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Microbial fabrics from Neogene cold seep carbonates, Northern Apennine, Italy
Palaeogeography, Palaeoclimatology, Palaeoecology 227 (2005) 143 – 155 www.elsevier.com/locate/palaeo
Microbial fabrics from Neogene cold seep carbonates, Northern Apennine, Italy Roberto Barbieri *, Barbara Cavalazzi Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita` di Bologna Via Zamboni 67, 40126 Bologna, Italy Received 16 April 2004; received in revised form 29 October 2004; accepted 11 April 2005
Abstract A number of hydrocarbon seep-carbonate masses are hosted within Miocene and Pliocene siliciclastic units along the Apennine Range and Sicily, and are mainly composed of micritic and microsparitic textures. Geomicrobiological analysis of some of these deposits shows that preserved microbial processes are few and the contribution of microbe-derived textures to their construction is volumetrically very limited. In the carbonate groundmass of two northern Apennine Miocene and Pliocene seep-carbonate deposits uncommon and well-preserved features suggest direct and indirect evidence of microbial activity. Some of these features are interpreted as the mineralization products of microbial textures, and include complex alveolar textures and fossilized biofilms that line and fill microconduits, pores and other voids, similar to living bacterial aggregations recovered in modern seep-carbonates. Other mineral structures, such as spheroids and rhombs, with concentric fabrics composed of calcite– dolomite alternations, abound as cement components and suggest a nucleation via bacterial sulfate reduction processes. A similar origin is also assumed for the acicular aragonite bundles that in the Miocene seep-carbonate mass enclose dense clusters of millimeter-size, filamentous morphologies of biological origin. D 2005 Elsevier B.V. All rights reserved. Keywords: Chemosynthesis; Hydrocarbon-seeps; Neogene; Authigenic carbonates; Italy
1. Introduction Biological communities sustained by hydrocarbon seepage have been described from a wealth of sites along modern continental margins, where the rapid
* Corresponding author. Tel.: +39 051 20 94 575; fax: +39 051 20 94 522. E-mail addresses: [email protected] (R. Barbieri), [email protected] (B. Cavalazzi). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.04.026
advection of low temperature fluids (cold seeps) is common (Paull et al., 1984; Campbell et al., 2002). The existence of these chemosynthesis-based communities has allowed the (re-)interpretation of a number of fossil assemblages, going back to the Silurian, where the most readily recognizable biotic components are bivalves and brachiopods (Campbell and Bottjer, 1995; Campbell et al., 2002; Barbieri et al., 2004). Hydrocarbons, especially methane, are the main components of cold seep fluids and the anaerobic
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oxidation of methane by archaea, connected with sulfate-reducing bacteria, leads to the precipitation of authigenic carbonates (Ritger et al., 1987; Peckmann et al., 2001). These carbonates form crusts, blocks, layers, lenses, and (exceptionally) mounds, and make up the geologic products of the seepage. Active methane advection and associated authigenic carbonates have been observed in the Mediterranean Sea (Colantoni et al., 1997; Aloisi et al., 2000), and fossil examples are well known in Oligo-Miocene and Pliocene-aged rocks of the Apennines and Sicily (Ricci Lucchi and Vai, 1994; Conti and Fontana, 1999). These fossil seep deposits are embedded in turbidite and hemipelagic units and host dense accumulations of mostly lucinid, vesicomyid, and modiolid bivalves (Taviani, 2001). These taxa are common components of modern seep and vent communities (Sibuet and Olu, 1998; Van Dover, 2000). A primary ecological attribute of cold seep sites is the role played by microbial communities. This includes mat-forming filamentous bacteria and bacterial endosymbionts hosted in gutless tubeworms and mollusks which are important biotic components at cold seep sites. The relationships between symbiotic microorganisms and their hosts characterize and distinguish a seep from surrounding ecosystems (Van Dover, 2000). In spite of their ecologic importance,
microbial cold seep fossils are rare and microbe-derived textures so far described from fossil seep carbonates include laminations, mineralized biofilms, crusts, tufts, peloidal fabrics, pyrite framboids, aragonite botryoids and crystal aggregates (Cavagna et al., 1999; Peckmann et al., 1999; Aharon, 2000; Clari and Martire, 2000; Peckmann and Thiel, 2004; Barbieri et al., 2004). The examination, under a reflected light microscope, of standard thin sections from a dozen seep-carbonates from the northern Apennine and Sicily shows that the carbonate groundmass consists mostly of homogeneous micritic and microsparitic textures, with planktonic foraminiferal tests as common biotic component, and various amounts of finegrained siliciclastic components derived from the enclosing rocks. The abundance of cavities, microconduits and other voids, as well as the pervasive presence of aragonite splays and other cements thought to be related to microbial processes were the reason for selecting two sites, Pietralunga and Stirone, for an investigation of microbe-derived textures. The Pietralunga paleoseep is Late Miocene in age from the Romagna Apennine and crops out along the Sintria River Valley. The Stirone paleoseep is Early Pliocene in age and crops out along the Stirone River, Emilia Apennine (Fig. 1). In both sites textures interpreted as microbe-derived are here documented from carbo-
Fig. 1. Location map of the seep-carbonate deposits (asterisks) of this study. Geologic map after Vai, 2001, modified.
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nates displaying strong 13C-depletion. The description and interpretation of these textures is the subject of the present study.
2. Characteristics of Apennine seep occurrences The stratigraphic relationships with the enclosing rocks are rarely preserved in Neogene Apennine seep occurrences. In many instances the carbonate masses are scattered in agricultural fields, where they lie as exhumed isolated bodies, so that the original stratigraphic position cannot be defined. Criteria for recognizing in situ versus displaced carbonate bodies have been proposed by Conti and Fontana (1998, 1999) and are based on field and compositional attributes. They also provided detailed outcrop descriptions. Whenever recognized, however, the host units consist of deep-water siliciclastic deposits. A comprehensive stratigraphic and tectonic scenario for these Apennine and Sicilian seep occurrences can be found in Ricci Lucchi and Vai (1994). They associated the seep carbonates with different foredeep basin types in which thrust-belt movements have produced the eastward compression of foredeep and satellite basins, with the expulsion of overpressured, methane-charged fluids as a result. Taviani (2001) provided a recent review on the geological, geochemical, and paleobiological aspects of these seep carbonates. The terrigenous nature of the host rocks is clearly reflected in the composition of the seep deposits. The light to dark grey hard carbonates, which make up most of the bodies, contain high proportions of silt- to sand-sized terrigenous materials; this is reflected by CaCO3 contents ranging from 46% to 60% (Barbieri, unpublished data). In broad terms, these seep-carbonates consist of microcrystalline calcite (micrite and microsparite), and the proportion of skeletal components may vary significantly even within a single accumulation. Planktonic foraminiferal tests are ubiquitous and abundant, and benthic foraminifers, sponge spicules, ostracods, pteropods, and fecal pellet concentrations are frequently embedded in the micritic carbonate. Chemosymbiotic megafauna includes abundant fragments of thick-shelled lucinids and/or other bivalve groups, and rare tube worm (vestimentiferans) remains (Barbieri and Cavalazzi, unpublished data).
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Microbial textures are uncommon in the broad variety of textural and compositional attributes recognized in the seep carbonates of the Apennine Range. In contrast, minerals potentially formed by the activity of microbial consortia are relatively common and include aragonite, barite, Mg-calcite, dolomite, goethite, and pyrite (Barbieri and Cavalazzi, unpublished data). Structures of a likely microbial origin include different kinds of laminated textures, peloidal/clotted fabrics, concentric structures, dark deposits encrusting skeletal remains (mollusks and tube worms), mineralized biofilms and crystal aggregates (e.g., bundles and dumbbells) (Cavagna et al., 1999; Peckmann et al., 1999, 2004; Clari and Martire, 2000). In general, peloidal/ clotted fabrics are the most ubiquitous ones. Since the study by Chafetz (1986), their relationship with bacteria and carbonate precipitation is well established, although not fully understood. These fabrics have been described in Miocene seep carbonates of northwest Italy (Cavagna et al., 1999; Clari and Martire, 2000), as well as in other seep deposits (Roberts et al., 1993; Campbell et al., 2002; Peckmann et al., 2002), where they are assigned to microbial degradation of methane. The scarcity of clearly documented remains of fossil microbial mats, such as the analogue of the mat-forming Beggiatoa that abound in modern seep sites at the sediment–water interface indicates a bias against their preservation. Peckmann et al. (2004) discussed this point and suggested the environmental conditions where Beggiatoa mats developed, that prevented carbonate formation, as explanation for preservation bias.
3. Pietralunga site The Pietralunga seep deposit (Figs. 1 and 2) is an isolated carbonate accumulation in the Sintria River Valley, exposed in agricultural fields developed on turbidite deposits of the Marnoso-arenacea Formation (see Marabini and Vai, 1985 for the regional geology context). These turbidites precede the onset of the evaporates of the Messinian Salinity Crisis. Seep-carbonate deposits of Langhian to Early Messinian age are relatively common in this area of the UmbriaRomagna Apennine and occur close to thrusts and fault systems developed during major tectonic phases of the Apennine orogeny (Ricci Lucchi and Vai, 1994; Terzi et al., 1994). Most of the Pietralunga deposit has
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thick. It is hosted by Early Pliocene Argille Azzurre Formation rocks, which are included in the foredeep succession of the southern margin of the Po River Basin (Ricci Lucchi et al., 1982; Argnani and Ricci Lucchi, 2001). At the outcrop scale, the rock is a grey to dark grey carbonate intermixed with terrigenous soft material filling the numerous centimeter-scale cavities in the carbonates. The carbonate outcrop displays concentrations of chemosymbiotic bivalves assigned to solemyid and lucinid species (Monegatti et al., 2001, Fig. 3). This megafaunal assemblage is embedded in homogeneous, grey and sandy carbonate with abundant glauconite, and consists mostly of bivalve and gastropod casts with poorly preserved Fig. 2. Outcrop of Pietralunga seep carbonate deposit, Sintria River valley, Romagna. The carbonate body is about 4 m in height. Note that the deposit has been quarried on the right side.
now been quarried, including those parts where concentrations of large-sized bivalves, gastropods, and other skeletal components were recovered and described by Terzi (1992). The remaining part (Fig. 2) is about 4 m in height and consists of tightly cemented, light to dark grey, homogeneous limestone where lucinid bivalves are rare or absent. Only this residual part of the deposit has been used for study. Six analyses of carbon and oxygen isotopes were performed on four different samples of carbonate (Table 1).
4. Stirone site The Stirone seep-limestone occurs within the Stirone River natural park (Fig. 1). The main carbonate mass is less than 10 m long and approximately 2 m Table 1 Results of stable isotope analysis, Pietralunga seep carbonate, carbon and oxygen isotope values Sample number PL PL PL PL PL PL
3A 3C1 3C1 3C6 3C6 4-1
Description
(1) (2) (1) (2)
Acicular aragonite bundles Light grey micrite (filaments) Dark grey micrite (filaments) Dark grey micrite (filaments) Dark grey micrite (filaments) Dark grey micrite (filaments)
Experimental precision: F0.1x.
d 13Cx (PDB)
d 18Ox (PDB)
37.7 40.0 42.0 42.8 41.3 32.1
1.7 2.1 1.9 2.5 2.5 5.3
Fig. 3. Outcrop (A) and hand sample (B) photographs from Stirone seep carbonate deposit, Stirone River, Emilia. The rock consists of bivalve-rich micrite.
R. Barbieri, B. Cavalazzi / Palaeogeography, Palaeoclimatology, Palaeoecology 227 (2005) 143–155 Table 2 Results of stable isotope analysis, Stirone seep carbonate, carbon and oxygen isotope values Sample number Description ST ST 2-1B (1) ST 2-1B (2) ST 2-3 ST ST ST ST ST
2-4 2-5 2-6A (1) 2-6A (2) 2-6A (3)
Lucinid shell Dark grey micrite Light grey micrite Dark grey and yellow micrite (microconduit margin) Light grey, porous micrite Light grey micrite Dark grey micrite Dark grey micrite Dark grey micrite (microconduit margin)
d 13Cx (PDB)
d 18Ox (PDB)
+1.1 23.5 17.6 23.9
+0.8 3.3 +3.7 +3.3
20.7 22.6 23.4 24.8 20.7
+3.3 +3.2 +3.3 +3.1 +3.6
Experimental precision: F0.1x.
shells. Another lithofacies is a dark grey brecciated carbonate with abundant millimeter-scale cavities and conduits. These cavities are filled either by siliciclastic material or by light colored cements (yellow Mgcalcite, see below). Nine analyses of carbon and oxygen isotopes were performed on six different samples of carbonate (Table 2). The unit enclosing the Stirone carbonate consists of sandy/silty shale with abundant dark green, glauconite pellets. Calcareous benthic foraminiferal assemblages are diverse and abundant, and are dominated by costate and hispid species (Uvigerina peregrina, U. proboscidea), planispiral-involute (Melonis pompilioides) and low trochospiral species (Cibicidoides dutemplei, C. pachyderma), and Globocassidulina subglobosa (Barbieri, unpublished data). Assuming the Stirone seeplimestone is in situ, benthic foraminiferal data suggest it formed at a paleodepth not exceeding 500 m.
5. Materials and methods Microfacies and microbial textures were investigated on petrographic thin sections (60 45 mm) with reflected light microscopy. Selected portions of thin sections and polished slabs were then examined using a JEOL JSM-5400 and a Philips XL30 Scanning Electron microscopes, equipped with EDS analyzer, for determining texture and elemental composition of the textural components. Mineral structures were investigated on a Philips PW 1480 X-ray diffractometer (XRD). Both light microscopy and SEM studies were
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partially conducted on HCl-etched thin sections. Etching was done by soaking samples in aqueous (deionized) solutions with 1% to 2% HCl concentration for 30 s to 2 min. Samples extracted under the binocular microscope from small carbonate chips were analyzed at the stable isotope laboratory of the Department of Earth Sciences at the University of Turin, Italy. Description of samples used for isotope analyses and data are summarized in Tables 1 and 2. Data were reported in conventional d-notation relative to the PDB standard. For both d 13C and d 13O the experimental precision was 0.1x. Samples include different micritic carbonates and aragonite bundles associated to the micritic facies. Measurements of microscopic structures and minerals were performed with a PyserSGI micrometer, scale 2 mm in 10 Am divisions (for reflected light).
6. Microbial structures 6.1. Pietralunga The hard limestone of the Pietralunga paleoseep consists of a microcrystalline groundmass composed by pure calcite and low-magnesian calcite. Terrigenous components from the host rock are suggested by the widespread presence of elements such as Si, Al, and K. In thin section the carbonate is less homogeneous and compact than at outcrop; false breccia (fragments fractured by veins) and frequent, irregularly distributed cavities indicate an original high porosity. These cavities often have concentrically laminated walls, are bordered by isopachous, fibrous aragonite fringes, and have drusy calcite and spar infill. In many instances, however, remainders of cavities/vugs and other pore spaces are filled with clusters of rhombohedral (Fig. 4) and spheroid crystal aggregates (Fig. 5) exhibiting fibrous and concentric fabrics. Rhombs associated with spar (Fig. 4A) have a diameter of 50–100 Am and the nucleus is pure calcium carbonate, whereas the concentric zones (Fig. 4B), which have grown around, consist of alternations of dolomite and calcium carbonate. A typical condition is shown in Fig. 4C, in which well-formed, zoned rhombs infill most of a partially empty cavity; aragonite splays departing from the wall cavities replace some of the rhombs. Rare barite accumulations have been found in associ-
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Fig. 4. Pietralunga seep carbonate, transmitted light micrographs of a petrographic thin section (A, B) and SEM micrograph of a polished surface (C). Well-formed zoned rhombs (r), composed of alternations of dolomite and calcium carbonate, and spar (s) fill cavities, which can be partially empty and filled with epoxy resin (e). Aragonite splays (a) partially reabsorb some of the rhombs. Light–dark alternations of rhombs are due to selective leaching. Note barite (b) accumulations. Scale bar: (A, C) = 200 Am; (B) = 50 Am.
ation with the rhomb textures or as thin and discontinuous laminae. Spheroids, with 20–30 Am average diameter (Fig. 5A), are more common than rhombs,
Fig. 5. Pietralunga seep carbonate, transmitted light micrograph of a petrographic thin section (A) and SEM micrographs of a etched polished surface (B, C). (A) Spherulitic textures (st), composed of alternations of dolomite and calcium carbonate, are common cavity infill. (B, C) Leaching is a common feature of these spherulites and is here emphasized by a slight HCl etching. Scale bar: (A) = 120 Am; (B) = 10 Am; (C) = 5 Am.
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which are the likely result of spheroids growth, and are associated with sparry calcite, the most common cement texture. The composition of spheroids is similar to that of rhombs. A more or less intense, selective leaching is a common feature for spheroids and rhombs (Fig. 4C) and locally it may lead to the almost destruction of these crystal habits (Fig. 5B and C). Dark, large filaments arranged in dense to loose clusters (Fig. 6A and B) are associated with both the microcrystalline (micrite and microsparite) groundmass and the cements (aragonite splays and spar) described above. Filaments have a 90–110 Am average diameter and 1–3 mm average length, although some of them can be several mm long (Fig. 6B). The distribution of these clusters is various; in places they are chaotically arranged in the micritic groundmass, whereas in others they mimic mat-forming arrangements. They often have such a dense concentration that they become the main rock component (Fig. 6B). Early diagenetic events have involved the filamentous structures, and acicular aragonite crystals may form an isopachous cement fringe around the filaments, which are then bound by sparry calcite (Fig. 6C). Locally, filamentous structures are embedded in siltsized, euhedral calcite crystals. Sectors of filamentrich microcrystalline carbonate often show spheroid fabrics. The original density of the filaments has only been preserved in the micritic groundmass, whereas in spar/aragonite-cemented areas part of these structures has been destroyed by later diagenetic processes. EDS analyzer revealed that the filamentous structures are composed of magnesian-calcite with local concentrations of iron oxides. Dense pyrite aggregates are often concentrated around filaments. A different reaction to a slight HCl etching (Fig. 6D) suggests possible mineralogical and/or textural difference between filaments and the embedding cements. Fig. 6. Pietralunga seep carbonate, transmitted light micrograph of petrographic thin sections (A, C); reflected light micrograph of a polished surface (B); SEM micrograph of a polished, slightly HCletched surface (D). Large filaments embedded in a microsparite groundmass (A) and arranged in dense concentrations (B). Aragonite splays (a) and blocky spar (bs) fill a residual cavity. (C) Aragonite cement fringe (a) around the filaments in cross section view. (D) Differences between tubules (f), in cross section view, and the embedding cements are evidenced by a different reaction to the etching of the rock surface. Scale bar: (A) = 1.5 mm; (B) = 500 Am; (C) = 200 Am; (D) = 50 Am.
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d 13C measurements from the samples collected at Pietralunga yielded negative values as low as 42.8x PDB. 6.2. Stirone Dark grey, brecciated carbonates with abundant cavities and conduits (Fig. 7A) contain the microbial textures that typify the Stirone paleoseep. This lime-
Fig. 7. Stirone seep carbonate, polished surface from a hand sample (A) and entire thin section view in transmitted light (B). In the hand sample (A) the light color network represents the yellow Mg-calcite mineral phase. Note cross sections of cavities/conduits (B) that are filled and rimmed with yellow Mg-calcite (YeCa). Dense pyrite aggregates and framboids (Py) are around conduits. Larger, dark grains spread in the groundmass is glauconite. Scale bar: (A) = 1 cm; (B) = 5 mm.
Fig. 8. Stirone seep carbonate, SEM micrographs of HCl-etched polished surfaces. Different magnification of the alveolar texture that makes up the yellow calcite. Note the multiple laminae that build this complex architecture. Scale bar = 5 Am.
stone consists mostly of a foraminiferal-rich micritic groundmass, with fine-grained siliciclastic components and numerous pyrite and glauconite grains. Millimeter- to centimeter-size cavities and conduits are surrounded by dense pyrite concentrations, and
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are partially or totally filled by yellow colored, microcrystalline Mg-calcite (Fig. 7B). Yellow Mg-calcite also rims and cements small fracture networks (Fig. 7A,B). The distribution of this mineral phase appears related to the porous nature of the sediment. In terms of mineral components both the dark grey carbonate and the yellow calcite consist of various amounts of calcite and Mg-calcite. d 13C values measured in these two lithofacies (Table 2) are approximately 18 to 25x PDB. The dark grey carbonate also consists of aragonite. Scanning electron microscopic observation of HCl-etched polished surfaces of these carbonate phases and the groundmass shows that the yellow Mgcalcite is made up of a three-dimensional, alveolar texture (Fig. 8), whereas the surrounding micritic groundmass lacks any clearly organized feature. This three-dimensional framework consists of irregular alveoli, ranging from about 5 to 10 Am in average diameter, which are bound by 1–2 Am thick walls (Fig. 8A). The walls may locally thicken up to
Fig. 9. Stirone seep carbonate, SEM micrographs of HCl-etched polished surfaces. (A) Sheaths and filaments trap grains. (B) Closeup of A. Scale bar: (A) = 10 Am; (B) = 1 Am.
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about 5 Am and exhibit a complex porous texture (Fig. 8A,B). The alveolar network is further subdivided by thin membranes, which are only locally preserved and exposed by the etching. Multiple laminae and filaments are the components that build the observed architecture (Fig. 8C). Irregular sheaths and filaments appear to have trapped and bound siliciclastic grains (Fig. 9) in different sectors of the dark grey groundmass adjacent to the sectors where yellow Mgcalcite developed.
7. Discussion and interpretation 7.1. Pietralunga The spheroids from the Pietralunga site are similar to those described by Terzi (1993), Terzi et al. (1994), Cavagna et al. (1999) and Clari and Martire (2000), from northern Apennine Miocene seep occurrences, and by Gunatilaka (1989), from hydrocarbon seepage areas of Kuwait. Peckmann et al. (2004) have described similar spheroids in the Pietralunga site. There are differences in composition: spheroids may consist of calcite–aragonite alternations (Terzi et al., 1994), dolomite (Gunatilaka, 1989; Cavagna et al., 1999; Clari and Martire, 2000), two rims of carbonate (probably calcite) cement (Peckmann et al., 2004), or calcite–dolomite alternations (this study). These carbonate minerals may have a biological origin and be related to microbial activity in hydrocarbon seep environments (Savard et al., 1996; Aharon, 2000), where they are a typical mineral component. Gunatilaka (1989) provided a convincing explanation for a possible nucleation of these structures via bacterial oxidation of hydrocarbons. Her hypothesis of an organic (bacterial) nucleus for spheroids is also corroborated by the experimental observations of Buczynski and Chafetz (1991), who refer to bacterial clumps as nuclei of calcium carbonate spheres and hemispheres that are similar in shape and size to those described here. In addition, Cavagna et al. (1999) described dumbbell-shaped cores in spheroids from Miocene seep carbonates. The dumbbell crystal morphology is commonly interpreted as bacteria-related (Buczynski and Chafetz, 1991). At Pietralunga spheroids and rhombs are intimately associated with splayed needles of aragonite, which are common as cement
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precipitates. This association of textural/cement features, coupled with mineral composition, suggests the intervention of microbial sulfate reduction processes. The presence of sulfate-reducing bacteria as a cause of dolomite formation at low temperature has also been experimentally demonstrated (Vasconcelos et al., 1995; Van Lith et al., 2003). The partial replacement of rhombs by aragonite needles (Fig. 4C) suggests different early diagenetic stages for these two mineralogies. The concentric structure of spheroids and rhombs and their location as cement textures filling cavities and residual pores suggest a common origin for the two fabrics. The filamentous structures (Fig. 6), which characterize a peculiar microfacies of this seep occurrence were previously figured by Barbieri et al. (2001) who hypothesized an origin from giant Beggiatoa-like bacteria arranged in microbial mats. Recently, Peckmann et al. (2004) provided a comprehensive bio-geochemical and petrographic study, which convincingly document their interpretation that these filamentous fossils are extraordinarily well-preserved Beggiatoatype filaments. A smaller average diameter (from 50 to 80 Am) of the filamentous structures described by Peckmann et al. (2004) is the only difference with the larger filaments here reported (average diameter 100–120 Am). This difference in size, however, does not substantially influence the explanation on the microbial (probably Beggiatoa) origin for these filamentous mats. An alternative interpretation that might be hypothesized for the origin of these large filaments is that they derive from sulfide-tolerant metazoan meiofaunas, including products of their metabolic activity, such as nematodes or other seep worms. This interpretation might be supported by shape and size of the filaments, that fit meiofauna, and lack of a visible segmentation that should be expected in well-preserved sulfur-oxidizing bacteria filaments (see the example by Bernhard and Buck, 2004, Fig. 7C). Some of these filaments are short and have lengthto-width ratio fitting rod-shaped morphologies and, thus, they resemble fecal pellets similar to the ones ejected by communities of worms, such as polychaetes (see example figured by Nowell et al., 1981). An interpretation as fecal remains of worm communities, however, cannot explain the prevailing presence of long filaments and their mat-like arrangement. The lack of segmentation in these filaments and their pre-
vailing diameter of 100 Am or more fit a nematode morphology. Nematodes, such as the gutless Astomonema southwardorum from North Sea methane seeps (Austen et al., 1993), are important components of seep meiofaunas. In seep/sulfide-rich habitats, however, nematodes do not exhibit the densely packed arrangement (Bernhard and Buck, 2004, Fig. 8; Levin, personal communication 2004) observed for the Pietralunga filaments. An origin by microbial oxidation of methane is suggested for the carbonate containing filaments by the abundance of aragonite cements and negative d 13C values (as low as 42.8x). Although methane can be assumed to have been the main carbon source, the co-occurrence of pyrite and rare, small aggregates of barite suggests that hydrogen sulfide is also a component of the cold fluid seepage. 7.2. Stirone The responsibility of microbial activity in the formation of the three-dimensional textures, which compose the yellow Mg-calcite of the Stirone paleoseep, is suggested by morphological comparisons with modern and fossil analogues of these textures. Because yellow calcite lines pores and conduits (Fig. 7), it should represent the mineral precipitate directly associated with the gas seepage. Reticulate features morphologically similar to those of the yellow calcite (Fig. 8) are promoted by microbial activity in a number of different environmental settings. Examples include the modern anastomosing network produced by microbial mats of filamentous cyanobacteria in coastal lagoons of the southern Australia (Bauld et al., 1993) and Tunisia (Gerdes et al., 2000), and in atoll lagoons of French Polynesia (Sprachta et al., 2001). In particular, the last mentioned example shows how microbial sheaths of filamentous cyanobacteria can decompose and coalesce into a three-dimensional organic framework (Sprachta et al., 2001, Figs. 7 and 8). Another modern morphologic analogue is the bspongyQ texture produced by mats of the sulfur-oxidizing chemotrophic Beggiatoa on the Peru slope (Williams and Reimers, 1983). Fossil examples of microbially induced, three-dimensional textures include residual deposits of hematite, filled by authigenic micrite, in a Silurian paleoseep, Morocco (Barbieri et al., 2004), carboniferous cyanobacterial textures from the Pan-
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ther Seep Formation, New Mexico (Chafetz et al., 1993), and Miocene stromatolites described from the Monterey Formation, California (Williams and Reimers, 1983; Williams, 1984). The Silurian and Miocene examples are interpreted as fossil equivalents of modern Beggiatoaceae mats. The porous nature of the Stirone carbonates, with centimeter-sized cavities filled by siliciclastic sediments, and the microconduits cemented by yellow Mg-calcite with scattered glauconite grains (Fig. 7B), document a close intermixture between seep carbonate and the surrounding loose sediment. This indicates that the precipitation of the authigenic mineral occurred beneath the sediment/water interface. A comparison with modern methane-derived carbonates having a similar fabric with abundant cavities, such as the carbonate precipitates of the Black Sea (Luth et al., 1999; Peckmann et al., 2001) and the Adriatic Sea (Barbieri and Cavalazzi, unpublished data), suggests that the abundant microbial films and colonies, which line the walls and fill the pores of these modern carbonate structures, may have as a fossil analogue the yellow Mg-calcite of the Stirone paleoseep. The negative d 13C values (as low as nearly 25x, Table 2), measured in the micrite, reveal that the mineralization of the organic (microbial) compounds occurred in conjunction with the precipitation of methane-derived mineral phases. These mineral phases are promoted by methanotrophic archaea through anaerobic oxidation processes. The abundant filamentous and membranous structures incorporating glauconite, pyrite, and siliciclastic grains in the dark grey micrite (Fig. 9) which compose most of the Stirone seepcarbonate, are interpreted as biofilm and mucilaginous compounds of microbial nature.
8. Conclusions The microbial fabrics described from the studied Apennine deposits were rarely reported, and some of them are newly described from seep-carbonate occurrences. Some of the fabrics are interpreted as fossilized microbial remains. For others the evidence of microbial imprints is indirect and can only be hypothesized. Fossilized remains of microbial communities are (i) the three-dimensional, alveolar textures recognized
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in the yellow Mg-calcite, and (ii) sheaths and filaments trapping detrital material at Stirone paleoseep. The alveolar textures represent an example of complex and organized fabric which have only recently been documented in ancient (Silurian) seep deposits. Because of their location, around and within conduits and fractures, these mineralized textures are interpreted as the fossil analogue of the bacterial aggregations that line and fill cavities in modern, porous seep carbonates. Structures induced by microbial activity include the spheroids and rhombs of Pietralunga paleoseep. These structures consist of alternations of dolomite and calcium carbonate, are commonly associated with aragonite splay cements, and are inferred to be related to processes mediated by sulfate-reducing bacteria. They are commonly associated with clusters of large filaments for which a bacterial origin (Beggiatoa-type filaments) has recently been documented.
Acknowledgements We thank James L. Goedert, Jo¨rn Peckmann and two anonymous reviewers for constructive critiques and suggestions. We also thank Paolo Ferrieri for his excellent assistance during SEM investigations, Giorgio Gasparotto for microanalysis, and Patrizia Ferraresi for drawings. This study was supported by the MIUR (Cofin 2002) program bTerrestrial geological analogues as models for Mars exopaleontologyQ.
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Microbial fabrics from Neogene cold seep carbonates, Northern Apennine, Italy Roberto Barbieri *, Barbara Cavalazzi Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita` di Bologna Via Zamboni 67, 40126 Bologna, Italy Received 16 April 2004; received in revised form 29 October 2004; accepted 11 April 2005
Abstract A number of hydrocarbon seep-carbonate masses are hosted within Miocene and Pliocene siliciclastic units along the Apennine Range and Sicily, and are mainly composed of micritic and microsparitic textures. Geomicrobiological analysis of some of these deposits shows that preserved microbial processes are few and the contribution of microbe-derived textures to their construction is volumetrically very limited. In the carbonate groundmass of two northern Apennine Miocene and Pliocene seep-carbonate deposits uncommon and well-preserved features suggest direct and indirect evidence of microbial activity. Some of these features are interpreted as the mineralization products of microbial textures, and include complex alveolar textures and fossilized biofilms that line and fill microconduits, pores and other voids, similar to living bacterial aggregations recovered in modern seep-carbonates. Other mineral structures, such as spheroids and rhombs, with concentric fabrics composed of calcite– dolomite alternations, abound as cement components and suggest a nucleation via bacterial sulfate reduction processes. A similar origin is also assumed for the acicular aragonite bundles that in the Miocene seep-carbonate mass enclose dense clusters of millimeter-size, filamentous morphologies of biological origin. D 2005 Elsevier B.V. All rights reserved. Keywords: Chemosynthesis; Hydrocarbon-seeps; Neogene; Authigenic carbonates; Italy
1. Introduction Biological communities sustained by hydrocarbon seepage have been described from a wealth of sites along modern continental margins, where the rapid
* Corresponding author. Tel.: +39 051 20 94 575; fax: +39 051 20 94 522. E-mail addresses: [email protected] (R. Barbieri), [email protected] (B. Cavalazzi). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.04.026
advection of low temperature fluids (cold seeps) is common (Paull et al., 1984; Campbell et al., 2002). The existence of these chemosynthesis-based communities has allowed the (re-)interpretation of a number of fossil assemblages, going back to the Silurian, where the most readily recognizable biotic components are bivalves and brachiopods (Campbell and Bottjer, 1995; Campbell et al., 2002; Barbieri et al., 2004). Hydrocarbons, especially methane, are the main components of cold seep fluids and the anaerobic
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oxidation of methane by archaea, connected with sulfate-reducing bacteria, leads to the precipitation of authigenic carbonates (Ritger et al., 1987; Peckmann et al., 2001). These carbonates form crusts, blocks, layers, lenses, and (exceptionally) mounds, and make up the geologic products of the seepage. Active methane advection and associated authigenic carbonates have been observed in the Mediterranean Sea (Colantoni et al., 1997; Aloisi et al., 2000), and fossil examples are well known in Oligo-Miocene and Pliocene-aged rocks of the Apennines and Sicily (Ricci Lucchi and Vai, 1994; Conti and Fontana, 1999). These fossil seep deposits are embedded in turbidite and hemipelagic units and host dense accumulations of mostly lucinid, vesicomyid, and modiolid bivalves (Taviani, 2001). These taxa are common components of modern seep and vent communities (Sibuet and Olu, 1998; Van Dover, 2000). A primary ecological attribute of cold seep sites is the role played by microbial communities. This includes mat-forming filamentous bacteria and bacterial endosymbionts hosted in gutless tubeworms and mollusks which are important biotic components at cold seep sites. The relationships between symbiotic microorganisms and their hosts characterize and distinguish a seep from surrounding ecosystems (Van Dover, 2000). In spite of their ecologic importance,
microbial cold seep fossils are rare and microbe-derived textures so far described from fossil seep carbonates include laminations, mineralized biofilms, crusts, tufts, peloidal fabrics, pyrite framboids, aragonite botryoids and crystal aggregates (Cavagna et al., 1999; Peckmann et al., 1999; Aharon, 2000; Clari and Martire, 2000; Peckmann and Thiel, 2004; Barbieri et al., 2004). The examination, under a reflected light microscope, of standard thin sections from a dozen seep-carbonates from the northern Apennine and Sicily shows that the carbonate groundmass consists mostly of homogeneous micritic and microsparitic textures, with planktonic foraminiferal tests as common biotic component, and various amounts of finegrained siliciclastic components derived from the enclosing rocks. The abundance of cavities, microconduits and other voids, as well as the pervasive presence of aragonite splays and other cements thought to be related to microbial processes were the reason for selecting two sites, Pietralunga and Stirone, for an investigation of microbe-derived textures. The Pietralunga paleoseep is Late Miocene in age from the Romagna Apennine and crops out along the Sintria River Valley. The Stirone paleoseep is Early Pliocene in age and crops out along the Stirone River, Emilia Apennine (Fig. 1). In both sites textures interpreted as microbe-derived are here documented from carbo-
Fig. 1. Location map of the seep-carbonate deposits (asterisks) of this study. Geologic map after Vai, 2001, modified.
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nates displaying strong 13C-depletion. The description and interpretation of these textures is the subject of the present study.
2. Characteristics of Apennine seep occurrences The stratigraphic relationships with the enclosing rocks are rarely preserved in Neogene Apennine seep occurrences. In many instances the carbonate masses are scattered in agricultural fields, where they lie as exhumed isolated bodies, so that the original stratigraphic position cannot be defined. Criteria for recognizing in situ versus displaced carbonate bodies have been proposed by Conti and Fontana (1998, 1999) and are based on field and compositional attributes. They also provided detailed outcrop descriptions. Whenever recognized, however, the host units consist of deep-water siliciclastic deposits. A comprehensive stratigraphic and tectonic scenario for these Apennine and Sicilian seep occurrences can be found in Ricci Lucchi and Vai (1994). They associated the seep carbonates with different foredeep basin types in which thrust-belt movements have produced the eastward compression of foredeep and satellite basins, with the expulsion of overpressured, methane-charged fluids as a result. Taviani (2001) provided a recent review on the geological, geochemical, and paleobiological aspects of these seep carbonates. The terrigenous nature of the host rocks is clearly reflected in the composition of the seep deposits. The light to dark grey hard carbonates, which make up most of the bodies, contain high proportions of silt- to sand-sized terrigenous materials; this is reflected by CaCO3 contents ranging from 46% to 60% (Barbieri, unpublished data). In broad terms, these seep-carbonates consist of microcrystalline calcite (micrite and microsparite), and the proportion of skeletal components may vary significantly even within a single accumulation. Planktonic foraminiferal tests are ubiquitous and abundant, and benthic foraminifers, sponge spicules, ostracods, pteropods, and fecal pellet concentrations are frequently embedded in the micritic carbonate. Chemosymbiotic megafauna includes abundant fragments of thick-shelled lucinids and/or other bivalve groups, and rare tube worm (vestimentiferans) remains (Barbieri and Cavalazzi, unpublished data).
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Microbial textures are uncommon in the broad variety of textural and compositional attributes recognized in the seep carbonates of the Apennine Range. In contrast, minerals potentially formed by the activity of microbial consortia are relatively common and include aragonite, barite, Mg-calcite, dolomite, goethite, and pyrite (Barbieri and Cavalazzi, unpublished data). Structures of a likely microbial origin include different kinds of laminated textures, peloidal/clotted fabrics, concentric structures, dark deposits encrusting skeletal remains (mollusks and tube worms), mineralized biofilms and crystal aggregates (e.g., bundles and dumbbells) (Cavagna et al., 1999; Peckmann et al., 1999, 2004; Clari and Martire, 2000). In general, peloidal/ clotted fabrics are the most ubiquitous ones. Since the study by Chafetz (1986), their relationship with bacteria and carbonate precipitation is well established, although not fully understood. These fabrics have been described in Miocene seep carbonates of northwest Italy (Cavagna et al., 1999; Clari and Martire, 2000), as well as in other seep deposits (Roberts et al., 1993; Campbell et al., 2002; Peckmann et al., 2002), where they are assigned to microbial degradation of methane. The scarcity of clearly documented remains of fossil microbial mats, such as the analogue of the mat-forming Beggiatoa that abound in modern seep sites at the sediment–water interface indicates a bias against their preservation. Peckmann et al. (2004) discussed this point and suggested the environmental conditions where Beggiatoa mats developed, that prevented carbonate formation, as explanation for preservation bias.
3. Pietralunga site The Pietralunga seep deposit (Figs. 1 and 2) is an isolated carbonate accumulation in the Sintria River Valley, exposed in agricultural fields developed on turbidite deposits of the Marnoso-arenacea Formation (see Marabini and Vai, 1985 for the regional geology context). These turbidites precede the onset of the evaporates of the Messinian Salinity Crisis. Seep-carbonate deposits of Langhian to Early Messinian age are relatively common in this area of the UmbriaRomagna Apennine and occur close to thrusts and fault systems developed during major tectonic phases of the Apennine orogeny (Ricci Lucchi and Vai, 1994; Terzi et al., 1994). Most of the Pietralunga deposit has
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thick. It is hosted by Early Pliocene Argille Azzurre Formation rocks, which are included in the foredeep succession of the southern margin of the Po River Basin (Ricci Lucchi et al., 1982; Argnani and Ricci Lucchi, 2001). At the outcrop scale, the rock is a grey to dark grey carbonate intermixed with terrigenous soft material filling the numerous centimeter-scale cavities in the carbonates. The carbonate outcrop displays concentrations of chemosymbiotic bivalves assigned to solemyid and lucinid species (Monegatti et al., 2001, Fig. 3). This megafaunal assemblage is embedded in homogeneous, grey and sandy carbonate with abundant glauconite, and consists mostly of bivalve and gastropod casts with poorly preserved Fig. 2. Outcrop of Pietralunga seep carbonate deposit, Sintria River valley, Romagna. The carbonate body is about 4 m in height. Note that the deposit has been quarried on the right side.
now been quarried, including those parts where concentrations of large-sized bivalves, gastropods, and other skeletal components were recovered and described by Terzi (1992). The remaining part (Fig. 2) is about 4 m in height and consists of tightly cemented, light to dark grey, homogeneous limestone where lucinid bivalves are rare or absent. Only this residual part of the deposit has been used for study. Six analyses of carbon and oxygen isotopes were performed on four different samples of carbonate (Table 1).
4. Stirone site The Stirone seep-limestone occurs within the Stirone River natural park (Fig. 1). The main carbonate mass is less than 10 m long and approximately 2 m Table 1 Results of stable isotope analysis, Pietralunga seep carbonate, carbon and oxygen isotope values Sample number PL PL PL PL PL PL
3A 3C1 3C1 3C6 3C6 4-1
Description
(1) (2) (1) (2)
Acicular aragonite bundles Light grey micrite (filaments) Dark grey micrite (filaments) Dark grey micrite (filaments) Dark grey micrite (filaments) Dark grey micrite (filaments)
Experimental precision: F0.1x.
d 13Cx (PDB)
d 18Ox (PDB)
37.7 40.0 42.0 42.8 41.3 32.1
1.7 2.1 1.9 2.5 2.5 5.3
Fig. 3. Outcrop (A) and hand sample (B) photographs from Stirone seep carbonate deposit, Stirone River, Emilia. The rock consists of bivalve-rich micrite.
R. Barbieri, B. Cavalazzi / Palaeogeography, Palaeoclimatology, Palaeoecology 227 (2005) 143–155 Table 2 Results of stable isotope analysis, Stirone seep carbonate, carbon and oxygen isotope values Sample number Description ST ST 2-1B (1) ST 2-1B (2) ST 2-3 ST ST ST ST ST
2-4 2-5 2-6A (1) 2-6A (2) 2-6A (3)
Lucinid shell Dark grey micrite Light grey micrite Dark grey and yellow micrite (microconduit margin) Light grey, porous micrite Light grey micrite Dark grey micrite Dark grey micrite Dark grey micrite (microconduit margin)
d 13Cx (PDB)
d 18Ox (PDB)
+1.1 23.5 17.6 23.9
+0.8 3.3 +3.7 +3.3
20.7 22.6 23.4 24.8 20.7
+3.3 +3.2 +3.3 +3.1 +3.6
Experimental precision: F0.1x.
shells. Another lithofacies is a dark grey brecciated carbonate with abundant millimeter-scale cavities and conduits. These cavities are filled either by siliciclastic material or by light colored cements (yellow Mgcalcite, see below). Nine analyses of carbon and oxygen isotopes were performed on six different samples of carbonate (Table 2). The unit enclosing the Stirone carbonate consists of sandy/silty shale with abundant dark green, glauconite pellets. Calcareous benthic foraminiferal assemblages are diverse and abundant, and are dominated by costate and hispid species (Uvigerina peregrina, U. proboscidea), planispiral-involute (Melonis pompilioides) and low trochospiral species (Cibicidoides dutemplei, C. pachyderma), and Globocassidulina subglobosa (Barbieri, unpublished data). Assuming the Stirone seeplimestone is in situ, benthic foraminiferal data suggest it formed at a paleodepth not exceeding 500 m.
5. Materials and methods Microfacies and microbial textures were investigated on petrographic thin sections (60 45 mm) with reflected light microscopy. Selected portions of thin sections and polished slabs were then examined using a JEOL JSM-5400 and a Philips XL30 Scanning Electron microscopes, equipped with EDS analyzer, for determining texture and elemental composition of the textural components. Mineral structures were investigated on a Philips PW 1480 X-ray diffractometer (XRD). Both light microscopy and SEM studies were
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partially conducted on HCl-etched thin sections. Etching was done by soaking samples in aqueous (deionized) solutions with 1% to 2% HCl concentration for 30 s to 2 min. Samples extracted under the binocular microscope from small carbonate chips were analyzed at the stable isotope laboratory of the Department of Earth Sciences at the University of Turin, Italy. Description of samples used for isotope analyses and data are summarized in Tables 1 and 2. Data were reported in conventional d-notation relative to the PDB standard. For both d 13C and d 13O the experimental precision was 0.1x. Samples include different micritic carbonates and aragonite bundles associated to the micritic facies. Measurements of microscopic structures and minerals were performed with a PyserSGI micrometer, scale 2 mm in 10 Am divisions (for reflected light).
6. Microbial structures 6.1. Pietralunga The hard limestone of the Pietralunga paleoseep consists of a microcrystalline groundmass composed by pure calcite and low-magnesian calcite. Terrigenous components from the host rock are suggested by the widespread presence of elements such as Si, Al, and K. In thin section the carbonate is less homogeneous and compact than at outcrop; false breccia (fragments fractured by veins) and frequent, irregularly distributed cavities indicate an original high porosity. These cavities often have concentrically laminated walls, are bordered by isopachous, fibrous aragonite fringes, and have drusy calcite and spar infill. In many instances, however, remainders of cavities/vugs and other pore spaces are filled with clusters of rhombohedral (Fig. 4) and spheroid crystal aggregates (Fig. 5) exhibiting fibrous and concentric fabrics. Rhombs associated with spar (Fig. 4A) have a diameter of 50–100 Am and the nucleus is pure calcium carbonate, whereas the concentric zones (Fig. 4B), which have grown around, consist of alternations of dolomite and calcium carbonate. A typical condition is shown in Fig. 4C, in which well-formed, zoned rhombs infill most of a partially empty cavity; aragonite splays departing from the wall cavities replace some of the rhombs. Rare barite accumulations have been found in associ-
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Fig. 4. Pietralunga seep carbonate, transmitted light micrographs of a petrographic thin section (A, B) and SEM micrograph of a polished surface (C). Well-formed zoned rhombs (r), composed of alternations of dolomite and calcium carbonate, and spar (s) fill cavities, which can be partially empty and filled with epoxy resin (e). Aragonite splays (a) partially reabsorb some of the rhombs. Light–dark alternations of rhombs are due to selective leaching. Note barite (b) accumulations. Scale bar: (A, C) = 200 Am; (B) = 50 Am.
ation with the rhomb textures or as thin and discontinuous laminae. Spheroids, with 20–30 Am average diameter (Fig. 5A), are more common than rhombs,
Fig. 5. Pietralunga seep carbonate, transmitted light micrograph of a petrographic thin section (A) and SEM micrographs of a etched polished surface (B, C). (A) Spherulitic textures (st), composed of alternations of dolomite and calcium carbonate, are common cavity infill. (B, C) Leaching is a common feature of these spherulites and is here emphasized by a slight HCl etching. Scale bar: (A) = 120 Am; (B) = 10 Am; (C) = 5 Am.
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which are the likely result of spheroids growth, and are associated with sparry calcite, the most common cement texture. The composition of spheroids is similar to that of rhombs. A more or less intense, selective leaching is a common feature for spheroids and rhombs (Fig. 4C) and locally it may lead to the almost destruction of these crystal habits (Fig. 5B and C). Dark, large filaments arranged in dense to loose clusters (Fig. 6A and B) are associated with both the microcrystalline (micrite and microsparite) groundmass and the cements (aragonite splays and spar) described above. Filaments have a 90–110 Am average diameter and 1–3 mm average length, although some of them can be several mm long (Fig. 6B). The distribution of these clusters is various; in places they are chaotically arranged in the micritic groundmass, whereas in others they mimic mat-forming arrangements. They often have such a dense concentration that they become the main rock component (Fig. 6B). Early diagenetic events have involved the filamentous structures, and acicular aragonite crystals may form an isopachous cement fringe around the filaments, which are then bound by sparry calcite (Fig. 6C). Locally, filamentous structures are embedded in siltsized, euhedral calcite crystals. Sectors of filamentrich microcrystalline carbonate often show spheroid fabrics. The original density of the filaments has only been preserved in the micritic groundmass, whereas in spar/aragonite-cemented areas part of these structures has been destroyed by later diagenetic processes. EDS analyzer revealed that the filamentous structures are composed of magnesian-calcite with local concentrations of iron oxides. Dense pyrite aggregates are often concentrated around filaments. A different reaction to a slight HCl etching (Fig. 6D) suggests possible mineralogical and/or textural difference between filaments and the embedding cements. Fig. 6. Pietralunga seep carbonate, transmitted light micrograph of petrographic thin sections (A, C); reflected light micrograph of a polished surface (B); SEM micrograph of a polished, slightly HCletched surface (D). Large filaments embedded in a microsparite groundmass (A) and arranged in dense concentrations (B). Aragonite splays (a) and blocky spar (bs) fill a residual cavity. (C) Aragonite cement fringe (a) around the filaments in cross section view. (D) Differences between tubules (f), in cross section view, and the embedding cements are evidenced by a different reaction to the etching of the rock surface. Scale bar: (A) = 1.5 mm; (B) = 500 Am; (C) = 200 Am; (D) = 50 Am.
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d 13C measurements from the samples collected at Pietralunga yielded negative values as low as 42.8x PDB. 6.2. Stirone Dark grey, brecciated carbonates with abundant cavities and conduits (Fig. 7A) contain the microbial textures that typify the Stirone paleoseep. This lime-
Fig. 7. Stirone seep carbonate, polished surface from a hand sample (A) and entire thin section view in transmitted light (B). In the hand sample (A) the light color network represents the yellow Mg-calcite mineral phase. Note cross sections of cavities/conduits (B) that are filled and rimmed with yellow Mg-calcite (YeCa). Dense pyrite aggregates and framboids (Py) are around conduits. Larger, dark grains spread in the groundmass is glauconite. Scale bar: (A) = 1 cm; (B) = 5 mm.
Fig. 8. Stirone seep carbonate, SEM micrographs of HCl-etched polished surfaces. Different magnification of the alveolar texture that makes up the yellow calcite. Note the multiple laminae that build this complex architecture. Scale bar = 5 Am.
stone consists mostly of a foraminiferal-rich micritic groundmass, with fine-grained siliciclastic components and numerous pyrite and glauconite grains. Millimeter- to centimeter-size cavities and conduits are surrounded by dense pyrite concentrations, and
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are partially or totally filled by yellow colored, microcrystalline Mg-calcite (Fig. 7B). Yellow Mg-calcite also rims and cements small fracture networks (Fig. 7A,B). The distribution of this mineral phase appears related to the porous nature of the sediment. In terms of mineral components both the dark grey carbonate and the yellow calcite consist of various amounts of calcite and Mg-calcite. d 13C values measured in these two lithofacies (Table 2) are approximately 18 to 25x PDB. The dark grey carbonate also consists of aragonite. Scanning electron microscopic observation of HCl-etched polished surfaces of these carbonate phases and the groundmass shows that the yellow Mgcalcite is made up of a three-dimensional, alveolar texture (Fig. 8), whereas the surrounding micritic groundmass lacks any clearly organized feature. This three-dimensional framework consists of irregular alveoli, ranging from about 5 to 10 Am in average diameter, which are bound by 1–2 Am thick walls (Fig. 8A). The walls may locally thicken up to
Fig. 9. Stirone seep carbonate, SEM micrographs of HCl-etched polished surfaces. (A) Sheaths and filaments trap grains. (B) Closeup of A. Scale bar: (A) = 10 Am; (B) = 1 Am.
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about 5 Am and exhibit a complex porous texture (Fig. 8A,B). The alveolar network is further subdivided by thin membranes, which are only locally preserved and exposed by the etching. Multiple laminae and filaments are the components that build the observed architecture (Fig. 8C). Irregular sheaths and filaments appear to have trapped and bound siliciclastic grains (Fig. 9) in different sectors of the dark grey groundmass adjacent to the sectors where yellow Mgcalcite developed.
7. Discussion and interpretation 7.1. Pietralunga The spheroids from the Pietralunga site are similar to those described by Terzi (1993), Terzi et al. (1994), Cavagna et al. (1999) and Clari and Martire (2000), from northern Apennine Miocene seep occurrences, and by Gunatilaka (1989), from hydrocarbon seepage areas of Kuwait. Peckmann et al. (2004) have described similar spheroids in the Pietralunga site. There are differences in composition: spheroids may consist of calcite–aragonite alternations (Terzi et al., 1994), dolomite (Gunatilaka, 1989; Cavagna et al., 1999; Clari and Martire, 2000), two rims of carbonate (probably calcite) cement (Peckmann et al., 2004), or calcite–dolomite alternations (this study). These carbonate minerals may have a biological origin and be related to microbial activity in hydrocarbon seep environments (Savard et al., 1996; Aharon, 2000), where they are a typical mineral component. Gunatilaka (1989) provided a convincing explanation for a possible nucleation of these structures via bacterial oxidation of hydrocarbons. Her hypothesis of an organic (bacterial) nucleus for spheroids is also corroborated by the experimental observations of Buczynski and Chafetz (1991), who refer to bacterial clumps as nuclei of calcium carbonate spheres and hemispheres that are similar in shape and size to those described here. In addition, Cavagna et al. (1999) described dumbbell-shaped cores in spheroids from Miocene seep carbonates. The dumbbell crystal morphology is commonly interpreted as bacteria-related (Buczynski and Chafetz, 1991). At Pietralunga spheroids and rhombs are intimately associated with splayed needles of aragonite, which are common as cement
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precipitates. This association of textural/cement features, coupled with mineral composition, suggests the intervention of microbial sulfate reduction processes. The presence of sulfate-reducing bacteria as a cause of dolomite formation at low temperature has also been experimentally demonstrated (Vasconcelos et al., 1995; Van Lith et al., 2003). The partial replacement of rhombs by aragonite needles (Fig. 4C) suggests different early diagenetic stages for these two mineralogies. The concentric structure of spheroids and rhombs and their location as cement textures filling cavities and residual pores suggest a common origin for the two fabrics. The filamentous structures (Fig. 6), which characterize a peculiar microfacies of this seep occurrence were previously figured by Barbieri et al. (2001) who hypothesized an origin from giant Beggiatoa-like bacteria arranged in microbial mats. Recently, Peckmann et al. (2004) provided a comprehensive bio-geochemical and petrographic study, which convincingly document their interpretation that these filamentous fossils are extraordinarily well-preserved Beggiatoatype filaments. A smaller average diameter (from 50 to 80 Am) of the filamentous structures described by Peckmann et al. (2004) is the only difference with the larger filaments here reported (average diameter 100–120 Am). This difference in size, however, does not substantially influence the explanation on the microbial (probably Beggiatoa) origin for these filamentous mats. An alternative interpretation that might be hypothesized for the origin of these large filaments is that they derive from sulfide-tolerant metazoan meiofaunas, including products of their metabolic activity, such as nematodes or other seep worms. This interpretation might be supported by shape and size of the filaments, that fit meiofauna, and lack of a visible segmentation that should be expected in well-preserved sulfur-oxidizing bacteria filaments (see the example by Bernhard and Buck, 2004, Fig. 7C). Some of these filaments are short and have lengthto-width ratio fitting rod-shaped morphologies and, thus, they resemble fecal pellets similar to the ones ejected by communities of worms, such as polychaetes (see example figured by Nowell et al., 1981). An interpretation as fecal remains of worm communities, however, cannot explain the prevailing presence of long filaments and their mat-like arrangement. The lack of segmentation in these filaments and their pre-
vailing diameter of 100 Am or more fit a nematode morphology. Nematodes, such as the gutless Astomonema southwardorum from North Sea methane seeps (Austen et al., 1993), are important components of seep meiofaunas. In seep/sulfide-rich habitats, however, nematodes do not exhibit the densely packed arrangement (Bernhard and Buck, 2004, Fig. 8; Levin, personal communication 2004) observed for the Pietralunga filaments. An origin by microbial oxidation of methane is suggested for the carbonate containing filaments by the abundance of aragonite cements and negative d 13C values (as low as 42.8x). Although methane can be assumed to have been the main carbon source, the co-occurrence of pyrite and rare, small aggregates of barite suggests that hydrogen sulfide is also a component of the cold fluid seepage. 7.2. Stirone The responsibility of microbial activity in the formation of the three-dimensional textures, which compose the yellow Mg-calcite of the Stirone paleoseep, is suggested by morphological comparisons with modern and fossil analogues of these textures. Because yellow calcite lines pores and conduits (Fig. 7), it should represent the mineral precipitate directly associated with the gas seepage. Reticulate features morphologically similar to those of the yellow calcite (Fig. 8) are promoted by microbial activity in a number of different environmental settings. Examples include the modern anastomosing network produced by microbial mats of filamentous cyanobacteria in coastal lagoons of the southern Australia (Bauld et al., 1993) and Tunisia (Gerdes et al., 2000), and in atoll lagoons of French Polynesia (Sprachta et al., 2001). In particular, the last mentioned example shows how microbial sheaths of filamentous cyanobacteria can decompose and coalesce into a three-dimensional organic framework (Sprachta et al., 2001, Figs. 7 and 8). Another modern morphologic analogue is the bspongyQ texture produced by mats of the sulfur-oxidizing chemotrophic Beggiatoa on the Peru slope (Williams and Reimers, 1983). Fossil examples of microbially induced, three-dimensional textures include residual deposits of hematite, filled by authigenic micrite, in a Silurian paleoseep, Morocco (Barbieri et al., 2004), carboniferous cyanobacterial textures from the Pan-
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ther Seep Formation, New Mexico (Chafetz et al., 1993), and Miocene stromatolites described from the Monterey Formation, California (Williams and Reimers, 1983; Williams, 1984). The Silurian and Miocene examples are interpreted as fossil equivalents of modern Beggiatoaceae mats. The porous nature of the Stirone carbonates, with centimeter-sized cavities filled by siliciclastic sediments, and the microconduits cemented by yellow Mg-calcite with scattered glauconite grains (Fig. 7B), document a close intermixture between seep carbonate and the surrounding loose sediment. This indicates that the precipitation of the authigenic mineral occurred beneath the sediment/water interface. A comparison with modern methane-derived carbonates having a similar fabric with abundant cavities, such as the carbonate precipitates of the Black Sea (Luth et al., 1999; Peckmann et al., 2001) and the Adriatic Sea (Barbieri and Cavalazzi, unpublished data), suggests that the abundant microbial films and colonies, which line the walls and fill the pores of these modern carbonate structures, may have as a fossil analogue the yellow Mg-calcite of the Stirone paleoseep. The negative d 13C values (as low as nearly 25x, Table 2), measured in the micrite, reveal that the mineralization of the organic (microbial) compounds occurred in conjunction with the precipitation of methane-derived mineral phases. These mineral phases are promoted by methanotrophic archaea through anaerobic oxidation processes. The abundant filamentous and membranous structures incorporating glauconite, pyrite, and siliciclastic grains in the dark grey micrite (Fig. 9) which compose most of the Stirone seepcarbonate, are interpreted as biofilm and mucilaginous compounds of microbial nature.
8. Conclusions The microbial fabrics described from the studied Apennine deposits were rarely reported, and some of them are newly described from seep-carbonate occurrences. Some of the fabrics are interpreted as fossilized microbial remains. For others the evidence of microbial imprints is indirect and can only be hypothesized. Fossilized remains of microbial communities are (i) the three-dimensional, alveolar textures recognized
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in the yellow Mg-calcite, and (ii) sheaths and filaments trapping detrital material at Stirone paleoseep. The alveolar textures represent an example of complex and organized fabric which have only recently been documented in ancient (Silurian) seep deposits. Because of their location, around and within conduits and fractures, these mineralized textures are interpreted as the fossil analogue of the bacterial aggregations that line and fill cavities in modern, porous seep carbonates. Structures induced by microbial activity include the spheroids and rhombs of Pietralunga paleoseep. These structures consist of alternations of dolomite and calcium carbonate, are commonly associated with aragonite splay cements, and are inferred to be related to processes mediated by sulfate-reducing bacteria. They are commonly associated with clusters of large filaments for which a bacterial origin (Beggiatoa-type filaments) has recently been documented.
Acknowledgements We thank James L. Goedert, Jo¨rn Peckmann and two anonymous reviewers for constructive critiques and suggestions. We also thank Paolo Ferrieri for his excellent assistance during SEM investigations, Giorgio Gasparotto for microanalysis, and Patrizia Ferraresi for drawings. This study was supported by the MIUR (Cofin 2002) program bTerrestrial geological analogues as models for Mars exopaleontologyQ.
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