How Do Surficial Lithic Assemblages Weather in Arid Environments? A Case Study from the Atacama Desert,Northern Chile

Archaeological sites composed only of surficial lithics are widespread in arid environments. Numerical dating of such sites is challenging, however, and even establishing a relative chronology can be daunting. One potentially helpful method for assigning relative chronologies is to use lithic weathering, on the assumption that the most weathered artifacts are also the oldest. Yet, few studies have systematically assessed how local environmental processes affect weathering of surficial lithics. Using macroscopic analyses, we compared the weathering of surficial lithic assemblages from seven mid‐to‐late Holocene archaeological sites sampled from four different microenvironments in the Atacama Desert of northern Chile. Changes in polish, texture, shine, and color were used to establish significant differences in weathering between two kinds of locations: interfluves and canyon sites. Lithics from interfluve sites were moderately to highly weathered by wind and possessed a dark coating, whereas canyon lithics were mildly weathered despite greater exposure to moisture, often lacked indications of eolian abrasion, and lacked dark coatings. Our results show that lithic weathering can be used as a proxy for relative age, but only after considering local environmental factors. The power of such chronologies can be improved by combining archaeological, paleoenvironmental, geomorphological, and taphonomic data.     

Age of the Raygorodok Au-bearing Gabbro—Monzodiorite Massif (Northern Kazakhstan)

Accessory zircon from the polyphase Raygorodok stock (Northern Kazakhstan) has been dated by local U–Pb analysis (SIMS and LA-ICP-MS). This Au-bearing intrusion has been dated to 442–447 Ma, suggesting its emplacement at the very end of the Ordovician and Early Silurian, allowing the stock’s correlation with the Stepnyak Complex of small intrusions. Thus, control of small intrusions of the Stepnyak type over gold mineralization has been corroborated permitting their use as a regional prospecting characteristic.     

Approximate Solutions For The Obukhov Length And The Surface Fluxes In Terms Of Bulk Richardson Numbers

Simple analytic approximate solutions arepresented for the set of equations that follows fromthe Monin–Obukhov flux-profile relationships using thestability functions of Dyer (unstable case) andBeljaars–Holtslag (stable case). Several publicationsare devoted to the same subject, however the currentapproach contains some new features, namely: (a) itappears to be more accurate for unstable situationsand (b) it applies also to the general case where windspeed (u) and potential temperature() are given at different levels. In order toillustrate the accuracy of the approach a comparisonwith the actual solutions is presented for someselected combinations of and u levelstypical for various practical applications.     

重庆夏季旱涝的欧亚环流特征分析

计算了重庆地区夏季区域降水指数．采用相关、合成、奇异值分解等多种技术方法分析了重庆地区夏季旱涝的同期夏季和前期冬季欧亚环流特征,在此基础上定义了关键区环流指数．并分析了其对重庆地区夏季降水预报的指示意义。     

Immunological studies on microbial mats from Solar Lake (Sinai)— A contribution to the organic geochemistry of sediments

Immunological and biochemical techniques were used to investigate organic matter in microbial mats from Solar Lake (Sinai, Egypt). Antibodies, elicited in rabbits against samples taken at different depths from a core of these mats, were used for the detection of immunological determinants preserved in these samples. Special attention was given to an antiserum directed against a top mat fraction (3–10 mm depth) and an antiserum against a fraction from a lower section of the core (380–390 mm depth). Both antisera, tested in a dot-blot immunobinding assay, were found to give positive reactions with organic matter from different depths of the sediment. An immunohistochemical experiment with the antisera points to antigenic determinants on Recent and subfossil sheaths of Microcoleus. In addition, extracts of sediment samples were submitted to electrophoresis in polyacrylamide gels in order to characterize the separated macromolecules by immunological means. The results showed a rapid decrease in the number of antigenic determinants from Recent to subfossil samples. However, even in the deepest layers of the sediment (690–700 mm depth) intact biopolymers were detected.     

The Oklo natural reactor: Cumulative fission yields and retentivity of the symmetric mass region fission products

Solid source mass spectrometry has been used to determine the relative cumulative fission yields of five elements in three samples of uranium ore from reactor zones in the Oklo mine site. Eighteen fission chains covering the mass range from105 ≤ A ≤ 130 have been measured for Pd, Ag, Cd, Sn and Te. These measurements have enabled a number of nuclear parameters to be calculated including the relative proportions of²³⁵U,²³⁸U and²³⁹Pu involved in the fission process.The concentration of the five elements in the Oklo samples have also been measured using the stable isotope dilution technique. These values have then been compared to the estimates of the amount of these elements produced by fission under the conditions that are appropriate to the three samples. This procedure enables the retentivity of the elements in the reactor zones to be evaluated. Our work confirms the fact that Pd and Te are retained almost in their entirety in the samples, whereas the other three elements have been partially lost from the reactor site. Almost all the Cd fission products have been lost, and more than 50% of the Ag and Sn fission-produced material has been removed.     

Multidirectional,synchronously‐triggered seismo‐turbidites and debrites revealed by X‐ray computed tomography (CT)

On 21 April 2007, a M_w 6·2 earthquake struck Aysén fjord (Chilean Patagonia) and caused onshore and offshore mass movements which triggered tsunamis and density flows in the fjord. To better understand the facies successions in, and the intercalation of, the density‐flow deposits, a study was made of the 2007 deposits in 22 short sediment cores taken in the inner Aysén fjord. By combining grain‐size analysis with X‐ray computed tomography scanning, it was possible to demonstrate that the encountered facies correspond to classical divisions of debrites and turbidites. The single‐event deposits consist of a succession of several sub‐deposits deposited under different flow directions and can be interpreted as stacked turbidites. Orientations of: (i) folds; (ii) imbricated mud clasts; (iii) backsets and foresets of climbing ripples; and (iv) asymmetrical convolute lamination were used to determine relative flow directions at the location of the cores. By assigning the basal flow of the stacked debrites and turbidites to the closest principal mass flow, the absolute flow directions of the sub‐deposits were determined which, in combination with multibeam basin‐floor morphology, allowed reconstruction of the 2007 density‐flow successions in Aysén fjord. Furthermore, alternating flow directions provide evidence for a seiche induced by the density flows. It was concluded that X‐ray computed tomography scans provide crucial information for reconstructing palaeoflows and can be a useful tool in marine and lacustrine sedimentology and palaeoseismology. The multidirectionality of sub‐deposits in turbidites is, next to differences in mineralogy, a new criterion to identify stacked turbidites. These multidirectional, stacked turbidites are an indication of simultaneous triggering of density flows and can therefore, in most cases, be attributed to earthquakes, ruling out other triggers, such as floods, storms or other sediment failures.     

The effect of conservation tillage on runoff erosivity and soil erodibility during concentrated flow

Crop residues in conservation tillage systems are known to cause both a reduction in the erosive runoff power and an increase in the topsoil erosion resistance. In this study, the relative importance of both mechanisms in reducing soil loss by concentrated flow erosion is examined. Therefore, a method to calculate the effective flow shear stress responsible for soil detachment in the presence of a residue cover is applied. The determination of effective flow shear stress is based on the recalculation of the hydraulic radius for residue treatments. The method was tested in a laboratory flume by comparing soil detachment rates of identical pairs of soil samples that only differ in the presence or absence of crop residues. This shear stress partitioning approach and a soil detachment correction were then applied to a dataset of soil detachment measurements on undisturbed topsoil samples from a no‐till field plot on a loess‐derived soil, sampled during one growing season. Results indicate that only a small fraction (10% on average) of the difference in soil detachment rate between conventional and conservation tillage can be attributed to the dissipation of shear forces on the residues. The remaining decrease in soil detachment during concentrated runoff after a two‐year application of conservation tillage can be explained by the increased dry bulk density and root and crop residue content in the topsoil that reduces soil erodibility. After correcting for the presence of residues, the temporal variability in soil detachment rates (D_r) during concentrated flow for a given flow shear stress (τ) for both treatments can be predicted fairly well (R² = 0·87) from dry soil bulk density (DBD, representing consolidation effects), soil moisture content (SMC, representing antecedent rainfall conditions), the dry mass of organic material (OM, representing root growth and residue decomposition) and saturated soil shear strength σ_{s, sat} using an equation of the form: This study is the first to show that the effect of conservation tillage on soil detachment rates is a result of soil property modifications affecting soil erodibility, rather than a result of the surface residue decreasing flow erosivity. Copyright © 2007 John Wiley & Sons, Ltd.     

Early Palaeozoic biotite Rb‐Sr dates in the Yilgarn Craton near Harvey,Western Australia

Biotite from granite and gneiss in the southwestern part of the Yilgarn Craton near Perth has been dated by the Rb‐Sr technique at about 430–500 Ma in a belt at the western edge of the craton. The belt widens southward from 30 km near Perth to 55 km east of Harvey. The eastern boundary of the belt passes along the east margin of the Saddleback greenstone belt. A transition zone 15–40 km wide separates the belt of young dates from an eastern chronological plateau where biotite dates, mainly 2300–2600 Ma, are marginally younger than regional Rb‐Sr whole rock dates which average about 2550 Ma. In contrast to biotite dates, whole rock dates greater than 2500 Ma persist to the western edge of the craton. The 430–500 Ma dates probably represent resetting during uplift in the Early Palaeozoic.