Abyssal Plain Research Articles

A comparative analysis of canyon and non-canyon populations of the deep-sea scavenging amphipod Paralicella caperesca

Existing population studies of deep-sea amphipods have focused on species that inhabit deep-sea vent or trench environments but few cosmopolitan species have been studied. Here we provide new insight into the life history and population ecology of the pan-oceanic scavenging amphipod Paralicella caperesca and discuss the influence of nutrient-rich submarine canyon environments on the growth and reproduction of this species. Data were collected through the dissection and measurement of 2997 P. caperesca from 14 samples taken from abyssal plains, continental slopes and submarine canyons in the North-East Atlantic. Sexual dimorphism was less pronounced than observed for other scavenging amphipod species but females were significantly larger and had shorter antennae than males. The size of oostegites in female P. caperesca varied considerably within size classes, ovaries contained a relatively large number of oocytes, and no empty ovaries were observed. These factors, in combination with absence of mature females, suggest that P. caperesca practices semelparity, a reproductive strategy that complements the feeding strategy of this obligate necrophage. Five male and seven female size-grouped cohorts were identified for P. caperesca. Cohorts from deep-sea submarine canyon populations showed consistently larger mean total body lengths than non-canyon cohorts. Individuals from canyon samples also expressed sexual characteristics at smaller sizes than non-canyon individuals. We hypothesize that these trends are indicative of nutrient-mediated growth, maturation, and reproduction in P. caperesca. The species is able to grow and reproduce relatively quickly in response to increased nutrient input in canyon environments and therefore dominates scavenging amphipod assemblages.

An Arctic and Subarctic ostracode database: biogeographic and paleoceanographic applications

A new Arctic Ostracode Database-2015 (AOD-2015) provides census data for 96 species of benthic marine Ostracoda from 1340 modern surface sediments from the Arctic Ocean and subarctic seas. Ostracoda is a meiofaunal, Crustacea group that secretes a bivalved calcareous (CaCO3) shell commonly preserved in sediments. Arctic and subarctic ostracode species have ecological limits controlled by temperature, salinity, oxygen, sea ice, food, and other habitat-related factors. Unique species ecology, shell chemistry (Mg/Ca ratios, stable isotopes), and limited stratigraphic ranges make them a useful tool for paleoceanographic reconstructions and biostratigraphy. The database, described here, will facilitate the investigation of modern ostracode biogeography, regional community structure, and ecology. These data, when compared to downcore faunal data from sediment cores, will provide a better understanding of how the Arctic has been affected by climatic and oceanographic change during the Quaternary. Images of all species and biogeographic distribution maps for selected species are presented, with brief discussion of representative species’ biogeographic and ecological significance. Publication of AOD-2015 is open-sourced and will be available online at several public websites with latitude, longitude, water depth, and bottom water temperature for most samples. It includes material from Arctic abyssal plains and submarine ridges, continental slopes, and shelves of the Kara, Laptev, East Siberian, Chukchi, Beaufort Seas, and several subarctic regions.

Coexistence of nitrous oxide undersaturation and oversaturation in the surface and subsurface of the western Arctic Ocean

AbstractThe nitrous oxide (N2O) distributions in a shelf‐slope‐basin section from the Chukchi Sea shelf to the Chukchi Abyssal Plain (CAP) in the western Arctic Ocean were observed for the first time during the fourth Chinese National Arctic Research Expedition (4th CHINARE) in late August 2010. The N2O concentrations were 11.4–16.9 nmol L−1, corresponding to saturation values of 92–118% in the surface water. In the surface, biological factor was negligible to N2O production, and physical process dominated the surface N2O distribution. The N2O undersaturation may be the result of dilution by sea ice meltwater. The oversaturation may be caused by the diffusing of N2O‐rich shelf bottom water. Below the surface, N2O concentrations ranged from 11.4 to 21.4 nmol L−1. On the Chukchi Sea shelf, N2O concentrations increased from the surface to the near‐bottom water, indicating sediment N2O produced by nitrification or denitrification emission may be a significant source. The subsurface maxima in the upper halocline layer (UHL) may receive partial contribution from nitrification production but was primarily caused by the spreading of shelf water.

From Basalt to Skeletons – the 200 million-year history of the Namibian margin uncovered by new seismic data

South of the prolific Angolan oil provinces and conjugate to the Southern Brasil margin, offshore Namibia could be the area focus for future exploration discoveries. The margin is adjacent to existing markets in South Africa such that hydrocarbon discoveries of significant quantity should prove to be commercial. Exploration offshore Namibia has yet to produce results as the expected potential remains elusive. Campaigns from the 1970s to 2000s have tested variety of plays from near shore shelfal carbonates to deep-water clastics. All exploration phases have approached the margin through the lens of the technologies and trends of the time, but only hydrocarbon discovery is the Kudu gas field discovered in 1973. More recent wells have successfully proven the constituent parts of a working petroleum system; mature source, reservoir, seal and trap; unfortunately the right combination to produce a large discovery and unlock a new petroleum province remains elusive. In order to understand the context of this margin it is important to see how the overall architecture of the Namibian offshore is fundamentally different from the provinces to the north and farther west in Brazil. On the West African margin the Walvis ridge is a significant boundary between the Cretaceous salt basins farther north and the more magmatic and clastic dominated systems further south. Thus, a different set of play concepts are required and are best understood within the regional setting of the margin. In 2014, ION acquired NamibiaSPAN, a 10,400 km survey of deep, long-offset, regional BasinSPAN seismic reflection lines covering the entire offshore Namibian margin from the shallow coastal waters to the abyssal plains (Figure 1). These data were acquired to intersect important wells and cross numerous basins, crustal domains and tectonic fabric. The objective of the survey was to image the full sedimentary and crustal section down to below the Moho. Several BasinSPAN surveys have been acquired across many of the world’s margins and prolific hydrocarbon provinces. This paper presents the PSTM data from the NamibiaSPAN survey. The PSDM data will be avail¬able in November 2015.

Different frequencies and triggers of canyon filling and flushing events in Nazaré Canyon, offshore Portugal

Submarine canyons are one of the most important pathways for sediment transport into ocean basins. For this reason, understanding canyon architecture and sedimentary processes has importance for sediment budgets, carbon cycling, and geohazard assessment. Despite increasing knowledge of turbidity current triggers, the down-canyon variability in turbidity current frequency within most canyon systems is not well constrained. New AMS radiocarbon chronologies from canyon sediment cores illustrate significant variability in turbidity current frequency within Nazaré Canyon through time. Generalised linear models and Cox proportional hazards models indicate a strong influence of global sea level on the frequency of turbidity currents that fill the canyon. Radiocarbon ages from basin sediment cores indicate that larger, canyon-flushing turbidity currents reaching the Iberian Abyssal Plain have a significantly longer average recurrence interval than turbidity currents that fill the canyon. The recurrence intervals of these canyon-flushing turbidity currents also appear to be unaffected by long-term changes in global sea level. Furthermore, canyon-flushing and canyon-filling have very different statistical distributions of recurrence intervals. This indicates that the factors triggering, and thus controlling the frequency of canyon-flushing and canyon-filling events are very different. Canyon-filling appears to be predominantly triggered by sediment instability during sea level lowstand, and by storm and nepheloid transport during the present day highstand. Canyon-flushing exhibits time-independent behaviour. This indicates that a temporally random process, signal shredding, or summation of non-random processes that cannot be discerned from a random signal, are triggering canyon flushing events.

Stress drop at the Kephalonia Transform Zone estimated from the 2014 seismic sequence

The Kephalonia Transform Zone (KTZ) is a seismically active dextral transform fault decoupling the extensional deformed area of the Ionian Abyssal Plain and the compressional deformed area of the Mediterranean Ridge. We estimate a prominent steady state strain rate of 225±20nstrain/year across the KTZ from the mean velocities of permanent Global Navigation Satellite System (GNSS) stations in East Italy and West Greece, which confirms previous estimates. Based on the regional statistical seismicity and using the local Gutenberg–Richter relation we investigate the energetic balance between stress accumulated as a consequence of the continuous dextral shear deformation, and the average stress released by shallow seismicity. If the maximum expected magnitude is Mw=7.4, the estimated a and b parameters of the local Gutenberg–Richter relation and the geodetically determined shear-strain rate set an upper limit to the regional stress drop ∆σ≤0.4MPa.We verify these values by analyzing a seismic sequence on the KTZ between January and February 2014, which culminated in the Mw=6.09 event of Jan. 26, and the Mw=6.14 event of Feb. 3. The estimated epicenters lie within a few kilometers from each other, on the western side of the Kephalonia Island, at a hypocentral depth≤10km. The measured coseismic displacements of GNSS stations in this area fit the expected surface dislocation, which can be predicted for an elastic half-space using the measured fault-plane solutions as input. If the hypocenters of the aftershocks are taken as indicator of optimal dextral shear-stress orientation, then a low regional deviatoric stress of 0.4MPa, that is comparable with the maximum regional stress drop estimated above, is required for the Coulomb stress to match the pattern of the aftershocks of the 2014 sequence. As a consequence, we conclude that the regional deviatoric stress and the seismically released shear stress are in close balance in the KTZ seismic province.

Biogenic silica in surface sediments of the South China Sea: Controlling factors and paleoenvironmental implications

We mapped the spatial distribution of biogenic silica (BSi) based on a total of 355 surface sediment samples from the South China Sea to provide a basin-wide BSi data set for paleoenvironmental reconstruction in the region. The results show that BSi relative abundance, calcium carbonate percentage, siliceous test total abundance, grain size and sediment magnetic susceptibility all show distinct spatial distribution patterns that are affected by environmental drivers. The gradual increase in BSi from the continental shelf to the abyssal plain suggests that decreased dilution of terrigenous sediments plays a greater role in BSi preservation than does increasing dilution by CaCO3, decreasing productivity or siliceous dissolution. The BSi correlation is weak for diatom abundance but relatively good for radiolarians, confirming that radiolarians are the primary contributor to BSi fluxes in the low-latitude South China Sea. In contrast, increased dissolution results in a decrease in CaCO3 with increasing water depth, reflecting the different preservation patterns of these two biogenic components. Correlation analysis also reveals a positive correlation between BSi and fine grain size, perhaps reflecting the ability of clay minerals to protect BSi from dissolution; volcanic ash and glass also can retard dissolution by increasing the pore-water saturation of SiO2 in abyssal basin sediments.

Latest Quaternary sedimentation in the northern Gulf of Mexico Intraslope Basin Province: I. Sediment facies and depositional processes

Very high-resolution seismic facies, classified, mapped, and interpreted from 3.5 kHz echograms, reveal that turbidity-current, mass-transport, and bottom-current depositional processes have all contributed to the regional sediment distribution in the intraslope basin province of the northwest Gulf of Mexico. Piston cores from these deposits confirm the interpretations of the processes. Turbidity currents transport sands into the intraslope mini-basins via channels and canyons. A few turbidity-current pathways, such as Bryant Canyon, allow extensive volumes of terrigenous sediment to bypass through many mini-basins and be deposited beyond the Sigsbee Escarpment to form large submarine fans, such as Bryant Fan. Bryant Fan is a large mud-rich fan that extends hundreds of kilometers from the mouth of Bryant Canyon but has only one meandering channel on the modern seafloor that extends down the length of the fan. In contrast, the much smaller Rio Grande Submarine Fan is deposited on a plateau area of the continental slope. Prolonged 3.5 echo character and numerous small, unleveed channels suggest this is a sand-rich fan with a braided channel system. These two submarine fans appear to present unique architectural and growth patterns not previously described in the numerous fan descriptions of W.R. Normark or other workers. Thus, these two fans appear to represent two new types of fans, which may be related to the complex structures of the intraslope basin province. Mass-transport deposits (MTDs) are ubiquitous throughout the mini-basins. Extensive areas affected by MTDs also occur along the upper continental slope and at the base of the eastern portion of the Sigsbee Escarpment. Piston cores confirm that the majority of MTDs are debris flows, which are characterized by mud clasts of variable size, shape, and color. Most have a muddy matrix, but sandy debris flows also occur in a few mini-basins. Some cores show deformation, folds, and faults that indicate slump or slide deposits. The East Breaks Slide Complex is the largest MTD complex and extends downslope from the shelf edge for >100 km off central Texas. The western portion of the complex is slump and/or slide blocks and debris flows. In contrast, the proximal part of the eastern portion of the complex is characterized by a modern leveed turbidite channel system. However, extensive MTDs underlie the channel-levee deposits and occur at the seafloor on the distal part of the eastern portion of the complex. Three large regions of migrating sediment waves occur on the Sigsbee Abyssal Plain and eastern Bryant Fan and appear to have been formed by circulation of the Loop Current. Sediment waves also occur locally at the base of the Sigsbee Escarpment in conjunction with previously reported erosional furrows.

Description of two ubiquitous species of Desmosomatidae (Isopoda: Asellota) from the Northwest Pacific Basin east of the Kuril-Kamchatka Trench.

Two new species of Desmosomatidae, Eugerdella kurabyssalis sp. nov. and Parvochelus serricaudis sp. nov., are described from the Northwest Pacific open abyssal plain to the east of the Kuril-Kamchatka Trench. These species constituted about one fourth of all collected desmosomatid specimens of the KuramBio expedition (2012), indicating it may be the most abundant species of the family in this area. E. kurabyssalis sp. nov. is rather similar to E. minutula Mezhov, 1986 and E. theodori Brix, 2007, resembling them in the shape of its body segments and appendages, including the absence of the mandibular palp and morphology of the pereopod I. The new species can be distinguished from the mentioned species by the number of cusps of the mandibular incisor process and lacinia mobilis, the relatively stouter pereopods and different setation of the pereopod I ischium. P. serricaudis sp. nov. is the second species of the genus Parvochelus Brix & Kihara, 2015 (see Brix et al. 2015) and the first record of the genus from the Pacific Ocean and from the Northern Hemisphere. P. serricaudis sp. nov. can be distinguished from P. russus Brix & Kihara, 2015 by its relatively stouter pereopod I, presence of the mandibular palp and the shape of the maxilliped palp articles and female operculum. A discussion of the taxonomic characters and the modified diagnosis of the genus Parvochelus are presented.

Observational analysis of the double-diffusive convection in the deep Canada Basin

The Canada Basin (CB) is the largest sub-basin in the Arctic, with the deepest abyssal plain of 3 850 m. The double-diffusive process is the possible passage through which the geothermal energy affects the above isolated deep waters. With the temperature-salinity-pressure observations in 2003, 500-m-thick transition layers and lower 1 000-m-thick bottom homogenous layers were found below 2 400 m in the central deep CB. Staircases with downward-increasing temperature and salinity are prominent in the transition layers, suggesting the doublediffusive convection in deep CB. The interface of the stairs is about 10 m thick with 0.001–0.002°C temperature difference, while the thicknesses of the homogenous layers in the steps decrease upward from about 60 to 20 m. The density ratio in the deep central CB is generally smaller than 2, indicating stronger double-diffusive convection than that in the upper ocean of 200–400 m. The heat flux through the deepest staircases in the deep CB varies between 0.014 and 0.031 W/m2, which is one-two orders smaller than the upper double-diffusive heat flux, but comparable to the estimates of geothermal heat flux.

Wind-driven interannual variability of sea ice algal production in the western Arctic Chukchi Borderland

Abstract. Seasonal and interannual variability in the biogenic particle sinking flux was recorded using multi-year bottom-tethered sediment trap mooring systems in the Northwind Abyssal Plain (Station NAP: 75° N, 162° W, 1975 m water depth) of the western Arctic Chukchi Borderland. Trapped particle flux at a median depth of 184 m had an obvious peak and dominance of sea ice-related diatom assemblages in August 2011. The observed particle flux was considerably suppressed throughout summer 2012. In the present study, the response of ice algal production and biomass to wind-driven changes in the physical environment was addressed using a pan-Arctic sea ice–ocean modeling approach. A sea ice ecosystem with ice algae was newly incorporated into the lower-trophic marine ecosystem model, which was previously coupled with a high-resolution (i.e., 5 km horizontal grid size) sea ice–ocean general circulation model. Seasonal model experiments covering 2-year mooring periods indicated that primary productivity of ice algae around the Chukchi Borderland depended on basin-scale wind patterns via various processes. Easterly winds in the southern part of a distinct Beaufort High supplied nutrient-rich water for euphotic zones of the NAP region via both surface Ekman transport of Chukchi shelf water and vertical turbulent mixing with underlying nutricline water in 2011. In contrast, northwesterly winds flowing in the northern part of an extended Siberian High transported oligotrophic water within the Beaufort Gyre circulation toward the NAP region in 2012. The modeled ice algal biomass during summer reflected the differences in nutrient distribution. The modeled sinking flux of particulate organic nitrogen (PON) was comparable with the time series obtained from sediment trap data in summer 2011. In contrast, lateral advection of ice algal patches of shelf origin during a great cyclone event may have caused a modeled PON flux bias in 2012. Sensitivity experiments revealed several uncertainties of model configurations of ice algal productivity, particle sinking speed, and grazing activities. Extending the year-long measurements is expected to help illustrate the more general features of ice-related biological processes in the Arctic Ocean.

Free-living nematode species (Nematoda) dwelling in hydrothermal sites of the North Mid-Atlantic Ridge

Morphological descriptions of seven free-living nematode species from hydrothermal sites of the Mid-Atlantic Ridge are presented. Four of them are new for science: Paracanthonchus olgae sp. n. (Chromadorida, Cyatholaimidae), Prochromadora helenae sp. n. (Chromadorida, Chromadoridae), Prochaetosoma ventriverruca sp. n. (Desmodorida, Draconematidae) and Leptolaimus hydrothermalis sp. n. (Plectida, Leptolaimidae). Two species have been previously recorded in hydrothermal habitats, and one species is recorded for the first time in such an environment. Oncholaimus scanicus (Enoplida, Oncholaimidae) was formerly known from only the type locality in non-hydrothermal shallow milieu of the Norway Sea. O. scanicus is a very abundant species in Menez Gwen, Lucky Strike and Lost City hydrothermal sites, and population of the last locality differs from other two in some morphometric characteristics. Desmodora marci (Desmodorida, Desmodoridae) was previously known from other remote deep-sea hydrothermal localities in south-western and north-eastern Pacific. Halomonhystera vandoverae (Monhysterida, Monhysteridae) was described and repeatedly found in mass in Snake Pit hydrothermal site. The whole hydrothermal nematode assemblages are featured by low diversity in comparison with either shelf or deep-sea non-hydrothermal communities. The nematode species list of the Atlantic hydrothermal vents consists of representatives of common shallow-water genera; the new species are also related to some shelf species. On the average, the hydrothermal species differ from those of slope and abyssal plains of comparable depths by larger sizes, diversity of buccal structures, presence of food content in the gut and ripe eggs in uteri.

Links between surface productivity and deep ocean particle flux at the Porcupine Abyssal Plain sustained observatory

Abstract. In this study we present hydrography, biogeochemistry and sediment trap observations between 2003 and 2012 at Porcupine Abyssal Plain (PAP) sustained observatory in the Northeast Atlantic. The time series is valuable as it allows for investigation of the link between surface productivity and deep ocean carbon flux. The region is a perennial sink for CO2, with an average uptake of around 1.5 mmol m−2 day−1. The average monthly drawdowns of inorganic carbon and nitrogen were used to quantify the net community production (NCP) and new production. Seasonal NCP and new production were found to be 4.57 ± 0.85 mol C m−2 and 0.37 ± 0.14 mol N m−2, respectively. The C : N ratio was high (12) compared to the Redfield ratio (6.6), and the production calculated from carbon was higher than production calculated from nitrogen, which is indicative of carbon overconsumption. The export ratio and transfer efficiency were 16 and 4 %, respectively, and the site thereby showed high flux attenuation. Particle tracking was used to examine the source region of material in the sediment trap, and there was large variation in source regions, both between and within years. There were higher correlations between surface productivity and export flux when using the particle-tracking approach, than by comparing with the mean productivity in a 100 km box around the PAP site. However, the differences in correlation coefficients were not significant, and a longer time series is needed to draw conclusions on applying particle tracking in sediment trap analyses.

Distribution and vertical fluxes of silicoflagellates, ebridians, and the endoskeletal dinoflagellate Actiniscus in the western Arctic Ocean

Spatial and temporal variations in major phytoplankton populations such as diatoms in the changing Arctic Ocean have been well studied, whereas only a few monitoring studies have been conducted on minor siliceous flagellates. To discern the relationship between hydrographic conditions and the spatio-temporal distribution of silicoflagellates, ebridians, and the endoskeletal dinoflagellate Actiniscus pentasterias, we analyzed seawater and bottom-tethered sediment-trap samples from the western Arctic Ocean. Silicoflagellates and ebridians were commonly observed in shelf waters around the southern Chukchi Sea in September–October during 2010 and 2013. However, one mesoscale patch with abundant silicoflagellates and ebridians was observed in the southwestern Canada Basin during September–October 2010. This offshore patch reflected an unusual occurrence of a mesoscale eddy deriving from the Alaskan Coastal Water. The active lateral transport of shelf materials by eddies was also evident in high silicoflagellate and ebridian fluxes at station Northwind Abyssal Plain (NAP) (75°N, 162°W, 1975-m water depth) in November–December during 2010 and 2011. The summer silicoflagellate flux at station NAP was mainly composed of Distephanus speculum. During the sea-ice cover period, except for July, silicoflagellates D. medianoctisol and D. octonarius were relatively abundant in the assemblage. The spike in D. speculum flux during July 2011 was observed with fecal pellets containing abundant silicoflagellates, suggesting a temporal silicoflagellate contribution to some kinds of zooplankton. The common occurrence of A. pentasterias in settling particles at station NAP during the winter may indicate their tolerance to cold water under sea ice.

The interplay between deformation, erosion and sedimentation in the deep‐water Mexican Ridges foldbelt, western Gulf of Mexico basin

AbstractWe analyse a regional 2D seismic section of the Mexican Ridges foldbelt (MRFB), western Gulf of Mexico, and construct excess‐area diagrams for each of the structures comprising the foldbelt to estimate shortening, the onset of folding and the degradation of the folded seafloor. From the chronostratigraphy, we derive rates of tectonic and superficial mass transport and illustrate how they change across the MRFB. The resulting tectonic transport in the MRFB is 11.8 km forming a train of twelve buckle folds above a detachment at a depth of ca. 6 s of two‐way travel time, with an average strain of ca. 10%. The fold train grew at a mean uplift rate of ca. 0.21 mm year−1. Cross‐sectional balancing demonstrates that shortening balances the down‐slip motion of the Quetzalcoatl extensional system (QES), suggesting that horizontal compaction, volume loss and other penetrative deformation mechanisms are negligible. By assuming steady‐state denudation, we are able to distinguish sediments derived locally from sediments transported from distant sources. The constant of mass diffusivity, a parameter controlling the degradation rate, is ca. 0.42 m2 year −1, which is characteristic of rapid, episodic, superficial mass movements. The combined sedimentation rate from both, local and distal sources is ca. 0.23 mm year −1. Those values are not constant; structures proximal to the continental shelf are rising rapidly and are being degraded more intensely than those in the distal part of the MRFB, where sedimentation outweighs tectonic uplift. Our results indicate deformation initiated up to 3 Myr earlier than estimated from stacking patterns. Moreover, we find deformation started synchronously during the Late Miocene throughout the MRFB and not in two episodes as the stacking relations suggest. The discrepancy can be explained by a delay in the sedimentary response to folding. During early fold growth, nearly constant thickness strata are deposited before a progressive unconformity and other converging geometries develop. The development of growth strata is fast in the folds near the QES, which are being uplifted rapidly and degraded vigorously. Under these conditions, the stratigraphic relations give only a broad estimate of the pretectonic/syntectonic limit when compared to the excess‐area method. On the other hand, the development of growth strata took twice as much time for folds near the abyssal plain, which are being uplifted at a slower rate and where degradation is less intense. Consequently, the delay takes more time, and the use of stratigraphic relations introduces an even more pronounced bias towards younger ages in the identification of the onset of folding.

Evolution of neodymium isotopic signature of seawater during the Late Cretaceous: Implications for intermediate and deep circulation

Neodymium isotopic compositions (εNd) have been largely used for the last fifty years as a tracer of past ocean circulation, and more intensively during the last decade to investigate ocean circulation during the Cretaceous period. Despite a growing set of data, circulation patterns still remain unclear during this period. In particular, the identification of the deep-water masses and their spatial extension within the different oceanic basins are poorly constrained. In this study we present new deep-water εNd data inferred from the Nd isotope composition of fish remains and Fe–Mn oxyhydroxide coatings on foraminifera tests, along with new εNd data of residual (partly detrital) fraction recovered from DSDP Sites 152 (Nicaraguan Rise), 258 (Naturaliste Plateau), 323 (Bellinghausen Abyssal Plain), and ODP Sites 690 (Maud Rise) and 700 (East Georgia Basin, South Atlantic). The presence of abundant authigenic minerals in the sediments at Sites 152 and 690 detected by XRD analyses may explain both middle rare earth element enrichments in the spectra of the residual fraction and the evolution of residual fraction εNd that mirror that of the bottom waters at the two sites. The results point towards a close correspondence between the bottom water εNd values of Sites 258 and 700 from the late Turonian to the Santonian. Since the deep-water Nd isotope values at these two sites are also similar to those at other proto-Indian sites, we propose the existence of a common intermediate to deep-water mass as early as the mid-Cretaceous. The water mass would have extended from the central part of the South Atlantic to the eastern part of proto-Indian ocean sites, beyond the Kerguelen Plateau. Furthermore, data from south and north of the Rio Grande Rise–Walvis Ridge complex (Sites 700 and 530) are indistinguishable from the Turonian to Campanian, suggesting a common water mass since the Turonian at least. This view is supported by a reconstruction of the Rio Grande Rise–Walvis Ridge complex during the Turonian, highlighting the likely existence of a deep breach between the Rio Grande Rise and the proto-Walvis Ridge at that time. Thus deep-water circulation may have been possible between the different austral basins as early as the Turonian, despite the presence of potential oceanic barriers. Comparison of new seawater and residue εNd data on Nicaraguan Rise suggests a westward circulation of intermediate waters through the Caribbean Seaway during the Maastrichtian and Palaeocene from the North Atlantic to the Pacific. This westward circulation reduced the Pacific water influence in the Atlantic, and was likely responsible for more uniform, less radiogenic εNd values in the North Atlantic after 80Ma. Additionally, our data document an increasing trend observed in several oceanic basins during the Maastrichtian and the Palaeocene, which is more pronounced in the North Pacific. Although the origin of this increase still remains unclear, it might be explained by an increase in the contribution of radiogenic material to upper ocean waters in the northern Pacific. By sinking to depth, these waters may have redistributed to some extent more radiogenic signatures to other ocean basins through deep-water exchanges.

Organic matter characterization and distribution in sediments of the terminal lobes of the Congo deep-sea fan: Evidence for the direct influence of the Congo River

The terminal lobe complex of the Congo River submarine fan sits on the abyssal Atlantic plain, at 5000m water depth, 760km offshore from the river mouth estuarine area. While most rivers deliver particulate material to the continental shelf, particulate matter from the Congo River largely bypasses the shelf and is transported by turbidity currents through the Congo submarine canyon system. We determined the quantity and quality of the organic matter reaching the terminal lobe complex at five sites with marked morphological differences that may influence the distribution of organic matter. A suite of bulk geochemical (% OC, δ13Corg, δ15N, C: N), 137Cs and palynofacies analyses were done on cores collected from the terminal lobe area. These results were also compared to the composition of sediments collected upstream at the Malebo Pool (Congo River).Distal lobe complex sediments contain high amounts of terrestrial organic carbon (3–5wt.%) that is homogeneously distributed in surficial (22cm) and deeper sediments (580cm) silty–clay facies. Strongly altered soil-derived organic matter with well-preserved land plant detritus from the Congo River predominates. A terrestrial soil origin for the particulate load was confirmed by the elevated 137Cs activity in lobe sediments. The vertical distribution of the 137Cs signal suggests that there has been a massive arrival of terrestrial sediments since 1963, consistent with a turbiditic origin. From the locations surveyed, we estimate a maximum accumulation of terrestrial organic carbon of ca. 1kg OC m−2 y−1 for the distal lobe. However, transport modifies the organic matter both in terms of quantity and quality. Observed differences were attributed to preferential degradation of nitrogenous matter during diagenesis and to the addition of highly remineralized marine organic matter.Results from our temporal reference site (E) suggest that organic matter may be preserved in turbidite facies for thousands of years. The good preservation state of the accumulated organic matter shows that turbiditic lobe complexes should be considered as a sink for terrestrial organic carbon in the deep ocean.

Ostracods: The ultimate survivors

Ostracods are tiny crustacean arthropods just a few millimetres long, with a bivalved carapace made of calcium carbonate that covers the whole body, and into which the animal can retreat from the world outside. Because of their diminutive size they are largely overlooked as fossils, but they have a fascinating history. Silent witnesses to life in the seas since the time of trilobites, they have a fossil record extending back to the Early Ordovician, and possibly the Cambrian. Ostracods have survived nearly 500 million years of Earth history including the ‘big five’ mass extinctions of the Phanerozoic Eon; they are true survivors. They are almost perfectly adapted for the aquatic environments in which they live, and can be found from the ocean abyssal plains to damp leaf litter. The ostracod carapace is a triumph of biological engineering that has been re‐configured into myriad different morphologies according to environment. Streamlined and agile species plough through the ocean water column, sometimes reaching a ‘giant’ size of a centimetre in length, whilst their tinier sea bottom cousins make elaborately ornamented carapaces to withstand the pressures of living at the seabed, or shape their carapaces into forms that facilitate burrowing into sediment. Ostracods are key components of aquatic ecosystems. As primary consumers they are food for larger animals both in seabed and planktonic habitats, and they recycle much of the organic detritus produced by larger animals and plants. Delve into the history of ostracods and it is possible to find pioneers who triumphed in the plankton, early colonisers of terrestrial aquatic ecosystems, and ostracods that literally conquered the land. And in more recent times, ostracods have even hitched rides on rockets into space.

ONR arctic acoustics 1978—1998

The Arctic Program Office of the Office of Naval Research ten Arctic field programs from 1978–1994 under the visionary leadership of program managers Dr. G. Leonard Johnson and Dr. Tom Curtin. During this period, over ten ice camps in both the western Arctic (Beaufort Sea) and the eastern Arctic (Nansen and Pole Abyssal Plains were manned and four ice breakers served as platforms in the marginal ice zone (Fram Straits). Since the cost of the support logistics for Arctic field programs is so very high, these experiments were multidisciplinary and almost all had an acoustic component. Some of the highlights were transoceanic reverberation, seismic reflection and refraction, random channels for time and Doppler spreading, target detection, matched field processing,ocean acoustic tomography, seismicity, and ambient noise were among the many topics examined. There were also robust efforts advancing data acquisition. Large, two dimensional horizontal arrays with both cabled and “WIFI” telemetry, large vertical arrays, precision sensor navigation, and sophisticated remote instrumentation buoys were deployed. With the end of the “Cold War” the last field program was in 1994 and the ONR Arctic program eventually was disestablished. Now, the retreat Arctic ice cover and Arctic Ocean warming has reinvigorated ONR's interest in the Arctic and after two decades ONR field programs are planned for the near future.

Shear Velocity Structure of Abyssal Plain Sediments in Cascadia

The sediments that form the upper layer of the oceanic crust vary significantly in thickness. As the crust ages, it accumulates more and more sediment, and when it is near a continental shelf, it can receive large deposits of continental sediment deposited by turbidity currents forming abyssal plains. Because of their soft, unconsolidated nature, the uppermost sediments tend to have very low shear velocities. These low velocities create significant delays in the arrival times of seismic waves moving through them, and these delay times can provide a significant source of error for body‐wave tomography. For surface‐wave tomography, these soft sediments can also provide a source of error by disproportionately slowing short‐period waves in a layer too thin to independently resolve. Correcting for sediments can significantly improve both body‐ and surface‐wave tomography. A deployment of ocean‐bottom seismometer (OBS) stations around the Juan de Fuca plate, the Cascadia Initiative is being deployed in four phases (Fig. 1; Toomey et al. , 2014). Year one focused on the northern portion of Juan de Fuca plate, and year two focused on the southern portion of the Juan de Fuca plate, including Gorda. Year two was coupled with the Blanco experiment, an independent National Science Foundation‐funded experiment, five stations of which have generously been made available to us by principal investigator John Nabelek. Year‐three stations largely reoccupied year‐one sites. Year‐four stations are currently deployed and their data are not yet available for analysis. Figure 1. A contour map of delay times. For each station identified as sitting on topography rising above the abyssal plains, the delay time was set at zero, and tie points with zero delay times were placed along the ridges. The triangles represent stations where we measured a sediment thickness, squares represent nonabyssal plain stations where we could not measure a sediment thickness, and …