Feldspar composition as an efficient tool for tephra identification: a case study from Holocene and Lateglacial lacustrine sequences (Jura, France)

Four volcanic ash layers have been identified in Lateglacial and Holocene sediments from three lakes in the Jura Mountains: the Vasset Tephra and the Nugere Tephra (Chaine des Puys), the Vedde Ash (Icelandic), and the Laacher See Tephra (LST; Eifel). Some of these are invisible to the naked eye and have been detected by magnetic susceptibility measurements and X-ray fluorescence analysis. These layers are composed of rare glass shards in association with feldspars and pyroxenes. In lacustrine carbonated sediment formations where glass shards are scarce and altered, the geochemical analysis of feldspar composition appears to be an effective substitute tool for characterization of the tephra. The identification of the Middle LST-C phase in our sites modifies the currently known spatial distribution of the LST (Eifel, Germany). The identification of two tephras from the Chaine des Puys volcanic sources in Jurassian sites, the trachytic Vasset Tephra and the trachy-andesitic Nugere Tephra, provides new time markers for the early Holocene and late AllerOd, respectively, thereby enhancing our knowledge of their distribution throughout Western Europe. More generally, these results confirm the major scientific value of lacustrine sequences from the Jura region for cryptotephra research, especially within Late Quaternary palaeoenvironmental studies.