Sedimentological, mineralogical, biogeochemical and micropaleontological parameters on sediment core PS69/849-2, Burton Basin, MacRobertson Shelf, East Antarctica

A multi-proxy study including sedimentological, mineralogical, biogeochemical and micropaleontological methods was conducted on sediment core PS69/849-2 retrieved from Burton Basin, MacRobertson Shelf, East Antarctica. The goal of this study was to depict the deglacial and Holocene environmental history of the MacRobertson Land-Prydz Bay region. A special focus was put on the timing of ice-sheet retreat and the variability of bottom-water formation due to sea ice formation through the Holocene. Results from site PS69/849-2 provide the first paleo-environmental record of Holocene variations in bottom-water production probably associated to the Cape Darnley polynya, which is the second largest polynya in the Antarctic. Methods included end-member modeling of laser-derived high-resolution grain size data to reconstruct the depositional regimes and bottom-water activity. The provenance of current-derived and ice-transported material was reconstructed using clay-mineral and heavy-mineral analysis. Conclusions on biogenic production were drawn by determination of biogenic opal and total organic carbon. It was found that the ice shelf front started to retreat from the site around 12.8 ka BP. This coincides with results from other records in Prydz Bay and suggests warming during the early Holocene optimum next to global sea level rise as the main trigger. Ice-rafted debris was then supplied to the site until 5.5 cal. ka BP, when Holocene global sea level rise stabilized and glacial isostatic rebound on MacRobertson Land commenced. Throughout the Holocene, three episodes of enhanced bottom-water activity probably due to elevated brine rejection in Cape Darnley polynya occured between 11.5 and 9 cal. ka BP, 5.6 and 4.5 cal. ka BP and since 1.5 cal. ka BP. These periods are related to shifts from warmer to cooler conditions at the end of Holocene warm periods, in particular the early Holocene optimum, the mid-Holocene warm period and at the beginning of the neoglacial. In contrast, between 7.7 and 6.7 cal. ka BP, brine rejection shut down, maybe owed to warm conditions and pronounced open-water intervals.

(Table 1) Grain size composition of recent sediments from the eastern Barents Sea shelf

Based on materials on geomorphology, hydrology, lithology, and sedimentation dynamics obtained during cruises of the P.P. Shirshov Institute of Oceanology in the Barents Sea, the author prepared a number of charts on content of the main particle size facies in shelf surface sediments, as well as a chart of lithologic types of sediments in the Barents Sea. Factors of sedimentation control and basic features of distribution of sedimentary material over its bottom area are taken into consideration.

Age determination of sediment cores from the shelf off Mauretania, West Africa

The 60 km wide shelf off Mauritania is cut by several submarine canyons. Its water-circulation is controlled by the cool Canary current and upwelling. Its Recent sediments show faunal associations remarkably related to the grain size distribution which in water depths between 40 and 80 m is strongly influenced by reworking of older coarse sand or sandstone. In this depth range a mixed biofacies originating from Pleistocene and Recent material is encountered. The present lateral faunistic and sedimentological facies change, including horizons of mixed provenance, can be recognized in vertical sequences taken by vibro-coring. This correlation combined with 14C-datations on molluscs enable the reconstruction of the history of the last glacial regression and transgression. Due to the arid climate, the emerging calcareous shelf sediments are indurated and, therefore, protected from subaerial and submarine erosion. During low sea level eolian sand migrates over the shelf, but only about 1/10 of this material remains there and is later incorporated into the sandy shelf sediments. The calculated average rate of total sedimentation during Holocene is 15 cm, and the production rate of carbonate is 5 cm/1000 years.

(Table 1) Conditions of vertical near-bottom profiling and the main parameters of the benthic boundary layer in several regions of the Black Sea shelf

Measurements of 14 vertical profiles of currents and hydrological parameters in the near-bottom layer with depth resolution of 0.1 m were carried out in several regions of the Black Sea shelf, at five points over the continental slope, and in three deep water regions. The upper boundary of the benthic boundary layer (BBL) was reliably determined at a point at distance from 5-7 to 35-40 m from the bottom where the gradients of density and current velocity changed. Experimental data obtained were used to determine the coefficient of bottom friction, friction velocity, coefficients of vertical diffusion of momentum and density, and vertical fluxes of temperature and salinity in the BBL.

Distribution of molluscs on the Atlantic continental shelf off Southern Spanish Sahara, West Africa

Two hundred and seventy five mollusc species from the continental shelf off Southern Spanish Sahara (depth: 32-60 m) were identified. Their distribution pattern is strongly influenced by the nature of the bottom (firm substrate, shelter, stability of sediment) rather than other factors at that depth interval. This faunal assemblage shows great affinity to the Mediterranean and Lusitanian faunas, and comprises only few (22 %) exclusively Senegalese and species living south of Senegal.

Sedimentology on cores off Filchner Shelf Ice, Weddell Sea, Antarctica

Sediment sampling with box corer and gravity corer was conducted along a profile parallel to the Filchner/Rønne Ice Shelf, from 48° to 61°W. Twenty-two sampling locations were determined after evaluation of 12 and 3.5 kHz sub-bottom profiling records. The sediment retrievals show a wide diversity, varying from very well sorted pure sands in the SE of the profile to heavily glacially influenced, pebbly muds close to the foot of the Antarctic Peninsula. In the middle part of the profile mainly soft sediments of muddy to sandy muds were found which were partially influenced by glacially derived dropstones or accumulations of pebble-sized material. The striking changes of surface sediments (marine to glacial) observed along the profile led to an attempt to investigate the concurrence of marine and glacial depositional processes controlling the accumulation of these recent sediments.

Sedimentology of surface sediments from the continental shelf and slope off West Africa (Table 1)

Samples from sutface sediments of the shell, continental slope, and adjacent deep sea regions off West Africa between 27° N and 15° N were investigated with respect to grain size distribution of the total samples, sand contents of the acid insoluble residues, carbonate content of the total samples, and the separate grain size fractions, specific surfaces, colours and mineralogical composition of the clay fractions. The grain size distributions of the total samples of the sediments of the shelf and the continental slope off Spanish-Sahara are controlled mainly by biogenic components. The supply of terrigeneous material in this area is very low. At deeper parts of the continental slope and in the deep sea areas, the relative amounts of carbonate minerals in the sediments are considerably reduced. The prevailing sand contents of the upper slope changes into clay dominance. On the shelf of Mauritania - represented by profiles extending down to 200 m water depth - the grain size is also controlled mainly by biogenic carbonates. Nevertheless, the admixture of fossil silicate components is important, too. The southern parts of the area is investigated are located in a region influenced by sediments of the Senegal River, which especially control the contents of silt and clay. The silicate sands, predominately of quartz, are fossil and form a mixed sediment with younger deposits. The carbonate contents of the different grain size fractions are formed either by sedimentation of carbonate and silicate particles of the respective grain size or by autochtonous disintegration of coarser sediment particles, as shown by the occurence of Mg-rich calcite and especially aragonite in the clay sized fraction. In the northern parts of the area investigated, which have very minute terrigeneous supply, the latter mechanism is the dominant factor, controlling the carbonate contents of the fine grain sized fractions. In the vicinity of the mouth of the Senegal the carbonate contents are influenced by extremely high dilution with terrigencous silicates. Mg-rich calcite and aragonite are produced preferentially in shallow slope and shelf areas up to 500 m of water depth. The specific surfaces of the carbonate-free clay fractions indicate that the clay fractions of the shelfareas with little terrigenous supply consits of relatively coarser particles. Very fine particles are removed and transported towards the deep sea. Lateral differentiation of this kind was not observed in the area off Senegal. The high surface areas, characterizing the clay fractions of this region, are thought to be due to high montmorillonite contents as was found for deep seas sediments. The mineralogical composition of the clay fraction from the southern parts of the area is characterized by high kaolinite and montmorillonite contents, while in the northern illite is predominating. At least two types of montmorillionites are present: in areas influenced by the Senegal mostly one type was found, which could swell to 17; on the shelves and slopes of the other regions the montmorillonite-group is represented by a montmorillonite-mica-type mixed-layer mineral. A "glauconite", found in the sand fraction, which had very similar properties to those of the montmorillonite-mica mixed-layer, is believed to be the source of this mixed-layer-type mineral. Palygorskite is present in all samples out of range of the Senegal supply. It may be an indicator of eolian transported material. The occurence of rich palygorskit deposits in the arid hinterlands emphasizes the terrestrial origin.

(Table 1) Heavy mineral content in sediments of the southern Mediterranean Ridge, Nile Cone and Egyptian Shelf

Petrographic analysis of Quaternary terrigenous sand layers in eastern Mediterranean cores reveals distinct mineralogical differences between the Egyptian Shelf-Nile Cone region and the southern part of the Mediterranean Ridge. A compositionally and texturally immature suite in Ridge cores, mixed with a Nile-derived assemblage, identifies a fresh non-recycled mineral component derived from proximal igneous and metamorphic surface or near-surface exposures, probably in the south-central Ridge area rather than from distal African sources. The presence of such basement terrains would be consistent with a compressive thrust-belt origin for this part of the Mediterranean Ridge.

Sedimentology of cores from the shelf of Lazarev Sea, East Antarctica

Distinct facies types, classified in radiocarbon-dated sediments from the shelf of the Lazarev Sea, East Antarctica, reveal a detailed history of processes that have controlled sedimentation during the deglaciation over the last 10,000 yr. The ice retreat on this part of the Antarctic shelf started 9500 yr BP, marked by the deposition of laminated sediments, deposited from a floating ice shelf. These laminites, which occur on top of diamictons laid down from a grounded ice sheet, are the basal sediments of the postglacial sequence. The intensity of the Antarctic Coastal Current (ACC), directed by shelf morphology, controlled sedimentation of the postglacial facies. A residual glaciomarine sediment with the fine fraction winnowed by strong currents developed from 9000-8000 yr BP in the western part of the investigation area and from 9000-5000 yr BP in the eastern part, closer to the prominent 'Fenno Deep' trough. Current velocities apparently decreased between 8000 and 2000 yr BP due to a deflection of the ACC by advancing ice tongues to the east of the investigation area during the 'Hypsithermal'. This led to a deposition of fine-grained sediments, and clay mineralogy suggests a continental source, possibly near the grounding line of the Nivl Ice Shelf, rather than a winnowing of sediments near the shelf break or advection from deeper water. Current velocities intensified after 2000 yr BP, removed fine material from these sediments and led to a relict sediment, consisting of coarse bryozoan and molluscan debris.