Robust date for the Bronze Age Avellino eruption (Somma-Vesuvius): 3945 ± 10 calBP (1995 ± 10 calBC)

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Abstract

We found Bronze Age lake sediments from the Agro Pontino graben (Central Italy) to contain a thin (2–3 cm) continuous tephra layer composed of lithics, crystals and minor volcanic glass. Tephrochronological and compositional constraints strongly suggest that this layer represents the Avellino pumice eruption, which has also been identified in Central Italian lake cores. Its provenance is corroborated by electron-microprobe analyses performed on juvenile pumice grains, showing that the tephra layer is probably the distal equivalent of the EU2 event of the Avellino eruption. We used multiple 14C age estimations of two lacustrine sequences with intercalated tephra layer, from the western border zone (Migliara 44.5) and the centre of the former lake (Campo Inferiore), for in tandem dating of this eruption, employing the OxCal code, which yielded a robust age of 3945 ± 10 calBP (1995 ± 10 calBC). To date, this is the only study providing both a terminus post and terminus ante quem of this precision, also demonstrating the advantage of dating distal tephra layers in a clear stratigraphic context over proximal deposits in sequences with major stratigraphic hiatuses. Our new results underscore the importance of the Avellino tephra layer as a precise time marker for studies on the Early Bronze Age of Central Italy.

Highlights

► Lacustrine deposits in the Agro Pontino hold thin tephra layer from Somma-Vesuvius. ► Tephra layer can be unequivocally attributed to Early Bronze Age Avellino eruption. ► In tandem dating by OxCal code provides first truly robust date: 3945 ± 10 calBP. ► Late-Holocene tephrochronology of Central Mediterranean needs some revision.

Introduction

The Avellino eruption of Somma-Vesuvius was a catastrophic Plinian event of similar magnitude as the famous AD 79 eruption (e.g. Lirer et al., 1973, Rolandi et al., 1993, Cioni et al., 2000; Sulpizio et al., 2010a). In the past decade, major Early Bronze Age sites and settlements were found underneath its proximal pyroclastic deposits. Some of these are exceptionally well preserved and represent early analogues of Pompeii (e.g. Albore Livadie et al., 1998, Albore Livadie, 2002, Di Vito et al., 2009).

Several eruption phases have been recognized from the proximal deposits (e.g. Cioni et al. 2000; Sulpizio et al., 2008): an opening magmatic phase (EU1), a Plinian magmatic phase (EU2, 3 and 4) and a phreatomagmatic phase (EU5). The fallout deposits of the Plinian-type units are by far the most voluminous and comprise geochemically distinct white (EU2) and grey (EU3) well-sorted pumice fragments. The distal ‘Avellino pumice layer’ is known as an important tephrochronological marker bed for the Early Bronze Age and has been recognized in many major Late-Holocene sediment cores from central and southern Italy (e.g. from Lago di Accesa, Magny et al., 2007; Lago di Mezzano, Ramrath et al., 1999; Lago di Albano and Lago di Nemi, Calanchi et al., 1996; Lago Grande di Monticchio, Wulf et al., 2008; Sulmona Basin, Giaccio et al., 2009), the Balkans (Lake Shkodra, Sulpizio et al., 2010c) as well as in marine cores from the Adriatic Sea (see Sulpizio et al., 2008 for a recent review). It is marked by highly vesicular porphyritic pumice, a relatively large crystal content (including sanidine, pyroxene, amphibole and garnet), and a mildly SiO2-undersaturated composition showing a transition from phonolitic for earlier deposited white pumice to tephri-phonolitic for subsequently deposited grey pumice. High Al2O3 (>23 wt.%) and low CaO (≤2 wt.%) contents in glasses are distinctive features relative to glass compositions of other major eruptions of Somma-Vesuvius (e.g. Santacroce et al., 2008, Giaccio et al., 2009).

Most information concerning the age of the eruption comes from 14C dating of organic matter associated with its deposits. Nearly all 14C dates pertain to organic carbon from paleosols, more or less carbonized plant/wood remains from archaeological sites, and skeletal material of animals or humans (e.g. Passariello et al., 2009, 2010). These materials originate from proximal ash sequences with major hiatuses. Passariello et al. (2009) reviewed all published 14C ages, which range from 3360 ± 40 (Vogel et al., 1990) to 3920 ± 50 (Watts et al., 1996). Passariello et al. (2010) concluded, also based on new dates of bones of goats killed by the eruption, that this eruption dates to ‘3550 ± 20 BP, which calibrates to 1935–1880 BC (1σ)’. However, in a slightly earlier review paper on the distal tephrostratigraphy of Somma-Vesuvius, Santacroce et al. (2008) concluded that the best ages available ‘yield an averaged age of 4365 ± 40 cal yr BP, which must be considered a maximum age’. Based on this conclusion, Sulpizio et al. (2008) stated that the Avellino eruption has ‘a best estimated maximum age of 3930 ± 20 14C yr BP (4370 ± 40 cal yr BP)’.

The uncertainty on the age of the Avellino eruption might be solved by 14C dating of continuous sedimentary sequences with an intercalated Avellino pumice layer, as for example encountered in the lakes of Monticchio, the Colli Albani and Mezzano. However, 14C ages from such sequences are scarce (Sulpizio et al., 2008) and inconclusive. This is exemplified by the AMS radiocarbon dating on Abies seeds overlying the TM-4 tephra (Avellino pumice layer) in the Monticchio record, which yielded an age of 3920 ± 50 BP (4514–4159 cal BP, 2σ error range; Watts et al., 1996). According to Wulf et al. (2004), the varve-based age of the layer is 4310 BP, given as 4310 ± 220 cal yr BP in Wulf et al. (2008). These ages are considerably higher than the age published in Passariello et al. (2010), but agree with the maximum age of Sulpizio et al. (2008).

These quoted ages illustrate the somewhat confusing way in which 14C dates have been presented in the extensive literature on the tephrochronology of Somma-Vesuvius. Also, conventions regarding the use of units for reported 14C ages were not always properly applied (see also materials and methods).

In this paper, we present a new series of 14C dates obtained from lacustrine sequences with an intercalated tephra layer in the Agro Pontino, which provide a robust date for the Avellino eruption.

Section snippets

Regional setting

The Agro Pontino in Southern Lazio, Central Italy, consists of a higher complex of Pleistocene marine terraces in the SW and an elongated, low-lying graben in the NE, the Agro Pontino s.s. (see Fig. 1). This graben contains Holocene sediments that initially filled in valleys formed during the preceding Glacial, and, with ongoing sea level rise and closure of the Holocene beach ridge near Terracina, gradually expanded over the earlier Pleistocene deposits. In the central part of the graben, the

Sites and materials

In the trench at Via Migliara 44.5 (see Fig. 2a), a thin (ca 2 cm) layer of fine sandy, non-differentiated greyish-white tephra (layer 3) is intercalated in black, pyritic clay (layers 2 and 4). This clay rests on greyish, less organic and non-pyritic reduced clay (layer 1). The layers 2 and 4 are composed of very fine laminae (<0.1 mm), highly indicative for a lacustrine origin. The tephra layer (layer 3) has sharp upper and lower boundaries, lacks internal stratification, is largely composed

Results

Grain-size distributions, presented in Fig. 3, demonstrate that the tephra layers at Migliara 44.5 and Campo Inferiore only partially consist of sand-size tephra particles (24 and 32%, respectively) and that the granulometry of the sand fraction is remarkably similar, pointing to a common origin. Amounts of organic matter, carbonates, pyrite and other materials that dissolve upon the standard pretreatment employed are considerable (18 and 35% resp.), as are amounts of clay to silt-size

Discussion

We will first address the origin of the tephra layers in the context of the depositional environment, then examine the compositional match with the Avellino eruption, and finally discuss our new age constraints for this major event.

Acknowledgements

We are indebted to the Soprintendenza archeologica per il Lazio, represented by inspector dott.ssa Nicoletta Cassieri and her local collaborator dott.ssa Carmela Anastasia, for the opportunity to make stratigraphical observations in the trial trenches at Campo Inferiore di Sezze. Together with the owner of the field, Ing. Mario Gaeta from Setia Solar Park company, they very graciously facilitated the research by keeping open and pumping out the excavation trench. The authors thank H.J.

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