Transition from arid to hyper-arid environment in the southern Levant deserts as recorded by early Pleistocene cummulic Aridisols

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ～2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ～1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (10⁵–10⁶ years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ¹⁸O and relatively high δ¹³C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ～1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 10⁶ yr.     

Abrasion-derived sediments under intensified winds at the latest Pleistocene leading edge of the advancing Sinai-Negev erg

Quaternary desert loess and sandstone-loessite relationships in the geological record raise questions regarding causes and mechanisms of silt formation and accretion. In the northern Sinai-Negev desert carbonate terrain, only sand abrasion in active erg could have produced the large quantities of quartzo-feldspathic silts constituting the late Quaternary northwestern Negev loess. In the continuum of source (medium to fine sand of dunes) to sink (silts in loess) the very fine sand is unaccounted for in the record. This weakens the sand abrasion model of silt formation as a global process. Here, we demonstrate that, as predicted by experiments, abrasion by advancing dunes generated large quantities of very fine sand (60-110 μm) deposited within the dune field and in close proximity downwind. This very fine sand was generated 13-11 ka, possibly synchronous with the Younger Dryas under gusty sand/dust storms in the southeastern Mediterranean and specifically in the northern Sinai-Negev erg. These very fine sands were washed down slope and filled small basins blocked by the advancing dunes; outside these sampling basins it is difficult to identify these sands as a distinct product. We conclude that ergs are mega-grinders of sand into very fine sand and silt under windy Quaternary and ancient aeolian desert environments.     

Diagenetic effects on the distribution of uranium in live and Holocene corals from the Gulf of Aqaba

We investigated the effects of diagenetic alteration (dissolution, secondary aragonite precipitation and pore filling) on the distribution of U in live and Holocene coral skeletons. For this, we drilled into large Porites lutea coral-heads growing in the Nature Reserve Reef (NRR), northern Gulf of Aqaba, a site close to the Marine Biology Laboratory, Elat, Israel, and sampled the core material and porewater from the drill-hole. In addition, we sampled Holocene corals and beachrock aragonite cements from a pit opened in a reef buried under the laboratory grounds. We measured the concentration and isotopic composition of U in the coral skeletal aragonite, aragonite cements, coral porewater and open NRR and Gulf of Aqaba waters.Uranium concentration in secondary aragonite filling the skeletal pores is significantly higher than in primary biogenic aragonite (17.3 ± 0.6 compared to 11.9 ± 0.3 nmol · g⁻¹, respectively). This concentration difference reflects the closed system incorporation of uranyl tri-carbonate into biogenic aragonite with a U/Ca bulk distribution coefficient (K_D) of unity, versus the open system incorporation into secondary aragonite with K_D of 2.4. The implication of this result is that continuous precipitation of secondary aragonite over ∼1000 yr of reef submergence would reduce the coral porosity by 5% and can produce an apparent lowering of the calculated U/Ca - SST by ∼1°C and apparent age rejuvenation effect of 7%, with no measurable effect on the calculated initial U isotopic composition.All modern and some Holocene corals (with and without aragonite cement) from Elat yielded uniform δ²³⁴U = 144 ± 5, similar to the Gulf of Aqaba and modern ocean values. Elevated δ²³⁴U values of ∼180 were measured only in mid-Holocene corals (∼5000 yr) from the buried reef. The values can reflect the interaction of the coral skeleton with ²³⁴U-enriched ground-seawater that washes the adjacent granitic basement rocks.We conclude that pore filling by secondary aragonite during reef submergence can produce small but measurable effects on the U/Ca thermometry and the U-Th ages. This emphasizes the critical importance of using pristine corals where the original mineralogy and porosity are preserved in paleooceanographic tracing and dating.     

Complex exposure histories of chert clasts in the late Pleistocene shorelines of Lake Lisan,southern Israel

Activities of ²⁶Al and ¹⁰Be in five chert clasts sampled from two beach ridges of late Pleistocene Lake Lisan, precursor of the Dead Sea in southern Israel, indicate low rates of chert bedrock erosion and complex exposure, burial, and by inference, transport histories. The chert clasts were derived from the Senonian Mishash Formation, a chert‐bearing chalk, which is widely exposed in the Nahal Zin drainage basin, the drainage system that supplied most of the material to the beach ridges. Simple exposure ages, assuming only exposure at the beach ridge sampling sites, range from 35 to 354 ky; using the ratio ²⁶Al/¹⁰Be, total clast histories range from 0·46 to 4·3 My, unrelated to the clasts' current position and exposure period on the late Pleistocene beach ridges, 160–177 m below sea level. Optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20·0 ± 1·4 ka and 36·1 ± 3·3 ka. These ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of Lake Lisan, which suggest that the lake reached its highest stand around 27 000 cal. years BP . If the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production, then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50–1300 ky and 390–3130 ky respectively. Alternatively, the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock. Back calculating such rates of rock erosion suggests values between 0·4 and 12 m My^?1. The relatively long burial periods indicate extended sediment storage as colluvium on slopes and/or as alluvial deposits in river terraces. Some clasts may have been stored for long periods in abandoned Pliocene and early Pleistocene routes of Nahal Zin to the Mediterranean before being transported again back into the Nahal Zin drainage system and washed on to the shores of Lake Lisan during the late Pleistocene. Copyright © 2003 John Wiley & Sons, Ltd.     

Life-bearing primordial planets in the solar vicinity

The space density of life-bearing primordial planets in the solar vicinity may amount to ～8.1×10⁴?pc^?3 giving total of ～10¹⁴ throughout the entire galactic disk. Initially dominated by H₂ these planets are stripped of their hydrogen mantles when the ambient radiation temperature exceeds 3?K as they fall from the galactic halo to the mid-plane of the galaxy. The zodiacal cloud in our solar system encounters a primordial planet once every 26 My (on our estimate) thus intercepting an average mass of 10³ tonnes of interplanetary dust on each occasion. If the dust included microbial material that originated on Earth and was scattered via impacts or cometary sublimation into the zodiacal cloud, this process offers a way by which evolved genes from Earth life could become dispersed through the galaxy.     

Microcracks in granite cores from the EPS-1 geothermal drill hole, Soultz-sous-Forêts (France): paleostress directions, paleofluids and crack-related Vp-anisotropies

For the modelling of Hot Dry Rock systems the interactions between macrofractures, prominent microcrack sets and the in situ stress field is of fundamental importance. In this study complete 3D analyses of microcrack orientations were carried out on granitoid samples from the Soultz geothermal borehole (EPS-1). The crack population is dominated by healed cracks in quartz forming three orthogonal sets (I, II, III) with strong preferred orientation, which probably result from superposition of internal thermal stresses during cooling and external tectonic stresses. The inferred orientation of paleo-σ_H is NE–SW. Based on the microthermometry of secondary fluid inclusions it can be assumed that cyclic crack/healing events occurred at P/T conditions roughly ranging between 1 and 2?kbar and 200–400?°C. A younger generation of open cracks in quartz which coincide with macroscopic fractures can be correlated with the Tertiary N–S direction of σ_H. The distribution of P-wave velocities (Vp) was measured by using the pulse transmission technique in 132 propagation directions at confining pressures up to 200?MPa. These Vp data and their variation with confining pressure are a suitable tool for detecting prominent crack patterns and to estimate the crack-controlled in situ properties. The observed bulk Vp anisotropy roughly reflects the composite microcrack fabric and the relative importance of individual sets. It can be assumed that the prominent crack sets represent planes of weakness and hence may cause a mechanically anisotropic behaviour dependent on their orientation with respect to the recent in situ stresses.     

The transparency of the Universe limited by Lyα clouds

The brightnesses of supernovae are commonly understood to indicate that cosmological expansion is accelerating due to dark energy. However the entire discussion presumes a perfectly transparent universe because no effects of reddening associated with the interstellar extinction law are seen. We note that with two kinds of dark matter (baryonic and nonbaryonic) strongly dominating the known mass of the universe, it is seriously premature to assume that these dark matter components have not reduced the transmission of the universe for cosmological sources. We show that the long‐known Lyα clouds, if nucleated by the population of baryonic dark matter primordial planetoids indicated by quasar microlensing, would act as spherical lenses and achromatically fade cosmologically distant sources. We attempt to estimate the amount of this cosmological fading, but ultimately the calculation is limited by lack of a satisfactory model for the tenuous outer parts of a primordial planetoid. We also consider the effects of such cosmological fading on the light of quasars. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)