The role of rare rainstorms in the formation of calcic soil horizons on alluvial surfaces in extreme deserts

Soils in similar geomorphic settings in hyperarid deserts (< 50 mm yr⁻¹) should have similar characteristics because a negative moisture balance controls their development. However, Reg soils in the hyperarid southern Negev and Namib deserts are distinctly different. Soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics. Annual rainfall amount can be similar (15-30 mm), but storm duration can drastically alter Reg soil properties in deserts. The cooler fall/winter and dry hot summers of the southern Negev Desert with a predominance brief (≤ 1 day) rainstorms result in gypsic-saline soils without any calcic soil horizon. Although the Namib Desert receives only 50-60% of the southern Negev annual rainfall, its rainstorm duration is commonly 2-4 days. This improves leaching of the top soil under even lower annual rainfall amount and results in weeks-long grass cover. The long-term cumulative effect of these rare rain-grass relationships produces a calcic-gypsic-saline soil. The development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil-moisture regime in deserts, and has important implications for the use of key desert soil properties as proxies in paleoclimatology.     

Sr and Nd isotopes as tracers in pedogenic studies: Evidence for Saharan dust contribution to the soils of Muravera (Sardinia,Italy)

Sr and Nd isotopes were applied to 5 soil profiles from the Muravera area, in south-eastern Sardinia.All the soils, which have developed during the Quaternary on the Lower Paleozoic metamorphic basement except for one on Eocene carbonates, are located far from major sources of pollution. Therefore, they are suitable for testing pedogenic processes and geochemical evolution to benefit for environmental studies.The Sr isotopic ratios range largely (δ⁸⁷Sr = 1.7–65.9‰), even in each soil profile. In particular, the observed increase of δ⁸⁷Sr with depth in the most of the metamorphic rock-based soils can be accounted for by the downward decrease of Sr contributions from organic matter and Saharan dust, both displaying lower isotopic ratios than the soil bedrocks. The carbonate rock-based soil exhibits δ⁸⁷Sr higher (1.7–18.1‰) than the bedrock, indicating a significant contribution of radiogenic Sr from the siliciclastic fraction of the soil, and probably from dust input. The Nd isotopic ratios are slightly variable through the profiles (ɛ_Nd from −7.8 to −14.5), confirming little mobility of Nd and Sm during the pedogenesis. Among the minerals present in the soils, phosphates, albite, and calcite are those important in providing low radiogenic Sr and Nd to organic matter of the soils.Lastly, this isotopic study has in particular allowed for evaluating the potential proportion of contribution of Saharan dust to south-eastern Sardinia, thus corroborating the findings of other studies related to soils from the central-western Mediterranean.     

Terrestrial stable isotope records of Late Quaternary paleoclimates in the eastern Mediterranean region

Terrestrial stable isotope records (¹³C/¹²C, ¹⁸O/¹⁶O, and D/H ratios) of late Quaternary paleoclimates in the eastern Mediterranean region are reviewed. Significant paleoclimatic reconstructions come from a variety of isotopic studies. Paleogroundwaters, although they cannot be accurately dated, show oxygen and hydrogen isotopic compositions highly depleted in heavier isotopes compared to modern meteoric waters in arid regions of southern Israel, Sinai and northeastern Africa and attest to a major difference in climatic regime some time in the Pleistocene. Th-U dating of land snails showing a similar ¹⁸O depletion indicates ages of 100,000 yr and ≥300,000 yr for this climatic regime in the Negev Desert of southern Israel. Carbon isotope records of organic matter have not been studied extensively in the region, except for the record of Holocene land snails in the Negev. These document a southward shift in pure C₃ plant communities in the middle and early Holocene relative to their present distribution and indicate wetter conditions at those times. An oxygen isotope curve for the Holocene, reflecting changes in the isotopic composition of precipitation, has been established from analysis of carbonate materials—land snail shells and speleothems. The curve indicates a depletion of ca. 2‰ in ¹⁸O centered around 7000 cal yr bp, with modern levels being reached by 5000 cal yr bp. Carbon isotope analysis of soil carbonates in paleosols developed in loess in the Negev show the existence of dramatic north–south climatic gradients at the times of formation of the soils (ca. 13,000, 28,000, and ≥37,000 ¹⁴C yr bp), as occur also today due to the waning influence of Mediterranean to the south. Some isotopic methods, widely used in other regions, have received little or no attention in the eastern Mediterranean region. These include oxygen and hydrogen isotopes in wood, phosphate oxygen and organic carbon in bones and teeth, and carbon in soil organic matter.     

Stable isotopic composition of soil calcite (O,C) and gypsum (S) overlying Cu deposits in the Atacama Desert,Chile: Implications for mineral exploration,salt sources,and paleoenvironmental reconstruction

Soils overlying two porphyry Cu deposits (Spence, Gaby Sur) and the Pampa del Tamarugal, Atacama Desert, Northern Chile were collected in order to investigate the extent to which saline groundwaters influence “soil” chemistry in regions with thick Miocene and younger sediment cover. Soil carbonate (calcite) was analyzed for C and O isotopes and pedogenic gypsum for S isotopes. Soil calcite is present in all soils at the Spence deposit, but increases volumetrically above two fracture zones that cut the Miocene gravels, including gravels that overlie the deposit. The C isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the Spence deposit; all δ¹³C_VPDB values fall within a narrow range (1.40–4.23‰), consistent with the carbonate having formed in equilibrium with atmospheric CO₂. However, δ¹⁸O_VPDB for carbonate over both fracture zones is statistically different from carbonate elsewhere (average δ¹⁸O_VPDB = 0.82‰ vs. −2.23‰, respectively), suggesting involvement of groundwater in their formation. The composition of soils at the Tamarugal anomaly has been most strongly affected by earthquake-related surface flooding and evaporation of groundwater; δ¹³C_VPDB values (−4.28‰ to −2.04‰) are interpreted to be a mixture of dissolved inorganic C (DIC) from groundwater and atmospheric CO₂. At the Spence deposit, soils only rarely contain sufficient SO₄ for S isotope analysis; the SO₄-bearing soils occur only above the fracture zones in the gravel. Results are uniform (3.7–4.9‰ δ³⁴S_CDT), which is near the middle of the range for SO₄ in groundwater (0.9–7.3‰). Sulfur in soils at the Gaby Sur deposit (3.8–6.1‰ δ³⁴S_CDT) is dominated by gypsum, which primarily occurs on the flanks and tops of hills, suggesting deposition from SO₄-rich fogs. Sulfate in Gaby Sur deposit gypsum is possibly derived by condensation of airborne SO₄ from volcanic SO₂ from the nearby Andes. At the Gaby Sur deposit and Tamarugal anomaly, pedogenic stable isotopes cannot distinguish between S from porphyry or redeposited SO₄ from interior salars.The three sites studied have had different histories of salt accumulation and display variable influence of groundwater, which is interpreted to have been forced to the surface during earthquakes. The clear accumulation of salts associated with fractures at the Spence deposit, and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process. The interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism.     

Isotopic evidence for the source of Ca and S in soil gypsum, anhydrite and calcite in the Atacama Desert, Chile

The origin of pedogenic salts in the Atacama Desert has long been debated. Possible salt sources include in situ weathering at the soil site, local sources such as aerosols from the adjacent Pacific Ocean or salt-encrusted playas (salars), and extra-local atmospheric dust. To identify the origin of Ca and S in Atacama soil salts, we determined δ³⁴S and ⁸⁷Sr/⁸⁶Sr values of soil gypsum/anhydrite and ⁸⁷Sr/⁸⁶Sr values of soil calcite along three east-west trending transects. Our results demonstrate the strong influence of marine aerosols on soil gypsum/anhydrite development in areas where marine fog penetrates inland. Results from an east-west transect located along a breach in the Coastal Cordillera show that most soils within 90 km of the coast, and below 1300 m in elevation, are influenced by marine aerosols and that soils within 50 km, and below 800 m in elevation, receive >50% of Ca and S from marine aerosols (δ³⁴S values > 14‰ and ⁸⁷Sr/⁸⁶Sr values >0.7083). In areas where the Coastal Cordillera is >1200 m in elevation, however, coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced. Most pedogenic salts from inland soils have δ³⁴S values between +5.0 to +8.0‰ and ⁸⁷Sr/⁸⁶Sr ratios between 0.7070 and 0.7076. These values are similar to average δ ³⁴S and ⁸⁷Sr/⁸⁶Sr values of salts from local streams, lakes, and salars (+5.4 ±2‰ δ³⁴S and 0.70749 ± 0.00045 ⁸⁷Sr/⁸⁶Sr) in the Andes and Atacama, suggesting extensive eolian reworking of salar salts onto the surrounding landscape. Ultimately, salar salts are precipitated from evaporated ground water, which has acquired its dissolved solutes from water-rock interactions (both high and low-temperature) along flowpaths from recharge areas in the Andes. Therefore, the main source for Ca and S in gypsum/anhydrite in non-coastal soils is indirect and involves bedrock alteration, not surficially on the hyperarid landscape, but in the subsurface by ground water, followed by eolian redistribution of ground-water derived salar salts to soils. The spatial distribution of high-grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt, supporting arguments for tropospheric nitrogen as the main source for soil nitrate.     

Spatial distribution of dust deposition within a small drainage basin: Implications for loess deposits in the Negev Desert

Loessial colluvial sediments and aeolian aprons are common deposits in the Negev Desert Highlands. In an attempt to monitor the amounts and distributional pattern of loess, monthly dust measurements were carried out during 2004 to 2006 in 10 cm diameter traps located at 18 stations along four slopes, north‐facing, south‐facing, east‐facing and west‐facing in a second‐order drainage basin near Sede Boqer, Negev Desert Highlands, Israel. Annual total dust depositions ranged between 110 g and 178 g m^?2 with an average of 151·1 g m^?2. The average annual dust deposition in the catchment was 23·5% higher than the average amount recorded at the hilltops (122·4 g m^?2) and may be a consequence of sheltering opportunities in the hilly topography. When analysed according to season and aspect, significantly higher monthly amounts were received during the wet rainy season of December to March (17·0 g m^?2), in comparison with the rest of the year (8·1 g m^?2). As for the aspect, while no significant differences characterized north‐facing and south‐facing slopes, east‐facing slopes received significantly higher amounts (by 43·3%) than west‐facing slopes, pointing to preferential dust deposition at the leeward slope. Concurring with the classical model that anticipates higher dust deposition at the leeside slope, but in disagreement with some reports published in the literature, the findings of this study were also supported by a field survey that showed preferential loess accumulation at the eastern and north‐eastern aspects. These findings may shed light on distributional patterns of colluvial sediments and aeolian aprons in the Negev, on soil‐forming processes and on past cycles of dust deposition.     

A threshold in soil formation at Earth’s arid-hyperarid transition

The soils of the Atacama Desert in northern Chile have long been known to contain large quantities of unusual salts, yet the processes that form these soils are not yet fully understood. We examined the morphology and geochemistry of soils on post-Miocene fans and stream terraces along a south-to-north (27° to 24° S) rainfall transect that spans the arid to hyperarid transition (21 to ∼2 mm rain y⁻¹). Landform ages are ? 2 My based on cosmogenic radionuclide concentrations in surface boulders, and Ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2.1 My. A chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall. In the arid (21 mm rain y⁻¹) soil, total mass loss to weathering of silicate alluvium and dust (−1030 kg m⁻²) is offset by net addition of salts (+170 kg m⁻²). The most hyperarid soil has accumulated 830 kg m⁻² of atmospheric salts (including 260 kg sulfate m⁻² and 90 kg chloride m⁻²), resulting in unusually high volumetric expansion (120%) for a soil of this age. The composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in-soil chemical transformation and loss. Long-term rates of atmospheric solute addition were derived from the ion inventories in the driest soil, divided by the landform age, and compared to measured contemporary rates. With decreasing rainfall, the soil salt inventories increase, and the retained salts are both more soluble and present at shallower depths. All soils generally exhibit vertical variation in their chemistry, suggesting slow and stochastic downward water movement, and greater climate variability over the past 2 My than is reflected in recent (∼100 y) rainfall averages. The geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold: in wetter soils, the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 10⁵ to 10⁶ years, while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion. The specific geochemistry of hyperarid soils is a function of atmospheric sources, and is expected to vary accordingly at other hyperarid sites. This work identifies key processes in hyperarid soil formation that are likely to be independent of location, and suggests that analogous processes may occur on Mars.     

Relationship between climate and vegetation and the stable carbon isotope chemistry of soils in the eastern Mojave Desert, Nevada

The relationship between the stable C-isotope composition of the soil environment and modern climate and vegetation was determined empirically along a present-day climatic transect in the eastern Mojave Desert. The δ¹³C of the soil CO₂ and carbonates decreased with increasing elevation and plant density, even though plant assemblages at all elevations were isotopically similar. Several factors, including differences in the ratios of pedogenic of limestone calcite and differences in past vegetation, were considered as explanations of this trend, However, it appears that in the sparsely vegetated Mojave Desert, the δ¹³C of pedogenic carbonate is controlled by differences in plant density and biological activity. This relationship may provide a tool for assessing past vegetational densities, as long as the vegetation is isotopically homogeneous.     

Desert Pavement—the most ancient surface or a more recent development?

Desert pavement comprises a surficial layer of gravel clasts and cobbles underlain by dust and silt that mantles mature alluvial surfaces in arid regions around the world. Once thought to originate from either deflation or expansion and contraction of wet desert soils, desert pavement is now known to form in place from the aeolian accumulation of dust beneath a surface layer of gravel. Desert pavement affects the type and distribution of desert plants on arid land surfaces, as well as inhibits the activities of small burrowing animals. Considerable debate surrounds the age of pavement surfaces and how well they survive changes in climatic conditions. Some researchers maintain that most desert pavements developed during the Holocene, whereas others indicate that some of the oldest surface soils on the planet are desert pavements in the Middle East.     

Transfer of litter-derived N to soil mineral–organic associations: Evidence from decadal 15N tracer experiments

Mineral–organic associations act as mediators of litter-derived N flow to the mineral soil, but the time scales and pathways involved are not well known. To close that gap, we took advantage of decade old ¹⁵N litter labeling experiments conducted in two European forests. We fractionated surface soils by density with limited disaggregating treatment and investigated organic matter (OM) characteristics using δ¹³C, δ¹⁵N and the C/N ratio. Mineral properties were studied by X-ray diffraction and selective dissolution of pedogenic oxides.Three types of associations were isolated: plant debris with few trapped minerals (<1.65 g/cm³), aggregates dominated by phyllosilicates (1.65–2.4 g/cm³), and single mineral grains and pedogenic oxides with little OM (>2.4 g/cm³). A small proportion of ¹⁵N tracer was rapidly attached to single mineral grains, while most of it moved from plant debris to aggregates of low density and progressively to aggregates of higher density that contain a more microbially processed OM. After a decade, 60% of the ¹⁵N tracer found in the investigated horizon was retained in aggregates, while plant debris still contained 40% of the tracer.We present a conceptual model of OM and N flow through soil mineral–organic associations, which accounts for changes in density, dynamics and chemistry of the isolated structures. It suggests that microbial reworking of OM entrapped within aggregates (1.65–2.4 g/cm³) causes the gradient of aggregate packing and, further on, controls the flow of litter-derived N through aggregates. For associations with denser material (>2.4 g/cm³), mineralogy determines the density of the association, the type of patchy OM attached to mineral surfaces and controls the extent of litter-derived N incorporation.     

Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert,Chile

Supergene enrichment of Cu deposits in the Atacama Desert has played a critical role in making this the prime Cu-producing province of the world. Previously, this has been believed to have occurred exclusively over a long period from the middle Eocene to the late Miocene, which ended when climatic conditions changed from arid to hyperarid. Here, we report U-series disequilibrium ages in atacamite-bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the Atacama Desert. ²³⁰Th–²³⁴U ages of gypsum intergrown with atacamite in supergene veins from Cu deposits cluster at ~240 ka (Chuquicamata), 130 ka (Mantos Blancos, Spence), and 80 ka (Mantos de la Luna, Michilla). When coupled with previous data, these results indicate that supergene enrichment of Cu deposits did not cease after the onset of hyperaridity. We propose that supergene enrichment in the Atacama region developed in two main stages. The main phase, caused by downward circulation of meteoric waters in a semi-arid setting, was active from 45 until ~9 Ma, with a last pulse ca. 5 Ma in the southern Atacama Desert. During this phase, atacamite-bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water. This was followed by a second stage starting at ~2–1.5 Ma and continuing until at least the late Pleistocene, when deep formation waters derived from the basement passed up through and modified the pre-existing supergene Cu oxide minerals. Atacamite has then been preserved in the prevailing hyperarid climate.     

Anticlockwise metamorphic paths at ca. 890–790 Ma from the NE Baidrag block,Mongolia, indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ～720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ～750 °C and ～9 kbar and decompression to ～650 °C and ～8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ～620 °C and 6 kbar, followed by further burial to ～590 °C and 8.5 kbar. The monazite data yield a continuum of ²⁰⁷Pb-corrected ²³⁸U/²⁰⁶Pb dates of ca. 926–768 Ma in the Grt-Sil-Ky gneiss, and ca. 937–754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32 °C/km is estimated at ca. 890–853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km dated at ca. 835–815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ～ 0.01–0.06 in Grt-Sil-Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice – probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.     

The formation of caliche in soils of the Mojave Desert,California

Radiocarbon and ²³⁰Th-²³⁴U dates of calcic horizons from calciorthid soil profiles in the Mojave Desert were used to calculate the rate of deposition of pedogenic CaCO₃. A major period of CaCO₃ deposition appears to have occurred about 20000 yBP forming calcic horizons below 100-cm depth during a climatic regime with greater effective rainfall than in the present. The overall rate of deposition has been 1.0 to 3.5 g CaCO₃/m²/yr during soil formation. This rate is consistent with present-day rates, assuming that the atmospheric deposition of Ca limits the process. Stable isotope ratios in calcic horizons indicate that CaCO₃ precipitated from a soil environment with CO₂ of ? 15.5%. ${}^{13}\text{C}{}^{12}\text{C}$ (vs. PDB) and H₂O of + 2.0%. ${}^{18}\text{O}{}^{16}\text{O}$ (vs. SMOW). These values suggest that CaCO₃ precipitates when seasonal drought simultaneously lowers soil pore pCO₂ and enriches soil water ¹⁸O by evaporation. The role of soil calcic horizons in the global geochemical cycle of carbon is discussed.     

Several distinct wet periods since 420 ka in the Namib Desert inferred from U-series dates of speleothems

The scarcity of numerical dates of the arid areas in southern Africa is a challenge for reconstructing paleoclimate. This paper presents a chronological reconstruction in the central part of the Namib Desert, Namibia, for the last 420,000 yr. It is based on ²³⁰Th/U dates (TIMS) from a large stalagmite and a thick flowstone layer in a small cave located in the hyper-arid central Namib Desert. The results provide for the first time evidence of three or possibly four succeeding wet periods of decreasing intensity since 420 ka through which speleothem deposited at approximately 420–385 ka, 230–207 ka and 120–117 ka following the 100-ka Milankovitch cycle. Speleothem growth was not recorded for the Holocene. These wet periods interrupted the predominantly dry climate of the Namib Desert and coincided with wet phases in deserts of the northern hemisphere in the Murzuq Basin, Sahara, the Negev, Israel, the Nafud Desert, Saudi Arabia, and the arid northern Oman, Arabian Peninsula.     

The Sha’it–Nugrus shear zone separating Central and South Eastern Deserts,Egypt: A post-arc collision low-angle normal ductile shear zone

The northerly dipping Sha’it–Nugrus shear zone (SNSZ) is the boundary separating the Central Eastern Desert from the South Eastern Desert of Egypt. The hangingwall of this shear zone is composed of low-grade metavolcanics and ophiolitic nappes of the Central Eastern Desert, while the footwall consists of South Eastern Desert high-grade metapsammitic gneisses (Migif-Hafafit gneissic complex). The SNSZ is about 700 m thick and represents the shear foliated lower parts of the hangingwall and upper parts of the footwall. A significant part of the SNSZ has been truncated by a later normal fault along Wadi Sha’it, however the SNSZ is well-preserved along Wadi Nugrus. Features of the SNSZ include shear-related schistosity (termed Ss), mylonite zones, sheared syn-kinematic granitoid intrusions, diverse metasomatism and metamorphic effects (higher T overprinting of hangingwall lithologies and retrogression of footwall lithologies). Shear-sense indicators clearly show top-to-N or NW displacement sense. SNSZ structures overprint arc collision related nappe structures (～680 Ma) and are therefore post-arc collision. SNSZ syn-kinematic intrusives have been dated at ～600 Ma. The SNSZ is deformed (regionally and locally folded and thrust dissected) during later NE–SW compressive tectonism. The SNSZ had an originally approximately E–W strike, low-angle N-dip and a normal shear sense, making this an example of a low-angle normal ductile shear (LANF) or detachment fault. The steep NE dip of Ss foliations and low-pitching slip lineations along Wadi Nugrus are due to NW–SE folding of the SNSZ, and do not indicate a sinistral strike-slip shear zone. The normal shear sense activity is responsible for juxtaposing the low-grade Central Eastern Desert lithologies against South Eastern Desert gneisses. A displacement of 15–30 km is estimated on the SNSZ, which is comparable to LANF displacements in the Basin and Range province of the western USA. Frictional resistance along this shear was probably reduced by high magmatic fluid pressure and hydrothermal fluid pressure. The vastness and diversity of the hydrothermal activity along this shear zone is a characteristic of other LANFs in the Eastern Desert, e.g. at Gabal El-Sibai, and may be Gabal Meatiq. The SNSZ formed during the Neoproterozoic extensional tectonic phase of Eastern Desert that began ～600 Ma, and followed arc collision and NW-ward ejection of nappes.     

A wind tunnel simulation and field verification of desert dust deposition (Avdat Experimental Station, Negev Desert)

The deposition of natural dust in an area of 53 ha, situated in the northern Negev desert, is investigated in detail both in the wind tunnel (dust storm simulations over a topographic scale model) and in the field. The wind tunnel results and the field results show a high degree of agreement, indicating that scale-model simulation may be considered an important technique for future loess and desert research. More dust settles on windward slopes than on leeward slopes, which is in contradistinction with the widespread wind shadow concept. Air-flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too. In the case of dust deposition on topographic scale models, a restricted height distortion of the model will not necessarily lead to serious problems. In addition, wind tunnel blockage percentages up to 13% may be allowed in order to obtain acceptable dust deposition patterns for the scale model. A mean gross dust deposition of about 200–250 g m^?2 year^?1 is calculated for the northern Negev desert for 1987. Thus, if the settled dust can be protected against erosion in the cultivated areas in the Negev, the dust content of the top soil will markedly increase with time. However, it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and, hence, to a higher dust content in the atmosphere. At any rate, a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields. From the air dust concentration data and the dust deposition data, a deposition velocity of 4.7cms^?1 can be calculated for Avdat dust.     

Fe(CO3)OH in goethite from a mid-latitude North American Oxisol: estimate of atmospheric CO2 concentration in the Early Eocene “climatic optimum”

Measured mole fractions (X) and δ¹³C values of the Fe(CO₃)OH component in pedogenic goethite from a mid-latitude Oxisol of Early Eocene age (≈52 Ma B.P.) range from 0.0014 to 0.0064 and −20.1 to −15.4‰, respectively. These values of X imply that concentrations of CO₂ gas in the paleosol were ≈7400 to ≈34,000 ppm. δ¹³C and 1/X are correlated and define a linear, soil-CO₂ diffusive mixing line with a positive slope. Such positive slopes are characteristic of mixing of two isotopically distinct CO₂ endmembers (atmospheric CO₂ and CO₂ from oxidation of soil organic matter). From the intercept of the mixing line, it is calculated that the δ ¹³C value of organic matter in the ancient soil was ≈−28.0‰. The magnitude of the slope implies an Early Eocene atmospheric CO₂ concentration of ≈2700 ppm.A simple model for forest soils suggests that a “canopy effect” may cause atmospheric CO₂ concentrations deduced from pedogenic minerals to underestimate the actual concentrations of atmospheric CO₂. If a significant forest canopy were present at the time of formation of pedogenic goethite in the Ione Fm, the concentration of 2700 ppm calculated for atmospheric CO₂ could be slightly low, but the underestimate is expected to be < ≈300 ppm (i.e., less than the analytical uncertainty). The relatively high concentration of 2700 ppm inferred for atmospheric CO₂ at ≈52 Ma B.P. would have been coincident with the Early Eocene climatic optimum. This result seems to support the case for an important role for variations of atmospheric CO₂ in the modification of global paleoclimate.     

Carbon storage in desertified lands: A case study from North China

Based on the organic and carbonate carbon levels of the top 1 m of soil from desertified soils of Northern People's Republic of China, climatic and vegetative cover zones have been derived for some 334000 km² of desertification-prone lands. Regional accumulations of pedogenic carbonates were examined relative to precipitation, altitude, and temperature. The largest accumulations of pedogenic carbonates were found in Calcic soils in warm, arid areas. Accumulated organic carbon predominated in soils under Betula platyphylla. In the naturally desertified lands of China, for example, the top 1.0-m soil layer contains some 7.84 Pg of organic carbon and 14.9 Pg of carbonate carbon. Total stored carbon, including carbonate carbon, is 1.8-fold more than organic carbon alone. The carbon released through land desertification in China may be an important factor affecting changes in concentrations of greenhouse gases worldwide.     

Field experiments of aeolian dust accumulation on rock fragment substrata

The effect of rock fragments and rock fragment cover on the accumulation of airborne dust was examined in a long-term field experiment in the Negev desert of Israel. Four parameters were studied: pebble eccentricity, pebble size, pebble flattening and cover density. The effect of these parameters on the accumulation of dust on the pebbles, on the accumulation of dust between and underneath the pebbles, and on total dust accumulation (pebbles+interpebble space) was measured separately. Accumulation on the pebbles increased as the pebbles became larger, less flattened and more elongated, and as cover density increased. Accumulation between and underneath the pebbles increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased (although the accumulation area available became smaller). Total dust accumulation increased as the pebbles became smaller, more flattened and more elongated, and as cover density increased. Rock fragments act as a dust trap as dust accumulation on pebble-covered surfaces is several tens of times larger than dust accumulation on similar but pebble-free surfaces. However, less than 20% of the initially settled dust is protected from further erosion. In the Negev desert, dust erosion by wind is of the order of 10 times larger than dust erosion by water. The rate of long-term dust accumulation in the Negev is of the order of 15–30 g.m^?2.yr^?1. This is considerably lower than the rates proposed elsewhere in the literature, which are based upon over optimistic dust retainment percentages.     

Radiocarbon ages of pedogenic carbonate nodules from Coimbatore region,Tamil Nadu

In this paper we discuss the limitation of radiocarbon dates on the pedogenic calcic nodules formed in situ within the vertisols in the upland region of Coimbatore, Tamil Nadu. The radiocarbon ages were obtained using low-level scintillation counters and the dates range from ∼24 to 31 ¹⁴C kyrs BP. The ages correlate with the marine isotope stage of Late MIS3. However, since the calcic nodules are pedogenised and formed in an open system, the ¹⁴C ages should only be considered as estimates and not absolute ages because of the possibility of open-system behaviour with respect to carbon. Thus, we express caution in the interpretation of these and other radiocarbon ages obtained on pedogenic carbonate nodules. Multiple sub-mm size subsamples could provide more reliable age estimates.