Effect of deposit feeders on migration of137Cs in lake sediments

Illite clay particles with adsorbed¹³⁷Cs were added as a submillimeter layer to the surface of silt-clay sediments contained in rectangular Plexiglas cells stored in a temperature-regulated aquarium, in order to trace the effect of the oligochaete, Tubifex tubifex, and the amphipod, Pontoporeia hoyi, on mass redistribution near the sediment-water interface. A well-collimated NaI gamma detector scanned each sediment column (～10 cm deep) at daily or weekly intervals for six months, depicting the time evolution of radioactivity with and without added benthos. In a cell with tubificids (～5 × 10⁴ m^?2), which feed below 3 cm and defecate on surface sediments, the labeled layer was buried at a rate of 0.052 ± 0.007 cm/day (20°C). When labeled particles entered the feeding zone,¹³⁷Cs reappeared in surface sediments creating a bimodal activity profile. In time, the activity tended toward a uniform distribution over the upper 6 cm, decreasing exponentially below to undetectable levels by 9 cm. In a cell with amphipods (～1.6 × 10⁴ m^?2) uniform activity developed rapidly (～17 days) down to a well-defined depth (1.5 cm). The mixing of sediments by Pontoporeia is described by a simple quantitative model of eddy diffusive mixing of sediment solids. The value of the diffusion coefficient, 4.4 cm²/yr (7°C) was computed from a least squares fit of theoretical to observed profile broadening over time. In a cell without benthos, small but measurable migration of¹³⁷Cs indicated an effective molecular diffusion coefficient of 0.02 cm²/yr.     

Geochemical investigations associated with the Wilga and Currawong massive sulphide deposits, Benambra, Victoria

The Wilga and Currawong copper-zinc massive sulphide deposits in northeastern Victoria occur within a sequence of Silurian volcanics and sediments. The Wilga deposit which was discovered in mid 1978 consists of a single lens while the Currawong deposit, discovered in early 1979, consists of at least two lenses.The first indication of the presence of base metal mineralization in the area was provided by an assessment of stream sediment geochemical data contained in open-file Exploration Licence reports at the Victorian Department of Minerals and Energy.The massive sulphide mineralization does not outcrop, but the ore horizons are weakly mineralized and give rise to stringer gossans as far as 150 m up dip from ore grade mineralization. These can be identified by their trace element chemistry (anomalous values of Bi, Fe, As, Au, Pb, Hg, Se, Co, Ag and Mn) corresponding to the trace element signature of both stringer and massive sulphides.Soils in the area are essentially skeletal and residual with some colluvial movement on the steeper slopes. The soils are highly anomalous in Cu, Pb, and Zn over the projected horizon of the Wilga mineralization and the No. 2 lens at Currawong.The stream sediment responses at both Wilga and Currawong result from a combination of chemical and elastic dispersion. Downslope from the surface expression of the Wilga mineralization a spring discharges directly into the Tambo River. The spring has a very low pH and is rich in base metals resulting in enhanced metal values in both stream water and stream sediments.Analyses of selected samples of the more prominent vegetation species have failed to show a clear relationship to the mineralization.     

High resolution scans of Comet Kohoutek in the vicinity of 5015, 5890, and 6563 Å

High resolution scans were made of Comet Kohoutek (1973f) using the McMath solar telescope at Kitt Peak National Observatory. The data were taken on January 1 and 4, 1974 UT, just after the comet perihelion. Hα emission (～4.1 × 10²⁷ photon sec^?1) was observed from the head of the Comet. An upper limit on the He I(5015) radiation was determined to be less than 2% of the observed Hα emission. The Na D1/D2 line intensities on both nights were approximately 0.5, indicating an optically thin emission region.     

Gas/solid carbon branching ratios in surface‐mediated reactions and the incorporation of carbonaceous material into planetesimals

We report the ratio of the initial carbon available as CO that forms gas‐phase compounds compared to the fraction that deposits as a carbonaceous solid (the gas/solid branching ratio) as a function of time and temperature for iron, magnetite, and amorphous iron silicate smoke catalysts during surface‐mediated reactions in an excess of hydrogen and in the presence of N₂. This fraction varies from more than 99% for an amorphous iron silicate smoke at 673 K to less than 40% for a magnetite catalyst at 873 K. The CO not converted into solids primarily forms methane, ethane, water, and CO₂, as well as a very wide range of organic molecules at very low concentration. Carbon deposits do not form continuous coatings on the catalytic surfaces, but instead form extremely high surface area per unit volume “filamentous” structures. While these structures will likely form more slowly but over much longer times in protostellar nebulae than in our experiments due to the much lower partial pressure of CO, such fluffy coatings on the surfaces of chondrules or calcium aluminum inclusions could promote grain–grain sticking during low‐velocity collisions.     

Reef Reservoir Identification by Wavelet Decomposition and Reconstruction: A Case Study from Yuanba Gas Field in China

The organic reef is a special type of carbonate reservoir which always dominates the spatial distribution, reserves and accumulations of natural gas. However, it is difficult to determine the organic reef’s internal structure and gas reservoirs due to numerous adverse factors such as the low resolution of seismic data, depth of burial, strong anisotropy, irregular spatial distribution and complex internal structure. A case study of wavelet decomposition and reconstruction technology applied to elucidate the features of organic reef reservoirs in the Changxing formation from Yuanba gas field shows that the seismic record reconstructed by high frequency signal can adequately describe the internal properties of organic reef reservoirs. Furthermore, the root mean square amplitude ratio of both low and high frequency data obtained from the reconstructed seismic data clearly show spatial distribution of gas and water in reef reservoirs.     

Picophytoplankton: A major contributor to planktonic biomass and primary production in a eutrophic,river-dominated estuary

Biomass and primary productivity of picophytoplankton (PP; phytoplankton <3 μm) and larger phytoplankton (>3 μm) were determined during an annual cycle along the salinity gradient in North Carolina’s Neuse River Estuary (NRE), a eutrophic, microtidal estuary. The PP were a major component of total phytoplankton biomass and productivity, contributing ∼35–44% of the total chlorophyll a (Chl a) and 42–55% of the total primary productivity. Chl a and productivity of PP decreased from the upper to lower estuary, although the PP contribution relative to larger phytoplankton remained nearly constant. Significant PP growth occurred in the spring, but PP productivity and biomass were maximal in summer. PP productivity and biomass were positively correlated with temperature and dissolved inorganic phosphorus concentrations, which were maximal in summer due to release from sediments. Biomass and productivity of PP and >3 μm phytoplankton were also positively correlated, suggesting that growth conditions favoring the onset of blooms of larger phytoplankton species will similarly affect PP. High PP productivity and biomass in the NRE support the notion that PP play an important role in the production and eutrophication potentials of this estuary. High PP productivity and biomass have been noted in several other temperate estuaries, all sharing a common feature with the NRE—long residence time. These findings challenge the assumption that PP relative importance should be minimal in eutrophic systems.     

Using atmospheric tracers to reduce uncertainty in groundwater recharge areas

A Monte Carlo-based approach to assess uncertainty in recharge areas shows that incorporation of atmospheric tracer observations (in this case, tritium concentration) and prior information on model parameters leads to more precise predictions of recharge areas. Variance-covariance matrices, from model calibration and calculation of sensitivities, were used to generate parameter sets that account for parameter correlation and uncertainty. Constraining parameter sets to those that met acceptance criteria, which included a standard error criterion, did not appear to bias model results. Although the addition of atmospheric tracer observations and prior information produced similar changes in the extent of predicted recharge areas, prior information had the effect of increasing probabilities within the recharge area to a greater extent than atmospheric tracer observations. Uncertainty in the recharge area propagates into predictions that directly affect water quality, such as land cover in the recharge area associated with a well and the residence time associated with the well. Assessments of well vulnerability that depend on these factors should include an assessment of model parameter uncertainty. A formal simulation of parameter uncertainty can be used to delineate probabilistic recharge areas, and the results can be expressed in ways that can be useful to water-resource managers. Although no one model is the correct model, the results of multiple models can be evaluated in terms of the decision being made and the probability of a given outcome from each model.     

Quantification of the 1998–1999 explosion sequence at Popocatépetl volcano,Mexico

The sequence of large Vulcanian explosions occurring at the andesitic Popocatépetl volcano, Mexico during November 1998 to April 1999 was studied. The size of 26 largest explosions was estimated from broadband seismic records at the distance of 4 km from the crater. The sequence began with the largest explosion (E = 2.6 × 10¹² J) occurring on 25 November at 08:05, and following largest daily explosions were characterized by gradual decrease in the energy. The energy of 20 large (E ≥ 10¹¹ J) explosions was distributed as Student's t-distribution with a geometrical mean Log E = 11.81 (J).