P. Clari

The role of bacteria in the formation of cold seep carbonates: geological evidence from Monferrato (Tertiary, NW Italy)

Abstract Methane-derived carbonate rocks (Lucina limestone and Marmorito limestone) crop out in Monferrato (NW Italy) and represent one of the first described examples of rocks produced at fossil cold seeps. These rocks, of Miocene age, consist of strongly carbonate-cemented siliciclastic sediments ranging in grain size from mud to coarse sand. The methane-related origin of Monferrato carbonates is based on: (a) outcrop-scale evidence: patchiness of cementation, chemosymbiotic fossil communities, presence of a network of polyphase carbonate-filled veins not related to tectonics; (b) isotope geochemistry: very depleted δ13C values, as low as −50‰ PDB; (c) peculiar petrographic features. Diverse microbial communities have been observed in present-day cold seeps. These communities include sulphate-reducing, sulphur-oxidizing and methane-oxidizing bacteria. The present work is focused on the identification and description of fossil evidence of such microbial activity in the Monferrato carbonates. Examples of fossilization of microbial structures are probably represented by pyritic rods and dolomite tubes referable to sulphur-oxidizing and to unspecified bacteria, respectively. Less direct but more abundant evidence has been found through petrographic and SEM studies of seep carbonates. Many features point to the presence of organic clumps or mats capable of trapping sediment and promoting carbonate precipitation: microcrystalline calcite peloids; dolomite crystals with irregular hollow cores; dolomite spheroids with dumbbell-shaped cores; laminated internal sediments lining cavities completely. All these features are interpreted to result from bacterially mediated, sedimentary and diagenetic processes and can therefore be considered as an additional evidence of ancient methane seeps.

Discontinuities in carbonate successions: identification, interpretation and classification of some Italian examples

Abstract The growing use of unconformities as bounding surfaces for new types of stratigraphic units seems not to be matched by a corresponding effort on process-oriented researches on unconformities themselves. This paper aims at giving a contribution to classification and understanding of discontinuities in carbonate successions from the outcrop perspective, based on some Italian examples. A number of integrated criteria (geometry, sedimentology, diagenesis, biostratigraphy) is proposed to recognise stratigraphic breaks. Old terms are discussed and sometimes redefined, while some new terms are introduced. The term discontinuity surface (DS) is proposed as a general one to indicate any stratigraphic interface where an interruption of sedimentation can be proved. A DS can be characterized as a firm ground or a rock ground depending on its coherence at the moment of the renewal of sedimentation. Polygenic and simple omission surfaces are separated within firm grounds if a recognizable gap is either present or not; a third kind of firm ground, revealed only by biostratigraphy, has been called hidden discontinuity surface. Two categories of rock grounds are also distinguished: hard grounds and inherited rock grounds if, respectively, generation of the DS and/or deposition of the overlying sediments took place in the same environment of the underlying sediments or not. A genetic interpretation for each type of discontinuity is proposed. The largest variety of DSs occurs in pelagic sediments. Some discontinuities (hard grounds and simple omission surfaces) are attributed to increased bottom-current activity during sea-level falls; others (polygenic omission surfaces and hidden DSs) are interpreted as submarine slide scars either with or without the overprint due to exposure on the seafloor. The DSs separating different facies (e.g. platform and pelagic ones) are interpreted to be due to tectonics. The relevance of the recognized DSs to sequence stratigraphy is briefly discussed.

Methane-derived carbonates and chemosymbiotic communities of Piedmont (Miocene, northern Italy): An update

Stable carbon isotope values of authigenic carbonate rocks in the Miocene terrigenous sediments of Piedmont indicate a methane-related origin. Some of these methane-derived carbonates (Lucina limestone) are characterized by the presence of abundant lucinid remains. Carbonate dissolution/precipitation and development of lucinid communities were related to bacterial methane oxidation, both aerobic and anaerobic. Anaerobic oxidation led to carbonate precipitation and production of sulfide, which sustained lucinid communities through chemosynthetic symbiotic bacteria. Aerobic oxidation of methane likely resulted in dissolution of carbonate skeletal grains. Several phases of carbonate precipitation, characterized by slightly different isotopic compositions, are recognizable in the limestones.

Authigenic carbonates in Upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy): Vestiges of an ancient gas hydrate stability zone?

A wide array of carbonate-rich rocks has been recognized in the Tertiary Piedmont Basin (NW Italy), hosted in lower Messinian slope deposits. Carbonate cements show negative δ 13 C values and positive δ 18 O values, suggesting that carbonate precipitation was induced by microbial degradation of methane produced from gas hydrate destabilization. Two groups of rocks have been distinguished: (1) Lucina -bearing mud breccias, representing the seafloor product of an ancient seepage site; and (2) Lucina -free concretions, originating below the sediment-water interface. Within this group, two subtypes have been further distinguished: stratiform concretions and cylindrical concretions. Stratiform concretions result from precipitation of dolomite in the pores of muddy sediments. Some of them display a brecciated structure; others show a network of septarian-like cracks that are empty, filled with sediments, or zoned carbonate cements. Their internal features are related to the formation of gas hydrates within the sediments and to their destabilization. Thus, these rocks mark a portion of the sedimentary column located within a (paleo) gas hydrate stability zone. Cylindrical concretions represent ancient fluid conduits related to the upward migration of CH 4 -rich fluids subsequent to gas hydrate destabilization. The carbonate-rich rocks of the Tertiary Piedmont Basin stand as one of the first examples of methane-derived rocks that record successive episodes of dissociation and re-formation of gas hydrates, and they provide precious elements to model the general evolution of a portion of the sedimentary column located within the hydrate stability zone.

Did Late Miocene (Messinian) gypsum precipitate from evaporated marine brines? Insights from the Piedmont Basin (Italy)

During the first stage of the Late Miocene Messinian salinity crisis (5.97–5.60 Ma), deposition of sulfates (the Primary Lower Gypsum) occurred in shallow silled peripheral subbasins of the Mediterranean undergoing restricted water exchange with the Atlantic Ocean. Fluid inclusions in Messinian selenite crystals from the Piedmont Basin (northwest Italy) have surprisingly low salinities (average of 1.6 wt% NaCl equivalent), suggesting that parent waters were depleted in Na + and Cl – compared to modern seawater. Modern gypsum from a Mediterranean salt work, in contrast, contains fluid inclusions with elevated salinities that match the normal evaporation trend expected for seawater. The salinity data indicate that the Messinian sulfate deposits from the Piedmont Basin formed from hybrid parent waters: seawater mixed with Ca 2+ and SO 4 2– enriched freshwaters that dissolved coeval marginal marine gypsum. Such mixed parent waters and complex recycling processes should be taken into account when explaining the genesis of other Messinian gypsum deposits across the Mediterranean Basin.

The Parona chaotic complex: a puzzling record of the Messinian (Late Miocene) events in Monferrato (NW Italy)

The Parona chaotic complex (PCC) from Monferrato (NW Italy) is a melange composed of carbonate-cemented blocks floating in a matrix of weakly consolidated mud breccias. It forms a minor (1 km2) chaotic sedimentary body enclosed in a larger Messinian melange that rests unconformably on older deposits and is followed, again unconformably, by Pliocene sediments. Three main facies have been recognized in the blocks. (a) Bioclastic rudstones and coquinoid grainstones of early Messinian age, composed of oligotypic assemblages of gastropods and the brackish-water bivalve Cerastoderma, mixed with shallow-water, marine biota (mollusks, corallinae algae, benthic foraminifers, echinoid fragments, rare ahermatypic corals). These sediments record shallow-water carbonate sedimentation with episodic brackish incursions and show a peculiar diagenetic overprint (selective dissolution of aragonitic grains and early dolomitic cements). (b) Monogenic breccias, composed of clasts of coquinoid grainstones floating in a dolomitic microcrystalline matrix. (c) Polygenic conglomerates, composed of rounded clasts of pre-Messinian pelagic sediments, of lower Messinian bioclastic carbonates and of Messinian evaporitic carbonates. Finally, a few blocks of strongly cemented micritic limestones, interpreted as methane-derived carbonates for the geochemical signature of the carbonate phases (strongly depleted in 13C), are present. The matrix enclosing the blocks is a mud breccia containing centimeter-sized clasts of marly sediments. Micropaleontological analyses have shown that lower Messinian and upper Tortonian planktic foraminifers are mixed with lower Miocene, Oligocene and Eocene forms. The PCC results from complex processes involving: (1) deposition of the peculiar sediments preserved in the blocks and (2) their dismemberment and mixing. As for the depositional history, composition of the blocks has allowed to sketch the following evolution. (a) Deposition of bioclastic rudstones and coquinoid grainstones that are interpreted as lower Messinian carbonate platform deposits for their strong compositional and diagenetical similarity with the lower Messinian carbonates cropping out in the Mediterranean region. The Monferrato bioclastic sediments are the northernmost example of Messinian shallow-water carbonates yet known. (b) Deposition of monogenic breccias. They formed during the evaporitic phase of the Messinian that was accompanied by a relative sea-level drop and by erosion of the previously deposited coquinas. Depositional and diagenetical processes during the evaporitic phase are also recorded by clasts of evaporitic carbonates in the polygenic conglomerates. (c) Deposition of polygenic conglomerates that are interpreted as upper Messinian post-evaporitic fan delta facies resulting from an important erosional phase. After deposition, this succession was dismembered in the blocks forming the present-day melange. Geometry, stratigraphic relationships, and internal characteristics of the PCC point to an origin related to gravity-driven phenomena, triggered by tectonics. However, the faunal mixing in the mud breccias and the occurrence of blocks of methane-derived carbonates suggest that mud diapirism also played a role for the genesis of the PCC. This latter mechanism, which has been poorly considered until now, could also have been effective in the genesis of Messinian chaotic deposits that extensively crop out in NW Italy.

Stress tolerant microfossils of a Messinian succession from the northern Mediterranean basin (Pollenzo section, Piedmont, Northwestern Italy)

High resolution quantitative data on calcareous nannofossil and foraminifer assemblages of the upper part of the Pollenzo section (Alba, Piedmont, northwestern Italy) are discussed in order to investigate the palaeoenvironmental evolution within the time interval immediately preceding the Messinian Salinity Crisis (MSC) and coeval to its onset. The studied succession, measuring approximately 24 m in thickness, consists of the seven uppermost cycles (Pm1-Pm7) of the Sant’Agata Fossili Marls. The latter are characterized by euxinic shale/ marl couplets that include a distinct carbonate-rich bed. Cyclical, orbitally controlled, fluctuations of warm/oligotrophic and temperate/ mesotrophic conditions are documented in the lower cycles Pm1-Pm4 on the basis of opposite abundances of planktonic microfossils. An increasingly stressed palaeoenvironment is recorded from cycle Pm5 upward by the higher abundance of stress tolerant taxa among the calcareous nannofossils (Reticulofenestra spp., Sphenolithus abies, Helicosphaera carteri, Umbilicosphaera rotula, and Rhabdosphaera procera), planktonic (Turborotalita quinqueloba) and benthic foraminifers (Bolivina spp., Bulimina echinata, and Cassidulina crassa). These data support the onset of the MSC within the time interval of the deposition of cycle Pm5. Small size planktonic foraminifers survived up to the lower part of cycle Pm7 while the more resistant calcareous nannofossils survive up to the upper part of cycle Pm7, positioned a few metres below the first gypsum bed (cycle Pg1). Micropalaeontological events (last peaks of Turborotalita multiloba and Orbulina universa, peak of S. abies) allow correlation of the lower Pm1-Pm4 cycles of the Pollenzo section with the upper cycles of the pre-evaporitic Mediterranean succession. RIASSUNTO [Microfossili tolleranti condizioni di stress ambientale del Messiniano pre-evaporitico di Pollenzo (Piemonte, Italia nordoccidentale] In questo studio vengono analizzati nuovi dati quantitativi sulle associazioni messiniane a foraminiferi e nannofossili calcarei della parte superiore delle Marne di Santa Agata Fossili nella sezione di Pollenzo (Alba, Piemonte, Italia nord-occidentale), costituita da alternanze cicliche di peliti laminate e di marne omogenee. Nell’intervallo studiato sono presenti anche livelli carbonatici cementati, e le associazioni diventano progressivamente impoverite, fino a scomparire a circa 1.5 m al di sotto del primo livello di gessi primari. Lo studio quantitativo ad alta risoluzione dei foraminiferi è stato condotto sui residui >125 μm e tra 63-125 μm. Nelle alternanze di peliti laminate e marne è stata evidenziata una iniziale ciclicità nelle variazioni percentuali dei microfossili calcarei. Le associazioni diventano poi dominate da taxa tolleranti condizioni di stress ambientale. Tra i nannofossili calcarei, taxa indicativi di condizioni fortemente eutrofiche e/o a salinità variabile sono rappresentati da Reticulofenestra spp., Helicosphaera carteri, Umbilicosphaera rotula e Rhabdosphaera procera, tra i foraminiferi da Turborotalita quinqueloba, Bulimina echinata, Cassidulina crassa e Bolivina spp. Drastici cambiamenti nelle associazioni a foraminiferi caratterizzano i cicli Pm4 e Pm5: scomparsa di esemplari e taxa >250 μm e >125 μm, impoverimento generale delle associazioni, aumento percentuale di C. crassa, Eponides cf. tumidulus, tipici di fondali ricchi di fitodetrito. Condizioni ambientali estremamente instabili e/o di stress sono inoltre suggerite dalle alte percentuali dei nannofossili calcarei Sphenolithus abies e di piccole Reticulofenestra spp. Foraminiferi planctonici (T. quinqueloba) di piccole dimensioni (<125 μm) sono presenti fino alla parte inferiore del ciclo Pm7, pochi metri al di sotto del primo banco di gessi (ciclo Pg1). I nannofossili calcarei risultano un poco più resistenti, sopravvivendo fino alla parte sommitale del ciclo Pm7. Gli ultimi picchi di abbondanza di Turborotalita multiloba e Orbulina universa nei cicli Pm1-Pm4 ed il picco d’abbondanza di S. abies alla base del ciclo Pm5 permettono la correlazione con i cicli sommitali della successione pre-evaporitica messiniana dell’area Mediterranea. Bollettino della Società Paleontologica Italiana, 52 (1), 2013 46 at basin margins, by primary sulphate evaporites of the Primary Lower Gypsum unit, representing here the Vena del Gesso Formation (Dela Pierre et al., 2011). In situ primary evaporites are not documented in the depocentral zones, but seismic data show that their time equivalent sediments are represented by shales (Irace et al., 2010). The transition between the shallow marginal area and the buried depocentre is exposed on the southern sector of the Piedmont Basin (Alba region) and was reconstructed in detail along a SW-NE transect. On the southern basin margins six Primary Lower Gypsum (PLG) cycles were recognised, composed of bottom grown-selenite layers; the sixth bed represents a distinct marker bed referred to as the Sturani Key-Bed (SKB) (Dela Pierre et al., 2011) and correlatable to the 6th PLG cycle (dated to about 5.84 Ma) (Lugli et al., 2010). Moving towards the NE (i.e., towards the depocentral zone), and in the Pollenzo section, only two gypsum beds are present below the SKB. They overlie a cyclical succession of euxinic shales, marls and carbonate-rich beds belonging to the Sant’Agata Fossili Marls. Because of their position with respect to the SKB, the uppermost three shales and marls cycles must be considered as the deeper water equivalent of the lower cycles of the PLG unit deposited in marginal setting (Dela Pierre et al., 2011). In the Pollenzo section, a thick slumped interval divides the Sant’Agata Fossili Marls into two portions. In the lower one, seven cycles were recognised; in the upper portion, above the slump, seven cycles (Pm1-Pm7) were recorded, each one including a distinct carbonate-rich bed. According to physical-stratigraphic position (with respect to the North of the Gello, Fanantello, Monte del Casino and Trave sections from central Italy (Negri et al., 1999; Manzi et al., 2007; Iaccarino et al., 2008; Roveri et al., 2008; Di Stefano et al., 2010), at latitude around 43° N. Recent studies in the area focused on the succession cropping out along the Tanaro River, near Alba (Lozar et al., 2010; Dela Pierre et al., 2011, 2012). This comprises the pre-MSC sediments, and overlying deposits, which testify the three stages of the MSC (Roveri et al., 2008), and evidence the lateral facies transition between marginal and distal settings. In the Pollenzo section the marine deposits, pertaining to the Sant’Agata Fossili Marls, are well exposed, separated in a lower and an upper interval by a thick slumped portion. Previously cited studies evidenced a precession-driven cyclicity, more evident in the interval overlying the slump, on the basis of lithological, petrographic, geochemical and micropalaeontological analyses. The upper part of the Sant’Agata Fossili Marls is overlain by the first local gypsum-bearing cycle, correlated to the 4th cycle of the Primary Lower Gypsum unit deposited in the marginal sector of the Messinian basin (Dela Pierre et al., 2011). Preliminary semiquantitative micropalaeontological analyses recorded very impoverished assemblages in the upper Sant’Agata Fossili Marls, dated to the calcareous nannofossil Zone MNN11b/c (Raffi et al., 2003) on the basis of Amaurolithus delicatus occurrence and to the foraminifer Non Distinctive Zone (Iaccarino, 1985) or to the Zone MMi13c (Lourens et al., 2004) on the basis of the occurrence of T. multiloba, of dominantly dextral coiling N. acostaensis and of Bulimina echinata (Lozar et al., 2010). A more detailed study of the sediments above the slump is here presented, in order to better understand the basin evolution in the time interval immediately preceding the MSC and coeval to its onset. A high resolution sampling and quantitative micropalaeontological analyses were performed on calcareous nannofossil and >125μm and 63-125μm foraminiferal assemblages. This procedure evidenced a cyclicity in the lowermost assemblages and a wider than expected foraminifer occurrence before the onset of the gypsum deposition as well as the importance of small planktonic and benthic taxa for the reconstruction of palaeoenvironmental changes. GEOLOGIC AND STRATIGRAPHIC SETTING During the Messinian, the TPB corresponded to a wide wedge top basin, developed above Alpine, Ligurian and Adria basement units (Rossi et al., 2009). The shallow marginal zones of the Messinian basin are presently exposed in the northern (Torino Hill and Monferrato) and southern (Langhe) sectors of the TPB, whereas the depocentral zones are buried below Pliocene and Quaternary sediments (Savigliano and Alessandria basins) (Fig. 1). The succession begins with deep water marine muddy sediments referred to as the Sant’Agata Fossili Marls (Tortonian-Lower Messinian) that record progressively more restricted conditions heralding the onset of the MSC (Sturani, 1978). This unit displays a precession-related cyclic stacking pattern, given by the repetition of euxinic shale/marl couplets and is followed, Fig. 1 Structural sketch of NW Italy (modified from Bigi et al., 1990). The arrow shows the location of the Pollenzo section. AM: Alto Monferrato; BG: Borbera-Grue; IL: Insubric Line; MO: Monferrato; TPB: Tertiary Piedmont Basin; TH: Torino Hill; SVZ: Sestri-Voltaggio zone. 47 D. Violanti et alii Stress-tolerant Messinian microfossils in NW Italy sample and by the Fisher’s diversity index (α) which takes into account the number of taxa as well as the number of specimens for each sample, using PAST ver. 1.77 (Hammer et al., 2008). Qualitative analyses were also carried out on the 45-63 μm fraction. The studied material is housed at the Museo di Geologia e Paleontologia dell’Università di Torino under repository numbers MGPT-PU134019 to MGPTPU134084.

Two Examples of the Geoheritage Potential of Piedmont (NW Italy): Verrua Savoia for the Present, Valle Ceppi for the Future

The aim of this article is to describe two “model” geosites of Torino province (Piedmont, NW Italy) in order to explain the state of the art of paleontological and stratigraphical geosite inventory and management in Piedmont. Collaboration between Torino University, local administrations, and industry resulted in the inauguration of the first geosite in Torino province, located in Verrua Savoia quarry, in October 2010. This site documents the geological record of the birth, life and death of a submerged mud volcano. Based on the success of this project, we propose the creation of another equipped area in Valle Ceppi, which constitutes an international point of reference for the study of Miocene molluscs. Piedmont is a region characterized by a heterogeneous patchwork of geological peculiarities that need to become public knowledge. Mismanagement of these areas may lead to their destruction, resulting in irreparable scientific and cultural damage. It should also be noted that the creation of equipped geosites plays an important role in the development of environmental awareness among the general public and local administrative entities and also increases the economic resources of the surrounding areas.

Virtual Tours Through Earth’s History and Palaeoclimate: Examples from the Piemonte (Northwestern Italy) Geoheritage (PROGEO-Piemonte Project)

A project developing virtual tours on the geoheritage of Piemonte aims to disseminate geological knowledge to the general public. Two of the most crucial geological themes, geological time and climatic and environmental change, are addressed. People will increase their awareness of Earth’s geodiversity and of the potential of Earth Sciences to improve our everyday lives. The tours will be developed with special attention to educational purposes and in lay language, and will also contribute to preserving vulnerable geoheritage.

Multimedia and Virtual Reality for Imaging the Climate and Environment Changes Through Earth History: Examples from the Piemonte (NW Italy) Geoheritage (PROGEO-Piemonte Project)

The dissemination of the knowledge of critical geological issues, such as past environmental and climate change and their correct placement in the geological time scale, is essential to rise the awareness of the huge potential impact of the Earth Sciences on the current and future generations. We selected two examples (the Marguareis and the Langhe and Monferrato areas) to disseminate these topics to a broad audience. Virtual tours, hosted on the PROGEO-Piemonte project website (www.progeopiemonte.it), have been developed with special care to educational purposes and made suitable for mobile devices. These allow us to overcome theoretical problems related to the complexity of geological items (spatial and evolutionary complexity, deep time dimension), with plain language explanations, detailed pictures and dynamic 3D models. The virtual visit overcomes practical difficulties, such as restricted access to high mountain or dangerous areas, and improves the access to detailed geological information at different scales, not readily available in the outcropping rocks. Dynamic or static information, stored on the website, are available on Google Earth or Google Maps in the field (mobile navigation) or at home (PC-based navigation). The virtual tours thus become an alternative to the visit on site, or a tool for deepening knowledge before or after the visit. This is especially helpful for schools, because virtual tours could be used in classroom with many different goals. The virtual tours, once accessed on-line by many visitors, will in turn push public administrators to realize geotouristic facilities on site and fund educational projects on these important issues.