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81.
Richard J. Herrington Philip M. Scotney Stephen Roberts Adrian J. Boyce Darrell Harrison 《Gondwana Research》2011,19(3):583-593
Whole-rock 87Sr/86Sr and δ18O analyses of volcanic rocks and 3He/4He analyses of sulphides and sulphates from mineralized rocks on Wetar, Indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted Australian craton to the south. These new data support the idea of progressive source contamination with precisely dated events showing that Wetar Island hosts the most extreme examples of crustal assimilation in the region. The increased continental contamination occurs during the Pliocene (Zanclian to Piacenzian) during distinct magmatic events between 5 and 4 Ma, and at 2.4 Ma when 87Sr/86Sr ratios in unaltered lavas, with whole-rock δ18O values between 5.7 and 9.6‰, increase from 0.707484 to extreme radiogenic values of 0.711656.The earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on Wetar. Samples from this yield radiogenic 3He/4He ratios between 0.5 and 1.4 R/RA, similar to the data from volcanic rocks on nearby Romang. The later magmatic event coincides with the arrival of the Australian Continental Margin at the subduction zone along the Banda arc. Progressive incorporation of continental-sourced components into the source region below the Wetar Island edifice coincides with the formation of gold-rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile. 相似文献
82.
In order to show that dolphins use estuary habitats differently depending on the season and tidal state, possibly in response to prey distribution, temperature, risk of stranding and accessibility, Indo-Pacific bottlenose dolphins (Tursiops aduncus) were observed year-round during a 3-year study in the Clarence River estuary (CR) and Richmond River estuary (RR) in northern New South Wales, Australia. Peak dolphin sightings occurred during the spring season and one or 2 h prior to high tide. The spatial distribution of the dolphins in each estuary was analysed using the distance in kilometres that the dolphins travelled upstream with seasons and tidal phase as determinants. A General Linear Model showed that in the CR the dolphin spatial distribution in the estuary was not determined by season (F = 0.434, df = 3, P = 0.729) but was by tidal phase (F = 9.943, df = 3, P < 0.001) and the interaction between season and tidal phase (F = 3.398, df = 9, P < 0.002). However, in the RR the spatial distribution of the dolphin use of the estuary was not determined by either season (F = 1.647, df = 3, P = 0.194) or tidal phase (F = 0.302, df = 3, P = 0.824). In the CR, the spatial distribution of the dolphins was largest on high and flood tides. This pattern of spatial distribution may occur because the CR is a relatively shallow estuary and this increased spatial distribution may reflect a lower stranding risk and an increase in accessibility of shallow areas during periods of higher tide. These areas could also provide access to their preferred prey items of sea mullet (Mugil cephalus) and sand whiting (Sillago ciliata). 相似文献
83.
Surface energy and water vapor fluxes observed in a desert plantation in central Iran 总被引:1,自引:0,他引:1
A. Oliphant P. Zawar-rezaG. Azizi A. DehghanpourJ. Harrison 《Journal of Arid Environments》2011,75(10):926-935
Summertime observations of surface radiation budget, energy balance and atmospheric surface layer meteorology were made on an arid valley floor planted with Haloxylon aphyllum to combat desertification in central Iran. The surface microclimate is characterized and compared with other arid regions and the role of ‘desert greening’ on surface fluxes is considered. A high surface albedo (0.265) and large longwave radiation loss produced relatively low net radiation. Energy partitioning was dominated by sensible and ground heat fluxes with opposing diurnal asymmetry governed by strong diurnal variability in eddy diffusivity. The Bowen ratio was 2.53, which fell inside the range of other vegetated arid surfaces. Surface temperature gradients were strong both in the atmospheric surface layer and in the substrate, with consistent lapse conditions by day and inversions at night. The wind regime included a moderate daytime regional wind which displayed Coriolis turning and weaker nocturnal slope flows. Actual evapotranspiration (1 mm dy−1) was only a small fraction of potential evapotranspiration. The diurnal pattern of AET indicates strong stomatal control. The desert greening effect of Haloxylon plantations provided atmospheric water and reduced sensible heat flux by up to 40%. 相似文献
84.
Suzanne?E.?HollinsEmail author Jennifer?J.?Harrison Brian?G.?Jones Atun?Zawadzki Henk?Heijnis Stuart?Hankin 《Journal of Paleolimnology》2011,46(4):579-596
In this study, we combined grain size and geochemical analyses with radioisotope analysis of lead-210 (210Pb), caesium-137 (137Cs) and radiocarbon (14C) ages to reconstruct the sedimentation history of two urbanised coastal lagoons in south-east Australia. Towradgi and Fairy
Lagoons were both found to exhibit slow initial sedimentation of less than 1 mm year−1 prior to anthropogenic influences. Land clearing in the catchments increased runoff and erosion in the creeks feeding into
the estuaries, and has resulted in progradation of fluvial material into the estuarine systems with a marked increase in sedimentation
to between 2 and 7 mm year−1. The upper 20–50 cm of the sediment column in both lagoons contained elevated concentrations of heavy metals such as Pb,
Cu, Ni and Zn. This pollution trend was found to be consistent with the history of industrialisation and urbanisation in the
region, which expanded rapidly post World War 2. The total metal concentrations were consistent with other urbanised/industrialised
estuaries around the world. Despite the fairly disturbed nature of these coastal lagoons, the use of 210Pb, 137Cs and 14C dating in combination with bulk geochemical analyses allowed detailed reconstruction of sedimentation history. 相似文献
85.
C. Rabouille D.J. Conley M.H. Dai W.-J. Cai C.T.A. Chen B. Lansard R. Green K. Yin P.J. Harrison M. Dagg B. McKee 《Continental Shelf Research》2008
We examined the occurrence of seasonal hypoxia (O2<2 mg l−1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m−2 d−1), and lower off the Rhône River (<1 g C m−2 d−1), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia. 相似文献
86.
Foggy air and clear air have appreciably different electrical conductivities. The conductivity gradient at horizontal droplet boundaries causes droplet charging, as a result of vertical current flow in the global atmospheric electrical circuit. The charging is poorly known, as both the current flow through atmospheric water droplet layers and the air conductivity are poorly characterised experimentally. Surface measurements during three days of continuous fog using new instrument techniques show that a shallow (of order 100 m deep) fog layer still permits the vertical conduction current to pass. Further, the conductivity in the fog is estimated to be approximately 20% lower than in clear air. Assuming a fog transition thickness of one metre, this implies a vertical conductivity gradient of order 10 fS m?2 at the boundary. The actual vertical conductivity gradient at a cloud boundary would probably be greater, due to the presence of larger droplets in clouds compared to fog, and cleaner, more conductive clear air aloft. 相似文献
87.
S. Harrison J. Molson H. Abercrombie J. Barker D. Rudolph R. Aravena 《Hydrogeology Journal》2000,8(6):608-622
Discovery of high contents of methane gas in coals of the Mist Mountain Formation in the Elk River valley, southeastern British
Columbia, Canada, has led to increased exploration activity for coal-seam gas (CSG). CSG production requires groundwater abstraction
to depressurize the coal beds and to facilitate methane flow to the production wells. Groundwater abstraction will have hydrodynamic
effects on the flow system, and an understanding of the groundwater flow system is needed to evaluate these effects. The purpose
of this paper is to describe the groundwater flow system in the area by means of a groundwater flow model and interpretation
of hydrochemical and isotopic analyses of groundwater and surface water.
Groundwater flow for the Weary Creek exploration area is modeled in two vertical sections. The model domains, based on classic
upland–lowland conceptual flow models, are approximately 10,000 m long and 4,000 m deep. Each consists of a fixed water-table
boundary and no-flow boundaries along the traces of major faults. Steady-state groundwater flow is calibrated to hydraulic-head,
streamflow, and groundwater-recharge data. Simulated steady-state velocity fields define regional and local flow components
consistent with the conceptual model.
The results are consistent with regional trends in δ2H, δ18O, tritium, and TDS, which define two distinct groundwater groups (A and B) and a third of intermediate composition. An active,
shallow, local flow component (group A) is recharged in beds cropping out along subdued ridges; this component discharges
as seeps along lower and mid-slope positions in the southern part of the study area. The waters are tritiated, relatively
enriched in δ2H and δ18O, and have low TDS. A deeper regional flow component (group B), which originates at a higher altitude and which discharges
to the Elk River valley bottom, is characterized by non-tritiated groundwater with relatively depleted δ2H and δ18O, and higher TDS.
Groundwater contributes less than 10% of the total direct flow to the Elk River, as indicated by flow measurements and by
the absence of group A and group B characteristics in the river water. Thus it is hypothesized that groundwater extraction
during CSG production will have little impact on the river. The groundwater flow model developed in this work is used in a
companion paper to further test this hypothesis.
Electronic Publication 相似文献
88.
Tropical paleoclimates at the Last Glacial Maximum: comparison of Paleoclimate Modeling Intercomparison Project (PMIP) simulations and paleodata 总被引:12,自引:2,他引:10
S. Pinot G. Ramstein S. P. Harrison I. C. Prentice J. Guiot M. Stute S. Joussaume 《Climate Dynamics》1999,15(11):857-874
Seventeen simulations of the Last Glacial Maximum (LGM) climate have been performed using atmospheric general circulation
models (AGCM) in the framework of the Paleoclimate Modeling Intercomparison Project (PMIP). These simulations use the boundary
conditions for CO2, insolation and ice-sheets; surface temperatures (SSTs) are either (a) prescribed using CLIMAP data set (eight models) or
(b) computed by coupling the AGCM with a slab ocean (nine models). The present-day (PD) tropical climate is correctly depicted
by all the models, except the coarser resolution models, and the simulated geographical distribution of annual mean temperature
is in good agreement with climatology. Tropical cooling at the LGM is less than at middle and high latitudes, but greatly
exceeds the PD temperature variability. The LGM simulations with prescribed SSTs underestimate the observed temperature changes
except over equatorial Africa where the models produce a temperature decrease consistent with the data. Our results confirm
previous analyses showing that CLIMAP (1981) SSTs only produce a weak terrestrial cooling. When SSTs are computed, the models
depict a cooling over the Pacific and Indian oceans in contrast with CLIMAP and most models produce cooler temperatures over
land. Moreover four of the nine simulations, produce a cooling in good agreement with terrestrial data. Two of these model
results over ocean are consistent with new SST reconstructions whereas two models simulate a homogeneous cooling. Finally,
the LGM aridity inferred for most of the tropics from the data, is globally reproduced by the models with a strong underestimation
for models using computed SSTs.
Received: 9 September 1998 / Accepted: 18 March 1999 相似文献
89.
Coastal sand dunes are considered among the most susceptible habitats to recreational use. The aim of this study was to monitor the impact of visitor use on soil and annual plants on long-established trails in the stabilised coastal dunes of the Sharon Park, Israel. The results indicate that:
- 1. The vegetation cover, height and species richness and diversity, as well as soil organic matter content were lower on trails subjected to high visitor use than that on trails under low use. However, soil compaction and moisture on high-use trails were higher than that on low use.
- 2. The rate of change in each of the vegetation properties moving outwards from the centre of the trail towards the undamaged area on its margins and beyond, was higher on trails under high visitor use than on low-use trails.
- 3. The impact of high visitor use is localised and limited to the trail boundaries and their immediate surroundings (6 m axis perpendicular to the trails), while the effect on low-use trails is dispersed over a larger area, apparently because the trail borders are less visually defined to the visitor.
90.
Harvey Karen L. Jones Harrison P. Schrijver Carolus J. Penn Matthew J. 《Solar physics》1999,190(1-2):35-44
Simultaneous measurements of the magnetic fields in the photosphere and chromosphere were used to investigate if magnetic
flux is submerging at sites between adjacent opposite polarity magnetic network elements in which the flux is observed to
decrease or `cancel'. These data were compared with chromospheric and coronal intensity images to establish the timing of
the emission structures associated with these magnetic structures as a function of height. We found that most of the cancelation
sites show either that the bipole is observed longer in the photosphere than in the chromosphere and corona (44%) or that
the timing difference of the disappearance of the bipole between these levels of the atmosphere is unresolved. The magnetic
axis lengths of the structures associated with the cancelation sites are on average slightly smaller in the chromosphere than
the photosphere. These observations suggest that magnetic flux is retracting below the surface for most, if not all, of the
cancelation sites studied. 相似文献