MINI-MACtm—A new generation dual—Band surveyor

Summary Litton Divisions presently produce both high accuracy GPS surveyors and low—cost GPS navigation sets. Aero Service'sMACROMETER ^R Interferometric Surveyors, have become the standard against which GPS surveying equipment is measured. Litton Aero Products has developed a highly digitized, low costL ₁,C/A code GPS card set. The integration of these technologies had led to the development of a low-cost, high-precision, GPS survey system which can be configured with or without a codelessL ₂ capability. TheMINI-MAC surveying system is the first member of the new generation of GPS survey systems resulting from this joint development. The system design is described in this paper, and initial survey test results using a prototypeMINI-MAC surveying system are presented.     

The role of stratification in the deoxygenation of Mobile Bay and adjacent shelf bottom waters

Oxygen depletion in the shallow bottom waters of Mobile Bay, Alabama, and in adjacent nearshore and continental shelf waters, is shown to be directly related to the intensity of water column stratification. Low winds speeds are coincidental with the onset of water column stratification and the occurrence of hypoxic events. Hourly, daily, and seasonal changes in the relationship between percent oxygen saturation or oxygen concentration in the bottom waters and surface-bottom density differences indicate that the oxidized materials are recently formed, and not relic or overwintering carbon sources. The influence of density structure (water column stratification) in other oxygen-depleted coastal water masses is compared to Mobile Bay.     

Ultimate removal mechanisms of elements from the ocean—a comment

Li (1981) has proposed a surface complexation model to rationalise the partitioning of the elements between the oceans and pelagic clays. In this comment we compare the resulting scattered correlations with the coherent and statistically significant correlations obtained from a simple electrostatic model.     

An experimental investigation of volatile exsolution in evolving magma chambers

Previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas. Turbulent transfer of heat between a layer of dense, hot and volatile-rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer. Small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing, followed by further exsolution of gas and explosive eruption. These processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of CO₂. We have investigated both the initial overturning produced by gas release in the lower layer, and the subsequent evolution of gas due to intimate mixing of the two layers. The latter experiments, in which the reactants remained isolated in the two layers until overturning occurred, demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat. This shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other. The magmas can coexist in the same stratified chamber, until their bulk densities become equal and they mix together. The processes illustrated in these experiments could occur in H₂O-bearing magmas such as in the calcalkaline association and in CO₂-bearing mafic magmas such as in silica undersaturated suites.     

Primary productivity and phytoplankton size fraction dominance in a temperate North Atlantic estuary

The composition, productivity, and standing crop of net (>20 μm) and nano-(<20 μm) phytoplankton of Peconic Bay, Long Island, New York was examined from June 1978 through May 1979. Nanoplankton, primarily small solitary flagellates, chlorophytes, and diatoms, dominated from May through September accounting for 88.5% of the productivity and 88.1% of the standing crop (measured as chlorophyll a). An apparent net plankton bloom began in December and continued through March. The dominant organism through most of the winter bloom was the chain-forming diatom Skeletonema costatum (Grev.) Cl. Net plankton at this time represented 66.4% of the standing crop. For both size fractions, productivity/chlorophyll a (g C per g chl a per d, integrated through the euphotic zone) was a function of light energy over the year with the exception of a few sampling dates during the post-winter bloom period. Assimilation numbers (g C per g chl a per h at saturating light intensities) were a function of temperature between 0 and 20°C. Nitrogen deficiency did not appear to be a factor in regulating phytoplankton growth rate through the euphotic zone, as ratios of ¹⁴C assimilation for dark bottles enriched with NH₃ and with no enrichment exhibited no relationship to environmental dissolved inorganic nitrogen concentrations. Zooplankton grazing pressure appeared to have been an important factor in regulating the upper limit of phytoplankton biomass and in influencing size fraction dominance. Dominance of one phytoplankton size fraction over the other on any given date was not based on physiological differences between the two groups since both fractions were composed of the same species. Apparent net phytoplankton blooms (in terms of productivity and chlorophyll a) were artifacts of increased chain lengths of nanoplankton diatoms such as Skeletonema costatum, and to a lesser extent, Thalassiosira nordenskioldii Cl. and Detonula confervacea (Cl.) Gran, rather than to the dominance of large, solitary cells.     

Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution

The ¹⁷⁶Lu-¹⁷⁶Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes ${}^{176}\text{Hf}{}^{177}\text{Hf}$ a powerful geochemical tracer. In general, proportional variations of ${}^{176}\text{Hf}{}^{177}\text{Hf}$ exceed those of ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}$ by factors of 1.5–3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in ${}^{176}\text{Hf}{}^{177}\text{Hf}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ and ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ diagrams, ${}^{176}\text{Hf}{}^{177}\text{Hf}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ display a single, linear isotopic variation in the suboceanic mantle. These discordant ${}^{87}\text{Sr}{}^{86}\text{Sr}$ relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased ${}^{176}\text{Hf}{}^{177}\text{Hf}$, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial ${}^{176}\text{Hf}{}^{177}\text{Hf}$. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find wide application in pure chronological studies.     

Trace element zoning in pyroxenes from ODP Hole 735B gabbros: diffusive exchange or synkinematic crystal fractionation?

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.     

The effects of organic removal treatment on the integrity of δ<Superscript>18</Superscript>O measurements from biogenic silica

Prior to oxygen isotope analysis of biogenic silica by fluorination techniques, sediments must be cleaned of all organic compounds. Despite the expanding volume of research utilising oxygen isotope ratios in biogenic silica in palaeoclimatology, very little is known about the effects of different standard preparation methods for the removal of organic matter on the isotope values. Here we compare a number of methods for the removal of organic matter in order to assess their effectiveness and influence on subsequent isotope measurements. Not all the methods described here effectively removed organic matter, the best treatments were H₂O₂ and ignition in air at 550°C. Ignition in air at 950°C, and heating in vacuum at 250 and 450°C results in marked alteration of isotope values, whereas use of hydrogen peroxide (H₂O₂), nitric acid (HNO₃) and plasma ashing have little or no effect, within the bounds of analytical error. Ignition in air at 550°C was the most effective technique, and induced no alteration of δ¹⁸O_silica, but practical constraints may limit the application of this method routinely. Our findings indicate that considerable caution is required when preparing sediment for isotope analysis of biogenic silica.