# Glacio-eustatic forcing on Middle Pleistocene fluvialu2013lacustrine depositional processes: a case history from the Tyrrhenian Sea margin of central Italy

G. M. Di Buduo, M. Gaeta, D. M. Palladino, F. Marra, S. Nomade, A. Pereira

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G. M. Di Buduo, M. Gaeta, D. M. Palladino, F. Marra, S. Nomade, et al.

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# Glacio-eustatic forcing on Middle Pleistocene fluvial–lacustrine depositional processes: a case history from the Tyrrhenian Sea margin of central Italy

G. M. Di Buduo<fnref n="1" /> · M. Gaeta<fnref n="2" /> · F. Marra<fnref n="3" /> · S. Nomade<fnref n="4" /> · D. M. Palladino<fnref n="2" /> · A. Pereira<fnref n="5" />

Received: 10 August 2024 / Accepted: 11 February 2025

© The Author(s) 2025

## Abstract

The complex stratigraphic setting of alternating volcanic and fluvial–lacustrine sedimentary deposits in the surroundings of Castel Cellesi village (Latium region, central Italy) offers an unprecedented opportunity to examine the influence of glacio-eustatic forcing on the depositional processes during Pleistocene times in the upper catchment of the Tiber River, in the inland sector of the Tyrrhenian Sea margin of Italy. Tight geochronologic constraints on the timing of sediment deposition have been established through precise $^{40}$Ar/$^{39}$Ar dating of intervening volcanic layers. These new age constraints reveal a clear chronological correlation between sediment aggradation phases and sea-level rises, as evidenced by the $\delta^{18}$O record and relative sea-level (RSL) curve. The analysis of pyroclastic-flow deposits within incised paleo-valleys further indicates a relationship between periods of erosion or non-deposition and sea-level lowstands. Specifically, twelve $^{40}$Ar/$^{39}$Ar dates have delineated three successive aggradational fluvial–lacustrine successions deposited during the sea-level rises of Marine Isotope Stages (MIS) 13, 11, and 9, as well as the erosional features associated with the sea-level drops of MIS 12 and 10.

**Keywords** Sea-level oscillations · Deglacial events · Aggradational successions · $^{40}$Ar/$^{39}$Ar geochronology · Quaternary stratigraphy

## Introduction

While there is broad consensus on the natural factors regulating the alternation of glacial and interglacial periods— among which effective insolation, since the formulation of Milankovitch's theory (Milankovitch 1941), is considered the principal forcing factor (e.g., Hays et al. 1976; Berger

1978; Imbrie et al. 1984; Huybers 2006)—no unanimously accepted theory explaining the occurrence of abrupt glacial terminations at specific moments in insolation history has been developed so far (e.g.: Muller and MacDonald 1997; Paillard 1998).

Establishing independent ages for glacial terminations is thus extremely valuable for comparing their chronology with that of the orbital parameters and proxies assumed as evidence of their occurrence (e.g., $\delta^{18}$O, Raymo et al. 1989), thereby facilitating the investigation of their triggers.

A robust independent chronology for the glacial terminations in the last million years has been established through the dating of the aggradational successions deposited by the Paleo-Tiber River in response to phases of sea-level rise in central Italy (Alvarez et al. 1996; Karner and Renne 1998; Karner and Marra 1998; Marra et al. 1998, 2008, 2016, 2017, 2021; Florindo et al. 2007, 2024; Pereira et al. 2020; Giaccio et al. 2021; Mazza et al. 2024). In addition to the numerous studies conducted in the paleo-Tiber delta near Rome, recent work has demonstrated the combined effect of volcano-tectonics and glacio-eustatic control on the sedimentary processes in the hydrographic basin of

<figure><img src="image_2.png" /></figure>

<footnote>F. Marra
fabrizio.marra@ingv.it</footnote>

<footnote n="1">¹ Museo Geologico e delle Frane di Civita di Bagnoregio, Viterbo, Italy</footnote>

<footnote n="2">² Dipartimento di Scienze della Terra, Sapienza-Università di Roma, Rome, Italy</footnote>

<footnote n="3">³ Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy</footnote>

<footnote n="4">⁴ CEA-CNRS, UMR 8212, Laboratoire des Sciences du Climat et de l'Environnement, CEA-UVSQ et Université Paris Saclay, Bat 714, Orme des Merisiers, 91191 Gif sur Yvette, France</footnote>

<footnote n="5">⁵ Laboratoire GEOPS, UMR 8148, Université Paris-Saclay, Bâtiment 504, 1 rue du Belvédère, 91400 Orsay, France</footnote>
the Paleo-Tiber River, which drains the eastern flanks of
the Vulsini Volcanic District in central Italy (Marra et al.
2019; Fig. 1). The age of intercalated volcanic layers has
constrained the time of deposition of a fluvial–lacustrine
succession exposed in different sections at varying altitudes
within the interval 345–322 ka. This time frame corresponds
to the period of sea-level rise during glacial termination IV
leading to MIS 9 highstand (e.g., Lisiecki and Raymo 2005;
Grant et al. 2014). Chronostratigraphic reconstructions by
Marra et al. (2019) showed that sediment aggradation of
a fluvial–lacustrine succession occurred synchronously in
this time frame at three distinct outcropping sections, pres-
ently forming three terraced successions at markedly dif-
ferent elevations of approximately 350 m, 270 m, and 210
m, indicative of post-sedimentary dislocation. In particu-
lar, fallout deposits dated to 345 ± 2 ka, correlated with the
Ponticello pumice eruption (Nappi et al. 1995), marked the
abrupt sedimentary transition from a basal layer of coarse
gravel (diameter ≥ 10 cm) to sandy clay deposits within the
fluvial–lacustrine succession of the lowest terrace. This
age is consistent, within associated errors, with the astro-
calibrated ages of glacial termination IV (337–335 ± 10
ka, Imbrie et al. 1984; Shackleton et al. 1990; Lisiecki and
Raymo 2005), supporting the sedimentary model of aggra-
dational successions proposed by Marra et al. (2008, 2016),
which accounts for a direct correlation between deglacial
events in the Apennine mountain chain and the emplacement
of thick gravel beds in the catchment basin of the Paleo-
Tiber River. Other fallout layers occurring within the fine-
grained portion of the fluvial–lacustrine succession in the
three terraces constrained its aggradation between 332 ± 1
and 322 ± 1 ka, coinciding with the sea-level rise during MIS

<figure><img src="image_3.png" /><figcaption>Fig. 1 DEM image (Tinitaly, property of INGV, Rome, Italy) show-
ing the investigated area in the surroundings of Castel Cellesi village,
located in the upper catchment of the Tiber River basin. The volcanic
districts of the Tyrrhenian Sea margin of central Italy are also shown</figcaption></figure>

10/9, and indicating the occurrence of post-sedimentary tec-
tonic displacement. These chronological constraints are also
in very good agreement with the age proposed for the same
termination by the ice core community (Bouchet et al. 2023).

In the present work, we present the results of a study con-
ducted in the same eastern Vulsini area, which allowed us to
precisely frame the aggradation of three sedimentary suc-
cessions within three consecutive sea-level oscillations dur-
ing MIS 14/13, MIS 12/11, and MIS 10/9. Besides provid-
ing further confirmation to the sedimentary model linking
phases of sediment aggradation within the river's catchment
basins to glacial terminations, this work should be regarded
as a case study to apply this methodological approach to the
detection and precise dating of glacial/interglacial transitions
in other similar regions of the world.

Geologic setting

The investigated area is located in the central portion of the
Tiber River catchment basin and corresponds to a sub-basin
on the hydrographic right which drains the eastern outer
flanks of the Bolsena Caldera in the Vulsini Volcanic Dis-
trict (Teverina region) (Fig. 1). This sector hosted the Plio-
Pleistocene marine basins formed by a back-arc extensional
process in the central Tyrrhenian Sea, induced by the west-
ward subduction of the Adrian microplate (Malinverno and
Ryan 1986; Patacca et al. 1990). Progressive uplift of this
area has occurred since 1.8 Ma, leading to its continentali-
zation, concurrent with magma uprising and the formation
of the volcanoes of the Roman Magmatic Province (Peccer-
illo 2017, and ref. therein), since 800–600 ka (Giaccio et al.
2015; Marra et al. 2020a). Throughout Middle-Late Pleis-
tocene, the geologic evolution of this region was dominated
by the interaction between volcanic activity, tectonics, and
glacio-eustasy, which controlled the sedimentary processes
(Marra et al. 2019; Di Buduo et al. 2020).

Explosive volcanic activity in the Vulsini District began
around 600 ka with the Paleo-Vulsini phase (Vezzoli et al.
1987; Palladino et al. 2010, and ref. therein) and was charac-
terized by a series of Plinian eruption, depositing thick fall-
out layers from approximately 590 through 570 ka, followed
by a period of inactivity marked by a deep paleosoil, which
preceded the deposition of large welded pyroclastic-flow
sheets since around 500 ka ("Nenfri" *Auctorum*) (Barberi
et al. 1994; Nappi et al. 1995).

Successively, discontinuous early activity in the so-called
Vulsini Fields preceded the highly explosive activity of the
Bolsena-Orvieto phase since around 350 ka (Ponticello
Pumice eruption; Nappi et al. 1995; Palladino et al. 2010;
Marra et al. 2020b), which included the major caldera-form-
ing eruption of the Orvieto-Bagnoregio Ignimbrite (333 ± 4
ka; Nappi et al. 1995).

<figure><img src="image_4.png" /></figure>
Moderate, mainly phreatomagmatic activity occurred from approximately 280 through 230 ka at the Montefias-cone center, while a climactic phase, characterized by the emplacement of several pyroclastic-flow and Plinian fall deposits, occurred at the Latera Caldera from approximately 250 ka through 150 ka (Palladino et al. 2010; Monaco et al. 2020). This was followed by a final Strombolian and effusive activity from several intra- and extra-caldera vents, with available data recording activity until approximately 110 ka (Marra et al. 2020a).

## Previous studies

Since pivotal studies in the late 90s (Alvarez et al. 1996; Karner and Renne 1998; Karner and Marra 1998; Marra et al. 1998), a vast literature documented the close inter-play between glacio-eustastic sea-level oscillations and sedimentary processes along the Tyrrhenian Sea coast of central Italy, thanks to the 40Ar/39Ar dating of volcanic layers intercalated with the sedimentary successions (e.g., Luberti et al. 2017, and ref. therein). In particular, a sedimentary model was developed which linked the abrupt shift from the basal coarse gravel layer to clayey sediments in coastal fining-upward successions with the occurrence of the glacial terminations ("aggradational successions", Marra et al. 2008, 2016; Giaccio et al. 2021). More recently, using 40Ar/39Ar ages of detrital sanidine, Marra et al. (2022) have shown that gravel deposition within the upper catchment of the Liri River basin (Latium, central Italy) was closely related with ice melting in the glaciers of the Apennine mountain range during glacial terminations V and IV, leading MIS 111 and MIS 9 highstands.

In the present study, we benefit from the particularly favorable geologic conditions occurring in the investigated area, located within the upper Tiber River basin draining the eastern flanks of the large Vulsini volcanic district and affected by regional uplift during the last 600 kyr (Marra et al. 2019; Di Buduo et al. 2020), to demonstrate the close coincidence between the timing of deposition of three embedded fluvial-lacustrine aggradational successions and the sea-level rises during MIS 14/13, MIS 12/11, and MIS 10/9.

Unlike previous studies in the Tiber delta, in which composite aggradational successions were reconstructed through the correlation of different, partial outcrops (e.g., Karner and Marra 1998; Marra et al. 2016, 2017, 2021), or far from the volcanic vents which used maximum ages from reworked volcaniclastic deposits (Marra et al. 2022), here a rather continuous volcanic activity in the near Vulsini district 580 through 320 ka provides a large number of tephra layers intercalated with the sedimentary deposits, allowing their strict geochronologic constraining. Moreover, the canyon-like geomorphologic features of this area, result of

a prolonged volcano-tectonic uplift, provide the almost continuous exposition of a suite of sedimentary successions, encompassing three glacio-eustatic cycles occurred in this time span.

## Methods

### $^{40}$Ar/$^{39}$Ar analysis

Nine samples were selected for $^{40}$Ar/$^{39}$Ar dating at the Laboratoire des Sciences du Climat et de l'Environnement (CEA, UMR 8212 CNRS-LSCE; Gif Sur Yvette, France). Pumice clasts from each sample were crushed, and sieved into three size fractions (2 mm, 1 mm, and 500 µm). The 1–0.5 mm fraction was washed repeatedly with distilled water and dried, as it contained the highest abundance of inclusion-free K-feldspar crystals based on binocular microscopic screening. Transparent K-feldspar crystals were hand-picked under a binocular microscope from this fraction. The selected crystals excluding samples with leucite were leached for 5 min in a 7% HF acid solution to remove any attached groundmass, followed by repeated rinsing in an ultrasonic bath with distilled water. Samples with leucite were leached for 5 min in a 10% HN03 acid solution. At least 30 individual crystals per sample were loaded into separate aluminum disks and irradiated for 1 h in the Cd-lined, in-core CLICIT facility of the Oregon State University TRIGA reactor in three distinct irradiations, IRR. CO-017 (CC2, CC12, CC13); IRR.CO-019 (CC15, CC14, CC5, and CC16; IRR.CO-022 (CC3bis and CC4). Interference corrections were applied based on nucleogenic production ratios. Following irradiation, individual crystals were loaded into a 133-pit copper sample holder in a Teledyne Cetac differential vacuum window connected to a customized extraction line. Crystals were fused using a 100 W CO₂ laser (Teledyne Cetac) at 2.5 W for 15 s, after a 15 s presweep at 0.3 W to remove surface contaminants. Extracted gases were purified by SAES GP-50 getters, first by a cold getter for 90 s, then two hot getters for 210 s. Argon isotopes (40Ar, 39Ar, 38Ar, 37Ar, 36Ar) were measured on a Thermo Scientific Argus VI multicollector noble gas mass spectrometer equipped with nine ATONA® amplifiers and one electron multiplier. Technical details are provided in Cox et al. (2020). $^{40}$Ar, $^{39}$Ar, and $^{38}$Ar were measured simultaneously on three ATONA® amplifiers, while $^{36}$Ar and $^{37}$Ar were measured sequentially on the electron multiplier, using 15 cycles of 20 s integrations. Data reduction was performed using ArArCALC V2.4 (Koppers 2002). *J* values were calculated by co-irradiating Alder Creek sanidine (ACS-2, 1.1891 ± 0.0008 Ma; Niespolo et al. 2017) and Renne et al. (2011) decay constants. At least 12 flux monitors bracketed unknowns. Mass discrimination was monitored daily and procedural blanks measured every three
analyses. Full analytical data are provided in supplementary material.

## Petrographic analysis

The texture, mineralogical composition, and petrographic classification were determined through observations with a polarizing microscope on thin sections with a thickness of 30 µm obtained from the three rock samples. The thin rock sections were prepared at the thin-section preparation laboratory of the Department of Earth Sciences, Sapienza University of Rome. The rock samples were cut using a diamond saw into parallelepiped blocks with dimensions of 40×20×10 mm. The samples were isotropic and did not require oriented cuts during block preparation. To prevent detachment or complete abrasion of particularly soft minerals, such as secondary ones, the three blocks were embedded in a two-component epoxy resin within a plexiglass container of adequate size. Once the resin hardened, one of the larger surfaces was temporarily glued onto a glass slide measuring 47×27×1.5 mm and subsequently flattened on a horizontal grinding wheel using abrasives of decreasing grit size, followed by polishing with diamond paste. The resulting polished surface was then permanently glued onto a microscope slide of dimensions (47×27×1.5 mm). The block, now permanently glued, was reduced to a thickness of approximately 80–100 µm using a specialized thickness-reducing machine equipped with electronic thickness control. Subsequently, through manual procedures, the thin section was further reduced to a thickness of approximately 40–50 µm and finally polished with diamond pastes to a final thickness of approximately 30 µm. The thin sections were then analyzed using a Leitz optical microscope, equipped with a rotating stage, polarizing lenses, objectives up to 100×, and a photographic imaging system.

## Results

### $^{40}$Ar/$^{39}$Ar dating

Results of the $^{40}$Ar/$^{39}$Ar analyses are summarized in Table 1. Probability age diagrams are reported in Fig. 2, and full analytical data are provided in Supplementary Material #1.

Excluding one sample CC12 that displays a complex multimodal probability diagram (Fig. 2) suggesting that this deposit is reworked all other samples have relatively simple probability diagrams corresponding to primary deposits.

CC4 and CC5 are the only two samples that have xenocrystal contamination. Such contamination is moderated with one and three xenocrystals for CC4 and CC5 respectively (Fig. 2). After excluding these older crystals, the remaining ones allow to calculate a robust weighted

<table><caption>**Table 1** $^{40}$Ar/$^{39}$Ar ages of samples dated in the present work and in Mazza et al. 2024(1)</caption><thead><tr><th>Sample</th><th>Age (ka)</th><th>± 2 sigma</th></tr></thead><tbody><tr><td>CC3bis</td><td>317.2</td><td>0.7</td></tr><tr><td>CC4</td><td>319.9</td><td>4.0</td></tr><tr><td>CC16</td><td>326.9</td><td>3.1</td></tr><tr><td>CC5</td><td>327.4</td><td>4.4</td></tr><tr><td>CC11(1)</td><td>329.2</td><td>0.8</td></tr><tr><td>CC10(1)</td><td>398.9</td><td>1.0</td></tr><tr><td>CC17(1)</td><td>401.6<sup>a</sup></td><td>1.2</td></tr><tr><td>CC12</td><td>435.8</td><td>1.5</td></tr><tr><td>CC9(1)</td><td>444.7<sup>a</sup></td><td>2.2</td></tr><tr><td>CC13</td><td>469.3</td><td>0.8</td></tr><tr><td>CC14</td><td>480.9</td><td>1.0</td></tr><tr><td>CC15</td><td>499.3</td><td>0.8</td></tr><tr><td>CC2</td><td>504.2</td><td>0.9</td></tr></tbody></table>

<sup>a</sup>Youngest crystal population age

mean age of 319.9 ± 4.0 and 327.4 ± 4.4 ka. All the other volcanic deposits (CC16, CC12, CC13, CC14, CC15, and CC2) display a single homogeneous juvenile population of crystals allowing to calculate weighted mean ages that will be interpreted as the age of deposition of these volcanic deposits (Table 1).

## Petrography

### CC13

One thin section of this dark gray, welded pyroclastic-flow deposit, characterized by the occurrence of large (cm- to dm-sized), black, sanidine-bearing scoriae with elongate vesicles, was observed.

The scoriae clasts display a porphyritic to glomeroporphyritic texture (phenocrysts = 5–10 vol%) formed by millimeter-sized sanidine and sub-millimeter-sized plagioclase, biotite, green, diopsidic clinopyroxene, and magnetite, with sanidine and magnetite being the most and least abundant mineral phases, respectively (Fig. 3a). Occasionally, felspars form mineral clusters (Fig. 3b) and anti-rapakivi texture, characterized by a plagioclase core and a mantle of alkali feldspar (Fig. 3c). The groundmass is glassy and vesicular. Vesicles are diffuse, strongly elongated (Fig. 3a), and frequently interconnected. The chemical composition of the glass, at the boundary between the trachytic and phonolitic fields of the TAS diagram, coupled with the mineralogical composition, indicates that sample CC1 is a trachytic scoria.

### CC5–CC16

Two thin sections of this dark gray, massive, semi-coherent ash-and-scoria pyroclastic-flow deposit were analyzed.
<figure><img src="image_5.png" /><figcaption>**Fig. 2** Age plots showing single crystal ages of the dated samples and the weighted mean ages with two sigma uncertainty. MSWD mean-squared weighted deviation, *p* probability</figcaption></figure>
**Fig. 3** Microphotographs of the CC13 sample at the optical microscope and parallel (a, b, d) and crossed nicols (c) light. a Diffuse, elongated in shape vesicles; b cluster of sanidine phenocrysts; c feldspar phenocryst featuring plagioclase core and alkali feldspar mantle (anti-rapakivi texture); d green, diopsidic clinopyroxene and magnetite

<figure><img src="image_6.png" /></figure>

The scoria clasts show a porphyritic texture (phenocrysts < 5 vol%) formed by scarce, millimeter-sized, anhedral olivine and colorless diopsidic clinopyroxene phenocrysts, and microphenocrysts (500–50 µm) of green diopsidic clinopyroxene, plagioclase, leucite, and rare

phlogopitic mica (Fig. 4). The groundmass is glassy and vesiculated with diffuse leucite microlites (< 50 µm). Vesicles are diffuse, ranging from spherical to irregular in shape, and frequently connected. Glass has completely altered to zeolites formed in subsolidus conditions, precluding its use

**Fig. 4** Microphotographs of the CC5 (a, b) and CC16 samples at the optical microscope and parallel light. a Millimeter-sized, colorless diopsidic clinopyroxene phenocryst with melt inclusions; b green and colorless, diopsidic clinopyroxenes in the glassy groundmass with spherical to irregular in shape vesicles; c millimeter-sized, anhedral olivine and d leucite microphenocryst in glassy groundmass with irregularly shaped vesicles

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
for rock classification. However, the presence of leucite microlites associated with olivine, diopsidic clinopyroxene, and plagioclase indicates that the CC5 and CC16 samples are tephritic leucitite scoria clasts.

## Chronostratigraphy

Results of the chronostratigraphic investigations are summarized in the geologic map of Fig. 5, along with the stratigraphic sketches of the sites where samples for $^{40}$Ar/$^{39}$Ar dating were collected.

More detailed stratigraphic sketches of the investigated sections are shown in Figs. 6, 7 and 8 and their correlations, based on the acquired geochronologic constraints, are discussed hereby.

### Site 1

A section ca. 30 m thick is exposed along the southern escarpment bordering the Castel Cellesi hamlet (Figs. 5, 6). The hillside is cut into the Paleovulsini volcanic succession, and a slope change marks the contact on top of the Plio-Pleistocene marine clay substrate at the foot of the cliff. Here, the top of the paleosol, which occurs between the early Paleovulsini fallout succession, dated to ~590–570 ka (Barberi et al. 1994; Nappi et al. 1995), and the "nenfri" succession, is exposed below a 5 cm thick, yellow pumice fallout underlying a dark gray, welded pyroclastic-flow deposit (Fig. 6A). A sample collected from the fallout layer (CC-13) yielded an age of $504 \pm 1$ ka, supporting field correlation with the Civitella d'Agliano pyroclastic-flow deposit ($505 \pm 6$ ka, Nappi et al. 1995). A several-meters-thick succession of bedded fallout deposits overlies the pyroclastic-flow deposit and is cut by an erosional surface on which well-bedded, silty clay lacustrine deposits with frequent volcaniclastic intercalations rest in onlap. A sample collected from a volcaniclastic layer (CC-3b) interbedded with the clayey succession yielded an age of $317 \pm 1$ ka (Fig. 6B). A lava sample (CC-4) collected from a wide plateau cropping out at the top of the hillside around 400 m a.s.l. yielded an age of $320 \pm 3$ ka which, given the large associated error, is compatible with its stratigraphic position sealing the sedimentary succession.

### Site 2

About 400 m east from the previous site, along the same escarpment, a ca. 2 m-thick pyroclastic-flow deposit crops out on the outskirts of Castel Cellesi village, at 360 m a.s.l. It displays the characteristic features of the "nenfri" with idiosyncratic cm- to dm-sized black, sanidine-bearing scoriae in a dark gray, welded ash-and-scoria matrix. However, a scoria sample (CC-13) yielded an age of $469 \pm 1$ ka,

representing a previously unknown, late eruption age for the "nenfri" units in the eastern Vulsini. Field and compositional features, considering the obtained age, suggest a preliminary correlation with the Piano della Selva unit, which is exposed in the Tuscania area, southern Vulsini, and is related to the late Paleovulsini activity (Palladino et al. 2010). The pyroclastic-flow deposit is followed conformably by a more than 10 m-thick succession of bedded fallout deposits and, at this site, overlies a ca. 2 m-thick lacustrine succession consisting of alternating bedded yellow silty clay and fallout deposits (Fig. 6C). A sample collected from the uppermost fallout layer (CC-14) yielded an age of $481 \pm 1$ ka.

### Site 3

An unpaved road descends along the escarpment bordering the Fosso delle Pianozze stream valley to the southwest, from Castell Cellesi to the valley bottom at the confluence with the Rio Chiaro stream. In the upper portion of the road, the bedded fallout deposits associated with the late nenfri succession are continuously exposed between 365 and 340 m a.s.l. At site 3, a lacustrine succession consisting of alternating bedded yellow silty clay, whitish carbonatic mud, and pyroclastic fall deposits crops out below the volcanic deposits (Fig. 7A). A sample collected from the second fallout deposit from the bottom (CC-15) yielded an age of $499 \pm 1$ ka, allowing the lateral correlation with the lacustrine succession exposed at a higher elevation at site 2.

### Sites 4–5

Discontinuous outcrops of whitish lacustrine silts, in which numerous volcaniclastic layers are intercalated, occur along the road between 335 and 325 m a.s.l. A massive, dark gray, semi-coherent ash-and-scoria pyroclastic-flow deposit is exposed along the road at a slightly lower elevation (Fig. 7B). Two samples collected from this volcanic deposit (CC5, CC-16) yielded ages of $327.4 \pm 4.4$ and $327 \pm 3$ ka. Also, whitish lacustrine silts are exposed on top of the pyroclastic-flow deposit, ca. 250 m south-eastward (site 5 in Fig. 7).

### Sites 6–7

The lower and upper portions of a lacustrine succession, geochronologically correlated with the sea-level rise during MIS 12/11 (Mazza et al. 2024), crop out at sites 6 and 7, respectively (Fig. 8).

At site 6, the escarpment of an alluvial terrace exposes a fluvial–lacustrine succession between 245 and 250 m a.s.l., consisting of alternating bedded yellow silty deposits and reworked volcaniclastic layers. A sample collected from one of these volcaniclastic layers (CC9) yielded the
<figure><img src="image_9.png" /><figcaption>**Fig. 5** a Geologic map of the investigated area; b stratigraphic sketches of the outcrops sampled for 40Ar/39Ar dating (see detailed sketches in Figs. 6, 7 and 8). Topographic base: sections 345021 and</figcaption></figure>

345034 of Carta Tecnica Regionale Numerica (CTRN) scale 1:5.000, Regione Lazio, Italy, 2014

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 6** Stratigraphic sketch and photographs of sampled volcanic deposits of sites 1 and 2 (see Fig. 5 for location)</figcaption></figure>

youngest crystal population, providing a maximum age of 445 ± 2 ka for the sedimentary succession (Fig. 8).

A detailed description of the stratigraphy exposed at Site 7, which corresponds to Campo della Spina locality, where the remains of a partial skeleton of *Palaeoloxodon antiquus* were recovered and museumized, is provided in Mazza et al. 2024. Here, two $^{40}$Ar/$^{39}$Ar ages constrain the deposition of a succession of alternating lacustrine and debris-flow layers between 402 ± 2 (sample CC17) and 399 ± 1 ka (sample CC-10) during the late MIS 11 highstand (Fig. 8A). A detailed analysis of this sequence, exposed between 265 and 270 m a.s.l., revealed a shift from arid (and possibly cooler) conditions, transitioning into an intermediate phase marked by periodic fluctuations between wet and dry cycles, and culminating in the establishment of wetter and warmer conditions (Mazza et al. 2024). At the top of this sequence, an unconformity boundary, marked by the deposition of coarse debris-flow deposits consisting of both volcanic rock and travertine fragments (Fig. 8A), indicates an increase in water transport leading to the erosional phase occurred during MIS 10 lowstand.

## Site 8

A thick succession of bedded fluvial–lacustrine deposits is exposed between 310 and 270 m a.s.l., below a thick package of reworked volcaniclastic deposits along the road descending the western cliffs of the Rio Chiaro stream valley. Several primary pyroclastic fall and reworked volcaniclastic layers are interbedded with the sedimentary deposits. A sample collected at 290 m a.s.l. from a white pumice fallout deposit (CC-11, Fig. 8B) yielded an age of 329 ± 1 ka (Mazza et al. 2024), allowing the correlation of the upper part of the fluvial–lacustrine succession with MIS 9. An unconformity boundary separating the MIS 9 and MIS 11 successions, characterized by the presence of debris-flow deposits, is observed a few meters below, along the same road (Fig. 8C).

## Site 9

A welded, orange ash-flow deposit with an erosional contact on the Plio-Pleistocene marine substrate occurs at site 9, in the northern, lowest portion of the cliff bordering the Rio Chiaro stream to the west (Fig. 8D). Discontinuous lenses
<figure><img src="image_12.png" /><figcaption>**Fig. 7** Stratigraphic sketch showing correlation among sites 2, 3, 4, 5, and 5b, and photographs of the sampled volcanic deposits (see Fig. 5 for location)</figcaption></figure>

of gravel occur above the erosional contact at the base of the volcanic deposit. A sample collected from the ash-flow deposit yielded an age of $437 \pm 1$ ka, allowing the correlation of the overlying lacustrine succession with MIS 11.

# Discussion

## Reconstruction of the aggradational phases and intervening tectonics

The lateral and vertical correlation of the sedimentary deposits, supported by the provided age constraints, permits the reconstruction of alternating aggradational and erosional phases step by step, and allows inferences about intervening tectonic displacement, as described in Figs. 9 and 10, and discussed below.

The earliest evidence of Pleistocene sedimentation in the investigated area is represented by the fluvial–lacustrine deposits cropping out between ~345 and 355 m a.s.l. at sites 2 and 3, which date to $499 \pm 1$ through $481 \pm 1$ ka during the late MIS 13.1 highstand (Fig. 9c). The base of this aggradational succession is not exposed, making it impossible to determine the precise onset of its deposition. However, the top of the succession is clearly deposited within a paleomorphological low, likely formed by erosional activity during the MIS 14 lowstand and possible concurrent fault displacement, as inferred from the stratigraphic setting at sites 1–2 (Fig. 9a). Indeed, no sedimentation occurred at site 1 between 570 (age of the Paleovulsini paleosol) and 469 ka, consistent with the higher elevation of the $469 \pm 1$ ka pyroclastic-flow deposit that caps the sediment aggradation at site 2 (Fig. 9d). Therefore, we assume that an early MIS 13.3 aggradational succession was emplaced from 535 ka below the exposed upper MIS 13.1 succession, within the

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 8** Stratigraphic sketch of sites 6, 7, 8, 8, and 9, and photographs of the sampled volcanic deposits (see Fig. 5 for location)</figcaption></figure>

paleoincision excavated during the sea-level fall from 575 through 535 ka (Fig. 9b).

Evidence for a new erosional phase occurring from 469 through 445/435 ka, concurrent with the MIS 12 sea-level fall, is indicated by the much lower elevation of the deposits correlated with MIS 11 at sites 6 and 9 (Fig. 9d), although intervening tectonic displacement may have enhanced the elevation gain.

However, a marked erosional phase is evidenced at site 9 by the direct emplacement of the 436 ka pyroclastic flow (an age remarkably coincident with the peak of MIS 12) on the Plio-Pleistocene substrate. The presence of gravel lenses above the erosional surface supports the occurrence of the glacial termination V during MIS 12/11 sea-level rise (Fig. 10). We remark that the apparently older age of 445 ± 2 ka provided by sample CC 9 to the early aggradational phase must be considered as a *terminus post quem*, since the sample was collected in a reworked volcaniclastic layer.

The tight match between sediment aggradation and sea-level rise during MIS 12/11 is evidenced by the age constraints on the fluvial-lacustrine deposits provided by Mazza et al. (2024) at site 7 (Fig. 10a). Notably, sedimentation

terminated soon after 399 ± 1 ka, as indicated by the emplacement of coarse debris-flow deposits on the clay sediments at sites 7 and 8 (Fig. 10b). A new marked erosional phase, concurrent with a long sedimentary hiatus, is evidenced at site 1 by the strongly unconformable contact of the sedimentary deposits dated to 317 ± 1 ka, while ages of 327 ± 3 and 329 ± 1 testify to sudden sediment aggradation during the MIS 10/9 sea-level rise, causing the rapid filling of the excavated paleo-valleys.

The significant elevation gain between the MIS 9 sedimentary deposits at site 1 and sites 4–8 is a decisive evidence for intervening tectonic displacement, likely associated with the volcanic activity in the Vulsini District from 589 through 110 ka (Marra et al. 2019, 2020a).

A tentative reconstruction of the tectonic displacement affecting the sedimentary successions and its timing is provided in Fig. 11.

No significant tectonism is inferred between 500 and 400 ka, assuming that two similar MIS 13 and MIS 11 aggradational successions were embedded within each other during the two consecutive glacio-eustatic cycles (Fig. 11a). However, moderate subsidence along fault A may explain
<figure><img src="image_15.png" /><figcaption>**Fig. 9** Evolution scheme reconstructing the alternating erosional and depositional phases and their correlation with the sea-level oscillations depicted in the δ¹⁸O record by Lisiecki and Raymo (2005) between 535 and 436 ka. Vertical red bars in the insets are the</figcaption></figure>

⁴⁰Ar/³⁹Ar dates of the volcanic deposits intercalated with the aggrada-
tional successions provided in the present work, allowing for correla-
tion. See text for further comment and explanation

an enhanced paleomorphology and continued sedimentation
until 481 ka following the MIS 13.1 highstand. Similarly,
incipient displacement along fault B can be regarded as the
cause of the lateral shift of the paleovalley (Fig. 11a).

Strong tectonic subsidence over a wide area since 340 ka
is suggested by the large thickness of the MIS 9 aggrada-
tional succession, its superposition on the MIS 11 aggrada-
tional succession, and the persisting sedimentation until 317
ka, well after the MIS 9 highstand (Fig. 11b).

Notably, such enhanced tectonism coincides with the
beginning of the main explosive phase in the NE sector of
the present Bolsena caldera from 345 through 320 ka (Marra
et al. 2020b), encompassing the Ponticello Pumice Plinian
eruption and the Orvieto-Bagnoregio caldera-forming erup-
tion (333 ± 4 ka).

Finally, a more local tectonic collapse (fault C in
Fig. 11c) is inferred to have occurred in the southeastern
sector of the investigated area after 317 ka, as indicated by

the continued accumulation of volcaniclastic deposits on the
MIS 9 fluvial–lacustrine succession, sealed by a large lava
flow and travertine lenses, consistent with the extensive fault
displacement affecting the MIS 9 terraced deposits in this
region (Marra et al. 2019).

Conclusions

The continuous volcanic activity occurred in the investi-
gated region throughout the Middle Pleistocene offered a
means to closely date the sedimentary deposits, thanks to
the dense intercalation of volcanic layers within the aggra-
dational successions.

Twelve 40Ar/39Ar ages presented in this work demon-
strate that sedimentation within the river catchment between
the coast and up to several tens of kilometers inland responds

<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 10** Evolution scheme reconstructing the alternating erosional and depositional phases and their correlation with the sea-level oscillations depicted in the δ¹⁸O record by Lisiecki and Raymo (2005) between 436 and 317 ka. Vertical red bars in the insets are the</figcaption></figure>

⁴⁰Ar/³⁹Ar dates of the volcanic deposits intercalated with the aggradational successions provided in the present work, allowing for correlation. See text for further comment and explanation

directly and synchronically to changes in sea level linked to glacio-eustatism.

Almost all sediment aggradation occurs exclusively during glacial terminations, in the time span between the negative and positive isotopic peaks, with a possible time-lag in the early regressive phase following the highstand, which seems triggered by local tectonic subsidence. However, it may also correspond to late sedimentary filling of the near coastal lagoon and marsh basins.

Sedimentation is completely absent during the regressive phase up to the maximum sea-level fall, a period characterized by erosion and lowering of the base level in the catchment.

Such features are particularly well displayed by the MIS 11 aggradational succession, bracketed by the ages of 436 ± 1 and 399 ± 1 ka yielded by primary volcanic deposits

at the very bottom and top. The oldest portion of the MIS 11 aggradational succession rests above a 436 ± 1 ka pyroclastic-flow deposit which overlies gravel lenses resting above the pre-aggradational substrate, directly, terminating a marked erosive phase. The youngest portion of the aggradational succession, in which a 399 ± 1 ka pumice fallout occurs, is immediately overlain by coarse debris deposits, evidencing the lowering of the base level in the catchment, linked with the incoming regressive phase terminating sediment aggradation.

Such chronostratigraphic constraints on the timing of an erosional phase during glacial MIS 12 starting at ~469 ka and lasting until 445/435 ka is key to characterize the lowstand of (or the progression toward) one of the most prominent glacial maxima of the last million years. This evidence agrees with the radiometrically constrained
<figure><img src="image_18.png" /><figcaption>**Fig. 11** Tentative reconstruction of the tectonic displacements affecting the sedimentary and volcanic deposits 504 through 317 ka, responsible for the observed present stratigraphic setting. See text for comments and explanation</figcaption></figure>

(U/Th) timing of particularly harsh climate conditions between ~457 and ~429 ka in southern Europe (Hu et al. 2024) and with a phase characterized by extended continental ice volume, which impacts global and Northern Hemisphere climate (see discussion in Hu et al. 2024).

The illustrated features of the sedimentary record, which constitutes a direct proxy of glacio-eustatic oscillations and deglacial events, are not exclusive of the investigated area and may occur in any coastal area in the world. Whenever these paleogeographic conditions are combined with the occurrence of continued volcanic activity (which is also a feature occurring in several coastal regions), the local near coastal stratigraphic record provides an extraordinary tool to investigate the climate change and its forcing mechanisms.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s00531-025-02494-2.

**Funding** Open access funding provided by Istituto Nazionale di Geofisica e Vulcanologia within the CRUI-CARE Agreement.

**Data availability** All the data used in the present paper are provided in the text and supplementary material.

## Declarations

**Conflict of interest** The authors have no relevant financial or non-financial interest to disclose. The authors have no competing interests to declare that are relevant to the content of this article.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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