Terrestrial age measurements using natural thermoluminescence of a drained zone under the fusion crust of Antarctic ordinary chondrites

Abstract— Miono et al. (1990) and Miono and Nakanishi (1994) have proposed that the build‐up of natural thermoluminescence (TL) in a drained layer directly below the meteorite fusion crust can be used to determine terrestrial ages of meteorites in the 40 to 200 ka range. We have measured the natural TL of the drained layer of 15 meteorites. The data indicate that this technique could be used to determine terrestrial ages of meteorites with ages <200 ka, after which TL equilibrium is reached. Comparison of TL build‐up with terrestrial ages for a suite of Antarctic meteorites suggests that the meteorites have been exposed to temperatures of 0 to 5 °C. The close correspondence between natural TL levels and surface exposure TL growth curves suggest that Allan Hills meteorites with ages <200 ka have spent a significant portion of their terrestrial history exposed on the ice surface, rather than being buried in the ice sheet. The technique is, however, sensitive to thermal history; and, for Antarctic meteorites with terrestrial ages <200 ka, natural TL of the drained zone largely reflects exposure on the ice surface.     

Geomorphologic and Palaeoenvironmental Development of Holocene Perched Coastal Dune Systems in Brittany, France

Two perched dune systems are investigated in the Baie d'Audierne (Finistère, Brittany, northwest France). A maximum age of 4545–4160 cal. yr bp for the onset of sand accretion is provided through radiocarbon (¹⁴C) analysis of an organic-rich basal palaeosol, corresponding closely to deceleration, but before stabilisation, of regional Holocene relative sea levels (RSLs). Molluscan analysis through the dune sand indicates an initial bare sand or sparsely vegetated surface, subsequently maturing and colonised by scrub vegetation, and finally returning to more open dune grassland conditions. These data suggest a three-stage Perched Dune Development Model (PDDM), which may be applicable to perched dunes on at least a regional scale: (1) initial sand sheet inundation as RSL rises; (2) stabilisation of RSL allowing dune to mature; and (3) sea-level stabilisation stimulates cliff formation, perching and progressive landward retreat of the dune shore. This study recognises that perched dunes hold greater potential for establishing chronologies for regional Holocene dune development than the more widely studied lowland dune systems, where evidence for early sand inundation may be obscured or lost by modest rises in sea level.     

Tag-encoded pyrosequencing analysis of bacterial diversity within different alpine grassland ecosystems of the Qinghai-Tibet Plateau,China

The Qinghai-Tibet Plateau is sensitive to climate change, with ecosystems that are important with respect to scientific research. Here high-throughput DNA pyrosequencing was used to assess bacterial diversity within different alpine grassland ecosystems of the Qinghai-Tibet Plateau, China. In total, 34,759 sequences were obtained for the three ecosystems––alpine cold swamp meadow (ASM), alpine cold meadow (AM), alpine sandy grassland (ASG), and 31 phyla and a small number of unclassified bacteria were detected. The bacterial community structures were different for each alpine grassland ecosystem. The Proteobacteria and Acidobacteria were the predominant phyla in all three ecosystems. Besides this, Actinobacteria and Chloroflexi were abundant in ASM, Bacteroidetes, Gemmatimonadetes and Verrucomicrobia were abundant in AM, and Actinobacteria were abundant in ASG. In addition, the functional bacterial genera also differed with each alpine grassland ecosystem. The ASM contained more nitrifying bacteria, methane-oxidizing bacteria and sulfur- and sulfate-reducing bacteria, whereas the ASG ecosystem contained more nitrogen-fixing bacteria. Pyrosequencing provided a greater insight into bacterial diversity within different alpine grassland ecosystems than previously possible, and gave key data for the involvement of bacteria in the protection of alpine grassland ecosystems of the Qinghai-Tibet Plateau, China.     

Rare-earth abundances in kimberlites from Greenland and Zambia

Ten rare-earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb and Lu) and Ta, Th and Hf contents in eight kimberlites and inclusions from Greenland and Zambia have been determined by instrumental neutron activation. All the samples have highly fractionated rare-earth (REE) distribution patterns. La/Yb ratios in the Greenland kimberlites (hypabyssal facies) vary from 111.8 to 188.4, and the total rare-earth contents range from 204.8 to 380.3 ppm. No europium anomaly is present. The Zambian kimberlites (diatreme facies) are altered and carbonated. Rare-earth patterns in these are also light REE-enriched. A significant difference is shown to exist between the diatreme and hypabyssal facies of kimberlites.     

Improved Platinum-Group Element Extraction by NiS Fire Assay from Chromitite Ore Samples Using a Flux Containing Sodium Metaphosphate

Many chromite-rich rocks contain relatively high concentrations of the platinum-group elements (PGE). In many cases, the phases carrying PGE occur as either platinum-group minerals (PGM) or as base metal sulfides in solid solution in sulfides. In some cases, such as the UG-2 unit of the Bushveld Complex, the PGM are occluded inside chromite grains. Chromites are notably difficult to dissolve in most fluxes and if the chromite contains some PGM the possibility exists that not all the PGE will be recovered during fusion. In this work, shortcomings in published methods of analysis based on the nickel sulfide fire assay procedure were investigated and a new procedure developed based on the addition of sodium metaphosphate to the fusion mixture. Optimum composition of the fusion mixture was found to be 10 g sodium metaphosphate and 9 g silica to 10 g sample, 15 g sodium carbonate, 30 g lithium tetraborate, 7.5 g nickel and 4.5 g sulfur to achieve complete dissolution of chromite grains. The new flux mixture was evaluated by the analysis of reference material CHR-Pt+ (which is known to contain PGM inside chromite grains) and no undissolved chromite grains were found in the glassy slag. Analysis of the nickel sulfide beads from this fire assay using neutron activation analysis showed similar results for Rh and Ru when compared with published conventional true (or accepted) values, while Au, Ir, Os, Pd and Pt values determined here were 10 to 30% higher than the corresponding published conventional true values. It was concluded that the addition of sodium metaphosphate improved chromite dissolution in the flux and appears to improve PGE recovery.     

全新世长江泥沙堆积的时空分布及通量估算

利用长江中下游、河口及口外、浙-闽沿岸陆架6个主要沉积盆地的40个晚第四纪钻孔及其年代学数据和长江口外、陆架的浅地层剖面,计算了全新世不同阶段各沉积盆地的沉积速率,并进行了近7 000年来泥沙堆积通量的估算。研究发现全新世早期距今10 000年至8 000年间长江口下切古河谷是长江泥沙的主要堆积中心,沉积速率可高达15m/ka。随着海平面上升,全新世中期长江中下游也成为长江泥沙的重要沉积盆地,其中江汉盆地的沉积速率可达10m/ka。近2 000年来,口外、陆架的堆积呈明显增加趋势,反映长江中下游盆地和河口可容空间日益减小。根据沉积速率估算,距今7 000年来长江中下游堆积泥沙约13 074×108 t,同期水下三角洲和陆架的泥沙堆积量约为9 470×108 t。研究还发现全新世以来有两个异常低沉积速率时期：距今8000-7 000年期间上述各沉积盆地沉积速率均显著低,未见长江泥沙的沉积中心; 距今4 000-2 000 年期间长江口呈现低沉积速率。 这两次异常的原因推测与海平面、气候波动事件密切相关。     

Development of Tide–Surge Interaction Model for the Coastal Region of Bangladesh

In this paper, a two-dimensional, vertically integrated hydrodynamic model is developed taking into account entrained air bubbles during storm surges as well as incorporating inverted barometer, and river and land dynamics. The model is specifically designed for the coastal region of Bangladesh. A nested scheme method with a fine mesh scheme (FMS), capable of incorporating the complex coastline and all major offshore islands accurately, nested into a coarse mesh scheme (CMS) covering up to 15° N latitude in the Bay of Bengal is used. To incorporate the small and big offshore islands in the Meghna estuarine region with its complex coastline accurately, a very fine mesh scheme (VFMS) is again nested into the FMS. Along the northeast corner of the VFMS, the Meghna river discharge is taken into account. The coastal and island boundaries are approximated through proper stair steps. The model equations are solved by a semi-implicit finite difference technique using a staggered C-grid. A stable appropriate tidal condition over the model domain is generated by applying tidal forcing with the four major tidal constituents M ₂, S ₂, K ₁, and O ₁ along the southern open boundary of the CMS. This tidal regime is introduced as the initial state of the sea for nonlinear interaction of tide and surge. The model is applied to simulate water levels due to the interaction of tide and surge associated with the cyclones April 1991 and Aila at different coastal and island locations along the coast of Bangladesh. The results are found to be quite satisfactory with root mean square error of ～0.50 m as calculated for both the storm events. Tests of sensitivities on water levels are carried out for air bubbles, offshore islands, river discharge, inverse barometer, and grid resolution. The presence of air bubbles increases simulated water levels a little bit in our model, and the contribution of air bubbles in increasing water level is found around 2 %. Further, water levels are found to be influenced by offshore islands, river discharge, inverse barometer as well as grid resolution.