The ratio and other parameters have been measured in fossil planktonic foraminifera from the Atlantic and Pacific Ocean basins in order to evaluate the ratio of seawater during the last 75 million years. Results on well-preserved samples indicate that the ratio has increased to its present value by 10–15% during the Cenozoic, and that minima occurred between 55-45 Ma and 10-5 Ma, when the ratio was 15–25% less than at present. The long-term increase may reflect either decreasing deposition of aragonite with a high ratio in shallow seas, or decreasing seafloor spreading rates and consequently decreasing hydrothermal supply of Ca during the Cenozoic. Other geologic evidence suggests that the Eocene minimum (near 50 Ma) may have resulted from increased aragonite sedimentation, while the Late Miocene minimum (between 10-5 Ma) may have been caused by an increased rate of seawater-basalt exchange when seafloor spreading rates increased on the East Pacific Rise near 10 Ma. 相似文献
In the Jebilet Palaeozoic inlier, 20 km north of Marrakech, there are extensive exposures of Carboniferous flysch deposits. Although there are some structural complications due to over-riding nappes with associated chaotic breccias, one clearly unbroken succession from basin-plain turbidites to shallow-marine deposits can be examined. The succession is more than 2 km thick and is dated as Upper Viscan in the uppermost part.The lowermost unit of B- and C-based turbidites shows no sequential organisation and is interpreted as a typical basin-plain association. Above this are similar turbidites arranged in thickening-upward sequences that may represent outer-fan or base-of-slope deposits. Succeeding these are thin-bedded turbidites with interbedded units formed by mass movement that represent a slope deposit. The overlying lenticular-bedded facies resembles previously described overflow deposits of submarine-fan channels, but is here interpreted as comprising storm-generated deposits on the outer shelf/upper slope. These deposits are genetically linked with the overlying parallel-laminated sandstones with irregular-rippled tops for which a storm-surge origin is suggested. The upper part of the succession shows cross-bedded, oolitic, bioclastic, sandy limestones with bipolar current structures sandwiched between low-energy siltstones containing thin-graded silt/sand beds. These are collectively interpreted as shelf deposits that formed under different depths due to transgressive-regressive events.The sequence differs from many described in the literature in that there is an absence of most submarine-fan facies. Locally a NNE-SSW basin strike is proposed with a basin margin to the ESE, but there is at present little control on regional palaeogeography. 相似文献
Long period body waves are examined to show that the Hamran (1972.9.3), Darel (1981.9.12) and Patan (1974.12.28) earthquakes in Kohistan had focal depths of about 8–10 km. All involved high angle reverse faulting (thrusting) and had seismic moments of about 2.2 to 2.7·1025 dyne cm. These shallow depths contrast with the deeper hypocentres found in the Hindu Kush and northeast Karakoram to the north and in Hazara to the south. The Hamran and Patan shocks were assigned depths of 45 km by the ISC, indicating that even well-recorded events in this region may have focal depths in error by 30 km 相似文献
Summary A comparison of 8 regional atmospheric model systems was carried out for a three-month late summer/early autumn period in
1995 over the Baltic Sea and its catchment area. All models were configured on a common grid using similar surface and lateral
boundary conditions, and ran in either data assimilation mode (short term forecasts plus data assimilation), forecast mode
(short term forecasts initialised daily with analyses) or climate mode (no re-initialisation of model interior during entire
simulation period). Model results presented in this paper were generally post processed as daily averaged quantities, separate
for land and sea areas when relevant. Post processed output was compared against available analyses or observations of cloud
cover, precipitation, vertically integrated atmospheric specific humidity, runoff, surface radiation and near surface synoptic
observations.
The definition of a common grid and lateral forcing resulted in a high degree of agreement among the participating model results
for most cases. Models operated in climate mode generally displayed slightly larger deviations from the observations than
the data assimilation or forecast mode integration, but in all cases synoptic events were well captured. Correspondence to
near surface synoptic quantities was good. Significant disagreement between model results was shown in particular for cloud
cover and the radiative properties, average precipitation and runoff. Problems with choosing appropriate initial soil moisture
conditions from a common initial soil moisture field resulted in a wide range of evaporation and sensible heat flux values
during the first few weeks of the simulations, but better agreement was shown at later times.
Received September 8, 2000 Revised April 3, 2001 相似文献
Implicit in the African Renaissance is the synergy between government, the private sector, the educated minority and the disadvantaged majority. For this concept to work, belief and commitment must arise first from the African individual, whatever his or her potential contribution may be. The geosciences in South Africa provide a currently vibrant example of such cooperation, which has the potential to contribute significantly to the upliftment of the country and its neighbouring states. Based largely on personal interviews with various role players, from the Presidency of South Africa, through ministerial levels, the corporate sector and down to the individual, we present a spectrum of viewpoints and initiatives which are starting to result in practical implementation of the African revival. An end to conflict and xenophobia, the entrenchment of democratic government and corporate expression of the entrepreneurial spirit are essential to provide the framework within which the individual African can become a “Renaissance Man or Woman”. 相似文献
This paper discusses the work carried out to obtain stereoscopic "photography" from airborne platforms with a Kodak DCS200 digital camera, as part of an investigation into the suitability of current digital camera technology for use in air survey. Details of the work, from the initial test flight in a Cessna light aircraft through to tests utilizing a Thruster microlight platform, are documented. The paper concludes with a summary of the present shortcomings in obtaining aerial photography for survey purposes with still video technology and discusses the future prospects. 相似文献
We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.3He/4He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 RA (RA is the atmospheric ratio of1.39 × 10−6), encompassing the range of previously reported values for MORB erupted away from high3He/4He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in3He/4He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in3He/4He along the ridge can be used to refine the hotspot source-migrating-ridge sink model.
MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by87Sr/86Sr of 0.7022, Pb isotopes close to the geochron and with206Pb/204Pb of 17.7, and3He/4He of 8.6–8.9 RA. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the3He/4He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of3He/4He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pbratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity.
Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations. 相似文献
A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth's magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature. 相似文献
3He/4He ratios in lavas erupted during the last 360 years at Mt. Vesuvius are between 2.2 and 2.7 RA (RA = atmospheric ratio of 1.39 × 10−6), and are among the lowest values measured in young volcanic rocks. They are also identical to values measured in summit crater fumaroles sampled during 1987–1991. This agreement indicates that the 3He/4He ratio in the crater fumaroles faithfully tracks the magmatic value. The relatively low and uniform 3He/4He ratio in the lavas reflects either a mantle source enriched in (U + Th)/3He, or a mixture of magmatic and crustal components. 相似文献