ALBERT

Description: THE rare deep-sea octopod Cirrothauma murrayi Chun 1910 was first described from a single specimen caught during the Michael Sars Expedition of 1910 (ref. 1). Until now it has been caught only four more times2. We describe here three specimens of this species that were recently caught during biological cruises of RRS Discovery (Fig. 1). All of these animals, including the Discovery ones, have been caught at depths of more than 1,500 m, except one that was dip-netted through the ice of the Arctic Ocean3.

Description: THE Sierra Leone Rise, located in the east equatorial Atlantic, forms a discontinuous chain of seamounts as shallow as 2 km extending with a general NE–SW trend from near the Sierra Leone coast of Africa, to the St Paul fracture zone near the Mid-Atlantic Ridge (Fig. 1). The origin of this feature has remained a topic of discussion. Sheridan et al.1 have hypothesised that the Sierra Leone Rise is a volcanic structure formed at the beginning of the opening of the Atlantic in the early Cretaceous period. The twin features of the Sierra Leone and the Ceara Rises are probably of oceanic origin and were created 80 Myr ago or later in their present-day position with respect to Africa and South America2. The Atlantic ocean exhibits several similar aseismic structures which appear symmetrically oriented with respect to the mid-oceanic ridge, such as the Walvis–Rio Grande Rise and the Iceland Faeroes–Iceland Greenland Ridges. These structures are volcanic edifices having a composition similar to that found in their associated islands3–7. Deep sea drilling of the Ceara Rise8,9 penetrated a basaltic basement of the upper Cretaceous period (Maestrichtian) (Leg 39, Site 354). Similarly, a DSDP hole (Leg 41, Site 366) on the Sierra Leone Rise, penetrated sediments of the same period, without reaching basement10. We report here the discovery of alkali-rich volcanics in an area of the Sierra Leone Rise. The sediment overlying the rock fragments is aged ∼45 Myr.

Description: Thick sediments (maximum of at least 4900 m) infill the Natal Valley, which lies between the coast of South Africa/Mozambique and the Mozambique Ridge. Eight physiographic provinces are recognised in the valley, and their boundaries can be related to well-defined sediment thickness or facies discontinuities within the basin infill. The acoustic stratigraphy of the area is established by reference to two regionally developed reflecting horizons, which, on the basis of previously published borehole and seismic data, are tentatively identified as mid-Cretaceous (McDuff) and mid-Cainozoic (Angus) hiatuses or important facies boundaries. Sedimentation began in the Natal Valley before mid-Cretaceous times and since then sediment dispersion has been strongly influenced by the disposition of large basement (? volcanic) highs: the approximately NE—SW Almirante Leite and Naudé ridges, and the larger N—S Mozambique Ridge. There is abundant evidence that sedimentation in the vicinity of these ridges has been current-controlled since at least mid-Cainozoic (Angus) times. The two main terrigenous sediment input points have been the Tugela and Limpopo rivers, which have large sediment cones adjacent to their mouths.

Description: The past 20,000 yr have witnessed tremendous climatic changes, a glacial maximum at about 18,000 yr BP and a climatic optimum centred on about 6,000 yr BP, both of which mark extreme situations for the Quaternary. This paper attempts to show that active sand dunes were extensive 18,000 yr ago. Conversely, it seems that sand dunes were generally dormant 6,000 yr ago. Thus the former textbook concept1,2 of an arid climatic optimum and a pluvially active glacial maximum is reversed.

Description: In studies in underwater acoustics,geophysics, and geology, the relations between soundvelocity and density allow assignment of approximate values of density to sediment and rock layers of the earth’s crust and mantle, given a seismicmeasurement of velocity. In the past, single curves of velocity versus density represented all sediment and rock types. A large amount of recent data from the Deep Sea Drilling Project (DSDP), and reflection and refraction measurements of soundvelocity, allow construction of separate velocity–density curves for the principal marine sediment and rock types. The paper uses carefully selected data from laboratory and i n s i t umeasurements to present empirical sound velocity–density relations (in the form of regression curves and equations) in terrigenous silt clays, turbidites, and shale, in calcareous materials (sediments, chalk, and limestone), and in siliceous materials (sediments, porcelanite, and chert); a published curve for DSDP basalts is included. Speculative curves are presented for composite sections of basalt and sediments. These velocity–density relations, with seismicmeasurements of velocity, should be useful in assigning approximate densities to sea‐floor sediment and rock layers for studies in marine geophysics, and in forming geoacoustic models of the sea floor for underwater acoustic studies.

Description: Five separate exposures of oceanic basalts were dredged in the vicinity of the Peru-Chile Trench between 9° and 27°S latitude. Each dredge is dominated by abundant pillow basalts. Approximately ten of the most unaltered, glassy and fine-grained samples were selected for detailed chemical and petrographic analyses from each dredge area. All basalts recovered in the Peru-Chile Trench are olivine and quartz-normative tholeiites that are believed to have formed at the now extinct Galapagos Rise 30–50 m.y. ago. Detailed chemical analyses of the basalts, including major and selected trace and rare earth elements, indicate that considerable compositional variability exists both within each of the dredged areas as well as between areas. Most of the inherent chemical variability observed within particular basement sections appears consistent with the concept of temporal evolution of magma bodies at a former spreading center by shallow-level fractional crystallization involving primarily plagioclase and olivine. In contrast, important chemical differences between the dredged areas suggest compositional heterogeneities in the mantle source regions. Our results indicate that although shallow-level fractionation has brought about large changes in composition of basalts in each area, compositional trends are distinct and appear to reflect original mantle-derived compositional differences.

Description: Thirty-four ash layers of Pleistocene and Pliocene age from DSDP Site 192, northwestern Pacific Ocean, have been subjected to detailed chemical and optical study to evaluate: (1) the chemical and optical variability in glass shards from deep-sea ash layers, and (2) secondary changes brought about by prolonged exposure to seawater. Glass shards from approximately half of the ash layers studied were found to have uniform compositions which approach the precision of the microprobe chemical analyses, whereas the remainder are compositionally diverse (e.g., SiO2, variations of 5–15% among shards from the same ash layer) and appear to be the eruptive products of compositionally zoned magma chambers. Optical studies of glass shards confirm the absence of devitrification or the formation of pervasive secondary alteration products. By contrast, chemical studies suggest that the glass shards have experienced progressive hydration with possible minor ion exchange of K, Mg, Ca and Si. The hydration occurs rapidly and leads to a rather uniform water content of 4.5–5% after several hundred thousands of years exposure to seawater. Step-wise heating dehydration experiments, optical effects, and published'oxygen isotope studies indicate that the water of hydration is incorporated uniformly within the glass. Systematic chemical differences between electron microprobe analyses of glass shard interiors and corresponding bulk chemical study by atomic absorption lead us to postulate that glass shard margins have undergone a minor chemical exchange with major cations in seawater. They have gained 0.10–0.20 wt. % K20, MgO, and CaO while losing a corresponding amount of Si2O. Although the glass shards from DSDP Site 192 are hydrated and may have experienced subtle, surficial ion exchange, we stress that they are the most chemically representative samples available of magmas that were explosively erupted from volcanic arcs.