Margin Off Research Articles

Multidecadal Changes in Marine Subsurface Oxygenation Off Central Peru During the Last ca. 170 Years

Subsurface water masses with permanent oxygen deficiency (oxygen minimum zones, OMZ) are typically associated with upwelling regions and exhibit a high sensitivity to climate variability. Over the last decade, several studies have reported a global ocean deoxygenation trend since 1960 and a consequent OMZ expansion. However, some proxy records suggest an oxygenation trend for the OMZ over the margins of the Tropical North East Pacific since ca. 1850. At the Tropical South East Pacific, the upper Peruvian margin is permanently impinged by a shallow and intense OMZ. In this study, we aim to 1) reconstruct the (multi)decadal oxygenation variability off central Peru, and 2) to identify the influence of both largescale and local factors and the potential underlying mechanisms driving subsurface oxygenation in the Eastern Pacific. We combined a multiproxy approach in multiple paleoceanographic records for the last ~170 years with instrumental records of subsurface oxygen concentrations since 1960. We analyzed benthic foraminiferal assemblages, redox-sensitive metals (Mo, Re, U), δ15N and contents of total organic carbon and biogenic silica in multiple sediment cores collected in the upper margin off Callao (180 m) and Pisco (~300 m). An OMZ weakening over the Peruvian central margin can be inferred from 1865 to 2004. The records can be divided in three major periods, based on responses of local productivity and subsurface ventilation: i) the mid to late nineteenth century, with enhanced siliceous productivity, a strong oxygen-deficient and reducing sedimentary conditions; ii) the late nineteenth century to mid-twentieth century, with less oxygen-deficient and reducing sedimentary conditions, superimposed to a slight decadal-scale variability; and iii) the late twentieth century until the early 2000’s, with a slight oxygenation trend. We attribute the centennial-scale oxygenation trend in the Tropical East Pacific to ventilation processes by undercurrents that decreased subsurface oxygenation even when during the same period an overall increase in export production was inferred off Peru. Unlike other upwelling areas in the Tropical East Pacific, subsurface oxygenation off Peru does not show a decrease in the last decades, instead a subtle oxygenation trend was observed close to the core of the OMZ at 200 m depth.

Development of regional downscaling capability in STEAMER ocean prediction system based on multi-nested ROMS model

ABSTRACT Oceanic regional downscaling capability was implemented into Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) developed by Japan Atomic Energy Agency to enable us to predict more realistically the oceanic dispersion of radionuclides at higher spatiotemporal resolutions for broader applications. The system consisted of a double-nested oceanic downscaling circulation model with tidal forcing and an oceanic radionuclide dispersion model. This system was used to comparatively examine downscaling and tidal effects on the dispersion of radionuclides hypothetically released from the Fukushima Daiichi Nuclear Power Plant in the colder season. The simulated dissolved 137Cs distribution was different from that obtained using coarser-resolution models because downscaling enhanced both horizontal and vertical mixing. The suppression of horizontal mixing and the promotion of vertical mixing by tidal forcing synergistically reduced offshore 137Cs transport. In addition, the submesoscale effects strengthened the three-dimensional 137Cs fluctuations by <10 times, while the tidal effects promoted slightly increased the intensity of three-dimensional 137Cs fluctuations by approximately 3%. This indicated that the submesoscale effects substantially surpassed tidal forcing in oceanic mixing in the coastal margin off Fukushima in the colder season.

(In)coherent multiproxy signals in marine sediments: Implications for high-resolution paleoclimate reconstruction

Accurate chronologies are the backbone of paleoclimate research, yet for marine sedimentary records the same age model is often applied to co-eval components that may have experienced different pre-depositional histories, leading to aliasing in corresponding proxy signals. Here we demonstrate 14C age discrepancies among several proxy carriers and use them to highlight spatio-temporal disparities among different components of the sediment. Total organic carbon (TOC), alkenones, and alkanoic (fatty) acids have older ages than co-occurring planktonic foraminifera in corresponding depth intervals of a sediment core retrieved from the so-called ‘Shackleton Sites’ on the southwest Iberian margin off Portugal. Temporal differences are explained by the addition of pre-aged material that is transported to the site by lateral advection, rivers or wind. We then modeled the age structure and relative abundance of the different pools that might have contributed to the older ages of TOC and alkenones. Results suggest the addition of a moderate (15-20%) proportion of very old allochthonous material (18,500-49,900 yr). This information was used to deconvolve the alkenone-derived sea surface temperature (SST) record and assess the impact of allochthonous inputs on paleoclimate signals.

Structural Evolution of the Rifted Margin off Northern Labrador: The Role of Hyperextension and Magmatism

AbstractHigh‐quality seismic reflection data from the offshore northern Labrador rifted margin allow imaging of the extended and rifted crust both along and across the continental margin and are described in conjunction with available seismic velocity and gravity data. The margin formed within cold, thick cratonic lithosphere. Both Archean continental basement and a discrete, undulating, high‐amplitude, deep reflection about 10 km below basement are observed. The deeper reflection can be correlated with the crust‐mantle boundary as measured on previous wide‐angle seismic data in the region. This reflection, termed here the L‐reflection, appears to be the equivalent to other top‐mantle detachments found elsewhere on magma‐poor rifted margins. However, normal mantle velocities have been observed to lie just below the reflection, suggesting that it may not be related to the formation of weak serpentinized mantle. A high‐velocity and density zone occupies the outer shelf seaward of the L‐reflection where basement is transparent, which may represent highly mafic crust or serpentinized mantle. A crustal reconstruction of this margin and its conjugate shows marked asymmetry, with a wider zone of crustal thinning on the Greenland margin. These crustal thinning profiles are comparable to those on other conjugate margins within cratonic lithosphere. While some of the attributes of this margin are those of a magma‐poor system, at the ocean‐continent transition, thick igneous crust created a magma‐rich zone in Paleocene time when a hot spot was active in the Davis Strait to the north. Thus, this margin exhibits characteristics of both magma‐rich and magma‐poor systems.

Elongate fluid flow structures: Stress control on gas migration at Opouawe Bank, New Zealand

High-resolution 2D and 3D seismic data from Opouawe Bank, an accretionary ridge on the Hikurangi subduction margin off New Zealand, show evidence for exceptional gas migration pathways linked to the stress regime of the ridge. Although the ridge has formed by thrusting and folding in response to a sub-horizontal principal compressive stress (σ1), it is clear that local stress conditions related to uplift and extension around the apex of folding (i.e. sub-vertical σ1) are controlling shallow fluid flow. The most conspicuous structural features are parallel and horizontally-elongated extensional fractures that are perpendicular to the ridge axis. At shallower depth near the seafloor, extensional fractures evolve into more concentric structures which ultimately reach the seafloor where they terminate at gas seeps. In addition to the ridge-perpendicular extensional fractures, we also observe both ridge-perpendicular and ridge-parallel normal faults. This indicates that both longitudinal- and ridge-perpendicular extension have occurred in the past. The deepest stratigraphic unit that we image has undergone significant folding and is affected by both sets of normal faults. Shallower stratigraphic units are less deformed and only host the ridge-parallel normal faults, indicating that longitudinal extension was limited to an older phase of ridge evolution. Present-day gas migration has exploited the fabric from longitudinal extension at depth. As the gas ascends to shallower units it ‘self-generates’ its flow pathways through the more concentric structures near the seafloor. This shows that gas migration can evolve from being dependent on inherited tectonic structures at depth, to becoming self-propagating closer to the seafloor.

Two new bairdiid ostracod species from the early Barremian–Hauterivian of the northern and central North Sea to the Atlantic margin off Norway

Abstract. Two new species of bairdiid Ostracoda are described from the lower Barremian – Hauterivian interval of the Valhall and Åsgard formations in the northern and central North Sea and Atlantic margin off Norway. The new species are Pontocyprella valhalla (lsid:zoobank.org:act:CA6B273F-CFF6-4C38-B9F4-18188225A711, 18 January 2018) and Bairdia asgarda (lsid:zoobank.org:act:4A4DC817-A028-45FB-9287-ABF3794F2FCB, 18 January 2018). These species dominate the ostracod assemblage that occurs abundantly in early Barremian–Hauterivian deep marine sediments of the northern and central North Sea and Haltenbanken area off Norway. Pontocyprella valhalla is restricted to this interval and because of its large size and distinct shape is a useful stratigraphic marker species, its last appearance being within the early Barremian.

Characterization of hydrate and gas reservoirs in plate convergent margin by applying rock physics to high-resolution seismic velocity model

Gas hydrates are widely distributed in the Kumano forearc basin, which is located above accretionary prism in the Nankai margin off the Kii peninsula, Japan. Bottom-Simulating Reflector (BSR) at the base of gas hydrate stability zone has been imaged as a strong acoustic impedance contrast on the reflection seismic profiles. In order to better define the accumulations of gas hydrates and free gases, we performed a high-resolution seismic velocity analysis to 3D seismic data using a method of conventional semblance spectra via automatic velocity picking algorithm. The results revealed that gas hydrate-bearing sediments above the BSR and free gas-bearing sediments below the BSR are characterized by P-wave velocities of 1900–2500 m/s and 1000–1800 m/s, respectively. Then, the velocity model was converted into gas hydrate and free gas saturation using rock physics approaches. The results indicated that saturation of gas hydrates ranges from 0% to 45% in the pore space, and highly concentrated around the outer ridge where faults are densely developed. Additionally, concentrations of free gas ranging from 0% to 20% in the pore space are widely distributed below BSRs and are considerably high above ridge structure generated by displacement of large fault splayed from the deep plate boundary décollement. Based on these results, we suggest that the gas hydrates concentrated due to the free gas influx which migrated upward through the steeply dipping strata and faults (or fractures) cutting through the basin. The accumulations of gas and/or hydrates are further controlled by fault movements in the accretionary prism beneath the forearc basin. Therefore, these factors generated by intensive tectonic movements in the plate subduction zone control the distribution and saturation pattern of gas hydrate and free gas formations.

Microseismicity Linked to Gas Migration and Leakage on the Western Svalbard Shelf

AbstractThe continental margin off Prins Karls Forland, western Svalbard, is characterized by widespread natural gas seepage into the water column at and upslope of the gas hydrate stability zone. We deployed an ocean bottom seismometer integrated into the MASOX (Monitoring Arctic Seafloor‐Ocean Exchange) automated seabed observatory at the pinch‐out of this zone at 389 m water depth to investigate passive seismicity over a continuous 297 day period from 13 October 2010. An automated triggering algorithm was applied to detect over 220,000 short duration events (SDEs) defined as having a duration of less than 1 s. The analysis reveals two different types of SDEs, each with a distinctive characteristic seismic signature. We infer that the first type consists of vocal signals generated by moving mammals, likely finback whales. The second type corresponds to signals with a source within a few hundred meters of the seismometer, either due east or west, that vary on short (∼tens of days) and seasonal time scales. Based on evidence of prevalent seafloor seepage and subseafloor gas accumulations, we hypothesize that the second type of SDEs is related to subseafloor fluid migration and gas seepage. Furthermore, we postulate that the observed temporal variations in microseismicity are driven by transient fluid release and due to the dynamics of thermally forced, seasonal gas hydrate decomposition. Our analysis presents a novel technique for monitoring the duration, intensity, and periodicity of fluid migration and seepage at the seabed and can help elucidate the environmental controls on gas hydrate decomposition and release.

Seismic lines Offshore Mount Etna (SOME): open database

The Seismic lines Offshore Mount Etna (SOME) database is presented. It consists of multichannel high-resolution seismic data acquired in 2005 off-shore Mount Etna (eastern Sicily). We describe first the acquisition of seismic lines and then the architecture of the data base. Finally we describe a very basic interpretation of some seismic lines to provide clear example of the potentiality of the seismic data sets in addressing relevant issues such as the occurrence of slope instabilities and the deformation style of the continental margin off shore mount Etna.

Recent reconstruction of deep-water macrofaunal communities recorded in Continental Margin sediments in the Balearic Basin

We present an initial reconstruction of recent (last few centuries) mud-bottom faunal communities on the upper slope (398–667m) of the continental margin off Catalonia (western Mediterranean), including periods free of any trawling impact. Radiometric dating of marine sediments and identification of faunal remains (e.g., fish otoliths, pteropod shells, coral sclerites) were performed to obtain a sediment geochronology in a 56cm sediment core (MC4) taken at 398m off the Ebro Delta in 2011. Core MC4 was especially rich in faunal remains, including, for example, 247 identifiable otoliths. A fine-scale chronology of MC4 was not possible due to sediment mixing. However, the depth of 210Pbxs penetration (20–22cm) identified sediments older (below 22cm depth) and younger (from core top to 22cm) than ca. 100 years. Mass Accumulation Rate (MAR) from the 210Pbxs profile was estimated as 0.23±0.02gcm−2 y−1. A significant peak of sclerites of the bamboo coral Isidella elongata was found between 4 and 8cm in MC4, with remains of the axes and bases of Isidella colonies exclusively found at core depths >8–10cm, which would correspond (MAR results) to the period 1980–1985. Such structures were not found in the 0–8cm layer, likely an effect of trawling that started in the area in the 1980s. Other changes both in benthos (corals and cirripedes) and zooplankton (pteropods) seemed to be related with Ebro river discharge, with changes coinciding with massive damming of the Ebro and tributary rivers in the 1950s and until 1965. Mesopelagic fish also showed temporal oscillations in MC4. Abundance of some myctophid remains (Lampanyctus croccodilus and Benthosema glaciale) was related with positive NAO periods and with rather high temperature in Levantine Intermediate Waters. By contrast, periods of higher dominance of Ceratoscopelus maderensis off Catalonian coasts could indicate lower salinity during the past and a progressive degree of eutrophication in intermediate waters in recent decades. A general decline in myctophids otolith abundance during the last ca. 100 years could be related to changes in the temperature and salinity of deep-water masses and a decline in ocean productivity that would also have affected open-water fish stocks.

Summer temperature evolution on the Kamchatka Peninsula, Russian Far East, during the past 20 000 years

Abstract. Little is known about the climate evolution on the Kamchatka Peninsula during the last glacial–interglacial transition as existing climate records do not reach beyond 12 ka BP. In this study, a summer temperature record for the past 20 kyr is presented. Branched glycerol dialkyl glycerol tetraethers, terrigenous biomarkers suitable for continental air temperature reconstructions, were analyzed in a sediment core from the western continental margin off Kamchatka in the marginal northwest Pacific (NW Pacific). The record suggests that summer temperatures on Kamchatka during the Last Glacial Maximum (LGM) equaled modern temperatures. We suggest that strong southerly winds associated with a pronounced North Pacific High pressure system over the subarctic NW Pacific accounted for the warm conditions. A comparison with an Earth system model reveals discrepancies between model and proxy-based reconstructions for the LGM temperature and atmospheric circulation in the NW Pacific realm. The deglacial temperature development is characterized by abrupt millennial-scale temperature oscillations. The Bølling–Allerød warm phase and the Younger Dryas cold spell are pronounced events, suggesting a connection to North Atlantic climate variability.

Neogene paleoceanography of the eastern equatorial Pacific based on the radiolarian record of IODP drill sites off Costa Rica

AbstractThe Integrated Ocean Drilling Program (IODP) Expedition 344 drilled cores following a transect across the convergent margin off Costa Rica. Two of the five sites (U1381 and U1414) are the subject of the present study. Major radiolarian faunal breaks and characteristic species groups were defined with the aid of cluster analysis, nodal analysis, and discriminant analysis of principal components. A middle‐late Miocene to Pleistocene age (radiolarian zones RN5 to RN16) was determined for the sites, which agrees with the nannofossil zonations and 40Ar/39Ar and tephra layers. Considering the northward movement of the Cocos plate (∼7.3 cm/yr), and a paleolatitude calculator, it is assumed that during the Miocene the two sites were located ∼1000 km to the southwest of their current position, slightly south of the equator. The radiolarian faunas retrieved were thus seemingly formed under the influence of different oceanic currents and sources of nutrients. Changes in the radiolarian assemblages at Site U1414 point at dissimilar environmental settings associated with the colder South Equatorial Current and the warmer Equatorial Countercurrent, as well as to coastal upwelling. These differences are best reflected by changes in the abundance of the morphotype Spongurus spp., with noticeably higher values during the Miocene, than in the Pliocene and the Pleistocene. Because Spongurus spp. is generally associated with cooler waters, these abundance variations (as well as those of several other species) suggest that during the Miocene the area had a stronger influence of colder waters than during younger periods. During the Pliocene and the lowermost Pleistocene, biogenic remains are scarce, presumably due to the terrigenous input, which could have diluted and affected the preservation of pelagic fossils, as well as to the displacement of the site to warmer waters. A typically tropical fauna characterized the Pleistocene, yet with widespread presence of colder water species, most probably indicative of the influence of coastal upwelling processes.

Widespread methane seepage along the continental margin off Svalbard - from Bj\xf8rn\xf8ya to Kongsfjorden

Numerous articles have recently reported on gas seepage offshore Svalbard, because the gas emission from these Arctic sediments was thought to result from gas hydrate dissociation, possibly triggered by anthropogenic ocean warming. We report on findings of a much broader seepage area, extending from 74° to 79°, where more than a thousand gas discharge sites were imaged as acoustic flares. The gas discharge occurs in water depths at and shallower than the upper edge of the gas hydrate stability zone and generates a dissolved methane plume that is hundreds of kilometer in length. Data collected in the summer of 2015 revealed that 0.02–7.7% of the dissolved methane was aerobically oxidized by microbes and a minor fraction (0.07%) was transferred to the atmosphere during periods of low wind speeds. Most flares were detected in the vicinity of the Hornsund Fracture Zone, leading us to postulate that the gas ascends along this fracture zone. The methane discharges on bathymetric highs characterized by sonic hard grounds, whereas glaciomarine and Holocene sediments in the troughs apparently limit seepage. The large scale seepage reported here is not caused by anthropogenic warming.

Distribution and mass inventory of mercury in sediment from the Yangtze River estuarine-inner shelf of the East China Sea

Mercury (Hg) was measured in 70 sediment samples from the Yangtze River estuarine-inner shelf of the East China Sea (ECS) to evaluate its occurrence, distribution, and deposition flux. Its concentrations were 10–92ng/g with a mean of 46±17ng/g. A decrease of Hg concentration with increasing distance offshore suggested a dominance of riverine input. The high levels of Hg observed at the southern inner shelf were partly due to the sorption affinity of fine-grained sediments. Hg concentration was significantly correlated with total organic carbon content and sediment grain size, implying that the nature of sedimentary organic matter and hydrodynamic forces could influence the Hg occurrence. A moderate correlation between Hg with high-molecular-weight polycyclic aromatic hydrocarbons in the YRE suggested that they shared a similar input pathway. The total deposition flux of Hg was estimated to be ~52t/y with a deposition rate of 6–120ng/cm2y, which indicated that the estuarine-inner shelf of the ECS was a major sink of Hg in the margins off China, and this area could play a significant role in the Hg biogeochemical cycle on a global scale.

Geochemical characterization of two distinctive systems with evidence of chemosynthetic activity, explored at the SE Pacific margin off Chile (46°S and 33°S)

This study presents the geochemical composition of superficial sediment under oxic and suboxic bottom water conditions along the Chilean continental margin (SE Pacific), where evidence for benthic chemosynthetic activity associated with diffuse seeping of chemically reduced fluids has been reported. The exploration was carried out at: (1) the Chilean Triple Junction (CTJ), at a water depth of ∼2900m, with the additional indication of hydrothermal activity near a methane-rich cold-seep area (46°S) (German et al., 2010); and (2) the El Quisco methane seep site (EQSS), at ∼340mwater depth (33°S) (Melo et al., 2007; Krylova et al., 2014). While the deeper CTJ is located within an oxic environment (dissolved oxygen in the bottom waters: 164μM), the shallower EQSS lies within a suboxic environment (dissolved oxygen in bottom water: 23μM), located within the lower limit of the SE Pacific oxygen minimum zone (OMZ).Pore water from short cores was analyzed for dissolved major, minor, and trace elements (Cl, Na, Mg, K, Ca, Sr, Si, B, P, Ba, Pb, Mn, Fe, Cd, U, and Mo), δ13DIC, sulfide, sulfate, and methane. The solid sediment fraction was likewise analyzed for total organic carbon (TOC), metals, and redox potential. Elevated sediment temperatures were found in superficial sediments (5–13°C) at the CTJ site, which could be due to warm fluids associated with the proximity of the ridge, where hydrothermal vents may occur. Reduced fluids were also present here, indicated by higher Mn fluxes toward the water column even in oxidized sediments (RPD>8cm), which contrasted with the lower fluxes in reduced sediments of the EQSS site (RPD∼2cm). 13C-depleted DIC, anomalously low pore water Cl (∼15ppb), and low concentrations of other major elements may be the result of dilution by fluid seeping and precipitation of major elements, producing authigenic enrichment (Ca, Mg, Sr). The fluid could also: (a) be diluted by pure water produced during methane hydrate dissociation, as observed in other cold-seep areas; and (b) correspond to clay mineral dehydration, as reported in plate subduction systems. The reducing conditions established at the CTJ conduct the Cd enrichment at a similar magnitude of that seen at the shallower suboxic site (EQSS). Evidence of chimney or vent fauna was not observed. At the EQSS, higher TOC and total sulfide contents were consistent with enhanced deposition of organic matter and reducing conditions developed in the OMZ, favoring the authigenic enrichment of Cd, U and Pb. The geochemical evidence, based only on methane concentrations and δ13DIC, is insufficient to establish the presence of methane seeps, as previously reported.

Mantle earthquakes beneath the South Iberia continental margin and Gulf of Cadiz – constraints from an onshore-offshore seismological network

The Gulf of Cadiz and the passive continental margin of southern Iberia to the west of the Strait of Gibraltar locally accommodate the presently ongoing convergence between Africa and Eurasia by widespread, rather diffusive, seismic activity. Seismicity of the northern Gulf of Cadiz was derived from an amphibious seismological network, including 24 temporary marine offshore stations, besides the permanent networks in Portugal, Spain, and Morocco. During the 6 month of the offshore network operation, in total 86 local earthquakes were located at six or more offshore stations with the majority of earthquakes occurring to the southwest of Iberia and along the Algarve continental margin off southern Iberia. The distribution of events along the Algarve margin mimics features reported for the Atlantic passive continental margins of both South and North America. Focal mechanisms at the Portimão Bank support that seismically active areas are associated with compression. Similar stress patterns are reported for the east coast of South and North America. However, while earthquakes along the American east coast occur at crustal levels, earthquakes in the northern Gulf of Cadiz occur both in the lower crust and upper mantle, with the majority of events rupturing within the mantle, including a number of well-located earthquakes beneath crust forming the continent-ocean transition zone. The large number of earthquakes in the mantle might be caused by the unique geological setting, where deformation occurs in cool lithosphere of Mesozoic age. We suggest that seismicity along the Algarve margin is caused by re-activation of pre-existing margin-parallel faults rather than corresponding to newly formed structures related to a new developing plate boundary.

The genus Polycope (Polycopidae, Ostracoda) in the North Atlantic and Arctic: taxonomy, distribution, and ecology

With almost 200 species described from all over the world, Polycope Sars, 1866 is the most specious and eurytopic genus of the exclusively marine family Polycopidae. Unlike other members of the subclass Myodocopa, polycopids are commonly found in fossiliferous rocks from strata as old as the Carboniferous, which makes them a useful tool in palaeoenvironmental reconstructions. The genus Polycope in particular is an opportunistic group and can putatively serve as an indicator of organic rich, fine grained sediment, reduced sea ice, and increased productivity, which is especially important when correlating biological data with the Quaternary climate changes that often requires accurate generic and species identifications. Polycopid taxonomy is relatively complicated because species tend to have conservative shell shapes and the soft body is extremely reduced, which can lead to numerous misidentifications, especially when fossil and Subrecent taxa are mixed and the shell is the only available source of taxonomic information. Here we study Polycope species collected during the first cruise of the IceAge project from the margin off Iceland. In total, four species have been recovered from 200 to 2747 m depth. They have been identified as P. pseudoinornata Chavtur, 1983, P. punctata Sars, 1869, P. yasuharai sp. nov., and P. sp. This is the first time since the middle of the 19th century that members of this genus have been studied in the North Atlantic and Arctic using both their soft parts and shells. It allowed us to reconsider numerous previous records of Polycope from this region. Beside a critical overview of the polycopid diversity in the North Atlantic and Arctic, and detailed taxonomic (re)descriptions of the newly collected specimens, we also provide maps of their distributions, analyse their ecological preferences, and offer a preliminary phylogeny of the North Atlantic species of Polycope based on 28S rDNA sequences.http://zoobank.org/urn:lsid:zoobank.org:pub:17EB9011-1715-4C1A-A854-FD0770F36E76

Interaction of down-slope and along-slope processes off Capo Vaticano (southern Tyrrhenian Sea, Italy), with particular reference to contourite-related landslides

The Late Quaternary architecture of the upper slope-shelf sector of the Calabro-Tyrrhenian continental margin off Capo Vaticano was reconstructed to decipher the interaction of along-slope and down-slope sedimentary processes, based on the analysis of very high-resolution single-channel seismic profiles and multibeam bathymetry. Both along-slope bottom currents and downslope gravity processes influenced the stratigraphic architecture of the margin during the last eustatic cycle, locally forming a mixed turbidite and contourite system. Along-slope processes formed elongated drifts located on the upper continental slope and outer shelf, while gravity-driven down-slope processes formed mass-transport deposits and turbidite systems with erosive channels, locally indenting the present-day shelf. Several slide events affected the elongated drifts and failure processes appear to be dominated by translational sliding that affect the upper 10–20m of the stratigraphic record, with glide plains developed within contourite deposits. The most striking feature is the Capo Vaticano slide complex, which displays a large spatial coverage (area of about 18km2), between ca −190m and −500m and is composed of several intersecting slide scars and overlapping deposits. These characteristics are peculiar for the Tyrrhenian continental margins, where slide events developed in open-slope areas are usually less complex and smaller in size. This would indicate that high sedimentation rates and occurrence of contourite deposits can represent a preconditioning factor for medium-large scale slope instability on the Tyrrhenian margins.

Structure of the Demerara passive-transform margin and associated sedimentary processes. Initial results from the IGUANES cruise

Abstract The IGUANES cruise took place in May 2013 on the R/V L'Atalante along the Demerara passive transform margin off French Guiana and Surinam. Seismic, multibeam and chirp acquisitions were made. Piston cores were collected for pore geochemistry and sedimentology. A mooring was deployed on the sea-bottom for 10 months (temperature, salinity, turbidity and current measurements). This new dataset highlights the lateral variability of the 350 km-long Guiana–Surinam transform margin due to the presence of a releasing bend between two transform segments. The adjacent Demerara Plateau is affected by a 350 km-long giant slide complex. This complex initiated in Cretaceous times and was regularly reactivated until recent times. Since the Miocene, contourite processes seem to be active due to the onset of the North Atlantic Deep Water (NADW) bottom current. A main NADW water vein flows towards SE, eroding slide headscarps and allowing the deposition of contourite drifts. Numerous depressions looking like comet tails or comet scours record this flow. Some of those were interpreted before the cruise as active pockmarks. Pore geochemistry and core analysis do not show any evidence of present-day gas seepage.

Organic matter pools, C turnover and meiofaunal biodiversity in the sediments of the western Spitsbergen deep continental margin, Svalbard Archipelago

We investigated organic matter (OM) quantity, nutritional quality and degradation rates, as well as abundance and biodiversity of meiofauna and nematodes along the deep continental margin off Spitsbergen, in the Svalbard Archipelago. Sediment samples were collected in July 2010 and 2011 along a bathymetric gradient between 600m and 2000m depth, and total mass flux measured at the same depths from July 2010 to July 2011. In both sampling periods sedimentary OM contents and C degradation rates increased significantly with water depth, whereas OM nutritional quality was generally higher at shallower depths, with the unique exception at 600m depth in 2010. Meiofaunal abundance and biomass (largely dominated by nematodes) showed the highest values at intermediate depths (ca 1500m) in both sampling periods. The richness of meiofaunal higher taxa and nematode species richness did not vary significantly with water depth in both sampling periods. We suggest here that patterns in OM quantity, C degradation rates, and meiofauna community composition in 2011 were likely influenced by the intensification of the warm West Spitsbergen Current (WSC). We hypothesize that the intensity of the WSC inflow to the Arctic Ocean could have an important role on benthic biodiversity and functioning of deep-sea Arctic ecosystems.