Planktonic foraminifera of the northern Indian Ocean: Distribution and preservation in surface sediments

In the northern Indian Ocean, planktonic foraminiferal tests accumulate in a wide variety of surface-water environments and depositional settings. This variability enables us to isolate the effects that surface-water ecology and differential dissolution have on the distribution of planktonic foraminifera from 251 geographically widespread surface sediment samples.Foraminiferal abundance varies from 0 to > 10⁴ whole foraminifera in the greater than 150 μm fraction per gram dry sediment. Values < 10 characterize the three deep basins of the equatorial Indian Ocean and the western Bay of Bengal. Foraminiferal tests are most abundant on carbonate covered Ninety-East and Carlsberg Ridges. Absolute abundance patterns are mainly controlled by non-ecological processes. Variations in dissolution resistant species (RSP) with water depth reveal that the foraminiferal lysocline (FL) varies regionally. The FL is deepest (3,800 m) in the equatorial region, rises abruptly to 3,300 m in the Arabian Sea, and varies from 2,600 m to near 2,000 m moving northward in the Bay of Bengal. Deep samples with anomalously low RSP (< 30%) suggest redeposition.Systematic geographic and depth-related variation is observed for the 17 most abundant foraminiferal species. Dissolution resistant species (G. menardii, G. tumida, G. dutertrei, P. obliquiloculata) generally exhibit a rapid and continuous increase in relative abundance at and below the FL. Susceptible species (G. ruber, G. bulloides, G. glutinata, for example) exhibit a rapid and continuous decrease in relative abundance at and below the FL. Moderately susceptible species (G. conglobatus, G. aequilateralis, G. conglomerata, for example) rapidly increase in abundance at the FL and systematically decrease with depth below the FL.Principal components analysis (PCA) of faunal data from minimally dissolved (< 30% RSP) samples reveals important ecologically related species intercorrelations. The major biogeographic gradient is the negative covarying relationship between aG. bulloides-G. glutinata species pair and a grouping ofG. sacculifer, G. conglobatus, G. aequilateralis, andG. ruber. PCA of all samples demonstrates how differential dissolution alters this and other species relationships. Species groupings that incorporateG. ruber, G. menardii, andG. dutertrei are particularly affected by dissolution.Comparison of average faunal data from minimally dissolved samples in the northern Indian Ocean with similar samples from other tropical regions suggests varying environmental factors produce distinct faunas within the tropical ocean. For example,G. bulloides, G. falconensis, andG. hexagona are significantly more abundant in northern Indian Ocean surface sediments while such species asG. ruber, G. sacculifer, G. dutertrei, andP. obliquiloculata dominate in other tropical regions.     

New biostratigraphic data from Cretaceous planktic foraminifera in Sahlabad province, eastern Iran

The foraminiferal content of two stratigraphic sections, located in eastern Iran within the Sahlabad province, between the Lut and Afghan blocks and ranging in age from Turonian to Campanian is investigated. Previous studies were general and only indicated the presence of planktonic foraminifera in this province. This paper presents a detailed study of planktonic foraminifera of the Shirshotor unit and establishes for the first time a local biostratigraphy consisting of five biozones. Biozones from the upper Turonian to lower Campanian are recognized, but the upper lower Campanian to lower upper Campanian strata are missing, as demonstrated by the lack of the Globotruncana ventricosa biozone. Tectonic activity in this region during the late early Campanian and mid-Campanian resulted in the presence of an unconformity together with debrites (debris flow deposits) in the lower upper Campanian. About twenty-five planktonic foraminiferal species are reported and illustrated. The largest faunal diversity is encountered in the upper Santonian. The planktonic foraminiferal biozones are precisely defined in selected stratigraphic sections and allow age determinations for the deepest marine sediments (pelagic limestones and bedded cherts) before the collision of the Lut and Afghan blocks.     

Ranking of dissolution susceptibility of planktonic foraminifera at high latitudes of the South Atlantic Ocean

Surface-sediment samples from the Maurice Ewing Bank (eastern Falkland Plateau), South Atlantic Ocean, have been analyzed to rank dissolution susceptibility of cool water planktonic foraminiferal species. A dissolution index is formulated from quantitative analyses of sedimentological properties (CaCO₃ content, frequency of planktonic foraminiferal fragments, radiolarians, mineral grains, and ratio of a dissolution-tolerant to a dissolution-susceptible planktonic foraminiferal species). This index is used to assess degrees of dissolution in the samples. Quantified differences in relative abundance of species between the two dissolution regimes (less prominent and stronger dissolution) formed the basis for differentiation. The species were ranked in the following order, from most resistant to least resistant: sinistrally coiled variety ofNeogloboquadrina pachyderma (antarctic variety of this species),Globorotalia inflata, G. truncatulinoides, Globigerina bulloides andGlobigerinita glutinata (a tie), dextralN. pachyderma (subantarctic variety),Globigerinita uvula, andGlobigerina quinqueloba. Fragmentation (frequency of damaged tests) increases with increasing dissolution in tolerant species, but not in susceptible species. This may be because susceptible species are completely dissolved under intense dissolution, whereas tolerant species, although damaged, remain and increase in abundance.     

Differential dissolution of Upper Cretaceous planktonic foraminifera from a temperate region of the South Atlantic Ocean

Differential dissolution susceptibility is well known in Quaternary planktonic foraminifera, but few quantitative dissolution studies have been carried out on pre-Quaternary species. A dissolution ranking scheme is proposed for 19 of the most common uppermost Cretaceous planktonic foraminifera from a temperate region of the South Atlantic Ocean. The study is based on quantitative faunal variations in an uppermost Maastrichtian segment (about 190 kyr long) from DSDP Site 527 (Leg 74) from the Walvis Ridge. A calcite dissolution index was formulated from degree of fragmentation of planktonic foraminiferal tests. Through this segment, relative abundance of benthonic foraminifera, which are generally known to be more dissolution-resistant than planktonic foraminifera, covary with degree of fragmentation, supporting that degree of fragmentation is a measure of dissolution. A prominent change from a state of moderate dissolution to a state of stronger dissolution takes place near the top of Magnetic Anomaly 30 (66.74 Ma). Some species decrease, whereas others increase, in relative abundance between these dissolution regimes, and dissolution susceptibility was ranked from quantified differences between dissolution regimes. The most susceptible species are in order, from least resistant to most resistant: Heterohelix globulosa, Globigerinelloides multispina, Rugoglobigerina rugosa, Pseudoguembelina costulata, Globotruncanita stuartiformis, and Heterohelix carinata. The following eight species are unaffected by dissolution in the sense that they show no significant difference between regimes: Heterohelix glabrans, Pseudotextularia browni, P. elegans, Racemiguembelina fructicosa, Planoglobulina brazoensis, Gublerina cuvillieri, Globotruncana arca, and Rosita contusa. The most resistant species were ranked in the following order, from least resistant to most resistant: Globigerinelloides subcarinata, Pseudoguembelina palpebra, P. polypleura, Globotruncanella havanensis, and Abathomphalus mayaroensis.     

Molecular evidence for an independent origin of modern triserial planktonic foraminifera from benthic ancestors

Gallitellia vivans is the only Recent representative of the triserial planktonic foraminiferal family Guembelitriidae. The origin and evolution of this interesting albeit poorly known family are enigmatic. To elucidate the phylogenetic relationships between G. vivans and other planktonic foraminifera, we sequenced the small subunit ribosomal DNA (SSU rDNA) for comparison to our extensive database of planktonic and benthic species. Our analyses suggest that G. vivans represents a separate lineage of planktonic foraminifera, which branches close to the benthic rotaliids Stainforthia and Virgulinella. Both genera resemble Gallitellia in general morphological appearance, having elongate triserial tests at least in their early ontogenic stages. The divergence time of G. vivans is estimated at ca. 18 Ma (early Miocene), suggesting an origin independent from the Cretaceous and Paleogene triserial planktonic foraminifera. Our study thus indicates that modern triserial planktonic foraminifera are not related to the Cretaceous–Paleogene triserial species, and that the sporadic occurrences in the fossil record are not the result of poor preservation, but reflect multiple transitions from benthic to planktonic mode of life.     

Isotopic and distributional evidence of a planktonic habit for the foraminiferal genusStreptochilus Bro¨nnimann and Resig, 1971

Oxygen and carbon isotope analyses show that the biserial foraminiferal genusStreptochilus, which was originally described from pelagic sediments on the Eauripik Rise and Ontong Java Plateau, lived deep in the upper water column within the oxygen minimum layer. The species ofStreptochilus average from 4 to 19% of the foraminiferal assemblages in which benthic forms compose less than 1 or 2%. Specimens ofStreptochilus are selectively dissolved when in contact with the bottom water mass. Their rapid evolutionary turnover of less than a few million years and their wide areal distribution in the equatorial Indo-Pacific are indicative of planktonic foraminifera. Aside from usefulness of the species ofStreptochilus as stratigraphic indices, these Neogene biserial planktonic foraminifera are potential indices of paleoceanographic stratification.     

南海北部琼东南盆地BD-2井中新世有孔虫生物地层及沉积环境

南海北部琼东南盆地BD-2井中新世地层沉积连续,是琼东南盆地中新统较典型的钻井剖面之一。该井中新世地层中含丰富的有孔虫化石,共鉴定有孔虫62属98种,其中浮游有孔虫13属41种,底栖有孔虫49属57种。根据有孔虫标志种及螺旋浮游有孔虫旋向优势度的变化,对该井的有孔虫生物地层进行了详细划分,从早中新世至晚中新世共识别出11个有孔虫化石带或联合化石带。探讨了下中新统与中中新统,中中新统与上中新统界线的有孔虫划分标志。依据有孔虫丰度、分异度及组合特征,讨论了BD-2井从早中新世至晚中新世沉积环境演化特征。     

Paleoceanographic implications of smaller benthic and planktonic foraminifera from the Eocene Pazin Basin (Coastal Dinarides,Croatia)

Summary Smaller benthic and planktonic foraminifera from the clastic sediments of the Pazin Basin (Istria, Croatia) were studied in order to obtain more data about paleoceanographic conditions that existed in the Middle Eocene Dinaric foreland basin. The succession investigated corresponds to the Middle Eocene planktonic foraminiferal zones Globigerapsis kugleri/Morozovella aragonensis (P11), Morozovella lehneri (P12), and Globigerapsis beckmanni (P13). Benthic foraminiferal assemblages from the clastic succession are dominated by epifaunal trochospiral genera suggesting oligotrophic to mesotrophic conditions and moderately oxygenated bottom waters. Planktonic foraminiferal assemblages indicate mesotrophic to eutrophic conditions of the surface waters, with increased eutrophication in the upper part of the section. Water depth, based on the ratio between planktonic and epifaunal benthic foraminifera and on the recognized species of cosmopolitan benthic foraminifera, was estimated to have been between about 900 and 1200 m. The basin was elongated and open to marine currents on both sides allowing good circulation and ventilation of the bottom water.     

Planktonic foraminiferal biostratigraphy and paleoenvironments at the Santonian-Campanian boundary of Northern Tunisia

The Santonian-Campanian boundary in Northern Tunisia has been studied in Oued el Khannga section, based on biostratigraphic and paleoecological analysis. The detailed biostratigraphic analysis allows us to define the stratigraphic distribution of diverse planktonic foraminifera and to establish a continuous biozonation. Sixty-six planktonic foraminiferal species belonging to 13 genera have been identified and two Tethyan planktonic foraminiferal zones have been organized, indicating the upper Santonian - lower Campanian age. These zones are: Dicarinella asymetrica (Total Range Zone) and Globotruncanita elevata/Globotruncana arca (Concurrent range zone). A major turn over event was recorded in this interval including the extinction of several species characterized by complex morphotypes with a double keel such as the genera of Dicarinella, Marginotruncana. In fact, the extinction of the index species D. asymetrica is the most important event which defines the Santonian-Campanian boundary. Many representative species of the genera Globotruncanita and Globotruncana occurred, for the first time, in the uppermost of the Santonian and thrived close to the Santonian-Campanian boundary. In addition some surviving species with a simple morphotype, which are considered like opportunistic taxa, crossed the boundary. This various responses of planktonic foraminifera and the fluctuation in diversity and in the number of species may be related to an increase in temperature and a deep outer shelf upper bathyal environment deposit.     

Standing stock,vertical distribution and flux of planktonic foraminifera in the Panama Basin

The relationships between planktonic foraminifera in the upper 2000 m of the water column and those in sediment traps and deep-sea sediments of the Panama Basin were investigated as part of the Composition, Flux and Transfer Experiments (CFATE) and the Sediment Trap Intercomparison Experiment (STIE) July–Dec. 1979. Planktonic foraminifera larger than 333 μm, sampled during the July–August trap deployment cruise, occurred most abundantly in the euphotic zone with maximal populations associated with the chlorophyll and primary production maxima both located in the upper thermocline just below the mixed layer. Juvenile abundances exceeded those of the >333 μm adults by 3–4 orders of magnitude and their depth distribution systematics indicated that most foraminifera reproduction occurs prior to sinking of the adults from the euphotic zone. Macroscopic aggregates and fecal pellets were identified as major carriers of smaller sized shells from the euphotic zone.During the July–August cruise, the species changed from an assemblage dominated byG. theyeri to one dominated byG. ruber; the liveG. theyeri population sank passively from the euphotic zone in early August with a mean settling speed of 150 m day^?1 while theG. ruber population grew in. The abundance of empty shells of all species in the upper 200 m varied by a factor of 40 during the deployment cruise and peaked on July 30, 1979. This variability was reflected in foraminifera collection rates of sediment traps deployed at 305 m for 24- and 6-h periods. The trap and watercolumn data were combined to calculate mean sinking speeds (>333 μm fraction) of several dominant foraminiferal species (G. ruber andG. theyeri, 500 m day^?1;G. dutertrei, 2000 m day^?1). Size and weight analysis of empty shells from plankton tows yielded values of empty shell density (calculated on a spherical basis using maximum shell dimension) ranging from 0.12 and 0.15 g cm^?3 (G. theyeri andG. ruber) to 0.60 g cm^?3 (G. dutertrei). These speeds and shell densities were found to be consistent with the laboratory sinking experiments of Berger and Piper (1972) and Fok-Pun and Komar (1983). It was concluded that shell fluxes through the water column may be calculated with reasonable accuracy using a modified version of the Bishop et al. (1977) settling model and empty shell size distribution data from pump and plankton tow collections. The Fok-Pun and Komar (1983) settling model is recommended for future modelling efforts but requires a more extensive set of measurements of each foraminiferal specimen to be made.The interpretation of foraminiferal flux profiles, based on the collections of sediment traps, plankton tows, and pumps deployed on time scales shorter than several days, must take into account the very short residence times of empty shells in the water column and short term temporal variability in empty shell production rates.Populations of >333 μm foraminifera sampled in Nov.–Dec. 1979 were reduced by a factor of six relative to the July–Aug. values. The cruise-to-cruise differences were due to reduced primary production and greater mixed layer depth.Planktonic foraminifera were well preserved in sediment traps deployed for 112 days at 665, 935 and 1770 m, although aragonitic pteropod shells were partially dissolved. The fluxes of many species into the 935 m trap could not be explained based on the standing stock data from the July–August cruise alone and it was noted that species fluxes during STIE were comparable to or exceeded maximal values measured by Thunell and Reynolds (1984) in bi-monthly time series traps deployed at the same location during the whole of 1980. The moored trap data could only be explained by higher mean foraminifera shell fluxes in the interval between trap deployment and recovery, consistent with the Bishop and Marra (1984) model of primary production and carbon flux at the STIE site.Primary differences between core top samples and trap samples, were explained by the loss from the sediments of dissolution prone species,G. theyeri andG. ruber leaving an assemblage dominated by the dissolution resistant species,G. dutertrei.     

Comparison of seasonal flux variations of planktonic foraminifera in sediment traps on both sides of the Ryukyu Islands,Japan

Variations in the fluxes of planktonic foraminifera were analyzed based on sediment trap data collected over a period of 10 months, from October 1994 to August 1995, at stations JAST01 and JAST02, located on both sides of the Ryukyu Islands, Japan. Station JAST01 (latitude 27°23′N, longitude 126°44′E) was deployed at a depth of 1000 m in the Okinawa Trough, East China Sea, along the flow axis of the Kuroshio Current, west of the islands, whereas station JAST02 (latitude 25°4′N, longitude 127°34′E) was deployed at a depth of 3000 m in the Ryukyu Trench, along the western margin of the northwest Pacific, east of the Islands. The total planktonic foraminiferal fluxes (TFFs) in the eastern station were high in winter, when the surface-water column was vertically well mixed. In contrast, the TFF peaks did not show significant seasonality in the western station, but values fluctuated with the northwest–southeast oscillation of the Kuroshio axis. Among the 36 planktonic foraminiferal species identified in the trap samples, Globigerina bulloides, Globigerinita glutinata, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerinoides ruber, Globigerinoides sacculifer and Globigerina falconensis exhibited a greater shell flux. On both sides of the Ryukyu Island Arc, the fluxes of G. ruber and G. sacculifer increased synchronously with the seasonal warming of surface waters, whereas that of G. falconensis increased during winter, when the water column was vertically well mixed as a result of the NW monsoon. In contrast to these species, which exhibited similar seasonal flux variation patterns at both stations, the species G. bulloides, G. glutinata, N. dutertrei, and P. obliquiloculata displayed different flux variations on both sides of the islands. These fluxes seem to be regulated by phytoplankton productivity, which is controlled by the vertical structure of the water column on the eastern Ryukyu Trench side and by the oscillation of the Kuroshio axis on the western Okinawa Trough side. Lateral transport of suspended planktonic foraminiferal shells to the Okinawa Trough might exist, but is not prominent enough to wipe out the original features of the planktonic foraminiferal flux. The species G. falconensis is an indicator of winter mixing on both sides of the Ryukyu Islands, and may possibly be used as a proxy to trace the intensity of the paleo-winter monsoon.     

Carbonate dissolution and planktonic foraminiferal assemblages observed in three piston cores collected above the lysocline in the western equatorial Pacific

Planktonic foraminiferal assemblages were analyzed in three piston cores recovered from depths above the recent sedimentary lysocline (3.5 km) in the western Pacific Ocean for the purposes of (1) evaluating the effects of carbonate dissolution on fossil assemblages and (2) the impact of dissolution on paleoceanographic reconstructions for the past 300 kyr. We used the perfect test ratio (PTR) of Globorotalia menardii as an indicator of carbonate dissolution. Down-core variations of the PTR indicate that significant dissolution occurred during reglaciation steps represented by isotope stages 7–6, 5–4, and 3–2. The results of principal component analysis (PCA) based on the relative abundances of dominant species of planktonic foraminifera indicate that down-core changes in factor 1 loadings correlate with variations in PTR and with variations in the percentage of coarse fraction (>63 μm) present in the sediments. These correlations indicate that foraminiferal assemblages were apparently altered by dissolution events despite the fact that they were deposited at water depths shallower than the modern lysocline. We also estimated variations in paleo-sea surface temperature (paleo-SST) using the transfer function FP-12E. A negative correlation was found in communality as calculated from PCA and factor 1 loadings; this trend is distinct for the case of communality lower than 0.9. Thus, we conclude that estimated paleo-SST values based upon analysis of planktonic foraminifera are biased by carbonate dissolution.     

Changes in quaternary oceanographic bottom conditions as documented by foraminifera in sediment cores from the slope of the exmouth plateau (off Western Australia)

Benthonic foraminifera from 15 surface- and 70 core-sediment samples (three cores) from the Western Australian continental margin (Exmouth Plateau) were quantitatively investigated. As far as possible, the occurrences of the species were correlated with the recent oceanographic conditions especially as related to water depths. The sampled sites are separated from the Australian Shelf by the Montebello Trough so that the taphocoenoses cannot become adulterated by foraminifera from the shelf. Only 1% or less of the foraminifera are benthonic species. The benthonic foraminiferal faunas of the surface sediment samples from 3800 to 1700 m are composed of about the same deep-water species as described from southeastern Indian Ocean basins by Corliss (1979a). They differ from one another only because different species dominate the faunas dependent on different water depth and/or water mass. Above 1700 m the character of the benthonic foraminiferal fauna is slowly changing from abyssal to bathyal. Downcore the benthonic foraminifera show marked changes in abundance patterns, which partly parallel the changes which can be observed with the “cool” and “warm” indicating planktonic foraminifera, partly show regular phase displacement with these changes. The different abundances of the same species in a core from, for instance, 3000 m or from 2000 m water depth, respectively, prevent a direct correlation between the cores. It also proved impossible to correlate the changes in the frequency patterns of certain species to certain paleo-oceanographic conditions. Examples for this are given.     

Experimental dissolution of a fossil foraminiferal assemblage (Paleocene–Eocene Thermal Maximum,Dababiya, Egypt): Implications for paleoenvironmental reconstructions

Dissolution experiments were carried out on a foraminiferal assemblage from the Paleocene–Eocene Thermal Maximum (PETM) at Dababiya, Egypt, in order to: 1) reveal the effects of differential dissolution on the composition of the foraminiferal assemblage and 2) develop objective criteria for the evaluation of dissolution in foraminiferal assemblages used in early Paleogene paleoenvironmental reconstructions, particularly with respect to neritic Midway-type assemblages from the Paleocene/Eocene transition. Our results confirm two general observations on modern foraminifera: 1) planktic foraminifera are much more vulnerable to dissolution than benthic foraminifera, leading to depressed P/B ratios and 2) dissolution susceptibility differs between size fractions, with the smaller specimens dissolving more rapidly than the bigger ones, leading to a larger average size of the remaining assemblage. Within a size fraction, wall structure and thickness are considered to be the main factors controlling differential dissolution susceptibility. We propose a ranking scheme for taxa with respect to dissolution resistance. Among the benthic taxa, Lenticulina is most resistant, followed by the agglutinated Gaudryina cf. ellisorae and Alabamina midwayensis. Biserial and triserial hyaline taxa and the porcelaneous Spiroloculina sp. are most susceptible to dissolution, whereas rotaliines, such as Cibicidoides and Anomalinoides have an intermediate susceptibility. This implies that mild dissolution of a Midway-type benthic assemblage leads to a relative enrichment in Lenticulina, Gaudryina and rotaliines. Amongst planktic foraminifera, the muricate taxa Acarinina and Morozovella are most resistant, followed by the cancellate Subbotina. The smooth and generally small Globanomalina and Zeauvigerina are least resistant to dissolution. Our data enable to objectively evaluate various degrees of dissolution in benthic and planktic foraminiferal assemblages retrieved from the lower Paleogene Tethyan outcrops. In this way taphonomic artifacts can be readily distinguished from paleoenvironmental signals affecting the primary composition of the assemblages. More generally, we propose that the combined use of foraminiferal numbers, P/B ratio and relative abundances of non-calcareous agglutinated taxa and Lenticulina may provide a powerful proxy for assessing dissolution in hemipelagic assemblages from Cenozoic and upper Cretaceous continental margins. In order to achieve more robust pre-Quaternary paleoenvironmental reconstructions based on quantitative foraminiferal data, application of dissolution proxies, like proposed here, or in slightly modified form, should become a more widely used micropaleontologic procedure. Particularly continental margin studies dealing with major biotic events (e.g. PETM) or employing P/B ratios for sea-level reconstructions should benefit from such an approach.     

Enregistrement des fluctuations du niveau marin dans l’Holocène supérieur du lac Retba (Sénégal) par les foraminifères

A study of sea level fluctuation records based on foraminiferal assemblages was carried out for the first time in a sediment core taken at the Lac Retba edge. A total of 37 foraminifera species were collected along the core, among which most are known in the modern estuaries and lagoons of Senegal. A succession of four assemblages dominated by Ammonia parkinsoniana and A. tepida were identified; they correspond to different stages of evolution of the lake. The first association, at the base of the core, indicates a lagoon slightly opened to the sea and bordered with vegetation, under relatively humid climate. The second has the richest and most diversified microfauna with a high proportion of coastal, marine benthic and planktonic species (about 10%) that indicate a small coastal gulf bordered with mangroves. The third association is oligospecific and typical of a closed and hypersaline lagoon under a dry climate. The last association again contains coastal, marine benthic and scarce planktonic species (3%) indicating a change to a saltier and more open lagoon under dry climate. The disappearance of planktonic foraminifera at the top of the core indicates the closure of the lagoon. The biocenotic indicators are evidence for two marine intrusions that are referred to the Dakarian (3000 years B.P.) and Saint-Louisian (2140-680 years B.P.) stages of the Upper Holocene of the senegalo-mauritanian stratigraphic scale. Evidence of the Lac Retba closure since 680 years B. P. appears in the core by high reduction of foraminifers’ abundance and diversity absence of planktonic species, and salts precipitated at the bottom of the lake.     

Differential dissolution of upper Cretaceous deep-sea benthonic foraminifers from the Angola Basin,South Atlantic Ocean

Material from the Deep Sea Drilling Project (DSDP) Site 527 from the Angola Basin, South Atlantic Ocean, has been analyzed to determine whether Upper Cretaceous benthonic foraminiferal taxa are differentially sensitive to calcite dissolution, and, if so, to rank their order of susceptibility. Two regimes of dissolution, established on the basis of the degree of planktonic foraminiferal fragmentation, representing stronger and less prominent dissolution within the section studied, were used as a framework for reference. A total of 60 calcareous and eight agglutinated benthonic foraminiferal taxa were identified at the species or genus level; of these, twenty-three calcareous and five agglutinated taxa were selected for the dissolution study. Relative abundance of each of the various taxa was tested statistically, using t-test, between dissolution regimes to assess the significance of the change induced by increased dissolution.Nuttallides truempyi andNeoeponides sp. intermediate form are interpreted as resistant to dissolution.Pullenia spp.,Alabamina sp. a,Anomalina sp. a,Praebulimina sp. fusiform, andNuttallinella sp. a are susceptible to dissolution. The majority of the taxa (16 in number) are unaffected by dissolution because they show no change between dissolution regimes. Among the agglutinated taxaGaudryina pyramidata is resistant to dissolution, whereas the remaining four taxa are unaffected by dissolution. At the level of superfamilies, one calcareous superfamily (Discorbacea) is resistant, two (Orbitoidacea and Cassidulinacea) are unaffected, and two (Nodosariacea and Buliminacea) are susceptible to dissolution.     

Patterns of planktonic foraminiferal abundance and diversity in surface sediments of the western North Atlantic

Seventy-two core tops and grab samples from the western North Atlantic were analyzed to determine what aspects of planktonic foraminiferal abundance and diversity are most closely related to ocean circulation. Some species appear to be reliable indicators of the Gulf Stream, a warm surface current. Both Globorotalia menardii and Globigerinoides sacculifer have their highest abundances under the main trend of the Gulf Stream. Globorotalia inflata reaches high abundances in the cold slope water north of the Gulf Stream but its distribution is not as continuous as the Gulf Stream indicators.Contoured values of species diversity, the Shannon diversity index, and species equitability also reflect surface circulation. A plot of species diversity (number of species) shows a poorly defined region of high diversity beneath the major trend of the Gulf Stream. Use of the Shannon diversity index enhances and clarifies this region of high diversity. A map of species equitability shows a broad belt of low species dominance (high equitability) beneath the Gulf Stream. North of the Gulf Stream, a tongue of high dominance (low equitability) corresponds to the increased relative abundance of Globorotalia inflata.High diversity of planktonic foraminifera in bottom sediments characterizes the warm shifting surface currents of the Gulf Stream; low diversity is typical of slope and Sargasso Sea waters. Low equitability (high species dominance) indicates either cold currents or gyre center waters. Maps of foraminiferal diversity and equitability for other intervals of geologic time may be useful in tracing the evolution of ocean circulation.     

Sensitivity of south‐east Atlantic planktonic foraminifera to mid‐Pleistocene climate change

Abstract: The last one million years are important in terms of climate development during the so‐called Mid‐Pleistocene Transition when amplification of the glacial–interglacial cycles occurred. This study describes abundance changes in fossil planktonic foraminifera in sediments from Core T89‐40, retrieved from the Walvis Ridge in the south‐east Atlantic, across this time period. Cycles between upwelling and subtropical planktonic foraminiferal assemblages are shown to mirror changes between glacial and interglacial periods, respectively. During interglacial marine isotopic stages (MIS) 9, 11 and 31, however, anomalously high abundances of the polar left‐coiled Neogloboquadrina pachyderma occur, presumably linked to unusual seasonal upwelling waters. The planktonic foraminiferal abundance record shows 41‐ky cyclic variations in the regional oceanography linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity). These orbitally induced oscillations in oceanographic change occurred throughout the entire record. The most conspicuous feature of the planktonic foraminiferal record is the near absence of left‐coiled Globorotalia truncatulinoides between 960 and 610 ka (MIS 26‐15). The abrupt disappearance of this species is synchronous with the onset of the Mid‐Pleistocene Transition in MIS 26.     

Integrated quantitative biostratigraphy of the latest Tortonian-early Messinian Pissouri section (Cyprus): An evaluation of calcareous plankton bioevents

A quantitative study was performed on planktonic foraminifera and calcareous nannofossils of the astronomically dated Late Miocene Pissouri section (Cyprus). Our results confirm the reliability of well-known planktonic foraminiferal events as Catapsydraxparvulus LO (Last Occurrence), sinistral coiling change of Globorotaliascitula, Globorotaliamiotumida group FRO (First Regular Occurrence), Globorotalianicolae FO (First occurrence) and LO, sinistral coiling change of Neogloboquadrinaacostaensis and also of several important calcareous nannofossil events (Amaurolithusprimus FO, Amaurolithusdelicatus FO, Reticulofenestrarotaria FO and FCO). Integrated planktonic foraminifera and calcareous nannoplankton data contribute to an enhanced time resolution of the Tortonian - early Messinian interval in the Levantine basin, and contribute to detailed correlations throughout the Eastern Mediterranean. In addition, we compare methodologies commonly used in calcareous plankton biostratigraphy, and shortly outline sources of bias that can influence the results of stratigraphic studies.     

Molecular evidence for an independent origin of modern triserial planktonic foraminifera from benthic ancestors

Gallitellia vivans is the only Recent representative of the triserial planktonic foraminiferal family Guembelitriidae. The origin and evolution of this interesting albeit poorly known family are enigmatic. To elucidate the phylogenetic relationships between G. vivans and other planktonic foraminifera, we sequenced the small subunit ribosomal DNA (SSU rDNA) for comparison to our extensive database of planktonic and benthic species. Our analyses suggest that G. vivans represents a separate lineage of planktonic foraminifera, which branches close to the benthic rotaliids Stainforthia and Virgulinella. Both genera resemble Gallitellia in general morphological appearance, having elongate triserial tests at least in their early ontogenic stages. The divergence time of G. vivans is estimated at ca. 18 Ma (early Miocene), suggesting an origin independent from the Cretaceous and Paleogene triserial planktonic foraminifera. Our study thus indicates that modern triserial planktonic foraminifera are not related to the Cretaceous–Paleogene triserial species, and that the sporadic occurrences in the fossil record are not the result of poor preservation, but reflect multiple transitions from benthic to planktonic mode of life.