Morphologies and transformations of celestite in seawater: The role of acantharians in strontium and barium geochemistry

0016~7037/92/$5.00 + .oO

Geochimica d Cosmochimica Ada Vol.56. pp. 3273-3279 Copyright Q 1992 Pcrgamon F’rcs Ltd.Rintcdin U.S.A.

Morphologies and transformations of celestite in seawater: The role of acantharians in strontium and barium geochemistry RENATE E. BERNSTEIN,ROBERTH. BYRNE, PETER R. BETZER,and ANTHONYM. GRECO Department of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701 USA (Received March 19, 199 1; accepted in revisedform December 16, 199 1)

Abstract-Free-drifting sediment traps deployed at 400, 1500, and 3200 m were used to collect particles near the US JGOFS Time-Series Station (3 1’49S’N and 64”08.2’W) in the Atlantic Ocean. Acantharian specimens isolated from our samples were abundant at the 400-m depth horizon and were rare to nonexistent in our 1500-m traps. No specimens were detected in the 3200-m traps. This trend parallels those noted for the Pacific and has been linked to the oceans’ Sr/Cl profiles. Our collections revealed the presence of myriad, heretofore undocumented, minute SrSO, particles. These particles are most likely related to the acantharian reproductive cycle. The extreme abundance of acantharians and acantharianderived particles may have implications beyond the oceans’ Sr budgets. Barium/strontium molar ratios in acantharian-derived celestite on the order of 3 X 10e3 indicate that acantharians may play an important role in oceanic Ba cycling. acantharians are frequently more abundant than their protozoan counterparts, the radiolarians and the foraminifem (BOTTAZZI et al., 1971; BEERSet al., 1975; BOTTA~~I and ANDREOLI, 1978; MICHAEL& 1988). Acantharian-derived celestite has been shown to contain a variety of trace elements including Ba ( ARRHENIUS,1963; RIEDERet al., 1982). Notably, the studies ofboth ARRHENIUS ( 1963) and RIEDERet al. ( 1982) report identical Ba/Sr ratios, (Bah/(Srh = 4 X 10e3. This ratio is considerably greater than the molar concentration ratios of Ba and Sr in the upper ocean, [Ba]=/[Sr], N 3 X 10e4 (BRULAND, 1983). Despite the fact that ocean waters are undersaturated with respect to both celestite and barite, celestite is abundant in the upper ocean; and barite crystals are ubiquitous throughout the water column. Although celestite precipitation in seawater is directly mediated by organisms, the mechanism for barite formation is uncertain ( DEHAIRSet al., 1980; BISHOP, 1988 ) . Proposed bar&e formation processes have generally involved elevated SO:- concentrations in decomposing organic-rich microenvironments such as fecal pellets (DEHAIRS et al., 1980; BISHOP, 1988). In this study, we have collected acantharian specimens using sediment traps deployed at the US JGOFS Time-Series Station in the Sargasso Sea. We will show that the vertical distributions of these specimens parallel those found in the Pacific by BERNSTEINet al. ( 1987), and further, that our results are consistent with the Sr/Cl profile presented by MACKENZIE( 1964 ) for the Sargasso. Additionally, we report the existence of previously undocumented highly abundant, minute, acantharian-derived celestite crystals. We will show that the abundance, range of morphologies, and chemical composition of acantharian-derived celestite, in conjunction with the physical-chemical characteristics of celestite and barite solubility equilibria, can create microenvironments highly conducive to barite formation.

INTRODUCI’ION THE mw QUANTITATIVE dataon Sr in seawater were presented by DESGREZand MEUNIERin 1926. This precipitated a number of investigations, involving a variety of techniques in which refined assessments of dissolved Sr in seawater were sought (e.g., CHOW and THOMPSON, 1955; SUGAWARAand KAWASAKI,1958; FABRICANDet al., 1966; CULKIN and Cox, 1966). Although Sr was initially thought to be a conservative element (e.g., CHOW and THOMPSON, 1955; FABRICANDet al., 1966), refined techniques indicated variations in Sr/Cl ratios (e.g., ANGINOet al., 1966; ANDERSENand HUME, 1968; ANDERSEN and JADAMEC, 197 1; BRASS and TUREKIAN, 1974). Strontiumlchlorinity ratios have been reported to exhibit temporal variations (e.g., BILLINGSet al., 1969), geographic variations (e.g., SUGAWARAet al., 1962; ANDERSEN et al., 1970; BERNAT et al., 1972; BRASS and TUREKIAN, 1974)) and depth dependences (e.g., MACKENZIE,1964; ANDERSENand JADAMEC, 197 1). The first report of variable Sr/Cl ratios with depth was made by MACKENZIE( 1964) for the Sargasso Sea. Although SUGAWARA and KAWASAKI ( 195 8 ) speculated as to the origin of nonconservative Sr distributions in the sea, MACKENZIE( 1964) was the first to suggest the possible linkage between acantharians and the observed Sr/Cl variations. This linkage was confirmed by BERNSTEINet al. ( 1987) for the eastern and western North Pacific. Acantharians are marine planktonic protozoans and are the only marine organisms that use Sr as a major structural component. Their skeletons and cysts consist of celestite (&SO,) (Fig. 1). Although very little is known about acantharian life cycles, cyst formation has been associated with the reproductive processes of particular groups of acantharians ( SCHEWIAKOFF,1926; HOLLANDEand ENJUMET, 1957; HOLLANDEet al., 1965; BOTTAWI, 1973). Previous work has shown that acantharians and their cysts are present in most of the world’s oceans ( BOTKZI et al., 1967; BOTT~I, 1973; BOTTAZZI and ANDREOLI, 1982a; BERNSTEINet al., 1987; MICHAEL&1988; SPINDLERand BEYER,1990). In fact,

METHODS AND MATERIALS Individually deployed Soutar-type sediment traps (S~UTARet al., 1977; BETZERet al., 1984; BERNSTEIN and BETZER,1991) were

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FIG. 1. (a) An acantharianskeleton.The skeleton is broken, revealing a number of the twenty equidistant spines characteristic of this protozoan group. Reference bar = 50 pm. (b) An acantharian cyst. This cyst is oriented to show the large pore present in many cysts. Reference bar = 50 rm.

suspended at 400, 1500, and 3200 m, where they sampled for oneday periods. Particulate samples were collected between March 30 and April 18, 1990, near the US JGOFS Time-Series Station in the Atlantic (3 I “49.5’N and 64”08.2’W). Our traps funneled particulates into a 3.8-cm diameter PVC collection cup located at the base of each cone. The collections made at 400 and 1500 m involved simultaneous deployments of two to three traps in close proximity at identical depths. To facilitate the partitioning of sample patticulates by size, collection cups were equipped with three screened PVC inserts (BERNSTEIN and BETZER, I99 I ). The Nitex-covered inserts roughly segregated passively settlingparticulates into 1500,500-253,253-63, and <63pm size ranges. This segregation of materials minimized filter clogging and thus facilitated rapid filtration. Immediately upon trap retrieval, sample material was filtered onto preweighed 47-, 25-, and I3-mm diameter, 0.4~pm pore size Nuclepore membranes. The contents from individual inserts were filtered onto separate 47-mm diameter Nuclepores, producing an initial sample categorization according to >500,500-253, and 253-63 pm size ranges. A portion ofthe remaining fine-fractioned material (~63 pm) was filtered onto 25-mm diameter Nuclepores for computerized particle analysis by scanning electron microscopy. The ability of the computer to resolve individual particles was optimized by filtering a range of sample volumes. To aid in identifying particle dispersions suitable for computerized analysis, following each filtration, subsamples were viewed under a Wild stereomicroscope (maximum magnification 187X). Thirteen-mm diameter Nuclepores were placed in

a Teflon “spot sampler” (BERNSTEIN and BETZER, I99 I ), and additional subsamples of fine-fractioned materials were filtered for highresolution electron microscopy. The remaining fine-fractioned material was filtered onto another 47-mm diameter Nuclepore. Ali filters were briefly rinsed to prevent the formation of salt crystals. Filters and particles, housed in partially open petri dishes were desiccated under vacuum (over silica gel) for a minimum of 24 h. Thereafter, our samples were refrigerated until they could be analyzed at a shore-based clean facility. To limit contamination possibilities, sample manipulations were conducted under a vertical-flow hood and/or in a climate-controlled clean room. Acantharian specimens in the >63-pm size range were identified under a Wild stereomicroscope (maximum magnification 187X). These specimens were transferred to a preweighed Nuclepore filter using a Japanese calligraphy brush slightly moistened with distilled-deionized water. After desiccation, the samples were weighed on a Perkin-Elmer autobalance. Weights were converted to SrSO, accumulation rates in mg/m’/day. Nuclepore filters (25-mm diameter) containing <63-pm particles were analyzed using a Lemont image analysis (IA) system interfaced to an ISI-DSI 30 scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDX ) system ( Kevex 7000). These systems provide individual particle analysis, including size and chemical composition for each particle analyzed. Thirty-two elements were monitored. Each particle was categorized according to an operatorspecified chemical classification scheme. Identical procedures were used for all sample analyses. Our procedures involved analyses at a specific magnification over similar sample areas. These analyses resulted in the characterization of 800-2000 particles per sample. High-resolution viewing of 13-mm diameter Nuclepore subsamples was accomplished on the upper stage of our dual-stage SEM system. Thus, we were able to scrutinize under high magnification the equivalent of what we were analyzing, via the SEM/EDX/IA systems, on the bottom stage. Acantharian cysts, which we had collected in the Pacific Ocean, were sent to the Department of Nuclear Engineering at North Carolina State University for neutron activation analysis. Our sample consisted of I .2 I2 + 0.0 IO mg of celestite and was irradiated for approximately 8 h. RESULTS AND DISCUSSION: ACANTHARIAN SPECIMEN ABUNDANCES AND CELESTITE MORPHOLOCIES

The acantharian specimens collected in our sediment traps in the >63-pm size category consisted almost exclusively of acantharian cysts (Fig. 1b). In the course of nine 400-m trap deployments, the S&O., accumulation rates represented by these specimens varied between 0. I3 mg/m’/day and 2.50 mg/m2/day (Table 1). These rates correspond to accumulations of 85-1420 acantharian specimens/m2/day. In eight deployments at the 1500-m depth horizon, our traps collected between zero and eight acantharian specimens/m’/day. The two 3200-m traps were devoid of any SrSO, particles. Studies of plankton distributions in the water columns (BEERS and STEWART, 1971; BEERS et al., 1975; BISHOP et al., 1977; BISHOP et al., 1978) show that, among the larger

microzooplankton, acantharians are abundant constituents in the euphotic zone. BEERS et al. ( 1975), for instance, reported acantharian concentrations ranging from 7200- 16,000 specimens/m3. These concentrations are greater than any they reported for radiolarians or foraminifera at all depths for six stations in the North Pacific. Previous studies do not appear to include acantharian cysts which, as we have noted, are abundant constituents in both the Atlantic and Pacific Oceans. Emerging data documenting cyst concentrations appear to indicate that acantharians are more important constituents within the marine environment and among the

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Geochemistry of Sr and Ba in acantharian particles TABLE 1.

SrSO, PARTICULATEDATA

SrSO, ACCUMULATION RATES (>63pm SIZE FRACTION)

DATE (DEPTH)

mdm2/d April 18 (400m) 1 2 3 April 16 (400m) 1 2 April 14 (4OOm) 1 2 April 12 (4OOm) 1 2 April 9 (1500m) 1 2 April 8 (1500m) 1 2 April 7 (1500m) 1 2 April 3 (1500m)

fragments

[ number/m2/d]

2.50 1.05 0.82

[ 14201 [ 7611 [ 3821

67.3 63.4 51.9

1.84 1.67

[1358] [ 13951

52.6 19.2

0.95 1.01

[ 6331 t JJll

38.3 56.7

0.88 0.13

[ 5051 [ 851

29.7

13.5

t t

[

21

2.3 5.7

t

t

21 81

0.9 0.3

[

51

0.2 0.1

[ - 31

21 April 2 (32OOm) 1 March 31 (3200m) 1 t Acantharian

PERCENTAGEOF TOTAL PARTICULATESAS SrSO, (<63pm SIZE FRACTION)

0.7

detected

plankton community than previously thought. Our acantharian specimens collected in the Sargasso consist primarily of cysts. Similarly, the majority of acantharian specimens collected by BERNSTEINet al. ( 1987) in the North Pacific consisted of cysts. A mass sedimentation of cysts was also noted in the Greenland Sea (V. Bodungen, pers. commun.). Computerized analyses of <63-pm sample size fractions indicated a high percentage of particles composed of Sr and S (Table 1). For our nine 400-m traps, the percentage of particles which were SrSO., rich ranged from 13.5-67.3%, with an average of 43.6%. The percentage abundance of SrSOo-rich particles in our 1500-m traps ranged from O-5.7%, with an average of 1.3%. No SrS04 particles were detected in either of two 3200-m traps. High-resolution scanning electron microscopy of our <63pm subsamples revealed myriad discrete minute particles which apparently have not been previously characterized (Fig. 2 ) . EDX analyses of these particles confirmed that strontium and sulfur were the particles’ dominant elemental components. To our knowledge, the photomicrographs presented in Fig. 2 represent the first documentation of the existence

of these abundant granules. During our analyses, we sometimes saw these granules in conjunction with acantharian cysts. In order to examine the association between the cysts and the minute granules, several cysts of different morphologies were mechanically broken and viewed on the SEM (Fig. 2b). Thousands of these particles were seen in the majority of cysts. They ranged from l-3 pm in length and displayed a remarkable morphological uniformity within each cyst. Their character and function is presently speculative. However, noting their association with cysts and the linkage between cyst formation and the reproductive cycle of certain acantharians ( SCHEWIAKOFF,1926; HOLLANDEand ENJUMET, 1957; HOLLANDEet al., 1965; BOTTAZZI, 1973), it is possible to speculate that these tiny SrS04 granules play a role in reproductive processes. Prior to reproduction, certain acantharians resorb their spines and undergo active !&SO4 metabolism to produce a &SO., shell or cyst that surrounds the acantharian cytoplasm. During encystment, the acantharian loses its buoyancy and begins to descend through the water column. The cytoplasm within the SrS04 shell eventually differentiates into numerous

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particles collected ules. &SO4

in our 400-m traps consist of SrSO., gran-

ACCUMULATION

RATES AND Sr/CI RATIOS

Our results from the Sargasso Sea can be directly compared to those of BERNSTEIN et al. ( 1987) in the eastern and western North Pacific Ocean, which were obtained using similar sample collection and processing techniques. A pronounced diminution of SrSO, accumulation rates below 400 m was observed in both study areas. BERNSTEIN et al. ( 1987) found that average SrS04 accumulation rates diminished by approximately a factor of 20 between 400 and 900 m. The decrease in accumulation rates between our 400-m and 1XXIm traps is nearly 100%. In the North Pacific data set, only one of seven samples obtained below 900 m contained detectable SrS04. The absence of particulate Sr in either of our 3200-m traps is consistent with the North Pacific results. In the North Pacific, BERNSTEIN et al. ( 1987) reported an inverse relationship between acantharian specimen abundances and Sr/Cl ratios. The incorporation of Sr into the skeletons and cysts of acantharians is reflected in decreasing Sr/Cl ratios. Conversely, the dissolution of acantharian-de-

FIG. 2. (a) Photomicrograph showing numerous SrSO, granules from the <63-pm size fraction. Reference bar = 10 pm. (b) A manually broken acantharian cyst containing uniformly sized SrSO,, granules. Reference bar = 50 pm.

flagellated spores or gametes. These spores/gametes subsequently exit the cyst through a pore (Fig. 1b). HOLLANDE and MARTOJA ( 1974) report the presence of a crystal of SrS04 inside the flagellated spores produced by certain radiolarians, and we observed numerous, approximately 10 pm long, SrS04 crystals inside a colonial radiolarian from the Family Collosphaeridae (Fig. 3). The close relationship between acantharians and radiolarians implies possible similarities in reproductive strategies. We propose that each acantharian spore/gamete contains a SrSO, granule (or granules) possibly to serve as a nucleus for subsequent skeletal growth and/or buoyancy adjustment. The l-3 pm long granules documented in this work can easily be incorporated into acantharian spores/gametes, which have been observed to be approximately 5 pm long ( HOLLANDE et al., 1965). Assuming that each SrS04 granule represents one spore/ gamete, then thousands of spores/gametes are associated with each acantharian cyst. Cyst densities, for the Antarctic, reported to be as high as thirty-two specimens/ m3 (SPINDLER and BEYER, 1990), would translate into a density of tens of thousands of gametes or spores per cubic meter. The potential abundance of these spores/gametes is consistent with our data wherein an average of nearly 50% of all the <63-Km

FIG. 3. (a) A group of colonial radiolarians from the Family Collosphaeridae. Backscattered imaging is sensitive to atomic number, allowing the SrSOI granules to be seen through the normally opaque siliceous shell. Reference bar = 100 pm. (b) A closeup view of a colonial radiolarian. Backscattered imaging contrasts the siliceous shell with the SrS04 crystalline granules. Reference bar = 10 pm.

3217

Geochemistry of Sr and Ba in acantharian particles rived Sr appears to cause increases in these ratios (BERNSTEIN et al., 1987). The rapid attenuation of particulate &SO4 accumulation rates with depth is mirrored in the dissolved Sr/ Cl maxima observed by BERNSTEINet al. ( 1987) between 700 and 1500 m from several stations. Our &SO4 accumulation rates in the Sargasso are consistent with the Atlantic Sr/Cl observations of MACKENZIE ( 1964) and the relationships between Sr/Cl ratios and SrS04 fluxes observed in the North Pacific (BERNSTEIN et al., 1987). The lowest Sr/Cl of MACKENZIE’S( 1964) ratios were noted in the upper water column (25-250-m depths). This can be attributed to the uptake of Sr by acantharians whose standing stocks have a near-surface maximum (MICHAEL&1988 ) . The reported Sr/Cl maximum between 500 and 800 m of MACKENZIE ( 1964) is consistent with the rapid attenuation of SrS04 accumulation rates below 400 m observed in the present work. Below the Sr/Cl maxima in the North Pacific, Sr/ Cl ratios decrease slightly and then tend to stabilize. Since only three Sr/Cl ratios below 800 m are presented for the Sargasso, a similar trend cannot be conclusively established. Nevertheless, a trend toward constancy with depth, in both the Atlantic and the Pacific, is consistent with the observation that most of the SrS04 particles in both oceans appear to dissolve at relatively shallow depths. The rates of SrS04 accumulation at 400 m are on the average much higher in the Sargasso Sea (1.21 mg/m2/day) than they are in the western and eastern North Pacific (0.75 mg/m2/day and 0.16 mg/m2/day). Additionally, our SrSO, accumulation rates at 400 m in the Sargasso exhibit a substantial range: 0.13-2.50 mg/m2/day. Although the variability in SrS04 accumulation rates between our simultaneous collections may indicate biases in trapping efficiency (GUST et al., 1992)) our variable 400-m SrS04 accumulation rates in the Sargasso are consistent with observations by BOTTAZZI and ANDREOLI( 1982b), who report nonuniform acantharian distributions there. Previous observations indicate the existence of geographical ( ANDERSENand HUME, 1968; BRASS and TUREKIAN, 1974) and temporal (BILLINGSet al., 1969) variations in Sr/Cl ratios. A qualitative assessment of these data appear consistent with the reported seasonal (e.g., BOTTAZZI and ANDREOLI, 1978; SPINDLERand BEYER, 1990) and geographical (e.g., BOTTAZZI and ANDREOLI, 1972) variability of acantharian specimen abundances.

The molar ratio of Ba and Sr in acantharians has been determined ( ARRHENIUS, 1963; RIEDER et al., 1982) as (Ba)T/(Sr)T = 4 X 10e3.

It is therefore clear that celestite particles exiting the upper ocean remove a larger fraction of dissolved Ba from surface waters than dissolved Sr. Since Sr/Cl depletions in the upper ocean are as large as 5% (BERNSTEINet al., 1987) and the Ba/Sr ratio in acantharians is ten times larger than the dissolved Ba/Sr ratio in surface ocean waters, a first-order assessment points to the possibility of significant acantharianmediated Ba depletions in surface waters. We have noted that acantharian cysts constitute a dominant portion of the celestite in our samples. Since previous measurements of the Ba/Sr ratio were conducted on acantharian skeletal material, we obtained the (Bah/( Sr)r ratio in acantharian cysts. Neutron activation analyses of our Pacific Ocean specimens produced the following result: (Bah/(Srh

= (30 X 1O-9 M)/(90 X 1O-6 M) = 3.3 X 10-4.

(I)

= (3.0 f 0.8) X 10m3.

(3)

This result compares well with those obtained spectrochemically by ( ARRHENIUS, 1963) and through EDXA ( RIEDER et al., 1982), further substantiating the role of acantharians in marine Ba geochemistry. The rapid dissolution of acantharian-derived celestite with depth substantially limits the export of Sr beyond intermediate waters. However, it is possible to advance arguments that the behavior of Ba during celestite dissolution differs distinctly from that of Sr. While the Sr in acantharianderived celestite appears to be rapidly recycled, it can be shown that celestite dissolution can produce conditions highly conducive to the formation of bar&e. The solubility products of SrS04( s) and BaS04( s) at 25°C and zero ionic strength are given as log K&( SrS04) = -6.50 + 0.05 and log Kfp(BaS04) = -9.96 + 0.03.

(4)

Consequently, using the Sr2+, Ba2+, and SO:- total activity coefficients of MILLERO and SCHREIBER(1982) at S = 35 and 25”C, the celestite and barite solubility products expressed in terms of total concentrations (S = 35, 25°C) are

INFLUENCE OF ACANTHARIANS ON BARIUM GEOCHEMISTRY To the extent that acantharian-derived celestite mediates strontium/chlorinity variations in the upper ocean, it can be argued that acantharians may play an even greater role in the marine geochemical cycle of barium. This contention can be initially evaluated by comparing dissolved Ba/Sr ratios in the upper water column with Ba/Sr ratios in acantharianderived celestite. The dissolved [ Ba2’lT/[ Sr2+lT molar ratio in surface water is ( BRULAND, 1983) as follows:

(2)

log K&(SrS04) = [ Sr2+lT[ SO:-],

= -4.80;

log K&(BaS04) = [ Ba2+lT[ SO:-],

= -8.19.

(5)

Since [SO:-lT = 0.0289 molar at S = 35, the saturation concentrations of Sr2+ and Ba2+ (25”C, S = 35) are =JSr2+lT = 5.5 X 10e4 M and &Ba2+lT = 2.2 X lo-’ M.

(6)

When compared to the reported surface ocean concentrations of Sr2+ and Ba2+ (BENDER et al., 1972; BRULAND, 1983), [Sr2+lT N 90 X 10m6M

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R. E. Bernstein et al.

and [Ba’+]* N 30 X lo-’ M,

(7)

the above saturation concentrations indicate that the surface ocean is approximately 16% saturated with respect to celestite and 14% saturated with respect to barite. In view of the substantial undersaturation of seawater with respect to celestite and barite, it is quite probable that precipitation of either mineral will occur only in oceanic microenvironments. The microenvironments which result from ingestion of acantharians by zooplankton are of special interest in this regard. Celestite remains have apparently not been reported in studies of zooplankton gut contents or fecal material. However, siliceous remains of radiolarians are frequent components in zooplankton gut contents and fecal material, and similarities between radiolarians and acantharians suggest that zooplankton are unlikely to reject acantharians as a food source (Hopkins, pers. commun.). Subsequent to ingestion, acantharian skeletons and cysts should dissolve rapidly due to the labile nature of celestite. Dissolution of a single acantharian or acantharian cyst weighing approximately 1 pg (5.4 X 10 -’ moles SrSO, ) can saturate a 10-PL volume of seawater with respect to SrSO.,: (90 x 1O-6 M) +

(a)

Using the celestite Ba/Sr molar ratio obtained in our study {(Ba)T/( Sr)T = 0.003 } , it follows that this lO+L microenvironment would then be supersaturated with respect to barite by a factor of more than seven, as shown in the following: (30 X lo-‘M)

+

162 X lo-” moles ’ I o x 1o-5 L

( .

= 1.65 X 10e6M = 7.5 X sat[Ba2+]T.

Acknowledgments-We

are grateful to the captain, Thomas R. Tyler, and the crew of the RV ENDEAVOR for their significant role, in the face of less than ideal circumstances, in bringing about a successful cruise. In particular, the bosun, Jack E. Buss, worked hard during

many long and extra hours to ensure our safety. We wish to thank the scientific team of Walter Bowles,Steven Giordano, GiselherGust, Chris Sabine, and Richard Young. Many risked life and limb during

trap deployments and retrievals. Additionally, the insightful critical comments provided by Drs. Neil Andersen and Fred MacKenzie were greatly appreciated. This research was supported by NSF grant OCE 88 13436.

5.4 X lo-’ moles 1 x 10_5L = 6.3 x 10m4M b =,[Sr2+lT..

of celestite morphologies produced by acantharians should provide a wide range of !&SO, dissolution rates, whereupon a substantial fraction of the celestite microenvironments in fecal pellets should be conducive to barite formation. Observations of variable and occasionally high mole fractions of Sr in marine barite ( DEHAIRSet al., 1980) are consistent with barite formation in microenvironments having a range of Sr concentrations. According to the mechanism we have proposed, Ba concentrations greater than sat[Ba2+]T would be obtained for microenvironmental Sr concentrations greater than approximately 28% of ,,[ Sr2+lT. Acantharian-derived celestite may exert a major influence on Ba geochemistry. Although biogenous celestite is not preserved in the geological record, it is nonetheless possible that a geological signature of acantharians is written in barite.

(9)

For lower temperatures in the surface ocean, saturation calculations (CHURCHand WOLGEMUTH, 1972) indicate that the degree of barite saturation will be even greater. Consequently, if acantharian-derived celestite saturates a microenvironment with Sr*+, the high (Ba)T/(Sr), molar ratio in celestite produces conditions highly favorable to barite formation. As such, not only formation but also dissolution of acantharians and their cysts may exert a substantial influence on the flux of Ba from the upper ocean. The formation of celestite by acantharians constitutes the most notable Sr concentrative mechanism in seawater. Celestite formation and dissolution appears to constitute an even more substantial concentrative mechanism for Ba. Elevated Ba concentrations in microenvironments present a much more plausible mechanism for barite formation than enhancement of sulfate concentrations alone. Barite particles have been frequently observed in fecal material. In the absence of Ba concentrations higher than those observed in the upper ocean ([ Ba2+lT N 30 X IO-’ molar), sulfate concentrations requisite to barite formation in fecal pellets would be implausibly high ([SO:-], k 0.20M). We suggest that acantharian specimens provide a reliable source of Ba in biogenie debris and that siliceous particles, among others, may play an important role as nucleation sites. The wide variety

Editorial handling: H. C. Helgeson

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