INTRODUCTION
Carbonate and evaporitic sediments can be laterally and vertically related as a result of changes in the basin hydrology and the degree of connection to the open sea, consequently a succession of deposits representing the change from normal marine to evaporitic conditions occurs (Rouchy et al., 2001). Examples of these types of deposits can be found in Messinian sediments of the Mediterranean area (Esteban, 1979; Rouchy, 1982;
Decima et al., 1988; Rouchy & Saint-Martin, 1992;
Esteban et al., 1996; Rouchy & Caruso, 2006). An open question is the Calcare di Base genesis. In particular whether these deposits were formed in hypersaline settings or they represent normal salinity marine sediments, formed before the main restriction and evaporative deposition (Rouchy et al., 2001).
Previous researches on the Calcare di Base were founded 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 not able to give a sound interpretation on the genesis of these peculiar 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 bacteria replace Ca-sulphate into carbonate, or subordinately as primary peloidal limestones (McKenzie, 1985; Bellanca et al., 1986, 2001; Bellanca & Neri, 1986; Decima et al., 1988; Decima & Wezel, 1971; Rouchy & Saint Martin, 1992; Pedley & Grasso, 1993). In both case these carbonate units are considered belonging to the Lower Evaporites succession.
The Calcare di Base is usually interpreted as formed in shallow waters with strongly fluctuating salinities. The brecciated facies is related to autobrecciation processes induced by dissolution of halite and gypsum intercalations 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 the brecciated character of the limestones is due to mass flow. They support this interpretation for the presence of particular stratigraphic and sedimentological features like erosional bases with load casts, overall normal gradation, upward transition to gypsarenite unit and clay chips. Unfortunately these researches developed mainly the stratigraphic, palaeogeographic and structural aspects ignoring almost totally the paleoecological
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 Messinian Calcare di Base Formation cropping out in the Rossano basin, northern Calabria. This approach allowed to elucidate the depositional conditions under which these carbonates formed, namely the physicochemical properties 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 and faecal 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 of molecular fossils indicative of anoxic or hypersaline environment as pregnanes and homopregnanes, extended hopanes (>C33), gammacerane and isorenieratane. The study of carbonaceous remains emphasizes a wide variety of the organic input. Geochemical data (Rock-Eval pyrolysis) indicate a mixed (marine and continental) organic input. These data have been confirmed by organic petrographic observations (palynofacies) that revealed the presence of phytoclasts derived from continental plant tissues, amorphous organic matter, and variable proportions of 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- nC28and 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 the interpretation 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, making difficult the interpretation of the causes which triggered the salinity crisis.
The Calcare di Base Formation, sampled in various localities of Calabria and Sicily, shows always the same dominant microfacies, which is constituted by peloidal/
coprolitic mudstone/wackestones with thrombolitic fabric (Guido et al., this volume). This microfacies records a single palaeoecological event, possibly not synchronous, prologue in southern Italy of the Messinian Salinity Crisis (Guido et al., this volume). With the aim to detect organic matter data on this microfacies we analyzed the Calcare di Base Formation cropping out near Cropalati village (Rossano basin, Northern Calabria). We selected this area since here the carbonate strata are preserved in their original mineralogy (aragonite) and microstructures and they do not show brecciated facies and/or evidence of transport.
We attempted a new approach analyzing in detail the sedimentary organic matter (palynofacies observation, Rock-Eval pyrolysis and GC/MS analysis).
Reconstruction of depositional conditions, based on biomarker data, is presently one of the main topics of organic geochemistry (Marynowsky et al., 2000).
Biomarkers are complex molecular fossils, derived from biochemicals produced by once-living organisms that provide information on organic matter origin and environmental conditions. Thus the detailed characteri- zation of biomarker assemblages allows assessing the major 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, especially bacteria. Therefore, the composition of an original com- munity (primary producers, zooplankton, aerobic and anaerobic bacteria, benthic microorganism, etc.) is diffi- cult to evaluate by traditional optical methods. Many studies 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 oils and show little or no change in structure from their par- ent organic molecules in living organisms. This approach allowed, for the first time, to characterize (microscopically) non-identifiable (non-preserved) organisms, in Calcare di Base sediments.
GEOLOGICAL AND STRATIGRAPHIC 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 crisis events in a complex basinal setting related to the fore-
land fold-thrust belt of Southern Italy orogen (Critelli, 1999).
Tortonian to Messinian sedimentary successions of northern Calabria represent the sedimentary response to the Neogene evolution of the Calabrian-Arc orogenic system. During the Upper Miocene the Calabrian Arc experiences abrupt uplift and rapid slip-rate eastward displacement, causing general accretionary tectonics along the Ionian border, and an extensional tectonics in the 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 basins of eastern Calabria are filled by Tortonian to Pleistocene dominantly clastic sedimentation interbedded with Messinian evaporite deposits.
The Tortonian sequence represents a characteristic transgressive system with an alluvial red conglomerate, passing into nearshore sediments and deep-marine turbidite strata, probably deposited during a low-stand system tract. Deep-marine sediments are followed, throughout an angular unconformity, by marls and diatomaceous shales including sulphate nodules, carbonates with decimetric intercalation of marl-clays and gypsumrudite-gypsumarenite deposits associated with clastic carbonates. These deposits pass, throughout a second angular unconformity, to arenites, marls, halite, gypsarenites and olistostromes of variegated clays
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 the Calcare di Base succession is constituted of two metric massive white to yellow fine grained calcareous beds interbedded with a decimetric light-brown laminated marls (Fig. 2). These calcareous beds overlay the diatomitic and marly layers (Tripoli Formation), which represent the first phases of restriction of the basin after the open marine condition represented by the Upper Tortonian/Early Messinian clays.
METHODS Samples selection
Samples for carbonates and organic matter analyses have been selected for their position in the studied sequence and for the features observed in the field. The texture and composition of these samples, observed under transmitted light microscopy, SEM and EDS, suggest that they have not been affected by strongly diagenetic processes.
We collected tree samples for each calcareous bed, numbered CB1-CB2-CB3 and CB4-CB5-CB6, from bottom to top respectively (Fig. 3). Several millimetric siltitic clasts (CB1S) have been separated from the carbonate samples CB1. Sample CB4 shows two facies:
detritic (CB4D) and stromatolitic (CB4M) which have been 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) layers have been selected for biomarker investigations.
Organic matter characterization
The organic matter quantity and quality was assessed by combining organic petrography (palynofacies), bulk geochemistry (Rock Eval T6 pyrolysis and Leco elemental analysis) and molecular analyses on lipids (Gas Chromatography - Mass Spectrometry).
Palynofacies
Organic petrography study was carried out on kerogen fraction, isolated from the carbonate and silicate phases of the sediment via the classical hydrochloric and hydrofluoric acid treatments (Durand & Nicaiese, 1980).
This study involves the identification of the different fractions of organic matter using transmitted light microscopy. The following procedure was employed: the samples have been pulverized and few grams (2-5 g) of dust were submitted to an acidic treatment with HCI (36%), to dissolve carbonates, and HF (50%), to remove silicates. The solid residue was washed with water and the supernatant was removed after centrifugation. The water washings were repeated several times, until a neutral pH was attained, then a first series of sections, named TS (Total Slide), has been carried out with few microlitres (300-500 µl) of residue. A further acidic treatment have been performed with KOH (10%) to put
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%) to oxidize the organic matter, to dissolve pyrite, and potassium salts formed. Then, after densimetric separation, a second series of sections, named RS (Residual Slide) have been performed.
Rock-Eval pyrolysis
Bulk organic matter geochemistry has been assessed by Rock Eval T6 pyrolysis (Espitalié et al., 1977;
Lafargue et al., 1998). This method allows the rapid quantitative and qualitative characterization of sedimentary organic matter. The Rock-Eval parameters used for this study are the followings: (1) Total Organic Carbon (TOC, %) accounts for the quantity of organic matter 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 OM maturity indicator in ancient sediments (Espitalié et al., 1985b), it is the temperature of the pyrolysis oven recorded at the top of peak S2, which corresponds to the maximum release of hydrocarbonaceous products during pyrolysis; (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 used for analysis, depending on the estimated OM content.
The pyrolysis program starts with an isothermal stage of 3 min at 200°C. Then, the pyrolysis oven temperature was raised at 30°C/min to 650°C, and held for 3 minutes at this temperature. The oxidation phase, performed in a second oven under an air stream, starts at an isothermal stage 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 to determine Total C, N and S contents.
Lipid biomarkers
The preparation for lipid analyses includes a step of extraction, then a separation, eventually a derivatisation step and finally the identification and quantitation of the compounds by gas chromatography (GC) or gas chromatography-mass spectrometry (GC-MS). 3g of powdered dry sediments were ultrasonically extracted three times with a mixture of dichloromethane/methanol (1:1). Samples were centrifuged following each extraction and the supernatant was collected. Combined extracts were dried under nitrogen. Because Leco analysis revealed very low concentrations of the elemental sulphur, only free lipids were analysed, without any desulphurization.
The acidic fraction was separated from the total extracts by solid phase extraction performed on amino- propyl bonded silica. Neutral compounds were eluted with dichloromethane/methanol (1:1), and acidic com- pounds were eluted with ether after acidification of the medium with ether:formic acid (9:1). Fatty acids were esterified using acetyl chloride in anhydrous methanol before analysis. The neutral fraction was further separat- ed by flash chromatography on deactivated silica (5%
water) with solvents of increasing polarity according to Ternois 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 GC and GC-MS analyses.
Fatty acids as their methyl esters and aliphatic and cyclic 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 achieved by Gas Chromatography/Mass Spectrometry (GC-MS).
GC-MS analyses were performed on a ThermoFinnigan TRACE-PolarisGCQ gas chromatograph-mass spec- trometer. The gas chromatograph was fitted with an Rtx®-5Sil MS capillary column (30 m x 0.25 mm i.d., 0.25 µm film thickness) with 5 m of guard column. The GC operating conditions were as follows: temperature hold at 40°C for 1 min, then increase from 40 to 120°C at 30°C/min, 120 to 300°C at 5°C/min with final isothermal hold at 300°C over 20 min. The sample was injected splitless, with the injector temperature set at 280°C. Helium was the carrier gas. The mass spectrom- eter was operated in the electron ionisation (EI) mode at 70 eV ionization energy and scanned from 50 to 650 Dalton. Compounds were identified by comparison with published mass spectra and relative retention times. Fatty acids, n-alkanes, pristane and phitane were quantified by GC-FID while alkylbenzenes, steranes, hopanes, ethers, ketones, alcohols and sterols have been identified and semi-quantified by GC-MS using the m/z 91+105 chromatograms for the alkylbenzenes, m/z 215+217+231
Fig. 3 - Simplified stratigraphy of Cropalati section with the location of the samples.
chromatograms for steranes, m/z 191 chromatogram for the hopanes, m/z 256 and m/z 236+255 chromatograms for ethers, m/z 96 chromatogram for ketones, m/z 55+97 chromatograms for alcohols and m/z 213+215 for sterols.
RESULTS Rock-Eval pyrolysis
The accumulation of organic matter (OM) has been estimated using TOC values. TOC ranges from 0.06% to 0.19% for the calcareous samples, from 0,21% to 0,88%
for the diatomitic and marly samples of the Tripoli Formation, and it is 0,27% for the marls interbedded between the calcareous layers. The low values in TOC suggest to treat with caution the Rok-Eval pyrolysis data becouse these are not much diagnostics. For this reason the values of Tmax cannot be considered as index of maturity but some indications can be extrapolate about the nature and the genesis of the preserved OM.
The TOC values do not vary significantly and do not exibit a particular trend, it can be only ascertained a greater content in TOC in the marly and diatomitic layers respect the calcareous bed (Fig. 4). HI values range from 306 to 417 mg HC/g TOC for the calcareous samples, from 22 to 143 mg HC/g TOC for the diatomitic and marly samples of the Tripoli Formation, and it is 67 mg HC/g TOC for the marls interbedded with the calcareous layers. OI values range from 105 to 312 mg CO2/g TOC for the calcareous samples, from 57 to 556 mg CO2/g TOC for the diatomitic and marly samples of the Tripoli Formation, and it is 185 mg CO2/g TOC for the marl interbedded with the calcareous layers (Fig. 4).
Rock-Eval pyrolysis data (HI; OI) for the carbonate samples, plotted in the pseudo Van Krevelen diagram (Tissot & Welte, 1984), put in evidence a transitional
composition between types II and III kerogens, revealing a mixture of marine and terrigenous organic matter (Guido et al., 2007). The diagram shows that HI decreases while OI increases upward within each carbonate layer and generally inside the whole section.
This tendency could be attributed to a relative lowering of the sea level with consequent larger continental input.
HI and OI values for the samples of the Tripoli Formation and for the marls interbedded with the Calcare di Base do not exhibit a peculiar trend. We only notice the lower HI values and the extremely variable OI values (Fig. 4).
Elemental nitrogen and sulphur have been detected at very low concentration. They show a general decreasing trend toward the top of each carbonate beds (Fig. 4).
Palynological observation
Palynofacies observations put in evidence that kerogen fraction is extremely low in the calcareous samples whereas it is abundant in the marly layers of the Tripoli Formation and in the marl-clays interlayered with the calcareous bed (Fig. 5).
The very low amount of extracted organic matter does not allow making a relative quantification of the different fractions; therefore it has been possible to perform 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 matter composition deduced by Rock-Eval analyses.
Palynofacies are constituted mainly by heterogeneous size elements, yellow-brown in colour, which show an intermediate fluorescence and rounded or tabular inclu- sions. The absence of structures in this material makes difficult 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 total sulphur 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 organic matter.
observed correlation between the presence of these par- ticles and the high values of the hydrogen index suggests an 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 OM families, structured particles of vascular-woody origin are also present. These components are subdivisible in two groups: translucid or semi-opaque elements (Figs.
5d, 5e), with reddish and sharp edges, and smaller dimension opaque elements with tabular or equidimen- sional shape (Fig. 5f). Generally internal biostructure are no visible, since they have been infilled and largely oblit- erated by gelification. Some of these elements exhibit relict structure of parallel fibres. The figurate elements are represented also by spores, pollens (Figs. 5a, 5b) and phytoplanktonic organisms (Fig. 5c). The presence of well preserved and bright-fluorescent spores and pollens indicates that these elements did not undergo degrada- tion and oxidation, suggesting a sedimentary environ- ment characterized by a stratified water column with periodic bottom dysoxic/suboxic conditions.
These organic facies are present in all studied samples, but in different proportions. Samples TR5 and TR6 are constituted mainly of amorphous OM and terrestrial origin materials. The latter are present in the sample TR5 with three different types: oxidized or burnt, gelificated and as pirofusinite. The sample TR6 shows the same kerogen composition but with an increase of amorphous organic matter, with particles of pedogenetic origin (AOM gelificated). In the terrestrial fraction the opaque particles decrease. Many small tabular siliceous fragments are also present and they could derive from diatom skeletons.
The palynofacies observations on the calcareous samples put in evidence a predominance of amorphous debris. It is possible to distinguish two main types of AOM: a dark-altered fraction and a brown immature fraction. The Calcare di Base extract is rich of immature algal debris. These algal debris are represented by transparent membranes with irregular or sub-spherical forms, almost not observable in transmitted light, but very fluorescent under UV excitation. Among organic fractions remarkable is the presence of well preserved exoskeletons fragments of arthropods, probably attributable to copepods (Fig. 5g, 5h). Carbonates samples show an increase of terrestrial debris from the bottom to the top of each carbonate layers, and generally through the whole section.
Kerogene of the sample CBM shows about the same composition of the tripolaceous marls but with a remarkable increase of terrestrial fraction and debris of pedogenetic origin.
The densimetric separate of the sample CB2 revealed numerous amorphous elements of gelatinous aspect with low reflectance and fluorescence. These amorphous elements have been interpreted as silica crystals linked together by soluble organic matter. The soluble organic matter has been pointed out by epifluorescence observations on Acridine orange stained thin sections
(Guido et al., this volume).
Biomarkers
Within the lipids extracted from our samples, we investigated fatty acids, aliphatic and cyclic hydrocar- bons, ethers, ketones, alcohols and sterols.
Fatty acids
Distribution - Fatty acid distribution are often dominated by a series of straight-chain components ranging from 14 to 32 carbon numbers (Fig. 6). Their distribution shows a strong predominance of even- carbon-number homologues and is bimodal with one maximum at nC16or nC18and the other within the range of 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 has been detected in significant proportions. Because this compound coelutes with nC18:2 in our analytical conditions, its presence was certified by GC-MS.
Evolution - Short chain homologues are the most abundant components in all samples (Fig. 6). Only in the millimetric siltitic clasts (CB1S) of the samples CB1, and in the marl (CBM), interlayered between the calcareous beds, they are subordinates to the long chains.
Among the short chains, the nC16 compound is the dominant in the tripolaceous samples, while the nC18 compound is more abundant in the Calcare di Base samples. These markers represent the characteristic fatty acids of samples CB4D and CB4M (Fig. 6). Long-chain fatty acids show the same distribution in the siltitic clasts of the samples CB1 and in the marl CBM. Another feature of the fatty acids distribution is the increase of the terrigenous long-chain compounds from the bottom to top of the two carbonate beds (Fig. 6).
Unsaturated fatty acids compounds maximize in the carbonate samples and intercalated marls (Fig. 7). They are significantly abundant in the samples CB4D and CB4M. nC18:1prevails and it is the only unsatured fatty acid in the sample CB3. nC18:2follow in abundance. Its peak superimposes to that of the phytanic acid and, for this reason, the abundance of the two components results added. These markers are significantly abundant in the stromatolitic/microbioalites (CB4M). nC16:1compound is less abundant but it is present in both tripolaceous and carbonates samples.
Interpretation - The distribution of fatty acids allow a clear discrimination between autochthonous (<C22) and allochthonous (>C22) sources. Short chain fatty acids found in sedimentary environments are generally attributed to autochthonous origin, since they are observed in aquatic organisms as bacteria and algae.
High molecular weight compounds are characteristic constituents of vascular plant waxes (Eglinton &
Hamilton, 1967).
The unsatured fatty acids distribution is characteristic of 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 are found in recent sediments from normal marine (Chuecas
& Riley, 1969; Grimalt & Albaigés, 1990; Volkman et al., 1980) to hypersaline environments (Barbé et al., 1990; Grimalt et al., 1992). The unsatured acids are generally lost after deposition (Rhead et al., 1971) and they 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 the Calcare di Base Formation constitutes one of these unusual cases, attesting the exceptional preservation of these sediments.
The survival of this algal/cyanobacterial signature is particularly significant in the carbonate sample CB4, which also exhibit clear biosedimentary features. In the tripolaceous samples, the fatty acid distributions of the autochthonous fraction, in which nC16 compound prevails, are mainly related to diatom contribution. On the 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, hopanes and steranes. A series of branched alkanes is also detected among long chain n-alkanes. Pristane and phytane, although poorly represented, were also examined.
n-Alkanes
Distribution - n-alkanes distribution ranges from nC16 to nC35 and shows close similarities for most of the samples (Fig. 8). This class of compounds exhibits two distributions: one in the short chains range, with maximum at nC18or nC20, the other occurs in the long chains range and shows a modal distribution centred on
the even numbered homologous nC26or nC28. In some samples this distribution is partly obscured by enhanced concentrations of the odd-numbered long-chain homologues (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). These samples are characterized by a unimodal distribution of n-alkanes ranging from nC20 to nC35 and centred on nC28. This distribution interferes with that of n-alkanes originating from terrestrial organic material, with dominant components in nC25, nC27, nC29 and nC31 (Figs. 8, 9). Components with short chains, ranging from nC16 to nC22, are less abundant but typified by a maximum at nC16. Carbonate samples show the same distribution with an increase in short chain homologues reaching a maximum at nC18 or nC20 (Fig. 9). In the high molecular weight region, the unimodal distribution of the even-chains shows a maximum at nC26. Odd- numbered homologues maximize in the samples with a significant siliclastic fraction (CB1S, CB4D and CBM) (Fig. 9). Sample CB4M (stromatolitic microbialite microfacies) provides an autochtonous signature only, although a terrestrial fraction is present in the corresponding detritic microfacies (CB4D).
The distribution of a series of branched alkanes elutes in the range nC22 to nC33 n-alkanes and maximizes among nC27to nC30(Fig. 8), however does not seem to have a particular evolutionary trend along the section.
Interpretation - The distribution of n-alkanes indicates a bimodal pattern, with one mode in the nC16-nC20 range attributed to algal- or bacterial-derived organic matter and the other in the nC27-nC31range attributed to higher vascular plants. This datum therefore confirms a mixed marine/continental organic matter input.
The modal distribution in nC26(hexacosane), without odd-even carbon number predominance, suggests a planktonic 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 of stromatolitic microbialite sample (CB4M), shows a strict modal distribution in n-C26, without terrigenous compounds, confirming its bacterial origin. The presence of isoalkanes, usually considered as bacterial markers, corroborates this interpretation (Connan et al., 1986; Baranger & Disnar, 1987; Baranger et al., 1989).
Steranes
Distribution - Steranes were detected and quantified by GC-MS on the m/z 215+217+231 specific ion chromatogram. The sterane distribution ranges from C27 to C30with distinct isomers (Fig. 10).
Evolution - The C27homologous is strongly dominant in all samples except CB1S where it is subordinate to the C29 and C30 homologues. Total C27, C28 and C29 components have been plotted on a ternary diagram
Fig. 7 - Distribution and relative abundance of the unsatured fatty acids in 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 the distribution 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 the regular steranes (Fig. 11). Two main groups can be identified in the diagram: the first is characterized by high C27 and low C28 and C29 regular steranes, the second is characterized by high C29 component. The samples of the Calcare di Base and those of the Tripoli formation constitute the first group. The second group is formed by the siliciclastic samples CB1S and CBM.
Another relevant datum is the absence of pregnanes and homopregnanes in the aliphatic fraction.
Interpretation - Steroid hydrocarbons found in marine sediments derive mainly from primary photosynthetic organisms thriving in the upper water column (Summons, 1993; Brassell, 1994). The dominance of the C27homologous reflects a phytoplanktonic contribution to the organic matter (Volkman, 1986). Similar distributions of steranes are reported in the samples from the Messinian of Italy (Schaeffer et al., 1995a; Sinninghe Damsté et al., 1995; Kening et al., 1995). These compounds might reflect the presence of dinoflagellates in 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 marine conditions for the Calcare di Base deposition. These compounds have been reported only from the evaporitic samples of the Messinian succession of the Northern Apennines (ten Haven et al., 1985).
Hopanes
Distribution - The m/z 191 mass chromatogram of the hydrocarbon fractions emphasizes the variations in hopanoid distributions (Guido et al., 2007). Generally hopanes are poorly represented or absent in the free fraction. Nevertheless, both αβ and βα isomers are present for most of the compounds, and the less stable ββ configuration is present for the C29 and C30 compounds. The hopanoids distribution is dominated by the non-extended hopanes, with the predominance of C30 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), and C30 (17β, 21β-hopane), C31 (17α, 21β and 17β, 21α- homohopane) and C32 (17α, 21β-bisomohopane). Small amounts of hopene were detected in samples TR5, TR6 and CBM.
Evolution - The hopanes are well represented in the tripolaceous samples. In the Calcare di Base the hopanes are detectable only in the samples with a significant siliciclastic component (CB1S and CBM). In the carbonate samples hopanes are poorly represented or absent (CB4D and CB6), and they decrease from the bottom to top of each carbonate bed. The hopanes distribution of the stromatolitic microbioalite microfacies (CB4M) is significant and clearly distinct this sample from the other carbonate samples
Interpretation - Although hopanes are not abundant, their distribution clearly indicates an organic matter input of bacterial origin (Ourisson et al., 1987). The stereochemical configuration of hopanes changes irreversibly with thermal stress from their biological configuration 17β, 21β (ββ), to a βα and αβ configuration; therefore the presence in our samples of compounds that have ββ configuration, indicates that they suffered very low thermal stress (Mackenzie et al., 1980; Seifert & Moldowan, 1980). Extended hopanes (>C33), gammacerane and isorenieratane, often associated with highly anoxic and/or hypersaline sediments (Moldovan et al., 1985, ten Haven et al., 1988;
Adam et al., 1993), have not been detected in our samples. These biomarkers have been recorded in the evaporitic sequences of Sicily and Northern Apennines, confirming their reliability as palaecological markers of stressed environment (Schaeffer et al., 1995a, 1995b;
Sinninghe Damsté et al., 1995; Kenig et al., 1995; Gelin et al., 1995; Keely et al., 1995; Schaeffer-Reiss et al., 1998). Their absence in the Calcare di Base samples suggests oxic to dysoxic marine condition at time of deposition. This result is in agreement with the existence of a reasonable community of organisms and further strengthens the hypothesis of a basinal environment not
Fig. 10 - Histogram showing the relative abundance of the main steranes.
Fig. 11 - Ternary diagram showing the relative abundance of C27, C28 and C29regular steranes in the lipidic fraction.
stressed. It makes unlikely an evaporitic deposition of these sediments.
Pristane and Phytane
Distribution - Isoprenoid hydrocarbons are poorly represented in the study sedimentary record and consist only of pristane and phytane (Fig. 8). The pristane/
phytane ratio is a commonly applied geochemical parameter used for assessment of depositional environment oxicity. It is based upon different type of reaction during phytol defunctionalisation, relative to the amount of oxygen available in the depositional environment (Didyk et al., 1978). Generally in oxic conditions the dominant product of defunctionalisation is pristane and in anoxic conditions phytane. However, there are some indications that this relationship is not so simple (ten Haven et al., 1987), and there are other sources than phytol for these two compounds.
Evolution - In our samples pristane and phytane do not show a particular trend of evolution. Phytane is more abundant than pristane in all samples except TR6 (Fig.
12). Pristane show a maximum in the sample CB4 while phytane is maximized in the sample CB1S, TR5 and CB4. This datum, even if collected at very low concentration, could suggest a dysoxic/suboxic deposition conditions.
Alkylbenzenes
Distribution - The m/z 91 + 105 chromatograms show a series of compounds characterized by a base peak at m/z 91 or m/z 105 and molecular ions at 232, 246, 260 or 274. Published data (Peters et al., 2005) allowed us interpreting the mass spectra of these compounds as alkylbenzene structures (Fig. 13). The alkylbenzenes distribution 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 both detritic and stromatolitic microbioalite microfacies (CB4D and CB4M) (Fig. 14). The alkylbenzenes with
molecular ion at m/z 260 are the most abundant and the methyl homologues dominate the series.
Interpretation - The presence of linear alkylbenzenes could 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 molecular ion at m/z 246 (sample CB4M).
These compounds may derive from green sulphur bacteria indicating episodic periods of anoxia (Summons
& Powell, 1988; Koopmans et al., 1996).
Ethers
The m/z 256 and m/z 236+255 mass chromatograms put in evidence high molecular-weight wax esters. The source of these wax esters has been assigned to zooplankton grazing on algal sterols (Wakeham, 1982).
Other authors suggest that diatoms may be a further source for these ethers.
Ketones
Ketones, identified in the mass chromatogram m/z 96, mostly consist of long-chain compounds. Their origin could be ascribed to a diagenetic intermediate in the microbiologically/chemically degradation of stenols to sterenes (Marlow et al., 2001). Long-chain ketones are ubiquitous in marine sediments. Unsaturated ketones from C37 to C39 (alkenones) are biomarkers for Haptophytes (Volkman et al., 1980; Marlowe et al., 1984; Conte et al., 1994). They have been observed in considerable quantity in the living coccolithophorids Emiliana huxleyi and Gephyrocapsa oceanica. Our chromatograms did not reveal the C37-C39homologues but the presence of Haptophytes in the depositional environment is testified by coccolithophorids moulds observed by SEM.
Alcohols
Three n-alcohols (C18, C22 and C28) have been recognized in the Cropalati section. The distribution of long-chain n-alcohols has a marked preference for even homologues, which is a typical distribution of land- derived organic matter (Eglinton & Hamilton, 1963; de Leeuw, 1986; Farrimond et al., 1990; Marlow et al., 2001).
Sterols
The identification of the sterols is based on relative retention times and comparison with published mass
spectra. The major observed components are the C27to C29sterols. The C27and C28sterols are considered the most abundant sterols in planktonic and marine invertebrates which are the principal marine source of organic matter (Huang & Meinschein, 1976, 1978;
Nishimura & Koyama, 1976, 1977; Nishimura, 1977, 1978). The C29 sterol is considered predominant in higher plants and animals (Huang & Meinschein, 1979;
Nishimura & Koyama, 1976, 1977). The presence of cholesterol+colestenol is likely to derive from the copepods, since most living copepods contain these compounds as the major sterols (Volkman et al., 1986).
Zooplankton is the major repository of colestenol in marine environment and it converts much of the sterols product by algae into colestenol. Also the observation that copepods excrete significant amounts of colestenol (up to 4ng/pellet; Volkman et al., 1980b) indicates that zooplankton faecal pellets are a major source of these sterols in the Calcare di Base Formation.
CONCLUSIONS
For the first time it was possible to perform a detailed study of the organic matter content scattered into Calcare di Base Formation, deposited in Northern Calabria. This approach permitted to elucidate the depositional conditions under which these carbonates formed and the role of microbes in the mineralization processes.
Rock-Eval pyrolysis data (HI; OI), plotted in the pseudo Van Krevelen diagram (Tissot & Welte, 1984), put in evidence a transitional composition between types II and III kerogens, revealing a mixture of marine and terrigenous organic matter. Hydrogen index (HI) decreases while oxygen index (OI) increases upward within each carbonate layer and generally inside the whole section. This tendency could be attributed to a relative lowering of the sea level with consequent larger continental input.
Palinofacies observations confirm the marine/
continental inputs and put in evidence amorphous organic matter (algal or bacterial origin), vascular plants debris, algae and arthropod exoskeletons. Arthropods (most probably copepods) could be one of the main producers of the faecal pellets observed in the microfacies. The presence of copepods in the sedimentary environment is also testified by the biomarkers, such as cholesterol+colestenol, since most living copepods contain these compounds as the major sterols (Volkman et al., 1986). In a previous research Volkman et al. (1980b) also demonstrated that copepods excrete significant amounts of colestenol (up to 4ng/pellet) confirming that copepods faecal pellets are the major source of these sterols in the Calcare di Base of Cropalati basin.
Lipids distributions confirm the marine/continental organic mixture. The n-alkanes distribution indicates the presence 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 carbon number predominance). Autochthonous acidic fraction (<C22) and relative unsatured compounds validate the algal and bacterial signature, also testified by predominant C27steranes (algal), hopanes and branched n-alkanes distributions (bacterial). High molecular weight acids and n-alcohols are characteristic constituents of vascular plant waxes.
The low organic carbon (OC) and sulphur contents of the Calcare di Base indicate unstressed environmental conditions for its deposition at least in the Rossano basin. Extended hopanes (>C33), gammacerane, isorenieratane, pregnanes and omopregnanes, often associated with highly anoxic and/or hypersaline sediments (Moldovan et al., 1985, ten Haven et al., 1988;
Adam et al., 1993), have not been detected in the study sediments. These biomarkers have been recorded in the evaporitic sequences of Sicily and Northern Apennines, confirming their reliability as palaecological markers of stressed environment (Schaeffer et al., 1995a, 1995b;
Sinninghe Damsté et al., 1995; Kenig et al., 1995; Gelin et al., 1995; Keely et al., 1995; Schaeffer-Reiss et al., 1998). Their absence in the Calcare di Base samples suggests oxic to dysoxic marine condition at time of deposition. This result is in agreement with the existence of a reasonable community of organisms and further strengthens the hypothesis of a basinal environment not stressed, making unlikely an evaporitic deposition of these sediments. The presence of coccolitophorid in fecal pellets (Guido et al., this volume) demonstrates definitely that Calcare di Base was deposited in normal marine conditions, even if the depositional environment became unstable for episodic freshwater inputs.
The thrombolitic facies, constituted of clotted peloidal
micrite, is a reliable clue for a biological induced deposition (Guido et al., this volume). Furthermore the constant and strong bacterial signal of the molecular fossils, represented by nC26-nC28 n-alkanes with no odd-even carbon number predominance, branched n- alkanes, hopanes and unsatured fatty acids, together with the widespread presence of amorphous organic matter in the palynofacies, corroborate the interpretation that dominant microfacies of the Calcare di Base represents a bacterial induced deposit.
The palaeoecological event, recorded in the Calcare di Base of the Rossano basin and characterized through the organic matter study, can have affected all basin and/or sub-basin involved in the messinian salinity crisis. This hypothesis is strongly corroborated by the observed microfacies uniformity of the Calcare di Base Formation in Calabria and in Sicily (Guido et al., this volume). This interpretation does not exclude that the carbonate beds, in some cases, could have later displaced through mass flow recorded by brecciated facies.
ACKNOWLEDGEMENTS - We thank the research group on organic matter of Institut des Sciences de la Terre d’Orléans for the technical support and the numerous suggestions to carry out 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 void among sedimentologist community. The authors wish thank S.
Conti (Università di Modena-Reggio Emilia) for his revisions and comments that greatly improved this paper.
Contribution to MIUR PRIN project 2004045107 (Palaeo- climatic forcing on building organism communities, carbonate productivity and depositional systems of some Italian Meso- Cenozoic shelf deposits). Bosellini A., coordinator.
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