Allan W. H. Bé

Modern coccolithophoridae of the atlantic ocean—I. Placoliths and cyrtoliths☆

Abstract Although there are more than 70 species of Coccolithophoridae living in the Atlantic only about 16 of these have adequate fossil records, mainly placoliths and to a lesser extent cyrtoliths. Biogeographic ranges determined from surface sediment and plankton samples show that living species have slightly broader distributional ranges than those preserved in oceanic sediments. This is attributed to rapid warming of the Atlantic since the last glacial age. Species distributions have been delineated by maximum position poleward of the limiting isotherm for warm-water species and maximum equatorward position of the limiting isotherm for cold water species. Dispersion beyond their present boundaries by ocean currents after death is negligible. Temperature studies based on cruise data and bimonthly sampling off Bermuda enabled the authors to determine maximum and optimum temperature ranges for each species. The majority are subtropical forms. A few are stenothermal, such as Umbellosphaera irregularis (21°–28°C) and Coccolithus pelagicus (7°–14°C) and they have proved useful in paleoecology. The species are grouped into five climatic assemblages: tropical, subtropical, transitional, subarctic, and subantarctic.

Vertical Distribution and Isotopic Composition of Living Planktonic Foraminifera in the Western North Atlantic

Thirteen species of planktonic foraminifera collected with vertically stratified zooplankton tows in the slope water, Gulf Stream cold core ring, and northern Sargasso Sea show significant differences in their vertical distributions in the upper 200 meters of these different hydrographic regimes. Gulf Stream cold core rings may be responsible for a southern displacement of the faunal boundary associated with the Gulf Stream when reconstructed from the deep-sea sediment record. Oxygen isotope analyses of seven species reveal that nonspinose species (algal symbiont-barren) apparently calcify in oxygen isotope equilibrium, whereas spinose species usually calcify out of oxygen isotope equilibrium by approximately –0.3 to –0.4 per mil in δ18O values. The isotope data indicate that foraminifera shells calcify in depth zones that are significantly narrower than the overall vertical distribution of a species would imply.

Temperature Dependence of Carbon Isotope Composition in Marine Plankton and Sediments.

Samples of marine plankton collected in high-latitude areas of the South Atlantic where surface water temperatures are near 0�C show a carbon-12 enrichment of 6 per mill relative to samples collected where temperatures are about 25�C. The organic carbon in sediments in the Drake Passage and Argentine Basin also shows a carbon-12 enrichment relative to warmer areas.

Surface circulation of the Indian Ocean during the last glacial maximum, approximately 18,000 yr B.P.

A seasonal reconstruction of the Indian Ocean during the last glacial maximum (∼18,000 yr B.P.) reveals that its surface circulation and sea surface temperature patterns were significantly different from the modern Indian Ocean. This reconstruction is based on the planktonic foraminiferal biogeography and estimated sea surface temperatures in 42 Indian Ocean samples. Compared to modern conditions, the polar front was 5° to 10° latitude further north during the last glacial maximum; the Subtropical Convergence was 2° to 5° latitude further north. The West Australian Current was more intense as part of the West Wind Drift was deflected northward along the coast of Australia. The Agulhas Current was cooler and weaker during the summer and more saline and subtropical during the winter. In general, the low latitudes underwent little temperature change. The western Arabian Sea was warmer which implies less upwelling and a weaker Southwest Monsoon. On the average, the Indian Ocean was 1.9°C cooler in February and 1.7°C cooler in August during the last glacial maximum.

Seasonal distribution of planktonic foraminifera in the western North Atlantic

Distinct and predictable patterns for 17 species, frequency distribution depends on phytoplankton, three major faunal assemblages (subarctic, transitional, subtropical)

Li, Sr, Mg, and Na in foraminiferal calcite shells from laboratory culture, sediment traps, and sediment cores

Abstract Constant-temperature laboratory culture experiments of the planktonic foraminiferal species Globigerinoides sacculifer (Brady) suggest that the ratios of Li and Sr to Ca in the shells are a function of these ratios in the culture solutions. Mg Ca and Na Ca in the shells did not vary with changes of these ratios in the culture solution. These are the first direct determinations of the relationship between foraminiferal shell chemistry and solution composition. The possibility of temperature dependence for the minor elemental composition of foraminiferal shells was also investigated in the laboratory and by analysis of several planktonic and one benthic foraminiferal species from sediment trap and sediment core samples. The Sr Ca , Mg Ca , and Na Ca ratios in the natural samples roughly correlate with calcification temperature, whereas differences in the Li/Ca ratios are small and not systematically related to temperature. However, laboratory culture experiments at 20°C and 30°C showed no variation in the Li Ca , Sr Ca , Mg Ca , and Na Ca ratios with calcification temperature for the planktonic foraminifera G. sacculifer and Orbulina universa. Therefore, observed differences in the Sr Ca , Mg Ca , and Na Ca ratios for the sediment trap and core foraminiferal samples cannot be ascribed to direct effects of calcification temperature, but may be due to some other environmental factor which is correlated with temperature.

Disappearance of pink-pigmented Globigerinoides ruber at 120, 000 yr BP in the Indian and Pacific Oceans

The planktonic foraminiferal species Globigerinoides ruber with pink-pigmented tests occupied a worldwide warm-water belt during much of the Pleistocene. This variety was exterminated from the Indian and Pacific Oceans at about 120,000 yr BP, based on the oxygen isotope stratigraphy in 11 Indo-Pacific deep-sea cores, but it continued to live on to the present in the North and South Atlantic Ocean and the Mediterranean Sea. The disparate Atlantic and Indo-Pacific records reflect faunal provincialism that has been developing in the two regions since the Pliocene.

Oxygen and carbon isotopic composition and biogeographic distribution of planktonic foraminifera in the Indian Ocean

Five planktonic foraminiferal assemblages were encountered along a north-south transect in the Indian Ocean with the following dominant species: Globorotalia truncatulinoides in the Southern Subtropical Assemblage; Globigerinoides ruber, Globigerinella aequilateralis and Globorotalia menardii in the Northern subtropical Assemblage; Globigerinoides sacculifer and Pulleniatina obliquiloculata in the Tropical Assemblage; Neogloboquadrina dutertrei and Globigerina bulloides in the Tropical Upwelling Assemblage; and Globigernoides conglobatus in the Tropical Arabian Sea Assemblage. Oxygen and carbon isotopic analyses were performed on three surface-dwelling species of planktonic foraminifera. In tropical and subtropical waters (22°–30°C) the δ 18O of Globigerinoides ruber and G. sacculifer varies with surface-water temperature and the slope of the isotope-temperature regression line is similar to that of Epstein et al. (1953) for marine molluscs. However, foraminiferal δ 18O values are significantly lighter than those of molluscs, indicating isotopic disequilibrium. Comparison between plankton tows taken within and below the mixed layer at the same stations shows that G. ruber has similar, low δ 18O values in both sets, suggesting that it secretes its shell within the mixed layer. By contrast, G. sacculifer has slightly higher δ 18O values below than within the mixed layer, because it continues to secrete calcite below the thermocline. No significant δ 18O variations with shell size were found. On the other hand, δ 13C values were strongly size-dependent, i.e., the smaller the size, the lighter the δ 13C. Comparison of living G. ruber and G. sacculifer with Recent specimens from the surface sediment shows that the living foraminifera are generally lighter in δ 18O than the fossils, supporting independent observations that the latter have secreted additional calcite during gametogenesis, when the organisms settle freely from the euphotic zone for about 12 h and pass through waters lower in temperature than the epipelagic waters where the foraminifera form the bulk of their shells.

Ecology of Recent Planktonic Foraminifera: Part 2: Bathymetric and Seasonal Distributions in the Sargasso Sea off Bermuda

Studies of the bathymetric distribution of planktonic Foraminifera indicate that they are most abundant in the euphotic zone of the oceanic waters around Bermuda. Seasonal fluctuations in the relative and absolute abundances of 15 species have been observed from 106 plankton samples, showing a regular succession in the foraminiferal populations. Globorotalia hirsuta, G. truncatulinoides, Globigerina inflata, and G. bulloides were most abundant in winter and spring, when temperatures within the euphotic zone ranged between 18 degrees and 23 degrees C. and salinities varied between 34.40%. and 36.60%. Globorotalia menardii, Globigerinoides sacculifer, and G. conglobatus reached their maximum concentrations in summer and fall, when temperatures ranged between 23 degrees and 27 degrees C. and salinities between 36.10% and 36.40%. The oceanographic and paleoecologic implications of these observations are discussed for the northern Sargasso Sea.

Oxygen-18 enrichment of planktonic foraminifera due to gametogenic calcification below the euphotic zone.

Empty shells of spinose planktonic foraminifera on the seabed are significantly enriched in oxygen-18 as compared with the shells of their living counterparts in surface waters. This enrichment is due to gametogenic calcification, which extracts calcium carbonate from deeper and colder waters as the shell sinks below the euphotic zone. JEAN-CLAUDE DUPLESSY