Jacques Giraudeau

Distribution of recent nannofossils beneath the Benguela system : southwest African continental margin

Abstract One hundred and forty eight surface samples recovered from the southwest African continental margin (35°-17°S) have been examined for their nannofossil content. Coccolith abundance is high in sediments from the deep water areas, but declines toward the coast under the main areas of upwelling. Five distinct bathymetric/oceanographic assemblages can be characterized by their nannofossil content: coastal upwelling, Orange Bank, intermediate, southern deep water, and northern deep water-Angola Current. These distinctions are based on the distribution of eight ecologically significant taxa. In order of decreasing abundance they are Gephyrocapsa oceanica, Helicopontosphaera kamptneri, Cyclococcolithina leptopora, Coccolithus pelagicus, Syracosphaera div. sp., Thoracosphaera div. sp., Rhabdosphaera div. sp. and Umbilicosphaera sibogae . Sea-surface temperature and salinity variations, which correlate with the extent and intensity of the main upwelling cells, are thought to be the most important controls on coccolith distribution, although nutrient availability and reduction of surface water transparency may be of local importance. The exclusion of three stratigraphical index species for the Quaternary ( Emiliania huxleyi, Gephyrocapsa caribbeanica , and G. ericsonii ) from the quantitative analysis will permit the future application of these results to paleoceanographic reconstructions of the Benguela system in Pleistocene core sections.

Coccolith evidence for instabilities in surface circulation south of Iceland during Holocene times

Abstract The coccolith-based micropaleontological investigation of an exceptionally thick deep-sea Holocene sediment core was conducted in order to document the pattern and timing of surface circulation changes south of Iceland, over the Gardar contour drift, during the last 12 000 years. Fluctuations in bulk carbonate content at the core location are primarily driven by the coccolith fraction. The observed overall correlation between bottom flow speed, as given by the ‘sortable silt’ mean size index, and the bulk coccolith concentration (abundance/g dry sediment) suggests that the accumulation of this fine carbonate fraction is, as a first order, controlled by processes of sediment redistribution by bottom current. The down-core variations in coccolith assemblage structure (species %) indicate that changes in properties of surface waters south of Iceland occurred as two distinct steps at ca. 11.2 and 6 ka. The onset of North Atlantic Drift water influence over the Gardar Drift after 11.2 ka was associated with excess export flux of coccolith carbonate in the vicinity of an active frontal system. The period from 10 to 6 ka saw the progressive warming of the study area, which culminated between 6 and 7 ka. Cooling of the surface waters after 6 ka, as indicated changes in the relative abundances of the dominant coccolith species, took place in two phases, the present hydrological regime being only established after a last cooling step between 3.5 and 2.8 ka. These long-term reorganisations of the surface hydrology are interpreted as the response of the North Atlantic to the combined force of the solar insolation and the waning Laurentide ice sheet. Millennial-scale perturbations of the surface hydrology are documented by changes in accumulation of the species Emiliania huxleyi . These successive decreases in the export fluxes of E. huxleyi exhibit a distinct millennial pacing, in phase with previously recorded Holocene advection of cool, ice-bearing waters from the Greenland–Iceland seas to the British islands. From a clear analogy with the most recent extreme variation in global ocean climate, we argue that mechanisms responsible for these observed millennial-scale perturbations might be found in long-term modulations of the atmospheric processes associated with the North Atlantic Oscillation (NAO).

Distribution and malformation of living coccolithophores in the northern Benguela upwelling system off Namibia

Abstract The water-column distribution of coccolithophores in the northern Benguela upwelling regime is investigated in a transect of five hydrographic and phytoplankton stations off Walvis Bay, Namibia. Hydrographic conditions indicate that upwelling was not in progress during the sampling period. Coccolithophore standing crops range from 0 to466×10 3 cells/l , the maximum cell density occurring in the shallow-water stations. Dissipation of turbulence and stratification of the system favour the dominance of coccolithophores over diatom and dinoflagellate communities which are normally associated with early stages of the upwelling cycles. Twelve species are identified with Emiliania huxleyi dominating in the nutrient depleted surface layers supporting a high cell density. Living Coccolithus pelagicus are reported, for the first time, in waters of the southern hemisphere. Vertical changes in species dominance are related to differences in nutrient preferences between E. huxleyi and Gephyrocapsa oceanica . Deep layers of the three offshore-most stations support a peculiar community which shows a high degree of skeletal deformation. The presence of these malformed coccolithophores, the first ever reported in South Atlantic surface waters, is not related to in-situ nutrient deficiency, or temperature anomalies. This malformed community could have been transported into the study area within a southward-moving, subsurface tongue of moderately saline tropical waters.

Sea-surface distribution of coccolithophores, diatoms, silicoflagellates and dinoflagellates in the South Atlantic Ocean during the late austral summer 1995

The sea-surface distribution of four selected fossilizable phytoplankton groups (coccolithophores, diatoms, silicoflagellates and dinoflagellates) has been studied along a transect from Cape Town (34°S) to South Sandwich Islands (57°S) during the late austral summer. The observed distribution of these groups shows that their biogeographical distribution is significantly constrained by the water masses and associated frontal systems of the Southern Ocean. Coccolithophores are the dominant group and show cell abundances up to 51×103 cells/l down to 57°S. Three restricted areas are marked by particularly high cell densities: the continental shelf of South Africa, the area between the Sub-Tropical Convergence and the Sub-Antarctic Front, and the southern border of the Antarctic Polar Front, where the highest abundances are recorded (>650×103 cells/l). The species composition of the various assemblages representative of the four groups defines distinct biogeographical zones bounded by marked sea-surface temperature gradients. This biogeographical distribution is confirmed by factor analysis of the coccolithophore (5 factors, 85% of the total variance) and diatom and silicoflagellate (7 factors, 87.5% of the total variance) populations. When compared with the distribution pattern of siliceous fossil assemblages in surface sediments, our data show a more accurate coupling between the various water-masses of the South Atlantic Ocean and the living siliceous population.

Planktonic foraminiferal assemblages in surface sediments from the southwest African continental margin

Abstract The relative abundance and distribution of planktonic foraminifera species on the southwest African continental margin have been determined from 152 surface sediment samples. Of the 23 recorded taxa, only 8 make a significant contribution to the foraminiferal assemblages. Individual species proportions are closely related to the surface water hydrography of the Benguela System and the associated upwelling phenomena. Three foraminiferal assemblages can be distinguished by correlation analysis on the abundances of the major taxa. Species composition of these assemblages range from polar taxa in the upwelling area (left-coiling Neogloboquadrina pachyderma and Globigerina quinqueloba ) to transitional taxa in the offshore oligotrophic environment ( Globorotalia inflata and Globorotalia truncatulinoides ). Globigerina bulloides and right-coiling Neogloboquadrina pachyderma are preferentially distributed in the highly productive intermediate zone of the Benguela System, where upwelled and oligotrophic offshore waters mix. Variability of the hydrological processes (upwelling intensity, strength of the frontal features) may be inferred from frequency variations of individual species, and diversity patterns.

A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa

Abstract The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200–250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.

Spatial dynamics of coccolithophore communities during an upwelling event in the Southern Benguela system

Abstract The spatial dynamics of coccolithophore communities were investigated in the top 100 m of a longitudinal transect off Hondeklip Bay, Southern Benguela region, during a period of active upwelling in late June 1993. This transect crossed five hydrological domains that were identified from the vertical sections of temperature and salinity, namely the upwelling zone, the upwelling front, the intermediate domain, the offshore divergence, and the oceanic region. Total coccolithophore standing crops ranged from less than 10 × 103 cells 1−1 to a maximum of 278 × 103 cells 1-1. Highest values were recorded in the top 25 m in the upwelling zone and at the offshore edge of the upwelling front. Fifty different taxa were identified with Emiliania huxleyi and Syracosphaera epigrosa being the dominant forms in the standing stock. The offshore divergence and upwelling front effectively isolated assemblages with distinct species compositions and diversity trends. The most successful assemblage in terms of both species diversity and standing crop was found in the intermediate domain where oceanic and upwelled water mix. Equally important was the control of the water-masses motion on the vertical distribution of coccolithophore communities. Shallow-dwelling communities thrived in the inshore upwelling and offshore divergence areas, whereas deep-dwelling communities were found in the intermediate domain, where surface waters downwell. No clear relationship was found between the concentrations in major nutrients, and the species diversity and total standing crop values. S. epigrosa was the only recorded species whose absolute abundance showed a positive relationship with the nitrate and phosphate surface-water concentrations.

A high-resolution time-series analyses of particle fluxes in the Northern Benguela coastal upwelling system: carbonate record of changes in biogenic production and particle transfer processes

Fluxes of biogenic particles at the shelf edge off Walvis Bay, Namibia, are investigated from a time-series sediment trap in order to evaluate the response of biogenic production to rapid changes in the dynamics of the upwelling process, as well as the importance of particle transfer processes on the nature of sediments accumulating at a high rate on the Namibian margin. Total mass flux displays a smooth trend of lowered values from the start to the end of the experiment despite evidence, from the SST and wind records, of a variable pattern of upwelling dynamics. Contributions of the various biogenic components (carbonates, opal, organic matter) to the total mass flux were relatively constant throughout the period of trapping, suggesting that changes in upwelling dynamics have no obvious implications on the nature of biogenic particles sedimenting at the trap location. A detailed examination of the carbonate fraction indicates a partitioning of its various contributors (coccolithophores and planktonic foraminifera) in terms of sources and mechanisms of transfer to depth. From these data, it is suggested that the bulk of the biogenic particles sedimenting on the slope is resuspended material from the outer shelf, and conversely that direct input from surface waters does not contribute to a high extent to the particle flux at depth. A highly partitioned, three-dimensional flow field is proposed to explain the observed pattern of particle flux as well as the suggested transfer processes affecting the biogenic components.

Agulhas leakage as a key process in the modes of Quaternary climate changes.

Heat and salt transfer from the Indian Ocean to the Atlantic Ocean (Agulhas leakage) has an important effect on the global thermohaline circulation and climate. The lack of long transfer record prevents elucidation of its role on climate changes throughout the Quaternary. Here, we present a 1,350-ka accumulation rate record of the planktic foraminiferal species Globorotalia menardii. We demonstrate that, according to previous assumptions, the presence and reseeding of this fauna in the subtropical southeast Atlantic was driven by interocean exchange south of Africa. The Agulhas transfer strengthened at glacial ice-volume maxima for every glacial-interglacial transition, with maximum reinforcements organized according to a 400-ka periodicity. The long-term dynamics of Agulhas leakage may have played a crucial role in regulating meridional overturning circulation and global climate changes during the Mid-Brunhes event and the Mid-Pleistocene transition, and could also play an important role in the near future.

Reconstruction of the late-Holocene changes in the Sub-Arctic Front position at the Reykjanes Ridge, north Atlantic

Changes in the dynamics of the North Atlantic subpolar gyre are involved in the modulation of the northward salinity and heat transport in the northern North Atlantic via the North Atlantic Current (NAC). Variations in the strength of the East Greenland Current (EGC) can influence the gyre dynamics by impacting deep convection in the Labrador Sea. Oxygen isotope data of three planktonic foraminiferal species (surface water Neogloboquadrina pachyderma dextral coiling and Globigerina bulloides; thermocline recorder Globorotalia inflata) from a site located close to the present Sub-Arctic Front at the Reykjanes Ridge suggest significant strengthening or shifting of the Sub-Arctic Front throughout the late Holocene. The oxygen isotope based inferences are supported by Mg/Ca-derived temperature reconstructions from Neogloboquadrina pachyderma dextral coiling, alkenone-derived sea surface temperature and other paleoclimatic proxy data. The late-Holocene strengthening/shift of the Sub-Arctic Front appears caused by an increasingly more defined and fresher EGC. The proposed subpolar gyre changes may modulate the northward heat transport, and explain the geographically different long-term climatic trends in the North Atlantic during the late Holocene, i.e. a cooling of the EGC-influenced regions and a warming of the NAC-influenced areas from c. 4 to 5 ka. This mechanism cannot, however, explain the simultaneous occurrence of millennial-scale events at c. 5.6, 3.9, 2.7, 1.3 ka and the ‘Little Ice Age’ in both areas. Noteworthy is the steadily increasing amplitude of these cold events at the Reykjanes Ridge, likely induced by drift ice and/or EGC-influence culminating in the ‘Little Ice Age’. A widespread pronounced warming at 2.0 ka seems to represent the ‘Roman Warm Period’ and reflects the warmest period of the late Holocene.