Spatial and temporal patterns in the demersal fish community on the shelf and upper slope regions of the Gulf of Alaska

We analyzed data from National Marine Fisheries Service bottom trawl surveys carried out triennially from 1984 to 1996 in the Gulf of Alaska (GOA). The continental shelf and upper slope (0–500 m) of the GOA support a rich demersal fish fauna dominated by arrowtooth flounder (Atheresthes stomias), walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus), Pacific halibut (Hippoglossus stenolepis), and Pacific Ocean perch (Sebastes alutus). Average catch per unit of effort (CPUE) of all groundfish species combined increased with depth and had a significant peak near the shelf break at 150–200 m. Species richness and diversity had significant peaks at 200–300 m. The western GOA was characterized by higher CPUEs and lower species richness and diversity than the eastern GOA. Highest CPUEs were observed in Shelikof Strait, along the shelf break and upper slope south of Kodiak Island, and on the banks and in the gullies northeast of Kodiak Island. Significant differences in total CPUE among surveys suggest a 40% increase in total groundfish biomass between 1984 and 1996. A multivariate analysis of the CPUE of 72 groundfish taxa revealed strong gradients in species composition with depth and from east to west, and a weak but significant trend in species composition over time. The trend over time was associated with increases in the frequency of occurrence and CPUE of at least eight taxa, including skates (Rajidae), capelin (Mallotus villosus), three flatfish species, and Pacific Ocean perch, and decreases in frequency of occurrence and CPUE of several sculpin (Myoxocephalus spp.) species. Results are discussed in terms of spatial and temporal patterns in productivity and in the context of their ecological and management implications.

Benthic foraminiferal distribution on the southeastern Brazilian shelf and upper slope

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Benthic foraminiferal distribution on the southeastern Brazilian shelf and upper slope

Foraminiferal data were obtained from 66 samples of box cores on the southeastern Brazilian upper margin (between 23.8A degrees-25.9A degrees S and 42.8A degrees-46.13A degrees W) to evaluate the benthic foraminiferal fauna distribution and its relation to some selected abiotic parameters. We focused on areas with different primary production regimes on the southern Brazilian margin, which is generally considered as an oligotrophic region. The total density (D), richness (R), mean diversity (H) over bar`, average living depth (ALD(X) ) and percentages of specimens of different microhabitats (epifauna, shallow infauna, intermediate infauna and deep infauna) were analyzed. The dominant species identified were Uvigerina spp., Globocassidulina subglobosa, Bulimina marginata, Adercotryma wrighti, Islandiella norcrossi, Rhizammina spp. and Brizalina sp.. We also established a set of mathematical functions for analyzing the vertical foraminiferal distribution patterns, providing a quantitative tool that allows correlating the microfaunal density distributions with abiotic factors. In general, the cores that fit with pure exponential decaying functions were related to the oligotrophic conditions prevalent on the Brazilian margin and to the flow of the Brazilian Current (BC). Different foraminiferal responses were identified in cores located in higher productivity zones, such as the northern and the southern region of the study area, where high percentages of infauna were encountered in these cores, and the functions used to fit these profiles differ appreciably from a pure exponential function, as a response of the significant living fauna in deeper layers of the sediment. One of the main factors supporting the different foraminiferal assemblage responses may be related to the differences in primary productivity of the water column and, consequently, in the estimated carbon flux to the sea floor. Nevertheless, also bottom water velocities, substrate type and water depth need to be considered.

Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro-Espinho (N Portugal)

The sedimentary unconsolidated cover of the Aveiro-Espinho continental shelf and upper slope (NW Portugal) records a complex interplay of processes including wave energy and currents, fluvial input, sediment transport alongshore and cross-shelf, geological and oceanographic processes and sediment sources and sinks. In order to study this record, a set of surface sediment samples was studied. Sediment grain size and composition, as well as the mineralogical composition (by XRD) of the fine (<63 mu m) and clay (<2 mu m) fractions and benthic microfaunal (foraminifera) data were analysed. Cluster analysis applied to the sedimentological data (grain size, sediment composition and mineralogy) allowed the establishment of three main zones corresponding to the: inner-, mid- and outer-shelf/upper slope. On the inner-shelf, the sedimentary coverture is composed of siliciclastic fine to very fine sand, essentially comprising modern (immature) terrigenous particles. The sediment grain size, as well as mineralogical and microfaunal composition, denote the high energetic conditions of this sector in which the alongshore transport of sand is predominantly southward and occurs mostly during the spring-summer oceanographic regime, when the main river providing sediments to this area, the River Douro, undergoes periods of drought. This effect may emphasize the erosive character of this coastal sector at present, since the Ria de Aveiro provides the shelf with few sediments. On the mid-shelf, an alongshore siliciclastic band of coarse sand and gravel can be found between the 40 m and 60 m isobaths. This gravelly deposit includes relic sediments deposited during lower sea-level stands. This structure stays on the surface due to the high bottom energy, which promotes the remobilization of the fine-grained sediments, and/or events of sediments bypassing. Benthic foraminifera density and "Benthic Foraminifera High Productivity" (BFHP) proxy values are in general low, which is consistent with the overall small supply of organic matter to the oceanic bottom in the inner- and mid-shelf. However, the Ria de Aveiro outflow, which delivers organic matter to the shelf, leaves its imprint mainly on the mid-shelf, identifiable by the increase in foraminifera density and BFHP values in front of the lagoon mouth. The higher values of BFHP along the 100 m isobath trace the present position of an oceanic thermal front whose situation may have changed in the last 3/5 ka BP. This zone marks a clear difference in the density, diversity and composition of benthic foraminifera assemblages. Here, in addition, sediment composition changes significantly, giving rise to carbonate-rich fine to medium sand in the deeper sector. The low bottom energy and the small sedimentation rate of the outer-shelf contributed to the preservation of a discontinuous carbonate-rich gravel band, between the 100 m and 140 m isobaths, also related to paleo-littorals, following the transgression that has occurred since the Last Glacial Maximum. The winter oceanographic regime favours the transport of fine grained sediments to the outer-shelf and upper slope. The inner- and mid-shelf, however, have low amounts of this kind of sediment and the Cretacic carbonated complexes Pontal da Galega and Pontal da Cartola, rocky outcrops located at the mid- and outer-shelf, act as morphological barriers to the cross-shelf transport of sediments. Thus a reduced sedimentation rate occurs in these deeper sectors, as indicated by the lower abundance of detrital minerals, which is compensated for the high sedimentary content of biogenic carbonates. The relatively high BFHP and Shannon Index values indicate water column stratification, high supply of organic matter and environmental stability, which provide favourable conditions for a diversified benthic fauna to flourish. These conditions also encourage authigenic chemical changes, favourable to glauconite formation, as well as illite and kaolinite degradation. Benthic foraminifera and clay mineral assemblages also reveal the effect of the internal waves pushing upward, and downslope losses of the sediments on the outer-shelf and upper slope.

Natural radionuclides distribution in the shelf and upper slope off southeast Brazil

In recent decades, Oceanography has been using a variety of radionuclides as tracers to understand the ocean dynamic processes, handling and disposal of sediments of seabed. In this context, the determination of natural radionuclides distributions (238U, 232Th and 40K) has been carried out with sediments samples from the shelf and upper slope off Southeast Brazil using a gamma spectrometry technique. The samples were sliced into strata of 2 cm, dried, ground and properly packed to be analysed. The concentration of activities was performed in a hyperpure Ge detector with a resolution of 1,9 keV for the peak of 1332,3 keV of 60Co, model GEM50P by EGG&ORTEC. The study area is located between latitudes 28°40'S and 23°00'S and extends from Cabo Frio (RJ) to Cabo de Santa Marta Grande (SC). The activity concentrations varied from 0,6 to 52,8 BqKg-1 for 238U, from 1,6 to 50,9BqKg-1 for 232Th and from 65,4 to 873,3 BqKg-1 for 40K. From these results it is possible to establish a correlation between the depositional area dynamics and the samples size parameters

Temperature and d18O measurements from different ODP Sites at the shelf and upper slope of the Peru Margin

The first experimentally determined temperature dependent oxygen-18 fractionation factor between dolomite and water at low temperatures [Vasconcelos et al. 1995 doi:10.1130/G20992.1] allows now the precise calculation of temperatures during early diagenetic dolomite precipitation. We use d18O values of early diagenetic dolomite beds sampled during ODP Legs 112 and 201 on the Peru continental margin (Sites 1227, 1228 and 1229) [Meister et al. 2007, doi:10.1111/j.1365-3091.2007.00870.x] to calculate paleo-porewater temperatures at the time of dolomite precipitation. We assumed unaltered seawater d18O values in the porewater, which is supported by d18O values of the modern porewater presented in this study. The dolomite layers in the Pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 °C lower than today. Since Sites 1228 and 1229 are located at 150 and 250 m below sealevel, respectively, their paleo-porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands. Thus, Pleistocene dolomite layers in the Peru Continental margin probably formed during glacial times. This finding is consistent with a model for dolomite precipitation in the Peru Margin recently discussed by Meister et al. [Meister et al. 2007, doi:10.1111/j.1365-3091.2007.00870.x], where dolomite forms episodically at the sulphate methane interface. It was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column, but dolomite layers may only form when the sulphate-methane interface stays at a fixed depth for a sufficient amount of time. We hypothesize that the sulphate-methane interface persists within TOC-rich interglacial sediments, while this zone is buried by TOC-poor sedimentation during glacial times. Thus, the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations. A similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a Milankovitch type pattern observed in the geological record, such as in the Miocene Monterey Formation.

Benthic foraminiferal distribution on the southeastern Brazilian shelf and upper slope

Foraminiferal data were obtained from 66 samples of box cores on the southeastern Brazilian upper margin (between 23.8A degrees-25.9A degrees S and 42.8A degrees-46.13A degrees W) to evaluate the benthic foraminiferal fauna distribution and its relation to some selected abiotic parameters. We focused on areas with different primary production regimes on the southern Brazilian margin, which is generally considered as an oligotrophic region. The total density (D), richness (R), mean diversity (H) over bar', average living depth (ALD(X) ) and percentages of specimens of different microhabitats (epifauna, shallow infauna, intermediate infauna and deep infauna) were analyzed. The dominant species identified were Uvigerina spp., Globocassidulina subglobosa, Bulimina marginata, Adercotryma wrighti, Islandiella norcrossi, Rhizammina spp. and Brizalina sp.. We also established a set of mathematical functions for analyzing the vertical foraminiferal distribution patterns, providing a quantitative tool that allows correlating the microfaunal density distributions with abiotic factors. In general, the cores that fit with pure exponential decaying functions were related to the oligotrophic conditions prevalent on the Brazilian margin and to the flow of the Brazilian Current (BC). Different foraminiferal responses were identified in cores located in higher productivity zones, such as the northern and the southern region of the study area, where high percentages of infauna were encountered in these cores, and the functions used to fit these profiles differ appreciably from a pure exponential function, as a response of the significant living fauna in deeper layers of the sediment. One of the main factors supporting the different foraminiferal assemblage responses may be related to the differences in primary productivity of the water column and, consequently, in the estimated carbon flux to the sea floor. Nevertheless, also bottom water velocities, substrate type and water depth need to be considered.

Morphology and surface sedimentation on the upper and middle continental slope off Uruguay

This paper presents for the first time a morphological and surface sediment characterization of the Uruguayan outer continental shelf and slope. The study is based on a high-resolution coverage using hydrographical, geomorphological and sedimentological sampling and several textural and productivity proxies. Along slope terraces and an important canyon system characterizes continental slope morphology, indicating that across- and down-slope sedimentary processes control large-scale sedimentation. Terraces represent the prolongation of the Argentinean Contouritic Depositional System that vanishes in the study area, presumably as a result of the dynamic of the Brazil-Malvinas confluence. Canyons incised in the upper slope are likely related to low-stand sea level conditions. At the outer shelf and shallow upper slope (170-250 m depth), off-shelf sand transport is inferred from the distribution of relict sand and reworked biogenic gravel. In the upper continental slope, the northern region is characterized by an erosive environment controlled by a steep slope and the southward flowing Brazil current. In the south, a depositional environment is enhanced by the presence of a gentler slope and seaward incised canyons and is mainly controlled by hemipelagic processes associated with nutrient-rich Sub-Antarctic Waters (SAW), by its confluence with South Atlantic Central Waters (SACW) and by the Rio de la Plata’s (RdlP) influence. Additionally, within the upper slope, the occurrence of igneous-metamorphic cobbles and pebbles in canyon and mound lag deposits suggests the influence of glacial fluvial discharge and/or iceberg transport processes. In the middle slope, sedimentation is controlled by thermohaline-induced deep-water bottom currents. The decreasing influence of the erosive Antarctic Intermediate Water (AAIW) is evident in a northward diminution in grain size. The variety of transport and sedimentary processes identified reflect the control of the Brazil-Malvinas confluence zone and the Rio de la Plata’s discharge.

Chartered Trawling on the Brazilian Slope

Commercial trawling on the Atlantic slope areas off Brazil intensified in the late 1990’s owing to the expansion of coastal trawling areas and the operations of a chartered foreign fleet. Between 2000 and 2003, 59 fishing trips conducted by 10 chartered trawlers were intensely monitored by observers and satellite vessel monitoring systems, totaling 9,069 tows and 30,085.2 trawling hours. Fishing operations occurred in northern, northeastern, southeastern, and southern sectors of the Brazilian coast in 60–1,173 m depths. Total retained and processed catch were 8,074.6 t and 6,479.8 t, respectively. Argentine hake, Merluccius hubbsi; and Argentine shortfin squid, Illex argentinus, were the primary species taken contributing to 41.1% and 28.6% of the overall catch, respectively. The silver John dory, Zenopsis conchifera; monkfish, Lophius gastrophysus; Brazilian codling, Urophycis mystacea; and the black grouper, Epinephelus nigritus, composed 23% of total processed catch, and the remaining 7.2% was composed of deep-sea shrimps (family Aristeidae) and other teleosts and elasmobranches. The occupation of slope areas included an early exploratory phase, followed by directed phases of the upper slope (300–500 m), aiming principally at the Argentine hake, and the lower slope (>700 m), targeting valuable concentrations of deep-sea aristeid shrimps. The role of chartering for slope trawling development was critically addressed. We conclude that chartered vessels were efficient explorers and were particularly important in areas not available to the technologically limited national fleet. Because the charters were market-oriented and had elevated profit demands, however, those vessels quickly turned from exploration to exploitation and competed with national trawlers in shallower areas and produced significant impacts on Brazil’s modest deep-sea resources.

Currents controlling sedimentation, deposits recording palaeo-hydrodynamics – lessons from the continental shelf-slope system off SE South America

The continental margin off SE South America hosts one of the world’s most energetic hydrodynamic regimes but also the second largest drainage system of the continent. Both, the ocean current system as well as the fluvial runoff are strongly controlled by the atmospheric circulation modes over the region. The distribution pattern of particular types of sediments on shelf and slope and the long-term built-up of depositional elements within the overall margin architecture are, thus, the product of both, seasonal to millennial variability as well as long-term environmental trends. This talk presents how the combination of different methodological approaches can be used to obtain a comprehensive picture of the variability of a shelf and upper-slope hydrodynamic system during Holocene times. The particular methods applied are: (a) Margin-wide stratigraphic information to elucidate the role of sea level for the oceanographic and sedimentary systems since the last glacial maximum; (b) Palaeoceanographic sediment proxies combined with palaeo-temperature indicating isotopes of bivalve shells to trace lateral shifts in the coastal oceanography (particularly of the shelf front) during the Holocene; (c) Neodymium isotopes to identify the shelf sediment transport routes resulting from the current regime; (d) Sedimentological/geochemical data to show the efficient mechanism of sand export from the shelf to the open ocean; (e) Diatom assemblages and sediment element distributions indicating palaeo-salinity and the changing marine influence to illustrate the Plata runoff history. Sea level has not only controlled the overall configuration of the shelf but also the position of the main sediment routes from the continent towards the ocean. The shelf front has shifted frequently since the last glacial times probably resulting from both, changes in the Westerly Winds intensity and in the shelf width itself. Remarkable is a southward shift of this front during the past two centuries possibly related to anthropogenic influences on the atmosphere. The oceanographic regime with its prominent hydrographic boundaries led to a clear separation of sedimentary provinces since shelf drowning. It is especially the shelf front which enhances shelf sediment export through a continuous high sand supply to the uppermost slope. Finally, the Plata River does not continuously provide sediment to the shelf but shows significant climate-related changes in discharge during the past centuries. Starting from these findings, three major fields of research should, in general, be further developed in future: (i) The immediate interaction of the hydrodynamic and sedimentary systems to close the gaps between deposit information and modern oceanographic dynamics; (ii) Material budget calculations for the marginal ocean system in terms of material fluxes, storage/retention capacities, and critical thresholds; (iii) The role of human activity on the atmospheric, oceanographic and solid material systems to unravel natural vs. anthropogenic effects and feedback mechanisms

Benthic foraminifera of surface sediments from the continental slope off Sierra Leone, North-West Africa

On "Meteor" cruise 30 (1973) 22 piston-cores were collected off Sierra Leone from water-depths between about 5000 m (Sierra Leone Basin) and 500 m (upper continental slope) with the objective to study the sediment composition and age as well as processes of sedimentation on the continental slope in a tropical humid region. Granulometric analysis and determinations of the carbonate contents of the sediment samples were carried out, as well as qualitative and quantitative analysis of the components of the grain size fractions > 63 µm and of the planktonic and benthonic foraminifera > 160 µm. Presently, the cold Canary Current influences the composition of the planktonic foraminifera within the northwestern area of investigation (profile A), whereas the planktonic fauna of the eastern area (profile C) seems to be truly tropical. In all Quaternary sediments from the continental slope off Sierra Leone, species of Globorotalia are less abundant than in truly pelagic sediments. For that reason, the zonation of the Pleistocene sediments based on the presence or absence of Globorotalia cultrata does not always agree with the climatic changes reflected in the sediments. Concerning past climates better results can be obtained by using the changes in percentage abundances of Globigerina sp. sp. and Globigerinoides sp. sp. as indicators for cool and warm temperatures. The Tertiary sediments contain a pelagic foraminiferal assemblage. In the Holocene sediments the benthonic foraminifera do not only serve as good paleodepth indicators, but their communities are also restricted to defined water masses, which change their positions in accordance with climatic changes. Thus, Cassidulina carinata in the area of investigation is an excellent indicator for sediments deposited during times, which were cooler than today; this is true for all cores from the continental slope off Sierra Leone independent of water-depth although this species presently abounds at water-depths around 600 m. The cores from the continental rise and from the Sierra Leone Basin (M30-261, M30-146, M30-147) were deposited below the calcium carbonate compensation depth. Only small sections of the cores consist of the original carbonate-free sediments, whereas the main part of the sediment column is redeposited material, rich in foraminifera, which normally live on the upper continental slope, or even on the shelf. From these cores only M30-261 can be subdivided into biostratigraphic zones ranging from zone V to zone Y. In all cores from the middle and upper continental slope of the eastern area of investigation (profile C; KL 230, 209-204) and in cores KL 183 and KL 184 from the northwestern area (profile A) we observed an undisturbed succession of sediments from the biostratigraphic zones X (partly), Y and Z. All cores from the central area (M30-181, M30-182, M30-262 to 264) and M30-187 from the upper slope of profile A show variable hiatuses in the sedimentary record. Locally, high velocity bottom currents were probably responsible for erosion, nondeposition or minimal sedimentation rates. These currents might have been initiated partly by the somewhat exposed position of this part of the continental slope, where the shelf edge bends from a northwest towards an eastern direction, and partly by very young tectonic movements. Fracture zones with vertically displaced fault blocs are frequent at Sierra Leone continental margin. According to seismic measurements by McMaster et al. (1975) the sites of the central area are located on an uplifted fault bloc explaining the reduced sediment rates or erosion. Unlike the central area, the eastern area (profile C) is situated on a downfaulted bloc with high sediment rates. The sediments from the cores of profile B as well as the turbiditic deep-sea sediments were deposited under a higher flow regime; therefore they are coarser than the extremely fine-grained sediments of the cores from profile C. Since the sand fraction (> 63 µm) is mainly composed of foraminifera, besides pteropods and light-colored fecal pellets, the carbonate content increases with the increasing percentage of the coarse grain fraction. Higher concentrations of quartz were only observed in core sections with considerable carbonate dissolution (mainly in the X-Zone), and, in general, in all sediments from the eastern area with higher terrigenous input including larger concentration of mica. Especially during times transitional between glacials and interglacials (or interstadials) the bottom currents were intensified. The percentages of coarse fraction and carbonate increase with increasing current velocities. Calcium carbonate dissolution becomes important in water depths > 3500 m. During cooler times the lysokline is depressed. Light-colored fecal pellets were redeposited from Late Neogene sediments (M30-187, M30-181). In the area of investigation they occur in the Holocene and mainly the Pleistocene sediments of the cores from the northwestern and central area because only here Tertiary sediments have been eroded at the uppermost continental slope. In the central area there are at least two periods of non-sedimentation and/or erosion which can be confined as being (1) not older than middle Pliocene and not younger than zone V and (2) younger than zone W. The local character of the erosion is documented by the fact that a complete Late Quaternary section is present in the cores of the northwestern and eastern area, each within less than 100 km from incomplete cores from the central area.

Distribution of planktic foraminifera in surface sediments and sedimentation rates of sediment cores from the Ibero-Moroccan continental rise and slope

Cores from the Atlantic Ibero-Moroccan continental rise and slope contain fine-grained Late Pleistocene and Holocene sediments. These young sediments cover the continental margin in large lensformed litho- and biostratigraphically well-defined units. The total sedimentation rates range from 4 cm/ 1000 yrs. to 27 cm/1000 yrs. off Portugal and from 6 cm/1000 yrs. to 14 cm/1000 yrs. off Morocco. Only a small proportion of these sediments usually consists of sand-sized particles (>0.063 mm) which are mostly dominated by foraminifera. Both planktonic and benthic foraminifera are much more abundant in Late Pleistocene and Holocene samples from the upper slope in comparison to those from the deeper slope and from the abyssal plains. Total sedimentation rates during cool and warm climatic stages are rather similar for both groups of foraminifera. For example, in Late Holocene sediments 7 x 10**3 benthic and 201 x 10**3 planktonic foraminifera (fraction 0.63 -0.20 mm) per 100 cm**2 and 1000 yrs. are preserved in the Tagus, 10-19 X 10**3 benthic and about 1.3 X 10**6 planktonic foraminifera are preserved in the Seine abyssal plain sediments. Values from the upper slope sediments are higher for benthic foraminifera by a factor of 60 off Portugal and 60 to 70 off Morocco. The values for planktonic ones differ by factors of 6-12 and 6 respectively. These numbers seem to reflect differences in production and preservation. Production rates of planktonic foraminifera generally seem to be somewhat higher during Holocene than during Late Pleistocene, and the rates of benthic foraminifera appear rather higher during Late Pleistocene than during Holocene.

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) Grain size distribution of sediments from core Ak-509, Black Sea continental slope

New results of geomorphological, seismoacoustic, and lithological investigations on the upper continental slope off the Arkhipo-Osipovka Settlement are presented. Here, a large submarine slump was discovered by seismic survey in 1998. The assumed slump body, up to 200 m thick, rises 50-60 m above the valley floor that cuts the slope. Recent semiliquid mud that overlies laminated slope sediments with possible slump deformations flows down in the valley thalweg. Radiocarbon age inversion recorded in a Holocene sediment section of shelf facies recovered from the upper slope points to the gravity dislocation of sediments.