南海北部陆缘OBS2018-H2测线地壳结构初步结果

南海北部陆缘洋陆转换带实施的OBS2018-H2测线的地壳速度结构, 将为探讨南海张裂-破裂机制提供重要证据。文章介绍了OBS2018-H2测线前期数据处理流程, 包括多道反射地震数据处理、海底地震仪OBS (Ocean Bottom Seismometer)数据格式转换、炮点和OBS位置校正, 以及OBS震相的初步识别, 并对地壳结构进行了初步分析。结果表明: 炮点和OBS位置校正效果良好; 多道反射地震数据为建立初始速度模型提供了良好约束; OBS综合地震剖面识别了多组清晰的P波震相, 包括Pw、Pg、PmP和Pn震相。根据测线西侧OBS36、OBS37两台站的震相分布特征初步估算台站下方地壳厚度约为6~7km, 与根据多道地震剖面LW3的双程走时估算的厚度6~9km大致相符。     

Spatial and temporal patterns of benthic macrofaunal communities on the deep continental margin in the Gulf of Guinea

Density, taxonomic composition at higher taxon level and vertical distribution of benthic macrofaunal communities and sediment characteristics (pore water, nitrogen, organic carbon, sulfur, C/N ratio, n-alcohol biomarkers) were examined at three deep sites on the Congo–Gabon continental margin. This study was part of the multidisciplinary BIOZAIRE project that aimed at studying the deep benthic ecosystems in the Gulf of Guinea. Sampling of macrofaunal communities and of sediment was conducted during three cruises (January 2001, December 2001 and December 2003) at two downslope sites (4000 m depth), one located near the Congo submarine channel (15 km in the south) and the other one far from the channel (150 km in the South). The third area located 8 km north of the Congo channel in the surroundings of a giant pockmark at 3160 m depth was sampled during one cruise in December 2003.At these three locations the macrofaunal communities presented relatively high densities (327–987 ind. 0.25 m⁻²) compared with macrofaunal communities at similar depths; that is due to high levels of food input related to the Congo river and submarine system activities that affect the whole study area. The communities were different from each other in terms of taxonomic composition at higher taxon level (phylum, class, order for all the groups except for the polychaetes classified into families). The polychaetes dominated the communities and were responsible for the increase in densities observed at both deep sites (4000 m) between January 2001 and December 2003 whereas the tanaidaceans, the isopods and the bivalves were the other most abundant taxa responsible for the spatial differences between these sites. The community at 3150 m differed from the two deep communities by higher abundances in bivalves, nemerteans and holothuroids. The composition of the polychaete community also differed among sites.In the vicinity of the Congo channel, the expected positive effect of the additional organic matter transported through the turbiditic currents on to the surrounding benthic communities was not observed, as the increase in densities during the study period was higher at the site located away from the Congo channel than near the channel (80% vs 30%). That may be due to the low food value of the organic matter of terrestrial origin carried through the turbidites, and/or to the disturbance caused by these turbidites. Conversely, far from the channel the macrofaunal communities benefit from organic matter of higher energetic value originating mainly from marine sources, but also from continental sources, carried by the Congo plume or by near-bed currents across or along the continental slope. Spatial and temporal variability in trophic and physical characteristics of the sediment habitat at both deep sites also affected the vertical distribution of the macrofaunal communities.The activities of the very active Congo system structure the deep macrofaunal communities on a large area in terms of densities, composition and vertical distribution. The food input is enhanced at regional scale as well as the heterogeneity of the sediment characteristics, mainly in terms of organic matter quality (marine vs terrigenous). In turn, the densities are enhanced as well as the regional diversity of the macrofaunal communities in terms of taxonomic composition and distribution.     

Factors influencing benthic bacterial abundance, biomass, and activity on the northern continental margin and deep basin of the Gulf of Mexico

As part of a larger project on the deep benthos of the Gulf of Mexico, an extensive data set on benthic bacterial abundance (n>750), supplemented with cell-size and rate measurements, was acquired from 51 sites across a depth range of 212–3732 m on the northern continental slope and deep basin during the years 2000, 2001, and 2002. Bacterial abundance, determined by epifluorescence microscopy, was examined region-wide as a function of spatial and temporal variables, while subsets of the data were examined for sediment-based chemical or mineralogical correlates according to the availability of collaborative data sets. In the latter case, depth of oxygen penetration helped to explain bacterial depth profiles into the sediment, but only porewater DOC correlated significantly (inversely) with bacterial abundance (p<0.05, n=24). Other (positive) correlations were detected with TOC, C/N ratios, and % sand when the analysis was restricted to data from the easternmost stations (p<0.05, n=9–12). Region-wide, neither surface bacterial abundance (3.30–16.8×10⁸ bacteria cm⁻³ in 0–1 cm and 4–5 cm strata) nor depth-integrated abundance (4.84–17.5×10¹³ bacteria m⁻², 0–15 cm) could be explained by water depth, station location, sampling year, or vertical POC flux. In contrast, depth-integrated bacterial biomass, derived from measured cell sizes of 0.027–0.072 μm³, declined significantly with station depth (p<0.001, n=56). Steeper declines in biomass were observed for the cross-slope transects (when unusual topographic sites and abyssal stations were excluded). The importance of resource changes with depth was supported by the positive relationship observed between bacterial biomass and vertical POC flux, derived from measures of overlying productivity, a relationship that remained significant when depth was held constant (partial correlation analysis, p<0.05, df=50). Whole-sediment incubation experiments under simulated in situ conditions, using ³H-thymidine or ¹⁴C-amino acids, yielded low production rates (5–75 μg C m⁻² d⁻¹) and higher respiration rates (76–242 μg C m⁻² d⁻¹), with kinetics suggestive of resource limitation at abyssal depths. Compared to similarly examined deep regions of the open ocean, the semi-enclosed Gulf of Mexico (like the Arabian Sea) harbors in its abyssal sediments a greater biomass of bacteria per unit of vertically delivered POC, likely reflecting the greater input of laterally advected, often unreactive, material from its margins.     

Physiographic features on the northern Gulf of Mexico continental slope

The continental slope of the northern Gulf of Mexico is diapirically controlled and is comprised of coalescing salt sheets, salt withdrawal basins, salt ridges, salt tongues and sills, and submarine canyons. Bathymetric information from single-beam data has resulted in several published maps. Many of the map areas have been remapped, using multibeam surveys, by the US National Ocean Service, and names have been given to the major physiographic features. The multibeam program was discontinued before complete coverage of the slope was accomplished. We provide charts of the remaining areas with names of features that have been accepted by the US Board of Geographic Names.     

Submarine faults and slides on the continental shelf,northern Gulf of Alaska

Abstract

Submarine faults and slides or slumps of Quaternary age are potential environmental hazards on the outer continental shelf (OCS) of the northern Gulf of Alaska. Most faults that approach or reach the seafloor cut strata that may be equivalent in age to the upper Yakataga Formation (Pliocene‐Pleistocene). Along several faults, the seafloor is vertically offset from 5 to 20 m. A few faults appear to cut Holocene sediments, but none of these shows displacement at the seafloor. Submarine slides or slumps have been found in two places in the OCS region: (1) seaward of the Malaspina Glacier and Icy Bay, an area of 1200 km² with a slope of less than 0.5°, and (2) across the entire span of the Copper river prodelta, an area of 1730 km², having a slope of about 0.5°. Seismic profiles across these areas show disrupted reflectors and irregular topography commonly associated with submarine slides or slumps. Potential slide or slump areas have been delineated in areas of thick sediment accumulation and relatively steep slopes. These areas include (1) Kayak Trough, (2) parts of Hinchinbrook Entrance and Sea Valley, (3) parts of the outer shelf and upper slope between Kayak Island and Yakutat Bay, and (4) Bering Trough.     

Shallow structures and evolution of the Ivory Coast and Ghana transform margin

Seismic profiles across the transform continental margin off the Ivory Coast and Ghana (Western Africa) illustrate the structural style resulting from the early Cretaceous phase of shear stress which leads to the final separation between the African and Brazilian continental margins in this area. Most of the characteristic tectonic features observed along this portion of margin (asymmetric grabens on the Ghanean platform, folds of the deep Ivory Coast basin, the Ivory Coast—Ghana marginal ridge) are believed to result from progressive transform contacts between the African and Brazilian continents as their margins were created during early Cretaceous time. A major tectonic unconformity inferred to be of upper Albian-lower Cenomanian age, may be a direct consequence of the final separation of the continental margins. The later evolution of the transform margin is chiefly explained by thermal subsidence.     

Seismic expression of sedimentary volcanism on the continental slope,northern Gulf of Mexico

High-resolution geophysical data define acoustically amorphous, mounded structures on the upper, middle, and lower continental slope of the northern Gulf of Mexico. Physical samples and observations within this unique seismic facies show gassy sediments, sometimes in hydrated form and, in places, as chemosynthetic communities. The geologic setting of these mounds suggests that the process of formation falls on the continuum of mud volcanoes to mud diapirs.     

Current-influenced depositional provinces, continental margin off Cape Hatteras, identified by petrologic method

Analyses of DSRV “Alvin” core samples on the Cape Hatteras margin indicate major textural and compositional changes at depths of about 1000 and well below 2500 m. The distribution patterns of petrologic parameters correlate well with water mass flow and suspended-sediment plumes measured on this margin by other workers. Our study also shows: (a) vigorous erosion and sediment transport at depths of less than 400 m resulting from the NE-trending Gulf Stream flow; (b) deposition, largely planktonic-rich sediment released from the Gulf Stream, on the upper- to mid-slope, to depths of about 800–1200 m; (c) winnowing, resuspension and deposition induced by periodically intensified slope currents on the mid-slope to uppermost rise, between about 1000 and 2500 m; and (d) prevailing deposition on the upper rise proper (below 2500 m), from transport by the SW-trending Western Boundary Undercurrent. Sediments moved by bottom currents have altered the composition and distribution patterns of material transported downslope by offshelf spillover; this mixing of gravity-emplaced and bottom-current-transported sediment obscures depositional boundaries. Moreover, reworking of the seafloor by benthic organisms alters physical properties and changes erodability of surficial sediments by bottom currents. Measurement of current flow above the seafloor and direct observation of the bottom are insufficient to delineate surficial sediment boundaries. Detailed petrologic analyses are needed to recognize the long-term signature of processes and define depositional provinces.     

Surficial sediments on the continental margin off the west coast of South Africa

A combination of climatic and oceanic factors has resulted in slow sedimentation rates on the continental shelf off the west coast of South Africa since Tertiary times. This has enabled a study to be made of the residual Late Tertiary, relict Pleistocene and Holocene sediments.Sediments on the continental shelf form rough belts parallel to the coast. Most of the coarse sediment is confined to the littoral zone and Holocene mud is concentrated at the base of a rocky nearshore platform. A veneer of Quaternary quartzitic sand seawards of the Recent mud belt wedges out onto a Tertiary erosion surface on the mid shelf. Residual glauconite and phosphorite sands derived by erosion during Tertiary sea-level fluctuations cover large parts of the mid shelf in the south. Most of the slope and parts of the outer shelf in the north are draped by Recent foraminiferal and coccolithophorid debris.     

Ocean swell variability along the northern coast of the Gulf of Guinea

This study analyses a 4.5 year (September 2009–March 2014) time-series of remotely-sensed data of altimeter significant wave heights to describe the temporal and spatial variability of ocean swells along the northern coast of the Gulf of Guinea. The NOAA WAVEWATCH III (NWW3) wave model data were used with altimeter data to determine the origin of the swells that occur along the coast of Côte d'Ivoire in West Africa. We show that the ocean swells along the northern coast of the Gulf of Guinea are generated in the Southern Ocean and then propagate from south to north in the South Atlantic Ocean, before turning south-west to north-east close to the coast. This finding corroborates previous studies in this area. The remotely-sensed and NWW3 significant wave height data captured the strong swells observed along the coast of Côte d'Ivoire from the period 28 August–3 September 2011, which were responsible for an extreme erosion event of more than 12?m along that country's coastline. This extreme event was triggered by a strong storm in the region between 40° and 60° S that occurred eight days previously in the South Atlantic. The waves propagated as swells at a speed of about 875?km day–¹ before reaching the northern African coast.     

Carbonate and organic carbon sedimentation on the continental margin off Southeastern Taiwan

Sediment and porewater data of two sediment cores from offshore southeastern Taiwan show important effects of the complex sea floor topography upon carbonate and organic carbon deposition. The variations in carbonate and organic carbon contents are attributed to the effects of carbonate compensation depths, glacial-interglacial history, and condition of anoxia. The study is of significance for environmental reconstruction and petroleum source bed evaluation of ancient analogues.     

Numerical model simulations of continental shelf flows off northern California

The three-dimensional circulation on the continental shelf off northern California in the wind events and shelf transport (WEST) experiment region during summer 2001 is studied using the primitive equation regional ocean modeling system (ROMS). The simulations are performed with realistic topography and initial stratification in a limited-area domain with a high-resolution grid. Forcing consists of measured wind-stress and heat flux values obtained from a WEST surface buoy. The general response shows a southward coastal upwelling jet of up to and a weakening or reversal of currents inshore of the jet when upwelling winds relax. Model results are compared to WEST moored velocity and temperature measurements at five locations, to CODAR surface current observations between Pt. Reyes and Bodega Bay, and to hydrographic measurements along shipboard survey lines. The model performs reasonably well, with the highest depth-averaged velocity correlation (0.81) at the inshore mooring (40 m water depth) and lowest correlation (0.68) at the mid-depth mooring (90 m depth). The model shows generally stronger velocities than those observed, especially at the inshore moorings, and a lack in complete reversal of southward velocities observed when upwelling winds relax. The comparison of surface velocities with CODAR measurements shows good agreement of the mean and the dominant mode of variability. The hydrography compares closely at the southern and northern edges of the survey region (correlation coefficients between 0.90 and 0.97), with weaker correlations at the three interior survey lines (correlation coefficients between 0.44 and 0.76). Mean model fields over the summer upwelling period show slight coastal jet separation off Pt. Arena and significant separation off Pt. Reyes. The cape regions also experience relatively strong bottom velocities and nonlinearity in the surface flow. Across-shelf velocity sections examined along the shelf reveal a double jet structure that appears just north of Bodega Bay and shows the offshore jet strengthening to the south. We examine the dynamics during an upwelling and subsequent relaxation event in May 2001 in which the WEST measurements show evidence of a strong flow response. The alongshelf variability in the upwelling and relaxation response introduced by Pt. Reyes is evident. Analysis of term balances from the depth-averaged momentum equations helps to clarify the event dynamics in different regions over the shelf. A clear pattern in the nonlinear advection term is due to the spatial acceleration of the southward jet around the capes of Pt. Arena and Pt. Reyes during upwelling. Results from a three-dimensional Lagrangian analysis of water parcel displacement show significant southward displacement in the coastal jet region, including a strong signal from the double jet. Alongshelf variability in parcel displacements and upwelling source waters due to the presence of Pt. Arena and Pt. Reyes is also apparent from the Lagrangian fields. A cyclonic eddy-like recirculation feature offshore of Pt. Arena prior to the upwelling event causes large patches of onshore-displaced parcels. Additionally, across-shelf variability in the response of water parcels along the D line includes decreased vertical displacement and increased alongshelf displacement in the offshore direction.     

Large Vesicomyidae (Mollusca: Bivalvia) from cold seeps in the Gulf of Guinea off the coasts of Gabon,Congo and northern Angola

Two new genera and three new species of large Vesicomyidae are described from cold-seep sites on pockmarks and other sulfide-rich environments in the Gulf of Guinea (tropical east Atlantic) off Gabon, Congo (Brazzaville) and northern Angola, from 500 to 4000 m depth: “Calyptogena” (s.l.) regab n. sp., Wareniconcha (n.g.) guineensis (Thiele and Jaeckel 1931), Elenaconcha guiness n.g. n. sp., and Isorropodon atalantae n. sp. For two other species already taken by the R/V Valdivia in 1898, Calyptogena valdiviae (Thiele and Jaeckel 1931) and Isorropodon striatum (Thiele and Jaeckel 1931) new localities were discovered, and the species are rediscussed. E. guiness n.g. n.sp. is also recorded from off Banc d’Arguin, Mauritania, collected by commercial fishing vessels. The vesicomyid species here treated were encountered in different depth ranges along the Gabon–Congo–Angola margin, between 500 and 4000 m depth, and it was found that, in comparison with the dredge samples taken by the Valdivia expedition off southern Cameroon and off Rio de Oro (both at 2500 m), the same species occur in other depth ranges, in some cases with a vertical difference of more than 1000 m. .That means that the species are not confined to a given depth thought being typical for them and that the characteristics of the biotope are likely to play a major role in the distribution of the vesicomyids associated to cold seeps or other reduced environments along the West African margin.     

Structure of oceanic crust adjacent to a transform margin segment: the Côte d’Ivoire–Ghana transform margin

The structure of the oceanic crust adjacent to the Côte d’Ivoire–Ghana transform margin is deduced from multichannel seismic reflection and seismic wide-angle data, showing crustal heterogeneities within oceanic basement; the oceanic crust adjacent to the transform margin is half as thick as standard Atlantic oceanic crust. Refraction data indicate a gradual velocity transition towards typical mantle velocities. Such an abnormal oceanic crustal structure appears quite similar to crustal structures known along transform faults. This crustal thinning may be related to thermal effects of the nearby continental crust, on the oceanic accretion processes. We did not find geophysical evidence for oceanic crust contamination by continental lithosphere.     

Deep crustal structure and rheology of the Gascoyne volcanic margin,western Australia

Interpretation of deep seismic reflection data across the Gascoyne Margin reveals six distinct seismic facies units related to the tectono-magmatic breakup history. On the outer Exmouth Plateau four large scale units are identified: (1) an extensively block-faulted upper crust; (2) a middle-crustal unit of discontinuous, undulatory reflectors; (3) a reflection-free deep crustal unit; and (4) a lower-crustal band of low-frequency, high-amplitude reflectors. Two additional units are found near the continent-ocean boundary (COB); (5) seaward-dipping reflectors (SDR); and (6) landward-dipping reflectors in the lower crust below the SDR. The lower-crustal high-reflectivity band, located near the top of a high-velocity unit (V_p > 7 kms^–1), is interpreted as magmatic underplating. There is a spatial correlation between the underplated area and the presence of extensive upper-crustal block-faulting and intrusive rocks in the shallow crust. The undulatory middle-crustal reflector unit is also only identified in the outer plateau area, and is interpreted as a zone in which the upper-crustal faults terminate. The inner parts of the margin consist of a deep basin showing little upper-crustal faulting and no evidence of middle crustal deformation or underplating. Theoretical modeling of the effect of rifting and magmatic underplating on crustal strength profiles suggests that the brittle-ductile transition may migrate at least 5 km upwards during several million years after the underplating event. Based on the seismic interpretation and crustal strength modeling we propose that the seismic structure of the outer Exmouth Plateau is severely modified by a transient change in the crustal rheological structure associated with magmatic underplating.     

Two fundamentally different types of submarine canyons along the continental margin of Equatorial Guinea

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme.The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (>150 m), dilute, turbidity currents. There is little evidence for erosion, mass-wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes.Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, highly aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass-wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.     

Vertical flux of biogenic matter during a Lagrangian study off the NW Spanish continental margin

Lagrangian experiments with short-term, drifting sediment traps were conducted during a cruise on RRS Charles Darwin to the NW coast of Spain to study the vertical flux and composition of settling biogenic matter. The cruise was split into two legs corresponding to (i) a period of increased production following an upwelling event on the continental shelf (3–10 August 1998) and (ii) an evolution of a cold water filament originating from the upwelled water off the shelf (14–19 August). The export of particulate organic carbon (POC) from the upper layer (0–60m) on the shelf was 90–240mgC.m⁻².d⁻¹ and off the shelf was 60–180mgC.m⁻².d⁻¹. Off shelf the POC flux at 200m was 50–60mg.m⁻².d⁻¹. A modest sedimentation of diatoms (15–30mgC.m⁻².d⁻¹) after the upwelling was associated with increased vertical flux of chlorophyll a (1.8–2.1mg.m⁻².d⁻¹) and a decrease of the POC:PON molar ratio of the settled material from 9 to 6.4. Most of the pico-, nano-, and microplankton in the settled material were flagellates; diatoms were significant during the on shelf and dinoflagellates during the off shelf leg. Off shelf, the exponential attenuation of POC flux indicated a strong retention capacity of the plankton community between 40 and 75m. POC:PON ratio of the settled particulate matter decreased with depth and the relative portion of flagellates increased, suggesting a novel, flagellate and aggregate mediated particulate flux in these waters. Export of POC from the euphotic layer comprised 14–26% of the integrated primary production per day during the on shelf leg and 25–42% during the off shelf leg, which characterises the importance of sedimentation in the organic carbon budget of these waters.     

Development of a passive transform margin: Côte d’Ivoire–Ghana transform margin – ODP Leg 159 preliminary results

The Côte d’Ivoire–Ghana transform margin is bounded to the south by a prominent marginal ridge. ODP Leg 159 shipboard analyses on sediments from four sites document three distinct transform margin sedimentary and tectonic stages of evolution: (1)?an intracontinental stage of transform faulting recorded in deformed lacustrine to marine siliciclastic sequences; (2)?a marginal ridge uplift stage, recorded by shallow water limestones, appears coeval with the passing of a hot, oceanic spreading center just south of the sediment wedge; and (3)?cool ing subsidence of the transform margin recorded in bathyal to abyssal sediments emphasizes a passive margin stage. These results are consistent with previously published models of evolution.