Madeira Abyssal Plain Research Articles

Mobility and Immobility of Redox-Sensitive Elements in Deep-Sea Turbidites During Shallow Burial

During the first few tens of thousands of years after an organic-rich turbidite is emplaced on an abyssal plain, surficial oxidation of the unit occurs by downwards diffusion of bottom water O2. From work on piston cores, it is known that a suite of redox-sensitive elements form characteristic relocation peaks in such units around the deepest reaches (upper few dm) achieved by oxidation (fossil oxic/post-oxic boundaries). Four individual turbidites now buried at depths of 130-230 m below seafloor (mbsf), obtained by ODP Leg 157 drilling on the Madeira Abyssal Plain (MAP), have been investigated to determine the effects of burial diagenesis on these geochemical signals over 3-7 My. Porewater data indicate that post-oxic conditions persist to around 130 mbsf on the MAP, below which sulphate reduction predominates. All four units have retained high total organic carbon (Corg) contents (∼1.4%) in their lower reaches, despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to ∼0.2%. The apparent inertness of the turbidite Corg contrasts with the reactivity of organic matter observed in hemipelagic sediment from other ODP investigations. In all four cases, low initial CaCO3 contents have been quantitatively removed along with Corg oxidation, with additional dissolution of carbonate by undersaturated bottom waters. Diagenetic fractions of the elements Se, Cd, Sb, Tl, and V are present in highly-localised peaks and show no evidence of migration. In different combinations, the elements Cu, Co, Ni, and Zn appear to have migrated over short (<5 cm) distances in thin (mm) dark-coloured sulphide bands, probably sourced by reduction of a Mn-oxyhydroxide host phase during early (70 ky) burial. Two examples of local diagenetic pyritisation (up to 20 wt% FeS2) are observed in the bodies of the two turbidites buried to ∼130 mbsf, confirming that sulphate reduction is active at this level. This diagenetic pyrite is associated with high concentrations of As, and lesser enrichments of Ni, Zn, Co, Se, Sb, and possibly Mo.

Postdepositional oxic degradation of alkenones: Implications for the measurement of palaeo sea surface temperatures

Free and “bound” long‐chain alkenones (C37∶2 and C37∶3) in oxidized and unoxidized sections of four organic matter‐rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 951B and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of U37k′. The profiles of both alkenones across the redox boundary show a preferential degradation of the C37∶3 compared to the C37∶2 compound. Because of the high initial U37k′ values and the way of calculating the U37k′ this degradation hardly influences the U37k′ profiles. However, for lower U37k′ values, measured selective degradation would increase U37k′ up to 0.17 units, equivalent to 5°C. For most of the U37k′ band‐width, much smaller degradation already increases U37k′ beyond the analytical error (0.017 units). Consequently, for interpreting the U37k′ record in terms of past sea surface temperatures, selective degradation needs serious consideration.

Latitudinal variation in invertebrate megafaunal abundance and biomass in the North Atlantic Ocean Abyss

Megafauna was collected by otter trawl at two widely separated abyssal sites in the eastern North Atlantic Ocean. The northern site, on the Porcupine Abyssal Plain (PAP, 4850 m), is subject to strong seasonal pulses of phytodetritus deposition, whereas the southern site, on the Madeira Abyssal Plain (OLIGO, 4500–4650 m), showed no indication of such deposition. Data from these two sites were compared with those from a third site (GME), also apparently not affected by phytodetritus, but on the Madeira Abyssal Plain 1200 km from OLIGO. Mean abundance and biomass of invertebrate megafauna at PAP were 72.6 individuals ha -1 and 1974 g ha -1 respectively. The corresponding values for OLIGO were 10.2 individuals ha -1 and 63.4 g ha -1 and for GME 21.7 individuals ha -1 and 112.9 g ha -1. Size-spectral curves of abundance and biomass based on PAP samples showed peaks in the 40–80 g wet weight class, thus confirming the megafauna as a functional entity. No evidence for seasonal variation of abundance or biomass was found. At OLIGO, abundance declined more or less regularly over the organism size range sampled, and biomass was spread fairly evenly across the larger size classes. Major differences in trophic structure among the three sites were evident, with OLIGO and GME more similar to one another than either were to PAP, with much of the higher biomass at PAP represented by particle-selective detritivorous holothurians. The contribution of invertebrates to overall megafaunal biomass at OLIGO (8%) and GME (30%) was lower than at PAP (48%), but a high proportion of fish biomass at OLIGO and GME, and almost all at PAP, belonged to macrophagous species trophically independent of benthic production.

Deep-sea benthic foraminiferal species diversity in the NE Atlantic and NW Arabian sea: a synthesis

We present a synthesis of species diversity data (Fishers’ alpha index, Shannon–Wiener (log 2), ES(100), Rank 1 Dominance) for “live” (stained) foraminifera from five bathyal (1340 m depth) and abyssal (4450–4950 m depth) sites in the NE Atlantic and a 3400 m-deep site in the Arabian Sea. Three Atlantic sites (Porcupine Seabight, BIOTRANS, Porcupine Abyssal Plain) are subject to seasonal phytodetritus inputs that support low diversity populations (8–17 species). In other respects the foraminifera are highly diverse. The meiofaunal fractions (>45 or >63 μm; including fragmented and phytodetritus species) of abyssal Atlantic samples yielded >110 and >170 species in the 0–1 cm and 0–10 cm layers, respectively; the Arabian Sea sample (0–1 cm layer only) yielded 232 species. In both cases, values for diversity measures were very high. Diversity was rather lower in bathyal Porcupine Seabight samples (0–1 cm layer), which yielded <100 species. The foraminiferal macrofauna (>500 μm; Porcupine and Madeira Abyssal Plains) was also speciose (113–133 species), but diversity measures were lower and dominance higher than for the meiofauna. All assemblages contained numerous undescribed species, many belonging to poorly known monothalamous, soft-bodied taxa. Sample diversity was influenced by several factors. Combining phytodetrital and sediment populations reduced diversity and increased dominance slightly; the inclusion of deeper sediment layers and finer sieve fractions had the opposite effect. The inclusion of fragments had more impact on macrofaunal than on meiofaunal diversity, although in both cases the effect was inconsistent (either positive or negative). Porcupine and Madeira Abyssal Plain multicore samples (>63 μm fraction) contained substantially more foraminiferal species than nematode species; the numbers of foraminiferal species in boxcore samples (>500 μm fraction) were comparable to, or greater than, literature values for macrofaunal taxa such as polychaetes and isopods. Few of the more abundant species at our Atlantic abyssal plain and Arabian Sea sites are endemic. This is consistent with literature evidence that many common deep-sea foraminiferal species are cosmopolitan and implies that global foraminiferal diversity may be more modest than the high sample diversity might suggest. Calcareous foraminifera, which are well-known taxonomically and have a good fossil record, may provide a model for diversity patterns among the deep-sea benthic biota in general.

Large, deep-sea agglutinated Foraminifera; two differing kinds of organization and their possible ecological significance

We consider two groups of deep-sea agglutinated foraminifera: 1) robust, tubular astrorhizaceans (Bathysiphon, Rhabdammina) and hippocrepinaceans (Hyperammina, Saccorhiza) which reach several centimeters or more in length and contain a substantial protoplasmic mass (sarcode); and 2) delicate komokiaceans with tests typically 1-5 mm in size and forming a branching tubule system containing accumulations of stercomata and sparse protoplasm, sometimes with fine sediment incorporated between the tubules to form a mudball structure. The tubular taxa occur in eutrophic settings, usually on continental margins, where the food supply originates from strong surface primary production, currents, or downslope transport. We speculate that they are opportunists, able to grow fairly quickly when food is available. The sarcode may provide a buffer against periods of food shortage, enabling these taxa to take advantage of a relatively high but fluctuating food input. Komokiaceans, on the other hand, have a large volume/low biomass organization and are most important in oligotrophic, central oceanic regions. We speculate that the combination of relatively body size and low protoplasmic content may reflect slow growth and longevity; it may also serve to reduce the food content of komokiaceans and hence their attractiveness to predators. In order to illustrate these contrasting groups, we briefly consider faunas from the Oman margin (3400 m depth) and NE Atlantic abyssal plains (4850-4950 m depth), distinguishing between: 1) oligotrophic, central oceanic regions rich in komokiaceans (Madeira Abyssal Plain); 2) central oceanic regions with seasonal phytodetritus inputs where komokiaceans coexist with small (

A case of post-depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the Madeira Abyssal Plain

A set of oxidized and unoxidized sediment intervals from the f-turbidite identified in two piston cores from the Madeira Abyssal Plain (MAP) was analyzed for calcium carbonate, total organic carbon (TOC), total nitrogen (TN), stable carbon composition of TOC ( δ 13C toc ), total hydrolyzable amino acids (THAA), total neutral sugars (TSUG), lignin phenols (LIG), two different lipid biomarkers of marine phytoplankton origin and vascular plantwax n-alkanes and n-acids. Comparison shows changes for all properties depicting the effect of post-depositional, aerobic oxidation. The oxidation process re-mineralized approximately 80% of the TOC and approximately 60% of the TN in the original deposit and left the residual organic matter depleted in 13C by 1.7–2.9‰ THAA and TSUG account for 10% or less of the TOC in all samples leaving approximately 90% of the TOC chemically unidentified. THAA, TSUG, LIG, marine phytoplankton biomarkers and plantwax n-acids were lost from the oxidized deposit to an extent as great as that for TOC. Two non-protein amino acids (β-alanine and γ-aminobutyric acid) and plant wax n-alkanes displayed much less degradation (9–37%) and concentrated 3.3–4.8 times relative to TOC by the oxidation process. These observations show that both marine and terrestrial components contribute to the re-mineralized TOC fraction. A binary mixing approach was used to model quantitatively the change in δ 13C toc and plantwax n-alkane concentration. Results suggest that terrestrial organic carbon contributes approximately 15% to the TOC content of the original turbidite, its abundance increases approximately 2–4 times as a consequence of the oxidation process and approximately 40 and 90% of the terrestrial and marine component of TOC in the original turbidite, respectively, was destroyed by the oxidation process. Although selective preservation of terrestrial relative to marine organic carbon is a well-documented phenomenon in sedimentary processes, this study represents the first attempt to assess the sensitivity of terrestrial organic matter to oxidative degradation in a sedimentary environment.

Preservation of organic-walled dinoflagellate cysts in different oxygen regimes: a 10,000 year natural experiment

The occurrence of organic-walled dinoflagellate cysts in (fossil) sediments depends on several factors, including as the ecological preferences of the cyst-forming dinoflagellates, cyst production, transport and preservation. Although laboratory experiments have shown that several cyst species are sensitive to chemical treatment, no information about the selective preservation of dinoflagellate cyst species in natural environments has previously been presented. Here, we present data on the effects of oxygen availability in bottom sediments on a cyst assemblage from the ungraded Madeira Abyssal Plain f-turbidite of which only the upper layer has been oxidized. Based on differences in species composition between the oxidized and underlying, unoxidized layers of this turbidite, the influence of oxygen availability on the preservation of individual species has been estimated. Cyst species have been classified in ascending order of resistance to oxygen availability in sediments as: (1) highly sensitive (cysts formed by Protoperidinium species), (2) moderately sensitive (e.g. Spiniferites species), (3) moderately resistant (e.g. Impagidinium paradoxum and Nematosphaeropsis labyrinthus) and (4) resistant (e.g. Impagidinium aculeatum).

Epifaunal and shallow infaunal foraminiferal communities at three abyssal NE Atlantic sites subject to differing phytodetritus input regimes

Multiple corer samples (0–1 cm sediment layer and overlying phytodetritus) obtained at three northeast Atlantic sites were analysed for Rose Bengal stained benthic foraminifera ( > 63 μm). Strong phytodetrital pulses have been documented at the northern site on the Porcupine Abyssal Plain (PAP; 48°50'N, 16°30'W) but not at the southern sites on the Madeira Abyssal Plain (MAP; 31°N, 20°W) and Cape Verde Abyssal Plain (CVAP; 21°N, 31°W). Foraminiferal densities reflected surface primary productivity estimates fairly closely, being higher at the PAP (mean: 800 per 25.5 cm 2 sample area= 314 per 10 cm z) than at the southern sites (mean: 502 per sample =197 per 10 cm Z). In part, this difference was attributable to the presence of a substantial phytodetrital deposit in two of the PAP samples. The phytodetrital aggregates provided organically-enriched microenvironments exploited by dense foraminiferal populations, which accounted for up to 22.4% of the entire foraminiferal fauna. They were of low diversity with a few dominant species (notably the rotaliid Epistominella exigua), a population structure typical of organically enriched systems. Most of the phytodetrital species were much less common in the sediments. The total (i.e. sediment + phytodetritus) populations were highly diverse (123–167 species); but southern site populations were somewhat more diverse than PAP populations, and the two PAP phytodetritus-rich cores had higher dominance values than other samples. Total species assemblages were broadly similar. However, in addition to the phytodetritus-dwelling species, some species of Reophax, Leptohalysis, Lagenammina and “Hyperammina” were more abundant at the PAP site, perhaps benefiting from degraded phytodetritus or from the associated bacterial populations. A few species, e.g. Subreophax aduncus and Lagenammina tubulata, were more abundant at the southern sites.

Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands

Research Article| March 01, 1996 Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands D. G. Masson D. G. Masson 1Southampton Oceanography Centre, European Way, Empress Dock, Southampton SO14 3ZH, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1996) 24 (3): 231–234. https://doi.org/10.1130/0091-7613(1996)024<0231:CCOTVI>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. G. Masson; Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands. Geology 1996;; 24 (3): 231–234. doi: https://doi.org/10.1130/0091-7613(1996)024<0231:CCOTVI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Landslides play an important role in the evolution of many volcanic islands, producing huge fields of blocky volcanic debris on their submarine slopes. Sidescan sonar images presented in this paper provide evidence for a large debris avalanche, El Golfo, on the northern flank of Hierro Island in the Canary Islands. Angular blocks, as much as 1.2 km across and 200 m high, cover the debris avalanche surface. El Golfo avalanche is related to both the Canary debris flow and a volcaniclastic turbidite found in the Madeira abyssal plain 600 km west of the Canaries. Dating of the turbidite and the failure scarp onshore indicates that the failure probably occurred between 13 and 17 ka. There appears to be a general correlation between volcaniclastic turbidites in the abyssal-plain sequence and landslides in the Canaries during the past 750 ka. Tentatively, this correlation suggests that seven major landslides have affected the Canaries in that time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Gran Canaria Volcanic Apron and Madeira Abyssal Plain drilled

The drilling program at Gran Canaria Volcanic Apron and Madeira Abyssal Plain had two distinct goals: to study the evolution of an oceanic island from sediments in the volcanic apron, and to study the history of sediment mass wasting in a deep‐sea basin by drilling abyssal‐plain turbidites. The abyssal plain receives sediment flows from the northwest African continental margin and the Canary Islands, thus providing a link between the studies, which were conducted by Leg 157 of the Ocean Drilling Program.

Magnetic measurements used to identify paleoxidation fronts in deep‐sea sediments from the Madeira Abyssal Plain

We describe the results of rock‐magnetic measurements of a sediment core from the Madeira Abyssal Plain which contains multiple turbidites. Previous studies of trace element (e.g. U) concentration and observations of sediment color changes have shown that turbidite emplacement has resulted in reductive diagenesis and the formation of paleoxidation fronts. We show that variations in sediment lithology and postdepositional sedimentary processes can be characterized very sensitively using magnetic measurements. Low concentrations of magnetic minerals occur in turbidites with a high organic content, reflecting the destruction of magnetic material via the process of reductive diagenesis. In addition, fossil redox fronts are readily identified by rock‐magnetic properties.

The Biogeochemistry of Sediments from the Madeira Abyssal Plain — Preliminary Results

AbstractThe organic biogeochemistry of surface sediments from the Madeira Abyssal Plain has been investigated. Total organic carbon contents, organic carbon to nitrogen ratios, concentrations of hydrolysable amino acids, hydrolysable carbohydrates and lipids have been determined. In one case, the detailed lipid compositions were also investigated.The sedimentary geochemistry appears to have been strongly influenced by the recent (ca. 1000 yrs.) deposition of a turbidite. Normally labile lipids are present in high concentrations through the turbidite and the numbers of the most abundant metazoan meiofauna (nematodes) closely correlates with the concentrations of the former. Concentrations of hydrolysable amino acids and carbohydrates, relative to total organic carbon, also decrease through the turbidite layer, but in the case of the carbohydrates tend to be higher in the underlying pelagic sediments. Deeper sediments are more oxidised than those at the surface. The concentration profile for manganese in one of the examined cores suggests that this anomaly arises through the action of manganese‐reducing bacteria that are present in pelagic sediments underlying the turbidite.

Micro‐ and Nanoforaminifera from Abyssal Northeast Atlantic Sediments: A Preliminary Report

AbstractRose Bengal stained benthic foraminifera which pass through a 63 μm mesh (microforaminifera and nanoforaminifera) have been extracted by handsorting the fine sieve residues (&gt; 45 μm, 31 μm, 28 μm, 20 μm, 15 μm) of abyssal sediment samples. The samples were collected using a multiple corer in four areas of the northeast Atlantic between 31° N and 59° N. The abundance of these minute foraminifera varied from 2 specimens per 1 cm2 (Madeira Abyssal Plain) to &gt; 110 per 1 cm2 (BIOTRANS area). They include a variety of taxa, the most common being certain rotaliid species, hormosinaceans and other multilocular agglutinated forms, the unilocular agglutinated genus Lagenammina, soft‐bodied agglutinated sphaeres and flasks (saccamminids and psammosphaerids) and allogromiids. Some specimens are &lt; 63 μm in maximum dimension but others belonging to elongate taxa are longer.Two samples taken 10 cm apart on the Porcupine Abyssal Plain suggest that minute foraminifera may be patchily distributed on a small scale. One sample, which was overlain by substantial amounts of phytodetritus, contained &gt; 100 stained specimens (&gt; 30 per 1 cm2) while the other, in which much less phytodetritus was present, yielded only 10 specimens (2.9 per 1 cm2). This observation suggests that some micro‐ and nanoforaminifera may flourish in the presence of decaying organic matter, perhaps consuming the associated bacteria. The presence of phytodetritus may also explain why two of our samples from the Madeira Abyssal Plain (MAP) contained an order of magnitude more stained tiny foraminifera than two other MAP samples in which phytodetritus was absent.

High-resolution chemostratigraphy of Quaternary distal turbidites: a case study of new methods for the analysis and correlation of barren sequences

Abstract The late Quaternary distal turbidites of the NE Atlantic Madeira Abyssal Plain (MAP), represent one of the most intensely studied sequences in the modern deep ocean. More than 160 sediment cores have been recovered from the 68 × 10 3 km 2 of the plain, and existing litho- and biostratigraphies, when integrated with oxygen stable-isotope data for other areas of the NE Atlantic, provide a tight spatial and temporal framework. We have used the MAP as a test-bed to assess the applications of inorganic geochemistry as a sedimentological and stratigraphic tool. Data for 22 major and trace elements were obtained in over 500 samples taken from representative cores. Chemostratigraphic sequences are highly uniform across the plain, and several turbidites have unique geochemical signatures, providing a means of establishing bed-by-bed correlations over distances of &gt; 500 km. Cluster and principal component analysis confirm the statistical validity of empirically derived correlations and groupings. Vertical and lateral geochemical trends within beds document the sedimentological evolution of flows, and may be used to establish palaeotransport pathways. Four compositional groups are interpreted as representing derivations from different source areas: (1) the margins of the NW Africa off Morocco; (2) off Western Sahara; (3) the Canary Islands; (4) the Cruiser-Great Meteor East seamount chains. The study amply demonstrates the viability of chemostratigraphy for the correlation and analysis of sedimentary sequences, providing a stratigraphic resolution and reliability which matches or exceeds that obtainable by other techniques.

Elemental and major biochemical changes across an oxidation front in a relict turbidite: An oxygen effect

The elemental and major biochemical compositions of the relict f-turbidite sampled in two cores from the Madeira Abyssal Plain were determined. This fine-grained, distal sequence occurs at ca. 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change. Oxygen diffused downward through sediments above this interface and in ca. 10 kyr destroyed 80% of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca. 140 kyr. These deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates. In both cores, a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers. The unoxidized sequences of both turbidities contained 0.93–1.02 wt% organic carbon (OC) and 0.10–0.11 wt% total nitrogen (TN). Approximately 20% of the initial OC and 40% of the initial TN remained in the oxidized horizons, with a consequent decrease in atomic C/N ratio from ca. 11 to ca. 5. All samples gave very low yields of lignin phenols and comparable OC-normalized yields of total aldoses and amino acids, and indicated predominantly marine organic matter (OM) and nonselective oxic degradation of these biochemical classes. Compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments, except that the oxidized horizons yielded markedly elevated (3–5X) percentages of nonprotein amino acids. This study clearly demonstrates that prolonged exposure to OZ can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone. In comparison to early diagenesis, however, alteration of the measured biochemicals was largely nonselective. Such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep-ocean environments.

Elemental and major biochemical changes across an oxidation front in a relict turbidite: An oxygen effect

The elemental and major biochemical compositions of the relict f-turbidite sampled in two cores from the Madeira Abyssal Plain were determined. This fine-grained, distal sequence occurs at ca. 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change. Oxygen diffused downward through sediments above this interface and in ca. 10 kyr destroyed 80% of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca. 140 kyr. These deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates.In both cores, a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers. The unoxidized sequences of both turbidities contained 0.93–1.02 wt% organic carbon (OC) and 0.10–0.11 wt% total nitrogen (TN). Approximately 20% of the initial OC and 40% of the initial TN remained in the oxidized horizons, with a consequent decrease in atomic C/N ratio from ca. 11 to ca. 5. All samples gave very low yields of lignin phenols and comparable OC-normalized yields of total aldoses and amino acids, and indicated predominantly marine organic matter (OM) and nonselective oxic degradation of these biochemical classes. Compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments, except that the oxidized horizons yielded markedly elevated (3–5X) percentages of nonprotein amino acids. This study clearly demonstrates that prolonged exposure to OZ can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone. In comparison to early diagenesis, however, alteration of the measured biochemicals was largely nonselective. Such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep-ocean environments.

Abyssal Megafaunal Necrophages: Latitudinal Differences in the Eastern North Atlantic Ocean

AbstractA pop‐up time‐lapse camera system (BATHYSNACK) was used to study megafaunal organisms attracted to bait at three abyssal localities in the eastern North Atlantic Ocean, a northern site on the Porcupine Abyssal Plain and two sub‐tropical sites on the Madeira Abyssal Plain. Fish and decapod crustaceans appeared on 47% of photographs taken during deployments of up to seven days duration. The fishes Coryphaenoides (Nematonurus) armatus and Pachycara bulbiceps and the decapod Munidopsis spp. dominated appearances at the northern locality whereas over 95% of records at the two subtropical localities were of the decapod Plesiopenaeus armatus.Cyclical variations in necrophage abundance did not correlate with tidal cycles of near‐bottom currents, although high P. armatus numbers were inversely related to current speed. Most of the frequently recorded entities showed contagious intraspecific distributions. In contrast, many of the interspecies relationships were non‐random, with co‐occurrences lower than expected, probably as a result of physical disturbance or potential predation. Population densities based on times of first arrival were calculated.Assemblages dominated by fish at higher latitudes and crustaceans at lower latitudes are discussed in terms of organic carbon supply to the deep sea and pelagic community structures.

Comparison of the Vertical Distribution of Nematodes from Two Contrasting Abyssal Sites in the Northeast Atlantic Subject to Different Seasonal Inputs of Phytodetritus

AbstractThe vertical distribution of nematodes in the sediments of three cores from the phytodetritus‐influenced Porcupine Abyssal Plain station is compared with three cores from the Madeira Abyssal Plain station in the DEEPSEAS programme. Nematode vertical distributions are compared with sediment organic chemistry data sampled at the same time from the same sites. The results support the two hypotheses erected by Thiel (1983) that the penetration of meiofauna into deep‐sea sediments might be correlated with the input of organic matter and that a greater biological activity of larger organisms would increase the penetration of nematodes into the sediment through increase downward transport of food material.

Determination of turbidity current erosional characteristics from reworked coccolith assemblages, Canary Basin, north‐east Atlantic

ABSTRACTTurbidites contain mixtures of sediments of various ages. These sediments can include both material that was eroded to form the initial turbidity current plus additions derived from erosion of the sea floor during transport. It has been possible to interpret the age range of sediments incorporated into individual turbidites from the Madeira Abyssal Plain by examining the proportions of microfossil (coccolith) species that they contain. The pelagic record of coccoliths is well known for the Quaternary period and shows a succession of dominant species or acmes each lasting a few tens of thousand years. Hence, erosion of sediment representing more than a few tens of thousand years will produce coccolith mixtures not seen in the pelagic record, but dependent upon the age range of the sediments that were eroded. This age range can be estimated by comparison with synthetic ratios of coccolith species created by combining ratios of species from successively older layers in the pelagic record. These can then be compared with the ratios found in individual turbidites until a match is found. The results show age ranges of 54–500 kyr for the sediment mixture in seven turbidites from the Madeira Abyssal Plain. Since the volumes of these turbidites are also known, and accumulation rates in their source areas can be estimated, it is possible to determine both the thickness and the area of the eroded sediment mass that generated the turbidity current. Minimum depths of erosion on the north‐west African continental margin vary from 8 to 50 m and minimum areas eroded from 1600 to 5800 km2. None of the turbidites examined contains a significant excess of surface sediment, suggesting that, once formed, the turbidity currents that transported them were virtually non‐erosional, and that they travelled several hundred kilometres in this state.

Variations in the invertebrate abyssal megafauna in the North Atlantic Ocean

Epibenthic sledge and otter trawl samples of invertebrate megabenthos were collected from the abyss of the northeastern Atlantic Ocean. Stations were selected to have contrasting overlying water column structure. Strong seasonal pulses of phytodetritus on the sea floor have been recorded on the Porcupine Abyssal Plain (PAP) but not on the Madeira Abyssal Plain (MAP). Sledge-and trawl-derived abundances of 13.73 and 8.13 individuals 10 3 m −2 were found at PAP, with corresponding wet biomass values of 169.4 and 189.2 g 10 3 m −2. Data from sledge hauls taken at MAP gave an abundance of 7.48 individuals and 5.15g 10 3 m −2. Sledge and trawl abundances at Great Meteor East (GME) were 2.91 and 2.19 individuals 10 3 m −2 and 4.31 and 11.30 g 10 3 m −2. Holothurians dominated the invertebrate biomass at PAP while Astoroidea and Decapoda Natantia were important taxa at MAP and GME. Otter trawl samples demonstrated fish to be the major component of total megadaunal biomass and suggest that small fixed-frame trawls give gross underestimates of fish abundance and biomass. Size spectra based on abundance and biomass data indicated the megafauna to be a functional group at PAP stationl only. Here peak invertebrate biomass occurred in the 40–80 g wet wt size class. In contrast, no large invertebrates (>20 g wet wt) were found at MAP of GME. Concurrent photographic records indicate higher faunal densities than do net catches. Faunal differences between stations were examined, and a weak relationship between surface productivity and megafaunal abundance was established. Data are inadequate to detect a similar relationship with biomass. Biomass values at PAP of 16–39 times those of MAP and GME appear to be explained by the deposition of phytodetritus at PAP but not at MAP or, probably, at GME. Among invertebrates, detritivores form the dominant feeding guild at all localities, but carnivores are important at MAP and GME. Phytodetritus appears to support the abundant surface-grazing holothurians at PAP.