Trace element distribution in the soils above deeply weathered pegmatites, Virginia, U.S.A.: implications for exploration

Chemical partitioning data are of fundamental interest to exploration geochemists. This paper is one of the few studies which has investigated the relative proportions of the rare elements in various soil extracts. The dispersion of trace elements from weathering pegmatites in Powhatan Country, Virginia, was found to be restricted to the immediate vicinity of the pegmatites. A sequential extraction procedure was used to measure the distribution of Be, La, Nb, Sn, U, Li, and Ni,among the following fractions of the B soil horizons: exchangeable, Fe and Mn oxyhydroxide, residual, and aqua regia digestion. The elements Sn, Be, Li, and U were found to be associated with soils over the complex Herbb No. 2 pegmatite, whereas La and Ni were generally associated with the background soils.A geochemical exploration model was developed using stepwise discriminant function analysis to determine the combination of elements and soil extracts that best differentiates between complex pegmatitic, simple pegmatitic, and background soils. Log-transformed aqua regia extract concentrations of Sn, La, U, and Li were the most effective variables when used to separate complex pegmatitic from simple pegmatitic soils.     

Late Quaternary sedimentation on the Leidy Creek fan, Nevada–California: geomorphic responses to climate change

Well-dated surface and subsurface deposits in semiarid Fish Lake Valley, Nevada and California, demonstrate that alluvial-fan deposition is strongly associated with the warm dry climate of the last two interglacial intervals, and that fans were stable and (or) incised during the last glaciation. Fan deposition was probably triggered by a change from relatively moist to arid conditions causing a decrease in vegetation cover and increases in flash floods and sediment yield. We think that this scenario applies to most of the other valleys in the southern Basin and Range. Radiocarbon, tephra, and a few thermoluminescence and cosmogenic ages from outcrops throughout Fish Lake Valley and from cores on the Leidy Creek fan yield ages of >100–50 ka and 11–0 ka for the last two periods of alluvial-fan deposition. Mapping, coring and shallow seismic profiling indicate that these periods were synchronous throughout the valley and on the proximal and distal parts of the fans. From 50 to 11 ka, fan deposition ceased, a soil formed on the older alluvium and the axial drainage became active as runoff and stream competence increased. Slow deposition due to sheet flow or aeolian processes locally continued during this interval, producing cumulic soil profiles. The soil was buried by debris-flow sediment beginning at about 11 ka, coincident with the onset of relatively dry and warm conditions in the region. However, ground-water discharge maintained a large freshwater marsh on the valley floor throughout the Holocene. Pulses of deposition during the Holocene are recorded in the marsh and fan deposits; some pulses coincided with periods of or transitions to warm, dry climate indicated by proxy climate records, whereas others may reflect local disturbances associated with volcanism and fires. Within the marsh deposits, much of the clastic material is probably desert loess. In addition, the deposition of coppice dunes within the fan deposits coincides with two dry periods during the late Holocene.     

Particulate dispersion from sources within a forest

Particulate dispersion from sources within a 10- to 13-m tall pine forest was studied experimentally at Brookhaven National Laboratory using stained ragweed pollen and other tracers ranging from 14 to 58 m in size. Forty-seven continuous point source releases lasting from 22 to 55 min were made at heights from 1.75 to 14.0 m from locations having a long fetch through the forest. In most experiments, differently colored ragweed pollen were emitted simultaneously from three locations. In other tests, several particle types were released from a single point. The sampling network consisted of 119 rotoslide samplers at heights from 0.5 to 21.0 m at 57 positions within and at the edge of the forest. Deposition to the ground was sampled by greased microscope slides at each position. Meteorological measurements were taken in and near the forest.Data were classified by particle characteristics, source height and meteorological parameters. Concentration patterns were illustrated on scale diagrams of the sampling grid. Changes in centerline and crosswind integrated concentrations, plume width and height, mass flux, deposition and deposition velocity were studied as a function of distance, particle size and wind speed. Results were compared to those obtained from similar releases over open terrain.In the forest, vertical predominates over lateral dispersion and considerable interchange occurs through the canopy. Flow is channelled somewhat by vegetation density differences but is generally in the direction of the mean wind above the forest. No systematic turning of the wind with height was observed. Most particles are lost to the foliage rather than to the ground and large particles are lost more rapidly than smaller ones. Rate of change in mass flux is similar to that over open terrain and is greater with light than with stronger wind speeds.This research was carried out under the auspices of the New York State Museum and Science Service and the U.S. Atomic Energy Commission (now Energy Research and Development Administration) and was partially supported by Research Grant No. R-800677 from the Division of Meteorology, U.S. Environmental Protection Agency.     

崎岖地形动植物栖息地生态环境遥感制图与应用

传统制图方法周期长、成本较高,影响了大面积精细动植物栖息地生态环境制图的生产。鉴此,本文采用IKONOS(VHR)图像对香港郊野公园崎岖地形的动植物栖息地生态环境制图进行了研究。由于香港植被的多样性,而且观察的动植物相互作用出现于结构的层面,所以对动植物栖息地的分类应以结构为基础而非以植物为基础。本文利用图像处理技术,运用决策树之多层次地物导向分割分类(MOOSC)方法绘制九种动植物栖息地类型图。MOOSC方法和其他现有的几种分类方法相比,其分类精度高、成本低。     

New and little‐known arietellidae (Copepoda: Calanoida) mainly from the south‐west pacific

Three species of Arietellidae are described from the south‐west Pacific: a new species of Paraugaptilus, a species which can not be separated from P. buchani, and Arietellus aculeatus. The identity of A. aculeatus is clarified by comparison of type material and material from the U.S. National Museum of Natural History, and an unidentified Atlantic species of Arietellus, evident from the comparison, is described.     

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C₁–C₄, CO₂]). Molecular ratios of LMWHC (C₁/[C₂ + C₃] > 1000) and stable isotopic compositions of methane (δ¹³C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C₁/C₂₊ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C₁–C₄, CO₂]) relative to the other gas types and the virtual lack of ¹⁴C–CH₄ indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C₁–C₄, CO₂)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of ¹⁴C–CH₄ (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.     

Geochemical processes controlling arsenic mobility in groundwater: A case study of arsenic mobilization and natural attenuation

The behavior of As in the subsurface environment was examined along a transect of groundwater monitoring wells at a Superfund site, where enhanced reductive dechlorination (ERD) is being used for the remediation of groundwater contaminated with chlorinated solvents. The transect was installed parallel to the groundwater flow direction through the treatment area. The ERD technology involves the injection of organic C (OC) to stimulate in situ microbial dechlorination processes. A secondary effect of the ERD treatment at this site, however, is the mobilization of As, as well as Fe and Mn. The concentrations of these elements are low in groundwater collected upgradient of the ERD treatment area, indicating that, in the absence of the injected OC, the As that occurs naturally in the sediment is relatively immobile. Batch experiments conducted using sediments from the site inoculated with an Fe(III)- and As(V)-reducing bacterium and amended with lactate resulted in mobilization of As, Fe and Mn, suggesting that As mobilization in the field is due to microbial processes.