Geochemical study of travertines along middle-lower Tiber valley (central Italy): genesis, palaeo-environmental and tectonic implications

Abstract

Several travertine deposits dating to the Pleistocene outcrop along the Tiber valley between Orte and Rome. Mineralogically, they are mainly composed of calcite; various lithofacies (stromatolitic, phytoclastic, and massive) were identified and relatively wide ranges of carbon (δ¹³C −8.11 to +11.42‰ vs. VPDB) and oxygen (δ¹⁸O +22.74 to +27.71‰ vs. VSMOW) isotope compositions were measured. The isotope and chemical compositions of water and free gases, in some cases associated with the travertines, were also measured. Carbon isotope data show that several samples fall in the typical range of thermogenic travertine, i.e., linked to the addition of deep inorganic CO₂. The oxygen isotope composition of the springs associated with the travertine deposits points to travertine precipitation by slightly thermal water of meteoric origin. In general, these travertines are in association with, or close to, mineralised groundwaters (with slightly acidic pH, low thermalism, and enrichment in sulphates or sodium chloride) and rich CO₂ gas emissions, the origin of which may be linked to decarbonation reactions. The travertine bodies are locally connected with crustal structural lineaments favouring the circulation of ascending deep CO₂-rich fluids. Conversely, some samples show isotopic connotations of meteogenic deposits, representing travertines formed mainly from soil biogenic or atmospheric carbon dioxide generally present in shallow groundwater or surface water. According to their morphology and isotope data, these travertines may be attributed to the sedimentary environment of waterfalls. These new geochemical and morphological data are integrated with those already available in the literature regarding the study area and contribute to shedding light on palaeo-environmental conditions in western-central Italy during the Quaternary.

Introduction

Active and fossil travertine deposits are widespread in central Italy (Panichi and Tongiorgi 1976; Minissale 2004): the formation of most of them was due to the massive CO₂ emissions which mainly occurred on the Tyrrhenian side of the region. The origin of this CO₂ has long been debated, although there is now a general consensus in favour of viewing it as the result of a mixture of mantle-degassing and limestone decarbonation (Minissale 2004; Chiodini et al. 2004). The study of such travertine deposits, being associated with CO₂ emission, can thus contribute to the debate on the origin of CO₂ in central Italy, as they potentially represent a valuable archive of several sources of CO₂ in places where gas emanations are no longer visible (Gibert et al. 2009). According to the origin of CO₂, Pentecost (2005) separated continental carbonate deposits into meteogene and thermogene travertines. In the former, CO₂ originates in the soil at low temperature and, being derived from the atmosphere and ‘soil respiration’, may ultimately be considered meteoric in origin. Although such deposits generally form in cold spring waters, they are occasionally found where water, circulating deep beneath the surface, is heated and rises to the surface as hot springs, containing negligible thermally generated CO₂ (e.g., Bormio, Italy; Pentecost 1995). Travertines, in which the carbon dioxide is derived mainly from hydrolysis, decarbonation, or magmatic degassing, are defined as thermogenic; the waters responsible for these deposits are frequently, but not necessarily, hot and rich in dissolved salts. This classification is useful to this study, because it is based on the origin of CO₂ and can easily be adopted using the carbon isotope composition, since thermogene and meteogene travertines are, respectively, enriched or depleted in ¹³C.

An alternative and widely used classification is based on an integrated approach, which involves the genetic environment and the presence of biota (Özkul et al. 2013; Capezzuoli et al. 2014; Gandin and Capezzuoli 2014). It comprises two main general classes of sub-aerial continental carbonate deposits: travertine and tufa. The former is generally hard and crystalline and related to abiotic processes in waters, typically hydrothermal in origin (Ford and Pedley 1996; Pedley 2009); they correspond more or less to the thermogene travertines defined by Pentecost (2005). ‘Tufa’ is more porous than travertine, and is rich in microphytes, macrophytes, and invertebrate and bacterial remains (Ford and Pedley 1996); it generally formed in waters of ambient temperature and may correspond to meteogene deposits. When this classification is applied, some uncertainties may occur regarding the interpretation of the deposits in cooled thermal waters, where tufa-like deposits may be precipitated when the water temperature decreases with distance from the spring source; the term ‘travitufa’ has been suggested for these types of travertine (Capezzuoli et al. 2014). In the present study, as we are mainly interested in the origin of carbon dioxide and in the distinction between deep and surface circulating fluids, it is convenient to discuss any carbonate deposits in terms of their thermogenic or meteogenic nature. In addition, this nomenclature allows us to describe and discuss these deposits without any preconceived ideas about precipitation conditions, such as water temperature (Claes et al. 2015).

The carbon and oxygen isotope compositions of travertines provide useful information which may reveal both the source of the CO₂ and precipitation conditions (e.g., temperature and isotope composition of precipitation water) (Friedman 1970; Panichi and Tongiorgi 1976; Gonfiantini et al. 1968). Panichi and Tongiorgi (1976) derived an empirical linear equation linking the carbon isotopic composition of CO₂ and travertine carbonate, and this approach was later applied elsewhere to calculate the isotopic composition of the parental CO₂ of a travertine and to identify the carbon source (Minissale et al. 2002; Minissale 2004; Piccardi et al. 2008; Kele et al. 2008, 2011; Sierralta et al. 2010; Rodríguez-Berriguete et al. 2012; De Filippis et al. 2012). In contrast, data on travertine oxygen isotopes have mainly been used in palaeohydrological and palaeoclimatological research (Pazdur et al. 2002; Minissale et al. 2002; Andrews 2006).

Several travertine deposits outcrop along the Tiber valley, representing an ideal archive for stratigraphic and geochemical studies. The valley is a morpho-tectonic depression of extensional origin, filled with syn-extensional sedimentary and volcanic successions of Pliocene–Quaternary age up to 1 km thick (Funiciello and Parotto 1978; Barberi et al. 1994; Cavinato et al. 1994; Mancini and Cavinato 2005; Funiciello and Giordano 2008). Earlier isotopic studies on these travertines date back to the 1970s (Manfra et al. 1976; Barbieri et al. 1979). More recently, Minissale et al. (2002) recognised in the Tiber valley the dividing line between a western sector, where deposits exhibit characteristics typical of a thermogenic (thermal spring) origin, and an eastern sector, in which they show meteogenic (low-temperature) characteristics. In their hydraulic reconstruction, the above authors also found that the travertine-precipitating waters east of the Tiber valley have shallower flow paths than those to the west (Minissale et al. 2002).

In this study, we present a morphological characterisation and describe the mineralogical and isotopic features of several Quaternary travertine deposits of the Tiber valley, in the section between Orte and Rome (Fig. 1). The isotope and chemical compositions of water and free gases, in some cases associated with both fossil and active depositing travertines, were also studied. The aim of this research was to shed light on the palaeo-environmental and geological conditions that allowed such extensive travertine deposition in this context during the Quaternary. The relationship between travertine deposits, tectonics, the volcanic activity of the Tyrrhenian sector, and climatic forcing are discussed, considering circulation and interaction between surface and deep waters and the carbon sources which contributed to the formation of travertine.

Fig. 1

Geological map of the Tiber valley (from SGI-ISPRA geoportal available at http://sgi.isprambiente.it/geoportal/catalog/main/home.page) showing location of travertine sites and sampling points (travertines, waters, and gas emissions)

Geological and hydrogeological setting

The study area (Fig. 1) corresponds to the middle and lower valley of the Tiber river between Orte and Rome, west of the central Apennines (central Italy), the structural setting of which results from the onset and eastward migration of an SW–NE directed extension related to the opening of the Tyrrhenian Sea back-arc basin (Patacca et al. 1990; Cavinato and De Celles 1999; Carminati and Doglioni 2012). The middle and lower stretches of the Tiber valley do in fact correspond to two adjoining NNW–SSE-directed, morpho-tectonic depressions of extensional origin: the Paglia-Tevere Graben and the Rome Basin, filled by syn-extensional sedimentary and volcanic successions of Pliocene–Quaternary age up to 1 km thick, at present crossed axially by the Tiber (Funiciello and Parotto 1978; Barberi et al. 1994; Cavinato et al. 1994; Mancini et al. 2004; Mancini and Cavinato 2005; Funiciello and Giordano 2008). These successions overlie with angular unconformity Meso-Cenozoic (Trias-late Miocene) pelagic carbonate and siliciclastic successions, piled into thrust sheets during the Tortonian–Messinian (Cavinato and De Celles 1999; Cosentino et al. 2010; Mattei et al. 2010). The basins are bounded to the east by the ridge of the Amerini–Sabini–Lucretili–Tiburtini Mountains, where the carbonate and siliciclastic substrate are exposed, and to the west and south by the Quaternary volcanic complexes of the Sabatini Mountains and the Alban Hills; the basins are also separated longitudinally by the alignment of the Soratte and Cornicolani Mountains, a 30-km-long carbonate ridge partly buried by the Plio-Quaternary cover. The structural style has thus been dominated since the late Zanclean by grabens and horsts bounded by major NNW–SSE-trending normal faults (the ‘Apennine’ trend), and longitudinally cut by minor NE–SW-running normal and transtensional faults (Funiciello and Parotto 1978; Faccenna and Funiciello 1993). Minor N–S-trending, right-lateral slip faults, and NNW–SSE-trending oblique normal faults have been identified in the areas of Mt. Soratte-Fiano and the Cornicolani Mts. (Faccenna 1994; De Rita et al. 1995).

The infill of the Paglia–Tevere and Rome basins is characterised by prevalent offshore marine and transitional (fluvio-deltaic) terrigenous deposits which filled the subsiding basins during the Zanclean–Calabrian interval, but which have been replaced since the late Early Pleistocene (1.4–1.0 Ma) by syn-uplift continental sediments and distal sub-aerial volcanic products from the Roman Magmatic Province (Mancini et al. 2004; Mancini and Cavinato 2005). The magmatic activity along the western margin of central Italy, the consequence of crustal thinning due to the extension of the Tyrrhenian area, generated the large stratovolcanoes and volcanic complexes of Vulsini (0.6–0.15 Ma), Vico-Cimino (0.4–0.1 Ma), Sabatini (0.6–0.04 Ma), and the Alban Hills (0.6–0.02 Ma), with the eruption of large volumes of potassic and ultrapotassic lavas and pyroclastics (De Rita et al. 1993; Barberi et al. 1994; Funiciello et al. 1994; Karner et al. 2001; Sottili et al. 2004; Peccerillo 2003; Funiciello and Giordano 2008; Marra et al. 2009). Intra-Apennine volcanism also occurred in the same period, and consists of some monogenic pyroclastic centres and lava flows, e.g., San Venanzo (0.3 Ma), Cupaello (0.6–0.3 Ma), and Polino (0.3 Ma), characterised by ultrapotassic melilititic (kamafugites) composition (Peccerillo 2003). Travertines are found at the top of, or interbedded with, fluvial deposits and pyroclastics (Figs. 2, 3).

Fig. 2

Geological cross sections showing the stratigraphic architecture of the middle Tiber valley, based on the geological map of Mancini et al. (2004). Cross sections highlight stratigraphic relations among travertine units and fluvial siliciclastic deposits and pyroclastics. It should be noted that the cross sections intercept some of the sites of sampling. Sampling site number in brackets

Fig. 3

Geological cross sections of the lower Tiber valley in the centre of Rome, based on the geological map of Funiciello and Giordano (2008) and borehole data from Ventriglia (2002). Geographical location of the cross sections is shown in Fig. 2. Sampling site number in brackets

The structural and stratigraphic complexity of the whole study area controls its hydrogeological setting, with complexes consisting of clastic heterogeneous marine to transitional deposits or alluvial deposits (Boni et al. 1988). The first complex is defined as poorly cemented sandstone and mud interbedded with gravels, conglomerates, and distal pyroclastics; the second consists of recent alluvial deposits (10–20 m thick for the tributaries and more than 60 m for the main Tiber valley; Mazza and La Vigna 2011) which hosts shallow groundwater circulation. An underlying carbonate unit of the Meso-Cenozoic substrate constitutes the main regional aquifer. Travertine deposits make up local hydrogeological complexes, with medium–high permeability due to porosity in phytoclastic/waterfall facies and local fracturing or karstification in massive travertine facies. Several thermal and cold mineral springs are scattered across the valley and along the boundaries of the limestone and volcanic formations (Capelli et al. 2012). The cold springs are generally composed of Ca(Mg)–HCO₃. The thermal or warm waters (T > 15 °C) commonly show compositions between Ca(Mg)–HCO₃ and Ca–SO₄ (the latter component deriving from the dissolution of the Triassic anhydrite layers at the base of the Mesozoic carbonate bedrock) or between Ca (Mg)–HCO₃ and Na–Cl (probably due to the dissolution of Miocene–Pliocene marine evaporites embedded in the shallower parts of the flow path) (Minissale et al. 2002; Minissale 2004). Currently, only a few springs precipitate travertine in central Italy (e.g., Tivoli, Viterbo, Saturnia; Minissale 2004): they are characterised by CO₂-rich gas emissions, temperatures higher than 20 °C (up to 65 °C at Viterbo spring), and Ca–HCO₃ composition.

Tiber valley travertines

There are many travertine outcrops in the middle and lower Tiber valley (Figs. 2, 3), the detailed lithochronostratigraphy of which is shown in the recent geological maps of the Paglia–Tevere and Rome basins (Mancini et al. 2004; Funiciello and Giordano 2008).

Middle Tiber valley

In the middle valley (Paglia–Tevere Graben), the oldest travertine units of Emilian age (approximately 1.4–1.2 Ma) are: (1) the travertine deposits of the Giove Formation (Mancini et al. 2007), which are up to 40-m thick, regressive on coastal marine sands, and outcrop at the south-western margin of the Amerini Mountains (Fig. 1); (2) travertines overlying the 1.3Ma ignimbrite produced by Cimino Mt. stratovolcano activity (Mancini et al. 2004). Both these units are fluvio-lacustrine in origin and are mostly composed of phytoclastic travertines with prevailing mudstone and wackestone.

Middle Pleistocene travertine deposits are more widespread in the Paglia–Tevere graben and are represented by two main lithostratigraphic units, the Grotte Santo Stefano Formation and the Fiano Formation, lying on the fluvial terrigenous terraces (Fig. 2), and by minor carbonate deposits interlayered within the alluvial formations (Mancini et al. 2004).

The Grotte Santo Stefano Formation outcrops near structural alignments, and is dated to 350–250 ka on the basis of its stratigraphic correlation with encasing pyroclastics (Mancini et al. 2004). It is mostly composed of phytoclastic and phytohermal travertines referable to the fluvial barrage depositional model proposed by Pedley (2009), e.g., at the ‘Poggio degli Ulivi’ site (Colle Verrucola) (Fig. 2). Older freshwater travertines (about 500–450 ka) are found on the topmost strata of the fluvial Graffignano Formation, and are referred to as fluvio-palustrine facies. This lithostratigraphic unit was sampled at the ‘Cava dei Preti’ (Fig. 2) and ‘Ponzano’ sites, which are characterised by cemented sands with calcified nodules and tubules, and at the ‘Ponte Sfondato’ site, which represents a floodplain environment typified by slightly cemented calcareous sand.

The Fiano Formation outcrops at: (1) the ‘Fiano Romano’ site (Fig. 2), where it is composed of 40-m-thick phytoclastic and hydrothermal travertines gently wedging out eastward; and (2) the Farfa-Tevere confluence, where encrusted biohermal travertines related to a waterfall environment (‘cascade model’ of Pedley 1990) overlie fluvial sands and gravels (e.g., at the ‘Cascata’ and ‘Ponte Ferrovia’ sites; Figs. 2, 5d). At the latter site, stromatolitic and phytoclastic facies both occur (Fig. 5a), with well-stratified stromatolitic travertines around 1 m in thickness characterising the summit of the outcrop (Fig. 5b); travertine presenting phytoclastic facies, in which the phytoclasts were deposited either in situ or after limited transport, is recorded in the lower middle part of the deposit. These structures exhibit a certain degree of orientation due to water flow (Fig. 5c). Some examples of travertine tubes are visible (Fig. 5d), consisting of protuberant cylinders of travertine forming natural spouts at the top of small waterfalls, and considered a sub-type of prograding cascades (Pentecost 2005). The association of these features can be attributed to the sedimentary environment of a waterfall. Oncolitic facies have also been identified, next to biohermal facies, suggesting moderately flowing water in a fluvial setting.

Smaller deposits are found buried under the Holocene alluvial plain, as at the ‘Piana Bella’ site, where a massive travertine (i.e., mudstone and wackestone, with a matrix-supported texture; Fig. 4d) has been identified, or the ‘Acqua Forte’ site, where phytoclastic travertines are actively depositing at the emergence of the spring, from mineralised-CO₂ degassing water.

Fig. 4

Field and hand specimen images of travertines from Tiber valley. a Gentle slope of the travertine outcrop, that can be interpreted as a slope deposit, possibly laterally or vertically transitional with waterfall and pool association, of ‘Poggio degli Ulivi’ site; b phytoclastic travertine from ‘Poggio degli Ulivi’ site; c site ‘Cascata’, phytoclastic travertine formed by tubular, coarse fragments mostly of encrusted reeds; d hand specimen of massive travertine from ‘Piana Bella’ site; e travertine deposit of ‘Valle Giulia’; f laminated travertines, with semi-translucent calcite veins, from ‘Valle Giulia’ site

Lower Tiber valley

Two well-defined depositional settings are known within the Rome Basin, one at Tivoli, due to tectonic basin infill (Faccenna et al. 1994, 2008) and the other in Rome, where travertines are interbedded with fluvial deposits and pyroclastics.

Near Tivoli, a town about 25 km east of Rome, near the Alban Hills volcano, is one of the largest Italian travertine deposits: the Lapis Tiburtinus, i.e., the stone of Tibur, the former name of Tivoli, from which the term travertine was derived in medieval times. Here, a remarkably thick (up to 90 m) plateau of compact, massive lacustrine, and hydrothermal travertines record the complex polycyclic infilling of the Acque Albule basin within the latest Middle Pleistocene–Holocene interval (Faccenna et al. 1994, 2008).

In Rome, continental carbonates are mainly due to the infilling of a multiple stack of fluvial-incised valleys, interbedded with well-dated pyroclastic units along the valleys of the Tiber and Aniene rivers. The oldest travertine deposits in Rome are known as the Santa Cecilia Formation, correlated with Marine Isotope Stages 16–15 (Funiciello and Giordano 2008; Marra and Florindo 2014), and are mostly found in subsoil. The prominent Valle Giulia Formation outcrops in the northern-central part of Rome (Fig. 3): it is correlated with Marine Isotope Stages 14–13 (about 550–500 ka), is mainly composed of phytoclastic and phytohermal travertines typical of a fluvial environment, and features large-scale cross bedding. Although travertine samples from the ‘Valle Giulia’ site are similarly characterised by the presence of plant fragments, they are also laminated and yellow-brownish in colour, sometimes with semi-translucent calcite veins (Fig. 4e, f). Minor travertine deposits, such as those at the ‘Prima Porta’ site, with a massive, compact aspect, are found interbedded within the late Pleistocene–Holocene alluvium.

Travertines and local structural setting

The travertines of the Middle and Lower Tiber valley are generally located along local tectonic structures. The oldest units of Early and Middle Pleistocene age (the Giove and Grotte Santo Stefano Formations) run along the main NNW–SSE-trending lineaments, such as the main boundary SW-dipping normal faults of the Amerini Mountains and the Soratte–Cornicolani ridge. They widen to the SW, dipping along the regional topographic slope and transversally to the axis of the Apennine Chain, and are interfingered with pyroclastic units.

The younger units of Middle–Late Pleistocene and Holocene age (~200 ka), such as the Fiano Formation and the Tivoli travertines, are related to N–S-trending dextral strike-slip faults and NNE–SSW-striking oblique faults cross-cutting the Mt. Soratte-Cornicolani horst and bounding the small tectonic depression of the Acque Albule Basin (Faccenna 1994; De Rita et al. 1995; Faccenna et al. 2010). The different geographical distribution of travertine deposits, from Early–Middle to Middle–Late Pleistocene, is thought to be related to a local change in the active tectonic regime in western-central Italy (at the periphery of the volcanic complexes) from a pure NE–SW-directed extension to a N–S-trending transtension. This change also controlled the last stages of volcanism and hydrothermal circulation (Faccenna et al. 2010).

Methods

The locations of travertine, water, and gas sampling are shown in Fig. 1, which includes both travertine samples reviewed from the literature and travertines, waters, and gases collected for this study and never examined previously.

The carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios of travertine were measured from ~0.2 mg powder samples made with the modified phosphoric acid method of McCrea (1950). Isotope analysis was generally carried out on bulk carbonate; in sites 4 and 21, nodules and tubules were selected for analysis. A Finnigan Kiel II—Carbonate Device interfaced with a Finnigan MAT 252 mass spectrometer was used for analyses.

The samples were also analysed for their mineralogical composition on a Philips PW1840 diffractometer (CuKa/Ni: 40 KV and 20 mA). Semi-quantitative mineral composition was determined with XPowder12 free-ware software (available online at http://www.xpowder.com).

Four locations were also characterised by a vigorous-free gas phase that was collected with the inverted bottle technique (Meents 1960) and stored in pre-evacuated 25-ml stainless steel canisters sealed with two vacuum stop-cocks. For rapid measurement of chemical composition, a Dräger X-am 7000 equipped with an IR sensor was used in the field. After off-line purification in a vacuum line, CO₂ was analysed for carbon isotope composition on a Finnigan MAT 252 mass spectrometer.

Chemical and isotopic analyses were also performed on five water samples, with temperature, pH, redox potential (Eh), and electrical conductivity (EC) measured in the field with a multi-parameter probe; alkalinity was measured by titration with 0.05 N HCl. In the laboratory, major anions (Cl and SO₄) and cations (Ca, Mg, Na, and K) were determined by ion chromatography (Metrohm) on filtered and filtered/acidified samples, respectively; the analytical precision, as %RSD based on replicate analysis of standard solutions, was <5%. The ¹⁸O/¹⁶O isotopic ratios of water samples were determined with the equilibration technique (Epstein and Mayeda 1953).

The results of isotope analyses on travertine, water, and CO₂-free gas are reported in the usual delta (δ) notation, representing relative deviation in parts per mil (‰) with respect to the following international standards: VPDB for carbon isotopes and VSMOW for oxygen isotopes. Standardisation for calcite samples was conducted with laboratory standards calibrated against NBS-18 and NBS-19 standards, and water samples according to laboratory standards calibrated against SMOW, GISP, and SLAP. The internal standard precision of isotopic analysis for both δ¹³C and δ¹⁸O was ±0.1‰.

Results

Travertine facies

In the Tiber valley, where travertines are located at varying topographic heights (from about 20 up to 400 m a.s.l.) (Table 2), four main facies were identified: phytoclastic, stromatolitic, coated grains (as described by Pedley 1990; Pentecost 2005; Ozkul et al. 2014), and massive travertine when there were no particular structures to be identified on a macroscopic basis. The association of these facies, integrated with field observation of the morphology of the deposits, indicates their depositional environment (Table 1).

Table 1 Overview of the Tiber valley travertine deposits examined for this study, with description of sites and samples. Stable isotope and mineralogical compositions based on XRD analyses are reported (tr trace amount)

Fig. 5

a General view of ‘Ponte Ferrovia’ site; b stromatolitic travertine with sub-horizontal laminae; c in the lower-middle part of ‘Ponte Ferrovia’ site is visible a more ‘chaotic’ system, with tubules and corrugates laminae; d from the same site of (c), a travertine tube not completely formed or eroded

The phytoclastic facies consist of fragments of carbonate encrustations on vegetable supports (Fig. 4a–c), with a detrital texture. The fragments include leaf imprints, stems (sometimes branching), twigs, tree trunks, and cylindrical clasts, which were encrusted during and/or after transport to their depositional setting (Pedley 1990). The dimensions of the phytoclasts vary from tree trunks about 5 cm in diameter to small twigs and leaves of a few millimetres; in some cases, large cavities characterise tree-trunk moulds. Phytoclastic facies formed in palustrine-to-shallow lacustrine conditions as well as several fluvial settings. These deposits can also be found in shallow braided rivers with a barrage system produced by the destruction of bryophytes and macrophytes. This facies also occurs in waterfall environments. In some cases, calcified stems were found oriented in life position (phytohermal facies).

The stromatolitic facies are composed of several laminae, with thicknesses of up to 1 cm for each lamina, and a sub-horizontal to convex-up laminated texture (Fig. 5b). The depositional environment of these dense, laminated deposits is generally associated with a sedimentary environment of fast-flowing water, such as stepped cascades in a fluvial setting.

Oncoids are the main component of coated grains facies; they are small grains (from <1 to 5 mm in diameter), spherical or spheroidal in shape, the growth of which is actively triggered by microbial activity. This facies is characterised by a grain-supported texture with a certain amount of matrix in the ‘Cascata’ and ‘Ponte Ferrovia’ sites, whereas the coated grains in the ‘Ponte Sfondato’ site are matrix-supported. Oncoids are typical of rivers and sluggish flow regimes.

‘Massive travertine’ consists of a hard, quite dense crystalline material, which appears structureless or with horizontally bedded and inclined layers, a few centimetres to decimetres thick. It may occur as mudstone and wackestone, with a matrix-supported texture. This kind of material is associated with low-energy environments, such as pools, shallow lakes (e.g., the ‘shallow lake-fill deposits’ at Tivoli, in Chafetz and Folk 1984), or floodplains in fluvial systems.

Stable isotope geochemistry and mineralogy of travertines

Table 1 lists the carbon and oxygen isotope values for the analysed travertine samples. The mean isotope values (Table 2) are plotted in Fig. 6, in which the new data are integrated with those already available in the literature regarding the study area (Manfra et al. 1976; Minissale et al. 2002; De Filippis et al. 2013).

Fig. 6

Stable oxygen and carbon isotope compositions of different travertine facies in the Tiber valley (error bars indicate the standard deviation). δ¹⁸O and δ¹³C of travertines of central-southern Italy (data after Ascione et al. 2013; Raimondi et al. 2015; Vignaroli et al. 2016; Minissale 2004 and reference therein) are represented by the field drawn with dashed line

Table 2 Mean values of stable oxygen and carbon isotope compositions and relative standard deviations (σ) of travertines in the Tiber valley

Isotopic values fall between −8.11 and +11.42‰ (average + 4.05‰) vs. VPDB for carbon and between +22.74 and +27.71‰ (average + 25.11‰) vs. VSMOW for oxygen. In Fig. 6, which reveals no evident correlation between δ¹³C and δ¹⁸O values, the Tiber valley travertines are grouped according to facies identified in the field or described in the literature. Samples of phytoclastic and stromatolitic facies, as well as calcified nodules and tubes, tend to group towards the most positive δ¹⁸O values in the diagram, whereas the travertines with massive travertine facies are characterised by more negative δ¹⁸O values. A large variation is shown by δ¹³C, which ranges between −8.11 and +11.42‰ VPDB, but no univocal relationship was found between the different facies and the carbon isotopes: although the massive travertine facies are characterised by positive values of δ¹³C (from +3.52 to +11.42‰), the phytoclastic facies span the entire range of carbon isotope values. For comparison, Fig. 6 also displays the field encompassing most of the isotopic values for travertines from central-southern Italy reported in the literature (data after Gonfiantini et al. 1968; Panichi and Tongiorgi 1976; Buccino et al. 1978; Ferreri and Stanzione 1978; Carrara 1994; Minissale et al. 2002; Minissale 2004; Ascione et al. 2013; Raimondi et al. 2015; Vignaroli et al. 2016). It shows that the Tiber valley travertines mostly fall in the field of Italian travertine, although their δ¹⁸O values are located towards the area of the most positive values and most carbon isotope data plot towards the higher end of the diagram.

Based on XRD analysis, the studied travertines are composed of almost pure calcite; aragonite was not detected in any sample (Table 1). In the samples from ‘Valle Giulia’ and ‘Ponte Sfondato’, a small amount of quartz (lower than 10%) was identified. The sample from ‘Ponzano’ is composed of quartz (~40%), calcite (~20%), feldspars (~30%), and phyllosilicates (~10%).

Water and gas geochemistry

The δ¹⁸O_w values, chemical compositions, and chemical-physical parameters (T, pH, Eh, EC) of water associated with travertines from ‘Acqua Forte’, ‘Prima Porta’, and ‘Tivoli’ are listed in Table 3. Groundwater can be classified as Ca–HCO₃ and Ca–SO₄–HCO₃, although the ‘Prima Porta’ sample shows slight enrichment in sodium and chloride; chemical data were used to calculate the Mg/Ca ionic ratio, which ranges between 0.17 and 0.93, because this parameter affects carbonate mineralogy. All waters displayed high electrical conductivity (up to 4250 µS/cm) and a slightly acidic pH, ranging between 6.1 and 6.5. On the basis of their temperatures, the waters from ‘Acqua Forte’ and ‘Prima Porta’ are classifiable as ‘cold’ (T < 20 °C) and the remaining samples as warm (T = 20–24 °C). The redox potential values (Eh) of the gas vent waters are negative (up to −350 mV in the ‘Cretone’ sample), with the exception of that from ‘Acqua Forte’, which shows a slightly positive value (28 mV). The δ¹⁸O_w values of groundwaters range from −6.2 to −7.3‰ and are compatible with the isotope composition of local meteoric water (Giustini et al. 2016).

Table 3 Chemical and oxygen isotope composition of water samples and carbon isotope composition of free gases associated with mineralised waters

The ‘Acqua Forte’, ‘Prima Porta’, and ‘Tivoli’ sites exhibit vigorous-free gas emission, identified as CO₂-dominated according to the chemical composition of free gases. The carbon isotopic composition of this CO₂ ranges from −0.51 to −2.86‰ vs. VPDB, with the δ¹³C_CO2 values of the ‘Acqua Forte’ and ‘Prima Porta’ gas vents slightly higher (−0.51 and −0.68‰, respectively) than those measured in the Tivoli area (~−2.8‰) (Table 3). Despite such differences, all these values are comparable with the δ¹³C of several CO₂-dominated gas vents in central Italy, the origin of which is generally ascribed to a mixture of mantle-degassing and limestone decarbonation in varying proportions (Chiodini et al. 2004; Minissale 2004).

Discussion

Carbon-stable isotopes and origin of CO₂

Stable isotope analyses have turned out to be important in improving our understanding of the genesis of travertines (Friedman 1970; Turi 1986; Andrews et al. 1997; Fouke et al. 2000; Minissale et al. 2002; Uysal et al. 2007; Kele et al. 2008, 2011). In particular, carbon and oxygen isotope data can help to determine the origin of parental CO₂ and precipitation conditions (temperature and isotope composition of precipitation water), respectively.

The carbon isotopic composition of travertine is primarily controlled by the δ¹³C values of possible CO₂ sources. These sources (and their δ¹³C values) may be atmospheric (δ¹³C ~ −8‰ VPDB), organic (generally lower than −20‰ VPDB; Cerling et al. 1991), or derived from limestone decarbonation, which generates CO₂ with δ¹³C = −1 to +2‰ VPDB (Turi 1986), and mantle/magma which releases CO₂ with δ¹³C = −5 to −8‰ VPDB (Javoy et al. 1986). As the δ¹³C ranges of the various CO₂ sources are greater than that induced by fractionation during travertine deposition, carbon isotopic compositions can provide insights into the origins of fossil travertine CO₂ (Minissale et al. 2002). However, problems such as incorrect identification may arise due to mixing; secondary processes may have a further impact on the carbon isotope composition of the precipitating carbonate, such as CO₂ degassing or biogenic activity, especially downstream of spring vents (Fouke et al. 2000; Kele et al. 2011). Such processes generally result in the preferential loss of light isotopes and the consequent increase in the δ¹³C of the carbonate rock (Fouke et al. 2000; Kele et al. 2011). The effect of microbial activity is still subject to debate and may produce carbon fractionation with either positive or negative shifts with respect to equilibrium value (Guo et al. 1996; Fouke et al. 2000; Andrews 2006); photosynthesis preferentially removes ¹²CO₂ from water, leaving calcite enriched up to 6‰ of ¹³C (Guo et al. 1996).

On the basis of carbon isotope data, and according to the classification of Pentecost (2005), both thermogene and meteogene travertines were identified along the Tiber valley (Fig. 6). The thermogenic type includes both massive and phytoclastic macroscopically distinguishable varieties of travertine, indicating that carbon isotopes are not closely connected with the lithofacies, whereas meteogene travertines consist of phytoclastic, stromatolitic facies and coated grains, nodules, and tubules, indicating the progressive input of organic CO₂. Thermogenic travertines with phytoclastic facies may also be defined as ‘travitufa’, representing samples precipitating from cooled hydrothermal waters, away from the spring orifice. They have a porous aspect and contain abundant fragments of plants, but their isotopic signature indicates carbon dioxide of deep origin. Most of the samples exhibit carbon isotopic compositions more positive than +7‰ VPDB, values which may be explained not only by the deep origin of carbon but also environmental factors, especially CO₂ degassing, in view of the fact that travertines are probably not always sampled close to the spring orifice.

The δ¹³C values of some samples fall in the intermediate range of thermogenic and meteogenic travertine (−2/+3‰ VPDB), suggesting that a mix of inorganic CO₂ and soil CO₂ generated by plant respiration (practically ubiquitous) may have occurred.

The empirical equation

$${\delta ^{{\text{13}}}}{{\text{C}}_{{\text{parental}}\;{\text{C}}{{\text{O}}_{\text{2}}}}}={\text{ 1}}.{\text{2 }} \times {\delta ^{{\text{13}}}}{{\text{C}}_{{\text{travertine}}}}-{\text{ 1}}0.{\text{5}}$$

(1)

proposed by Panichi and Tongiorgi (1976) calculates the δ¹³C of CO₂ released from water during travertine deposition (${\delta ^{{\text{13}}}}{{\text{C}}_{{\text{parental}}\;{\text{C}}{{\text{O}}_{\text{2}}}}}$) at the spring orifice. The calculated carbon isotopic compositional range for parental CO₂ varies between ~−20 and ~+3‰ (Table 2; Fig. 7). The position of the spring orifice of most of the studied travertines is unknown, i.e., we cannot exclude the possibility that they were not affected by secondary effects (which generally tend to increase the δ¹³C of the travertine), so that Eq. (1) generally overestimates the δ¹³C of the parental carbon dioxide. Despite this limitation, thermogenic travertines show values of parental CO₂ in the range of −6 to +3‰, matching the δ¹³C of CO₂ of the present-day gas vents in the Tyrrhenian sector of central Italy (δ¹³C −9 to +2‰ VPDB; Minissale 2004), whose origin may be ascribed to varying proportions of both mantle/magma degassing and limestone decarbonation (Chiodini et al. 2004; Minissale 2004). The δ¹³C of free gases associated with the ‘Prima Porta’, ‘Acqua Forte’ and ‘Tivoli’ sites also falls within the range of the values of the gas vents of central Italy (Table 3; Fig. 7); the first two show ¹³C-enrichment probably due to a lower percentage of mantle-degassing and/or biogenic CO₂ input, compared with the CO₂ gas emissions of the upper Tiber valley (Vaselli et al. 1997) and the Tivoli area. Comparing ${\delta ^{{\text{13}}}}{{\text{C}}_{{\text{parental}}\;{\text{C}}{{\text{O}}_{\text{2}}}}}$ with the isotope composition of free gases associated with the ‘Prima Porta’ and ‘Tivoli’ sites (Table 3; Fig. 7), a significant difference appears (δ¹³C_{parental CO2} is about 4‰ enriched in ¹³C with respect to free gases). This discrepancy may partly be attributed to the overestimation above-mentioned for Eq. (1), indicating that the samples were located at a certain distance from the spring orifice (Fouke et al. 2000; Kele et al. 2008, 2011). In the case of the ‘Acqua Forte’ site, where the travertine sample was collected from the spring orifice, the ¹³C enrichment is lower (~1.5‰).

Fig. 7

Carbon isotopic composition of parental CO₂ calculated by Panichi and Tongiorgi equation (1976) (white diamonds), and free gases associated with ‘Prima Porta’, ‘Acqua Forte’, and ‘Tivoli’ sites (grey squares). Grey diamond represents the parental CO₂ calculated for the spring’s orifice sample from ‘Acqua Forte’ site. Ranges of δ¹³C values of possible CO₂ sources are from Clark and Fritz (1997)

Variable amounts of soil CO₂ are considered to be responsible for the observed variations in travertine ${\delta ^{{\text{13}}}}{{\text{C}}_{{\text{parental}}\;{\text{C}}{{\text{O}}_{\text{2}}}}}$ < −10‰ VPDB. In fact, the biogenic CO₂ originating in the soil may be transferred to shallow groundwater, due to the high permeability of the alluvial deposits of the Tiber valley. In addition, even a small contribution of this organic CO₂ can greatly affect the final isotopic composition of CO₂ because of its low ¹³C content (δ¹³C_organic < −20‰ VPDB; Cerling et al. 1991).

Oxygen stable isotopes and calculation of palaeo-temperature of water

The oxygen isotope composition of the Tiber valley travertines has a relatively large variability (δ¹⁸O = +21.90 to +27.71‰ VSMOW), although most of the samples fall within a narrower range between +24 and +25‰ VSMOW. Thermogenic travertines show greater variability than meteogenic deposits, although the mean δ¹⁸O values of the two groups are very similar, at +25.05‰ and +25.35‰ VSMOW, respectively (Fig. 6). The phytoclastic, stromatolitic facies, and samples of coated grains, nodules, and tubules are generally characterised by higher oxygen values than massive travertines, indicating that phytoclastic stromatolitic facies form at lower temperatures.

In general, the relatively wide-ranging δ¹⁸O values of the travertines indicate precipitation from isotopically different waters at varying temperatures. Precipitation temperature can be calculated if the oxygen isotope compositions of the carbonate and the water of deposition are known values, which in this case must be assumed. The water oxygen isotope value may be that typical of meteoric origin, represented by the water analysed at ‘Acqua Forte’, ‘Prima Porta’, and the Tivoli area, i.e., −6‰/−7‰ VSMOW (Table 3). Crucial assumptions must be made about isotopic equilibrium and the selection of one of the calcite–water equations relating oxygen isotopes to temperature. It is currently unclear which, if any, of the published calibration equations, represents true equilibrium. It is also unclear precisely in which travertine facies calcite precipitates at equilibrium. Kele et al. (2015) contend that very little or no fractionation due to kinetic effects occurs close to the spring vents or in pools; vent and pool travertines show higher mineral-water oxygen isotope fractionation than the values reported by Kim and O’Neil (1997), whereas many of them fit the curves published by Coplen (2007), Tremaine et al. (2011), and Affek and Zaarur (2014). In the present study, one of the equations reported by Kele et al. was selected to calculate the range of water temperatures at the time of precipitation by means of the oxygen isotopic composition of travertine:

$${{\text{1000ln}}{\alpha _{{\text{CaC}}{{\text{O}}_{\text{3}}} - {{\text{H}}_{\text{2}}}{\text{O}}}}={\text{17}}\frac{{{\text{1}}{{\text{0}}^{\text{3}}}}}{T} - {\text{26}}},$$

(2)

where ${\alpha _{{\text{CaC}}{{\text{O}}_{\text{3}}} - {{\text{H}}_{\text{2}}}{\text{O}}}}$ is the fractionation factor between calcite and water, and T is the temperature in K. The temperature ranges from 5 to 35 °C, with an average of about 20 °C (Table 4). This approach does not take into account the different facies represented by the samples of this study, i.e., kinetic effects and enrichment of ¹⁸O downstream because of water evaporation (especially evident in sub-aerial precipitation). These effects would produce a higher isotopic composition of the carbonate and consequently a lower temperature. The temperatures listed in Table 4 must be considered possible minimum estimates of the true values. A comparison of the calculated temperatures and those measured at ‘Acqua Forte’, which was the only active depositing spring sampled in this study, reveals that Eq. (2) estimates this context quite well: the oxygen isotope compositions of the spring water and travertine, which were −6.82 and +25.49‰, respectively, give a deposition temperature of 16.4 °C, which matches the directly measured temperature of 17 °C.

Table 4 Palaeo-temperatures of formation of Tiber valley travertines

The calculated temperatures are approximately consistent with those recorded in mineralised groundwaters sampled in the Tiber valley. A temperature as low as 5 °C, that recorded for the ‘Prima Porta’ spring, which is associated with a typically thermogenic fossil travertine, may be the result of those isotopic effects which lower the true temperature.

Of note is the striking correspondence of the temperatures calculated and directly measured for the hydrothermal springs of the Tivoli area (also called the Acque Albule springs) (Table 4), which contributed to the precipitation of the large ‘tiburtinus’ plateau. Several hydrogeological and geochemical studies have been carried out in this context (e.g., Petitta et al. 2011; Carucci et al. 2012), highlighting the fact that deep high-salinity groundwater, rising along tectonic lines, mixes with shallow fresh groundwater, decreasing the temperature of the water in the travertine aquifer. A similar model may be depicted for the Tiber valley because of the large capacity of the shallow aquifer hosted in the alluvial deposits.

The above palaeo-temperature calculations do not take into account the possibility that the water cycle in the area may have varied in the past, particularly during colder periods when the δ¹⁸O values of palaeo-spring waters were lower than those presently existing as a temperature effect on isotope fractionation in precipitation. In this case, the calculated temperatures may be further overestimated, taking into account that a 1‰ difference in δ¹⁸O_water corresponds to a difference in water temperature of about 5 °C, according to the equation of Kele et al.

The range of temperatures calculated for the Tiber valley travertines indicates that the thermogenic travertines were formed by slightly thermal water (at a temperature more than 5 °C above the mean annual air temperature, according to the definition of Goldscheider et al. 2010) and not by hot springs (>37 °C; cfr. Pentecost 2005), which are often related to thermogenic deposits.

A further constraint for the water temperature at the time of precipitation is the mineralogical composition of the travertines, which are almost pure calcite. Several studies (Friedman 1970; Sturchio 1990; Folk 1994; Fouke et al. 2000) have demonstrated that aragonite usually forms at temperatures ranging from 30 to 60 °C, although particular cases of pure calcite precipitation have been detected at high water temperatures (~67 °C), especially when the precipitation rate was observed to be high (Jones et al. 1996; Kele et al. 2008). In the Tiber valley travertines, the absence of aragonite indicates that carbonate precipitation occurred from waters with temperatures lower than 30 °C, as also indicated by the isotope data. In addition, according to Fischbeck and Müller (1971), aragonite precipitation becomes significant when the water Mg/Ca ratio is ~2.9; in our water samples, this ratio is lower than 0.9 (Table 3).

Travertines, tectonic activity, and palaeo-environmental remarks

The geochemical features and depositional characteristics of travertines are controlled by the balance between several factors, including tectonic/seismic activity, climatic variations, fluid discharge, chemical composition of travertine-depositing waters, mixing of deeply derived waters and shallow waters, CO₂ levels, and microbiological activity.

Travertine deposition is common in areas affected by extensional tectonics, where the circulation of fluids which have the potential for travertine deposition is controlled by damage zones resulting from normal faulting (Altunel and Hancock 1993; Çakır 1999; Brogi 2004; Altunel and Karabacak 2005; Uysal et al. 2007; Mesci et al. 2008; Selim and Yanık 2009; Kele et al. 2011; De Filippis et al. 2012; Brogi et al. 2012, 2014; Özkul et al. 2013, 2014; Van Noten et al. 2013). In contrast, travertine precipitation along major strike-slip fault zones is less common and/or studies examining travertines related to strike-slip faulting are limited (Faccenna et al. 2008; Temiz et al. 2013; Çolak Erol et al. 2015). To clarify the relationship between tectonics and travertines (and consequently between tectonics and the origin of CO₂-rich fluids) along the Tiber valley, Fig. 8 shows the spatial distribution of meteogenic and thermogenic travertines in the study region. It is evident from this geological sketch that most of the thermogenic travertines (δ¹³C > +0.5‰) are distributed along the border of the main volcanic districts and are aligned along the NNW–SSE-oriented normal faults which compose the structure of horsts/grabens in this area (Funiciello and Parotto 1978; Faccenna and Funiciello 1993). Out of this aligned distribution, the ‘Fiano Romano’ and ‘Prima Porta’ thermogenic travertines seem to be directly connected to the Pleistocene N–S strike-slip shear zone, south of the Monte Soratte structure, already partially described by Faccenna (1994). In contrast, the ‘Cerdomare’ deposit (site 5, Table 2) is the most eastern thermogenic travertine and outcrops for about 4 km in the Apennine direction, along the southern edge of an intramontane basin within the Meso-Cenozoic limestone of the Sabini–Lucretili–Tiburtini Mountains, and probably related to the rise of deep fluids migrating through a normal buried fault (Manfra et al. 1976).

Fig. 8

Location map of meteogene (δ¹³C from −7.8 to +0.5‰, yellow squares) and thermogene (δ¹³C from +0.5 to +11.2‰, red stars) travertines and of low-magnitude earthquakes (M < 5) epicenters recorded in the 2005–2015 period at shallow and deep crustal levels (up to 30-km depth) (Italian Seismological Instrumental and parametric Data-basE ISIDE, http://iside.rm.ingv.it, last accessed May 2015)

In the present study, matching other cases in the literature, most of the travertine deposits occur along the normal faults of the Tiber graben, although a few interesting examples (‘Prima Porta’, ‘Fiano Romano’, and the well-known ‘Tivoli’ deposit) seem to be related to strike-slip faulting.

The close spatial relationship between faults, thermogenic travertine deposits, current gas emissions, and mineralised groundwaters, in general, have demonstrated that tectonic discontinuities, which are common in deformed carbonate bedrock, are natural pathways allowing meteoric waters to descend into the subsurface and hydrothermal fluids to rise to the surface. As a consequence, the presence of thermogenic travertine may indicate the migration of deep-seated gases, probably associated with warm, saline waters, that is generally conveyed towards the surface by tectonic discontinuities. The precipitation of mineral species such as calcite, oxides, and silica from supersaturated hydrothermal fluids may take place over hundreds of years, decreasing the permeability of these tectonic structures (Sibson 1987); however, fluid circulation can be guaranteed by persistent faulting and fracturing, which progressively open new conduits for fluid flow, as implied by tectonic activity (Brogi et al. 2010, 2012). In this regard, the fact that the Tiber valley springs have remarkable saline contents would imply that the tectonic fissures could be sealed by mineral deposition; this sealing process may be effectively contrasted by the recent activity of the related faults. The diffuse present-day seismicity recorded by the low-magnitude earthquakes (M < 5) in the study area (Fig. 8) may corroborate the view that tectonic activity allowed tectonic fissures to remain open and maintain mineralised fluid circulation.

The relationship between the Tiber valley travertine deposition, volcanic activity, and glacial/interglacial cycles was explored by comparing the ages of the travertines with the main eruptive events of the nearby volcanic districts, as well as with palaeo-climatic indicators (Fig. 9). As previously mentioned, the travertines sampled in this study belong to Middle Pleistocene lithostratigraphic units and were stratigraphically positioned at about 550–500 and 400–200 ka; the most recent travertine outcrops have been referred to the Late Pleistocene and Holocene (Mancini et al. 2004; Fig. 9). Radiometric U-Th dating is available in the literature for the ‘Civita Castella’, ‘Fiano Romano’, and ‘Tivoli’ deposits (133, 170, and 134.8–219.5 ka, respectively; Faccenna and Funiciello 1993; Tuccimei et al. 2001; Minissale et al. 2002) (Table 2).

Fig. 9

Comparison between travertine dating (Faccenna and Funiciello 1993; Tuccimei et al. 2001; Minissale et al. 2002; Mancini et al. 2004), volcanic activity, and palaeoclimate indicators. Curves I and II represent atmospheric carbon dioxide-level variations (Luthi et al. 2008 ) and LR04 benthic isotope stack record (Lisiecki and Raymo 2005); Marine Isotope Stages (MIS) were defined by Lisiecki and Raymo (2005). Time-frame of volcanic activity of the Roman Province (Peccerillo 2003) and intra-Apennine volcanic centres (Laurenzi et al. 1994) is shown; square symbols represent the age of main eruptive events (Perini et al. 2004; Marra et al. 2009; Palladino et al. 2010; Sottili et al. 2010)

Figure 9 shows the variation in atmospheric CO₂ levels, the δ¹⁸O climatic stack curve from deep sea cores, the time range of large-scale volcanic events in the Roman Province (distinguished by volcanic districts), and the depositional time range of the studied travertines for the last 600 ka. It is evident that the time-frame of travertine formation is so approximate that it barely allows definite associations between climatic/volcanic events and travertine deposition to be identified. However, Fig. 9 does provide certain indications. First, the most recent travertines are entirely thermogenic; their formation is correlated with the last intense Sabatini event and does not show any clear correspondence with climatic phases, which alternate between glacial and interglacial from 200 ka towards the Last Glacial Maximum. As regards the travertines deposited during the periods 350–250 and 550–500 ka, both thermogenic and meteogenic deposits were recorded, corresponding to the intense volcanic phases of most of the Latium volcanic complexes, especially the main eruptions of the Sabatini complex, indicating a relationship between events in that area and the development of travertine springs. The climatic relationship again seems uncertain, since the time-span of formation of any travertine corresponds to both glacial and interglacial climatic conditions (travertines formed between 350–250 ka encompass the MIS 8 and 9 glacial/interglacial, and those formed between 550–500 ka the MIS 13–14 glacial/interglacial couple). Recent studies (Frank et al. 2000; Faccenna et al. 2008; De Filippis et al. 2013) have shown that the formation of travertine depends on climatic factors, particularly variations in aridity and humidity. These studies contend, for instance, that thermogenic travertine deposition may be reduced in arid glacial periods, due to scarce water seepage at deeper levels and consequent decreased hydrothermal circulation. Conversely, in the warm humid periods of interglacial stages, travertine deposition may be favoured by the increased availability of water which facilitates deep-shallow water mixing, as in the case of the Tivoli deposits. However, this general response of travertine development to climate is less obvious in our study.

Considering the above evidence, Fig. 10 shows a possible palaeo-environmental reconstruction of the middle Tiber valley. Corresponding to the structural discontinuities, which intersect the carbonate bedrock and define the Tiber graben, CO₂-rich fluids of deep origin (metamorphic and partly mantle/magma degassing, as revealed by carbon isotopes) found suitable routes for their ascent to the surface, where they mixed with shallow groundwaters. The thermal nature of the deep fluids may be partly obscured by large volumes of cold regional, alluvial, and volcanic aquifer waters, as indicated by the temperature estimation based on travertine oxygen isotopes, and by direct measurement of mineralised spring waters dispersed in the valley; these spring waters consequently often show a hypothermal character. At the points of spring emergence, large travertine bodies were formed by precipitation from supersaturated, CO₂ degassing waters, and eventually grew to form natural dams, upstream of which lacustrine and palustrine pools formed and the related lithofacies were deposited. Some deposits were later buried under the recent alluvial deposits and are thus hidden along the valley.

Fig. 10

Schematic palaeo-environmental reconstruction (not to scale) of the middle Tiber valley. 1 Alluvial deposits, 2 Plio-Pleistocene marine and continental deposits, 3 pyroclastic rocks, 4 Meso-Cenozoic limestone, 5 travertine, 6 travertine body buried under the recent alluvial deposits, 7 travertine deposits east of valley (e.g., ‘Poggio Moiano/Cerdomare’, site n.5, Table 2) related to the rising of deep fluids

In this reconstruction, meteogenic travertines were solely formed at the palaeo-confluence of the Farfa and Tiber rivers, on the present-day left bank of the Tiber, at the point where it bends (Fig. 10). These travertines are located on both banks of the Farfa at higher elevations than the present-day banks, and represent a high-energy environment: the association of facies (Fig. 4) attributes these travertines to a sedimentary waterfall environment. The fault bordering the graben probably generated a vertical topographic displacement which developed into a waterfall system suitable for carbonate precipitation, the steep topography subsequently being self-generated by travertine development.

Conclusions

Field observations and geochemical analyses of groundwaters, gas emanations, and travertines along the Tiber valley (central Italy) have highlighted both the origin of CO₂-rich fluids, from which the travertines were deposited, and their relationship with the tectonic setting.

The carbon isotopic composition of the travertines, ranging from −8.11 to +11.42‰ vs. VPDB, identifies two groups of deposits and consequently two different sources of CO₂. Thermogenic travertines (δ¹³C ~ >0.5‰) are related to CO₂ of prevailingly inorganic origin, mainly from limestone decarbonation. These travertines are clearly aligned along, or located close to, the main tectonic structures and active faults, allowing the rapid rise of fluids circulating at depth. In addition, the close spatial relationship between thermogenic travertines, gas emissions, mineralised groundwaters, and faults indicates that the tectonic discontinuities represent local routes of intense deep fluid circulation. According to our data, the ultimate origin of these travertines was affected by the contribution of deeply derived CO₂-dominated fluids to meteorically derived recharge waters, which diluted the solute contents and reduced the deep water temperature; the δ¹⁸O values of these travertines in fact indicate that the calcite was predominantly formed in warm waters, rarely exceeding 30 °C. The spatial distribution of the epicenters of present-day seismic events is consistent with the locations of the most important faults in the study area and with the locations of thermogenic travertines, gas emissions, and mineralised groundwaters, indicating that tectonic activity ensures the ongoing permeability of faults and fractures. Comparison of the time of travertine deposition with the occurrence of the main volcanic and climatic events taking place during the Pleistocene reveals that volcanism, especially the Sabatini volcanic events, may have had an influence in travertine formation that climate may have not, at least at the level of our dating resolution.

In the meteogenic travertines (δ¹³C ~ <0.5‰), which are located at the confluence of the Tiber with the Farfa, variable amounts of soil CO₂ are considered to be responsible for the observed depletion in ¹³C. A waterfall system was probably the original sedimentary environment of these travertines, the growth of this morphology probably being favoured by the vertical displacement of a fault bordering the Tiber graben. Oxygen isotopes show that both types of travertines, meteogenic and thermogenic, were formed in warm or slightly thermal waters.