ALBERT

Description: Using the fault plane mechanisms of the shallow earthquakes occurring along the Hellenic arc and the extent of the intermediate seismic belt, we make a quantitative estimate of the relative motion occurring between the Hellenic arc and the adjacent sea floor. This estimate is then used to evaluate the deformation in the Aegean area and to reconstruct the pattern of motion over the Eastern Mediterranean region for the last 13 m.y. It is shown that this pattern is compatible with the neotectonic and seismicity studies in Aegea. We then discuss the dynamics of the area and propose that, since Serravallian-Tortonian time, Aegea has been spreading gravitationally in front of the southwestward advancing Turkey. The reasons for this gravitational spreading are discussed.

Description: Data gathered by recent “Islas Orcadas” cruises reveal the seafloor spreading pattern for a region south of the Agulhas/Falkland fracture zone system. The presence of a magnetic anomaly bight about the Agulhas Plateau indicates that the Agulhas Plateau may have developed at the site of a tectonic plate triple junction during the Late Cretaceous. A westward jump in the seafloor spreading center during the Late Maestrichtian (anomaly 34−31) reduced the offset across the Falkland/Agulhas fracture zone system and resulted in the formation of two conjugate aseismic ridges here described as the Meteor and Islas Orcadas Rises. The magnetic lineation pattern in the Agulhas Basin suggests that a tectonic plate (Malvinas Plate) existed during Campanian to Maestrichtian times. Relative rates of motion are calculated for Antarctica, South America, and Africa for the Late Cretaceous.

Description: We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Ceara and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present-day positions with respect to South America and Africa.

Description: This chapter discusses the carbon turnover, calcification, and growth in coral reefs. Carbon turnover within a total reef community is a function of two distinct, biochemically interacting cycles. The first is the metabolic cycle consisting of the photosynthetic fixation of CO2 and the release of CO2 by respiration and decomposition processes. Superimposed on this are the direct incorporation of organic compounds (dissolved or particulate; living or non-living) originating outside the reef systems (in the adjacent ocean waters), and the loss of organic compounds from the reef system into the out-flowing water. The second is the inorganic carbonate cycle involving the biological and non-biological precipitation and dissolution of carbonates. Superimposed on this is the loss of particulate carbonates in suspension in the out-flowing water. The main chemical component of a coral-reef system is calcium carbonate, which occurs either as high-Mg calcite, aragonite, or low-Mg calcite. The mean calcification values in various environments at One Tree Reef are presented in the chapter. These data may be converted to an implied vertical growth rate potential assuming that accrual is dominantly aragonite (density = 2.89 g cm–3) and that there is 50% porosity after normal compaction.

Description: This paper concerns the linear response of the ocean to forcing at a specified frequency and wave number in the absence of mean currents. It discusses the details of the forcing function, the general properties of the equations of motion, and possible simplifications of these equations. Two representations for the oceanic response to forcing are described in detail. One solution is in terms of the normal modes of the ocean. The vertical structure of these modes corresponds to that of the barotropic and baroclinic modes; their latitudinal structure corresponds to that of inertia‐gravity and Rossby waves. These waves are eigenfunctions of Laplace's tidal equations (LTE) with the frequency as eigenvalue. The description in terms of vertically standing modes is particularly useful if the forcing is nonlocal, because only these modes can propagate into undisturbed regions. The principal result is that it is extremely difficult for baroclinic (but not barotropic) disturbances to propagate horizontally away from a forced region. Instabilities of the Gulf Stream excite disturbances that are confined to the immediate neighborhood of the current; disturbances due to instabilities of equatorial currents do not propagate far latitudinally. A second representation of the oceanic response to forcing is in terms of vertically propagating, or vertically trapped, latitudinal modes. These modes are eigenfunctions of LTE with the equivalent depth h (not the frequency) as eigenvalue. Both positive and negative eigenvalues h are necessary for completeness. The modes with h 〉 0 consist of an infinite set of inertia‐gravity waves and a finite set of Rossby waves which either propagate vertically or form vertically standing modes. The latitudinally gravest modes are equatorially trapped and have been observed in the Atlantic and Pacific oceans. The modes with h 〈 0 are necessary to describe the oceanic response to nonresonant forcing. In the vertical this response attenuates with increasing distance from the forcing region. Because of the shallowness of the ocean the large eastward traveling atmospheric cyclones in mid‐latitudes and high latitudes force a response down to the ocean floor. Interaction with the bottom topography will result in smaller‐scale disturbances and will affect the frequency spectrum of the response when bottom‐trapped waves are excited.

Description: A tsunami earthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep-sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October 20, 1963, and on June 10, 1975, respectively. Both can be regarded as an aftershock of the preceding larger events. Their tsunami heights and seismic wave amplitudes are compared with those of the preceding events. The results show that the time constants involved in the tsunami earthquakes are relatively long but not long enough to explain the observed disproportionality between the tsunamis and the seismic waves. The process times are estimated to be less than 100 s. The spatio-temporal characteristics of the two events suggest that they represent a seaward and upward extension of the rupture associated with a great earthquake which did not break the free surface at the coseismic stage. The amplitude and phase spectra of long-period surface waves and the long-period P waveforms indicate that this extension of the rupture did not take place entirely along the lithospheric interface emerging as a trench axis. It rather branched upward from the interface in a complex way through the wedge portion at the leading edge of the continental lithosphere. This wedge portion consists in large part of thick deformable sediments. A large vertical deformation and hence extensive tsunamis result from such a branching process. A shallowest source depth, steepening of rupture surfaces, and a deformable nature of the source region all enhance generation of tsunamis. The wedge portion ruptured by a tsunami earthquake is usually characterized by a very low seismic activity which is presumably due to ductility of the sediments. We suggest that this portion fractures in a brittle way to generate a tsunami earthquake when it is loaded suddenly by the occurrence of a great earthquake and that otherwise it yields slowly. Upward branching of the rupture from the lithospheric interface produces permanent deformation of the free surface which is relative uplift landward and relative subsidence trenchward of the zone of surface break. This surface break zone geomorphologically corresponds to the lower continental slope between the deep-sea terrace and the trench. Such a mode of permanent deformation seems to be consistent with a rising feature of the outer ridge of the deep-sea terrace and a depressional feature of the trench. This consistency implies a causal relationship between great earthquake activities and geomorphological features near the trench.

Description: A tsunami earthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep-sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October 20, 1963, and on June 10, 1975, respectively. Both can be regarded as an aftershock of the preceding larger events. Their tsunami heights and seismic wave amplitudes are compared with those of the preceding events. The results show that the time constants involved in the tsunami earthquakes are relatively long but not long enough to explain the observed disproportionality between the tsunamis and the seismic waves. The process times are estimated to be less than 100 s. The spatio-temporal characteristics of the two events suggest that they represent a seaward and upward extension of the rupture associated with a great earthquake which did not break the free surface at the coseismic stage. The amplitude and phase spectra of long-period surface waves and the long-period P waveforms indicate that this extension of the rupture did not take place entirely along the lithospheric interface emerging as a trench axis. It rather branched upward from the interface in a complex way through the wedge portion at the leading edge of the continental lithosphere. This wedge portion consists in large part of thick deformable sediments. A large vertical deformation and hence extensive tsunamis result from such a branching process. A shallowest source depth, steepening of rupture surfaces, and a deformable nature of the source region all enhance generation of tsunamis. The wedge portion ruptured by a tsunami earthquake is usually characterized by a very low seismic activity which is presumably due to ductility of the sediments. We suggest that this portion fractures in a brittle way to generate a tsunami earthquake when it is loaded suddenly by the occurrence of a great earthquake and that otherwise it yields slowly. Upward branching of the rupture from the lithospheric interface produces permanent deformation of the free surface which is relative uplift landward and relative subsidence trenchward of the zone of surface break. This surface break zone geomorphologically corresponds to the lower continental slope between the deep-sea terrace and the trench. Such a mode of permanent deformation seems to be consistent with a rising feature of the outer ridge of the deep-sea terrace and a depressional feature of the trench. This consistency implies a causal relationship between great earthquake activities and geomorphological features near the trench.

Description: The usually high concentrations of Zn, Pb, Cd, and Cu in the most recently accreted portions of ferromanganese nodules from the western Baltic Sea are thought to reflect increased metal input due to anthropogenic mobilization. If so, the point of increase represents a time horizon within the structure of the nodule. Similar trace metal distributions of radiometrically dated sediments from the same area suggest that the ferromanganese nodules have grown in thickness between 0.02 and 0.16 mm yr−1. From this growth rate anthropogenic Zn flux to the nodule surface was calculated to be 80 mg m−2 yr−1.

Description: Microbial decomposition of organic matter in recent sediments of the Landsort Deep—an anoxic basin of the central Baltic Sea—resulted in the formation of a characteristic assemblage of authigenic mineral precipitates of carbonates, sulfides. phosphates and amorphous silica, The dominant crystalline phases are a mixed Mn-carbonate [(Mn0.85Ca0.10Mg0.05)CO3]. Mn-sulfide [MnS] and Fecarbonate [FeCO3]. Amorphous Fe-sulfide [FeS]. Mn-phosphate [Mn3(PO4)2] and a mixed Fe-Ca-phosphate [(Fe0.86Ca0.14)3(PO4)2] were identified by their chemical compositions only. The variability in composition of these solid phases and their mode of occurrence as a co-existing assemblage constrains the conditions and solution composition from which they precipitated. Estimates of activities for dissolved Fe. Mn. PO4, CO3 and S in equilibrium with such an assemblage are close to those found in recent anoxic interstitial water-sediment systems. It is important to have detailed knowledge of the composition and stability conditions of these solid precipitates in order to refine stoichiometric models of interstitial nutrient regeneration in anoxic sediments.

Description: Goban Spur lies on the continental margin of northwest Europe, southwest of Ireland. It is a marginal plateau underlain by Hercynian granites and Palaeozoic sediments, which form large horsts, grabens and tilted fault blocks with a trend that is approximately parallel to the main Celtic/Armorican Shelf edge. The spur is thought to be a westward continuation of the buoyant Cornubian Ridge, and is bounded to the north and south by large fault lines which probably represent reactivation of Hercynian structural trends. The continental basement is further divided longitudinally into a low-lying outer zone (Intermediate Zone) and a high inner region (Goban Spur proper). The ocean/continent boundary is thought to lie at the outer edge of the Intermediate Zone, Sea-floor spreading anomalies immediately west of the Intermediate Zone suggest that the adjacent ocean crust was created prior to anomaly 33 (say at ca. 90 m.y. B.P.). The sedimentary sequence on Goban Spur can be subdivided into four layers which can be tentatively correlated with the stratigraphic succession on the Meriadzek Terrace (IPOD sites). Such a correlation suggests that the lowermost sediment layer (?Jurassic-Albian) represents a tectono-sedimentary rift infill, and that Layers 2–4 (Upper Cretaceous to Quaternary) were deposited during slow epeirogenic downwarping of the plateau and that the sedimentary processes involved both draping and strong current moulding. The relatively buoyant nature of Inner Goban Spur has inhibited the accumulation of a thick post-rifting sedimentary sequence (700–1000 m), and has shielded the Intermediate Zone from downslope mass movements of material. This has resulted in the accumulation of an anomalously thin post-rift stage outer margin sediment prism (ca. 1000 m) compared to most continent rises (up to 10 km).