摘要:

Published reports of the presence of radioactive debris from the Chernobyl reactor accident in snow on the Greenland ice sheet raised the strong prospect that such debris might constitute a valuable time stratigraphic marker all over the ice sheet. Large volume snow samples to test this possibility were collected from 7 snowpits as part of a wide ranging regional snow chemistry survey conducted during 1987 and 1988. Snow “labeled” with Chernobyl derived radioactivity was detected in all of the pits. However, the total amount of radioactive debris found at the different locations varied over a 20 fold range. The variability in total fallout showed no clear large scale spatial pattern that could be related to the presumed progress of the radioactive plume over Greenland, suggesting that small scale differences in precipitation pattern and reworking of the snow by wind were predominantly responsible for the patchy preservation of the Chernobyl “layer” on the Greenland ic

ISSN:0094-8276    

DOI:10.1029/GL016i009p00987

年代:1989

数据来源: WILEY

摘要:

We present evidence suggesting that only a portion of the dissolved organic carbon (DOC) in the seawater analyzed previously by Williams and Druffel (1987) was oxidized by the UV‐radiation method. High temperature catalytic (HTC) methods (Sugimura and Suzuki, 1988) used to reoxidize the central North Pacific gyre water samples reveal that the total DOC (DOCHTC) is about twice that of the UV‐oxidizable DOC (DOCuv). Indications from the original study suggest that this additional DOC contains higher concentrations of radiocarbon than in the DOCuv(Williams and Druffel, 1987). This evidence implies that DOC is older and thus more refractory with respect to biological utilization, yet is more chemically reactive with respect to photooxidation, than the fraction resistant to UV (DOCres).We report accelerator mass spectrometry (AMS) Δ14C measurements of humic, fulvic and hydrophilic acid fractions isolated from water collected at 180 m in the North Pacific (19°N, 158°W) using XAD macroreticular resins. Δ14C values of the humic material are less than those of DOCuvfrom a similar depth 1200 km further north (Williams and Druffel, 1987) indicating that these humic substances are part of the ‘old’ recycled DOCuvi

ISSN:0094-8276    

DOI:10.1029/GL016i009p00991

年代:1989

数据来源: WILEY

摘要:

As one component of the Arctic Environmental Drifting Buoy, two thermistor strings were installed through the ice to measure ice temperatures and determine oceanic heat fluxes as the buoy drifted from the Arctic basin into the Greenland Sea. Ice temperature data between 14 Dec 1987 and 2 Jan 1988 were retrieved. During this period the AEDB progressed from approximately 81°N 4°E to 77°N 5°W. This constituted the most rapid displacement of the entire drift, coinciding with the entry of the floe into the marginal ice zone of Fram Strait. Once in the MIZ, water temperatures increased, most notably at a depth of 16 m, where values changed from −1.8°C to more than 2°C. Bottom ablation rates of 34 mm/day were observed between 21 and 28 Dec. During this excursion into warmer water, the oceanic heat flux increased by a factor of 18, from 7 W/m² to

ISSN:0094-8276    

DOI:10.1029/GL016i009p00995

年代:1989

数据来源: WILEY

摘要:

The Spaced Antenna (SA) technique is applied to measuring ocean surface currents for the first time. This allows the measurement of the ocean surface current transverse to the radar beam as well as the radial current and thus eliminates the requirement of two separate radars to measure the full current vector as with present ocean surface current radar systems. Therefore, this technique offers considerable advantages for ocean current mapping in terms of simplicity for real‐time operational systems and could have wide applications, for example at sites where it is not practical to have two separate radar

ISSN:0094-8276    

DOI:10.1029/GL016i009p00999

年代:1989

数据来源: WILEY

摘要:

The electrical conductivity of sea floor materials is a strong function of porosity, pore geometry, fluid content, temperature and percentage of partial melt. These material properties are particularly volatile near the midocean ridge. Tracking electromagnetic eddy currents generated on the sea floor as they diffuse across the ridge can provide images of the subaxial electrical and through interpretation the geological structure. We have computed numerically in 2D the response of a crustal magma chamber located beneath the East Pacific Rise to excitation in two fundamental orthogonal modes, rise‐normal (TE) and rise‐parallel (TM) electric field. A finite element representation of the electromagnetic (EM) diffusion equations is solved in the Laplace domain and the results inverted into the time domain via the Gaver‐Stehfest algorithm. A sequence of snapshots for each mode for the time range 0.1 to 15 s shows the complete progression of the eddy currents into, through and around the magma chamber. The TE and TM signals at a receiver site above the conductive magma chamber are delayed and attenuated in comparison with those expected for normal layered oceanic

ISSN:0094-8276    

DOI:10.1029/GL016i009p01003

年代:1989

数据来源: WILEY

摘要:

Measured under‐ice acoustic profile data and several large‐scale geometric parameters of ice keels are used to construct and partition an ensemble of large‐scale relief features into two subsets: a subset of keel‐like features (e.g., ice keels) and a subset of nonkeel‐like features. The draft data of each feature are regarded as a realization of a nonstationary random process while the draft increment data are regarded as a realization of a stationary zero mean random process. A maximum likelihood estimator technique (MLE) and a technique based on the variance function are used to calculate fractal dimensions for keel‐like and nonkeel‐like features. It is shown that for the same feature, the MLE technique and the variance function based technique yield similar values for the fractal dimension (D) and that for keel‐like features 1.2

ISSN:0094-8276    

DOI:10.1029/GL016i009p01007

年代:1989

数据来源: WILEY

摘要:

Gravity accelerations estimated from satellite altimetric mean sea surfaces (Seasat and Geos‐3) are compared to ship gravity measurements. Ship gravity are closer to an estimate based on least squares collocation, orbit perturbations, altimetry and terrestrial gravity than to an estimate based on Fourier transforms, orbit perturbations and altimetry only. Both altimetric estimates yield a smoothed picture of the geostrophic component of sea surface currents in the North Atlantic when gravity acceleration data from only nine cruises are subtracted from the altimetric gravit

ISSN:0094-8276    

DOI:10.1029/GL016i009p01011

年代:1989

数据来源: WILEY

摘要:

Fast large aperture photometers directed at the zenith on clear nights near Minneapolis have recorded many light pulses in the millisecond time range, but aside from man‐made events these were almost entirely due to Rayleigh scattered distant lightning, with a residual very low rate (<0.1/hr) of unidentified pulses. We believe that 1 ms light pulses seen in several previous experiments (FAP's) may also be mostly Rayleigh scattered lightning, rather than fluorescent light due to electron precipitation from lightning‐induced whistlers as previously discus

ISSN:0094-8276    

DOI:10.1029/GL016i009p01015

年代:1989

数据来源: WILEY

摘要:

During the early morning of January 16, 1989 the Arecibo 430 MHz incoherent scatter radar and the University of Illinois CEDAR Na lidar were operated simultaneously for several hours at the Arecibo Observatory, Puerto Rico (18°N, 67°W). At approximately 0425 LST a sporadic E (Es) layer began forming near 101 km altitude and moved steadily downward reaching an altitude of 99 km by 0500 LST. The electron abundance of the Eslayer increased rapidly from approximately 3×109cm−2at 0510 LST to approximately 6×109cm−2by 0530 LST. During this same time period a sporadic Na (Nas) layer formed at the same altitude as the Eslayer. Both the Esand Naslayers moved upwards together between 0530 and 0600 LST before dissipating at an altitude of 101 km. The high correlation between the altitude and abundance variations of the two layers strongly suggests that the Naslayer formation is related to the Eslayer and may involve Na cluster ion ch

ISSN:0094-8276    

DOI:10.1029/GL016i009p01019

年代:1989

数据来源: WILEY

摘要:

In situmeasurements above the polar cap by the DE satellites show asymmetric, field‐aligned electron velocity distributions in the photoelectron energy range (5–60 eV). The formation of these anisotropic distributions is described by a collisional kinetic equation that includes the effects of Coulomb collisions (via a Fokker‐Planck operator), those of a parallel electric field, and the magnetic mirror effects of the geomagnetic field. We solve this equation using a Monte Carlo simulation method. The simulation illustrates the formation of the portion of the observed electron distributions originating in the ionosphere, and permits us to evaluate the significance of the suprathermal electrons at higher altitude. Both observations and calculations indicate that these electrons possess an energy flux which, by its effect on the polar wind electric field, may influence the dynamics of the polar wind ou

ISSN:0094-8276    

DOI:10.1029/GL016i009p01023

年代:1989

数据来源: WILEY