high salinity variability during the early messinian … 40/7bosellini2.pdf · 2018. 1. 23. ·...

INTRODUCTION

In the geological record coral reefs are usuallyconsidered indicators of past tropical sea surfacetemperatures (SSTs), “normal” salinity (36‰), lownutrient concentration and a high carbonate super-saturation, because these factors promote rapid coralgrowth and carbonate precipitation, a prerequisite ofrimmed, steep-sided carbonate platforms (Hallock &Schlager, 1986; Schlager, 1992). The present-dayMediterranean is devoid of coral reefs but during theLate Miocene they were prolific, which implies surfacewater cooling. In Late Miocene reefs, low genericdiversity and low annual growth rates of corals indicateSSTs close to the lower threshold of reef growth(Rouchy et al., 1986; Esteban, 1996; Brachert et al.,2006b). Moreover, reef growth was not continuous overlong time spans but interrupted by intercalatedbryozoan-bivalve or coralline red algal carbonates, apattern interpreted by many to indicate SSTsepisodically below the critical threshold for reefs incertain areas (Martín & Braga, 1994).

Apart from temperature, stable isotope data frommicrofauna and inorganic carbonate grains documentchanges in evaporation and rainfall or river discharge,which substantially affected salinity distributions andcirculation patterns of the Mediterranean on orbital time-scales prior to the Messinian Salinity Crisis (MSC)(Bellanca et al., 1986; Rohling et al., 2002).

Nonetheless, so far no evidence of salinity stress onreefal, shallow-marine biota has been detected, althoughthe paleoecological significance of microbialites in coralreef frameworks remains problematic (Riding et al.,1991).

In this paper, we address the bearing of early marinecarbonate cement and carbonate build-ups other thancoral reefs to the understanding of Late MioceneMediterranean shallow water paleoecology: (1) build-ups formed by the vermetid Petaloconchus (vermetidreef; VR) in the intertidal zone and downslope, and (2)bioherms formed of segments from the green algaHalimeda (Halimeda segment reefs; HSR) at depths ofseveral tens of meters (Braga et al., 1996). VRs andHSRs are described from a large rimmed carbonateplatform of the central Mediterranean exposed to theopen sea (Salento Peninsula, Southern Italy) and small,potentially semi-enclosed basins of the Eastern Mediter-ranean (Crete, Greece) (Fig. 1). A conspicuous commonaspect in these contrasting paleogeographic settings isthe presence of thick crusts or botryoids of early marinearagonite cements, which in HSRs were shown torepresent an integral part of the primary constructionalfabric (Braga et al., 1996). Stable isotope data (δ18O,δ13C) of these cements are highly positive and are hereinterpreted as a consequence of high sea surface salinity(SSS), in contrast to the normal marine fauna of theframeworks. We discuss whether this conflictingevidence could be related to high SSS episodes and how

Geologica Romana 40 (2007), 51-66

HIGH SALINITY VARIABILITY DURING THE EARLY MESSINIANREVEALED BY STABLE ISOTOPE SIGNATURES FROM VERMETID

AND HALIMEDA REEFS OF THE MEDITERRANEAN REGION

Thomas C. Brachert*, Alessandro Vescogni**, Francesca R. Bosellini**,Markus Reuter° & Regina Mertz-Kraus*

* Institut für Geowissenschaften, Johannes Gutenberg-Universität, Becherweg 21, 55099 Mainz, Germany** Dip. di Scienze della Terra, Università di Modena e Reggio Emilia, largo S. Eufemia 19, 41100 Modena, Italy

° Institut für Erdwissenschaften, Karl-Franzens-Universität, 8010 Graz, [email protected]

ABSTRACT - The early Messinian deep-water record of the Mediterranean region reveals increasing evidenceof significant salinity stress prior to the Messinian Salinity Crisis (MSC). In shallow reef environments such apattern has not yet been documented, however, the paleoecological interpretation of some sedimentarycomponents, such as stromatolites in coral reef frameworks, has remained ambiguous. We present data from openand semi-restricted reef settings of southern Italy and Greece (Crete), where stable isotope analyses have beenmade on early marine carbonate cements from vermetid reefs and Halimeda bioherms. δ18O compositions ofmarine cements translate into sea surface salinity (SSS) peaking at 50 to 60‰, certainly too high for most shallowwater biota (i.e. zooxanthellate corals). It seems, therefore, that recurrent events of high salinity have occurredcausing events of community replacement eventually ending up in “abiotic” episodes. These events, however, weretoo short to be recorded as discrete beds in 4th order depositional sequences. Because the cements reflecting highsalinity occur equally in the platform margin and reefal slope facies, salinity build-up was not linked to evaporativedraw-downs of sea level. Such a scenario fits concepts of the MSC starting synchronously over great part of theMediterranean as a deep brine pool.

KEY WORDS: Vermetid reef, Halimeda reef, aragonite cements, stable oxygen isotope composition, high salinity, LateMiocene, Italy, Greece.

these salinity inputs could have affected the Medi-terranean basin as a prelude of the MSC.

METHODS

In the field, outcrops were described according tostratal geometries and corresponding distributions ofsedimentary facies. Samples were taken according to thesearch sampling method described by Flügel (2004).Polished slabs and thin-sections were stained withFeigl’s solution to reveal the distribution of aragoniteand calcite. Under the SEM, broken and polishedsurfaces were analysed for ultrastructures. X-raydiffraction for aragonite and calcite was carried outusing a Seifert XRD 3000 diffractometer. Samples werescanned from 2θ of 20° to 60°. Carbonate powders forstable isotope analysis and X-ray diffraction wereproduced using a hand-held microdrill equipped with a0.6 mm steel drill bit. Samples for stable isotope analysiswere taken as single samples or along transects over theentire crusts of early marine cements. Carbonatepowders were reacted with 100% phosphoric acid(density >1.9, Wachter & Hayes, 1985) at 75°C using aKiel III online carbonate preparation line connected to aThermoFinnigan 252 mass spectrometer. All values arereported in per mil relative to V-PDB by assigning aδ13C value of +1.95‰ and a δ18O value of -2.20‰ toNBS19. Reproducibility was checked by replicateanalysis of laboratory standards and is better than ± 0.07for δ18O and +0.06 for δ13C (1σ). Stable oxygen isotopecompositions found in marine aragonite cement whereused to calculate salinity according to the equationδ18

water[SMOW] = 0.27 * S - 8.9 (Pierre, 1999) wereδwater was calculated making assumptions for watertemperatures according to the relationship T [°C] = 20 -4.42 * (δ18Osample - δ18Owater) (simplified from Böhm etal., 2000).

LOCAL GEOLOGICAL SETTINGSAND STRATIGRAPHY

Salento Peninsula (S-Italy)

The Salento Peninsula (Fig. 1) is thesubaerial portion of the ApuliaPlatform, a vast carbonate platformwhich persisted from Jurassic to LateTertiary. The Apulia Platformextended from the southeasternAbruzzi region across Apulia andprobably across the Strait of Otrantoto the Greek islands of Cephalonia andZante (Bosellini A., 2002). Theeastern part of the Apulia Platformrepresents the foreland of theApennine thrust chain and is thereforeonly mildly deformed. The westernpart instead is downfaulted and buried

beneath the foredeep and the adjacent Apennine chain.The Cretaceous margin of the platform and its transitionto the adjacent Ionian basin crops out in the GarganoPromontory (Bosellini A. et al., 1999b), whereas theTertiary margin occurs on the eastern coast of theSalento Peninsula, where Eocene, Oligocene andMiocene reefal units are spectacularly exposed(Bosellini F.R. & Russo, 1992; Bosellini F.R. et al.,2001, 2002) (Fig. 2).

After the Oligocene, the area of the Salento Peninsulasubsided rapidly as a direct result of the emplacement oftwo thrust loads on either sides of the Adriatic sea,causing accentuated lithospheric depression andforeland basin development (Bosellini A. et al.,1999a).However the area was too far from the growing chains toreceive any siliciclastic influx and became a true starvedplateau, block-faulted and swept by currents. The resultwas the deposition of the Pietra Leccese Formation,represented in the eastern margin of the platform by athin pelagic unit (Aturia Level) which covers a long timespan (about 11 My), from late Burdigalian to earlyMessinian (Mazzei, 1994).

The early Messinian Novaglie Formation

On Salento Peninsula, the early Messinian time periodis documented by the depositional system of the earlyMessinian Novaglie Formation and Andrano Calcarenite(Fig. 2). The reef complex of the Novaglie Formation isexposed along the eastern coast of the peninsula from thesmall town of Tricase Porto to Capo S. Maria di Leuca(Fig. 1). The lower contact of the formation is un-conformable with various pre-Miocene units and theAturia Level. The age of the Novaglie Formation hasbeen derived from benthic foraminifera (Buliminaechinata zone) and ostracod associations (Bosellini A. etal., 1999a; Bosellini F.R. et al., 2001). The geological-stratigraphical setting of the reef complex and its reef-building assemblages have been described in detailelsewhere (Bosellini A. et al., 1999a; Vescogni, 2000;Bosellini F.R. et al., 2001, 2002). A series of non-reefalcarbonate units with a patchy distribution along thepresent-day coast completes the Neogene stratigraphy ofthe area (Bosellini A. et al., 1999a).

BRACHERT et al.52 Geologica Romana 40 (2007), 51-66

Fig. 1 - Location maps for the study sites in southern Italy (Salento Peninsula) and southernGreece (Crete).

The Novaglie Formation represents a succession offringing reefs accommodated in paleo-embayments ofthe original Messinian rocky coast. It consists of threesuperimposed depositional sequences (4th order) sepa-rated by erosion surfaces; lower boundaries of the twolast sequences are colonized by vermetid bioconstruc-tions (Bosellini A. et al., 1999a; Bosellini F.R. et al.,2001, 2002) (Fig. 3). Only the first sequence is preservedas a complete coral reef tract with an associated clinos-tratified forereef slope. The platform margin is repre-sented by reefal frameworks formed by dense stands ofmassive and upright branching Porites with minor con-tributions by Tarbellastraea and coralline algal crusts.Seaward, the growth forms of Porites become increas-

ingly platy and then grade into patchy frameworks andaccumulations of the vermetid Petaloconchus with fewintermixed out-of-place Porites. Halimeda reefs andsheet-like Halimeda beds formed at the base of the prox-imal slope (Fig. 3). Further downslope, talus and mas-sive slope breccias become increasingly abundant. TheNovaglie Formation exhibits extensive fissure systemsfilled with varicoloured micrite and carbonate cements.Multiple generations of infill and various cross-cuttingrelationships document continual brittle deformation inrelation with differential compaction of slope sedimentsor structural definition of platform architecture (cf.Playford et al., 1989). A detailed description of the vari-ous reef facies has been given by Bosellini F.R. et al.(2002).

Crete

The Neogene basins of Crete formed in response to thegeodynamic evolution of the eastern Mediterranean.Subduction of the African plate beneath the European -Asian plate is active since the pre-Neogene (Rahl, 2004).Extension and drowning of the Aegean landmass,including small segments of Crete, occurred as aconsequence of rapid subduction zone roll-back duringthe Late Oligocene and Early Miocene (Rögl &Steininger, 1984). Consequently, Neogene basins occurscattered all over the island of Crete and show a highdegree of similarity with regard to lithofacies and historyof subsidence and inversion (Meulenkamp et al., 1979).In response to southward subduction zone rollback,systems of east-west trending graben are generallybelieved to have subsided from the Middle Mioceneonward (ten Veen & Postma, 1999; Fassoulas, 2001).

HIGH SALINITY VARIABILITY DURING THE EARLY ... 53Geologica Romana 40 (2007), 51-66

Fig. 2 - Simplified stratigraphic architecture of the eastern SalentoPeninsula - southern Italy (modified from Bosellini A. et al., 1999a).Reef units are represented by the Torre Specchialaguardia Limestone(Priabonian), the Castro Limestone (lower Chattian) and the NovaglieFormation (lower Messinian).

Fig. 3 - Depositional model of the lower Messinian reef complex of the Salento Peninsula (Novaglie Formation), showing internal distribution ofvermetid and Halimeda facies (modified from Bosellini F.R. et al., 2001). Presence of aragonitic cements is indicated.

During the Late Miocene (early Messinian), maximumextensional stresses rotated in an east-west directionrelated to a westerly movement of the Anatolian Block.From the Late Pliocene until the present, a combinationof subduction zone rollback and Anatolian tectonicescape is reflected in a multidirectional extension system(Fassoulas, 2001).The present-day Iraklion Basin is formed by a series ofnorth-south trending halfgrabens. It is situated inbetween the Ida Mountains in the west, the DictiMountains in the east and the Messara Plain in the south.To the north, the basin continues into the present CretanSea. The substrate of the basin is formed by pre-Neogene rocks of the Cretan nappes: Mesozoiclimestone (Tripolitza Unit, Plattenkalk Unit) andPaleogene sandstone and mudstone (Tripolitza Unit)with large Mesozoic limestone olistoliths. Basinfill ofthe western Iraklion Basin is formed by clastics,carbonates and evaporites of Middle Miocene toPliocene age (Meulenkamp et al., 1979). During the LateMiocene, the topography and subsidence patterns weregoverned by a complex mosaic of tilted blocks thatcontrolled patterns of facies distribution.Synsedimentary block movements, however, were slowas compared to global sea level changes, becausestacking patterns of depositional sequences attributed toscales of third and fourth order are consistent overindependent blocks and within various segments of thebasin (Reuter et al., 2006).

Late Miocene rocks of CreteMiddle to Late Miocene sedimentary environments werebrackish lagoons, marginal and offshore marine settings(ten Veen & Postma, 1999; Moisette et al., 1993).Subsequent to a phase of non-marine and marginallymarine sedimentation at the end of the Serravallian andearly Tortonian, the basin became an open marineenvironment during the early Tortonian. Coarse clasticsediments formed along the coastlines of the basin andbecame intermittently colonized by colonial corals(Porites, Tarbellastraea with minor Acanthastraea andSiderastraea) building laterally extensive biostromes.Extensional tectonics during the late Tortonian triggered

rotational uplift of blocks in positions distal to the basinmargin and gave rise to pure offshore carbonateenvironments. In these isolated settings, coral reefs wereassociated with sheer cliffs and gentle ramps withlaterally extensive level bottom coral communities.Around the Tortonian-Messinian transition, mostshallow-water environments drowned during a pulse ofrelative basin subsidence and hinterland uplift. EarlyMessinian coral growth remained restricted to a narrowzone fringing the Ida Mountains (Reuter et al., 2006;Reuter & Brachert, 2007) (Fig. 4). Although most ofthese shallow-water deposits of early Messinian agewere destroyed by post-Miocene erosion, slope and toe-of-slope deposits have been found containing HSRs,large reworked fragments of reef corals (Porites,Tarbellastrea) and blocks of VRs. The main aspect of thesections, however, is an alternation of decimetricpackages of homogeneous and laminated calcareousmudstone (Fig. 5). In a distal position, intercalated withthis sedimentary unit towards its top is a package oflaminated gypsum dated to be Messinian in age(Meulenkamp et al., 1979) (Fig. 4).

MICROFACIES AND PALEONTOLOGYOF EARLY MESSINIAN VERMETID REEFS

AND HALIMEDA SEGMENT REEFS

Vermetid reefs (VRs)

Vermetid reefs of the Salento Peninsula occur withintwo different settings of the Messinian reef complex: (1)along the shallower seaward portion of the platform edgeand (2) in the upper part of the slope, in the transitionzone between Porites colonies and bioclastic accumula-tions (Fig. 3). These reefs consist of relatively small bio-constructions (width about 10-25 m, maximum measur-able length about 100 m and maximum thickness about70 cm) characterized by dense and chaotic aggregates ofvermetids, serpulids, binding organisms such as bry-ozoans and coralline algae, together with matrix andcements (Bosellini F.R. et al., 2001, 2002) (Fig. 6).Vermetids are entirely represented by the genus


Fig. 4 - Late Miocene stratigraphy of central Crete showing sedimentary setting of early Messinian Halimeda reefs and debris flow deposits/slumpswith blocks of vermetid reef. Units 7 and 8 shown with a gray shading are of early Messinian age. Black circles = proximal facies, white circles =distal facies. Schematic and not to scale. From Reuter et al. (2006), modified.

Petaloconchus, with shells characterized by closelycoiled early whorls and by terminal, unwound “feedingtubes” (1.5-2 cm long and about 1.6 mm wide). They arecommonly encrusted by serpulids, dominated by thegenus Spirobranchus (Fig. 6c) and coralline algae(Lithophyllum). Associated organisms are represented bysmall benthic foraminifera, echinoids, ostracods and raregastropods. The interskeletal matrix consists of a wacke-stone-packstone with abundant peloidal and micriticaggregations commonly organized in millimetric to sub-millimetric laminae; in places these microbial crusts areabsent and vermetid and serpulid tubes are directly coat-ed by the thick aragonitic fibrous cements. Upper slopevermetid colonizations are usually less dense in compar-ison with their shelf edge analogues, and have also beenrecognized to be the source of a great amount of bioclas-tic sediment (cemented vermetid blocks and packstones-

wackestones with reworked and fragmented vermetidsand serpulids), thus representing an important compo-nent of the Messinian reef slope.

On Crete, vermetid colonizations show comparablefeatures, even if the platform edge “trottoirs” are miss-ing, due to the young erosion of the upper part of the reefsystem. Vermetids have been observed in the slope faciesas reworked blocks of VRs extremely similar to thecoeval Salento examples, especially with regard to biot-ic composition and overall textures (Fig. 6b). They con-sist of monogeneric Petaloconchus chaotic aggregates,together with a variable amount of serpulids (Spiro-branchus) and coralline algae (Lithophyllum), with thinaragonitic crusts around and/or between vermetids andserpulids (Fig. 6d). As in Salento, the framework can bestrengthened by such thick acicular cements. The matrixcommonly consists of a wackestone/packstone rich of


Fig. 5 - Early Messinian slope or toe-of-slope deposits, western Iraklion Basin, central Crete. A. road section, Asites - Kroussonas. B. Kroussonasarcheological site.

fragments of the various “reef-building” componentstogether with some barnacles, rare bryozoans and smallbenthic foraminifers. Some small fragments of Poritesare present. The VRs represent a common component ofdecimeter size in debris flow deposits and chaotic slumpmasses accumulated in a slope environment (Reuter etal., 2006) (Fig. 5).

Halimeda reefs (HSRs)

In the Early Messinian platform systems, segments ofthe green alga Halimeda are present forming discrete,sheet-like beds or mounded structures in a slopeenvironment (Reuter et al., 2006). On Crete, themounded structures represent rather small, localconcentrations of Halimeda segments with a flat base,more or less bell-shaped upper surface and weredescribed briefly by Moisette et al. (1993) (Fig. 7b). Oneof these mounds is perfectly exposed along the road

from Asites to Kroussonas. It is 1.8 m wide and 0.6 mthick and consists of a chaotic aggregation ofdisarticulated Halimeda segments that constitute 80-90% of the skeletal component (Fig. 7d). The associatedfauna is represented by benthic foraminifers, corallinered algae, serpulids, decapod crabs and ostracods as wellas rare vermetids. Agglutinated tubes with a near verticalorientation measuring 1-1.5 cm in diameter mayrepresent tubes of terebellid worms. The bioclasticmaterials form a grain-supported texture leaving openlarge irregular shelter pores similar to those describedfrom Bahamian slopes (James & Ginsburg, 1979),however, a dense micritic sediment with abundantpeloids fills in most void spaces (Fig. 7d). Remainingopen spaces (20%), i.e. shelter pores, exhibit thickmarginal crusts of isopachous fibrous cements and a lastgeneration of laminated micritic internal sediment infill.

In the lower Messinian reef complex of the SalentoPeninsula slightly lenticular structures (about 30 m long


Fig. 6 - Facies of vermetid reefs. Left column Salento, right column Crete. A. Hand specimen showing the typical dense chaotic aggregates of thevermetid “trottoirs”, at the top of the 1st sequence (Novaglie Formation). Scale bar = 1 cm. B. Outcrop close-up of the vermetid limestone with denseaggregates of vermetid conchs. C. Internal structure and biotic composition showing dominance of the vermetid gastropod Petaloconchus. The ser-pulid Spirobranchus is visible at the lower right corner; thin section. D. Aggregates of vermetids (Petaloconchus) and coralline algae; thin section.

and 4-5 m thick) have been recognized in the mid slope(Bosellini F.R. et al. 2001, 2002). They consist of chaoticaggregations of closely packed disarticulated Halimedasegments (Fig. 7a). Like on Crete, Halimeda segmentsconstitute about 80-90% of the bioclastic component andare generally very well preserved and rarely broken, inplaces encrusted by bryozoans and coralline algae (Fig.7c). The associated fauna is represented by some smallbenthic foraminifers, bivalves, bryozoans, echinoids,coralline algae, ostracods and rare vermetids. Halimedasegments are commonly embedded within a wackestonematrix with abundant peloids. Shelter cavities andresidual voids are filled by sparry calcite (Fig. 7c, 7e).

EARLY MARINE CEMENTS

A conspicuous element of VRs and HSRs is thepresence of significant volumes of fibrous cement (Fig.7e, 7f, 8). The fibre-like crystals range in thickness from3 to 15 µm and 200 - 500 µm in length, however, themaximum length of the crystals is not exactly known(Fig. 8g, 8h). According to staining with Feigl’s solutionand confirmed by X-ray diffraction, the fibrous cementsand botryoids are composed of aragonite (calcite belowdetection limits of X-ray diffraction, i.e. <1%). In SEMview, the aragonite fibres exhibit grooved crystal facesorientated parallel to the axis of maximum growthreflecting a compact growth style without significantintercrystal porosity (Fig. 8g, 8h). Fibrous cementcrystals forming epitaxial overgrowths represent denselinings of equal thickness with individual crystals beingorientated normal to nucleation surface or radiallystructured botryoids. In VRs, cement makes a significantcontribution to the rock volume (40 - 60%), where it fillsin framework cavities formed by chaotic aggregations ofvermetids, serpulids and minor secondary binders suchas bryozoans and microbial crusts (Bosellini F.R. et al.2002). In HSRs, cements either exist between Halimedasegments filling up interparticle porosity, or within large(several cm), irregular shelter voids (Fig. 7e) which areidentical in architecture with those in Halimedaaccumulations in modern Bahamian reefal slopes (James& Ginsburg, 1979).

Fibrous cements are 0.5-7 mm (typically 5 mm) thick,representing dense linings of equal thickness with indi-vidual crystals orientated normal to nucleation surface.Botryoidal fans have a maximum size of <50 mm andmay occur as isolated splays within a pore or as dense,coalescing mamelons. Fine sediment particles andpeloids filtering into the cavities were incorporated intothe cements forming thin “dust lines” sensu Grammer etal. (1993), that grade laterally into fossiliferous micriteor pelletoidal internal sediment (Fig. 9). Fibrous andbotryoidal cements, therefore, formed at the same timewith pelletoidal sediments and at an early marine diage-netic stage in contact with seawater. Acicular aragonitespetro-graphically identical to those described here havebeen documented from HSRs of early Messinian age in

SE-Spain (Braga et al., 1996; Martín et al., 1997). A syn-despositional seafloor lithification by these cements inconjunction with peloidal microbial crusts was found theprerequisite for syndepositional stabilizations of flanksin HSRs (Braga et al., 1996; Martín et al., 1997).Interestingly, age determinations for botryoidal andisopachous acicular aragonite cements intergrown withhigh-Mg calcite peloids occurring in intergranular poresand various irregular cavities in Bahamian marginalslope deposits have documented rapid and syndeposi-tional growth (Ginsburg & James, 1976; Aïssaoui, 1985;Brachert & Dullo, 1991; Grammer et al., 1993).Although botryoidal aragonites eventually may occurwithin terrestrial cave systems (Ostermann et al., 2007),early Messinian acicular aragonite cements more plausi-bly formed coeval with the accumulation of fossiliferouscryptocrystalline carbonate sediment within smallsynsedimentary cavities (growth-framework porosity ofvermetid reef and shelter porosity in Halimeda reef) andduring a very early stage of diagenesis in contact withsea water.

A second generation of cement that fills in remaininginterparticle pore spaces is a clear calcite forming crustsof equant to bladed crystals (<1 mm thickness) on thefirst generation of aragonite cement (Fig. 7e).

Petrographically identical, clear spar cement alsooccurs in moulds formed by dissolution of originallyaragonitic biota (i.e. the vermetid shells and some of theHalimeda segments) that form the substrate of the earlyaragonite cements. Fabrics related to dissolution offibrous aragonite cement are minor and confined to basaltips of the crystals or radial channels <1 mm widefollowing longitudinal crystal boundaries. Calcite sparcement also occurs within the channels, although someof them have remained open. The high resistance todissolution of the aragonite cements relative to vermetidshells and Halimeda plates has been assigned to thedense crystal fabrics in the cements. Dissolution ofskeletal aragonite, therefore, took place after theprecipitation of acicular and botryoidal aragonites butwell before the formation of calcite spar cements.Petrographically, isopachous equant to bladed calcitecements can be assigned to a meteoric-phreaticdiagenetic environment (Longman, 1980). Dissolutionof aragonite occurred between episodes of early marineand freshwater cementation, and therefore, mostplausibly reflects subaerial exposure and weathering.

A peculiarity to the Novaglie Formation is the pres-ence of extensive subvertical fissure systems. The dikescrosscut primary sedimentary facies, including earlyaragonite cements, at straight lines reflecting rather latebrittle deformation. Multiple phases of infill by vari-coloured micrites and various generations of cementsreflect a complex history of deformation and fissurerejuvenation. According to thin-section observation,micrite infills pre- and postdate dissolution of aragonitegrains, i.e. vermetid tubes, and postdate also rims ofbladed calcite in secondary cavities formed by dissolu-tion (i.e. molds of vermetids). Nonetheless, in some of



Fig. 7 - Facies of Halimeda reefs. Left column Salento, right column Crete. A. Outcrop view of the Halimeda reef with chaotic aggregation of close-ly packed disarticulated segments, slope facies of the reef complex (Novaglie Formation). B. Field aspect of Halimeda reef situated on top of a debrisflow deposit. The debrite contains large angular lithoclasts of vermetid limestone, fragments of Porites and Halimeda segments. The reef is coveredby laminated marl. Road section, NE Asites, central Crete. C-D. Thin section micrographs showing internal structure of reefs and well-preservedHalimeda segments. E. Halimeda segments coated by two generations of cement; thin-section. F. Thin-section micrograph of HSR. Halimeda seg-ment (H) covered by botryoidal (B) and isopachous (I) linings of fibrous cement and peloidal micrite (M1). Isopachous linings have a cover of micrite(M2). Arrows denote “dust line” of peloids incorporated into botryoid. Varvara Formation, Gorgolaini monastry, Crete.


Fig. 8 - Early diagenetic cements in early Messinian limestone. Left column Salento, right column Crete. A-B. Field aspect of vermetid facies andearly diagenetic aragonite cements (A: diameter of the coin is 1.9 cm). C-D. Polished slabs of vermetid reef showing distributions of vermetids, earlycements and partially lithified micritic internal sediment. Note surprising overall similarity of texture in limestone from Salento and Crete. E.Coalescing mamelons of botryoidal aragonite cement showing typical undulose extinction. Sediment inclusions form dust lines that mimic growthzonations. Dark dots represent small holes in thin-section. Crossed nicols. F. Crust of fibrous aragonite cement in vermetid boundstone. Micritic inter-nal sediment partially infills vermetid tubes. G-H. Transverse and longitudinal sections of aragonite fibres showing well developed crystal faces andlongitudinal grooves. Minor dissolution may be inferred from rounded grooves in H (SEM photographs).

the dikes we observed thick palisades of acicular arago-nite rim cement (<45 mm) and large aragonite botryoids(<60 mm) intercalated with fossiliferous pelletoidal car-bonate and red dike-filling micrite (Fig. 10).

Stable isotope composition of carbonate cementsand micrite

Stable oxygen and carbon isotopes were measuredfrom botryoidal, isopachous aragonite crusts and blockycalcite cements. No trace of calcite has been detected byX-ray diffraction (detection limits of the method <1%) inacicular cements selected for stable isotope analysis. Theaverage isotope composition of individual cement crustsand botryoids was found to be highly variable fromsample to sample, both with regard to δ18O (range+2.44‰ to +5.06‰, mean +4.07‰) and δ13C (range+2.66‰ to +4.68‰, mean +4.07‰) (Fig. 11). Incontrast, stable oxygen and carbon isotope variability ofa given sample is surprisingly uniform: cement crustssampled in transects at sub-millimeter incrementsexhibit an insignificant variation with an averagecomposition of δ18O = +4.41‰ (+0.26‰ 1σ) and δ13C= +4.25‰ (+0.12‰ 1 σ), a pattern also detected in othermarine botryoidal cements (Grammer et al., 1993) (Fig.12). Equant calcite cement, on the other hand, iscompositionally highly negative and variable in δ18O = -3.11‰ (+0.56‰ 1 σ) and δ13C = -7.19‰ (+2.50‰ 1 σ)(Fig. 13). In matrix micrite, δ18O and δ13C are variableas well but more positive than in calcite cement (averageδ18O = -0.87‰, δ13C = -2.51‰). A highly significantcorrelation of δ18O and δ13C in both cement and micrite,however, suggest micrite samples to represent variousstages of diagenetic alteration and transformation intocalcite (Fig. 13).

DISCUSSION

Sedimentary environments

Vermetid reefsVermetids are sessile gastropods distributed in tropical

and subtropical seas (44°S to 44°N latitude), usuallyclose to coral reefs diffusion boundaries, with a mini-mum temperature threshold estimated to be around 14°-16°C (Safriel, 1966; Laborel, 1986; Antonioli et al.,1999). Present-day vermetids, build dense aggregates inhigh-energy environments (called reef, caps or “trot-toirs”). They consist mainly of the vermetid genusDendropoma, together with coralline algae, serpulidsand encrusting foraminifers. For this reason, Dendro-poma colonizations are frequently used as paleobathy-metric markers, strictly related to the intertidal or shal-low subtidal zones (Antonioli et al., 1999; Silenzi et al.,2004). Concerning the use of other “reef-building” ver-metid genera as paleobathymetric indicators, caution issuggested by some authors, due to the lack of knowled-ge of their ecological demands (Schiapparelli et al.,2006). Even if the Salento stratigraphical setting pointsto a paleodepth of 0-50 m, studies are in progress tocheck the real precision of fossil Petaloconchus reefs assea level indicators.

Halimeda reefsThe Halimeda reef studied on the island of Crete is

intercalated with a succession of homogeneous andlaminated marl with scattered debrite and calciturbiditebeds which is indicative of a slope environment. Thepresence of angular clasts of Mesozoic basementlimestone, lithoclasts of VRs, reworked reef corals(Porites, Tarbellastraea), and Halimeda segments are areflection of a coastal and/or shallow water source areaand some transport of the debrites and calciturbidites.The HSR rests in situ on top of a breccia.

Consequently, it formed distally from shore in a ratherdeep low-light environment. The upper surface of theHSR is curved convex upward and marked by a sharp


Fig. 9 - Photomicrograph of thin-section illustrating the spheruliticgrowth of botryoidal aragonite. Layers of peloids and small skeletalparticles reflect mammilary growth. Identical pelletoidal marinesediment covers the aragonite crystals (VR, Novaglie Formation,Salento).

Fig. 10 - Large aragonite botryoids interlayered with reddish to whitemicrite filling a dike within proximal slope deposits of the NovaglieFormation (Salento) (diameter of the coin is 2.3 cm).

lithological change into finely laminated dark marl. Themarl drapes over the mound, thereby documenting itsoriginal topography. Fine lamination and scarcity ofmacrofossils both point to a quiet and restrictedenvironment. The HSR described from Salento issurrounded by poorly stratified bioclastic calcarenitesand breccias as well, and sedimentary structuresindicative of transport are poorly developed pointing outaccumulation in place (Bosellini F.R. et al., 2001, 2002).

In present-day marine environments, at depths ofseveral tens-of-meters (commonly 30-50 m), segmentsof the udoteacean green alga Halimeda can accumulatemore or less in place forming structures called mounds,banks, bioherms or “segment reefs” (Hine et al., 1988;Marshall & Davies, 1988; Roberts et al., 1988; Orme &Riding, 1995). The first Miocene example has beendescribed from early Messinian reef systems of SorbasBasin, southeastern Spain (Braga et al., 1996; Martín etal., 1997). The HSRs of the Salento Peninsula (Bosellini

F.R. et al., 2001, 2002) and Crete, although considerablyreduced in size, show strong similarities with thosedescribed in Spain and are both interpreted to have beenformed in down-slope environments and documentbenthic carbonate production and rapid cementation inthe lower photic zone (Reuter et al., 2006). Comparisonswith present-day analogues point out a paleo-waterdepthof ~50 m for both Crete and Salento HSRs.

Diagenetic environments

Petrographic evidence documents two generations ofcement, an early phase of botryoidal and/or isopachousacicular aragonite, and a second, late phase of calcitespar following an episode of dissolution of aragonite.The stable isotopic composition (δ18O, δ13C) issignificantly different in the two generations of cementand fully consistent respectively with a marine andfreshwater phreatic diagenetic environment (Figs. 11,13). The highly positive composition in both δ18O and


Fig. 11 - Stable isotope composition (δ18O, δ13C) of botryoidal and isopachous cements. Botryoidal aragonites from Belize and Bahamas accordingto Grammer et al. (1993), Red Sea from Taviani & Rabbi (1984) and Aïssaoui (1985), Ouvrea from Aïssaoui (1985), Cyprus from Follows (1992).

Fig. 12 - Stable isotope compositional variability (δ18O, δ13C) withinmamelons of botryoidal aragonite cement. Gray shading in photo-graph shows vermetids and blocky calcite spar forming a second gen-eration of cement. Diamonds represent δ13C (V-PDB). Salento penin-sula (sample 12/17).

Fig. 13 - Stable isotope composition (δ18O, δ13C) of equant to bladedcalcite spar and micrite from vermetid reefs and Halimeda segmentreefs.

δ13C found for botryoidal and isopachous crusts ofacicular aragonite supports petrographic evidence for amarine origin. Recently, botryoidal aragonites have beenshown to be closely associated with marine methaneseeps (Peckmann, 2001), however, the positive δ13Csignature in the Messinian cements rules out anypotential genetic relationship with hydrocarbon venting,although seep carbonates play a significant role in thelate Miocene geological record of the Mediterraneanregion (Peckmann, 1999; Pierre & Rouchy, 2004).

Interestingly, oxygen stable isotope data (δ18O) frombotryoidal aragonites described from secondary cavitieswithin early Messinian reefs of Sicily (+6.8‰) are evenmore positive than those analysed in this study (Esteban& Prezbindowski, 1985). Although speleothems formedof masses of botryoidal aragonite have been describedfrom some cave systems and rockfall breccias, δ18O =+6.8‰ clearly rules out a formation within freshwatercave systems (cf. Ostermann et al., 2007), however, mayrepresent a reflection of precipitiation from percolating,highly saline Messinian brines within secondary cavities(Esteban & Calvet, 1983; Esteban & Prezbindowski,1985). Although such an interpretation is in goodagreement with the cements being restricted to“secondary cavities” in older reef rock, it is notconsistent with the sequence of diagenetic eventsobserved at Salento and Crete, where formation ofsecondary voids and precipitation of freshwater cementsclearly postdate aragonite cements. In addition, we inferan intra-early Messinian age of aragonite cements fromlithoclasts of intraformational breccias from Crete andSalento. Lithoclasts of reworked VRs usually show aninternal structure heavily cemented by aragonite, butcements terminate bluntly along clast surfaces (Fig. 14).Moreover, coral fragments embedded within matrixsupported breccia (debrites) exhibit rims of aragonitecement which implies precipitation of aragonite prior toredeposition.

The contrasting stratigraphic settings and stableisotope compositions of botryoidal aragonite from Sicily

and Salento - Crete may point, therefore, to the existenceof two separate phases of precipitation of botryoidalaragonite, one of which occurred during the earlyMessinian when the coral reef systems formed, the othersomewhat later and in conjunction with the MSC. It mayfind its reflection in some of the dike filling botryoidalaragonites, however, more data are needed in support ofsuch a classification.

Environmental constraints from stable isotopes

The vermetid, Halimeda and coral reef assemblagesall lived in shallow waters, and, thus, represent excellentgeochemical archives of ambient water temperatures(Silenzi et al., 2004; Felis & Pätzold, 2004; Brachert etal., 2006a). In Salento and on Crete precipitation ofaragonite fibrous cement took place early in a marineenvironment. Because the cements formed inequilibrium with ambient sea water, the stable oxygenisotopic composition of the early marine cements can beconsidered a proxy data set of the paleoenvironment(SST, SSS), plus a variable seawater componentreflecting global ice build-up (Ginsburg & James, 1976;Aïssaoui, 1985; Brachert & Dullo, 1991; Grammer et al.,1993) (Fig. 11).

Oxygen stable isotope compositions in fibrouscements from Salento and Crete exhibit significant,however, very similar variation at both sites (Crete∆δ18O = 1.75‰, Salento ∆δ18O = 1.66‰). Providedmodern systems apply roughly to the Late Miocene,such a compositional variation is equivalent with a 7.7°Cand 7.3°C SST spectrum on Crete and Salento,respectively (according to temperature equation given byBöhm et al., 2000), or a 6.5‰ and 6.1‰ salinityvariation (according to salinity - δ18Owater relationshipby Pierre, 1999) in the central and eastern Mediterraneanduring the early Messinian (Fig. 11). Althoughpetrographic and stable isotope data are fully consistentwith a marine origin of the aragonite cements, theinterpretation of δ18O signatures remains problematic interms of separating the temperature and salinitycomponents. If one accepts reconstructions of winterSSTs of 18°C and average annual SSTs of 23°C basedon vermetid and coral biofacies (Rosen, 1999; BoselliniF.R. et al., 2002; Brachert et al., 2006a, Brachert et al.,2006b), then the measured stable oxygen isotopecomposition has a significant evaporative component.Based on the modern temperature - δwater relationshipfound for modern coralline sponge aragonite andinorganic aragonite (Böhm et al., 2000) and δwater -salinity relationship (Pierre, 1999) salinity amounts to 45to 54‰. In order to account for global Late Mioceneseawater δwater effects, a correction factor of +1‰ maybe added to the measured isotope ratios (Feary et al.,1991; Billups & Schrag, 2003), however, this procedureadds an additional 3.7‰ on calculated salinity (SSS = 48to 58‰). Salinity estimates tend to be lower, whenannual SSTs were lower than assumed in this study.However, the temperature effect on calculated salinity


Fig. 14 - Block of vermetid reef from slope breccia. Inset: vermetidframework stabilized by linings of fibrous aragonite cement truncatedat contact with matrix. Novaglie Formation, Salento.

requires unrealistically low SST (0 to 5°C) to arrive atopen seawater salinity.

Significance of stable isotope data

Sea surface salinity which here is estimated in theorder of 48 to 58‰, however, represents a major restric-tion to most benthic biota and raises the question of thenature of the early Messinian neritic environments docu-mented on Crete and Salento Peninsula. Mediterraneanrestriction starting early during the Late Miocene is wellconstrained by geochemical (Flecker & Ellam, 1999)and foraminifer δ18O records (Kouwenhoven et al.,1999). Declining benthic and planktonic foraminiferdiversity and mass occurrences of phytoplankton adapt-ed to high salinity in the Mediterranean are all evidenceof fluctuating and increasing salinity stress prior to theMSC (Santarelli et al., 1998; Kouwenhoven et al., 1999).Aplanktonic beds re-occurring on precessional time-scales reported from various Miocene basins of theentire Mediterranean area imply events of high SSSbeyond relevant paleo-ecological thresholds(Kouwenhoven et al., 2006). Non-skeletal aragonite pre-cipitates forming the matrix of some aplanktonic beds(Bellanca et al., 1986) are identical to those from the“aplanktonic zone” of the Red Sea, where some 50 to55‰ SSS were sufficient to wipe out most calcareousbenthos and plankton during Last Glacial Maximum andto switch carbonate production into a non-skeletal modetaking the shape of aragonite spherulites and microbialcrusts (Brachert, 1999).

Sea surface salinity of more than 50‰ represents amajor restriction to most macro-invertebrates of reefenvironments. Among modern zooxanthellate corals,Porites are most resistant to high salinity and tolerate amaximum of 45-48‰ (Kinsman, 1964). With regard tothe early Messinian neritic environments of Salento andCrete, the dilemma encountered between the paleonto-logical and geochemical salinity reconstructions may beresolved by assuming high-frequency salinity variabilitybeyond the resolution of 4th order depositionalsequences. In this scenario salinity increases were geo-logically short episodes peaking at 12 to 22‰ above nor-mal values. We suggest that in an environment undergo-ing high salinity stress on short, i.e. precessional time-scales or less, community replacement must have beenthe norm. The problematic dominance of stromatolitesover other stenohaline encrusters in Messinian coral reefframeworks (including VRs and HSRs) (Riding et al.,1991) may, therefore, reflect community replacementlinked to environmental fluctuations such as suggestedfor stromatolites in Holocene open-marine coral frame-works (Camoin et al., 1994).

Previously, early Messinian salinity variability wasascribed to isolation related changes in the evaporation/precipitation (or runoff) balance of small, closed orsemi-closed basins (Bellanca et al., 1986). A semi-isolated setting may apply to our sites on the island ofCrete, but reef systems of the Novaglie Formation on

Salento Peninsula were exposed to the open sea and,therefore, imply high frequency salinity variations oflarge parts of Mediterranean surface waters, which werein the same order of magnitude as those from small,semi-restricted basins. One might speculate, therefore,that salinity changes affected the entire Mediterranean,and that salinity build-up paralleled evaporative sealevel. However distribution of aragonite cements fromslope to platform margin (Fig. 3) show that episodes ofaragonite precipitation and high salinity were notcoupled to sea level lowstands, i.e. to an evaporative“draw-down” of sea level. In order to compensate forisolation driven evaporative losses, we must infer mutualchanges in evaporative “draw-in” of Atlantic surfacewater and/or river discharge. Such inferences support theconcept of the MSC starting as a deep brine pool andinvolve the occurrence of possible refugia forstenohaline biota to survive and to recover after episodesof environmental restriction.

CONCLUSIONS

Reef complexes of early Messinian age in southernGreece (Crete) and southern Italy (Salento peninsula)exhibit surprising similarity with respect to reef-buildingbiota. In both locations vermetid reefs and Halimedabioherms occur in close relationship with Porites coralreefs, where they played an important role in thedepositional system. Moreover, vermetid reefs andHalimeda bioherms from Crete and Salento both showlarge amounts of thick aragonite fibrous cements(isopachous and botryoidal acicular aragonite).

Field observations (cemented vermetid blocksembedded in the slope sediment - Fig. 14), thin sectionanalysis (sediment particles incorporated in the cementcrusts, laterally grading into fossiliferous sediment - Fig.9) and comparison with present-day tropical examples(Ginsburg & James, 1976; Aïssaoui, 1985; Brachert &Dullo, 1991; Grammer et al., 1993) indicate an earlymarine origin of aragonite cements, which formed incontact with sea water.

Stable isotope compositions of aragonite cement(δ18O, δ13C) at both localities are surprisingly similar(Fig. 11) and must reflect paleotemperature andpaleosalinity. On the basis of the reef-building biotarequirements, we assume an average winter SST of 20°Cand an average annual SST of 23°C (Rosen, 1999;Bosellini F. R. et al., 2002; Brachert et al., 2006a,2006b). For this reason, stable oxygen isotopecompositions (δ18O = +2.44 to + 5.06 V-PBD; Fig. 11)must have a strong evaporative component anddocument strongly hypersaline environments of 48 to58‰ which are lethal to most of the shallow marinecalcareous biota.

As a possible explanation to this discordant paleo-environmental information we infer the paleoceano-graphic changes to have been short and beyondstratigraphic resolution of the shallow water record.


Consequently, indicators of “abiotic” episodes couldhave been obscured by time-averaging, preventing themto be recorded in 4th order depositional sequences.

Paleogeographically, the carbonate platforms of Cretepotentially formed within semi-enclosed basins, andtherefore, reflect local climate events. The platforms ofSalento, however, faced the open sea in the CentralMediterranean (Fig. 1), and the equal presence ofcements recording high salinity inputs in reef and slopedeposits (Fig. 3) suggest that salinity events occurred inlarge parts of the Mediterranean and were not coupled toevaporative draw-downs of sea level. The latter observa-tion implies the possibility of some refugia for stenoha-line biota persisting over periods of high salinity andallowing for rapid re-colonization.

ACKNOWLEDGMENTS - C. Fassoulas and G. Illiopoulos(Iraklion, Crete) are thanked for their continued interest in our

work and stimulating discussions. Michael Maus (Mainz)performed the microsampling of carbonate cements for stableisotope analyses, and H. Becker (Mainz) prepared the thin-sections. Stable isotope analyses were carried out by M.M.Joachimski (Erlangen) and J. Fiebig (Frankfurt). X-raydiffraction analyses were performed by H.-D. Werner (Mainz).We are also grateful to Charlotte Schreiber for her suggestionsand constructive remarks on the manuscript draft. TCB andMR acknowledge funding by the Deutsche Forschungs-gemeinschaft (DFG Br 1153/8 and 9) and TCB thanks forsupport by Forschungsfonds der Universität Mainz. Studies byFRB and AV were funded through grant PRIN 2004.

Contribution to MIUR PRIN project 2004045107 (Palaeo-climatic forcing on building organism communities, carbonateproductivity and depositional systems of some Italian Meso-Cenozoic shelf deposits). Bosellini A., coordinator.


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