Distribution of forms of sulfur in peats from saline environments in the Florida Everglades

Although there is published information on the mechanism of sedimentary pyrite formation resulting from bacterial reduction of sulfate, little is known about the distribution of forms of sulfur, including organic, in peats. In order to increase understanding of the geochemistry of sulfur in coals, we have determined distributions of forms of sulfur in five cores of saline peat and one of brackish peat, all from areas near the Shark River and Whitewater Bay in the coastal swamps of south Florida. All concentrations vary widely with depth. Total sulfur concentrations range up to 6% of dry solids. Minor amounts of sulfur are present as H₂S, S⁰, SO₄²⁻, and acid-soluble sulfide, but the principal forms are usually pyritic and organic. The ratio, organic/pyritic, is highly variable, but at a majority of levels in a profile is considerably greater than unity. It is inferred that topochemical factors are important in determining the distribution of sulfur forms in any element of volume.     

土壤质量评价国外研究进展

土壤是人类赖以生存的根本资源。土壤质量涉及到动植物生产力,大气、水的质量和人类的健康,随着人们对自然资源和环境保护意识的增强,越来越多的国家和地区已将土壤质量评价工作作为实现可持续发展的一个重要方面。通过土壤质量评价,了解土壤管理和利用的现状,为土壤质量的改善及可持续利用提供理论依据和技术支撑。根据国外近10年的研究,总结了主要的评价指标体系和评价方法。评价指标主要包括土壤物理、化学和生物性质3个方面的指标。评价方法分为定性和定量评价,定量评价应用较广,包括土壤质量指数、土壤质量模型、多元线性回归、相对土壤质量法等。掌握和了解国外相关研究的动态,对于开展土地质量地球化学评估及土地的分等定级工作有着重要的意义。     

Accretion and exhumation at a Variscan active margin, recorded in the Saxothuringian flysch

The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370?Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380?Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380?Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30?mm per year, and rates of exhumation of ca. 3?mm/a, and possibly more.     

Biostratigraphy,sedimentology and sequence stratigraphy of the Tournaisian–Viséan transitional strata in South China (Guangxi)

The biostratigraphy and sedimentological evolution of the Tournaisian–Viséan (T–V) transitional strata in South China (Guangxi) have been investigated. The sediments were deposited on a carbonate platform and in slope and basinal environments. In the T–V transitional strata, six foraminiferal associations have been distinguished which allow correlation between the shallow and deep water deposits. A careful examination of the evolutionary stages of the foraminifer Eoparastaffella provides a more accurate criterion for the definition of the T–V boundary, but does not significantly modify the historical one. The distinction of two morphotypes is based on the elevation of the last whorl and the peripheral outline. Tournaisian specimens of Eoparastaffella have a well rounded periphery (morphotype 1) contrasting with the subangular periphery of younger Viséan specimens (morphotype 2). A coefficient can be deduced from simple biometric measurements for more precisely defining the T–V boundary. The sequence stratigraphy of the T–V strata in South China has been reconstructed by combining biostratigraphical and sedimentological data. It allowed the correlation of the T–V transitional strata between the platform area and the slope and basinal locations. Late Tournaisian strata were deposited during a highstand systems tract. Near the end of the Tournaisian, a major drop in relative sea-level led to the development of an unconformity in the platform area. Lowstand deposits formed during latest Tournaisian time in the basin where a detailed biostratigraphic framework has been devised. Sediments deposited during the ensuing transgressive systems tract overlie the late Tournaisian highstand sediments in the platform area and the latest Tournaisian lowstand deposits in the basin. A major drop in relative sea-level near the end of the Tournaisian has been recognized worldwide. Therefore, the possibility of using the sequence stratigraphy of the T–V strata in South China for worldwide correlations should be investigated. © 1997 John Wiley & Sons, Ltd.     

Provenance of sandstones from the Neoproterozoic Bombouaka Group of the Volta Basin,northeastern Ghana

The provenance and tectonic setting of sandstones from the Bombouaka Group of the Voltaian Supergroup, in the northeastern part of Ghana, have been constrained from their petrography and whole-rock geochemistry. Modal analysis carried out by point-counting sandstone samples indicates that they are quartz arenites. The index of compositional variability values and SiO₂/Al₂O₃, Zr/Sc, and Th/Sc values indicates that the sediments are mature. The sandstones are depleted in CaO and Na₂O. They are, however, enriched in K₂O, Ba, and Rb relative to average Neoproterozoic upper crust. These characteristics reflect intense chemical weathering in the source region as proven by high weathering indices (i.e., CIA, PIA, and CIW). In comparison with average Neoproterozoic upper crust, the sandstones show depletion by transition metals and enrichment by high field strength elements. They generally show chondrite-normalized fractionated light rare-earth element (LREE) patterns (average La_N/Sm_N = 4.40), negative Eu anomalies (average Eu/Eu* = 0.61), and generally flat heavy rare-earth elements (HREE) (average Gd_N/Yb_N = 1.13). The sandstones have La/Sc, Th/Sc, La/Co, Th/Co, Th/Cr, and Eu/Eu* ratios similar to those of sandstones derived from felsic source. Mixing calculations using the rare-earth elements (REE) suggests 48% tonalite–trondhjemite–granodiorite and 52% granite as possible proportions for the source of the sandstones. Both the petrographic and whole-rock geochemical data point to a passive margin setting for the sandstones from the Bombouaka Group.     

Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period

The Holocene climate evolution in Northern Africa is studied in a 9000-yr-long transient simulation with a coupled atmosphere–ocean–vegetation model forced by changes in insolation and atmospheric greenhouse gas concentrations. The model simulates in the monsoonal domains a significant decrease in precipitation under influence of the orbitally forced reduction in summer insolation. In the Western Sahara region, the simulated mid-Holocene transition from humid to arid conditions (the termination of the African Humid Period) is highly non-linear with the occurrence of centennial-scale climate fluctuations due to the biogeophysical feedback between precipitation and vegetation cover. This result is in agreement with proxy data from the Western Sahara region. The other monsoonal regions experience a more gradual climate evolution that linearly follows the insolation forcing, which appears in disagreement with available lake level records.     

A model of lunar evolution

There have been many models describing the evolution of our sister planet. As information from the intensive exploration by the Apollo program has accumulated, more constraints on these models have emerged. We specifically consider a hypothesis in which there is a present day asthenosphere, a heat flow between 24 and 32 ergs cm⁻² s⁻¹ and a crust which developed early in the Moon's history by melting of the outer 100 to 200 km. We have also introduced a constraint which keeps the deep interior below the Curie point of iron for the first 1 to 1.5 b.y. so that it is able to carry the memory of an early field which magnetized the cold interior. The magnetized mare basalts and breccias cooled in this field from above the Curie point of iron (≈800°C.) and acquired a thermoremanent magnetization. While fully recognizing that some of these constraints are subject to other interpretations, it is nevertheless instructive to consider the thermal history that follows from such a model. First, the initial temperature must be high enough to cause melting in the outer 100–200 km, while the interior temperature must be cool enough to be below the Curie point of iron. Second, the crust in this model cools off so rapidly that the mare basalts could not be developed as late as indicated in lunar history. Rather we propose that the mare basalts result from local remelting associated with giant impacts. Third, the Moon's deep interior must have warmed up enough to erase the memory of the ancient magnetic field from the deep interior and to develop the asthenosphere which has been detected seismically. Fourth, if this asthenosphere is real, the viscosity of the Moon as a function of temperature must be high enough to have prevented convective cooling until the temperature increased to a value near the solidus temperature. At this temperature, the Moon would then likely cool by convection in the solid state. It is, therefore, a consequence of this model that solid body convection tool place late in lunar history. This may well have contributed to the lunar center of figure and center of mass offset, to the low order terms in its gravity field and to, its disequilibrium moment of inertia differences.     

An experimental investigation of LNAPL migration in an unsaturated/saturated sand

Accidental spills of hydrocarbons, such as Light Non-Aqueous Phase Liquids (LNAPLs), are one of the most common sources of subsurface contamination. Migration of LNAPL in a porous medium is influenced by various factors such as the number of fluids present in the unsaturated/saturated zones and the proportion of pores occupied by each fluid. The results for relationship between matric suction and degree of saturation are presented in this paper for water–air, water–LNAPL and LNAPL–air systems in a sand. A simple and reliable setup using Buchner funnel was designed to obtain these relations. It was found that the relationship between matric suction head and degree of saturation is hysteretic for all the fluid systems (water–air, water–LNAPL and LNAPL–air). Furthermore, the amount of hysteresis depended upon the fluid system, with the maximum hysteresis occurring for water–air system. The results suggest that the amount of trapped air depends upon the reversal degree of saturation from drying to wetting.