Southeastern Atlantic Ocean Research Articles

Seasonal Variations in the Southern Hemisphere Storm Tracks and Jet Streams as Revealed in a Reanalysis Dataset

Abstract Regional characteristics of the climatological seasonal variations in Southern Hemisphere (SH) storm tracks are examined based on a reanalysis dataset. When differences in vertical structure between the subpolar and subtropical jet streams (SPJ and STJ, respectively) are considered, the regional characteristics can be interpreted reasonably well from a potential vorticity perspective of baroclinic eddy growth and downstream development of a baroclinic wave packet. Eddy activity in the upper and lower tropospheres is strongest in the core region of the SPJ over the eastern South Atlantic and Indian Ocean throughout the year, even in austral winter when the intense STJ forms over the Indian Ocean and South Pacific. Showing its eddy-driven nature, the SPJ accompanies the strong surface westerlies along a well-defined baroclinic zone above an intense oceanic frontal zone. In this core region of the storm track, low-level eddy activity is strongly correlated with the local near-surface baroclinicity, ...

A palynological reconstruction of the Agulhas Retroflection (South Atlantic Ocean) during the Late Quaternary

Here, we reconstruct the varying influence of the Agulhas Current (AgC), the South Atlantic Current and the Antarctic Circumpolar Current on the Agulhas Retroflection (AgR) in the eastern South Atlantic Ocean for the last 160,000 years on the basis of the dinoflagellate cysts, pollen and spores present in a sediment core (GeoB 3603-2) from the southeastern Cape Basin offshore South Africa, where the Agulhas Current enters the Atlantic Ocean. Our analyses reveal strong orbital forcing on the heat exchange between the Indian Ocean and the South Atlantic Ocean during the Late Quaternary. Maxima in local productivity appear to be primarily related to a strengthening of the ocean circulation as a result of the high seasonal contrast during precession maxima. During precession minima, seasonal contrast was low and stratified, oligo- to mesotrophic conditions prevailed, notably when these minima coincided with the glacial terminations. The clear presence of periodicities on a sub-Milankovitch scale as well as modulations of the primary frequencies demonstrate that the Agulhas Retroflection furthermore is modulated substantially by complex interactions between the subtropical, ‘precession-driven’, climate and ocean circulation systems, and the southern, ‘obliquity-driven’, high latitudes.

Further constraints on the African superplume structure

It is well established that there is a large-scale low velocity structure in the lowermost mantle beneath Africa, extending from the Southeastern Atlantic Ocean to the Southwestern Indian Ocean with a volume greater than 10 billion km 3 (>7000 km long, 1000 km across and 1200 km high) [Earth Planet. Sci. Lett. 206 (2003) 119]. This low velocity structure is often called the African superplume. Various studies also require sharp boundaries for the plume. However, as for its height and shear velocity reduction, there has been some controversy, especially concerning the velocities at the core–mantle–boundary (CMB). Here, we present an assortment of phases involving S diff, SKS, S and S cS with both vertical and horizontal paths sampling a 2D corridor through the structure. Travel time and waveform modeling of these seismic phases argues for a model with shear velocity reduction of approximately 3% within the superplume (which is basically a 200 km thick layer low velocity layer beneath the Southern Atlantic Ocean, and a 1200 km high structure beneath South Africa), and against a model of a substantially reduced low velocity layer (up to 10%, 300 km) beneath the superplume. We also analyzed P diff and the differential times of P cP–P and compared them with S diff and S cS–S observations along the same great circle paths. The P-velocity is not very anomalous, at most −0.5%, much smaller than −1% as expected from a thermal anomaly with −3% lower S-velocity [Geophys. Res. Lett. 27 (2000) 421], thus again arguing for a chemical origin which was suggested from the modeling of African superplume sharp sides [Science 296 (2002) 1850].

Upper-layer circulation in the eastern Equatorial and South Atlantic Ocean in January–March 1995

The upper-layer circulation in the eastern basin of the South Atlantic was studied from hydrographic and direct velocity measurements along WOCE lines A11, A13 and A14. A13 and A14 provide quasi-meridional samplings of the equatorial, subequatorial and subtropical circulation regimes. A13 was carried out along the African coast at about 600 km offshore from it, and A14 along the nominal longitude 9°W. A11 intersects the Cape Basin between 46°S in the west and 30°S in the east. Transport estimates were derived from direct velocity measurements and a box inverse model. In the equatorial eastern Atlantic, the Equatorial Undercurrent (EUC) transport decreases from 25×10 6 m 3 s −1 at 9°W to 13×10 6 m 3 s −1 at 5°E. Re-circulations of the EUC into the northern and equatorial branches of the South Equatorial Current (SEC) are evidenced at 5°E and quantified. In the tropical Atlantic, we estimate 7.5×10 6 and 4.2×10 6 m 3 s −1 for the transports at 9°W of the South Equatorial Undercurrent (SEUC) and South Equatorial Countercurrent (SECC), respectively. Both the SEUC and SECC extend vertically down to intermediate depths and contribute to the northern limb of the Angola Gyre. The Angola Current transport is estimated to be 16±5×10 6 m 3 s −1 for σ 1<32.1 at 13°S. South of the Angola Gyre, the transports show an apparent cyclonic circulation, developed mostly at the intermediate level. The water mass properties suggest that it is, at least partially, a re-circulation of the Benguela Current. Further south and for the subtropical gyre, we estimate 10±5×10 6 m 3 s −1 for the transport of the South Atlantic Current across 9°W and 28±4×10 6 m 3 s −1 for the transport of the Benguela Current at 10°E for σ 1<32.1.

On the deep water circulation of the eastern South Atlantic Ocean

The main flow patterns and transports of North Atlantic Deep Water (NADW) in the eastern South Atlantic and round southern Africa were inferred from two quasi-meridional WOCE hydrographic lines at 9°W and 5°E nominal longitudes, and from segments of other sections in the Atlantic-to-Indian Oceans transition region. An export of 11±4 Sv of NADW from the Atlantic was estimated near southern Africa inshore of the Subtropical Front. Most of this flow can be traced back across the Cape Basin to passages south of 28°S in Walvis Ridge, and farther upstream to an entry in the eastern basin at 20–25°S. Other transfers of NADW from the western boundary current to the eastern boundary were confirmed near the equator and at 22°S near the Namib Col of Walvis Ridge, yet a cyclonic recirculation of the former in the Angola Basin, and the weakness of the latter (4±2 Sv), make their direct contributions to the export of NADW minor ones. As another consequence, the estimated transport of the deep southward boundary current against the western continental slope of Africa shows important latitudinal variations (12±9 Sv at 7°30′S, 3±5 Sv at 22°30′S, 11±4 Sv at 35°S), with even a suggestion of its reversal in the northern Cape Basin. To the southwest of Cape Town, the deep boundary current is influenced by the temporal variability of the Agulhas retroflection region, and most of it separates from the slope to enter the Indian Ocean in the deep return flow of the Agulhas Current. Only a 2–3 Sv remnant against the continental slope was estimated at 30°E to the east of the Agulhas Plateau.

Habitat selection of two gobies ( Microgobius gulosus, Gobiosoma robustum): influence of structural complexity, competitive interactions, and presence of a predator

Herein I compare the relative importance of preference for structurally complex habitat against avoidance of competitors and predators in two benthic fishes common in the Gulf of Mexico. The code goby Gobiosoma robustum Ginsburg and clown goby Microgobius gulosus (Girard) are common, ecologically similar fishes found throughout the Gulf of Mexico and in the southeastern Atlantic Ocean. In Florida Bay, these fishes exhibit habitat partitioning: G. robustum is most abundant in seagrass-dominated areas while M. gulosus is most abundant in sparsely vegetated habitats. In a small-scale field survey, I documented the microhabitat use of these species where their distributions overlap. In a series of laboratory experiments, I presented each species with structured (artificial seagrass) versus nonstructured (bare sand) habitats and measured their frequency of choosing either habitat type. I then examined the use of structured versus nonstructured habitats when the two species were placed together in a mixed group. Finally, I placed a predator ( Opsanus beta) in the experimental aquaria to determine how its presence influenced habitat selection. In the field, G. robustum was more abundant in seagrass and M. gulosus was more abundant in bare mud. In the laboratory, both species selected grass over sand in allopatry. However, in sympatry, M. gulosus occupied sand more often when paired with G. robustum than when alone. G. robustum appears to directly influence the habitat choice of M. gulosus: It seems that M. gulosus is pushed out of the structured habitat that is the preferred habitat of G. robustum. Thus, competition appears to modify the habitat selection of these species when they occur in sympatry. Additionally, the presence of the toadfish was a sufficient stimulus to provoke both M. gulosus and G. robustum to increase their selection for sand (compared to single-species treatments). Distribution patterns of M. gulosus and G. robustum likely result from a synthesis of various biotic and abiotic filters, including physiological tolerances to environmental factors, dispersal ability of larvae, and availability of food. Selection for structural complexity, competition, and presence of predators may further define the resulting pattern of distribution observed in the field.

Reconstructing pathways of aeolian pollen transport to the marine sediments along the coastline of SW Africa

The distribution of pollen in marine sediments is used to reconstruct pathways of terrigenous input to the oceans and provides a record of vegetation change on adjacent continents. The wind transport routes of aeolian pollen is comprehensively illustrated by clusters of trajectories. Isobaric, 4-day backward trajectories are calculated using the modelled wind-field of ECHAM3, and are clustered on a seasonal basis to estimate the main pathways of aeolian particles to sites of marine cores in the south-eastern Atlantic. Trajectories and clusters based on the modelled wind-field of the Last Glacial Maximum hardly differ from those of the present-day. Trajectory clusters show three regional, and two seasonal patterns, determining the pathways of aeolian pollen transport into the south-eastern Atlantic ocean. Mainly, transport out of the continent occurs during austral fall and winter, when easterly and south-easterly winds prevail. South of 25°S, winds blow mostly from the west and southwest, and aeolian terrestrial input is very low. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in marine surface sediments and the occurrence of the source plants on the adjacent continent. The northern Angola Basin receives pollen and spores from the Congolian and Zambezian forests mainly through river discharge. The Zambezian vegetation zone is the main source area for wind-blown pollen in sediments of the Angola Basin, while the semi-desert and desert areas are the main sources for pollen in sediments of the Walvis Basin and on the Walvis Ridge. A transect of six marine pollen records along the south-western African coast indicates considerable changes in the vegetation of southern Africa between glacial and interglacial periods. Important changes in the vegetation are the decline of forests in equatorial Africa and the north of southern Africa and a northward shift of winter rain vegetation along the western escarpment.

Eocene to Miocene terrigenous inputs and export production: geochemical evidence from ODP Leg 177, Site 1090

Changes in ocean circulation and climate during the Cenozoic led to the development of the Antarctic Circum-polar Current (ACC) and permanent Antarctic ice sheets. However, the timing of the opening of the Drake Passage and the establishment of the ACC is poorly constrained. We present geochemical proxies of terrigenous inputs and export production at a single site in the southeastern Atlantic Ocean that suggest the Drake Passage had opened to intermediate and deep water by 32.8 Ma. Two separate styles of sedimentation and geochemical composition of terrigenous material are observed, with the change in sediment geochemical characteristics occurring at 32.8 Ma. Middle to late Eocene records are highly variable, while latest Eocene to early Miocene records exhibit cyclic variations in elemental records and terrigenous material. Based on Al/Ti ratios, metal sources change from continental crust to oceanic crust sources at the Eocene/Oligocene transition, which we suggest reflects a change in deep-water circulation. Phosphorus/metal ratios indicate that there are two distinct intervals of enhanced export production, one in the middle Eocene and one throughout the Eocene/Oligocene transition. Elevated Al/Ti ratios, greater than any lithic source, in the middle Eocene provide evidence of particulate scavenging and thus increased export production. Barium ratios further support changes in productivity in the middle Eocene and at the Eocene/Oligocene transition. Permanent changes in the Ba concentration record in the early Oligocene further support a change in deep-water circulation due to the opening and deepening of the Drake Passage at 32.8 Ma.

Late Pleistocene deep-water circulation in the subantarctic eastern Atlantic

We present three new benthic foraminiferal δ 13C, δ 18O, and total organic carbon time series from the eastern Atlantic sector of the Southern Ocean between 41°S and 47°S. The measured glacial δ 13C values belong to the lowest hitherto reported. We demonstrate a coincidence between depleted late Holocene (LH) δ 13C values and positions of sites relative to ocean surface productivity. A correction of +0.3 to +0.4 [‰ VPDB] for a productivity-induced depletion of Last Glacial Maximum (LGM) benthic δ 13C values of these cores is suggested. The new data are compiled with published data from 13 sediment cores from the eastern Atlantic Ocean between 19°S and 47°S, and the regional deep and bottom water circulation is reconstructed for LH (4–0 ka) and LGM (22–16 ka) times. This extends earlier eastern Atlantic-wide synoptic reconstructions which suffered from the lack of data south of 20°S. A conceptual model of LGM deep-water circulation is discussed that, after correction of southernmost cores below the Antarctic Circumpolar Current (ACC) for a productivity-induced artifact, suggests a reduced formation of both North Atlantic Deep Water in the northern Atlantic and bottom water in the southwestern Weddell Sea. This reduction was compensated for by the formation of deep water in the zone of extended winter sea-ice coverage at the northern rim of the Weddell Sea, where air–sea gas exchange was reduced. This shift from LGM deep-water formation in the region south of the ACC to Holocene bottom water formation in the southwestern Weddell Sea, can explain lower preformed δ 13C DIC values of glacial circumantarctic deep water of approximately 0.3‰ to 0.4‰. Our reconstruction brings Atlantic and Southern Ocean δ 13C and Cd/Ca data into better agreement, but is in conflict, however, with a scenario of an essentially unchanged thermohaline deep circulation on a global scale. Benthic δ 18O-derived LGM bottom water temperatures, by 1.9°C and 0.3°C lower than during the LH at deepest southern and shallowest northern sites, respectively, agree with the here proposed reconstruction of deep-water circulation in the eastern South Atlantic Ocean.

Reconstruction of primary productivity from the barium contents in surface sediments of the South Atlantic Ocean

The aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity. For this purpose, 190 surface sediment samples from the South Atlantic Ocean were analysed for their barium and aluminium concentrations. Biogenic barium is estimated by subtracting the calculated terrigenous barium (obtained from the terrigenous Ba/Al ratio and the amount of Al in the sample) from the total Ba content in the sample. Based on the accumulation rates of biogenic barium, export production is estimated using three different algorithms proposed by [Paleoceanography 7 (1992) 163; Global Biogeochem. Cycles 9 (1995) 289; Geomar. Report 38 (1995) 105]. Primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters. Only the primary productions calculated on the basis of the algorithm of [Paleoceanography 7 (1992) 163] yield productivity values comparable to those existing in ocean surface waters. This study further reveals that it is not sufficient to use a constant, generally applicable organic carbon/biogenic barium ratio, as is postulated by [Global Biogeochem. Cycles 9 (1995) 289]. This ratio has to be assessed regionally. For the sediments of the Cape Basin in the eastern South Atlantic Ocean, a new algorithm is developed which gives plausible primary productivities for the overlying surface waters.

Genetic divergence and reproductive isolation between Anisakis brevispiculata and Anisakis physeteris (Nematoda: Anisakidae)s

In order to assess the taxonomic status of Anisakis brevispiculata Dollfus, 1966 population samples of this taxon from central and south-eastern Atlantic ocean were compared at 22 enzymatic loci with samples belonging to Anisakis physeteris Baylis, 1923 from the Mediterranean sea and central-eastern Atlantic ocean. Very low interpopulational genetic divergence was observed both within A. brevispiculata (average D Nei=0.008) and within A. physeteris ( D Nei=0.009) despite the geographic distance among the samples, indicating high levels of gene flow in both taxa. On the other hand, the average genetic distance between A. brevispiculata and A. physeteris was found to be D Nei=0.80, a value generally observed between well differentiated congeneric species. The reproductive isolation between A. brevispiculata and A. physeteris is indicated by the following observations: (1) no F 1 hybrids or recombinant genotypes were until now observed; and (2) the two Anisakis species do not seem to share their definitive hosts. The main definitive host of A. brevispiculata is the pygmy sperm whale ( Kogia breviceps), while for A. physeteris it is the sperm whale ( Physeter catodon). Only adult males differ slightly in spicule length, while females and larval stages are not differentiated morphologically. Both A. brevispiculata and A.physeteris show a type II larva. The correct recognition of A. brevispiculata from A. physeteris and from other Anisakis species studied, in either sexes and at any life stage, is made easy by allozyme markers (e.g. Icdh, Gapdh, Sod-1, Np, Aat-2, Adk-2, fEst-2, PepB, PepC-2, Mpi). Diagnostic keys, which can be used for routine identification in the field of these Anisakis worms, based on genetic markers, are given.

Translation, decay and splitting of Agulhas rings in the southeastern Atlantic Ocean

All Agulhas rings that were spawned at the Agulhas retroflection between 1993 and 1996 (a total of 21 rings) have been monitored using TOPEX/Poseidon satellite altimetry and followed as they moved through the southeastern Atlantic Ocean, decayed, interacted with bottom topography and each other, or dissipated completely. Rings preferentially crossed the Walvis Ridge at its deepest parts. After having crossed this ridge they have lower translational speeds, and their decay rate decreases markedly. Half the decay of long‐lived rings takes place in the first 5 months of their lifetimes. In addition to the strong decay of rings in the Cape Basin, about one third of the observed rings do not seem to leave this region at all but totally disintegrate here. The interaction of rings with bottom topography, in particular with the Vema Seamount, is shown frequently to cause splitting of rings. This will enhance mixing of the rings' Indian Ocean water into that of the southern Atlantic. This localized mixing may well provide a considerable source of warm and salty Indian Ocean water into the Atlantic overturning circulation.

Complex Shear Wave Velocity Structure Imaged Beneath Africa and Iceland.

A model of three-dimensional shear wave velocity variations in the mantle reveals a tilted low velocity anomaly extending from the core-mantle boundary (CMB) region beneath the southeastern Atlantic Ocean into the upper mantle beneath eastern Africa. This anomaly suggests that Cenozoic flood basalt volcanism in the Afar region and active rifting beneath the East African Rift is linked to an extensive thermal anomaly at the CMB more than 45 degrees away. In contrast, a low velocity anomaly beneath Iceland is confined to the upper mantle.

Three Agulhas rings observed during the Benguela Current Experiment

A field program to study the circulation of the Benguela Current and its extension into the southeastern Atlantic Ocean has completed the survey and instrument deployment phase. We report here new observations of three Agulhas rings north and west of Cape Town, South Africa. Three mesoscale anticyclonic rings initially identified by means of TOPEX/POSEIDON altimetry were surveyed with expendable bathythermographs, conductivity‐temperature‐depth‐oxygen profiles, direct current measurements from a lowered acoustic Doppler current profiler, a hull‐mounted acoustic Doppler current profiler, and satellite‐tracked surface drifters. Characteristics of the rings are presented, and their origins are discussed. Two are typical Agulhas rings surveyed at different times after their generation; the third Agulhas ring has an anomalous water mass structure whose most likely origin is the Subtropical Front.

FIRST FINDING OF THE PELAGIC CRAB PLANES MARINUS (DECAPODA: GRAPSIDAE) IN THE SOUTHWESTERN ATLANTIC

ABSTRACT Most grapsid crabs are benthic species that live in intertidal or subtidal marine or brackish water. Columbus crabs, Planes spp., which live associated with living or inanimate floating objects in the open ocean are an exception. A group of crabs was found on flotsam stranded on a beach near Mar Chiquita (37°45'S, 57°26'W), Argentina, and identified as Planes marinus. This is the first report of this oceanic species in the southwestern Atlantic Ocean. The crab was previously collected in the Pacific Ocean, from British Columbia to New Zealand, and at St. Helena Island (southeastern Atlantic Ocean). Surface oceanic currents are postulated as a possible transport mechanism between the highly isolated collection sites. This paper includes morphometrical data that allowed the identification of the species and several biological characteristics (sex ratio, relative growth and sexual dimorphism, incidence of limb autotomy and regeneration, epibiosis, and fecundity). Both P. marinus and most littoral marine grapsids have strong sexual dimorphism (male chelae are larger), a high incidence of autotomy, and a similar egg size and number (the latter related to female size). This information suggests that the life history and behavior of P. marinus, a typical pelagic crab, is similar to its benthic littoral relatives.

Living planktonic foraminifera in the eastern South Atlantic during spring: Indicators of water masses, upwelling and the Congo (Zaire) River plume

Planktonic foraminifera were collected from pumped surface-water samples and net tows over the upper 150 m of the water column in the eastern South Atlantic Ocean during October and November 1989. Of the 24 recorded species, 12 occur abundantly and form 6 foraminiferal assemblages. The assemblages correlate with the physico-chemical properties of the 6 vapor surface-water masses. The correlation of species with low salinity is due to properties co-variant with salinity. At several locations, high concentrations of planktonic foraminifera were observed caused by frontal mixing or reproduction. The G. sacculifer-G. ruber white assemblage marks the South Equatorial Under Current. minor contribution of G. menardii and N. dutertrei to this assemblage reflects the subsurface Equatorial Under Current. The N. dutertrei-G. siphonifera assemblage reflects the South Equatorial Counter Current and Equatorial Under Current. The N. pachyderma-G. bulloides assemblage reflects the coastal upwelling over the shelf and the filaments of mixed oceanic and upwelled water in the coastal of the Benguela Current (BC). The Angola-Benguela Front, at the interface of the warm currents and the BC, is reflected by the G. inflata-G. bulloides assemblage. An unusual coincidence of G. sacculifer-G. bulloides in one assemblage results from the interaction of the subtropical gyre and the oceanic branch of the BC. The G. rubber pink- N. dutertrei assemblage is attributed to the warm low-saline plume in front of the Congo (Zaire) River.

The influence of the Benguela upwelling system on Namibia's marine biodiversity

Namibia's marine environment falls within the Benguela system, an eastern boundary current upwelling system in the south eastern Atlantic Ocean. Conditions within much of this environment change continuously as a consequence of the upwelling of nutrient-rich water into the surface zone. In addition, irregular anomalies in temperature, oxygen concentration and salinity occur, particularly in shelf waters. These fluctuations, which are inherent in the functioning of the Benguela system, tend to favour the persistence of few, generalist species, while at the same time high productivity supports large abundances. This trend is evident in all the major marine habitats off Namibia, where diversity is often lower than in comparable habitats in the southern Benguela system off the west coast of South Africa. Namibia's marine environment is considered 'relatively pristine', although threats to biodiversity are posed by both natural and anthropogenic phenomena.

Evidence for Decreased NADW Flux During Glacial Stages 2 and 4 from Nd Isotopes in Southern Ocean Sediments

It has been suggested that North Atlantic Deep Water (NADW) production was reduced during the Last Glacial Maximum thereby altering the distribution of heat around the globe, and the partitioning of CO2 between the ocean and atmosphere (Broecker and Denton, 1989). Although the thermohaline circulation concept has great appeal in explaining much of the evidence for climate changes that have taken place in the past, currently employed proxies have produced conflicting results regarding the changes in NADW production in the late Quaternary. In the Southern Oceart ~5C measurements on benthic foraminifera have been interpreted to indicate a switching off of NADW during glacial times. However, this interpretation is in conflict with interpretations based on Cd/Ca measurements of benthic foraminifera (Boyle, 1992). In support of the Cd/Ca results, 23 ipaj230Th data from core top and late glacial marine sediments have been interpreted to indicate that export of deep water formed in the North Atlantic was similar to modem conditions (Yu et al., 1996). The conflicting interpretations based on commonly employed proxies highlights the need for new tracers. We report here on work in progress to validate and apply the Nd isotope composition of marine authigenic precipitates as a proxy for variation in deep water mass composition. The Nd isotope composition of deep seawater varies systematically with water mass properties. The unradiogenic character of Nd in the North Atlantic reflects the ancient shield rocks surrounding the area of NADW formation. The very young rocks surrounding the Pacific Ocean impart a more radiogenic signature to Pacific waters. In the Atlantic Ocean the Nd isotope composition of today's deep water appears to be the product of mixing of Southern Component Waters and Northern Component waters. The observation that the Nd isotope composition of present-day water conforms to water mass properties is an important line of evidence in evaluating the potential for Nd isotopes as a tracer of water mass mixing in the past. However, it is also necessary that there be some extractable phase that sequesters seawater Nd in marine sediment in order to make use of this observation. Published work has demonstrated that ferromanganese precipitates, specifically crusts and nodules, generally record the approximate eNd of the bottom water mass that bathes them, with only a few exceptions (e.g. Albarbde and Goldstein, 1992). Although ferromanganese crusts have been demonstrated to provide a reliable measure of bottom water Nd composition, and recent studies have used these substrates to examine changes in seawater composition over time, crusts are limited in their availability and growth rate, generally integrating over l0 thousand to 1 million years per subsample. Accordingly, we are in the process of testing the application of leaching methods to extract the ferromanganese phase from marine sediments in order to examine secular variation in Nd isotope composition of seawater at glacial to interglacial timescales. A preliminary test of leaches from a suite of core tops has yielded positive results that are entirely overlapping with seawater measurements. Core RCll-83 (41.36~ 9.48~ 4718 m) is located in the Cape Basin of the southeastern Atlantic Ocean. Extensive stable isotope data have been reported for this core (Charles et al., 1996). Charles et al. (1996) have interpreted the large variations in the ~5C to reflect a reduced contribution of NADW to this region during Stage 2 and Stage 4. RC11-83 also records a significant glacial to interglacial contrast in the Nd isotopic composition of the ferromanganese component (Fig. 1). A core top sample and a sample from isotope Stage 3 provide Nd isotope ratios similar to nearby water measurements. In contrast, values much closer to those of the modem Pacific are found in two samples measured from Stage 2. Two measurements come from samples that were taken from transitional intervals based on stable isotope measurements, and these samples also provide intermediate Nd isotope values.

Recent benthic foraminifera from the eastern South Atlantic Ocean: Dependence on food supply and water masses

Sixty surface sediment samples from the eastern South Atlantic Ocean including the Walvis Ridge, the Angola and Cape basins, and the Southwest African continental margin were analysed for their benthic foraminiferal content to unravel faunal distribution patterns and ecological preferences. Live (stained with Rose Bengal) and dead faunas were counted separately and then each grouped by Q-mode principal component analysis into seven principal faunal end-members. Then, multiple regression technique was used to correlate Recent assemblages with available environmental variables and to finally differentiate between four principal groups of environmental agents acting upon the generation of benthic foraminiferal assemblages: (1) seasonality of food supply and organic carbon flux rates, together with oxygen content in the pore and bottom waters; (2) lateral advection of deep-water masses; (3) bottom water carbonate corrosiveness; and (4) energetic state at the benthic boundary layer and grain size composition of the substrate. Food supply and corresponding dissolved oxygen contents in the pore and bottom waters turned out to be the most important factors which control the distribution pattern of the Recent benthic foraminifera. At the continental margin, in the zone of coastal upwelling and its mixing area, benthic foraminiferal assemblages are dominated by stenobathic high-productivity faunas, characterized by elevated standing stocks, low diversities and a large number of endobenthic living species. At the continental shelf and upper continental slope the live assemblages are characterized by Rectuvigerina cylindrica, Uvigerina peregrina s.1., Uvigerina auberiana and Rhizammina spp. while the dead assemblages are characterized by Cassidulina laevigata, Bolivina dilatata, Bulimina costata and B. mexicana. At the lower continental slope strong influence of high organic matter fluxes on the species composition is restricted to the area off the Cunene river mouth, where the live assemblage is dominated by Uvigerina peregrina s.1., the corresponding dead assemblage by Melonis barleeanum and M. zaandamae. In the adjacent areas of the lower continental slope the biocoenosis is characterized by Reophax bilocularis, and Epistominella exigua which becomes dominant in the corresponding dead assemblage. At the Walvis Ridge and in the abyssal Angola and Cape basins, where organic matter fluxes are low and highly seasonal, benthic foraminiferal assemblages reflect both the oligotrophic situation and the deep and bottom water mass configuration. The top and flanks of the Walvis Ridge are inhabited by the Rhizammina, Psammosphaera and R. bilocularis live assemblages, the corresponding dead assemblages are dominated by G. subglobosa on the ridge top and E. exigua on the flanks. Within the highly diverse E. exigua dead assemblage several associated epibenthic species coincide with the core of NADW between about 1600 and 3700 m water depth. These species include Osangularia culter, Cibicidoides kullenbergi, Melonis pompilioides, Bolivinita pseudothalmanni and Bulimina alazanensis. The assemblages of the abyssal Cape and Angola basins are characterized by Nuttallides umbonifer and a high proportion of agglutinated species. These species are adapted to very low organic matter fluxes and a carbonate corrosive environment.

Multiple episodes of aridity in southern Africa since the last interglacial period

There is generally a dearth of evidence of the nature of Quaternary climate change within desert systems, which has limited previous interpretations of past environmental change at low latitudes. The Last Glacial Maximum has previously been identified as the peak of Late Quaternary aridity, when desert systems expanded to five times their present extent1,2,3, and low-latitude aridity has been described for previous glaciations4. But little evidence has been derived directly for large desert basins, particularly southern Africa. Here we report new chronological (optical dating) evidence of arid episodes recorded in aeolian sediments from the Mega Kalahari sand sea. Episodic aeolian activity is recorded at the northeastern desert margin, whereas more sustained activity is evident from the southwestern desert core. Several significant arid events are apparent since the last interglacial period, with dune-building (arid) phases at ∼95–115, 41–46, 20–26 and 9–16 kyr before present. Existing atmospheric general circulation model simulations and independent palaeoclimate data indicate that the changes in aridity are related to changes in the northeast–southwest summer rainfall gradient, which are in turn related to sea surface temperatures in the southeastern Atlantic Ocean.