Optically stimulated luminescence dating as a tool for calculating sedimentation rates in Chinese loess: comparisons with grain-size records

Understanding loess sedimentation rates is crucial for constraining past atmospheric dust dynamics, regional climatic change and local depositional environments. However, the derivation of loess sedimentation rates is complicated by the lack of available methods for independent calculation; this limits interpretation of the environmental changes revealed by the loess record. In particular, while the Quaternary/Neogene Chinese loess and Red Clay sequences have the potential to provide detailed records of past sedimentation and climate change, there is great uncertainty concerning: (i) the influences on sediment grain‐size and accumulation; and (ii) their relationship through time and across the depositional region. This uncertainty has led to the widespread use of assumptions concerning the relationship between sedimentation rate and grain‐size in order to derive age models and climate reconstructions. To address this uncertainty, detailed independent age models, based on optically stimulated luminescence dating, undertaken at 10 to 40 cm intervals at five sections across the Loess Plateau in China, have been used to calculate sedimentation rates and make comparisons with grain‐size changes over the late Pleistocene and Holocene. The results demonstrate that sedimentation rates are site specific, extremely variable over millennial timescales and that this variation is often not reflected in grain‐size changes. In the central part of the Loess Plateau, the relationship between grain‐size and sedimentation rate appears most complex, suggesting an interplay between local conditions at source and sink and a changing emplacement mechanism. This observation further undermines the common use of loess sedimentation age models that rely on a derived relationship between grain‐size and sedimentation rate from a type section. The results also highlight the difficulty in assigning specific environmental causes to sedimentation rate changes and, to a lesser extent, grain‐size shifts.     

Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800{degrees}C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions

We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H₂O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H₂O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO₂, Na₂O, and K₂O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al₂O₃, Na₂O, andK₂O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al₂O₃/[Na₂O + K₂O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10^–15m²/s and (0·6–2·4) x 10^–14 m²/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10^–11m²/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H₂O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H₂O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis     

Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006

During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10⁻³ to 5.2 × 10⁻³ at 450 nm and from 0.3 × 10⁻³ to 2.5 × 10⁻³ at 700 nm. Groupings of k values could be attributed to different source regions.     

Impact of the Arctic Fox Alopex lagopus on nesting success of geese in southeast Svalbard, 1989

Goose populations, with special emphasis on Light-bellied Brent Geese Branta bernicla hrota , were censured in Tusenøyane and Tjuvfjorden, southeast Svalbard, July-August 1989. A total of 425 non-breeding Brent Geese, 210 Barnacle Geese Branta leucopsis and 421 Pink-footed Geese Anser bra-chyrhynchus and 11, 2 and 3 families of the three species, respectively, were counted. Brent Geese attempted to nest on 6 of the 20 islands surveyed, and were successful on four. Barnacle Geese attempted to nest on three islands and were successful on two. Many islands known to be traditional nesting sites were entirely void of geese. Pink-footed Geese were only seen in Tjuvfjorden. The breeding failure on the islands was linked to the presence of the Arctic Fox Alopex lagopus which probably caused the geese to completely give up the attempt to nest. On one island the fox had apparently arrived after nest initiation and ravaged 45 Brent Goose and Barnacle Goose nests. It is likely that the foxes stranded on the islands during ice break-up. As a consequence of the presence of foxes on most of Tusenøyane, more than half of the potential breeding segment of the Svalbard Brent Goose population failed to nest in the summer of 1989.     

Pyrite geochemistry in the Toarcian Posidonia Shale of south‐west Germany: Evidence for contrasting trace‐element patterns of diagenetic and syngenetic pyrites

Authigenic pyrite grains from a section of the Lower Toarcian Posidonia Shale were analysed for their trace‐element contents and sulphur‐isotope compositions. The resulting data are used to evaluate the relationship between depositional conditions and pyrite trace‐element composition. By using factor analysis, trace‐elements in pyrite may be assigned to four groups: (i) heavy metals (including Cu, Ni, Co, Pb, Bi and Tl); (ii) oxyanionic elements (As, Mo and Sb); (iii) elements partitioned in sub‐microscopic sphalerite inclusions (Zn and Cd); and (iv) elements related to organic or silicate impurities (Ga and V). Results indicate that trace‐element contents in pyrite depend on the site and mechanism of pyrite formation, with characteristic features being observed for diagenetic and syngenetic pyrites. Diagenetic pyrite formed within anoxic sediments generally has a high heavy metals content, and the degree of pyritization of these elements increases with increasing oxygen deficiency, similar to the degree of pyritization of reactive Fe. The highest gradient in the increase of the degree of trace element pyritization with bottom‐water oxygenation was found for the elements Ni < Cu < Mo = As < Tl. In contrast, syngenetic pyrite formed within a euxinic water column typically is enriched in As, Mo and Sb, but is low in heavy metals, and the geochemical variation reflects changes in sea water composition.     

The Holocene history of the southwestern Baltic Sea as reflected in a sediment core from the Bornholm Basin

A study of changes in siliceous microfossil assemblages and chemical analyses in a well-dated offshore sediment core from the Bornholm Basin, southwestern Baltic Sea, is carried out with the objective of increasing knowledge of the Holocene history of the area. The core covers about 11 300 calendar years from the brackish phase of the Yoldia Sea stage to the present. The first weak marine influence in the Ancylus Lake stage is recorded about 10 100 cal. yr BP (c. 8900 ¹⁴C BP), indicating a complex transition to the Litorina Sea with different phases of brackish-water inflow. The lithology, organic carbon content and C/N and C/S ratios indicate no major changes in the sedimentary environment during the Litorina-Post-Litorina Sea stages. A high productivity event recorded in the Post-Litorina Sea stage around 950 cal. yr BP correlates with the Medieval warm event. A biostratigraphical change indicating a colder climate is recorded in the sediment at about 800 cal. yr BP, which might mark the beginning of the Little Ice Age.     

Noble gases in chondrules and associated metal‐sulfide‐rich samples: Clues on chondrule formation and the behavior of noble gas carrier phases

Abstract— Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide‐rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial ²⁰Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial ³⁶Ar. Our preferred interpretation is that‐in contrast to CAIs‐the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial ²⁰Ne and ³⁶Ar show low presolar‐diamond‐like ³⁶Ar/²⁰Ne ratios. In contrast, the metal‐sulfide‐rich coatings generally show higher gas concentrations and Q‐like ³⁶Ar/²⁰Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar‐carrying phase Q became enriched in the metal‐sulfide‐rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne‐carrying presolar diamonds survived the melting event leading to the low observed ³⁶Ar/²⁰Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar‐type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.     

Studies using an airborne laser altimeter to measure landscape properties

Abstract

Vertical surface properties of the landscape were measured using a laser altimeter mounted in a small twin-engine aeroplane. The laser altimeter makes 4000 measurements per second with a vertical recording precision of 0.05 m for a single measurement. These airborne laser measurements were analysed to provide information on topography, vegetation canopy and stream and gully cross-sections. Laser altimeter data were used to measure small (less than 0.20 m deep) and large gullies and stream cross-sections. Vegetation canopy heights, cover, structure and distribution were determined in studies in Texas and Arizona. Laser measurements of vegetation cover and height were significantly correlated with ground measurements made with line-intercept methods. While conventional ground-based techniques may be used to make all these measurements, airborne laser altimeter techniques allow the data to be collected in a quick and efficient way over large and inaccessible areas. The airborne laser altimeter data can also help quantify various land surface parameters needed for natural resource and landscape management or required by hydrological simulation models. Measurements of landscape properties over large areas provide a better understanding of landscape functions and can lead to the development of better management plans to conserve and improve the productivity of natural resources.