Study of the temperature dependence of the hyperfine parameters in two orthopyroxenes by 57Fe Mössbauer spectroscopy

⁵⁷Fe Mössbauer measurements were performed on two natural orthopyroxenes in the temperature range between 17 and 490 K. The temperature variations of the center shifts and of the quadrupole splittings have been interpreted using the Debye model for the lattice vibrations and the crystal-field model respectively. Two approaches have been applied to evaluate the crystal field. The first one, which is commonly used by Mössbauer spectroscopists, emanates from the approximative and simplified symmetry of the ferrous sites, whereas the second one takes into account the real C ₁ symmetry of the ferrous sites, thus leading to a pointcharge calculation. For comparison, analogous calculations have been carried out on literature data for an iron-rich orthopyroxene (specimen XYZ).Industrial Highschool BME, Voskenslaan, B-9000, Gent, Belgium     

An 57Fe Mössbauer effect study of poorly crystalline γ-FeOOH

Four synthetic lepidocrocite samples with a significant difference in crystallinity have been studied by the ⁵⁷Fe Mössbauer effect technique at temperatures ranging between 12 K and 360 K. In the magnetically ordered region, the spectra display broad hyperfine field distributions, the profile of which could be derived numerically. In a broad transition region of approximately 20 K, an additional doublet must be included in the fitting model. From Mössbauer thermoscanning measurements at zero-velocity and with and without the application of an external magnetic field, it is found that the doublet is due to Fe³⁺ species in a paramagnetic state. Neither the hyperfine field nor the Néel temperature distributions are markedly affected by the crystallinity. In the pure paramagnetic state, the broadened doublets are best described by a distribution of quadrupole splittings in the range 0.3–1.9 mm/s. By numerical manipulations of the derived probability profiles it was possible to distinguish between the iron species in the outer surface layers of the γ-FeOOH particles and those in the innermost layers, the relative amount of each being correlated with the measured surface areas. The centre shift is quite uniform and its temperature dependence, determined in detail for one of the samples, is perfectly described by the Debije approximation for the second-order Doppler shift. From the obtained Debije temperature, a Mössbauer fraction f of 0.79 at room temperature was calculated, which is in good agreement with the results obtained for a reference lepidocrocite/hematite mixture. Finally, line shape simulations aimed to explain the external field spectrum at 180 K provided strong indications that both the sign and the asymmetry parameter of the electric field gradient are non-uniform as well.     

Statistics for Modeling Heavy Tailed Distributions in Geology: Part I. Methodology

Many natural phenomena exhibit size distributions that are power laws or power law type distributions. Power laws are specific in the sense that they can exhibit extremely long or heavy tails. The largest event in a sample from such distribution usually dominates the underlying physical or generating process (floods, earthquakes, diamond sizes and values, incomes, insurance). Often, the practitioner is faced with the difficult problem of predicting values far beyond the highest sample value and designing his system either to profit from them, or to protect against extreme quantiles. In this paper, we present a novel approach to estimating such heavy tails. The estimation of tail characteristics such as the extreme value index, extreme quantiles, and percentiles (rare events) is shown to depend primarily on the number of extreme data that are used to model the tail. Because only the most extreme data are useful for studying tails, thresholds must be selected above which the data are modeled as power laws. The mean square error (MSE) is used to select such thresholds. A semiparametric bootstrap method is developed to study estimation bias and variance and to derive confidence limits. A simulation study is performed to assess the accuracy of these confidence limits. The overall methodology is applied to the Harvard Central Moment Tensor catalog of global earthquakes.     

Late Pleistocene and Holocene palaeogeography of the Lower Tagus Valley (Portugal): effects of relative sea level, valley morphology and sediment supply

Because of the deep glacial incision, the Lower Tagus Valley hosts a sedimentary record since 20 000 cal BP, making this a unique site along the European Atlantic margin with respect to palaeogeographic and sea level changes. Based on nine cross-sections and 55 radiocarbon dates together with a newly created relative sea level curve, we constructed five palaeogeographic maps of the infill of the Lower Tagus Valley since 20 000 cal BP. We illustrate that relative sea level rise and fluvial sediment supply were the prime forcing factors determining the depositional history and palaeogeographic changes. Around 20 000 cal BP a deeply incised braided river existed, which was directly connected to the ocean across the narrow continental shelf. After that (12 000 cal BP) the gradually moister and warmer climate caused a change to a single-channel river. During the following period (12 000–7000 cal BP) relative sea level rise resulted in a transgression in the Lower Tagus Valley and the establishment of extensive tidal environments. After relative sea level rise had ended (7000 cal BP) the valley was progressively filled by a fluvial wedge and tidally influenced bayhead delta. Since 1000 cal BP the valley-fill history was dominated by increased sediment input due to human-induced degradation of catchment slopes. Generally, climate was of subordinate importance during the entire studied period, merely causing a single-channel river resulting from the change from the cold Heinrich event 1 to the temperate Bölling–Allerød interstadial. Despite the tectonic activity in the region, neotectonic uplift or subsidence was limited, as supported by the horizontal relative sea level curve since 7000 cal BP. Neotectonics played a minor role due to the large distance from the Fennoscandian ice sheet and the narrow continental shelf, which prohibited strong glacio- and hydro-isostatic movements.     

Temperature dependence of the hyperfine parameters of maghemite and al-substituted maghemites

Synthetic aluminum-substituted maghemite samples, -(Al _y Fe_1-y )₂O₃ with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mössbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: _A=_B-0.12 mm/s. From the temperature dependence of _B it was possible to determine the characteristic Mössbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing Al content, at least up to 10 mole%. The temperature dependence of the A-and B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: J _AB =-25 K; J _AA =-18 K and J _BB = -3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the constancy of this quantity for the B sites, suggest that the Al cations occupy the B sites.Research Director, National Fund for Scientific Research, Belgium     

早、中更新世东亚冬季风强度的快速变化

对黄土高原中部洛川黄土堆积进行了系统的粒度和磁化率测量,以黄土－古土壤中的粗颗粒组分,即＞３０μｍ颗粒的百分含量作为冬季风强度代用指标,根据一个独立的时间标尺,发现在１４５～１６５、２４０～２８０、３２０～３５０、３９０～４４０、６００～６４０、８６０～８９０、９００～９３０和１３３０～１４００ｋａＢ．Ｐ．东亚冬季风强度存在千年尺度快速变化的特征,冬季风加强事件的持续时间在１．４～７．２ｋａ之间,变化幅度也不相同;周期分析结果表明,这些古气候变化存在１．８９～４．０ｋａ之间的周期,由于时间标尺的精度还有待于进一步提高,在这里没有把冬季风强度变化与北大西洋沉积物记录的古气候事件进行对比,但是,实验结果证明在早、中更新世的某些阶段东亚冬季风强度存在快速变化的现象     

Human and climate impact on catchment development during the Holocene — Geul River, the Netherlands

Interest in the Holocene development of small to medium-sized river catchments in Western and Central Europe in relation to changes in land use and climate has increased over the past years. In this study we reconstruct the Holocene landscape development and fluvial dynamics of the Geul River (The Netherlands) and the main forcing mechanisms of environmental change. Field studies were carried out and we used OSL and ¹⁴C dating methods to reconstruct the Holocene valley development. Our study shows that 2 periods of deforestation (during the Roman Period and the High Middle Ages) led to severe soil erosion and increased floodplain sedimentation in the catchment of the Geul River, possibly combined with periods of increased wetness during the High Middle Ages. Alluvial fans have been active since the Roman deforestation phase. Our results show that the Geul catchment is highly sensitive to changes in land use.     

A medieval settlement caught in the sand: Optical dating of sand-drifting at Pulle (N Belgium)

Close to the village of Pulle (N Belgium), six trenches were opened for archaeological research. This resulted in the discovery of a dense spread of traces of human settlement. Only a relatively small number of artefacts was found, but these allowed attributing the traces to habitation during the Iron Age and Early Middle Ages. In one of the trenches, the level with the archaeological finds was covered by a drift-sand dune. The accumulation of these aeolian sediments presumably put an end to the human occupation. Hence, dating these sediments might provide essential information as to the chronology of the occupational history of the site.Quartz-based optical dating was applied to constrain the timing of sand-drifting at Pulle. All samples showed satisfactory luminescence characteristics and yielded an internally consistent set of optical ages. Within analytical uncertainty, the age results show no increase with depth and do not allow differentiating between distinct phases of sand-drifting. An average age of 1.2 ± 0.1 ka was obtained, which is interpreted as an upper age limit for human occupation at the site. The results also illustrate (the limit on) the time resolution that can be achieved through conventional quartz single-aliquot optical dating techniques; considering these limitations, we conclude that the sand-drifting at Pulle occurred during a single, fairly short event in the Early Middle Ages.     

Possible controlling factors in the development of seasonal sand wedges on the Ordos Plateau,North China

Wedge-like structures filled with silty sand penetrate Quaternary fluvial and aeolian sediments and, in places, Tertiary bedrock on the Ordos Plateau, North China. The wedges reflect thermal contraction cracking of either permafrost or seasonal frost during the Late Pleistocene and early Holocene. Wedges of about 1 m in depth form polygonal nets of 2-3 m in diameter(type B). They contrast with wedges of 3-4 m in depth that form polygons of 10-15 m in diameter(type A).This review focuses upon the highly variable size of the inferred polygon nets and discusses the problem of differentiating between seasonally and perennially frozen ground, or between seasonal frost and permafrost.