Calcareous nannoplankton coccolithophores and discoasters

Calcareous nannoplankton are a heterogeneous group of calcareous forms, including coccoliths, discoasters and nannoconids, ranging in size from 0.25 to 30 |im. In the fossil record they are found in fine-grained pelagic sediments and can be sufficiently abundant to become rock-forming, for example the Upper Cretaceous chalk. Coccolithophores are unicellular planktonic protozoa with chrysophyte-like photosynthetic pigments, but they differ from most other Chrysophyta in having two flagella of equal length and a third whip-like organ called a haptonema. The group is an important constituent of the oceanic phytoplankton, providing a major source of food for herbivorous plankton. Tiny calcareous scales called coccoliths (3-15 |im in diameter) form around these cells as a protective armour that eventually falls to the ocean floor to build deep sea ooze and fossil chalks. Being both abundant and relatively easy to recover from marine sediments, coc-coliths are used for biostratigraphical correlation of post-Triassic rocks and in palaeoceanographic studies.

The stellate calcareous nannofossils, the discoasters, are an extinct group that are exceedingly useful in the biostratigraphy of the Tertiary. Their taxonomy is based on the number of rays and ornamentation in plan view.

Nannoconids are minute, cone-shaped microfossils (5-30 |im) constructed of closely packed, calcite wedges that form a spiral. A canal penetrates the axis of the cone. Up to 12 specimens make up the skeleton of a single organism arranged in a petal-like structure (Trejo 1960). Nannoconids are useful in Cretaceous biostratigraphy in the absence of other groups.

The living coccolithophore

A coccolithophore is generally a spherical or oval unicell, less than 20 |im in diameter, equipped with two golden-brown pigment spots with a prominent nucleus between, two flagella of equal length and a haptonema. The small, calcite coccoliths are formed in vesicles within the cell under the stimulus of light. These eventually move to the outside of the cell where old coccoliths are shed. Reproduction is mostly asexual, by simple division of the mother cell into two or more daughter cells. In some living genera there is also an alternation between a motile and a non-motile planktonic or benthic stage. The motile stage has a flexible skeleton with coccoliths embedded in a pliable cell membrane, but in the non-motile cysts, calcification of the membrane can take place, thereby forming a rigid shell called a coccosphere (Fig. 14.1).

Flagellum Haptonema Vacuole

Chloroplast Cell membrane Mitochondria

Sketch Vacuole

Mitochondria Nucleus

Vacuole

Fig. 14.1 Sketch of a living coccolithophore cell. During the non-motile stage the flagella are absent and the cell is covered by coccoliths. (After Siesser in Lipps 1993, figure 11.14 (with permission).)

Mitochondria Nucleus

Vacuole

Fig. 14.1 Sketch of a living coccolithophore cell. During the non-motile stage the flagella are absent and the cell is covered by coccoliths. (After Siesser in Lipps 1993, figure 11.14 (with permission).)

Coccoliths

Coccolith morphology is the basis for classification of both living and fossil members of the group. Two basic modes of construction are known from electron microscope studies: holococcoliths are built entirely of submicroscopic calcite crystals, mostly rhombohedra, arranged in regular order; heterococcoliths are usually larger and built of different submicroscopic elements such as plates, rods and grains, combined together into a relatively rigid structure. As holococcoliths invariably disintegrate after they are shed, it is the hetero-coccoliths that provide the bulk of the microfossil record. Heterococcoliths vary considerably in form and

Coccoliths

Fig. 14.2 Coccoliths (a) Recent coccolithophore in non-motile stage, X2780. (b) Side view of Cyclococcolithina, with cross-section. (c) Pseudoemiliania distal view, X3600. (d) Same as (c) from proximal side. (e) Helicopontosphaera, X2930. (f) Zygodiscus, X5340. (g) Prediscosphaera proximal and side views, X4000. (h) Braarudosphaera, X2140. (i) Rhabdosphaera side view, X4000. (j) Discoaster, X1000.

Fig. 14.2 Coccoliths (a) Recent coccolithophore in non-motile stage, X2780. (b) Side view of Cyclococcolithina, with cross-section. (c) Pseudoemiliania distal view, X3600. (d) Same as (c) from proximal side. (e) Helicopontosphaera, X2930. (f) Zygodiscus, X5340. (g) Prediscosphaera proximal and side views, X4000. (h) Braarudosphaera, X2140. (i) Rhabdosphaera side view, X4000. (j) Discoaster, X1000.

Coccolithophores

Fig. 14.3 Electron photomicrographs of living coccolithophores. Scale bar = 1 |m. (a) Emiliania huxleyi var. huxleyi (Pleist.-Rec.). (b) Discosphaera tubifera (Pleist.-Rec.). (c) Braarudosphaera bigelowii (Jur.-Rec.). (d) Scyphosphaera apsteinii f. apsteinii (Eoc.-Rec.). ((a)-(d) From Winter & Siesser 1994 (with permission).)

Fig. 14.3 Electron photomicrographs of living coccolithophores. Scale bar = 1 |m. (a) Emiliania huxleyi var. huxleyi (Pleist.-Rec.). (b) Discosphaera tubifera (Pleist.-Rec.). (c) Braarudosphaera bigelowii (Jur.-Rec.). (d) Scyphosphaera apsteinii f. apsteinii (Eoc.-Rec.). ((a)-(d) From Winter & Siesser 1994 (with permission).)

construction. The majority comprise discs of elliptical or circular outline (shields) constructed of radially arranged plates, enclosing a central area which may be empty, crossed by bars, filled with a lattice or produced into a long spine. The outward-facing (distal) side of the shield is often more convex with a prominent sculpture and may be provided with a spine, whilst the other proximal face is flat or concave and may have a separate architecture (Fig. 14.2).

Coccolithophores have provided a major source for carbonate ooze since the Early Mesozoic and thus the biomineralization of coccoliths is a globally significant rock-forming process, yet little is known about the mechanism of formation of coccoliths (for a review see Piennar, in Winter & Siesser 1994, pp. 13-39). Coccolithophores grown in laboratory cultures produce coccoliths of calcite with small amounts of aragonite and vaterite, however fossil coccoliths are exclusively composed of low magnesium calcite. The Golgi body, reticular body and nucleus are all instrumental for the formation of the coccoliths and it appears that not all groups produce coccoliths in the same way. The simplest method seems to be the secretion of scales and coccoliths in the Golgi body followed by extrusion to the cell surface. In Coccolithus pelagicus scales are first produced in the Golgi body, are extruded and then form the nucleation sites for the later development of the coccolith between the cell membrane and an organic pedicle that develops around the cell. Emiliania huxleyi (Fig. 14.3a) produces coccoliths in a vesicle adjacent to the nucleus and reticular body by the precipitation of calcite, controlled by an organic matrix. The base of the coccolith is precipitated first followed by the upward and lateral development of the shields. On completion the coccoliths are extruded to form the interlocking external skeleton (Westbroek et al. 1984).

It is thought that coccoliths are formed for a number of reasons including protection from intense sunlight, to concentrate light, to provide a site for the disposal of toxic calcium ions or as supporting armour which stabilizes and acts as ballast for the cell.

Some species of coccolithophores are known to be dimorphic, for example Scyphosphaera apsteinii (Fig. 14.3d) and Pontosphaerea japonica are known to occur on the same coccosphere as do Helicosphaera carteri and H. wallichi. Some living coccolithophores (e.g. Scyphosphaera, Fig. 14.3d) produce two layers of morphologically distinct coccoliths (dithecism). Pleomorphism can also occur with hetero- and holo-coccolith bearing coccospheres being produced in different phases of the life cycle of a single species. All these phenomena have led to different fossil coccoliths being placed in separate form species when they should have been described as a single species, and as a consequence estimates of coccolith diversity though time may have been grossly overestimated.

Ecology of coccolithophores

Coccolithophores are predominantly autotrophic nannoplankton (i.e. 5-60 |m in size), utilizing the energy from sunlight for photosynthesis. Living cells are therefore largely restricted to the photic zone of the water column (0-200 m depth) with the lighter, smaller cells living near the surface and heavier cells living lower down. As such the distribution of cocco-lith species is under the direct control of climate. They thrive in zones of oceanic upwelling or of pronounced vertical mixing, as it is here that vital trace minerals are most readily available.

Although a few species are adapted either to fresh or brackish waters, the majority of species are marine. Nannofloras do not typically show nearshore-offshore differentiation though members of the Braarudo-sphaeraceae (Box 14.1) are found exclusively in inshore waters. Marked seasonal variation occurs in the abundance of some species including E. huxleyi, though in most cases the rhythmic millimetre scale laminations in deep sea sediments accumulate over thousands of years and do not reflect annual cycles. The relative abundance of complete coccoliths to

Box 14.1 Family level classification of coccoliths with diagrams of typical terms (sketches from photomicrographs in Siesser in Lipps 1990 and Perch-Nielsen in Bolli et al. 1985)

Kingdom CHROMISTA Infrakingdom CHROMOBIOTA Phylum HAPTOPHYTA Class PATELLIFERA Order COCCOSPHAERALES

Ahmuellerellaceae (Reinhardt 1965). Elliptical coccoliths with a wall of inclined crystal elements and a central area spanned by a cross, aligned with the axis of the ellipse. Trias./E. Jur.-L. Cret/Palaeog.

Ahumuellerella

Arkhangelskiellaceae (Bukry 1969). Elliptical coccoliths with a complex rim consisting of three to five elements. L. Jur.-L. Cret.

Arkhangelskiella

Arkhangelskiella

Biscutaceae (Black 1971). Circular to elliptical coccoliths consisting of two closely appressed shields composed of petal-shaped elements. E. Jur.-Palaeog.

Biscutum

Biscutum

Braarudosphaeraceae (Deflandre 1947). Pentagon-shaped coccoliths. E. Cret.-Rec.

Braarudosphaera

Calciosoleniaceae (Kamptner 1927). Rhomboidal coccoliths with calcite laths extending inwards from the walls. E. Cret.-Rec.

Anaplosolenia

Anaplosolenia

Calyculaceae (Noël 1973). Elliptical to subcircular coccoliths with a central area covered in a grid; cup-like in side view. E.-L. Jur.

Calyculus

Calyculus

Calyptrosphaeraceae (Boudreaux & Hay 1969). Holococcoliths with a highly variable morphology. L. Jur.-Rec.

Zygrhablithus

Zygrhablithus

Ceratolithaceae (Norris 1965). Horseshoe-shaped coccoliths. Neog.-Rec.

Ceratolithus

Ceratolithus

Chiastozygaceae (Rood et al. 1973). Elliptical coccoliths with an X- or H-shaped central structure. Trias./Jur.-Palaeog.

Chiastozygus

Chiastozygus

Coccolithaceae (Poche 1913). Elliptical coccoliths with a distal shield of radiating, petal-shaped elements. Proximal shield usually birefringent between cross polars, distal shield is larger and not birefringent. L. Cret.-Rec.

Coronocyclus

Coronocyclus

Crepidolithaceae (Black 1971). Elliptical coccoliths consisting of a ring of elements lacking imbrication. A large distal process may be present. Palaeog.-Neog.

Conusphaera

Discoasteraceae (Tan 1927). Star-or rose-shaped nannofossils. Palaeog.-Neog.

Discoaster

Discoaster

Eiffellithaceae (Reinhardt 1965). Elliptical coccoliths, distal shield with slightly overlapping elements, proximal shield with radially arranged elements. E. Jur.

Eiffellithaceae (Reinhardt 1965). Elliptical coccoliths, distal shield with slightly overlapping elements, proximal shield with radially arranged elements. E. Jur.

Eiffelithus

Eiffelithus

Fasciculithaceae (Hay & Mohler 1967). Cylindrical nannoliths with a promial column and a distal disc or cone. Palaeog.

Fasciculithus

Fasciculithus

Goniolithaceae (Deflandre 1957). Pentagon-shaped coccoliths with a wall composed of vertical elements enclosing a granular centre. L. Cret.-Palaeog.

Goniolithus

Goniolithus

Helicosphaeraceae (Black 1971). Spiral-walled coccoliths, usually with a flange. Central area open, spanned by a bridge or rarely closed. Palaeog.-Rec.

Helicosphaera

Helicosphaera

Heliolithaceae (Hay & Mohler 1967). Cylindrical nannofossils with a short proximal column and one or two distal cycles of elements. Palaeog.-Rec.

Heliolithus

Heliolithus

Lithostromationaceae (Deflandre 1959). Triangular, hexagonal or nearly circular nannofossils covered in symmetrical arranged depressions. Palaeog.-Neog.

Lithostromation

Microrhabdulaceae (Deflandre 1963). Cylindrical, rod- or spindle-shaped nannofossils. L. Jur.-L. Cret.

Lithoraphidites

Lithoraphidites

Nannoconaceae (Deflandre 1959). Conical nannofossils with a thick wall of wedge-shaped elements perpendicular to and spirally surrounding an axial canal. L. Jur.-L. Cret.

Nannoconus

Nannoconus

Podorhabdaceae (Noel 1965). Elliptical coccoliths with a rim consisting of two to three cycles of elements. The wide central area spanned by a variety of structures. E. Jur.-L. Cret.

Cretarhabdus

Cretarhabdus

Polycyclolithaceae (Forchheimer 1972). Cylinder-, block-, star- or rosette-shaped nannofossils. E.-L. Cret.-Palaeog.

Eprolithus

Eprolithus

Pontosphaeraceae (Lemmermann 1908). Coccoliths with a raised wall, of varying height, consisting of two cycles of elements and a large central area. Palaeog.-Rec.

Pontosphaera

Pontosphaera

Prediscosphaeraceae (Rood et al. 1971). Circular or elliptical coccoliths, almost always with 16 elements in each of two shields. E.-L. Cret.

Prediscosphaera

Prinsiaceae (Hay & Mohler 1967). Circular to elliptical coccoliths, distal shield is birefringent between crossed polars. L. Cret.-Rec.

Gphyrocapsa

Gphyrocapsa

Rhabdosphaeraceae (Lemmermann 1908). Nannofossils with a base consisting of a varying number of cycles of elements. A central process rises from the base. Palaeog.-Rec.

Rhabdosphaeara Rhagodiscaceae (Hay 1977). Elliptical coccoliths with a wall composed of inclined elements with a granular central area. L. Jur.-L. Cret.

Rhagodiscus

Rhagodiscus

Schizosphaerellaceae (Deflandre 1959). Nannofossils consisting of two overlapping hemispheres. Trias.-L. Jur.

Schizopharella

Schizopharella

Sollasitaceae (Black 1971). Elliptical coccoliths with two shields and a large central opening occupied by a grid or bars, lacking a central process. E. Jur.-Palaeog.

Sollasites

Sphenolithaceae (Deflandre 1952). Nannoliths with a proximal shield or column above which are disposed tiers of radiating lateral elements. Palaeog.-Neog.

Sphenolithus

Stephanolithiaceae (Black 1968). Circular, elliptical or polygonal coccoliths. The outer wall has vertically arranged elements and may bear lateral spines. E. Jur.-L. Cret.

Stephanolithus

Syracospaeraceae (Lemmermann 1908). Coccoliths with a complex wall and a central area partially closed by laths. Neog.-Rec.

Syracosphaera

Thoracosphaeraceae (Schiller 1930). Spherical or ovoid nannofossils composed of interlocking polygonal elements. L. Jur.-Rec.

Tharacosphaera

(g)

Triquetrorhabdulaceae (Lipps 1969). Spindle-shaped rods constructed of three blades. Triradial cross-section. Palaeog.-Neog.

Triquetrorhabdulus

Zygodiscaceae (Hay & Mohler). Coccoliths with one or two cycles of inclined elements in the wall and a bridge aligned with the short axis of the ellipse. E. Jur.-Palaeog.

Glaucolithus

broken coccoliths and coccolith flour changes with depth (Fig. 14.4).

In the Atlantic Ocean nannofloral provinces are delimited by temperature (Fig. 14.5) with different assemblages indicating subglacial, temperate, trans itional, subtropical and tropical latitudes. It is in tropical areas where they are most abundant and their numbers may reach as many as 100,000 cells per litre of sea water. A similar latitudinal differentiation occurs in the Pacific Ocean but the greatest diversity occurs at

Relative units of CaCO suspended matter

1000

Relative units of CaCO suspended matter

1000

2000-

3000 J

Fig. 14.4 Vertical distribution of coccoliths and coccolith-derived carbonates in the Pacific Ocean. (After Lisitzin in Funnell & Riedel 1971, figure 11.4.)

2000-

3000 J

Fig. 14.4 Vertical distribution of coccoliths and coccolith-derived carbonates in the Pacific Ocean. (After Lisitzin in Funnell & Riedel 1971, figure 11.4.)

50°N. Depth stratification also occurs in the Pacific Ocean (see Honjo & Okada 1974; Honjo in Ramsay 1977, pp. 951-972). Of the 10 species cultured by Mclntyre et al. (1970) E. huxleyi had the broadest temperature tolerance (1-31°C) and tropical species (e.g. Discosphaera, Fig. 14.3b) the narrowest range (20-30°C). There also appears to be a narrowing of temperature tolerance in species living offshore.

Production of coccoliths is strongly but not completely controlled by light. Whilst E. huxleyi increases abundance with increasing nutrients (in culture and in the oceans), most subtropical, oceanic species do not (Brand, in Winter & Siesser 1994, pp. 39-51).

Coccoliths and sedimentology

After death coccolithophores sink through the water column at about 0.15 m per day and the coccoliths fall away. With increasing depth these scales tend to dissolve or disaggregate into finely dispersed carbonate matter (Fig. 14.4), this process operating first on holococcoliths or delicate heterococcoliths. Therefore coccolith assemblages from sediments deeper than 1000 m are not truly representative of the original nannoflora. At depths of over 3000-4000 m, few coccoliths remain as most of the CaCO3 has gone into solution, at these depths coccolith oozes are replaced by the less-soluble diatom or radiolarian oozes, or by red clays. Many factors may cause this dissolution, including high hydrostatic pressures, high CO2, low O2, low pH, low temperatures, low CaCO3 precipitation by organisms, or sluggish recycling of CaCO3 from the land. Honjo (1976) and Philskaln & Honjo (1987) showed, however, that coccoliths (and even whole coccospheres) can reach ocean depths intact by settling rapidly within the faecal pellets of copepod crustaceans. The proportion of coccolithic material in Recent oceanic carbonates is greatest in subtropical and tropical regions underlying waters with high organic productivity. Here they may average 26% by weight of the sediment (Fig. 14.5). Coccoliths are likewise an important constituent of Cretaceous and Tertiary chalks. They are fewest in sediments from subglacial waters (about 1%) where both productivity and preservation conditions are unfavourable.

Unfortunately, there is a tendency for calcite overgrowths or recrystallization to occur in coccoliths, obscuring their morphology. Solution of elements critical to the identification of fossil coccoliths may also present problems. Yet another disadvantage to the biostratigrapher is the ease with which coccoliths are reworked into younger sediments without showing outward signs of wear. The role of coccolithophores in sedimentation is reviewed by Honjo (1976) and Steinmetz (in Winter & Siesser 1994, pp. 179-199).

Classification

Kingdom CHROMISTA Infrakingdom CHROMOBIOTA Phylum HAPTOPHYTA Class PATELLIFERA

Calcareous Nannoplankton

Fig. 14.5 Coccolith concentrations in near-surface sediments of the Atlantic Ocean plotted as percentages. Superimposed are major surface currents and calcareous nannoplankton provinces. Black dots are Deep Sea Drilling Project locations. Roman numerals in the figure correlate with the assemblages that follow. I - Tropical: Umbellosphaera irregularis, Calcidiscus annulus, Oolithotus fragilis, Umbellosphaera tenuis, Discosphaera tubifer, Rhabdosphaera stylifer, Helicosphaera carteri, Gephyrocapsa oceanica, Emiliania huxleyi, Calcidiscus leptoporus. II - Subtropical: Umbellosphaera tenuis, Rhabdosphaera stylifer, Discosphaera tubifer, Calcidiscus annulus, Gephyrocapsa oceanica, Umbilicosphaera sibogae, Helicosphaera carteri, Calcidiscus leptoporus, Oolithotus fragilis. III - Transitional: Emiliania huxleyi, Calcidiscus leptoporus, Gephyrocapsa ericsonii, Rhabdosphaera stylifer, Gephyrocapsa oceanica, Umbellosphaera tenuis, Coccolithus pelagicus. IV - Subarctic: Coccolithus pelagicus, Emiliania huxleyi, Calcidiscus leptoporus. V - Subantarctic: Emiliania huxleyi, Calcidiscus leptoporus. (After McIntyre & McIntyre in Funnell and Reidel 1971.)

Neither botanists nor palaeontologists have agreed on how to classify the coccolithophores and their relatives. Cavalier-Smith (1993) proposed they be placed in the kingdom Chromista; based upon the nature and location of the chloroplast and 18sRNA phylogenetic studies. He regarded them as belonging to the phylum Haptophyta because they are unicellular, golden-brown algae with two equal flagella and a coat of scales. Traditional micropalaeontological classification schemes retain the coccolith-bearers in the division Chrysophyta, class Coccolithophyceae. Beyond this recent schemes are based on the ultrastructure of coccoliths and their arrangement about the cell, little of which can be seen without the aid of an electron microscope.

Box 14.1 outlines the familial level classification and shows illustrations of eponymous taxa. The following genera exemplify some of the main types of heterococ-colith. Cyclococcolithina (Olig.-Rec., Fig. 14.2b) has a disc comprising two circular or elliptical rings (termed proximal and distal shields) built of overlapping radial plates arranged around a central, tubular pillar. Such

Fig. 14.6 Species diversity of described coccoliths through time. (Based on Tappan & Loeblich 1973.)

Fig. 14.6 Species diversity of described coccoliths through time. (Based on Tappan & Loeblich 1973.)

Tappan Shield

an arrangement, with two shields connected by a central tube, is called a placolith. In Pseudoemiliania (U. Plioc.-L. Pleist., Fig. 14.2c), the radial plates of the two shields do not overlap and are arranged around a central space. The radial plates of Helicopontosphaera (Eoc.-Rec., Fig. 14.2e) are distinctively arranged into a single elliptical central shield surrounded by a spiral flange, also of radial elements. The coccolith of Zygodiscus (U. Cret.-Eoc., Fig. 14.2f) comprises an elliptical ring built of steeply inclined and overlapping staves spanned by a cross bar. An open ring built of 16 quadrangular grains spanned by cross bars is characteristic of Prediscosphaera (M.-U. Cret., Fig. 14.2g). This genus contributed greatly to the deposition of the Cretaceous chalk. Braarudosphaera (Cret.-Rec., Figs 14.2h, 14.3c) has five plates arranged with pentaradial symmetry. The solid spine of Rhabdosphaera (Plioc.-Rec., Fig. 14.2i) arises from a basal disc of fine and complex construction. Such rhabdoliths probably serve to reduce sinking of the cell below the photic zone. Simpler in plan are the stellate coccoliths of the discoasters. Discoaster (U. Mioc.-Plioc., Fig. 14.2j) had a star-like disc up to 35 |im in diameter, built from 4-30 radiating arms of variable shape. The upper and lower surfaces also differ slightly in appearance. Discoasters are mostly found in fossil deep sea carbonates, especially from warmer latitudes, and play an important role in Cenozoic biostratigraphy.

General history of coccolithophores

Being both a primary source of food in the oceans and a significant producer of atmospheric oxygen, the history of coccolithophores has a bearing on the overall history of life (see Tappan & Loeblich 1973; Tappan 1980). Palaeozoic records are few and dubious. The first generally accepted fossil coccoliths are rare and reported from upper Triassic rocks. Their diversification in the Early Jurassic was a remarkable event that parallels the radiation of the peridinialean dinoflagellate cysts and both may be related to oceano-graphic changes connected with the opening of the Atlantic Ocean at this time. Their numbers and taxonomic diversity increased steadily until the Late Cretaceous period when there was a major marine transgression and a further, explosive radiation of many planktonic groups (Fig. 14.6). These conditions led to the deposition of chalk over vast areas of the continental platforms. The vast majority of coccoli-thophores became extinct at the K-T boundary, many of their habitats being filled by the diatoms during the Early Cenozoic. Coccolithophores have since regained their dominance in tropical and temperate waters but are significantly less diverse than in the Mesozoic.

There was another resurgence of forms in the Eocene, including the discoasters, many of them rosette-shaped with numerous rays. The latter died out at the end of the Eocene after which time there was a general dwindling in the diversity of coccoliths and discoasters, leading to the extinction of the dis-coasters at the end of the Pliocene. This may have been due to climatic cooling and regression. Certain of the placolith-bearing coccolithophores, however, thrived in the cooler waters of the Quaternary Era.

Applications of coccoliths

The biostratigraphical value of coccoliths and discoasters is unrivalled in the Mesozoic and Cenozoic and they have become the standard biostratigraphical index fossils for the Cenozoic. Mesozoic and Cenozoic biostratigraphical zonations are summarized in Bown (1998) and Perch-Nielsen (in Bolli et al. 1985, pp. 329-554). Examples of coccolith and discoaster evolution are given by Prins (in Bronnimann & Renz 1969, vol. 2, pp. 547-559), Gartner (1970), Bukry (1971) and Siesser (in Lipps 1993, pp. 169-203).

The increasingly large database relating coccolith assemblages to modern day water masses and latitudinal provinciality means coccoliths are extremely important in oceanographical studies. The distribution of coccolithophores has changed significantly over time. In the Cretaceous they were cosmopolitan (Tappan 1980) and abundant in both coastal and oceanic waters and from the poles to the tropics. Now the highest diversity is found in the subtropical gyres or in areas of nutrient-rich upwelling. Most species live in stratified water and the degree of stratification affects abundance (Winter 1985; Verbeek 1989; Brand 1994, in Winter & Siesser 1994, pp. 39-51; Roth 1994, in Winter & Siesser 1994, pp. 199-219).

During the last glacial maximum (c. 18,000 BP) North Atlantic water masses and their constituent nan-nofloras shifted 15 degrees southwards of their present location. Vertical changes in nannofloras in sediment cores from cool- to warm-water assemblages reflect the glacial-interglacial cycling of the Pleistocene climate (Fig. 14.7). Similar whole-scale shifts in nannofloras have also been documented from the Miocene though the direct climatic implications are poorly understood

Fig. 14.7 North Atlantic Polar Front migrations during the last 225,000 years. (After McIntyre et al. 1972.)

kyr BP 78 N

25 50 75 100 125 150 175 200 225

[ I Polar I I Subpolar I I Transitional

Subtropical

Fig. 14.7 North Atlantic Polar Front migrations during the last 225,000 years. (After McIntyre et al. 1972.)

(Haq 1980). Haq & Lohmann (1977) have plotted the apparent migrations of 'warm' and 'cold' coccolith assemblages through the Cenozoic and estimated from this the changes in palaeotemperature.

Coccolith morphology is also known to vary with temperature. The cold-water variety of E. huxleyi has a solid proximal shield whereas in warm water this shield is open and the rim is composed of many more elements. The ratio between coccoliths of warm and cool water type (e.g. Discoaster, Chiasmolithus) is a useful tool for indicating the changing palaeotemperat-ure through Late Cenozoic time (see Bukry 1973, 1975) but becomes decreasingly reliable for more remote periods. Worsley (1973) discussed similar palaeoclimatic aspects and the determination of depo-sitional depth in coccolith-bearing sediments.

The analysis of stable isotopes from calcareous nannoplankton is hampered by their small size and problems caused by diagenetic overgrowths; typically bulk sediment samples are analysed. Anderson & Arthur (1983) and Steinmetz (in Winter & Siesser 1994, pp. 219-231) have reviewed the difficulties and provide case examples. In general stable oxygen isotope values in the CaCO3-living coccolithophores reflects the influence of temperature and vital effects. Experiments in culture have shown that many species do not grow in chemical equilibrium with the sea water. Despite these problems there is a strong correlation between the S18O values from planktonic foraminifera and coccolithophores through the Pleistocene (Fig. 14.8). The progressive enrichment in S18O values from benthic to planktonic forams to coccolithophores probably reflects their depth of growth. Margolis et al. (1975) noted the S13C profile from coccolithophores paralleled curves derived from benthic and planktonic forams. Data from Cretaceous and Cenozoic DSDP cores indicate S13C values from coccolithophores are a better indicator of surface water chemistry and reflect surface productivity (Kroopnick et al. 1977).

Further reading

Good general introductions to all aspects of calcareous nannoplankton can be found in Siesser (in Lipps 1993, s"o

-400

-800

-1000

Fig. 14.8 Oxygen isotopic analyses on the Pleistocene Caribbean core P6304-4. A, Globigerinoides sacculcifer, B, coccolith size fraction (3-25 |m; data from Steinmetz & Anderson 1984). Shaded areas are glacials. (Based on Steinmetz in Winter & Siesser 1994, figure 3.)

pp. 169-203) and Haq (1983) and coccolithophores in Winter & Siesser (1994). Further information on collection, examination and identification to generic level can be found in Hay (in Ramsay 1977, pp. 1055-1200). Identification of genera and species may also be assisted by reference to Farinacci (1969 to date). Some aspects of their classification, ecology, distribution and

Coccolithophores Distribution

Fig. 14.8 Oxygen isotopic analyses on the Pleistocene Caribbean core P6304-4. A, Globigerinoides sacculcifer, B, coccolith size fraction (3-25 |m; data from Steinmetz & Anderson 1984). Shaded areas are glacials. (Based on Steinmetz in Winter & Siesser 1994, figure 3.)

evolution are brought together in a chapter by Haq in Haq & Boersma (1998). A comprehensive biostrati-graphical treatment of the Mesozoic and Cenozoic in Britain can be found in Bown (1998). Perch-Nielsen (in Bolli et al. 1985, pp. 329-554) provides a taxo-nomic and biostratigraphical synthesis of Cenozoic nannofossils and can be used for identification.

Hints for collection and study

Fossil coccoliths are abundant in Mesozoic and Cenozoic chalks and marls and are not uncommon in fossiliferous shales and mudstones. To extract them for study is relatively simple. Pulverize about 5-50 g of fresh sample (as in method A, see Appendix) and pour the liquid into a glass container to a depth of about 20 mm. After vigorous shaking allow the liquid to separate for about 2 minutes and then pipette some of the supernatant liquid on to a glass slide. For a temporary mount, add a cover slip and examine the slide at 800X magnification (or higher) with highly condensed transmitted light under a petrographic microscope. The light should be polarized with crossed nicols so that rotation of the stage (or the slide) brings out the position of the small wheel-like coccoliths with black cross optical figures. Permanent mounts can be prepared from strews dried on glass slides: add a drop of Caedax or Canada Balsam to the cover slip and place this over the strew mount.

REFERENCES

Anderson, T.F. & Arthur, M.A. 1983. Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems. In: Arthur, M.A., Anderson, T.F., Veizer, J. & Land, L.S. (eds) Stable Isotopes in Sedimentary Geology, SEPM Short Course No. 10, pp. 1-151.

Bolli, H.M., Saunders, J.B. & Perch-Nielsen, K. 1985. Plankton Stratigraphy. Cambridge University Press, Cambridge.

Bown, P.R. (ed.) 1998. Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society, Kluwer Academic Publishers, Dordecht.

Bronnimann, P. & Renz, H.H. (eds) 1969. Proceedings of the First International Conference on Planktonic Micro-fossils, Geneva 1967, vols 1, vol. 2. E.J. Brill, Leiden.

Bukry, D. 1971. Discoaster evolutionary trends. Micropalae-ontology 17, 43-52.

Bukry, D. 1973. Coccolith and silicoflagellate. stratigraphy, Tasman Sea and southwestern Pacific Ocean. 21, 885-891.

Bukry, D. 1975. Coccolith and silicoflagellate stratigraphy, northwestern Pacific Ocean, DSDP Leg 32. 32, 677-701.

Cavalier-Smith, T. 1993. Kingdom Protoza and its 18 phyla. Microbiological Review 57, 953-994.

Farinacci, A. 1969 to date. Catalogue of Calcareous Nannofossils. Edizioni Tecnoscienza, Rome.

Funnel, B.M. & Riedel, W.R. (eds) 1971. The Micro-palaeontology of Oceans. Cambridge University Press, Cambridge.

Gartner Jr, S. 1970. Phylogenetic lineages in the lower Tertiary coccolith genus Chiasmolithus. Proceedings. National American Paleontological Convention 1969, Part G, 930-957.

Haq, B.U. 1980. Biogeographic history of Miocene calcareous nannoplankton and paleoceanography of the Atlantic Ocean. Micropalaeontology 26, 414-443.

Haq, B.U. (ed.) 1983. Calcareous nannoplankton. Benchmark Papers in Geology 78, 338.

Haq, B.U. & Boersma, A. (eds) 1998. Introduction to Marine Micropaleontology. Elsevier, Amsterdam.

Haq, B.U. & Lohmann, G.P. 1977. Calcareous nannoplank-ton biogeography and its paleoclimatic implications. Cenozoic of the Falkland Plateau (DSDP Leg 36) and Miocene of the Atlantic Ocean. 36, 745-759.

Honjo, S. 1976. Coccoliths: production, transportation and sedimentation. Marine Micropalaeontology 1, 65-79.

Honjo, S. & Okada, H. 1974. Community structure of coc-colithophores in the photic layer of the Mid Pacific. Micropaleontology 20, 209-230.

Kroopnick, P.M., Margolis, S.V. & Wong, C.S. 1977. 13C variations in marine carbonate sediments as indicators of the CO2 balance between the atmosphere and the oceans. In: Andersen, N.R. & Malahouf, A. (eds) The Fate of Fossil Fuel CO2 in the Ocean. Plenum Press, New York, pp. 295321.

Lipps, J. (ed.) 1993. Fossil Prokaryotes and Protists. Blackwell Scientific Publications, Oxford.

McIntyre, A. & Bé, A.W.H. & Roche, M.B. 1970. Modern Pacific coccolithophorida: a paleontological thermometer. Transactions of the New York Academy of Science 32, 720-731.

McIntyre, A., Ruddiman, W.F. & Jantzen, R. 1972. Southward penetrations of the North Atlantic Polar Front: faunal and floral evidence for large-scale surface water mass movements over the last 225,000 years. Deep sea Research 19, 61-77.

Margolis, S.V., Kroopnick, P.M., Goodney, D.E., Dudley, W.C. & Mahoney, M.E. 1975. Oxygen and carbon isotopes from calcareous nannofossils as paleoceanographic indicators. Science 189, 555-557.

Philskaln, C.H. & Honjo, S. 1987. The fecal pellet fraction of biogeochemical particle fluxes to the deep sea. Global Biogeochemical Cycles 1, 31-48.

Ramsay, A.T.S. (ed.) 1977. Oceanic Micropalaeontology, 2 vols. Academic Press, London.

Steinmetz, J.C. & Anderson, T.F. 1984. The significance of isotopic and palaeontologic results on Quaternary calcareous nannofossil assemblages from Caribbean core P6304-4. Marine Micropalaeontology 8, 403-424.

Tappan, H. 1980. The Paleobiology of Plant Protists. W.H. Freeman, New York.

Tappan, H. & Loeblich Jr, A.R. 1973. Evolution of the ocean plankton. Earth Science Reviews 9, 207-240.

Trejo, M.H. 1960. La Familia Nannoconidae y su alcance estratigrafico en America (Protozoa, Incertae saedis). Boletin. Asociatio'n Mexicana de Géologos Petroleros XII, 259-314.

Verbeek, J.W. 1989. Recent calcareous nannoplankton in the southernmost Atlantic. Polarforschung 59, 45-60.

Westbroek, P., De Jong, E.W., Van Der Wal, P., Borman, A.H., De Vrind, J.P.M., Kok, D., De Bruijn, W.C. & Parker, S.B. 1984. Mechanism of calcification in the marine alga Emiliania huxleyi. In: Miller, A., Phillips, D. & Williams, R.J.P. (eds) Mineral Phase in Biology. Royal Society, London, pp. 25-34.

Winter, A. 1985. Distribution of living coccolithophores in the California Current System, southern California Borderland. Marine Micropalaeontology 9, 385-393.

Winter, A. & Siesser, W.G. (eds) 1994. Coccolithophores. Cambridge University Press, Cambridge.

Worsley, T.R. 1973. Calcareous nannofossils: Leg 19 of Deep Sea Drilling Project. 19, 741-750.

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Responses

  • dalila lombardo
    How does calcareous nannoplankton contribute to fine sediment?
    8 years ago
  • CORA BOLGER
    When did calcareous nannofossils become extinct?
    6 years ago
  • eglantine
    Are coccolithophores the same as calcareous nannoplankton?
    2 years ago

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