Ocean Margin Systems

Sedimentation on continental margins bears the strong imprint of the tectonic setting, changes of sea-level and many local processes, including human intervention in sediment supply. Carbonate sediments are mainly created within the depositional basin whereas clastics supplied from upland areas are susceptible to enhancement due to deforestation or cut-off due to damming. Quaternary oscillation of sea-level has occurred with a frequency that has generally prevented supply and dispersal systems coming to equilibrium. Thus shelf shape reflects low-stand of sealevel and sediments dispersed from outer shelves/upper slopes were mainly emplaced at low stand — only in cases of maj or deltas has delivery overcome post-glacial sea-level rise. Dispersal processes of waves, tidal currents, wind-driven currents, oceanic currents and slope currents, and gravity flows down slopes and in canyons are briefly outlined. Major areas of uncertainty remain in budgets where flood-plain storage is generally unknown, the amount (and grain size distribution) trapped on shelves versus escaping to the ocean is unknown, and the magnitude/frequency structure of modern mass flows in canyons is poorly known.

The ocean margin, including the continental shelf, slope and rise, constitutes an essential boundary between the continents and the ocean basins and represents about 20% of the surface area of the marine system. It is characterized by an enhanced productivity and biological activity due to the input of nutrient from rivers and more importantly, from the transfer of nutrient-rich deep ocean waters. This transfer results from upwelling under favourable local wind conditions or from turbulent mixing at the shelf break due to wind-stress or internal waves mostly of tidal origin. Despite the great complexity of the ocean margin system, compilation of the literature provides coherent data sets indicating that the global primary production on the shelf may reach 6 – 7 GTC yr-1 and about 5 GTC yr^-1 on the adjacent slope, as compared to 28 GTC yr^-1 for the open ocean. Results on ¹⁵N incorporation experiments or nutrient budget calculations indicate that the f-ratio is most probably around 0.3 5 – 0.4. The export production estimates, mainly obtained from flux measurements in sediment traps deployed on the slope, are often lower than those for the new production, likely due to the rapid degradation of organic matter in the intermediate and deep water column and to the low trapping efficiency. Furthermore, organic matter may be exported from the shelf to the open ocean in surface waters or within the benthic boundary layer by resuspension and off-shore transport. These fluxes may not be recorded by the sediment traps. Exchange of CO₂ with the atmosphere at the margins is complicated by the competition between high inputs of this-component from deep ocean water and river water, and its rapid removal by photosynthesis in a very productive area. In addition, a tentative mean global cycle for nitrogen on the shelf is presented and discussed. It confirms on a global scale the major role of deep open ocean water transfer versus continental input of this element. Finally the data collected in the literature demonstrate that the continental margins are privileged areas of production, deposition and burial of CaCO₃, a component which has been often poorly considered in existing global carbon cycle assessments in the oceans.

In regions of complex geology, the corresponding seismic time sections show diffraction events, distorted positions of reflectors, smeared lateral discontinuities and uncertain amplitudes of seismic reflections with the consequence that the interpreter cannot automatically reconstruct and identify the cross-section through the earth from the seismic time section. The purpose of migration (imaging of seismic data) is to reconstruct structural depth sections from seismic time sections and to produce a geological image of the subsurface. ‘Poststack migration’ is an imaging process that uses stacked averaged seismic data. Stacking reduces the amount of data drastically, and thus subsequent migration of stacked data is less expensive than prestack migration. On the other hand, severe problems occur where the standard stacking procedure can no longer be applied, e.g. when the reflection surfaces are strongly curved or there are strong variations of velocities. The ‘prestack seismic migration process’ is a reconstruction of subsurface structures directly from measurements at the surface of the earth. The advantages of this process particularly appear in faulted steep dip regions with extreme flexures. Furthermore, this process allows an iterative estimation of the migration velocity field (macro velocity field), which is needed as an important parameter field for the migration process itself. ‘Depth migration’ is a transformation, mapping, and imaging of measured seismic time data into a migrated depth section. It yields a reliable image of the subsurface, provided that the migration velocity field (macro velocity field) is correct. ‘Time migration’, in principle, represents a special type of depth migration, where the depth scale τ is scaled by a pseudo-time scale using a replacement velocity. Most time migration schemes do not take into account the refraction of waves at the boundaries, hence, these schemes fail to image data from laterally inhomogeneous media. On the other hand, time migration schemes show low sensitivity to migration velocity errors and can be used as an intermediate step in seismic data processing for imaging seismic data. The conventional seismic 2-D technique provides an erroneous image of the subsurface, if the subsurface varies perpendicularly to the 2-D seismic recording direction. In this case, a 3-D technique has to be applied. The 3-D migration process plays a fundamental role in the 3-D processing sequence. It provides a reliable image of the earth and avoids possible misinterpretation which may occur when 3-D structures are explored with 2-D methods. 3-D poststack time migration is often used in practical work because this first result is needed for first interpretation and to check the effect of 3-D processing and 3-D imaging. Nevertheless, this type of imaging fails to give a reliable image of the subsurface in the case of complex geological areas, e.g. complex overburden structures. 3-D poststack depth migration needs the same computing time as 3-D prestack time migration, but yields a reliable depth image of the subsurface provided that the conditions for 3-D stacking are given and a reliable 3-D macro velocity field is available. 3-D prestack depth migration has long been recognized as the appropriate imaging procedure for complex structures and complex velocities. It requires that the velocity field is estimated using the migration procedure itself in an iterative manner, however, therefore this migration process consumes an enormous amount of computing time. In this paper, we focus on the description of a 3-D imaging process which uses a special type of 3-D prestack depth migration only to estimate the 3-D macro velocity field and then applies a 3—D poststack depth migration scheme. This scheme is applied to a 3-D data set acquired at the Costa Rica convergent margin.

The sampling of sediments from the sea floor is an integral part of marine geological research. However, as the sea floor is not directly accessible, marine geologists have to rely on instruments which they have to lower to the sea floor from seagoing vehicles, including e.g. vessels and drilling platforms. This paper aims to summarise the present state of the art of these sampling methods and tries to give an outlook on future prospects of scientific coring and drilling of marine sediments.

Processes that determine the environmental structure of ocean margins are calling for multidisciplinary observations. Current knowledge has been gained mainly through ship board observational programs. But this approach is not sufficient to account for the variability of the processes involved in the ocean margin environment. For instance the importance of intense events of short duration compared to slowly but permanently ongoing processes is not well understood up to now. Therefore the main focus of future observational program will be on improving the spatial and temporal coverage of the measured parameters and enhancing the long-term capability of the system.This paper describes already existing or upcoming technologies that will allow for more intensive investigations of water column properties. Moored profiling platforms that are able to host a complete suit of physical, biological and chemical sensors are introduced as efficient observational tools to fulfil the envisaged goals. Common to all described systems are the need for adequate energy supply, autonomous operation and bi-directional data exchange to enable event triggered sampling. The tradeoffs between different technological approaches addressing the mentioned scientific needs and the consequences for their applicability are described.

Autonomous Underwater Vehicles (AUVs) are fast becoming accepted as very useful data gathering platforms within the marine science community throughout the world, as the range and depth envelopes are being pushed, by developments in battery technology, propulsive efficiency, and pressure vessels technologies. It is already accepted that AUVs can bring great benefits in data quality and cost, in for example geophysical surveys for oil and gas exploration. But within the science community there is the perception that AUVs are expensive, complex and risky to use. Is this a fair representation, or is it based on outdated prejudices? This paper examines the advantages and disadvantages of the use of AUVs as platforms for Ocean Margin surveys, compared to conventional towed instruments, drawing on examples of AUVs currently being used throughout the world. It illustrates the development and use of a scientific AUV, Autosub, during the past four years. How has it developed to overcome technological problems, such as launch and recovery, and achieving greater depth and range, and how have the engineers coped with the integration of many different types of sensor? It discusses some possible reasons why AUVs are not more generally used for ocean surveys.

Ocean margins are very productive areas and consequently they are interesting both for scientific and socio-economic reasons. Their economical importance was the main reason to support integrated projects to understand and quantify the processes responsible for high biological productivities, in order to create the scientific knowledge required for its management. For long time it was believed that high productivity of ocean margin areas was a consequence of the discharge of nutrients form the continents. As the scientific knowledge of the processes taking place on those areas increased, it was shown that biological productivity of the ocean margin is mainly a consequence of the complexity of the physical processes taking place on those areas and only in semi-enclosed areas (e.g. estuaries) a consequence of continental discharge. This conclusion has enhanced the importance of the development of integrated studies involving fieldwork and modelling.The complexity of physical problems taking place on ocean margins is a consequence of local depth gradients (e.g. continental slope and submarine canyons), but also of the wide range of forcing mechanisms driving the flow — wind, density and tides. The combination of these forcing mechanisms lead to an even more wide range of phenomena like, upwelling, fronts, internal waves, surface gravity waves, etc. To understand processes going on, process-oriented models can be used. However the final product for modelling processes in coastal areas must be an integrated model based on the primitive equations for mass and momentum. For management purposes this model has to couple physical and biological processes. In this paper a general modelling framework is described. This tool is developed to accommodate models for physics, biology and sediment transport. Numerical solutions, processes and results for the Iberian margin and for the Tagus Estuary (Portugal) are described.

Ocean margins have become the focus of numerous geophysical and environmental surveys, because of their economic, scientific and oceanographic significance. These surveys deliver increasingly larger volumes of data, acquired by many types of techniques and sensors. Despite their importance, most of these data are still interpreted visually and qualitatively by skilled interpreters. Human interpretation is time-consuming and difficult to standardise; and, in certain conditions, it can be error-prone. Current research in data processing is shifting toward computerbased interpretation techniques, and in particular seafloor classification. After a brief review of the main characteristics of ocean margins, the different notions of classification will be presented, along with the desired aims. These will be followed by a review of non-acoustic and acoustic (mainly sonar) classification techniques, supplemented with actual examples when applicable. Seabed classification, in general and at ocean margins, is fast becoming a major tool in seafloor surveying and monitoring. The last section will assess the latest tendencies and the technical developments that can be expected in the near future.

Processes in the benthic boundary layer (BBL) at different continental margins are described and the importance of lateral particle transport, particle aggregation, biological mediated particle deposition and resuspension within the BBL is discussed. Theoretical and methodical aspects of BBL research are demonstrated and examples of modern continental margin studies are given.

The early diagenetic processes occuring in ocean margin sediments are discussed with an emphasis on the carbon, nitrogen and oxygen cycle. We first use simple mass-balance assumptions to derive a relationship between the fluxes at the sediment-water interface and the processes that occur in the sediments. This allows amongst others relating the sediment community oxygen consumption rate (SCOC) to total organic carbon deposition. It is shown, based on literature data that, overall, SCOC decreases significantly with depth, equivalent with a decline of organic carbon flux from about 35 g C m^-2 yr¹ at 100 m to a flux of 4 g C m^-2 yr ¹ at 4000 m depth. A simple 0-dimensional and two more complex 1-dimensional models are then used to demonstrate the applicability of diagenetic modelling in interpreting the species distribution vertically in the sediment and quantifying the processes occurring in sediments. Finally, the different view of biochemists and biologists of the sediment is discussed. It is concluded that much can be gained from a combined biochemical and biological analysis of the sediments.

Giant submarine landslides occur on almost every contintental margin. Individual slides involve up to 20,000 km³ of slope material and cover an area of up to 113,000 km². Their wide spread distribution and their large dimensions make them important geological features, particularly as many of them are located within hydrocarbon exploration areas. The factors that are controlling slope stability are still poorly understood in spite of significant research efforts, and there are only few landslides for which the trigger is known with certainty. It appears that ground motion due to earthquakes, rapid sedimentation, and slope destabilization by gas hydrates are among the most important factors, whereas slope angles seem to be less important.

Processes at the ocean margins are described and the importance of sedimentary settings, the carbon cycle, the benthic boundary layer, canyons and mass movements is discussed. Mayor outstanding problems are formulated and new enabling technologies are demonstrated.

The transfer of solutes and gases across the sediment-water interface through fluid flow at vents and seeps are complementing the inputs of nutrients, methane and xenobiotics through riverine and atmospheric input and diffusive fluxes from the seafloor to the water column of the continental margin. Fluid advection through sediments provide an efficient mechanism for the upward transport of reactive components and trace gases like methane and carbon dioxide, otherwise remineralised or precipitated within the sediment without impact on the chemistry of the ocean or the biota at the seafloor. Studies on submarine groundwater discharge and fluid venting along accretionay ridges emphasise the importance of fluid flow for hydrological budgets, biogeochemical cycles, and physical properties of sediments. Offshore plankton blooms, methane and trace element plumes, and release of large amounts of fresh water reveal the ecological and economic significance of submarine fluid discharge in the coastal zone. Along accretionary ridges, vent sites are characterised by the unique consortium of benthic organisms, formations of massive authigenic carbonates, methane plumes, and occurrences of gas hydrate. These features are associated with fluid flow in sediments. Techniques and tracers for localisation of discharge sites and quantification of discharge rates are introduced. The similarities between these two environmental settings characterised by fluid flow are considered and potential needs for future studies are discussed.

The impact of macrobenthic activity on the geochemistry of surface sediments is reviewed to provide conceptual insights on animal-sediment relations for benthic ecologists, paleoceanographers applying paleo-redox proxies and geochemists interested in the broad area of early diagenesis. It is pointed out that conceptual models for the geochemical implications of macrobenthic activity are relatively well understood but that quantitative approaches are largely lacking. Consequently, particular attention is directed to in situ and ex situ methods to derive rates of macrobenthic activity. From this literature study it becomes clear that benthic fauna studies and geochemical studies have rarely been integrated. However, this is essential to fully understand the impact of the temporal and spatial variable benthic assemblages on important issues such as organic matter mineralization and metal mobilization in ocean margin sediments. The effects of macrobenthic activity are highly diverse and concern dissolved and solid phase distributions. With respect to nutrient cycling and organic matter mineralization the most important effects arise from bioirrigation. Burrows and tubes are flushed with oxic bottom water which increases the total surface area for aerobic respiration, nitrification and denitrification. In addition, active pumping increases the efflux of dissolved species and creates radial diffusion which is not accounted for when fluxes are quantified from (vertical) pore water profiles by means of molecular diffusion. Since metal diagenesis is ultimately related to solid phase redistributions, e.g. across redox boundaries, bioturbation plays an important role. The depth distribution of bioturbatory activity depends on the feeding strategy of the prevailing fauna which varies significantly.

This paper is intended to give a brief overview covering the main aspects of mineralization and preservation of organic carbon in continental margin sediments. It is not meant to be a comprehensive overview of the whole subject. Instead, we will summarise the relevant subjects, present data from a number of well studied sites from different areas of the world ocean and focus on the aspects of lateral sediment advection, the role of oxygen minimum zones and the preservation/ dissolution of calcium carbonate. We also summarise data compiled in different studies and compare it to global estimates to be able to better evaluate the role of mineralization in ocean margin sediments for the world oceans.

In this paper an overview is given about numerical modelling of transport processes in the subsurface. Numerical models enable a better process understanding, can determine local or global budgets and are able to make predictions for changing conditions. The physical and mathematical model concepts are introduced for subsurface transport processes in a single phase, water, and, briefly in two phases, water and gas. The last model concept is required to simulate flow and transport processes when e.g. free gas occurs. In many cases, flow and transport processes must be considered in two or three dimensions. Finite-Difference, Finite-Element and Finite-Volume Methods are discussed together with stabilization techniques which are required because of the advection / dispersion character of the basic equations. The authors give recommendations to the different methods. In the future the models can be further developed to simulate non-isothermal multiphase / multicomponent processes, which occur e.g. around gas hydrates.

Submarine subsurface fluid flow is ubiquituous with rates varying enormously with space and time. Therefore, fluid flow most probably is of paramount importance for the transport of matter and heat as well as to control fluxes between the subsurface and the ocean. However, rates to date still are only very poorly understood. In this study we therefore first identify major fluid flow systems including fault transport at active and passive margins, gas hydrate affected flow systems, or submarine groundwater discharge. Then, we describe geophysical and geochemical methods which are capable to better and quantitatively understand interacting submarine fluid flow systems and lastly, propose strategies for future research.

Deep water environments along the continental margin show mainly depth-related, but also significant along-slope, patterns in the composition, biomass and diversity of the benthic biota. Superimposed on this broad-scale pattern, local-scale habitat variability, including nature of the sediment, hydrodynamics and topography, can also be detected. However, along-slope variability is poorly understood, because only a relatively small number of sites have as yet been investigated and because sampling effort is generally sparse and poorly replicated. We are unable to predict benthic biology with a high enough resolution to provide useful precision for specific areas. Furthermore, the benthic biology associated with highly localised conditions, such as areas of seafloor fluid seepage or canyons, remains largely unknown, while knowledge of patterns in smaller benthic size classes is less well developed than for larger size classes. Progress in knowledge of benthic biodiversity on the continental margin is severely constrained by present taxonomic deficiency, with an increasingly large imbalance between available taxonomic capability and the extraordinarily rich, yet significantly undescribed, biodiversity. There is an urgent need for the application of molecular genetic tools to problems of understanding vertical and horizontal ranges of organisms. Little is presently known, even though studies of morphological variability have been applied to some groups of animals. Understanding of relevant processes operating both at (1) ecological time scales in maintaining high local-scale species richness, and at (2) historical time scales influencing speciation, needs to be developed before meaningful assessments of long-term anthropogenic impacts can be made. Yet, such impacts, particularly those from presently unregulated deep-sea trawling for non-quota fish stocks along the European continental margin may have already changed the ecosystem from its previously pristine state. Deep-sea trawling causes a direct physical impact, which is known already to have damaged slow-growing cold-water corals in some areas. Deep-sea fishing also may have an effect on the background ecosystem, including the benthos, as a result of return to the sea of biomass discarded from the catch. Although care needs to be taken in interpreting the outcome of specific anthropogenic impacts, knowledge of (1) likely sensitivities of species and (2) overall ecosystem resilience, will be informed by studies of responses to natural disturbance.

In the present paper, genetic studies on organisms that are associated with continental slopes and slope-like habitats were reviewed. This indicated that few slope-dwelling species exhibit homogenous spatial genetic structure over wide geographic scales. The species that do exhibit homogenous genetic population structures tend to inhabit isolated topographic features such as seamounts, plateaus and mid-ocean ridges. Such a genetic structure may reflect the success of species with a dispersive life history in colonising such fragmented habitats. Most studies on slope dwelling species indicate genetic differentiation between populations on oceanic, regional and more local scales. In such cases topographic and hydrographic factors have been considered as important in structuring populations. Sometimes strong temporal variance in allele frequencies may have been a factor in causing differences in allele frequencies between areas. Sibling or cryptic species are commonly found amongst deep-sea taxa especially when comparisons of samples from different depths are made. The reasons for this are unclear but may be associated with speciation driven by selection exerted by increasing pressure or other factors correlated with depth. Alternatively, it is likely that historical processes have been important in speciation processes on the continental slope. More detailed studies are required of both intraspecific and interspecific genetic variation of slope species. Such studies should be on the scale of species distributions and should include the collection of associated biological and environmental data that is of relevance when interpreting genetic structure. The increasing availability of genome sequence data and genomic technology may allow scientists to study significant genes that have played a role in speciation processes in the deep-sea. Extensive phylogenetic work across multiple taxa may be necessary to identify common historical factors that have influenced speciation in the deep-sea.

The European margin has given rise to a series of sampling programmes that have provided an insight into the reproductive biology of deep-sea species. From these observations it is apparent that no one reproductive pattern dominates. The observed life history pattern is a function of phylogenetic constraint, whilst the timing of reproduction in some species is determined by the local input of surface-derived material. Little is known of the larvae of deep-water species from any depth. A few larval types have been described, but we know little of the larval ecology. Experimental analysis of larval development has shown that embryos of certain taxa have wider temperature/pressure tolerances than the adult and, as a result, may allow a mechanism by which shallow-water species could invade the deep-sea. It is imperative that there is a better understanding of reproductive processes because anthropogenic impacts may have sub-lethal effects on the benthos by disrupting those processes without killing the adult. We need to understand the natural processes of reproduction in the deep-sea so that the effects of man on this environment can be modelled and predicted.

Soft bottom macrofauna (benthos) in the deep-sea is influenced by a wide variety of environmental factors. In this review I will shortly evaluate the importance of some factors on different parameters of the benthos. Studies on macrobenthic communities from the Skagerrak in the north to the Iberian continental margin in the south are compared. Not one factor can be indicated as the factor controlling the benthic fauna at the European continental margin. Nearly all factors are coupled and directly or indirectly influencing the benthic community, indicating the complexity of this ecosystem. The overall pattern in decrease in total density and biomass with increasing water depth is mainly determined by the food input, but special topographic structures can highly influence this pattern. In canyons very high densities were found, whereas on very steep slopes and on sea mounts densities were very low. Not only quantity, but also quality, reliability and source of the food are important for the benthos. High flow velocities can resuspend the organic matter again and make it more available for suspension-feeders, whereas the fauna can change the flow and actively capture food that otherwise would pass. Extreme flow conditions can periodically disturb the fauna, allowing only deep living fauna to maintain and favor rapid colonizers, whereas a stable environment allows the development of a ’climax’ community in which biotic interactions become very important. As we still know only very little about the biology of the deep-sea fauna, we also can say very little about the elasticity and carrying capacity of this remote, but probably very important ecosystem. The highly complex nature of the continental margins, with strong difference in appearance, steep slopes with rocky outcrops, smooth sedimental plateaus, bights, troughs, seamounts and canyons, results in high local variability. We need to know more about the actual life-history strategies, feeding-types and mobility of the deep-sea fauna. Long-term observations and experiments would give an idea about the reactions of the benthos on different environmental events, as changes in flow velocities, food falls, sedimentation, even pollution and disturbances, but also reactions on biological events, as predators, competitors, etc. DNA- analyses can give information about evolution and distribution of the fauna and probably allow us a better understanding of biodiversity.

The ocean margins contain a great variety of habitats and biological communities. Recent discoveries, such as deep-water coral reefs, show that these communities are poorly described and understood. However, observations have already indicated that benthic communities on ocean margins show high levels of spatial and temporal variation at all scales. The European continental margin is increasingly exploited for both biological resources (fisheries) and non-biological resources (oil, gas, minerals). Environmental management of the exploitation of continental margins requires an understanding of natural levels of variation inherent in biological communities that are potentially impacted by such activities. This paper presents a synthesis of the present knowledge of the spatial and temporal variation of slope communities. Priorities for future research and its technological development are discussed. The aim of this research is to provide a scientific basis for the environmental management of the continental slopes of Europe.

The small sized organisms including prokaryotes (bacteria and archaea), protozoa and metazoan meiofauna (< 250 µm) are the driving forces for biogeochemical fluxes in surficial deepsea sediments under oxic conditions. The relative proportion of small sized organisms increases along trophic gradients from eutrophy to oligotrophy or from the continental margin towards the mid oceanic deep-sea. They can consume up to 10% of freshly sedimented organic matter per day. The small sized fauna consumes and respires the largest part of organic matter, while macrofauna is instrumental in incorporating fresh detritus into the sediment, structuring the environment and thus facilitating microbial processes. Small organisms, in particular prokaryotes, can adapt to amount and quality of organic matter input. Under nutrient starvation probably a large proportion of the prokaryotic community is dormant and is reactivated during sedimentation events. On time scales of 7–10 days (metabolism) to 2–3 weeks (biomass increase) they can react to pulses of deposition of organic material. However, the history of food supply influences the speed of adaptation and effectiveness of growth. At stations close to continental margins estimates of organic matter input from sediment traps largely disagree with measurements of benthic respiration, carbon turnover or estimates obtained from geochemical modelling. This discrepancy is much smaller at mid-oceanic stations. Lateral inputs from productive shelf seas into the deep-sea are suspected to cause this discrepancy.

Carbonate mounds from the geological record provide ample evidence of microbial mediation in mound buildup and stabilization. Advanced models argue for the prominent role which biofilms may have played at the interface between the fluid and mineral phases. While up to the early nineties, there was little evidence of mud-mound formation from Late Cretaceous times onwards, recent investigations have increasingly reported occurrences of large mound clusters on modern ocean margins, in particular in basins rich in hydrocarbons. Mound provinces are significant ocean margin systems, up to now largely overlooked. How do such recent mound provinces relate to the fossil examples, and do the modern mound provinces provide a new window on the microbiota that were instrumental in building giant mounds throughout Phanerozoic times? These are burning questions, and the answer will only come through a new dialogue between experts of the past, explorationists of the recent ocean, and microbiologists. An example is given of the power of new exploration tools, which can highlight controls on mound nucleation and patterns of early diagenesis — typically microbially driven processes. New insights can pave the way for new sampling opportunities, both by targeted surface sampling and controlled subsurface sampling through drilling.

As the major biological sink of methane in marine sediments, the microbially mediated anaerobic oxidation of methane (AOM) is crucial in its role of maintaining a sensitive balance of our atmosphere’s greenhouse gas content. Although there is now sufficient geochemical evidence to exactly locate the “hot spots” of AOM, and to crudely estimate its contribution to the methane cycle, a fundamental understanding of the associated biology is still lacking, consequently preventing a thorough biogeochemical understanding of an integral process in the global carbon cycle. Earlier microbiological work trying to resolve the enigma of AOM mostly failed because it was largely focussed on the simulation of AOM under laboratory conditions using cultivable candidate organisms. Now again, understanding the biological and biochemical details of AOM is the declared goal of several interational research groups, but this time in a combined effort of biogeochemists and microbiologists using novel analytical tools tailored for the study of unknown microbes and habitats. This review gives an overview on very recent progress in the study of AOM that dramatically advanced this ~ 30-yr-old field. New insights on the quantitative significance of AOM are combined to refine older estimates.

The zone of continental margins is most important for the ocean’s productivity and nutrient budget and connects the flow of material from terrestrial environments to the deep-sea. Microbial processes are an important “filter” in this exchange between sediments and ocean interior. As a consequence of the variety of habitats and special environmental conditions at continental margins an enormous diversity of microbial processes and microbial life forms is found. The only definite limit to microbial life in sedimentary systems of continental margins appears to be high temperatures in the interior earth or in fluids rising from the interior. Many of the catalytic capabilities which microorganisms possess are still only incompletely explored and appear to continuously expand as new organisms are discovered. Recent discoveries at continental margins such as the microbial life in the deep sub-seafloor, microbial utilization of hydrate deposits, highly specialized microbial symbioses and the involvement of microbial processes in the formation of carbonate mounds have extended our understanding of the Earth’s bio- and geosphere dramatically. The aim of this paper is to identify important scientific issues for future research on microbial life in sedimentary environments of continental margins.