Arsenic sequestration by sorption processes in high-iron sediments

High-iron sediments in North Haiwee Reservoir (Olancha, CA), resulting from water treatment for removal of elevated dissolved arsenic in the Los Angeles Aqueduct system, were studied to examine arsenic partitioning between solid phases and porewaters undergoing shallow burial. To reduce arsenic in drinking water supplies, ferric chloride and a cationic polymer coagulant are added to the aqueduct upstream of Haiwee Reservoir, forming an iron-rich floc that scavenges arsenic from the water. Analysis by synchrotron X-ray absorption spectroscopy (XAS) showed that the aqueduct precipitate is an amorphous hydrous ferric oxide (HFO) similar to ferrihydrite, and that arsenic is associated with the floc as adsorbed and/or coprecipitated As(V). Arsenic-rich floc and sediments are deposited along the inlet channel as aqueduct waters enter the reservoir. Sediment core samples were collected in two consecutive years from the edge of the reservoir along the inlet channel using 30- or 90-cm push cores. Cores were analyzed for total and extractable arsenic and iron concentrations. Arsenic and iron speciation and mineralogy in sediments were examined at selected depths by synchrotron XAS and X-ray diffraction (XRD). Sediment-porewater measurements were made adjacent to the core sample sites using polyacrylamide gel probe samplers. Results showed that sediment As(V) is reduced to As(III) in all cores at or near the sediment-water interface (0-4 cm), and only As(III) was observed in deeper sediments. Analyses of EXAFS spectra indicated that arsenic is present in the sediments mostly as a bidentate-binuclear, inner-sphere sorption complex with local atomic geometries similar to those found in laboratory studies. Below about 10 cm depth, XAS indicated that the HFO floc had been reduced to a mixed Fe(II, III) solid with a local structure similar to that of synthetic green rust (GR) but with a slightly contracted average interatomic Fe-Fe distance in the hydroxide layer. There was no evidence from XRD for the formation of a crystalline GR phase. The release of dissolved iron (presumably Fe²⁺) and arsenic to solution, as monitored by in situ gel probes, was variable but, in general, occurred at greater depths than arsenic reduction in the sediments by spectroscopic observations and appears to be near or below the depth at which sediment GR was identified. These data point to reductive dissolution of the sorbent iron phase as the primary mechanism of release of sorbed arsenic to solution.     

An exercise in symbolic programming: Computation of general normalized inclination functions

Celestial Mechanics and Dynamical Astronomy - Computation of the general normalized inclination functionsA k lm(i) defined by R. H. Gooding is studies as an example of a representative problem in...     

Preservation of near-solar neon isotopic ratios in Icelandic basalts

Neon isotopic ratios measured in olivine and basaltic glass from Iceland are the most primitive observed so far in terrestrial mantle-derived samples. Ratios were measured in gas released from olivine and basaltic glass from a total of 10 samples from the Reykjanes Peninsula, Iceland, and one sample from central Iceland. The neon isotopic ratios include solar-like, mid-ocean ridge basalt (MORB)-like and atmospheric compositions. Neon isotopic ratios near the air–solar mixing line were obtained from the total gas released from glass separates from five samples. MORB-like neon isotopic compositions were measured in the total gas released from olivine and glass separates from four samples. Although there is clear evidence for a solar neon component in some of the Icelandic samples, there is no corresponding evidence for a solar helium ratio (320R_a>³He/⁴He>100R_a). Instead, ³He/⁴He ratios are mainly between 12±2(R_a) and 29±3(R_a), similar to the range observed in ocean island basalts, indicating that the He–Ne isotopic systematics are decoupled. The mantle source of Icelandic basalts is interpreted to be highly heterogeneous on a local scale to explain the range in observed helium and neon isotopic ratios. The identification of solar-like neon isotopic ratios in some Icelandic samples implies that solar neon trapped within the Earth has remained virtually unchanged over the past 4.5 Ga. Such preservation requires a source with a high [Ne_solar]/[U+Th] ratio so that the concentration of solar neon overwhelms the nucleogenic ²¹Ne* produced from the decay of U and Th in the mantle over time. High [Ne_solar]/[U+Th] ratios are unlikely to be preserved in the mantle if it has experienced substantial melting. An essentially undegassed primitive mantle component is postulated to be the host of the solar neon in the Icelandic plume source. Relatively small amounts of this primitive mantle component are likely to mix with more depleted and degassed mantle such that the primitive mantle composition is not evident in other isotopic systems (e.g. strontium and neodymium). The lower mantle plume source is inferred to be relatively heterogeneous owing to being more viscous and less well stirred than the upper mantle. This discovery of near-solar neon isotopic ratios suggests that relatively primitive mantle may be preserved in the Icelandic plume source.     

Mineralogical and textural changes accompanying ageing of silica sinter

Twenty nine samples of silica sinter, ranging in age from modern to Miocene, record temporal changes in both mineralogy and texture. When first deposited, sinters consist largely of noncrystalline spheres (<1–8 μm diameter) of opal-A exhibiting varying degrees of close-packing. Particle densities range from 1.5 to 2.1 g cm⁻³, total water 4–10 wt%, and porosities 35–60%. Changes over ∼10,000 years following deposition are slight although the spheres may be invested by an additional film of secondary silica. For the next 10,000 to ∼50,000 years, the silica incrementally crystallises to become poorly crystalline opal-CT and/or opal-C; spherical particles of thin-bladed crystals (lepispheres) replace opal-A particles and coalesce in microbotryoidal aggregates (∼10–30 μm diameter). Amygdaloidal fibrous clusters occur with lepispheres. As silica lattice ordering becomes enhanced, total water content drops to <7 wt%, particle density increases to ∼2.3 g cm⁻³, and porosity reduces to <30%. The change from opal-A to opal-C takes place over a briefer periods (∼50 years) in silica sinters that contain other materials (e.g. calcite, sulfur, alunite, plant remains). Sinters older than ∼50,000 years have recrystallised to microcrystalline quartz. With the onset of quartz crystallisation at ∼20,000 years, total water is <0.2 wt%, particle density approximates quartz (2.65 g cm⁻³), and porosity is <4%. The progressive changes in silica species and texture yield ageing profiles for sinters that may serve as guides to the paleohydrology of geothermal systems and/or epithermal ore deposits in areas where surface thermal activity has declined or ceased. Received: 18 November 1998 / Accepted: 6 July 1999     

The late Holocene paleohydrology of Pine Lake, Alberta: a multiproxy investigation

This paper reports on a high-resolution, multi-proxy, late-Holocene study from a lake in the Aspen Parkland of southern Alberta, Canada. A sediment core spanning the last 4000+ yrs from Pine Lake was analyzed for charcoal, granulometry, grain roundness, tephra content, geochemistry, mineralogy and pollen. This multi-proxy record indicates: (1) increasing anoxia causing a shift in S deposition from gypsum to pyrite due to increasing moisture availability in the late Holocene; (2) a decrease in Mg flux into the lake due to the development of the aspen forest, which reduced water flow through the Mg-rich shallow sand aquifer; the aspen forest expansion was in turn induced by the extirpation of plains bison prior to settlement; and (3) a change in the upland fire regime from frequent low-biomass grass fires to less frequent but higher biomass under-story fires, also as a result of the expansion of the aspen forest. Not only are the different proxies sensitive to different rates and magnitudes of change, they also show different sensitivities to different types of hydrological change: the mineralogy and geochemistry are sensitive to changes in water level and redox potential, and to changes in the relative strengths of the aquifers feeding the lake, while the granulometry is sensitive to total hydrological balance. Thus, apparently contradictory proxy results should be viewed as complementary.     

Evidence for transverse spread in Leonid meteors

We report here evidence for significant transverse spread of the light production region in bright Leonid meteors. One Leonid meteor has an apparent spread in the light production region of about 600 m perpendicular to the flight path for the meteor, that transverse spread persisting for at least 0.3 s. We have also detected short-duration, jet-like features emanating from a bright Leonid meteor recorded in 1998. These jet-like features have maximum spatial dimensions up to 1.9 km. While we cannot definitively rule out instrumental artefacts as a cause for these jet-like features, they may be evidence of motion contributing to the observed spatial spread in the light production region.     

A multimethod approach to inform epikarst drip discharge modelling: Implications for palaeo‐climate reconstruction

Resolving the hydrological processes that form speleothems and the palaeo‐climate archives that they contain is difficult. Typical approaches to hydrological investigation are not suited to karst landscapes, geophysics are seldom applied, drip monitoring and modelling have limitations, and ignoring potential hydrological impacts can result in a proxy record that does not reflect the external environment. We aim to understand the processes and controls that have created a palaeo‐climate proxy record preserved in a speleothem (JC001) in the “Grotto of Oddities,” part of the Jersey Cave at the Yarrangobilly Caves, Australia, to infer the likely nature and resolution of this record. Electrical resistivity tomography (ERT), traditional surveying, and drip discharge monitoring (April 2013 to February 2015) were used to investigate the structure and hydrology of the epikarst overlying the Grotto of Oddities. Data collected through these methods were then used to construct a physically informed and parsimonious drip hydrology model. Geophysics showed that changes in hillslope above the Grotto of Oddities are collocated with a region of low resistivity, which forms an epikarstic reservoir acting to supply enhanced discharge to the speleothem. Drip monitoring showed hysteretic behaviour with a distinct threshold response, and a simple drip classification indicated that the speleothem associated with the drip has the potential to record palaeo‐seasonality or an annual–decadal signal. Discharge modelling indicated discharge was comprised of quick and slow flow, and that discharge is probably perennial. These multimethod results together indicate that the speleothem likely represents a palaeo‐climate record of a length and resolution unprecedented for nonglacial areas of the Southern Hemisphere and for Australia in particular and will significantly enhance current knowledge of the climate of southeast Australia. Although ERT methods have previously been applied in the karst landscape, to our knowledge, this represents the first application of these multiple methods in combination as an approach to assess the fidelity of a speleothem, based on an understanding of the hydrological processes for palaeo‐climate reconstruction.     

From Vegetable Box to Seafood Cooler: Applying the Community-Supported Agriculture Model to Fisheries

Community-supported fisheries (CSF) projects show signs of rapid growth. Modeled on community-supported agriculture (CSA) projects, CSFs share objectives of reducing social and physical distance between consumers and producers and re-embedding food systems in social and environmental contexts. This article offers a comparison of CSF and CSA, situated in the differences between seafood and agricultural products, and fishing and farming. We draw on economic and resource theory, past research on CSA, and a member survey from a case study CSF. Survey results show CSF members are interested in accessing high-quality, fresh, local seafood, and in supporting fishing communities, and they believe that participating in a CSF achieves both. They are less certain that a CSF can address environmental concerns, and few identify environmental motives as their primary reason for participating. The latter contrasts with CSA research results, and we contextualize these findings in our broader comparison.     

Timing and source constraints on the relationship between mafic and felsic intrusions in the Emeishan large igneous province

Several I- and A-type granite, syenite plutons and spatially associated, giant Fe-Ti-V deposit-bearing mafic-ultramafic layered intrusions occur in the Pan-Xi (Panzhihua-Xichang) area within the inner zone of the Emeishan large igneous province (ELIP). These complexes are interpreted to be related to the Emeishan mantle plume. We present LA-ICP-MS and SIMS zircon U-Pb ages and Hf-Nd isotopic compositions for the gabbros, syenites and granites from these complexes. The dating shows that the age of the felsic intrusive magmatism (256.2 ± 3.0-259.8 ± 1.6 Ma) is indistinguishable from that of the mafic intrusive magmatism (255.4 ± 3.1-259.5 ± 2.7 Ma) and represents the final phase of a continuous magmatic episode that lasted no more than 10 Myr. The upper gabbros in the mafic-ultramafic intrusions are generally more isotopically enriched (lower ε_Nd and ε_Hf) than the middle and lower gabbros, suggesting that the upper gabbros have experienced a higher level of crustal contamination than the lower gabbros. The significantly positive ε_Hf(t) values of the A-type granites and syenites (+4.9 to +10.8) are higher than those of the upper gabbros of the associated mafic intrusion, which shows that they cannot be derived by fractional crystallization of these bodies. They are however identical to those of the mafic enclaves (+7.0 to +11.4) and middle and lower gabbros, implying that they are cogenetic. We suggest that they were generated by fractionation of large-volume, plume-related basaltic magmas that ponded deep in the crust. The deep-seated magma chamber erupted in two stages: the first near a density minimum in the basaltic fractionation trend and the second during the final stage of fractionation when the magma was a low density Fe-poor, Si-rich felsic magma. The basaltic magmas emplaced in the shallow-level magma chambers differentiated to form mafic-ultramafic layered intrusions accompanied by a small amount of crustal assimilation through roof melting. Evolved A-type granites (synenites and syenodiorites) were produced dominantly by crystallization in the deep crustal magma chamber. In contrast, the I-type granites have negative ε_Nd(t) [−6.3 to −7.5] and ε_Hf(t) [−1.3 to −6.7] values, with the Nd model ages () of 1.63−1.67 Ga and Hf model ages () of 1.56−1.58 Ga, suggesting that they were mainly derived from partial melting of Mesoproterozoic crust. In combination with previous studies, this study also shows that plume activity not only gave rise to reworking of ancient crust, but also significant growth of juvenile crust in the center of the ELIP.