Ralf Schiebel

Modern planktic foraminifera

Planktic foraminifers are marine protozoans with calcareous Shells and chambered tests. They first appeared in the mid-Jurassic and spread since the mid-Cretaceous over all the world’s oceans. Modern planktic foraminifers evolved since the early Tertiary, when the first spinose species occurred. Most species live in the surface to sub-thermocline layer of the open ocean, and in marginal seas like the Mediterranean, Caribbean, South China Sea, and Red Sea. Planktic foraminifers are absent in shallow marginal seas, for example, the North Sea. Planktic foraminifers respond to food, temperature and chemistry of the ambient seawater. Species abundance varies according to seasons, water masses, and water depths. Symbiont-bearing species depend on light and are restricted to the euphotic zone. Planktic foraminifers constitute a minor portion of total Zooplankton, but are major producers of marine calcareous particles (shells) deposited on the ocean floor where they form the so-called foraminiferal ooze.Planktic foraminifers contribute substantially to the fossil record of marine Sediments and are of high ecologic, paleoceanographic, and stratigraphic significance since the mid-Cretaceous. Radiocarbon (14C) gives an absolute age of shell formation within late Pleistocene and Holocene Sediments. Factors that determine the modern faunal composition are applied to Interpretation of the fossil assemblages, for example, by multiple regression techniques (transfer functions) to yield an estimate on ancient environmental parameters. The chemical composition of the calcareous shell (stable isotopes and trace elements) holds clues to the chemical and physical State of the ambient seawater and is useful in the reconstruction of temperature, chemical State, and biological productivity of the ancient marine environment.KurzfassungPlanktische Foraminiferen sind kalkschalige, marine Protozoen mit gekammerten Gehäusen. Sie sind seit dem mittleren Jura (∼170 Millionen Jahre) fossil überliefert. Seit der mittleren Kreide sind planktische Foraminiferen im marinen Pelagial weit verbreitet. Die meisten Arten sind an der Kreide / Tertiär-Grenze ausgestorben. Die modernen Arten, mit den spinösen -stacheltragenden- Arten, haben sich seit dem Tertiär entwickelt. Der Lebensraum der meisten Arten ist die euphotische Deckschicht bis knapp unterhalb der saisonalen Thermokline. Wenige Arten leben in der Tiefsee. Auch in tiefen Randmeeren leben planktische Foraminiferen, wie etwa im Mittelmeer, der Karibik, dem Südchinesischen Meer und dem Roten Meer. In flachen Randmeeren, wie der Nordsee, sind planktische Foraminiferen nicht heimisch. Das Artenspektrum variiert entsprechend des Futterangebotes, der Temperatur und des Chemismus des umgebenden Wassers. Symbiontentragende Arten sind lichtabhängig und an die euphotische Zone gebunden. Planktische Foraminiferen bilden nur einen geringen Teil der planktischen Biomasse, sind aber hauptsächlich an der Produktion und Sedimentation des marin-pelagischen, partikulären Karbonates beteiligt.Planktische Foraminiferen bilden den sogenannten Foraminiferen-Schlamm am Meeresboden und tragen substantiell zum fossilen Inhalt mariner Sedimente bei. Seit der mittleren Kreide sind planktische Foraminiferen stratigraphische Leitfossilien und wichtige ökologische und paläoozeanographische Indikatoren. Radiokohlenstoff (14C) der Foraminiferengehäuse wird zur absoluten Altersdatierung pleistozäner und holozäner Sedimente genutzt. Daten zur Ökologie rezenter Faunen werden mit multiplen Regressionsverfahren (Transferfunktionen) auf fossile Faunen übertragen und damit paläo-ökologische, -ozeanographische und -klimatologische Rekonstruktionen ermöglicht. Die chemische Zusammensetzung (stabile Isotope und Spurenelemente) der kalkigen Foraminiferenschale repräsentiert den chemischen und physischen Zustand des umgebenden Meerwassers und liefert wichtige Daten zur Rekonstruktion von Temperatur, chemischer Zusammensetzung und biologischer Produktivität vergangener Ozeane.

Reconstructing past planktic foraminiferal habitats using stable isotope data: a case history for Mediterranean sapropel S5

Abstract A high-resolution stable O and C isotope study is undertaken on all planktic foraminiferal species that are reasonably continuous through an Eemian sapropel S5 from the western side of the eastern Mediterranean. The data are considered within a context of high-resolution isotope records for two further S5 sapropels from the central and easternmost sectors of the basin, alkenone-based sea surface temperature records for all three sapropels, and planktic foraminiferal abundance records for the same sample sets through all three sapropels. Results are compared with similar data for Holocene sapropel S1. The adopted approach allows distinction between species that are most suitable to assess overall changes in the climatic/hydrographic state of the basin, including depth-related differentiations and the main seasonal developments, and species that are most affected by variable biological controls or local/regional and transient physico–chemical forcings. It is found that a-priori assumptions about certain species’ palaeohabitats, based on modern habitat observations, may become biased when non-analogue conditions develop. In the case of Mediterranean sapropel S5, these consisted of enhanced freshwater dilution, elevated productivity, shoaling of the pycnocline between intermediate and surface waters, and stagnation of the subsurface circulation. Under these conditions, some species are found to ‘shift’ into habitat settings that differ considerably from those occupied today. The present multiple-species approach can identify such ‘anomalous responses’, and thus offers a sound background for further shell-chemistry investigations and quantitative interpretation of the isotopic profiles. We capitalise on the latter potential, and offer the first quantitative estimates of monsoon flooding into the Mediterranean during the deposition of Eemian sapropel S5.

Planktic foraminiferal production stimulated by chlorophyll redistribution and entrainment of nutrients

During September and October 1996 planktic foraminifers and pteropods were sampled from the upper 2500 m of the water column in the BIOTRANS area (47°N, 20°W), eastern North Atlantic, as part of the JGOFS program. Hydrography, chlorophyll fluorescence, and nutrient content were recorded at high spatial and temporal resolution providing detailed information about the transition time between summer and fall. At the beginning of the cruise a shallow pycnocline was present and oligotrophic conditions prevailed. Over the course of the cruise, the mixed layer depth increased and surface water temperature decreased by 1.5°C. Both chlorophyll-a dispersed in the upper 50 m by vertical mixing and chlorophyll-a concentrations at the sea surface increased. The nitracline shoaled and nutrient enriched waters were entrained into the mixed layer. Planktic foraminifers and pteropods closely reflected the changes in the hydrography by increased growth rates and changes in species composition. Three main groups of planktic foraminiferal species were recognized: (1) a temperate and low-productivity group dominated by Neogloboquadrina incompta characterized the shallow mixed layer depths. (2) A temperate and high-productivity group dominated by Globigerina bulloides characterized the period with wind-induced dispersal of chlorophyll-a and entrainment of nutrient-enriched waters. (3) A warm water group containing Globigerinoides sacculifer, Orbulina universa, Globigerinoides ruber (white), and Globigerinella siphonifera was most common during the first days of sampling. Synchronous with the hydrographic change from summer to fall, planktic foraminiferal and pteropod growth was stimulated by redistribution of chlorophyll-a and entrainment of nutrient-enriched waters into the mixed layer. In addition, the seasonal change in the eastern North Atlantic resulted in a transition of the epipelagic faunal composition and an increased calcareous particle flux, which could be used to trace seasonality in fossil assemblages and allow for better paleoceanographic interpretation of the boreal Atlantic.

Population dynamics of the planktic foraminifer Globigerina bulloides from the eastern North Atlantic

Abstract A cumulative data set from the eastern North Atlantic was compiled and analysed to study the population dynamics of Globigerina bulloides . Data were generated from samples collected with a multiple opening and closing plankton net from the upper ocean (0–500 m). We analysed the total assemblage > 125 μm. The habitat of G. bulloides in the eastern North Atlantic is restricted mostly to the upper 60 m of the water column and depends on the availability of its food resources and, therefore, on the general hydrographic pattern. The temporal distribution of tests at different depths reveals a systematic succession that is related to the lunar cycle. We suggest that G. bulloides reproduces mainly within the upper 60 m of the ocean. Gametogenesis is unusual in test size classes below 125 μm but frequent in specimens larger than 150 μm. Reproduction takes place during the first week after new moon. Maturation of specimens takes place during the second half of waxing moon and during waning moon. Large numbers of mature specimens [gametogenic calcification (GAM) specimens > 250 μm] occur during the time of main reproduction.

Interannual variability of planktic foraminiferal populations and test flux in the eastern North Atlantic Ocean (JGOFS)

Planktic foraminiferal assemblages vary in response to seasonal fluctuations of hydrographic properties, between water masses, and after periodical changes and episodic events (e.g. reproduction, storms). Distinct annual variability of the planktic foraminiferal flux is also known from sediment trap data. In this paper we discuss the short-term impacts on interannual flux rates based on data from opening-closing net hauls obtained between the ocean surface and 500 m water depth. Data were recorded during April, May, June, and August at around 47°N, 20°W (BIOTRANS) in 1988, 1989, 1990, 1992, 1993, and during May 1989 and 1992 at 57°N. 20-22°W. Species assemblages closely resemble each other when comparing the mixed layer fauna with the fauna of the upper 100 m and the upper 500 m of the water column. In addition, species assemblages > 100 μm are almost indistinguishable from assemblages that are > 125 μm in test size. The standing stock of planktic foraminifers at BIOTRANS can vary by more than one order of magnitude over different years; however, species assemblages may be similar when comparing corresponding seasons. Early summer assemblages (June) are distinctly different from late summer assemblages (August). Significant variations in the species composition during spring (April/May) are independent of the mixed layer depth. Spring assemblages are characterized by high numbers of Globigerinita glutinata In particular, day-to-day variations of the number of specimens and in species composition may have the same order of magnitude as interannual variations. This appears to be independent of the reproduction cycle. Species assemblages at 47°N and 57°N are similar during spring, although surface water temperatures and salinities differ by up to 10°C and 0.7 (PSU). We suggest that the main factors controlling the planktic foraminiferal fauna are the trophic properties in the upper ocean productive layer. Planktic foraminiferal carbonate flux as calculated from assemblages reveals large seasonal variations, a quasi-annual periodicity in flux levels, and substantial differences in timing and magnitude of peak fluxes. At the BIOTRANS station, the average annual planktic foraminiferal CaCO 3 fluxes at 100 and 500 m depth are estimated to be 22.4 and 10.0 g m -2 yr -1 , respectively.

Planktic foraminifers and hydrography of the eastern and northern Caribbean Sea

The distribution of living planktic foraminifers and their relation to the hydrography of the Caribbean Sea was investigated in plankton net tows and surface sediment samples taken along the Antilles island arc during April/May 1996. The planktic foraminiferal community was strongly influenced by spatial variations in salinity that were largely due to the influx of Orinoco River water into the southeastern Caribbean Sea and inflowing Sargasso Sea water in the north. Along the Antilles island arc, Globigerinoides ruber was the dominant species in the surface waters throughout. In the southeastern Caribbean Sea, where Orinoco River outflow influences the planktic community, standing stocks of planktic foraminifers (>100 μm) between 4 and 50 specimens m−3 were medium to low. The southeastern faunas between Tobago and Guadeloupe were characterized by increased proportions of Neogloboquadrina dutertrei. Highest standing stocks of 159 specimens m−3 in the upper 20 m of the water column were recorded in the northeastern Caribbean Sea and the assemblages were characterized by high proportions of Globigerinita glutinata, associated with cyclonic eddies. In the Anegada Passage, where Sargasso Sea water flows into the Caribbean Sea, low standing stocks of 18 specimens m−3 indicate oligotrophic conditions. Together with the oligotrophic surface waters, the Subtropical Underwater enters the Caribbean Sea through the Anegada Passage in water depths between 100 and 300 m. These waters are characterized by higher concentrations of Globorotalia truncatulinoides relative to the adjacent water masses. Comparison of the living planktic foraminiferal fauna with empty test assemblages from the water column and from surface sediments shows that differences in the faunal composition mostly correspond to the distribution of water masses and to the differential dissolution of species. In the vicinity of islands Globigerinoides ruber reaches higher relative frequencies than in the open ocean, pointing towards a higher tolerance of this species towards neritic conditions than in other species.

Size distribution of Holocene planktic foraminifer assemblages: biogeography, ecology and adaptation

The size of any organism is influenced by the surrounding ecological conditions. In this study, we investigate the effects of such factors on the size spectra of planktic foraminiferal assemblages from Holocene surface sediments. We analyzed assemblages from 69 Holocene samples, which cover the major physical and chemical gradients of the oceans. On a global scale, the range of sizes in assemblages triples from the poles to the tropics. This general temperature-related size increase is interrupted by smaller sizes at temperatures characteristic of the polar and subtropical fronts, at 2°C and 17°C, respectively, as well as in upwelling areas. On a regional scale, surface water stratification, seasonality and primary productivity are highly correlated with the size patterns. Such environmentally controlled size changes are not only characteristic for entire assemblage, but also for the dominant single species.

Distribution, biomass and diversity of benthic foraminifera in relation to sediment geochemistry in the Arabian Sea

The distribution, biomass, and diversity of living (Rose Bengal stained) deep-sea benthic foraminifera (>30 [mu]m) were investigated with multicorer samples from seven stations in the Arabian Sea during the intermonsoonal periods in March and in September/October, 1995. Water depths of the stations ranged between 1916 and 4425 m. The distribution of benthic foraminifera was compared with dissolved oxygen, % organic carbon, % calcium carbonate, ammonium, % silica, chloroplastic pigment equivalents, sand content, pore water content of the sediment, and organic carbon flux to explain the foraminiferal patterns and depositional environments. A total of six species-communities comprising 178 living species were identified by principal component analysis. The seasonal comparison shows that at the western stations foraminiferal abundance and biomass were higher during the Spring Intermonsoon than during the Fall Intermonsoon. The regional comparison indicates a distinct gradient in abundance, biomass, and diversity from west to east, and for biomass from north to south. Highest values are recorded in the western part of the Arabian Sea, where the influence of coastal and offshore upwelling are responsible for high carbon fluxes. Estimated total biomass of living benthic foraminifera integrated for the upper 5 cm of the sediment ranged between 11 mg Corg m-2 at the southern station and 420 mg Corg m-2 at the western station. Foraminifera in the size range from 30 to 125 [mu]m, the so-called microforaminifera, contributed between 20 and 65% to the abundance, but only 3% to 28% to the biomass of the fauna. Highest values were found in the central and southern Arabian Sea, indicating their importance in oligotrophic deep-sea areas. The overall abundance of benthic foraminifera is positively correlated with oxygen content and pore volume, and partly with carbon content and chloroplastic pigment equivalents of the sediment. The distributional patterns of the communities seem to be controlled by sand fraction, dissolved oxygen, calcium carbonate and organic carbon content of the sediment, but the critical variables are of different significance for each community.

Primary production in the Arabian Sea during the last 135 000 years

Abstract Variations in primary productivity (PP) have been reconstructed in eutrophic, mesotrophic and oligotrophic parts of the Arabian Sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages. Temporal variation in paleoproductivity is most pronounced in the mesotrophic northern (NAST site) and oligotrophic eastern (EAST site) Arabian Sea, and comparatively weak in the western eutrophic GeoB 3011-1 site in the upwelling area off Oman. Higher PP during interglacials (250–320 g C m−2 year−1) than during cold stages (210–270 g C m−2 year−1) at GeoB 3011-1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities. At NAST, during interglacials, PP is estimated to exceed 250 g C m−2 year−1, and during glacials to be as low as 140–180 g C m−2 year−1. These fluctuations may result from a (1) varying impact of filaments that are associated to the Oman coastal upwelling, and (2) from open-ocean upwelling associated to the Findlater Jet. At EAST, highest productivity of about 380 g C m−2 year−1 is documented for the transition from isotope stage 5 to 4. We suggest that during isotope stages 2, 4, 5.2, the transition 5/4, and the end of stage 6, deep mixing of surface waters was caused by moderate to strong winter monsoons, and induced an injection of nutrients into the euphotic layer leading to enhanced primary production. The deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep-dwelling planktic foraminiferal species. A high-productivity event in stage 3, displayed by estimated PP values, and by planktic foraminifera and radiolaria flux and accumulation rate, likely resulted from a combination of intensified SW monsoons with moderate to strong NE monsoons. Differential response of Globigerina bulloides, Globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale.

Impacts of storms on Recent planktic foraminiferal test production and CaCO3 flux in the North Atlantic at 47 °N, 20 °W (JGOFS)

Planktic foraminiferal assemblages are well known to vary in accordance with seasonal fluctuations in ocean properties, periodic reproduction cycles, and variations between water masses. Here we report that storms also can significantly influence foraminiferal assemblages. During the RV Meteor cruise 21 to the Northeast Atlantic Ocean (biotrans area), from March to May 1992, planktic foraminifera were sampled using a multiple opening-closing net. While sampling, two storms with wind forces up to 12 Beaufort caused intensified surface layer mixing with shifts in the depth of the upper ocean mixed-layer from 20–40 m to 170–240 m. Subsequently, planktic foraminiferal growth rates increased, resulting in an elevated quantity of small (100–150 μm) tests (Phase 1). When the wind strength increased a second time, the mixed-layer deepened to a depth below the former position of the pycnocline, and again the abundance of small tests increased (Phase 2). During Phase 2, the weight of calcite in specimens of the productive zone reached its maximum. In the export zone, an associated increase in empty tests occurred with a lag time depending on the test sinking velocity. In the upper export zone, down to 700 m water depth, CaCO3 flux increased from 9.3 to 49.8 mg CaCO3 m−2 d−1 after the first storm and from 8.9 to 19.9 mg CaCO3 m−2d−1 after the second storm. In the 700 to 2500 m depth interval, the flux increased from 5.1 mg CaCO3 m−2 d−1 to about 9.2 mg CaCO, m−2 d−1. Thus, the standing stock of living foraminifera and export of empty tests from the productive zone increased after the storms, leading to pulses of CaCO3 exported from the surface to deep water.