Coccolith Sedimentation by Fecal Pellets: Laboratory Experiments and Field Observations
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Coccolith Sedimentation by Fecal Pellets: Laboratory Experiments and Field Observations P. H. ROTH ^ M. M. MULLIN > Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037 W. H. BERGER J ABSTRACT FEEDING EXPERIMENTS Copepods readily ingested coccospheres during feeding experi- Experimental Methods ments. Fecal pellets contained coccoliths in a state of excellent preservation, although some mechanical breakage occurred, espe- Planktonic copepods, Calanus pacificus and Rhincalanus cially among delicate forms. Fecal pellets collected in deep waters nasutus were captured by net off La Jolla, California, in April 1974 of the eastern tropical Pacific contained mainly siliceous plankton and held in the laboratory at 15° to 20°C until use within 24 hr remains but also some coccoliths showing good to moderate pres- later. Unialgal suspensions of coccolithophorids were made up in ervation. Some coccolith-bearing fecal matter apparently reached filtered sea water from cultures grown in an enriched sea-water the sea floor, even well below the carbonate compensation depth. medium. The clones employed were Cricospbaera elongata The implications of fecal transfer to paleontology and geochemis- (Droop, 1955), originally isolated by I. J. Pinter of the Haskins try may be of the greatest importance. Key words: deep-sea Laboratory, New York; Cricospbaera carterae (Braarud and Fager- sedimentation, fecal pellets, coccolith preservation. land, 1946), probably from the North Pacific Central Gyre, iso- lated by J. B. Jordan; and Emiliania huxleyi (Lohmann, 1902) INTRODUCTION from Oslo Fjord, isolated by E. Paasche. Copepods were rinsed in filtered sea water and transferred by pipette to the suspensions of The transfer of calcareous and siliceous skeletal particles from coccolithophorids. After 24 to 48 hr, fecal pellets were picked from near-surface waters where they are produced to the sea floor where the vessels, rinsed in filtered sea water, and prepared for electron they dissolve or become buried is a process of paramount impor- microscopy. Cells in aliquots of the cultures used to make up the tance to the geochemistry of the oceans. The available information suspensions were similarly prepared, so that the coccolith structure on this process is exceedingly sparse, however. Bramlette (1961), in the fecal pellets could be compared to that of the particular clone and many workers since, recognized two paradoxes regarding the the copepods had ingested. transfer: (1) very fine particles appear to drift but little from the place of production to that of deposition, and (2) coccoliths and Ultrastructure of the Coccoliths Used in the Experiments diatoms accumulate on the sea floor despite a presumably long settling time spent in undersaturated waters. Accelerated sinking is The two clones of Cricospbaera used have identical coccolith usually invoked to reconcile these observations (see Smayda, 1970, morphology and could not be distinguished in the scanning elec- 1971, for reviews). Schrader (1971) presented direct evidence for tron microscope (SEM). The coccosphere of Cricospbaera elongata such accelerated sinking by fecal transport in the sedimentation of is covered with about 200 eliptical coccoliths approximately 2 /ira diatoms. long, 0.5 pm wide, and 0.2 /am thick (Fig. 1A). An organic mem- Similar fecal transport must be assumed for coccoliths. In addi- brane covers the coccoliths and obscures most of the ultrastructural tion to the problems of drift and dissolution during settling, it is details of the coccoliths. There are some indications, especially obvious that the amount of coccoliths suspended in the water col- from broken specimens in fecal pellets, that the individual coccolith umn below the photic zone (Hasle, 1959; Marshall, 1968; Okada is composed of two closely appressed cycles, which are about 0.1 and Honjo, 1970), when combined with Stokesian settling, is fim wide and composed of approximately 12 nonoverlapping ele- insufficient to provide the flux necessary to account for observed ments. The lower proximal cycle is only about half as thick as the sedimentation rates in areas where coccoliths are well preserved. distal cycle. The illustrations and descriptions of Hymenomonas The fluxes, therefore, must be provided by much larger particles. (Cricospbaera) carterae published by Braarud and others (1952), Such particles are low in concentration due to their short transit Manton and Leedale (1969), and Outka and Williams (1971) agree times and are rarely sampled therefore. Analogous problems arise quite well with the forms here either identified as Cricospbaera in the transfer of organic matter (Menzel, 1967). Large particles elongata or C. carterae; some (for example, Manton and Leedale, could be provided by fecal pellets, especially copepod pellets, since 1969) show forms of Hymenomonas ( = Cricospbaera) carterae copepods are the dominant component of zooplankton. with somewhat broader cycles of elements (shields), but forms with From these considerations, we decided to check whether coc- narrower shields very similar to the ones observed in our clones coliths can survive ingestion by copepods and whether coccoliths have also been illustrated (Outka and Williams, 1971). Modern are indeed present in fecal pellets recovered from the deep sea — in electron micrographs of Cricospbaera elongata have not been pub- the water and on the sea floor. We believe that our results, although lished, and therefore it is not possible to decide whether the clones of a preliminary nature, will encourage a greatly intensified effort originally identified as C. elongata (Fig. 1A) and C. carterae (Fig. in this neglected but important field of deep-ocean sedimentation. IB) are indeed identical, as would be suspected from the coccolith Geological Society of America Bulletin, v. 86, p. 1079-1084, 3 figs., August 1975, Doc. no. 50805. 1079 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/8/1079/3443813/i0016-7606-86-8-1079.pdf by guest on 02 October 2021 Figure 1. A. Cricosphaera elongata, isolated by I. J. Pinter. Collapsed coccosphere on millepore. filter, from original culture. B. Cricosphaera carterae, clone isolated by Jordan from central North Pacific gyre. Coccosphere from original culture on millepore filter collapsed during drying. C. Coccosphere of C. elongata in fecal pellet. Most of coccoliths are still attached to organic membrane; others have come off. D. Fecal material from copepod with numerous coccospheres, isolated coccoliths, and fragments of Cricosphaera elongata, mixed in with organic debris. E. Strongly fragmented coccoliths of Cricosphaera elongata from fecal pellet. Elements of two cycles of coccoliths are more visible than in the original samples; perhaps part of the organic coating has been removed in digestive tract of copepods. F. Whole coccoliths and fragments of Cricosphaera carterae from fecal pellet of copepod. Scale bar on all pictures indicates 1 /urn. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/8/1079/3443813/i0016-7606-86-8-1079.pdf by guest on 02 October 2021 COCCOLITH SEDIMENTATION BY FECAL PELLETS 1081 ultrastructure. The coccoliths of the clone of Emiliania huxleyi was preserved (Fig. 2E), whereas the elements of the shield were have the usual structure, which has been well described by many broken. This would indicate that mechanical damage, rather than authors (for example, Mclntyre and Be, 1967). However, some dissolution, caused the fragmentation. Delicate forms like unusual variations in shape of the coccoliths were observed. Some Scapholithus sp. and a fragment of Rhabdosphera clavigera were of the coccospheres are covered by elliptical and circular coc- still preserved (Figs. 2F, 3A). Another fecal mass from the gut of an coliths; others have only elliptical or only circular coccoliths (Fig. ostracod contained both well-preserved coccoliths of E. huxleyi 1A). Circular forms of Emiliania huxleyi have to our knowledge (Fig. 3C) and specimens of Umbilicosphaera sibogae that showed not been described from natural assemblages in the open ocean, signs of etching. and apparently they are aberrant forms due to prolonged culturing. A fecal pellet from inside the gut of a brittle star collected at a Light microscopy of the coccoliths of E. huxleyi shows that they depth of 3,600 m, several hundred metres below the regional car- are well calcified, because the usual birefringence is observed (Fig. bonate compensation depth (CCD), contained very well pre- 2C). served coccoliths, such as Gephyrocapsa oceanica, with a pre- served central grille (Fig. 3E). Isolated shields of C. leptopora (Fig. Fecal Pellets from Feeding Experiments 3F) and fragments of other species were also common. The domi- nant components of the fecal pellet from the brittle star gut, how- The fecal pellets produced by the two species of copepods are ever, were diatoms, fragments, and clay minerals. The sediment rod-shaped, about 2mm long, and 0.5 mm in diameter (Fig. 2D). within the gut of the brittle star, but not enclosed in a fecal pellet, An organic membrane encloses the content. Slight pressure with a contained mostly clay and only rare, poorly preserved coccoliths. paint brush or needle breaks the organic membrane, and the con- All the fecal pellets and gut contents studied so far were taken in tent of the fecal pellets can then be spread on a cover slip and pre- the water column at great depth or from the sea floor. They might pared