South Atlantic Basin Research Articles

Tectono-sedimentary controls on Cretaceous black shale deposition along the opening Equatorial Atlantic Gateway (ODP Leg 159)

Abstract The Cretaceous Equatorial Atlantic Gateway between the Central and South Atlantic basins is of interest not only for palaeoceanographic and palaeoclimatic studies, but also because it provided particularly favourable conditions for the accumulation and preservation of organic-rich sediments. Deposition of carbonaceous sediments along the Côte d’Ivoire-Ghana Transform Margin (Ocean Drilling Program Leg 159) was intimately linked to the plate tectonic and palaeoceanographic evolution of this gateway. Notably, the formation of a marginal basement ridge on the southeastern border of the transform margin provided an efficient shelter of the landward Deep Ivorian Basin against erosive and potentially oxidizing currents. Different subsidence histories across the transform margin were responsible for the development of distinct depositional settings on the crest and on both sides of the basement ridge. Whereas the southern, oceanward flank of the basement ridge was characterized by rapid, continuous deepening since the latest Albian-early Cenomanian, marine sedimentation on the northern, landward flank was interrupted by a period of uplift and erosion in the late Albian, and rapid subsidence started after the early Coniacian. Organic-rich sediments occur throughout almost the entire Cretaceous section, but hydrogen-rich marine black shales were exclusively recovered from core sections above an uplift-related unconformity. These black shales formed when separation of Africa and South America was sufficient to allow permanent oceanic midwater exchange after the late Albian. Four periods of black shale accumulation are recorded, some of them are correlated with the global oceanic anoxic events: in the latest Albian-earliest Cenomanian, at the Cenomanian-Turonian boundary, during the middle Coniacian-early Campanian, and in the mid-Maastrichtian. These periods were characterized by increased carbon fluxes to the seafloor, induced by enhanced palaeoproductivity and intensified supply of terrestrial organic matter. Black shale deposition appears to be intimately linked to periods of rising or maximum eustatic sea level and to the expansion of the oxygen minimum zone, as indicated by foraminiferal biofacies. Intervals between black shale units, in contrast, indicate a shrinking oxygen minimum zone and enhanced detrital flux rates, probably related to lowering sea level. Upper Cretaceous detrital limestones with high porosities may provide excellent hydrocarbon reservoirs, although their areal extent appears to be limited. Palaeogene porcellanites, capped by Neogene pelagic marls and clays, extend over a wider area and may provide another target for hydrocarbon exploration.

An interdecadal climate cycle in the South Atlantic and its links to other ocean basins

A singular value decomposition analysis and a combined complex empirical orthogonal function analysis are performed on 80 years of monthly sea surface temperature (SST) and sea level pressure (SLP) data from the South Atlantic region. The analyses reveal the existence of interdecadal fluctuations in the coupled atmosphere‐ocean system with a period of around 20 years. The SST anomalies are observed to propagate anticyclonically around the South Atlantic basin following the subtropical gyre circulation. At the same time, a westward propagation of SLP anomalies across the basin generates changes in the atmospheric circulation that appear to reinforce such SST anomalies through anomalous exchanges of heat. It is thus proposed that the dominant physical processes involved in this interdecadal cycle include the horizontal advection of heat by the ocean currents and changes in the atmosphere‐ocean heat fluxes through local air‐sea interactions. The global SST and SLP patterns that accompany the different phases of the South Atlantic cycle are also presented. They show similarities with other well‐known interdecadal signals observed by several investigators in other ocean basins (e.g., the low‐frequency part of the North Atlantic Oscillation). This suggests that the South Atlantic signal described in this study may be a regional aspect of global interdecadal variability.

South Atlantic Ocean circulation: Simulation experiments with a quasi‐geostrophic model and assimilation of TOPEX/POSEIDON and ERS 1 altimeter data

Simulation experiments of South Atlantic Ocean circulations are conducted with a 1/6°, four‐layered, quasi‐geostrophic model. By means of a simple nudging data assimilation procedure along satellite tracks, TOPEX/POSEIDON and ERS 1 altimeter measurements are introduced into the model to control the simulation of the basin‐scale circulation for the period from October 1992 to September 1994. The model circulation appears to be strongly influenced by the introduction of altimeter data, offering a consistent picture of South Atlantic Ocean circulations. Comparisons with observations show that the assimilating model successfully simulates the kinematic behavior of a large number of surface circulation components. The assimilation procedure enables us to produce schematic diagrams of South Atlantic circulation in which patterns ranging from basin‐scale currents to mesoscale eddies are portrayed in a realistic way, with respect to their complexity. The major features of the South Atlantic circulation are described and analyzed, with special emphasis on the Brazil‐Malvinas Confluence region, the Subtropical Gyre with the formation of frontal structures, and the Agulhas Retroflection. The Agulhas eddy‐shedding process has been studied extensively. Fourteen eddies appear to be shed during the 2‐year experiment. Because of their strong surface topographic signature, Agulhas eddies have been tracked continuously during the assimilation experiment as they cross the South Atlantic basin westward. Other effects of the assimilation procedure are shown, such as the intensification of the Subtropical Gyre, the appearance of a strong seasonal cycle in the Brazil Current transport, and the increase of the mean Brazil Current transport. This last result, combined with the westward oriention of the Agulhas eddies' trajectories, leads to a southward transport of mean eddy kinetic energy across 30°S.

Study of ozone formation and transatlantic transport from biomass burning emissions over West Africa during the airborne Tropospheric Ozone Campaigns TROPOZ I and TROPOZ II

Using the results obtained over the Ivory Coast during the airborne Tropospheric Ozone campaign (TROPOZ) I (December 1987) and over West Africa and South America during the meridional airborne campaign TROPOZ II (January 1991), a study is made of biomass burning emissions in the northern African tropics, a region which is much less well documented than that of southern Africa. The analysis of data for O3 and its precursors (CO, CH4, NO, and NOy) confirms the existence of important ozone formations over northern Africa during the Boreal winter with photochemical O3 formation rates of 15–35 ppbv O3 d−1. The distributions obtained over the Ivory Coast show the presence of different layers associated with the Inter Tropical Convergence Zone (ITCZ), with an ozone‐ and precursor‐laden Harmattan layer embedded between the southerly depleted monsoon layer below and the free troposphere easterly circulation above. In front of the ITCZ discontinuity, air parcels of different ages are identified in biomass burning plumes resulting from the transfer to altitude of compounds emitted by fires followed by ascending motions of this polluted air above the monsoon layer. Using a chemical box model, the chemical evolution of concentrations in these air parcels is simulated, confirming the explanations already proposed from the study of the dynamic context and history of the air parcels along their trajectories. The ozone measurements performed during TROPOZ II on both sides of the central Atlantic (Brazil, West Africa) support the hypothesis that because of the prevailing meteorology, smoke‐laden air masses emanating from the burning regions in northern Africa can enter a meteorological regime which permits them to be transported out over the Atlantic where they can influence budgets over the south Atlantic Basin during the Austral summer.

A sigma-coordinate primitive equation model for studying the circulation in the South Atlantic Part II: Meridional transports and seasonal variability

The mean and seasonal variability of the circulation and meridional heat transport in the South Atlantic are investigated using a set of numerical experiments. The primitive equation model uses a topography-following (sigma) coordinate. The model domain is limited to the South Atlantic basin. Artificial boundaries at Drake Passage, between Brazil and Angola, and between South Africa and Antarctica are treated as open boundaries. Finally, recent and self-consistent estimates of seasonal fluxes are used to define a model-dependent atmospheric forcing. Quasi-diagnostic simulations forced by constant climatological winds are first conducted to determine the sensitivity of model solutions to bottom topography smoothing, and to diagnose meridional fluxes from a mass field that is relaxed to the annual climatology of Levitus (1982). Model results show good agreement with known climatological circulation features in this basin, especially in the Confluence Region, where coarse resolution models usually give smooth structures. Sensitivity studies show that the more detailed features of the circulation are influenced by the model bathymetry. The model simulates a meridional circulation whose upper branch (the return flow that balances the southward flow of North Atlantic Deep Water) is composed of Intermediate (IW) and Thermocline (TW) Waters. The transport of IW is found to be predominant, and the value of meridional heat transport consequently falls within the low estimates. We notice that the meridional heat balance is sensitive to the position of the Confluence. When this region occurs too far south, the amount of IW contributing to the return flow of the overturning cell is reduced. Prognostic simulations forced by seasonal winds and heat fluxes are studied to quantify the impact of wind forcing on the circulation in the South Atlantic. Particular attention is focused on meridional transports at 30°S. Analysis of the mean annual circulation confirms that the upper branch of the meridional circulation is predominantly composed of IW (9 Sv), rather than TW (5.3 Sv). The mean transport of the lower branch is 16 Sv, in agreement with recent estimates by Schlitzer (1996). The annual meridional heat transport (0.29 PW) is still within the low estimates, but agrees well with other estimates that give a dominant role to IW (Rintoul, 1991). Original results also concern the variability of the upper branch of the meridional circulation. It is shown that the wind creates seasonal variability in the Subtropical Gyre, which has a marked impact on the water mass balance in the South Atlantic. In winter, a large convergence to the north of the Subtropical Gyre (27°S) reduces the northward flow of IW, whereas stronger Ekman pumping favors an equatorward transport of TW. In summer, this convergence disappears and a larger transport of IW is allowed. Thus a more complex scheme is proposed for the meridional circulation, in which local wind forcing in the South Atlantic Basin has a significant role in preconditioning the surface waters of the global overturning cell.

A sigma-coordinate primitive equation model for studying the circulation in the South Atlantic. Part I: Model configuration with error estimates

This paper describes the configuration of a topography-following ( sigma) coordinate, numerical ocean model for studying the circulation in the South Atlantic. An analysis is performed (i) to ensure that the model configuration does not introduce a numerical bias in the model solution and (ii) to give estimates of numerical errors. The model is the Semi-spectral Primitive Equation Model (SPEM) from Rutgers University (Haidvogel et al., 1991). Two important issues relating to the sigma-coordinate are investigated: the pressure gradient calculation and the diffusion of tracers. Errors in the pressure gradient calculation are investigated by simulating an ocean at rest, and the choice is made to reduce errors by smoothing the bathymetry. A smoothing criterion is derived that permits a limitation of the errors in the pressure gradient calculation to an acceptable level (i.e. maximum errors on velocities below a millimeter per second). It is applied to define the model bottom topography. Errors in the tracer fields, induced by a diffusion scheme operating along constant sigma surfaces, generates large unrealistic velocities (of the order of 10 cm/s). A rotation of the diffusion tensor into geopotential coordinates is proposed. Tests show that errors are then reduced to an insignificant level. The rotation of the diffusion tensor is therefore retained. The numerical treatment of the open boundaries and the flux conditions that yields the most realistic circulation is also described. Open boundary conditions are based on radiation conditions and relaxation to climatology. They appear to be numerically robust, and to be able to bring into the South Atlantic basin the necessary information from the outer oceans. A configuration of the SPEM model to study the large scale circulation in the South Atlantic is then obtained. Errors due to model configuration are shown to be small compared to the signal one wants to simulate, and their spatial pattern is known, which will facilitate the interpretation of the model simulations presented in following papers.

Abstract: The Post-rift Paleogeographic Evolution of the South Atlantic Basins of Brazil and West Africa and the Influence of Hinterland Uplift on Drainage and Sedimentary Depocenters 

Abstract: The Post-rift Paleogeographic Evolution of the South Atlantic Basins of Brazil and West Africa and the Influence of Hinterland Uplift on Drainage and Sedimentary Depocenters 

Origin of ozone and NOx in the tropical troposphere: A photochemical analysis of aircraft observations over the South Atlantic basin

The photochemistry of the troposphere over the South Atlantic basin is examined by modeling of aircraft observations up to 12‐km altitude taken during the TRACE A expedition in September–October 1992. A close balance is found in the 0 to 12‐km column between photochemical production and loss of O3, with net production at high altitudes compensating for weak net loss at low altitudes. This balance implies that O3 concentrations in the 0–12 km column can be explained solely by in situ photochemistry; influx from the stratosphere is negligible. Simulation of H2O2, CH3OOH, and CH2O concentrations measured aboard the aircraft lends confidence in the computations of O3 production and loss rates, although there appears to be a major gap in current understanding of CH2O chemistry in the marine boundary layer. The primary sources of NOx over the South Atlantic Basin appear to be continental (biomass burning, lightning, soils). There is evidence that NOx throughout the 0 to 12‐km column is recycled from its oxidation products rather than directly transported from its primary sources. There is also evidence for rapid conversion of HNO3 to NOx in the upper troposphere by a mechanism not included in current models. A general representation of the O3 budget in the tropical troposphere is proposed that couples the large‐scale Walker circulation and in situ photochemistry. Deep convection in the rising branches of the Walker circulation injects NOx from combustion, soils, and lightning to the upper troposphere, leading to O3 production; eventually, the air subsides and net O3 loss takes place in the lower troposphere, closing the O3 cycle. This scheme implies a great sensitivity of the oxidizing power of the atmosphere to NOx emissions in the tropics.

Chemical characteristics of continental outflow over the tropical South Atlantic Ocean from Brazil and Africa

The chemical characteristics of air parcels over the tropical South Atlantic during September – October 1992 are summarized by analysis of aged marine and continental outflow classifications. Positive correlations between CO and CH3Cl and minimal enhancements of C2Cl4 and various chlorofluorocarbon (CFC) species in air parcels recently advected over the South Atlantic basin strongly suggest an impact on tropospheric chemistry from biomass burning on adjacent continental areas of Brazil and Africa. Comparison of the composition of aged Pacific air with aged marine air over the South Atlantic basin from 0.3 to 12.5 km altitude indicates potential accumulation of long‐lived species during the local dry season. This may amount to enhancements of up to two‐fold for C2H6, 30% for CO, and 10% for CH3Cl. Nitric oxide and NOx were significantly enhanced (up to ∼1 part per billion by volume (ppbv)) above 10 km altitude and poorly correlated with CO and CH3Cl. In addition, median mixing ratios of NO and NOx were essentially identical in aged marine and continental outflow air masses. It appears that in addition to biomass burning, lightning or recycled reactive nitrogen may be an important source of NOx to the upper troposphere. Methane exhibited a monotonic increase with altitude from ∼1690 to 1720 ppbv in both aged marine and continental outflow air masses. The largest mixing ratios in the upper troposphere were often anticorrelated with CO, CH3Cl, and CO2, suggesting CH4 contributions from natural sources. We also argue, based on CH4/CO ratios and relationships with various hydrocarbon and CFC species, that inputs from biomass burning and the northern hemisphere are unlikely to be the dominant sources of CO, CH4, and C2H6 in aged marine air. Emissions from urban areas would seem to be necessary to account for the distribution of at least CH4 and C2H6. Over the African and South American continents an efficient mechanism of convective vertical transport coupled with large‐scale circulations conveys biomass burning, urban, and natural emissions to the upper troposphere over the South Atlantic basin. Slow subsidence over the eastern South Atlantic basin may play an important role in establishing and maintaining the rather uniform vertical distribution of long‐lived species over this region. The common occurrence of values greater than 1 for the ratio CH3OOH/H2O2 in the upper troposphere suggests that precipitation scavenging effectively removed highly water soluble gases (H2O2, HNO3, HCOOH, and CH3COOH) and aerosols during vertical convective transport over the continents. However, horizontal injection of biomass burning products over the South Atlantic, particularly water soluble species and aerosol particles, was frequent below 6 km altitude.

Ozone and aerosol distributions and air mass characteristics over the South Atlantic Basin during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the tropical South Atlantic during the NASA Global Tropospheric Experiment (GTE)/Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) field experiment conducted in September–October 1992. Gases from extensive fires in Brazil were transported by convective storms into the upper troposphere where tropospheric ozone (O3) was photochemically produced and advected eastward over the South Atlantic. In central Africa, the fires were widespread, and in the absence of deep convection, the fire plumes were advected at low altitudes (below ∼6 km) over the Atlantic. There was a positive correlation between O3 and aerosols found in the plumes that were not involved in convection. High O3 (>75 parts per billion by volume (ppbv)) was observed in the low‐altitude plumes, and also in the upper troposphere where O3 often exceeded 100 ppbv with low aerosol loading. The average tropospheric O3 distributions were determined for the following: Brazil and western South Atlantic, eastern and central South Atlantic, central and east coast of Africa, and the entire South Atlantic Basin. The tropopause heights and O3 columns across the troposphere were calculated for individual flights and for the average O3 distributions in the above regions. A maximum tropospheric O3 column of 56 Dobson units (DU) was found over the biomass burning region in Zambia and in the subsidence region over the central South Atlantic. The high O3 region over the South Atlantic from 4° to 18°S corresponded with the latitudinal extent of the fires in Africa. In situ and laser remote measurements were used to determine the frequency of observation and chemical composition of nine major air mass types. Biomass burning emissions contributed to most of the air masses observed over the South Atlantic Basin, and biomass burning was found to contribute up to half (28 DU) of the O3 column across this region.

Methane in the tropical South Atlantic: Sources and distribution during the late dry season

Methane (CH4) mixing ratios in the South Atlantic basin were sampled from a DC‐8 aircraft during the TRACE A expedition over the months of September and October 1992. This high‐precision (±0.1%), high‐resolution data set (independent measurements every 5 s) includes a total of 67,335 observations and ranges from 1586.6 to 2152.8 parts per billion by volume (ppbv). The observed values were influenced by emissions from biomass burning and local urban/industrial areas, stratosphere‐troposphere exchange, active convective mixing, and possible long‐range transport of pollution. Average mixing ratios increased from the atmospheric mixed layer (0–2 km) to the free troposphere (2–6 km) and were greatest at altitudes above 6 km. A longitudinal trend observed between 5°N and 40°S latitude suggests that CH4 inputs were greater on the South American side of the basin. A latitudinal trend in values between the near‐surface and 11 km exhibits the expected gradient from north to south, but differs from the latitudinal gradient observed in surface level clean air at this time. Higher concentrations at altitude, at least at this time of year, indicate that ground‐based atmospheric monitoring sites may underestimate pollution inputs to the region.

Factors influencing the upper free tropospheric distribution of reactive nitrogen over the South Atlantic during the TRACE A experiment

This paper evaluates the potential sources of the enhanced levels of NO that were observed throughout the upper troposphere over the equatorial and tropical South Atlantic. During September/October 1992 NOx (NO + NO2) mixing ratios in the 8‐ to 13‐km region averaged 150 parts per trillion by volume (pptv) and were greatly affected by what appeared as spatially large “plumes” (100 to 1000 km) with NO enhancements of over 800 parts per trillion by volume. Other trace gases were also enhanced within these plumes (e.g., CO, CO2, CH4, CH3Cl, C2H2, C2H6, C3H8, and PAN). However, for these tracers of surface emissions, inconsistent patterns of enhancement were found with respect to one another and to NO. We analyzed these plumes for indications of coherent relationships between the enhanced levels of NO and the enhanced levels of biogenic and combustion‐related tracers. This analysis indicated that the tracer relationships were primarily produced by their common injection via deep convection into the upper troposphere. A corollary analysis using a combustion tracer reference frame in combination with meteorological analysis indicates a longer than expected lifetime of NOx in the upper troposphere. This analysis also suggests that an efficient mechanism may exist in the upper troposphere for recycling HNO3 back into NOx with a rate comparable to that predicted for the HNO3 formation. During the Transport and Atmospheric Chemistry Near the Equator Atlantic study period this in‐situ formation of NOx is estimated to provide the equivalent of approximately 0.7 TgN/yr of NOx within the South Atlantic basin's upper troposphere. This magnitude of local in situ source is estimated to be comparable to the combined inputs from lightning and biomass burning, which are both injected via deep convection. Our analysis also suggests that lightning can contribute as much as half of the external input of NOx into this region of the upper troposphere with biomass burning possibly representing the remainder.

A meteorological overview of the TRACE A period

General meteorological conditions over the South Atlantic basin during the Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) experiment period are described. TRACE A occurred during one of the longest continuous El Nino‐Southern Oscillation (ENSO) events of the twentieth century. Effects of this ENSO event included warmer than normal temperatures concurrent with a severe drought in southern Africa and warm, dry conditions over northeastern Brazil. Large‐scale flow patterns are described, and the TRACE A period is compared to climatological normals. Significant meteorological features affecting the TRACE A flights included a northward moving middle‐latitude cold frontal system over Brazil, eastward moving middle‐latitude frontal waves crossing the South Atlantic basin and southern Africa, and deep convection over central Africa. The Brazilian cold front helped induce organized convective systems which led to vertical transports of biomass burning related chemical species. A well‐defined middle‐latitude frontal wave was traversed during the transit flight from Rio de Janeiro to Johannesburg, along with the penetration of a tropopause fold of stratospheric air. Deep surface‐based mixed layers and frequent diurnal convection over central Africa were responsible for vertical transports over that continent. Middle‐latitude frontal waves that crossed the southern region of Africa slightly displaced the shear axes and col zones that often were present off the west coast of central Africa.

Impact of biomass burning emissions on the composition of the South Atlantic troposphere: Reactive nitrogen and ozone

In September/October 1992 an instrumented DC‐8 aircraft was employed to study the composition and chemistry of the atmosphere over the southern tropical Atlantic Ocean. Analysis of measurements, which included tracers of biomass combustion and industrial emissions, showed that this atmosphere was highly influenced by biomass burning emissions from the South American and African continents. Marine boundary layer was generally capped off by a subsidence inversion and its composition to a large degree was determined by slow entrainment from aloft. Insoluble species (such as PAN, NO, hydrocarbons, CO) were enhanced throughout the troposphere. Soluble species (such as HNO3, HCOOH, H2O2) were minimally elevated in the upper troposphere in part due to scavenging during cloud (wet) convection. Ozone mixing ratios throughout the South Atlantic basin were enhanced by ≈20 ppb. These enhancements were larger in the eastern South Atlantic (African emissions) compared to the western South Atlantic (South American emissions). In much of the troposphere, total reactive nitrogen (NOy) correlated well with tracers of biomass combustion (e.g., CH3Cl, CO). Although NOx (NO + NO2) correlated reasonably with these tracers in the lower (0–3 km) and middle troposphere (3–7 km), these relationships deteriorated in the upper troposphere (7–12 km). Stratospheric intrusions were found to be a minor source of upper tropospheric NOx or HNO3. Sizable nonsurface sources of NOx (e.g., lightning) as well as secondary formation from the NOy reservoir species (such as HNO3, PAN, and organic nitrates) must be invoked to explain the NOx abundance present in the upper troposphere. It is found that HNO3, PAN, and NOx were able to account for most of the NOy, in the middle troposphere (3–7 km); but a significant shortfall was present in the upper troposphere (7–12). This shortfall was also most pronounced in air masses with low HNO3. The reasons for the upper tropospheric reactive nitrogen shortfall is probably due to instrumental uncertainties and the presence of unidentified organic and inorganic nitrogen species.

World Ocean Density Ratios

Abstract In this report, the potential for salt finger instability in the central water of the World Ocean is examined. The form of the temperature– salinity relationship determined from the Levitus climatological data and the density ratio of this relationship are used as a proxy to identify the regions that are susceptible to salt finger activity. The analysis indicates that most of the North and South Atlantic basins, and southeastern Indian and southwestern South Pacific Central Waters have density ratios smaller than 2.0. This is an indication that enhanced vertical salinity fluxes due to salt fingers can be an additional process affecting the thermocline freshwater budget. This study also indicates that most of the ocean's central water T–S curves are better described by a constant density ratio T–S curve than by a straight line connecting near-surface and intermediate water types.

An Assessment of ENSO-Scale Secular Variability

Abstract Secular changes in the spatial and temporal structures of the El Nino/Southern Oscillation (ENSO) cycle using four different versions of global sea surface temperature (SST) analysis are examined. The assessments were made for both multidecadal climate means and multidecadal measures of variability by separating the SST variations into low-frequency (periods longer than 30 years) and high-frequency components. The reliability of these estimates is also addressed. This study substantiates a conceptual framework that views the multidecadal, low-frequency variations as a varying climate “base state” upon which ENSO-scale variability is superposed. The secular changes of the climate base state were quantified both in space and in time. The analysis suggests that multidecadal SST variability has been concentrated in the South Atlantic and Indian Ocean Basins. The Pacific is dominated by the ENSO-scale variability. The analyses reveal that variations in the climate base state and ENSO-scale variability...

Gravity-Driven Structures and Rift Basin Evolution: Rio Muni Basin, Offshore Equatorial West Africa

Offshore Equatorial Guinea, west Africa, gravity-driven nappes, more than 1 km thick and 15 km from head to toe, provide key evidence in reconstructing the late synrift evolution of this part of the South Atlantic margin basin system. Furthermore, Aptian-Cenomanian carbonate and clastic rocks in the nappes' allochthonous hanging walls are attracting interest as a new exploration play in west Africa. The nappes exhibit a range of geometries that suggest they share many of the same deformation processes as thin-skin thrust and linked extensional fault systems. Not only are these structures significant in their own right, representing a rare example of gravity tectonics in the virtual absence of major halokinesis, but their presence may record an otherwise undetectable proce s active during the transition from a rift basin to a passive continental margin. A review of Equatorial Guinea in its pre-Atlantic configuration, alongside neighboring basins in Brazil (the Sergipe-Alagoas basin) and Gabon, suggests that gravity gliding was sustained by a relatively steep, westward paleoslope promoted by eastward offset of the locus of thermal uplift from the rift basin (i.e., a simple shear model of basin formation). In contrast to gravity-driven structures in most postrift settings, the Equatorial Guinea nappes developed at the close of the Aptian-Albian synrift episode in response to a growing bathymetric deep caused by rapid subsidence outpacing restricted sedimentation.

Chlorofluorocarbon uptake in a world ocean model: 1. Sensitivity to the surface gas forcing

The uptake and redistribution of chlorofluorocarbons (CFCs) CFC‐11 and CFC‐12 are studied in a series of world ocean model experiments. In part 1 of this study the sensitivity of the simulated CFC distributions to the model parameterization of air‐sea CFC fluxes is examined within a control experiment. The control experiment represents a low‐resolution ocean model with global coverage and a proper seasonal cycling in surface thermohaline and wind stress conditions. The specification of a surface ocean CFC concentration that is instantaneously in saturated equilibrium with the atmosphere is found to flux too much CFC into the model. Signatures of CFC‐11 are found to be grossly overestimated in regions of deep and bottom water formation, both in the surface mixed layer and at depth. The use of a classical air‐sea gas exchange formula (even one with a simplified gas transfer velocity that is independent of wind speed) is seen to greatly improve the CFC simulations at depth. In addition, the model reproduces many of the observed trends in surface CFC concentrations; namely, undersaturation in regions of deep convective overturn and near‐surface upwelling and supersaturation in the summer mixed layer. In further sensitivity experiments, we consider the effect of sea ice cover in limiting air‐sea gas exchange in polar waters. It is found that bottom water in the Arctic Ocean and around the Antarctic continent is significantly reduced in CFC content once regions covered with sea ice are limited to fractional air‐sea gas exchange. This more physically meaningful framework is found to further reduce the spurious uptake of CFC‐11 and CFC‐12 found under a “saturated surface” assumption. In a final sensitivity experiment the gas exchange rate is parameterized using a complete wind speed and Schmidt number dependence. The wind speed dependent gas forcing increases the surface CFC equilibration rate under the subpolar westerlies. On the other hand, the polar and tropical oceans witness reduced CFC uptake under a wind speed dependent flux regime. Simulated ocean CFC concentrations are compared directly with observational data in certain key areas for deep and bottom water formation. It is found that a reasonable representation of oceanic CFC is achieved in the convected water column in the Weddell and Labrador Seas. In contrast, deep waters that have left the convective area with the model ocean currents are found to be deficient in CFC‐11 in the North Atlantic Ocean. This is because the model advective timescale for North Atlantic Deep Water (NADW) outflow across the equator is too long compared with observed ocean estimates. The long timescale is not due to unrealistically sluggish deep currents. Rather, the path of NADW outflow includes a loop eastward from the Labrador Sea into the Northeastern Atlantic Basin, effectively increasing the required outflow journey by around 4000 km. This ages the water mass by almost 10 years, thereby yielding significantly lower CFC concentrations in the NADW extension. In addition, the outflow signature spreads too far into the eastern North Atlantic, presumably because the advective process is too broad and the horizontal diffusion too strong at depth. Contrasting the North Atlantic, bottom water CFC ventilation in the Southern Ocean is found to be too strong, even when significant levels of surface undersaturation are simulated in polar waters. CFC‐tagged waters flowing into the deep South Atlantic basin (from the Weddell Sea formation zone) are too enriched in CFC‐11, even when the deep signatures adjacent to the Antarctic shelf remain close to observations. This suggests that the advective timescale for bottom water ventilation is too rapid in the Southern Ocean. In addition, too much convective overturn persists in the Southern Ocean at 55°S–70°S, with unrealistically deep CFC‐11 penetration noted at particular longitudes. This is because not enough older (CFC‐deprived) water recirculates and upwells into the Southern Ocean. For example, more upwelled circumpolar deep water in the Southern Ocean would weaken the CFC‐11 concentrations by contributing to a lower CFC mixture and by suppressing the convective activity in the region. Bottom and deep level CFC signatures are broad and diffuse compared with the real ocean. The broadness of the CFC imprint is due, in part, to the model resolution, which gives any convective event a spatial extent of at least 3.75° longitude by 4.5° latitude and a bottom level CFC signal thickness in excess of 800 m. An important finding of our study is that the vertical convection of unstable waters acts as the efficient tracer ventilator of the ocean system. This has significant implications for numerical studies of the world's climate, since the meridional overturning has traditionally been considered the reason for the ocean's moderating influence during global warming scenarios. Our study suggests that the vertical convection would play a much greater role over the typical timescale for anthropogenic climate change.

Timing and Paleoceanography of Oceanic Dysoxia/Anoxia in the Late Barremian to Early Aptian (Early Cretaceous)

Normal marine deposition in the early Aptian was interrupted by an episode of ocean-wide dysoxia/anoxia. This event is recorded by the occurrence of organic carbon-rich sediments in land sections from Europe and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) sites in the North and South Atlantic, Indian and Pacific Ocean Basins. To elucidate the origin, and spatial and temporal relationships of these carbonaceous sediments, we have conducted an integrated biostratigraphic, lithostratigraphic and geochemical investigation of fourteen sections from a range of geographic and oceanographic settings

Redfield ratios of remineralization determined by nutrient data analysis

A nonlinear inverse method is applied to nutrient data upon approximately 20 neutral surfaces in each of the South Atlantic, Indian, and Pacific basins, between 400 and 4000 m depth. By accounting for the gradients in nutrients due to the mixing of “preformed” concentrations of the major water masses, the nutrient changes due to biological activity are examined, and the time‐mean, basin‐wide Redfield ratios calculated. It is found that the P/N/Corg/−O2 ratios of nutrient regeneration between 400 and 4000 m (corrected for the effect of denitrification) are approximately constant with depth and basin, at a value of 1/16 ± 1/117 ± 14/170 ± 10. These ratios agree with those of fresh organic matter, suggesting that the flux of organic material to the deep ocean may be dominated by fast‐sinking matter produced by sporadic, high‐productivity events. Sedimentary denitrification reduces the N/P utilization ratio to 12 ± 2 between 1000 and 3000 m. In the Indian and Pacific basins the Corg/Cinorg regeneration ratio decreases from approximately 7 ± 3 at 400 m to 3 ± 1 at 1000 m and to 1 ± 0.5 at 4000 m, suggesting a significant amount of calcium carbonate dissolution above the calcite lysoclines in the Indian and Pacific oceans.