Porcupine Abyssal Plain Research Articles

Temporal variability in polychaete assemblages of the abyssal NE Atlantic Ocean

Temporal variability in deep-sea polychaete assemblages was assessed at the Porcupine Abyssal Plain Sustained Observatory, NE Atlantic, over a 9-year period (eight cruises between August 1989 and September 1998). The polychaete communities were characterized by large number of individuals (abundance) and high family richness. The highest abundances occurred in the upper 1 cm sediment layer (53.2% of total abundance). The most abundant families were the Cirratulidae, Spionidae, Opheliidae and Paraonidae. Surface deposit feeders were the dominant trophic group (67.4% of total abundance). Significant temporal variability was evident in polychaete abundance with significant differences in polychaete abundance between sampling periods (cruises; p<0.01). Stepwise increases in abundance in September 1996 and March 1997 coincided with similar increases in abundance in large invertebrates (megafauna) in the same area (known as the ‘Amperima Event’ after a species of holothurian that increased in abundance by over three orders of magnitude). Similar patterns were observed for abundances across different layers of the sediment, main families and trophic groups showing significant differences between cruises (p<0.05). A comparison of samples taken (1) before the ‘Amperima Event’ (1989–1994) and (2) during the ‘Amperima Event’ (1996–1998) showed significant differences in the polychaete abundance in the upper 3 cm of the sediment. There were significant differences in some trophic groups (predators, deposit feeders and burrowers) and the dominant families (Cirratulidae, Spionidae and Opheliidae). Not all elements of the polychaete community showed a response (e.g. the Paraonidae). Changes in surface deposit feeders were particularly evident. The temporal variability is likely to be related to seasonal and interannual variability in organic matter input. Greater food supply in some years may allow the growth and development of deposit-feeding polychaetes.

A new and exceptional species of Bradya Boeck, 1873 (Copepoda: Harpacticoida:Ectinosomatidae) from the abyssal plain of the Angola Basin and the variabilityof deep-sea Harpacticoida

Bradya kurtschminkei sp. nov. is described from deep-sea samples collected from the Angola Basin (Southeast Atlantic) during the DIVA 1 campaign in 2000, the Guinea and Cape Basins (Southeast Atlantic) during the DIVA 2 campaign in 2005, and the Porcupine Abyssal Plain (Northeast Atlantic) during the RRS ‘‘Challenger’’ cruise 111 in 1994. B. kurtschminkei sp. nov. is exceptional because of many characters, hitherto unknown from other Bradya species. B. kurtschminkei sp. nov. can be distinguished from its congeners by its unique habitus as the body is slightly flattened dorso-ventrally and the female cephalothorax is almost as long as the free pedigerous somites and the urosome together. The rostrum’s length is a third of the cephalothorax length and it tapers to a point. The antennule is 5-segmented, the basis of maxillule is fused with exopod and endopod, and the endopod has only four setae. The endopod of maxilla has only one large claw, the exopod of the P5 is short, its distal end reaches to the distal end of the baseoendopod, and the outer seta of the baseoendopod has a double tip. The armature formula of P1–P4 is exceptional as there are in total seven setae less on the endopods than in all other species of Bradya. Like many other deep-sea Harpacticoida, B. kurtschminkei sp. nov. shows compared with shallow waters species a remarkable, as yet unknown morphological variability.

Abyssal NE Atlantic benthic foraminifera during the last 15 kyr: Relation to variations in seasonality of productivity

Benthic foraminiferal faunas (> 63 μm) and stable isotopes from the last 15 kyr were studied in BENGAL programme (high-resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality) kasten core 13078#16 from the Porcupine Abyssal Plain, NE Atlantic (48°49.91 N, 16°29.94 W, water depth 4844 m). Changes occurred in the accumulation rates, species composition, diversity, and stable isotopes during the last 15 kyr. Today, the area is strongly influenced by seasonal inputs of phytodetritus following the spring blooms in surface water primary productivity. Variations in the relative abundance of the two most abundant species, Epistominella exigua and Alabaminella weddellensis, which today show significant increases in abundance with the presence of phytodetritus on the sea-floor, are interpreted as resulting from changes in the seasonality of productivity. Seasonal productivity was higher during the Holocene than during the last deglaciation and Younger Dryas, probably coinciding with the retreat of the polar front to higher latitudes. This hypothesis is consistent with simultaneous decreases in the percentage of the polar planktic foraminifera Neogloboquadrina pachyderma (s), and increases in the percentage of Globigerina bulloides, a warmer water planktic foraminifera indicative of phytoplankton blooms and enhanced productivity. The relative abundance of the ‘phytodetritus species’ ( E. exigua and A. weddellensis) covary between 14.7 and 8.1 kyr, but not between 7.8 and 1.2 kyr. Major decreases in the numbers per gram and accumulation rates of planktic and benthic foraminifera occurred at ∼ 12–8.5 kyr and at ∼ 4 kyr which correspond to decreases in the % sediment coarse fraction and published data on inorganic carbon contents suggesting that dissolution may have increased at these times. Relationships between benthic foraminiferal faunas and benthic stable isotope records suggest no simple relationship between faunal abundances and test isotope chemistry. For example, the abundances of phytodetritus species do not show strong correlations with either the δ 13C values of E. exigua or the Δ δ 13C E. exigua − P. wuellerstorfi record, which have previously been suggested as indicative of seasonality of productivity.

Particle export from the euphotic zone: Estimates using a novel drifting sediment trap, 234Th and new production

We describe the first results obtained using a novel free-drifting neutrally buoyant sediment trap called PELAGRA (Particle Export measurement using a LAGRAngian trap). The trap uses an APEX float (Autonomous Profiling EXplorer) to maintain its location at a predetermined depth or density horizon and is designed to be deployed in the depth range 100–600 m for periods of up to a week. PELAGRA was deployed into a well characterised region of the Northeast (NE) Atlantic over the Porcupine Abyssal Plain (PAP) during the summers of 2003, 2004, 2005 and 2006 in post-bloom conditions. In parallel, measurements were made of total and new primary production and estimates of particle export based on 234Th deficits in the upper water column. Samples of sinking material from PELAGRA were recovered almost uncontaminated by “swimmers”. The material collected differed from that obtained using water bottles and an in situ filtration system and this indicated selective export of centric diatoms. Fluxes of 234Th into the traps were less than that calculated from the deficit of this isotope in the upper water column. It is concluded that this reflects export events that had occurred prior to these deployments. Fluxes of organic carbon into the PELAGRA traps were similar to those expected from measurements of new production in the upper mixed layer. During 2006, simultaneous PELAGRA deployments at depths of 150 and 250 m provided a direct measurement of the decrease in flux with depth. This decrease was substantially more than that predicted by the often-used “Martin equation” and yielded a “ b value” of 1.7. Using this value in the NE Atlantic about 14% of the total production reaches a depth of 150 m during the post-bloom period under nutrient limitation.

Comparison of the nematode fauna from the Weddell Sea Abyssal Plain with two North Atlantic abyssal sites

Abstract This study provides for the first time data on the nematode community composition from the Weddell Sea Abyssal Plain. In addition, new data on the nematode communities from the Cape Verde Abyssal Plain and the Porcupine Abyssal Plain are provided. Three stations along an east–west transect in the Weddell Sea were sampled during the ANDEEP II campaign and compared with two north-east Atlantic abyssal sites, characterized by a contrasting food supply, in terms of densities, community structure and biodiversity. Local total nematode densities on the Weddell Sea Abyssal Plain (station 138) rank among the highest values recorded from abyssal plains worldwide, pointing to a locally high availability of food for meiofauna. Minor differences in the proportion of dominant genera were observed between the Weddell Sea Abyssal Plain and both North Atlantic sites. The nematode communities at the Weddell Sea Abyssal Plain were dominated by Thalassomonhystera and Acantholaimus, which is comparable to the North Atlantic abyssal plains. No endemic genera for the Southern Ocean were found. However, the higher abundance of the genera Microlaimus and Dichromadora seems to be typical for the Southern Ocean deep-sea, and might be related to the availability of fresh food.

Dissolved organic matter in abyssal sediments: Core recovery artifacts

We report measurements of pore‐water dissolved organic carbon (DOC), dissolved organic nitrogen, total dissolved carbohydrates, dissolved free monosaccharides, and ammonium in recovered deep‐sea sediments from the Porcupine Abyssal Plain (PAP), Northeast Atlantic. There were distinct maxima close to the sediment‐water interface of these constituents at all times of the year. The very high diffusive effluxes calculated from these porewater distributions were not compatible with simultaneous sediment trap measurements of particulate organic carbon, nitrogen, and carbohydrate fluxes toward the seafloor. Effluxes calculated from pore‐water DOC distributions in recovered cores from another Atlantic deep‐sea site, showing almost identical maxima as those at PAP, were more than an order of magnitude greater than simultaneous in situ chamber DOC flux measurements. We suggest that the dissolved organic matter maxima are predominantly artifacts induced by lysis of, or leakage from, mainly bacterial biomass resulting from decompression and/or warming during recovery of the sediment cores from the abyssal seafloor. Temperature elevation during core recovery from the abyss gives a N2 saturation of about 150%, and the combined effect of warming and decompression results in a CO2 saturation of about 135%, which together plausibly are associated with bubble formation creating cell bursting. Previous estimates of microbial biomass in abyssal sediments may be underestimates because of the difficulty of counting lysed bacterial cells. Since exoenzymes are inducible, previous measurements of their activities in recovered abyssal sediments may be overestimates.

Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 341:37-44 (2007) - doi:10.3354/meps341037 Seasonal development of a deep pelagic bioluminescent layer in the temperate NE Atlantic Ocean E. J. V. Gillibrand1,3,*, A. J. Jamieson1, P. M. Bagley1, A. F. Zuur2, I. G. Priede1,** 1Oceanlab, University of Aberdeen, Main Street, Newburgh, Aberdeen AB41 6AA, UK 2Highland Statistics Ltd, 6 Laverock Road, Newburgh, Aberdeen AB41 6FN, UK 3Present address: Robert Gordon University, Schoolhill, Aberdeen AB10 1FR, UK *Née Battle**Corresponding author. Email: i.g.priede@abdn.ac.uk ABSTRACT: Vertical distribution of bioluminescent organisms throughout the water column in the Porcupine Seabight and Porcupine Abyssal plain area of the NE Atlantic Ocean was measured using a free-falling autonomous vehicle equipped with a high-sensitivity video system viewing stimulated flashes of light from impacts on a mesh screen. Data were recorded from 500 m depth down to the sea floor at depths from 740 to 4808 m during spring and autumn of 2001 and 2002. Bioluminescent organisms decreased in abundance from a maximum of 80 m–3 at 600 m depth to a mean of 0.5 to 1.2 m–3 at depths greater than 3000 m. During autumn, a seasonally enhanced peak of abundance (mean 27.19 m–3, compared with 7.52 m–3 in spring) centred at 1420 m within the depth range of occurrence of Mediterranean intermediate water. It is hypothesised that this bioluminescent layer reflects seasonal increase in deep pelagic biomass fed by downward transport of organic matter following the spring peak of primary production in the surface layers. The bioluminescent layer is a large-scale regional phenomenon extending over 100s of kilometres. KEY WORDS: Bioluminescence · Deep pelagic · NE Atlantic Ocean · Mediterranean intermediate water · Pelagic–benthic flux Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 341. Online publication date: July 04, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Organic and siliceous protistan scales in north-east Atlantic abyssal sediments

We report the occurrence of a high diversity of minute (∼1 μm diameter) organic and siliceous protistan scales in small samples (total volume ∼35 μl) of superficial sediment from the Porcupine Abyssal Plain (PAP), north-east Atlantic (4850 m water depth). Many exhibit characters by which they can be identified to species. The organic scales belong to the haptophyte genera Chrysochromulina (8–9 species), Chrysocampanula and Dolichomastix (1 species each). The siliceous scales belong to the chrysophytes Paraphysomonas vestita and Meringosphaera sp. and to the heterotrophic flagellate genus Thaumatomastix (T. dybsoeana, T. formosa, Thaumatomastix sp.). As far as we are aware, this is the first time that non-calcareous protistan scales have been observed in deep-sea sediments (although siliceous skeletal plates and cysts are reported). All scales probably originated from the upper water column and were delivered to the deep-sea floor on rapidly sinking detrital aggregates. However, naked heterotrophic flagellates are known to thrive in abyssal sediment habitats and so the possibility that some scale-bearing protists also live in benthic deep-sea environments cannot be eliminated. Many species identified at the PAP site are common in coastal marine waters around Europe; some occur as far afield as Tasmania and New Zealand. Five Chrysochromulina species are known from central oceanic areas, including parts of the North Atlantic, while another species, C. pringsheimii, is reported from a British freshwater lake. We retrieved ∼15% of the 55 named Chrysochromulina species (∼8% of the estimated total number of species in this diverse group) in the ∼35 μl of abyssal sediment. Because the scales can persist and be identified after cell death, they may provide useful time- and space-averaged information about the distribution of protist species in marine habitats. The long-term fate of the scales on the sea-floor is unknown. It is possible that at least some of the organic scales are preserved as microfossils in deep-sea sediments.

The influence of changing food supply on the lipid biochemistry of deep-sea holothurians

The biochemical response of three species of deep-sea holothurian ( Oneirophanta mutabilis, Pseudostichopus villosus, and Psychropotes longicauda) to temporal variation in food supply at the Porcupine Abyssal Plain (PAP; ∼4850 m water depth in the NE Atlantic) was studied over a period of 22 months. Lipid contents of P. longicauda showed a strong positive correlation with the contents of lipids in the surficial sediments (0–5 mm; Spearman rank correlation, R s=1.0, P<0.001). O. mutabilis did not show the same trend in total lipid, but there was an apparent enrichment of sterol in both particulate organic matter (POM) arriving at the sea floor in September–October 1997 and the tissues of O. mutabilis, suggesting that this species can respond to changing availability of food resources. Lipid contents of P. villosus did not vary temporally, probably because this species feeds on deeper layers of sediment, which show little or no temporal variation in lipid composition or concentration. The biochemical response of holothurians to variations in food supply appears to depend on their feeding mode. Changes in the quantity and quality of organic matter have the potential to change deep-sea benthic community structure.

A new North‐East Atlantic species of Bacescomysis (Mysidacea, Crustacea) with original moth‐like antennules

A new Mysidacea, Bacescomysis papilio, with a unique, supernumerary complement of sensilla on the antennules, is described from the Porcupine Abyssal Plain (4848 m) in the North‐East Atlantic. Comparisons are made with the species of Bacescomysis and of the related genus Hansenomysis. Biological and biogeographical remarks on these two genera are provided, i.e. role of antennule, adaptations to deep water, and correlations between depth range of species and latitude.

Nitrogen cycling in deep-sea sediments of the Porcupine Abyssal Plain, NE Atlantic

Rates of transformation, recycling and burial of nitrogen and their temporal and spatial variability were investigated in deep-sea sediments of the Porcupine Abyssal Plain (PAP), NE Atlantic during eight cruises from 1996 to 2000. Benthic fluxes of ammonium (NH 4) and nitrate (NO 3) were measured in situ using a benthic lander. Fluxes of dissolved organic nitrogen (DON) and denitrification rates were calculated from pore water profiles of DON and NO 3, respectively. Burial of nitrogen was calculated from down core profiles of nitrogen in the solid phase together with 14C-based sediment accumulation rates and dry bulk density. Average NH 4 and NO 3-effluxes were 7.4 ± 19 μmol m −2 d −1 ( n = 7) and 52 ± 30 μmol m −2 d −1 ( n = 14), respectively, during the period 1996–2000. During the same period, the DON-flux was 11 ± 5.6 μmol m −2 d −1 ( n = 5) and the denitrification rate was 5.1 ± 3.0 μmol m −2 d −1 ( n = 22). Temporal and spatial variations were only found in the benthic NO 3 fluxes. The average burial rate was 4.6 ± 0.9 μmol m −2 d −1. On average over the sampling period, the recycling efficiency of the PON input to the sediment was ∼94% and the burial efficiency hence ∼6%. The DON flux constituted ∼14% of the nitrogen recycled, and it was of similar magnitude as the sum of burial and denitrification. By assuming the PAP is representative of all deep-sea areas, rates of denitrification, burial and DON efflux were extrapolated to the total area of the deep-sea floor (>2000 m) and integrated values of denitrification and burial of 8 ± 5 and 7 ± 1 Tg N year −1, respectively, were obtained. This value of total deep-sea sediment denitrification corresponds to 3–12% of the global ocean benthic denitrification. Burial in deep-sea sediments makes up at least 25% of the global ocean nitrogen burial. The integrated DON flux from the deep-sea floor is comparable in magnitude to a reported global riverine input of DON suggesting that deep-sea sediments constitute an important source of DON to the world ocean.

&amp;lt;i&amp;gt;Hyperammina micaceus&amp;lt;/i&amp;gt; sp. nov.: a new foraminiferan species (Protista) from the Porcupine Abyssal Plain, northeast Atlantic

Abstract. The paper describes Hyperammina micaceus sp. nov., a distinctive new deep-water agglutinated foraminiferan from the Porcupine Abyssal Plain (PAP), northeast Atlantic (4850 m water depth). The new species is tiny (&lt;1 mm long and 20–30 μm wide), with an oval proloculus which merges smoothly into the long, tubular part of the test. The test wall is composed of a single layer of plate-like mineral grains. Hyperammina micaceus occurs in most of the PAP cores collected between 1989 and 2002 and represents up to 4% (in one sample 18%) of the live foraminiferal assemblage in the 0–1 cm layer (&gt;63 μm size fraction). It tends to be more common in samples collected during the late spring and summer (May to September), prior to the spring bloom, than in samples obtained during the autumn (October) and the earlier part of the year (March, April), before the spring bloom. Live specimens are concentrated in the 0–0.5 cm sediment layer but are uncommon in the overlying phytodetrital deposits and virtually absent below 1 cm depth.

Long-term changes in reproductive patterns of the holothurian Oneirophanta mutabilis from the Porcupine Abyssal Plain

Time-series studies (1989–2002) in the NE Atlantic have shown large-scale changes in the composition and structure of the benthic community on the Porcupine Abyssal Plain. Radical changes in the abundance of some species in 1996 led to a significant shift in the way in which organic matter was reprocessed at the seabed. This article examines the reproductive processes of the holothurian Oneirophanta mutabilis collected during the time series. The reproductive biology of O. mutabilis is reviewed. No males were evident in any of the samples. The sex ratio (females: ‘no gender’) was biased significantly towards no-gender individuals. The maximum egg size was 650 μm and there was no evidence of synchrony in reproduction. Significant changes in the oocyte-size distribution and the fecundity of O. mutabilis were noted through time, coinciding with the time of greatest faunal change in the benthic community. There was an increase in the proportion of previtellogenic oocytes and a decrease in the proportion of mature vitellogenic oocytes in 1997 and 1998, in parallel with a significant decrease in fecundity of the post-1996 samples. Samples from 2002 showed a reversal in the reproductive trends, with an increase in the proportion of mature vitellogenic oocytes and fecundity. The results are discussed in relation to large changes in abundance of the epibenthos on the Porcupine Abyssal Plain. It is suggested that the superabundance of certain megafaunal species on the abyssal seafloor affected the availability of trophic resources for O. mutabilis, leading to the changes in its reproductive effort.

Conqueria laevis gen. and sp. nov., a new soft-walled, monothalamous foraminiferan from the deep Weddell Sea

Conqueria laevis gen. and sp. nov., a new monothalamous agglutinated foraminiferan, is described from core samples collected in the abyssal western Weddell Sea. The species is characterized by a very elongate, almost cylindrical test that usually follows a more or less curved course and has a single terminal aperture located at the end of a short neck. The wall has a very smooth outer surface and is composed of tiny (&lt;5 μm) agglutinated particles. Very similar and presumably congeneric morphotypes occur at northern hemisphere sites, including Arctic fjords around Svalbard and the Porcupine Abyssal Plain. Molecular phylogenetic analyses, based on small subunit rRNA gene sequences, indicate that the new Weddell Sea species forms an independent lineage branching among monothalamous foraminiferans as a sister group to the clade of Psammophaga.

P4.013 Effects of donepezil on behavioral andpsychological symptoms in Korean patients with Alzheimer's disease

We analyzed patterns of α- and β-diversity in deep-sea bivalves collected by epibenthic sleds from the western North Atlantic south of New England, and from the eastern North Atlantic in the Rockall Trough, Porcupine Seabight and Porcupine Abyssal Plain. In the western North Atlantic, species diversity, measured as the normalized expected number of species, shows a unimodal bathymetric trend peaking at mid-bathyal depths. In the eastern North Atlantic, diversity increases monotonically with depth reaching a maximum at abyssal depths. We used Baselga's (2010) metrics to distinguish two separate components of β-diversity along depth gradients, species dissimilarity among sites due to spatial replacement (turnover) and species loss leading to nestedness. We also examined the rank order of nestedness with depth using Rodríguez-Gironés and Santamaría's (2006) BINMATNEST. The primary difference in β-diversity between west and east centers on the composition of abyssal communities. In the western North Atlantic, abyssal assemblages are nested subsets of bathyal assemblages. In the eastern North Atlantic, turnover dominates at all depths. These very fundamental differences in community structure between the basins may be attributable to differences in food supply, which is greater in the eastern North Atlantic region sampled. POC-flux to abyssal depths in the east may not reach levels low enough to depress species diversity as it does in the west. In the west, the abyssal fauna is largely an impoverished nested subset of the bathyal fauna that shows less endemism and may be maintained partly by source–sink dynamics.

Growth, reproduction and possible recruitment variability in the abyssal brittle star Ophiocten hastatum (Ophiuroidea: Echinodermata) in the NE Atlantic

Growth was studied from skeletal growth markers in the cosmopolitan abyssal brittle star Ophiocten hastatm. Samples for analysis were taken at five sites located in the southern (2900 m) and central (2000 m) Rockall Trough, at ca. 3000 and 4000 m in the Porcupine Seabight, and at 4850 m on the Porcupine Abyssal Plain. Growth bands were assumed to reflect an annual cycle in skeletal growth. Band measurements on arm vertebrae, standardised to disc diameter, were used to provide size-at-age data and size-increment data that took into account overgrowth of early bands in older individuals. The Richards growth function marginally provides best fit to pooled size-at-age data, although the asymptote-less Tanaka function and the Gompertz growth function also provided good fit to size-at-age data which showed a rather linear growth pattern with little indication of a growth asymptote. Loge transformed size-increment data were linearised by applying the Ford–Walford method to approximate Gompertz growth so that growth could be compared at the five sites. Grouped linear regression and analysis of covariance showed no significant differences between growth at the sites and a common fitted regression. However, pairwise comparisons suggest growth differences with increasing bathymetric separation. Oocyte size frequencies measured from histological preparations of the gonad of specimens from the Porcupine Abyssal Plain indicate marked reproductive periodicity, with spawn-out in late winter that is likely followed by planktotrophic early development in spring with benthic settlement in summer. Although usually rare in the trawl and epibenthic sled samples, several years of successful recruitment followed by a period when recruitment was low or absent might explain size structure observed in a single unusually large sample from the Rockall Trough. This is consistent with previous observations during the late 1990s of a large population increase on the Porcupine Abyssal Plain. Analysis of growth bands of these specimens sampled in 1997 suggest the population increase derives from a single or small number of year classes recruited during the early 1990s.

Recycling and burial of organic carbon in sediments of the Porcupine Abyssal Plain, NE Atlantic

Rates of degradation, recycling and burial of organic carbon (OC) and their temporal and spatial variability were investigated in deep-sea sediments of the Porcupine Abyssal Plain (PAP) in the Northeast Atlantic Ocean (depth 4800–4850 m) during six cruises in August 1996—April 1999. Benthic fluxes of total dissolved inorganic carbon (CT) and alkalinity (AT) were measured in situ using benthic chambers of a free-vehicle benthic lander on four of the cruises. To obtain the OC oxidation rate (Cox), the CT flux in each chamber was corrected for CaCO3 dissolution. Burial of OC was determined by using downcore profiles of OC (n=20) in the solid phase of the sediment and sediment accumulation rates estimated from a model age based on 14C dating of foraminifera. Results obtained showed that during 1996–1999 Cox rates in PAP sediments varied between deployments from 0.21±0.04 to 0.86±0.23 mmol m−2 d−1 with a total mean of 0.46 mmol m−2 d−1 (SD±0.37, n=31). No statistically significant spatial (between stations) or temporal (seasonal as well as interannual) variation in the Cox rates could be detected during the sampling period. The mean burial rate was determined to be 0.03±0.01 mmol m−2 d−1. On average over the sampling period, ∼94% of the particulate organic carbon (POC) deposited was recycled as CT to the overlying water and ∼6% was buried. The calculated mean POC demand during 1996–1999 of 0.49±0.37 mmol m−2 d−1 (the sum of corrected CT and burial fluxes) was not significantly different from the annual integrated mean POC supply (1997–1999) of 0.24±0.06 mmol m−2 d−1 estimated from calibrated sediment traps 1800 mab at the same locality.

Recycling and burial of organic carbon in sediments of the Porcupine Abyssal Plain, NE Atlantic

Rates of degradation, recycling and burial of organic carbon (OC) and their temporal and spatial variability were investigated in deep-sea sediments of the Porcupine Abyssal Plain (PAP) in the Northeast Atlantic Ocean (depth 4800–4850 m) during six cruises in August 1996—April 1999. Benthic fluxes of total dissolved inorganic carbon ( C T) and alkalinity ( A T) were measured in situ using benthic chambers of a free-vehicle benthic lander on four of the cruises. To obtain the OC oxidation rate ( C ox), the C T flux in each chamber was corrected for CaCO 3 dissolution. Burial of OC was determined by using downcore profiles of OC ( n=20) in the solid phase of the sediment and sediment accumulation rates estimated from a model age based on 14C dating of foraminifera. Results obtained showed that during 1996–1999 C ox rates in PAP sediments varied between deployments from 0.21±0.04 to 0.86±0.23 mmol m −2 d −1 with a total mean of 0.46 mmol m −2 d −1 (SD±0.37, n=31). No statistically significant spatial (between stations) or temporal (seasonal as well as interannual) variation in the C ox rates could be detected during the sampling period. The mean burial rate was determined to be 0.03±0.01 mmol m −2 d −1. On average over the sampling period, ∼94% of the particulate organic carbon (POC) deposited was recycled as C T to the overlying water and ∼6% was buried. The calculated mean POC demand during 1996–1999 of 0.49±0.37 mmol m −2 d −1 (the sum of corrected C T and burial fluxes) was not significantly different from the annual integrated mean POC supply (1997–1999) of 0.24±0.06 mmol m −2 d −1 estimated from calibrated sediment traps 1800 mab at the same locality.

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ∼0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (∼5 210Pb half lives) northward currents prevailed within the lowest ∼1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean.

Evidence for a sedimentary fingerprint of an asymmetric flow field surrounding a short seamount

Physical oceanographic modeling and field studies have shown that kilometer-scale seafloor elevations of comparable breadth and width (abyssal hills, knolls, seamounts) are surrounded by complex flow fields. Asymmetric flow fields, reversed flow and closed streamlines around the topographic feature (Taylor caps), and resonantly amplified tidal currents around the seamount rim potentially control near-bottom particle dynamics, particle deposition at the seafloor and, consequently, the formation of the sedimentary record. We combine numerical modeling and field data to study how such topographically controlled flow-field features are reflected in the sedimentary record. Sediment deposition on a topographically isolated abyssal knoll (height: 900 m) on the Porcupine Abyssal Plain in the Northeast Atlantic (water depth above the abyssal plain: 4850 m) was studied, (1) by comparing the spatial distribution of 210Pb fluxes, calculated from inventories of sedimentary excess 210Pb, with 210Pb input from the water column as recorded by sediment traps; and (2) by comparing sedimentary grain-size distributions and Zr/Al ratios (an indicator for contents of the heavy mineral zircon) at slope, summit and far-field sites. Given Rossby numbers ≥0.23, a fractional seamount height of ∼0.2, and the absence of diurnal tides it is concluded that an asymmetric flow field without Taylor cap and without amplified tidal currents around the seamount rim is the principal flow-field feature at this knoll. The results and conclusions are as follows: (1) Geochemical and grain-size patterns in the sedimentary record largely agree with the predicted pattern of flow intensity around the topographic elevation: with increasing current strength (erosiveness) there is evidence for a growing discrepancy between water column-derived and sediment-derived 210Pb fluxes, and for increasing contents of larger and heavier particles. The topographically controlled flow field distorts a homogeneous particle-flux input signal from the ocean interior and results in kilometer-scale differences of the amount and composition of the deposited material. (2) The fact that, at the summit, the sediment-derived 210Pb flux is lower than the water-column-derived 210Pb flux indicates that the passing water is partly advected around and partly advected over the knoll. (3) The orientation of the sedimentary pattern indicates that at least during the past 100 years (∼5 210Pb half lives) northward currents prevailed within the lowest ∼1000 m of the water column on the Porcupine Abyssal Plain. The fact that the modelled spatial current-velocity distribution shows a better match with sedimentary velocity (erosiveness) proxies at higher than at lower inflow velocities suggests that mean far-field current velocities might have been higher in at least the past 100 years as compared to today. More comprehensive studies of this kind could provide information on paleo-changes of the orientation and current velocity of flow fields in the deep ocean.