摘要:

The distribution of dissolved gaseous mercury (DGM) was determined at four hydrographic stations in the tropical Pacific Ocean. These vertical profiles of DGM, to a depth of 400 meters, are the first obtained for the open ocean. Dissolved gaseous Hg concentrations generally increased with depth. The major portion of DGM in surface sea water was of an inorganic gaseous form, probably elemental Hg (Hgo). In two instances, organic gaseous Hg, presumably dimethyl Hg [(CH3)2Hg], was observed in the deeper waters. These results suggest that bacteria may be responsible for producing Hgoand (CH3)2Hg at depth in this ocean region.

ISSN:0094-8276    

DOI:10.1029/GL015i001p00040

年代:1988

数据来源: WILEY

摘要:

Fission‐product nuclides134Cs,137Cs and103Ru originated from the Chernobyl accident have been detected in sediment traps deployed at mid‐water depths ranging from 110 to 780 m in the N. Pacific and the Bering Sea. The detected radioactivities, originally associated with fine airborne particles, have apparently been incorporated into much larger aggregates of predominantly biogenic material formed in the surface ocean, and transferred downward through the water column with velocities of the order of 100 m/

ISSN:0094-8276    

DOI:10.1029/GL015i001p00044

年代:1988

数据来源: WILEY

摘要:

Skewnesses of the marine and MAGSAT magnetic anomalies of the Atlantic ocean are explained in terms of the remanent magnetization gradually acquired by the oceanic lithosphere during its cooling. Magnetizations of the lower crust and the upper mantle are about 20–50 and 10–30 percent of the magnetization of the oceanic basaltic layer 2A, respectively. The oceanic upper mantle controls the skewness of the MAGSAT anomalies, whereas the skewness of the marine anomalies arise primarily from the oceanic lower cr

ISSN:0094-8276    

DOI:10.1029/GL015i001p00048

年代:1988

数据来源: WILEY

摘要:

Paleomagnetic data from Miocene to Recent deep sea sediments recovered during Leg 94 of the Deep Sea Drilling Project (DSDP) in the North Atlantic exhibit a systematic shallowing of inclinations with depth at some sites. This shallowing is coincident with a downhole decrease in water content, and is observed only in sediments with a carbonate content consistently greater than 80%. Compaction due to overburden pressure is believed to be responsible for the shallow inclinations observed in these unconsolidated sediments. At sites where the carbonate content of the sediments is consistently below about 80% no significant downcore decreasing trends in the water content or inclination record are observed.

ISSN:0094-8276    

DOI:10.1029/GL015i001p00052

年代:1988

数据来源: WILEY

摘要:

Available paleomagnetic data show approximately 100° of clockwise rotation for Permian and Triassic strata of the Eastern Klamath terrane. Jurassic strata of this terrane are rotated approximately 60° clockwise, which is comparable to rotations reported for Jurassic plutons that occur elsewhere in the Klamath Mountains province. Paleomagnetic data obtained during the present study from the Shasta Bally belt of Cretaceous plutons indicate 25.7° ± 13.6° of clockwise rotation for the province since Early Cretaceous time (≃ 136 Ma). The waning stages of rotation at the time of emplacement of the Shasta Bally belt plutons, which was closely followed by deposition of basal strata (Lower Cretaceous) of the Great Valley sequence, probably represents completion of accretion of the province to cratonic North A

ISSN:0094-8276    

DOI:10.1029/GL015i001p00056

年代:1988

数据来源: WILEY

摘要:

New tomographic images of the Hellenic subduction zone demonstrate slab penetration in the Aegean Upper Mantle to depths of at least 600 km. Beneath Greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern Aegean. Schematically we derive minimum time estimates for the duration of the Hellenic subduction zone that range from 26 to 40 Ma. This is considerably longer than earlier estimates which vary between 5 and about 13 Ma.

ISSN:0094-8276    

DOI:10.1029/GL015i001p00060

年代:1988

数据来源: WILEY

摘要:

In this study, the power spectra of short‐period P waves were obtained from a network of 17 stations in order to measure P‐wave attenuation in the entire mantle. A Q‐depth relationship was estimated from the use of a Southern Alaskan earthquake and a Novaya Zemlya nuclear explosion. For the determination of t* values from stations, the power spectra were obtained using the instantaneous maximum likelihood method for frequencies of 0.5, 1.0 and 1.5 Hz. Attempts to obtain a satisfactory Q model by inversion of the differential travel times and t* values were unsuccessful. An indirect approach was therefore used to obtain the Q‐depth relationship. The resulting Q model was then compared with existing long‐period Q models. This short‐period Q model shows a high‐Q zone near the middle of the mantle. These short‐period Q values were higher than those Q values at

ISSN:0094-8276    

DOI:10.1029/GL015i001p00064

年代:1988

数据来源: WILEY

摘要:

Two dimensional thermal convection of a fluid layer overlayed by a conductive lid is studied. Lateral thermal conductivity variation is assumed within the rigid lid, in order to see how an increase of the thermal conductivity within the lithosphere can stabilize a downwelling plume beneath it. Calculations are performed for a constant viscosity fluid with Rayleigh numbers (Ra) up to 4 × 105. Different lid thicknesses (δ) and thermal conductivity (k) values are considered. The numerical results reveal a coupling between the zone of high thermal conductivity and the downwelling plumes. The adjustment time of the convective circulation decreases when the values of parameters (Ra, δ, k) increase. The lowest value for the adjustment time (500 My), is obtained for Ra = 4 × 105. The calculations suggest that an increase of the effective thermal conductivity causes the anchoring of downwelling mantle jets below primitive continental nuclei. The presence of such sinking plumes attached to the cratonic lithosphere may protect them against disruption by mantle flow. The increase in the effective thermal conductivity of the cratonic lithosphere is attribued to thermal conductivity anisotropy of olivine crystal. Such a mechanism can explain the stability of old continental shie

ISSN:0094-8276    

DOI:10.1029/GL015i001p00068

年代:1988

数据来源: WILEY

摘要:

Estimation of the Rayleigh number of the inner core suggests that this region is convecting. The flow pattern is likely to produce crystallographic preferred orientation of the elastically anisotropic hexagonal closest packed (ϵ) phase of iron, such that compressional‐wave velocities would be greater in the axial relative to the equatorial direction by roughly 1 percent. This result is in agreement with seismological evidence that the inner core is elastically anisotrop

ISSN:0094-8276    

DOI:10.1029/GL015i001p00072

年代:1988

数据来源: WILEY

摘要:

The inner core of the earth most likely consists of ϵ (hcp) iron and shows anisotropy of seismic velocities which could be explained by crystallographic preferred orientation developing during convective flow. This paper summarizes deformation mechanisms in hcp metals and applies the Taylor theory of polycrystal deformation to analogs of ϵ‐iron for different strain histories. The pattern of preferred orientation is then used to calculate elastic properties of deformed polycrystals with special emphasis on anisotr

ISSN:0094-8276    

DOI:10.1029/GL015i001p00076

年代:1988

数据来源: WILEY