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Dolomite Mountains and the origin of the dolomite rock of whichthey mainly consist: historical developments and newperspectives

JUD ITH A. MCKENZIE and CRISOGO NO VA SCONCELOSETH-Zurich, Geological Institute, 8092 Zurich, Switzerland (E-mail: [email protected])

ABSTRACT

Beginning in the late 18th Century, the Dolomite Mountains in Northern Italyhave been the location for major sedimentological developments, from thediscovery of the mineral dolomite to the formulation of the coral-reef hypothesis to explain the origin of the massive dolomite structures thatdefine the splendid scenery of the region. Further, the Dolomite Mountainshave inspired voluminous research into the origin of dolomite, questioningwhether dolomite is a primary precipitate or a secondary replacement product.Recently, with the recognition that microbes can mediate dolomiteprecipitation, a new geomicrobiological approach, combining the study of modern natural environments with bacterial culture experiments, is now beingused to calibrate or interpret microbial evidence derived from the dolomiterock record. This three-pronged methodology applied to the study of dolomiteformation holds great promise for future research into the ‘Dolomite Problem’and provides a new impetus to revisit the Dolomite Mountains in the 21stCentury.

Keywords Dolomieu, dolomite, Dolomite Mountains, microbial meditation.

INTRODUCTION

Dolomite [CaMg(CO3)2] is a common carbonatemineral in sedimentary rocks throughout thegeological record, especially in Precambrian carbonate rocks where it is abundant and oftenfound in association with microbial structures; but it is rarely found forming in modern carbonateenvironments. Because of its rare occurrence inmodern sediments, as well as the apparentinability to synthesize it under low-temperatureconditions in the laboratory, the origin of dolo-mite has remained a long-standing enigma

in sedimentology, often called the ‘DolomiteProblem’.

Numerous publications reviewing the DolomiteProblem have appeared during the past 100 years, beginning with the classic paper of Van Tuyl(1916). More recent publications (Machel &Mountjoy, 1986; McKenzie, 1991; Warren, 2000;Machel, 2004) document and discuss in detail themany dolomite-forming models. Based on fieldobservations and theoretical considerations, thesemodels are constructed so that they include the

various conditions that may overcome the ther-

modynamic and kinetic barriers to dolomiteprecipitation. In this work, however, the aim isnot to review the Dolomite Problem, per se, but totrace the historical developments of fundamentalconcepts related to the origin of dolomite aspertaining to the important contributions of theMediterranean Realm, in particular the DolomiteMountains of Northern Italy, to this enduringsedimentological conundrum. Additionally, anew geo-microbiological approach to dolomiteinvestigations is presented, which promises toprovide innovative insights for the Dolomite

Problem in the 21st Century.

DOLOMITE: THE MINERAL, ROCK ANDMOUNTAINS

The mountains of the Southern Tyrol Alps of theMediterranean Realm are acknowledged widelyto be the geographical area where dolomitewas discovered. The Italian geologist GiovanniArduino (1713 to 1795) is credited with the first

Sedimentology (2009) 56, 205–219 doi: 10.1111/j.1365-3091.2008.01027.x

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists 205



documented identification of dolomite as a dis-tinct carbonate mineral in 1779, as reported invon Morlot (1847); but the name of the mineral,rock and mountains is unquestionably associatedwith one individual, the French geologist andmineralogist De odat de Dolomieu (1750 to 1801).During field excursions in the region, Dolomieu

recognized the occurrence of an unusual carbon-ate rock that resembled limestone but onlyweakly effervesced with acid (de Dolomieu,1791; Zenger et al., 1994). Dolomieu publishedhis observations in the Journal de Physique and,one year later, in the same journal, Nicolas-The -odore de Saussure (1767 to 1845) provided achemical analysis of the rock, which was named‘dolomie’ after his colleague Dolomieu (de Saus-sure, 1792). Within 10 years of its discovery, theEnglish version of the name, dolomite, was in useand it had become a commonly identified rock

type.Because of the high resistance of the mineraldolomite to chemical weathering, dolomite rock

is a prominent geomorphological feature in theEastern Alps. Thus, it is easy to understand whythe extremely rugged mountains found in North-eastern Italy have been designated the DolomiteMountains most probably beginning with thepublication of a book by Gilbert & Churchill(1864), who described their excursions through

the district during a period of three years. Gilbertand Churchill documented the magnificence of the mountainous scenery with beautiful water-colours illustrating the most exceptional features,such as the Rosengarten Group shown in Fig. 1.Summarizing the 19th Century research in theDolomite Mountains, those authors wrote that thequestion, ‘‘What then is the origin of this strictly‘family group’ of mountain masses?’’, had twoparts: ‘‘First, what is the origin of these moun-tains, as such? And then, What is the origin of theDolomite rock of which they mainly consist?’’

(Gilbert & Churchill, 1864). More than 200 yearsafter the discovery by Dolomieu, these two ques-tions essentially remain the central focus of

Fig. 1. Watercolour by Josiah Gilbert illustrating the Southern Tyrol landscape (Gilbert & Churchill, 1864). Thisromantic 19th Century image of the Rosengarten Group is one of the watercolours and etchings printed in their traveljournal, which details the authors’ ‘‘excursions through Tyrol, Carinthia, Carniola, & Friuli in 1861, 1862, & 1863including a geological chapter, and pictorial illustrations from original drawings on the spot’’.

206 J. A. McKenzie and C. Vasconcelos

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology , 56, 205–219



sedimentological investigations into carbonateplatform evolution and the origin of dolomite atthe beginning of the 21st Century, although theyare formulated scientifically in other terms.

ORIGIN OF THE DOLOMITE MOUNTAINS

In the 19th Century, Baron Ferdinand F. vonRichthofen (1833 to 1905) produced the firstsystematic stratigraphical analysis of the moun-tains in the Southern Tyrol. The outstandinggeological research of von Richthofen led to theformulation of a coral-reef hypothesis to explainthe topographical peculiarities of the region (vonRichthofen, 1860). Based on detailed mappingand stratigraphical investigations of the region(Fig. 2), von Richthofen used the ‘Schlern Dolo-mite’ to illustrate this hypothesis. Von Richtho-

fen (1860) wrote that ‘‘the Schlern is a coral reef,and the entire formation of the ‘Schlern Dolo-mite’ has in like manner originated throughanimal activity’’. The cross-section includedwith the geological map in Fig. 2 diagrammati-cally shows the great difference in elevation between the Schlern mass and the surroundinglandscape. The Schlern with its slopes fallingsteeply on all sides is isolated from similarmasses in the vicinity, such as the Langkofel.The undisturbed flat-lying beds upon which theSchlern rests and the equally undisturbed bedsof the overlying Raibl Group, containing fauna

indicative of shallow waters, suggested to vonRichthofen that the deposits are nearly in theiroriginal depositional position.

The coral-reef theory of Charles Darwin (1809to 1882), first published in 1842 (Darwin, 1842),was accepted readily by biologists and geologistsin the following years. The clear exposition of Darwin on coral growth and coral-reef buildingaround volcanic edifices in the Pacific Ocean andthe evolution of these barriers or encircling reefsinto lagoon islands or atolls with subsequent slowsubsidence of the mid-ocean islands provided

von Richthofen with the appropriate model toencompass all of his field observations. Thethick, isolated masses of dolomite and dolomiticlimestone of Southern Tyrol, with occasionallypreserved corals, compellingly resembled thecoral reef islands of Darwin. Von Richthofen(1860) was convinced that these masses could best be accounted for if they were considered to be of coralline origin having accumulated duringlong periods of continued subsidence. In otherwords, draining the Pacific Ocean would reveal a

scene similar to the unique topography observedin the modern Southern Tyrol panorama.

Edmund Mojsisovics von Mojsva r (1833 to1905) maintained and developed further thecoral-reef hypothesis of von Richthofen in thor-ough studies of the Dolomite Mountains. Thephotographs and accompanying cross-sections of

Mojsisovics von Mojsva r that illustrate the Tri-assic reefs of the Southern Tyrol, together with adetailed analysis of the sedimentological andstratigraphical character of the outcrop sections,firmly established the concept that the structureswere formed originally as isolated carbonateplatforms in an open marine environment(Fig. 3; Mojsisovics von Mojsvar, 1879). Therecontinued to be, however, opponents of thecoral-reef hypothesis considering evidence basedon the general absence of corals in the dolomite,with calcareous algae and echinoderms being the

most common fossils, and the apparent thinning-out of the dolomite layers into siliciclastics dueto faulting, which was also used to account forthe shape of the masses (Skeats, 1905). Morerecently, it has been proposed that the carbonateplatforms may actually have formed as microbialmud-mounds based on the predominance of bio-induced micritic precipitate comprisingundolomitized boulders redeposited from theplatforms and preserved in the surroundingsiliciclastics (Russo et al., 1997). Furthermore,major amounts of bio-induced micrite can still be recognized in the pervasively dolomitized

slope sediments, indicating the importance of this process in the production of carbonate forthe platform evolution (Keim & Schlager, 1999).

Overall, in the intervening years since vonRichthofen and Mojsisovics published the resultsof their comprehensive research, the Triassiccarbonate platforms in the Dolomite Mountainshave provided geologists with spectacular out-crops to study the evolution of carbonate plat-forms. See Schlager and Keim (2009) for anoverview or Bosellini et al. (2003) for a brief introduction to the geology of the Dolomite

Mountains.

ORIGIN OF THE DOLOMITE ROCK

As reported by von Morlot (1847), Arduino in1779 was the first to attempt an explanation forthe origin of Southern Tyrol dolomite via thealteration of the local limestone by reaction withmagnesium-enriched hydrothermal solutions.Approximately 50 years later, Leopold von Buch

Dolomite Mountains and the origin of the dolomite rock 207




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(1774 to 1853) expanded on this theme in a seriesof letters published in 1824 (von Buch, 1824).Von Buch proposed that the Southern Tyrol

mountains were uplifted by volcanic forces andwere converted from limestone into dolomite bythe action of ‘magnesia’ vapours penetrating from

A

B

Fig. 3. (A) Photograph of the view from the southern plateau of the Schlern with the Rosengarten in the background(reproduced from von Mojsvar, 1879) and (B) north-south longitudinal cross-section through the Rosengarten–Schlern Mountains (Stratal dip in the background of the Thierser Valley; reproduced from Mojsisovics von Mojsvar,1879). Although the two mountains are tectonically and orographically separated, the profile illustrates the strikingconsistency of the total thickness (900 to 1000 m) for the massive Wengen Dolomite (g), suggesting a parallelformation history. The Raibler Schichten (k) caps the Schlernmassiv.





the molten volcanic rocks, as succinctly de-scribed by von Morlot (1847). The theory of vonBuch on the origin of dolomite was consideredprobably to be incorrect, as dolomite was found inmany areas without a trace of porphyritic rocks.More recent research defining hydrothermal do-lomitization plumes in carbonate platforms, such

as the Latemar buildup (Newton Wilson et al.,1990), suggest that a modified version of the vonBuch model may be applicable to some dolomiteconstructions. In any case, after 1860, the generalacceptance of the von Richthofen coral-reef hypothesis to explain the origin of the carbonateedifices forming the Dolomite Mountains hadunforeseen implications for determining theorigin of the dolomite.

In a major volume on Coral and Coral Islands(1872), James Dwight Dana (1813 to 1895) elabo-rated on the discovery of dolomite in elevated

coral limestone on Metia or Aurora Island in thePaumotu Archiplago, South Pacific (Fig. 4). Thedolomite, with a reported Ca:Mg ratio of 62:38,was found in an uplifted coral sand or mud bodywhich appeared to have been deposited in thelagoonal area of the island. Dana (1872) postu-lated that the source of the Mg for the dolomitewas normal sea water and that the dolomitiza-tion occurred under low temperatures, perhaps,within a contracting and evaporating lagoon. Wasthis replacement process likewise the mechanism

that transformed the Dolomite Mountains fromthe original organic calcium carbonate buildupinto their present configuration?

In 1860, von Richthofen had already more orless adopted the marine alteration theory toexplain the formation of the dolomitic reefs inSouthern Tyrol. Von Richthofen recognized the

need for a chemical solution to a geologicalproblem and pointed out, for example, that theoriginal calcium carbonate shell of Ammonitesglobosus found embedded in the Schlern had been dissolved and the remaining mould infilledwith dolomite. Von Richthofen imagined that thisprocess could account for the transformation of the entire mountain. Von Richthofen commentedon the chemical reactions of Wilhelm Haidinger(1795 to 1871), which were formulated toillustrate the transformation of limestone, via aone-to-one exchange of Mg ions with the Ca ions,

to dolomite and gypsum, an association thatwas observed often in the rock record (Fig. 5;Haidinger, 1845).

Haidinger was aware that, under ordinary Earthsurface conditions, the reverse reaction wasknown to occur, that is, dedolomitization in thepresence of a solution of CaSO4 converted dolo-mite into MgSO4 and CaCO3. Considering the firmevidence in the rock record for the occurrence of replacement dolomitization, it had, however,never actually been observed. Thus, Haidinger

Metia, or Aurora Island

Fig. 4. Sketch of Metia or Aurora Island in the Paumotou Archipelago, South Pacific (reproduced from Dana, 1872).Dana reported that the elevated portion of the island, up to 250 ft, ‘‘consists largely of this kind of white, compactcoral-made limestone, which appears to correspond to the interior of the original lagoon of the island; it exemplifiesthe kind of rock-making which is going forward in most coral-island lagoons’’. To account for the dolomite found inthe uplifted coral limestone, Dana proposed that the magnesia of circulating sea water might have affected thealteration.





assumed that the replacement reaction must takeplace at great depths and under considerablepressure. Haidinger first pointed out the possibil-ity of secondary replacement dolomitization in1827 (as cited in von Morlot, 1847). It was

subsequently formulated mathematically byBeaumont (1836, as cited in Van Tuyl, 1916),who calculated that the replacement of one out of every two CaCO3 with an equivalent of MgCO3

should lead to an increase in porosity, as thevolume of the rock would decrease by ca 12%.Beaumont believed that this change in volumewith dolomitization could account for thecavernous character of the dolomite in SouthernTyrol. This same increased porosity is the char-acteristic that frequently makes dolomite bodiesvery good reservoir rocks for hydrocarbons orhydrothermal ores.

In the Transactions of the Royal Society of Edinburgh, 19 March 1827, Haidinger reported onthe ‘parasitic’ nature of dolomite formation fromlimestone stating that: ‘‘The chemical change ishere very distinctly indicated, part of the carbon-ate of lime is replaced by carbonate of magnesia,so as to form in the new species a compound of one atom of each. How this change was broughtabout is a difficult question to resolve, though thefact cannot be doubted as we have in the speci-men described a demonstration of it, approachingin certainty to ocular evidence’’ (1827, as cited in

von Morlot, 1847). Indeed, the replacement dolo-mite process remains a topic of uncertainty today.

By the beginning of the 20th Century, the vonRichthofen coral-reef hypothesis with the associ-ated corollary on secondary replacement dolo-mitization had become a well-establishedexplanation for the origin of both the DolomiteMountains and the dolomite rock of which theymainly consist. Skeats (1905) exemplified thisacceptance in a perceptive paper entitled ‘On thechemical and mineralogical evidence as to the

origin of the Dolomites of Southern Tyrol’. Thestated view of Skeats ‘‘is that the Schlern dolo-mite originated first as a limestone, composed of organisms, in a slowly subsiding area. Dolomiti-zation of the limestone in superficial waters kept

pace with the slow subsidence, so that the wholethickness of 3000 feet or more of rock wascontinuously and uninterruptedly converted intodolomite during the Triassic Period’’.

PRIMARY PRECIPITATION VERSUSSECONDARY REPLACEMENT

Van Tuyl (1916), in an article entitled ‘The originof dolomite’, provides an excellent and compre-hensive compilation of the knowledge and com-peting theories on dolomite formation at the

beginning of the 1900s. One of a number of theories competing with the marine alterationtheory was the chemical theory wherein dolomiteis a primary precipitate. According to Van Tuyl(1916), there were a number of previous publica-tions proposing that the dolomite of the SouthernTyrol had a primary origin because it was oftendevoid of fossils but preserved fine detailedsedimentary structures. Further, both the dolo-mite and the limestone of the region could bechemical precipitates as they often appear tograde into each other. Gilbert & Churchill (1864)

had noted that the dolomite rock is observed tohave ‘‘a variety of state, and under differentconditions. The proportion of the magnesiancarbonate, too, varies almost indefinitely. Whena small percentage only is present – say, less thanten or twelve per cent., the rock is called Dolo-mitic limestone. Sometimes it has a sugar-like,crystalline structure, at other times it is compact,with a chonchoidal fracture. To these two statesthe term Dolomite is often restricted’’. Appar-ently, there was petrographical and stratigraphi-

Fig. 5. First formulated in 1845, thegraphical equations of WilheimHaidinger depict the secondaryreplacement formation of dolomiteand gypsum by the reaction of a

magnesium sulphate solution withlimestone. This equation undoubt-edly represents one of the firstattempts to illustrate a sedimentarygeochemical process as a chemicalreaction. Reproduced fromvon Morlot (1847).





cal evidence for both primary precipitation andsecondary replacement dolomite in the DolomiteMountains, as the following two equations illus-trate:

1. Primary precipitation

Ca2þ þ Mg2þ þ 2CO23 Ð CaMgðCO3Þ2

2. Secondary replacement

2CaCO3 þ Mg2þ Ð CaMgðCO3Þ2 þ Ca2þ

In the last 40 to 50 years, the discovery of dolomite forming in specific environments, suchas beneath the Abu Dhabi sabkhas, UAE, and inthe ephemeral lakes of the South Australian

Coorong Lagoon, has provided new insights intothe physico-chemical controls on the process.Although minor in scope, these discoveries fur-nished modern analogues that were used toevaluate ancient dolomite formations, such asthose found in the Dolomite Mountains, based onthermodynamic and kinetic considerations, aswell as defining the hydrology of the dolomitizingenvironment. Numerous hydrological modelswere developed to interpret the variety of dolo-mite found in the rock record. There was a basicconsensus, however, that: (i) most dolomites wereformed by secondary replacement of meta-stable

calcium carbonates, such as aragonite and high-Mg calcite; and (ii) sea water is the ideal sourcefor dolomitizing solutions because of its highconcentration of magnesium ions. Ironically,although modern sea water is also supersaturatedwith respect to dolomite, neither massive dolo-mite precipitation nor dolomitization has beenobserved to occur. Even though the modernenvironments presented the possibility to studythe geochemical parameters promoting dolomiteprecipitation, attempts to simulate the observedconditions in laboratory experiments based on

these actualistic studies proved to be of limited orno success in precipitating dolomite under Earthsurface conditions (Land, 1998).

MICROBIAL DOLOMITE

At the turn of the 20th Century, Georgi A. Nadson(1867 to 1940), a Russian microbiologist, reportedthat small amounts of fine-grained dolomite hadprecipitated in anaerobic culture experiments

with sulphate reducing bacteria isolated from asalt lake. Nadson proposed that ‘‘Understandingthe essential role played by this bacterial pheno-menon may be the solution to the DolomiteProblem and the problems of the Mg cycle inthe ocean’’ (translation from Nadson, 1928). Areport by Nadson, entitled Die Mikroorganismens

als geolog. Faktoren, was first published in StPetersburg in 1903. Apparently, because theseresults were published in a Russian journal,Nadson felt that they were not receiving wideenough recognition. For this reason, in 1928,Nadson republished this work in a Germanjournal with greater circulation to reach a widercommunity. The experimental results of Nadsonhad, however, been noted and were included inthe geology textbook entitled Geschichte der Erdeund des Lebens (Walther, 1908), wherein Walthercomments that bacteria even were proposed as

the mediator of dolomite precipitation. Walther(1908) speculated that bacteria living in the seawater filling the pore spaces influence the trans-formation of the reef calcium carbonate to dolo-mite by promoting the entry of MgCO3 intothe crystal structure, thereby accomplishing thedolomitization that Dana (1872) had observed onMetia Island in the South Pacific. Regardless, itremained unclear whether the microbial-pro-duced dolomite represented a replacement prod-uct or a primary precipitate. The role of bacteriain dolomite formation and the mechanism for thistransformation were not developed further and

the idea faded into obscurity awaiting thedevelopment of new technologies to study themicrobial activity and biomineralization.

In recent years, however, a new approachusing anaerobic culture experiments has pro-vided fundamentally new data to understand themechanisms that may be involved in the primarydolomite precipitation under Earth surface con-ditions (Vasconcelos et al., 1995; Warthmannet al., 2000) and has inspired the developmentof the microbial dolomite model based on astudy of a modern dolomite-forming hypersaline

coastal lagoon, Lagoa Vermelha, Brazil (Vasco-ncelos & McKenzie, 1997). Apparently, in speci-fic modern hypersaline environments, sulphate-reducing bacteria induce dolomite precipitation(Vasconcelos & McKenzie, 1997; van Lith et al.,2002, 2003a,b; Wright & Wacey, 2005). Also,culture experiments using aerobic halophilicheterotrophic bacteria and methanogenes indi-cate that other types of metabolic activity may,likewise, play an important role in dolomiteprecipitation (Roberts et al., 2004; Sa nchez-





Roma n et al., 2009). All of these studies demon-strate the importance of microbial processes incarbonate mineral formation with the presenceof the active cells, and associated biofilms, beingan essential ingredient. The culture experimentssuggest the need to add a microbial factor to thegeochemical equation for primary dolomite pre-

cipitation, for example, the addition of bacterialsulphate reduction as exemplified in the follow-ing equation:

Ca2þ þ Mg2þ þ 2SO24 þ 2CH3COOH

! CaMgðCO3Þ2 þ 2H2S þ 2H2O þ 2CO2

Undoubtedly, the ability of microbes to overcomethe kinetic factors inhibiting dolomite precipita-tion is the indispensable component driving thereaction.

MICROBIAL-MEDIATED DOLOMITE INTHE DOLOMITE MOUNTAINS

As previously mentioned, at the beginning of the20th Century, the opponents of the coral-reef hypothesis used the general absence of corals inthe dolomite masses as one line of evidence tosupport their rejection. In fact, the most commonmacrofossils observed in the reefs are calcareousalgae and echinoderms but the reefs can also bedescribed as buildups of microbialites, that is,constructions of microbial origin (Burne & Moore,

1987). Using the term microbialite implies agenetic connotation but is consistent with therecognition that bio-induced micrite is a majorcomponent of the carbonate platforms in theDolomite Mountains (Russo et al., 1997; Keim &Schlager, 1999). Thus, the recent research intomicrobial dolomite formation and the associatedmicrobial dolomite model may have importantimplications for the origin of the Dolomite Moun-tains and the dolomite rock of which they mainlyconsist.

Vasconcelos et al. (2006) examined the micro-

bial processes inducing carbonate precipitation inlithifying microbial mats (Fig. 6A) in the coastalhypersaline lagoon of Lagoa Vermelha, Brazil,which is a site of modern dolomite formation.These authors proposed that the precipitation of very high-Mg calcite in the uppermost layers of the active microbial mat might be an essentialstep in the dolomite precipitation processes.At depths of < 1 cm within the microbial mat,precipitates of both high-Mg calcite and Ca-dolomite are found encased in a microbial biofilm

(Fig. 6B). Within a few centimetres below the matsurface, the carbonate becomes primarily dolo-mite. This transformation suggests a complexsymbiosis of metabolic reactions within thedynamic microbial community leading to under-saturation with respect to (Mg-) calcite whilemaintaining supersaturation with respect to dolo-

mite (Moreira et al., 2004; Vasconcelos et al.,2006; Sa nchez-Roma n et al., 2009). Althoughthere are many processes involved in dolomiteformation, geomicrobiological studies of microbial mats in modern dolomite-forming environ-ments provide analogues for a possible microbialdolomite transformation process, which mayhave been involved during the formation of somemassive dolomite found in the Dolomite Moun-tains.

Particularly suitable for a microbial dolomiteinterpretation is the massive dolomite found in

the Norian Dolomia Principale or HauptdolomitFormation, comprising up to 1000 m of a dolo-mitized succession of cyclic subtidal and peri-tidal inner platform facies with restricted fauna(Iannace & Frisia, 1994). Recently, Mastandreaet al. (2006) have investigated microbialitic faciesfound in the carbonate platform of the NorianDolomia Principale of Northern Calabria (South-ern Italy). These authors describe microbialitesfrom the peritidal dolomite facies consisting of stromatolites, thrombolites and aphanitic dolo-mites. Based on microscopic observations, thedolomite is interpreted as having a microbial

primary origin. By analogy with the Lagoa Ver-melha microbial mats, the very fine, dark lami-nations of the microbial boundstones (Fig. 6C)comprise micritic dolomite crystals (Fig. 6D),< 5 lm in size and rich in organic matter.

A more specific indicator of microbial biomin-eralization is the occurrence of distinctive crystalmorphologies and the preservation of microbialfossils in the dolomite, observable using high-resolution scanning electron microscopy. Riding(2000) has suggested that morphology (micro-fabric) studies can provide insight regarding

biomineralization processes, as well as on theenvironmental conditions where the dolomiteprecipitates. Regardless of bacterial type (auto-trophic or heterotrophic; marine or non-marine),carbonate morphologies mediated by micro-organisms are distinguishable from inorganicprecipitation (Krumbein, 1979; Buczynski &Chafetz, 1991; Knorre & Krumbein, 2000; Brais-sant et al., 2003). Specific morphological features,such as spheres and dumb-bells, have been foundin dolomite produced in culture experiments





(Warthmann et al., 2000; Sa nchez-Roma n et al.,2009). Figure 7 illustrates the progressive develop-ment of the dumb-bell morphology from discretedolomite nanocrystals associated with sulphate-reducing bacteria to larger and larger dolomitedumb-bells. Apparently, carbonate spheruliteswill be the final stage of the dumb-bell growth(Buczynski & Chafetz, 1991; Warthmann et al.,2000).

Microbial dolomite with spheroid morphologyalso has been found in diverse modern environ-ments (von der Borch & Jones, 1976; van Lithet al ., 2003a,b) and in geological dolomite sam-ples (Folk, 1993; Nielsen et al., 1997). Thespecific morphologies of biominerals can beevaluated using very high-resolution imagingtechnologies, which enable the visualization of microbial carbonates. For example, Figs 8A and Bshow putative nanobacteria associated withdolomite precipitation in an anaerobic culture

experiment, illustrating the possible entombmentof bacteria to form spherical structures (Vascon-celos et al., 1995). The growing nanobacteria, asindicated by the apparent cell division (Fig. 8A),appear to be encrusted in nanocrystals creatingspherical structures in the dolomite precipitate(Fig. 8B). Such nanoglobular structures poten-tially are signals of microbially produced nucleithat are found commonly in dolomite.

Searching for analogous microbial tracers in thedolomite rocks of the Dolomite Mountains willprovide the necessary basis to determine theimportance of this microbial phenomenon in theevolution of these Triassic carbonate platforms.For example, dolomite spheres observed in a LagoaVermelha microbial mat, which is incorporatedinto a living stromatolite (Fig. 8C), may be analo-gous to nano-scale spheroids associated withmicrobial primary dolomite within the microbialitic facies of the Upper Triassic carbonate

cm

10 µm

5 µm1 mm

A B

C D

Fig. 6. Comparison of macrostructures and microstructures in modern microbial mats with Upper Triassic microbialite. (A) Cross-section of a microbial mat from Lagoa Vermelha (modified after Vasconcelos et al., 2006). Stratifiedlayers of carbonate precipitation (white layers) alternate with non-lithified organic layers. (B) Scanning electronmicroscopy (SEM) photomicrograph of mixture of high-Mg calcite and dolomite nanocrystals encased in biofilm fromthe white layer at ca 1Æ5 cm in microbial mat (A). Scale = 10 lm. (C) Photomicrograph of stromatolite lamination indolomitic microbialites from the Upper Triassic Dolomia Principale Formation, Southern Italy (modified afterMastandrea et al., 2006). Scale = 1 mm. (D) SEM photomicrograph of micritic euhedral dolomite crystals from thevery fine dark laminae of the stromatolite (C) (modified after Mastandrea et al., 2006). Scale = 5 lm.





platforms of Southern Italy (Fig. 8D). Mastandreaet al. (2006) tentatively interpreted these spher-oids as fossilized bacteria. These two examples of spheroids associated with microbial primary dolo-mite indicate the potential applicability of themicrobial dolomite model to interpreting the ori-gin of some of the dolomite in the DolomiteMountains. In particular, micritic dolomite withfine laminations and stromatolitic structures are

potential targets to test the microbial dolomitemodel.

DOLOMITE MOUNTAINS AND TRIASSICSEA WATER CHEMISTRY

Sea water is deemed to be the primary source of Mg ions for either primary dolomite precipitationor replacement dolomitization (Morrow, 1982;Land, 1985). According to the extensive compila-

tion of dolomite abundance as a function of stratigraphical age throughout the Phanerozoic(Given & Wilkinson, 1987), the Triassic was aperiod of maximum dolomite formation (Fig. 9C).Furthermore, Given & Wilkinson (1987) calcu-lated that there was a sufficient flux of Mg ions tothe oceans to make all of the dolomite in the rockrecord, even during times of maximum dolomiteabundance. Thus, it is interesting to consider

whether there was something unusual about Tri-assic sea water chemistry that would promotemassive dolomite formation in the carbonateplatforms of the Dolomite Mountains.

Secular changes in Phanerozoic sea water have been recognized as an oscillation between periodsdominated by non-skeletal aragonite versus non-skeletal calcite carbonate precipitation and have been designated as times of aragonite seas orcalcite seas, respectively (Sandberg, 1983; Hardie,1996; Fig. 9B). The aragonite seas are associated

1 µm 5 µm 10 µm

10 µm 10 µm

20 µm 20 µm

Fig. 7. Scanning electron microscopy photomicrographs of a sulphate-reducing bacteria strain associated with theprecipitation of microbial dolomite with dumb-bell morphology. Nanocrystal growth appears to begin with a closespatial relationship to bacterial cells inside larger aggregates of bacteria. The later development of the dolomitedumb-bells occurs gradually with transformation to larger cauliflower structures. Modified from Warthmann et al.(2000).





with the formation of MgSO4 evaporites, while thecalcite seas are linked with the formation of KClevaporates (Fig. 9B). Hardie (1996) proposed thatthese secular changes are controlled by fluctua-

tions in mid-ocean ridge hydrothermal brine fluxwhich, in turn, reflect changes in the rates of ocean crust production. Furthermore, the secularchanges in the rate of sea floor generation onPhanerozoic time scales have been associated withlong-term changes in eustatic sea-level (Gaffin,1987). Based on these compilations, the Triassicsea water chemistry and its impact on the forma-tion of the Dolomite Mountains can be considered.

During the Triassic period, global sea-level ap-pears to have remained relatively low, being equiv-alent to a nearly modern value (Vail et al., 1977;

Fig. 9A). In contrast, the Triassic ocean was under-going a transition from being an aragonite sea to acalcite sea. Also, the mole ratio of Mg/Ca appears toshow a transition from values of ca three to two(Hardie, 1996), possibly approaching one (Wilkin-son & Algeo, 1989), reflecting a decrease in the Mgconcentration of sea water (Fig. 9D). As the loweustatic sea-level indicates relatively low rates of ocean crust production, the decrease in sea waterMg/Ca cannot be attributed fundamentally to thetake-up of Mg during basalt alteration with

enhanced hydrothermal circulation through themid-ocean ridges. Thus, could the massive amountof dolomite formation during the Triassic be theculprit drawing down the oceanic Mg concentra-

tion?Burns et al. (2000) proposed that massive dolo-

mite formation might be related intrinsically tothe redox state of the ocean. These authors basedthis hypothesis on the general association of highdolomite abundance with high global sea-levelstands, i.e. increased sea floor hydrothermalcirculation (Figs 9A and C). The abundantTriassic dolomite is the notable exception to thissecular correlation. The Triassic period does,however, show a relationship with reduced levelsof atmospheric oxygen, with values lower than

modern (Lasaga, 1989; Fig. 9E), which implieslower levels of dissolved oxygen in sea water. Thecorresponding lack of significant ironstone depos-its (Van Houton & Bhattacharyya, 1982; Fig. 9F)indicates, however, that conditions in the Trias-sic period were not conducive to the aqueoustransport of iron as Fe2+ from the continents to beoxidized and deposited as Fe3+ in ironstones as,for example, observed in the Jurassic. Overall,when compared to the postulated secular trendsthroughout the Phanerozoic (Fig. 9), the Triassic

1 µm 1·0 µm

A

C

B

D

Fig. 8. Scanning electron microscopy (SEM) photomicrographs of microbial dolomite precipitated in anaerobicculture experiments showing: (A) close-up of twinned nanobacteria possibly imaged in the process of reproductionvia cell division; and (B) close-up of knobby carbonate surface with attached sub-spherical nanobacteria/nano-globules encrusted by nanocrystals of dolomite, possibly in the process of becoming entombed (modified fromVasconcelos et al., 1995). Scales = 100 nanometres. (C) SEM photomicrograph of spherical dolomite structureencased in biofilm within Lagoa Vermelha microbial mat growing on a stromatolite surface. Scale = 1 lm. (D) SEMphotomicrograph of sub-spherical bodies, interpreted as fossilized bacteria, found in dolomitic microbialite from theUpper Triassic Dolomia Principle Formation, Southern Italy (Mastandrea et al., 2006). Scale = 1 lm.





oceans appear to represent a transition phase between the two end-member conditions of aragonite versus calcite seas under a slightlydecreased atmospheric oxygen state, possiblyleading to oxygen-deficient oceans.

In fact, Iannace & Frisia (1994) observed achange in the dolomitization style of the Upper

Triassic carbonate platform successions in thepre-Mediterranean Alpine region from massiveearly and complete dolomite formation in theNorian (221 to 210 Ma) to lesser amounts of dolomitization in the Rhaetian (210 to 206 Ma).The widespread pervasive and fabric-preservingNorian dolomitization was on a non-uniformitar-

ianism scale, for which a modern equivalent doesnot exist; whereas a Florida–Bahamas type modelrelated to large-scale fluid circulation couldaccommodate the patchy Rhaetian dolomitiza-tion. Iannace & Frisia (1994) attributed themassive Norian dolomite formation to specificpalaeoceanographic/tectonic/climatic conditionsin the Tethyan region. Low rates of oceanic ridgeactivity lead to increased Mg/Ca ratios, while awarm, arid climate and restricted oceanic circu-lation promoted hypersalinity and stratifiedconditions, respectively. Iannace & Frisia (1994)

postulated that these conditions in the Norianocean were essential to overcome the kinetic barriers to dolomite precipitation. The authors of the present paper would add that these sameenvironmental factors are also ideal to promotethe specific anaerobic halophilic microbial activ-ity, such as bacterial sulphate reduction, whichhas been observed to mediate dolomite precipita-tion in cultural experiments and natural environ-ments. As previously documented (Mastandreaet al., 2006), the Norian Dolomia Principale orHauptdolomit Formation within the Mediterra-nean realm is a very suitable ancient example in

which to test the validity of microbial dolomiteprecipitation as a significant geological process.Studying these ancient analogues would be theperfect complement to the research evolving fromlaboratory experiments and modern environ-ments.

CONCLUSIONS

From the late 18th Century to the beginning of the20th Century, the Dolomite Mountains of South-

ern Tyrol have been the focus of fundamentalresearch into the origin of dolomite, which hasremained a long-standing problem in sedimento-logy. Until recently, dolomite formation modelshave neglected the microbial factor in construct-ing geochemical equations, i.e. primary precipi-tation versus secondary replacement, which werepostulated to represent dolomite formation pro-cesses. Now, it is appreciated generally thatmicrobial activity under specific environmentalconditions can mediate the precipitation of

A

B

C

D

E

F

Fig. 9. Phanerozoic cycles of: (A) eustatic sea-level(after Vail et al., 1977); (B) oscillating mineralogy of evaporites (MgSO4 = blue versus KCl = green) andprimary calcium carbonate precipitates from sea waterproducing aragonite (open areas) versus calcite (shadedareas) seas (after Sandberg, 1983 & Hardie, 1996);(C) dolomite abundance as per cent total carbonate rock(after Given & Wilkinson, 1987); (D) sea water Mg/Cafrom Hardie (1996) (blue curve) and Wilkinson & Algeo(1989) (red curve); (E) modelled atmospheric O2 levelsin 1018 mol (from Lasaga, 1989; dashed line is modernvalue); and (F) occurrences of oolitic ironstones (after

Van Houton & Bhattacharyya, 1982). The Triassic per-iod is highlighted in red. Modified after Burns et al.(2000; figs 3 and 4).





significant quantities of dolomite at low tempera-tures. How this microbial mediation of a biomin-eral can be translated into the production of gigantic structures, such as those in the DolomiteMountains, remains unclear but the influence of these micro-organisms cannot be ignored. Today,the world lacks the giant microbial environments

that undoubtedly existed in the geological past.Such environments, which may have existedwithin the multifacies geometry of the Noriancarbonate platforms, would have been conducivefor the microbial production of micritic carbonate,possibly even the primary precipitation of dolo-mite and, thus, would have been capable of supplying massive amounts of fine-grained carbonate to undergo geochemical and physicaltransformation into carbonate structures. In thefuture, the search for microbial evidence, whichcan be gleaned from ancient dolomite rocks, will

undoubtedly provide new information to betterunderstand the processes that created the excep-tional dolomite platforms found in the Mediter-ranean realm.

ACKNOWLEDGEMENTS

We kindly acknowledge our colleagues in theGeomicrobiology Laboratory, ETH-Zurich, espe-cially Rolf Warthmann, who have contributedto our understanding of the processes involved inthe formation of microbial dolomite. We

thank Maurice Tucker, Peter Swart and DanielBernoulli for constructive reviews of an ear-lier version of this manuscript. The SwissScience National Foundation (SNF) is gratefullyacknowledged for generous financial supportthrough Grant Nos 20-067620 and 20-105149.

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Manuscript received 1 February 2008; revisionaccepted 4 September 2008



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