Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of ²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, ²³⁰Th excesses and ²²⁶Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of ²²⁶Ra, this inverse correlation between ²³⁰Th and ²²⁶Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, ²²⁶Ra and ²³⁰Th excesses also vary with lava composition: ²²⁶Ra excesses are negatively correlated with Na₈ and La/Yb and positively correlated with Mg#. Conversely, ²³⁰Th excesses are positively correlated with Na₈ and La/Yb and negatively correlated with Mg#. Th/U, ²³⁰Th/²³²Th, and ²³⁰Th excesses are also variable and correlated to one another. ²³¹Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na₈. The isotope ratios ¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁸Pb/²⁰⁶Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, ²³⁰Th, and ²²⁶Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.     

Riverine dunes on the Coastal Plain of Georgia, USA

Relict eolian dunes on the Georgia Coastal Plain line the north and east sides of many SE-trending streams and contain important paleoenvironmental information. A detailed analysis of four dune fields characterizes dune geomorphology, stratigraphy, sedimentology and soils. Georgia's inland dunes range from parabolic to infilled parabolic to irregular in form, typically merge to form long chains that parallel source streams and can occur as distinct bands. Typically, the dunes are composed of >95% quartz sand grains, with more than half the sand falling in the medium sand fraction. Dunes overlie flood plain and pointbar sands, backswamp and peat deposits, fluvial terraces and other dunes. In places, organic deposits occur on top of dunes. Cross-bedding is observable in dunes more than about 4 m thick and indicates eastward dune migration. Paleosols found within some dunes indicate Holocene reworking. Conditions favoring dune formation likely included at least seasonal aridity. Postdepositional modification of the dunes includes a smoothing of the dune surface, a clay increase in the C horizon, spodic horizon development, bioturbation, removal of weatherable minerals and fluvial incision.     

Reconstruction of urban stormwater contamination of an estuary using catchment history and sediment profile dating

Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history (1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling. Pollen and¹⁴C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960). Heavy metal concentrations, particle size,¹³⁷Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.¹³⁷Cs profiles did not reflect the historical fallout pattern, but deposition of¹³⁷Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater¹³⁷Cs concentrations are likely due to varying contributions from¹³⁷Cs-rich topsoil and¹³⁷Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes, and rapid mangrove colonization of formerly bare intertidal sediments.     

Siderophile element constraints on the formation of metal in the metal-rich chondrites Bencubbin, Weatherford, and Gujba

Laser ablation inductively coupled plasma mass spectrometry was used to measure abundances of P, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt, and Au in metal grains in the Bencubbin-like chondrites Bencubbin, Weatherford, and Gujba to determine the origin of large metal aggregates in bencubbinites. A strong volatility-controlled signature is observed among the metal grains. The refractory siderophiles Ru, Rh, Re, Os, Ir, and Pt are unfractionated from one another, and are present in approximately chondritic relative abundances. The less refractory elements Fe, Co, Ni, Pd, and Au are fractionated from the refractory siderophiles, with a chondritic Ni/Co ratio and a higher than chondritic Pd/Fe ratio. The moderately volatile siderophile elements Ga, Ge, As, Sn, and Sb are depleted in the metal, relative to chondritic abundances, by up to 3 orders of magnitude. The trace siderophile element data are inconsistent with the following proposed origins of Bencubbin-Weatherford-Gujba metal: (1) condensation from the canonical solar nebula, (2) oxidation of an initially chondritic metal composition, and (3) equilibration with a S-rich partial melt. A condensation model for metal-enriched (×10⁷ CI) gas is developed. Formation by condensation or evaporation in such a high-density, metal-enriched gas is consistent with the trace element measurements. The proposed model for generating such a gas is protoplanetary impact involving a metal-rich body.     

Comment on G. Wansard and F. Mezquita, The response of ostracode shell chemistry to seasonal change in a Mediterranean freshwater spring environment

In a study of the minor element chemistry of ostracode shells, Wansard and Mezquita (2001) concluded that the Mg/Ca and Sr/Ca of the ambient water controlled the respective ratios in ostracode calcite. Contrary to their conclusion that minor element chemistry is not influenced by temperature, we find a very strong statistical correlation (r = 0.97) between temperature and shell chemistry in their data. This discrepancy apparently arises due to their use of a partition coefficient model of shell chemistry that masks a number of strong correlations in their data. We argue here that the partition coefficient model is not appropriate for this biologically-mediated carbonate, and that a broader range of possible controls on shell chemistry needs to be tested.     

Attitude determination by integration of MEMS inertial sensors and GPS for autonomous agriculture applications

Integration of Global Positioning System (GPS) and Inertial Navigation System (INS) technologies, which has widespread usage in industry, is also regarded as an ideal solution for automated agriculture because it fulfils the accuracy, reliability and availability requirements of industrial and agricultural applications. Agriculture applications use position, velocity and heading information for automated vehicle guidance and control to enhance the yield and quality of the crop, and in order to vary the application of fertilizer and herbicides according to soil heterogeneity at sub-field level. A loosely coupled GPS/INS integration algorithm known as “AhrsKf” is introduced for automated agriculture vehicle guidance and control utilizing MEMS inertial sensors and GPS. The AhrsKf can produce high-frequency attitude solutions for the vehicle’s guidance and control system, by using inputs from a single survey grade L1/L2 antenna, eliminating the need for the previous two antenna solutions. Given its agricultural application, the AhrsKf has been implemented with some specific design features to improve the accuracy of the attitude solution including, temperature compensation of the inertial sensors, and the aid of plough lines of farm lands. To evaluate the AhrsKf solution, two benchmarking tests have been conducted by using a three-antenna GPS system and NovAtel’s SPAN-CPT. The results have demonstrated that the AhrsKf solution is stable and can correctly track the movement of the farming vehicle.     

An investigation into the efficiency of particle size separation using Stokes’ Law

The accuracy of gravimetric fractionation as a means of obtaining size fractions from marine sediments has been explored. Analysis of the particle size distribution and sediment properties of fractions obtained using this method was undertaken. This has highlighted the extent to which experimental artefacts rather than variations in sediment characteristics may adversely affect the efficiency of the fractionation process. Copyright © 1999 John Wiley & Sons, Ltd.