Congo Fan Research Articles

Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan

The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates “from above”. As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.

Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan

This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1–4), 93–109]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600 yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.

U/Th dating of cold-seep carbonates: An initial comparison

Authigenic carbonates from hydrocarbon seeps are unique archives of past seepage and associated environmental parameters. In order to constrain the ages of a set of seep carbonates and the time involved in carbonate formation, we applied Uranium/Thorium (U/Th) dating to samples from the Gulf of Mexico, the Congo Fan, and the Black Sea. The resulting U/Th ages indicate that environmental conditions must have been favorable for enhanced methane-rich fluid seepage during the time intervals of 53.4 to 1.7kaBP for the Gulf of Mexico and 45.5 to 3.0kaBP for the Congo Fan. The seep carbonates from the Black Sea formed at 1.6 to 1.1kaBP. Our results suggest that enhanced fluid flow during these time intervals was closely related to 1) sea-level variations associated with glacial/interglacial cycles and 2) environmental alterations in the course of Late Quaternary climate change, including variations in bottom-water temperatures that affected the stability of gas hydrate reservoirs.

Vegetation response to glacial–interglacial climate variability near Lake Malawi in the southern African tropics

Pollen records from Lake Malawi, Africa spanning the last 135 kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity between 135 and 75 ka. Peaks in both the relative abundance and total production of Podocarpus pollen define the first two of these drought episodes. From 135 to 127 and again from 117 to 105 kyr BP, Podocarpus percentages remain above 16% with peak values as high as 38% indicating a period marked by a cool climate resulting in expansion of montane forest taxa to lower elevations. Marine palynological records from the Angola Margin and Congo Fan show similar peak Podocarpus percentages at this time (oxygen isotope stage 5d) indicating a similar climate across the African continent at this latitude. From 105 to 90 ka, continuing drought resulted in total pollen accumulation rates in Lake Malawi to fall to less than 300 grains/cm 2/yr of predominately grass pollen. This episode in African history was severe enough to cause the disappearance of pteridophytes and forest taxa such as Uapaca and Brachystegia as well as montane taxa ( Podocarpus, Olea spp. and Ericaceae) within the pollen source area of Lake Malawi. These taxa all remain nearly absent from the surrounding vegetation for the next 18,000 years. The resultant semi-desert vegetation would have been inhospitable for early humans living within or traveling through the Lake Malawi region. Increasing moisture following these arid intervals allowed expansion, creation and maintenance of a more diverse landscape vegetation mosaic around Lake Malawi including Zambezian miombo woodland, humid evergreen woodland and afromontane forests. The relative abundance of each fluctuated in response to either cooling (i.e. afromontane expansion from 60 to 56 ka) or moisture balance (i.e. increasing humid evergreen woodland between 75 and 65 ka). Notably there was no significant change in vegetation composition during the Last Glacial Maximum (LGM) (30–15 ka) as compared to the previous 20,000 years.

Authigenic carbonates from methane seeps of the northern Congo fan: Microbial formation mechanism

Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113 m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The δ 13C values of the Hydrate Hole carbonates range from −62.5‰ to −46.3‰ PDB, while the δ 13C values of the Diapir Field carbonate are somewhat higher, ranging from −40.7‰ to −30.7‰ PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of 18O ( δ 18O values as high as 5.2‰ PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO 3-treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in δ 13C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux.

Middle Miocene oxygen minimum zone expansion offshore West Africa: Evidence for global cooling precursor events

Research Article| August 01, 2009 Middle Miocene oxygen minimum zone expansion offshore West Africa: Evidence for global cooling precursor events S. Kender; S. Kender * 1British Geological Survey, Keyworth, Nottingham NG12 5GG, UK *E-mail: sev.kender@bgs.ac.uk. Search for other works by this author on: GSW Google Scholar V.L. Peck; V.L. Peck 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar R.W. Jones; R.W. Jones 3BP, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK Search for other works by this author on: GSW Google Scholar M.A. Kaminski M.A. Kaminski 4Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (8): 699–702. https://doi.org/10.1130/G30070A.1 Article history received: 26 Jan 2009 rev-recd: 04 Mar 2009 accepted: 23 Mar 2009 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. Kender, V.L. Peck, R.W. Jones, M.A. Kaminski; Middle Miocene oxygen minimum zone expansion offshore West Africa: Evidence for global cooling precursor events. Geology 2009;; 37 (8): 699–702. doi: https://doi.org/10.1130/G30070A.1 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 Three dissolution events ca. 16 Ma, 15.5 Ma, and 14.3 Ma ago have been identified in sediments from the Congo Fan. Multiproxy benthic foraminiferal and sedimentary records suggest an expanded oxygen minimum zone consistent with enhanced upwelling at these times. Marine carbonate records from adjacent North Africa indicate coincident episodes of increased continental weathering, suggesting that an intermittently stronger polar front strengthened west African offshore winds, increasing surface water productivity, and enhanced North African weathering during these events. We propose that Columbia River Flood Basalt volcanism, estimated to have released 106 Tg CO2 and 106 Tg SO2 between 16 and 15.6 Ma ago, may have influenced these climatic changes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Four-dimensional analysis of the Sembo relay system, offshore Angola: Implications for fault growth in salt-detached settings

Subsurface mapping of several relay ramps from a raft-related fault array in the lower Congo Basin, offshore Angola, reveals a full spectrum of fault linkage styles. A comprehensive three-dimensional geometric and kinematic appraisal of a more complex relay system within the fault array, the Sembo relay system (SRS) highlights differences with current structural models of relay ramp genesis, evolution, and breaching. The SRS is a rearward (upper ramp) breached relay system with a maximum fault overlap and spacing of 9500 and 3600 m (~31,000 and 11,800 ft), respectively. This system is characterized by a structural geometry that becomes increasingly complex with depth as the relay system is gradually assimilated into the structural architecture of a separate and structurally deeper fault system. A detailed kinematic appraisal of the SRS indicates that the throw patterns on the frontal and rearward segments are intrinsically different. Throw backstripping reveals different initiation ages for the rearward and frontal segments of about 5.5 and 2.5 Ma, respectively. Mutual overlap is estimated to first occur about 2.5 Ma at the 5.0 Ma structural level, with fault linkage and ramp breaching occurring subsequently. The SRS therefore represents a complex amalgamation of faults that have each developed independently at different times. The genesis and evolution of the SRS have been governed through time by both salt withdrawal and associated fault detachment histories, in conjunction with increased Congo Fan progradation and sedimentation rates and phases of tectonic tilting of the underlying salt detachment surface.

Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption

Abstract A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO3, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds. The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5 mmol m−2 d−1 at 1300 and 3100 m depth, respectively, and between 1.9±0.3 and 2.4±0.2 mmol m−2 d−1 at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1 mmol m−2 d−1, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel–levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5–8 times larger than the vertical flux recorded in traps. Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y−1) found on this site. The Lobe region could receive as high as 19 mol C m−2 y−1, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.

Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering

The combined use of Lu–Hf and Sm–Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf–Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (ε Nd ~ − 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by ε Hf values (from − 1.1 to + 1.3) far more radiogenic than associated sediments (from − 7.1 to − 12.0) and turbidite sands (from − 27.2 to − 31.6). ε Hf values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between ε Hf of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf–Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the ‘seawater array’ (i.e. the correlation defined by deep-sea Fe–Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the ‘seawater array’, which we refer to as the ‘zircon-free sediment array’ (ε Hf = 0.91 ε Nd + 3.10). Finally, we show that the Hf–Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (ε Hf = 1.80 ε Nd + 2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust.

Petroleum geology and resources in West Africa: An overview

Abstract The passive continental margin of West Africa is becoming an attractive area for hydrocarbon exploration, and it experienced four tectonic stages, i.e. pre-rift, syn-rift, transitional and post-rift (drifting) stages. The syn-rift and post-rift stages deposited deep lacustrine hydrocarbon sources and marine hydrocarbon sources of anoxic sediment, respectively, and the transitional stage deposited very thick evaporate rocks, which are very important for forming large traps. The outer basins in the passive margin are the main hydrocarbon-producers in West Africa. Several large rivers formed huge deltas and turbid fans such as Niger Delta, Congo Fan, Kwanza Fan, which contributed large amount of petroleum. Niger Delta and Lower Congo Basin are the most attractive areas in West Africa; Gabon Basin's and Rio Muni Basin's remained hydrocarbon resources are limited and the key problem is to find new targets; The exploration in the Kwanza Basin's deep sea is rare and has a certain risk.

Early to middle Miocene foraminifera from the deep-sea Congo Fan, offshore Angola

Analysis of a 630m section of an exploration well penetrating the distal part of the Congo Fan (~2000m water depth) yielded high abundance and diversity assemblages of agglutinated and calcareous benthic foraminifera. Planktonic foraminifera constrain the age to Early – Middle Miocene, and 18O records reveal the Mi1 (~16.3 Ma) isotopic shift. Relatively few taxonomic studies of deep-water calcareous and agglutinated benthic foraminifera exist from this time period in this locality. All species encountered are therefore taxonomically described and documented using SEM photography (over 170 species), along with 27 species of planktonic foraminifera. Faunas show close affinities to those of the eastern Venezuela Basin, Gulf of Mexico and Central Paratethys. Seven assemblages are defined and analysed using morphogroup analysis and Correspondence Analysis, documenting the response of benthic foraminifera to three primary environmental-forcing factors; energy levels in the benthic boundary layer, oxygen levels relating to changing surface water productivity, and fluctuations in the level of the CCD. Near the top and bottom of the studied section both foraminiferal abundance and diversity decrease, corresponding with increased sand content implying greater energy levels and environmental disturbance. The majority of the section consists of shales with very low percentage sand, high foraminiferal abundance and diversity, and high sedimentation rates of ~10cm/kyr. Morphogroup analysis reveals a major switch in the fauna at around oxygen isotope event Mi1, with the transition from an epifaunal-dominated Cibicidoides assemblage to shallow infaunal-dominated Bulimina assemblage. We regard this as likely due to expansion of the oxygen minimum zone (paleobathymetric estimates are ~1000m) related to increased surface-water productivity and global cooling. Shifts in calcareous foraminiferal percentage over the studied interval overprint these signals and are believed to be related to a shoaling CCD, linked to reduced oceanic acidity and global atmospheric CO2 levels during the early Middle Miocene Monterey Carbon Isotope Excursion.

Pockmarks in the Northern Congo Fan area, SW Africa: Complex seafloor features shaped by fluid flow

Three pockmarks named “Hydrate Hole”, “Black Hole”, and “Worm Hole” were studied in the northern Congo Fan area at water depths around 3100 m. The cross-disciplinary investigations include seafloor observations by TV-sled, sampling by TV-guided grab and multicorer as well as gravity coring, in addition to hydroacoustic mapping by a swath system, a parametric sediment echosounder and a deep-towed sidescan sonar. The pockmarks are morphologically complex features consisting of one or more up to 1000 m wide and 10–15 m deep depressions revealed by swath-mapping. High reflection amplitudes in the sediment echosounder records indicate the presence of a 25–30 m thick shallow sediment section with gas hydrates, which have been recovered by gravity corer. Hydrates, chemosynthetic communities, and authigenic carbonates clearly indicate fluid flow from depths, which we propose to be mainly in the form of ascending gas bubbles rather than advection of methane-rich porewater. Evidence for seepage at the seafloor is confined to small areas within the seafloor depressions and was revealed by characteristic backscatter facies. Small meter-scale sized depressions signified as “pits” exist in or close to the pockmarks but seafloor observations did not reveal evidence for the presence of typical seep organisms or authigenic carbonates. Areas of intermediate backscatter were inhabited by vesicomyid clams in soft sediments. High backscatter was associated with vestimentiferan tubeworms (Siboglinidae) and authigenic carbonates. We discuss the three different environments “pits”, “vesicomyid clams”, “vestimentifera/carbonate” in the light of differences in the geochemical setting. Pits are probably formed by escaping gas bubbles but seepage is too transient to sustain chemosynthetic life. Vesicomyid clams are present in sediments with gas hydrate deposits. However, the hydrates occur several meters below the surface indicating a lower flux compared to the vestimentifera/carbonate environment. In the latter environment, accumulated carbonates and clam shells indicate that fine grained particles have been eroded away. Gas hydrates were found in this environment at depths below about 50 cm suggesting the highest supply with methane compared to the other environments.

西アフリカ大水深における探鉱成果に基づく考察―ガボン・アンゴラのタービダイト砂岩を探る―

Deep water offshore West Africa is the most prolific petroleum exploration and production area in the last decade. “Golden Triangle” consists of US Gulf of Mexico, Brazil Campos and West Africa and is well known as three world class most prolific and promising deep water E&P areas. Especially, deep offshore Niger delta and Congo fan are the most prolific amongst them. 3D seismic data associated with advanced interpretation and visualization technique supported by recent powerful computer capability as well as sequence stratigraphy concept have contributed very efficiently to deep water success in West Africa. An approach utilizing seismic multi-attribute is the essential element for reservoir delineation and risk minimization because turbidite sand is significantly heterogeneous and thus development well spacing/positioning should be optimized by integrated reservoir study. A seismic-well log study example in deep water offshore Gabon is briefly introduced in this paper.

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.

Micropaleontological characterization of Cenozoic deep-sea fan deposits, Congo Fan, offshore Angola

Abstract

Worm tube fossils from the Hollard Mound hydrocarbon-seep deposit, Middle Devonian, Morocco: Palaeozoic seep-related vestimentiferans?

The core facies of the Hollard Mound (southeastern Morocco) contains a dense cluster of worm tubes and a high-abundance but low-diversity assemblage of bivalve molluscs. The tube worms and bivalves lived at an Eifelian hydrocarbon-seep. They are enclosed in microbial carbonates, which are similar to those that form modern seep limestones. These carbonates show δ13C values as low as −21‰. Likely carbon sources include thermogenic methane or petroleum. Hydrothermalism was possibly a factor in the generation of hydrocarbons in the Early Devonian sedimentary succession of the Hamar Laghdad with its submarine volcanics, but it is considered unlikely that the Eifelian seep fluids were hydrothermal. The Hollard Mound worm tube fossils share some characteristics with living seep vestimentiferan worms, but differ in having calcitic tube walls. Deformed Hollard Mound tubes indicate that the primary tube wall material may originally have been flexible. Comparative analyses of tubes of modern vestimentiferan worms from Gulf of Mexico and Congo Fan seeps reveal that early taphonomic processes lead to the calcification of primary organic and flexible tube walls. A very specific fabric found in both modern taphonomically altered vestimentiferan tubes and the Hollard Mound tubes is created by the delamination of individual layers of the tube wall induced by cement growth. A particular carbonate phase of the Hollard Mound microbialites is the so-called ‘carbonate with spheres’. The thin rims of the sub-mm-sized spheres are made of apatite, which is thought to have precipitated by the action of microbes that were degrading fluid droplets (e.g., petroleum at the Eifelian seep site).

High-resolution last deglaciation record from the Congo fan reveals significance of mangrove pollen and biomarkers as indicators of shelf transgression

High abundances of mangrove pollen have been associated with transgressive cycles on tropical margins, but the detailed relations between systems tracts and the taphonomy of the pollen are unclear. We report here the occurrence and high abundance of Rhizophora pollen, in association with taraxerol, a Rhizophora-sourced biomarker, from a high-resolution Congo fan core covering the last deglaciation. An age model based on 14C dates enables the temporal changes in taraxerol content and the percentage frequencies and flux (pollen grains (pg) cm −2 (10 3 yr) −1) of mangrove pollen to be compared quantitatively with the lateral rate of transgression across the flooding surface (derived from glacio-hydro-isostatic model output and the bathymetry of the margin). Rhizophora pollen concentrations and taraxerol content of the sediment are very strongly positively correlated with the lateral rate of transgression and indicate, independently of any sequence stratigraphic context, that mangrove pollen spikes are associated with the transgressive systems tract rather than the highstand systems tract or maximum flooding surface. Lower-resolution longer-term records from this margin indicate an association between taraxerol concentrations and transgressive rather than regressive phases. The flux of these materials to the Congo fan is interpreted as a function of the erosion of flooded mangrove swamp on the shelf and, less importantly, changing extent of mangrove habitat, during sea-level rise. Congo River palaeoflood events also result in reworking of mangrove pollen and supply to the fan, but this mechanism is subdominant. Rhizophora pollen has been underestimated in many palynological studies undertaken on cores from the African margin because of inappropriate sieve mesh size used during laboratory preparation.

Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin

The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245–0 ka) from the Niger and Congo deep-sea fans, however, reveal that C org/N tot ratios and isotopic signatures of bulk organic matter (δ 13C org) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased C org/N tot and δ 13C org (up to −17‰) were driven by generally nitrogen-poor but 13C-enriched terrigenous plant debris and SOM from C 4/C 3 vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C 3/C 4 vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower C org/N tot and δ 13C org (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and 12C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of C org/N tot and δ 13C org for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO 2.

Response of tropical African and East Atlantic climates to orbital forcing over the last 1.7 Ma

Abstract Records of organic matter accumulation, organic carbon isotopic composition and iron content covering the last 1.7 Ma are presented for the Congo Fan Ocean Drilling Program (ODP) Site 1075, and are compared with their counterparts from ODP Site 663 in the equatorial upwelling region. They are discussed with regard to variations in African precipitation and Congo River discharge and in the context of changes in trade-wind-driven marine productivity for the tropical Atlantic at periodicities typical of Milankovitch forcing. On the Congo Fan, elevated total organic carbon mass accumulation rates (TOC MAR) and Fe intensities occur predominantly during interglacial periods when the African monsoon was most intense. Band-pass filtering applied to TOC MAR shows distinct precessional variations, indicating that the African climate was largely controlled by low-latitude insolation changes. Only for the last 0.6 Ma, an interval of enhanced glacial-interglacial climate changes, is the precessional TOC MAR signal superimposed by a strong 100 ka oscillation. In contrast, variations in terrestrial iron input to the Congo Fan indicate pronounced 100 ka variance already well before global glacial-interglacial cycles increased in amplitude between 0.9 and 0.6 Ma. Obliquity cycles in the Fe signal are strongly expressed for the last 0.9 Ma. The highest amplitudes in the precessional variance of fluvial Fe input occur when amplitudes in the 100 ka oscillation were at intermediate levels and reveal a 800 ka cycle in phase shift with respect to precessional forcing. Together with a pronounced 800 ka signal in the 100 ka amplitude variations during the last 1.7 Ma, the Congo Fan iron record therefore suggests that eccentricity modulation of the low-latitude insolation directly influenced the equatorial African monsoon system and probably the weathering conditions on land. It further suggests that low-latitude precessional forcing and monsoonal response in the tropics might have played an important role for 100 ka cycles in global climate well before huge continental ice sheets existed.

Morphogenesis of Congo submarine canyon and valley: implications about the theories of the canyons formation

The bottom and edges of the Congo submarine canyon are made of deposits which show morphologies of superposed terraces associated with lense side slope. Using 3D seismic, 3.5 kHz acoustic profiles and core datas, these geometries are interpreted as being mainly the result of the polyphase processes due to the activity of turbidity currents. All the turbidite deposits located below the canyon floor and the actual Congo fan belong to a unique depositional sequence which begun at the early Pliocene. Probably, this sequence has been initiated by an important uplift of the West Africa margin.