Abyssal Plain Site Research Articles

Biogeography and Connectivity Across Habitat Types and Geographical Scales in Pacific Abyssal Scavenging Amphipods

Recently, there has been a resurgent interest in the exploration of deep-sea mineral deposits, particularly polymetallic nodules in the Clarion-Clipperton Zone (CCZ), central Pacific. Accurate environmental impact assessment is critical to the effective management of a new industry and depends on a sound understanding of species taxonomy, biogeography, and connectivity across a range of scales. Connectivity is a particularly important parameter in determining ecosystem resilience, as it helps to define the ability of a system to recover post-impact. Scavenging amphipods in the superfamilies Alicelloidea Lowry and De Broyer, 2008 and Lysianassoidea Dana, 1849 contribute to a unique and abundant scavenging community in abyssal ecosystems. They are relatively easy to sample and in recent years have become the target of several molecular and taxonomic studies, but are poorly studied in the CCZ. Here, a molecular approach is used to identify and delimit species, and to investigate evolutionary relationships of scavenging amphipods from both abyssal plain and deep (>3000 m) seamount habitats in three APEIs (Areas of Particular Environmental Interest, i.e., designated conservation areas) in the western CCZ. A total of 17 different morphospecies of scavenging amphipods were identified, which include at least 30 genetic species delimited by a fragment of the cytochrome c oxidase subunit I (COI) barcode gene. The scavenging communities sampled in the western CCZ included the most common species (Abyssorchomene gerulicorbis (Shulenberger and Barnard, 1976), A. chevreuxi (Stebbing, 1906), Paralicella caperesca Shulenberger and Barnard, 1976, and P. tenuipes Chevreux, 1908) reported for other ocean basins. Only four morphospecies, representing five genetic species, were shared between APEIs 1, 4, and 7. The two abyssal plain sites at APEIs 4 and 7 were dominated by two and three of the most common scavenging species, respectively, while the APEI 1 seamount site was dominated by two species potentially new to science that appeared to be endemic to the site. The presence of common species in all sites and high genetic diversity, yet little geographic structuring, indicate connectivity over evolutionary time scales between the areas, which span about 1500 km. Similar to recent studies, the differences in amphipod assemblages found between the seamount and abyssal sites suggest that ecological conditions on seamounts generate distinct community compositions.

Net community oxygen production derived from Seaglider deployments at the Porcupine Abyssal Plain site (PAP; northeast Atlantic) in 2012–13

As part of the OSMOSIS project, a fleet of gliders surveyed the Porcupine Abyssal Plain site (Northeast Atlantic) from September 2012 to September 2013. Salinity, temperature, dissolved oxygen concentration and chlorophyll fluorescence were measured in the top 1000 m of the water column. Net community production (N) over an annual cycle using an oxygen-budget approach was compared to variations of several parameters (wind speed, mixing layer depth relative to euphotic depth, temperature, density, net heat flux) showing that the main theories (Critical Depth Hypothesis, Critical Turbulence Hypothesis, Heat-flux Hypothesis) can explain the switch between net heterotrophy to net autotrophy in different times of the year, The dynamics leading to an increase in productivity were related to shifts in regimes, such as the possible differences in nutrient concentration. The oxygen concentration profiles used for this study constitute a unique dataset spanning the entire productive season resulting in a data series longer than in previous studies. Net autotrophy was found at the site with a net production of (6.4 ± 1.9) mol m−2 in oxygen equivalents (or (4.3 ± 1.3) mol m−2 in carbon equivalents). The period exhibiting a deep chlorophyll maximum between 10 m and 40 m of depth contributed (1.5 ± 0.5) mol m−2 in oxygen equivalent to the total N. These results are greater than most previously published estimates.

Basal monothalamous and pseudochambered benthic foraminifera associated with planktonic foraminiferal shells and mineral grains from the Porcupine Abyssal Plain, NE Atlantic

We present a survey of ‘live’ (stained) and dead monothalamous (single-chambered, mainly spherical) and pseudochambered (chain-like) foraminifera associated with planktonic foraminiferal shells and mineral grains, based on two samples from one abyssal plain site (F2, 4,880 m water depth) and one abyssal hill site (H4, 4,330 m water depth) on the Porcupine Abyssal Plain (PAP), northeast Atlantic. Our study is the first to focus on this poorly known component of abyssal foraminiferal faunas and highlight their abundances and diversity at the PAP. In both samples these monothalamids and pseudochambered forms represented 27–35 % and 18–23 %, respectively, of the entire ‘live’ and dead foraminiferal assemblage (>150 μm, 0–1 cm sediment layer). Among 1,078 stained and dead specimens we recognise a total of 18 distinct morphotypes on the basis of test characteristics. Another 144 specimens could not be assigned to any morphotype and are regarded as indeterminate. Most of the monothalamids are small (<150 μm), although some incorporate planktonic foraminiferal shells to create larger structures. In absolute terms, stained and dead individuals of these morphotypes were more abundant at the abyssal hill site, although data from additional samples are needed to confirm if this is representative of differences between abyssal hills and the surrounding abyssal plain at the PAP. Agglutinated spheres and domes similar to some of our abyssal forms have been reported from shelf and slope settings, but they are generally much larger. Small agglutinated spheres are very common in the abyssal Pacific, at depths close to or below the carbonate compensation depth (CCD). However, they are composed largely of siliceous particles, including mineral grains, radiolarians and diatom fragments. In contrast, carbonate oozes at the PAP, situated above the CCD, are rich in coccoliths and planktonic foraminiferal shells, which are used in the construction of agglutinated spheres and domes. Our results underline the important contribution made by largely underestimated foraminiferal taxa to abyssal communities.

Reconciliation of the carbon budget in the ocean’s twilight zone

Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean. The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage. It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude. Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink.

The relative contribution of fast and slow sinking particles to ocean carbon export

Particulate organic carbon (POC) generated by primary production and exported to depth, is an important pathway for carbon transfer to the abyss, where it is stored over climatically significant timescales. These processes constitute the biological carbon pump. A spectrum of particulate sinking velocities exists throughout the water column, however numerical models often simplify this spectrum into suspended, fast and slow sinking particles. Observational studies suggest the spectrum of sinking speeds in the ocean is strongly bimodal with &gt;85% POC flux contained within two pools with sinking speeds of &lt;10 m day−1 and &gt;350 m day−1. We deployed a Marine Snow Catcher (MSC) to estimate the magnitudes of the suspended, fast and slow sinking pools and their fluxes at the Porcupine Abyssal Plain site (48°N, 16.5°W) in summer 2009. The POC concentrations and fluxes determined were 0.2 μg C L−1 and 54 mg C m−2 day−1 for fast sinking particles, 5 μg C L−1 and 92 mg C m−2 day−1 for slow sinking particles and 97 μg C L−1 for suspended particles. Our flux estimates were comparable with radiochemical tracer methods and neutrally buoyant sediment traps. Our observations imply: (1) biomineralising protists, on occasion, act as nucleation points for aggregate formation and accelerate particle sinking; (2) fast sinking particles alone were sufficient to explain the abyssal POC flux; and (3) there is no evidence for ballasting of the slow sinking flux and the slow sinking particles were probably entirely remineralised in the twilight zone.

The sustained observatory over the Porcupine Abyssal Plain (PAP): Insights from time series observations and process studies

Oceanographic research in the past 20 years has highlighted the importance of understanding decadal-scale variation in the oceans in order to predict how the ocean will respond to climate change. Climate factors control many of the processes that regulate the input of nutrients to the euphotic zone, primary productivity, zooplankton responses, the downward flux of organic matter, the biomass and composition of seabed communities and, ultimately, the burial of carbon in deep-sea sediments. A critical part of our understanding of the interdependence of these many elements comes from long-term eulerian observations, such as the Hawaii Ocean Time series (HOT) (Karl et al., 2003), the Bermuda Atlantic Time Series (BATS) (Steinberg et al., 2001), the NE Pacific Station M (Smith and Druffel, 1998) and the NE Atlantic Porcupine Abyssal Plain site (PAP) ([Billett and Rice, 2001] and [Lampitt et al., 2001]). Each time series site is considered as representing a different oceanic setting and each has a different suite of observations and history. However, the unifying theme of them all is to understand the time varying properties and processes of the oceanic environment. Only at Station M and PAP have water column and seabed observations been integrated, a feature of some considerable advantage. This special issue presents the latest work at PAP.

Local and regional species diversity of benthic Isopoda (Crustacea) in the deep Gulf of Mexico

Abstract Recent studies of deep-sea faunas considered the influence of mid-domain models in the distribution of species diversity and richness with depth. In this paper, I show that separating local diversity from regional species richness in benthic isopods clarifies mid-domain effects in the distribution of isopods in the Gulf of Mexico. Deviations from the randomised implied species ranges can be informative to understanding general patterns within the Gulf of Mexico. The isopods from the GoMB study contained 135 species, with a total of 156 species including those from an earlier study. More than 60 species may be new to science. Most families of deep-sea isopods (suborder Asellota) were present, although some were extremely rare. The isopod family Desmosomatidae dominated the samples, and one species of Macrostylis (Macrostylidae) was found in many samples. Species richness for samples pooled within sites ranged from 1 to 52 species. Because species in pooled samples were highly correlated with individuals, species diversity was compared across sites using the expected species estimator ( n =15 individuals, ES 15 ). Six depth transects had idiosyncratic patterns of ES 15 , and transects with the greatest short-range variation in topography, such as basins and canyons, had the greatest short-range disparity. Basins on the deep slope did not have a consistent influence (i.e., relatively higher or lower than surrounding areas) on the comparative species diversity. ES 15 of all transects together showed a weak mid-domain effect, peaking around 1200–1500 m, with low values at the shallowest and deepest samples (Sigsbee Abyssal Plain); no longitudinal (east–west) pattern was found. The regional species pool was analyzed by summing the implied ranges of all species. The species ranges in aggregate did not have significant patterns across longitudes, and many species had broad depth ranges, suggesting that the isopod fauna of the Gulf of Mexico is well dispersed. The summed ranges, as expected, had strong mid-domain patterns, contrasting with the local species richness estimates. The longitudinal ranges closely matched a randomized pattern (species ranges placed randomly, 1000 iterations), with significant deviations in the east attributable to lower sampling effort. The depth pattern, however, deviated from the mid-domain model, with a bimodal peak displaced nearly 500 m shallower than the mode of the randomized distribution. The deviations from random expectation were significantly positive above 1600 m and negative below 2000 m, with the result that regional species richness peaked between 800 and 1200 m, and decreased rapidly at deeper depths. The highest species richness intervals corresponded to the number of individuals collected. Residuals from a regression of the deviations on individual numbers, however, still deviated from the randomized pattern. In this declining depth-diversity pattern, the Gulf of Mexico resembles other partially enclosed basins, such as the Norwegian Sea, known to have suffered geologically recent extinction events. This displaced diversity pattern and broad depth ranges implicate ongoing re-colonization of the deeper parts of the Gulf of Mexico. The Sigsbee Abyssal Plain sites could be depauperate for historical reasons (e.g., one or more extinction events) rather than ongoing ecological reasons (e.g., low food supply).

Impact of large-scale natural physical disturbance on the diversity of deep-sea North Atlantic nematodes

Nematode alpha diversity from 3 physically disturbed sites in the deep North Atlantic was compared with reference sites. Nematode diversity at the HEBBLE benthic storm site was statis- tically, and significantly, lower than at reference sites. Nematode diversity at the Madeira Abyssal Plain site, which was subject to a turbidite dated at 930 BP, also showed a significantly lower diver- sity than reference sites. However, limited data suggest that diversity was not low at a Venezuela Basin turbidite site. The difference in nematode diversity between the 2 turbidite sites is ascribed to a long term change in sediment conditions at the Madeira site. The Venezuela Basin turbidite site has a sedimentation rate greater than the Maderia site by 1 to 2 orders of magnitude, and this was reflected in the sediment profiles. Another possibility is that the Venezuela Basin turbidite is consid- erably older, by at least 1000 yr, than the Madeira turbidite, allowing more time for recolonisation. The data suggest that deep-sea nematode diversity may be affected by physical disturbance but that deep-sea nematodes, like their shallow counterparts, are more robust than macrofauna such as poly- chaetes to such impacts.

Early diagenesis of lipid biomarker compounds in North Atlantic sediments

Near‐surface sediments (0–45 mm depth interval) from the Iceland Basin (59°N, 21°W, 3070‐m water depth) and Biscay Abyssal Plain (48°N, 17°W, 4105‐m water depth) were sectioned at millimeter‐scale resolution to assess alterations in key lipid biomarkers during early diagenesis. Inventories (µg cm−2 in the topmost 45 mm) of biomarkers from presumed terrestrial sources (&gt;C20 linear n‐alkanes, n‐alkanoic acids, n‐alkanols) and from phytoplankton sources (phytosterols, long‐chain alkenones and alkenoates) were 1.5–4 times more abundant in the Iceland Basin core than in the Biscay Abyssal Plain core, indicating greater sedimentary inputs and/or preservation at the Iceland Basin site. Biomarker concentrations varied significantly over millimeter‐scale depth intervals. Pronounced subsurface concentration maxima were present at both sites. At the Biscay Abyssal Plain site, steep concentration gradients in the uppermost few mm of the sediment column indicate extensive diagenetic losses at or near the sediment/water interface. Strong covariance was seen in the concentrations of various compounds within each class (e.g., long‐chain n‐alkanes, n‐alkanols, n‐alkanoic acids and long‐chain alkenones and alkyl alkenoates), indicating that individual compounds within each class are associated with the same organic matrix and have similar degradation and mixing rates. Thus biomarker indices for assessment of land‐derived inputs (CPI, C29/C31, ACL) and sea surface temperature (IPT) remain largely unaltered by early diagenetic processes in oxic, abyssal sediments in spite of extensive bioturbation and degradative losses within the uppermost few centimeters of the sediment column.

Authigenic uranium in Atlantic sediments of the last glacial stage — a diagenetic phenomenon

Cores from three different Atlantic localities (equatorial, Cape Verde Rise and Porcupine Abyssal Plain) are shown to have anomalous high U contents (5–8 ppm total) in sediments laid down during the last glacial stage (12–24 ky BP). Radiocarbon data demonstrate that the sediments hosting the peak U levels were accumulated at rates similar to those immediately above and below. All the cores exhibit maximum Mn levels, characteristic colour changes, and maximum U levels in the same sequence with increasing depth in core. On the evidence of the similarities between the cores, and pore water U data from a Porcupine Abyssal Plain site, it is proposed that the authigenic U enrichments are syndiagenetic and possibly active. No correlation is observed between sediment authigenic U and C org contents. The source of enrichment is bottom water U which has diffused downwards into the sediments to be sorbed at a particular redox level, located 10–30 cm below the oxic/post-oxic boundary marked by the colour change. The magnitude of the enrichments is caused by the persistence of this boundary at a particular level as a result of the decrease in mean sediment accumulation rate between the last glacial stage (5.2 up to at least 19.1 cm ky −1) and the Holocene (2.2–4.1 cm ky −1). Similar accumulation rate contrasts are expected to be widespread in the Atlantic, and the implications for previous reported work, particularly from the Mid-Atlantic Ridge, are discussed.

Modeling ocean-bottom reflection loss measurements with the plane-wave reflection coefficient

Ocean-bottom reflection loss has been measured at an abyssal plain site in the northeast Pacific Ocean. The layer of unconsolidated sediment at this site is thin (less than 200 m), and the near-surface sediments are composed of strongly reflecting sandy turbidites. In consequence, the surficial sediments and the relatively shallow substrate must be accounted for in describing the bottom loss measurements. The bottom loss has been modeled in this paper using the plane-wave reflection coefficient calculated for a geoacoustic model of the bottom developed from seismic profiling experiments carried out at the site. In the model the sediment column is described by fluid layers with a relatively small value of attenuation and a large value of sound-speed gradient in the surficial layer, and the substrate is assumed to be a solid. The bottom loss measurements are presented in 1/3 octave bands at 40, 80, and 160 Hz. The results are well-modeled at small grazing angles (θ≤20°) where the interaction is confined to the surficial sediments. At larger grazing angles the importance of including a solid substrate is demonstrated.

Comparison of spatial coherence of bottom‐interacting sound for Abyssal Plain and Abyssal Hills sites

Acoustic reflectivity experiments were conducted over two different ocean bottom types in the northeast Pacific. Explosive sources were dropped at ranges up to 50 km from a vertical hydrophone array that was suspended near midwater depth. The experimental geometry allowed time isolation of the bottom‐interacting arrival. The bottom‐interacting arrival received by the array was processed to obtain spatial coherence for frequencies in the 20‐ to 250‐Hz band. The behavior of spatial coherence vs frequency, grazing angle, and sensor separation is distinctly different for the two sites. At the Abyssal Plain site, coherence is nearly 1.0 at grazing angles 10° to 25°, decreases with increasing grazing angle until about 60°, and then increases after 60°. At the Abyssal Hills site, coherence is low (about 0.3 for 280‐m sensor separation) and variable for 10° to 60°, and increases after 60°. The results are consistent with the combined effects of scattering from a rough interface and changing sensor geometry with grazing angle. [Work supported by NORDA and Naval Electronic Systems Command.]

In-place measurement of deep sea soil shear strength: A. F. Richards, V. J. McDonald, R. E. Olson and G. H. KellerASTM Spec. Techn. Publ. No. STP 501, 55–68 (1972)

A vane shear device was developed for in-place measurement of the undrained strength of clays in the marine environment. It was designed to operate at a water depth of 4·6 km (15,000 ft) and to penetrate nearly 3 m (10 ft) below the mudline in steps of about 0·3 m (1 ft). The device contained its own power supply. Data were wire telemetered to the ship for recording and analysis. Experiments in several locations in the Atlantic Ocean and the Gulf of Mexico demonstrated the capability of the apparatus. The shear strength measured in place generaly exceeded the strengths measured on soil samples collected from shallow water, where the sedimentation rate was relatively rapid. There was little difference between shear strength measured in place and on cores from the Gulf of Mexico abyssal plain site, where the sedimentation rate was slow.