Les échos de la phase aquitano-burdigalienne dans la coupe de Dellaca Bianca (Val Scrivia,bassin Liguro-piémontais,Italie)

The geologic section of the Torrente Scrivia in 061 the vicinity of Rigoroso is generally reported as an conformable sequence in Oligocene and Miocene strata.In fact, it has suffered important reworkingsduring Lower Miocene and this event is not only evident in sedimentology but also in paleontology. Locally, these events reflect tectonic motions more strongly marked in the north-eastern part of the ligurian piemontese basin.Consequently, the result is that the geologicsection concerned could not be satisfactory referring to Lower Miocene.     

Coccolith crystals ofPleurochrysis carterae: Crystallographic faces, organization, and development

Summary The crystallographic and morphological configuration of the mineral ring associated with the coccoliths ofPleurochrysis carterae was determined by transmission electron microscopy and electron diffraction. Mature Pleurochrysis coccoliths consist of an oval organic base plate, a distal rim of interlocking calcite crystals, and a narrow ribbon of organic material which tethers the mineral ring to the base plate. Crystals of two distinct forms (R and V units) alternate about the rim in a quasi regular manner; their crystallographicc-axes are aligned parallel to and inclined about 63° to the coccolith plane, respectively. The mineral ring has four platelike elements: the distal-shield and outer-tube elements which form the V unit, and the proximal-shield and inner-tube elements which form the R units. The platy surfaces of both tube elements correspond to the common (10 4) rhombohedral faces of calcite, and the plates of the proximal-shield element are prismatic (2 0) faces. The plates of the distal-shield element are rather curved and their orientation does not correspond to a favorable calcite face; however, for convenience they are described as approximately ( 108) faces, faces which rarely, if ever, develop in inorganic sources of calcite. During coccolith development the earliest habits observed for both V and R units correspond to rectangular parallelepipeds. Outgrowth from the initial V unit begins by expansion of (10 4) faces which form the platy surfaces of the outer-tube element. Throughout this period of development the mineral ring is flexible, at least in an isolated state. Subsequent outgrowth of the inner-tube and proximal-shield elements from the initial R unit produces a rigid interlocking ring. The unusual ( 108) faces of the distal-shield element develop after the crystals are locked in place. Organic structures in intimate association with the mineral phase during its nucleation and growth include the coccolith ribbon, the calcium-polyanion particles, and the membrane of the coccolith vesicle. These structures are described in reference to their putative functions in regulating the development of V and R units.Abbreviation PS polysaccharide     

Physiological regulation of carbon fixation in the photosynthesis and calcification of coccolithophorids

Emiliania huxleyi and Gephyrocapsa oceanica are the predominant coccolithophorid species that produce blooms in the ocean and affect the global environment. These species are capable of carbon fixation by both photosynthesis for organic matter production and by intracellular calcification for coccolith production. Both processes were strongly affected by the nutrient status in a laboratory culture. The coccolith production was stimulated by the addition of a high concentration of sodium bicarbonate and by the depletion of phosphate. Interestingly, when the calcification was stimulated, the increase in cell number during algal growth was greatly suppressed and then the cell volume increased. When the growth rate was increased under nutrient-sufficient conditions, the cells became very small in size and most of them bore few or no coccoliths. The data from laboratory experiments show that the cell growth and calcification proceeded apparently independently at different phases. We, therefore, assume that the coccolithophorid blooms in the ocean might be separated into two phases; firstly, the increase in cell population might be triggered by an adequate supply of nutrients to enhance algal growth and then the calcification might subsequently be stimulated when the nutrients become depleted by substantial algal growth.     

Crepidolithus cantabriensis nov. sp., a new calcareous nannofossil (Prymnesiophyceae) from the Lower Jurassic of northern Spain

A new calcareous nannofossil, Crepidolithus cantabriensis nov. sp., is described from the Lower Jurassic marls and marly limestones of the Cantabrian Range (Northern Spain). The new species is characterized by medium-sized, normal to narrowly elliptical coccoliths with open central-areas. In the light microscope, under crossed nicols and in distal view, it displays a characteristic bicyclic extinction pattern. Its inner rim edge shows maximum birefringence colours, which vary from white to yellow, whereas the faintly outer rim margin exhibits low birefringence, tending to grey, and they are separated by a dark and sigmoidal isogyre. C. cantabriensis nov. sp. seems closely related to Crepidolithus crassus on the basis of shared morphological characters. Both of them have a distal shield composed of around thirty sub-vertical, dextrally imbricating laths. The central-area width of C. cantabriensis nov. sp. is approximately half of the coccolith width, whilst it is reduced to one or two tiny lenticular slits in C. crassus. Nevertheless, the phyletic relationships between C. cantabriensis nov. sp. and Crepidolithus cavus are not clear, since they show different rim structures, and C. cavus has a delicate bar aligned along the minor axis of its central area. The first specimens of C. cantabriensis nov. sp. have been recognized within the Raricostatum AZ in the Tudanca section. This species seems to be more abundant in the Reinosa area, which occupied a more distal position with respect to the Iberian Massif during the studied time interval. It is hypothesized here that C. cantabriensis nov. sp. could have inhabited the lower-photic zone.     

Strontium/Calcium ratio, carbon and oxygen stable isotopes in coccolith carbonate from different grain-size fractions in South Atlantic surface sediments

In this study, the Strontium/Calcium (Sr/Ca) ratio, and the carbon and oxygen isotopic compositions of coccoliths are investigated in three different grain-size fractions (<20 μm, 15-5 μm, <5 μm) of 17 surface sediment samples from the Equatorial and South Atlantic. The results are compared to environmental parameters in order to assess the factors controlling the observed coccolith geochemical patterns. Isotopic and geochemical composition of coccolith species in surface sediment samples from the South Atlantic greatly varies according to the different grain-size fractions. However, even if the absolute values show a great offset, the general trends are comparable. The δ¹⁸O values show a decreasing trend with increasing temperature. The δ¹³C and Sr/Ca ratio are mainly influenced by productivity of coccolithophores, which is in turn controlled by different factors, such as temperature, nutrient supply and productivity of other phytoplankton groups. Dilution and dissolution are negligible factors in these open marine samples. Therefore, coccolith abundance in bulk sediment is the best approximation for productivity of coccolithophores. The various coccolith species fractionate Sr differently, as is best shown by the 5-15 μm fraction where three species (Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus) predominantly occur.     

Variations in coccolithophorid production in the Eastern Equatorial Pacific at ODP Site 1240 over the last seven glacial–interglacial cycles

The calcareous nannofossil assemblage from ODP Site 1240 in the equatorial upwelling of the Eastern Pacific was analysed for the last 560 Ka. The chronological framework was set with a combination of isotopic stratigraphy, nannofossil biostratigraphy and one paleomagnetic event.     

Structural and physiological effects of calcium and magnesium in Emiliania huxleyi (Lohmann) Hay and Mohler

In organisms which perform both photosynthesis and calcification, the fact that calcification proceeds faster in the light than in the dark has led to the long-established view that photosynthesis and calcification are closely coupled. It is now clear that calcification does not promote photosynthesis, but an enhancement of calcification by photosynthesis could still explain why calcification is faster in the light. To test this, the kinetics of the two processes were monitored over a wide range of calcium concentrations (0-50 mM) in the coccolithophore Emiliania huxleyi. The addition of 50 mM calcium strongly inhibited both processes, but when incubated in lower concentrations, rates of calcification increased up to 20 mM calcium whilst those of photosynthesis remained constant over the same range of calcium concentrations. So, rates of calcification are able to rise without a concomitant increase in photosynthetic rates. In addition, calcification rate and coccolith morphology responded similarly to changes in calcium concentrations; low calcification rates were associated with poor coccolith structure (undercalcification) and high calcification rates with perfectly formed coccoliths. Calcium concentration thus strongly influences calcification affecting both crystal structure and rate of calcite deposition. A similar structural analysis of coccoliths from cells grown in different magnesium concentrations showed that this ion is also essential for calcification, since strong signs of coccolith malformation and undercalcification were apparent at both low and high magnesium concentrations. In contrast with the calcium results, coccoliths were flawless only in the normal seawater concentration of 58 mM magnesium. We conclude that photosynthesis and calcification are not closely coupled and that calcification depends on a precise balance of both calcium and magnesium concentrations.     

Regulation of CaCO(3) formation in coccolithophores

Coccolithophores impact the ocean carbon cycle principally through the generation of CO₂ during CaCO₃ production. Coccolithophore biomineralization has been examined most extensively in Pleurochrysis carterae and Emiliania huxleyi both of which produce mineralized scales—coccoliths—composed of elaborate calcite crystals attached to an underlying organic base plate. Calcification of preformed base plates is mediated by acidic polysaccharides and occurs in Golgi-derived structures known as mineralizing vesicles. In Pleurochrysis a high capacity calcium-binding polysaccharide PS2 is required for efficient nucleation of calcitic protocrystals. A galacturonomannan PS3 is required for the growth and transformation of the protocrystals into a massive double disc of calcite. The genes that regulate expression of the glycans have not yet been identified. In addition to the coccolith-bearing diploid phases, Pleurochrysis and Emiliania possess both haploid and diploid non-calcifying stages, which are self-perpetuating via binary fission. One non-calcifying Pleurochrysis phase fails to synthesis PS2 and spontaneously reverts to the mineralizing morphotype in laboratory cultures. As yet, there is little information on environmental factors that effect the expression or silencing of calcifying genes or favor the growth of calcifying over non-calcifying phases. These issues will need extensive investigation, if we are to appreciate the role of coccolithophores in the regulation of atmospheric CO₂ levels.     

Cold Stress Stimulates Intracellular Calcification by the Coccolithophore, Emiliania huxleyi (Haptophyceae) Under Phosphate-Deficient Conditions

Intracellular calcification by the coccolith-producing haptophyte Emiliania huxleyi (NIES 837) is regulated by various environmental factors. This study focused on the relationship between cold and phosphate-deficient stresses to elucidate how those factors control coccolith production. (45)Ca incorporation into coccoliths was more than 97% of the total (45)Ca incorporation by whole cells. In a batch culture, orthophosphate in the medium (final concentration, 28.7 muM) was rapidly depleted within 3 days, and then extracellular alkaline phosphatase (AP) activity, an indicator of phosphate deprivation, increased during the stationary growth phase. The increase in AP activity was slightly higher at 20 degrees C than at 12 degrees C. The calcification started to increase earlier than AP activity, and the increase was much higher at 12 degrees C than at 20 degrees C. Such enhancement of calcification was suppressed by the addition of phosphate, while AP activity was also suppressed after a transient increase. These results suggest that phosphate deprivation is a trigger for calcification and that a rather long induction period is needed for calcification compared to the increase in AP activity. While calcification was greatly stimulated by cold stress, other cellular activities such as growth, phosphate utilization, and the induction of AP activity were suppressed. The stimulation of coccolith production by cold stress was minimal under phosphate-sufficient conditions. The high calcification activity estimated by (45)Ca incorporation was confirmed by morphological observations of coccoliths on the cell surface under bright-field and polarization microscopy. These results indicate that phosphate deprivation is the primary factor for stimulating coccolith production, and cold stress is a secondary acceleration factor that stimulates calcification under conditions of phosphate deprivation.     

Seasonal occurrence of coccoliths in sediment traps from West Caroline Basin, equatorial West Pacific Ocean

Coccolith fluxes were investigated by sediment trap studies in the West Caroline Basin, which is located in the equatorial western Pacific. The investigation was conducted from June 1991 to March 1992 at two water depths, 1592 and 3902 m, as part of the Northwest Pacific Carbon Cycle Study (NOPACCS) program. Two seasonal maxima of coccolith fluxes were observed during September–early October and late December–January. The average coccolith and coccosphere fluxes at the depth of the shallow trap were 1800×10⁶ coccoliths m⁻² day⁻¹ and 1.9×10⁶ coccospheres m⁻² day⁻¹, respectively. The flux of coccoliths followed the same trend as the total flux, and was closely correlated with the flux of organic matter flux. Florisphaera profunda, Gladiolithus flabellatus, Gephyrocapsa oceanica, Umbilicosphaera sibogae var. sibogae, Emiliania huxleyi, and Oolithotus fragilis were the most abundant species together comprising more than 85% of the total flora. Observed seasonal changes of the species composition of coccolith flora, as well as analysis of the R-mode cluster, revealed that during the summer, the assemblage was marked by the dominance of G. oceanica and U. sibogae. However, during the winter, the assemblage was dominated by E. huxleyi and O. fragilis. These assemblage changes were influenced by monsoonal events, which were observed off the New Guinea coast. F. profunda dominated the community in the shallow trap throughout most of the year; peak values of this species were recorded during the winter. The coccosphere assemblage was dominated by G. oceanica at both water depths. In the deep trap, the sedimentation pattern was similar to that observed at the shallow depth. Mean coccolith and coccosphere fluxes at the deep trap were 2000×10⁶ coccolith m⁻² day⁻¹ and 0.08×10⁶ coccospheres m⁻² day⁻¹, respectively. The increase in coccolith flux with water depth suggests a lateral influx. The estimated average daily mass of CaCO₃ flux in coccoliths and coccospheres was 16.6 mg m⁻² day⁻¹ at the 1592 m trap and 17.9 mg m⁻² day⁻¹ at the 3902 m trap, respectively. These calculated values contributed only 23.3% to the total CaCO₃ flux at the shallow trap and 27.9% at the deep trap.