Southwest African Continental Margin Research Articles

Toward a global calibration for quantifying past oxygenation in oxygen minimum zones using benthic Foraminifera

Abstract. Oxygen minimum zones (OMZs) are oceanic areas largely depleted in dissolved oxygen, nowadays considered in expansion in the face of global warming. To investigate the relationship between OMZ expansion and global climate changes during the late Quaternary, quantitative oxygen reconstructions are needed but are still in their early development. Here, past bottom water oxygenation (BWO) was quantitatively assessed through a new, fast, semi-automated, and taxon-independent morphometric analysis of benthic foraminiferal tests, developed and calibrated using WNP (western North Pacific, including its marginal seas), ENP (eastern North Pacific), and ESP (eastern South Pacific) OMZ samples. This new approach is based on an average size and shape index for each sample. This method, as well as two already published micropalaeontological techniques based on benthic foraminiferal assemblages' variability and porosity investigation of a single species, was calibrated here based on availability of new data from 45 core tops recovered along an oxygen gradient (from 0.03 to 2.88 mL L−1) from the WNP, ENP, EEP (eastern Equatorial Pacific), ESP, SWACM (southwest African continental margin), and AS (Arabian Sea) OMZs. Global calibrated transfer functions are herein proposed for these methods. These micropalaeontological reconstruction approaches were then applied to a palaeorecord from the ENP OMZ to examine the consistency and limits of these methods, as well as the relative influence of bottom and pore waters on these micropalaeontological tools. Both the assemblage and morphometric approaches (which are also ultimately based on the ecological response of the complete assemblage and faunal succession according to BWO) gave similar and consistent past BWO reconstructions, while the porosity approach (based on a single species and its unique response to a mixed signal of bottom and pore waters) showed ambiguous estimations.

Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling

Abstract To understand the structure of the Southwest African continental margin, a lithosphere-scale 3D structural model has been developed, covering the marginal Cretaceous-Cenozoic Orange, Luderitz, Walvis and Namibe basins, the easternmost Walvis Ridge offshore. Onshore, the model includes two late-Proterozoic Owambo (Etosha) and Nama basins. This 3D model integrates published thickness maps (sediment isopach maps), shallow seismic and well data as well as published deep seismic information and has been additionally constrained by 3D gravity and thermal modelling. Using 3D gravity modelling, the first order configuration of the crystalline crust has been resolved with respect to the location of the continent–ocean boundary. The distribution of a high-density lower crustal layer indicates a continuous body extending below the Cretaceous-Cenozoic depocentres and aligned parallel to the coast line. In addition, high-density zones within the continental crystalline crust had to be included in the model to fit observed and calculated gravity. The obtained Moho topography correlates with the major tectonic units of this continental margin. The results of the 3D thermal modelling indicate that there is a clear relationship between the location of thickened sediments and areas with increased temperatures within the upper 10 km of the 3D model. This indicates that the low thermal conductivity of the sediments increases heat storage within the areas covered by thick sediments. Within the deeper crust, the main feature of the temperature distribution is the transition across the continental margin from the relatively cold oceanic part to the warm continental one. This regional pattern is controlled by the thickness of the crystalline continental crust, which is characterized by an increased radiogenic heat production. At a depth of 80–90 km, the temperature becomes higher beneath the oceanic domain than beneath the continent, reflecting the configuration of the lower thermal boundary which is represented by an isothermal lithosphere–asthenosphere boundary.

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5 m length indicates a maximum age of about 60,000–80,000 years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0 < δ13C ‰ V-PDB < –40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4 < δ18O ‰ V-PDB < +6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ18O values might reflect the contribution of 18O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca2+ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ18O–δ13C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.

Crustal structure beneath the Orange Basin, South Africa

Although the development of passive margins has been extensively studied over a number of decades, significant questions remain on how mantle and crustal dynamics interact to generate the observed margin geometries. Here, we investigate the Orange Basin, located on the south-west African continental margin. The basin fill is considered to comprise a classic rift-drift passive margin sequence recording the break-up of Gondwana and subsequent opening of the South Atlantic Ocean. Based on interpreted seismic reflection data, a 3D geological model was first constructed. Subsequently, an isostatic calculation (Airy′s model) using a homogeneous middle and lower crust was applied to this geological model to determine the position of the Moho for an isostatically balanced system. Isostatic sensitivity tests were applied to the model and their gravity response was validated against different crustal structures for the basin. The best-fit model requires dense, presumably mafic material, in the middle and lower crust beneath the basin and an abrupt change to less dense material near the coast to reproduce the observed gravity field.

Glacial/interglacial changes in southern Africa: Compound‐specific δ13C land plant biomarker and pollen records from southeast Atlantic continental margin sediments

This is part 2 of a study examining southwest African continental margin sediments from nine sites on a north‐south transect from the Congo Fan (4°S) to the Cape Basin (30°S) representing two glacial (MIS 2 and 6a) and two interglacial stages (MIS 1 and 5e). Contents, distribution patterns, and molecular stable carbon isotope signatures of long‐chain n‐alkanes (C27‐C33) and n‐alkanols (C22‐C32) as indicators of land plant vegetation of different biosynthetic types were correlated with concentrations and distributions of pollen taxa in sediments of the same time horizons. Selected single pollen type data reveal details of vegetation changes, but the overall picture is best illustrated by summing pollen known to predominantly derive from C4 plants or C4 plus CAM plants. The C4 plant signals in the biomarkers are recorded in the δ13C data and in the abundances of C31 and C33n‐alkanes, and the C32n‐alkanol. Calculated clusters of wind trajectories for austral summer and winter situations for the Holocene and the Last Glacial Maximum afford information on the source areas for the lipids and pollen and their transport pathways to the ocean. This multidisciplinary approach provides clear evidence of latitudinal differences in leaf wax lipid and pollen composition, with the Holocene sedimentary data paralleling the current major phytogeographic zonations. The northern sites (Congo Fan area and northern Angola Basin) get most of their terrestrial material from the Congo Basin and the Angolan highlands dominated by C3 plants. Airborne particulates derived from the western and central South African hinterland dominated by deserts, semideserts, and savannah regions are rich in organic matter from C4 plants. As can be expected from the present and glacial positions of the phytogeographic zones, the carbon isotopic signatures of n‐alkanes and n‐alkanols both become isotopically more enriched in 13C from north to south. In the northern part of the transect the relative importance of C4 plant indicators is higher during the glacials than in the interglacials, indicating a northward extension of arid zones favoring grass vegetation. In the south, where grass‐rich vegetation merges into semidesert and desert, the difference in C4 plant indicators is small.

A north to south transect of Holocene southeast Atlantic continental margin sediments: Relationship between aerosol transport and compound‐specific δ13C land plant biomarker and pollen records

We examined near‐surface, late Holocene deep‐sea sediments at nine sites on a north‐south transect from the Congo Fan (4°S) to the Cape Basin (30°S) along the Southwest African continental margin. Contents, distribution patterns and molecular stable carbon isotope signatures of long‐chain n‐alkanes (C27‐C33) and n‐alkanols (C22‐C32) are indicators of land plant vegetation of different biosynthetic types, which can be correlated with concentrations and distributions of pollen taxa in the same sediments. Calculated clusters of wind trajectories and satellite Aerosol Index imagery afford information on the source areas for the lipids and pollen on land and their transport pathways to the ocean sites. This multidisciplinary approach on an almost continental scale provides clear evidence of latitudinal differences in lipid and pollen composition paralleling the major phytogeographic zonations on the adjacent continent. Dust and smoke aerosols are mainly derived from the western and central South African hinterland dominated by deserts, semi‐deserts and savannah regions rich in C4 and CAM plants. The northern sites (Congo Fan area and northern Angola Basin), which get most of their terrestrial material from the Congo Basin and the Angolan highlands, may also receive some material from the Chad region. Very little aerosol from the African continent is transported to the most southerly sites in the Cape Basin. As can be expected from the present position of the phytogeographic zones, the carbon isotopic signatures of the n‐alkanes and n‐alkanols both become isotopically more enriched in 13C from north to south. The results of the study suggest that this combination of pollen data and compound‐specific isotope geochemical proxies can be effectively applied in the reconstruction of past continental phytogeographic developments.

Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa

Research Article| October 01, 2002 Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa Zvi Ben-Avraham; Zvi Ben-Avraham 1Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel, and Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa Search for other works by this author on: GSW Google Scholar George Smith; George Smith 2Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa Search for other works by this author on: GSW Google Scholar Moshe Reshef; Moshe Reshef 3Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel Search for other works by this author on: GSW Google Scholar Eric Jungslager Eric Jungslager 4Petroleum Agency SA, 151 Frans Conradie Drive, Parow 7500, South Africa Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (10): 927–930. https://doi.org/10.1130/0091-7613(2002)030<0927:GHAMVO>2.0.CO;2 Article history received: 07 Mar 2002 rev-recd: 13 Jun 2002 accepted: 21 Jun 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Zvi Ben-Avraham, George Smith, Moshe Reshef, Eric Jungslager; Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa. Geology 2002;; 30 (10): 927–930. doi: https://doi.org/10.1130/0091-7613(2002)030<0927:GHAMVO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Widespread occurrence of bottom-simulating reflectors (BSRs) has been detected in multichannel seismic profiles on the upper continental slope in the southern periphery of the Orange River delta, probably indicating the presence of large quantities of gas hydrate in this area. This report is the first to show the presence of BSRs on seismic records on the southwest African continental margin south of the Walvis Ridge. Another remarkable feature in the area is the occurrence of a large number of mud volcanoes. The distribution of the BSRs and the location of the mud volcanoes are controlled by the locations of active faults. The gas hydrate in this region may consist of a mixture of microbial and thermogenic gas, whereas much of the gas flowing through the mud volcanoes probably originated from deep-seated Aptian source shales. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

Holocene climatic optimum in Southwest Africa—evidence from the marine clay mineral record

Sediment composition, grain size and clay mineral record of a high-resolution sediment core from the continental slope off Namibia was investigated to gain information on the deposition of terrigenous matter in this part of the Southwest African continental margin during the last 18 k.y. The depositional processes involved are fluvial input by the Kunene River and eolian input from the Namib and Kalahari deserts, each supplying characteristic mineral suites. During low sea level, erosion of the exposed shelf yields additional material. The amount of eolian or fluvial matter depends on the strength of the transport process, which is a function of aridity or humidity of the source area, thus allowing paleoclimatic interpretations. Arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short-distance transport by southerly winds. A dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition, which is attributed to increased runoff of the Kunene River and fluvial erosion of shelf sediments. This period coincides with a strengthening of the monsoonal system during a precessional minimum, which is observed in numerous sites in Central Africa and indicates an intensified influence of the monsoon at the Kunene headwaters. A distinct shift in clay mineralogy towards river-derived material marks a second period of increased river runoff—during high sea level—from 9 to 5 ka, indicating maximum humidity in the source area from 6 to 5 ka. This corresponds to the Holocene climatic optimum observed in the arid belts of Northern Africa. The present balance between fluvial and eolian input was reached approximately at 4 ka.

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

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.

Distribution of Recent nannofossils beneath the Benguela system: Southwest African continental margin

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.

Subsidence of Aseismic Ridges: Evidence from Sediments on Rio Grande Rise (Southwest Atlantic Ocean)

Rio Grande Rise is a large aseismic ridge in the southwest Atlantic Ocean. Though the nature of its basement only can be inferred from geophysical evidence, shallow-water sediments recovered from various locations along its flanks show that it was a large towering 2 km above sea level during Santonian-Campanian times (75 to 85 m.y. ago) and that it slowly submerged since that time. Lithic fragments of basaltic material present with shallow-water sediments suggest that this island has a volcanic base of Santonian/pre-Santonian age. From the locations of shallow-water sediment components, subsidence rates of former shorelines were reconstructed. Application of the sedimentation rates from DSDP drill sites allowed estimation of the subsidence of the volcanic basemen and backtracking of the paleodepth of deposition of the pelagic-sediment facies recovered from the flanks of the rise. These facies can be correlated to similar sediments on the adjacent Brazilian continental margin (Sao Paulo Plateau) and on the southwest African continental margin.