Recent stromatolites in landlocked pools on Aldabra, Western Indian ocean

Palaeogeography, Palaeoclimatology, Palaeoecology, 69 (1989): 145 165

145

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

RECENT STROMATOLITES IN LANDLOCKED POOLS ON ALDABRA, WESTERN INDIAN OCEAN C. J. R. B R A I T H W A I T E 1, J. C A S A N O V A 2, T. F R E V E R T 3 and B. A. W H I T T O N 4 1Department of Geology, The University, Dundee DD1 4HN, Scotland (U.K.) zC.N.R.S., Laboratoire de Gdologie du Quaternaire, Facultd des Sciences de Luminy, 13288 Marseille (France) 3Fresenius Consult GmbH, 6204 Taunusstein-Neuhof (F.R.G.) 4Department of Botany, University of Durham, Durham DHI 3LE (U.K.) (Received February 9, 1988; revised and accepted September 27, 1988)

Abstract Braithwaite, C. J. R., Casanova, J., Frevert, T. and Whitton, B. A., 1989. Recent stromatolites in landlocked pools on Aldabra, western Indian Ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 69:145 165. Stromatolites occurring in three areas of landlocked pools on Aldabra show distinctive morphologies, petrography and biological characteristics related both to water chemistry and the physical characteristics of sites. They are formed apparently by indirect precipitation of high-magnesium calcite in association with growths of cyanobacteria and Cladophora. The three contrasted areas are dominated respectively by Phormidium, Lyngbya and Pleurocapsa, by Entophysalis, Lyngbya and Pleurocapsa, and by Pleurocapsa, Dichothrix gypsophila and Schizothrix, reflecting an increasing concentration and increasing dissolved carbonate activity of pool waters. Centimeters to decimeters of carbonate encrustations are made up of irregular laminate columns, porous layered coatings of planar or cerebroid form (including oncolites) and finely laminate branching columns. Laminae are typically alternations of' dense and porous micrite with, in one locality, radiating fibrous sparry crystal growths. Although exclusively associated with cyanobacteria, calcification typically begins several millimeters beneath living cells, with few examples of carbonate-coated unicells or filaments. Micrite consists of randomly arranged particles of varying size, products of sparse homogeneous nucleation followed by both overgrowth and dissolution. Coarse cements of contrasted morphologies develop within internal cavities, resembling the endostromatolites described by others. Chemical data indicate that waters in all three pools are supersaturated with respect to CaCO 3 (calcite). This is adequate, but is not in itself sufficient, to bring about precipitation of carbonate which is site-specific. Cyanobacteria are not directly calcified but metabolic activities may promote precipitation, through mechanisms such as extraction of C()~ or by the generation of specific organic molecules which promote nucleation, or colonies may provide a closed passive environment in which other bacteria or inorganic processes bring about crystallization. The precise path is not known but the reduction of possibilities carries implications both for the accretion of other stromatolites and the crystallization of carbonate in deposits such as tufas.

Introduction T h e v a r i e t y of s t r u c t u r e s which can be described as ~ s t r o m a t o l i t i c " includes b o t h biogenic and a b i o g e n i c forms g e n e r a t e d in e n v i r o n m e n t s r a n g i n g from m a r i n e t h r o u g h fresh w a t e r s to soils and springs (see articles in Walter, 1976). A l t h o u g h not always easy to 0031-0182/89/$03.50

differentiate, stromatolites are r e g a r d e d as specifically biogenic and as r e s u l t i n g from the i n t e r a c t i o n b e t w e e n g r o w t h of b e n t h i c microbial c o m m u n i t i e s ( d o m i n a n t l y c y a n o b a c t e r i a ) and sediment deposition. B u r n e and M o o r e (1987) h a v e discussed this i n t e r p l a y in more detail and h a v e proposed the use of a general t e r m microbialite to include s t r u c t u r e s formed

i("~ 1989 Elsevier Science Publishers B.V.

146 by trapping and binding or by direct or indirect calcification by microbes. A few authors (for example Monty, 1973a) have argued that stromatolites are inherently structures which actively precipitate carbonate, phosphate, sulphate or other mineral. However, there is little support for this view and Golubic (1973) suggested in contrast t h a t carbonate precipitation is not in fact a genetic attribute of any cyanobacterium. Thus, further evidence on the relationship between organisms and precipitates in stromatolites is of interest. For some time, descriptions of fossil and Holocene stromatolites were dominated by those formed in the marine intertidal zone. However, similar features form in a range of environments (Monty, 1973b) and many, like those described here, are non-marine. Contemporary freshwater examples described from areas including Lake Annecy in France (Casanova, 1986) and the freshwater marshes of Florida and the Bahamas (Monty and Hardie, 1976) are of low magnesium calcite. In contrast, those from hypersaline lakes, such as the Great Salt Lake, Utah (Halley, 1976) and coastal hypersaline lagoons such as Baja California (Horodyski and Vonder Haar, 1975 and Margulis et al., 1980) and southern Sinai (Kushnir, 1981) include deposits of aragonite, high-magnesium calcite, dolomite and hydromagnesite. There have been few reports of contemporary stromatolites from the Indian Ocean. Some examples from Aldabra were noted by Braithwaite et al. (1973). In addition, parts of the lagoon floor here are sites of deposition of microbially laminated sediments. These were also called ~'stromatolites" by Potts and Whitton (1980), but d o not contain precipitated carbonate and are unlikely to be preserved. The purpose of this study has been to establish the distribution and characteristics of contemporary stromatolites on Aldabra, to examine the biology of associated organisms and relate these to the chemistry of pool waters. In addition since the water bodies containing stromatolites are small, preservation, if it occurs, will be within a generally

terrestrial lithosome. Stromatolite-like forms associated with palaeosols (Braithwaite et al., 1973; Braithwaite, 1975), are common in the late Pleistocene succession on Aldabra. These resemble both the pedogenic brown laminated crusts described elsewhere by Multer and Hoffmeister (1968) and normal aqueous stromatolites. It is therefore important to examine contemporary stromatolites to see which features separate them from pedogenic structures.

General setting Aldabra is a largely uninhabited island in the western Indian Ocean (9°24'S, 45°20'E), about 420 km northwest of Madagascar and 640 km from the East African coast, on the western fringes of the Seychelles group. It is a raised atoll, about 34 km long, east to west, and 14.5 km wide, consisting of a narrow rim of Pleistocene limestone surrounding a shallow central lagoon (Fig.l). The geology of the island was described by Braithwaite et al. (1973) and the petrography and sedimentology of non-marine Pleistocene rocks by Braithwaite (1975). Fryer .(1910) and Stoddart et al. (1971) reported on the geomorphology. Although a ~'high" island for the region, much of Aldabra is low-lying, with the largest area formed by an old lagoon floor, now a 4 m terrace, with an 8 m terrace forming a narrow discontinuous rim around it. Dissolution and precipitation accompanying late Pleistocene sea-level changes have produced a dense impermeable limestone surface, although the sediment beneath retains much of its original composition and porosity. This is significant since, following heavy rain, water lies in pools on the surface and is only able to drain away through small channels to feed an extremely attenuated groundwater lens. Permeability is restricted because it relies on the intercommunication of conduits developed by dissolution during periods of lowered sea-level (see Braithwaite et al., 1973). Thus, some permanent pools relatively distant from the sea or lagoon react rapidly to tidal movements, while others are strongly damped and move-

147

b

(

~'~ " "~' ql,0 . ikm8 ~

'" " ' "

CINQCASES ~ENZIE

Fig.1. Aldabra, location and stromatolite sites.

ments are both smaller t han and lag behind the tidal response. However, there is no simple relationship between the amount of tidal influence (in terms of motion) and the salinity of pools, which is determined by four factors: (1) relative proportions of marine and fresh water, dependent upon the degree of interconnection and mixing of the marine and island water bodies; (2) net evaporation, determining concentration; (3) composition of recharge waters, which provide a continuing supply of calcium carbonate; (4) unresolved contribution of the biota. There are no p e r m a n e n t freshwater pools (total dissolved solids <5%o) on Aldabra, but some apparently pe r m a ne nt waters in areas such as Cinq Cases have very low salinities (MacKenzie, 1971; Donaldson and Whitton, 1977a). Aldabra lies in the most arid sector of the Western Indian Ocean (Farrow, 1971), with a marked seasonality of rainfall. Rain falls over a period of 5-6 months, with a total of about 670 mm. However, droughts of three months or more are recorded, and in 1967/8 42% of the rain fell in only five days (Farrow, 1971). Summer maximum temperatures average 32°C at the western end of the atoll but may be 3 ° lower elsewhere (there is no permanent weather station). The winter minimum averages 22~C. Highest and lowest shade temper-

atures are 36.6°C and 19.5°C, respectively. Thus, for most of the year, there is a high potential for evaporation. Most of the observations on stromatolites reported here were made in Jul y and August 1984. (Previous observations of Braithwaite, 1975, relate to the same season.) However, although this is a limited period, and measured conditions probably only reflect those of about 20% of the year, we believe, for reasons given below, t hat they are the significant conditions in the formation of the stromatolites. An extensive survey of oceanic, lagoonal and land-locked pools of various salinities revealed only three sites on Aldabra where lithified stromatolites are present. The three separate pool groups, all in the south-eastern part of the atoll, do not all have formal names but, in order of increasing c o n c e n t r a t i o n are referred to as Bassin Profond, the Cinq Cases group and Bassin MacKenzie. Their approximate positions are shown on Fig.1. The grid references given are from the British Directorate of Overseas Surveys maps published in 1964. Pool waters

All three pool areas retain some water t h r o u g h o u t the year. However, they have their largest volume, and are probably freshest, in the wet season, and are smallest, and presumably most saline (concentrated), at the end of the dry season. Present measurements relate to

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the dry season which is also the cooler part of the year, but Donaldson and Whitton (1977a) and Potts and Whitton (1979) monitored selected pools in the wet seasons of 1972-1973 and 1974-1975. The chemistry of pool waters must play an integral part in the processes of crystallization of carbonates. Large scale variations in ionic strengths result from seasonal changes in rainfall and evaporation. Donaldson and Whitton (1977a) showed that in smaller pools cations increase seasonally together with chlorides. In small pools variations in potassium, phosphate, combined nitrogen (ammonia) and nitrate levels reflect relatively high proportions of bird and tortoise excrement. Many chemical reactions in pools are controlled by the metabolism of organisms and Donaldson and Whitton (1977a) record hourly and diurnal changes in calcium and magnesium in response to microbial activity. Present investigations have assumed relatively high levels of concentration of ions and have emphasized factors related to carbonate activity and possible controls on carbonate precipitation. Direct measurements of the physico-chemical properties of waters have been supplemented by simple dissolution experiments, which have allowed us to define an approximate solubility for the calcite which forms the

stromatolites (Table I). This empirical approach has given an operational ion concentration product, which seems to relate to the observed chemical precipitates of the pools. Conductivity provided a convenient measure of total ion concentration (Wagner, 1980). However, it could not be translated as salinity since values of chlorinity suggest a lower marine contribution than it would otherwise indicate. For example, in Bassin MacKenzie, conductivity of 87.47 mS cm- 1 (equivalent to a salinity nearly double that of seawater) is associated with a chlorinity of only 26%o. We calculated CO32 from measured pH values and the thermodynamic protolysis constants K'1 and K~ of carbonic acid (corrected using Davies, 1967, approximation). The pH values were regarded as relative deviations from the NBS buffers for a given temperature. Thus, 10 - p ~ = [ H 3 0 +] tool/1. However, pool water densities change seasonally, and there are changes in diffusion potentials affecting the function of the reference half cell used in pH measurements. NBS buffers verify ionic strengths of up to 0.1mol/kg 1, creating a potential inaccuracy for pH measurements in solutions of different ionic strengths (Bates, 1982). But, since the strengths determined correspond with those of the calibration buffers, we have made no corrections. Davies (1967)

TABLE I Results of dissolution experiments with fresh and biologically infested (see appendix) rock. Titration acidity (Acy) to pH 8.4, t i t r a t i o n alkalinity (Alk) to pH 4.3, Ca 2÷ is by EDTA t i t r a t i o n with calcon indicator. The Mg 2+ content can be calculated as total h a r d n e s s (TH) less Ca z +. Samples were m a i n t a i n e d at 25 ° and m e a s u r e d at 7 and 22 days to monitor the equilibrium between solid and solution phases, p = - l o g l 0 [ m o l l - l ] . Kt=specific conductivity in mS c m - 1. I= ionic s t r e n g t h Rock

I

T °C

pH

pTH

pAcy

pAlk

pCa

pK'2 temp

Kt

Fresh Infested

0.006 0.035

26.5 26.7

9.34 7.39

3.16 2.41

3.56

2.85 2.39

3.56 2.60

10.21 10.08

375 2249

Fresh Infested

0.006 0.039

25.9 25.8

9.08 7.31

3.29 2.38

4.38

3.08 2.33

3.51 2.57

10.21 10.07

386 2395

Fresh Infested

0.007 0.057

25.0 25.0

8.35 6.93

3.19 2.28

5.00

2.85 2.11

3.33 2.44

10.21 9.75

358 3100

149

approximation applies within a 10% error for monovalent ions with ionic strengths up to 0.5 m o l/k g - 1 (Stumm and Morgan, 1981). Since measured strengths range from 0.006 to 2.3, the greatest discrepancy arises between calculated K'1(2) values and the thermodynamically relevant K'1(2), where ionic strengths are higher than 1.0. Measurements completed in situ in the field are given in Table II. These were complemented by laboratory analyses of water samples, tak en at the same sites and completed within 48 h, for titr a t i on alkalinity and acidity, total hardness, and Ca 2 ÷ (Frevert, 1983). The biota of pools has not been examined in detail although there are both direct and indirect influences on water chemistry noted above. In addition to the microbial communities considered here, MacKenzie (1971) studied entomostracan faunas. Donaldson and Whitton (1977b) noted the probable influence of grazing ostracods on microbial floras and the grazing activities of Cardisoma and Geograpsus although none of these seem to have a p a r t i cu lar influence in the present pools. The giant tortoise Geochelone has been observed to eat some oscillatorians and Spirogyra but we saw no evidence of their impact on stromatolites and topography excludes them from sites such as Bassin Profond and, probably, Bassin MacKenzie.

Field relationships and stromatolite morphology Since stromatolites in the three sites examined have proved to be morphologically, bio-

logically and petrographically distinct they are described separately in order of increasing concent rat i ons of waters.

Bassin Profond (Grid ref. 3745 0965) This relatively inaccessible pool lies nort h of the Cinq Cases locality. It occupies a solution pit approximately 50 m wide in an open area. Pit walls are formed by cliffs about 2 m high. Waters about 2 m deep lap against these and deepen towards the pool centre. Arcuate joints around the pool margins suggest inwards collapse towards some deeper hole, during a period of lowered sea-level. Although the water is brackish and of relatively low salinity (this is the only stromatolite pool with conductivity below that of seawater, see Tables II and III), there is a very rapid tidal response, showing changes of level of at least 50 cm within a few hours.

Morphology Stromatolites occur within a shallow u n d e r c u t on the eastern margin of the pool (Fig.2). They form a ledge about 70 cm wide tapering for several meters at either end (Fig.3). The upper surface of this platform was a few centimeters below water level when first examined but subsequently water depth increased by at least 50 cm. Encrusting carbonate extends at least 70 cm below the initial water level but decreases with depth so that the outer margin overhangs. This implies t hat deposition is faster at the water surface. The overhung submerged margin prevented

TABLE II S a t u r a t i o n indices r e s u l t i n g from dissolution experiments, and [OH ], [H2CO3"] and OH /H2CO3 * c o n c e n t r a t i o n ratios in A l d a b r a s t r o m a t o l i t e ponds p[OH ]

p[H2C03* ]

p[C032 ]

OH-/H2C03*

SI fresh rock

SI infested rock

5.20

5.41 5.86 5.80 4.87 4.66

3.86 2.94 3.65 3.59 4.16

1.6 37.2 10.7 0.54 0.11

2.69 3.69 2.76 2.34 1.77

2.31 3.31 2.38 1.96 1.39

Cinq Cases 4.29 MacKenzie Profond

4.77 5.14 5.63

150 TABLE III Results of physico-chemical measurements of waters from Aldabra stromatolite ponds. Dissolved oxygen (DO) in mg l-1 (salinity corrected). Kt in mS cm-1. TH= total hardness. I = ionic strength. Dissolved oxygen was determined using an Orbisphere Sensor, Model 2714, H2S resistive, Specific conductivity with a PTI-10 Mini-digital meter (Analyses August, 1984)

Cinq Cases MacKenzie Profond

Weather

I

T(°C) pH

pTH

pAcy

pAlk

pCa

pK'2 temp

Kt

DO

Sun Sun Sun Cloud Sun

2.28 2.02 2.00 1.27 0.66

22.5 31.2 27.3 29.1 25.9

1.04 0.75 0.99 1.05 1.39

n.d2 n.d. n.d. n.d. 0.01

2.60 2.30 2.68 2.22 2.40

1.55 1.55 1.71 2.15 z.15

10.11 10.04 10.07 10.08 10.13

133.75 147.05 131.32 87.47 41.88

9.0 8.8 8.7 14.0 n.d.

8.88 9.51 9.15 8.72 8.38

an.d.= not detectable.

Fig.2. General view of Bassin Profond, Acrosticum fern in foreground, 60 cm high.

sampling of any but the upper parts of the edifice. The upper surface of the stromatolite ledge is relatively flat, but locally a series of narrow, sinuous plates 2-3 cm high rise vertically from it. The inner (shaded) surfaces of such plates are dense and smoothly botryoidal. They have a distinctive laminated structure with branching columns beneath botryoids (Fig.4). Outer surfaces, and the interior of the ledge, are typically unstructured and porous.

Biology The biota associated with these stromatolites is more diverse than at other sites. It consists of Lyngbya, Phormidium, Schizothrix, Spirulina and Pleurocapsa, diatoms and the filamentous green algae Cladophora and Rhi-

Fig.3. Stromatolite ledge on shore of Bassin Profond, scale lm.

zoclonium. The smooth inner surfaces of the plates noted above have a mat consisting mostly of Lyngbya confervoides with Schizothrix and Phormidium. They are typically brown, but colour is thought to reflect an organic or inorganic precipitation and not coloured microbial sheaths. The outer surfaces are covered with scattered tufts of green algae. In the surface layers, only two species showed indications of calcification. The walls

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Fig.4. Columnar structure of stromatolites from Bassin Profond. Negative print of section 30 mm across. of Cladophora are birefringent, and cells of Pleurocapsa are s u r r o u n d e d by c a r b o n a t e 1 2 mm below the surface. However, irrespective of surface diversity, sections t h r o u g h s t r o m a t o l i t e s only locally show preserved s h e a t h s of Pleurocapsa and, less a b u n d a n t l y ,

Schizothrix. Petrography No c o n t a c t with the P l e i s t o c e n e was recovered here. Dense c a r b o n a t e e n c r u s t a t i o n s are c o n s p i c u o u s l y l a m i n a t e d with sharplydefined i n c r e m e n t s w h i c h m a y be only 70 pm thick. M a n y a r e a s consist of closely-packed columns, 1-2 mm wide a n d up to 5 mm long, g r a d u a l l y c o a l e s c i n g to form a c o n t i n u o u s surface (Fig.4). One m a j o r h i a t u s was noted, with new c o l u m n s established on its surface. This, h o w e v e r , was s m o o t h and c o n t r a s t s with the p r e s e n t surface sculpted and e t c h e d by erosion. L a m i n a e consist of a l t e r n a t i o n s of r a d i a l l y fibrous calcite and a u t h i g e n i c or a l l o g e n i c micrite with only s c a t t e r e d c o a r s e r grains. R a d i a l l y fibrous calcite o c c u r s as two r e l a t e d varieties. A p p a r e n t l y fresh m a t e r i a l is d a r k b r o w n with p r o m i n e n t m i c r o l a m i n a e 2-25 pm thick. It forms small o u t w a r d s - f a c i n g domes with radial fibres c r o s s - c u t t i n g l a m i n a e (Fig.5). Bundles of fibres g r o w from the surfaces of older l a m i n a e or from micrite, and pellets m a y act as nuclei. W i t h i n domes, some l a m i n a e thin

Fig.5. General view of dome masses of radially fibrous calcite resembling "Rivularia" colonies in Bassin Profond stromatolites. Photograph 1,4 mm across. l a t e r a l l y and s t r u c t u r e s m a y cease g r o w t h and be re-colonized or m a y expand to o v e r g r o w older margins. In the last case, fibres can point l a t e r a l l y a w a y from cores or m a y even be directed, fanwise, b a c k w a r d s (Fig.6). P r e s e r v e d filaments are only c o m m o n locally. Unidentiffed t u b u l e s of 1 pm d i a m e t e r parallel some dome surfaces. O t h e r s are u n r e l a t e d in distrib u t i o n either to the s t r u c t u r e or individual i n t e g r i t y of domes and are p r o b a b l y endolithic. N e i t h e r c o a t e d filaments nor unicell moulds h a v e been r e c o g n i z e d in S E M samples. B r o w n fibrous domes are p a t c h i l y d i s t r i b u t e d and m a n y show an a p p a r e n t loss of both c o l o u r and l a m i n a t i o n . However, a l t h o u g h some domes are colourless, c o l o u r l e s s a r e a s more com-

Fig.6. Close up of radially fibrous calcite in Bassin Profond stromatolite. Note endolithic filaments. Photograph 750 pm across.

152 monly form laterally continuous sheets. In both of these varieties of calcite SEM examination shows only poorly defined crystals with ragged i n t e r p e n e t r a n t boundaries (Fig.7). These are analogous to the spherulitic crystals described by Davies et al. (1978) and Ferguson et al. (1978), and domes in general show a remarkable resemblance to the spherules produced experimentally by these authors. The strong colour observed could reflect dissolved organic molecules present, and t h e loss of colour a bleaching due to oxidation. The identity of individual fibres could be lost by integration analogous to t hat seen in tufa deposits (Braithwaite, 1979). In any event, fibrous domes are important since, notwithstanding similarities to spherules and the d e a rth of preserved filaments, they resemble radially fibrous forms attributed to cyanobacteria (Rivulariaceae). Greyish micrite forms thin diffuse laminae or ovoid peloidal masses, about 75 ~m diameter, associated with domes. Larger areas in interior samples define fenestrae with crude linear vertical channels. I nt e r veni ng columns (Fig.8) are dominantly aggregated micrite but there

Fig.7. SEM of ragged interpenetrant crystal boundaries in radially fibrous area of Bassin Profond stromatolite. Bar scale 10 ~m.

Fig.8. Columns of micrite in Bassin Profond stromatolites. Negative print of section 35 mm across. are locally interlaminated or poorly ordered clusters of laminate fibrous sheets. Channel margins often appear eroded. SEM studies show t h a t micrite consists of disordered nannograins lacking crystal form (Fig.9). Similar micrites have been attributed to precipitation by cyanobact eri a such as Phormidium but there are no preserved cell moulds. Pelletal and apparently crustose layers of micrite might also reflect generation by microbial

Fig.9. SEM of disordered micrite in stromatolitic column from Bassin Profond. Bar Scale 10 pm.

153

activity, but there is no independent evidence of such organisms. Small gastropods, foraminifera (4-5 species) and bone fragments have been noted in interstices between micrite columns, but such components are less common th an in stromatolites from ot he r localities. Since much of the calcite within stromatolites here is fibrous crystals, it is difficult to identify cements. Many cavities from porous interior samples are lined with a laminate fibrous or g r an u lar calcite. This resembles the calcite in colourless fibrous domes but is also similar to flowstone deposits and there are analogies with the endostromatolites of Monty and Maurin (1982). Surfaces of this stalagmitic cement are bored by tubules of 40 ~m diameter, and filaments of about 1 pm seem to occur within crystals. Other surfaces are overlain by micritic internal sediments. Cavities within foraminifera may be lined with either spherular or g r a n u l a r cements. In some gr a nul a r areas within micrite, crystals show a sweeping strain extinction and may be neomorphic. Bulk XRD analyses show Bassin Profond samples are dominated by high-Mg calcite, but magnesium is about 11-12 mol% (estimated from the position of the highest intensity peak), lower t h a n in the other sites described. Small amounts of aragonite may be present.

Fig.10. General view of southern shore of Bassin MacKenzie, scale given by figure on left.

direct surface influx of freshwater was observed, but during the wet season (observation B.A.W.) there is recharge by surface wash from the local catchment. The response to this is rapid and water levels rise and fall daily and seasonally to reflect rainfall. In the dry season the primary control is evaporation. We detected no movements attributable to tides and infer substantial independence from local marine w a t e r s Chemical data are given in Tables I! and III. Conductivity is about 50% more than that of seawater.

Morphology Bassin MacKenzie (grid ref. 3372 0525) This pool (site of sample 42 in MacKenzie, 1971, and pool T3 in Potts and Whitton, 1979) lies a few hundred meters south of the landmark of T a k a m a k a Grove, one of the few surviving areas of hardwood trees on Aldabra. It is about 40m E - W and 35m N-S, and is effectively a dissolution pit rimmed by rock walls about 1 m high (Fig.10). It is floored by more th an a meter of soft spongy sediment. When examined, the n o r t h e r n half of this was exposed and desiccated, while the remainder was covered with water up to 40 cm deep. The sediment is fine-grained and organic-rich and that around the water margin was carpeted with bright pink phototrophic bacteria. No

The stromatolites occur along the southern rim of the basin and are best developed in well lit areas. For several meters they form a shelf projecting about 5 c m from the rock face (Fig.11) and up to 2 c m above the observed water surface. This implies t hat significant growth is related to this surface, r e f e c t i n g the volume and presumably chemistry of dry season residual waters. It is not effective when stromatolites are either emersed or covered by more dilute waters. One isolated dome was seen but additions to upwards-facing surfaces are generally only a few millimeters thick, while those below an overhang may reach 4 5 cm. Upwards-facing surfaces tend to be smooth while others are wrinkled or have a cerebroid folding.

154

white masses beneath them consisting largely of calcite crystals. Thin sections of the calcified layer below the mat surface show apparently live cells of Pleurocapsa but also filaments of Hyella, which is abundant on vertical surfaces and locally common elsewhere. At greater depths abundant Pleurocapsa sheaths are embedded in the calcareous matrix, usually with narrow oscillatoriacean sheaths, and sometimes with the remains of Hyella filaments. Here, however, Dichothrix was either not present or not preserved.

Petrography

Fig.ll. Close up of stromatolite platform on south shore of Bassin MacKenzie, scale 1 m.

Biology Most of the stromatolites have a conspicuous surface mat of live cyanobacteria which ranges from red-brown to dark green. Pleurocapsa, Dichothrix gypsophila and Schizothrix are the predominant organisms. Surface mats tend to be soft where the first two are dominant, and tougher and more leathery where the last is most abundant. The hemispherical colonies of Dichothrix are most frequent on ridges of crenulate surfaces. The innermost parts of the living layer of this have an almost continuous calcareous matrix and incorporate a number of organisms associated with H2S. These organisms include Beggiatoa and Thiothrix with S-granules inside the cells, and mucilaginous masses of pink cells about 1 ~m diameter, which are probably phototrophic bacteria. Calcite crystals are obvious in the sheaths of most superficial growths, but the living colonies are not heavily calcified. In Pleurocapsa, calcification is only obvious around cells 2-3 mm below the surface and deeper. Some larger Dichothrix colonies have

The stromatolitic carbonate accretions rest on an eroded surface of Pleistocene rock. This includes neomorphosed marine limestones associated with areas of palaeosol packed with calcified angiosperm roots (cf. Braithwaite, 1975). Stromatolites typically consist of a series of irregular closely-spaced laminate columns 1-1.5 mm in diameter and 10 mm long. Larger forms with diameters of 1 2.5 cm are present locally (Fig.12). Groups of columns may merge laterally to form sheets. However, in some areas columns are poorly defined and stromatolites have a disordered fenestral structure. Laminae are about 1 mm thick, each consisting of porous inner and denser outer portions. Sometimes these couplets are grouped into major packets or cycles but, while the cyclicity in growth which this indicates obviously reflects some periodic environmental

Fig.12. Cross section of irregular columnar stromatolites, Bassin MacKenzie.

155 change, the controlling factors have not been determined. Growth was not a continuous process, and periods of accretion were interspersed with periods of non-deposition and/or erosion. Some laminae wrap around t r u n c a t e d (eroded) ends of earlier generations. Porous zones consist predominantly of a flocculent aggregated micrite. Some also incorporate foraminifera (several species), ostracods, fragments of mollusc shell and whole gastropods up to 3.5 mm diameter and other allochthonous grains, as well as faecal pellets and peloids of 30 pm maximum diameter. Typically, the total volume of such allochems is small and foraminifera, in particular, are probably not autochthonous. They may have been carried to the pools by birds or tortoises. The most commonly preserved organic structures are filaments. These, however, are generally sparse and randomly-orientated, occurring in a dispersed micritic matrix. In contrast, in dense zones, some areas of micrite lacking coarser grains are associated with closely-packed tubules. These may take several forms. Masses of upright parallel tubules (about 5 ~m diameter) form arching sheets or cushions connected by irregularlyspaced pillars to the sediment below. Unfortunately, most such tubes are empty, probably as a result of attack by bacteria, and/or oxidation of organic material. Where preserved, filaments have thick, brownish, birefringent walls which increase their overall diameter to about 15 ~m. Other tufts consist of similar upright tubules of about 30 ~m diameter. In one sample tubes of 150 pm diameter lie within the bases of cushions, parallel to laminae. These are far larger th an cyanobacterial filaments and might have been formed by metazoans such as insect larvae, but these have not been identified. In some cases, the earliest filaments within mats are extensions of endolithic tubules present in the substrate. A few empty tubules have been identified with the scanning electron microscope (SEM), but these are not common (Fig.13). In some thin sections patches of micrite contain clouds of granules of about 5 pm diameter. These are always dispersed

Fig.13. SEM of Bassin MacKenzie stromatolite showing empty tubules. Bar scale 10 pm. within authigenic carbonate and may represent unicells (see below) but haw~ not been identified in SEM samples. Thus, most of the sediment consists of dense micrite (Fig.14) lacking any evidence of organic structures. Although porous samples contain a number

Fig.14. SEM of coalescivenannograins in micrite of Bassin MacKenzie stromatolite. Bar scale 10 ~m.

156 of morphologically distinctive coarser crystals, most of these only occur locally and adjacent cavities may have different linings. Referring to these as cements suggests t h a t they are distinct from supposed authigenic micrite, although both crystallize from solution, however, it underlines a difference in morphology and time of emplacement. Locally, granular calcite crystals of about 12.5 pm diameter form a cement lining pores. Needle-like crystals of calcite about 10 pm long occur within foraminifera. More importantly, a few large cavities contain a fringe lining of coarse prismatic calcite with crystals as much as 84 ~m long. Some of these have scalenohedral terminations, implying growth submerged in fluid, but others have blunt endings against a common surface, indicating limitation of growth within a water film, t h a t is, vadose deposition. Small areas of radially fibrous zoned cements, some overlain by greyish micrite, resemble stalagmitic coatings. These may have represented relatively open channels within the rock but some stalagmite cements contain sparse 2 pm diameter filaments which could be significant since both Monty (1982) and Monty and Maurin (1982) attributed similar laminar features to microbial activity and called them endostromatolites. The importance of these vadose cements is to indicate t h a t this carbonate deposition took place above the observed water level and is thus a dry season event. In addition to cements there are diffuse patches of granular calcite with crystals about 10 pm diameter. These could be neomorphic or could represent overgrowths on loosely-aggregated micrite, no distinction could be made. Bulk Xray diffraction (XRD) analyses indicate that both micrites and cements are high-Mg calcite, with magnesium ranging from 12-15mo1.~o (estimated from the position of the highest intensity peak). Some aragonite may be present, but volumes can only be small.

Bassins Cinq Cases (Grid Ref. 3770 0605) These pools form an interconnected chain (Fig.15) a few hundred meters south of the

extensive lagoonal channel system of Bras Cinq Cases. Tributaries of this drain wide areas of mangroves and the pools lie within the Lumnitzera racemosa vegetation zone. During the rainy season flooding forms an extensive freshwater cover. During the study, maximum water depths were from centimeters to a few decimeters and once more deposition of stromatolites is closely related to the observed water surface. Water levels varied by several centimeters, but changes lagged several hours behind tidal movements and were of much lower amplitude. This indicates only a tenuous connection to marine waters, although these are the most saline pools examined. Three sets of measurements describe water chemistry (Tables II and III). Stromatolites are far more extensive here than in other locations and extend over an area of about 40,000 m 2 (Fig.15). Living cyanobacterial mats occur up to 10 cm above the observed water surface and calcified stromatolites were found at slightly higher levels, some in pools which contain no active mats. The emergent mats may reflect extreme tidal or seasonal changes in water volume, while the dead calcified structures suggest longer term climatic changes. As in Bassin MacKenzie it appears t h a t most growth occurs at or near dry season levels. Carbonate precipitation, though perhaps not microbial activity generally, seems to be reduced or absent during wet season floods and is clearly limited by emersion. Pools are generally depressions in a low lying rock surface (Fig.16). Pool sediments, like those of Bassin MacKenzie, are light brown, fine-grained, and organic rich. They are again at least a meter thick. Emergent surfaces show desiccation polygons and glisten with salt, but around pool margins, in an intermittently wet zone about 20 cm wide, the surface is bright pink with phototrophic bacteria. Laminated carbonate encrustations are several centimeters thick. Four morphological groups are recognized: (1) Planar encrustations are thin, laterally continuous coatings on surfaces (Fig.16). They

157

Pt 57

®

@

®

:..'.::'.

: -

®

----

Track

E~

Pleistocene marine limestones

POols

Fine sediments

0 I

l

l

l

l

l

50m

@

Unlithified algal mats ®

Oncolites Plane incrustations Cerebroid facies

St romatolites

Pebble incrustations

Fig.15. Distribution map of stromatolitic structures within Cinq Cases pools.

Fig.16. General view of Cinq Cases pools showing platelike stromatolitic encrustations in the foreground, with 30 cm scale.

predominantly face NW SE wind directions with growth and deposition influenced by wave agitation. Coated areas may be several meters long and tens of centimeters wide. The cyano-

bacterial mat (see below) forms a soft spongy surface layer with a crumb-like t ext ure when broken. Beneat h this several centimeters of porous, laminate, authigenic carbonate rest on an eroded surface of Pleistocene limestone. Laminae, varying from millimeters to a centimeter thick, consist of layers of white carbonate separated by dense dark green or black organic films. Some planar encrustations are marked by conspicuous vertical rills. These resemble fluted lapies surfaces, but the intervening ribs are additions to the rock substrate. Morphology nevertheless suggests a relationship to surface drainage and t hat rills reflect concentrations of water flow derived from the living surface mat. (2) Cerebroid bodies form individual mounds 20 40 cm in diameter, but may coalesce laterally. They are best developed in areas exposed to

158 waves. T h e o r g a n i c s u r f a c e is soft a n d flocculent, a n d c a n be d e t a c h e d by gentle pressure. Lithified m a t e r i a l b e n e a t h this is s e v e r a l c e n t i m e t e r s thick, o v e r l y i n g a core of Pleistocene limestone. T h e o u t e r s u r f a c e is c h a r a c t e r ized by low d o m e s w i t h c e n t i m e t e r - s p a c e d i n v o l u t i o n s , h e n c e '~cerebroid" (Fig.17). D o m e s c o n t a i n a s u r f a c e - p a r a l l e l l a m i n a t i o n w h i c h is t h i c k e s t on c r e s t s a n d t h i n s to t r u n c a t e a g a i n s t the m a r g i n s of i n v o l u t i o n s (Fig.18). T h i s implies t h a t g r o w t h ( a c c r e t i o n ) is p r i n c i p a l l y on top of h i g h areas. C e r e b r o i d bodies o c c u r w i t h i n a p e r i o d i c a l l y e m e r g e n t zone, a n d t h e i r m o r p h o l o g y m a y be a biotic r e s p o n s e to t h e p h y s i c o - c h e m i c a l stress of drying. G r a z i n g m a y also be a l i m i t a t i o n , b u t t h e r e a r e no living m o l l u s c s and few a r t h r o p o d s in t h e a r e a a n d

Fig.17. Cerebroid Stromatolites in Cinq Cases pools, scale 30 cm.

we s a w no e v i d e n c e of d a m a g e w h i c h m i g h t h a v e b e e n p r o d u c e d by them. (3) Oncolites ( s u b s p h e r i c a l or ovoid bodies f r o m 2 to 7 cm in d i a m e t e r ) a r e p a r t i c u l a r l y c o m m o n in i n t e r p o o l c h a n n e l s (Fig.19), generally on a r o c k y or c o a r s e s e d i m e n t s u b s t r a t e . T h e y r e p r e s e n t a n e x t e n s i o n of s u r f a c e coloniz a t i o n a r o u n d small loose objects, a n d irregular substrates allow both upwards and d o w n w a r d s - f a c i n g s u r f a c e s to b e c o m e c o a t e d with cyanobacteria. Upper surfaces are pink a n d g r o w t h is g r e a t e s t on these, l a t e r a l margins a n d l o w e r s u r f a c e s a r e d a r k green. In cross-section t h i c k c a r b o n a t e l a m i n a t i o n s (Fig.20) c i r c u m s c r i b e the nucleus, w h i c h c a n be a pebble of P l e i s t o c e n e l i m e s t o n e or o t h e r object (see below). A s y m m e t r i e s of l a m i n a e indicate that growth vectors sometimes change a n d t h a t o n c o l i t e s o v e r t u r n but, a p a r t f r o m d i s t u r b a n c e by g i a n t t o r t o i s e s (which m u s t be rare), t h e r e is no o b v i o u s m e c h a n i s m for this. S u r f a c e l a y e r s of g r o u p s of o n c o l i t e s t e n d to coalesce, f o r m i n g a c o n t i n u o u s m a t w h i c h p r e v e n t s m o v e m e n t . H o w e v e r , s u c h coalesc e n c e is confined to the c y a n o b a c t e r i a l c o v e r a n d no lithified e x a m p l e s h a v e b e e n seen. (4) Other encrustations m a y o c c u r on fragm e n t s of wood, o c c a s i o n a l shells of d e a d Terebralia or pebbles of 20-40 cm d i a m e t e r . T h e s e objects a r e also m o s t c o m m o n in interpool c h a n n e l s . E n c r u s t a t i o n s on t h e l a r g e s t bodies a r e confined to u p p e r s u r f a c e s a n d a r e t h i c k e s t on up-wind sides. W o o d f r a g m e n t s a r e

Fig.18. Vertical sections through cerebroid stromatolites, Cinq Cases.

Fig.19. General view of oncolites in interpool channel at Cinq Cases, refer to Fig.20 for scale.

159

of cerebroid stromatolites. Most of this is H. balani, but some samples from side faces include long filaments of H. caespitosa. The differences in community structure and relative abundance of microbes with depth in representative cerebroid and oncolitic stromatolites are shown in Table IV.

Petrography

Fig.20. V e r t i c a l s e c t i o n s t h r o u g h o n c o l i t e s from Cinq Cases.

often coated concentrically and, where they project upwards, may produce hollow tubes as interiors decompose. In all cases cyanobacterial coatings are smooth and show no par t i c ul a r ornamentation.

Biology In this area cyanobacterial mats associated with calcified stromatolites form on moist or submerged hard substrates. Alongside these, however, both rock substrates and submerged or i n t er mitten tly wetted sediments may bear morphologically similar biotic communities lacking any lithification. On hard substrates the biota is predominantly cyanobact er i a but diatoms are ab u n d a nt on sediments. The soft superficial mats on stromatolites range from orange-pink to dark green, the former dominated by Entophysalis, the latter by Lyngbya (10 pm trichome diameter) or Pleurocapsa. The various shades of pink or orange-brown on submerged sediments are due to combinations of narrow oscillatoriaceans (Lyngbya and Schizothrix), diatoms and phototrophic bacteria. Only two organisms are associated with calcification: Entophysalis, which shows many small crystals growing amongst confluent gelatinous sheaths, and Pleurocapsa. The latter is domin a n t in the outermost layers but substantial lithification only occurs 2-3 mm below the surface. The endolithic Hyella frequently occurs in the outermost parts of the calcareous layer and is sometimes a bunda nt on the sides

Notwithstanding the wide range of surface morphologies in this area and the variations in the biota, stromatolites are petrographically uniform. Carbonate crusts are often centimeters thick. They rest on Pleistocene limestones which are typically coral- and Halimeda-bearing marine deposits, locally preserving some aragonite, with patches of palaeosol containing angiosperm rootlets. Surfaces have been sculpted by dissolution and are often extensively bored. Laminate surface crusts range from smooth sheets on planar surfaces and oncolites, to deeply furrowed cerebroid crusts, but there are no major textural differences. Carbonate laminae consist predominantly of micrite aggregates and form layers T A B L E IV R e l a t i v e a b u n d a n c e s of c y a n o b a c t e r i a a n d a l g a e at specified d e p t h s w i t h i n r e p r e s e n t a t i v e s of cerebroid a n d oncolitic s t r o m a t o l i t e s from Cinq C a s e s pools. Note t h a t t h o s e at t h e s u r f a c e a r e live, as a r e s o m e Pleurocapsa cells at 3 m m , o t h e r s c o r e s refer e n t i r e l y to o b v i o u s s h e a t h r e m a i n s . A b u n d a n c e scale 1 5. 1 = rare, 5 = v e r y a b u n d a n t Species

CEREBROID Pleurocapsa Lyngbya digueti L. martensiana Lyngbya 8 12 pm s h e a t h Hyella balani Entophysalis major ONCOLITE Pleurocapsa Lyngbya digueti L. martensiana Entophysalis ( P u r p l e S-bacteria)

D e p t h from s u r f a c e (mm) 0

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9

12

15

18

5 2

5 2

5

5 1 2

5 2

5 2

5 2

5 2 1

5 2 1

3 4 2 3 5 2 2 2 3

5 1 1

5 3 1

160 a b o u t 1 mm thick. Some, p a r t i c u l a r l y in oncolites, i n c o r p o r a t e micrite pellets and d e t r i t a l grains such as i n t r a c l a s t s and forams, b u t these are not typical. Two o r g a n i c s t r u c t u r e s are preserved. Random filaments are locally a s s o c i a t e d with b o t h micrite and g r a n u l a r calcite. G e n e r a l l y t h e r e is no evidence t h a t these are responsible for p r o d u c i n g a defined t e x t u r e , b u t t h e r e are areas with p a t c h e s of m i c r i t e c o a t i n g v a c a n t tubules, and some i n t e r w o v e n bundles of poorly-preserved filaments form u p r i g h t tufts or c u s h i o n s w i t h i n c a r b o n a t e . H o w e v e r , few filaments h a v e been r e c o g n i z e d in S E M samples (Fig.21). L a r g e n u m b e r s of unicells of a b o u t 7.5 ~m d i a m e t e r o c c u r locally (Fig.22). These resemble cell moulds figured by Horodyski and V o n d e r H a a r (1975), and are attrib u t e d to Entophysalis. As the last two figures illustrate, f r a c t u r e surfaces show no well defined grains. T h e micrite is c e r t a i n l y granular and c r y s t a l l i n e b u t m a g n i f i c a t i o n s of 2000 times fail to define b o u n d a r i e s of i n d i v i d u a l crystals. In m o r e p o r o u s areas (Fig.23) grains look more like a g g r e g a t e d d e t r i t a l p a r t i c l e s t h a n growing crystals. As in Bassin M a c K e n z i e samples, t h e r e are

Fig.21. SEM of filament moulds in Cinq Cases stromatolites. Bar scale 100 ~m.

Fig.22. SEM of moulds of algal unicells in Cinq Cases stromatolites. Bar scale 10 pm. y o u n g e r cements. Well-defined s c a l e n o h e d r a l crystals line pores deep w i t h i n one sample, t o g e t h e r w i t h small p a t c h e s of fibrous split crystals. G r a n u l a r calcite c e m e n t is p r e s e n t w i t h i n some enveloped forams. Small areas of needle-fibre c e m e n t with crystals a b o u t 40 ~m

Fig.23. SEM of aggregated micrite in stromatolites from Cinq Cases. Bar scale 10 ~m.

161 long are also present lining pores. In one cerebroid sample, laminae are truncated abruptly against the margin of a 3 cm dome and are overlain by a granular scalenohedral (70 gm) cement which binds allochthonous pellets onto the surface. These last indicate that crystallization can occur a t the free surface. Finally, small areas of spherular calcite resemble the spherular cements in Bassin Profond samples but appear to be fragments rather than in situ growths. There are again areas of granular calcite which may be overgrowths on existing micrite or be neomorphic. Bulk XRD analysis indicates that samples consist predominantly of high-Mg calcite, here estimated at 14 15 molto Mg, with only traces of aragonite. Discussion and conclusions

Notwithstanding the wide variety of moist carbonate-bearing environments available, only three localized sites on Aldabra have been found to contain stromatolites associated with living cyanobacterial mats. All are in landlocked pools, but only one (Bassin Profond) has a salinity below that of normal seawater. Conductivity of waters in Bassin MacKenzie and the Cinq Cases ponds point to concentrations of dissolved salts at least double t h a t of seawater, although salts present are not simply derived from this source. Substantial dilutions occur in wetter periods, but seasonal changes are not regarded as important since deposition of stromatolites is related to dry season water levels. Although there is considerable morphological variation, and close association with cyanobacterial and algal communities, the structures generated in all three sites show features similar to those produced by passive precipitation (not induced by direct microbial activity) in freshwaters of caves and around springs. The operational saturation index used indicates that all the waters examined are highly supersaturated with respect to CaCO 3 (calcite) irrespective of the supposed marine contribution. In a strict thermodynamic sense, this is

necessary but not sufficient for calcite precipitation. The biogenic nucleation and crystallization kinetics of calcite are pH dependent (Wiechers et al., 1975) and precipitation only becomes common where pH>8.3 (Novitsky, 1981). Precipitation is kinetically controlled by the removal rate of either CO z or H2CO3" from the solution with a minimum CO32- activity (concentration) of 105mol 1 1 in the solution (Michaelis et al., 1985). In open pools, C02 should be in equilibrium with the atmosphere, but, as shown by Donaldson and Whitton (1977a), levels are controlled by biotic activity. Above pH 7.5 (Sj6berg, 1976) the dissolution kinetics of the solution depend upon calcite surface area as well as undersaturation. In Aldabra pools, the in situ carbonate surface is so large t h a t this can be neglected. Thus, from a chemical viewpoint, precipitation depends upon the presence of C a 2 + and CO3 2 ions in solution and the concentrations of OH , H2CO3" and/or CO2 at the solid/solution interface. Assuming a completely mixed liquid phase, these interfaeial conditions are approximately the concentration ratios of the bulk solutions. However, since crystals only form within or beneath microbial colonies, this is an oversimplified view. Table III highlights important differences between pools, indicated by OH /H2CO3" activity ratios, ranging from >>1 in the more saline pools of Cinq Cases to values of <1 in the "fresher" waters of Bassin MacKenzie and Bassin Profond. In the less concentrated waters of Bassin MacKenzie and Bassin Profond, spontaneous precipitation is inhibited if CO 2 exceeds OH at reaction surfaces. The gross reaction Ca 2 + + 2HCO 3 -+ (1a03 ~s)+ H2C03" is unlikely since the carbonic acid released cannot be neutralized. If, however, OH is dominant then the system stabilizes by (extremely rapid) neutralization. H2CO 3 + OH- = H20 + HCO3 Such neutralizations c a n n o t be influenced by the biota, but biological control is possible, mediated by photosynthetic or respiratory

162 influence on the H2CO 3 activity (concentration) at the liquid/solid interface. Therefore, it seems that with sufficient saturation if OH-/H2CO ~ < 1 and if

CO3

2-

< 10 5 mo1 1- 1

precipitation could be controlled by the photosynthetic uptake of CO2 and/or the release of OH-, while dissolution could occur by release of either CO2 in respiration or of other protolysing (organic) compounds. In practice, the well buffered calcite supersaturation of the bulk solution prevents dissolution of precipitated CaCO3, but the implied balance may favour neomorphism. The Aldabra stromatolites are not, therefore, simply the products of bulk solution conditions. The associated microbial communities generate microenvironments and build up interfacial concentration gradients, and while not all cyanobacterial mats promote calcification, surfaces which lack them also lack carbonate crusts at all three sites. The living surface cover of the stromatolites is dominated by cyanobacteria, but only four species are associated with carbonate deposition, Entophysalis, Pleurocapsa, Dichothrix gypsophila and Cladophora. Carbonate is not formed in the surface layer of any of these. Beneath this, however, Pleurocapsa cells, and in Cinq Case samples Entophysalis trichomes, which are presumed to have been living, are seen embedded in carbonate. Locally a few narrow oscillatoriaeean sheaths appear in carbonate in the older parts of crusts while Hyella infests substrate surfaces. However, generally large areas of carbonate contain no trace of cells and these were either never present or have been lost by oxidation and bacterial decomposition as well as obscured by carbonate crystallization. Thus, calcification is only present within the inner region of or beneath the live cell cover. This could indicate t h a t deposition only occurs in older material. It might be seasonal, paralleling some unrecognized variation in species composition, and occur in the wetter October April period when temperatures are higher. In fact, stromatolite morphology sug-

gests t h a t accretion/growth is related to dry season water levels and that some cements also form at this time, indicating a chemically favourable environment. Thus, the relationships observed probably do reflect depositional conditions. The key remaining issue is the mechanism(s) by which cyanobacteria control their microenvironment and bring about the precipitation of carbonate. There are several possibilities. The crystals (including micrite) are not intracellular and filament sheaths, although birefringent, are not calcified. In a saturated system specific molecular groupings within sheath surfaces could promote nucleation and crystal growth. Such a template mechanism is common in biomineralization (as in Wilbur and Watabe, 1963). However, relatively few crystals are seen encrusting filaments and the latter are rarely preserved. Ferguson et al. (1978) showed that some nucleation may involve specific reactions of functional groups on organic molecules (such as cyanobacterial mucilage) with sites on the carbonate surface. This is essentially the same argument, although one stage removed from direct cellular contact. Ferguson et al. (1978) also indicated that several high molecular weight organic compounds can promote crystal growth by hydrophylic/hydrophobic reactions and Mitterer and Cunningham (1985) further emphasized a generalized role for dispersed organic compounds. Finally, Goldsworthy (1986) records minute electrical charges generated by cells which could segregate ions and encourage the formation of critical nuclei. None of these mechanisms is necessary as the chemical data show how the photosynthetic uptake of CO2 would increase the OH-/H2CO ~ ratio of included waters, increase the effective saturation, and promote heterogeneous nucleation on any suitable surface. The implied indirect contribution of the microbial community explains the distribution of the carbonate, although it does not explain why only some species are able to effect precipitation. Beneath colonies we expect to find a concentration gradient in which diffusion and

163 transport of ions are limited, having only restricted communication with open pool waters. Such an environment, maintained within stromatolites, can also account for the growth of cements. The variations in crystal morphology could reflect substrate control, as discussed in Schroeder (1972), but could also indicate chemical differentiation. The lack of well-defined crystal faces may reflect inhibition of particular sites. Ferguson et al.'s (1978) experiments produced several different grain morphologies, and in general higher concentrations of organic molecules favoured the growth of fewer larger crystals. They also repeated the observations of others, implicating magnesium in the inhibition of precipitation and modification of crystal growth. Much of the granular calcite recorded may simply reflect overgrowth on existing micrite. As such, it would not be expected to show a well-defined geometric selection. The growth of granular cements on lateral surfaces of stromatolites is dependent on the presence of a chemical barrier, presumably a mucilaginous coating, in areas not occupied by living cells. Lastly, solutions contained within stromatolites were not always oversaturated since there is local evidence of dissolution of high surface-free energy areas (corners and face edges) of crystals. The radially fibrous calcite growths of Bassin Profond generally lack either preserved filaments or unicells, although they are indistinguishable from bodies claimed to have been formed by cyanobacteria. The structures also closely resemble spherules generated experimentally by Davies et al. (1978), and columnar structures figured by Folk et al. (1985). In the former, crystallization is determined by solutions containing humic acids, which bring about the formation of membranous coatings beneath which crystallization takes place. Unspecified '~algal mucilage", used in the same experiments, consistently produced spherulites. In these the hemispherical surface is defined because it represents the thermodynamically most stable shape, having the smallest ratio of volume to surface area. Some analogous mechanism may be responsible here,

controlled by the brown organic precipitate observed. However, the distinction between crystal growth beneath a mucilaginous membrane without filaments, and that within a coalescent mucilage packed with microbial cells, is only one of degree. Pentecost and Riding (1986) have discussed various styles of calcification in cyanobacteria, in particular, differences between carbonate forming in surface crusts o n sheaths and that generated w i t h i n them. We see no evidence here of either direct association and the illustrations of surface encrustations given by these authors in fact only indicate the availability of filaments as sites for nucleation and do not prove that surfaces were instrumental in bringing nucleation about. The stromatolites on Aldabra are seen to reflect microchemical environments produced by the activities of specific microbial communities. Pool water chemistry results from run-off, substrate solubility, evaporation and mixing with marine ground waters. It is, however, modified within an envelope defined by colony growth. Variations in the biota reflect differences in pool composition and the physical characteristics of sites. These variations are expressed in turn by the diversity of carbonate crystallization products, most obvious in the Bassin Profond samples. The precise path to crystallization is not known; identification would require detailed long term biochemical experiments on a wide variety of organisms and conditions. However, the restriction of possibilities carries important implications both for the accretion of other stromatolites and for the "passive" crystallization of carbonates in tufa deposits of caves and streams. Like Golubic (1973) we see no direct link between microbes and carbonate precipitation. We find no specific association with any terrestrial deposits and conclude that these structures are unlike '~stromatolites" in the Aldabra Pleistocene.

Acknowledgements Dr C. J. R. Braithwaite wishes to acknowledge, with appreciation, the generous support

164

of the Carnegie Trust for the Universities of Scotland, Dr J. Casanova the support of the C.N.R.S. of France.

Appendix Dissolution experiments established an empirical saturation index. In these, two kinds of rock (predominantly high-Mg calcite) were used, one fresh, the other with an epilithic and endolithic biota. Rock fragments were added to about 300ml distilled water in dark closed containers, shaken for about 14 mins and allowed to settle. Subsamples of the waters were decanted and analysed at intervals, with the results given in Table I. The alkalinity and acidity data may be referred to the bicarbonate and carbonic acid concentrations of the solutions. The buffer intensity (pH) in the pH range 4.3-8.4 indicated only the pK' 1 of carbonic acid. Below this range a buffer peak might be due to HSO42-, while above it there is a peak which may reflect NH~/NH 3. A buffering area between pH 9.5-12.5 relates to the pK'2 of carbonic acid and the system changes as CaCO 3 and Mg(OH): are precipitated. The differences in values of the variables measured during the experiments suggest t h a t the system did not reach equilibrium. However, because there was a significant drop in pH in later measurements we assumed our earlier results to be closest to equilibrium and used these to calculate saturation indices.

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