Tephra layers from Holocene lake sediments of the Sulmona Basin, central Italy: implications for volcanic activity in Peninsular Italy and tephrostratigraphy in the central Mediterranean area
B. Giaccio a , * , P. Messina a , A. Sposato a , M. Voltaggio a , G. Zanchetta b , F. Galadini c ,
S. Gori c , R. Santacroce b a Istituto di Geologia Ambientale e Geoingegneria, CNR, Area della Ricerca RM1-Montelibretti, Via
Salaria km 29,300, 00016 Monterotondo Stazione (Roma), Italy b Dipartimento di Scienze della Terra, Universita` di Pisa, Pisa, Italy c Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Milano-Pavia, Milano, Italy abstract
We present a new tephrostratigraphic record from the Holocene lake sediments of the Sulmona basin, central Italy. The Holocene succession is represented by whitish calcareous mud that is divided into two units, SUL2 (ca 32 m thick) and SUL1 (ca 8 m thick), for a total thickness of ca 40 m. These units correspond to the youngest two out of six sedimentary cycles recognised in the Sulmona basin that are related to the lake sedimentation since the Middle Pleistocene. Height concordant U series age determinations and additional chronological data constrain the whole Holocene succession to between ca
8000 and 1000 yrs BP. This includes a sedimentary hiatus that separates the SUL2 and SUL1 units, which is roughly dated between <2800 and ca 2000 yrs BP. A total of 31 and 6 tephra layers were identified within the SUL2 and SUL1 units, respectively. However, only 28 tephra layers yielded fresh micropumices or glass shards suitable for chemical analyses using a microprobe wavelength dispersive spectrometer. Chronological and compositional constraints suggest that 27 ash layers probably derive from the Mt. Somma-Vesuvius Holocene volcanic activity, and one to the Ischia Island eruption of the
Cannavale tephra (2920 _ 450 cal yrs BP). The 27 ash layers compatible with Mt. Somma-Vesuvius activity are clustered in three different time intervals: from ca 2000 to >1000; from 3600 to 3100; and from 7600 to 4700 yrs BP. The first, youngest cluster, comprises six layers and correlates with the intense explosive activity of Mt. Somma-Vesuvius that occurred after the prominent AD 79 Pompeii eruption, but only the near-Plinian event of AD 472 has been tentatively recognised. The intermediate cluster (3600–
3100 yrs BP) starts with tephra that chemically and chronologically matches the products from the
‘‘Pomici di Avellino’’ eruption (ca 3800_ 200 yrs BP). This is followed by eight further layers, where the glasses exhibit chemical features that are similar in composition to the products from the so-called
‘‘Protohistoric’’ or AP eruptions; however, only the distal equivalents of three AP events (AP3, AP4 and
AP6) are tentatively designated. Finally, the early cluster (7600–4700 yrs BP) comprises 12 layers that contain evidence of a surprising, previously unrecognised, activity of the Mt. Somma-Vesuvius volcano during its supposed period of quiescence, between the major Plinian ‘‘Pomici di Mercato’’ (ca

9000 yrs BP) and ‘‘Pomici di Avellino’’ eruptions. Alternatively, since at present there is no evidence of a similar significant activity in the proximal area of this well-known volcano, a hitherto unknown origin of these tephras cannot be role out. The results of the present study provide new data that enrich our previous knowledge of the Holocene tephrostratigraphy and tephrochronology in central Italy, and a new model for the recent explosive activity of the Peninsular Italy volcanoes and the dispersal of the related pyroclastic deposits.
1. Introduction
Tephra studies in distal settings of the central Mediterranean and continental European regions have in the last decade increased significantly (e.g. Wastegård et al., 2000; Siwan et al., 2002; Siani et al., 2001, 2004; Turney et al., 2004;Wulf et al., 2004; Munno and
Petrosino, 2007; Blockley et al., 2008; Giaccio et al., 2008a; Paterne et al., 2008; Turney et al., 2008; Zanchetta et al., 2008 ). The reasons for this growing interest in tephrostratigraphy are its great potential as a relevant, and sometimes particularly precious, tool of investigation for both volcanological and Quaternary studies.
Within the volcanological sphere of interest, the recognition and mapping of the products of known explosive eruptions in remote areas provides information for determining some important eruption parameters, such as magnitudes and intensities, the knowledge of which has particular implications for hazard evaluation
(e.g. Pyle et al., 2006; Giaccio et al., 2007 ). Furthermore, long and continuous marine and lacustrine sedimentary successions have great potential in the preservation of the complete record of the explosive history of adjacent volcanoes, including the lesser events that might have been missed in the stratigraphic records of a volcanic area (e.g. Wulf et al., 2008 ). This, again, can have significant implications for the assessment of eruptive recurrence and provide improvements to broaden our knowledge of the behaviour of volcanoes over time (e.g. Paterne et al., 1990; Paterne and Guichard, 1993; McGuire et al., 1997 ).
The contributions of tephrostratigraphy to the wide spectrum of the Quaternary sciences are also significant. Well-dated tephra markers can provide us with formidable tools for refining the chronology of Quaternary sedimentary successions, and more importantly, for synchronising paleo-environmental records across the regions and integrating the stratigraphies of different events
(e.g. Giaccio et al., 2006; Fedele et al., 2008; Hoffecker et al., 2008;
Lowe et al., 2008 ). Indeed, these aspects are crucial in the current debate on the role, temporal/spatial origins, amplification and propagation of climate change (e.g. Lowe et al., 2008 ).
To be really effective, distal tephrostratigraphy evidently needs to have a robust stratigraphic, chronological and geochemical reference framework. The recurrent explosive activity of the Quaternary

volcanic centres in Italy ( Fig. 1 ) potentially makes the central
Mediterranean region one of the most suitable areas for the application of tephrostratigraphic studies. Indeed, for this area, significant reference datasets from studies with both proximal and distal settings are already available (e.g. Siani et al., 2004;Wulf et al., 2004;
Peccerillo, 2005 ; Di Vito et al., 2008 ; Insinga et al., 2009 ; Turney et al., 2008; Wulf et al., 2008 ). However, the general framework is still fragmentary, and more records, and especially continental ones, are required to improve our knowledge of tephrostratigraphy and climatic events in this part of the Mediterranean basin. Within this perspective, paleo-environmental archives that are characterised by continuous and high sedimentation rates, that contain rich tephra records, and that are suitable for multi-proxy paleoclimatic studies are of primary relevance.
Here, we present a new tephrostratigraphic record from the
Holocene calcareous sediments that autcrop in the Sulmona intermountain basin of the central Apennines ( Fig. 1 ). The ca 40-mthick exposed lake succession contains 37 tephra layers that document the explosive history of the Campanian volcanoes between ca 8000 and 1000 yrs BP. Of note, in addition to the most prominent tephra layers that have been commonly recognised in several distal archives (e.g. the ‘‘Pomici di Avellino’’ of ca
3800 yrs BP from the Mt. Somma-Vesuvius volcano), we document tephra layers that have derived from minor eruptions of the Mt.
Somma-Vesuvius and Ischia Island volcanoes. Moreover, a surprising number of tephra layers would demonstrate for the first time previously unrecognised activity of the Mt. Somma-
Vesuvius volcano between the major Plinian eruptions of the
‘‘Pomici di Mercato’’ (ca 9000 yrs BP) and the ‘‘Pomici di Avellino’’; alternatively a different, hitherto unknown origin has to be invoked.
2. Geological setting
The Sulmona basin ( Fig. 2 ) is one of the intermountain tectonic depressions of the Apennine chain thatwas formed during the Plio-
Quaternary extensional tectonic phase. This was driven by the system of NW–SE trending normal faults (e.g. Cavinato et al., 1993,
1994; Cavinato and De Celles, 1999; Bosi et al., 2003; Centamore et al., 2003; Galadini et al., 2003; Galadini and Messina, 2004 ) that dissected the earlier orogenetic, compressive structures (e.g.
Patacca et al., 1990; Doglioni, 1995; Cipollari et al., 1999; Patacca and Scandone, 2001; Cosentino et al., 2003 ).
In particular, the formation of the Sulmona basin was driven by the movements of the Mt. Morrone normal fault system, a Quaternary
( Cavinato and Miccadei, 1995; Miccadei et al., 1998 ) and currently active tectonic structure (e.g. Galadini and Galli, 2000;
Gori et al., 2007 ), which represents the NE border of the depression
( Fig. 2 ). Its Quaternary sedimentary infilling has been described in

several studies (e.g. Demangeot, 1965; Radmilli, 1984; Bosi and
Locardi,1992; Cavinato and Miccadei,1995; Carrara,1998; Miccadei et al., 1998; Ciccacci et al., 1999 ). According to these, the depression has seen continental deposition since the Early Pleistocene, with a culmination in the Middle Pleistocene period, during which the thickest lacustrine succession was deposited ( Cavinato and
Miccadei, 1995, 2000; Miccadei et al., 1998 ). Following this phase, the continental sedimentation was dominated by Upper Pleistocene alluvial fan deposits and minor Holocene colluvial episodes; no lacustrine deposits younger than those of the Middle Pleistocene have been documented in previous studies.
3. Materials and methods
3.1. Stratigraphic and geomorphological investigations
The stratigraphic and geomorphological analyses were carried out by routine field investigations and aerial photography interpretations.
The field investigations included the whole Sulmona basin, and they were particularly detailed in the northern sector of the basin ( Figs. 2 and 3 ).
3.2. Tephra analyses
Tephra layers were generally collected as bulk samples along the exposed section. For the tephras exhibiting an internal stratigraphy, several sub-samples were collected and treated as separate subunits.
The chemical compositions of the tephra layers were determined on micro-pumice fragments and/or glass shards (grain size
>125 mm), which were mounted on polished and carbon-coated thin sections. These analyses were carried out at the Istituto di
Geologia Ambientale e Geoingegneria (CNR, Rome, Italy) using a Cameca SX50 electron microprobe that was equipped with a fivewavelength dispersive spectrometer. The operating conditions were as follows: accelerating voltage, 15 kV; beam current, 15 nA; beam diameter, 10–15 mm; counting time, 20 s per element. The following standards were used: wollastonite (Si and Ca), corundum
(Al), diopside (Mg), andradite (Fe), rutile (Ti), orthoclase (K), jadeite
(Na), phlogopite (F), KCl (Cl), baritina (S), and metals (Mn). The Ti content was corrected for the overlap of the Ti and Ka peaks.
In order to evaluate the accuracy of the analyses, three international secondary standards (Kakanui augite, Iceladic Bir-1 and rhyolite RLS132 glasses from USGS) were measured prior to any series of measurements. The mean precision was about 1% for SiO2,
2% for Al2O3, 6% for K2O, CaO and FeO, and 8–10% for the other elements.
3.3. Uranium series disequilibrium methods
U series disequilibrium methods were used for age determination of ten bulk samples of carbonate sediments (eight samples of calcareous mud, plus one of travertine) and one tephra layer. The samples of carbonate sediment were dated by the 230Th/234U method, applying the age-correction procedure of Ku and Liang

(1984) . The tephra layerwas dated by the 226Ra/230Th agemethod for young potassic volcanic rocks, as described by Voltaggio et al. (1995) .
3.3.1. 230Th/234U dating
The calcareous mud and travertine samples to which the
230Th/234U method was applied appeared very homogeneous.
A preliminary gamma spectrometry investigation of several subsamples from the same total sample showed that the 238U/232Th ratio variation was not more than 10%. Therefore, the total-sample dissolution and isochronic technique proposed by Ku (1979) , and experimentally assessed by Bischoff and Fitzpatrick (1991) , was not used here. This technique requires a 238U/232Th variation higher than 10% to get a good dispersion of the points in the isochron diagram and low age errors. Therefore, the partial dissolution method was used, which followed the correction procedure of Ku and Liang (1984) and calculated the relative correction equations for the 230Th–234U–238U contents of the pure carbonate component.
The samples were dissolved in 1 M HNO3, and after separation of the insoluble residue, they were spiked with a known amount of solution containing 232U and 228Th (228Th/232U activity ratio¼ 1.027). The U and Th were co-precipitated with iron hydroxide by adding NH4OH. The precipitate containing Fe, U and
Th was dissolved in 10 M HCl and passed through a column of anionic Dowex 1 _8 resin (10 M HCl conditioned). The eluate contained the Th and it was evaporated to dryness. The U was eluted from the column with 0.1 M HCl and evaporated to dryness.
The U and Th fractions were each redissolved in 7 M HNO3 and separately purified by passage through anionic Dowex 1 _8 resin columns (7 MHNO3 conditioned) and successive elution with 0.1 M
HCl. After selective extraction with TTA, the U and Th fractions were deposited on U-free and Th-free steel planchets and counted by alpha spectrometry. The insoluble residues were dissolved in
HFþ HClO4 and analysed in the same way as the eluates.
Table 1 gives the U series data of the samples together with the data and ages calculated according to the Ku and Liang (1984) equations. The main assumptions of the
Ku and Liang’s (1984) method are the simultaneous absence of: (i) differential leaching between U and
Th; and (ii) differential adsorption of these radionuclides onto detrital minerals during the acid attack of the samples. In particular, and as shown by Taddeucci and Voltaggio (1988) , this method fails when Th-adsorbing phases occur in the detrital component. To test for preferential leaching of U over Th from detrital minerals and for differential adsorption onto detrital minerals, we plotted the
230Th/234U activity ratios of the eluates and their relative residues, from all of the samples analysed ( Fig. 4 ). A negative correlation here would be seen when U is preferentially leached or differentially adsorbed over Th during the carbonate dissolution. However, a positive trend is seen, thus providing a diagnostic indication of the absence of significant fractionation between these two

elements.
3.3.2. 226Ra/230Th dating
The 226Ra/230Th dating was achieved by a method originally proposed by Rubin and Macdougall (1990) , and modified by Voltaggio et al. (1995) , using K instead of Ba as the stable analogue for
Ra. This particular version of the method has been applied to young potassic rocks of the Volcano island and Mt. Vesuvius ( Soligo et al.,
2000; Carafa et al., 2001 ). One bulk sample of the tephra layer
SUL2-7 was analysed to determine the 226Raradiogenic/230Th ratio of the Th-enriched, 226Rainitial-depleted mineral phases that crystallized at a time close to the eruption age. To obtain a diagram of the
226Ra/228Ra versus K/228Ra ratios of the original sample before and after each elution step, six elution steps of the original sample were carried out using strong HCl and analysed by gamma spectrometry following the procedure of Voltaggio et al. (1995) ( Fig. 5 ). The intercept of the best-fit straight line represents the 226Raradiogenic/
228Ra ratio of the Th-enriched/226Ra-depleted phases. The fitting parameters were calculated using a regression algorithm
(Model 1-Yorkfit) as implemented in ISOPLOT/Ex, version3 ( Ludwig,
2003 ). The 226Raradiogenic/230Th ratio was obtained by dividing the intercept value for the 230Th/232Th value of the original sample
(analysed and measured by alpha spectrometry, according to the procedure described in Section 3.3.1
) and the crystallization age was calculated through the following equation: t ¼ _ln½ð226Ra=230ThÞ _ 1_=l226
3.3.3. 226Ra/230Th measurements
Four tephra layers with the highest glass contents were selected to measure their present 226Ra/230Th activity ratio. The high content of glass assures that the measured ratio has not resulted by mineralogical fractionation during the transport but that it reflects the magmatic value, allowing a comparison with values measured for
Holocene Italian volcanic samples. The data were obtained by dividing the 226Ra/228Ra activity ratio, measured by high resolution gamma spectrometry, and the 230Th/228Th activity ratio,measured in unspiked samples by alpha spectrometry, assuming secular equilibrium between 228Ra and 228Th, i.e. 228Ra activity¼ 228Th activity.
4. Results
4.1. A brief geological framework of the whole Quaternary succession of the Sulmona basin
Integrated morphological and stratigraphic investigations carried out principally in the north-eastern sector of the Sulmona basin ( Fig. 3 ) are supported by new radiocarbon and tephrochronological age determinations. These allow us to recognize and map six main lake depositional units, which are labelled from the oldest as Sulmona 6 to the most recent as Sulmona 1 (SUL6–
SUL1) in Fig. 3 a, b. Seven correlated alluvial fan units are also recognised
(data not shown).

Here we focus on the tephrostratigraphy of the two youngest lake units, and therefore only briefly summarise the whole of the
Quaternary succession.
The six lacustrine units are separated by erosional surfaces and/ or paleosols. The presence of well preserved, terraced, depositional top surfaces makes the SUL1, SUL2, SUL4 and SUL5 units further distinguishable and easily traceable throughout wide portions of the Sulmona basin ( Fig. 3 b). The lithology of the lake sediments is represented by relatively uniform light coloured calcareous mud.
This has only slight chromatic, textural and grain-size variations, which makes the deposits of the different units barely distinguishable from each other.
A further common feature shared by these six units is the occurrence of numerous cm-thick tephra layers. Several of these have very idiosyncratic stratigraphical and textural characteristics, which make their field recognition very straightforward. Indeed, a number of these layers, or clusters of layers, were used as local stratigraphic markers for recognising the lake units and tracing reliable correlations throughout the basin. A thick and coarsely grained tephra marker present in the SUL5 unit was also correlated to its proximal equivalent ( Freda et al., 2008 ), corresponding to the
‘‘Pozzolane Rosse’’ eruption from the Colli Albani volcano, central
Italy, dated at 456 _3 ka BP. This provided the lowest chronological control-point for the whole succession studied.
Additional chronological indications for dating the later units were provided by a rich Paleolithic sequence of the Svolte di Popoli open-air site ( Radmilli, 1984 ). This was associated with the sediments of an alluvial fan stratigraphically and morphologically correlated to the SUL4 unit, and attributed to the late Acheulean-
Mousterian technocomplexes ( Tozzi, 2003 ). According to these findings, the SUL4 unit should not be older than the Last Interglacial
(ca 130,000–110,000 yrs BP).
4.2. Geomorphology and lithology of the SUL1 and SUL2 units
The deposits of the SUL1 and SUL2 units correspond to the last two lacustrine depositional cycles recognised in the north-eastern sector of the basin ( Fig. 3 a, b). The depositional top surfaces of both the SUL2 and SUL1 units define well preserved terraces that show appreciable variations in their elevation due to the different depositional settings. In marginal lake areas, close to the slope carved into the Mesozoic carbonate rocks (e.g. the Gagliano locality; Fig. 3 a), the terrace of the SUL2 unit reaches a maximum height of ca 325 m a.s.l. It gradually drops, with a gentle and constant inclination of ca 3_, down to ca 275 m a.s.l. towards the original lake depocentre ( Fig. 3 a, b). Similarly, the elevation of the terrace associated with the SUL1 unit ranges from ca 300 m a.s.l. to ca 265 m a.s.l. This represents the lowest recognised terrace, located ca 15–50 m above the present plain (ca 250 m a.s.l.).

SUL2 is the main unit of the two here discussed units, as it comprises ca 29 m of outcropping massive whitish calcareous mud
( Fig. 6 a). A further 3 m of sediment (32 m in total) was exposed by excavating a trench, which, however, did not reach the base of the unit ( Fig. 7 , site 1). In the field, the SUL2 unit was never seen as a complete 32-m-thick sequence in any single outcrop. However, a composite section was reliably reconstructed by merging several partially overlapping sections, each one of which documented different stratigraphic intervals ( Fig. 7 ). The occurrence of several distinctive tephra or clusters of tephra allowed the correlation of these key sections, and made the resulting reconstruction of the whole succession reliable ( Fig. 7 ).
The ca 8-m-thick SUL1 unit comprises the usual whitish calcareous mud, which shows, however, a distinctive fine – possibly annual – lamination. This sedimentological feature can be regarded as a distinguishing character of this unit ( Fig. 6 c). It is separated from the earlier Holocene unit by an erosional boundary that affected the surface of the SUL2 unit, which was marked by dm- to m-thick levels of well rounded fluvial gravels and sands, capped by an immature paleosol. This indicates a temporary disappearance of the lake or a substantial drop in its level ( Fig. 7 , site 7).
4.3. Lithology and chemistry of the tephra layers
There are 31 and 6 ash layers within the lacustrine sediments of the SUL2 and SUL1 units, respectively ( Fig. 7 ). The tephra layers are here identified by a label made up of the short form of the relative unit (SUL2 or SUL1) followed by a number that refers to the relative stratigraphic position, where 1 is the most recent, at the top of the succession ( Fig. 7 ). With the exception of a dm-thick reworked layer that was at the base of the SUL1 unit, and other sporadic evidence of minor bioturbations, the tephra layers show sedimentological characteristics that indicate a primary, almost completely undisturbed, ash fallout origin ( Fig. 6 b). These features include: (i) sharp boundaries at the base and the top of the ash layers; (ii) no evidence of substantial mixing with lake calcareous mud; (iii) repetition of the same distinctive internal stratigraphy and constant thickness over considerable distances; and (iv) absence of cross-bedding that would indicate lateral transport.
On the whole, the tephra layers are rather well preserved and yielded relatively fresh material that was suitable for both lithological and chemical analyses. However, the surface particles and the lowtotals of glass composition of some tephra layers showevidence of weathering to various degrees. In particular, the micro-pumices from nine layers ( Fig. 7 ) are wholly altered and unsuitable for microprobe analyses.Averaged data aresummarised in Table 2 , while the whole dataset is available as online supplementary material .
Several analysed tephra layers have a relative low analytical total ( Table 2 ). However we consider these data reasonably reliable because no appreciable relation seems to exist between the normalised

concentrations of each elements and totals ( Fig. 8 ). This indicates that there was no significant differential leaching and the eventual chemical alteration did not change significantly the original proportion of each element. This is particularly true for the alkali which, being among the most mobile elements, should be the best indicators of any eventual differential leaching.
A basic lithological description and the chemical composition of the 28 tephra layers analysed is provided below.
SUL2-31 – This is the lowest tephra layer (ca 31 m depth from the top) and one of the thickest of the ash layers of the SUL2 unit (ca
5 cm thick). It is a finely grading ash layer that is entirely composed of whitish, extremely vesicular micro-pumices and y-shaped transparent glass shards. It has a rather homogeneous phonolitic composition, with a low K2O/Na2O ratio ( Table 2 ; Fig. 9 a).
SUL2-29 – This is a relatively coarse-grained ash layer, of ca
3.5 cm thick, which is composed of whitish–greyish, moderately vesicular and porphyritic micro-pumices, which includes feldspar, clinopyroxene and leucite crystals, with a homogeneous tephriphonolite/ phonolitic composition ( Table 2 ; Fig. 9 a).
SUL2-27 – This is a 4-cm-thick, finely grained ash layer of whitish, moderately vesicular micro-pumices, which includes sparse feldspar and mica crystals with a homogeneous phonolitic composition ( Table 2 ; Fig. 9 a).
SUL2-25 – This is a relatively coarse-grained and thick (ca 6 cm thick) ash layer that is made up of whitish–greyish, moderately vesicular and porphiritic micro-pumices, which includes feldspar, clinopyroxene, leucite, and mica crystals. It has a heterogeneous tephriphonolitic to phonolitic composition ( Table 2 ; Fig. 9 b).
SUL2-23 – This is a 3-cm-thick, finely grained ash layer that includes both whitish, sub-aphyric, very vesicular and elongated micro-pumices, with a phonolitic composition, and a minor amount of tephriphonolitic greyish leucite-bearing micro-scoria
( Table 2 ; Fig. 9 b).
SUL2-22 – This is a thick tephra layer (ca 7 cm thick) that is made up of white, aphyric and very vesicular micro-pumices that have loose phenocrysts of alkali feldspar, clinopyroxene and mica with few volcanic lithics. It has a heterogeneous phonolitic composition with a variable alkali ratio ( Table 2 ; Fig. 9 b).
SUL2-20 – This is a dark and thin (ca 1 cm thick) layer of very fine ash that is composed of blocky, leucite-bearing, brown to yellowish micro-scoria, which is characterised by a tephriphonolitic composition ( Table 2 ; Fig. 9 c), with loose crystals of feldspar and leucite.
SUL2-19 – This is composed of a basal mm-thick sub-layer of whitish phonolithic micro-pumices, followed by a thicker upper
SUL2-18 – This is a mm-thick layer composed of very fine whitish, vesicular micro-pumices and glassy, brown-greenish leucite-bearing micro-scoria with abundant loose crystals of mica,

alkali feldspar and leucite. The composition varies from phonotephrite
(glassy dark scoria) to phonolite (pale pumices).
SUL2-17 – This is a very thin layer (crypto-tephra) of very fine ash composed of transparent and elongated glass shards with sparse sanidine crystal and phonolitic composition ( Table 2 ; Fig. 9 c).
SUL2-16 – This layer has a thickness of about 2 cm and a peculiar make-up, which consists of mm-sized, white, very vesicular and aphyric pumices, which are embedded in a matrix of very fine aphyric ash (_125 mm). The chemical analyses revealed a different composition for the two young components, which clustered in the phonolitic and tephriphonolitic ranges ( Table 2 ; Fig. 9 c).
SUL2-15 – This is the thickest (ca 8 cm thick) and one of the coarsest grained ash layers. This tephra layer is arranged in two sub-layers, with a basal horizon of white, very vesicular and aphyric micro-pumices, and an upper level enriched in greyish, moderately porphyritic micro-pumices, volcanic lithic clasts and loose crystals of alkali feldspar, clinopyroxene and mica. Its phonolitic chemical composition is particularly homogeneous, with a slight variation in the alkali ratio ( Table 2 ; Fig. 9 d).
SUL2-12 – This relatively thin layer (ca 2 cm thick) is a massive aggregate of very fine grained (_125 mm) white and extremely vesicular micro-pumices, which occasionally includes relatively large phenocrysts of sanidine. It has a rather homogeneous, phonolitic composition, with a high alkali total and a relative low alkali ratio ( Table 2 ; Fig. 9 d).
SUL2-11 – This is a stratified, thick layer (ca 6 cm thick) of relatively coarse-grained, moderately porphyritic and barely vesicular yellowish micro-pumices, with loose crystals of leucite, alkali feldspar and clinopyroxene. Its chemical composition is particularly heterogeneous, ranging from tephriphonolitic and phonolitic, with a low alkali ratio ( Table 2 ; Fig. 9 d).
SUL2-10 – This is a stratified thick (ca 6 cm thick) layer of coarse ash that is made up of porphyritic and blocky greyish micropumices with loose crystals of leucite, feldspar, clinopyroxene and mica, and sparse sedimentary and volcanic lithics. Its composition ranges from phonotephrite to tephriphonolitic ( Table 2 ; Fig. 9 e).
SUL2-9 – This is a thin (ca 2 mm thick) and finely grained layer including two different types of juvenile clasts, represented by white, aphyric and moderately vesicular micro-pumices and blocky, leucite-bearing, brown glassy micro-scoria with abundant loose crystals of feldspar, clinopyroxene and mica and sparse sedimentary and volcanic lithics. It exhibits a marked double chemical composition, ranging from tephrite-phonotephrite
(brown blocky glass) to tephriphonolite-phonolite (white micropumices)
( Table 2 ; Fig. 9 e).
SUL2-8 – This is a mm-thick layer of white, very vesicular, aphyric and fine-grained micro-pumices, with a rather homogeneous phonolitic composition ( Table 2 ; Fig. 9 e).

SUL2-7 – This well stratified thick layer (ca 4.5 cm thick) has a very peculiar internal stratigraphy that defines three sub-units: (i) a basal layer of white, very vesicular, aphyric and relatively coarsegrained, up to 1-mm-sized, micro-pumices; (ii) a mid horizon of greyish, barely vesicular and porphyritic micro-scoria, grading upward to (iii) a very fine-grained grey ash. The chemical composition is phonolitic for the white basal micro-pumices, and tephriphonolitic–phonolitic for the grey micro-pumices ( Table 2 ;
Fig. 9 f).
SUL2-6 – This is a finely grained and thin (ca 2 mm thick) ash layer of whitish, porphyritic micro-pumices (clinopyroxeneþ feldspar) that are associated with loose crystals of biotite. It has a homogeneous phonolitic composition ( Table 2 ; Fig. 9 f).
SUL2-4 – This is a stratified, cm-thick layer with a basal level of white, very vesicular, aphyric and fine-grained micro-pumices, graded into a very fine-grained upper layer of whitish ash. It has a particularly homogeneous phonolitic composition, with variable alkali ratio ( Table 2 ; Fig. 9 g).
SUL2-2 – This is a thin (ca 3 mm thick) and finely grained layer of greyish and porphyritic micro-scoria with homogeneous tephritic– phonotephritic composition ( Table 2 ; Fig. 9 g).
SUL2-1 – This is the uppermost layer of the SUL2 unit, which was detected at a depth of about 4 m from the depositional top surface ( Fig. 7 ). It is a cm-thick layer (ca 3 cm thick) that is made up of relatively coarse-grained ash that is characterised by a notable amount of loose feldspar crystals that are associated with fragments of very vesicular grey micro-pumices. Its composition is peculiar ( Table 2 ; Fig. 9 g), as it is the unique trachyte ash layer of the whole tephrostratigraphic record of the Sulmona Holocene series.
SUL1-6 – This is the unique, considerably reworked layer of the whole Holocene succession, which is represented by up to 0.5 m of ash, which occurs just at the top of the paleosol separating the SUL2 and the SUL1 units ( Fig. 7 ). It is made up of green, porphyritic and finely grained micro-scoria that has a foiditic composition ( Table 2 ;
Fig. 9 h).
SUL1-5 – This is a thin layer (ca 1 cm thick) of finely grained, grey–green ash that contains brown, blocky, leucite-bearing microscoria and lighter coloured and moderately vesicular micropumices.
The chemical composition ranges from foidite (blocky scoria) to phonolite (vesicular scoria) ( Table 2 ; Fig. 9 h).
SUL1-4 – This is a thin layer (ca 1 cm thick) of very fine greyish ash that has a tephritic composition ( Table 2 ; Fig. 9 h).
SUL1-3 – This is a ca 3-cm-thick and relatively coarse-grained layer of white–grey micro-pumices, which contains an appreciable amount of clinopyroxene and feldspar crystals. It has a slightly variable chemical composition, from phonolite to tephriphonolite
( Table 2 ; Fig. 9 h).

SUL1-2 – This is a finely grained ash layer, of ca 1.5 cm in thickness, and made up of greyish, barely vesicular and porphiritic micro-pumices. It has a homogeneous tephritic composition ( Table
2 ; Fig. 9 h).
SUL1-1 – This uppermost layer of the SUL1 unit that occurs at less than a 1 m from the depositional top surface ( Fig. 7 ) is a finely grained ash layer, ca 2 cm in thickness, made up of greyish, barely vesicular and aphyric micro-pumices. It has a tephritic composition
( Table 2 ; Fig. 9 h).
4.4. Age measurements and 226Ra-excess
Height 230Th/234Uagemeasurementswere carried outon samples of calcareous mud (six) and travertine (one) sediments from SUL2
(seven samples) and SUL1 (one sample) units. The seven samples from SUL2 unit were distributed along the whole sequence, and yielded stratigraphically concordant ages between 7550_ 600 and
2950_1100 yrs BP ( Fig. 7 ; Table 1 ). This defines a sedimentation rate ranging between 5 mm/yr and 9 mm/yr, with an appreciable increase at ca 3600 yrs BP ( Fig. 10 ). The sample from SUL1 unit, collected around itsmidpoint ( Fig. 7 ), provided an age of 1700_2100 yrs BP. A further 230Th/234U measurement was performed on the calcareous sediments of the SUL4 unit, yielding an age of 94_16 ka BP.
Following the procedure proposed by Voltaggio et al. (1995) ; see also Section 3 , a 226Ra/230Th measurement was carried out on the
SUL2-7 tephra layer. Although not statistically distinguishable from the topmost 230Th/234U determination, this measurement gives a slightly younger age (2450 _ 200 yrs BP) compared to the
230Th/234U-based chronological framework ( Fig. 10 ).
Finally four radiocarbon measurements were performed on both the lake calcareous and organic sediments of the SUL2 unit and on the organic material from the paleosol that separated the
SUL2 and SUL1 units ( Fig. 7 , site 7). Although stratigraphically concordant – at least those performed on the organic fraction – these measurements yielded ages substantially older than those obtained with the U series disequilibrium methods, ranging from
36,000 to10,000 14C yrs BP ( Table 3 ). These chronological discrepancies are discussed below.
The 226Ra/230Th isotope measurements were performed on bulk samples of the tephra layers SUL2-31, SUL2-16, SUL2-12 and
SUL2-4, occurring at depth of 31.15, 23.3, 13.55 and 6.75 m from the top of the unit SUL2. The results indicate a significant 226Raexcess
(i.e. 226Ra/230Th> 1) for all the four Sulmona tephra layers
( Fig. 11 ). The chronological significance of these results is discussed below.
5. Discussion
5.1. Chronology
The chronological framework of the SUL2 and SUL1 units is here defined on the basis and consistency of several temporal

constraints, including: (i) the morphological and stratigraphic relationships with other Sulmona sedimentary units (Section 4.1
);
(ii) the 230Th/234U and radiocarbon age determinations and
226R/230Th activity ratio (Section 4.4
); (iii) the consistency of the age measurements with the stratigraphic constraints, including the tephra chemistry; and (iv) the archaeological and historical evidence.
As mentioned in Section 4.1
, the SUL2 and SUL1 units are morphologically and stratigraphically embedded within the older
Sulmona sedimentary units, including the SUL4 lake unit that is correlated to the alluvial fan deposits that comprise the ‘‘Le Svolte di Popoli’’ Palaeolithic open-air site. This provides a terminus post quem for the SUL2 unit, as not older than the Last Interglacial–
Interpleniglacial (ca 125,000–40,000 yrs BP) (cfr. Palma di Cesnola,
1999; Tozzi, 2003 ). This temporal interval is supported by the
230Th/234U age of 94,000_16,000 yrs BP that was obtained for the equivalent lake sediments of the SUL4 unit ( Table 2 ).
With regard to the age measurements, the marked discrepancy between the U series and the radiocarbon determinations ( Fig. 7 ) suggests that one of the two datasets is not reliable. Two compelling arguments indicate a substantial inaccuracy in the radiocarbon dates. First, the significant 226Ra/230Th disequilibrium which shows the basal tephra layer SUL2-31 as well as the stratigraphically upper layers SUL2-16, SUL2-12 and SUL2-4, indeed confirm that the whole SUL2 spans a temporal interval within the Holocene ( Fig. 11 ).
In fact, if a volcanic rock initially displays a 226Ra-excess, its
226Ra/230Th activity ratio trends towards unity on a time that depends on l226 and on the initial 226Ra/230Th activity ratio ( Fig.11
).
Considering that in Italy the highest initial 226Ra/230Th activity ratio was detected in the volcanic rocks from the recent historical activity of Somma-Vesuvius (1631–1944) ( Voltaggio et al., 2004 ) we assume that value (ca 11.3) as the highest reasonably possible initial one for the examined tephras ( Fig. 11 ). Even by assuming such a high initial value, second only to the Oldoinyo Lengai carbonatites
( Voltaggio et al., 2004 and reference therein), the basal tephra layer SUL2-31 cannot be older than 9000 yrs BP, whilst any lower initial value would implicate a younger age (e.g. ca 7 ka BP if the initial 226Ra/230Th activity ratio was ca 5; Fig. 11 ).
Second, if reliable, the radiocarbon age measurements of the
SUL2 lake sediments would indicate that deposition of the sequence studied here occurred between >36,000 and <10,000
14C yrs BP ( Fig. 7 ; Table 3 ). However, this temporal interval is not consistent with the largely predominant, Na- and Al-rich tephrophonolitic to phonolitic composition of the glass shards from the tephra layers ( Fig. 9 ; Table 2 ). Indeed, none of the Italian volcanoes active in this time span were fed by ultra-alkaline magmas. On the contrary, the Mediterranean tephrostratigraphic records indicate that this time interval was dominated by the

deposition of numerous trachytic tephra layers, mostly derived from eruptions of the Campania volcanoes, including some widespread markers, such as the Campanian Ignimbrite or Y5 (ca
40,000 yrs BP), Y3 (ca 30,000 yrs BP) and Neapolitan Yellow Tuff
(c. 14,000 yrs BP) (e.g. Paterne et al., 1988; Calanchi et al., 1998;
Narcisi and Vezzoli, 1999; Wulf et al., 2004; Giaccio et al., 2008a,b;
Zanchetta et al., 2008 ). None of the tephra from both SUL2 and
SUL1 units have any geochemical affinity with these marker layers.
On this basis, we consider the U series dataset as being much more reliable than the radiocarbon ages; the latter are likely to have been affected by considerable hard-water/reservoir effects. Indeed, the basin hosting the lake sediments is carved into Mesozoic limestone rocks; old carbon that derives from the dissolution of these deposits may have substantially altered the carbon isotope composition of the lake water and may account for the aging of the related sediments that is seen. In addition, the most important water system in the area is represented by the Pescara springs, which are fed by the huge karst groundwater (Gran Sasso massif), which is possibly characterised by a strong hard-water effect.
Based on the 230Th/234U chronometric determinations, we have developed a simple age model that provides a preliminary age estimation for both SUL2 and SUL1 units and the related tephra layers, according to which the end of the SUL2 lake sedimentation should be dated at about 2800 yrs BP ( Fig. 10 ). For this purpose, we have excluded the age obtained from the travertine sample ( Fig. 7 ), because it contained some Mesozoic limestone clasts that will have inevitably altered the measurement, yielding an overestimated age.
Although in general agreement with the 230Th/234U determinations, we also did not take into account the age obtained by the
226Ra/230Th procedure. Indeed, in contrast to the 230Th/234U procedure, the validity of which is by now fully tested, the accuracy of this 226Ra/230Th dating method has been verified only in relatively few studies (e.g. Voltaggio et al., 1995; Soligo et al., 2000;
Carafa et al., 2001 ). Being aware of its potential limitation, we have applied this method with the intention of having an additional chronological indication to confirm the attribution of the SUL2 unit to the Holocene period.
As to the age of the SUL1 unit, the 230Th/234U age determination of 1700 _2100 years BP indicates that its middle part cannot be older than ca 3800 years BP. The age of this unit can, however, be better constrained by considering its stratigraphic relationship with the older and well-dated SUL2 unit, as well as the history of the Popoli village which is located on a part that was previously occupied by the paleo-lake. The linear interpolation of the
230Th/234U age determinations of the uppermost part of the unit
SUL2 suggests an age for the base of the SUL1 unit of less than ca
2800 yrs BP ( Fig. 10 ). On the other hand, the upper sediments can

be attributed to an age older than the 11th–12th centuries AD, corresponding to the period of the foundation of the Popoli village.
In summary, the deposition of both the SUL2 and SUL1 units, and of the related tephrostratigraphic record, can be placed between ca 7600 and 1000 yrs BP, with a century-long sedimentary hiatus that is more recent than 2800 yrs BP, and possibly older than
2000 yrs ago ( Fig. 10 ).
5.2. Origin of the tephra layers
5.2.1. Chronological and geochemical constraints
Among the potential tephra sources, the volcanoes of the Roman
Province (Colli Albani, Sabatini, Cimini,Vulsini and Vico) should be considered, given their geographic proximity to the Sulmona basin
( Fig. 1 ) and their favourable position with respect to the prevailing eastward dispersion of the tephras. However, the most recent activities of these volcanoes have been dated to several tens of thousands of years ago (e.g. ca 36 ka Colli Albani; ca 90 ka Sabatini;
Karner et al., 2001; Marra et al., 2004; Freda et al., 2006; Giaccio et al., 2007, 2009 ) before the beginning of the deposition of the outcropping SUL2 (ca 8000 yrs BP). Although regarded unreliable
(see discussion above), even by considering the 14C chronology, the activities of these volcanoes, which reached their maxima between
450 and 250 ka BP (e.g. Marra et al., 2004 ), appear chronologically wholly incompatible to be regarded as potential sources for the
Sulmona tephras.
Few volcanoes were active in Italy over the time span during which the succession investigated here was deposited. Those that were are all located in southern Italy ( Fig. 1 ) and include: Campi
Flegrei (e.g. Di Vito et al., 1999 ), Ischia island (e.g. Orsi et al., 1996 ) and Mt. Somma-Vesuvius (e.g. Santacroce, 1987; Ayuso et al., 1998;
Branca and Del Carlo, 2004; Santacroce et al., 2008 ), in Campania;
Mt. Etna (e.g. Branca et al., 2004 ), in Sicily; and the islands of
Stromboli (e.g. Gillot and Keller, 1993 ), Vulcano ( De Astis et al.,
1997; Gioncada et al., 2003 ) and Lipari (e.g. Calanchi et al., 2002;
Gioncada et al., 2003 ) of the Aeolian volcanic arc, in the southern
Tyrrhenian Sea (for a recent review of the active Italian volcanoes, see also Santacroce et al., 2003 ) ( Fig. 1 ).
In addition to the chronology, a further constraint towards the discrimination of the tephra sources is provided by their chemical compositions. On the basis of the geochemical data ( Fig. 9 ; Table 2 ), two main compositional groups can be distinguished within the
Sulmona tephrostratigraphic record: (i) the first, which is the predominant group, comprises 27 of the 28 ash layers analysed, and it can be classified as ultra-alkaline, intending with this term the volcanic rocks that in the total alkali silica (TAS) diagram plot between the tephrite and the phonolite fields or into the foidite field ( Fig. 12 ); (ii) the second group only includes the tephra at the top of the SUL2 unit (SUL2-1). This can be classified as alkaline which includes the rocks with higher silica content and a relatively

lower alkali concentration, plotting between the trachybasalt and trachyte fields of the TAS diagram ( Fig. 12 ; Table 2 ).
For the origin of the ultra-alkaline tephra layers, we can surely exclude Mt. Etna and the volcanoes of the Aeolian arc, with the latter characterised by Na-alkaline (trachybasalts, hawaiites and minor benmoreites and trachytes) and calc-alkaline, high-K calcalkaline to shoshonitic rocks, respectively (e.g. Peccerillo, 2005 ), which in the TAS diagram occupy fields completely different from those that characterise this predominant group of the Sulmona tephra layers ( Fig. 12 ). Similarly, the Campi Flegrei (e.g. D’Antonio et al., 1999 ) and Ischia (e.g. Civetta et al.,1991 ) volcanoes, with their shared K-alkaline affinity, cannot be considered as possible sources for the ultra-alkaline group ( Fig. 12 ). Indeed, among all of the chronologically consistent sources, only the Mt. Somma-Vesuvius pyroclastic deposits have a comparable ultra-alkaline affinity
( Fig. 12 ) (e.g. Ayuso et al., 1998; Santacroce et al., 2008 ). Remarkably, due to the differentiation processes that involve extensive
K-feldspar crystallization ( Santacroce et al., 2008 ), the glass shards from the Mt. Somma-Vesuvius pyroclastics have a peculiar Na2O and Al2O3 enrichment; a characteristic that is shared with the
Sulmona tephra layers. In conclusion, within the whole Italian volcanic framework, the unique known volcano encompassing both the chronological and geochemical constraints that are required as a source for the Sulmona ultra-alkaline tephra layers is Mt. Somma-
Vesuvius.
As for the single trachytic ash layer of the K-alkaline group at the top of the SUL2 unit, the relatively low alkali ratio and low contents of CaO and FeO strictly replicate those of the Holocene explosive products of the Ischia Islands ( Fig. 12 b), from which, in all probability, the tephra derives.
5.2.2. Proximal and/or distal equivalents
5.2.2.1. The tephra from Ischia Island. The Holocene volcanism of
Ischia Island was punctuated by a high number of explosive eruptions, which were particularly concentrated during the last three millennia (e.g. Chiesa and Poli, 1988; Orsi et al., 1996; Santacroce et al., 2003 ), and were rather uniform in the chemical compositions of their products (e.g. Poli et al., 1987 ). This thus makes the identification of the distal equivalents problematic.
However, some Holocene distal tephra layers from Ischia Island have been identified recently, within the well-dated Monticchio
Lake tephrostratigraphic record ( Wulf et al., 2008 ). Remarkably, layer TM-2-1 of the Monticchio record, dated as 3040 _150 yrs BP, which Wulf et al. (2008) correlated to the Ischia Cannavale tephra, has the same peculiar enrichment in loose sanidine crystals that characterise layer SUL2-1 here, dated according the age model of
Fig. 10 at ca 3000 _1100 yrs BP. Such a lithological match and the good chronological agreement allow us to correlate layer SUL2-1 to the TM-2-1 Monticchio tephra ( Fig. 13 e), and therefore to the Ischia

Cannavale tephra dated at 2800 _180 14C yrs BP ( Orsi et al., 1996 ), which corresponds to 2920 _ 450 cal yrs BP ( Reimer et al., 2004 ).
Noteworthy, currently this is the most distal occurrence of the
Cannavale tephra.
5.2.2.2. Prominent, lesser and possibly previously unknown tephra layers from Mt. Somma-Vesuvius. The Holocene explosive history of the Mt. Somma-Vesuvius volcano is commonly subdivided into three main temporal intervals that are bounded by the products from the major Plinian eruptions of ‘‘Pomici di Mercato’’ (ca
8900 yrs BP), ‘‘Pomici di Avellino’’ (ca 3800 yrs BP) and ‘‘Pompeii’’
(AD 79), which in proximal areas act as important stratigraphic markers (e.g. see Santacroce et al., 2008 , for a detailed review particularly for glass shard compositions). The three Holocene stages – Mercato-Avellino (M-A), Avellino-Pompeii (A-P) and
Pompeii-20th century (P-20th) – differ among these in terms of either the frequency of the related inter-Plinian episodes or the silica undersaturation degree. In particular, in the sections close to the vent, the M-A period would appear to be represented only by the same bounding Plinian events of the ‘‘Pomici di Mercato’’ and
‘‘Pomici di Avellino’’ deposits, both mildly undersaturated phonolites
(e.g. Santacroce et al., 2008 ). The A-P interval is similarly represented by mildly undersaturated products, although, in contrast, it records the occurrence of several phreato-Plinian to violent Strombolian/Vulcanian explosive events, known as ‘‘Protohistoric’’
( Rolandi et al., 1998
) or ‘‘Avellino-Pompeii’’ (AP;
Andronico and Cioni, 2002; Santacroce et al., 2008 ) eruptions. The third temporal interval (P-20th) results from a significant shift in the chemical composition of the feeding magmas, from mildly undersaturated to highly undersaturated (e.g. Ayuso et al., 1998; Santacroce et al., 2008 ). This last interval is punctuated by numerous
Strombolian to near-Plinian explosive eruptions, including the prominent AD 472 and AD 1631 events (e.g. Rolandi et al., 1998;
Santacroce et al., 2008 ).
A comparison between the 27 ultra-alkaline tephra layers distributed in both the Holocene succession investigated and the available data on the Mt. Somma-Vesuvius pyroclastic deposits, from both proximal and distal records, reveals that only a few of the Sulmona tephra layers can be unquestionably correlated to well-known Holocene explosive eruptions or distal tephra layers.
From the oldest deposits, the first of these layers is the SUL2-12 tephra, which occurs in close stratigraphic proximity to the
230Th/234U age determination of 3550_900 yrs BP ( Fig. 10 ). Both the age and chemical composition match those of the ‘‘Pomici di
Avellino’’ eruption ( Table 4 ; Figs. 13 a, 14 ), one of the largest
Vesuvius Plinian and pyroclastic-flow-forming events, which has been dated between 3600 and 4000 yrs BP (e.g. Rolandi et al.,
1993; Mastrolorenzo et al., 2006; Santacroce et al., 2008 ). In particular, the occurrence of some silica-rich trachytic glass shards

in SUL2-12 ( Table 2 ), which are distinctive of the final phreatomagmatic pyroclastic-flow deposits ( Sulpizio et al., 2008 ) ( Table 4 ), suggests that this layer can be correlated to the final stage of the
‘‘Pomici di Avellino’’ eruption. Indeed, during this phase, the main pyroclastic density current deposits and related distal ash were predominately dispersed northward, as testified by their recognition in the Latium and Tuscany regions, in central Italy, as far as
400 km north of the vent ( Sulpizio et al., 2008 ). Distal ash deposits that are related to the Plinian phase have been recognised in several localities of southern Italy and of the Adriatic Sea ( Sulpizio et al., 2008 , and reference therein). This correlation is further supported by the high 226Ra-excess characterising the layer SUL2-
12, which is comparable to that exhibited by the white pumices of the Plinian deposits of the ‘‘Pomici di Avellino’’ eruption ( Voltaggio et al., 2004 ; Fig. 15 ).
The cluster of eight tephrophonolitic to phonolitic layers from
SUL2-11 to the SUL2-2, which are dated between ca 3600 and
3100 yrs BP ( Fig. 10 ), and which were deposited between the
‘‘Pomici di Avellino’’ and the Cannavale distal equivalents (SUL2-12 and SUL2-1), is chronologically and compositionally comparable to the products of the Protohistoric or AP eruptions. According to
Andronico and Cioni (2002) , the A-P interval includes at least six major events, indicated as AP1–AP6, and two minor eruptions, as
AP3a and AP4a, respectively lying over AP3 and AP4. The previous study by Rolandi et al. (1998) reported three main Protohistoric eruptions, with the last eruptive unit subdivided into five members, which probably correspond to the AP3–AP6 succession of Andronico and Cioni (2002) . In distal settings, the AP-equivalent tephra layers have been recognised in the succession of Monticchio, southern Italy (AP2, 3, 4 and 5) ( Wulf et al., 2004, 2008 ), and of the
Adriatic Sea (AP3) ( Calanchi and Dinelli, 2008 ).
The studies carried out at the Mt. Somma-Vesuvius volcano and in distal areas generally agree in the recognition of a slight compositional shift towards more mafic terms, starting from the most evolved AP1 pyroclastics. However, a closer examination of the available chemical data, which was obtained with the same micro-analytic procedure (i.e. SEM–EDS), reveals appreciable differences in the composition of the single AP deposits, which suggests notable variability within the same eruptive units. This partially hinders the recognition of the distal tephra layers related to the single AP eruptions. Indeed, in our opinion, any possible correlation at present is strongly conditioned by the choice of one, rather than another, of the available datasets.
However, starting from these uncertainties, some aspects need to be discussed. The AP-equivalents identified at Sulmona coincide numerically with those recognised in the proximal area. On this basis, the layer-per-layer correlation of all of the eight distal tephra layers with the complete succession of the AP eruptions (AP1–AP6,

plus AP3a and AP4a) appears to be logical. However, the chemical compositions of the first two layers (SUL2-11 and SUL2-10) subsequent to the ‘‘Pomici di Avellino’’ equivalent, hardly match those of the deposits from the first AP1 and AP2 eruptions. Indeed, a correlation with the products from the AP3 and AP4 events would appear much more plausible ( Fig. 13 b, c). These possible AP3 and
AP4 distal equivalents are followed by two tephra layers that have chemical compositions that again do not match those theoretically expected immediately above; i.e. the products related to the AP5 and AP6 eruptions. In particular, the SUL2-9 layer is characterised by a marked bimodal composition ( Fig. 9 e; Table 2 ), which is never found in the AP proximal or distal series. A tentative correlation with AP6 may, however, be proposed for the subsequent SUL2-7 layer, the peculiar lithological features of which are comparable to those described by Andronico and Cioni (2002) for the zoned deposits derived from the AP6 eruption. Finally, the potential AP6- equivalent layer is followed by three further AP-related tephra layers that cannot be associated to any known proximal deposits.
Remarkably, the phonolitic SUL2-4 layer, which shows an
‘‘Avellino-like’’ composition, and the phonotephritic SUL2-2 layer exhibit chemical compositions that have never been identified in the AP proximal or distal products.
In summary, the Sulmona tephra layers that chronologically and stratigraphically coincide with the Mt. Somma-Vesuvius A-P inter-
Plinian interval probably include, from the base: (i) the AP3 and
AP4 distal equivalents; (ii) two layers that derive from unknown
Mt. Somma-Vesuvius eruptions; (iii) the AP6 equivalent; and (iv) three further layers that also derived from unknown Mt. Somma-
Vesuvius explosive events. This potential correlation would imply the occurrence of AP events not previously recognised in the proximal area or in other distal archives, including three eruptions that were more recent than the AP6 one.
The recognition of the numerous tephra layers possibly from Mt.
Somma-Vesuvius that occurred below the ‘‘Pomici di Avellino’’ distal equivalent, which are dated between ca 7600 and 4700 yrs BP
( Figs. 6, 7 ) and are temporally and stratigraphically coincident with the last 3–4 millennia of the M-A interval (ca 9000–4000 yrs BP), is even more surprising. Based on the available proximal data, this interval has to be considered a quiescent period, which was apparently interrupted by only two eruptions, designated as MA1 and MA2, which have not yet been conclusively attributed at Mt.
Somma-Vesuvius activity ( Santacroce et al., 2008 ). However, in the
Monticchio distal record, at least four tephra layers ascribed to Mt.
Somma-Vesuvius activity occur immediately after the ‘‘Pomici di
Mercato’’ distal equivalents (
Wulf, 2000 ). These Monticchio layers show strong chemical analogies with the potential MA equivalents recognised at Sulmona. In particular, the potential Sulmona MA equivalents show continuous oscillations between the phonolitic

and tephripholitic compositions ( Table 2 ; Fig. 9 a–d), with the phonolitic terms approaching the composition of either the
‘‘Pomici di Mercato’’ or the ‘‘Pomici di Avellino’’ deposits (
Fig. 13 d).
This is particularly evident for the SUL2-22 and SUL2-15 layers, the compositions of which differ from the ‘‘Pomici di Mercato’’ deposits for only slight variations in the silica content and the alkali ratio. In addition, the determined 226Ra-excess of two of these potential MA appears consistent with the temporal trend expected between the
‘‘Pomici di Mercato’’ and ‘‘Pomici di Avellino’’ eruptions (
Fig. 15 ).
For the succession of the six layers from the SUL1 unit, which were dated between ca 2000 yrs BP and 12th century, the occurrence of two foiditic tephra layers at its base (SUL1-6 and SUL1-5;
Fig. 9 h) suggests a chronological attribution subsequent to the
Pompeii eruption. Indeed this eruption marks the major change in the feeding system of the Mt. Somma-Vesuvius volcano, from the moderately undersaturated silica magmas to the strongly undersaturated ones (e.g. Ayuso et al., 1998; Santacroce et al., 2008 ).
However, with the exception of the major AD 472 and AD 1631 explosive events (e.g. Santacroce et al., 2008 ) and a few other minor ‘‘Ancient Historic’’ and ‘‘Medieval’’ eruptions ( Rolandi et al.,
1998 ), some of which have also been recognised in the Monticchio distal record ( Wulf et al., 2008 ), the products from the continuous violent Strombolian activity of this period are poorly known and still not well characterised in terms of the chemical compositions of the glasses. Although this, again, hinders conclusive correlations, for at least the SUL1-5 layer some hypotheses can by formulated. This layer has, in fact, a regular chemical trend from the foidite to the phonolite fields ( Fig. 9 h), with the most evolved terms approaching the typical composition of the products from the AD 472 eruption.
5.3. Potential implications
The discovery of the potential, intense explosive activity of Mt.
Somma-Vesuvius between the ‘‘Pomici di Mercato’’ and ‘‘Pomici di
Avellino’’ events and of additional unknown AP eruptions that have never been seen in either proximal or distal records has significant implications for our improved knowledge of the Holocene explosive history of this famous volcano, and for the assessment of the eruptive recurrence and related hazards in the surrounding, densely populated areas. However, although both chronological and geochemical data exclude any other known potential origin of these tephras, since at present there is no evidence of a similar intense Vesuvius activity, a firm, unquestionable correlation cannot be proposed at this time.
On the other hand, irrespectively of their origin, the recognition of these previously unknown Holocene tephra layers ( Fig. 16 ) significantly enriches our previous knowledge of the distal Holocene tephrostratigraphy in central-southern Italy, and possibly in the wider area of the central Mediterranean basin. A comparison

with previous studies reveals that the Sulmona record adds at least
22 potential marker tephra layers to the most complete available tephrostratigraphic framework (e.g. Wulf et al., 2008 , and references therein). These new tephra layers are clustered in three different time intervals between 7600 and 4800 (12 layers), 3800 and 3000 (5 layers) and 2000 and >1000 (5 layers) yrs BP. On the other hand, some tephra markers from prominent Campi Flegeri eruptions appear to be lacking at Sulmona; e.g. that of Agnano
Monte Spina ( de Vita et al., 1999 ) that was dated to 4690–
4300 yrs BP ( Blockley et al., 2008 ), and which has been recognised in several distal sequences (e.g. Calanchi et al., 1998; Siani et al.,
2004; Wulf et al., 2004 ; Magny et al., 2007 ). However, it might be represented by the chronologically consistent SUL2-14 (ca
4500_1000 yrs BP; Fig. 10 ), which unfortunately did not yield fresh glass shards for chemical analyses here.
The recognition of some previously undetected tephra layers
(e.g. SUL2-4, SUL2-15, SUL2-16 and SUL2-22) that are, at certain extent, compositionally hardly distinguishable from the other most common and prominent pyroclastics (i.e. ‘‘Pomici di Avellino’’ and
‘‘Pomici di Mercato’’) ( Tables 2 and 4 ) is particularly relevant from the tephrostratigraphic perspective. Indeed, this substantial compositional similarity might lead to erroneous attributions when dealing with incomplete or badly dated distal sequences, with evident stratigraphic and chronological repercussions.
Although the exceptional conditions of preservation of these unknown tephra layers are in all probability due to the particular local conditions of the Holocene Sulmona basin (high sedimentation rate, with the consequent rapid sealing of the volcanic ash that protected them from erosion, bioturbation, and mixing processes), these distal volcanic deposits should inevitably be found elsewhere.
In this regard, the geochemical ( Fig. 9 ; Table 2 ) and chronological
( Figs. 7, 10 ) datasets presented in the present study should represent a reference for future investigations.
6. Concluding remarks
In the present study, we report a new tephrostratigraphic record from a 40-m-thick Holocene succession in the Sulmona basin in central Italy. This succession is divided into two different sedimentary cycles of the lake (SUL2, ca 32 m thick, and SUL1, ca
8 m thick). Height U series age determinations, in conjunction with other chronological constraints, allow us to date the whole
Holocene succession between ca 8000 and 1000 yrs BP, with a interruption of the sedimentation between <2800 and
>2000 yrs BP, which separates the SUL2 and SUL1 units. Twentyeight of the total of 37 ash layers recognised were suitable for both lithological and chemical analyses. The age and chemical compositional constraints indicate that 27 of them derive possibly from the Holocene explosive activity of Mt. Somma-Vesuvius, while a single layer (SUL2-1) is correlated to the Cannavale tephra

(2920 _ 450 cal yrs BP) from Ischia Island.
Six tephra layers from the most recent SUL1 unit (ca 2000 yrs BP to >12th century) are broadly related to the post-AD 79 eruptive activity of Mt. Somma-Vesuvius. However, only for the SUL1-5 layer, which is attributed to the AD 472 eruption, is a plausible, precise correlation proposed ( Fig. 16 ).
Among the 21 tephra layers from the SUL2 unit (7600–
2800 yrs BP) that are attributable to Mt. Somma-Vesuvius, only the
SUL2-12 layer, which is dated to 3550 _ 900 yrs BP, is compellingly correlated to a well-known explosive event. This event is represented by the prominent ‘‘Pomici di Avellino’’ eruption of ca
3800 _ 200 yrs BP ( Fig. 16 ), the distal tephra layers of which acts as an important marker for several terrestrial, lacustrine and marine successions. The ‘‘Pomici di Avellino’’ equivalent is followed by a group of eight tephra layers that are stratigraphically, chronologically and chemically comparable to the products from the socalled
AP eruptions, a cluster of eight minor events that occurred between the Plinian events of ‘‘Pomici di Avellino’’ and ‘‘Pompeii’’
(AD 79). However, although the number of the Sulmona APequivalents is the same as the proximal ones, the chemical and stratigraphic data do not support a simple layer-per-layer correlation with the complete proximal set, with only the distal equivalents of the AP3, AP4 and AP6 eruptions tentatively recognised. In particular, three of the five undefined AP-equivalent tephra layers occur above the possible AP6-correlative layer, suggesting the existence of further AP events more recent than the youngest recognised in the proximal area ( Fig. 16 ).
Finally, the Sulmona tephrostratigraphic record indicates the occurrence of a minimum of 12 – or perhaps 17 – tephra layers geochemically compatible with an explosive activity of the Mt.
Somma-Vesuvius that date between ca 7600 and 4800 yrs BP ( Fig.15
), i.e. within the five-millennial-long period of relative quiescence experienced by this volcano between the ‘‘Pomici di Mercato’’ (ca
8900 yrs BP) and the ‘‘Pomici di Avellino’’ (ca 3800 yrs BP) eruptions.
However, although the present knowledge on the Italian Holocene volcanoes leads us in recognising the Mt. Somma-Vesuvius as the unique possible source of these tephras, the lacking of any evidence in the proximal setting of this volcano – among the best known in Italy – does not allow us to rule out a different and completely unknown origin.
In conclusion, the results of the present study indicate that our knowledge of the Holocene tephrostratigraphy in central Italy is far from conclusive. Many of the tephra layers from the Sulmona basin cannot be associated to any proximal or distal counterparts, and a reconsideration of the Holocene tephrostratigraphy in the central
Mediterranean area, and perhaps of the recent eruptive history of
Mt. Somma-Vesuvius, is now required. Our findings confirm the value of the distal archives as fundamental integrative sources of

data for volcanological studies, and they also reveal that even the tephra layers from minor explosive events can be traced over long distances and can act as potential, relevant temporal/stratigraphic markers on a wide regional scale.
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