Intercomparison of interannual variability and potential predictability: an AMIP diagnostic subproject

 The inter-annual variability and potential predictability of 850 hPa temperature (T ₈₅₀), 500 hPa geopotential (φ₅₀₀) and 300 hPa stream function (ψ₃₀₀) simulated by the models participating in the Atmospheric Model Intercomparison Project (AMIP) are examined. The total inter-annual variability is partitioned into a potentially predictable component which arises from the forcing implied by the prescribed SST and sea-ice evolution, or from sources internal to the simulated climate, and an unpredictable low frequency component induced by “weather noise”. There is wide variation in the ability to simulate observed inter-annual variability, both total and weather-noise induced. A majority of models under simulate seasonal mean φ₅₀₀ variability in DJF and JJA and over simulate ψ₃₀₀ variability in JJA. All but one model simulates less T ₈₅₀ total inter-annual variability than in the analysed data. There is little apparent connection between gross model characteristics and the corresponding ability to simulate observed variability, with the possible exceptions of resolution. Received: 7 July 1996 / Accepted: 8 January 1998     

Mantle convection with a brittle lithosphere: thoughts on the global tectonic styles of the Earth and Venus

Plates are an integral part of the convection system in the fluid mantle, but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults. The conditions for plate tectonics are studied by considering brittle behaviour, using Byerlee's law to limit the maximum stress in the lithosphere, in a mantle convection model with temperature-dependent viscosity.
When the yield stress is high, convection is confined below a thick, stagnant lithosphere. At low yield stress, brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation; surface deformation occurs in localized regions close to upwellings and downwellings in the system. At intermediate levels of the yield stress, there is a cycling between these two states: thick lithosphere episodically mobilizes and collapses into the interior before reforming.
The mobile-lid regime resembles convection of a fluid with temperature-dependent viscosity and the boundary-layer scalings are found to be analogous. This regime has a well defined Nusselt number–Rayleigh number relationship which is in good agreement with scaling theory. The surface velocity is nearly independent of the yield stress, indicating that the 'plate' motion is resisted by viscous stresses in the mantle.
Analysis suggests that mobilization of the Earth's lithosphere can occur if the friction coefficient in the lithosphere is less than 0.03–0.13—lower than laboratory values but consistent with seismic field studies. On Venus, the friction coefficient may be high as a result of the dry conditions, and brittle mobilization of the lithosphere would then be episodic and catastrophic.     

Volcanoclastics of the Walvis Ridge

The paper generalizes the distribution of volcanoclastic material in the Cenozoic sedimentary cover of the Walvis Ridge, made on the basis of the DSDP (Deep Sea Drilling Projects) and ODP (Ocean Drilling Program). The cycles of volcanoclastic accumulation have been distinguished. It has been proved that the distribution of the material in the Paleogene primary reflects the dynamics of volcanism of the ridge itself. The sources of volcanoclastics have been determined. The possibility of the existence of Early Eocene submarine volcanoes in the central part of the ridge has been shown. The dynamics of volcanism of the ridge has been compared with the variability of major climatic markers in sediments, indicating the unity of volcanic processes in the region and processes that led to an increase in the index of ¹³C content in sediments and CO₂ content in the atmosphere.     

Cyclic explosive activity of the Iceland plume in the quaternary

Data on the volcanic ash layers in 70 DSDP and ODP Sites and 100 cores obtained during cruises of the R/V Akademik Kurchatov and Mikhail Lomonosov were used for compiling tephrostratigraphic scale and schematic distribution maps of the pyroclastic material in the Quaternary sediments of the North Atlantic and Norwegian-Greenland Basin. It is revealed that the distribution of pyroclastic material through this region is characterized by cyclic and spatially irregular patterns. Based on their petrochemical and geochemical properties, these ashes are compared with the volcanics of Iceland and the Jan Mayen islands. The relations between the extreme climatic and cyclic explosive events are discussed.     

Basic intrusives and hydrocarbonic potential of the South-East Baltic

The Paleozoic rocks of Kaliningrad oblast and the adjacent areas of the SE Baltic Sea have recently became exploration targets. The small size and number of prospective structures, as well as the small size of the oil reserves in the onshore areas, have resulted in a decline in production and a decrease in exploration activity. At the same time, previous studies conducted by the Institute of Oceanology of the Russian Academy of Sciences and other institutions show that the SE Baltic Sea contains numerous yet-to-be-explored structures, which could be attractive exploration targets. Recent studies indicate that the region was intensely intruded by numerous diabase sills, which cut across Paleozoic reservoir rocks. This fact may have both positive and negative implications for the petroleum potential of the basin and should be taken into account in outlining the possible future directions for resource exploration. This study presents data on the distribution, sizes, ages, composition, and volumes of the mafic intrusions and discusses their thermal effects on the petroleum potential of the SE Baltic Basin.     

Melilitites in the alkaline volcanic succession of the Gorringe bank,Southwestern Portugal

Melilitites were found for the first time in the alkaline volcanics of the Gorringe bank. With allowance for previous studies, this indicates the existence of a common melilitite-nephelinite-phonolite alkaline series, which is widely spread as lava fields, dikes, and diatremes in the continental magmatic provinces of Europe, Africa, Asia, and America, often accompanying the formation of carbonatite complexes. This is considered as one of the evidence in support of the continental nature of the alkaline volcanism of the bank. The fact that the volcanics contain fragments of plutonic rocks, which are usually observed in the carbonatitebearing alkaline complexes (melteigites, ijolites, nepheline syenites), makes it possible to suggest the presence of alkaline intrusions of such type at a shallow depth.     

Peculiarities of the magmatism and tectonics of the Knipovich Ridge

Petrological-geochemical data were obtained for intrusive rocks (gabbroids) recovered on the eastern flank of the Knipovich Ridge by deep-sea site 344 (DSDP, Leg 238). It was found that these rocks are similar to basalts and basaltic glasses studied in the adjacent sections of the ridge rift zone [7, 8]. This indicates that the intrusive rocks and erupted lavas are comagmatic. The gabbroids, basalts, and their quenched glasses were derived by differentiation in different-depth chambers and feeder channels. The petrochemical features of the gabbros and basalts (low Fe content, oxidized magnetic minerals) caused their weak magnetic properties. Owing to the multidirectional movements of the oceanic blocks, the bodies of the intrusive and effusive rocks their lost strict linearity and produced the mosaic anomalous magnetic field of the Knipovich Ridge.     

Magmatism: The Jan Mayen hotspot,Arctic Atlantic Ocean

Original and published material was used to examine the history of eruptive magmatism at the Jan Mayen hotspot, although the scientific community is not unanimous in recognizing it as part of the Iceland plume. It is shown that occurrences of alkaline magmatism that is characteristic of present-day volcanoes on Jan Mayen Island started in the Early Eocene and were caused by the passage of eastern Greenland near the plume. Magma was supplied to the Jan Mayen hotspot via deep-seated faults and channelways from relict and new magma chambers during the Oligocene, as well as the Miocene, Pliocene, and Quaternary periods. These chambers were probably replenished by cycles of magmatism at the Iceland plume and as a result of local magma generation in the Jan Mayen transform fault zone; the Jan Mayen volcanoes are currently observed to migrate towards this fault.