DOES THE CALCARE DI BASE (MESSINIAN, NORTHERN … 40/12 GUIDO.pdf · brecciated facies and/or evidence of transport. We attempted a new approach analyzing in detail the sedimentary

INTRODUCTION

Carbonate and evaporitic sediments can be laterallyand vertically related as a result of changes in the basinhydrology and the degree of connection to the open sea,consequently a succession of deposits representing thechange from normal marine to evaporitic conditionsoccurs (Rouchy et al., 2001). Examples of these types ofdeposits can be found in Messinian sediments of theMediterranean area (Esteban, 1979; Rouchy, 1982;Decima et al., 1988; Rouchy & Saint-Martin, 1992;Esteban et al., 1996; Rouchy & Caruso, 2006). An openquestion is the Calcare di Base genesis. In particularwhether these deposits were formed in hypersalinesettings or they represent normal salinity marinesediments, formed before the main restriction andevaporative deposition (Rouchy et al., 2001).

Previous researches on the Calcare di Base werefounded mainly on classical sedimentological,stratigraphic, and geochemical studies (McKenzie,1985; Bellanca & Neri, 1986; Decima et al., 1988;Rouchy & Saint Martin, 1992; Pedley & Grasso, 1993;Roveri et al., 2006b; Manzi et al., 2007a; 2007b; Rouchy& Caruso, 2006). These researches however were notable to give a sound interpretation on the genesis of thesepeculiar carbonates and to verify if these carbonates

represent the actual onset of the Salinity Crisis. The Calcare di Base Formation has been interpreted as

evaporitic deposits, in which sulphate reducing bacteriareplace Ca-sulphate into carbonate, or subordinately asprimary peloidal limestones (McKenzie, 1985; Bellancaet al., 1986, 2001; Bellanca & Neri, 1986; Decima et al.,1988; Decima & Wezel, 1971; Rouchy & Saint Martin,1992; Pedley & Grasso, 1993). In both case thesecarbonate units are considered belonging to the LowerEvaporites succession.

The Calcare di Base is usually interpreted as formed inshallow waters with strongly fluctuating salinities. Thebrecciated facies is related to autobrecciation processesinduced by dissolution of halite and gypsumintercalations during phases of water dilution (Ogniben,1963; Decima et al., 1978; Pedley & Grasso, 1993).

Furthermore, some researchers (Roveri et al., 2006b;Manzi et al., 2007a, 2007b) hold that sometimes thebrecciated character of the limestones is due to massflow. They support this interpretation for the presence ofparticular stratigraphic and sedimentological featureslike erosional bases with load casts, overall normalgradation, upward transition to gypsarenite unit and claychips. Unfortunately these researches developed mainlythe stratigraphic, palaeogeographic and structuralaspects ignoring almost totally the paleoecological

Geologica Romana 40 (2007), 129-146

DOES THE CALCARE DI BASE (MESSINIAN, NORTHERN CALABRIA)REPRESENT A BACTERIAL INDUCED DEPOSIT?

Adriano Guido*, Jérémy Jacob°, Pascale Gautret°, Fatima Laggoun-Défarge°,Adelaide Mastandrea* & Franco Russo*

* Dip. di Scienze della Terra, Università della Calabria, Via Bucci Cubo 15b I–87036 Rende (CS), Italy° ISTO, UMR 6113 du CNRS - Université d’Orléans, Bâtiment Géosciences, 45067 Orléans, France

ABSTRACT - Geochemical and petrographic studies of organic matter have been carried out on the MessinianCalcare di Base Formation cropping out in the Rossano basin, northern Calabria. This approach allowed toelucidate the depositional conditions under which these carbonates formed, namely the physicochemicalproperties of the water column and the possible role of microbes in the mineralization processes.

The biological evidence in the microfacies, like thrombolites mainly constitute of clotted peloidal micrites andfaecal pellets, the primary microstructure, the absence of mould and pseudomorphs after evaporitic minerals,exclude an evaporitic deposition for these sediments as well as the result of diagenetic processes replacing Ca-sulphates into carbonates (Guido et al., this volume). These considerations are confirmed by the absence ofmolecular fossils indicative of anoxic or hypersaline environment as pregnanes and homopregnanes, extendedhopanes (>C33), gammacerane and isorenieratane. The study of carbonaceous remains emphasizes a wide varietyof the organic input. Geochemical data (Rock-Eval pyrolysis) indicate a mixed (marine and continental) organicinput. These data have been confirmed by organic petrographic observations (palynofacies) that revealed thepresence of phytoclasts derived from continental plant tissues, amorphous organic matter, and variable proportionsof zooclasts, pollens, spores, phytoplanktonic organisms and filaments, dubitatively attributable to cyanobacteria.The constant and strong bacterial signal of the molecular fossils, represented by the n-alkanes with mode in nC26-nC28 and with no odd-even carbon number predominance, branched alkanes, hopanes and unsatured fatty acids,together with the widespread presence of amorphous organic matter in the palynofacies, corroborates theinterpretation that the clotted peloidal micrite represents a bacterial induced deposit.

KEY WORDS: Calcare di Base Formation, organic matter, molecular fossils, bacterial mineralization, Messinian, Calabria, Italy.

characteristics of sedimentary environments, makingdifficult the interpretation of the causes which triggeredthe salinity crisis.

The Calcare di Base Formation, sampled in variouslocalities of Calabria and Sicily, shows always the samedominant microfacies, which is constituted by peloidal/coprolitic mudstone/wackestones with thromboliticfabric (Guido et al., this volume). This microfaciesrecords a single palaeoecological event, possibly notsynchronous, prologue in southern Italy of the MessinianSalinity Crisis (Guido et al., this volume). With the aimto detect organic matter data on this microfacies weanalyzed the Calcare di Base Formation cropping outnear Cropalati village (Rossano basin, NorthernCalabria). We selected this area since here the carbonatestrata are preserved in their original mineralogy(aragonite) and microstructures and they do not showbrecciated facies and/or evidence of transport.

We attempted a new approach analyzing in detail thesedimentary organic matter (palynofacies observation,Rock-Eval pyrolysis and GC/MS analysis).

Reconstruction of depositional conditions, based onbiomarker data, is presently one of the main topics oforganic geochemistry (Marynowsky et al., 2000).

Biomarkers are complex molecular fossils, derivedfrom biochemicals produced by once-living organismsthat provide information on organic matter origin andenvironmental conditions. Thus the detailed characteri-zation of biomarker assemblages allows assessing themajor contributing source species.

A broad variety of microorganisms, whether pho-totrophic or not, are implicated in the carbonatogenesis.However their direct characterization in ancient sedi-mentary system is limited by the extremely low fos-silization potential of most microorganisms, especiallybacteria. Therefore, the composition of an original com-munity (primary producers, zooplankton, aerobic andanaerobic bacteria, benthic microorganism, etc.) is diffi-cult to evaluate by traditional optical methods. Manystudies have confirmed that organic geochemical tech-nique can be used to trace a former presence of microor-ganism by the recognition of molecular fossils, the bio-markers (Michaelis & Albrecht, 1979; Tissot & Welte,1984; Mycke et al., 1987; Hefter et al., 1993).

Biomarkers occur in sediments, rocks, and crude oilsand show little or no change in structure from their par-ent organic molecules in living organisms. Thisapproach allowed, for the first time, to characterize(microscopically) non-identifiable (non-preserved)organisms, in Calcare di Base sediments.

GEOLOGICAL ANDSTRATIGRAPHIC SETTING

The study Messinian Calcare di Base Formation out-crops in the Rossano Basin (Northern Calabria) (Fig. 1).The Rossano basin records the Messinian salinity crisisevents in a complex basinal setting related to the fore-

land fold-thrust belt of Southern Italy orogen (Critelli,1999).

Tortonian to Messinian sedimentary successions ofnorthern Calabria represent the sedimentary response tothe Neogene evolution of the Calabrian-Arc orogenicsystem. During the Upper Miocene the Calabrian Arcexperiences abrupt uplift and rapid slip-rate eastwarddisplacement, causing general accretionary tectonicsalong the Ionian border, and an extensional tectonics inthe nascent Tyrrhenian Sea back-arc region (Malinverno& Ryan, 1986; Patacca et al., 1990; Sartori, 1990;Funiciello et al., 1997; Critelli, 1999; Van Dijk et al.,2000; Mattei et al., 2002). Neogene sedimentary basinsof eastern Calabria are filled by Tortonian to Pleistocenedominantly clastic sedimentation interbedded withMessinian evaporite deposits.

The Tortonian sequence represents a characteristictransgressive system with an alluvial red conglomerate,passing into nearshore sediments and deep-marineturbidite strata, probably deposited during a low-standsystem tract. Deep-marine sediments are followed,throughout an angular unconformity, by marls anddiatomaceous shales including sulphate nodules,carbonates with decimetric intercalation of marl-claysand gypsumrudite-gypsumarenite deposits associatedwith clastic carbonates. These deposits pass, throughouta second angular unconformity, to arenites, marls, halite,gypsarenites and olistostromes of variegated clays

GUIDO et al.130 Geologica Romana 40 (2007), 129-146

Fig. 1 - Simplified geological map showing the study area.

(Critelli, 1999). The study samples have been collected from an

outcrop located near the Cropalati village. In this area theCalcare di Base succession is constituted of two metricmassive white to yellow fine grained calcareous bedsinterbedded with a decimetric light-brown laminatedmarls (Fig. 2). These calcareous beds overlay thediatomitic and marly layers (Tripoli Formation), whichrepresent the first phases of restriction of the basin afterthe open marine condition represented by the UpperTortonian/Early Messinian clays.

METHODS

Samples selection

Samples for carbonates and organic matter analyseshave been selected for their position in the studiedsequence and for the features observed in the field. Thetexture and composition of these samples, observedunder transmitted light microscopy, SEM and EDS,suggest that they have not been affected by stronglydiagenetic processes.

We collected tree samples for each calcareous bed,numbered CB1-CB2-CB3 and CB4-CB5-CB6, frombottom to top respectively (Fig. 3). Several millimetricsiltitic clasts (CB1S) have been separated from thecarbonate samples CB1. Sample CB4 shows two facies:detritic (CB4D) and stromatolitic (CB4M) which havebeen analyzed separately. Light-brown laminated marls,interbedded with the calcareous layers, have also been

collected (sample CBM) (Fig. 3).Six samples have been collected from the Tripoli

Formation. The diatomitic (TR5) and marly (TR6) layershave been selected for biomarker investigations.

Organic matter characterization

The organic matter quantity and quality was assessedby combining organic petrography (palynofacies), bulkgeochemistry (Rock Eval T6 pyrolysis and Lecoelemental analysis) and molecular analyses on lipids(Gas Chromatography - Mass Spectrometry).

Palynofacies

Organic petrography study was carried out on kerogenfraction, isolated from the carbonate and silicate phasesof the sediment via the classical hydrochloric andhydrofluoric acid treatments (Durand & Nicaiese, 1980).

This study involves the identification of the differentfractions of organic matter using transmitted lightmicroscopy. The following procedure was employed: thesamples have been pulverized and few grams (2-5 g) ofdust were submitted to an acidic treatment with HCI(36%), to dissolve carbonates, and HF (50%), to removesilicates. The solid residue was washed with water andthe supernatant was removed after centrifugation. Thewater washings were repeated several times, until aneutral pH was attained, then a first series of sections,named TS (Total Slide), has been carried out with fewmicrolitres (300-500 µl) of residue. A further acidictreatment have been performed with KOH (10%) to put

DOES THE CALCARE DI BASE (MESSINIAN, NORTHERN ... 131Geologica Romana 40 (2007), 129-146

Fig. 2 - Panorama of the study section showing diatomites and marly layers of the Tripoli Formation and carbonate beds of the “Calcare di Base”Formation.

in solution the humic matter, and with HNO3 (63%) tooxidize the organic matter, to dissolve pyrite, andpotassium salts formed. Then, after densimetricseparation, a second series of sections, named RS(Residual Slide) have been performed.

Rock-Eval pyrolysis

Bulk organic matter geochemistry has been assessedby Rock Eval T6 pyrolysis (Espitalié et al., 1977;Lafargue et al., 1998). This method allows the rapidquantitative and qualitative characterization ofsedimentary organic matter. The Rock-Eval parametersused for this study are the followings: (1) Total OrganicCarbon (TOC, %) accounts for the quantity of organicmatter present in the sediment; (2) Hydrogen Index (HI,in mg HC/g TOC) is the amount of hydrocarbonaceous(HC) products released during pyrolysis (S2 peak)normalized to TOC; (3) Tmax is a well-known OMmaturity indicator in ancient sediments (Espitalié et al.,1985b), it is the temperature of the pyrolysis ovenrecorded at the top of peak S2, which corresponds to themaximum release of hydrocarbonaceous products duringpyrolysis; (4) Oxygen Index (OI, in mg O2/g TOC),which gives the oxygen content of the OM.

Between 50 and 100mg of dried sediments were usedfor analysis, depending on the estimated OM content.The pyrolysis program starts with an isothermal stage of3 min at 200°C. Then, the pyrolysis oven temperaturewas raised at 30°C/min to 650°C, and held for 3 minutesat this temperature. The oxidation phase, performed in asecond oven under an air stream, starts at an isothermalstage at 400°C, followed by an increase to 850°C at

30°C/min and held at final temperature for 5 minutes.Elemental analyses have been performed on bulk

material with a CNS-2000 LECO® apparatus in order todetermine Total C, N and S contents.

Lipid biomarkers

The preparation for lipid analyses includes a step ofextraction, then a separation, eventually a derivatisationstep and finally the identification and quantitation of thecompounds by gas chromatography (GC) or gaschromatography-mass spectrometry (GC-MS). 3g ofpowdered dry sediments were ultrasonically extractedthree times with a mixture of dichloromethane/methanol(1:1). Samples were centrifuged following eachextraction and the supernatant was collected. Combinedextracts were dried under nitrogen. Because Lecoanalysis revealed very low concentrations of theelemental sulphur, only free lipids were analysed,without any desulphurization.

The acidic fraction was separated from the totalextracts by solid phase extraction performed on amino-propyl bonded silica. Neutral compounds were elutedwith dichloromethane/methanol (1:1), and acidic com-pounds were eluted with ether after acidification of themedium with ether:formic acid (9:1). Fatty acids wereesterified using acetyl chloride in anhydrous methanolbefore analysis. The neutral fraction was further separat-ed by flash chromatography on deactivated silica (5%water) with solvents of increasing polarity according toTernois et al. (1998). Six fractions comprising aliphatics,aromatics, ethers, ketones, alcohols and sterols were col-lected by this mean. 5a-cholestane was added prior to GCand GC-MS analyses.

Fatty acids as their methyl esters and aliphatic andcyclic hydrocarbons were quantified by gas chromato-graphy using a GC TRACE (ThermoFinnigan). For sev-eral compounds that coeluted or were in too low abun-dance, the identification and quantitation was achievedby Gas Chromatography/Mass Spectrometry (GC-MS).GC-MS analyses were performed on a ThermoFinniganTRACE-PolarisGCQ gas chromatograph-mass spec-trometer. The gas chromatograph was fitted with anRtx®-5Sil MS capillary column (30 m x 0.25 mm i.d.,0.25 µm film thickness) with 5 m of guard column. TheGC operating conditions were as follows: temperaturehold at 40°C for 1 min, then increase from 40 to 120°Cat 30°C/min, 120 to 300°C at 5°C/min with finalisothermal hold at 300°C over 20 min. The sample wasinjected splitless, with the injector temperature set at280°C. Helium was the carrier gas. The mass spectrom-eter was operated in the electron ionisation (EI) mode at70 eV ionization energy and scanned from 50 to 650Dalton. Compounds were identified by comparison withpublished mass spectra and relative retention times. Fattyacids, n-alkanes, pristane and phitane were quantified byGC-FID while alkylbenzenes, steranes, hopanes, ethers,ketones, alcohols and sterols have been identified andsemi-quantified by GC-MS using the m/z 91+105chromatograms for the alkylbenzenes, m/z 215+217+231


Fig. 3 - Simplified stratigraphy of Cropalati section with the locationof the samples.

chromatograms for steranes, m/z 191 chromatogram forthe hopanes, m/z 256 and m/z 236+255 chromatogramsfor ethers, m/z 96 chromatogram for ketones, m/z 55+97chromatograms for alcohols and m/z 213+215 forsterols.

RESULTS

Rock-Eval pyrolysis

The accumulation of organic matter (OM) has beenestimated using TOC values. TOC ranges from 0.06% to0.19% for the calcareous samples, from 0,21% to 0,88%for the diatomitic and marly samples of the TripoliFormation, and it is 0,27% for the marls interbeddedbetween the calcareous layers. The low values in TOCsuggest to treat with caution the Rok-Eval pyrolysis databecouse these are not much diagnostics. For this reasonthe values of Tmax cannot be considered as index ofmaturity but some indications can be extrapolate aboutthe nature and the genesis of the preserved OM.

The TOC values do not vary significantly and do notexibit a particular trend, it can be only ascertained agreater content in TOC in the marly and diatomitic layersrespect the calcareous bed (Fig. 4). HI values range from306 to 417 mg HC/g TOC for the calcareous samples,from 22 to 143 mg HC/g TOC for the diatomitic andmarly samples of the Tripoli Formation, and it is 67 mgHC/g TOC for the marls interbedded with the calcareouslayers. OI values range from 105 to 312 mg CO2/g TOCfor the calcareous samples, from 57 to 556 mg CO2/gTOC for the diatomitic and marly samples of the TripoliFormation, and it is 185 mg CO2/g TOC for the marlinterbedded with the calcareous layers (Fig. 4).

Rock-Eval pyrolysis data (HI; OI) for the carbonatesamples, plotted in the pseudo Van Krevelen diagram(Tissot & Welte, 1984), put in evidence a transitional

composition between types II and III kerogens, revealinga mixture of marine and terrigenous organic matter(Guido et al., 2007). The diagram shows that HIdecreases while OI increases upward within eachcarbonate layer and generally inside the whole section.This tendency could be attributed to a relative loweringof the sea level with consequent larger continental input.

HI and OI values for the samples of the TripoliFormation and for the marls interbedded with theCalcare di Base do not exhibit a peculiar trend. We onlynotice the lower HI values and the extremely variable OIvalues (Fig. 4).

Elemental nitrogen and sulphur have been detected atvery low concentration. They show a general decreasingtrend toward the top of each carbonate beds (Fig. 4).

Palynological observation

Palynofacies observations put in evidence that kerogenfraction is extremely low in the calcareous sampleswhereas it is abundant in the marly layers of the TripoliFormation and in the marl-clays interlayered with thecalcareous bed (Fig. 5).

The very low amount of extracted organic matter doesnot allow making a relative quantification of thedifferent fractions; therefore it has been possible toperform only qualitative observations.

The kerogen shows two major component types:amorphous organic matter and terrestrial origin materials(lignaceous debris, spores and pollen). Generally paly-nological observations confirm the mixed organic mattercomposition deduced by Rock-Eval analyses.

Palynofacies are constituted mainly by heterogeneoussize elements, yellow-brown in colour, which show anintermediate fluorescence and rounded or tabular inclu-sions. The absence of structures in this material makesdifficult identifying its origin and nature. However the


Fig. 4 - Stratigraphic distributions of total organic carbon (TOC), Tmax, hydrogen index (HI), oxygen index (OI), total nitrogen content (N) and totalsulphur content (S).


Fig. 5 - Main organic debris of the analysed samples. (a-b) pollens; (c) phytoplanktonic organisms; (d-e) plant-derived ligno-cellulosic tissues; (f)oxidized or burnt terrestrial debris; (g-h) exoskeleton fragments of arthropod; (i) altered amorphous organic matter; (h) preserved amorphous organicmatter.

observed correlation between the presence of these par-ticles and the high values of the hydrogen index suggestsan algal and/or bacterial genesis. It is possible to distin-guish three different types of this amorphous OM: dark,brown and yellow (Figs. 5i, 5l). In addition to these OMfamilies, structured particles of vascular-woody originare also present. These components are subdivisible intwo groups: translucid or semi-opaque elements (Figs.5d, 5e), with reddish and sharp edges, and smallerdimension opaque elements with tabular or equidimen-sional shape (Fig. 5f). Generally internal biostructure areno visible, since they have been infilled and largely oblit-erated by gelification. Some of these elements exhibitrelict structure of parallel fibres. The figurate elementsare represented also by spores, pollens (Figs. 5a, 5b) andphytoplanktonic organisms (Fig. 5c). The presence ofwell preserved and bright-fluorescent spores and pollensindicates that these elements did not undergo degrada-tion and oxidation, suggesting a sedimentary environ-ment characterized by a stratified water column withperiodic bottom dysoxic/suboxic conditions.

These organic facies are present in all studied samples,but in different proportions. Samples TR5 and TR6 areconstituted mainly of amorphous OM and terrestrialorigin materials. The latter are present in the sample TR5with three different types: oxidized or burnt, gelificatedand as pirofusinite. The sample TR6 shows the samekerogen composition but with an increase of amorphousorganic matter, with particles of pedogenetic origin(AOM gelificated). In the terrestrial fraction the opaqueparticles decrease. Many small tabular siliceousfragments are also present and they could derive fromdiatom skeletons.

The palynofacies observations on the calcareoussamples put in evidence a predominance of amorphousdebris. It is possible to distinguish two main types ofAOM: a dark-altered fraction and a brown immaturefraction. The Calcare di Base extract is rich of immaturealgal debris. These algal debris are represented bytransparent membranes with irregular or sub-sphericalforms, almost not observable in transmitted light, butvery fluorescent under UV excitation. Among organicfractions remarkable is the presence of well preservedexoskeletons fragments of arthropods, probablyattributable to copepods (Fig. 5g, 5h). Carbonatessamples show an increase of terrestrial debris from thebottom to the top of each carbonate layers, and generallythrough the whole section.

Kerogene of the sample CBM shows about the samecomposition of the tripolaceous marls but with aremarkable increase of terrestrial fraction and debris ofpedogenetic origin.

The densimetric separate of the sample CB2 revealednumerous amorphous elements of gelatinous aspect withlow reflectance and fluorescence. These amorphouselements have been interpreted as silica crystals linkedtogether by soluble organic matter. The soluble organicmatter has been pointed out by epifluorescenceobservations on Acridine orange stained thin sections

(Guido et al., this volume).

Biomarkers

Within the lipids extracted from our samples, weinvestigated fatty acids, aliphatic and cyclic hydrocar-bons, ethers, ketones, alcohols and sterols.

Fatty acids

Distribution - Fatty acid distribution are oftendominated by a series of straight-chain componentsranging from 14 to 32 carbon numbers (Fig. 6). Theirdistribution shows a strong predominance of even-carbon-number homologues and is bimodal with onemaximum at nC16 or nC18 and the other within the rangeof the long-chain fatty acids. Monounsatured nC16(C16:1) and nC18 mono- (nC18:1) and diunsatured(nC18:2) fatty acids are also detected in the acid fraction.Among branched fatty acids, only phytanic acid hasbeen detected in significant proportions. Because thiscompound coelutes with nC18:2 in our analyticalconditions, its presence was certified by GC-MS.

Evolution - Short chain homologues are the mostabundant components in all samples (Fig. 6). Only in themillimetric siltitic clasts (CB1S) of the samples CB1,and in the marl (CBM), interlayered between thecalcareous beds, they are subordinates to the long chains.Among the short chains, the nC16 compound is thedominant in the tripolaceous samples, while the nC18compound is more abundant in the Calcare di Basesamples. These markers represent the characteristic fattyacids of samples CB4D and CB4M (Fig. 6). Long-chainfatty acids show the same distribution in the siltitic clastsof the samples CB1 and in the marl CBM. Anotherfeature of the fatty acids distribution is the increase ofthe terrigenous long-chain compounds from the bottomto top of the two carbonate beds (Fig. 6).

Unsaturated fatty acids compounds maximize in thecarbonate samples and intercalated marls (Fig. 7). Theyare significantly abundant in the samples CB4D andCB4M. nC18:1 prevails and it is the only unsatured fattyacid in the sample CB3. nC18:2 follow in abundance. Itspeak superimposes to that of the phytanic acid and, forthis reason, the abundance of the two components resultsadded. These markers are significantly abundant in thestromatolitic/microbioalites (CB4M). nC16:1 compoundis less abundant but it is present in both tripolaceous andcarbonates samples.

Interpretation - The distribution of fatty acids allow aclear discrimination between autochthonous (<C22) andallochthonous (>C22) sources. Short chain fatty acidsfound in sedimentary environments are generallyattributed to autochthonous origin, since they areobserved in aquatic organisms as bacteria and algae.High molecular weight compounds are characteristicconstituents of vascular plant waxes (Eglinton &Hamilton, 1967).

The unsatured fatty acids distribution is characteristicof algae or cyanobacteria (Chuecas & Riley, 1969;



Fig. 6 - Distribution and relative abundance of the fatty acids in the analysed samples.

Russel et al., 1997). Unsatured species of fatty acids arefound in recent sediments from normal marine (Chuecas& Riley, 1969; Grimalt & Albaigés, 1990; Volkman etal., 1980) to hypersaline environments (Barbé et al.,1990; Grimalt et al., 1992). The unsatured acids aregenerally lost after deposition (Rhead et al., 1971) andthey are not generally found in sediments as old as 6 My.However, in some cases, they can be preserved (Parker,1969; Russel et al., 1997) and their occurrence in theCalcare di Base Formation constitutes one of theseunusual cases, attesting the exceptional preservation ofthese sediments.

The survival of this algal/cyanobacterial signature isparticularly significant in the carbonate sample CB4,which also exhibit clear biosedimentary features. In thetripolaceous samples, the fatty acid distributions of theautochthonous fraction, in which nC16 compoundprevails, are mainly related to diatom contribution. Onthe contrary in the sample CB4 of the Calcare di Base,the quoted distributions are related to bacterial activity,with C18:0 as the dominant compound (Volkman et al.,1980; Russel et al., 1997).

Aliphatics hydrocarbon composition

Aliphatic hydrocarbons are mainly composed of n-alkanes and subordinately of alkylbenzenes, hopanesand steranes. A series of branched alkanes is alsodetected among long chain n-alkanes. Pristane andphytane, although poorly represented, were alsoexamined.

n-Alkanes

Distribution - n-alkanes distribution ranges from nC16to nC35 and shows close similarities for most of thesamples (Fig. 8). This class of compounds exhibits twodistributions: one in the short chains range, withmaximum at nC18 or nC20, the other occurs in the longchains range and shows a modal distribution centred on

the even numbered homologous nC26 or nC28. In somesamples this distribution is partly obscured by enhancedconcentrations of the odd-numbered long-chainhomologues (nC25, nC27, nC29, nC31).

Evolution - Tripolaceous samples (TR5 and TR6)reveal n-alkanes distribution dominated by the long-chain with odd-numbered homologues (Figs. 8, 9). Thesesamples are characterized by a unimodal distribution ofn-alkanes ranging from nC20 to nC35 and centred onnC28. This distribution interferes with that of n-alkanesoriginating from terrestrial organic material, withdominant components in nC25, nC27, nC29 and nC31(Figs. 8, 9). Components with short chains, ranging fromnC16 to nC22, are less abundant but typified by amaximum at nC16. Carbonate samples show the samedistribution with an increase in short chain homologuesreaching a maximum at nC18 or nC20 (Fig. 9). In thehigh molecular weight region, the unimodal distributionof the even-chains shows a maximum at nC26. Odd-numbered homologues maximize in the samples with asignificant siliclastic fraction (CB1S, CB4D and CBM)(Fig. 9). Sample CB4M (stromatolitic microbialitemicrofacies) provides an autochtonous signature only,although a terrestrial fraction is present in thecorresponding detritic microfacies (CB4D).

The distribution of a series of branched alkanes elutesin the range nC22 to nC33 n-alkanes and maximizesamong nC27 to nC30 (Fig. 8), however does not seem tohave a particular evolutionary trend along the section.

Interpretation - The distribution of n-alkanes indicatesa bimodal pattern, with one mode in the nC16-nC20range attributed to algal- or bacterial-derived organicmatter and the other in the nC27-nC31 range attributed tohigher vascular plants. This datum therefore confirms amixed marine/continental organic matter input.

The modal distribution in nC26 (hexacosane), withoutodd-even carbon number predominance, suggests aplanktonic or bacteria source (Thiel et al., 1997;Baranger & Disnar, 1987; Baranger et al., 1989;Meinschein, 1969; Johnson & Calder, 1973; Tissot &Welte, 1978). The biosedimentary feature ofstromatolitic microbialite sample (CB4M), shows a strictmodal distribution in n-C26, without terrigenouscompounds, confirming its bacterial origin. Thepresence of isoalkanes, usually considered as bacterialmarkers, corroborates this interpretation (Connan et al.,1986; Baranger & Disnar, 1987; Baranger et al., 1989).

Steranes

Distribution - Steranes were detected and quantifiedby GC-MS on the m/z 215+217+231 specific ionchromatogram. The sterane distribution ranges from C27to C30 with distinct isomers (Fig. 10).

Evolution - The C27 homologous is strongly dominantin all samples except CB1S where it is subordinate to theC29 and C30 homologues. Total C27, C28 and C29components have been plotted on a ternary diagram


Fig. 7 - Distribution and relative abundance of the unsatured fatty acidsin the analysed samples.


Fig. 8 - Partial m/z 57+71+85 (GC/MS) mass fragmentogram of the hydrocarbon fraction of sample TR5, CB1, CBM and CB4M showing thedistribution of n-alkanes, iso-alkanes and isoprenoid hydrocarbons.


Fig. 9 - Distribution and relative abundance of the n-alkanes in the analysed samples.

illustrating the composition and probable origin of theregular steranes (Fig. 11). Two main groups can beidentified in the diagram: the first is characterized byhigh C27 and low C28 and C29 regular steranes, thesecond is characterized by high C29 component. Thesamples of the Calcare di Base and those of the Tripoliformation constitute the first group. The second group isformed by the siliciclastic samples CB1S and CBM.Another relevant datum is the absence of pregnanes andhomopregnanes in the aliphatic fraction.

Interpretation - Steroid hydrocarbons found in marinesediments derive mainly from primary photosyntheticorganisms thriving in the upper water column(Summons, 1993; Brassell, 1994). The dominance of theC27 homologous reflects a phytoplanktonic contributionto the organic matter (Volkman, 1986). Similardistributions of steranes are reported in the samples fromthe Messinian of Italy (Schaeffer et al., 1995a; SinningheDamsté et al., 1995; Kening et al., 1995). Thesecompounds might reflect the presence of dinoflagellatesin the medium at time of deposition (Minale & Sodano,1977; Volkman et al., 1980b).

The absence of pregnanes and omopregnanes in our

samples corroborates the hypotesis of normal marineconditions for the Calcare di Base deposition. Thesecompounds have been reported only from the evaporiticsamples of the Messinian succession of the NorthernApennines (ten Haven et al., 1985).

Hopanes

Distribution - The m/z 191 mass chromatogram of thehydrocarbon fractions emphasizes the variations inhopanoid distributions (Guido et al., 2007). Generallyhopanes are poorly represented or absent in the freefraction. Nevertheless, both αβ and βα isomers arepresent for most of the compounds, and the less stableββ configuration is present for the C29 and C30compounds. The hopanoids distribution is dominated bythe non-extended hopanes, with the predominance ofC30 member (17α, 21β-hopane). The other compounds,listed in order of abundance, are the following: C29(17α, 21β-norhopane), C27 (17β and 17α-trisnorho-pane), C29 (17β, 21α and 17β, 21β-norhopane), andC30 (17β, 21β-hopane), C31 (17α, 21β and 17β, 21α-homohopane) and C32 (17α, 21β-bisomohopane). Smallamounts of hopene were detected in samples TR5, TR6and CBM.

Evolution - The hopanes are well represented in thetripolaceous samples. In the Calcare di Base the hopanesare detectable only in the samples with a significantsiliciclastic component (CB1S and CBM). In thecarbonate samples hopanes are poorly represented orabsent (CB4D and CB6), and they decrease from thebottom to top of each carbonate bed. The hopanesdistribution of the stromatolitic microbioalitemicrofacies (CB4M) is significant and clearly distinctthis sample from the other carbonate samples

Interpretation - Although hopanes are not abundant,their distribution clearly indicates an organic matterinput of bacterial origin (Ourisson et al., 1987). Thestereochemical configuration of hopanes changesirreversibly with thermal stress from their biologicalconfiguration 17β, 21β (ββ), to a βα and αβconfiguration; therefore the presence in our samples ofcompounds that have ββ configuration, indicates thatthey suffered very low thermal stress (Mackenzie et al.,1980; Seifert & Moldowan, 1980). Extended hopanes(>C33), gammacerane and isorenieratane, oftenassociated with highly anoxic and/or hypersalinesediments (Moldovan et al., 1985, ten Haven et al., 1988;Adam et al., 1993), have not been detected in oursamples. These biomarkers have been recorded in theevaporitic sequences of Sicily and Northern Apennines,confirming their reliability as palaecological markers ofstressed environment (Schaeffer et al., 1995a, 1995b;Sinninghe Damsté et al., 1995; Kenig et al., 1995; Gelinet al., 1995; Keely et al., 1995; Schaeffer-Reiss et al.,1998). Their absence in the Calcare di Base samplessuggests oxic to dysoxic marine condition at time ofdeposition. This result is in agreement with the existenceof a reasonable community of organisms and furtherstrengthens the hypothesis of a basinal environment not


Fig. 10 - Histogram showing the relative abundance of the mainsteranes.

Fig. 11 - Ternary diagram showing the relative abundance of C27, C28and C29 regular steranes in the lipidic fraction.

stressed. It makes unlikely an evaporitic deposition ofthese sediments.

Pristane and Phytane

Distribution - Isoprenoid hydrocarbons are poorlyrepresented in the study sedimentary record and consistonly of pristane and phytane (Fig. 8). The pristane/phytane ratio is a commonly applied geochemicalparameter used for assessment of depositionalenvironment oxicity. It is based upon different type ofreaction during phytol defunctionalisation, relative to theamount of oxygen available in the depositionalenvironment (Didyk et al., 1978). Generally in oxicconditions the dominant product of defunctionalisationis pristane and in anoxic conditions phytane. However,there are some indications that this relationship is not sosimple (ten Haven et al., 1987), and there are othersources than phytol for these two compounds.

Evolution - In our samples pristane and phytane do notshow a particular trend of evolution. Phytane is moreabundant than pristane in all samples except TR6 (Fig.12). Pristane show a maximum in the sample CB4 whilephytane is maximized in the sample CB1S, TR5 andCB4. This datum, even if collected at very lowconcentration, could suggest a dysoxic/suboxicdeposition conditions.

Alkylbenzenes

Distribution - The m/z 91 + 105 chromatograms showa series of compounds characterized by a base peak atm/z 91 or m/z 105 and molecular ions at 232, 246, 260or 274. Published data (Peters et al., 2005) allowed usinterpreting the mass spectra of these compounds asalkylbenzene structures (Fig. 13). The alkylbenzenesdistribution ranges from C16 to C20 total carbon atoms.Each molecular ion is constituted by five compounds,i.e. pentyl-alkylbenzene, butyl-alkylbenzene, propyl-alkylbenzene, ethyl-alkylbenzene and methyl-alkyl-benzene.

Evolution - Total alkylbenzenes are maximized in bothdetritic and stromatolitic microbioalite microfacies(CB4D and CB4M) (Fig. 14). The alkylbenzenes with

molecular ion at m/z 260 are the most abundant and themethyl homologues dominate the series.

Interpretation - The presence of linear alkylbenzenescould confirm a contribution of bacterial organic matter.


Fig. 12 - Histogram showing the pristane and phytane distribution.

Fig. 13 - Mass spectra of the series of alkylbenzenes with molecularion at m/z 246 (sample CB4M).

These compounds may derive from green sulphurbacteria indicating episodic periods of anoxia (Summons& Powell, 1988; Koopmans et al., 1996).

Ethers

The m/z 256 and m/z 236+255 mass chromatogramsput in evidence high molecular-weight wax esters. Thesource of these wax esters has been assigned tozooplankton grazing on algal sterols (Wakeham, 1982).Other authors suggest that diatoms may be a furthersource for these ethers.

Ketones

Ketones, identified in the mass chromatogram m/z 96,mostly consist of long-chain compounds. Their origincould be ascribed to a diagenetic intermediate in themicrobiologically/chemically degradation of stenols tosterenes (Marlow et al., 2001). Long-chain ketones areubiquitous in marine sediments. Unsaturated ketonesfrom C37 to C39 (alkenones) are biomarkers forHaptophytes (Volkman et al., 1980; Marlowe et al.,1984; Conte et al., 1994). They have been observed inconsiderable quantity in the living coccolithophoridsEmiliana huxleyi and Gephyrocapsa oceanica. Ourchromatograms did not reveal the C37-C39 homologuesbut the presence of Haptophytes in the depositionalenvironment is testified by coccolithophorids mouldsobserved by SEM.

Alcohols

Three n-alcohols (C18, C22 and C28) have beenrecognized in the Cropalati section. The distribution oflong-chain n-alcohols has a marked preference for evenhomologues, which is a typical distribution of land-derived organic matter (Eglinton & Hamilton, 1963; deLeeuw, 1986; Farrimond et al., 1990; Marlow et al.,2001).

Sterols

The identification of the sterols is based on relativeretention times and comparison with published mass

spectra. The major observed components are the C27 toC29 sterols. The C27 and C28 sterols are considered themost abundant sterols in planktonic and marineinvertebrates which are the principal marine source oforganic matter (Huang & Meinschein, 1976, 1978;Nishimura & Koyama, 1976, 1977; Nishimura, 1977,1978). The C29 sterol is considered predominant inhigher plants and animals (Huang & Meinschein, 1979;Nishimura & Koyama, 1976, 1977). The presence ofcholesterol+colestenol is likely to derive from thecopepods, since most living copepods contain thesecompounds as the major sterols (Volkman et al., 1986).Zooplankton is the major repository of colestenol inmarine environment and it converts much of the sterolsproduct by algae into colestenol. Also the observationthat copepods excrete significant amounts of colestenol(up to 4ng/pellet; Volkman et al., 1980b) indicates thatzooplankton faecal pellets are a major source of thesesterols in the Calcare di Base Formation.

CONCLUSIONS

For the first time it was possible to perform a detailedstudy of the organic matter content scattered into Calcaredi Base Formation, deposited in Northern Calabria. Thisapproach permitted to elucidate the depositionalconditions under which these carbonates formed and therole of microbes in the mineralization processes.

Rock-Eval pyrolysis data (HI; OI), plotted in thepseudo Van Krevelen diagram (Tissot & Welte, 1984),put in evidence a transitional composition between typesII and III kerogens, revealing a mixture of marine andterrigenous organic matter. Hydrogen index (HI)decreases while oxygen index (OI) increases upwardwithin each carbonate layer and generally inside thewhole section. This tendency could be attributed to arelative lowering of the sea level with consequent largercontinental input.

Palinofacies observations confirm the marine/continental inputs and put in evidence amorphousorganic matter (algal or bacterial origin), vascular plantsdebris, algae and arthropod exoskeletons. Arthropods(most probably copepods) could be one of the mainproducers of the faecal pellets observed in themicrofacies. The presence of copepods in thesedimentary environment is also testified by thebiomarkers, such as cholesterol+colestenol, since mostliving copepods contain these compounds as the majorsterols (Volkman et al., 1986). In a previous researchVolkman et al. (1980b) also demonstrated that copepodsexcrete significant amounts of colestenol (up to4ng/pellet) confirming that copepods faecal pellets arethe major source of these sterols in the Calcare di Baseof Cropalati basin.

Lipids distributions confirm the marine/continentalorganic mixture. The n-alkanes distribution indicates thepresence of three main biological signatures: algal


Fig. 14 - Trend of variation of the total alkylbenzenes.

(mode in nC18-nC20), terrestrial (mode in nC27-nC29),and bacterial (nC26-nC28 with no odd-even carbonnumber predominance). Autochthonous acidic fraction(<C22) and relative unsatured compounds validate thealgal and bacterial signature, also testified bypredominant C27 steranes (algal), hopanes and branchedn-alkanes distributions (bacterial). High molecularweight acids and n-alcohols are characteristicconstituents of vascular plant waxes.

The low organic carbon (OC) and sulphur contents ofthe Calcare di Base indicate unstressed environmentalconditions for its deposition at least in the Rossanobasin. Extended hopanes (>C33), gammacerane,isorenieratane, pregnanes and omopregnanes, oftenassociated with highly anoxic and/or hypersalinesediments (Moldovan et al., 1985, ten Haven et al., 1988;Adam et al., 1993), have not been detected in the studysediments. These biomarkers have been recorded in theevaporitic sequences of Sicily and Northern Apennines,confirming their reliability as palaecological markers ofstressed environment (Schaeffer et al., 1995a, 1995b;Sinninghe Damsté et al., 1995; Kenig et al., 1995; Gelinet al., 1995; Keely et al., 1995; Schaeffer-Reiss et al.,1998). Their absence in the Calcare di Base samplessuggests oxic to dysoxic marine condition at time ofdeposition. This result is in agreement with the existenceof a reasonable community of organisms and furtherstrengthens the hypothesis of a basinal environment notstressed, making unlikely an evaporitic deposition ofthese sediments. The presence of coccolitophorid infecal pellets (Guido et al., this volume) demonstratesdefinitely that Calcare di Base was deposited in normalmarine conditions, even if the depositional environmentbecame unstable for episodic freshwater inputs.

The thrombolitic facies, constituted of clotted peloidal

micrite, is a reliable clue for a biological induceddeposition (Guido et al., this volume). Furthermore theconstant and strong bacterial signal of the molecularfossils, represented by nC26-nC28 n-alkanes with noodd-even carbon number predominance, branched n-alkanes, hopanes and unsatured fatty acids, together withthe widespread presence of amorphous organic matter inthe palynofacies, corroborate the interpretation thatdominant microfacies of the Calcare di Base represents abacterial induced deposit.

The palaeoecological event, recorded in the Calcare diBase of the Rossano basin and characterized through theorganic matter study, can have affected all basin and/orsub-basin involved in the messinian salinity crisis. Thishypothesis is strongly corroborated by the observedmicrofacies uniformity of the Calcare di Base Formationin Calabria and in Sicily (Guido et al., this volume). Thisinterpretation does not exclude that the carbonate beds,in some cases, could have later displaced through massflow recorded by brecciated facies.

ACKNOWLEDGEMENTS - We thank the research groupon organic matter of Institut des Sciences de la Terre d’Orléansfor the technical support and the numerous suggestions to carryout this paper. We are deeply grateful to prof. Claudio Neri,Dipartimento di Scienze della Terra, Università della Calabria,for his critical comments. His untimely death left a great voidamong sedimentologist community. The authors wish thank S.Conti (Università di Modena-Reggio Emilia) for his revisionsand comments that greatly improved this paper.

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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