1,721,042 research outputs found
Modern deep-sea benthic foraminifera: a brief review of their biodiversity and trophic diversity
No full textMost fossil deep-sea foraminifera are multichambered and have relatively robust, calcareous or agglutinated shells. Modern assemblages, on the other hand, include many fragile monothalamous (single-chambered) forms and komokiaceans (a superfamily of protist currently placed within the foraminifera) with soft test walls. These groups are poorly known and most of the hundreds of morphospecies recognized in deep-sea samples are undescribed. The relative abundance of robust and fragile taxa varies with water depth and food supply. Calcareous and other hard-shelled species tend to predominate in relatively eutrophic areas, particularly on continental margins, but decrease as a proportion of the ‘entire’ live fauna (i.e. including soft-shelled species) with increasing water depth, even above the CCD (carbonate compensation depth). Most of the species on which the foraminiferal proxies used in palaeoceanography are based live in these bathyal regions. At abyssal depths, and particularly below the CCD, faunas are largely agglutinated and dominated by monothalamous forms. These assemblages have a much lower fossilization potential than those found on continental margins. In addition to carbonate dissolution, these patterns probably reflect adaptations to increasingly oligotrophic conditions on the ocean floor with increasing depth and distance from land. Bathyal species include herbivores and opportunistic deposit feeders (omnivores) that consume labile organic material, in addition to deep-infaunal deposit feeders, and must contribute significantly to carbon cycling. Many abyssal monothalamous foraminifera, in constrast, accumulate stercomata (waste pellets composed of fine sediment particles) and probably ingest sediment, associated bacteria and more refractory organic matter. Some monothalamous species without stercomata may be bacteriovores. Although they probably process organic carbon at a slower rate than calcareous species, the shear abundance of monothalamous taxa at abyssal depths suggests that they are important in carbon cycling on a global scale. The loss of a substantial proportion of foraminiferal biomass and biodiversity from the fossil record should be considered when using foraminifera to reconstruct palaeoproductivity, for example, by using the Benthic Foraminiferal Accummulation Rate (BFAR). Different dietary preferences among calcareous species have implications for the stable carbon isotope signal preserved in their shel
Monothalamous soft-shelled foraminifera at an abyssal site in the North Pacific: a preliminary report
No full textSimple foraminifera flourish at the ocean's deepest point
No full textExtreme water depths make it very difficult to sample the bottom of deep-ocean trenches. As a result, almost nothing is known about small sediment-dwelling organisms (meiofauna) living in these environments, which are among the most remote on Earth. During a study of western Pacific trenches (97000 m water depth), we discovered abundant foraminifera (shelled protists) living in the Challenger Deep, the deepest place (10,896 m) in the world ocean. The fauna is dominated by morphologically simple species with organic walls. These distinctive taxa seem to be characteristic of the deepest ocean depths
‘Live’ benthic foraminifera at an abyssal site in the equatorial Pacific nodule province: Abundance, diversity and taxonomic composition
No full textReplicate sediment samples were obtained from 3 closely spaced stations in the Kaplan East (KE) area of the abyssal eastern Equatorial Pacific (15°N, 119°W; 4100 m water depth), just below the carbonate compensation depth. At each site, 2 (Stns 827, 838) or 3 (Stn 824) complete cores (57 mm i.d.) were subsampled using 2–3 cut-off syringes of 6.6 cm3 cross-sectional area. The 0–1 cm sediment layers (>32 ?m fraction) of these 20 subsamples together yielded 12,513 small, rose-Bengal stained benthic foraminifera dominated by agglutinated taxa, most of them morphologically simple monothalamous types or komokiaceans. Almost two-thirds (65%) of specimens were either obvious fragments, mainly of komokiaceans and tubular foraminifera, or single chambers or small groups of chambers believed to be fragments of very fragile komokiaceans. The remaining 4438 specimens (35%) were considered to be complete individuals. Most (78%) of these complete tests were indeterminate agglutinated spheres (termed ‘psammosphaerids’) that constituted 27.6% of all specimens (complete plus fragments). Complete individuals that could be assigned to either described or undescribed species accounted for 983 specimens (22% of complete tests=7.6% of all specimens); only 26 specimens (0.59% of complete individuals) were calcareous and these had invariably lost their tests through dissolution. Some groups exhibited considerable spatial heterogeneity. For example, 45% of the 3455 indeterminate psammosphaerids and 45% of the 3087 Komokiacean-like chambers occurred in single subcores. A total of 252 morphospecies was recognised; 168 were represented by complete individuals and 84 by fragments. There are clear differences between these Pacific assemblages and those from other oceans; in particular, psammosphaerids and isolated komokiacean chambers appear to be much more prevalent in the Pacific compared to the Atlantic Ocean. Some morphospecies present in Kaplan samples are known from the Atlantic but many are not. Such species may either (1) be ubiquitous but undersampled because they are rare or (2) have geographically patterned distributions. Without further sampling, there is no way to distinguish between these 2 possibilities. Fossilisable tests represent a very small component of the KE assemblage. Many of the delicate, monothalamous species that have little fossilisation potential, including the komokiaceans, accumulate stercomata (waste pellets) and may consume organic material and bacteria associated with sediment. Because of their enormous abundance at abyssal depths, these poorly known taxa probably play a substantial role in carbon cycling over vast areas of the Pacific seafloor. <br/
Traces of dissolved particles, including coccoliths, in the tests of agglutinated foraminifera from the Challenger Deep (10,897 m water depth, western equatorial Pacific)
No full textWe examined four multilocular agglutinated foraminiferan tests from the Challenger Deep, the deepest point in the world's oceans and well below the depth at which biogenic and most detrital minerals disappear from the sediment. The specimens represent undescribed species. Three are trochamminaceans in which imprints and other traces of dissolved agglutinated particles are visible in the orange or yellowish organic test lining. In Trochamminacean sp. A, a delicate meshwork of organic cement forms ridges between the grain impressions. The remnants of test particles include organic structures identifiable as moulds of coccoliths produced by the genus Helicosphaera. Their random alignment suggests that they were agglutinated individually rather than as fragments of a coccosphere. Trochamminacean sp. C incorporates discoidal structures with a central hole; these probably represent the proximal sides of isolated distal shields of another coccolith species, possibly Hayaster perplexus. Imprints of planktonic foraminiferan test fragments are also present in both these trochamminaceans. In Trochamminacean sp. B, the test surface is densely pitted with deep, often angular imprints ranging from roughly equidimensional to rod-shaped. The surfaces are either smooth, or have prominent longitudinal striations, probably made by cleavage traces. We presume these imprints represent mineral grains of various types that subsequently dissolved. X-ray microanalyses reveal strong peaks for Ca associated with grain impressions and coccolith remains in Trochamminacean sp. C. Minor peaks for this element are associated with coccolith remains and planktonic foraminiferan imprints in Trochamminacean sp. A. These Ca peaks possibly originate from traces of calcite remaining on the test surfaces. Agglutinated particles, presumably clay minerals, survive only in the fourth specimen (‘Textularia’ sp.). Here, the final 4–5 chambers comprise a pavement of small, irregularly shaped grains with flat surfaces and no obvious intervening cement. Our observations suggest that (1) small biogenic particles can reach the deepest parts of the ocean intact in rapidly sinking phytodetrital aggregates or faecal pellets and (2) some agglutinated foraminifera living at extreme hadal depths construct a test from biogenic or detrital particles, which subsequently dissolve, leaving imprints and other remnants in the organic matrix of the test.<br/
New organic-walled Foraminifera (Protista) from the ocean's deepest point, the Challenger Deep (western Pacific Ocean)
No full textWe describe four new species and a new genus of very small (< 500 µm) Foraminifera from the Challenger Deep, the deepest point in the world's oceans (10 896 m water depth). All have transparent, mainly organic test walls that incorporate some minute agglutinated mineral particles of various shapes and compositions. Nodellum aculeata sp. nov. has an elongate proloculus with a pointed proximal end followed by a long, tubular section. The genus Resigella is represented by two species: in R. laevis sp. nov., the test comprises 3–4 elongate, oval to cylindrical chambers while R. bilocularis sp. nov. has an oval proloculus followed by a second, larger globular chamber. The fourth species, Conicotheca nigrans gen. et sp. nov., is characterized by a tiny, elongate, conical test filled with dark stercomata. Except in C. nigrans, the test wall has a brownish tinge; energy-dispersive spectroscopy (EDS) suggests the presence of organically bound Fe in all species including C. nigrans. Scanning electron microscopy (SEM) combined with EDS reveals distinctive wall structures. In N. aculeata, the proloculus is strewn with tiny (< 0.7 µm), elongate grains. In this species and in R. laevis, the test surface (except for the proloculus) is covered with a carpet of minute (?0.1 µm) finger-like projections, rather similar to the organic cement of agglutinated Foraminifera. In R. bilocularis, the larger second chamber often has a partial veneer of fine mineral grains of varying composition, as well as organic areas consisting of meshed strands. SEM images of these three species reveal flat, plate-like features that we interpret as clay particles. In C. nigrans, the wall is relatively featureless except where the surface is raised into hummocky mounds and scale-like features, again probably clay particles. We suggest that these species represent a distinctive group of 'agglutinated' Foraminifera in which the test is predominately organic
A minute new species of Saccammina (monothalamous Foraminifera; Protista) from the abyssal Pacific
No full textAbstract. Saccammina minimus sp. nov., a spherical agglutinated organism presumed to be a foraminiferan, is described from the Kaplan Central site in the abyssal eastern Equatorial Pacific (5042 m water depth). The new species is minute in size (<100 μm diameter) with a test wall composed of a jumble of small mineral grains and a few relatively larger diatom fragments. Occasional specimens, often droplet-shaped rather than spherical, have a simple aperture. The new species is by far the most abundant organism in sieve fractions >32 μm of samples collected at the KC site. In six subcores (6.6 cm2 surface area, 0–1 cm layer) from two cores obtained during a single deployment of a multiple corer, it represented 59% of all stained foraminifera in the samples. However, it had an extremely patchy small-scale distribution on a scale of centimetres; for example, 3, 285 and 1090 specimens were extracted from three subcores. A separate study has reported similar patterns among other minute indeterminate monothalamous foraminifera at the Kaplan East site (4032–4089 m water depth) located to the east, c. 1200 km from our study site. The reason for these very patchy distributions is not clear.</jats:p
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
