- Email: [email protected]
The role of sulphate-reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the Coorong region, South Australia
ELSEVIER
Sedimentary Geology 126 (1999) 147–157
The role of sulphate-reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the Coorong region, South Australia David T. Wright * Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Received 4 May 1998; accepted 3 March 1999
Abstract The dolomitic distal ephemeral lakes of the Coorong region of South Australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input. During late evaporation, active bacterial sulphate reduction occurs in uppermost, anoxic lake sediments as rising salinity and then H2 S-toxicity kills off the macrobiota. These hypersaline lakewaters are characterised by high pH and elevated concentrations of carbonate and sulphate ions. Magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly-electrolytic, alkaline lake brines from lake-margin stromatolites and mats during desiccation, and may explain the remarkable increase in Mg levels recorded at this time. Continued evaporation of the lakewaters produces a halite crust overlying the dolomite: sulphate-free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate-reducing bacteria. Dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters, and both the morphology and size distribution of dolomite grains is identical to bacterial cells. It is not proven exactly when dolomite precipitation occurs, but the submicron-sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate, which is known to inhibit dolomite formation. Bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation, and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation. In adjacent non-dolomitic lakes, sulphate is present throughout evaporation, so that other minerals, principally aragonite and hydromagnesite, are precipitated. 1999 Elsevier Science B.V. All rights reserved. Keywords: dolomite; bacterial sulphate reduction; kinetic inhibitors; cyanobacteria
1. Introduction The Coorong area of South Australia is well known for the occurrence of modern dolomite, and has been the subject of many studies (Alderman, Ł E-mail:
[email protected]
1959; Von der Borch, 1965, 1976; Von der Borch and Lock, 1979; Lock, 1982; Warren, 1988). It occurs in several of a string of small, distal ephemeral lakes, characteristically elevated a minimum of 1 m above sea level, located to the north of the Coorong Lagoon (Fig. 1). The dolomite is not replacive, and has been interpreted as a primary precipitate (Von
0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 3 7 - 8
148
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 1. Locality map showing distal ephemeral lakes of the Coorong region near Salt Creek, South Australia. Dolomitic study lakes appear in bold type. After Warren (1988).
der Borch and Lock, 1979; Rosen et al., 1988; Wright, 1999), but the proposed hydrological and evaporative models for dolomite formation, though popular, are inconsistent with empirical data and do not explain the contrasting mineralogy of the most recent sediments in adjacent lakes in the chain.
The size and shape of the dolomite grains resemble bacterial bodies, suggesting that they become mineralised during precipitation of dolomite. Benthic microbial communities, especially sulphate-reducing bacteria, dominate the lake ecosystem during the latest stages of evaporation, and their metabolic ac-
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
tivities can have profound effects on ambient water chemistry. This paper provides morphological and geochemical evidence for an essential role for microbes in dolomite formation in distal ephemeral lakes.
2. Geological setting and lake stratigraphy In the early Holocene, the site of the present distal ephemeral lakes was connected to the sea and formed part of an estuarine to lagoonal environment. Deposition of calcareous sediments began to choke the waterway as coastal beach ridges formed a barrier, and the lagoon began to break up into the chain of ephemeral lakes seen today. Lake deposits, which accumulated throughout most of the Holocene in interdunal depressions, comprise a stratigraphically differentiated range of mostly non-skeletal carbonate minerals, reflecting changing depositional environments during continuing marine regression. Cored sections from dolomitic and non-dolomitic lakes all record a shallowing-upwards vertical sequence, beginning with a 6500 year old basal lagoonal facies of mixed aragonite and calcite muds with a restricted marine fauna sharply overlying calcretised marine Pleistocene sands (Von der Borch and Lock, 1979; Warren, 1988). Isolation of the lakes led to the deposition in each of a stratigraphically correlatable suite of carbonate minerals with similar textures; however, mineralogical variability is observed in lakes above the lagoonal carbonate muds. In dolomitic lakes, the lagoonal muds are succeeded by proximal ephemeral lacustrine calcitic and protodolomitic facies, which are capped by a ca. 1 m thick dolomite=magnesite or pure dolomite unit of the distal ephemeral lacustrine facies (Fig. 2). It is this capping dolomite which forms the basis of this study. In the non-dolomitic distal ephemeral lakes near Salt Creek, this uppermost unit comprises aragonite and hydromagnesite (Von der Borch, 1965; D.T. Wright, unpubl. data).
3. Lake ecology The distal ephemeral lakes are fed by rainfall and by seaward-flowing, continental groundwaters from a regional unconfined aquifer which rises above the
149
lake floor in winter, but become isolated during desiccation in the dry summer months as the water table falls (Von der Borch, 1976). The plant Ruppia maritima and an aquatic macrofauna including the brine shrimp Paratemia zietziena, ostracods, cerithid gastropods and nematode worms colonise the lakes during winter and early spring, but are progressively killed off by the rising salinity of increasingly concentrated brines, and toxicity of H2 S. Abundant tests of small gastropods, which thrive when lakewaters are brackish, are blown lagoonwards as the lakes dry out, thus removing fossils (and calcium) from the lake (Von der Borch, 1965; Warren, 1988). Microbes flourish during these late evaporative stages (Von der Borch, 1976; Wright, 1999); important benthic microbial components include surficial cyanobacterial mat and stromatolites, and sulphate-reducing bacteria within the sediments, the latter including Desulfovibrio desulfuricans (L.L. Robbins, pers. commun., 1996; see Section 4). Small, isolated, filmy, conical stromatolites 2–3 cm high, occur in Dolomite and McFaiden lakes, but are completely degraded during desiccation and do not participate directly in mineralisation processes. Single and colonial forms of the cyanobacterium Synechococcus dominate surface waters, but decline in abundance with depth. Other autotrophs, and the green bacteria Chloroflexus, were observed. Desiccation cracks may be colonised during later stages of evaporation by cyanobacteria, supported by capillary-driven water supply, and=or dew.
4. Methods Core samples were obtained from the capping sediments of dolomitic Dolomite, Pellet and McFaiden lakes, and from non-dolomitic South Stromatolite Lake, using 4-cm-diameter plastic piping and=or sample tubes. Most were immersed in 4% glutaraldehyde, which was also added to lakewater samples. Lakewater samples obtained by the author were filtered on site using 0.45 mm filters and duplicates were analysed at CSIRO (Adelaide). pH was measured in situ in the field, thus avoiding sample contamination or alteration during conventional laboratory analysis, and in the laboratory for comparison, using an Orion A290 pH meter and Ross ‘sureflow’ electrode.
150
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 2. Simplified carbonate mineral stratigraphy of a typical dolomitic distal ephemeral lake, as determined by XRD analysis. Carbonate mineralogy is related to changing environments, reflected in successive facies changes during progressive isolation of the lakes; modern dolomite occurs only in ephemeral lakes which experience seasonal desiccation (after Von der Borch and Lock, 1979).
Mg and Ca concentrations were obtained by atomic absorption, sulphate by gas chromatography, and carbonate alkalinity by titration. Mineralogy was determined by XRD: carefully disaggregated powders were mounted and run on a Philips HT Generator Model PW 13 20=00 at the Department of Earth Sciences, University of Oxford, using a Diffractor PW 1050. Microbes were identified from sediments by culture using Bergey’s manual of determinative bacteriology (Holt, 1993); and by electron
microscopy, by Prof. L.L. Robbins, University of South Florida, Tampa, Florida, USA.
5. Dolomite textures in uppermost lake sediments The top few centimetres of the capping dolomite of the dolomitic distal ephemeral lakes has a gelatinous consistency reminiscent of yoghurt, and is associated with organic detritus from degraded Ruppia
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
151
forming by the coalescing of the submicron-sized grains (Fig. 5).
6. Nature and timing of dolomite formation
Fig. 3. SEM photograph of dolomite sediment from Dolomite Lake. Grains are mostly submicron-sized, with larger grains resulting from aggregation of smaller grains. Close inspection of the smallest grains reveals a subspherical or elliptical, bacterial-shaped core often with an overgrowth of white dolomite. Grains are interpreted to be the result of mineral encapsulation of bacterial cells.
plants and cyanobacteria. The dolomite, which becomes more plastic and stiffer with depth, has a structureless, aphanitic texture comprising submicron-sized grains and spherular aggregates which range in size from <5 µm to >63 µm, though pellets in Pellet Lake may be >500 µm. Under the scanning electron microscope (SEM), subspherical and rod-shaped individual dolomite grains (Figs. 3 and 5) are seen to be morphologically identical to bacteria (e.g. De´farge et al., 1996; Folk, 1997; Gournay et al., 1997), whereas aragonite and hydromagnesite sediments from adjacent non-dolomitic lakes comprise elongate crystalline prisms and platelets (Fig. 4). Both the shapes and size distributions of the dolomite grains and clusters are typical of bacterial precipitates (Buczynski and Chafetz, 1991; Folk, 1993). Discrete grains show very limited morphological variation and no regular crystal habit. This suggests that the bacteria have been mineralised by encapsulation while alive (e.g. Castanier et al., 1989; Chafetz and Buczynski, 1992). The smallest spheres are identical in shape and size to so-called ‘nannobacteria’ (Folk, 1993). Dolomite collected at 5 cm depth from Lake McFaiden has apparently undergone recrystallisation, with individual rhombic crystals typical of crystalline dolomite apparently
Radiocarbon age dating of surface dolomites is 300 š 250 years (Von der Borch, 1976), proving that they are recent. The absence of evidence for replacement of calcium carbonate by dolomite supports the contention that the dolomite is a primary precipitate. The dolomite was not found to be intergrown with any other carbonate phase. Dolomite forms in situ — there is no dolomite present in interdune flats or calcareous dunes — and must therefore be related in some way to lakewater and=or porewater chemistry. In the dry season, the lakes are perched above the water table, and apart from rare showers, there is no input of water. This seasonal hydrological isolation of the lakes means that chemical evolution of lakewaters is driven by evaporation and internal factors: the lakes form dynamic systems in which the concentrations of solutes generally increase as evaporation continues, but changes in alkalinity must be related to internal processes, of which sulphate reduction is known to be important. When exactly dolomite precipitation occurs is still questionable. It is highly probable that precipitation is rapid, as suggested by the micron-sized grains, lattice disorder of cations and closely spaced lattice defects (Rosen et al., 1988; Wright, 1999). Chemical analysis of lakewaters (Tables 1 and 2), together with experimental evidence that sulphate inhibits precipitation of dolomite (Baker and Kastner, 1981; Slaughter and Hill, 1991), suggests that the dolomite precipitates when all sulphate is removed, just prior to total desiccation. Analytical test-strips detected sulphate levels at between 0 and 200 mg=l in residual wet patches in Dolomite Lake just prior to complete evaporation.
7. Results and discussion Tables 1 and 2 show that all lakes record high pH, carbonate alkalinity and sulphate concentrations relative to surface seawater. Given the calcareous nature of the surrounding dune aquifers (Von der Borch, 1965), it is surprising that Ca levels are so low (be-
152
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 4. (a, b) SEM photographs of euhedral aragonite crystals and platelets of hydromagnesite from sediment North Stromatolite Lake. In the general view (a) the abundant larger grains are non-microbial in form, and few submicron grains reminiscent of bacteria are present (note presence of rare possible organism at centre-left). Enlarged detail of the sediment comprising prisms and platelets appears in (b).
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
153
Table 1 Chemical analyses of Coorong distal lakewaters (late October) Lake=ion
Mg2C (mg=l)
Ca2C (mg=l)
Mg=Ca
CO23 (mg=l)
SO24 (mg=l)
pH
Mineralogy
Pellet Dolomite S. Stromatolite
1730 957 134
66 115 54.1
26 8 2.5
23.24 34.67 28.12
5,000 3,500 35,788
8.470 9.260 8.780
dolomite dolomite aragonite=hydromagnesite
CO23 (mg=l)
SO24 (mg=l)
pH
Mineralogy
21.68
9650 <200 73400 2688
8.461 8.512 7.863 7.9
dolomite dolomite aragonite=hydromagnesite
Table 2 Chemical analyses of Coorong distal lakewaters (early December) Lake=ion
Mg2C (mg=l)
Ca2C (mg=l)
Pellet Dolomite S. Stromatolite Seawater
5,280 165 32 Dried out, except for residual wet patches 28,720 112 256 1,313 400 3.2
Mg=Ca
low seawater concentrations). A possible reason for this may be the precipitation of calcium carbonate as biotic skeletal elements in Pellet and Dolomite lakes, where dolomite is the sole non-skeletal sediment, rather than deficiency of supply. Comparison of Tables 1 and 2, and reference to seasonal chemical analyses of several distal lakes of varying mineralogy in the Coorong chain reported by Von der Borch (1965), show that concentrations of Mg dramatically increase late in the evaporative cycle. This peak cannot be attributed solely to concentration by evaporation in Pellet Lake because Mg=Ca ratios increase significantly, and the only carbonate phase present is dolomite. Cyanobacteria are known to concentrate Mg preferentially in their sheaths by up to four times its abundance in seawater, and in experiments were shown to contain 60% MgO (Gebelein and Hoffman, 1973). In South Stromatolite Lake, the dramatic increase in Mg=Ca ratio follows desiccation of lakemargin stromatolites, suggesting that Mg stored in stromatolitic cyanobacteria throughout their growth is released rapidly during late evaporation. Data from the non-dolomitic South Stromatolite lake (Table 2) clearly show that high Mg=Ca ratios alone cannot be a controlling factor in dolomite formation. These modern, massive dolomites and other carbonates in the lacustrine surficial sediments have been interpreted as typically evaporative in origin (e.g. Warren, 1988). However, there are a number
12.4 16.2
of problems associated with this view. Experiments involving evaporation of the ephemeral lakewaters under laboratory conditions have consistently failed to produce dolomite (Von der Borch, pers. commun., 1995, 1996). The dominant carbonate mineral precipitated in such experiments was in fact aragonite, which is in line with experiments performed by Borchert and Muir (1964). Aragonite is not found in any of the lake sediments where modern dolomite occurs. Furthermore, although elevated sulphate concentrations were recorded during evaporation, gypsum is not found in any of the studied lakes. The precipitation of halite crusts in desiccated lakes negates the possibility that gypsum was somehow flushed from the system: it never formed. Lippmann (1973) argued that evaporation would not favour dolomitisation despite consequent high Mg=Ca ratios, because of the accompanying decrease in the CO23 -free ion. The already low activity of the CO23 ion would fall further, due to its complexing to form MgCO03 neutral ion pairs while Mg2C would also complex with sulphate, giving and MgSO04 , thus further enhancing kinetic inhibitions. The apparent interdependence of dolomite with evaporation is therefore misleading. Clearly, other processes must be involved in the precipitation of dolomite. Modern evaporitic environments are often colonised by benthic microbial communities (BMCs), principally cyanobacteria, and a genetic link be-
154
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 5. (a, b) SEM photographs of dolomite sediment at 5 cm depth from Lake McFaiden. The uppermost sediment comprises submicron-sized grains of bacterial affinity overlying more structured grains with developing crystal faces (a). There appears to be an intimate relationship between clusters of these mineralised, bacterial-shaped nanno-grains and crystalline dolomite: where the two grain types are in physical contact, lower surfaces of nanno-grains can be seen partially incorporated into crystalline dolomite. Remnant outer surfaces of nanno-grains can be seen within crystal faces where they become almost totally enveloped by the larger crystal — clearly seen at higher magnification in (b). The loss of identity of the nanno-grains at the expense of the larger crystals may represent diagenetic self-organisation of mineral grains driven by neomorphic crystal growth from encapsulated, mineralised bacterial spheres.
tween BMCs and dolomite has been observed in both modern and ancient sediments (e.g. Compton, 1988; Wright, 1993, 1994, 1997, 1999; Vasconcelos et al., 1995; Vasconcelos and McKenzie, 1997). Cyanobacteria, in the form of stromatolites and as free-floating autotrophs, were recorded in all the studied lakes, and the yoghurt-like consistency of the carbonate muds is due in part to microbial binding (see also Rosen et al., 1988). Microbial populations are thus abundant in the dolomitic muds of the Coorong distal ephemeral lakes, and sulphate-reducing bacteria are particularly active in shallow subsurface lake sediments during the later stages of evaporation. The spherular dolomite resembles the grains produced in microbial experiments by Vasconcelos et al. (1995) through the mineralisation of organic material in a sulphate-reducing medium. This suggests a similar origin for the Coorong dolomite in shallow subsurface sediments: anoxic organic degradation driven by sulphate-reducing bacteria can cause profound
chemical changes to porewaters, and by diffusion to overlying lakewaters, leading to elevated pH and carbonate concentrations. The removal of sulphate from overlying lakewaters may proceed by diffusion to exhaustion, so that dolomite may then precipitate both authigenically within pre-existing dolomitic sediments and from modified lakewaters. Within the zone of sulphate reduction, organic matter is consumed, sulphate removed, and sulphide released into ambient waters with metabolic CO2 (Berner, 1980): 2CH2 O C SO24 ! H2 S C 2HCO3 The sulphide may combine with available iron to produce pyrite and=or pyrrhotite (e.g. Last and De Decker, 1990), and pyrite-rich laminae occur in sediments of the north shore of Pellet Lake. Presentday cyanobacteria contain nitrogen-rich proteins and fatty acids (Huc, 1980). Sulphate-reducing bacteria and other microbes oxidise cyanobacterial and
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
155
Fig. 5 (continued).
other organic matter to support their metabolism, producing ammonia from enzymatic breakdown of proteins, which is rapidly absorbed by ambient waters, thereby increasing pH and carbonate alkalinity to levels necessary for dolomite formation (Berner, 1980; Durand, 1980; Slaughter and Hill, 1991): 2 2NH3 C CO2 C H2 O ! 2NHC 4 C CO3
NH3 C H2 O ! NHC 4 C OH OH C HCO3 ! H2 O C CO23 These reactions help explain the elevated pH values obtained from lakewaters and sediment porewaters. Sulphate reduction may also lead to the release of free Mg2C ions (Slaughter and Hill, 1991): 2CH2 O C
MgSO04
! 2HCO3 C H2 S C Mg
conditions favourable to dolomite precipitation, by increasing the activity of carbonate and magnesium ions and removing sulphate. The chemical data from Dolomite and Pellet lakes would appear to be consistent with this hypothesis. In South Stromatolite Lake on the other hand, sulphate concentrations are much higher, and persist throughout evaporation. It is known that calcite as well as dolomite is inhibited by the presence of sulphate, which may explain why aragonite is the preferred mineral carbonate phase (e.g. Bathurst, 1975; Walter, 1986), while the remaining Mg is precipitated as hydromagnesite.
8. Conclusion
2C
The sharp increase in lakewater Mg concentrations concomitant with desiccation of cyanobacteria indicates that cyanobacterial degradation may be an important source of unhydrated Mg ions (Murata et al., 1972; Gebelein and Hoffman, 1973; Lyons et al., 1984). Sulphate-reducing bacteria in association with cyanobacterial degradation can thus create
Sulphate-reducing bacteria and other microbes play an essential role in dolomite precipitation in distal ephemeral lakes of the Coorong, by modifying ambient waters as a consequence of their metabolism so that kinetic inhibitors to dolomite formation are overcome. Bacterial sulphate reduction in shallow subsurface sediments of the dolomitic lakes reaches a peak during late evaporation, and Mg
156
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
previously concentrated in cyanobacterial sheaths is released rapidly during desiccation of lake-margin stromatolites into the highly alkaline, strongly electrolytic, sulphate-depleted brines. This combination of biochemical processes facilitates dolomite precipitation around the numerous bacterial cells found in ambient waters and sediments during this time. Carbonate mineralogy is probably controlled by the degree of biochemical modification of ambient waters, so that in non-dolomitic lakes, where sulphate remains in porewaters even in very low concentrations throughout evaporation, aragonite along with hydromagnesite or magnesite will precipitate rather than dolomite.
Acknowledgements I am greatly indebted to Lisa Robbins (University of South Florida, USA) for advice and technical support in salty waters and to Christ Von Der Borch of Flinders University, Adelaide, for logistical and scientific support during field data acquisition in the Coorong. I would like to record my sincere thanks to the editor, Gilbert Camoin, for his encouragement, and to my reviewers, one of whom was Jean-Paul Perthuisot, for their constructive role in the completion of this paper. I also acknowledge with gratitude the support of the Royal Society for travel to Australia.
References Alderman, A.R., 1959. Aspects of carbonate sedimentation. J. Geol. Soc. Aust. 6, 1–10. Baker, P., Kastner, M., 1981. Constraint on the formation of sedimentary dolomite. Science 213, 214–216. Bathurst, R.G.C., 1975. Carbonate Sediments and their Diagenesis. Elsevier, Amsterdam. Berner, R.A., 1980. Early Diagenesis: a Theoretical Approach. Princeton University Press, Princeton, NJ. Borchert, H., Muir, R.O., 1964. Salt Deposits. Van Nostrand Reinhold, London. Buczynski, C., Chafetz, H.S., 1991. Habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy. J. Sediment. Petrol. 61, 226–233. Castanier, S., Maurin, A., Perthuisot, J.-P., 1989. Production expe´rimentale de corpuscules carbonate´s sphe´roı¨daux a` structure fibro-radiaire. Re´flexion sur la de´finition des ooı¨des. Bull. Soc. Ge´ol. Fr. 5, 589–595.
Chafetz, H.S., Buczynski, C., 1992. Bacterially induced lithification of microbial mats. Palaios 7, 277–293. Compton, J.S., 1988. Sediment composition and precipitation of dolomite and pyrite in the Neogene Monterey and Sisquoc formations, Santa Maria Basin area, California. In: Shukla, V., Baker, P.A. (Eds.), Sedimentology and Geochemistry of Dolostones. Soc. Econ. Paleontol. Mineral. Spec. Publ. 43, 53–64. De´farge, C., Trichet, J., Jaunet, A.-M., Michel, R., Tribble, J., Sansone, F.J., 1996. Texture of microbial sediments revealed by electron microscopy. J. Sediment. Res. 66, 935–947. Durand, B., 1980. Sedimentary organic matter and kerogen. Definition and quantitative importance of kerogen. In: Durand, B. (Ed.), Kerogen. Technip, Paris, 27, pp. 13–34. Folk, R.L., 1993. SEM imaging of bacteria and nannobacteria in carbonate sediments and rocks. J. Sediment. Petrol. 463, 990–999. Folk, R.L., 1997. Nannobacterial mediation in carbonate diagenesis. IAS–ASF–IGCP 380 Int. Workshop ‘Microbial Mediation in Carbonate Diagenesis’, Chichilliane (abstr.). Publ. Assoc. Sedimentol. Fr. 26, 23. Gebelein, C.D., Hoffman, P., 1973. Algal origin of dolomite laminations in stromatolitic limestone. J. Sediment. Petrol. 43, 603–613. Gournay, J., Folk, R.L., Kirkland, B.L., 1997. Evidence for nannobacterially precipitated dolomite in Pennsylvanian carbonates. IAS–ASF–IGCP 380 Int. Workshop ‘Microbial Mediation in Carbonate Diagenesis’, Chichilliane (abstr.). Publ. Assoc. Sedimentol. Fr. 26, 33. Holt, J.G., 1993. Bergey’s Manual of Determinative Bacteriology, 9th ed. Williams & Wilkins, Baltimore, 787 pp. Huc, A.Y., 1980. Origin and formation of organic matter in recent sediments and its relationship to kerogen. In: Durand, B. (Ed.), Kerogen. Technip, Paris, 27, pp. 445–474. Last, W.M., De Decker, P., 1990. Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia. Sedimentology 37, 967–981. Lippmann, F., 1973. Sedimentary Carbonate Minerals. Springer, Berlin. Lock, D.E., 1982. Groundwater Controls on Dolomite Formation in the Coorong Region of South Australia and in Ancient Analogues. Ph.D. Thesis, The Flinders University of South Australia, Adelaide. Lyons, W.B., Long, D.T., Hines, M.E., Gaudette, H.E., Armstrong, P.B., 1984. Calcification of cyanobacterial mats in Solar Lake, Sinai. Geology 12, 623–626. Murata, K.J., Friedman, I., Cremer, M., 1972. Geochemistry of diagenetic dolomites in Miocene marine formations in California and Oregon. U.S. Geol. Surv. Prof. Pap. 724-C. Rosen, M.R., Miser, D.E., Warren, J.K., 1988. Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia. Sedimentology 35, 105–122. Slaughter, M., Hill, R.J., 1991. The influence of organic matter in organogenic dolomitisation. J. Sediment. Petrol. 61, 296–303. Vasconcelos, C., McKenzie, J.A., 1997. Microbial mediation of modern dolomite precipitation and diagenesis under anoxic
D.T. Wright / Sedimentary Geology 126 (1999) 147–157 conditions (Lagoa Vermelha, Rio de Janeiro, Brazil). J. Sediment. Petrol. 67, 378–390. Vasconcelos, C., McKenzie, J.A., Bernasconi, S., Grujic, D., Tien, A., 1995. Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures. Nature 377, 220–222. Von der Borch, C.C., 1965. The distribution and preliminary geochemistry of modern carbonate sediments of the Coorong area, South Australia. Geochim. Cosmochim. Acta 29, 781– 799. Von der Borch, C.C., 1976. Stratigraphy and formation of Holocene dolomitic carbonate deposits of the Coorong area, South Australia. J. Sediment. Petrol. 46, 952–966. Von der Borch, C.C., Lock, D., 1979. Geological significance of Coorong dolomites. Sedimentology 26, 813–824. Walter, L.M., 1986. Relative efficiency of carbonate dissolution and precipitation during diagenesis: a progress report on the role of solution chemistry. In: Gautier, D.L. (Ed.), Roles of Organic Matter in Sediment Diagenesis. Soc. Econ. Paleontol.
157
Mineral. Spec. Publ. 38, 1–11. Warren, J.K., 1988. Sedimentology of Coorong dolomite in the Salt Creek region, South Australia. Carbonates Evaporites 3, 175–199. Wright, D.T., 1993. Studies of the Cambrian Eilean Dubh Formation of North-Western Scotland. Ph.D. Thesis, University of Oxford, Oxford. Wright, D.T., 1994. The role of benthic microbial communities in widespread dolomite formation (abstr.). In: Awramik, S.M. (Ed.), Death Valley International Stromatolite Symposium, Laughlin, NV, p. 95. Wright, D.T., 1997. An organogenic origin for widespread dolomite in the Cambrian Eilean Dubh Formation, northwestern Scotland. J. Sediment. Res. 67, 54–64. Wright, D.T., 1999. Benthic microbial communities and dolomite formation in marine and lacustrine environments — a new dolomite model. In: Glenn, C.R., Lucas, J., Prevot-Lucas, L. (Eds.), Marine Authigenesis: From Global to Microbial. Soc. Econ. Paleontol. Mineral. Spec. Publ. 65 (in press).
Sedimentary Geology 126 (1999) 147–157
The role of sulphate-reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the Coorong region, South Australia David T. Wright * Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Received 4 May 1998; accepted 3 March 1999
Abstract The dolomitic distal ephemeral lakes of the Coorong region of South Australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input. During late evaporation, active bacterial sulphate reduction occurs in uppermost, anoxic lake sediments as rising salinity and then H2 S-toxicity kills off the macrobiota. These hypersaline lakewaters are characterised by high pH and elevated concentrations of carbonate and sulphate ions. Magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly-electrolytic, alkaline lake brines from lake-margin stromatolites and mats during desiccation, and may explain the remarkable increase in Mg levels recorded at this time. Continued evaporation of the lakewaters produces a halite crust overlying the dolomite: sulphate-free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate-reducing bacteria. Dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters, and both the morphology and size distribution of dolomite grains is identical to bacterial cells. It is not proven exactly when dolomite precipitation occurs, but the submicron-sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate, which is known to inhibit dolomite formation. Bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation, and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation. In adjacent non-dolomitic lakes, sulphate is present throughout evaporation, so that other minerals, principally aragonite and hydromagnesite, are precipitated. 1999 Elsevier Science B.V. All rights reserved. Keywords: dolomite; bacterial sulphate reduction; kinetic inhibitors; cyanobacteria
1. Introduction The Coorong area of South Australia is well known for the occurrence of modern dolomite, and has been the subject of many studies (Alderman, Ł E-mail:
[email protected]
1959; Von der Borch, 1965, 1976; Von der Borch and Lock, 1979; Lock, 1982; Warren, 1988). It occurs in several of a string of small, distal ephemeral lakes, characteristically elevated a minimum of 1 m above sea level, located to the north of the Coorong Lagoon (Fig. 1). The dolomite is not replacive, and has been interpreted as a primary precipitate (Von
0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 3 7 - 8
148
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 1. Locality map showing distal ephemeral lakes of the Coorong region near Salt Creek, South Australia. Dolomitic study lakes appear in bold type. After Warren (1988).
der Borch and Lock, 1979; Rosen et al., 1988; Wright, 1999), but the proposed hydrological and evaporative models for dolomite formation, though popular, are inconsistent with empirical data and do not explain the contrasting mineralogy of the most recent sediments in adjacent lakes in the chain.
The size and shape of the dolomite grains resemble bacterial bodies, suggesting that they become mineralised during precipitation of dolomite. Benthic microbial communities, especially sulphate-reducing bacteria, dominate the lake ecosystem during the latest stages of evaporation, and their metabolic ac-
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
tivities can have profound effects on ambient water chemistry. This paper provides morphological and geochemical evidence for an essential role for microbes in dolomite formation in distal ephemeral lakes.
2. Geological setting and lake stratigraphy In the early Holocene, the site of the present distal ephemeral lakes was connected to the sea and formed part of an estuarine to lagoonal environment. Deposition of calcareous sediments began to choke the waterway as coastal beach ridges formed a barrier, and the lagoon began to break up into the chain of ephemeral lakes seen today. Lake deposits, which accumulated throughout most of the Holocene in interdunal depressions, comprise a stratigraphically differentiated range of mostly non-skeletal carbonate minerals, reflecting changing depositional environments during continuing marine regression. Cored sections from dolomitic and non-dolomitic lakes all record a shallowing-upwards vertical sequence, beginning with a 6500 year old basal lagoonal facies of mixed aragonite and calcite muds with a restricted marine fauna sharply overlying calcretised marine Pleistocene sands (Von der Borch and Lock, 1979; Warren, 1988). Isolation of the lakes led to the deposition in each of a stratigraphically correlatable suite of carbonate minerals with similar textures; however, mineralogical variability is observed in lakes above the lagoonal carbonate muds. In dolomitic lakes, the lagoonal muds are succeeded by proximal ephemeral lacustrine calcitic and protodolomitic facies, which are capped by a ca. 1 m thick dolomite=magnesite or pure dolomite unit of the distal ephemeral lacustrine facies (Fig. 2). It is this capping dolomite which forms the basis of this study. In the non-dolomitic distal ephemeral lakes near Salt Creek, this uppermost unit comprises aragonite and hydromagnesite (Von der Borch, 1965; D.T. Wright, unpubl. data).
3. Lake ecology The distal ephemeral lakes are fed by rainfall and by seaward-flowing, continental groundwaters from a regional unconfined aquifer which rises above the
149
lake floor in winter, but become isolated during desiccation in the dry summer months as the water table falls (Von der Borch, 1976). The plant Ruppia maritima and an aquatic macrofauna including the brine shrimp Paratemia zietziena, ostracods, cerithid gastropods and nematode worms colonise the lakes during winter and early spring, but are progressively killed off by the rising salinity of increasingly concentrated brines, and toxicity of H2 S. Abundant tests of small gastropods, which thrive when lakewaters are brackish, are blown lagoonwards as the lakes dry out, thus removing fossils (and calcium) from the lake (Von der Borch, 1965; Warren, 1988). Microbes flourish during these late evaporative stages (Von der Borch, 1976; Wright, 1999); important benthic microbial components include surficial cyanobacterial mat and stromatolites, and sulphate-reducing bacteria within the sediments, the latter including Desulfovibrio desulfuricans (L.L. Robbins, pers. commun., 1996; see Section 4). Small, isolated, filmy, conical stromatolites 2–3 cm high, occur in Dolomite and McFaiden lakes, but are completely degraded during desiccation and do not participate directly in mineralisation processes. Single and colonial forms of the cyanobacterium Synechococcus dominate surface waters, but decline in abundance with depth. Other autotrophs, and the green bacteria Chloroflexus, were observed. Desiccation cracks may be colonised during later stages of evaporation by cyanobacteria, supported by capillary-driven water supply, and=or dew.
4. Methods Core samples were obtained from the capping sediments of dolomitic Dolomite, Pellet and McFaiden lakes, and from non-dolomitic South Stromatolite Lake, using 4-cm-diameter plastic piping and=or sample tubes. Most were immersed in 4% glutaraldehyde, which was also added to lakewater samples. Lakewater samples obtained by the author were filtered on site using 0.45 mm filters and duplicates were analysed at CSIRO (Adelaide). pH was measured in situ in the field, thus avoiding sample contamination or alteration during conventional laboratory analysis, and in the laboratory for comparison, using an Orion A290 pH meter and Ross ‘sureflow’ electrode.
150
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 2. Simplified carbonate mineral stratigraphy of a typical dolomitic distal ephemeral lake, as determined by XRD analysis. Carbonate mineralogy is related to changing environments, reflected in successive facies changes during progressive isolation of the lakes; modern dolomite occurs only in ephemeral lakes which experience seasonal desiccation (after Von der Borch and Lock, 1979).
Mg and Ca concentrations were obtained by atomic absorption, sulphate by gas chromatography, and carbonate alkalinity by titration. Mineralogy was determined by XRD: carefully disaggregated powders were mounted and run on a Philips HT Generator Model PW 13 20=00 at the Department of Earth Sciences, University of Oxford, using a Diffractor PW 1050. Microbes were identified from sediments by culture using Bergey’s manual of determinative bacteriology (Holt, 1993); and by electron
microscopy, by Prof. L.L. Robbins, University of South Florida, Tampa, Florida, USA.
5. Dolomite textures in uppermost lake sediments The top few centimetres of the capping dolomite of the dolomitic distal ephemeral lakes has a gelatinous consistency reminiscent of yoghurt, and is associated with organic detritus from degraded Ruppia
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
151
forming by the coalescing of the submicron-sized grains (Fig. 5).
6. Nature and timing of dolomite formation
Fig. 3. SEM photograph of dolomite sediment from Dolomite Lake. Grains are mostly submicron-sized, with larger grains resulting from aggregation of smaller grains. Close inspection of the smallest grains reveals a subspherical or elliptical, bacterial-shaped core often with an overgrowth of white dolomite. Grains are interpreted to be the result of mineral encapsulation of bacterial cells.
plants and cyanobacteria. The dolomite, which becomes more plastic and stiffer with depth, has a structureless, aphanitic texture comprising submicron-sized grains and spherular aggregates which range in size from <5 µm to >63 µm, though pellets in Pellet Lake may be >500 µm. Under the scanning electron microscope (SEM), subspherical and rod-shaped individual dolomite grains (Figs. 3 and 5) are seen to be morphologically identical to bacteria (e.g. De´farge et al., 1996; Folk, 1997; Gournay et al., 1997), whereas aragonite and hydromagnesite sediments from adjacent non-dolomitic lakes comprise elongate crystalline prisms and platelets (Fig. 4). Both the shapes and size distributions of the dolomite grains and clusters are typical of bacterial precipitates (Buczynski and Chafetz, 1991; Folk, 1993). Discrete grains show very limited morphological variation and no regular crystal habit. This suggests that the bacteria have been mineralised by encapsulation while alive (e.g. Castanier et al., 1989; Chafetz and Buczynski, 1992). The smallest spheres are identical in shape and size to so-called ‘nannobacteria’ (Folk, 1993). Dolomite collected at 5 cm depth from Lake McFaiden has apparently undergone recrystallisation, with individual rhombic crystals typical of crystalline dolomite apparently
Radiocarbon age dating of surface dolomites is 300 š 250 years (Von der Borch, 1976), proving that they are recent. The absence of evidence for replacement of calcium carbonate by dolomite supports the contention that the dolomite is a primary precipitate. The dolomite was not found to be intergrown with any other carbonate phase. Dolomite forms in situ — there is no dolomite present in interdune flats or calcareous dunes — and must therefore be related in some way to lakewater and=or porewater chemistry. In the dry season, the lakes are perched above the water table, and apart from rare showers, there is no input of water. This seasonal hydrological isolation of the lakes means that chemical evolution of lakewaters is driven by evaporation and internal factors: the lakes form dynamic systems in which the concentrations of solutes generally increase as evaporation continues, but changes in alkalinity must be related to internal processes, of which sulphate reduction is known to be important. When exactly dolomite precipitation occurs is still questionable. It is highly probable that precipitation is rapid, as suggested by the micron-sized grains, lattice disorder of cations and closely spaced lattice defects (Rosen et al., 1988; Wright, 1999). Chemical analysis of lakewaters (Tables 1 and 2), together with experimental evidence that sulphate inhibits precipitation of dolomite (Baker and Kastner, 1981; Slaughter and Hill, 1991), suggests that the dolomite precipitates when all sulphate is removed, just prior to total desiccation. Analytical test-strips detected sulphate levels at between 0 and 200 mg=l in residual wet patches in Dolomite Lake just prior to complete evaporation.
7. Results and discussion Tables 1 and 2 show that all lakes record high pH, carbonate alkalinity and sulphate concentrations relative to surface seawater. Given the calcareous nature of the surrounding dune aquifers (Von der Borch, 1965), it is surprising that Ca levels are so low (be-
152
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 4. (a, b) SEM photographs of euhedral aragonite crystals and platelets of hydromagnesite from sediment North Stromatolite Lake. In the general view (a) the abundant larger grains are non-microbial in form, and few submicron grains reminiscent of bacteria are present (note presence of rare possible organism at centre-left). Enlarged detail of the sediment comprising prisms and platelets appears in (b).
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
153
Table 1 Chemical analyses of Coorong distal lakewaters (late October) Lake=ion
Mg2C (mg=l)
Ca2C (mg=l)
Mg=Ca
CO23 (mg=l)
SO24 (mg=l)
pH
Mineralogy
Pellet Dolomite S. Stromatolite
1730 957 134
66 115 54.1
26 8 2.5
23.24 34.67 28.12
5,000 3,500 35,788
8.470 9.260 8.780
dolomite dolomite aragonite=hydromagnesite
CO23 (mg=l)
SO24 (mg=l)
pH
Mineralogy
21.68
9650 <200 73400 2688
8.461 8.512 7.863 7.9
dolomite dolomite aragonite=hydromagnesite
Table 2 Chemical analyses of Coorong distal lakewaters (early December) Lake=ion
Mg2C (mg=l)
Ca2C (mg=l)
Pellet Dolomite S. Stromatolite Seawater
5,280 165 32 Dried out, except for residual wet patches 28,720 112 256 1,313 400 3.2
Mg=Ca
low seawater concentrations). A possible reason for this may be the precipitation of calcium carbonate as biotic skeletal elements in Pellet and Dolomite lakes, where dolomite is the sole non-skeletal sediment, rather than deficiency of supply. Comparison of Tables 1 and 2, and reference to seasonal chemical analyses of several distal lakes of varying mineralogy in the Coorong chain reported by Von der Borch (1965), show that concentrations of Mg dramatically increase late in the evaporative cycle. This peak cannot be attributed solely to concentration by evaporation in Pellet Lake because Mg=Ca ratios increase significantly, and the only carbonate phase present is dolomite. Cyanobacteria are known to concentrate Mg preferentially in their sheaths by up to four times its abundance in seawater, and in experiments were shown to contain 60% MgO (Gebelein and Hoffman, 1973). In South Stromatolite Lake, the dramatic increase in Mg=Ca ratio follows desiccation of lakemargin stromatolites, suggesting that Mg stored in stromatolitic cyanobacteria throughout their growth is released rapidly during late evaporation. Data from the non-dolomitic South Stromatolite lake (Table 2) clearly show that high Mg=Ca ratios alone cannot be a controlling factor in dolomite formation. These modern, massive dolomites and other carbonates in the lacustrine surficial sediments have been interpreted as typically evaporative in origin (e.g. Warren, 1988). However, there are a number
12.4 16.2
of problems associated with this view. Experiments involving evaporation of the ephemeral lakewaters under laboratory conditions have consistently failed to produce dolomite (Von der Borch, pers. commun., 1995, 1996). The dominant carbonate mineral precipitated in such experiments was in fact aragonite, which is in line with experiments performed by Borchert and Muir (1964). Aragonite is not found in any of the lake sediments where modern dolomite occurs. Furthermore, although elevated sulphate concentrations were recorded during evaporation, gypsum is not found in any of the studied lakes. The precipitation of halite crusts in desiccated lakes negates the possibility that gypsum was somehow flushed from the system: it never formed. Lippmann (1973) argued that evaporation would not favour dolomitisation despite consequent high Mg=Ca ratios, because of the accompanying decrease in the CO23 -free ion. The already low activity of the CO23 ion would fall further, due to its complexing to form MgCO03 neutral ion pairs while Mg2C would also complex with sulphate, giving and MgSO04 , thus further enhancing kinetic inhibitions. The apparent interdependence of dolomite with evaporation is therefore misleading. Clearly, other processes must be involved in the precipitation of dolomite. Modern evaporitic environments are often colonised by benthic microbial communities (BMCs), principally cyanobacteria, and a genetic link be-
154
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
Fig. 5. (a, b) SEM photographs of dolomite sediment at 5 cm depth from Lake McFaiden. The uppermost sediment comprises submicron-sized grains of bacterial affinity overlying more structured grains with developing crystal faces (a). There appears to be an intimate relationship between clusters of these mineralised, bacterial-shaped nanno-grains and crystalline dolomite: where the two grain types are in physical contact, lower surfaces of nanno-grains can be seen partially incorporated into crystalline dolomite. Remnant outer surfaces of nanno-grains can be seen within crystal faces where they become almost totally enveloped by the larger crystal — clearly seen at higher magnification in (b). The loss of identity of the nanno-grains at the expense of the larger crystals may represent diagenetic self-organisation of mineral grains driven by neomorphic crystal growth from encapsulated, mineralised bacterial spheres.
tween BMCs and dolomite has been observed in both modern and ancient sediments (e.g. Compton, 1988; Wright, 1993, 1994, 1997, 1999; Vasconcelos et al., 1995; Vasconcelos and McKenzie, 1997). Cyanobacteria, in the form of stromatolites and as free-floating autotrophs, were recorded in all the studied lakes, and the yoghurt-like consistency of the carbonate muds is due in part to microbial binding (see also Rosen et al., 1988). Microbial populations are thus abundant in the dolomitic muds of the Coorong distal ephemeral lakes, and sulphate-reducing bacteria are particularly active in shallow subsurface lake sediments during the later stages of evaporation. The spherular dolomite resembles the grains produced in microbial experiments by Vasconcelos et al. (1995) through the mineralisation of organic material in a sulphate-reducing medium. This suggests a similar origin for the Coorong dolomite in shallow subsurface sediments: anoxic organic degradation driven by sulphate-reducing bacteria can cause profound
chemical changes to porewaters, and by diffusion to overlying lakewaters, leading to elevated pH and carbonate concentrations. The removal of sulphate from overlying lakewaters may proceed by diffusion to exhaustion, so that dolomite may then precipitate both authigenically within pre-existing dolomitic sediments and from modified lakewaters. Within the zone of sulphate reduction, organic matter is consumed, sulphate removed, and sulphide released into ambient waters with metabolic CO2 (Berner, 1980): 2CH2 O C SO24 ! H2 S C 2HCO3 The sulphide may combine with available iron to produce pyrite and=or pyrrhotite (e.g. Last and De Decker, 1990), and pyrite-rich laminae occur in sediments of the north shore of Pellet Lake. Presentday cyanobacteria contain nitrogen-rich proteins and fatty acids (Huc, 1980). Sulphate-reducing bacteria and other microbes oxidise cyanobacterial and
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
155
Fig. 5 (continued).
other organic matter to support their metabolism, producing ammonia from enzymatic breakdown of proteins, which is rapidly absorbed by ambient waters, thereby increasing pH and carbonate alkalinity to levels necessary for dolomite formation (Berner, 1980; Durand, 1980; Slaughter and Hill, 1991): 2 2NH3 C CO2 C H2 O ! 2NHC 4 C CO3
NH3 C H2 O ! NHC 4 C OH OH C HCO3 ! H2 O C CO23 These reactions help explain the elevated pH values obtained from lakewaters and sediment porewaters. Sulphate reduction may also lead to the release of free Mg2C ions (Slaughter and Hill, 1991): 2CH2 O C
MgSO04
! 2HCO3 C H2 S C Mg
conditions favourable to dolomite precipitation, by increasing the activity of carbonate and magnesium ions and removing sulphate. The chemical data from Dolomite and Pellet lakes would appear to be consistent with this hypothesis. In South Stromatolite Lake on the other hand, sulphate concentrations are much higher, and persist throughout evaporation. It is known that calcite as well as dolomite is inhibited by the presence of sulphate, which may explain why aragonite is the preferred mineral carbonate phase (e.g. Bathurst, 1975; Walter, 1986), while the remaining Mg is precipitated as hydromagnesite.
8. Conclusion
2C
The sharp increase in lakewater Mg concentrations concomitant with desiccation of cyanobacteria indicates that cyanobacterial degradation may be an important source of unhydrated Mg ions (Murata et al., 1972; Gebelein and Hoffman, 1973; Lyons et al., 1984). Sulphate-reducing bacteria in association with cyanobacterial degradation can thus create
Sulphate-reducing bacteria and other microbes play an essential role in dolomite precipitation in distal ephemeral lakes of the Coorong, by modifying ambient waters as a consequence of their metabolism so that kinetic inhibitors to dolomite formation are overcome. Bacterial sulphate reduction in shallow subsurface sediments of the dolomitic lakes reaches a peak during late evaporation, and Mg
156
D.T. Wright / Sedimentary Geology 126 (1999) 147–157
previously concentrated in cyanobacterial sheaths is released rapidly during desiccation of lake-margin stromatolites into the highly alkaline, strongly electrolytic, sulphate-depleted brines. This combination of biochemical processes facilitates dolomite precipitation around the numerous bacterial cells found in ambient waters and sediments during this time. Carbonate mineralogy is probably controlled by the degree of biochemical modification of ambient waters, so that in non-dolomitic lakes, where sulphate remains in porewaters even in very low concentrations throughout evaporation, aragonite along with hydromagnesite or magnesite will precipitate rather than dolomite.
Acknowledgements I am greatly indebted to Lisa Robbins (University of South Florida, USA) for advice and technical support in salty waters and to Christ Von Der Borch of Flinders University, Adelaide, for logistical and scientific support during field data acquisition in the Coorong. I would like to record my sincere thanks to the editor, Gilbert Camoin, for his encouragement, and to my reviewers, one of whom was Jean-Paul Perthuisot, for their constructive role in the completion of this paper. I also acknowledge with gratitude the support of the Royal Society for travel to Australia.
References Alderman, A.R., 1959. Aspects of carbonate sedimentation. J. Geol. Soc. Aust. 6, 1–10. Baker, P., Kastner, M., 1981. Constraint on the formation of sedimentary dolomite. Science 213, 214–216. Bathurst, R.G.C., 1975. Carbonate Sediments and their Diagenesis. Elsevier, Amsterdam. Berner, R.A., 1980. Early Diagenesis: a Theoretical Approach. Princeton University Press, Princeton, NJ. Borchert, H., Muir, R.O., 1964. Salt Deposits. Van Nostrand Reinhold, London. Buczynski, C., Chafetz, H.S., 1991. Habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy. J. Sediment. Petrol. 61, 226–233. Castanier, S., Maurin, A., Perthuisot, J.-P., 1989. Production expe´rimentale de corpuscules carbonate´s sphe´roı¨daux a` structure fibro-radiaire. Re´flexion sur la de´finition des ooı¨des. Bull. Soc. Ge´ol. Fr. 5, 589–595.
Chafetz, H.S., Buczynski, C., 1992. Bacterially induced lithification of microbial mats. Palaios 7, 277–293. Compton, J.S., 1988. Sediment composition and precipitation of dolomite and pyrite in the Neogene Monterey and Sisquoc formations, Santa Maria Basin area, California. In: Shukla, V., Baker, P.A. (Eds.), Sedimentology and Geochemistry of Dolostones. Soc. Econ. Paleontol. Mineral. Spec. Publ. 43, 53–64. De´farge, C., Trichet, J., Jaunet, A.-M., Michel, R., Tribble, J., Sansone, F.J., 1996. Texture of microbial sediments revealed by electron microscopy. J. Sediment. Res. 66, 935–947. Durand, B., 1980. Sedimentary organic matter and kerogen. Definition and quantitative importance of kerogen. In: Durand, B. (Ed.), Kerogen. Technip, Paris, 27, pp. 13–34. Folk, R.L., 1993. SEM imaging of bacteria and nannobacteria in carbonate sediments and rocks. J. Sediment. Petrol. 463, 990–999. Folk, R.L., 1997. Nannobacterial mediation in carbonate diagenesis. IAS–ASF–IGCP 380 Int. Workshop ‘Microbial Mediation in Carbonate Diagenesis’, Chichilliane (abstr.). Publ. Assoc. Sedimentol. Fr. 26, 23. Gebelein, C.D., Hoffman, P., 1973. Algal origin of dolomite laminations in stromatolitic limestone. J. Sediment. Petrol. 43, 603–613. Gournay, J., Folk, R.L., Kirkland, B.L., 1997. Evidence for nannobacterially precipitated dolomite in Pennsylvanian carbonates. IAS–ASF–IGCP 380 Int. Workshop ‘Microbial Mediation in Carbonate Diagenesis’, Chichilliane (abstr.). Publ. Assoc. Sedimentol. Fr. 26, 33. Holt, J.G., 1993. Bergey’s Manual of Determinative Bacteriology, 9th ed. Williams & Wilkins, Baltimore, 787 pp. Huc, A.Y., 1980. Origin and formation of organic matter in recent sediments and its relationship to kerogen. In: Durand, B. (Ed.), Kerogen. Technip, Paris, 27, pp. 445–474. Last, W.M., De Decker, P., 1990. Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia. Sedimentology 37, 967–981. Lippmann, F., 1973. Sedimentary Carbonate Minerals. Springer, Berlin. Lock, D.E., 1982. Groundwater Controls on Dolomite Formation in the Coorong Region of South Australia and in Ancient Analogues. Ph.D. Thesis, The Flinders University of South Australia, Adelaide. Lyons, W.B., Long, D.T., Hines, M.E., Gaudette, H.E., Armstrong, P.B., 1984. Calcification of cyanobacterial mats in Solar Lake, Sinai. Geology 12, 623–626. Murata, K.J., Friedman, I., Cremer, M., 1972. Geochemistry of diagenetic dolomites in Miocene marine formations in California and Oregon. U.S. Geol. Surv. Prof. Pap. 724-C. Rosen, M.R., Miser, D.E., Warren, J.K., 1988. Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia. Sedimentology 35, 105–122. Slaughter, M., Hill, R.J., 1991. The influence of organic matter in organogenic dolomitisation. J. Sediment. Petrol. 61, 296–303. Vasconcelos, C., McKenzie, J.A., 1997. Microbial mediation of modern dolomite precipitation and diagenesis under anoxic
D.T. Wright / Sedimentary Geology 126 (1999) 147–157 conditions (Lagoa Vermelha, Rio de Janeiro, Brazil). J. Sediment. Petrol. 67, 378–390. Vasconcelos, C., McKenzie, J.A., Bernasconi, S., Grujic, D., Tien, A., 1995. Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures. Nature 377, 220–222. Von der Borch, C.C., 1965. The distribution and preliminary geochemistry of modern carbonate sediments of the Coorong area, South Australia. Geochim. Cosmochim. Acta 29, 781– 799. Von der Borch, C.C., 1976. Stratigraphy and formation of Holocene dolomitic carbonate deposits of the Coorong area, South Australia. J. Sediment. Petrol. 46, 952–966. Von der Borch, C.C., Lock, D., 1979. Geological significance of Coorong dolomites. Sedimentology 26, 813–824. Walter, L.M., 1986. Relative efficiency of carbonate dissolution and precipitation during diagenesis: a progress report on the role of solution chemistry. In: Gautier, D.L. (Ed.), Roles of Organic Matter in Sediment Diagenesis. Soc. Econ. Paleontol.
157
Mineral. Spec. Publ. 38, 1–11. Warren, J.K., 1988. Sedimentology of Coorong dolomite in the Salt Creek region, South Australia. Carbonates Evaporites 3, 175–199. Wright, D.T., 1993. Studies of the Cambrian Eilean Dubh Formation of North-Western Scotland. Ph.D. Thesis, University of Oxford, Oxford. Wright, D.T., 1994. The role of benthic microbial communities in widespread dolomite formation (abstr.). In: Awramik, S.M. (Ed.), Death Valley International Stromatolite Symposium, Laughlin, NV, p. 95. Wright, D.T., 1997. An organogenic origin for widespread dolomite in the Cambrian Eilean Dubh Formation, northwestern Scotland. J. Sediment. Res. 67, 54–64. Wright, D.T., 1999. Benthic microbial communities and dolomite formation in marine and lacustrine environments — a new dolomite model. In: Glenn, C.R., Lucas, J., Prevot-Lucas, L. (Eds.), Marine Authigenesis: From Global to Microbial. Soc. Econ. Paleontol. Mineral. Spec. Publ. 65 (in press).