death Valley, California) for the measurement of palaeotemperature. When available, the exact methods of physical dating and isotope geochemistry are very helpful in fixing palaeoclimatic events and changes (Geyh and Jäkel, 1974; Delibrias et al., 1976; Gasse et al., 1987; Heine, 1990; Yadav, 1997; Geyh and Eitel, 1998; Hofmann and Geyh, 1998; von Grafenstein et al., 1998; Dutkiewicz et al., 2000; Glennie and Singhvi, 2002; Juyal et al., 2003). Isotopic ratios may characterize palaeoenvironments (Hoefs, 1987; Tyson, 1986; Sharp, 2007). The concentration of total organic carbon (TOC) represents the amount of organic matter preserved after sedimentation, which depends upon initial production and the degree of degradation (Meyers and Teranes, 2001; Leng et al., 2005). The source of organic matter can be differentiated by the ratio of organic carbon and total nitrogen (C/N) and δ¹³C. Organic matter derived from lacustrine phytoplankton has a lower C/N (<10) value compared to organic matter derived from terrestrial plants (>10) (Prasad et al., 1997). δ¹³C of organic matter indicates the source of HCO₃– and dissolved CO₂ (i.e. derived from C4 or C3 plants by ground water inflow, organic matter deposited as methane in the lake bottom, catchment carbonate deposits, and atmospheric CO₂). Similarly, δ¹⁸O in inorganic material (carbonates and silicates) indicates palaeo–temperature during formation of the mineral phases (Leng et al., 2005). In water samples, deviation of both δ¹⁸O and δD from the Global Meteroric Water Line suggests kinetic fractionation due to evaporation. In warmer and tropical water, both isotopes have higher δ values compared to colder and polar rainfall (Leng and Marshall, 2004).

Organic material is also used for ¹⁴C measurements (Geyh, 2005) to reconstruct the chronology of the identified events. In the absence of organic material, which is the case in many Quaternary and even Tertiary arid environments, luminescence (TL, OSL) (Zöller, 1995; Rögner et al., 1999) or ESR techniques (Dhir et al., 2004) may help.

2.4. Regional distribution of drylands

Pans, playas and palaeolakes of the great dryland areas have been the objects of interests in palaeoclimatic studies. In these regions, enrichment of (earth) alkalies, iron and silica are reported. Iron enrichment is documented in (semi–) aridic ferricretes of former swamps (Nahon, 1976; Felix–Henningsen, 2000, 2004). Silica enrichment in highly alkaline lakes is reported for one of the two main silcrete formations (i.e. allochthonous silcrete, while the autochthonous type represents a soil formation) by Summerfield (1983), Young (1985), Joachim (1988) and Thomas and Shaw (1991). Salt enrichment is documented by Derbyshire et al. (1995), Mehrshabi et al. (2003), and Schütt (2004). Development of soils, pans, and desertic landscape of the world's largest desert, the Sahara, is reported by Petit–Maire (1986, 1987, 1991) and Baumhauer (1990). Guo et al. (2000) have compared the Sahara with east Asian deserts, including the Gobi and other large deserts in China, Tibet, and Mongolia (Kukla, 1987; Fang, 1991; Hofmann, 1993; Derbyshire et al., 1995; Jäkel, 1995; Lehmkuhl, 1995; Liu and Fu, 1996; Qin and Yu, 1998; Wünnemann et al., 1998; Grunert et al., 2000; Grunert and Lehmkuhl, 2004). While the deserts of Iran (Mehrshabi et al., 2003) and Arabia (Glennie and Singhvi, 2002; Barth, 2003) are rarely studied, the Indian Desert (Thar) located in the northwestern Indian state of Rajasthan has received significantly more attention from geoscientists (Wasson et al., 1983, 1984; Kar, 1990; Jain and Tandon, 2003; Juyal et al., 2003; Singhvi and Kar, 2004; Sinha et al., 2006; Roy et al., 2001, 2006, 2008b, 2009). This is also true for the world's oldest desert, the Namib, in South Africa (Rust, 1989; Vogel, 1989; Heine, 1998, 2004; Eitel, 2000; Eitel et al., 2004). Relative to the Namib, a relatively small desert, Sinai in Egypt, has received some attention by Nir (1974), Rögner and Smykatz–Kloss (1991), Rögner et al. (1999, 2004), and Smykatz–Kloss et al. (1998, 1999/2000, 2004). American deserts have been studied by Fahey and Mrose (1962), Bradley and Eugster (1969), Gaylord (1990), Monger and Daugherty (1991), Bischoff et al. (1997), Metcalfe et al. (2002), and Roy et al. (2010). The Australian desert has been studied by Young (1985), Holland et al. (1988), Nanson and Tooth (1989), and English et al. (2001).

 

3. Mineralogical and geochemical criteria and examples

3.1. Minerals as criteria

Among the common cations, the solution and precipitation behaviour of Na⁺ and Mg²⁺ (and Fe²⁺ and Mn²⁺ in Eh–negative environments) are very suitable (Holser, 1979, Mason and Moore, 1982). Sodium is generally highly soluble (abundant in water bodies of arid environments) and dissolves even when water activity is low. Suitable environments for preservation of Na⁺ are (semi–) deserts, e.g. the drylands of Northern Africa as Wadi Natron in Egypt (Wenigwieser, 1992), Playa Oum el Krialate in Tunisia (Perthuisot, 1976; Koessl, 1988), and similar environments in Libya, Morocco, and Mauritania. In these environments, besides halite (NaCl), Na+ precipitates as the highly soluble mirabilite (Na₂SO₄•10H₂O), but only in contact with water (e.g. in pools, see Smykatz–Kloss et al., 1992). After its formation, mirabilite transforms to thenardite (Na₂SO₄) on contact with air. In the limnic sediments of Wadi Natron, idiomorphic thenardite occurs in crystals with diameters up to a few centimetres.

Among the places where a large number of disequilibrium products occur in a short period, the Playa Oum el Krialate is one of the most favourable and well–studied localities. A short period spans a few weeks up to a few months, until the next rain occurs and dissolves the Na–salts. But during dry periods, i.e. times of low water activity, a number of semi–finished products ("precursors") are observed, even locally as main constituents, many of which contain either very little water or no water at all. These are in the process of becoming thermodynamically reasonable and stable end products and minerals such as mirabilite (Na₂SO₄•10H₂O), hexahydrite (MgSO₄•6H₂O), pentahydrite (MgSO₄•5H₂O), leonhardtite (MgSO₄•H₂O), nahcolite (NaHCO₃), trona (Na₃(CO₃)(HCO₃)•2H₂O), pirssonite (Na₂Ca(CO₃)₂•2H₂O), gaylussite (Na₂Ca(CO₃)2•5H₂O), or burkeite (Na₆(CO₃)(SO₄)₂) (see Table 1).

Among these evaporites that partially occur as main constituents, some are very rare compounds and occur as traces, i.e. glauberite, polyhalite, carnallite, bassanite, rozenite and szomolnokite (Spencer et al., 2003) (see Table 1). Bassanite has been found in aridic soils by Akopodje (1985) and Smykatz–Kloss et al. (1985) on the surface of sunny walls in South Tunisia. Very recently, this mineral has been observed as a main component on the ground of the "White Desert" in Egypt, where it is mainly covered by a few centimetres of wind–blown carbonates and anhydrite. This cover of wind–blown weathering products is the reason for the occurrence and longer persistence of bassanite.

The very rare and instable Fe²⁺–sulfates rozenite and szomolnikite (FeSO₄•4H₂O and FeSO₄•H₂O, respectively) are found on the way from the gypsum karst, east of Tatahouine, to the Oum el Krialate playa, associated with organic material (Koessl, 1988; Smykatz–Kloss et al., 1992). The preservation of organic material is the reason for the negative Eh environment, which is necessary for forming Fe²⁺–sulfates. However, the occurrence of bassanite, rozenite, and szomolnokite in the deserts of Egypt and Tunisia is exceptional among sulfate evaporites, as their occurrence requires some shelter from the (rarely abundant) rain events.

Most of the other evaporites are double salts (i.e. combinations of Na, Mg, Ca, and K; see Table 1). These occur only for short periods and in traces; such is the case for glauberite, kainite, carnallite, polyhalite (Smykatz–Kloss et al., 1992), burkeite, trona, pirssonite, and gaylussite (Fahey and Mrose, 1962; Bradley and Eugster, 1969; Holser, 1979). Trona has been a known mineral to Humanity for millennia, conspicuous in the sediments of the Wadi Natron in Egypt (Wenigwieser, 1992).

There is another special tropical environment for the preservation of Na–minerals, co–precipitated with silica, although only when very high amounts of Na and Si in the brine. Such is the case along the East African Rift Valley (East African Graben), e.g. in Kenya and Tanzania, and especially where high temperature and weathering rates of abundant sodic volcanics (Na–carbonatites) feed the playa lakes, such as Lake Magadi and other highly alkaline lakes (pH ~ 12) in the grabens. The alkali (Na) silicates precipitate from oversaturated brines diluted by rain water. The product is magadiite (NaSi₇O₁₃(OH)₃•3H₂O; see Eugster, 1969), that forms only in highly alkaline environments (e.g. Lake Magadi, with pH = 12 and concentrations of Na >100000 ppm). As pH decreases (i.e. by dilution with rain water), magadiite transforms to kenyaite (NaSi₁₁O_26.5(OH)₄•3H₂O) and finally to chalcedony (SiO₂) (Eugster, 1969). The formation of magadiite is also reported from a few other (similar) localities in California.

The other highly soluble cation, Mg²⁺, forms evaporites in very special (semi–) aridic environments, and such Mg–bearing evaporites are somewhat more stable than the Na–bearing ones. Mg²⁺ compounds occur periodically (e.g., during evaporation in Mg–containing playa lakes) in recent playa lakes in Spain (de la Peña et al., 1982; Ordóñez et al., 1994) and Tunisia (Koessl, 1988) as Mg sulfates of the kieserite–epsomite series (Table 1), and form very soluble intermediate products such as hexahydrite (MgSO₄•6H₂O), pentahydrite (MgSO₄•5H₂O) or leonhardtite (MgSO₄•4H₂O). These minerals are found as efflorescences on dolomitic limestones in Libya as well, together with bloedite in half caves (with stalactites and flowstones) of the Jabal Nafusah (Tripolitania; Smykatz–Kloss et al., 1992). The next rain dissolves these products rather quickly, unless they are covered with sediments or other authigenic formations.

There are a few more persisting evaporates of this soluble cation. There are Mg–silicates that form even in contact with water, in playa lakes and sabkha environments. This includes the occurrence of palygorskite ((Mg,Al)₂(OH/SiO₁₀)•4H₂O) (Millot, 1964; Yaalon and Wieder, 1976; Singer, 1979, 1984; Monger and Daugherty, 1991; Eitel, 1994), Mg–montmorillonite, (Mg₃–x(OH)₂Si₄O₁₀ xAlx•nH₂O, Na₂–x) (Stengele and Smykatz–Kloss, 1995), or talc (Mg₃(OH)₂Si₄O₁₀) in special cases (mainly metasomatically formed). These minerals form more quickly and re–dissolve when the environmental conditions change. Thus, they represent remnants of the former arid history of that special limnic sediment (Eitel, 1994; Eitel and Zöller, 1996).

More common than simple Na– or Mg– minerals is the case of double salts (like bloedite) (Braitsch, 1971; Koessl, 1988) and carbonates. Sedimentary dolomites are reported from playa lakes in Russia, China, India, North Africa (Roy et al., 2001, 2006). Meta–stable proto–dolomite (Ca>Mg) typically occurs instead of dolomite in limnic and aridic regions (Smykatz–Kloss and Goebelbecker, 1992), where small amounts of Fe and Mn are incorporated into the dolomite structure (Roy et al., 2009).

3.2. Geochemical criteria

The behaviour of highly soluble elements like Na⁺, Mg²⁺ (or K⁺) is contrary to that of hydrolysates, i.e. Ti⁴⁺, Al³⁺, Fe³⁺ (Mason and Moore, 1982), which are resistant to normal weathering solutions. In contact with water, the soluble elements dissolve and are transported away, while the hydrolysates (interacting with solutions) become increasingly enriched in the sediment. Due to its relatively larger ionic radius and partial immobility, the behaviour of the soluble cation K⁺ is different. It dissolves (like Na⁺) but is adsorbed quickly, forming new compounds (e.g. illite) (Reheis, 1990; Pandarinath et al., 1999). Thus, Na/K continuously decreases with time. However, the ratios Na/Ti, Na/Al, Na/Fe, Mg/Al, Mg/Ti, and Mg/Fe (the last three, only in carbonate–free environments) strongly decrease with increasing water activity (and vice versa). Nesbitt and Young (1982) were pioneers in the use of such ratios for palaeoclimatic considerations. Other authors continued in characterising palaeoclimatic environments by using some of these relations (Sirocko, 1995; Gallet et al., 1996; Smykatz–Kloss et al., 1998, 2004; Rögner et al., 2004; Schütt, 2004; Roy et al., 2006, 2008b, 2009; Sinha et al., 2006).

Sponholz et al. (1993) observed intermittence between palaeoclimatic events in desert lakes and surrounding geomorphology (e.g., the water level of these lakes) when they identified relics of fulgurites (concretions in the Sahara sand produced by lightnings at former lake levels), which enabled them to reconstruct the desert lake level with time. Similarly, Roy and Smykatz–Kloss (2007) studied the REE geochemistry of evaporites and the degree of roundness of rock fragments in the clastic fractions of sediments in order to reconstruct the palaeo–fluvial conditions in the Thar Desert. Fromm et al. (2005) used Fe–Mn vein mineralisations and the different stages of calcrete formation as indicators of the development of the Tunisian desert. Roy et al. (2009, 2010) used geochemical signatures of chemical weathering and mineralogical composition of playa lake sediments to reconstruct late Holocene hydrological changes at the margins of the Thar Desert (India) and late Pleistocene–Holocene palaeo–environmental conditions at Sonora Desert (Mexico).

Impressive walls (up to 60 m high) of loess–like sediments are found in the Feiran Valley (Sinai, Egypt) (Nir, 1974; Rögner and Smykatz–Kloss, 1991; Rögner et al., 1999, 2004; Smykatz–Kloss et al., 1999/2000, 2003, 2004). Their origin has been related to fluvial terraces (Büdel, 1982), lake sediments (Nir, 1974), flood products, fluvial–torrential sediments, and alluvial loess (Rögner et al., 2004). The sequences consist of 3–20 cm thick loess layers intercalated with 2–8 cm thick polygonal soil layers. Table 2 presents the geochemical composition of 17 different loess–soil pairs found throughout several metres in a profile from the Feiran Oasis (data compiled from Knabe, 2000, and Rögner et al., 2004). The palaeoecological information is provided by the simple ideas that (1) loessian material is a product of desertic environment, and (2) subsequent soil formation requires water (humidity).

In each loess layer, the weathering of clastic minerals present in the uppermost part (feldspars, micas and, to a lesser degree, amphiboles and pyroxenes) led to the formation of the overlying soil. During this process, dissolution and transportation (i.e. removal) of soluble elements (Na, Mg, and K) occurs in the uppermost part of each loess layer and thus changes the ratios of soluble to (relatively insoluble) hydrolysate (TiO₂, Al₂O₃, Fe₂O₃) elements in the soil. Humid conditions lower these ratios and increasing humidity (or intensive contact of water and loess) is mirrored by an increase in the differences of the mentioned ratios (e.g., amount of Na₂O/TiO₂ in loess minus the amount of Na2O/TiO2 in the overlying soil). These values are listed in Table 2 and shown in Figure 1.

Figure 1 shows two distinct shifts to very arid conditions (the 7^th and 15^th loess–soil pairs) and three smaller deviations to (semi–) aridity (the 4^th, 12^th, and 17^th loess–soil pairs). Exact age determinations for these arid events are most desirable. The TL age (Rögner et al., 1999) suggests that the studied profile covers 25.7 ± 3.9 ka and more than 25 m of the profile is unexposed.

Five different loess–soil pairs from the Wadi Feiran profile (Table 3) are quite similar to those of the 17 different loess–soil pairs from Feiran Oasis (Figure1) and represent another profile in the same valley. The chemical compositions (Table 3) show that the soils are mostly enriched in Fe₂O₃ and Al₂O₃ compared to the underlying loess. The MgO contents seem to be quite constant throughout the profile. Based on the difference in the ratio between soluble and insoluble elements, two distinct aridic events were identified (Figure 2), i.e. the 2nd, 3rd, and 5^th loess–soil pairs. Both these examples show the use of geochemical ratios in identifying arid events in the profiles of alluvial loess near the Feiran Oasis (Smykatz–Kloss et al., 1998; Knabe, 2000; Rögner et al., 2004). Two more examples of the application of geochemical ratios are given below, representing the relations in the playa lakes of the Thar Desert (Roy et al., 2006, 2008b). Table 4 presents the geochemical compositions in the zones (I: 0–10 cm, II: 10–130 cm, III: 130–150 cm) in a shallow profile from the Pachapadra playa lake (Roy et al., 2008b). The last example compares the geochemical relations between a semi–humid (desert margin) Sambhar playa and semi–arid Didwana playa (Figure 3 and Table 5).

 

 

4. Conclusions

This paper stresses the applicability of mineralogical and geochemical methods for characterising palaeoclimatological environments in tropical regions, especially in arid environments that lack preservation of biological proxies such as pollen, diatoms, and ostracods. Mineralogically, the persistence of (rare) Na– and (more common) Mg–evaporite minerals may prove the development of a former playa lake (via evaporation and transformation to more stable mineral products in the evaporitic crusts and hardpans). Transformed products, like Mg–salts or Na–, Mg–silicates such as magadiite, kenyaite, and palygorskite are reported from various palaeo–lakes.

Geochemically, the alteration of limnic sediments (e.g., alluvial loess in the Sinai and salty silts in the Thar) and formation of a soil cover by weathering of the underlying layer in contact with water suggest a shift from arid to (semi–) humid climate. The intensity of the humidification is observed in the evaluated differences in geochemical relations between underlying loess and overlying soils. Especially during the humid period, Na–silicates are dissolved and partly transported and removed, while Al³⁺, Ti⁴⁺, and Fe³⁺ show to be nearly unaffected. Thus, the ratio of soluble (Na₂O) to hydrolysate elements (TiO₂, Al₂O₃, and Fe₂O₃) and their comparison report the (hydro–) geochemical development of the soil cover from the underlying loess. In suitable environments, a ¹⁴C or luminescence age determination will be desirable, but in many cases the sediments (or soils) lack organic carbon for an exact age determination by ¹⁴C method. Similarly, the younger sediments in arid regions are constantly mobilized by wind activity and are partially stripped of their OSL signal to be correctly dated by luminescence methods.

 

Acknowledgements

We are very grateful to Nadine Smykatz–Kloss and Andy Jones (Stroud, England) for helping in English corrections, and to Maria Tannhäuser (University of Karlsruhe) for helping with the preparation of the manuscript.

 

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