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961.
Reconstruction of contaminant trends in a salt wedge estuary with sediment cores dated using a multiple proxy approach 总被引:2,自引:0,他引:2
The Taunton River is a partially mixed tidal estuary in southeastern Massachusetts (USA) which has received significant contaminant inputs, yet little information exists on the history of discharge and the subsequent fate of these contaminants. Three sediment cores taken along a transect were analyzed, reconstructing the spatial and temporal trends of pollution in the estuary. A combination of radiometric dating, contaminant markers, and storm layers from major hurricanes were used to establish age models and sedimentation rates. Age estimates obtained from the different dating methods compared well, establishing an accurate history of contaminant release to the estuary. Polycyclic aromatic hydrocarbons (PAHs) were present in one core at depths corresponding to the early 1860s, earlier than previously established dates of introduction. Temporal and spatial trends of Cr, Cu, Hg and Pb indicated multiple sources of varying input to the river. Polychlorinated biphenyls (PCBs) were present in each of the cores from the 1930s onward, with elevated levels still present in surficial sediments at several sites. A unique organic compound, Topanol, which was produced locally was used as a tracer to track contaminant transport in the river. Tracer data indicates that contaminants are still being transported and deposited to surficial sediments at high concentrations well after their discharge. This reconstruction demonstrates the utility of using multiple dating proxies where often the sole use of radiometric dating techniques is not an option and provides insights into the fate of contaminants discharged decades ago but continue to represent environmental risks. 相似文献
962.
Precipitation of heavy metals in produced water: influence on contaminant transport and toxicity 总被引:1,自引:0,他引:1
Azetsu-Scott K Yeats P Wohlgeschaffen G Dalziel J Niven S Lee K 《Marine environmental research》2007,63(2):146-167
Produced water undergoes changes in its physical chemistry including precipitation of heavy metals after being discharged and mixed with ambient seawater. Potential impacts of the precipitation of heavy metals on their transport and toxicity were studied using samples from offshore oil production sites on the Scotian Shelf off eastern Canada. Concentrations of aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel and zinc were measured in total, particulate and dissolved fractions together with Microtox tests for assessment of toxicity. Heavy metals in produced water were transformed from dissolved to particulate phase in a period of hours under oxygenated conditions, and aggregated to larger particles that settle rapidly (>100 m/day) over a few days. In addition, there was production of buoyant particles comprised of heavy metal precipitates sequestered onto oil droplets that were transported to the surface. The particulate fraction was generally more toxic than the dissolved fraction. This was evident at the mixing interface between produced water and seawater where elevated particulate and toxicity levels were observed. Laboratory studies suggest an increase in the toxicity of discharged produced water over time. Time-series experiments showed a sustained toxic response for more than a week following the oxidation of freshly discharged produced water that initially elicited little or no toxic response in the Microtox test. Chemical processes identified in this study, namely precipitation of heavy metals and consequent settling and rising fluxes of particles, will influence the toxicity, the fate and the transport of potential contaminants in the produced water. Therefore, these processes need to be considered in assessment of the environmental impact associated with offshore oil and gas operations. 相似文献
963.
Anthony D. Del Genio John M. Barbara Andrew P. Ingersoll Ashwin R. Vasavada Carolyn C. Porco 《Icarus》2007,189(2):479-492
We apply an automated cloud feature tracking algorithm to estimate eddy momentum fluxes in Saturn's southern hemisphere from Cassini Imaging Science Subsystem near-infrared continuum image sequences. Voyager Saturn manually tracked images had suggested no conversion of eddy to mean flow kinetic energy, but this was based on a small sample of <1000 wind vectors. The automated procedure we use for the Cassini data produces an order of magnitude more usable wind vectors with relatively unbiased sampling. Automated tracking is successful in and around the westward jet latitudes on Saturn but not in the vicinity of most eastward jets, where the linearity and non-discrete nature of cloud features produces ambiguous results. For the regions we are able to track, we find peak eddy fluxes and a clear positive correlation between eddy momentum fluxes and meridional shear of the mean zonal wind, implying that eddies supply momentum to eastward jets and remove momentum from westward jets at a rate . The behavior we observe is similar to that seen on Jupiter, though with smaller eddy-mean kinetic energy conversion rates per unit mass of atmosphere (). We also use the appearance and rapid evolution of small bright features at continuum wavelengths, in combination with evidence from weak methane band images where possible, to diagnose the occurrence of moist convective storms on Saturn. Areal expansion rates imply updraft speeds of over the convective anvil cloud area. As on Jupiter, convection preferentially occurs in cyclonic shear regions on Saturn, but unlike Jupiter, convection is also observed in eastward jet regions. With one possible exception, the large eddy fluxes seen in the cyclonic shear latitudes do not seem to be associated with convective events. 相似文献
964.
Thomas Pape André Bahr Janet Rethemeyer John D. Kessler Heiko Sahling Kai-Uwe Hinrichs Stephan A. Klapp William S. Reeburgh Gerhard Bohrmann 《Chemical Geology》2010,269(3-4):350-363
Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C1–C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (δ13C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C1–C4, CO2]) relative to the other gas types and the virtual lack of 14C–CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1–C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C–CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area. 相似文献
965.
Solubility of manganotantalite, zircon and hafnon in highly fluxed peralkaline to peraluminous pegmatitic melts 总被引:2,自引:0,他引:2
Marieke Van Lichtervelde Francois Holtz John M. Hanchar 《Contributions to Mineralogy and Petrology》2010,160(1):17-32
The behavior of tantalum and zirconium in pegmatitic systems has been investigated through the determination of Ta and Zr
solubilities at manganotantalite and zircon saturation from dissolution and crystallization experiments in hydrous, Li-, F-,
P- and B-bearing pegmatitic melts. The pegmatitic melts are synthetic and enriched in flux elements: 0.7–1.3 wt% Li2O, 2–5.5 wt% F, 2.8–4 wt% P2O5 and 0–2.8 wt% B2O3, and their aluminum saturation index ranges from peralkaline to peraluminous (ASILi = Al/[Na + K + Li] = 0.8 to 1.3) with various K/Na ratios. Dissolution and crystallization experiments were conducted at
temperatures varying between 700 and 1,150°C, at 200 MPa and nearly water-saturated conditions. For dissolution experiments,
pure synthetic, end member manganotantalite and zircon were used in order to avoid problems with slow solid-state kinetics,
but additional experiments using natural manganotantalite and zircon of relatively pure composition (i.e., close to end member
composition) displayed similar solubility results. Zircon and manganotantalite solubilities considerably increase from peraluminous
to peralkaline compositions, and are more sensitive to changes in temperature or ASI of the melt than to flux content. A model
relating the enthalpy of dissolution of manganotantalite to the ASILi of the melt is proposed: ∆H
diss (kJ/mol) = 304 × ASILi − 176 in the peralkaline field, and ∆H
diss (kJ/mol) = −111 × ASILi + 245 in the peraluminous field. The solubility data reveal a small but detectable competitivity between Zr and Ta in the
melt, i.e., lower amounts of Zr are incorporated in a Ta-bearing melt compared to a Ta-free melt under the same conditions.
A similar behavior is observed for Hf and Ta. The competitivity between Zr (or Hf) and Ta increases from peraluminous to peralkaline
compositions, and suggests that Ta is preferentially bonded to non-bridging oxygens (NBOs) with Al as first-neighbors, whereas
Zr is preferentially bonded to NBOs formed by excess alkalies. As a consequence Zr/Ta ratios, when buffered by zircon and
manganotantalite simultaneously, are higher in peralkaline melts than in peraluminous melts. 相似文献
966.
Ben Buse John C. Schumacher R. Stephen J. Sparks Matthew Field 《Contributions to Mineralogy and Petrology》2010,160(4):533-550
Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa,
Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase
assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet
and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F−, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite
and hydrogarnet are estimated to form at temperatures of ca. 350–250°C, which are similar to those for serpentinisation. Alteration
within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency
of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation
and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites
from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages
within kimberlite deposits. 相似文献
967.
Marlon M. Jean John W. Shervais Sung-Hi Choi Samuel B. Mukasa 《Contributions to Mineralogy and Petrology》2010,159(1):113-136
The middle Jurassic Coast Range Ophiolite (CRO) is one of the most important tectonic elements in western California, cropping
out as tectonically dismembered elements that extend 700 km from south to north. The volcanic and plutonic sections are commonly
interpreted to represent a supra-subduction zone (SSZ) ophiolite, but models specifying a mid-ocean ridge origin have also
been proposed. These contrasting interpretations have distinctly different implications for the tectonic evolution of the
western Cordillera in the Jurassic. If an SSZ origin is confirmed, we can use the underlying mantle peridotites to elucidate
melt processes in the mantle wedge above the subduction zone. This study uses laser ablation–inductively coupled plasma–mass
spectrometry (LA–ICP–MS) to study pyroxenes in peridotites from four mantle sections in the CRO. Trace element signatures
of these pyroxenes record magmatic processes characteristic of both mid-ocean ridge and supra-subduction zone settings. Group
A clinopyroxene display enriched REE concentrations [e.g., Gd (0.938–1.663 ppm), Dy (1.79–3.24 ppm), Yb (1.216–2.047 ppm),
and Lu (0.168–0.290 ppm)], compared to Group B and C clinopyroxenes [e.g., Gd (0.048–0.055 ppm), Dy (0.114–0.225 ppm), Yb
(0.128–0.340 ppm), and Lu (0.022–0.05 ppm)]. These patterns are also evident in orthopyroxene. The differences between these
geochemical signatures could be a result of a heterogeneous upper mantle or different degrees of partial melting of the upper
mantle. It will be shown that CRO peridotites were generated through fractional melting. The shapes of REE patterns are consistent
with variable degrees of melting initiated within the garnet stability field. Models call for 3% dry partial melting of MORB-source
asthenosphere in the garnet lherzolite field for abyssal peridotites and 15–20% further partial melting in the spinel lherzolite
field, possibly by hydrous melting for SSZ peridotites. These geochemical variations and occurrence of both styles of melting
regimes within close spatial and temporal association suggest that certain segments of the CRO may represent oceanic lithosphere,
attached to a large-offset transform fault and that east-dipping, proto-Franciscan subduction may have been initiated along
this transform. 相似文献
968.
Near liquidus experiments on peridotite and other olivine normative compositions from 1.7 to 6 GPa confirm the applicability of exchange-based empirical models of Ni and Co partitioning between olivine and silicate liquids with compositions close to the liquidus of peridotite. Given that most estimates of lunar bulk composition are peridotitic, the partitioning models thus lend themselves to calculation of olivine compositions produced during the early stages of magma ocean crystallization. Calculation of olivine compositions produced by fractional crystallization of a model lunar magma ocean, initially 700 km deep, reveals a prominent maximum in Ni concentration versus fraction crystallized or Mg’ (molar MgO/(MgO + FeO)), but a pattern of monotonically increasing Co concentration. These patterns qualitatively match the puzzling patterns of Ni and Co concentrations observed in lunar rocks in which forsteritic olivines in magnesian suite cumulates have lower Ni and Co abundances than do less magnesian olivines from low-Ti mare basalts, and olivines from the ferroan anorthosite suite (FAS) have lower Ni, but similar Co to mare basalt olivines.The Ni and Co abundances in olivines from the magnesian suite cumulates can be reconciled in terms of fractional crystallization of a deep magma ocean which initially produces a basal dunite comprised of the hottest and most magnesian olivine overlain by an olivine-orthopyroxene (harzburgite) layer that is in turn overlain by an upper zone of plagioclase-bearing cumulates. The ultramafic portion of the cumulate pile overturns sending the denser harzburgite layer, which later becomes a portion of the green glass source region, to the bottom of the cumulate pile with Ni- and Co-rich olivine. Meanwhile, the less dense, but hottest, most magnesian olivines with much lower Ni and Co abundances are transported upward to the base of the plagioclase-bearing cumulates where subsequent heat transfer leads to melting of mixtures of primary dunite, norite, and gabbronorite with KREEP (a K-REE-P enriched component widely believed to be derived from the very latest stage magma ocean liquid). These hybrid melts have Al2O3, Ni, and Co abundances and Mg’ appropriate for parent magmas of the magnesian suite. Ni and Co abundances in the FAS are consistent with either direct crystallization from the magma ocean or crystallization of melts of primary dunite-norite mixtures without KREEP. 相似文献
969.
Transition metal stable isotope signatures can be useful for tracing both natural and anthropogenic signals in the environment, but only if the mechanisms responsible for fractionation are understood. To investigate isotope fractionations due to electrochemistry (or redox processes), we examine the stable isotope behavior of iron and zinc during the reduction reaction + 2e− = Mmetal as a function of electrochemical driving force, temperature, and time. In all cases light isotopes are preferentially electroplated, following a mass-dependent law. Generally, the extent of fractionation is larger for higher temperatures and lower driving forces, and is roughly insensitive to amount of charge delivered. The maximum fractionations are δ56/54Fe = −4.0‰ and δ66/64Zn = −5.5‰, larger than observed fractionations in the natural environment and larger than those predicted due to changes in speciation. All the observed fractionation trends are interpreted in terms of three distinct processes that occur during an electrochemical reaction: mass transport to the electrode, chemical speciation changes adjacent to the electrode, and electron transfer at the electrode. We show that a large isotope effect adjacent the electrode surface arises from the charge-transfer kinetics, but this effect is attenuated in cases where diffusion of ions to the electrode surface becomes the rate-limiting step. Thus while a general increase in fractionation is observed with increasing temperature, this appears to be a result of thermally enhanced mass transport to the reacting interface rather than an isotope effect associated with the charge-transfer kinetics. This study demonstrates that laboratory experiments can successfully distinguish isotopic signatures arising from mass transport, chemical speciation, and electron transfer. Understanding how these processes fractionate metal isotopes under laboratory conditions is the first step towards discovering what role these processes play in fractionating metal isotopes in natural systems. 相似文献
970.
John Overton 《Geoforum》2010,41(5):752-762
Geographers have developed a keen interest in the social production of space - in the way meanings and values are ascribed to places as a result of changing social, cultural and political processes. There is a need to explore this approach further, seeing how social and economic values of places are inter-related and how these values are constructed in often deliberate and concerted ways. There is also a need to explore how such values are consumed: how the values in place are traded, appropriated and redistributed, both through the products of places and in land markets. This paper examines the New Zealand wine industry where certain wine regions have been identified and developed in ways which attempt to emphasise their distinctiveness in terms of wine quality and thus enhance the value of the wine produced. Different strategies have been employed in this process of place construction and this reflects the differential role of capital, striving on the one hand to increase the price and marketability of the products of distinctive places but, on the other, careful not to over-inflate land values and thus restrict further expansion. The paper suggests that the issue of consumption of space, involving a complex relationship amongst land, capital and place, is worthy of further exploration. 相似文献