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.

Shell preservation of Limacina inflata in surface sediments of the Central and South Atlantic

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inflata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass configurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inflata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inflata per sample were found). This is most probably due to extensive "metabolic" aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inflata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between different water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived fine aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble surficial calcium carbonate data and delta13C and carbonate ion concentration profiles in the water column. Moreover, preservation of L. inflata within AAIW and UCDW improves significantly to the north, whereas carbonate corrosiveness diminishes due to increased mixing of AAIW and UNADW. The close relationship between LDX values and aragonite contents in the sediments shows much promise for the quantification of the aragonite loss under the influence of different water masses. LDX failure and uncertainties may be attributed to (1) aragonite dissolution due to bottom water corrosiveness, (2) aragonite dissolution due to additional CO2 release into the bottom water by the degradation of organic matter based on an enhanced supply of organic matter into the sediment, (3) variations in the distribution of L. inflata and hence a lack of supply into the sediment, (4) dilution of the sediments and hence a lack of tests of L. inflata, or (5) redeposition of sediment particles.

(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.

Age model and stable isotopes of sediment cores from north-central Chile and south-central Chile

Terrigenous sediment supply, marine transport, and depositional processes along tectonically active margins are key to decoding turbidite successions as potential archives of climatic and seismic forcings. Sequence stratigraphic models predict coarse-grained sediment delivery to deep-marine sites mainly during sea-level fall and lowstand. Marine siliciclastic deposition during transgressions and highstands has been attributed to sustained connectivity between terrigenous sources and marine sinks facilitated by narrow shelves. To decipher the controls on Holocene highstand turbidite deposition, we analyzed 12 sediment cores from spatially discrete, coeval turbidite systems along the Chile margin (29° - 40°S) with changing climatic and geomorphic characteristics but uniform changes in sea level. Sediment cores from intraslope basins in north-central Chile (29° - 33°S) offshore a narrow to absent shelf record a shut-off of turbidite deposition during the Holocene due to postglacial aridification. In contrast, core sites in south-central Chile (36° - 40°S) offshore a wide shelf record frequent turbidite deposition during highstand conditions. Two core sites are linked to the Biobío river-canyon system and receive sediment directly from the river mouth. However, intraslope basins are not connected via canyons to fluvial systems but yield even higher turbidite frequencies. High sediment supply combined with a wide shelf and an undercurrent moving sediment toward the shelf edge appear to control Holocene turbidite sedimentation and distribution. Shelf undercurrents may play an important role in lateral sediment transport and supply to the deep sea and need to be accounted for in sediment-mass balances.

Global compilation of benthic data sets I

Approaches to quantify the organic carbon accumulation on a global scale generally do not consider the small-scale variability of sedimentary and oceanographic boundary conditions along continental margins. In this study, we present a new approach to regionalize the total organic carbon (TOC) content in surface sediments (<5 cm sediment depth). It is based on a compilation of more than 5500 single measurements from various sources. Global TOC distribution was determined by the application of a combined qualitative and quantitative-geostatistical method. Overall, 33 benthic TOC-based provinces were defined and used to process the global distribution pattern of the TOC content in surface sediments in a 1°x1° grid resolution. Regional dependencies of data points within each single province are expressed by modeled semi-variograms. Measured and estimated TOC values show good correlation, emphasizing the reasonable applicability of the method. The accumulation of organic carbon in marine surface sediments is a key parameter in the control of mineralization processes and the material exchange between the sediment and the ocean water. Our approach will help to improve global budgets of nutrient and carbon cycles.

Annotated record of the detailed examination of ferromanganese concretions from the Svalbard shelf in the Barents Sea

Ferromanganese concretions from ten stations in the Barents Sea have been analysed for 24 elements. The deposits occur as discoidal and flat concretions and as coatings, in the latter case on lithified or detrital material or as extensive pavements on the Svalbard shelf. The concretions are compositionally similar to Baltic concretions but differ considerably from deep-ocean nodules, particularly in Cu, Ni and Co contents. Statistical analyses reveal distinct correlations between Mn, Na, Ba, Ni and Cu; the Mn-rich coatings showed enrichment of Mo, Zn and possibly Co in a Mn-phase. The iron phase holds high concretions of P and As. Two iron-rich concretions with high contents of P, Ca, Sr, Y, Yb and La were found east and northeast of Spitsbergen Banken, probably indicating upwelling of nutrient-rich, cold polar water along the Svalbard shelf.

Follow-up of the sponge and asteroid populations at the station Larsen A south (PS69/724-1 and PS77/253-1), Antarctic Peninsula, between 2007 and 2011

Over 30% of the Antarctic continental shelf is permanently covered by floating ice shelves, providing aphotic conditions for a depauperate fauna sustained by laterally advected food. In much of the remaining Antarctic shallows (<300 m depth), seasonal sea-ice melting allows a patchy primary production supporting rich megabenthic communities dominated by glass sponges (Porifera, Hexactinellida). The catastrophic collapse of ice shelves due to rapid regional warming along the Antarctic Peninsula in recent decades has exposed over 23,000 km**2 of seafloor to local primary production. The response of the benthos to this unprecedented flux of food is, however, still unknown. In 2007, 12 years after disintegration of the Larsen A ice shelf, a first biological survey interpreted the presence of hexactinellids as remnants of a former under-ice fauna with deep-sea characteristics. Four years later, we revisited the original transect, finding 2- and 3-fold increases in glass sponge biomass and abundance, respectively, after only two favorable growth periods. Our findings, along with other long-term studies, suggest that Antarctic hexactinellids, locked in arrested growth for decades, may undergo boom-and-bust cycles, allowing them to quickly colonize new habitats. The cues triggering growth and reproduction in Antarctic glass sponges remain enigmatic.

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

The surface layer of bottom sediments on the Barents Sea shelf has an irregular but generally very low abundance of diatoms. Tests of species belonging to present-day diatom flora were absent in nearly half of samples; their abundance was only a few shells per gram of dry sediment in 30% of the samples, it was up to 100 shells per gram in 9% of the samples, and was in thousands of shells per gram in only 13% of the samples. The lowest abundances of diatom shells were found in sediments of the eastern and northeastern parts of the sea owing to unfavorable sedimentation conditions and deficiency of dissolved silica in water. But distribution of diatom species on the surface of bottom sediments is strictly consistent with their present-day ranges. About 30% of the samples contained re-deposited Cretaceous and Paleogene diatoms indicating that bottom sediments have largely formed by scouring and re-deposition of underlying material.

(Table 2) Age determination of sediment cores from the continental shelf of Mac Roberston Land, East Antarctica

Geochemical records are presented for five sediment cores from basins on the continental shelf of Mac. Robertson Land, East Antarctica. The cores contain 2-4 m thick sequences of hemipelagic, siliceous mud and ooze (SMO) deposited under seasonally open marine conditions. The inner and middle shelf SMO sequences are massive dark olive green material, whereas the outer shelf SMO sequences are dark olive material interspersed with light olive green layers ~1-10 cm thick. The biogenic material is dominated by marine diatoms including Fragilariopsis curta, Fragilariopsis cylindrus, and Chaetoceros spp. in the dark-colored SMO and Corethron criophilum in the light-colored layers. Radiocarbon dates suggest that the cores provide continuous accumulation records extending from < 1 kyr before present (B.P.) back as far as 4-15 kyr B.P., with estimated accumulation rates of 0.07-5 mm/yr. The three core records from the middle and outer shelf suggest six episodes of increased accumulation of biogenic material at ~5.5 kyr B.P. (all three cores), 1, 2, and 6.2 kyr B.P. (two of the three cores), and 3.8 and 10.8 kyr B.P. (one core), most of which coincide with Corethron layers. We interpret these features as the result of enhanced diatom production over the outer shelf, possibly related to climatic warm periods. The absence of such features in the inner shelf core records is thought to reflect a relatively constant level of seasonal diatom production in adjacent waters maintained by a coastal polynya.

Last glacial maximum sediment record of the southern Weddell Sea shelf

Abstract: The history of grounded ice-sheet extent on the southern Weddell Sea shelf during the Last Glacial Maximum (LGM) and the timing of post-LGM ice-sheet retreat are poorly constrained. Several glaciological models reconstructed widespread grounding and major thickening of the Antarctic Ice Sheet in the Weddell Sea sector at the LGM. In contrast, recently published onshore data and modelling results concluded only very limited LGM-thickening of glaciers and ice streams feeding into the modern Filchner and Ronne ice shelves. These studies concluded that during the LGM ice shelves rather than grounded ice covered the Filchner and Ronne troughs, two deep palaeo-ice stream troughs eroded into the southern Weddell Sea shelf. Here we review previously published and unpublished marine geophysical and geological data from the southern Weddell Sea shelf. The stratigraphy and geometry of reflectors in acoustic sub-bottom profiles are similar to those from other West Antarctic palaeo-ice stream troughs, where grounded ice had advanced to the shelf break at the LGM. Numerous cores from the southern Weddell Sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments. These data sets give evidence that grounded ice had advanced across the shelf during the past, thereby grounding in even the deepest parts of the Filchner and Ronne troughs. Radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited, but indicate that the ice grounding occurred at the LGM and that ice retreat started before ~15.1 corrected 14C kyrs before present (BP) on the outer shelf and before ~7.7 corrected 14C kyrs BP on the inner shelf, which is broadly synchronous with ice retreat in other Antarctic sectors. The apparent mismatch between the ice-sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles (similar to those of "ice plains") that had advanced through Filchner Trough and Ronne Trough at the LGM. Considering the global sea-level lowstand of ~130 metres below present, a low surface slope of the expanded LGM-ice sheet in the southern Weddell Sea can reconcile grounding-line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland. This scenario implies that ice-sheet growth in the Weddell Sea sector during the LGM and ice-sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea-level fluctuations during these times.

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

The sediments of 14 box cores and 7 gravity cores, mainly taken directly in front of the Filchner(-Ronne) ice shelf northwest of Berkner Island (Weddell Sea), allowed to distinguish six sediment types. On the one hand,the retreat of the at first grounded and then floated ice from the last glacial maximum is documented. On the other hand,the sediments give an insight into extensive Holocene sediment deposition and remobilization northwest of Berkner Island. The ortho till was deposited directly by the grounded ice sheet and is lacking any marine influence. After floating of the ice shelf, partly very weIl stratified, partly unstratified, non-bioturbated paratill is deposited beneath the ice shelf. Lack of IRD-content in the paratill immediately above the orthotill indicates freezing at the bottom of the ice, at least for a short period after the ice became afloat. The orthotill and paratill contain small amounts of fragmented Tertiary diatoms, which allow the conclusion, that glacial-marine sediments in the accumulation area of the Ronne ice shelf will be eroded and later deposited by ice in the investigation area. Starting of bioturbation and therefore change in sedimentation from paratill to bioturbated paratill,is caused by the retreat of the ice shelf to its actual position. Isostatic uplift of the sea-bed after the Ice Age causes minor water depths with higher current velocities. The fine-fraction is eroding and mean particle-size will increase. Maybe, also isostatic uplift is responsible for repeated great advances of the floated ice shelf as shown in an erosional horizon in some cores containing bioturbated paratill. Postglacial sediment-thicknesses exceed 3 m. Assuming floating of the ice 15.000 YBP, accumulation rates reach nearly 20cm/lOOO years. Following the theories about sediment input in front of wide ice shelves, this was not expected. In the shallower water depths of Berkner Bank, the oscillations of the ice shelf are recorded in the sediments. Sorting and redistribution by high current velocities from beneath the ice up to the calving line, lead to the deposition of the weIl to very weIl sorted sandy till. In front of the calving line the finer fraction will settle down. Remobilization is possible by bioturbation and increasing current-velocity. According to the intensity of mixing of the sandy till with the fine fraction, modified till or muddy till results.