Palaeotsunami deposits at the Tiber River mouth (Ostia Antica, Italy): Do they really exist?
Résumé
In this paper, we test the recent hypothesis of the occurrence of five to seven tsunami generations, that would have struck the ancient harbour basin of Ostia (Italy), and the lower channel of the Tiber River during the last three millennia. Because these steady disaster events would have deep implications on our knowledge of the history of Rome, we reviewed the pluridisciplinary data available at the Tiber River mouth. Considering sedimentological, geomorphological, micropalaeontological, geochemical, chronological and historical evidence, there is currently no conclusive palaeoenvironmental evidence to suggest past tsunami inundations near Ostia . River mouths are not the best context in which to identify tsunami deposits. High fluvial and coastal mobility generated by regular floods and storms hardly record single High-Energy Events (HEE) from floods, storms, or tsunamis. Sediments are regularly reworked at the river mouth both in the river channel and on the close shoreface. Mixed fluvial and marine influences and the seasonal formation of a salt wedge at the mouth of the Tiber create specific estuarine assemblages for micro- and macrofauna. The layer called High-Energy Event 1 (HEE-1) on the palaeo-shoreface close to the river mouth and HEE-4/5 in the point bars of the Tiber channel are most probably layers reworked several times by fluvio-coastal events (storms and/or floods). HEE-3 sealing the Republican harbour of Ostia is clearly related to flood deposits. Complementary analyses would be necessary to definitively identify the origins of the HEE-2 and HEE-7 in the harbour, and HEE-6 in the palaeochannel or floodplain. Based on the data available, we show how other processes than tsunami inundations could be just as accountable for these coarse-grained sediment layers (storm deposit, flood deposit, or riverbank deposit). This review puts into question the use of pluridisciplinary proxies to identify palaeotsunami deposits. In addition, we demonstrate how high Pb concentrations constitute a robust proxy to definitively refute the presence of palaeotsunami deposits. As such, this study will be beneficial to a large community of specialists in coastal research.
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