摘要:

Record low ozone was measured by balloon‐borne ozonesondes (40 flights) at McMurdo Station, Antarctica (78°S) during the 1993 austral spring. Total column ozone declined by 55% from an initial 275 Dobson Units (DU) on 30 August to a minimum of 130±7 DU on 2 October. Ozone within the 12–20 km showed a 95% decrease from an initial 138 DU in August to a record low 7 DU on 19 October. Probable causes of the 1993 record low ozone, based on balloon‐borne observations at McMurdo include: the presence of the Pinatubo volcanic aerosol layer between 11 and 16 km (though decreased from the 1992 season); a colder than normal stratosphere over McMurdo (183 K minimum); and a relatively stable polar vortex which delayed the intrusion of high levels of ozone from outside the polar vortex wall until after 22 October. These conditions provided an optimum environment for the formation of polar stratospheric clouds (PSCs), essential to the heterogeneous chemistry that subsequently leads to the catalytic destruction of ozone by reactive c

ISSN:0094-8276    

DOI:10.1029/94GL02993

年代:1995

数据来源: WILEY

摘要:

The Marlborough fault system comprises a series of major right‐lateral strike‐slip faults that link the Alpine fault to the Hikurangi subduction zone in the Pacific/Australia plate boundary zone in New Zealand. All of the major active faults of the Marlborough fault system have continuous traces except for the Clarence fault which terminates abruptly near the Ward syncline. Paleomagnetic data from upper Miocene and lower Pliocene sedimentary rocks between the Marlborough faults indicate a consistent post‐early Pliocene regional clockwise rotation of about 20°. An additional rotation of about 10° is evident at many localities near the termination of the Clarence fault. It is proposed that the additional rotation, relative to the rest of the region, is caused by a velocity gradient that likely exists between the active Awatere fault and the termination of the Clarence fault. The existence of such a velocity gradient is consistent with inferences from geodetic strain data which suggest that one‐third of the displacement in the Marlborough fault system occurs between the Awatere and Clarence faults. The kinematics of rotation can be modelled by treating the area as a rigid block that pivots about the termination of the Clarence fault. The block rotation model satisfies field constraints on the styles of deformation observed at the boundaries of the block and is consistent with available paleomagnetic and geod

ISSN:0094-8276    

DOI:10.1029/94GL02582

年代:1995

数据来源: WILEY

摘要:

Self‐potential (SP) observation was carried out in the summit area of Unzen, one of the active volcanoes in Kyushu island, Japan. We found a positive SP anomaly in the vicinity of the newly extruded lava dome. The potential difference across the anomaly exceeds 1000mV per 500m. Streaming potentials associated with subsurface hydrothermal convection seem to be the most reasonable mechanism for the positive anomaly. In association with the first emergence of the lava dome a sharp increase of SP was detected, which is considered to be a result of the growth of the hydro‐thermal sys

ISSN:0094-8276    

DOI:10.1029/94GL03077

年代:1995

数据来源: WILEY

摘要:

A permanent Global Positioning System receiver at Casa Diablo Hot Springs, Long Valley Caldera, California was installed in January, 1993, and has operated almost continuously since then. The data have been transmitted daily to the Jet Propulsion Laboratory for routine analysis with data from the Fiducial Laboratories for an International Natural sciences Network (FLINN) by the JPL FLINN analysis center. Results from these analyses have been used to interpret the on going deformation at Long Valley, with data excluded from periods when the antenna was covered under 2.5 meters of snow and from some periods when Anti Spoofing was enforced on the GPS signal. The remaining time series suggests that uplift of the resurgent dome of Long Valley Caldera during 1993 has been 2.5±1.1 cm/yr and horizontal motion has been 3.0±0.7 cm/yr at S53W in a no‐net‐rotation global reference frame, or 1.5±0.7 cm/yr at S14W relative to the Sierra Nevada block. These rates are consistent with uplift predicted from frequent horizontal strain measurements. Spectral analysis of the observations suggests that tidal forcing of the magma chamber is not a source of the variability in the 3 dimensional station location. These results suggest that remotely operated, continuously recording GPS receivers could prove to be a reliable tool for volcano monitoring throughout the

ISSN:0094-8276    

DOI:10.1029/94GL02968

年代:1995

数据来源: WILEY

摘要:

Microearthquake waveforms and spectra are well known to be strongly influenced by the shallow subsurface conditions at the recording site. We make use of this fact to determine optimum site models from SH‐wave microearthquake seismograms using a non‐linear waveform inversion technique based on simulated annealing. Site models are optimized in terms of thicknesses, S‐wave velocities, and densities of the uppermost layers (<100 m) for 1D plane layer models. In addition, the source parameters in terms of Brune model source time functions are optimized simultaneously. The synthetic seismograms are calculated using a modification of Haskell's plane SH‐wave algorithm. Using aftershock recordings of the Loma Prieta earthquake, a 1D plane layer model has been obtained for a selected site in Central California which has been occupied during the aftershock experiment. For this dataset, cross‐correlation coefficients between optimized synthetics and observed records reach values above 0.8. The site model at the selected station for varying sources is fairly stable, especially for the uppermost layers. We observe systematic variations of layer parameters with back‐azimuth which would be in agreement with slightly dipping layer

ISSN:0094-8276    

DOI:10.1029/94GL02970

年代:1995

数据来源: WILEY

摘要:

ODP Site 762 (eastern Indian Ocean) includes a section of sediment that spans the Cenomanian‐Turonian Boundary (CTB) and was deposited along a continental margin during a period of widespread oceanic O2deficiency. The rare earth element (REE) content of pre‐ and post‐boundary sediment is similar to that of present‐day continental slope material deposited in well‐oxygenated seawater, whereas the CTB section is characterized by significantly depleted REE abundances, a bulk Ce anomaly that increases to maximum of 1.0, and a REE pattern that resembles that of present‐day fluvial material. We suggest the change in REE patterns reflects release of scavenged REEs upon reductive dissolution of authigenic Fe‐Mn oxyhydroxides, such that sediment deposited during the CTB is dominated by the lithogenous REE fraction. These results are consistent with recent models concerning pervasive fractionation of Mn and Fe and redirection of Mn in pelag

ISSN:0094-8276    

DOI:10.1029/94GL03047

年代:1995

数据来源: WILEY

摘要:

Satellite data from the Geosat altimeter and the Advanced Very High Resolution Radiometer (AVHRR) are used to show the large‐scale structure of the surface circulation of the California Current System in summer. These data show the connection between an equatorward jet and temperature front off Oregon that lies within 100 km of the coast, similar to that first observed in the 1960's and 1970's, and a jet that meanders along the convoluted offshore edge of a temperature front off California, as repeatedly observed in the 1980'

ISSN:0094-8276    

DOI:10.1029/94GL03011

年代:1995

数据来源: WILEY

摘要:

Data from Geosat and TOPEX altimeters are used to infer the structure of the Peru‐Chile Countercurrent, a jet that flows from at least as far north as 10°S (historical data suggests 7°S) to 35°–40°S, maintaining its position between approximately 100–300 km offshore. Although the annual mean current cannot be determined from altimeter observations, the nearly antisymmetric patterns in spring and fall, combined with historical observations, suggest that the countercurrent is poleward at most times and is maximum in spring and minimum in fall. Previous studies have linked the offshore countercurrent at 7°S to the Equatorial Undercurrent west of the Galapagos Islands, suggesting that the countercurrent is part of a continuous flow that extends from the western equatorial Pacific to the region off Chile betwee

ISSN:0094-8276    

DOI:10.1029/94GL02807

年代:1995

数据来源: WILEY

摘要:

Carbonyl sulfide (OCS) photoproduction rates were measured at selected wavelengths of ultraviolet light between 297 and 405 nm in sea water samples from the southern Pacific Ocean. Near‐surface and column production rate spectra for natural sunlit waters were calculated using sea‐surface sunlight data measured near the austral summer solstice. These plots show that photoproduction rates are at a maximum at 313 nm in tropical waters and at 336 nm in Antarctic waters. Tropical surface and column rates were found to be 68 pM/day and 360 nmol/m²/day, respectively, and Antarctic surface and column rates were found to be 101 pM/day and 620 nmol/m²/day, respectively. A high degree of variability was observed between photoproduction rates from different ocean regions, with coastal rates being the highest, suggesting that natural environmental variability is an important factor. Photoproduction rates at 297 nm were found to be constant at individual locations with increasing irradiation time. Relative photoproduction rates from this work are compared to previously measured rates from coastal sea

ISSN:0094-8276    

DOI:10.1029/94GL03000

年代:1995

数据来源: WILEY

摘要:

Hexachlorocyclohexanes (HCHs) are the most abundant organochlorine pesticides in the arctic atmosphere and ocean surface water. A compilation of measurements made between 1979–93 from stations in the Canadian and Norwegian Arctic and from cruises in the Bering and Chukchi seas indicates that atmospheric concentrations of α‐HCH have declined significantly (p<0.01), with a time for 50% decrease of about 4 y in summer‐fall and 6 y in winter‐spring. The 1992–93 levels of about 100 pg m−3are 2–4 fold lower than values in the mid‐1980s. The trend in γ‐HCH is less pronounced, but a decrease is also suggested from measurements in the Canadian Arctic and the Bering‐Chukchi seas. HCHs in ocean surface water have remained relatively constant since the early 1980s. The decline in atmospheric α‐HCH has reversed the net direction of air‐sea gas exchange to the point where some northern waters are now sources of the pesticide to the a

ISSN:0094-8276    

DOI:10.1029/94GL02990

年代:1995

数据来源: WILEY