A Comparison of the Geochemical Features of Some Loess Sections in New Zealand and China

A comparison of the geochemical features of loesses of New Zealand and China indicates that the distributions of the elements and their variations reflect the fluctuations of climate which can be compared with the oxygen isotope stages and glacial periods.New Zialand loess is different in source from Chinese loess.Therefore ,some differences are also noticed in their chemical compositions .Loess accumulation in New Zealand is later than that in China.Because of more rainfall in New Zealand and different distributions of loess the elements in loess have suffered stronger leaching than in China.     

On the Effect of Planetary Rotation on Shear and Density Layers in Stratified Flows

The effect of the Coriolis forces on the dynamicsof shear and density layers instratified flows is investigated, an effectthat has not been taken into account inmost previous studies of turbulence-meanfield or wave-mean field interactions.For instance, recent studies have shownthat shear and density layers can grow inthe presence of turbulence in a stronglystratified fluid but the effect of planetaryrotation was not taken into account. Toaddress this problem, wave-mean flowinteraction in a stratified fluid is hereinvestigated in the presence of rotation usingdirect numerical simulation. The results showthat the wave-mean flow interactionand the formation of layers is less intensewhen rotation is present because thehorizontal mean motions are deviated by theCoriolis forces, which tends to reducethe distortion of the wave field, and thusthe wave-induced fluxes of buoyancy andmomentum. This effect appears even when therotation rate is weak.     

Mineralogy of fine-grained sediment by energy-dispersive spectrometry (EDS) image analysis – a methodology

. The application of energy-dispersive X-ray (EDS) spectrometry image analyses to geological, geochemical, and environmental studies is facilitated by recent increases in computer speed, storage capacity, and connectivity. EDS microbeam analyses provide digital element maps of mineral grains, with brightness directly proportional to element abundance. Using computer-based image analysis protocols, combinations of EDS element images can characterize grain mineralogy and quantify mineral abundance. This approach is well suited to the analyses of fine sand and silt-sized grains (0.425–0.010 mm), a size fraction not amenable to quantification by other mineralogical analysis methods. For this size range, 50 to more than 300 grains can be readily mapped, permitting mineral identification for concentrations >1% to be reliably distinguished.     

A Simulation of the Climatic Conditions Associated with the Collapse of the Maya Civilization

It has been speculated that the collapse of the Maya civilization in the Yucatan region of Mexico around 900 AD was caused by drought. A 10,000-year simulation with the CSIRO Mark 2 coupled global climatic model has been used to investigate such a possibility. The model replicates sporadic, severe drought over the Yucatan consistent with the above speculation. It was found that these droughts were specifically constrained to the Central American area, with no obvious linkages to other regions. An investigation of the mechanisms associated with rainfall fluctuations over the Yucatan indicates that these were not caused by sea surface temperature variations. Fluctuations in the intensity of the topographically constrained meridional wind systems located on both the western and eastern coasts of the Americas were found to be the dominant influence. The sensitivity of the Yucatan to drought episodes arises from its location at the convergence zone of these wind systems. It is concluded that the severe drought episodes in this region are a consequence of stochastic fluctuations of these wind systems and that external influences are not necessary.     

The effects of nutrients and zooplankton community structure on phytoplankton growth in a subtropical Australian reservoir: An enclosure study

Patterns of zooplankton–phytoplankton interactions in subtropical lakes of the Southern Hemisphere may deviate from those established for north-temperate lakes. We tested the responses of phytoplankton growth to different community structures of zooplankton and nutrient enrichment in a subtropical Australian reservoir for the prediction of potential outcomes of lake biomanipulation. Two zooplankton communities were created in lake enclosures over 4 weeks: a rotifer-dominated community developed in the presence of planktivorous fish (Hypseleotris spp.) and a Ceriodaphnia-dominated community developed in the absence of fish. Biomass gradients of both communities were established in 20 L containers and several separate containers received no additions (controls) or were enriched with nitrogen and/or phosphorus. The growth rate of total phytoplankton significantly increased in response to nutrient enrichment, indicating nutrient limitation. Most phytoplankton taxa were not markedly affected by grazing of either zooplankton community. However, both communities had significant stimulatory effects on the growth of inedible chlorophytes. The ability of zooplankton grazing to negatively affect phytoplankton growth during the summer was counteracted regardless of zooplankton community structure, possibly by nutrients regenerated by zooplankton. We hypothesise that in the subtropical system studied, changes in food web nutrient recycling may be more important for the outcome of biomanipulation than grazing impacts.     

Effect of citrate-bicarbonate-dithionite treatment on fine-grained magnetite and maghemite

Mineral magnetic properties of soils and parent materials have been interpreted in terms of paleoclimate and rates of soil formation but it is important to understand which minerals contribute to the mineral magnetic signal. Citrate-bicarbonate-dithionite (CBD) treatment has been used to determine the amounts of fine-grained, often pedogenic, ferrimagnetic minerals relative to coarse-grained, often inherited, magnetic minerals. However, questions have been raised about the effect of particle size on the efficacy of CBD in dissolving magnetite and maghemite grains. In this paper we use magnetic susceptibility and its frequency dependence, and the low-temperature behavior of a saturation isothermal remanent magnetization, to track the dissolution of carefully sized magnetite grains. We found that the standard CBD procedure dissolves fine magnetite particles (ca. < 1 μm) but leaves larger particles (ca. > 1 μm) essentially intact. Thin oxidized coatings, presumably maghemite, are also dissolved by the CBD procedure. These results support previous interpretations that the CBD procedure can be used to distinguish between pedogenic and lithogenic magnetic grains, assuming that most pedogenic magnetic grains are sufficiently small (ca. < 1 μm) and most lithogenic magnetic grains are sufficiently large (ca. > 1 μm). These results also show that the standard procedure is too harsh to differentiate between 1 μm grains of magnetite and maghemite. A modified CBD extraction that uses half as much dithionite reduces the magnetic susceptibility of 1 μm magnetite grains by only 10%. This method may be useful in distinguishing between magnetite and maghemite grains in this size range.     

A precise U---Pb zircon age for the Archaean Pongola Supergroup volcanics in Swaziland

U---Pb dating of single abraded zircon grains from a Pongola Supergroup rhyodacite sample collected in southern Swaziland yields a crystallization age of 2985±1 Ma. This date is significantly older than our previously reported U---Pb age determined on highly discordant and unabraded zircons from the same sample. The new zircon age indicates that the formation of the Pongola volcanics occurred not much later than the emplacement of the youngest granitoids thought to be broadly coeval with the Dominion Group (Robb et al., 1991), and before deposition of the Lower West Rand Group (Barton et al., 1989).     

Interannual variability of the simulated hydrology in a climatic model-implications for drought

A 10-year simulation of the atmosphere has been performed with a two-level global general circulation model. The model takes account of realistic topography and the hydrological cycle, and computes its own cloud cover, snowfall and sea ice distributions. The seasonally varying sea surface temperatures are specified from climatology. The results presented in this paper are restricted to hydrological aspects of the integration, particularly the soil moisture variations. The model exhibits considerable temporal and regional variability in its simulation of the hydrological cycle, generally reproducing quite fair agreement with observations. Some geographical regions were found to have very noticeable interannual variability, with the occurrence of annual or multi-annual drought a feature at a number of locations. The implications of such naturally occurring drought are discussed.