Continental Slope

Metazoan meiofauna are ubiquitous in marine soft sediments and play a pivotal role in diagenesis of particulate organic matter. However, the relative importance of meiofauna to the function of deep-sea benthic boundary layer communities has not been resolved. Here, meiofauna biomass, respiration, and grazing on aerobic heterotrophic bacteria were estimated and compared to standing stocks and fluxes of other benthic components (e.g., bacteria and macrofauna). Biomass and respiration declined with depth. Highest biomass and respiration occurred in the proximity of the Mississippi River on the upper continental slope of the central Gulf of Mexico. Meiofauna required 7% of their biomass per day to meet their metabolic energy budget, compared to approximately 24% day À1 in shallow water. Respiration accounted for 8-22% of whole sediment community respiration (SCOC), reflecting the importance of meiofauna in diagenesis, deep-sea carbon budgets, and global biogeochemical cycles.

The ecology and status of Risso's dolphins Grampus griseus worldwide are poorly known. In the Mediterranean Sea, modern field studies of cetaceans only began in the late 1980s and this has resulted in rapid advances in knowledge of some species, but not Risso's dolphin. This paper reviews available information on the distribution and ecology of Risso's dolphins in the Mediterranean and identifies factors that may negatively affect them in this region. Risso's dolphins occur in continental slope waters throughout the Mediterranean basin and around many of the region's offshore islands and archipelagos. No synoptic estimate of abundance is available for the Mediterranean region, but densities and overall numbers are low in comparison to some other small odontocetes. Diet consists primarily of cephalopods, with a clear preference for mesopelagic squid. The principal known threat to populations in the Mediterranean is entanglement in pelagic drift gillnets. Other potential problems for Risso's dolphins in the Mediterranean include noise disturbance and ingestion of plastic debris. Conservation actions to mitigate the risk of entanglement in fishing gear are likely to benefit Risso's dolphins; specifically, the existing driftnet ban in EU waters should be strictly enforced and extended to the high seas and to waters under non-EU State jurisdiction. More and better data are needed on abundance, distribution, movements, population dynamics and trends in Risso's dolphin populations, and better information on threats (e.g. bycatch in fishing gear) is needed to inform conservation efforts.

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.

The study of many slope channel systems has led to the development of rules in the form of observations, measurements, and hypotheses. For example, we hypothesize that high abandonment relief can strongly influence the location of the subsequent channel element and will result in an organized channel stacking pattern in which the path of the younger channel element approximates the path of the former element. The rules were developed with the objective of constructing forward models of petroleum reservoirs that are internally consistent, reproducible, and quantifiable. Channelized turbidite deposits can be interpreted to be the product of multiple cycles of waxingewaning flow energy at multiple scales. Systematic changes in the volume and caliber of turbidity flows through time trigger a fall of the equilibrium profile, which drives erosion and sediment bypass across the slope, followed by a rise of the equilibrium profile, which allows deposition on the slope of increasingly mud-rich sediments through time. In most turbidite successions, at least three scales of waxingewaning cyclicity can be interpreted: element, complex set, and sequence. The stacking pattern of channel elements within a complex setscale cycle tends to be sequential: (1) erosion and sediment bypass; (2) amalgamation of channel elements associated with a low rate of aggradation; (3) a disorganized stacking pattern of channel elements associated with a moderate rate of aggradation; and (4) an organized stacking pattern of channel elements associated with a high rate of aggradation. Stages 1 and 2 may be absent or minor in mud-rich systems but prominent in sand-rich systems. Conversely, stage 4 may be prominent in mudrich systems but absent in sand-rich systems. Event-based forward modeling, utilizing rules, can produce realistic architectures, such as the four stages described above. Multiple realizations and multiple alternative models can be constructed to quantitatively examine the probability of specific parameters of interest such as pore volume and connectivity.

Keywords: Late Quaternary stratigraphy siliciclastic shelf architecture source-to-sink sediment path biogeography peat formation Sea-level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late Quaternary times. Fluctuations on millennial time-scales produce locally complex deposits in coasts and on shelves, associated with short-term influence on species development. This article reviews the sedimentary and biogeographic history of the tropical siliciclastic Sunda Shelf as an end-member of continental shelves regarding extreme width, an enormous sediment supply, and highest biodiversity in response to rapid sea-level fluctuations. We describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data, field observations, and calculation of hydro-isostatic adjustment effects on changing relative sea level. These segments are characterized by individual sedimentary processes and deposits, and by a specific potential for material storage and re-mobilization. Long-term regressive intervals led to overall sigmoidal-promoting, extremely thick, and wide succeeding units. In contrast, rapid lateral shifts of defined depocentres over long distances took place in response to short-term sea level fluctuations. Fully isolated small-scale sediment bodies formed when sea level changed at exceptionally high rates. As a result of the high availability of organic-rich sediments, mangrove and freshwater peats formed frequently over late Quaternary times. The appearance of thick, massive and widespread peats is mainly linked to time intervals of a sea-level rise at slow rates, whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level. The preservation potential of the regressive units is generally high due to highest initial sediment supply, stabilizing soil formation during exposure and rapid subsidence. Preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change. Besides complex channel incision, an overall lowering of the sediment surface related to erosion, as deep as 20 m or more, over wide areas took place mainly during sea level lowering. The final export of shelf material is documented by enormous mass-wasting packages on the associated continental slope. From a palaeogeographic perspective, the rapid formation or disappearance of special habitat zones, such as mangrove fringes and extended mud flats, led to species establishment or truncation in distribution. In addition, the opening or closure of ocean passages, as narrow bridges allowing limited species crossing or as fully colonized corridors, had severe impact on eco-fragmentation and the expansion or contraction of species. Independent of such particular conditions, sea-level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast-related ecosystems.

Harpacticoid copepod community structure was analyzed at 43 stations in the northern Gulf of Mexico deep-sea to test regional and bathymetric patterns of diversity in relation to environmental variables and topographic complexity of the continental slope. Depth, longitude, and proximity to the Florida Escarpment significantly affect average phylogenetic diversity, but basins and canyons do not. Multivariate analysis reveals a significant inverse relationship between diversity and POM flux, which is confirmed by significant region-scale depth and longitude differences. Although species richness declines linearly with increasing depth, the expected number of species (rarefraction) is maximized at approximately 1200 m, and average taxonomic and phylogenetic diversity continue to increase with depth, suggesting greater morphological or functional harpacticoid diversity with increasing depth. Most stations have unique species compositions, suggesting high regional (2200 species) and global (10 5 -10 6 species) diversity by extrapolation. Therefore, processes maintaining harpacticoid diversity in the northern Gulf of Mexico deepsea seem to rely on both small-scale dispersal and large-scale food supply mechanisms.

This paper describes multiple layers of ice-rafted debris and an ice-scour structure in Early Permian marine strata within the Shoalhaven Group of the Sydney Basin, New South Wales, Australia. Strata were deposited on a highlatitude, glacially influenced continental margin during the final stages of Late Palaeozoic glaciation between 277 and 260 Ma. Poorly sorted ice-rafted debris, forming layers up to 1.25 m thick, is common within inner to outer shelf and continental slope facies of the Shoalhaven Group. More than 120 layers can be identified. These are argued to record the enhanced delivery of debris by icebergs drifting north from marine-based Antarctic ice margins. Deformation structures in associated sediments are interpreted as iceberg scours and ice keel turbates. Associated glendonites record early diagenesis of organic-rich sediment and bottom water temperatures close to freezing.

The present paper is mostly a synthesis of work conducted on the continental margin of the Gulf of Ca´diz using a broad database collected by several cruises and projects supported by the Spanish Research Council and the NRL and Naval Oceanographic Office (USA), including: bathymetry, sidescan sonar imagery, seismic profiles, sediment cores, submarine photographs and physical oceanographic data. These data have enabled us to establish a detailed understanding of the morphologic development of the margin, its Pliocene and Quaternary stratigraphy, and a full characterization of the contourite depositional system (CDS) generated by the Mediterranean outflow water (MOW). The northern margin of the Gulf of Ca´diz shows the following distinct features: (a) an active compressive framework where the ''Cádiz Allocthonous Unit'' provides an unstable substratum for Late Miocene, Pliocene and Quaternary sedimentation; (b) a relative lack of submarine canyons, except in the western area of the Algarve margin; (c) a very broad continental slope that lacks a marked continental rise; (d) a middle slope dominated by along-slope processes driven by the MOW, which has generated a complex CDS during the Pliocene and Quaternary; and (e) an irregular lower slope and abyssal plain region dominated by down-slope processes that is partly detached from an upper slope source region. The CDS is composed of both depositional and erosive features. The main depositional features are characterised by sedimentary wave fields, sedimentary lobes, mixed drifts, plastered drifts, elongated mounded, and separated drifts and sheeted drifts. The main erosive features are contourite channels, furrows, marginal valleys and moats. These various depositional and erosive features have a specific location along the margin, and their detailed distribution is essential to understand the present (and past) interaction of the MOW ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$ -see front matter r (F.J. Herna´ndez-Molina).