Undamaged sedimented coccolithophorids in a deep environment (continental slope of the Gulf of Lions)

Marine

Geology

123 (1995) 239-252

Undamaged sedimented coccolithophorids in a deep environment (continental slope of the Gulf of Lions) Catherine Riaux-Gobin

a, Marie-Joskphe ChrCtiennot-Dinet

a, Chantal Descolas-Gros b

a CNRS, Laboratoire Arago, 66650 Banyuls-sur-mer, France b CNRS, Laboratoire d’Hydrobiologie, UniversitCMontpellier II, 34095 Montpellier ckdex, France Received 24 February 1994; revision accepted 13 December 1994

Abstract At different periods of the year, undisturbed sediment samples from the continental slope of the Gulf of Lions were collected with a multiple gravity corer. The overlying water (O-20 cm above the sediment) and the surface water masses were also collected. The following stations were sampled: Rq, an interfluve (a platform between two canyons), 760 m deep; Rg, at the junction of two canyon floors, 1540 m and R6, at the top of the wall, 1280 m. At all stations, the top sediment layer and the overlying water were enriched with coccoliths. During a late spring cruise (March 22-April 5, 1991) a thin microphyte-enriched layer at the water-sediment interface of station R5 contained encysted diatoms, dinophytes and coccolithophorids. Several of the coccolithophorid species were in good preservation, among which Emiliania huxleyi was dominant and showed a considerable range of morphological variations in coccolith length and degree of calcification. The presence of tintinnids that use coccoliths to build their loricae reflects the species composition in the water masses from which this biodeposit originated. Homogeneous hydrological conditions favoured the accumulation of this deep biodeposit.

1. Introduction

Understanding the process by which phytoplankton is deposited to the deep-sea floor will help to reconstruct palaeoenvironments (hydroclimatology, continental water discharge, sedimentation rates, currents and their role in dispersion of particles) and may improve data on the vertical fluxes, pelagic cycle and the evaluation of recent organic carbon trapped and buried in these deep sediments. Some pelagic microphytes are more suitable than others as biological tracers in sediments. Organic thecate dinocysts from deep sediments are being intensively studied by geologists as paleoenvironmental indicators partly because the methods required are similar to those used by 0025-3227/95/$9.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0025-3227(94)00126-X

palynologists. Calcareous microphytes in recent sediments (mainly coccolithophorids, calcareous dinophytes and dinocysts) have been less studied due to the difficulties in isolating them and quantifying their abundance. Coccolithophorids are particularly interesting because of their environmental and biostratigraphic importance and the apparent fragility of their ornamentation. The coccosphere preservation may reflect the recent history of the cell. For example, Emiliania huxleyi (Lohmann) Hay and Mohler in culture reaches its maximum cell plating during the logarithmic growth phase and the shedding of coccoliths increases in the early stage of the stationary phase (Balch et al., 1992). According to Honjo (1976) and Cadee (1985), isolated coccospheres probably never reach the sea

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bottom, but are recycled in the euphotic zone (i.e. pelagic loop). Nevertheless, grazing may aid the degraded cells and dissociated coccoliths to reach the bottom (Honjo, 1976). More or less degraded cells have been observed embedded in faecal pellets (Honjo, 1976; Honjo and Roman, 1978). Moreover, if the conditions become abruptly unfavourable (low nutrients, turbulence), formation of phytoplanktonic floes may occur, followed by a rapid sinking (up to 300 m day- ‘; Cushing, 1992) such a process is described as a “seeding strategy” by Smetacek (1985). In these specific conditions, some microphytes, such as dinophytes and diatoms, encyst themselves as a survival strategy, while some coccolithophorid species (i.e. E. huxZeyi; Cadee, 1985) are able to develop another strategy: They form macroaggregates, preserving the structure of the cell, and rapidly sinking (100 m day-‘, Smayda, 1971). These well preserved cells rapidly reach the sea bottom, then they become part of the benthic cycle, either to become buried (carbon sink), or to return to the photic zone after being resuspended. Except in sediment traps (Samtleben and Bickert, 1990), the presence of such well preserved, or still living encysted cells in the deep environments is rarely reported. The deep sediments of western the Mediterranean continental slope and the canyons (locally named “rechs”) consist of clay with a large percentage of isolated coccoliths (20-50%, depending on the sampling site; Plate I, 1). The sedimentation rate in the sampling area is around 0.1 cm yr-’ (Zuo et al., 1991). The aim of the MEDIMAR program in the Gulf of Lions (1990-1992), was to study the carbon fluxes at the water-sediment interface on the continental slope. During a 1991 spring cruise an unusual layer, rich in sedimented microphytes was collected. The microphyte assemblage was dominated by calcareous dinophytes and living encysted diatoms which are usually rare. A preliminary and rough description of this phenomenon has been reported previously (Riaux-Gobin and Descolas-Gros, 1992) while the taxonomy of the encysted diatoms and dinophytes will be described elsewhere. This biodeposit was also enriched with various undamaged coccolithophorid genera. The present study lists and describes the coccolitophorid species, their

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degree of preservation, and the hydrological conditions that prevailed at the time of collection and what probably favoured such a fast sedimenting assemblage. Several tintinnid species using coccoliths to build their loricae were also present in the biodeposit and in the upper layer of the sediment. The wellpreserved loricae also show that they were recently deposited and their composition reflects the living assemblages from which these tintinnids originate.

2. Material and methods Various stations have been sampled during the (Fig. 1). Station R,, a platform located between two canyons (760 m: 42”26’.43N, 3”40’.88E), has been regularly sampled as a reference station (every two months from November 1990 to December 1991, and again in May 1992). Three additional stations were sampled during the MEDIMAR-2 oceanographic cruise (R.V. Suroit, March-April, 1991): (1) Station R5 at the junction MEDIMAR program

between two canyon floors, the Lacaze-Duthiers and Pruvot rechs (1540 m: 42”21’.91N, 3”55’.50E); (2) Station R, on the edge of the Pruvot rech wall (1280 m: 42”23’.58N, 3”55’.09E); and (3) Station R, (42”20’.94N, 3”58’.63E) which was sampled only for hydrological parameters. A CTD “SEA-BIRD” equipped with an in vivo fluorometer was used for reading water parameters. Chl a concentrations were measured from water samples by spectrofluorometry (Neveux and Lantoine, 1993). Sediments were collected with the 12-core “Wuttke” multiple gravity corer [described by Barnett et al. (1984) and produced by Fa. Wuttke, Hamburg]. This corer allowed the study of an undisturbed watersediment interface (whereas the use of a boxcorer usually disturbs the interface). Furthermore, during the MEDIMAR-2 cruise, observation by epifluorescence microscopy permitted some in vivo tests on board. In the laboratory on land, the sediment samples (the first few millimeters scraped and preserved in formalin on board) were dehydrated with alcohol, mounted in Hyrax medium,

Fig. 1. Sampling locations (Gulf of Lions): Stations R,, R,, R, and R,

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and observed with an Olympus BH2 microscope equipped with a Nomarsky differential interference contrast. For scanning electron microscopy (SEM), samples were filtered on Nuclepore filters (1 pm pore diameter), rinsed with distilled water, oven dried (6O”C), gold coated and observed with a Hitachi S 520 scanning electron microscope. One litre samples were collected at each station for phytoplankton counts from four depths: 5, 10, 35 and 50 m. They were fixed with formaldehyde taking care of the pH so as to avoid coccolith dissolution. After sedimentation of at least one week, the bottom 200 ml were used for light microscopy and/or scanning electron microscopy (SEM). Cell counts were made using sedimentation chambers (usually 50 ml) on an inverted microscope equipped with phase contrast at a magnification of x 400. SEM preparations were achieved by gentle filtration of 50 ml samples on Nuclepore membranes (0.8 pm), rinsed with distilled water to remove salts and coated with gold. Observations were made as previously mentioned.

Table 1 Coccolithophorid species from the Gulf of Lions sediment surface observed as loose coccohths, arranged in decreasing frequency order

3. Results

The following chapters mainly focus on this bioclastic greenish layer. In contrast, at stations R, and R6 (same cruise, same period; see Fig. l), this specific biodeposit was absent and undamaged cells and cysts were rarely encountered on the surface sediment. Furthermore, the tests of fluorescence, positive at R,, were negative at R, and poorly significant at Rg.

3.1. Sediment samples Solitary coccoliths from about 15 species were common in these deep muds, whatever the season, but undamaged cells were scarce. The coccoliths were more or less broken and showed varying degrees of dissolution (Plate I, 1). The mixed, dissociated plates may have diverse origins, including long-term transport by currents and sediment resuspension of sub-fossil species as well as recent vertical sedimentation, thus unlikely to give consistent information. Among these loose coccoliths (see Table l), the more common species were: Emiliania huxleyi (Lohmann) Hay and Mohler, Helicosphaera carteri ( Wallich) Kamptner, Calcidiscus Zeptoporus (Murray and Blackman) Loeblich and Tappan. Table 1 shows the high species diversity. Some of these species are illustrated in Plate I along with a general view of the bioclastic muds. During a 1991 spring cruise, a specific and spectacular biodeposit was sampled at R, (Fig. 1).

Emiliania huxleyi (Lohmann) Hay and Mohler Helicosphaera carteri (Wallich) Kamptner var. carteri Calcidiscus leptoporus (Murray and Blackman)

common common common

Loeblich and Tappan Gephyrocapsa oceanica Kamptner/G.

muellerue

occasional

Breheret Syracosphaera pulchra Lohmann Scyphosphaera apsteinii Lohmann var. apsteinii Rhabdosphaera clavigera Murray and Blackman (f. clavigera and f. sfyfi$ra see Kleijne, 1992) Coccolithus pefagicus ( Wallich) Schiller Umbilicosphaera sibogae ( Weber-van Bosse) Gaarder var. sibogae Anoplosolenia brasiliensis (Lohmann) Deflandre Calyptrolithina wettsteinii (Kamptner) Kleijne Coronosphaera mediterranea (Lohmalm) Gaarder Discosphaera tubifera (Murray and Blackman)

occasional occasional occasional occasional occasional rare rare rare rare

Ostenfeld Pontosphaera syracusana Lohmann Umbellosphaera tenuis (Kamptner) Paasche (type

rare rare

II in Kleijne, 1993)

3.2. Biodeposit at station R, Samples collected at station R5 ( l/04/1991 ; Fig. 1) contained an unusual brownish biodeposit (2-3 mm thick), floating above the surface of the sediment of each core of the sampler. This biodeposit was enriched with degraded oceanic phytoplankton composed of undamaged empty diatom frustules (large centric diatoms and delicate cells with large setae, still in their chain), encysted diatoms (for the most part still in their mothercell), dinophytes and coccolithophores. The assemblage (Riaux-Gobin and Descolas-Gros, 1992) was dominated by the calcareous dinophytes Thoracosphaera heimii (Lohmann) Kamptner and

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244

PLATE II

C. Riaux-Gobin et al.jMarine Geology 123 (1995) 239-252

Sphaerodinella cf. tuberosa Kamptner, and encysted diatoms mainly belonging to the genera Chaetoceros Ehrenberg and Ditylum Bailey. Observation with the fluorescence microscope showed that the encysted diatoms were still alive. However, it was not possible to identify and test the viability of the smaller cells (i.e. flagellates) because of their small cell size and the fact that they were embedded in fine particles. Faecal pellets were scarce in the R, biodeposit. Plate II ( 1) illustrates a faecal pellet densely packed with coccoliths belonging to various species. Plate II (2) illustrates some damaged coccospheres of Emiliania huxleyi, with only a few, more or less broken, coccoliths, that may also have derived from a faecal pellet. The majority of the coccospheres present in this biodeposit are intact (Plate II, 3-5) and do not show mechanical signs of grazing. However, these undamaged cells were almost always found as solitary cells and not in macroaggregates. Intact coccospheres were observed at station Rs, both in the biodeposit and the sediment (Table 2). It was not possible to concentrate them as was done for dinoflagellates (double filtration), nor to do a quantitative analysis, because of their small dimensions (E. huxleyi and Gephyrocapsa muellerae were often smaller than 5 pm in diameter), and because they were embedded in fine particles and debris. The relative abundances, quoted in Table 2, were estimated from SEM examination of 10 subsamples from the R, sediments, 10 sub-samples from the R, biodeposit and 5 sub-samples from the R5 overlying water.

3.3. Taxonomic notes Emiliania huxleyi Plates II (3-5) and III illustrate the undamaged coccolithophorid species encountered (listed in

245

Table 2). Among the coccolithophorids, Emiliania huxleyi was the dominant type and was present in different morphotypes. All degrees of coccolith calcification occurred in the samples, as observed by Young and Westbroek (1991), but poorly calcified undamaged cells (Plate II, 4-5) were present in very low percentages. A great range of variation existed in cell length and number of coccoliths. According to previous descriptions, the less calcified cells seem to be wider, with larger coccoliths. Cell diameters of heavily calcified cells (Plate II, 3) ranged from 5 to 6.3 pm (mean= 5.84) with coccolith lengths from 2.86 to 3.3 pm; they are close to type A of Young and Westbroek ( 1991). Cell diameters of poorly calcified cells ranged from 6 to 7 pm (3.6-3.9 pm coccolith diameter), which is smaller than type C but within the limit size range for type B. They are also close to the lower length limit of E. pujosae Verbeek (Verbeek, 1990; synomym of type B, Young and Westbroek, 1991). The tintinnid shells confirm the dominance of heavily calcified (type A) E. huxleyi coccoliths (Plate IV), corroborated by SEM observation of coccoliths in the water column. Other species

Other undamaged species that were encountered are listed in Table 2 and illustrated on Plate IV. Certain species were found as dissociated spheres, probably due to the fragility of the coccolith arrangement (Table 2, species marked with an asterisk). These cells may also have been disrupted during filtration or during rinsing with distilled water. The two forms of the species Rhabdosphaera clavigera (f. stylifera with thin process, and f. clavigera with a coarser process) occur together (Plate I, 7-8) which confirms the results of Kleijne ( 1992).

PLATE II l-5. Coccospheres and faecal aggregations at Station R,. 1. Faecal aggregation of coccoliths belonging to different species. 2-5. Coccospheres of Emiliania huxleyi. 2. Three coccospheres at different stages of calcification in the same sample (probably a faecal aggregation). 3. Heavily cacified coccosphere (type A in Young and Westbroek, 1991). 4-5. Low calcification of two coccospheres (type B in Young and Westbroek, 1991). Scale bars equal 20 pm ( 1) and 2 pm (2-5).

246

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Table 2 Coccolithophorid species from the Gulf of Lions (R, sampling station; March-April 1991 cruise) sediment surface and the overlying water arranged in decreasing frequency order. Only species that have been observed more or less undamaged are listed. (+ + + numerous, + + present, + rare; *coccospheres more or less dissociated) Sediment surface

Emiliania huxleyi (Lohman) Hay and Mohler Calcidiscus leptoporus (Murray and Blackman) Loeblich and Tappan Gephyrocapsa muellereae B&&et** *Helicosphaera carteri (Wallich) Kamptner Coccolithus pelagicus ( Wallich) Schiller *Rhabdosphaera clavigera Murray and Blackman *Syracosphaera pulchra Lohmann Gephyrocapsa ericsonii McIntyre and Be *Algirosphaera robusta (Lohmann) Norris *Syracosphaera exigua Okada and McIntyre

+++ ++ ++ ++ + + + t

Overlying water biodeposit

(O-10 cm)

+++ ++ ++ ++ + +

+t _ _ _ _

+ _ +

+ _

**(= G. margereli Breheret).

Coccoliths associated with tintinnids

One species of tintinnids (Plate IV, l-2) used Calcidiscus leptoporus to build the median part of its lorica (see also Delgado and Fortuno, 1991, plate XCIII, a,b), while another tintinnid used Helicosphaera carteri (Plate IV, 3). Coccoliths of Umbellosphaera tenuis (Kamptner) Paasche type II (Kleijne, 1993) were observed on the lorica of some tintinnids. This species was present in the sediment but never as undamaged cells (probably due to the fact that the coccoliths of this species are loosely connected). Coccoliths of some species such as Syracosphaera anthos (Lohmann) Janin, Gaarderia corolla (Lecal) Kleijne (Kleijne, 1993) (Plate IV, l-2) or Helicosphaera pavimentum

Okada and McIntyre (Plate IV, 4) were present on the lorica of tintinnids, but these species were never observed in the sediment nor in the biodeposit. 3.4. Comparison with the livingphytoplankton assemblage at station R5

Large diatoms dominated the surface waters ( 5- 10 m) while dinoflagellates were more abundant between 35 and 50 m. Coccolithophorids were present at all considered depths with a maximum of 7.7 x lo4 cells 1-l at 20 m. Emiliania huxleyi was the dominant species of this group with a relative percentage of 90% of the total coccolitho-

PLATE III 1-13. 1, 3. 2. 4. 5, 6. I. 8. 9. 10. 11. 12. 13. Scale

Coccospheres and coccoliths at Station R, Gephyrocapsa muellerae (1: intact coccosphere; 3: coccosphere with partly dissolved bridges resembling G. margereli). Coccosphere of Gephyrocapsa ericsonii. Dissociated coccosphere of Algirosphaera robusta. Coccosphere of Coccolithus pelagicus (5: Light microscopy; 6: SEM). Coccoliths of Syracosphaeru sp. (type C in Kleijne, 1993). Endothecal coccoliths of Syracosphaera sp. (type D in Kleijne, 1993). Endothecal coccoliths of Syracosphaeru pulchra. Coccosphere of Calcidiscus leptoporus. Coccosphere embedded in sediment. Coccosphere of Helicosphaera carteri. Dissociated rhabdoliths of Rhabdosphaera clavigera. bars equal 2 pm (1-3, 7-8) and 5 urn (4-6,9-13)

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248

PLATE IV

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C. Riaux-Gobin et aLlMarine Geology 123 (1995) 239-252

phorid count except at 20 m, where Syracosphaera spp. were also abundant. As the smaller cells (down to 3 pm) were also counted, the total cell number in the samples at station R, reached lo6 cells 1-l for the 20 and 50 m depths. SEM examination of the 5 m sample resulted in counts of E. huxleyi coccospheres at 4 x lo4 1-l and loose coccoliths at about lo6 l-i, indicating that Emiliania was abundant before the sampling date. Coccolithophorid assemblages found in this area and dominated by E. huxleyi are otherwise close to the A-2 assemblage defined by Kleijne (1993). Compared with the Mediterranean phytoplanktonic species quoted by Delgado and Fortuno ( 199 1), Umbellosphaera tenuis and Scyphosphaera apsteinii are absent from the sediment or only present as solitary coccoliths. Only the holococcolithophorid species Calyptrolithina wettsteini is represented, but it is known that holococcoliths rapidly dissolve in the water column and rarely appear in sediments (Tappan, 1980). Oolithotus fragilis (Lohmann) Okada and McIntyre, described by Bernard (1942), under the name Coccolithus fragilis as the most abundant species in Mediterranean waters, and probably illustrated by Delgado and Fortuno under the name Calcidiscus leptoporus (Delgado and Fortuno, 199 1, plate LXXIII, b,c), was not recorded in this sampling nor found in Kleijne’s samples ( 1993).

4. Discussion The coccolith assemblage encountered in the station R, biodeposit (Tables 1 and 2) is very

PLATE

249

similar to that described by Bartolini (1970, table 3) from recent sediments south of Spain (western part of the Alboran Sea, 1490 m deep). These first investigations which need further confirmation, may attest that, even if hydroclimatic parameters and sedimentological rates have changed over the late 14,000 years in the western part of the Mediterranean Sea, coccolithophorid species assemblages remained unchanged. However, as noted by Bartolini (1970), it seems to be difficult to relate coccolithophorid species to paleotemperatures, at least in this area. Before the MEDIMAR-2 cruise (spring 1991) Stations R,, Rs and R, were not documented for hydrological parameters, whereas Station R, had been regularly sampled (Lantoine, pers. commun.; present study). Furthermore, Jacques et al. ( 1969) and Jacques (1970; Station E) give information on a station somewhat more close to the coast in the same area. In these open sea stations, the thermocline takes place during late spring (end of May, June) and disappears in autumn (November). Two phytoplankton blooms generally take place: the first in early spring lasting for one month and a shorter one in late autumn. Chlorophyll a concentrations rarely reach 1.0 pg 1-i in these areas. Jacques (1970) noticed that, in April-May, chlorophyll a concentrations in surface waters may be higher in open sea stations than in coastal waters. During the sampling at Station R, in April 1991, the water column was hydrologically homogeneous (temperature: 12.9”-13.3”C; salinity: 38.3%; Fig. 2) while the phytoplankton biomass reached a maximum at 25 m depth (up to 2.7 pg 1-l Chl a) (C. Descolas-Gros, shipboard report and Fig. 2).

IV

Tintinnid loricae covered by coccoliths of different species (Station R,): Calcidiscus leptoporus (I), Helicosphaera carteri (Z), Emiliania huxleyi (3), Syracosphaera anthos (4), Gaarderia corolla (5), Gephyrocapsa sp. (6), Helicosphaera pavimentum (7) Umbellosphaera tenuis (8) and Rhabdosphaera clavigera (9). l-2. Loricae of a tintinnid, Dictyocystis sp. 1, covered by different types of coccoliths, with Calcidiscus leptoporus and Helicosphaera carteri on the median part of the lorica. Lorica of a tintinnid, Codonella sp., almost entirely covered by coccoliths of Helicosphaera carteri. 3. Detail of the lorica of a tintinnid, Dictyocystis sp. 2: assemblage of different types of coccoliths, with dominance of 4. Helicosphaera carreri. Scale bars equal 10 pm (l-3) and 5 pm (4). 1-4.

250

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Temperature

“C

Salinity 37.00

38.00

250 -

19

Chl al(g

Fluorescence

ir

.ocI

1

-

300

“IoO

I-’

1.oo 2.00 ~.~

Station R4

1

0

50 IOO-

I

g tso& 200 -

6

i

250 300

Station Rs

I

0 50-

Fig. 2. Hydro-biological conditions, during the and fluorescence (relative) and Chl. a pg 1-l).

-

MBDIMAR-2

cruise, at Stations R4, R5 and R, (CTD data: temperature “C, salinity %O

These biomasses were exceptionally dense. Possibly, the lack of a thermocline favoured fast sedimentation of floes originating from this intense phytoplankton bloom represented in the bio-

deposit with only a few species of diatoms and dinoflagellates (Riaux-Gobin and Descolas-Gros, 1992). This caused the sedimentation of the coccolithophores represented by a diverse assemblage.

C. Riaux-Gobin et al.IMarine Geology 123 (1995) 239-252

Support of this hypothesis comes from the fact that the majority of the species identified in the biodeposit where also found in the overlying water column. The lack of intense grazing, as suggested by the presence of very few faecal pellets, may also have favoured the good preservation of these microphytes. Another indication of the planktonic and superficial origin of this biodeposit is the 613C that reached -31%0 (I. Bentheleb, pers. commun.). The existence of this deep assemblage associated with these hydrobiological features demonstrates the importance, for this continental slope, of fast sedimentation of local blooms, as compared to the advective transport that is generally mentioned (Monaco et al., 1990). The Station R5 biodeposit assemblage may correspond to a restricted period of sedimentation, so that it is difficult to generalize over the whole year. Other species may be transported undamaged to the sediment surface at other periods of the year as well. It is interesting to note that during the other cruises (same year; unpubl. results) such an enriched layer has never been recorded.

251

sional mass sedimented material (Thiel et al., 1988-1989). The multiple corer, as opposed to sediment traps, provides the opportunity to collect and study a natural biodeposit without disturbances (no artificial or secondary aggregation or degradation of particles due to the sampler). This biodeposit reached the sediment surface, whereas particles collected in sediment traps may have never reached the sea-bottom. Furthermore, the multiple corer, if compared to box-corers, allows the sampling of small undisturbed surfaces, and permits immediate duplicates for spatial heterogeneity study. Intact coccospheres may reach the bottom only by way of faecal pellets, macro-aggregates which increase the settling rate (Cadee, 1985) or embedded in floes. Our observations show that undamaged fossil coccospheres originated in similar hydrobiological events. This phenomenon is probably not exceptional but limited in time and space. Moreover, the marked geographical heterogeneity observed in samples collected at the sedimentwater interface during spring 1991 may have its origin in the overlying phytoplankton patchiness.

5. Conclusion Acknowledgements The main point of this study is that under specific hydrographic (i.e. lack of thermocline) and biological conditions, such as intense phytoplankton bloom and reduced grazing, a part of the coccolithophorid assemblage can rapidly reach the deep bottom layers as entire cells. This organic carbon is often underestimated in flux evaluations. Moreover, sediments from the Mediterranean continental slope are often considered as containing low biological presence, whereas numerous macrofauna1 burrows, mainly from crustaceans related to the Thalanassidae (A. Dinet, pers. commun.), were observed during the March-April 1991 cruise at R,, the lower part of the canyon, and R6, the canyon wall. Monaco et al. ( 1990) also referred to the presence of such burrows but mostly on canyon floors. Such a dense macrofauna implies that an important carbon stock is available. The turnover of this organic material is certainly fast since these biodeposits are rarely encountered. An intense grazing may suppress this pulsed or occa-

Thanks are due to the helpful crews of the R.V. Suroit and Georges Petit, to P. Le Doz (multiple corer assistance; IFREMER, Brest), to D. Gorand Microcopy assistance; Electron (Scanning University of Perpignan), to Dr J. Neveux and F. Lantoine (spectrofluorometry; CNRS, Banyulssur-mer), to N. Briand and M. Panouse (hydrology, CTD; CNRS, Banyuls/mer). Thanks are also due to Drs Hay, Kleijne and Knappertsbusch for their constructive criticisms of the manuscript. Financial support was provided by the CNRS EcoM.4nGEprogram.

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