摘要:

We present a method for normalizing sedimentary records for estimating relative paleointensity of the geomagnetic field, similar to that successfully used to obtain absolute paleointensity from thermally blocked remanences. It has the advantages that it is more effective in removing unwanted viscous remanence, thereby improving agreement among various records and that it allows the estimation of the uncertainty in the relative paleointensity calculated.

ISSN:0094-8276    

DOI:10.1029/95GL03166

年代:1995

数据来源: WILEY

摘要:

The magnetostratigraphy has been established in two sections of middle and late Oxfordian age from the Iberian Range (Spain). The polarity sequences correlate well with the magnetozones in sections at Aguilón and Tosos [Juárez et al., 1994]. A composite magnetostratigraphic sequence has been derived for all the sections in the Iberian Range. It correlates with the Handschumacher et al. [1988]extension of the geomagnetic polarity time scale, and ties the oceanic magnetic anomaly sequence to the Oxfordian sedimentary section

ISSN:0094-8276    

DOI:10.1029/95GL02848

年代:1995

数据来源: WILEY

摘要:

Chondrite normalized (La/Sm)nratio variations from 66 basalt glasses dredged along the Mid‐Atlantic Ridge (MAR) from 40–54.5°S reveal two distinct geochemical anomaly highs coupled with gravity and bathymetric highs and crest‐like rather than rifted ridge‐axis morphology. The Discovery anomaly (45–49°S), located opposite the Discovery hotspot track, is irregular and shows large local variation. The Shona anomaly (50–52.5°S) is smoother and shows less local variation. These results suggest that the influential Shona mantle plume may be located on or very close to the ridge. In contrast, the dispersal of the Discovery plume, located well off the ridge, is affected in a complex manner by the Agulhas fracture zone and the asthenospheric flow field associated with the

ISSN:0094-8276    

DOI:10.1029/95GL02665

年代:1995

数据来源: WILEY

摘要:

Self‐organized systems are often used to describe natural phenomena where power laws and scale invariant geometry are observed. The Piton de la Fournaise volcano shows power‐law behavior in many aspects. These include the temporal distribution of eruptions, the frequency‐size distributions of induced earthquakes, dikes, fissures, lava flows and interflow periods, all evidence of self‐similarity over a finite scale range. We show that the bounds to scale‐invariance can be used to derive geomechanical constraints on both the volcano structure and the volcano mechanics. We ascertain that the present magma bodies are multi‐lens reservoirs in a quasi‐eruptive condition, i.e. a marginally critical state. The scaling organization of dynamic fluid‐induced observables on the volcano, such as fluid induced earthquakes, dikes and surface fissures, appears to be controlled by underlying static hierarchical structure (geology) similar to that proposed for fluid circulations in human physiology. The emergence of saturation lengths for the scalable volcanic observable argues for the finite scalability of complex naturally self‐organized critical systems, including

ISSN:0094-8276    

DOI:10.1029/95GL01786

年代:1995

数据来源: WILEY

摘要:

We present accurate numerical calculations for the opening and extension of tensile wing cracks driven by sliding on an oblique crack with Coulomb friction. In contrast to previous analyses, the portions of the crack system that are open or closed and slipping or stuck are determined as part of the solution. Calculations reveal that reduction of the compressive normal stress accompanying opening of the wing cracks causes considerable opening of the sliding crack providing an additional component of macroscopic dilatancy. Immediately upon unloading the central portion of the planar crack is locked but there is closure of the open portions of the crack causing a decrease in inelastic volume strain as observed in laboratory experiments. Reverse slip begins near the open portion of the crack and spreads toward the center. The rate of decrease of inelastic volume strain with axial stress remains small, causing an apparent “dead band” until the axial stress is sufficiently reduced to cause reverse slip on the entire crack. This apparent “dead band” is small because opening reduces the compressive normal stress on the planar crack. These calculations demonstrate that the behavior of this model is consistent with measurements of inelastic volume strain and acoustic velocity on laboratory rock sp

ISSN:0094-8276    

DOI:10.1029/95GL02072

年代:1995

数据来源: WILEY

摘要:

It is usually assumed in geodetic studies that measurement errors are independent from one measurement to the next and that the rate of deformation (velocity) is constant over the duration of the experiment. Any temporal correlation between measurements can substantially affect the uncertainty in this velocity estimate when it is determined from the time series of measurements. One source of possible long‐term correlation is motion of the geodetic monument with respect to the “deep” crust. Available measurements suggest that this motion introduces errors that have the form of a random walk process. We show how such errors affect the uncertainty of velocity estimates. For a geodetic experiment of set duration we calculate the velocity uncertainty as a function of the number of observations and of the relative amount of correlated and uncorrelated noise. We find that 1) neglecting long‐term temporal correlations makes the uncertainty in the estimated velocities much too small, and that 2) when the correlated and independent noise sources are of similar magnitude, the expected improvement in uncertainty from having more measurement is not realized; there is almost no improvement in some cases. We have also examined the effect of outliers (“blunders”) on the velocity uncertainty; for a frequency of outliers typical of geodetic field campaigns, the previous two conclusions remain unchanged. These results suggest that long‐term correlations have a large effect on estimating deformation rates; unless these correlations are small, frequent observations give little advantage. If frequent observations are planned, the amount of correlated noise due to monument instability must be kept small if the full capabilities of the measurement technique are t

ISSN:0094-8276    

DOI:10.1029/95GL02661

年代:1995

数据来源: WILEY

摘要:

The end of the Little Ice Age (LIA) in Svalbard (76–81°N), a climate‐sensitive region at the northern extreme of strong poleward heat transfer, was marked by an abrupt increase in mean annual air temperature of up to 5°C around 1920. Glacier mass balance has been consistently negative since this time, and large cumulative net losses of mass have occurred at most glaciers. Energy‐balance modelling confirms the sensitivity of Svalbard glaciers to climate change, predicting a negative shift in net mass balance of up to 0.8 m a−1(water equivalent) per degree temperature rise. This climate‐related shift in glacier mass balance has reduced the intensity of glacier surge activity in Svalbard. One glacier, known to have surged since the end of the LIA, has since failed to accumulate the mass required to re‐initiate the surge cycle, and is also now cold at its base and incapable of rapid flow by basal sliding. Three overviews of the total number of actively‐surging glaciers in Svalbard between 1936–90 show a decrease from 18 to 5. This is significant compared with the expected numbers of surges based on LIA conditions. Post‐LIA climate change in Svalbard has therefore affected not only glacier extent, but also ice dynamics. This is trend will probably continue given CO2‐in

ISSN:0094-8276    

DOI:10.1029/95GL02821

年代:1995

数据来源: WILEY

摘要:

While the possible instability of the West Antarctic ice sheet has been widely recognized for some time as a potential source of sea‐level rise in an enhanced greenhouse warming, the stability of the East Antarctic ice sheet has only recently become the subject of such a conjecture. We approach an aspect of this issue using a numerical experiment with an atmospheric general circulation model (GCM) coupled to a 3‐dimensional ice‐sheet model. Response of the ice sheet model to the external forcing generated by an atmospheric GCM due to doubling the CO2concentration does not show any appreciable changes in the horizontal extent of the ice sheet due to normal creep and topographic instabilities, suggesting that an Antarctic collapse resulting from these factors is unlikely. The mechanics of basal sliding, non‐isothermal effects, and ice shelves are as yet too poorly understood to make quantitative estimates of possible instabilities due to these pr

ISSN:0094-8276    

DOI:10.1029/95GL02935

年代:1995

数据来源: WILEY

摘要:

Three sub‐grid‐scale mixing parameterizations (lateral/vertical; isopycnal; Gent and McWilliams, 1990) are used in a global ocean model in an attempt to determine which yields the best ocean climate. Observed CFC‐11 distributions, in both the North and South Atlantic, are used in evaluating the model results. While the isopycnal mixing scheme does improve the deep ocean potential temperature and salinity distributions, when compared to results from the traditional lateral/vertical mixing scheme, the CFC‐11 distribution is worse in the upper ocean due to too much mixing. The Gent and McWilliams (1990) parameterization significantly improves the CFC‐11 distributions when compared to both of the other schemes. The main improvement comes from a reduction of CFC uptake in the southern ocean where the ‘bolus’ transport cancels the mean advection of tracers and hence causes the Deacon Cell to disappear. These results suggest that the asymmetric response found in CO2‐increase experiments, whereby the climate over the southern ocean does not warm as much as in the northern hemisphere, may be due to the particular mixi

ISSN:0094-8276    

DOI:10.1029/95GL02651

年代:1995

数据来源: WILEY

摘要:

In order to estimate the concentrations of atmospheric nitrous oxide (N2O) during the last 250 years, air samples were extracted from an Antarctic ice core, H15, using a dry extraction system and were then analyzed with a precision of ±2 ppbv. The results obtained were clearly less scattered and much tighter than those of the previous studies. Our data showed that the concentrations of atmospheric N2O in the 18th century were about 276 ppbv on average. It was also obvious that the N2O concentration began to increase in the mid‐19th century and reached approximately 293 ppbv around 1965, the trend of the concentration increase correlating quite well with the direct atmospheric measurements at the South Pole. Such an increase in the atmospheric N2O concentration is thought to be of anthropogenic orig

ISSN:0094-8276    

DOI:10.1029/95GL02822

年代:1995

数据来源: WILEY