Showing papers on "Foraminifera published in 1988"

Deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation

Abstract: The degree of similarity of the ∂13C records of the planktonic foraminiferal species N. pachyderma and of the benthic foraminiferal genus Cibicides in the high-latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle. Whereas continuous formation of deep water is recognized in the southern ocean, the Norwegian Sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation. However, deep water formed in the north Atlantic south of the Norwegian Sea during the last climatic cycle as early as isotope substage 5d, and this area was also the only active northern source during stages 4–2. A detailed reconstruction of the geographic distribution of ∂13C in benthic foraminifera in the Atlantic Ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean, whereas the Glacial North Atlantic Deep Water cannot be traced south of 40°N. At shallower depth an oxygenated 13C rich Intermediate Water mass extended from 45°N to 15°S. In the Pacific Ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700–2600 m and is best explained by stronger formation of Intermediate Water in high northern latitudes.

A response by benthic Foraminifera to the deposition of phytodetritus in the deep sea

Abstract: A recent major discovery has been the rapid sedimentation of phytodetritus to the deep-sea floor1–3 Although benthic mega-faunal invertebrates appear to seek out this relatively fresh food source1,4, and its seasonal arrival on the sea floor may synchronize reproduction in some echinoderms5, a convincing response by ben-thic organisms to phytodetritus has not been demonstrated3. Here I present evidence that certain small benthic Foraminifera (within the meiofaunal size-range) react dramatically to the presence of phytodetritus. Fresh aggregates of this material harbour abundant, low-diversity populations of these protists. The three commonest species are usually poorly represented in the more diverse assemblages inhabiting the underlying sediment. These findings suggest that some deep-sea benthic Foraminifera, like their shallow-water relatives6–8, are specialist feeders that bloom opportunistically when the appropriate food (phytodetritus and associated micro-organisms) becomes available, while others remain unaffected by the organic influx.

Morphotype patterns of Norwegian Sea deep-sea benthic foraminifera and ecological implications

Abstract: Deep-sea benthic foraminifera from Norwegian Sea surface sediments are classified into morphotypes on the basis of test shape and nature of test coiling and show distinct patterns with water depth. The morphotype data are used to determine microhabitat patterns of the foraminifera, which are suggested to be related to the organic-carbon content of the surficial deep-sea sediments.

Correction of accelerator mass spectrometry 14C ages measured in planktonic foraminifera: paleoceanographic implications

Abstract: Carbon 14 dates obtained by accelerator mass spectrometry (AMS) on foraminiferal samples from deep-sea sediment cores must be corrected for the difference in 14C composition between the atmosphere and the sea surface. In the modern ocean, the “apparent age” of carbonate shells formed in surface waters varies between 300 and 1200 years and depends mainly on latitude. The time variation of this parameter during climate oscillations of the last 40,000 years may have been significant: there should have been small changes for most of the ocean between 40°S and 40°N, but an increase of the apparent age by several hundred years should be expected at high latitudes in response to subpolar/subtropical front movements. The North Atlantic is likely to have experienced the most significant changes, due to large variations in the mode and rate of North Atlantic Deep Water production. These hypothetical changes may be measured by coupled AMS 14C dating of contemporaneous planktonic foraminifera and terrestrial organic matter (pollen, charcoal, wood, etc.) which occur in the same core or are stratigraphically linked by the same volcanic ash layer. The Δ(14C atmosphere, 14C sea surface) can be viewed as a new paleoceanographic tracer which may provide additional information about high latitude surface waters complementary to those obtained with 13C/12C and Cd/Ca ratios measured in planktonic foraminifera.

Benthic foraminifera from the Adriatic Sea : principles of phenotypic variation

Abstract: The distribution and morphology of the benthic foraminifera in the Adriatic Sea appear to be strongly dependent on two primary controlling environmental parameters, oxygen concentration and food availability. These factors are both governed by the runoff from the Po and other Italian rivers, and by the subsequent distribution of the fluvial discharge products by the system of surface currents. The area most strongly influenced by runoff products, which is characterized by ample food availability and low oxygen concentrations at the bottom, is a strip parallel to the Italian coast at a water depth of between 20 and 60 m. In the present study we describe in detail the changes in the benthic foraminiferal faunas along transects perpendicular to this zone. The faunas along these transects react essentially in two different ways to the changes in environmental conditions. On the one hand we see qualitative and quantitative changes in the composition of the faunas, and on the other hand a number of species show a high degree of ecophenotypic variation. Statistical analysis of the faunal patterns resulted in the recognition of eight faunal clusters. One of these clusters, occurring in the near-shore zone, is consisting of obviously reworked specimens. The other seven clusters are separated on the basis of differences in the tolerances for stressed conditions and in the preferences for particular feeding niches of the various taxa. In the centre of the zone influenced by run-off products the faunas are dominated by a cluster with Bulimina marginata forma denudata and Nonionella turgida. Obviously the taxa of this cluster have the greatest tolerance for the sometimes stressed conditions found in this area. In the taxa Ammonia parkinsoniana, Elphidium granosum and Elphidium poeyanum, comparable successions of three fundamental morphologies are demonstrated. Compact morphotypes are typical for the near-shore, relatively nutrient-poor and oxygen-rich environment, in which there is a minimum influence of runoff products. In the somewhat deeper, food-enriched environment with lower oxygen concentrations, inflated morphologies occur. Still deeper, in the central part of the strip influenced by run-off products, where oxygen-poor conditions occur, these types are found with supplementary characteristics. The study of the variation in Bulimina marginata suggests that in this taxon the morphology reflects the life position of the animal. On the basis of the distribution of different morphologies in this and in similar species, it is concluded that the degree of niche separation in benthic foraminifera is dependent on the thickness of the oxygen-containing layer. If this thickness is minimal, the inbenthic microhabitat is ruled out, and only taxa adapted to an epibenthic microhabitat are found. In the final chapter some possibilities to apply the results of our study are discussed. It is argued that the bathymetrical distribution of benthic foraminifera in the Adriatic Sea is largely influenced by variations in productivity. Although the Recent situation can be described in great detail, one should be careful about using these data for estimating paleo-depths in other basins and/or other time-slices. Far more promising is the potential use of the present results for the interpretation of benthic foraminiferal faunas from deltaic environments in terms of productivity. It is demonstrated that not only variations in the productivity itself can be distilled, but also the source of such variations, such as variations in run-off, changes in circulation patterns and sea level fluctuations. It is suggested that benthic foraminifera should be used more often in pollution studies, because they can provide an accurate description of the distribution patterns of pollution phenomena in the bottom environments.

Calcareous nannofossil biostratigraphy and assemblages of the Cenomanian-Turonian boundary interval: Implications for the origin and timing of oceanic anoxia

Abstract: A detailed nannofossil zonation has been derived for the Cenomanian-Turonian boundary interval consisting of three zones, two subzones, and nine additional biohorizons. This biostratigraphy, based on the investigation of numerous sections from Europe, North America, and Africa, allows accurate correlation of sections from shelf and deep-sea environments. The increased resolution helps solve several pertinent paleoceanographic and sedimentologic problems in this time period. Facies patterns are variable in this interval for numerous reasons. A widespread hiatus occurred in the latest Cenomanian and earliest Turonian in much of northwest Europe and may have been an indirect result of peak marine transgression. In parts of the deep Tethys, dissolution of carbonate took place in undersaturated bottom waters or during early diagenesis. Only a few boundary sections, mostly within the Western Interior, United States, are characterized by continuous carbonate deposition. In many sections a ∥13C excursion (which has been associated with the burial of large amounts of marine organic matter) coincides with a facies change, lower ratios of coccolith carbonate to micrite, and poorer nannofossil preservation. This excursion therefore may be strongly overprinted by diagenesis. Numerous nannofossil events indicate that the carbon excursion is slightly diachronous between sequences of the Western Interior, and that the shift occurred distinctly later in this region than in northwest Germany. The data obtained do not invalidate the concept of a Cenomanian-Turonian ocean-wide anoxic event but show how local tectonic, climatic, and oceanographic perturbations have masked the global scenario. In particular, they indicate that carbon shifts may be basinal and not always oceanic phenomena, a conclusion which can be rationalized with overall sluggish middle Cretaceous circulation. Nannofossil assemblages in almost all sequences observed contain exceptionally high abundances of two dissolution-resistant taxa in particular samples: Eprolithus floralis and Broinsonia sp.. However, the overall composition of assemblages does not change dramatically in the boundary interval, in contrast to the macrofossils and foraminifera, which suffered widespread extinctions.

Preliminary estimates for the radiocarbon age of deep water in the glacial ocean

Abstract: Radiocarbon ages for benthic and planktonic foraminifera from the late glacial sections of two Atlantic and two Pacific cores are reported. The differences for benthic-planktonic pairs suggest that the radiocarbon age for deep Atlantic water was somewhat larger than today's (i.e., 600±250, as opposed to 400 years) and that the radiocarbon age for deep Pacific water was also slightly larger than today's (2100±400, as opposed to 1600, years). Our results suggest that during glacial time, the deep Pacific was, as it is today, significantly depleted in radiocarbon relative to the deep Atlantic. As many questions remain unanswered regarding the reliability of this approach, these conclusions must be considered to be preliminary.

Glacio-eustatic sea-level control on Red Sea salinity

Abstract: Previous studies of the Red Sea demonstrated that glacial surface-and bottom-water salinities in the basin were significantly higher than at present. The very low abundance of planktonic foraminifera, the so-called 'aplanktonic zone', during the last glacial indicates that surface-water conditions approached or exceeded the tolerance limits of this plankton group1,2. Glacial sediments are also characterized by high concentrations of magnesian calcite3,4, dolomite3, inorganically precipitated aragonite4,5 and benthic foraminifera typical of hypersaline environments3,6. Additional evidence from oxygen isotope records of planktonic2,7 and benthic foraminifera8,9, as well as pteropods2, demonstrate that glacial–interglacial contrasts in the Red Sea have an amplitude much larger than typically observed in open ocean records. Here we use both oxygen isotope data and a 'fractional overmixing' model to estimate the impact of the most recent (18,000 yr BP) Pleistocene glacio-eustatic sea-level lowering on Red Sea salinity. We estimate that during the last glacial maximum, surface salinities in the central Red Sea were more than 10.0‰ higher than at present. Deep-water salinities were also higher during the last glaciation and remained unusually high through deglaciation. The combination of very high bottom salinities and the onset of pluvial conditions during deglaciation in the Red Sea region prevented ventilation of Red Sea bottom waters and resulted in the accumulation of organic-rich sediments.

Bathyal meiobenthos of the western Coral Sea: distribution and abundance in relation to microbial standing stocks and environmental factors

Abstract: The distribution and abundance of meiobenthos in relation to microbial densities and environmental factors were examined at 24 stations in a bathyal (298–1610 m) region of the western Coral Sea. Densities of metazoan meiofauna were low (x = 57; range: 19–170 individuals 10 cm−2) compared with other bathyal communtiesm but when densities of living Foraminifera (x = 560; range: 0–3410 individuals 10 cm−2) were included, total faunal densities were high (x = 610 10 cm−2) and ranged from 73 to 3465 individuals 10 cm−2. Soft-bodied (non-chitinous) taxa (e.g. turbellarians) were not detected beyond the continental slope. Densities of all metazoan taxa, excluding nematodes, decreased significantly with bathymetric depth. When the effect of ocean depth was held constant, only a few significant correlations of meiobenthos with microbes and sediment characteristics were found. Our data, coupled with earlier findings of low bacterial densities and organic conditions, suggest that low densities of metazoan meiobenthos in the western Coral Sea are due to low rates of detrital input. However, densities of Foraminifera and other protozoans increased with bathymetric depth (Alongi, 1987, Deep-Sea Research, 34, 1245–1254), indicating their ability to exploit oligotrophic conditions and to gain numerical dominance in benthic food webs of the deep sea.

Buliminid foraminifera from inner neritic sand and mud facies of the Papuan Lagoon, New Guinea

Abstract: Fifty eight species of agglutinated foraminifera are recorded from 125 samples collected from the Papuan Lagoon, on southeast coast of New Guinea. The samples come from five different physiographic settings: o back-reef flat of Motupore Island (medium-coarse sand; <1.5 m water depth); o eastern channel between Motupore Island and mainland (medium-coarse sand; 10 m water depth); o fore-reef slope of the fringing reef along the western side of Motupore Island (sand to mud downslope; 10-20 m); o lagoon basin to the north and west of Motupore Island (mud; 15-38 m); o and the northwest slope of Horseshoe Reef which forms part of the Papuan Lagoon (medium-coarse sand; 16-53 m). Agglutinated taxa comprise only a small percentage of the fauna, rarely making up more than 1% of the total assemblage. Only on the northwest slope of Horseshoe Reef does the relative abundance rise in excess of this, reaching values of up to 17%. This overall low level of agglutinated foraminifer abundance is typical of tropical foraminiferal faunas, which are commonly dominated by calcareous taxa. With one exception, all recovered taxa are calcareous-cemented. The greatest diversity of agglutinated taxa is concentrated in the lagoon basin to the north and west of Motupore Island, and on the northwest slope of Horseshoe reef. No-one species is dominant, although taxa belonging to the Textulariidae are well represented. The lowest diversities are recorded at samples sites on the back-reef flat of Motupore Island. The distribution of more common species is examined and links between species distribution and the bathymetry and mud content of the sediment at the sample sites is explored. Taxonomy of recovered taxa is discussed and, if necessary, classification is revised in line with the latest classification of the group. This study compliments previous studies which documented miliolid and buliminid foraminiferal taxa respectively from the same area.

Comparison between radiocarbon ages obtained on coexisting planktonic foraminifera

Abstract: As a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera, measurements have been made on coexisting species of planktonic foraminifera While ideally no differences should exist, we do find them In this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic-planktonic age differences

Norwegian sea deep water variations over the last climatic cycle: Paleo-oceanographical implications

Abstract: Oxygen and carbon isotope analyses of planktonic and benthic foraminifera from five Norwegian Sea sediment cores demonstrate that the Norwegian Sea was an active area of deep water formation not only during full interglacial conditions (isotopic stages 1 and 5e) but also during the early part of the glaciation (isotope stages 4 and 5a–5d).