Global and Planetary Change 63 (2008) 299–308

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Late Pleistocene depositional cycles of the Lapis Tiburtinus travertine (Tivoli, CentralItaly): Possible influence of climate and fault activity

Claudio Faccenna, Michele Soligo, Andrea Billi ⁎, Luigi De Filippis, Renato Funiciello,Claudio Rossetti, Paola TuccimeiDipartimento di Scienze Geologiche, Università Roma Tre, Largo S. L. Murialdo 1, Rome, 00146, Italy

⁎ Corresponding author. Tel.: +39 0657338016; fax: +E-mail address: billi@uniroma3.it (A. Billi).

0921-8181/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.gloplacha.2008.06.006

A B S T R A C T

A R T I C L E I N F O

Article history:

The depositional and erosio Received 23 November 2007Accepted 25 June 2008Available online 3 July 2008

Keywords:travertinepaleoclimatefaultsuranium-series methodthree-dimensional methods

nal history of the Lapis Tiburtinus endogenic travertine located circa 25 km to theeast of Rome, Central Italy, near the Colli Albani quiescent volcano, is interpreted through three-dimensionalstratigraphy and uranium-series geochronology. Analyses of large exposures located in active quarries and ofcores obtained from 114 industrial wells reveal that the travertine deposit is about 20 km2 wide and 60 mthick on average. The travertine thickness is over 85 m toward its western N–S-elongated side, wherethermal springs and large sinkholes occur aligned over a seismically-active N-striking fault. The travertineage was calculated using the U/Th isochron method. Results constrain the onset and conclusion of travertinedeposition at about 115 and 30 ka, respectively. The three-dimensional study of the travertine shows that thisdeposit is characterized by a succession of depositional benches grown in an aggradational fashion. Thebenches are separated by five main erosional surfaces, which are associated with paleosols, conglomerates,and karstic features. This evidence shows that the travertine evolution was mostly controlled by water tablefluctuations. Chronological correlations between travertine evolution and paleoclimate indicators suggestthat the travertine deposition was partly modulated by climate conditions. Other influencing factors mayhave been fault-related deformation and volcanic events.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Travertines and tufa are the product of calcium carbonate precipita-tion under hydrothermal and near ambient conditions, respectively, incontinental areas (Pentecost and Viles, 1994; Ford and Pedley, 1996;Pentecost, 2005). Several parameters may control the formation oftravertines and tufa (e.g., Chafetz et al., 1991; Dreybrodt et al., 1994;Pentecost, 1995). Climate conditions (i.e., humid climate with markedincrease in plant growth), for instance, have been recognized as veryinfluential in the deposition of tufa (Harmon et al., 1979; Henning et al.,1983; Pazdur et al., 1988; Baker et al., 1993; Goudie et al., 1993;Pentecost, 1995, 2005; Ford and Pedley, 1996; Roberts et al., 1998;Dramis et al., 1999; Soligo et al., 2002). The influence of climateconditions is, in contrast, less well-documented and understood for thecase of endogenic travertines (Goff and Shevenell, 1987; Sturchio et al.,1994; Pentecost, 1995; Frank et al., 2000; Rihs et al., 2000), which aretravertine deposits formed through chemical precipitation fromground-water with at least a deeply-derived significant componentregardless of the water temperature (Crossey et al., 2006). It is alsopoorly understood how climate conditions may combine with other

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factors such as a high heat flow, deep fluids, and high-permeabilityconduits through the crust to control the formation of endogenictravertines. It is, in fact,well established that thedeposition of endogenictravertines often occurs around thermal springs, whose location andsupply are usually controlled by active fault-related deformation (Fordand Pedley, 1996; Brogi, 2004; Brogi and Capezzuoli, in press);otherwise, fracture healing by solute precipitation would quickly sealmost fluid pathways (Sibson, 1987; Altunel and Hancock, 1993; Bartonet al., 1995; Oliver, 1996; Curewitz and Karson, 1997; Hancock et al.,1999;Micklethwaite and Cox, 2004; Altunel and Karabak, 2005; Newellet al., 2005; Crossey et al., 2006; Uysal et al., 2007).

The aim of this paper is to contribute to the knowledge of the factorsthat control the depositional cycles of endogenic travertines. To do so,we studied the Lapis Tiburtinus travertine, which is exposed near Tivoli,circa 25 km to the east of Rome, Central Italy (Fig. 1), and provides anexceptional three-dimensional view of an endogenic travertine formedduring late Pleistocene time over an active strike-slip fault nearby theColli Albani quiescent volcano (Chafetz and Folk, 1984; Pentecost andTortora, 1989; Pentecost, 1995; Gasparini et al., 2002; Chiodini et al.,2004). This travertine has been quarried for more than two millenniaand, therefore, spectacular exposures as well as a wealth of technicaldata connected with the quarry activities (e.g., well logs and cores) areavailable. We used three-dimensional stratigraphic and uranium-series

Fig. 1. (a) Geologicalmap of theRoman area (Central Italy). Faults underlying the volcanic edifices are inferred from indirect and direct evidence such as alignments offluid emergences andfracture fields (De Rita et al., 1988, 1995). The Lapis Tiburtinus travertine is located circa 25 km to the east of Rome. Note the location of the Castiglione Crater where the pollen sequencecurvewas determined (Tzedakis et al., 2001). (b) Geological map of the study area including the Acque Albule basinwhere the Lapis Tiburtinus travertine deposited during late Pleistocenetime. The fault beneath the Lapis Tiburtinus travertine is seismically active (i.e., 0.5-to-1.5 km deep earthquakes during the 2001 seismic sequence; Gasparini et al., 2002).

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dating methods to unravel the depositional cycles of the travertine andunderstand their possible causes or influencing factors.

2. Geological setting

2.1. Regional setting

The Lapis Tiburtinus travertine is located in the inner sector of theCentral Apennine fold-thrust belt. This belt consists of Meso-Cenozoiccarbonate thrust sheets developed and migrated toward the east inNeogene time during the subduction of the Adriatic plate toward thewest. During late Neogene time, the Tyrrhenian side of the Apenninebelt was extended under the backarc tectonic regime, while, toward theeast, accretionwas still active at the front of thewedge (Malinverno andRyan, 1986; Patacca et al., 1992). In the Tyrrhenian side of the Apenninebelt, reduced thickness of the lithosphere, volcanism, extensional basins,and high heat flow are the results of the Neogene-Quaternary backarcextensional process (Funiciello et al., 1976; Barchi et al., 1998; Jolivetet al.,1998; Chiodini et al., 2004;Acocella and Funiciello, 2006; Billi et al.,2006a). The Tyrrhenian side of theCentral Apennines is characterized bya system of NW-striking normal faults and associated basins. The basinsare laterally bounded byNE-striking transfer faults. The age of the basin-filling deposits reveals that the growth of normal faults and associatedbasins occurred during late Miocene-early Pleistocene time (Faccennaet al., 1994a,c; Acocella and Funiciello, 2006).

In theRomanarea (Fig.1a), large explosive volcanic districts (i.e., ColliAlbani and Sabatini) became active in mid-Pleistocene time (i.e., sinceabout 700 ka) and remained intermittently active until recent times (DeRita et al.,1988,1995). Inparticular, the last dated episodeof volcanism isthe emplacement of a lahar during Holocene time from the Colli Albanivolcano (Funiciello et al., 2003).

Evidence of late Pleistocene–Holocene tectonic activity is wide-spread in the area of Rome (Faccenna et al., 1994b; Marra et al., 1995,

2004;Mara,1999). This recent tectonic history differs from the previousextensional regime because it is mainly characterized by N-strikingright-lateral and NE-striking transtensional-to-normal faults (Alfonsiet al., 1991; Faccenna et al., 1994a). These structures have partiallycontrolled the latest stages of volcanism and related hydrothermaloutflows (Faccenna et al., 1994b).

2.2. Structural setting and hydrologic framework

To the west of Tivoli (Fig. 1b), the so called Acque Albule (meaningwhite waters) basin is a morphological depression gently dippingtoward the south. The topographic surface of the Acque Albule basinlies at an altitude of about 70 m. The basin is filled by the LapisTiburtinus travertine, which lies on Plio-Pleistocene alluvial, lacus-trine, and epivolcanic deposits. The substratum of these depositsconsists of a thick (4–5 km at least) succession of Meso-Cenozoicmarine carbonate rocks. Travertine deposition started just after orconcurrently with the last phase of volcanic activity (late Pleistocenetime) in the region. Near Tivoli, on the eastern margin of the AcqueAlbule basin (Fig. 1b), a small tufa body precipitated from the fresh-waters of the Aniene River falls. This deposit overlies the LapisTiburtinus travertine. The southernmargin of the Acque Albule basin isbordered by at least four pyroclastic flows of the Colli Albani volcano,emplaced between 500 and 350 ka (De Rita et al.,1995). To the north ofthe Acque Albule basin, Plio-Pleistocene marine clay deposits areexposed. These deposits are overlain by lower Pleistocene sandy andfanglomeratic sequences. The above-depicted sedimentary and volca-nic deposits are transgressive over the Jurassic–Miocene shallow-water carbonates, which form the thrust sheets of the Lucretili,Tiburtini, Cornicolani, and Sabini Mountains (Fig. 1).

The Acque Albule basin is an area rich of active thermal springs,sinkholes, and other karstic features (Pentecost and Tortora, 1989;Minissale et al., 2002; Salvati and Sasowsky, 2002; Billi et al., 2006b).

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Most springs and sinkholes occur along a N-striking, right-lateral,seismically-active fault (Gasparini et al., 2002), which is partly coveredby the travertine body (Figs.1 and 2). In 2001, a swarm of shallow low-magnitude (M≤3) earthquakes occurred along this structure. Thelocation of earthquake foci defines a NNW-striking fault zone seis-mically active between about 1.5 and 0.5 kmof depth beneath the LapisTiburtinus travertine (Gasparini et al., 2002). Previous structuralstudies (Mattei et al., 1986; Maiorani et al., 1992; Faccenna et al.,1994a,b,c; Sagnotti et al.,1994; Billi et al., 2006b) showed that this faultand the associated brittle deformation are well exposed toward thenorth in the Jurassic–Miocene marine carbonate strata of theCornicolani Mountains (Fig. 1). In these strata, mesoscopic deforma-tions associated with the fault mainly consist of N-striking, right-lateral faults and N30–40°-striking, transtensional-to-normal faults. AN40°-striking set of joints is also widely developed along and near themaster fault. Fault-related deformation also affects the overlying rocks.In particular, a fewmesoscopic faults kinematically consistentwith theN-striking master fault occur in the Plio-Pleistocene clays and in theupper Pleistocene volcanic rocks. The N40°-striking set of joints has

Fig. 2. (Above) Isochore map of the Lapis Tiburtinus travertine in the Acque Albule basin.interpolating travertine thickness data obtained from the interpretation of 114 drilled cores,thickening along the N-striking right-lateral fault (i.e., drawn with a dashed line after Faccenconsidered for the three-dimensional model shown in Fig. 4 is shownwith a dashed rectangldifferent vertical and horizontal scales. Cross-section tracks are shown in themap above. Eroslarge exposures within active quarries. Strongly cohesive travertines occur below the S1 ero

been observed in the clays, in the volcanic rocks, and also in thetravertine overlying themaster N-striking fault (Faccenna et al., 1994c;Sagnotti et al., 1994). Calcite and sulfate precipitates are frequent overthe fault surfaces and within the joints, where evidence of multiplecycles of precipitation have been observed. Isotope analyses on theseprecipitates showed that the originating fluids were of mixed nature,including meteoric waters combined with some deeper fluids thatoriginated from or interacted with thermally-active volcanic bodies(Maiorani et al., 1992; Billi et al., 2006b).

The isotope analysis of the CO2 contained in the Lapis Tiburtinustravertine provided δ13C-values between about 0 and 4 (‰ PDB). Theseresults are consistent with those above mentioned and suggest ahydrothermal origin for the fluids that originated the travertine deposit.ACO2 contribution fromthemantle is absent or very limited (Minissale etal., 2002). The isotope analysis of the oxygen contained in the travertineprovided δ18O-values of about 25 (‰ SMOW) (Minissale et al., 2002).These values are typical of marine carbonate rocks and indicate that theLapis Tiburtinus travertinemostly formed fromthe chemical alterationofthe Meso-Cenozoic marine carbonates typical of Central Italy (Fig. 1).

Isochores (i.e., lines of equal vertical thickness of the travertine deposit) are drawn bywhose location is displayed with small white dots. Note thermal springs and travertinena et al., 1994c and Gasparini et al., 2002), which is covered by the travertine. The areae. (Below) Two geological cross-sections across the Lapis Tiburtinus travertine. Note theional surfaces are sketched on the cross-sections as inferred fromwell core data and fromsional surface, whereas young, non-cohesive tufa (“testina”) occurs above this surface.

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The present hydraulic circuit of the Acque Albule basin involvesgroundwater that infiltrates through the fractured carbonate massifssurrounding the Acque Albule basin (Capelli et al., 1987). At deep levels,thesewatersmix with warmer fluids, whose temperature and chemicalcompositiondependon interactionswith the adjacent, thermally-active,volcanic complexes. The deep circulation of these waters mostly occurswithin fractured carbonate rocks. The thermal waters eventually risealong active faults and emerge in the Acque Albule basin (Figs. 1 and 2)with anaverage temperature of 23 °C and pHvalues between 6.0 and 6.2(Manfra et al., 1976; Capelli et al., 1987; Pentecost and Tortora, 1989;Maiorani et al., 1992; Pentecost, 1995; Billi et al., 2006b). The complete

Fig. 3. Photographs showing travertine exposures within active quarries in the Acque Albule bshowing the S2 erosional surface and the overlying strata onlapping S2. (c) Photograph showindeveloped along preexisting joints (e.g., Billi et al., 2006b). (d) Photograph (kindly provided by FThe sinkhole occurs in the Lapis Tiburtinus travertine above the seismically active faults (Fig. 2paleosol development. The karstic processes (i.e., cavity formation and subsequent collapse)occurring in Plio-Pleistocene clays underlying the Lapis Tiburtinus travertine and overlying the sGuidonia (Fig. 1b). Joints are partly filled by multiple generations of calcite precipitates interpr

chemical composition of these waters is provided by Giggenbach et al.(1988) and Minissale et al. (2002).

3. Methods and results

3.1. Three-dimensional stratigraphy

We reconstructed the three-dimensional stratigraphic setting ofthe Lapis Tiburtinus travertine by analyzing all travertine exposuresfrom the existing quarries and 114 drill cores from stratigraphic wellsmade available by the local mining industry (Figs. 2–4). Except for a

asin. (a) The S-dipping erosional surfaces S1, S2, S3, and S4 are indicated. (b) Photographg a karstic cavity below the S2 erosional surface. The cavity and other karstic features oftenrancesco Poggi) showing a large karstic sinkhole beneath the present topographic surface.). (e) Photographic evidence for the temporal relationship between karstic processes andprecede and are concurrent with the development of paleosols. (f) Photograph of jointseismically-active fault shown in Fig. 2 (Gasparini et al., 2002); this exposure is located neareted as tectonically-driven (Sagnotti et al., 1994).

Fig. 4. Lateral viewof the three-dimensionalmodel showing the spatial relationship between the S1, S2, S3, S4, and S5 erosional surfaces across the Lapis Tiburtinus travertine. Volumesof travertine are obtained from the three-dimensional model. B1, B2, B3, and B4 are the depositional benches between, respectively, S1 and S2, S2 and S3, S3 and S4, and S4 and S5.

Table 1Uranium and thorium activity ratios and age of travertine deposits deduced fromisochron diagrams (Fig. S1)

Sample (234U/238U) (230Th/234U) Age (kyr)

1 1.056±0.073 0.233±0.023 29±42 1.026±0.062 0.265±0.036 34±53 1.115±0.072 0.323±0.022 42±34 1.281±0.025 0.350±0.013 46±25 1.094±0.035 0.408±0.037 57±76 1.084±0.051 0.402±0.024 55±47 1.093±0.051 0.564±0.045 89±118 1.054±0.058 0.600±0.019 99±59 Errorchron ⁎

10 1.081±0.075 0.533±0.039 82±911 + 1.041±0.037 0.652±0.027 114±812 § 0.958±0.063 0.512±0.073 79±1713 Errorchron⁎14 1.027±0.008 0.504±0.024 76±515 1.006±0.002 0.654±0.039 116±1216 1.107±0.058 0.424±0.041 59±8

The entire dataset is provided in Table S1.⁎ Excess of data scatter in the isochron.+ This age has been calculated using a single subasample because the 230Th/232Thactivity ratio was equal to about 20.§ See text, for age interpretation.

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few preserved historical exposures of Roman and Baroque ages, thehistorical quarries have been progressively incorporated in themodern and larger quarries (De Filippis and Massoli-Novelli, 1998).

In the central sector of the Acque Albule basin, the travertinethickness is 60 m on average and gently thins toward the east, thenorth, and the south (Fig. 2). Toward the west, the travertine thickensup to a maximum of almost 90 m coincident with the underlying N-striking active fault and with the associated emergences of thermalwaters and sinkholes (Maxia, 1950). To the west of the fault, thetravertine thickness reduces rather abruptly.

On exposures and in well cores, we observed that the travertine iscomposed of a sequence of benches separated by erosional surfaces(Fig. 3). Both the benches and the erosional surfaces gently dip towardthe south (Fig. 3a and b) (Chafetz and Folk,1984). The bench thickness isvariable within a maximum of about 8–10 m. The travertine benchesconsist offinely laminated carbonateswith alternations of bacterialmudtufa-like travertines and laminated stromatolites (Chafetz and Folk,1984). These sedimentological features are indicative of a travertinegrowth in restricted pools a few meters deep (i.e., less than about 1–2m). The occurrence of thick (about 7mm) and thin (about 0.1–0.5mm)laminations is ascribed, respectively, to annual and diurnal cycles ofgrowth (Chafetz and Folk, 1984). The geometry and attitude of thebenches show that the travertine deposit grew in a southwardaggradational fashion. The basal strata of each travertine bench onlapthe sloping erosional surface, onwhich they rest (Fig. 3b). This evidenceindicates the occurrence, after each erosional episode, of a new cycle oftravertine aggradational deposition in sub-horizontal pools and terracesoccurring over the inclined erosional surface.

Within the travertine deposit, we distinguished five main erosionalsurfaces named S1, S2, S3, S4, and S5, from the youngest surface to theoldest one (Figs. 3 and 4). B1, B2, B3, and B4 are the depositional benchesbetween, respectively, S1 and S2, S2 and S3, S3 and S4, and S4 and S5(Fig. 4). The upper and youngest erosional surface (S1) truncates theprevious ones andmarks a clear change in thedepositional environment.S1 is, in fact, the basal surface of the loose-to-poorly cohesive youngesttufa. The erosional surfaces are marked by thin (less than about 1 m)paleosols (Fig. 3b) and by pockets of conglomerates. Karst features arealso frequent immediately below the erosional surfaces (Fig. 3c and e).Whereas these karstic features occur solely within about 1–2 m belowthe erosional surfaces, to which they are associated through the entiretravertine deposit, some greater karstic features such as large sinkholes(Fig. 3d) are present only in the elongated portion of the travertinedeposit overlying the active fault, i.e., where most thermal springs occur(Fig. 2). This evidence suggests that the formation of the karstic featuresassociatedwith the erosional surfaces ismostly connectedwith exogenicwaters during erosional events, whereas the formation of the largerkarstic features (i.e., the sinkholes) is mostly connected with endogenicwaters (Salvati and Sasowsky, 2002; Billi et al., 2006b).

To obtain information about the temporal evolution of thetravertine depositional rate, through the ArcGIS 9.2 mapping softwareproduced by ESRI, we constructed a three-dimensional model of thetravertine erosional surfaces and related benches (Fig. 4). From themodel, volumes of travertine benches included between the erosional

surfaces are computed and expressed as percentages of the totalvolume (i.e., volume of the travertine above S5): 2.7% for B1, 26.5% forB2, 42.6% for B3, and 28.2% for B4.

3.2. U-series geochronology

Wemeasured radiometric ages of 16 travertine samples (Table 1) toconstrain the age of the travertine benches, their deposition rate, theage of the erosional surfaces, and the age of calcite precipitates fillingsome joints and shear fractures observed within the Plio-Pleistoceneclays lying just north of the travertine deposit. By following the total-sample-dissolution technique (TSD) for impure carbonates (Bischoffand Fitzpatrick,1991), we analyzed 3 or 4 coeval subsamples fromeachsample by the U-series method (Tables 1 and S1). We choose thismethod rather than the leachates alone (Schwarcz and Latham, 1989),because the leaching method provides reliable results only in the caseof selective dissolution of the carbonate fraction without any removalof the U- and Th-isotopes from the detritic component, or whenuranium and thorium are leached without any fractionation. Usually,these conditions are not verified because uranium and thorium areoften fractionated and readsorbed onto the residual component.Moreover, in the analysis of samples consisting of simple mixtures ofcarbonate and detritic components, the use of the TSD technique ispreferable for age determinations because the sample is totally dis-solved and, therefore, no preferential leaching or readsorption canoccur (Luo and Ku, 1991; Ludwig and Titterington, 1994).

To apply successfully this technique, it is necessary that theanalyzed coeval subsamples have different U/Th ratios, but the same230Th/232Th activity ratio at the time of deposition.

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The travertine age was calculated by measuring the 230Th/232Th,234U/232Th, and 238U/232Th activity ratios of several coeval subsamplesto obtain the value of the 230Th/234U and 234U/238U activity ratios inthe pure carbonate fraction. These values were calculated from theslope of the regression lines in the 230Th/232Th versus 234U/232Th and234U/232Th versus 238U/232Th isochron plots (Fig. S1). Calculationswere done using ISOPLOT, a plotting and regression software forradiogenic-isotope data (Ludwig, 2003). Errors affecting the travertineages (about 10–15%; Table 1) were determined by alpha counting(Ludwig, 2003) and are connected with the low content of uranium ofmost samples (i.e., except samples 12 and 16; Table S1). Alternatively,large errors may also be connected with alignment of subsamples inthe isochron plots (Fig. S1).

Sampleswere analyzed according to standard chemical proceduresto separate the uranium and thorium isotopic complexes (Edwards,1988). Subsequently, samples were alpha-counted using high resolu-tion ion implanted Ortec silicon surface barrier detectors. Subsamplesfrom 13 out of 16 samples provide reliable results (Table 1 and Fig. 5).Three samples (11, 12, and 15) collected at the bottom of the travertinedeposit were analyzed to define the beginning of the travertinedeposition. In particular, samples 12 and 15 were collected fromdrilled cores, whereas sample 11 was collected on a travertineexposure within a deep quarry. Sample 12 provides a young agepossibly connected with post-deposition fluid circulation and 234Uleaching as also shown by the 234U/238U activity ratios, which arelower than 1 (Table 1). Samples 11 and 15 show that the onset of thetravertine deposition occurred at about 115±9 ka (Fig. 5). Thisrepresents a minimum age for the onset of deposition. In the westernnarrow deeper portion of the basin, in fact, the travertine depositionmay have started earlier. The end of travertine deposition, asconstrained by the analysis of sample 1 collected in non-consolidatedtufa lying at the top of the Acque Albule travertine body, is 29±4 ka(Fig. 5). Faccenna et al. (1994c) found a similar age for the youngest

Fig. 5. (Above) Cross-section from A-A' (Fig. 2) showing the erosional surfaces and samplewhereas the other samples are from travertine exposures. (Below) Age versus elevation diagfor ages are shown (Table 1). S1, S2, S3, S4, and S5 indicate ages of the five erosional surfaces athe south, the travertine grew by lateral aggradation interrupted by erosional episodes protravertines (e.g., samples 3 and 4) may lay at a lower elevation of older travertines (e.g., sam

tufa deposited in the Collefiorito area (Fig. 2). All together the aboveradiometric data constrain the age of the Acque Albule travertinebetween about 115 and 30 ka (Fig. 6).

To estimate the travertine depositional rate, we measured the age ofeach erosional surface by dating samples of travertine located immedi-ately belowor above the erosional surfaces (Fig. 5). Results show that theerosional surfaces are dated as follows: S1≈34±5 ka (sample 2), S2≈44±4 ka (samples 3 and 4), S3≈56.5±8 ka (samples 5 and 6), S4≈82±9 ka(sample 10), and S5≈99±5 ka (sample 8) (Figs. 5 and 6).

In the travertine, isochronous horizons are gently dipping towardthe south owing to the southward aggradational pattern of thetravertine benches (Figs. 3 and 4). This implies that older travertines inthe north lie at an altitude greater than the altitude at which youngertravertines lie in the south (Fig. 5). The observed maximum verticaloffset of isochronous horizons due to the aggradational geometry isabout 30 m for the S2 and S3 erosional surfaces.

By combining radiometric data (Table 1) and volumes of thetravertine benches as computed from the three-dimensional strati-graphic model (Fig. 4), we obtained the following minimum deposi-tional rates (i.e., neglecting erosion): 80±48 m3/yr for the benchbetween S1 and S2 (B1), 650±487m3/yr for the bench between S2 andS3 (B2), 481±222 m3/yr for the bench between S3 and S4 (B3), and488±287 m3/yr for the bench between S4 and S5 (B4) (Fig. 6). Theseestimates represent minimum accumulation rates due to the effects oferosion (e.g., Gaillardet et al., 1997; Plan, 2005).

To constrain the age of faulting and related deformations affectingthe Lapis Tiburtinus travertine and older rocks (Faccenna et al., 1994c;Sagnotti et al., 1994), we analyzed and dated the calcite filling a veinfrom a set of joints affecting the Plio-Pleistocene clays, which liebeneath the travertine (Fig. 3c). These joints are exposednearGuidoniaalong the northern prolongation of the seismically-active fault lyingbeneath the travertine (Fig. 2). Results provide an age of 59±8 ka(sample 16; Tables 1 and S1). This age is consistent with previous

location in the Lapis Tiburtinus travertine. Samples 12, 14, and 15 are from well cores,ram for the travertine samples analyzed with uranium-series dating method. Error barss inferred from the age of samples collected near these surfaces (see also Fig. 6). Towardven by the occurrence of erosional surfaces. Because of the lateral aggradation, youngple 5).

Fig. 6. Comparison between available travertine and paleoclimate data. (a) Pollen record during late Pleistocene time (Tzedakis et al., 2001). The curve represents the ratio (i.e., inpercentage) between arboreal and non-arboreal pollens. It follows that peaks can be interpreted as corresponding to relatively humid times (more arboreal pollens), whereasdepressions represent relatively dry times (less arboreal pollens). Data are from the Castiglione Crater site, which is located only about 10 km to the south of the Acque Albule basin(see Fig.1a for the site location). (b) SPECMAPmarine paleoclimatic δ18O record during late Pleistocene time (Martinson et al., 1987). Glacial times correspond to depressions, whereasinterglacial times correspond to peaks. (c) Minimum deposition rates of the Lapis Tiburtinus travertine for the B1, B2, B3, and B4 benches during late Pleistocene time. Shadingrepresents the error bar. The deposition rate is the minimum one because volumes of eroded travertine are unknown and are therefore ignored. Note that the plotted values areaverage estimates (i.e., time-integrated values) over the periods of time included between two successive erosional surfaces. Possible peaks or minima of the depositional rate duringthese periods cannot be determined with the available data. (d) Travertine features: S1, S2, S3, S4, and S5 are ages of erosional surfaces with the related error bars represented asshading; B1, B2, B3, and B4 are the depositional benches. Isotopic stages: G indicates main glacial times, whereas g indicates cool sub-stages (stadials) during the warm stage 5.

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geochronological measurements on calcite precipitates filling thesame set of joints (i.e., 49±8 ka; Faccenna et al., 1994c).

4. Discussion

Geochronological, sedimentary, and three-dimensional strati-graphic evidence shows that the Lapis Tiburtinus travertine evolvedin a cyclical manner by alternating depositional and erosional stagesbetween about 115 and 30 ka. In particular, the occurrence ofalternated depositional benches and erosional surfaces, which areassociated with paleosols, conglomerates, and karstic features, showsthat the cyclical evolution of the travertine deposit was controlled byfluctuations of the water table in the Acque Albule basin. Erosionoccurred under subaerial conditions during low stands of the watertable, whereas deposition occurred during high stands, when thetravertine deposit grew by onlapping the underlying erosional surfaceand by aggrading toward the south (Fig. 7).

Fluctuations of the water table in low and near-coastal areas suchas the Acque Albule basin (Fig. 1) can be modulated by several factors.The first and most viable hypothesis is that paleoclimate conditions(i.e., average temperature and humidity) influenced the elevation ofthe water table in the Acque Albule basin by modulating the level ofboth the sea and the continental waters (e.g., Noe-Nygaard andHeiberg, 2001; Prokopenko et al., 2005). To explore this hypothesis, in

Fig. 6, we compare the evolution of some travertine features (i.e., theage of erosional surfaces and the average rates of travertinedeposition) with the available global and local paleoclimatic indica-tors for the late Pleistocene time (i.e., the SPECMAP marine δ18Orecord and the pollen record). Fig. 6 suggests a climate control at leaston the onset and end of travertine deposition. The deposition onset atabout 115 ka coincides with the end of the cold isotopic stage 6 andbeginning of the warm isotopic stage 5 (Fig. 6b). The beginning of thestage 5 is also a humid period as demonstrated by the abundance ofarboreal pollens in the Castiglione Crater site (Fig. 6a). A warm andhumid timemay have favored fluid circulation, the rise of the sea level(i.e., because of deglaciation) and that of the water table in the AcqueAlbule basin. Although these conditions are favorable for travertinedeposition, they cannot be invoked as the only cause for the onset oftravertine deposition; otherwise, the travertine should have depositedalso during the warm and humid isotopic stage 7 (Fig. 6b and d).

Analogously to the onset of deposition, the end of the Lapis Tiburtinustravertine deposition at about 30 ka may have been influenced bypaleoclimatic conditions. The endof deposition, in fact, coincideswith theonset of a cold and dry period (i.e., the isotopic stage 2; Fig. 6), whichmayhave induced a lowering of the water table in the basin and a limitedcirculation of fluids. Also in this case, paleoclimatic conditions cannot beinvoked as the only cause for the end of travertine deposition; otherwise,deposition should have started again during the warm and humid

Fig. 7. Conceptual model showing the relationship between cycles of travertine deposition and erosion and fluctuations of the water table in the basin. Photographs are from Fig. 3(e)(above) and Fig. 3(b) (below) and show some real features sketched in the model (i.e., karstic features and erosional surfaces).

306 C. Faccenna et al. / Global and Planetary Change 63 (2008) 299–308

isotopic stage 1 (Fig. 6). It follows that proper climate conditions areprobably necessary but not sufficient alone to explain the deposition ofthe Lapis Tiburtinus travertine. This is also supported by the average ratesof travertine deposition (Fig. 6c). We observe that, except for the B1bench, these rates are rather homogeneous for the bulk of the travertinedeposit (i.e., B2, B3, and B4 benches), this evidence suggesting thatpaleoclimatic conditions had no remarkable effects on the depositionsystem, at least over a time scale of about 10–15 kyr, which is approxi-mately the duration of a travertine depositional stage between twosubsequent erosional surfaces (Fig. 6). The deposition rates are, in fact,computed by considering the volume of travertine included between twoerosional surfaces and the time included between the ages of thesesurfaces. As such, these rates are average values for each bench andcannot be used to infer possible influences of paleoclimatic conditions ona time scale smaller than thebenchduration (i.e.,10–15kyrat least, Fig. 6).

The above-presented considerations about the relationshipbetween travertine cycles and paleoclimatic conditions suggest thatthe travertine deposition and erosion should have been influenced by

some other factors in addition to paleoclimate conditions. We hypo-thesize that fault-related deformation exerted an important control onthe formation of the travertine and on its depositional rate (e.g., Franket al., 2000; Rihs et al., 2000). The causal link between travertinedeposition and faulting is indeed supported by the following evidence:(1) the age of the calcite filling some fault-related fractures is coevalwith travertine deposition; (2) the maximum thickness of thetravertine deposit coincides with the underlying seismically-activefault; and (3) thermal springs, large sinkholes, and fractures inside thetravertine deposit are well aligned over the same fault. We suspectthat, regardless of the paleoclimatic conditions and amount of fluids,the flow of endogenic waters toward the Acque Albule basin shouldhave been regulated and limited by the available conduits, whichweremostly provided by fault-related deformation (e.g., Curewitz andKarson, 1997; Hancock et al., 1999; Micklethwaite and Cox, 2004;Anderson and Fairley, 2008). This process may explain why thedepositional rates are rather homogeneous (Fig. 6c). If this hypothesisis true, then the low depositional rate associated with the formation of

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the B1 bench may be explained by a reduced activity of the faultbeneath the travertine and, therefore, by limited pathways for fluids.On the other hand, it is also possible that faulting and climate condi-tions interacted and that a humid climate promoted fault slip (e.g., Saarand Manga, 2003; Husen et al., 2007). Capelli et al. (1987) estimatedthe carbonate recharge area draining toward the Acque Albule basin(i.e. the carbonate massifs surrounding the basin; Fig. 1) as wide asabout 150 km2. They also estimated an average porosity of 5% for theshallow fractured carbonate rocks (Billi et al., 2006b), which are theaquifer of the area surrounding the Acque Albule basin. By consideringthis porosity and a piezometric gradient between the Acque Albulebasin and the surrounding carbonate massifs of about 0.5% (Capelliet al.,1987), we estimate that a fluctuation of thewater table by 30m inthe Acque Albule basin (i.e., as shown by the vertical offset of someisochronous horizons; Fig. 5) corresponds to a fluctuation of at least100 m in the surrounding carbonate massifs. Such a fluctuation of thewater table (i.e. ∼100 m) can produce a fluid pressure increase ofapproximately 1MPa. According to the Coulomb criterion, at a depth of500 m, which is the minimum hypocentral depth for the 2001 seismicswarm along the fault beneath the Acque Albule basin (Gasparini et al.,2002), an increase of fluid pressure by 1MPa can produce a decrease offault strength byabout 10% (i.e. about 0.6MPa). Studies on earthquakesinduced artificially by fluid pressure variations show that stresschanges as low as 0.01-to-0.1 MPa are sufficient to trigger seismicityand fault rupture onpre-existing, critically-stressed faults (Audin et al.,2002; Saar and Manga, 2003). Stress changes observed for the bulk ofinduced seismicity in fluid injection experiments are in the order of1 MPa (Zoback and Harjes, 1997). It is therefore plausible to hypothe-size a causal link between climate and faulting in the Acque Albulebasin and their cooperation in controlling the deposition of the LapisTiburtinus travertine.

Eventually, we cannot rule out that fluid circulation, water tablefluctuations, and travertine deposition in the Acque Albule basin mayhave been influenced by sudden seismic or volcanic phenomena. It isknown, for instance, that the flow of karstic sources can radicallychange (i.e., either increasing or decreasing) immediately prior than orafter large nearby earthquakes (e.g., Mercalli, 1883; Baratta, 1901).Analogously, the level of the water table in volcanic areas may drop byseveral meters or tens of meters prior than a large eruption (e.g., seethe case of the Vesuvius eruptions in Mercalli, 1883). Although there isno evidence to infer similar phenomena in the Acque Albule basinduring late Pleistocene time, the occurrence of an active fault beneaththe travertine deposit and the presence of an adjacent quiescentvolcano (i.e., the Colli Albani volcano) do not allow us to exclude theoccurrence of earthquake- or eruption-related sudden changes in thefluid regime and therefore in the travertine deposition.

5. Conclusions

The Lapis Tiburtinus travertine deposit evolved during late Pleisto-cene time (i.e., between about 115 and 30 ka) by alternating stages ofdeposition and erosion, which were controlled by fluctuations of thewater table level in the Acque Albule basin. Paleoclimate cycles (i.e.,cold-to-warm and humid-to-dry oscillations) possibly contributed tomodulate the water table in the Acque Albule basin and therefore thetravertine depositional-erosional cycles. However, paleoclimate oscilla-tions alone are not sufficient to fully explain the observed travertinecycles and travertine deposition onset and end. Further influencingfactors such as fault-related deformation and volcanic activity at thenearby district must be postulated and better explored in the future.

Acknowledgments

We thank E. Anzalone, A. Brogi, B. D'Argenio, V. Ferreri, L. Lombardi,M. Mattei, A. Minissale, and A. Taddeucci for stimulating discussions.F. Lippiello and the “Centro per la Valorizzazione del Travertino

Romano” are thanked for letting us work in the travertine quarries.S. Cloetingh, L. Crossey, J. Gosse, B. Murphy, D. Newell, W. Sharp, andanonymous reviewers are thanked for constructive reviews andcomments.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.gloplacha.2008.06.006.

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