Aragonite laminae in hot water travertine crusts, Rapolano Terme, Italy

Sedimentology (1992) 39, 1067-1079

Aragonite laminae in hot water travertine crusts, Rapolano Terme, Italy

L I G U O and R O B E R T R I D I N G

Department of Geology, University of Wales College of Cardiff; CardizCFI 3 YE, UK

ABSTRACT

Small (5-30 pm) aggregates of aragonite needles occur in calcite crystal crusts of present day hot water slope travertines at Rapolano Terme in Tuscany, Italy. The aggregates are mainly concentrated in irregular, wispy and dark laminae which cross-cut calcite crystal feathers to create a pervasive millimetre scale banded appearance in the deposit; they also occur less commonly scattered irregularly through the calcite layers. The aragonite needle aggregates are in the form of crosses, fascicles (sheaf shaped bundles, or dumbbell shaped), rosettes and spherulites. Locally, irregular masses of needles also occur. The fascicles, rosettes and spherulites have hollow centres which resemble microbial components (?fungal spores, bacterial colonies and pollen), suggesting that the aragonite crystals are biotically nucleated. The lamination is interpreted to reflect diurnal control. Stimulation of microbial activity during daylight concentrates cells in laminae and promotes aragonite calcification. Calcite feather crystals, although traversed by the aragonite aggregate laminae, have a clear appearance under the light microscope. They form more or less continuously through the diurnal cycle by abiotic precipitation. The constant association of aragonite with organic nuclei, irrespective of whether the latter are in laminae or scattered through the calcite layers, supports a biotic control on aragonite formation. Lamination in Pleistocene travertines is superficially similar to that in the present day deposits, but is diagenetically altered. In the Pleistocene deposits, the calcite feathers appear dark under the light microscope and the aragonite aggregates, where they are not altered to dark calcite, are dissolved, together with parts of the adjacent spar calcite, and therefore appear light coloured.

INTRODUCTION

Many non-marine carbonates deposited from flowing water are characterized by millimetric light-dark bands. These are observed in tufa, oncoids, speleo- thems and travertines and they presumably reflect fluctuations in environmental factors. Another distinctive feature in the formation of these essentially crystalline deposits is the intimate interaction of biotic and abiotic influences on precipitation.

Millimetric light-dark bands in present day hot water slope travertines at Rapolano Terme, Tuscany, Italy, show alternations of thin micritic aragonite laminae and thick feather crystal calcite crusts. Similar millimetric bands in travertine slope deposits from Italy have been interpreted by Folk et al. (1985) to reflect daily changes in precipitation related to variation in the activity of photosynthetic bacteria. However, their work was based on old travertines (from quarries), and they did not report the presence of aragonite aggregates.

The aragonite laminae in the present day material are dominated by small sheaf-like fascicles, rosettes and spherulites composed of aragonite needles which formed on now destroyed nuclei interpreted to have been organic objects (possibly fungal spores, bacteria and pollen grains). Precipitation of these aragonite aggregates was biotically influenced. The aragonite laminae traverse coarsely crystalline calcite feathers which appear to be abiotic in origin. The resulting delicate layering, which is a general feature of these slope travertines, is thus bimineralic and appears to be due to contrasting biotic and abiotic nucleation. However, early diagenetic dissolution destroys the evidence for the origin of the micrite and in older Quaternary deposits the aragonite aggregates are absent, although their original presence is revealed by sheet-like pores, mainly cement filled, which preserve the characteristic primary lamination.

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1068 L. Guo and R. Riding

LOCATION AND GENERAL DESCRIPTION

Rapolano Terme is located about 65 km south- southeast of Florence, Italy (Fig. 1). Quaternary travertines, reported to be up to 10000 years old (Barazzuoli et al., 1988), occur extensively in several patches up to 3 km in diameter (Fig. 1) and 50 m thick near Rapolano (Barazzuoli et al., 1987). The term travertine is used here for layered deposits of CaCO,, with moderate to low primary porosity, commonly with a dendritic macrofabric, which is produced by precipitation associated with warm springs (Riding, 1991). The travertines exhibit a wide variety of lithological types related to complex variations in depositional environment. However, they can be broadly grouped into two varieties: (i) light coloured, often white; and (ii) relatively dark coloured, usually in shades of yellow and brown (Barazzuoli et al., 1988). The light coloured travertines mainly formed on slopes, including terrace systems with pools, and typically consist of thick crystal crusts. The dark coloured travertines generally have gently sloping to horizontal stratification, and were mainly deposited in depressions, such as ponds and bases of slopes, and contain an abundance of allochthonous material. In the Rapolano area, the travertines forming today, which occur most extrensively around Terme San

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Giovanni (Fig. l), are mainly restricted to light coloured (slope) varieties. The older, quarried travertines exhibit many examples of both types.

Terme San Giovanni, 1.3 km south-west of Rapo- lano Terme town centre (Fig. l), has Holocene travertines associated with a hot spring. Diversion of water to the thermal baths has reduced the flow to the natural deposits (Barazzuoli et al., 1987), which are limited to a fissure ridge (for terminology see Bargar, 1978; Chafetz & Folk, 1984) and adjacent shallow pools where the flow of water is low and episodic. Water temperature is in the range 34-39C (Barazzuoli et al., 1987). Waters from the baths are periodically piped down the Borro Canotoppa stream valley, to the south of the Terme, where they deposit thick masses of laminated, brilliantly white crystalline crusts (Fig. 2). However, these travertine slope deposits when examined were dry and no longer the site of active deposition.

Cava la Chiusa, located 800m south of Serre di Rapolano (Fig. l), is one of the largest quarries in the area. The travertine sequence is up to 25 m thick, was deposited on Pliocene deposits and is thinly covered by Pleistocene argillaceous pebbly sands (Barazzuoli et al., 1988). Light coloured terrace and slope facies dominate the eastern side of the quarry and pass westward down the palaeoslope into darker fan (Chafetz & Folk, 1984), spring mound and pool

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i Fig. 1. Location of travertine deposits, outlined in bold, near Rapolano Terme, Tuscany. Recent travertine occurs extensively at Terme San Giovanni. Cava la Chiusa is one of several quarries in the older travertines of the area. After Barazzuoli et at. (1987).

Hot water travertine crusts, Italy

sw - fissure ridge

drainage channel crystalline

slope crusts

Borro

r

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Fig. 2. Schematic section of present day travertine (indicated by arrows) deposited on older travertine (in black) at Terme San Giovanni. Hot spring waters which were piped from the travertine fissure ridge to thermal baths were drained to the stream valley where they formed thick, white, feather-like crystalline crusts on the eastern slope.

Fig. 3. Cava la Chiusa quarry face showing Pleistocene crystalline crusts as the main components of travertine terrace and slope deposits.

deposits. The feather crystal crusts (Folk et nl., 1985) are principal components of terrace and slope facies (Fig. 3).

METHODOLOGY

Recently formed travertine crystalline crusts were sampled along the slopes of the Borro Canotoppa

Fig. 4. Crystalline crust consisting of masses of feather crystals orientated perpendicular to the surface of deposition and crossed at 90" by thin, dark coloured aragonite laminae. Modern travertine, Borro Canotoppa steam valley near Terme San Giovanni.

stream valley (Fig. 2); their Pleistocene counterparts were collected from the quarry Cava la Chiusa. Mineralogy was determined by X-ray diffraction (XRD) using a Debye-Scherrer camera. Unetched specimens of the crusts for scanning electron micro- scopy (SEM) observation and electron-probe micro-

1070 L. Guo and R . Riding

analysis (EPMA) were examined on either gold- or carbon-coated broken surfaces.

Giovanni (Fig. 2). They develop on the steep (30-90") slope of the valley as dense, white, laminated deposits, which consist of feather-like crystals orientated perpendicular to the depositional surface and crossed by

CRYSTALLINE SLOPE CRUSTS

Present day travertine, Terme San Giovanni

thin, closely spaced micritic laminae formed by needle aggregates. The laminae give the rock a regularly laminated appearance (Fig. 4). The surface is commonly corrugated into microterraces (see Schreiber et

The crystalline crusts on the north side of the Borro Canotoppa stream valley form when water is released through an open drain from the baths at Terme San

a/., 1981; Chafetz & Folk, 1984) with a relief of 5-20 mm. The deposit is typically soft and powdery below a harder surface.

Fig. 5. Calcite feather crystal crusts. (A) Thin section photomicrograph of an aragonite lamina (arrow) traversing feather crystals. The lamina consists of irregular clusters of spherulitic aggregates. In detail the masses of feather crystals (see Fig. 4) can be seen here to consist of numerous individual feathers branching from either side of a main axis. (B,C) SEM photomicrographs of individual crystal feathers consisting of rhombohedra1 (B) and chevron-shaped (C) crystals. Modem travertine, Borro Canotoppa stream valley near Terme San Giovanni.

Hot water travertine crusts, Italy 1071

branching from either side of a central axis (Fig. 5A). They have been termed ray crystals (Chafetz & Folk, 1984; Folk ef a/ . , 1985) and also have been described as feathery (Folk et al., 1985). Commonly one side is

Calcite feathers

Feather crystals are commonly 1-10 cm high and typically have a bladed appearance with crystals

Fig. 6. SEM photomicrographs of aragonite needle crystal aggregates. (A) Fascicle with a hollow centre suggested to be the site of a bacterial cell. (B,C) Clusters of fascicles. (D) A cross formed by two bundles which taper distally. Modern travertine, Borro Canotoppa stream valley near Terme San Giovanni.


better developed, giving a 'cedar-tree' appearance (Kitano, 1963; Folk et al., 1985). Feather crystals are common features of travertine slopes and terrace rims where they characteristically form dense deposits. The feathers consist of elongated rhombohedra1 (Fig. 5B) and chevron-shaped crystals which are regularly superimposed on each other (Fig. 5C). Irregular micritic crystals may form an external fringe to feathers. Aragonite needle aggregates are concentrated in laminae which cross-cut the feathers, but some aragonite needle aggregates also occur scattered through the feather crystal layers.

Mineralogy. The feather-like crystals making up the major part of the crystalline crusts, together with associated smaller rhombs (see description below), are calcite. This is confirmed by XRD analysis and also indicated by their shape in SEM images and by magnesium incorporation shown by EPMA.

Aragonite needle aggregates

The needles form distinctive aggregates (fascicles, rosettes, crosses, spherulites), and also occur as irregular crusts. The larger aggregates are noticeable in thin section but are only clearly seen in SEM

images. There appears to be a sequence of develop- ment from fascicles to rosettes and finally spherulites.

Fascicles. These are 5-8 pm long, elongate bundles of needles, expanded at each end into a sheaf-like appearance. Their 2-4 pm centres, some hollow, are elliptical (Fig. 6A), spherical or cylindrical in shape. The ends of sheaves expand in radial arrays of needles up to 5 pm in overall diameter.

Rosettes. These are 5-10 pm diameter aggregates of fascicles (Fig. 6B,C).

Crosses. Crosses consist of two similar sized bundles of needles, 5-10 pm in length, which cross at 90". The bundles differ from fascicles in that they do not expand distally but instead taper slightly (Fig. 6D).

Spherulites. These typically have internal diameters of 10 pm and consist of an external radial crust of needle crystals 5-10 pm thick (Fig. 7). Generally, a thin layer of tiny spherical and subspherical grains, 0.15 pm in size, occurs between the needle rim and the hollow nucleus (Fig. 7 ) and formed the substrate on which the needles grew. The inner surface of the crust is always at least slightly irregular, suggesting that a nuclear body was present which may have been

Fig. 7. SEM photomicrograph of a broken spherulite showing the hollow centre (suggested to be an originally organic nucleus) and external radial crust of needle crystals. A thin layer of small grains (arrow) occurs at the base of the needle crust. Modern travertine, Borro Canotoppa stream valley near Terme San Giovanni.


Fig. 8. SEM photomicrographs revealing the various shapes of the hollow centres of spherulites. (A) A composite spherulite interpreted to have formed on several juxtaposed nuclei. (B) A spherulite crossed by a central rod-like projection (arrow). (C) Remains of (?)collapsed organic material within a spherulite. (D) A cylindrical central cavity. Modern travertine, Borro Canotoppa stream valley near Terme San Giovanni.

collapsing as crystals nucleated on its external surface. by a rod-like projection approximately 4 pm in The hollow centres are either spherical and subspher- diameter, which may be hollow (Fig. 8B). In some ical or are composite and formed by several spherical specimens, collapsed ?organic bodies can be observed hollows (Fig. 8A). Solid spherulites, i.e. those with in the otherwise hollow centres (Fig. 8C). Spherulites centres occupied by crystals, have not been observed. are occasionally associated with tubular forms with a Composite spherulites have centres which are crossed central cylindrical hole (Fig. 8D). Two or more


Fig. 9. SEM photomicrograph of several aggregated spherulites sharing internal walls (arrows). Modern travertine, Borro Canotoppa stream valley near Terme San Giovanni.

spherulites, linked together into aggregates up to 50 pm in size, share an internal wall (Fig. 9). Needle crystals of the spherulite rims are straight, 0.2-0.4 pm in diameter, 5-10 pm in length and have either blunt or sharp terminations.

Scattered needles and needle crusts. Individual needles, 5-10 pm long and 0.3-0.5 pm wide, occur in dense irregular sheets or as isolated individuals. Commonly, these needles encrust or are scattered on, and may be partly incorporated within, associated rhombic crystals (Fig. 10; see Rhombic crystals, below). They are generally not closely associated with the fascicles and spherulites and many of these needles are only partly covered by encrusting calcite. This indicates that the formation of needle crystals was not restricted to isolated nuclei.

Mineralogy. The morphology of the needle crystals which constitute the aggregates (fascicles, rosettes, crosses, spherulites) seen in SEM resembles aragonite. This is confirmed by XRD analysis using the Debye- Scherrer camera. Interestingly, for a non-marine aragonite, SEM EPMA also indicated slight (approximately 1%) strontium incorporation. Evaporites within the subsurface may be the source of this strontium, which is preferentially incorporated into the aragonite lattice.

Fig. 10. SEM photomicrograph of aragonite needles scattered on rhombic calcite crystals. Modern travertine, Borro Canotoppa stream valley near Terme San Giovanni.

Rhombic crystals

Less commonly occurring rhombic calcite crystals, 10-25 pm in size, are associated with the needle

Hot water travertine crusts, Ztaly 1075

Fig. 11. SEM photomicrograph of rhombic crystals forming overgrowths on spherulites. Modern travertine, Borro Can- otoppa stream valley near Terme San Giovanni.

aggregates and can form overgrowths on them (Fig. ll), generally with the c-axis of the rhombs parallel to the long-axes of the needles. Initially, small pyramidal crystals are precipitated on the surfaces of spherulites and fascicles, and gradually increase in size to form rhombs. The underlying needles and aggregates are progressively enveloped by the rhombs, although occasionally needles can be seen protruding from the early formed rhombs. This encrustation incorporates the spherulites and other aggregates into the bases of the calcite feather-like crystals. Thus, these rhombic crystals constitute the transition to the feather crystals.

Lamination

In hand specimen, the laminae appear as thin lines, approximately 0.5 mm thick, which are slightly different in colour from the feather crystals. Generally, they are evenly spaced at distances of 1-2 mm and are accentuated by weathering, creating a pervasive millimetre scale lamination in the deposit. In places, however, the laminae are obscure or absent.

In thin section, the laminae are dark coloured. They are revealed to be irregular and composed of concen- trations of aggregates in irregular wispy layers which may traverse calcite crystal feathers (Fig. 5A), and

also occur as delicate festoon-like patches between feathers (Fig. 4).

Older analogues, Cava la Chiusa

Feather-like crystal crusts in the Pleistocene travertine quarries of the Rapolano area, originally deposited on the rims of terrace pools and on steep slopes (Folk et al., 1985), are very similar in occurrence and hand specimen appearance to their present day analogues which formed on the slope of the Borro Canotoppa stream valley near Terme San Giovanni. However, there are significant microscopic differences. In thin section, the Pleistocene feather crystals are dark coloured and their outlines are generally obscure. They are similar in appearance to some shrubs from horizontal travertine deposits. The micritic laminae are either not readily distinguished from the feathers or are absent and appear to have been dissolved, together with parts of adjacent calcite crystals. This dissolution has created irregular, discontinuous voids which crudely parallel the primary layering and thus preserve a semblance of the original lamination (Fig. 12). The space has usually been filled by clear crystalline cement. XRD analyses reveal only calcite in these crusts. Nevertheless, macroscopically these

Fig. 12. Thin section photomicrograph of Pleistocene crystalline crusts from Cava la Chiusa showing that the feather crystals are dark coloured, and the micritic laminae have been dissolved and left open or infilled by clear crystalline cements.


older deposits retain the distinct fine laminations seen in the present day deposits.

DISCUSSION

Origins of needle crystal aggregates

Spherulites and fascicles (also referred to as sheaf- like, and dumb-bell shaped aggregates) similar to those described in this study have been reported from laboratory experiments by Oppenheimer (1961), Greenfield (1 963), McCallum & Guhathakurta (1970), Lippmann (1973, p. 118), Krumbein & Cohen (1977), Krumbein et al. (1977), Castanier et al. (1989) and Buczynski & Chafetz (1991). They have also been observed in Holocene sediments, including Solar Lake mats (Krumbein et al., 1977), Gulf of Aqaba beachrock (Krumbein, 1978), Mammoth Hot Spring travertines (Pentecost, 1990) and travertines in south-west Colorado (Chafetz et al., 1991).

Several of these studies (Oppenheimer, 1961 ; McCallum & Guhathakurta, 1970; Krumbein et ul., 1977; Krumbein, 1979; Castanieretal., 1989; Buczyn- ski & Chafetz, 1991) have directly linked the formation of the fascicles and spherulites to bacterial activity. This has supported earlier studies (Monaghan & Lytle, 1956) and suggests that bacterial activities can concentrate Ca2+ and Mgz+ on the cell surfaces and create microenvironments favouring calcium carbonate precipitation. In addition, abiotic origins for fascicles and spherulites have also been suggested by Lippmann (1973, p. 118) and Pentacost (1990).

In our study, the presence of spherulites, fascicles and rosettes all with hollow centres indicates that these aggregates formed on and around small nuclei. The nuclei are now hollow, suggesting they were originally organic objects or gas bubbles.

Carbonate-encrusted bubbles within travertine are common. Both calcitic (Schreiber et al., 198 1 ; Chafetz & Folk, 1984) and aragonitic (Kitano, 1963) examples have been described from travertine pool deposits, and bimineralic examples (Chafetz et al., 1991) are known to be associated with microbial mats inmodern travertines. However, all these carbonate-encrusted bubbles are of relatively large size, generally a few millimetres in diameter (Chafetz et al., 1991), have regular spherical centres (Schreiber et ul., 1981 ; Chafetz et at., 1991), smooth inner surfaces (Kitano 1963) and do not present such complicated variations in shape as those described in this study. Particularly, the central rod-like projection (Fig. 8B), the collapsed remains (Fig. 8C), the cylindrical hole (Fig. 8D) and

the internal walls (Fig. 9) are very unlikely to have been produced by gas bubbles.

Although compelling evidence is lacking to allow confident interpretation of an organic origin, the hollow centres do resemble microbes, such as fungal spores, bacteria and pollen, in size and morphology. It is well known that it can be difficult to identify microbes, especially where they have been calcified (Jones & MacDonald, 1989). Krumbein et al. (1977) have demonstrated that bacteria or cyanobacteria are deflated and completely destroyed within hours by crystallites surrounding them. The small hollow nuclei (2-5 Fm in size) of the fascicles (Fig. 6A) resemble bacterial nuclei of rosettes of aragonite crystals which McCallum & Guhathakurta (1970) observed from precipitation of calcium carbonate by marine isolates from Bahama Bank sediment. The rosettes (Fig. 6C) are similar to bacterial rosette-shaped aragonite which Krumbein (1979) obtained from laboratory precipitation of carbonates by bacteria isolated from beachrock. The rosettes and spherulites are also similar to bacterial spherulites which Krumbein & Cohen (1977) found from laboratory cultures of a heterotroph rod- shaped bacterium isolated from Solar Lake algal mats. Most spherulites in our study are comparable to bacterially induced spherulites which Oppenheimer (1961) obtained from laboratory experiments. Our hollow spherulites generally show a very thin inner layer consisting of fine spherical grains (Fig. 7), which is comparable to the aragonite-encrusted cyanobacter- ium Aphanothece displaying a deflated and hollow centre (Krumbein et al., 1977). The irregular central structure (Fig. 8C) is similar to collapsed fruiting bodies which Videtich (1985) documented from Pleistocene-Tertiary limestones. Cylindrical holes surrounded by needles (Fig. 8D) may represent an organic filament. Uncalcified spherical bodies attributed to bacteria or fungal spores from Holocene cave deposits (Jones & MacDonald, 1989) are similar to the smallest of the (unpreserved) nuclei observed in our study. The varied shapes of the hollow nuclei may be due to a variety of microbes, as suggested by Greenfield (1963), who related shapes of aragonite aggregates to crossed, rosetted and aggregated bacterial nuclei. In all the above reported cases it has been suggested that the hollow nuclei are organic.

Periodicity of laminae

Regular lamination is conspicuous in these present day slope deposits and their older equivalents in the


travertine quarries at Rapolano Terme. Folk et al. (1985) interpreted this to reflect diurnal changes in precipitation related to variations in the activity of photosynthetic bacteria. They used further evidence for the daily origin of the laminae from modern travertines in Mammoth Hot Springs, Yellowstone National Park, and other sites (Folk et al., 1985) to support this interpretation. The periodicity of the alternation which generates the lamination could be diurnal because the scale of the lamination is so fine. However, Folk et al. (1985) based their interpretation of a bacterial origin for the bands on similarities with travertines from Idaho interpreted as bacterial. They did not report spherulites and fascicles from the Italian travertines they studied. Although their hand specimens show identical lamination to those described here, in thin section they observed shrub-like growths rather than crystal feathers alternating with thin micritic laminae. In contrast, our specimens show aragonite needle aggregate laminae associated with possibly microbial nuclei alternating with long calcite feathers.

Controls on lamina formation

The aragonite mineralogy of the laminae indicates a control on precipitationdifferent from that facilitating the formation of the calcite feather crystals. What specific diurnal variables could be responsible for this alternation and what processes did they control?

It is unlikely that variations in water chemistry from the hot springs occur so regularly and frequently as to produce the observed laminations. Other possible factors determining the mineralogy of the CaCO, precipitate are variations in temperature (both of water and air), degree of carbonate mineral supersaturation (COT * ion concentration) in the waters and microbial activity. These could all vary diurnally.

High temperature as a factor favouring aragonite precipitation in preference to calcite has been suggested by Lowenstam (1954), Wray & Daniels (1957), Kitano et a/ . (1962), Lippmann (1973), Burton & Walter (1987) and Folk (1990). The water temperature at Terme San Giovanni (34-39C; Barazzuoli et al., 1987) is slightly below the 40-45C threshold believed necessary to promote aragonite over calcite in hot springs (Kitano, 1963; Friedman, 1970; Folk, 1990). Possibly, water temperature can be increased after leaving the vent (Chafetz et al., 1991) due to direct summer sunlight on shallow streams of water, and this sunlight can raise the temperature to the threshold of aragonite precipitation. Unfortunately, the fact that

the crystalline crusts examined came from dry slopes which are no longer the site of active travertine formation prevents assessment of conditions that produced the different mineralogies. Nevertheless, if the mineralogy is controlled by temperature, then aragonite precipitation should be enhanced during the Tuscan summer, because the temperatures of the waters are almost constant annually (see Barazzuoli et al., 1987, pp. 79-82). Conversely, aragonite precipitation should be reduced and even disappear during the winter. However, no cyclical annual variation in lamina mineralogy was observed to support this hypothesis.

The degree of carbonate supersaturation may control the mineralogy of the precipitates (Given & Wilkinson, 1985). Chafetz et al. (1991) have demonstrated that with very high supersaturation levels, stellate aragonite crystals were the first precipitates surrounding bubbles and that as the supersaturation levels declined, calcite rhombohedra precipitated. Buczynski & Chafetz (1991) found that dumbbell shaped aragonite crystals precipitated in a more highly supersaturated fluid media, whereas at lower supersaturation states, calcite formed. Furthermore, Buc- zynski & Chafetz (1991) indicated that loss of COz, driving the degree of supersaturation with respect to CaCO, minerals to higher levels, could be due to microbial activities. The stellate clusters of aragonite crystals(Chafetzetal., 1991) and thedumbbellshaped aragonite crystals (Buczynski & Chafetz, 1991) are similar to the fascicles of aragonite observed in our study, which may indicate that these aragonite aggregates were controlled by the degree of supersaturation of waters influenced by biotic activity. Moreover, the aragonite spherulites observed in our study not only appear concentrated in the laminae, but are also scattered on the calcite feathers. This suggests that growth of each spherulite could occur in suspension, with precipitation related to highly supersaturated conditions (Steinen et al., 1987).

The view that microbial activity specifically promotes aragonite rather than calcite precipitation is supported both by observation and by experimental work (see Origins of needle crystal aggregates, above). If this is correct, then the lamination could relate to factors controlling the organic growth of microbes which in turn facilitate aragonite rather than calcite precipitation. In a general sense, microbial activity is greater during daylight due to both a light and temperature stimulus. If light is the major control, then lamination, reflecting alternation in the rate of microbial activity, should be present throughout these


deposits, irrespective of the season. Our preliminary observations indicate that laminations are universally present, suggesting that light is the principal control.

Diagenesis

Comparison of present day laminated slope travertines with their ancient counterparts shows that although similar in hand specimen they are different in thin section. In the Pleistocene examples, the feather crystals are dark coloured and the micritic laminae are either not readily distinguished from the feathers or are absent. In SEM studies of older travertines, the spherulites, rosettes and other needle aggregates are not present and no aragonite was found during XRD analyses. The absence of the micritic laminae and the aragonite aggregates in old travertines are attributed to diagenetic dissolution (or replacement). Somewhat similar phenomena have been demonstrated from the micritic layers in other travertine deposits (Love & Chafetz, 1988), where micritic calcitic algal laminae have been neomorphosed to sparry calcite. Addition- ally, the dark, almost micritic, appearance of the older feather crystal crusts from Rapolano Terme results in them resembling bush-like fabrics which are common elsewhere in travertines (Chafetz & Folk, 1984).

CONCLUSIONS

Thin, micritic, subparallel layers are largely responsible for the fine lamination in dense white crusts in present day hot water slope travertines near Rapolano Terme, Italy. The micritic laminae are irregular and consist of fascicles and spherulitic aggregates of aragonite needles. Generally, these aggregates are 5-30pm in size and appear to have organic nuclei which may include fungal spores, pollen and/or bacterial colonies. Calcite feather crystals form the major part of the crusts and are traversed at 90" by the thin aragonitic micritic layers.

The regular, fine scale lamination may be due to diurnal variation in light, and possibly also in temperature, stimulating microbial activity and con- centrating cells(?) in thin layers and pockets on top of and beside the abiotically precipitated calcite feathers. Aragonite formation may be influenced by elevated temperatures and also by high levels of calcium carbonate supersaturation. However, there is a con- sistent correlation between organic nuclei and aragonite which suggests a specific biotic influence on mineralogy. Organic activity possibly produced high

degrees of supersaturation which resulted in aragonite precipitation in these deposits. Inconstrast, the feather crystals appear to have grown continuously by abiotic processes and form palisades of vertically elongate crystals which are little affected by the aragonitic laminae which pass through them.

Absence of spherulites and fascicles in nearby Holocene-Pleistocene feather crystal crusts is due to dissolution or replacement which has destroyed the aragonite. The micritic laminae are now either not readily distinguished from the dark calcite feathers, or are absent and appear to have been dissolved. These early diagenetic processes have maintained the laminar appearance of these deposits but destroyed the evidence of microbially influenced aragonite precipitation.

ACKNOWLEDGMENTS

This work was funded by TOTAL, Paris, and we thank AndrC Maurin for his support and encourage- ment. Anna Gandin and Armando Constantini generously provided advice and information on the field-area. L.G. is indebted to Steve Kershaw for help and stimulating discussion in the field, and to Arabella Sestini for logistical support in Rapolano. Critical comments by Hank Chafetz and Allan Pentecost helped to improve the manuscript.

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(Manuscript received 16 March 1992; revision received 27 August 1992)

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Aragonite laminae in hot water travertine crusts, Rapolano Terme, Italy