Using stable isotopes in deciphering climate changes from travertine deposits: the case of the Lapis Tiburtinus succession (Acque Albule Basin, Tivoli, Central Italy)

1 Introduction

The global triggers of glacial and interglacial cycles during the Quaternary have been largely established. However, little is still known about local responses to such global controlling factors, as well as the relationships between the latter and regional causes (Regattieri et al., 2015). Travertine deposits, controlled by climate, tectonic, and volcanic activity, thus represent the best archive for investigating these dynamics (Mancini and Capezzuoli, 2024). The term “travertine” is derived from the Italian travertino. It is related to the Latin words Lapis Tiburtinus, meaning “stone of Tibur,” representative of the Roman name for the present-day village of Tivoli. Such material was quarried by the Romans in the Tivoli area and was mostly used as ornamental and building stone (e.g., the Colosseum, AD 70–80) from the first century BC. Travertine corresponds to calcium carbonate deposits related to non-marine waters supersaturated in calcium bicarbonate-rich waters and associated with hydrothermal fluid circulation (Chafetz and Folk, 1984; Pedley, 2009; Jones and Renaut, 2010; Capezzuoli et al., 2014; Mancini et al., 2019a; Mancini et al., 2019b; Mancini et al., 2019c; Mancini et al., 2021). In recent decades, travertine deposits have attracted a variety of scientific interest for different reasons as they are widespread in continental settings and important archives of palaeo-environmental proxies and also because they offer the possibility of reconstructing past ecosystems, tectonic settings, and sedimentary regimes of the environment in which they developed (Capezzuoli et al., 2014; Brogi et al., 2018). Moreover, these deposits could also be used to address the relationships between deposition and related bacterial activity as a proxy for geogenic/biogenic CO₂ fluxes associated with the precipitation of calcium carbonate deposits and as reservoir analogues for hydrocarbon reservoirs (Claes et al., 2015, 2016; Ronchi and Cruciani, 2015; Schröder et al., 2016; Mancini et al., 2019a; Mancini et al., 2020). However, identifying in detail the impact of climate changes in different depositional settings and understanding the interplay between global, regional, and local trends that potentially affect a depositional system are always a challenging frontier for geosciences. Hence, travertine deposits represent a particularly interesting case study, as their deposition is controlled by a complex set of physical, chemical, and biological processes (Della Porta, 2015; Della Porta et al., 2017a), particularly influenced by the local tectonic setting in which they develop (Hancock et al., 1999; Capezzuoli et al., 2014; Brogi et al., 2021a; Brogi et al., 2021b). Despite their complexity, recent studies have hypothesized climatic control of travertine precipitation (Frank et al., 2000; Crossey et al., 2006; Uysal et al., 2007; Faccenna et al., 2008; Uysal et al., 2009; Brogi et al., 2010; Anzalone et al., 2017; Ricketts et al., 2019; Giustini et al., 2020).

Faccenna et al. (2008) noticed that the onset of the Lapis Tiburtinus travertine deposition in the Acque Albule Basin was coeval with the last interglacial of the Late Pleistocene (MIS5 sensu Shackleton, 1969; Ruddiman, 2006), starting at the onset of the last interglacial ca 115 ka and ending at the beginning of the last glaciation, 30 ka ago. Moreover, Anzalone et al. (2017), analysed the oxygen isotope record on a 30-m thick Lapis Tiburtinus travertine core and noted a precessional control on the travertine deposition.

Travertine deposition can be influenced by global changes in climate (Ricketts et al., 2019). However, despite the singular attempt by Ricketts et al. (2019) to correlate the oxygen isotope record of travertine to global trends, no other studies have investigated the potential of travertine systems to record climate changes since most of the abundant literature focuses on the endogenic causes leading to travertine deposition (Crossey et al., 2006; Uysal et al., 2007, 2009; Brogi et al., 2010; Faccenna et al., 2008; De Filippis et al., 2013; Castorina et al., 2023) than on external forcing (Frank et al., 2000; Mancini et al., 2021).

In addition, travertine systems are extremely heterogeneous in terms of lateral facies changes, even considering the relatively spatially limited settings of precipitation. This characteristic feature of travertine often hampers the possibility of correlating different records even over small distances and probably discourages the development of significant paleoclimate studies on these peculiar settings. In this paper, we address this frontier challenge, focusing our attention on the Lapis Tiburtinus travertine succession of the Acque Albule Basin, Tivoli, central Italy (Faccenna et al., 2008; Della Porta et al., 2017b; Mancini et al., 2021), to explore the sensitivity of travertine systems to global climate changes. In particular, the aims of the paper are threefold: 1) to evaluate the potential of travertines to record global oxygen isotope shifts and, thus, glacial and interglacial cycles; 2) to identify the relationship between global forcing and the local controlling factors on travertine precipitation through the comparative study of oxygen and carbon isotope ratios; 3) to evaluate the carbon isotope record associated with the groundwater level changes compared with CO₂ flux and volcanic activity of the Colli Albani volcanic complex.

2 Geological setting

The Acque Albule Basin represents the eastern part of the Roman Basin, bounded to the north and east by the Neogene Apennine fold and thrust belt (i.e., Cornicolani–Lucretili–Tiburtini Mountains), by the Aniene River and Pleistocene Colli Albani volcanic complex to the south, and by the Tiber River valley to the west (Figure 1A). The extensional tectonic activity that occurred during the Pliocene–Quaternary interval controls the basin’s evolution (Conato et al., 1980; Faccenna et al., 2008; De Filippis et al., 2013; Della Porta et al., 2017b; Cardello et al., 2022). The eastern side of the Apennine belt is characterized by Meso-Cenozoic carbonate rocks that form thrust sheets with a piggy-back sequence from the Late Miocene to the Pliocene epochs, while the western side is influenced by extensional tectonic activity occurring from the Middle Miocene to the Pliocene and related to the opening of the back-arc basin (Tyrrhenian Sea) (Funiciello et al., 1976; Chiodini et al., 2004; Acocella and Funiciello, 2006; Rossetti et al., 2007; Mancini et al., 2014). The Acque Albule Basin (Figure 1B) is located 30 km east of Rome and is characterized by a flat topography (Faccenna et al., 2008; De Filippis et al., 2013; Di Nezza et al., 2015; Anzalone et al., 2017). The morphological depression of the Acque Albule Basin is influenced by moderate subsidence of 0.4 mm/yr (Faccenna et al., 2008; De Filippis et al., 2013) and is controlled by Pleistocene–Holocene tectonic activity mainly associated with N–S-striking right-lateral and transtensional to normal faults (Cosentino and Parotto, 1986; Corrado et al., 1992; Faccenna et al., 1994; Gasparini et al., 2002; Billi et al., 2006; Faccenna et al., 2008; De Filippis et al., 2013). The succession filling the Acque Albule Basin is related to Plio-Pleistocene alluvial, lacustrine, and epivolcanic deposits that accumulated on top of a Meso-Cenozoic marine carbonate succession (De Rita et al., 1995; La Vigna et al., 2013a; La Vigna et al., 2013b). On top the Plio-Pleistocene deposits a travertine succession developed. Known as Lapis Tiburtinus travertine, this carbonate developed during the Late Pleistocene (115–30 ka) and covers an area of 28 km² with a maximum thickness of 90 m (Faccenna et al., 2008). The Lapis Tiburtinus travertine succession is characterized by bedded to sub-horizontal units with onlapping geometries on the underlying erosional surfaces dipping toward the south, east, and west and separated by erosional surfaces covered by palaeosoils and conglomerate deposits (less than 1 m) (Faccenna et al., 2008; De Filippis et al., 2013). The Acque Albule Basin and its hydrogeological regime are influenced by the presence of the Meso-Cenozoic carbonate succession to the north, northeast, and east of the basin (Cornicolani–Lucretili–Tiburtini Mountains) (Carucci et al., 2012; La Vigna et al., 2013a, La Vigna et al., 2013b) and by the damage zone associated with the N–S striking faults (Billi et al., 2006) that affect these deposits. Three different hydrostratigraphic units comprise the local groundwater conceptual flow model (Carucci et al., 2012; La Vigna et al., 2013a; La Vigna et al., 2013b). From base to top, 1) the first is related to the Meso-Cenozoic limestone of the Apennine belt, cropping out in the surrounding area of the basin and lowered by the activity of normal faults, 2) the second unit, developed on top of the Meso-Cenozoic limestone, is characterized by Pliocene–Pleistocene sand-claystone marine deposits, and 3) the third is represented by the fractured travertine deposits, strongly affected by anthropogenic activities. Several CO₂-rich thermal springs, mainly located close to the N–S-striking right-lateral faults, are characterized by a temperature of 23 °C and pH values between 6.0 and 6.2, while the Aniene River, located to the south of the basin, is the main run-off destination of all the superficial thermal waters (Pentecost and Tortora, 1989; Minissale et al., 2002; Minissale, 2004; Carucci et al., 2012; La Vigna et al., 2013a; La Vigna et al., 2013b; Di Salvo et al., 2013).

Figure 1

Figure 1. (A) Geological map of Central Italy with an indication of the study area (square) and a digital elevation model of the study area (modified from Tarquini et al., 2023). (B) Geological map of the Acque Albule Basin (modified from Mancini et al., 2021).

3 Materials and methods

Two boreholes (Sn1 and Sn2) crossing the entire Lapis Tiburtinus travertine succession of the Acque Albule Basin (Figures 2A, B) were drilled with a Beretta T47S. The Sn1 borehole was drilled in the north-west of the quarried area to a depth of 70 m, while the Sn2 borehole, located in the south of this area, reached a depth of 42 m. The quarried area lies in a topographic depression, with a topographical height of 31–78 m above sea level. The top of the Sn1 borehole occurs at 68.2 m above sea level, while the Sn2 borehole is situated at 57.4 m. A detailed core description of both the Sn1 and Sn2 cores was carried out which focused on macroscopic characteristics. Nine different lithofacies associations were recognized: six related to travertine deposits and four to non-travertine deposits (Figures 3A–I). For lithofacies identification and description, we refer to Mancini et al. (2021). The age constraints used for dating and mutual correlations are from Faccenna et al. (2008), while the geogenic CO₂ fluxes associated with travertine depositional units are based on Mancini et al. (2020). The CO₂ discharge rate was obtained by dividing the CO₂ discharge by the area of the hydrological basin that feeds the springs. From both cores, a total of 59 (118) pure calcite powder samples, collected every 2–2.5 m on average, were acquired by a dental drill. Visible crystalline cements were avoided. Stable oxygen and carbon isotopic signatures were determined at the Friedrich-Alexander-Universität (Erlangen–Nürnberg, Germany). Carbonate powders were analysed with a Thermo Finnigan V and mass spectrometer (Thermo Scientific Inc.), coupled with a Gasbench II device, after attacking them with 100% phosphoric acid at 70 °C. The analytical standard deviations for δ¹³C and δ¹⁸O are 0.04‰ and 0.05‰, respectively. The values obtained are reported in per mil (‰), relative to Vienna Pee Dee Belemnite (V-PDB), by assigning a δ¹³C value of +1.95‰ and a δ¹⁸O value of −2.20‰ to the standard NBS19.

Figure 2

Figure 2. (A) Stratigraphic log and pictures of Sn1 and Sn2 boreholes showing the lithofacies, divided in travertine and non-travertine deposits, and depositional units; (B) map showing positions of Sn1 and Sn2 boreholes (modified from Google Earth, 2023).

Figure 3

Figure 3. (A–I) Pictures showing the different lithofacies related to travertine and non-travertine deposits. (A) Laminated deposit; (B) shrubs; (C) intraclast deposit; (D) crystalline crust; (E) coated reed and phytoclast; (F) non-laminated deposit; (G) palaeosoil; (H) sand, sandstone, and conglomerate; (I) claystone and marlstone deposits; (J,K) pictures showing the characteristics of the karstified travertine deposits observed in the Sn2 borehole.

4 Results

4.1 Lithofacies

The classification and characterization of the different deposits observed in the Sn1 and Sn2 cores are reported in Table 1. The different lithofacies types are subdivided into “travertine” and “non-travertine” deposits (see also Figures 2, 3).

Table 1

Table 1. Description of the travertine and non-travertine lithofacies associations observed in the Sn1 and Sn2 cores.

4.2 Depositional units, travertine volume, and geogenic CO₂ fluxes

The Acque Albule Basin covers 28 km², while travertine deposits have a total volume of 1.1 km³ and can be divided into 10 different depositional units labelled “U1” to “U10” from base to top (Table 2 and also Figure 2) (Faccenna et al., 2008; De Filippis et al., 2013). The travertine succession studied in the Sn1 and Sn2 cores develops on top of different clastic deposits. In the Sn1 core, travertine succession occurred on top of claystone and marlstone deposits related to lacustrine and alluvial plain depositional environments, while the travertine succession of the Sn2 core developed on top of fluvial sand, sandstone, and conglomerates related to a fluvial depositional environment in relation to the activity of the Aniene River. The 10 depositional units were correlated in both Sn1 and Sn2 cores based on Erthal et al. (2017), Della Porta et al. (2017b), and Mancini et al. (2021) and were integrated with the previous subdivision and age dating reported by Faccenna et al. (2008). The following units were differentiated:

- Unit 1 is the first travertine depositional event in the studied boreholes. In the Sn1 core, this unit is 20 m thick, while Sn2 is only 3 m thick. Claystone layers rich in pyrite are interdigitated with non-laminated lithofacies in the Sn1 core, while, in Sn2, Unit 1 is composed exclusively of non-laminated travertine lithofacies. Such lithofacies association testifies to deposition in a low-energy setting such as a pool or shallow lake with freshwater input which was sometimes affected by anoxic conditions. This unit was deposited between 115 ± 5 and 99 ± 5 ka with a travertine volume deposit of 0.16 km³ associated with a geogenic CO₂ flux of 1.11 × 10⁶ mol a⁻¹ km⁻².

- Unit 2 has a thickness of 13 m in Sn1 and 11.2 m in Sn2. Non-laminated deposits vertically alternating with laminated and intraclast deposit lithofacies occurred in Sn1, while coated reed and phytoclast, laminated deposit, intraclast deposit, non-laminated deposit, shrub, crystalline crust, and claystone and marlstone deposits are observable in Sn2. This lithofacies association suggest a subaqueous depositional environment in the Sn1 borehole and a palustrine setting with freshwater input in Sn2.

- Unit 3 has a thickness of 1.3 m in the Sn1 core and 2.2 m in Sn2. Laminated deposits associated with intraclast deposits and coated reed and phytoclast lithofacies are predominant in Sn1, while crystalline crust and intraclast deposits occur in Sn2. The lithofacies association suggests subaqueous conditions in Sn1 and a slope setting in Sn2.

- Unit 4 has a thickness of 5 m in Sn1 and 4.5 m in Sn2. Laminated deposits interbedded with intraclast lithofacies occur in Sn1, while, in Sn2, laminated deposit is interbedded with crystalline crust, intraclast deposit, coated reed, and phytoclast lithofacies. Based on the lithofacies associations, the deposition occurred in a subaqueous environment (Sn1) passing to a slope setting (Sn2).

- Unit 5 has a thickness of 9.6 m in Sn1 and 10.4 m in Sn2. In Sn 1, it is composed of laminated deposit, shrub, non-laminated deposit, intraclast deposit, and crystalline crust lithofacies association. In Sn2, however, the lithofacies association is characterized by the presence of laminated, intraclast claystone, and marlstone deposits. In both cores, the lithofacies association suggests a depositional environment mainly characterized by subaqueous conditions and a slope setting.

- Unit 6 is 3 m thick in Sn1 and 5 m in Sn2. Laminated deposits, associated with intraclast deposit and coated reed and phytoclast lithofacies, are observable in Sn1, while the presence of non-laminated deposits, and coated reed and phytoclast laminated deposits, as well as claystone and marlstone deposits, characterize Unit 6 in Sn2. Both lithofacies associations of Sn1 and Sn2 attest to subaqueous deposition.

Table 2

Table 2. Travertine depositional units of Sn1 and Sn2 boreholes, with thickness, lithofacies description, age, volume, and geogenic CO₂ fluxes calculated. The latter are reported based on Mancini et al. (2020).

Units 2–6 were deposited, according to Faccenna et al. (2008), from 99 ± 5 to 82 ± 9 ka, with a travertine volume of 0.31 km³ associated with a geogenic CO₂ flux of 2.02 × 10⁶ mol a⁻¹ km⁻².

Units 7–10 were only recognised in Sn1 since these travertines had already been quarried at the Sn2 location.

- A thickness of 2 m characterizes Unit 7 in Sn1. Laminated and intraclast deposits as well as claystone and marlstone are the predominant lithofacies; this association attests to deposition in subaqueous and slope environments.

- Unit 8 is 1.9 m thick in Sn1. Laminated deposit is the predominant lithofacies, attesting to subaqueous deposition. Units 7 and 8 were deposited according to Faccenna et al. (2008) at a time interval between 82 ± 9 ka and 56.1 ± 1 ka, with a travertine volume of 0.21 km³ associated with a geogenic CO₂ flux of 9.14 × 10⁵ mol a⁻¹ km^-2.

- Unit 9 is 1.5 m thick in Sn1 and is composed of laminated deposits, attesting to a subaqueous environment. This unit was deposited between 56.1 ± 1 ka and 44 ± 4 ka (Faccenna et al., 2008) with a travertine volume of 0.08 km³ associated with a geogenic CO₂ flux of 7.11 × 10⁵ mol a⁻¹ km⁻².

- The youngest stratigraphic unit, Unit 10, is 4.8 m thick in Sn1 and is mainly composed of shrub, laminated, intraclast, and non-laminated deposits, attesting to a subaqueous low-energy depositional setting (Scalera et al., 2021). This unit was deposited between 44 ± 4 ka and 34 ± 5 ka (Faccenna et al., 2008) with a total volume of 0.34 km³ and is associated with a geogenic CO₂ flux of 2.70 × 10⁶ mol a⁻¹ km⁻².

4.3 Carbon and oxygen stable isotopes

Figure 4 illustrates the δ¹³C and δ¹⁸O values of the Sn1 and Sn2 travertines.

Figure 4

Figure 4. Carbon and oxygen isotope ratios measured for Sn1 and Sn2 cores (for the colour legend, see Figure 2).

In Sn1, the oxygen isotope ratios (Figure 4) are entirely negative, varying between −8.66‰ and −5.02‰. The base of the core shows a less depleted spike, with values rising from −7.26‰ at 63.25 m to −5.02‰ at 60.55 m, followed by a longer negative trend where the values lower to −8.06‰ at 45.95 m. A second less depleted spike is then recorded, with values up to −6.64‰ at 42.73 m, followed by a depletion trend until 31.07 m, where a δ¹⁸O of −8.66‰ is recorded, being the lowest of the entire section. Then, the highest portion of the core is characterized by a less depleted trend, with values rising to −5.47‰ at 15 m to mildly lower in the subsequent uppermost portion of the core, reaching −6.73‰ at 13.20 m. There, it remains rather stable up to the top of the core, where the last sample, recovered at −4.46 m, is characterized by a δ¹⁸O of −6.71‰.

The carbon isotope (Figure 4) ratios in Sn1 are, instead, all significantly positive, spanning 8.70‰–11.36‰. The lowest portion of the core shows values scattered between 10‰ and 11‰, with no evident trend. Conversely, a trend can be identified between 55.58 m and 52.45 m, where the δ¹³C evolves from 10.98‰ to 9.50‰ to subsequently oscillate between 9‰ and 10‰ up to 32.09 m. Then, a two-stepped shift is recorded, with values rising to 10.47‰ and 10.51‰, respectively at 28.10 m and 27.26 m, and then further up to 11.36‰ at 23.32 m, where the highest values are recorded. This positive shift is followed by a decrease in the carbon isotope ratios, which is lower to 9.34‰ at 13.20 m. Lastly, in the uppermost portion of the core, a sharp positive shift is recorded, characterized by values up to 10.88‰ at 8.36 m and then down to 8.70‰ at the top of the core.

In Sn2, the oxygen isotope ratios (Figure 4) are entirely negative, and span from −8.06‰ to −5.03‰. The base of the core shows an oscillation between −8.06‰ and −6.53‰. A wide shift towards less depleted values is then recorded, with values reaching −5.03‰ at 26.65 m. Then, a long negative shift is recorded up to 18.60 m, where a δ¹⁸O of −6.79‰ is attested. A second minor isotope shift towards less depleted values is recorded in the upper part of the core, where the values rise to −5.86‰ at 13.59 m, followed by an isotope ratio decrease to −7.41‰ at 4.81 m. Lastly, the uppermost portion of the core shows again a rising trend to −6.81‰.

The carbon isotope ratios (Figure 4) are entirely positive, from 8.43‰ to 12.29‰. At the base of the core, the values oscillate between 9.17‰ and 10.38‰. From 29.90 m, a marked positive isotope shift starts, with values rising to 12.29‰ at 26.65 m to subsequently evolve to 9.01‰ at 20.98 m. Then, a second minor, positive carbon isotope shift is recorded, with values rising to 10.68‰ at 18.60 m. Then, a general decrease towards lighter values is attested, with values down to 8.43‰ at 8.70 m. The uppermost portion of the core does not show particular trends, where wide oscillations of the δ¹³C values between 8.43‰ and 9.90‰ can be seen.

5 Discussion

Reported carbon and oxygen isotope ratios of the two analysed Sn1 and Sn2 cores agree with previously published isotope data on the Lapis Tiburtinus travertine succession/hydrothermal system (Minissale et al., 2002; Anzalone et al., 2017; Della Porta et al., 2017b). In particular, the oxygen isotope record attests to a hydrothermal origin, probably influenced by precipitation temperatures of thermal water and original isotopic water composition (Della Porta et al., 2017b). Such values are coherent with those measured in Central Italian travertine deposits (Minissale et al., 2002; Della Porta et al., 2017b). Furthermore, the carbon isotope signature of the Lapis Tiburtinus travertine succession is significantly heavier than known Pleistocene–Neogene Italian travertine systems, as already observed by Minissale et al. (2002) and Della Porta et al. (2017b). The 2‰–4‰ heavier carbon isotope values of the Lapis Tiburtinus travertine succession are related to different factors, such as pH, isotopic composition of the dissolved inorganic carbon, mixing with meteoric water percolating through soil, carbon isotope values of the carbonate bedrock, the distance from the hydrothermal springs, and by downstream CO₂ degassing processes (Della Porta et al., 2017b). Moreover, according to Minissale et al. (2002), the heavier carbon isotope values measured for the Lapis Tiburtinus travertine succession reflect the typical CO₂ signal related to the metamorphism of the Meso-Cenozoic marine limestone substrate and/or derived from magmatic to mantle sources. The mantle source derivation is also testified by the ³He/⁴He mantle signature of the gas vents measured in the western sector of the Tiber Valley. Such a signal derives from a long-term rock–fluid interaction and limestone decarbonation favoured by high temperatures at depth. These processes lead to significantly heavier carbon isotope values than the circulation of shallower fluids, where CO₂ shows a biogenic signature mostly due to soil development and interaction. This heavy signature has been significantly overlooked in the literature, where different studies attest to this characteristic feature of the Lapis Tiburtinus travertine succession but never particularly focus on it.

However, despite the general signature, the analysed Sn1 and Sn2 cores record cyclical shifts, which, according to Anzalone et al. (2017), can be correlated with global Milankovitch cycles and thus with palaeoclimate changes (Ruddiman, 2003; Shackleton et al., 2003; Ruddiman, 2006). According to Anzalone et al. (2017), a 30-m-thick travertine-core drilled in the north-western sector of the Acque Albule Basin shows a stacking pattern of the Lapis Tiburtinus travertine succession characterized by a hierarchy of higher to lower frequency cycles. Such a pattern is controlled and influenced by millennial-scale solar perturbations (i.e., short-term cycles) to sub-Milankovitch (i.e., medium-term cycles) and Milankovitch (i.e., long-term cycles) periodicities. Anzalone et al. (2017) suggest that the higher cycles are related to water table fluctuations associated with millennial-scale climatic changes and solar perturbation, while the lower-frequency cycles are associated with medium- and longer-term precession-driven periodicities. Moreover, according to Anzalone et al. (2017), this hypothesis is also justified by the hierarchical organization of the travertine shown in the travertine depositional units, affected only by discontinuous surfaces observable both along the quarry walls and in the analysed cores. Nevertheless, the hydrothermal origin of the travertines obviously masks the precise water temperature changes related to climatic shifts, as compared to the marine limestones usually investigated in this context. However, a potential climatic control, among the endogenic ones, on the travertine deposition has already been found by different studies and based on sedimentological organization and stable isotope values (Faccenna et al., 2008; Anzalone et al., 2017; Castorina et al., 2023). As already found by Dabkowski and Limondin-Lozouet (2022) and by Sancho et al. (2015), the calcareous tufa deposits are able to record climatic conditions, especially during the Holocene and Pleistocene. Stage MIS5 was particularly characterized by warmer and wetter conditions than today (Sancho et al., 2015). Although the examples described by previous research are not travertine deposits but calcareous tufa, the example described in the Denizli area in Turkey is representative of travertine deposits. According to Özkul et al. (2013), these deposits, coeval with the Lapis Tiburtinus travertine, are independent of climatic changes, and the most important controlling factor is represented by tectonic activity. As also observed by Faccenna et al. (2008), the Lapis Tiburtinus travertine and the Acque Albule Basin are controlled by tectonic activity. However, Mancini et al. (2021) suggested that the Lapis Tiburtinus travertine deposits, while influenced by tectonic activity, are also mainly influenced and controlled by climate changes. In this view, tectonic activity controls only the location of the spring at basin scale. For these reasons, the Denizli Basin and the Acque Albule Basin travertine deposits (Lapis Tiburtinus) are, in our opinion, comparable only in terms of depositional conditions, lithofacies, and geobody geometries.

Three main controlling factors and processes are addressed below: 1) Pleistocene glacial-interglacial cyclicity, and the MIS-5 record in particular; 2) the alternation of arid and humid phases; 3) changes in the groundwater table height and its influence on CO₂ degassing.

5.1 The MIS-5 record

In this framework, in fact, the two Sn1 and Sn2 cores record isotopic shifts that represent the precessional cyclicity identified during the warm MIS-5 (Figure 5A). The Sn1 core records two positive oxygen (5d and 5b) and two negative oxygen (5c and 5a) shifts, respectively, indicative of cool and warm sub-Milankovitch cycles between 115 and 50 Ka (Shackleton, 1969; Ruddiman, 2006). The Sn2 core, instead, records only 5c and 5a warm phases, separated by the 5b oxygen shift towards less depleted values, which identifies a colder phase (Figure 5A). Interestingly, similar shifts have been recorded and observed in two palaeo-lacustrine carbonate records of Central Italy (Fucino and Sulmona lake records) (Giaccio et al., 2015; Regattieri et al., 2015), attesting to the overall regional climatic sensitivity of the Central Apennines during the Late Pleistocene. The climatic control on these shifts is also demonstrated by the fact that they are present in both cores despite the relative distance from the springs and the different lithofacies.

Figure 5

Figure 5. (A) Oxygen isotope ratios of the Sn1 and Sn2 cores correlated with climate curves reconstructed by Shackleton (1987) and Ruddiman (2006). The oxygen isotope curve of Sn2 shows less depleted (5d and 5b) and more depleted (5c and 5a) shifts related to cool and warm sub-Milankovitch cycles of 115–50 Ka. The Sn1 oxygen isotope curves reconstructed instead show only 5c and 5a warm phases, separated by the 5b oxygen less depleted shift and related to a cold phase. (B) Carbon isotope curves of Sn1 and Sn2 correlated with pollen stratigraphy of the Castiglione Crater (Magri, 1989; Tzedakis et al., 2001) and CO₂ fluxes calculated for the depositional units of the Lapis Tiburtinus travertine succession. Carbon isotope variations are interpreted to relate to water table fluctuations and increased CO₂ degassing associated with the Colli Albani volcanic complex activity.

5.2 Influence of arid and humid conditions

However, despite the overall precessional control from which is inferred the pace of these shifts, several superimposed controlling factors might have played a role in determining the recorded oxygen isotope signature. In fact, peak 5d in Sn1 results in a much wider peak than the second positive peak since it developed during an arid period (Figure 5A; Tzedakis et al., 2001). Arid conditions favour evaporation and also attest to low meteoric precipitation rates, which would lower the oxygen isotope ratios in carbonate precipitates. Conversely, peak 5b corresponds to the less depleted shift recorded in the Sn2 core within laminated deposits and crystalline crust lithofacies. In this case, the regional pollen records of Magri (1989) and Tzedakis et al. (2001) attest to overall humid conditions, but the facies in which the widest shift is recorded are those more controlled by evaporation and fast degassing processes.

Therefore, the oxygen isotope record of this complex travertine system proved to be better controlled by cyclic climate shifts, but numerous local factors, such as the influence of meteoric waters, stagnation, movement, and the depth of the supersaturated fluids from which travertines precipitated, acted to hamper or enhance global trends.

5.3 Influence of the groundwater table on CO₂ degassing

Carbon isotope ratios of travertines are mostly controlled by degassing and interaction with organic matter, which affects fractionation with both photosynthesis and respiration. CO₂ degassing deprives the water of light ¹²C, leaving inorganic carbonates relatively enriched in the heavy ¹³C isotope (Pentecost, 2005; Kele et al., 2008; Della Porta, 2015). In the analysed cores (Figure 5B), two positive carbon isotope shifts can be identified. The first, in Sn1, is recorded within non-laminated deposits during the cool and arid 5d phase, as attested by the pollen record of the Castiglione Crater site, located 10 km from the Acque Albule Basin (Tzedakis et al., 2001). In this case, this positive carbon isotope signature is likely mostly controlled by the arid climate phase which determined the lowering of the groundwater table and the enhancement of light CO₂ degassing. Conversely, the second positive carbon isotope shift, recorded within laminated deposits of the Sn2 core, occurred during the 5b cool phase; unlike the 5d, this attests to increased humid conditions in Central Italy (Tzedakis et al., 2001). However, it recorded a moment of increased CO₂ degassing in the Lapis Tiburtinus travertine systems (Mancini et al., 2020). The Lapis Tiburtinus travertine deposition commenced after the beginning of the warm period (isotopic stage 5) from 115 ka, and its end coincided with the cold and dry period related to isotopic stage 2 about 30 ka (Faccenna et al., 2008). Mantle degassing phenomena, a variation of CO₂ fluxes associated with the Lapis Tiburtinus travertine deposition, occurred in the last 115 ka and can be associated with the Colli Albani volcanic complex, as already indicated by Mancini et al. (2020). In fact, the variation in travertine deposition could be influenced by climatic changes, thus affecting changes in deposition rates due to varied CaCO₃ solubility in the amount of water available in the aquifer (Figure 6) and by changes in the deep flux of CO₂. Travertine volume variations and the related calculated CO₂ fluxes reveal that, within a warm period, some larger depositional hiatus followed by a continuous travertine deposition and CO₂-associated flux occurred (82–44 ka). Instead, at the end of a cold period, travertine deposition and associated CO₂ fluxes increase. Such an increase, occurring between 44 and 30 ka, cannot be justified by significant climatic variation, as also demonstrated by the pollen content measured in the Castiglione Crater which is similar to previous periods (Magri, 1989). The calculated volume and associated CO₂ flux more than doubled compared to the previous period, which is likely related to variations in the water table and a changed degassing rate related to the Colli Albani volcanic activity. Moreover, the maximum CO₂ degassing activity (2.70 × 10⁶ mol a⁻¹ km⁻²) associated with travertine deposition occurred 44 to 30 ka, which also corresponds with the last phase of volcanic activity in the Colli Albani volcanic complex (56.5 ka–30 ka) (De Rita et al., 1995, De Rita et al., 2002; Gaeta et al., 2000; Karner et al., 2001; Giordano et al., 2010).

Figure 6

Figure 6. Schematic model showing the possible cause of the carbon isotope shifts measured for the Lapis Tiburtinus travertine succession. The positive shifts are associated with arid phases, with a low groundwater level leading to an increase in CO₂ degassing phenomena (A), while the humid periods are characterized by inhibited CO₂ degassing associated with a groundwater level rise (B) (modified from Mancini et al., 2021).

Increased degassing controlled an increased fractionation effect of the carbon isotopes and relative enrichment of the heavier isotope in the supersaturated fluids. Furthermore, humid conditions might have controlled organic matter blooms and soil developments, which could also have led to enhanced carbon isotope fractionation. In this framework, laminated deposits, strongly influenced by evaporation, are less prone to organic matter blooms than other facies, such as non-laminated deposits characterised by higher water depth and stagnation, or soils. Obviously, the extreme latter variation of travertine facies in a relatively small space hampers the possibility of correlating all the isotopic shifts at a basin-scale level, but it does provide useful insights into the overall carbon cycle dynamics in such a complex environment (Figure 6). Moreover, Brasier et al. (2011) suggested that the primary δ¹⁸O values can be altered due to dissolution and reprecipitation phenomena, especially if these processes occurred in fluids with different temperatures and/or compositions, leading finally to an isotopic homogenization of the bulk stable isotopes and removal of differences between micrite and sparite stable isotopes (Allan and Matthews, 1982; Beck et al., 2005; Watkins et al., 2014). However, the differences between oxygen-stable isotopic values of the cement and micrite of most samples, tested before sample preparation and analysis, are significantly large, implying at least partial preservation of the isotopic signatures. Carbon isotopic signature instead possesses a better preservation potential due to carbon buffering (Brasier et al., 2011). In this view, the δ¹³C signatures are the result of continuous re-equilibration with new fluids, starting from syn-depositional conditions (Erthal et al., 2017; Daëron et al., 2019). Consequently, due to the limited time available for the Late Pleistocene–Holocene (Faccenna et al., 2008) cementation of this deposit, the diagenetic resetting is considered relatively minor (Figure 7).

Figure 7

Figure 7. Boxplot showing the relationship between carbon and oxygen stable isotopes measured in the Sn1 and Sn2 cores, displaying some correlation. However, some value shifts are observable (see values indicated by A, B, and C). “A” values are representative of “laminated deposit lithofacies,” developed in a low-energy setting affected by evaporation. The “B” value relates to “coated reed and phytoclast lithofacies,” attesting to some stagnant environmental conditions affected by freshwater input. Such a shift could be indicative of a biotic effect. The “C” value is instead indicative of “crystalline crust lithofacies,” attesting to a high-energy setting characterized by rapid degassing phenomena. In this view, as also reported in Brasier et al. (2011), a limited diagenetic effect could have influenced the travertine deposits of the Lapis Tiburtinus.

In summary, this record shows that the fractionation of carbon isotopes in travertine systems is far more complex than in other carbonate systems and is related to multiple factors which are not always easily detectable. However, this record shows that positive carbon isotope shifts occur preferentially during relatively cold phases and high CO₂ degassing.

6 Conclusion

This study demonstrates that travertine precipitation is controlled by a complex set of physical, chemical, and biological processes and is marked by the overall heterogeneity of facies even over small distances. However, the detailed analysis of stable C- and O-isotope records from travertine deposits can still provide insights into the sensitivity of travertine systems to climate changes and into the relationship between the global, regional, and local controlling factors of their deposition. The stable isotope record of the analysed cores testifies to a travertine system sensitive to global climate changes, the signature of which was only partially hampered by local factors. The oxygen isotope record, on the one hand, can be correlated with precessional cycles within the warm MIS5 stage even if evaporation processes significantly amplify the wideness of the shifts. On the other hand, the carbon isotope ratios are mostly controlled by CO₂ degassing. When the latter increases, either due to endogenic factors or reduced pressure in the aquifer during arid phases, the fractionation effect among carbon isotopes increases, leading to a major degassing of light carbon in the atmosphere and relative enrichment in heavy ¹³C in the inorganic carbonate. In this view, travertine deposits, largely used to constrain and reconstruct the tectonic activity of the area where they develop, could also be used as a proxy to investigate climatic changes and groundwater table variations based on variations in oxygen and carbon stable isotopes.

Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author contributions

AM: conceptualization, investigation, supervision, and writing–original draft. IC: conceptualization, methodology, validation, and writing–original draft. JL: data curation, investigation, and writing–original draft. EC: supervision, validation, and writing–review and editing. RS: project administration, resources, supervision, validation, and writing–review and editing. MB: conceptualization, visualization, and writing–review and editing.

Funding

The authors declare that financial support was received for the research, authorship, and/or publication of this article. Financial support was received for the research activity from Shell, Total, and Petrobras in the frame of the TraRAS project. The funders were not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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