Angola Dome Research Articles

Modeling Lithospheric Thickness Along the Conjugate South Atlantic Passive Margins Implies Asymmetric Rift Initiation

AbstractThe lithospheric architecture of passive margins is crucial for understanding the tectonic processes that caused the breakup of Gondwana. We highlight the evolution of the South Atlantic passive margins by a simple thermal lithosphere‐asthenosphere boundary (LAB) model based on onset and cessation of rifting, crustal thickness, and stretching factors. We simulate lithospheric thinning and select the LAB as the T = 1,330°C isotherm, which is calculated by 1D advection and diffusion. Stretching factors and margin geometry are adjusted to state‐of‐the‐art data sets, giving a thermal LAB model that is especially designed for the continental margins of the South Atlantic. Our LAB model shows distinct variations along the passive margins that are not imaged by global LAB models, indicating different rifting mechanisms. For example, we model up to 200 km deep lithosphere in the South American Santos Basin and shallow lithosphere less than 60 km in the Namibe Basin offshore Africa. These two conjugate basins reflect a strong asymmetry in LAB depth that resembles variations in margin width. In a Gondwana reconstruction, we discuss these patterns together with seismic velocity perturbations for the Central and Austral Segments of the margins. The shallow lithosphere in the Namibe Basin correlates with signatures of the Angola Dome, attributed to epeirogenic uplift in the Neogene, suggesting an additional component of post‐breakup lithospheric thinning.

Frontogenesis of the Angola–Benguela Frontal Zone

Abstract. A diagnostic analysis of the climatological annual mean and seasonal cycle of the Angola–Benguela Frontal Zone (ABFZ) is performed by applying an ocean frontogenetic function (OFGF) to the ocean mixing layer (OML). The OFGF reveals that the meridional confluence and vertical tilting terms are the most dominant contributors to the frontogenesis of the ABFZ. The ABFZ shows a well-pronounced semiannual cycle with two maximum (minimum) peaks in April–May and November–December (February–March and July–August). The development of the two maxima of frontogenesis is due to two different physical processes: enhanced tilting from March to April and meridional confluence from September to October. The strong meridional confluence in September to October is closely related to the seasonal southward intrusion of tropical warm water to the ABFZ that seems to be associated with the development of the Angola Dome northwest of the ABFZ. The strong tilting effect from March to April is attributed to the meridional gradient of vertical velocities, whose effect is amplified in this period due to increasing stratification and shallow OML depth. The proposed OFGF can be viewed as a tool to diagnose the performance of coupled general circulation models (CGCMs) that generally fail at realistically simulating the position of the ABFZ, which leading to huge warm biases in the southeastern Atlantic.

Basin‐scale inputs of cobalt, iron, and manganese from the Benguela‐Angola front to the South Atlantic Ocean

We present full‐depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin‐scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N2O, and O2, as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid‐type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin‐scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.

Synoptic transects on the distribution of trace elements (Hg, Pb, Cd, Cu, Ni, Zn, Co, Mn, Fe, and Al) in surface waters of the Northern- and Southern East Atlantic

Surface seawater samples were taken in the framework of the GEOTRACES program on “POLARSTERN” expedition ANT XXIII/1 in the Eastern Atlantic in 2005 to study the distribution of the trace elements Hg (mercury), Pb (lead), Cd (cadmium), Cu (copper), Ni (nickel), Zn (zinc), Co (cobalt), Mn (manganese), Fe (iron), and Al (aluminium). With the exception of Hg, results were compared to earlier datasets from 1989 to 1990. The particulate fraction averaged over the transect was calculated to be 49% for Cd, 23% for Mn and 50% for Fe indicating a release of these TEI's (trace elements and their isotopes) from a leachable SPM fraction in the stored and acidified samples. Total Pb concentrations ranged between 5 and 20 pmol kg − 1 in 2005 with highest values in the ITCZ (intertropical convergence zone). In 1989 Pb concentrations were twice as high in the region of the ITCZ, while by a factor of 10–15 higher values were obtained in the North Atlantic. Total Cd and Co are dominated, by different seasonal upwelling regimes (Equatorial upwelling, Guinea Dome, Angola Dome). Total Cu, Ni, Fe, Mn and Al show nearly identical concentrations in 1990 and 2005. For total manganese and aluminium strong maxima (3–4 nmol kg − 1 and 55 nmol kg − 1 respectively) are observed between 23°N and 0°, while the Fe maximum (6–9 nmol kg − 1 ) is located at 7°N. Total Hg concentrations ranged between 0.5 and 4.5 pmol kg − 1 .

Seasonal and Interannual Variations of Oceanic Conditions in the Angola Dome

Abstract Using outputs from a high-resolution OGCM, seasonal and interannual variations of the Angola Dome (AD) are revisited. Although the AD was previously considered to be one large cold tongue extending from the West African coast, it is shown that two cold domes exist. These two domes have remarkably different mechanisms for their seasonal variation. The weak dome, whose center is located at 6°S, 1°E, develops from May to September owing to the divergence of heat transport associated with upwelling. The strong dome, on the other hand, extends from the west coast of Africa between 20° and 15°S, and develops from April to August by the surface heat flux. The interannual variation of the weak dome is strongly influenced by the Atlantic Niño. An unusual relaxation of easterly wind stress in the central equatorial Atlantic Ocean associated with the Atlantic Niño triggers second baroclinic downwelling equatorial Kelvin waves, which propagate eastward along the equator and poleward along the coast after reaching the African coast as coastal Kelvin waves. Then, downwelling Rossby waves radiate away from the coast and cause significant warming in the weak dome region. The interannual variation of the South Equatorial Undercurrent may be associated with that of the AD; its transport decreases by 0.6 Sv, and its core shifts equatorward by 0.2° when the AD is anomalously weak.

A 190 ky record of lithogenic grain-size on the Namibian slope: Forging a tight link between past wind-strength and coastal upwelling dynamics

We inferred wind-strength variations at the Namibian coast and its effect on upwelling intensity over the last 190 ky, by comparing grain-size data and alkenone-based sea surface temperatures (SSTs) from one sediment core located on the upper slope off Lüderitz (25°6′S, 13°38′E). The wind-strength proxy was defined by partitioning terrigenous grain-size distributions (GSDs) into two sub-populations owing to a simple statistical model. The fine, poorly sorted component is assumed to include both fine aeolian dust and hemipelagic mud carried by rivers or winnowed from the shelf. Those two fractions are assumed to have been identically sorted during settling in the water column and are not distinguished by our approach. The mode of the coarse, well-sorted sub-population displays rapid, high amplitude variations through time. We ascribe a strictly aeolian origin to this component, and consider that low-altitude winds are able to carry coarse-grained particles to the ocean, as shown by grain-size measurements of present dust material from the adjacent continent [ Stuut, J.B.W., Prins, M.A., Schneider, R.R., Weltje, G.J., Jansen, J.H.F., Postma, G., 2002. A 300-kyr record of aridity and wind-strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic. Mar. Geol. 180, 221–233]. Variations in the coarse-grained dust diameter are closely correlated with SST changes. Past SSTs records from the Namibian upper slope have been previously hypothesized as reflecting upwelling intensity combined with occasional injection of warm-water from the Angola Dome. We challenge this view in considering that past long-term SSTs variations are primarily induced by the wind-strength and witness coastal upwelling intensity. Early MIS 6 is the only period with evidences of warm, poleward water intrusion at the core site. The windiest conditions occurred during isotopic stages 3 and 4. The wind-strength proxy defined in this study can be used confidently as an independent proxy for past upwelling intensity over the last two climatic cycles.

Cross-front hydrography and fish larval distribution at the Angola–Benguela Frontal Zone

The hydrography and the ichthyoplankton abundance and composition was surveyed along a meridional transect off Angola. It extended from near the equator, cut the western edge of the Angola Dome, and ended off northern Namibia, south of the Angola–Benguela Frontal Zone (ABFZ). The meridional gradient of the ABFZ was much stronger inshore than farther offshore, but both reacted within days to changing wind situations. It exhibited changes in hydrographical morphology, as well as in the distributional limits of surface tropical and subtropical species. In contrast, the faunistic boundaries of midwater species and water masses were locationally more persistent. The cross frontal features were consistent, real and persistent in their meridional gradients, in spite of a Benguela Niño situation. North of the ABFZ, fish larval abundances were six times higher than south of the ABFZ, and three times higher than in the adjacent open ocean. Vertically, larvae occurred from the surface to at least 200 m, but preferentially above 100 m. The maximum concentration of both chlorophyll and fish larvae coincided roughly with the pycnocline and deepened with it at the front. The number of taxa declined southward from 36 species per station at 6°S by about one species per degree of latitude. Gross number of taxa did not differ much across the boundaries of the Angolan Gyre. However, the community structure changed drastically across the front, with maxima of diversity within it. The faunal structures north and south of the ABFZ had a similarity of only 22%. Off Angola, larvae of mesopelagic fish species that are adapted to oxygen deficits dominated.

Biological and physical signatures in the tropical and subtropical Atlantic

The variability of sea surface chlorophyll concentration in the tropical and subtropical Atlantic during the first year of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) imagery is examined. An Ocean General Circulation Model (OGCM) is used, along with TOPEX/Poseidon dynamic height observations and global gridded wind stress data sets, to explain the physical forcing of surface ocean color signals. Regions of high surface chlorophyll are strongly correlated with mesoscale and large‐scale physical processes such as the strong upwelling off the west coast of Africa, the relatively high oceanic production within the Guinea Dome region, and the generation and propagation of large anticyclonic eddies along the coast of South America, north of the equator. The major river outflows (Amazon, Orinoco, and Congo) have strong signatures with plumes of apparently high Chl a in excess of 10 mg m−3 near their deltas. The fall bloom in the eastern tropical Atlantic observed by the Coastal Zone Color Scanner (CZCS) was absent in 1997, whereas a bloom was observed in this region in July‐September 1998, which was not observed by the CZCS. We attribute these apparent anomalies to the projection of the 1997–1998 El Niño event into the tropical Atlantic basin; these signals are correlated with sea surface temperature anomalies known to be associated with ENSO. The SeaWiFS images show that there are seasonal blooms within the hydrographic provinces of the Guinea and Angola domes. These hydrographic provinces are characterized by the dynamic uplift of the thermocline at the North Equatorial Current southern boundary (Guinea Dome) and the Benguela Current eastern boundary (Angola Dome). Within these domes, the Ekman pumping and transport are significant due to the strong trade winds at the surface. The Ekman drift plays a major role in the spreading of surface blooms. The spreading of the oceanic bloom at 12°N, 30°W, the Congo River plume, and the areal extent of the upwelling blooms off the coast of Africa, parallels the strength and extent of the Ekman surface drift. Upwelling, when broadly defined to include large scale vertical excursions of the thermocline, explains virtually all of the surface chlorophyll observations in excess of 0.5 mg m−3, except in the river plumes.

A biomarker perspective on paleoproductivity variations in two Late Quaternary sediment sections from the Southeast Atlantic Ocean

Sedimentary inventories of selected molecular biomarkers in two sediment sections from the African continental margin below highly productive surface waters in the Southeast Atlantic ocean, each spanning the last 76 kyr, were investigated and compared to each other. A principal component analysis of the stratigraphic data separated groups of biomarkers, representing different assemblages of biological sources and/or different diagenetic histories, related to their chemical structures and their metabolic role in the food chain. The concentrations of long-chain n-alkenones, n-diols, the major n-ketool, and loliolide and its epimer isololiolide, are correlated with paleoproductivity estimates that were calculated from organic carbon contents. This group of relatively refractory compounds very likely represents those planktonic communities, which contributed significantly to surface water productivity and subsequent burial of organic carbon during the considered time interval. Concentrations of (dust-transported) terrigenous long-chain fatty acids (C 24 to C 28) are interpreted to be related to the intensity of upwelling-driving winds and are also significantly correlated with high contents of organic matter. Concentrations of cholesterol and the major short-chain fatty acids are not related to the marine paleoproductivity signal, most likely as a consequence of their nonspecific origin and their metabolic role during heterotrophic feeding processes. Abundances of 24-ethylcholest-5-en-3 β-ol and its saturated counterpart, 24-ethyl-5 α-cholestan-3 β-ol, the major steroid alcohols in both sedimentary environments, appear not to be related to any of the above mentioned components and probably represent the contribution of algal species adapted to nutrient conditions different from those of other major primary producers. Differences between both environments, the Angola Dome and the Benguela coastal upwelling, are evident in the relative distribution of major components and the stratigraphic behavior of selected compounds relative to other variables. These differences are discussed in relation to specific characteristics of both oceanographic environments.© 1999 Elsevier Science Ltd. All rights reserved.

Simulation of the Tropical Thermal Domes in the Atlantic: A Seasonal Cycle

A general ocean circulation model is used to analyze seasonal variations in the Guinea Dome and Angola Dome regions. The cold Guinea Dome develops from June through September due to active divergence of heat transport. The cooling is related to upwelling generated by the local wind stress curl associated with the northeast trade winds. The Guinea Dome thus provides an active mechanism of absorbing heat from the atmosphere. The coastal Guinea region experiences semiannual warming in April–July and November–December due to intrusion of coastal downwelling Kelvin waves from the equatorial region. The Angola Dome in the Southern Hemisphere is found to be cooled from March through August. The surface flux plays a major role in the seasonal variation of the heat budget in contrast to the situation of the Guinea Dome in the Northern Hemisphere. The Angola front located at the northern border of the cold Angola Dome becomes distinguishable particularly during the boreal fall because of the intrusion of warm water from the equatorial region. The coastal warming occurs twice a year in both hemispheres mostly due to intrusion of warm water accumulated in the eastern border of the Gulf of Guinea. The semiannual relaxation of the trade winds east of 30°W as well as the semiannual intensification of the eastward component of the West African monsoon is responsible for this remarkable oceanic phenomenon.

Temporal variation in intensity of upwelling off southwest Africa

Abstract Sediments recovered at DSDP Sites 362 and 532 on Walvis Ridge Abutment Plateau and 530 in the southeastern Angola Basin record long-term waxing and waning of upwelling. The amounts of opaline silica and organic carbon in the sediments increase from latest Miocene to latest Pliocene then decline to present. During the late Pliocene, opaline silica accumulated ten times faster than during the late Pleistocene. If the area of accumulation also expanded to include the Abutment Plateau, this region would have been the global silica sink, consuming the equivalent of the entire silica output of the present ocean. The sediments contain light-dark cycles; the dark cycles contain more terrigenous material and on this basis have been interpreted as representing glacials. Before the Pliocene the maximum biological productivity in this region occurred during the glacials, but since then it has occurred during the interglacials, an effect of sea-level change on productivity opposite to that in most parts of the world. The most important factors causing the changes in sedimentation are judged to be threefold. (1) Closing of the Central American Isthmus resulted in salinization of the North Atlantic and caused increased production of NADW, AAIW and AABW. The increased rate of production of these water masses resulted in differentiation of nutrients among them and may have resulted in a shallower pycnocline facilitating upwelling of nutrient-rich water. The production of relatively nutrient-rich AAIW was a prerequisite for the long term increase in productive upwelling recorded by the late Miocene to latest Pliocene sediments. (2) During the Late Miocene northward migration of the subtropical high and ITCZ in response to growth of the Antarctic ice cap may have initiated productive upwelling over the Abutment Plateau. Subsequent southward migration of the ITCZ as the Earth changed from unipolar to bipolar glaciation would have enhanced the Angola Coastal Current and upwelling from the Angola Thermal Dome. A stronger Angola Coastal Current may have carried productive waters from the Angola Dome and focussed advection of productive coastal waters upwelled along the Namibian shelf over the Walvis Ridge Abutment Plateau. The decline in strength of upwelling since 1.7 Ma recorded by the sediments on the Abutment Plateau may reflect southward shift of the upwelling center to its present location near Lüderitz in response to the growth of northern hemisphere ice caps. (3) The desiccation of the Mediterranean drew the ITCZ to its maximum northern position and the reflooding corresponded to the time of change from maximum upwelling productivity coinciding with glacials to its coinciding with interglacials. During glacials, when NADW production was reduced, AAIW was replaced by nutrient-poor NAIW, so that the upwelled water did not become productive. Another possible cause of the general decline in productivity since 1.7 Ma is the that increased Mediterranean saline outflow has resulted in progressive expansion of NAIW into the South Atlantic during glacials.

Periodic processes in free pseudo-linear oscillatory systems

The hydrography and the ichthyoplankton abundance and composition was surveyed along a meridional transect off Angola. It extended from near the equator, cut the western edge of the Angola Dome, and ended off northern Namibia, south of the Angola–Benguela Frontal Zone (ABFZ). The meridional gradient of the ABFZ was much stronger inshore than farther offshore, but both reacted within days to changing wind situations. It exhibited changes in hydrographical morphology, as well as in the distributional limits of surface tropical and subtropical species. In contrast, the faunistic boundaries of midwater species and water masses were locationally more persistent. The cross frontal features were consistent, real and persistent in their meridional gradients, in spite of a Benguela Niño situation.North of the ABFZ, fish larval abundances were six times higher than south of the ABFZ, and three times higher than in the adjacent open ocean. Vertically, larvae occurred from the surface to at least 200 m, but preferentially above 100 m. The maximum concentration of both chlorophyll and fish larvae coincided roughly with the pycnocline and deepened with it at the front. The number of taxa declined southward from 36 species per station at 6°S by about one species per degree of latitude. Gross number of taxa did not differ much across the boundaries of the Angolan Gyre. However, the community structure changed drastically across the front, with maxima of diversity within it. The faunal structures north and south of the ABFZ had a similarity of only 22%. Off Angola, larvae of mesopelagic fish species that are adapted to oxygen deficits dominated.