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71.
T. Baussant R.K. Bechmann I.C. Taban B.K. Larsen A.H. Tandberg A. Bjørnstad S. Torgrimsen A. Nævdal K.B. Øysæd G. Jonsson S. Sanni 《Marine pollution bulletin》2009,58(12):1796-1807
Mytilus edulis and Chlamys islandica were exposed to nominal dispersed crude oil concentrations in the range 0.015–0.25 mg/l for one month. Five biomarkers (enzymatic and cellular responses) were analysed together with bioaccumulation of PAHs at the end of exposure. In both species, PAH tissue residues reflected the exposure concentration measured in the water and lipophylicity determined the bioaccumulation levels. Oil caused biomarker responses in both species but more significant alterations in exposed C. islandica were observed. The relationships between exposure levels and enzymatic responses were apparently complex. The integrated biomarker response related against the exposure levels was U-shaped in both species and no correlation with total PAH body burden was found. For the monitoring of chronic offshore discharges, dose- and time-related events should be evaluated in the selection of biomarkers to apply. From this study, cellular damages appear more fitted than enzymatic responses, transient and more complex to interpret. 相似文献
72.
Kirti Sharma Stephen Self Stephen Blake Thorvaldur Thordarson Gudrun Larsen 《Journal of Volcanology and Geothermal Research》2008
The explosive rhyolitic eruption of Öræfajökull volcano, Iceland, in AD 1362 is described and interpreted based on the sequence of pyroclastic fall and flow deposits at 10 proximal locations around the south side of the volcano. Öræfajökull is an ice-clad stratovolcano in south central Iceland which has an ice-filled caldera (4–5 km diameter) of uncertain origin. The main phase of the eruption took place over a few days in June and proceeded in three main phases that produced widely dispersed fallout deposits and a pyroclastic flow deposit. An initial phase of phreatomagmatic eruptive activity produced a volumetrically minor, coarse ash fall deposit (unit A) with a bi-lobate dispersal. This was followed by a second phreatomagmatic, possibly phreatoplinian, phase that deposited more fine ash beds (unit B), dispersed to the SSE. Phases A and B were followed by an intense, climactic Plinian phase that lasted ∼ 8–12 h and produced unit C, a coarse-lapilli, pumice-clast-dominated fall deposit in the proximal region. At the end of Plinian activity, pyroclastic flows formed a poorly-sorted deposit, unit D, presently of very limited thickness and exposed distribution. Much of Eastern Iceland is covered with a very fine distal ash layer, dispersed to the NE. This was probably deposited from an umbrella cloud and is the distal representation of the Plinian fallout. A total bulk fall deposit volume of ∼ 2.3 km3 is calculated (∼ 1.2 km3 DRE). Pyroclastic flow deposit volumes have been crudely estimated to be < 0.1 km3. Maximum clast size data interpreted by 1-D models suggests an eruption column ∼ 30 km high and mass discharge rates of ∼ 108 kg s− 1. Ash fall may have taken place from heights around 15 km, above the local tropopause (∼ 10 km), with coarser clasts dispersed below that under a different wind regime. Analyses of glass inclusions and matrix glasses suggest that the syn-eruptive SO2 release was only ∼ 1 Mt. This result is supported by published Greenland ice-core acidity peak data that also suggest very minor sulphate deposition and thus SO2 release. The small sulphur release reflects the low sulphur solubility in the 1362 rhyolitic melt. The low tropopause over Iceland and the 30-km-high eruption column certainly led to stratospheric injection of gas and ash but little sulphate aerosol was generated. Moreover, pre-eruptive and degassed halogen concentrations (Cl, F) indicate that these volatiles were not efficiently released during the eruption. Besides the local pyroclastic flow (and related lahar) hazard, the impact of the Öræfajökull 1362 eruption was perhaps restricted to widespread ash fall across Eastern Iceland and parts of northern Europe. 相似文献
73.
Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers 总被引:1,自引:0,他引:1
Bergrún Arna Óladóttir Olgeir Sigmarsson Gudrun Larsen Thor Thordarson 《Bulletin of Volcanology》2008,70(4):475-493
The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the
nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the
long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy
from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The
section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were
analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR)
derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year
record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K2O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations
in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution
of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic
component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions
of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on
the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous.
The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the
plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might
be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene,
each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing
system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted
by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber,
represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within
the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during
magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing
system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and
relatively low eruption frequency. 相似文献
74.
The present contribution gives an overview of current knowledge of a comprehensive and steadily growing research field. The first section deals with water pumping and particle retention mechanisms in ciliary and muscular filter feeders. The second section examines the biological filter pumps in order to assess adaptation to the environment. Filter-feeding benthic invertebrates have evolved filter pumps to solve common basic problems. This has led to a large degree of similarity between otherwise distant standing species, which makes comparative studies interesting and important. The present review of zoobenthic filter feeding aims at accentuating such recognition. 相似文献
75.
Toomre J. Christensen-Dalsgaard J. Howe R. Larsen R.M. Schou J. Thompson M.J. 《Solar physics》2000,192(1-2):437-448
The variation of rotation in the convection zone over a period of two years from mid-1996 is studied using inversions of SOI–MDI data. We confirm the existence of near-surface banded zonal flows migrating towards the equator from higher latitudes, and reveal that these banded flows extend substantially beneath the surface, possibly to depths as great as 70 Mm (10% of the solar radius). Our results also reveal apparently significant temporal variations in the rotation rate at high latitudes and in the vicinity of the tachocline over the period of study. 相似文献
76.
Surface gravity waves have been used to probe flows in the two megameters beneath the photosphere using the techniques of
timedistance helioseismology. The results suggest that supergranule velocities are smaller than at the surface. The outward
flow outside a sunspot penumbra (the moat) is observed, as is an inward flow in the region beyond the moat. 相似文献
77.
The Plio-Pleistocene wedge off mid Norway is characterized by a pronounced increase in sonic velocities versus depth. At its base, a velocity inversion of 0.55 km s−1 exists with respect to the underlying unit. We propose that the inversion may be a result of the stage margin evolution, in which both the pre-depositional physiography of the margin and the depositional history of the wedge itself have been key elements. The glaciation of the uplifted mainland and the cyclic advancement of glaciers over the shelf led to the formation of the Plio-Pleistocene prograding megasequence. Underlying sequences may have been overpressured due to the rapid deposition of the wedge and because shales at the base of the wedge, deposited in a distal deltaic environment, represented a seal for water drainage. Overpressure provides sediment instability, and is an attractive mechanism to explain intra-sequence deformation and partly lateral mass movements. 相似文献
78.
W. R. Boynton J. D. Hagy J. C. Cornwell W. M. Kemp S. M. Greene M. S. Owens J. E. Baker R. K. Larsen 《Estuaries and Coasts》2008,31(4):623-651
Multi-year nitrogen (N) and phosphorus (P) budgets were developed for the Patuxent River estuary, a seasonally stratified
and moderately eutrophic tributary of Chesapeake Bay. Major inputs (point, diffuse, septic, and direct atmospheric) were measured
for 13 years during which, large reductions in P and then lesser reductions in N-loading occurred due to wastewater treatment
plant improvements. Internal nutrient losses (denitrification and long-term burial of particulate N and P) were measured in
tidal marshes and sub-tidal sediments throughout the estuary as were nutrient storage in the water column, sediments, and
biota. Nutrient transport between the oligohaline and mesohaline zones and between the Patuxent and Chesapeake Bay was estimated
using a salt and water balance model. Several major nutrient recycling terms were directly and indirectly evaluated and compared
to new N and P inputs on seasonal and annual time-scales. Major findings included: (1) average terrestrial and atmospheric
inputs of N and P were very close to the sum of internal losses plus export, suggesting that dominant processes are captured
in these budgets; (2) both N and P export were a small fraction (13% and 28%, respectively) of inputs, about half of that
expected for N based on water residence times, and almost all exported N and P were in organic forms; (3) the tidal marsh-oligohaline
estuary, which by area comprised ~27% of the full estuarine system, removed about 46% and 74% of total annual upland N and
P inputs, respectively; (4) recycled N and P were much larger sources of inorganic nutrients than new inputs during warm seasons
and were similar in magnitude even during cold seasons; (5) there was clear evidence that major estuarine processes responded
rapidly to inter-annual nutrient input variations; (6) historical nutrient input data and nutrient budget data from drought
periods indicated that diffuse nutrient sources were dominant and that N loads need to be reduced by about 50% to restore
water quality conditions to pre-eutrophic levels. 相似文献
79.
Søren Jessen Flemming Larsen Dieke Postma Pham Hung Viet Nguyen Thi Ha Pham Quy Nhan Dang Duc Nhan Mai Thanh Duc Nguyen Thi Minh Hue Trieu Duc Huy Tran Thi Luu Dang Hoang Ha Rasmus Jakobsen 《Applied Geochemistry》2008
To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta’s Quaternary geological development was reviewed. The water samples (n = 30) were analyzed for As, major ions, Fe2+, H2S, NH4, CH4, δ18O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15–20 km wide and 50–60 m deep Holocene incised valley. The NE part of the transect consists of 60–120 m of Pleistocene yellowish alluvial deposits underneath 10–30 m of estuarine clay overlain by a 10–20 m veneer of Holocene sediments. The distribution of δ18O-values (range −12.2‰ to −6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe2+ (0.03–2.0 mM), Mn (0.7–320 μM), SO4 (<2.1 μM–0.75 mM), H2S (<0.1–7.0 μM), NH4 (0.03–4.4 mM), and CH4 (0.08–14.5 mM). Generally, higher concentrations of NH4 and CH4 and low concentrations of SO4 were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013–11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH4, CH4 and SO4. Low concentrations of As (?0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe2+ and H2S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19–65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO4 was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration. 相似文献
80.
Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska 总被引:1,自引:0,他引:1
Finney Benjamin; Turner Simon; Hawkesworth Chris; Larsen Jessica; Nye Chris; George Rhiannon; Bindeman Ilya; Eichelberger John 《Journal of Petrology》2008,49(5):857-884
Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km3) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H2O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO2 with 0·8–3·3wt % K2O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (18O 4·4–4·9,87Sr/ 86Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(230Th/ 238U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc 相似文献