1,721,080 research outputs found
The niche of benthic foraminifera, critical thresholds and proxies
Full text linkEcological studies of benthic foraminifera are carried out to explain patterns of distribution and the dynamics of communities. They are also used to provide data to establish proxy relationships with selected factors. According to niche theory, the patterns of distribution of benthic foraminifera are controlled by those environmental factors that have reached their critical thresholds. For each species, in variable environments, different factors may be limiting distributions both temporally and spatially. For a species or an assemblage to be useful as a proxy its abundance must show a strong correlation with the chosen factor. Since numerous factors influence each species, it is only in those environments where the majority of factors show little variation but one particular factor shows significant variation that the proxy relationship for that factor can be determined. On theoretical grounds, the reliability of using foraminiferal abundance as a proxy of a selected environmental factor should be restricted to the range close to the upper and lower thresholds. For oxygen, foraminifera are potential proxies for the lower limits but once oxygen levels rise to values of perhaps >1 or 2 ml l-1, there is no longer a relationship between oxygen levels and abundance. By contrast, the flux of organic matter over a large range shows a sufficiently close relationship with foraminiferal assemblages so that transfer functions can be derived for the deep sea. However, the relationship at species level is far less clear cut. Much more accurate estimates of primary productivity and modern organic flux rates are required to improve the determination of past flux rates
AMS 14C dating of Holocene estuarine deposits: consequences of high energy and reworked foraminifera
No full textBenthic foraminiferal species were used to AMS date Holocene sediments adjacent to trangressive overlap boundaries (TOB) in two high-energy estuaries in northern Spain. It had previously been recognized that the foraminifera could be divided into indigenous and exotic components. Whereas it was known that the exotic forms had been transported into the estuaries from the adjacent inner shelf, it was assumed that the indigenous forms were in situ. The AMS dates based on indigenous forms obtained from pairs of samples adjacent to the TOB showed an inverse relationship in three out of four boreholes (i.e., the age of the higher sample was older than that of the lower). The most probable explanation of this unexpected result is that there had been significant reworking of indigenous foraminifera especially associated with the transgressive episode. It is concluded that, in high energy estuaries, AMS dating on foraminiferal shells is influenced by transport and reworking even of the indigenous component. Therefore, if only foraminifera are available for dating, we recommend that closely spaced pairs of samples should be dated in order to assess the reliability of the results
Mortality, protoplasm decay rate, and reliability of staining techniques to recognise 'living' foraminifera: a review.
No full textNon-vital staining, especially with rose Bengal, has been widely used in ecological studies to differentiate between the tests of dead(unstained) foraminifera from those presumed to be living at the time of collection (stained). Doubts have been expressed about staining methods because of the possibility that dead individuals may retain undecayed protoplasm for weeks or months after death; when stained, such individuals would be recorded as living. To assess the importance of such false positives, it is necessary to examine rates of mortality, and the modes of generation of empty tests, i.e., whether due to reproduction, growth stages (leaving empty tests during growth) or death. It can be argued that reproduction, ontogeny, and death through predation lead to tests devoid of protoplasm. Whereas reproduction may affect only a small proportion of the population of each species (due to high pre-reproductive mortality), predation in oxygenated environments may be responsible for the major part of that pre-reproductive mortality. In oxygenated environments, disease or adverse environmental conditions are most likely to lead to dead individuals having tests containing protoplasm. In dysaerobic/anoxic environments, predation by macrofauna may be excluded, so foraminifera die through other causes and thus more tests with dead protoplasm may be potentially available for staining. Therefore, for most other environments, the problem of staining dead individuals is almost certainly overstated. Furthermore, from comparative studies, it seems that the most commonly used technique (staining with rose Bengal) is as reliable as others. Now that new vital staining techniques, especially the use of fluorescent probes, are being introduced, it is timely for further objective comparative studies of all techniques to be made in order to evaluate data already gathered and to develop the best strategies for future ecological studies according to whether they are field-based or experimental
Patterns in the cumulative increase in species from foraminiferal time-series
No full textTime-series studies are essential for defining short and long-term variability in environments and without such records it is difficult to make well-founded judgments about faunal or environmental change. Datasets of intertidal benthic foraminifera from the Exe and Hamble estuaries, England, intertidal zone of Bahrain, Arabian Gulf, and subtidal Lim channel, Adriatic, have been analysed to provide information on patterns of species richness. Living assemblages comprise one or two dominant species plus two or three subsidiary species and a variable number of ephemeral rare species, the latter being the main control of species richness. The cumulative increase in species from month to month follows a pattern which approximates a log series. None of the datasets follows a broken stick or log normal pattern. The final plateau in cumulative species increase is reached in periods ranging from 5–10 months (Lim channel) to 22–23 months (estuaries). The cumulative maximum number of species gives an estimate of the carrying capacity for species, i.e. the maximum number of species for the environment. The species present in each area represent the species pool related to that environment (e.g. estuarine intertidal zone) together with introductions from a species pool in a different environment nearby (e.g. inner shelf). The time-averaged accumulation of successive live assemblages that form the dead assemblage is not identical with the cumulative total. Some live taxa are unrepresented in the dead assemblage either through taphonomic loss or the introduction of new species that have not yet left a dead record. In addition, the Exe estuary dead assemblage includes a large number of dead tests transported in from the adjacent shelf. It does not conform to any of the three possible patterns. At present the few time-series studies of benthic foraminifera that are available are either intertidal or very shallow water. There remains a need for further studies in a wide range of environments in order to understand more fully the faunal response to dynamic change.<br/
Ecology and applications of benthic foraminifera
No full textIn this volume John Murray investigates the ecological processes that control the distribution, abundance, and species diversity of benthic foraminifera in environments ranging from marsh to the deepest ocean. To interpret the fossil record it is necessary to have an understanding of the ecology of modern foraminifera and the processes operating after death leading to burial and fossilisation. This book presents the ecological background required to explain how fossil forms are used in dating rocks and reconstructing past environmental features including changes of sea level. It demonstrates how living foraminifera can be used to monitor modern-day environmental change. Ecology and Applications of Benthic Foraminifera presents a comprehensive and global coverage of the subject using all the available literature. It is supported by a website hosting a large database of additional ecological information (www.cambridge.org/0521828392) and will form an important reference for academic researchers and graduate students in Earth and Environmental Sciences
Biodiversity of living benthic foraminifera: how many species are there?
No full textIn ecological studies involving the analysis of 2.4 million living (stained) individual tests, to date 2140 species of benthic foraminifera have been recorded. Of these 602 species are agglutinated, 341 porcelaneous and 1197 hyaline. The numbers of species in the major environments are: marginal marine 701 (in 1.5 million individuals), shelf 989 (in 0.6 million individuals) and deep sea 831 (in 0.3 million individuals). 381 species occur in more than one major environment. Overall 33% have abundance of > 10% while 67% are of minor abundance (< 10%). The majority of species are rare, most are endemic and very few are cosmopolitan (5% or less). To estimate the potential total number of living species the following factors need to be quantified: the proportion of species already named (here considered unlikely to be less than 50% of the potential total), the number of species currently known to be dead but for which living representatives may yet to be found (assumed to be 5% = 107 species), and the proportion of species that are synonyms (10–25% = 214 to 535 species). Assuming that 50% of species have already been named (2140 + 107 = 2247), the potential total ranges from 3959 to 4280 species for 10% synonymy to 3210 to 3531 species for 25% synonymy. <br/
An illustrated guide to the benthic foraminifera of the Hebridean shelf, west of Scotland, with notes on their mode of life.
No full textThe Hebridean shelf presents a contrast in substrate type between higher energy open shelf sands, which are influenced by storm waves and lower energy muddy sands in depositional sinks called 'deeps'. The latter reach outer shelf depths (>100 m) even when situated close to land (e.g., Muck Deep). The primary purpose of this paper is to illustrate the majority of the benthic foraminifera. For most species, information is provided on whether they are epifaunal or infaunal, based on their distribution in rose Bengal stained samples. Since the redox boundary is shallow in this area (less than 4-5 cm), infaunal taxa are most abundant in the top 1 cm of sediment and decrease in abundance down to 2 or 3 cm with no subsurface maxima as recorded elsewhere. Some dead tests are infilled with glauconite which preserves the form of the species even when the shell is lost. The organic-cemented agglutinated fauna was concentrated by treating the samples with dilute acid to dissolve the calcareous forms. The species diversity of the resultant acid-treated assemblage (ATA) has been compared with that of the original dead assemblage (ODA). The pattern for the Hebridean shelf matches that recorded from other northwest European shelf seas. This procedure has allowed the following agglutinated species to be recorded from the area for the first time: Cuneata arctica, Eggerella europea, Eggerelloides medius, Morulaeplecta bulbosa, Portatrochammina murrayi, and Recurvoides trochamminiformis. In addition, the following calcareous taxa are also newly recorded from the area: Cornuloculina balkwilli, Ammonia falsobeccarii, Nonionella iridea, Robertina subcylindrica and Rosalina anomala
Some trends in sampling modern living (stained) benthic foraminifera in fjord, shelf and deep sea: Atlantic Ocean and adjacent seas
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The roles of elevation and salinity as primary controls on living foraminiferal distributions: Cowpen Marsh, Tees Estuary, UK
No full textBenthic foraminiferal assemblages in subrecent deposits are commonly used to reconstruct past sea level. Interpretations are generally made by comparison with either modern dead or total (live plus dead) assemblages. In both cases there will have been post-mortem changes that have differentially affected preservation. It is therefore important to establish the primary ecological controls by analysis of the living assemblages. We have determined the spatial and temporal variability of intertidal benthic foraminifera in the surface (0–1 cm) sediments from a time series survey of 31 sampling stations at Cowpen Marsh, for a period of 12 months. We counted 112,067 live foraminifera assigned to 28 species. The fauna was dominated by two agglutinated species (Jadammina macrescens and Trochammina inflata) on the high and middle marshes, and three calcareous species (Elphidium williamsoni, Haynesina germanica and Quinqueloculina spp.) on the low marsh and tidal flat.The standing crop of the whole intertidal zone, including the high, middle, low marsh and tidal flat habitats, and the individual species varied both temporally and spatially. The standing crop of the intertidal zone as a whole was greatest in the summer months and showed a positive correlation with elevation. The standing crops of the high and middle marshes showed similar temporal variation with peaks in summer and autumn and a trough in winter. The low marsh showed numerous peaks and troughs of standing crop during the year, whereas the tidal flat showed a single peak in summer. The standing crops of Jadammina macrescens and Trochammina inflata on the high and middle marshes peaked from April to May and August to October with troughs in winter. These agglutinated species showed a strong correlation with elevation. Haynesina germanica peaked in May to August and November to January on the low marsh, whereas on the tidal flat there was a single peak in July. The standing crops of E. williamsoni on the low marsh and tidal flat were relatively high in June and May, and July, respectively. Quinqueloculina spp. peaked in May to July on the low marsh and July on the tidal flat. The species was also found in the middle marsh from July to May and high marsh from September to November. Haynesina germanica showed a strong negative correlation with elevation, whereas the other two dominant calcareous species demonstrated weak negative correlations with both elevation and salinity.Reconstructing former sea level depends primarily on the recognition of high and middle marsh assemblages and in this study these are shown to be strongly controlled by elevation rather than salinity. Caution may be needed in interpreting low marsh and tidal flat data as salinity plays a more important role here. <br/
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